Prospects for maintaining strength of paper and paperboard products while using less forest resources: A Review

Hubbe, M. A. (2014). "Prospects for maintaining strength of paper and paperboard products while using less forest resources: A Review,"BioRes. 9(1), 1634-1763.

Abstract

Paper production requires large amounts of cellulosic fiber, whereas the world’s forested lands and croplands have a finite capacity to supply such resources. To deal with likely future pressure on forest resources, as well as to hold down costs of materials, publications examined in the preparation of this review suggest that the paper industry will need to implement several concurrent strategies. In particular, the industry can be expected to view recycling as a central part of its activities. Basis weights of various paper-based products can be expected to decrease over the coming decades, and more of the fiber content will be replaced with fillers such as calcium carbonate. Such trends will place intense demands upon chemical-based strategies to enhance the bonding within paper and paperboard. Based on the literature, further progress in reducing the amount of new forest resources used to meet a given set of paper product requirements will require a combined approach, taking into account various fiber attributes, nanostructures, novel concepts in bond formation, and advances in the unit operations of papermaking.

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Prospects for Maintaining Strength of Paper and Paperboard Products While Using Less Forest Resources: A Review

Martin A. Hubbe

Keywords: Decreased virgin fiber utilization; Forest resources; Paper recycling; Papermaking; Dry strength; Filler content; Bonding agents

Contact information: Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005, USA; E-mail: hubbe@ncsu.edu

This review article is motivated by a deceptively simple question: “Can paper products be successfully manufactured with substantially less freshly-obtained cellulosic fibers?” To the extent that this question can be answered in the affirmative, there is potential either to save costs or to decrease future pressure on the world’s forests and other fibrous plants while meeting the requirements of papermakers and their customers. The words “freshly obtained” are included in the question due to the fact that fibers recovered from used paper products can displace some of the need to harvest wood. Emphasis in this review will be placed upon wood resources due to the fact that about 96% of the world’s paper production currently involves fibers from wood (RISI 2012), and therefore a great deal of research attention has been focused on wood-derived fibers. There does not seem to be serious cause to be concerned regarding over-use of certain non-wood fiber sources such as sugarcane bagasse (Khakifirooz et al. 2013) or wheat straw (Singh et al. 2011). In all cases there is still a motivation to increase the amount and strength of paper that can be made from a given amount of fiber resource.

The organisation of this article, after the Introduction, entails four main sections, which deal with (a) factors that limit or hurt the strength of paper, (b) various things that “need to go right” in a papermaking operation in order that the strength of paper approaches its expected level, (c) strategies that show promise to go beyond the present established practices relative to achieving strength goals with less dependence on virgin fibers, and (d) ways in which the unit operations of papermaking might be changed in order to boost strength attributes and allow decreases in the amount of fresh fiber employed.

Whether or not the question posed in the first paragraph is in fact the best question to be asking is a matter that mainly will be left unexamined until near the end of this article. There it will be shown that the published literature is somewhat conflicted regarding the environmental impacts of fiber utilization, including whether recycling of waste paper is environmentally preferable to tree harvesting. Whether and to what extent the utilization of wood resources for papermaking are likely to exceed replacement through photosynthesis are not considered in this article. Evidence suggests that forest resources will continue to be sufficient to meet the near-term needs of papermakers, though not in all regions of the world. While pulping and papermaking practices can have various environmental consequences (Diaz-Balteiro et al. 2011; Demel et al.2012), the underlying technologies are based on efficient utilization of renewable materials. In principle, the manufacture of paper can be highly sustainable, from an environmental perspective. Nevertheless, it is reasonable to examine what options may be available to avoid future overutilization of fiber resources, beyond the level at which they can be replaced through environmentally responsible forestry and agricultural practices.

Main Goals

As has been shown by the authors of publications to be considered in this review article, answers to the question of how to achieve paper strength with less use of virgin fibers will turn out to be complex. The required strength characteristics of various paper-based products often pose challenges if one’s goal is to reduce the amounts of freshly prepared cellulosic material, i.e. virgin fibers. Meeting strength requirements can become even more of a challenge if one considers various ongoing trends, such as increased use of fibers that have been recycled multiple times, greater use of higher-yield, less bondable fibers, greater use of mineral fillers, and decreasing basis weights of various paper and paperboard products (Wielema and Brouwer 2003).

Environmentally friendly approaches

In addition to just avoiding over-use of freshly prepared cellulosic fibers, one also needs to be concerned about net environmental impacts. Paper, when it is manufactured according to conventional practices, is quite durable, easily recyclable, and usually compostable (Hubbe et al. 2010; Kumar 2012). Though replacing paper with plastic or metal could be considered as a way to reduce the usage of virgin fibers, such an approach would not address all the underlying environmental issues. The environmental impacts of various strength-enhancing treatments also will need to be considered on a case-by-case basis.

Ways to use less new forest resources

Because it is to their potential economic advantage, papermakers have been striving – practically since paper’s invention over 2000 years ago – to achieve comparable strength performance with as little as practical of the cellulosic fibers, which usually have represented a major cost item. One of the most straightforward ways to save on fibers is to reduce the basis weight, i.e. the mass of paper per unit area (Gutmann et al. 1993; Nordström and Norman 1995c; Hekkert et al. 2002; Kerman et al. 2009; Li et al. 2012b). Notably, all of the articles just cited, except for that of Hekkert et al. (2002), had a primary emphasis on how to maintain sufficient product strength when the weight is reduced. Though a great deal of research has been focused on the tensile strength of paper, it is important to keep in mind that other attributes of paper strength can be more important, depending on the application. Many end-uses of paper depend on stiffness and/or compression strength. To maintain stiffness, papermakers often attempt to reduced paper’s weight without sacrificing either its thickness or its modulus of elasticity. In the case of paperboard grades, the shear strength within the sheet is often critically important.

Another way to save forest resources is by increasing the yield of the pulping process. In principle if one can use a higher proportion of the raw material in a paper product, then one should expect a corresponding decrease in the demand for the raw material. There are two main categories of pulping processes, mechanical pulping and chemical pulping (Biermann 1996). The pulp from mechanical pulping processes such as thermomechanical pulping (TMP), groundwood pulping, among others, typically comprises over 90% of the starting material, on a solids basis (Sundholm 1998; Kappel 1999). By contrast, the kraft chemical process, which is by far the most dominant pulping method used in the world today, achieves yields in the range of about 40 to 60% of the starting wood material, depending on the wood species and the grade of pulp being made (Biermann 1996; Gullichsen and Fogelholm 1999). Kraft pulping can be very good for the strength of the resulting paper because chemical decomposition and subsequent solubilization of the lignin holding adjacent fibers together in wood makes it possible to separate the fibers from each other in a relatively undamaged condition, conserving the full fiber length. As will be discussed, kraft fibers also tend to have greater ability to form strong inter-fiber bonds compared to most mechanically pulped fibers. The lignin and other materials released from the wood during pulping can be incinerated in a kraft recovery boiler, a process that provides steam energy to help run the pulp and paper operations. By contrast, mechanical pulping operations require huge amounts of electrical energy (Viljakainen 2006).

Increasing the proportion of mineral fillers is a popular strategy that may allow decreased usage of cellulosic fibers. Because mineral products such as calcium carbonate and clay are often less expensive than the fibers that they replace, levels of filler use in various paper grades have tended gradually to increase over a time-scale of decades (Fairchild 1992; Brouillette et al. 2010; Doelle and Amaya 2012). Since the intrinsic densities of CaCO₃ and clay minerals are substantially higher than those of cellulosic fibers, and also because the mineral products often can pack together more tightly, paper having a high mineral content tends to have a high mass per unit volume – a factor that is often considered undesirable by papermakers. To counter such effects, papermakers have adopted various strategies to keep the apparent density of paper relatively low, depending on the grade of paper being made (Moberg 1985).

The recovery of fibers from used paper products is yet another key strategy that papermakers have employed to conserve forest resources or to save costs. The proportion of recovered fiber that is being used throughout the world for paper manufacture has reached about 54% (Pöyry 2011). In some grades of paper such as containerboard and newsprint paper, the rate of recycling is yet higher. However, it turns out that the process of making kraft fibers into paper, using the paper, and then recycling the used papers, often has the effect of rendering the fibers less conformable when wet, depending on the details of their preparation (Weise 1998). Less conformability can contribute to greater bulkiness of the recycled sheet, which is often just what the papermaker wants when making such grades as printing papers, file folders, and the like. But less conformability also can manifest itself as lower inter-fiber bonding strength.

As noted by Niskanen and Kärenlampi (1998), paper tends to break at locations of greatest weakness. The next section will provide some background on factors that tend to either weaken or limit the strength of paper. Later sections of this article then will examine different approaches that have been evaluated for increasing the strength of paper – thus making it possible to achieve more with less fresh fiber. An underlying theme here is that many things need to be done well simultaneously, since a sheet of paper can fail at many different points and for different reasons.

Critical Mechanical Properties

Before considering factors that either limit or contribute to paper’s strength, it is important to consider the widely different physical requirements of different types of paper. This subject has been well covered in the following sources (Scott and Abbot 1995; Mark 2002; Niskanen 2012). Most paper products are required to simultaneously meet several different mechanical requirements. Though tensile strength is among the most commonly evaluated properties, other attributes can be more important with respect to different grades of paper. For example, the mechanical performance of linerboard often can be improved by increasing the tensile stiffness, without necessarily increasing tensile strength, compression strength, or z-directional strength (i.e. delamination resistance). Resistance to cracking during folding is another property that is critical for linerboard, and there is often little correlation between such resistance and other strength properties. Bending resistance can be a critical requirement for paper carton board; often such resistance can be achieved most effectively by use of a low-density middle ply. Also, different paper grades can have very different requirements for the ratio of various strength properties in the machine-direction (MD) versus the cross-direction (CD).

FACTORS LIMITING OR HURTING THE STRENGTH OF PAPER

Factors that Tend to Limit Paper’s Maximum Strength

The subject of paper strength has been considered in great detail in some previous monographs and key articles. Fundamental aspects, focusing on the structure of fibers and how they attach together during the formation of paper, have been considered (Clark 1973, 1978a,b, 1984; McKenzie 1978; Robinson 1980; Fellers 1986; Htun 1986a). Other sources have dealt with the physics underlying paper strength, focusing on the use of models and equations to represent paper structures and to predict their attributes (Page 1969a; de Ruvo et al. 1986; Niskanen et al. 1998).

Based on the types of analyses mentioned above, it appears that one of the most pervasive limitations governing paper strength lies in the inherently finite strength of its component fibers. As has been explained by Page (1969a), the strength of paper can depend on the strength of the fibers as well as the strength of the bonds connecting them. Thus, even if one has done a superb job in bonding the fibers together (more on that aspect later), the paper still can fail if the applied tensile or other force exceeds the inherent strength of the fibers, e.g. the “zero-span tensile force”. Page (1969a) showed that the tensile breaking force (expressed as the length of paper that will break due to its own weight) can be expressed as,

1/T = 9/(8Z) + 12 Ag / [bPL (RBA)] (1)

where T is the tensile breaking length (km), Z is the zero-span breaking length (km, usually taken as an indication of the tensile strength of individual fibers), A is the average fiber cross-sectional area, is the density of the fiber material, g is gravitational acceleration, b is the shear bond strength per unit area of intimate contact, L is the length of a fiber, and RBA is the relative bonded area. The RBA is essentially the ratio between the area of fiber surfaces that are intimately attached to adjacent fibers, compared with the total outer surface area of fibers within the sheet. Notably, the first term in Eq. 1 deals with failure due to the strength of individual fibers, whereas the second term deals with failure due to separation of bonded areas.

Interestingly, Mayhood et al. (1962) observed a similar limiting strength when evaluating the shear bond strength per unit of bonded area between fibers that had been pressed strongly together in the wet state and allowed to dry. The shear strength per unit of bonded area was found to be almost constant, independent of the type of fibers or how they had been prepared. Thus it appears that there may be an inherent maximum in the strength of a well-formed joint between adjacent fibers in paper. More research of this type, quantifying fiber-to-fiber joint strengths, was reported by Görres et al. (1995) and by Fischer et al. (2012). Joshi et al. (2011) reviewed various studies of this type; their tabulation of results indicates that shear bond strengths based on “fiber crossing” failure tests generally fall within a range between 1.5 and about 8 MPa. The tabulated findings also suggest that different fibers achieve different levels of shear bond strength. Also, differences among the results of different studies tend to cast doubt on whether laboratory-prepared crossings between pairs of fibers are equivalent to what can be achieved within a sheet of paper under realistic conditions of papermaking. There is doubt regarding whether the loading of the individual bond, up to the point of failure, is at all representative of what happens when a paper strip is undergoing a tensile test. Indeed, Joshi et al. (2011) obtained higher values of shear bond strength, within the range of 14 to 38 mPa, by using the innovative approach of weakening the fibers in paper sheets by controlled exposure to acid gas. The treatment level was varied such that some of the treated paper showed 100% fiber breakage as a mode of failure along the line of separation.

Ways paper can fail

Paper can fail in different ways, including tensile failure (Page 1969a; Jayaraman and Kortschot 1998), compressive failure such as by crushing or buckling (Seth et al. 1979; Fellers 1983, 1986; Paulapuro and Thorp 1983), and by delamination (Fellers 1986; Skowronski 1991; Kang et al. 2004). These failure mechanisms each can involve both fiber failure and inter-fiber bond failure. As noted by de Ruvo et al. (1986), “Paper is a material with an enormously high degree of heterogeneity containing a seemingly infinite number of cracks of different dimensions.” Thus, the fact that paper can fail is no surprise; rather the challenge is to attempt to come closer and closer to the ideal strength represented by paper’s component parts.

Several authors have considered the concept of “stress concentration,” whereby application of stress to a piece of paper leads to highly nonuniform stresses within the nanostructure of the sheet (Page 1985; de Ruvo et al. 1986; Htun 1986a; Tryding and Gustafsson 2000; Kotik et al. 2005; Considine et al. 2012). As noted by Htun (1986a), the stresses that are manifested at the nano scale, even before force has been applied to the dry sheet of paper, can be attributed to three main sources, the in-plane tensions resulting from drying, additional stresses due to nonuniformities in drying rates, and deformation effects, with emphasis on plastic deformation of the material. One reason why drying tends to impart localized residual stresses within paper is that chemically pulped fibers shrink much more in their cross-direction than in their lengths when they are dried (Salminen et al. 1996). Nissan and Sternstein (1964) proposed that only about 10% or less of the hydrogen bonds within a sheet of paper are under a high degree of stress at any time when tensile stresses are imposed on a sample due to the nonuniformity of paper’s structure and the various orientations of the individual bonds. Kato and Cameron (1999) proposed that as the microstructure within paper becomes more organized, the sheet would be expected to become more vulnerable to failure when subjected to a selected degree of expansion, i.e. strain. By contrast, a more disorganized structure tends to be better able to absorb the energy of deformation in an elastic manner, not giving rise to localized failure. As proposed by Mayhood et al. (1962), it is likely that the initiation of failure involves the detachment of some individual contacts between fibers somewhere within the paper structure. The cited authors noted that such failure was evident when sufficient stress had been applied to approach the end of the elastic region, within which a mainly reversible and quasi-linear relationship between applied stress and observed strain was observable. Acoustic emissions tests have provided one way in which researchers have sought to detect the early events preceding general failure of a stressed paper sample (Yamauchi et al. 1990; Gradin et al. 1997; Salminen et al. 2003; Isaksson and Hägglund 2007). Also, it has been shown that most breakage of individual fibers occurs across the fracture line, and almost no fibers break elsewhere in the stressed sheet (Chiam et al. 2013).

Viscoelastic properties and creep

Although the strength components of typical paper products are usually evaluated at temperatures well below their glass transition points, T_g, aspects of viscous flow still have been shown to be very important in certain of paper’s applications. As reported by Back and Salmén (1982), the T_g values of both lignin and hemicellulose are strongly depressed by the presence of moisture. Thus it was observed that moist wood-based materials can display viscoelastic behavior at temperatures far below the boiling point of water. By contrast, rigid materials, for which the ambient temperature is below the respective T_g values of the components, can be expected not to show any flow behavior and to experience brittle failure (Wool 2008).

The term “creep” is used to denote a progressive distortion of paper’s original shape when tension or compression is applied for a protracted period. Creep behavior can be important when a paper or paperboard product has to sustain a load or stress during its use. For example, the progressive bulging of juice cartons during storage can lead consumers to suspect spoilage of the contents, even if the cause may be more related to the performance of the paperboard. Corrugated containers can gradually sag and eventually buckle when loaded on top of each other in a warehouse (Boonyasarn et al. 1992). DeMaio and Patterson (2007) showed that the creep behavior of paper can be modeled according to the conventional “spring and dashpot” models that are widely used in understanding the behavior of other materials. Best fits to data were found when it was assumed that overall creep behavior was due to parallel processes governing initial creep, primary creep, and secondary creep. The authors attributed the non-elastic component of creep to the flow of the matrix components of cellulosic fibers – the hemicellulose and lignin. Htun (1986a) showed that relaxation of an applied stress, at constant length, could be modeled by parallel processes, one governed by an exponential decay law and the other by a power law. Similarly, Kolseth and Ehrnrooth (1986) showed that creep phenomena could be fitted to a combination of initial and logarithmic rate processes.

An especially worrisome type of creep behavior has been observed in some cases when paper-based products are subjected to continuous stress when the humidity is repeatedly cycled back and forth between low and high values (Fellers 1986; Thorpe and Choi 1991). Thorpe and Choi (1991) estimated that containers kept in typical warehouses are able to support only 25% of the loads that would be expected under constant-humidity conditions. Compressive failure of boxes subjected to such conditions was attributed to progressive bending of fibers until build-up of a critical stress at some point within the structure. Haslach (2000, 2009) has reviewed the use of mathematical models and their application in predicting the failure of boxes. Haslach (2009) found that most such failures were ultimately precipitated by fiber pullout and fiber swing, with the release of localized compressive stresses.

Clark (1942) proposed that the flexible, compliant nature of the bonds between fibers may play an important role relative to the toughness and ultimate breaking strengths of paper materials. The idea is that if paper is connected by a series of flexible joints, then an applied load can be transferred throughout the structure and be shared more evenly. Thus, plastic flow may help to explain why paper does not fail earlier than it does (de Ruvo et al. 1986). In support of this view, de Ruvo et al. (1986) found that there was a low variation of stress within typical paper when placed under tension.

Gimåker et al. (2007) showed that short-time creep can be reduced by favorable selection of the wet-end polymer additive used for strength enhancement. Although cationic starch was shown to enhance other strength attributes of sheets made from softwood kraft pulp, it did not reduce instantaneous or time-dependent deformation. By contrast, polyallylamine hydrochloride, a highly charged cationic linear polyelectrolyte, reduced the amount of creep in comparison to the untreated control sheets.

Factors that Reduce Paper’s Strength Potential

The factors considered up to this point might be considered as a “best-case scenario,” dealing with the limitations on paper strength that are inherent in the fiber materials themselves – even if they are of premium type, quality, and preparation. But to be realistic, a majority of the fibers used by papermakers worldwide are somehow deficient in at least one attribute related to paper strength. To begin with, over half of the mass of pulp used in papermaking comes from the recovery of fiber from wastepaper (Pöyry 2011). Other factors to consider include defects in fibers, losses in the molecular mass of cellulose within the fibers, losses in fibers’ ability to swell when placed back into water, the debonding effects of fillers, effects of contaminants, and changes in paper properties due to humidity.

Strength attributes of fibers recovered from wastepaper

Many studies have documented strength losses when cellulosic fibers obtained from the kraft or sulfite pulping processes are made into paper, then redispersed back into water and formed into recycled paper (Pfaler 1933; Brecht 1947; McKee 1971; Cildir and Howarth 1972; Horn 1975; Gurnagul et al. 2001; Wan et al. 2011). Fuller lists of such studies are provided in reviews of paper recycling (Howard 1990; Ellis and Sendlachek 1993; Nazhad and Pazner 1994; Hubbe et al. 2003, 2007c; Nazhad 2005). The greatest relative losses in the bonding ability of such chemical pulp fibers tend to occur during the first cycle of recycling (Göttsching and Stürmer 1978b; Klungness and Caulfield 1982; Van Wyk and Gerischer 1982; Bobalek and Chaturvedi 1989; Hubbe et al. 2007c; Yamauchi and Yamamoto 2008; Hamzeh et al. 2012). By contrast, mechanical pulps, such as thermomechanical pulp or groundwood pulp used in newspaper production, do not tend to change much from cycle to cycle (Bovin et al. 1973; Howard and Bichard 1992; Alanko et al. 1995; Law 1996). An exception to this rule is that high-yield pulps that have been chemically treated in some way may also show losses of physical properties of later generations of paper (Ma et al. 2012). As noted by Gerspach et al. (1993), there is potential for a given fiber to be recycled multiple times, leading to a complex distribution of age of fibers within recycled paper products.

The physical, chemical, or surface-chemical changes responsible for the differences in bonding properties between virgin and recycled fibers appear to be rather subtle (Wistara and Young 1999; Wistara et al. 1999). According to the cited studies, recycling of bleached kraft pulps appears to entail a modest reduction in hemicellulose content, which may at least partly explain a reduced swelling ability. No substantial changes in surface chemistry or crystallinity were observed. Thus, one might conclude that physical changes, such as kinks, microcompressions, and pore closure, as well as effects of various contaminants, must be responsible for the reported lower strength of recycled kraft fibers.

Refining has been found to be somewhat effective for restoring the bonding ability of used fibers, except that the additional refining can have a strongly adverse effect on dewatering rates (Bovin et al. 1973; Ellis and Sendlachek 1993; Bawden and Kibblewhite 1997; Zhang et al. 2004a). The decreased drainage rates appear to be mainly associated with higher levels of cellulosic fine matter (Hubbe and Heitmann 2007), especially in cases where recycled fibers are subjected to a lot of additional refining in an attempt to reach challenging strength targets (Bawden and Kibblewhite 1997). To make matters yet more challenging, if a relatively high level of refining has been applied during the initial cycle of papermaking, then one can expect there to be less possibility to restore bonding within the pulp during subsequent cycles of papermaking (McKee 1971). Cellulosic fines obtained from recovered paper are sometimes considered “dead fines” due to their reduced contribution to inter-fiber bonding (Laivins and Scallan 1993, 1995; Joseleau et al. 2012). Some authors have even suggested removing some fines and discarding them as a solution to this dilemma (Cardwell and Alexander 1977; Doshi 1998). However, such a discarding of fine material is seldom done intentionally by papermakers due to concerns about costs of materials and a desire to maximize yields from the process.

The best-known and arguably the dominant cause of strength deficiencies in recycled chemical pulp fibers involves a loss in ability of such fibers to swell when placed into water (Lyne and Gallay 1950; Szwarcsztajn and Przybysz 1976). The popular name for such changes is “hornification” (Jayme 1944). The topic has been discussed by many researchers (Lindström and Carlsson 1982; Guest and Weston 1990; Howard 1990; Nazhad and Pazner 1994; Horn 1995; Weise and Paulapuro 1996; Bawden and Kibblewhite 1997; Cao et al. 1998; Scallan 1998; Wiese 1998; Weise and Paulapuro 1999; Hubbe et al. 2003, 2007c; Diniz et al. 2004; Nazhad 2005; Welf et al. 2005; Garg and Singh 2006; Yamauchi and Yamamoto 2008). Gurnagul et al. (2001) proposed that a main effect of hornification involves reductions of the shear bond strength, whereas other researchers have more often attributed the effects to reduced relative bonded area. The severity of hornification effects has been found to increase depending on the extent and intensity of drying (Lundberg and de Ruvo 1978a,b; Pycraft and Howarth 1980; Guest and Voss 1983; Hubbe et al. 2003; Welf et al. 2005), though the main effect has been observed regardless of the conditions of drying of chemical pulp fibers. Baker (2000) notes that the drying of paper, in contrast to the drying of the unrefined pulp, tends to have a more severe negative effect on the reswellability of kraft fibers. Most paper products are dried to the range 2 to 6% moisture content, whereas during the production of dry-lap pulp, the final moisture content can be 10% or higher.

Various factors can influence the susceptibility of different papermaking fibers to the effects of hornification. In the first place, the removal of lignin and some of the hemicellulose from wood material during chemical pulping yields a highly porous structure on a nano-scale. Such pores are susceptible to closure during drying (Stone and Scallan 1966; Berthold and Salmén 1997). Also, a relatively low pH during drying and a relatively low content of residual hemicellulose tend to make the loss of swellability more severe (Lindström and Carlsson 1982). As shown by Hamzeh et al.(2012), fibers from juvenile wood (produced when the tree is younger than a critical age) tend to suffer hornification to a greater degree than mature-wood fibers when subjected to the same conditions of preparation, refining, and papermaking. Jahan (2003) and Sheikhi et al. (2010) showed that effects of hornification can be seen in non-wood fibers. Maloney et al. (1997) showed that some portion of the observed reduction in swellability associated by hornification may already be imparted by wet-pressing, even before the fibers have been subjected to drying. Aarne et al. (2012b) found that cationic polyelectrolytes tended to promote irreversible closure of pores in the cell wall having widths greater than about 20 nm; the results were attributed to the suppression of osmotic swelling effects by adsorption of a cationic polyelectrolyte on negatively charged cellulosic surfaces.

De Ruvo and Htun (1983) described the effects of hornification as being essentially the reversal of refining, a process of locking adjacent fibrillar surfaces within the fiber back together after they had been partly delaminated from each other. Such changes might possibly be associated with the increase in size of cellulose crystalline domains during drying, and the effect appears to be brought about by the coalescence or “aggregation” of adjacent cellulose crystallite surfaces (Iyer et al. 1991; Newman and Hemmingson 1997; Scallan 1998; Kato and Cameron 1999; Newman 2004; Pönni et al. 2012). Newman’s (2004) work with ¹³C nuclear magnetic resonance appears to provide the best evidence that such coalescence of adjacent cellulose crystalline domains can take place during the drying of kraft fibers. Many researchers have concluded that pores within the cell walls of chemical pulp fibers close up in the course of drying, and only some of the pores are readily opened again if the fibers are later placed back in water (Stone and Scallan 1966; Guest and Voss 1983; Berthold and Salmén 1997; Maloney et al. 1997; Park et al. 2006). Such pore closure effectively collapses the reticular structure within the water-swollen cellulose (Stone and Scallan 1966; Maloney et al. 1998a). The closing up of porosity has been confirmed by reduction in the amounts of cationic polymers that can be adsorbed on the fibers from aqueous solution (Gruber et al. 1996; Lee and Joo 2000; Hubbe et al. 2003). Reductions in the wet flexibility of kraft fibers that have been dried at least once have been documented (Ehrenrooth et al.1977; Paavilainen 1993; Tschirner et al. 1998; Cao et al. 1999; Dulemba et al. 1999; Zhang 2004a). All of these changes can be understood based on the inherent difference between a wooden stick and a cotton rope (Hubbe 2010). Both such structures contain partly-aligned cellulosic fibrillar elements. As noted in Fig. 1, in the case of the stick these elements are fused together due to the lignin between the fibers. In the case of the rope, the cotton fibers are able to slide past one another, thus explaining the much greater flexibility of a rope than a stick.

Fig. 1. The difference in behavior between a stick and a rope, which both can be comprised of cellulosic fibers, can be attributed to whether or not the component fibers are fused together.

Wang et al. (2003) proposed a mechanism whereby refining in some cases may be expected to render chemically pulped fibers more resistant to loss of swellability upon drying. The authors found that refining of oven-dried bleached kraft softwood fibers yielded a surface area (by the nitrogen adsorption method) that was 7 to 14% lower compared to never-dried pulp exposed to the same refining treatments. When refined to give the same amounts of water in the cell walls, the previously dried pulps exhibited about 12% less cumulative volume of pores smaller than 10 nm, but a correspondingly larger proportion of volume from pores in the intermediate range of 10 to 500 nm. A mechanism that was proposed to explain these findings is illustrated in Fig. 2. As shown, in the native wood one can picture the cellulose micro- or nanofibrils as being held apart by the non-crystalline polymers hemicellulose and lignin. Upon removal of the lignin and a lot of the hemicellulose during pulping, such fibers become susceptible to fibrillar coalescence upon drying (Rebuzzi and Evtuguin 2006; Pönni et al.2012). Refining can be expected to disturb some of the regularity of alignment of nanofibrils within the structure, thus possibly inhibiting coalescence. Evidence to support such a mechanism involves measurements of the crystalline domain sizes (Wang et al. 2003; Newman 2004). The described mechanism is also supported by measured changes of the wet flexibility of dried and reslurried unbleached softwood kraft fibers as a function of their refining history. Zhang et al. (2004a) reported that the relative decrease in flexibility of the refined fibers was much less than the corresponding decrease in the case of unrefined fibers.

Fig. 2. Schematic illustration of hypothesis to account for an effect of refining the sometimes renders kraft fibers less susceptible to relative loss of flexibility upon drying and rewetting

An alternative or supplementary mechanism to account for hornification involves the possible formation of lactones or ester bonds upon drying of chemical pulp fibers (Diniz et al. 2004). The concept is based on the fact that the biosynthesis of each and every monomeric group within cellulose involves lactone formation. It was proposed that formation of additional lactone or ester groups (Ruffini 1966; Slavic and Kucerova 1967) may serve to cross-link and embrittle cellulose-based structures that are heated (Diniz et al.2004). A serious problem with such a mechanism is that it requires there to be a substantial quantity of carboxylic acid groups present in their protonated form in the fibers. Though Lindström and Carlsson (1982) observed substantial hornification of bleached kraft pulps that were prepared in their protonated form, such hornification did not seem to depend on the content of carboxylic acid groups. And though, in principle, esters can form between carboxylic acid groups and –OH groups within a paper sheet, such reactions generally require temperatures that are well above those that are typically used during the drying of paper (Caulfield 1994; Mao and Yang 2001).

Attempts to overcome or avoid hornification by chemical pretreatments of fibers before the paper is dried have been only marginally successful, even in cases where the chemical treatments produced substantial strength benefits due to their action as dry-strength agents (Higgins and McKenzie 1963; Grau et al. 1996; Sjöström and Ödberg 1997; Gruber and Weigert 1998; Zhang et al. 2001b, 2002, 2004b; Hubbe and Zhang 2005; Mocchiutti et al. 2006; Duker et al. 2008). Early work of this nature carried out by Higgins and McKenzie (1963) established that changes in the swellability and bonding properties of kraft fibers could be minimized if the paper was dried from a concentrated sugar solution. Other, more technically practical types of treatment failed to prevent loss of swelling upon drying and rewetting. One of the most innovative approaches that have been reported in an attempt to overcome the effects of hornification on bonding involved the application of polyelectrolyte multilayers onto dried fibers, thus reinforcing the bonding zones with substantial quantities of adhesive polyelectrolytes (Torgnysdøtter and Wågberg 2006). In principle, if a sufficiently large amount of adhesive is employed, then even rough and non-compliant fibers can be effectively bonded together (Heerman et al. 2006). Many approaches involving polyelectrolyte treatments will be considered later in this article as strategies to make up for and possibly surpass the negative effects of hornification on paper strength.

Strength losses involving cellulose molecular mass

When papermakers wish to predict the quality of a given batch of pulp, one of their most widely used tests is the “viscosity” test, which provides a measure of the molecular mass or “degree of polymerization” (DP) of cellulose within the sample (Nazhad and Pazner 1994). The viscosity of cellulose within fibers is known to decrease during conventional pulping (Suckling et al. 2001) and conventional bleaching practices (Fuhrmann et al. 1996). As shown by Suckling et al. (2001), conventional kraft pulping of pine chips reduced the weight-averaged DP of the cellulose from about 8800 to about 6000 at a final yield of 40 to 50%. With respect to bleaching, Fuhrmann et al. (1996) found that the viscosity of the cellulose in softwood kraft pulp fell from 920 to 850 mL/g in the course of a relatively selective bleaching cycle, as the kappa number was decreased from 15 to 2. A more aggressive bleaching sequence reduced the viscosity to 600 mL/g at a kappa number of 2. The corresponding drop in zero-span tensile test, which is representative of fiber strength, was about 15% in relative terms. To provide some perspective on these values, the DP of cellulose within the crystalline regions of cellulose, obtained after acid hydrolysis, lies between 100 and 400 (Battista et al. 1956; Kolseth and de Ruvo 1986). Meng et al.(2013) showed that related effects can be imparted by the conditions employed during the deinking of pulps; though the molecular mass was not evaluated, the nanostructure of the pulp was profoundly affected.

The cellulose DP also can fall during the aging of paper, especially if the paper has been prepared under acidic conditions (Klungness and Caulfield 1982; Welf et al. 1998; Baty et al. 2010; Zervos 2010). Though it is not possible to “keep all things equal” when evaluating the effects of cellulose DP on paper strength, it is well known that extensive cleavage of cellulose chains during the accelerated aging of paper results in very poor resistance to folding, i.e. extremely brittle paper (McComb and Williams 1981). Kato and Cameron (1999) noted that when paper has been weakened by aging, failure zones tend to be straight and abrupt, consistent with the embrittlement and breakage of fibers, rather than the bonds between them. The fact that paper becomes brittle upon aging, rather than just being weak and limp, suggests a process of coalescence between adjacent fibrils (Newman 2004; Baty et al. 2010; Pönni et al. 2012). To the extent that such a mechanism is involved, it is likely that a reduced molecular mass of the cellulose, due to hydrolysis, can accelerate the process of microfibrillar coalescence by temporarily increasing the mobility of the individual chains.

Elevated temperature – often with control of humidity – is used in so-called “accelerated aging” tests to determine the relative suitability of different paper products for archival use (Zervos 2010). Pronounced decreases in the degree of polymerization have been observed in some cases, especially during the accelerated aging of papers having acidic pH of the water extract (Klungness and Caulfied 1982; Stephens et al. 2008; Zervos 2010). Kato and Cameron (1999) did an analysis to determine whether or not a similar effect might result during the ordinary drying of paper, which entails heating the wet paper in the neighborhood of the boiling point of water. The cited authors concluded that not much chain scission is able to take place during the conventional drying of paper due to the relatively short times during which the paper web is exposed to heat. These conclusions were corroborated by Welf et al. (2005). Some results from the latter study are shown replotted in Fig. 3. The data labeled as “wet heating” in the figure corresponds to 15 minutes of exposure of damp kraft fibers to the specified temperatures within a pressure vessel.

Fig. 3. Effects of temperature on the polymerization degree (as indicated by viscosity) of cellulose in kraft fibers subjected to heating during drying (dotted line, upper) or heating while kept wet (dash-dot line, lower)

Removal of hemicellulose

Another contribution to the loss of the bonding ability of fibers, occurring during various pre-extraction, pulping, and bleaching processes, is the loss of hemicellulose content from the fibers. Pre-extraction of wood chips to remove some of the hemicellulose has been suggested as a potentially beneficial practice, leading to savings in bleaching costs (Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008). Also, the idea is that the extracted hemicelluloses could be somehow converted into liquid fuels or chemical feedstocks. However, especially if a high proportion of the hemicellulose is removed, researchers have observed substantial losses in the bonding strength of the fibers after subsequent pulping and bleaching procedures had been completed (Oksanen et al. 1997; Molin and Teder 2002; Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008). Thus, there appears to be an inherent conflict of interest between bleaching costs, biofuel opportunities, and the bondability of fibers resulting from a pulping process that incorporates pre-extraction of hemicellulose. To the extent that such systems become implemented, they will tend to increase challenges associated with inter-fiber bonding and susceptibility of the paper to brittle failure.

Defects in fibers

In addition to any loss of swelling, conformability, and bondability, i.e. hornification, papermaking fibers also can experience significant mechanical damage at various points during their processing. Such defects, relative to an ideal hypothetical fiber, can include curl, kinks, microcompressions, and weak points (Page 1969b, 1985; Karnis 1993; Omholt 1999). According to Page (1985), a creased or bent section of a fiber cannot be expected to take it full part in transmitting of tensile loading applied to a paper sample. As noted by the cited authors, many such defects can be at least partly reversed by additional refining to swell the fibers and “remove latency”. In other words, the fibers are straightened out to some degree (Le Ny and Messmer 2007). The term “microcompressions” is sometimes used to denote abrupt, localized interruptions in the straightness or smoothness of fibers (Nazhad and Pazner 1994; Niskanen and Kärenlampi 1998). Jayaraman and Kortschot (1998) noted that none of the well-established models used to predict paper’s breaking strength have attempted to incorporate the effects of fiber defects.

Kontturi and Vuorinen (2009) carried out an innovative study demonstrating a mechanism by which stresses occurring during the drying of paper can induce damage to cellulose nanocrystals within the fiber structure. Such stresses were found to make the nanocrystals more susceptible to hydrolytic attack when the pulp fibers were subjected to concentrated sulfuric acid, using conditions suitable for the preparation of nanocrystalline cellulose. It was proposed that the localized stresses during drying induced strains, possibly within disordered regions between cellulosic crystal domains, thus weakening the material at a nano scale.

Mineral particles (fillers) and paper strength

The adverse effects of mineral additives, such as calcium carbonate and clay particles, on paper strength are well known (Bown 1985a, 1998; Tanaka et al. 2001; Li et al. 2002). In general terms, such effects have been attributed to the fact that mineral particles at the surfaces of fibers can get in the way of potential bonding interactions between adjacent fibers. It has been found that relatively small filler particles, which can cover more area of fiber surface with a given mass of mineral, tend to have a greater negative effect on strength (Fairchild 1992; Bown 1998a; Li et al. 2002; Hubbe 2004). In addition, the interface between the cellulosic material and a mineral surface is always abrupt, never involving intermixing of macromolecule segments and microfibrils from both adjacent surfaces into a three-dimensional adhesive zone.

The tendency of fillers to hold fiber surfaces apart, thereby increasing the amount of air spaces within a paper sheet, can play a distinctly beneficial role in those grades of paper that require increased levels of opacity or brightness (Bown 1998b; Hubbe et al. 2008a). Thus, rather than regarding fillers as some kind of “defect”, their role in debonding paper is to some extent unavoidable, at least in the case of paper products that require effective scattering of the incident light. Thus, papermakers frequently face the challenge of providing enough dry-strength additive and enough refining action to achieve both goals – a desired bulking effect, which tends to reduce bonded area in the paper, and also sufficient dry strength to meet product requirements.

Contamination

Issues involving “contaminants” are similar, in a way, to the effects of fillers: Something gets in the way of bonding sites, and there is potential for negative effects on paper’s strength. But in addition, there are problems due to variability. Levels of contaminants present in a paper machine system are unlikely to remain steady, especially if there are variations in the pulp supply, deinking operations, pulping, or bleaching, etc. As noted by Kuys and Zhu (1994), one approach to dealing with variability is to make measurements and attempt to compensate in some way. To give one example, the cited authors recommended the use of online titrations of charge demand of recovered fiber suspensions having variable content of anionic polymeric and colloidal substances. Issues related to charge demand relative to the performance of dry-strength additives and the general operations of paper machine systems have been reviewed elsewhere (Hubbe et al. 2012). Strength variations can, in principle, be compensated for by adjustments in refining, by changing the ratio of fiber types, or by adjustment of wet-end additives. In addition, the process engineers who oversee papermaking operations look for deficiencies in washing operations, process control strategies, and alternative sources of supply in order to minimize problems with contaminants.

Monomeric salts, such as sodium sulfate, can adversely affect the performance of wet-end additives such as cationic starch, which is the most widely used dry-strength agent added to the fiber slurry (Beaudoin et al. 1995; Ulbrich et al. 2012). Such effects become important as the electrical conductivity rises above about 2000 µS/cm. High levels of salt also suppress the swelling of fibers, with a corresponding negative effect on strength (Fält and Wågberg 2003). Though such effects sometimes can be reduced by improvements in washing efficiency (Desharnais et al. 2002; Lind 2011), there is a pervasive trend towards higher salt levels in many paper mills. This is because of efforts to reduced usage of fresh water, i.e. by multiple reuse of the process water within the paper machine system (Chandra 1997; Huhtamäki 2003; Lee et al. 2006). It should be noted, however, that the colloidal and surface-active components often have been found to decrease paper strength properties to a greater extent than inorganic salts (Lee et al. 2006).

Surfactants can have either negative or positive effects on paper strength (Touchette and Jenness 1960; Lindqvist et al. 2009). Negative effects, when observed, can be attributed partly to a tendency of surfactants to accumulate at phase boundaries – some of which are likely to be the potential bonding sites between fibers during the formation of paper. Also, the reduced surface tension, due to the presence of surfactant molecules at the water surface, will tend to decrease capillary forces that draw fiber surfaces together as the paper is dried. Likely sources of surfactants in a paper machine system include deinking processes, carry-over from pulping and bleaching operations, and a variety of stabilizers used in the formulation of chemical additives. The most serious strength decreases have been observed when cationic surfactants are added to paper machine systems (Touchette and Jenness 1960). This finding is not surprising, given the fact that certain cationic surfactants also are also used as fabric softeners – an application in which they help make cotton cloth feel soft, rather than acting like they are tightly matted together. Certain cationic surfactants have been optimized for the purpose of reducing inter-fiber bonding in certain paper tissue and towel products (Asakura 2003; I’Anson et al. 2006). Such debonding agents appear to act by forming hydrophobic patches on the fiber surfaces, so that the net area of bonding is systematically reduced. A similar mechanism, but in an opposite sense, seems to be involved when thermomechanical (TMP) pulps are treated with alkaline solutions (Wegner 1982). Results presented in the cited study are consistent with the removal of hydrophobic wood resins from the TMP fiber surfaces, thus permitting bonding to occur more readily between the more hydrophilic underlying parts of the TMP fibers.

Certain surfactants can have a positive effect on paper strength as a result of their positive effect on water removal during papermaking (Lindqvist et al. 2009). The enhanced dewatering was attributed to a lower surface tension, making it easier for suction forces to remove capillary water from between the fibers in paper as the wet web passes over vacuum flatboxes. Also, with less water held within the web before a wet-press nip, the pressing can be more effective in densifying the paper. The surfactant needs to be carefully chosen to avoid stabilization of foam bubbles and entrained air in the system.

Non-linear effects of basis weight reductions

One of the most challenging aspects of attempts to reduce paper’s mass per unit area, or “basis weight,” while maintaining strength properties, is that the effects tend to be nonlinear. For example, the stiffness of an ideal, uniform, smooth, and isotropic material of constant density can be expected to be proportional to the third power of its mass per unit area (Carson and Popil 2008). Due to the nonidealities of paper, such as the fact that its surface is not completely smooth and contiguous, exponents lower than 3 can be expected to give better fits to stiffness data. Compression strength is another highly non-linear strength property; work reported by Fellers (1983, 1986) has shown that the mechanism tends to shift from one of crushing to one of buckling as basis weight is reduced in a critical range. In both of these instances it would be necessary to increase the elastic modulus (or some other intrinsic measure of strength of the material) by a greater relative amount compared to the desired reduction in basis weight. For instance, one can calculate that to maintain stiffness while decreasing the basis weight by 10%, the Young’s modulus would need to be increased by a factor of about (1/0.9)³ = 1.37, i.e. about a 37% increase in modulus of elasticity. Another option would be to keep the thickness and modulus constant and increase the specific bonding strength such as to be able to use 10% less material. In theory, an increase of about 36% in the bonding strength might make this possible.

Humidity and moisture

High humidity situations pose some particularly difficult challenges relative to paper’s strength (Zauscher et al. 1996, 1997). Even in cases where exposure of the paper to liquid water is avoided, high-humidity conditions can cause the paper to lose strength, as well as to stretch or curl as it is used in various applications (Rutland 1992; Taylor 1999; Green 2000).

According to Salmén (1986), all of the main components of wood exhibit rigid behavior under strictly dry conditions unless temperatures are raised above 180 ^oC. However, in the presence of water the lignin becomes rubbery within the temperature range 80 to 90 ^oC, and hemicellulose is already swollen by water at room temperature. High humidity conditions are sufficient to bring about large changes. The viscoelastic nature of the hemicellulose and lignin within wet or moist wood material at moderate levels of heating make it possible to carry out such processes as thermomechanical pulping (Li et al. 2011) and the steam-bending of wood (Wright et al. 2013). Meanwhile, though it would be reasonable to expect amorphous regions within the cellulose structure also to swell, evidence suggests that such regions, to the extent that they exist, must be no longer than 4 to 5 anhydroglucose units (Nishiyama et al. 2003). Major distortions in the length dimension of cellulosic materials are prevented by the extensive presence of crystalline domains, through which most of the cellulose polymer chains must pass at least once.

Even when paper is dry, under typical conditions of relative humidity, it often contains between about 5% and 8% of moisture content. Some researchers have suggested that the association of such water within the fibers of paper is like a hydrate, something analogous to hydrated forms of minerals (Campbell 1934; Joubert et al. 1959; Clark 1984). The term “hydrate” is also supported by the generation of heat when water is added to dry fibers (Maloney et al. 1998b). It is not clear to what extent such tightly bonded water can participate in the plasticization of hemicellulose and lignin within papermaking fibers.

The transverse shrinkage and reswelling of a kraft or sulfite fiber during drying and rewetting is typically within the range of 10 to 30% (Page and Tydeman 1962; Larsson and Wågberg 2008). Meanwhile, the length of fibers shrinks by only 1 to 2% upon drying. This contrast in shrinkage tendencies gives rise to some unique effects in paper due to the multiple and intimate contact zones between fibers oriented in different directions from each other. Lateral shrinkage of one fiber has been shown to “crimp” and thus reduce the effective length of a fiber to which it is becoming bonded during the drying process (Page and Tydeman 1962; Baum 1991; Niskanen and Kärenlampi 1998; Kiiskinen et al. 2000). Figure 4 illustrates the crimping of fibers within bonded areas, as well as suggesting the effects of such operations as delignification, pulping, and refining on the conformability and ribbon-like shape of kraft fibers in a sheet of paper.

Fig. 4. Illustration of the layered structure of fibers in wood, as well as changes in the shape of fibers in response to kraft pulping, refining, and the drying of a sheet of paper

Drying-induced distortions in the shapes of fibers, including the development of “skirts” adjacent to bonding zones, have been documented (Nanko and Ohsawa 1989; Uesaka and Qi 1994). Such distortions were explained schematically by Lindström et al. 2005). Schmied et al. (2013) observed that kraft fibers that had been dried in 90-degree contact with each other, then detached by normal force, tended to be smoother in formerly bonded areas, presumably because the adjacent fibers tended to restrain cross-directional shrinkage and related wrinkling. They also observed a higher amount of dangling fibrils in the transition zone between bonded and non-bonded areas after separation of the fibers. The fact that these dangling fibrils had been involved in inter-fiber bonding was evident from the force-distance curves, which indicated progressive failure of bridging elements, one after another. The participation of fibrils in reinforcing inter-fiber bonding is also supported by high-resolution microscopic images of paper surfaces (Nanko and Ohsawa 1989; Fatehi et al. 2009b; Schmied et al. 2013). Fatehi et al. (2009b) observed many fibrillar attachments among fibers in paper formed from bleached sulfite softwood fibers that had been refined. Remarkably, no such fibrillar attachments could be seen in a micrograph representing the same system, but with treatment of the fibers with a high-charge cationic polymer before sheet formation. Presumably the cationic polymer neutralized the charge of the dangling fibrils, causing them to lie down on the wet fiber surfaces.

Some of the dimensional stability problems associated with paper can be traced to the fact that the extent of crimping, as well as its relaxation, are affected by changes in humidity. Indeed, as shown by Uesaka and Qi (1994), the crimping mechanism helps to explain why paper having a high degree of interfiber bonding tends to be more dimensionally unstable and subject to curling when the humidity changes. As noted by Kershaw (1980), a pronounced shrinkage of paper in the course of drying commences just as the amount of water remaining in the paper reaches the fiber saturation point, i.e. almost all of the remaining water is within the cell walls of fibers. An additional contribution to the dimensional instability of paper is contributed by the cellulosic fines; according to Przybysz and Czechowski (1985) the dimensions of fines generally have a greater proportional response to changes of humidity in comparison to fibers.

CONTRIBUTIONS TO STRENGTH IN CONVENTIONAL PAPER

Taking Inspiration from the Structure and Performance of Wood

Think for a moment about how truly remarkable are the strength and durability of a typical tree in a forest. Stand next to one that has a trunk about the same size as your own, and you will notice that it towers way above your height. It can stand there patiently, without seemly becoming tired for years and years, while being exposed to occasional strong winds and alternations between freezing cold (in temperate climates), to sweltering heat, and from parched dryness to long stretches of heavy rain. Try holding your arm out perpendicular from your body for even 30 minutes; the tree can do the same for 30 years or more. How does the tree manage to achieve such feats of strength? Clearly it has a lot to do with the details of construction at many different dimensional levels. And because most paper is largely composed of wood-based material, some of what can make paper strong must come from the detailed construction of the woody material itself.

The challenge of understanding and explaining the strength of paper has been taken up in a series of notable articles that generally fall under the classification of “paper physics” (Forman 1950; Page 1969a; de Ruvo et al. 1986; Fellers 1986; Kolseth and de Ruvo 1986; Deng and Dodson 1994; Jayaraman and Kortschot 1998; Niskanen et al. 1998; Retulainen et al. 1998). As noted by these authors, some of the essential aspects that enable the preparation of paper with suitable strength attributes for various applications include macroscopic dimensions, such as fiber length, coarseness, and cell wall thickness. Aspects of the nanostructure are important, including microfibril angle, pore structure, fibrillation (both internal and external), and various organizational aspects such as the uniformity of formation, the presence of fiber flocs, preferential orientation of fibers, and the arrangements of fibrils and cellulosic fines in paper.

To summarize some general findings from the sources cited above, it can be hypothesized that a suitably strong, stiff, but resilient sheet of paper, capable of meeting the requirements of a range of applications, can be created as long as the following criteria are fulfilled:

Extensive crystallinity within the cellulose component of the fibers contributes to rigidity and resistance to flow; these are attributes that are important in many paper products.
Amorphous materials, such as the hemicellulose component of fibers contribute sufficient compliance to reduce the susceptibility of paper to brittle failure. It is also reasonable to expect that disordered nanodomains within the cellulose component can contribute to the flexibility of the material.
There is a well-adapted structure with suitable orientation of cellulose chains, fibrils, and organization of the “matrix” components of lignin and hemicellulose to achieve resistance to failure when stresses are applied to fibers within paper in different directions.
There is a suitable degree of fiber lumen collapse (so that fibers can be more ribbon-like when present in paper), as well as sufficient straightness of the fibers (except when one wants to prepare paper having high extensibility).
There is an effective bonding system capable of joining the fibers together with sufficient strength and flexibility, noting that such bonding may involve such factors as the compliance and fibrillation of the fibers, a contribution from the hemicellulose component, and contributions from various dry-strength additives.

Macrostructure

The effects of the dimensions and arrangements of fibers within paper, relative to measurable properties, have been considered in several key studies (Van den Akker 1959, 1969; Page 1969a; Robinson 1980; Fellers 1986; Rigdahl and Hollmark 1986; Jayaraman and Kortschot 1998; Niskanen and Leskela 1997; Yan and Kortschot 1997; Niskanen et al. 1998; Considine et al. 2012). Some issues to consider initially are fiber straightness, the activation of fiber segments, and visco-elastic effects. The cited studies have shown that it is useful to consider paper as being composed of layers of flattened ribbons arranged mainly in two dimensions. According to Page (1969a), the usual number of crossing points of other fibers on one selected fiber is in the range of about 20 to 40. Thus, even though an individual fiber-to-fiber bond is expected to be much weaker than a fiber, the accumulation of multiple bonds in a well-bonded sheet of paper can be sufficient so that fiber breakage can occur as a major mechanism during tensile failure of a well-bonded sheet. When a strip of paper is being stretched, the force acting along an individual fiber in the loaded network depends strongly on location relative to the fiber’s mid-point. The force typically rises from zero at each free end of the fiber to approach a broad maximum at the middle of the fiber. An increase in the number of fiber-fiber bonds will decrease the distance along the fibre required to approach the asymptote. The original derivation is due to Cox (1952).

Fiber dimensions

Depending on the species, genetic differences, the maturity of a tree, and various aspects of growing conditions, the fibers can be quite different in size. To some extent such differences can be the deciding factor governing the selection of a suitable fiber composition for different types of paper product. Softwood fibers, which are often 3 to 4 mm in length and about 30 µm in thickness (Biermann 1996; Nanko et al. 2005), can be a good choice when making products that need to resist tearing during their use. By contrast, hardwood fibers, which are often about 1 mm in length and about 20 µm in thickness, are generally preferred for products that need to be smooth and relatively uniform, such as fine printing papers. Fiber length can be critical in light of the multiple contacts between fibers, as mentioned in the previous paragraph. If fibers are too short, then they can be more easily pulled away from each other when tensile stress is applied to the paper. But there is also a penalty for high fiber length; longer fibers are more prone to tangling with each other and forming flocs, which hurt the uniformity of paper. Another important attribute is the cell wall thickness. Thin-walled fibers tend to be more conformable than thick-walled fibers, often leading to greater relative bonded area and higher inter-fiber bonding strength. But thicker-walled fibers tend to be tougher, more tear-resistant, and more capable of withstanding physical abuse. Extensive information about the dimensions of commercially available pulp products is provided by Nanko et al. (2005).

Fiber straightness

The relative straightness of the fibers can be expected to play a big role relative to how paper responds to stresses (Page 1985). In a tensile mode, the force required to straighten a fiber can be expected to be much less than the strength required to break the fiber. In addition, when subject to compression, bent fibers will tend to buckle rather than to be crushed (Fellers 1986). In applications where paper might fail in a tensile mode, curled fibers are known to bring about failure of the sheet at lower applied force (de Ruvo and Htun 1983; Rigdahl and Hollmark 1986). Fellers et al. (2001) showed that straight bleached kraft fibers were much more effective for the reinforcement of mechanical pulp paper in comparison to curled fibers of the same type. The straight fibers developed higher tensile breaking length than the curly fibers used for reinforcement. Fracture mechanics theory was used to explain the results. Paper products that need to be stiff during their application – such as printing papers – are held under considerable tension during drying by control of the rotational speeds of various rolls and dryer cans (Halme 1967; Maus 1967). Typically the velocity of a paper sheet at the dry end of a paper machine is 5 to 9 percentage points higher than the corresponding value at the forming table (Kershaw 1980); the difference is due to stretching of the sheet during pressing operations and as a result of machine-directional tension during drying (Baum 1991). Likewise, Halme (1967) estimated that paper typically is stretched by 6 to 8% between the forming section and the exit of the final press nip. Such processes tend to straighten out papermaking fibers during the formation of paper, especially in the case of fibers oriented predominantly in the machine direction.

Page (1985) proposed that kinks and microcompressions within fibers tend to be straightened out in the process of refining fibers that previously had been subjected to drying. The straightening was attributed to both the swelling of the fibers during beating and the tensile forces exerted on individual fibers. Page cited evidence that the strength benefits achieved by refining can be greater than what can be attributed to the observed improvements in bonding strength between fibers in the sheet. A pair of micrographs of a single bleached softwood kraft fiber, comparing the shape of the same fiber before and after refining, showed that the kinks or microcompressions had been effectively removed.

It should be noted, however, that curled fibers can be advantageous when the intent is to prepare relatively weak, bulky structures such as tissue paper or fluff pulp fibers to be used in disposable diapers; for such purposes it is a common practice to employ fresh-dried kraft fibers, which tend to be much more curled in comparison to fibers tried under tension in sheet form (de Ruvo and Htun 1983). As demonstrated by work reviewed by Page (1969b), microcompressions, leading to higher extensibility of the resulting paper, can be induced by carrying out refining at higher consistency levels. Steenberg (1949) noted that paper formed from fibers that are “creased” so that they include a bend between adjacent crossing points with other fibers, can be expected to withstand a greater degree of shock. Thus, when papermakers set out to make tough bags to hold cement and similar products, the processing conditions are optimized to maximize the extensibility of the paper (Edwards and Edmunds 1968; Vishtal and Retulainen 2012; Zeng et al. 2013). Such products are typically prepared with a minimum of tension applied during drying, thus allowing the paper structure to shrink and giving it more potential to be later stretched without breakage. Also, tiny bends can be introduced to the paper web during paper processing to achieve related effects (Clupak Inc. 1967).

In the case of cross-machine dimensions, it is possible to somewhat decrease shrinkage of paper during the drying process by control of tension of dryer fabrics, which hold the damp sheet firmly, but intermittently against the steam-heated cylinders (Kiiskinen et al.2000; Linna et al. 2002). In ordinary cases the sheet can be expected to shrink in width by at least 2% (Clark 1978c), and in theory by an amount in the range of 1 to 10% (Heikkilä and Paltakari 2000), during drying. Meanwhile the thickness of the paper, which is not subject to any restraint, shrinks by about 30 to 40% (Heikkilä and Paltakari 2000). Hansson et al. (1989) showed that cross-directional stiffness and compression strength properties could be improved by an innovative device that prevented cross-directional shrinkage by attaching the edges of a paper web to belts at each side of a paper machine dryer system. A more commonly applied approach, which has been motivated by a desire to minimize web breaks, has involved use of a single dryer fabric, especially in the first dryer section of a paper machine (Kuhasalo et al. 2000). As a paper web passes through such a dryer section it remains in contact with just one fabric, rather than having an open draw between each successive dryer can. Such a system at least offers the possibility that friction or temporary adhesion to the fabric surface can inhibit cross-directional shrinkage.

Applied tensions, especially in the machine direction, are believed to minimize the amount of crimping and skirt formation at crossing points between fibers in the paper (Uesaka and Qi 1994). Giertz (1964) introduced the term “activation” to describe a process in which a combination of applied tension, together with the natural shrinkage of fibres during drying, can result in an increase in the proportion of fibers in paper that are straight enough in their segments between crossing points that they fully participate in bearing stress loads. Lobben (1975) used the concept of activation to explain the tensile stiffness of paper. Page (1985) suggested that activation may partly explain the strength benefits of refining once-dried kraft fibers. Vainio and Paulapuro (2007a,b) confirmed the activation concept and showed that fiber segments tend to become straightened when the web is constrained from shrinking during the drying process. Vainio et al. (2007) showed that fines from thermomechanical pulp promoted activation by effectively shortening the distances between fiber crossing points within paper. In addition, the TMP fines contributed to more resilient attachments within the wet web, making it easier to strain and thus activate the network without causing too much damage to the fiber-to-fiber bonds. All of these effects promote load-sharing within paper and contribute to higher tensile strength values. Likewise, Retulainen et al. (1998) used the term “the Jentzen effect” to describe the irreversible changes within a paper sheet when tension is applied during drying; such effects can include the straightening of fibers and the pulling out of localized wrinkles. Jenztzen (1964) considered the effects of drying on the properties of individual fibers when dried under tension. Such effects can increase the tensile modulus of paper, especially in the machine direction.

Development of bonded area

One of the most unique aspects of the papermaking process is the manner in which adjacent fibers become bonded very tightly to each other over wide areas when the damp sheet of paper is dried. Key theoretical aspects underlying the process of bond formation were explained already by Campbell (1947, 1959) and Forman (1950). The most important aspect of the theory is illustrated in Fig. 5. As shown, one envisions a meniscus of water at a crossing point between two of the fibers, which are represented as planes.

Fig. 5. Diagram of ideal meniscus between smooth planar parallel surfaces, showing the two radii or curvature corresponding to the Young-Laplace equation (Eq. 2).

As water gradually evaporates from the meniscus in the course of drying, a very strong capillary force of attraction draws the fibers so close together that hydrogen bonds can be established (Forman 1950; Campbell 1959; Davison 1980; Robinson 1980). The maximum force can be estimated based on the following assumptions:

For the sake of simplicity, one treats the two fiber surfaces as if they were perfectly flat and smooth. This turns out to be not a bad model, since the capillary forces are generally large enough to overcome effects of roughness on the surfaces, bringing them into intimate contact. Also, the refining operation renders chemically pulped fibers sufficiently compliant that the lumen spaces collapse, resulting in a ribbon-like shape (Jang and Seth 1998).
The interfacial tension that can be measured by standard techniques is assumed to apply. Though pure water has a value of about 72 mNm^-1 at room temperature, lower values can be expected under the situations of interest due to the presence of surface-active substances and higher temperatures.
It is assumed that the liquid readily wets the surfaces, so that the contact angle is significantly less than 90 degrees. Sometimes a value of equal to zero is assumed as an approximation.
The negative pressure within the meniscus is assumed to be equal and opposite to elastic forces acting within the solid material. Such forces act to restore the initial shape that the fiber surfaces had before being subjected to high compression in the wet press and resulting from evaporation of water from each meniscus. Such forces first become prominent when the wet web emerges from the first press nip, such that the capillary forces continue to hold adjacent fibers close together, squashing the original topographies of the two surfaces.
Finally it is assumed that the outer circumferential border of the meniscus zone exhibits considerable resistance to slippage. Such behavior is consistent with the generally water-loving nature of cellulosic materials, along with the effects of roughness and porosity of the surfaces. Also, as shown by Moss and Retulainen (1997), fiber fines also can play a role in preventing retraction of the bonded area during the drying process.

To the extent that Fig. 5 can be used to model what happens during the drying of paper, the negative pressure (or “vacuum”) within the meniscus zone can be expressed by the Young-Laplace equation (Page 1993; Chen et al. 2006),

where is the air-water interfacial tension, is the contact angle, as measured through the liquid phase, R₁ is the tight radius at the edge of the meniscus, and R₂ is the much larger radius around the outside of the meniscus. One assumes that R₂ becomes very much greater than R₁, especially as water evaporates and the film of water gets very thin. In addition, for simplicity, the value of is set to zero. On this basis it is justified, as an approximation, to simplify Eq. (2) as,

The negative pressure predicted by Eq. 3 tends toward infinity as the last water is evaporated from the meniscus, causing the parameter R₁to approach zero. Whether the actual pressure differential becomes as high as the predicted maximum value will depend on such factors as the elastic response of the materials, as they attempt to restore the original somewhat rough topographies of the fiber surfaces in the zone of close contact. If one makes the further assumption that the thickness of the meniscus near to its edge is twice the value of R₁, it follows that,

where x is the thickness of the wetted film. Though it is reasonable to expect that Eq. 4 ceases to be accurate long before the last water has evaporated, the following evidence supports the existence of extremely strong capillary forces drawing fibers together:

The crimping action described by Lyne and Gallay (1950) requires that the fibers must for some reason be very strongly joined together already before the sheet is dried.
Studies have shown considerable damage and delamination throughout zones of inter-fiber bonding in cases where the fibers were forcefully detached from each other in the dry state (Page 1960; Sachs and Kuster 1980; Nanko et al. 1989; Stratton and Colson 1993). Such damage indicates that molecular contact had been established over wide areas.
Capillary forces within nano-sized channels have been shown to be sufficiently strong to cause mechanical failure (Tas et al.2010).
It is hard otherwise to account for the wet-web strength of paper, even before evaporative drying has commenced.
Assessments of the relative bonded area carried out using visible light of different wavelengths have been found to all give the same results (Page 1960). This observation is consistent with a process that fully closes up any gaps between the adjacent cellulosic surfaces.

In addition to the Young-Laplace contribution to capillary forces drawing cellulosic fibers together after the wet-pressing of paper and during drying, other components of capillary force also have been considered relative to the forming of paper (van de Ven 2008; Tejado and van de Ven 2010). For instance, Fig. 6 illustrates a component of capillary force that helps to explain the initial densification of a paper web on a Foundrinier section of a paper machine up to the point at which air is being pulled through the sheet by application of vacuum from below.

Since no curvature of the meniscus is assumed, the capillary force is given by (van de Ven 2008),

where is a coefficient with a value near to 1, is the interfacial tension, and D is the fiber diameter. When summing up all the expected fiber crossings in paper, this component of force was considered by van de Ven (2008) and judged to be at least ten times too small to play a major role in the development of inter-fiber bonding.

Fig. 6. Illustration of capillary forces acting to densify a wet web of paper as water is being drained out by vacuum and suction from below

Regardless of the detailed mechanisms, the effects of drying on paper properties are dramatic. Figure 7, which is redrawn from the data of Nordman, Levin and Visti, as reported by Kuhasalo et al (2000), shows an exponential rise in paper’s tensile strength in the dry solids content range between 40% (after pressing) and about 92% (not far from the completion of drying). The right-hand frame of the figures shows that the toughness of the paper, as represented by the tensile energy absorption, went through a distinct maximum at a dry solids content of about 85%. Such behavior is consistent with the decreasing stretchability of paper as it approaches dryness.

Fig. 7. Tensile stress to breakage (left) and tensile energy absorption to breakage (right) as a function of dry solids content of paper (redrawn from Kuhasalo et al. 2000)

Factors affecting the compliance and bondability of fibers

In addition to the critical role played by capillary forces, some other key factors in the formation of bonded areas during papermaking are wet-pressing, the relatively compliant nature of cellulosic fibers that have been prepared by kraft pulping, the further increases in wet-flexibility brought about by refining, and ultimately, the substantial plasticization of hemicellulose and possibly the disordered regions within the cellulose component when exposed to near-boiling temperatures as the damp paper web passes through the dryer section of a paper machine.

The importance of the wet-compliance of a fiber surface in achieving strong bonding has been supported by several studies. As noted by Guest and Weston (1990), paper made from recycled kraft fibers – which tend to be less compliant when wet than freshly-prepared fibers of the same type – generally yield weaker, bulkier sheets, if all other factors are kept constant. In the course of research carried out by Zhang et al. (2004b) some unpublished micrographs were obtained showing a pronounced twisting of kraft fibers after they had been formed into paper, then rewetted with water (see Fig. 8). When such fibers were formed again into paper, without further refining, they yielded bulkier and weaker sheets.

A number of authors have reported strong correlations between paper’s density and its bond-related strength properties (Robinson 1980; Rigdahl and Hollmark 1986; Guest and Weston 1990; Yamauchi and Hatanaka 2002; Andreasson et al. 2003; Asakura 2003). Batchelor and Kibblewhite (2006) found that there is a linear relationship between bonded area and sheet density, provided that fiber cross-sectional area is kept constant. The relationship between sheet density and bonding is considered in detail by Niskanen and Kärenlampi (1998), who showed that greater increases in bonding can be achieved when the paper’s apparent density is increased by increasing refining, rather than by application of higher wet-pressing loads. In addition, the densification of dry paper in a calendering nip, as a means of achieving a smoother surfaces, generally does not contribute to bonding. In fact, damage to bonded areas and/or the fibers themselves during calendering tends to reduce the strength of the paper (Hestmo et al. 2002; Burnett and l’Anson 2003).

Fig. 8. Micrograph of unbleached softwood kraft fibers that had been made into paper, oven-dried for 12 hours at 105 ^oC, then subjected to standard repulping but no further refining (from the work of Zhang et al. 2004b, but not published at the time)

The relationship between the porous structure of pulp fibers and paper strength was considered by Andreasson et al. (2003). Their study documented increases in the pore volume within wet papermaking fibers as a consequence of lignin removal during pulping. A maximum in pore radius was found at 46% yield when pulping softwood fibers by the kraft process. Further pulping tended to reduce pore sizes, suggesting the commencement of a general relaxing and contraction of the cell wall structures upon removal of most of the lignin and high proportions of hemicellulose from the never-dried fibers. Greatest paper strength was likewise observed at intermediate yield. The authors concluded, however, that the higher strength of such sheets was at least partly attributable to the greater negative surface charge density at the intermediate yield levels. The presence of carboxylic acid groups in the hemicellulose component promotes the swelling of pulp – which is another means by which fibers become flexible and compliant.

Refining and fiber contact

Refining is clearly one of the most important tools to increase the bonding between cellulosic fibers. Already in 1934 Campbell noted that passing a slurry of fibers between a rotor and stator, thereby applying compressing and shearing forces, has the effect of increasing the swelling or “hydration” of fibers. Thode and Ingmanson (1959) found that the strength of paper tended to increase with increases in the swollen specific volume and external surface area of the fibers. Studies have shown that refining tends to increase the swelling of fibers with water, i.e. the water retention value; such swelling has been correlated with increased conformability of the fiber surfaces, leading to higher inter-fiber bond strength (Jayme 1944; Ingmanson and Thode 1959; Zhang et al. 2004a).

Based on microscopic inspection, one of the most apparent changes that occurs upon refining of kraft fibres is the unraveling of fibrils from the fiber surface. Clark (1978b) proposed that such fibrils play a dominant role in the formation of inter-fiber bonds. Such a view has been partly refuted more recently by studies in which fibers still retained their bonding ability when cellulase enzyme treatment was applied under suitable conditions to remove much of the external fibrils (Bhardway et al. 1997; Mansfield and Saddler 1999). Thus, it is likely that a more important aspect of refining is its ability to impart so-called “internal fibrillation” to fibers (Retulainen et al. 1998). By internally delaminating the fibers, they naturally become more flexible, as was mentioned earlier. Page (1985) proposed that at least some of the improvement in strength that results from refining of previously-drived fibers may be related to fiber straightness. On the one hand, conventional refining was observed to remove some kinks and curl from fibers, possibly as a consequence of increased swelling. In addition, the subsequent greater shrinkage during the drying of refined fibers might provide greater opportunity for the fibres to be drawn into a straight condition by the time their shape becomes set in the sheet.

Because the cellulose chains and fibrils that contribute to the thin outer-most layers of a woody fiber (i.e. the P and S1 layers) generally wrap themselves around the circumference, they tend to restrain swelling of individual fibers suspended in water. So another key function of refining, from the papermaker’s standpoint, is that it essentially breaks up those layers, thus allowing the fibers to take up water and become susceptible to internal delamination. It follows that substantial external fibrillation, often leading to the release of fine material, can be considered as an inherent step in the process of getting fibers ready to be formed into paper.

Wet-pressing and the development of bonding

There are many similarities to the effects of refining and of wet-pressing operations during papermaking operations. Wet-pressing, which has been well described elsewhere (Atkins 1979; Back 1987; Mathur et al. 1987; Reese 1991; Paulapuro 1993), can be viewed as another approach that results in densification of the sheet and increases in relative bonded area between fibers. An attractive feature of increased wet-pressing, as a strategy to increase paper strength, is that it can result in a lower moisture content of the paper web, which often implies a lower drying energy and a higher rate of production. By contrast, increasing the level of refining can be expected to increase the fines content of the furnish. In the case of a drainage-limited paper machine a slower drainage leads to a lower production rate. The extent of pressing on a given paper machine is often constrained by mechanical factors. In such cases there are a variety of upgrades that have been used in newer paper machines and rebuilds. As will be discussed later in this article, these options include entended-nip presses that provide a longer time for dewatering to take place, while not exceeding levels of pressure that would damage the paper. Conventional wet presses may apply as much as 200 kN/m of linear load, whereas extended nip presses (shoe press) with linear loads up to 1500 kN/m are in industrial use (O’Brian 2002).

Effects of fines on bonding

The degree to which cellulosic fines contribute to paper strength is a somewhat complex subject, in light of fines’ strongly negative effect on dewatering when present at relatively high levels in the production high basis weight paper and paperboard (Hubbe and Heitmann 2007; Chen et al. 2009). The effects of fines on sheet properties also can depend strongly on the type and bondability of the fines (Niskanen and Kärenlampi 1998). Przybysz and Czechowski (1985) proposed that fines can intensify the capillary effects described by Campbell, thus leading to development of stronger bonds. Some of the most persuasive and interesting evidence supporting the importance of fines was reported by Brecht (1947). His work showed that whereas freshly prepared fines from the refining of kraft pulps can have a strong beneficial effect on paper strength, the fines present in recycled furnish generally do not. This effect was demonstrated by fractionating recycled stock and replacing the “old fines” with “new fines”. Properties equivalent to those obtained with virgin kraft pulp in many respects were obtained. Similarly large contrasts in the properties of freshly prepared versus recycled fines have been reported more recently by others (Laivins and Scallan 1993; Olejnik et al. 2012).

Formation uniformity

The importance of the uniformity of formation on paper strength has been quite well demonstrated, a least on a scale associated with fibers and fiber flocs. Fundamental works and review articles have described key aspects (Linhart et al. 1987; Deng and Dodson 1994; Niskanen and Leskela 1997; Niskanen and Kärenlampi 1998). Also, progress has been achieved in terms of mathematical models to bridge the gap between paper structure and the damage leading to breakage that occurs on a local level within the structure (Korteoja et al. 1997; Niskanen and Leskela 1997; Kotik et al. 2005; Hagglund and Isaksson 2006). Certain studies that set out to demonstrate benefits of dry-strength additives have shown, probably to the surprise of the researchers, that some effects of the additives could be accounted for based on changes in the uniformity of the resulting paper (Roberts et al. 1986; Kim and Hubbe 2010). A study by Linhart et al. (1987) provides a particularly persuasive demonstration showing how the addition of retention aids, depending on their type and dosage, reduced the tensile strength of handsheets in a manner that was highly correlated with changes in the degree of fiber flocculation in the sheets. Another way to carry out such a demonstration is to vary the time between agitation of fibers in a handsheet deckle and the drainage of water to form the sheet; delayed drainage produces a much more flocculated sheet that has much lower strength (Norman 1965; Niskanen et al. 1998). Retulainen et al. (1998) observed that certain effects of formation uniformity on paper strength may become amplified when paper is strained in the wet condition. As was noted earlier, paper tends to be stretched lengthwise by several percentage points between when it leaves the formation section and when it is reeled up (Halme 1967; Kershaw 1980).

Despite the evidence cited above, the relationship between the uniformity of formation and paper strength can be complex. In general a favorable effect of uniform formation is observed when the formation is improved by reducing the consistency of the headbox furnish. This has been shown in the twin-wire roll forming of kraft pulp (Hallgren and Lindström 1989) and recycled pulp (Nordström 2003f). Improved formation through self-healing during dewatering is also associated with an increase in tensile strength (Nordström 2003d). By contrast, tensile properties can be adversely affected when the improved uniformity is achieved by means of pulsations caused by the dewatering blades used in certain twin-wire forming systems (Nordström and Norman 1996; Nordström 2003a; Nordström 2006) or by means of adjusting the jet-to-wire speed difference (Nordström and Norman 1994). The latter effects may be attributed to the partial disruption of the deposited mat of fibers by the imposed flow pulsations, impairing the subsequent establishment of fiber-to-fiber bonds. The flow pulsations may also interfere with the straightness of fibers and their orientation relative to the plane of the sheet. The adverse effects apparently over-ride any favorable effect of blade-induced flows and jet-to-wire adjustments on formation uniformity.

In the case of twin-wire roll-blade forming, the adverse effects of blade dewatering changes the response of the system to forming consistency, as controlled by the headbox flow rate (Nordström 2003b). Formation uniformity and strength both were found to improve with decreasing consistency, but only up to a certain point. Further decrease in consistency beyond that point yielded deterioration in both formation and strength. The latter result was attributed to an increased influence of the blade-induced dewatering, which can be expected to harm the intertwined structure of the wet web.

At a given forming consistency, twin-wire roll forming had been shown to produce substantially better formation that does Fourdrinier forming (Nordström 2003c). The superior results do not depend on an optimization of the jet-to-wire speed difference to achieve the higher strength performance. Rather, the effect may be attributed to the much faster dewatering that takes place in twin-wire forming, making it possible to take advantage of the superior formation uniformity that can be achieved at the outlet of a modern hydraulic headbox. By contrast, the wet web of a Fourdrinier paper machine takes several times as long before the fibers become immobilized as a sheet, giving more time for reflocculation to occur.

Certain aspects of paper strength appear to be somewhat forgiving of nonuniformities in paper that manifest themselves as visible flocs or a cloudy appearance when one holds a sheet of paper up to the light. For instance, Niskanen and Kärenlampi (1998) found that the elastic modulus of paper tends to be self-averaging. Likewise, resistance to delamination, as measured by the internal bond test, seems to be relatively insensitive to the presence of fiber flocs (Odell 2000). By contrast, the force required to break paper in tensile mode has been found to be strongly dependent on the strength of the weakest segments of the paper (Roberts et al. 1986; Linhart et al. 1987; Korteoja et al. 1997; Niskanen and Leskela 1997). Thus, the average tensile strength of paper is decreased if a longer sample from the same source is being tested (Hristopulos and Uesaka 2003; Ora 2012). Values closer to the ultimate potential of the material can be obtained by use of dumbbell-shaped paper strips, forcing breakage to mainly occur within a narrow range of distance (Norman 1986).

Conventional tests of formation uniformity do not appear to be telling the full story with respect to the development of paper strength. For instance, Norman (1986) noted that in certain cases a sheet of paper that appears more floccy, as a result of less hydrofoil action during its formation on a paper machine, can actually exhibit higher strength; such findings were interpreted to mean that something important must be happening relative to sheet formation at a scale smaller than what can be detected by conventional formation analysis methods. It can be expected, for instance, that superior paper strength requires a suitably aligned or interwoven organization of microfibrils or nanofibers in the contact zones between fibers even before the paper is dried. The importance of establishing the detailed structure of overlapping and crossed fibers within paper before drying was made evident by Alince et al. (2006); these researchers showed that when pairs of wet blotter papers were pressed together without drying, the strength of the resulting joint was substantially less at the same moisture content than in the case of sheets formed by reslurrying the individual fibers and forming them into a fresh sheet of damp paper having the same solids content.

Fiber preferential alignment

Depending on the type of forming equipment that is used, papermakers have some ability to affect the distribution of orientations of fibers within the plane of the sheet. A tendency for a higher proportion of fibers to be aligned in the machine direction (MD) is called orientation anisotropy. Factors responsible for such isotropy have been considered elsewhere (Norman 1991; Nordström 2003c; Hubbe 2007b). Certain products such as newsprint paper and certain web-offset printing grades require a high degree of MD orientation in order to withstand the high tensions placed on the web during printing at high speeds. Other products, such as certain photocopy papers, tend to perform better when the orientation is close to neutral. In traditional hand-made papermaking such adjustments could be made by optimizing the side-to-side and front-to-back sloshing of the sheet mold during dewatering (Hubbe and Bowden 2009). Certain relatively early models of Fourdrinier paper machines likewise incorporated horizontal shaking of the forming section in an attempt to improve formation uniformity and to encourage more of the fibers to be aligned in the cross direction (CD) (Robertson and Bailey 1934). In modern Fourdrinier papermaking an MD preference can be achieved by a modest mismatch in the jet and wire speeds, though such results are highly dependent on the details of the equipment and furnish being used (Nordström 2003c). In twin-wire roll forming, MD anisotropy can be induced by the elongational strain in the headbox nozzle (Nordström and Norman 1994, 1995b). If high MD anisotropy is desired in roll forming, then a headbox with a high nozzle contraction ratio should be used. Also, long softwood fibers show a larger response to elongational strain in the headbox in comparison to the shorter hardwood fibers (Nordström 2003e). If low anisotropy is desired in roll forming, then a headbox with a low contraction ratio should be used.

Nanostructure

In some respects the nanometer-scale structure of paper may be the most difficult aspect to manage during the papermaking process. The nano-scale organization of fibers already is substantially determined by the growth of the plant, and then to some degree by such processes as pulping, bleaching, and refining. It can be argued that paper owes much of its versatility and resilience to the way in which the cellulose, hemicellulose, and lignin are organized at a nano scale, but the details seem to be largely outside of the ability of papermakers to control what happens. The nano-scale structure of papermaking fibers has been well described (Stone and Scallan 1968; Lindström 1986b). Further, Nissan and Batten (1997) made an attempt to bridge the apparent theoretical gap between macroscopic aspects of paper strength and what happens at a molecular scale. Authors also have explored the implications of models of the fiber surface that involve nano-scale fibrillation (Pang and Gray 1998; Pelton 1993).

Given the central importance of the crystalline regions within the cellulose component of papermaking fibers, it is remarkable how little has been reported about the details of their organization. The degree or crystallization of cellulose within the cell wall has been determined to be somewhere in the range of 45 to 92%, depending on the sample and the method of evaluation (Kolseth and de Ruvo 1986). Cellulose nanocrystals, such as those obtained from cotton by digestion with strong acid or enzymatic treatments, typically have dimensions of about 100 to 300 in length and 8 to 10 nm in width (de Souza Lima and Borsali 2004). A model involving a reticular organization of cellulose (Stone and Scallan 1968) appears to account for many of the observed properties of fibers, especially following their delignification. However, it has not been shown whether or not the crystalline domains of cellulose correspond to the parts of the nanostructure that remain lined up with the fiber axis during the course of such changes as delignification, swelling, and drying.

An even greater degree of uncertainty still seems to persist relative to the nano-scale layout of the non-crystalline parts of cellulose in a fiber. The fact that the average molecular mass of native cellulose in wood is much larger than what has been determined for the molecular mass of cellulose within nanocrystals (Kolseth and de Ruvo 1968) is consistent with a model in which cellulose chains are continuous between crystalline domains and the adjacent disordered regions. Such evidence supports a “fringed micelle” model in which the cellulose nanocrystals within wood are mutually connected by somewhat disorganized zones of cellulose chains (Nishiyama et al.2003). Kolseth and de Ruvo (1986) have suggested that hemicellulose acts as a coupling agent between the cellulose and lignin within a native fiber. The isolation of lignin-carbohydrate complexes (Lawoko et al. 2005) supports this view. The layout on a nano-scale does not seem to have been determined.

It has been well demonstrated that mechanical damage to cellulosic fibers under dry conditions is able to convert crystalline cellulose into non-crystalline cellulose. As an example of this, the extensive ball-milling of dry, delignified fibers is capable of reducing crystallinity to a low level (Maier et al. 2005; Avolio et al. 2012). Kontturi and Vuorinen (2009) found that the drying of paper can induce stress within cellulose fibrils, leading to smaller nanocrystals when the cellulose is digested in strong acid solution. Such damage to cellulose nanocrystals may be just another manifestation of the same interplay between crystallinity and mechanical stressing.

Fibrillar angles within cellulosic fibers

In a manner analogous to the importance of fiber straightness, the predominant orientation angle of cellulose microfibrils within a fiber has been found to play a huge role with respect to strength and dimensional stability of paper (Salmén 1986; Nissan and Batten 1997; Retulainen et al. 1998; Courchene et al. 2006). Because the S2 sublayer within a fiber typically contributes most of the mass, at least in the case of the wood species most widely used for papermaking, the microfibril origination within that layer has a major effect on the fibers’ suitability for papermaking. Courchene et al. (2006) found a very strong correlation between the average microfibril angle obtained from otherwise similar loblolly pine trees and the breaking strength, stretch to breakage, and modulus of elasticity of the resulting paper. At the other extreme, Hänninen et al. (2011) observed unusual stretching ability in paper sheets made from kraft pulp fibers from juniper trees, a species that is known for having a predominant fiber angle in the range of 22 to 37 degrees from the fiber axis, i.e.high deviations from co-linearity. In general, the longitudinal elastic modulus of a fiber can be expected to increase with decreasing microfibril angle in its S2 sublayer (Retulainen et al. 1998).

Chemical Aspects of Inter-fiber Bonding

This section will consider what happens at a molecular scale during and after the events described above. The combined effects of pulping, refining, wet-pressing, and capillary forces all can contribute to bringing the fiber surfaces close enough together so that bonding is possible. But without some interaction at the molecular level, the process would not be completed.

Dominance of hydrogen bonding

It is widely agreed that hydrogen bonding plays a central role in holding cellulosic fibers together in a sheet of paper (Forman 1950; Nissan and Sternstein 1964; Nissan et al. 1985; Batten and Nissan 1987; Nissan and Batten 1990; Zauscher et al. 1996, 1997; Higgins 2002). As explained by Forman (1950), as the last water evaporates from between a pair of fibers, the residual valencies of the hydroxyl groups on the adjacent fibers become mutually satisfied by each other, thus forming hydrogen bonds that join the two fibers together. Additional hydrogen bonds already are at work, acting between and within cellulosic molecular chains in each fiber (Kolseth and de Ruvo 1986; Gross and Chu 2010). Nissan and Batten (1997) showed that macroscopic aspects of paper strength, such as stress-strain curves, could be explained in terms of the stretching of numerous hydrogen bonds throughout the cellulose-based structure. However, due to the different orientations of hydrogen bonds and their varied locations within a paper structure, Nissan and Sternstein (1964) estimated that only about 10% of the hydrogen bonds present would be likely to participate in opposing an applied tensile force. Yuan et al. (2013) reported results suggesting that the proportions of hydrogen bonds playing different roles within cellulosic fibers are sensitive to refining. Zauscher et al. (1996) showed that a model of paper’s elastic modulus based on pervasive hydrogen bonding also was able to account quantitatively for the effects of moisture content and humidity on the measured modulus.

One of the pieces of evidence that is often cited in support of the dominant contribution of hydrogen bonding to paper strength is the fact that extensive incorporation of hydrophobic groups at fiber surfaces tends to decrease bonding strength (Robinson 1980). For instance, acetylation at the 20% level was found to be sufficient to completely eliminate bonding between cellulosic fibers suspended in aqueous solution (Nissan and Higgins 1959). However, this explanation needs to be viewed with caution. As noted by McKenzie (1987) the effect easily could be due to the less swollen and therefore stiffer nature of the derivatized fibers when placed in water. Indeed, it has been found that low levels of acetylation can actually improve the bonding between recycled fibers (Ehrnrooth et al. 1977); the affect was attributed to disruption of the regularity of intra-fiber hydrogen bonds.

Various attributes of hydrogen bonds can help to explain paper strength, including paper’s sensitivity to moisture and the fact that it is generally easy to recycle. First of all, hydrogen bonding is reversible. Placement of paper back into water allows the hydrogen bonds connecting adjacent fibers to be quickly replaced by fiber-water hydrogen bonds. Second, though hydrogen bonds are roughly a factor of ten weaker than most covalent bonds, based on energy content, they are considerably stronger than the van der Waals interactions upon which materials such as polyethylene depend on so critically. And thirdly, hydrogen bonds are direction-specific. The adjacent groups to be bonded not only need to be close enough, but they also need to be aimed in suitable orientations in order to achieve effective bonding. When this happens as a result of biosynthesis, say during the growth of a tree, it is reasonable to assume that various cellular proteins and organelles play a role in arranging the nanocomponents of wood into a favorable structure to take optimum advantage of hydrogen bonding. When two fibers are thrust together in the course of papermaking, it is not known whether or not nano-scale features have a tendency to align themselves such as to favor a high density and regular organization of hydrogen bonding.

Self-assembly, microfibril coalescence

Campbell (1934) may have been among the first to consider that formation of cellulose chains into crystallites might be involved in the formation of bonding when cellulosic fibers are dried in contact with each other. He stated, for instance, that adjacent cellulose crystallites might “unite if sufficiently close to each other” during the process of evaporation. Newman (2004) presented evidence in support of fibril coalescence and enlargement of crystalline domains in the course of drying and deswelling of kraft fibers during papermaking. Pönni et al. (2012) recently reviewed the subject of nano-scale coalescence of cellulose. The high regularity of the cellulose macromolecule can be expected to favor a zipper-like cooperative effect (Lindström 1986b) when cellulose chains happen to be lined up with each other when they are dried in contact. One of characteristics of crystallites, if they are large enough, is that such a structure is unlikely to come apart all at once, even when placed in water.

Coalescence of adjacent crystallites does not appear to play a significant role in joining papers together during the course of ordinary papermaking. If it did, then one would expect to observe substantial development of wet strength even in the absence of wet-strength agents. Since wet-strength development ordinarily requires the use of chemical additives (Espy 1995), it follows that there must be some mechanism tending to inhibit the type of cellulose crystal coalescence that would bind adjacent fibers together in a water-resistant fashion. It is likely that the opportunities for formation of crystallites in the zone of adhesion between adjacent fibers is limited by the presence of materials other than pure cellulose at the surfaces of the adjacent fibers, and also the stringent requirements for alignment in order for adjacent crystallites to “heal together,” forming a single unit (Wool 2008). Much more extensive formation of crystallites might be possible if a fully amorphous cellulose, which has to be carefully prepared in the laboratory, were placed in water (Wadehra and Manley 1965). Work by Kontturi et al. (2011) showed, however, that certain amorphous cellulose materials can resist conversion to a crystalline form even when they are exposed to water.

Another way to envision a process of self-assembly, possibly in addition to the crystallite-formation process just described, is as an inter-mingling between hydrophilic gel phases. As noted by Pelton (2004), if two water-swollen hydrogel phases are pressed together and then dried in that position, strong bonding will result. The adhesion in such a case can be attributed to some three-dimensional mixing of polymeric chains extending from each of the adjacent surfaces. Such an interaction is consistent with an inter-diffusion mechanism as described by Robinson (1980) and by McKenzie (1984). Pelton and coworkers (2000) later found strong support for such a theory by evaluating the strength adhesion between surfaces that were treated with polymers that were either the same or quite different in terms of their solubility characteristics. The highest strength of bonding was achieved when cellulosic fibers in a slurry all were treated with the same cationic dextran, a very hydrophilic polymer. Quite effective bonding also was achieved if all the fiber surfaces were treated with a somewhat hydrophobic version of the cationic dextran. The lowest bonding strength within the paper was observed when half of the fibers were treated with each of the two contrasting polyelectrolytes. The latter effect is consistent with mutual insolubility of the contrasting types of polymer segments.

The role of conventional dry-strength agents

Because the subject of conventional dry-strength agents such as cationic starch and acrylamide products was covered in a previous review article (Hubbe 2006), only highlights will be included here. Some history of the subject and additional information is provided in earlier reviews of the topic (Clark 1942; Davison 1980; Reynold and Wasser 1980; Ketola and Andersson 1999). Various authors have shown cases in which the addition of dry-strength additives to papermaking furnish has helped them to achieve the stated goal of the present article – to maintain strength properties with less reliance on virgin cellulosic fibers. For instance, dry-strength additives have made it possible to use higher levels of mineral products as fillers in paper (Fineman and Lindström 1985; Tanaka et al. 2001; Brouillette et al. 2010). Alternatively, the strength of products made from recycled furnish has been improved by such additives (Eichinger 1981; Gaspar 1982; Strazdins 1984; Hipple 1991; Pelton 2004; Mocchiutti et al. 2006; Fatehi et al. 2010).

To summarize some main points from the articles just cited, dry-strength additives appear to enhance the extent of hydrogen bonding between fiber surfaces. To perform well in this role, the following attributes appear to be important: (a) a highly hydrophilic character with capability of hydrogen bonding; (b) suitable electrostatic charge (either cationic or a combination of cationic and anionic additives) to enable efficient retention at fiber surfaces during papermaking; and (c) sufficiently high molecular mass so that the additive remains at the surface of fibers and does not have sufficient time to become tightly matted down on the surfaces or to permeate into the porosity of the cell wall before the paper is formed. Because many of the effects of dry-strength agents are parallel to those that can be achieved by refining – yielding a stronger, denser, and less compliant sheet – the term “chemical hydration” is sometimes used (Delgado-Fornué et al. 2011).

Types of dry-strength agents

Cationic starch is the most widely used dry-strength agent, and many articles have characterized starch’s ability to strengthen paper (Roberts et al. 1986, 1987; Howard and Jowsey 1989; Alince et al.1990; Glittenberg 1993; Formento et al. 1994; Beaudoin et al. 1995; Ulbrich et al. 2012). Brännvall et al. (2007) suggested an innovative approach in which unrefined bleached kraft fibers were pretreated with starch to enhance the bonding properties of dry bales of pulp. To place such wet-end applications in context, it is important to keep in mind that starch products also are frequently applied to the surface of paper in solution form, either at a size press or in a coating operation mixed with clay, calcium carbonate, and other additives of a coating formulation. The usage of cationic starch at the wet end of the paper machine is usually no higher than about 1.5% of the dry weight of solids, whereas the amounts applied at the surface are often larger in such products as printing papers. Cationic starch also is frequently used in combination with other wet-end additives. In particular, sequential addition of cationic starch and colloidal silica products, i.e.nanoparticles, can provide a combination of dry-strength improvement and increased dewatering (Swerin et al. 1995; Hubbe 2005b; Khosravani and Rahmaninia 2013).

Polysaccharides other than starch also have found use as dry-strength additives (Cushing and Schuman 1959; Rojas and Neuman 1999; Myllytie et al. 2009). As noted by Myllytie et al. (2009), though some differences have been found between different types of polysaccharides, the overall trends can be expected to be similar. Such findings support the statement given earlier about three key criteria for selection of effective dry-strength agents. Thus, the detailed composition of the material appears to be less important than the combination of hydrophilic character, having an ionic charge, and having a sufficiently high molecular mass.

Synthetic polyelectrolytes, such as acrylamide products, are also used as dry-strength agents. In certain cases the synthetic additives have been reported to achieve high performance (Linke 1968; Chan 1976; Nealy et al. 1989; Bhardwaj et al. 1997; Carlsson et al. 1997; Yamauchi and Hatanaka 2002; Mihara et al. 2008; Kerman et al.2009; Sakaemura and Yamauchi 2011). More recently, polyvinylamine and polyallylamine products have come into use (Mocchiutti et al. 2011; Marais and Wågberg 2012). Some of the most promising results have been achieved when using amphoteric acrylamide products, i.e. polymers that have both positive and negative ionic groups (Tanaka et al. 1976; Song et al. 2006; Hubbe et al. 2007a,b). Such systems will be considered in more detail in a later section.

Carboxymethylcellulose (CMC), despite not being nearly as widely used as a dry-strength additive compared to starch products, has received a great deal of research attention (Horsey 1947; Beghello et al. 1997; Laine et al. 2000, 2002, 2003a,b; Watanabe et al. 2004; Duker and Lindström 2008; Duker et al. 2008; Kontturi et al. 2008; Aarne et al. 2012a). Because CMC has a negative charge, it tends to be repelled from untreated cellulosic fibers in suspension. Therefore, two main approaches have been used to retain CMC as a strength agent. On the one hand, CMC can be added sequentially with a high-charge cationic additive (Gärdlund et al. 2003; Hubbe et al. 2005; Lofton et al. 2005; Fatehi et al. 2009a). On the other hand, the CMC treatment can be optimized in terms of the degree of substitution (relatively low charge), ionic strength (relatively high), temperature (relatively high), and time (relatively long) to overcome the electrostatic repulsion (Laine et al. 2002, 2003a; Watanabe et al.2004; Duker and Lindström 2008; Aarne et al. 2012a). Since each of these approaches offers potential to go beyond the strength performance offered by currently commercialized systems, further discussion will be given later in this article.

According to Zhao and Kwon (2011), dry-strength agents can contribute in two main ways to the effectiveness of inter-fiber bonding within paper, by establishing molecular bonds between the adjacent fibers and by changing the viscoelastic nature of the adhesive joints. It follows that by design and selection of the additives to be used one can achieve effects ranging from strength enhancement to debonding (as might be used to promote softness and bulk in tissue products). Further information about traditional and state-of-the-art wet-end chemical strategies to increase paper strength has been reviewed (Reynolds and Wasser 1980; Scott 1996; Lindström et al. 2005; Hubbe 2006).

STRATEGIES TO ENHANCE PAPER STRENGTH

This section focuses on publications evaluating strategies to reach yet higher strength than is ordinarily achieved in current production of paper. In general, papermakers understand that they need to select a suitable mix of fiber types consistent with the strength attributes and other properties required in the respective product. Thus, unbleached softwood kraft fibers would be suitable for making corrugated boxes and paper bags, which need to be very strong, but not for products such as copy paper, which also need to be smooth, fine-textured, and colorless. For cost reasons papermakers often settle for the use of fibers that might be regarded as deficient in some respects; then other measures can be taken to make up for the deficiencies. For instance, it is possible to upgrade recycled fibers and regain some strength performance by such measures as low-intensity refining, alkaline treatments, fractionation to remove some of the fine matter, mixing with some premium-quality fibers, and addition of cationic starch (Nazhad 2005). In the subsections that follow, such approaches will be considered, starting with the macrostructure of paper, then factors affecting the nanostructure of paper, and finally chemical approaches to increasing paper strength beyond what can be achieved with widely commercialized practices.

Table A, which appears in the Appendix, lists the relative increases in paper strength that have been reported in a large number of studies using a wide variety of approaches. When using such information, readers are urged to also consult the cited original publications. That is because the default or “control” conditions considered in different studies can be very different from each other. Some of the most dramatic relative increases in strength have been observed in comparison to weak paper as the default condition. For instance, there may be a low degree of refining or earlier cycles of drying of the fibers. Also, one should bear in mind that various of the individual strategies represented in Table A may be overlapping, mutually exclusive, or inherently incapable of addressing certain other “weak links” that limit the strength of paper.

Macrostructure

The main items to be included under “macrostructure” will be the selection of the fibers to be used, the conditions employed for pulping and bleaching the fibers, considerations in selection of mineral products, surface applications (e.g. use of a size press or coater), and ply construction in manufacture of paper or paperboard.

Fiber selection

Fibers can break during the tensile failure of paper, especially in cases where the fibers within the paper are well bonded to each other (van den Akker et al. 1958; Helle 1963, 1965; Page 1969a). Information about the strength potential of different kinds of commercially available pulp fibers has been presented by Nanko et al. (2005). Looking to the long term, there are opportunities to improve fiber properties by means of tree breeding programs or genetic engineering. This is especially the case for plantation-grown fibers, which in the case of Eucalyptus species can have harvest cycles as short as six years (McNabb and Wadouski 1999; Thomas et al. 2009). Via et al. (2004) reviewed aspects of tree breeding relative to the potential to influence fiber length, coarseness, and pulp quality. Noting that tree breeders often strive for increased density of the wood, it was found that high wood density often correlates with lower stiffness and other strength attributes of the resulting paper. It follows that a new paradigm of tree selection, based on an optimization of other attributes, rather than just a maximization of density, holds promise for improved fiber quality in the distant future.

Microfibril angle, which refers to the orientation of cellulose chains and fibrils within the predominant S2 sublayer of fibers, appears to be a primary determinant of the suitability of different cellulosic fibers for papermaking. For the mainstream of paper products, such as for packaging and printing, species and individual trees having low values of microfibril angle are widely preferred, mainly due to the higher elastic modulus of the resulting paper (Retulainen et al. 1998; Courchene et al. 2006). In principle, a lower microfibrillar angle could be achieved by selective breeding, by genetic engineering, and by selection and planting of premium trees (Courchene et al. 2006).

Older trees, which can be expected to have a higher ratio of mature wood to juvenile wood, are often preferred based on attributes of the fibers (Severo et al. 2013). Juvenile wood fibers, which are those that are produced during the first several years of a tree’s life, often tend to have higher microfibril angles, thinner cell walls, and somewhat lower pulping yield in comparison to fibers produced later in a tree’s life. It follows that sawmill residues, which mainly come from the outermost layers or tops of trees, can be considered as a promising source of fibers for papermaking. The time of transition between juvenile and mature wood is highly variable, not only between different tree species, but also dependent on growing condition (Nemeth et al. 2008; Mansfield et al. 2009). This issue may be of particular importance for short-rotation species such as plantation-grown eucalyptus trees. However, the variability of microfibril angle with age of eucalyptus species (e.g. 35 degrees for juvenile vs. 10 degrees for mature) tends not to be as great as that associated with softwoods (e.g. 15 degrees for juvenile vs. 10 degrees for mature) (French et al. 2000). One of the goals of some selective breeding programs has been to decrease the microfibril angle in the juvenile wood fibers (Courchene et al. 2006). Work reported by Hamzeh et al.(2012) showed that juvenile wood also can be more susceptible to strength loss upon its use and recycling.

Pulping

The pulping of cellulosic materials to prepare fibers for papermaking is a huge subject, which fortunately has been well reviewed elsewhere (Biermann 1996; Gullichsen and Fogelholm 1999; Hon 2001; Santos et al. 2013), and only some key aspects will need to be highlighted here. For example, it has been found that the potential to achieve certain paper property goals often can be predicted based on the properties of the pulp fibers (Kellogg and Thykeson 1975).

Kraft pulp fibers are generally considered to be superior in strength properties to other kinds of delignified fibers, though the reasons are not completely clear (Page 1983; Young 1994). In the cited reports it is suggested that the difference may be due to the transformation of some partially crystalline regions of cellulose into more fully amorphous micro-domains in the freshly-pulped fibers prior to drying. In other words, a higher ratio of disordered to fully crystalline cellulose content might render the fibers more capable of forming strong inter-fiber bonds upon drying. Further research would be needed both to support the validity of such a hypothesis and also to attempt to somehow engineer superior pulp fibers by manipulating their crystallinity. More recent work by Kontturi and Vuorinen (2006) suggests that low-molecular-mass cellulose at the surface of sulfite pulps and certain other pulps can be a cause of lower tear strength.

Another aspect of kraft pulps that may partly account for their superior strength, compared to various other kinds of pulp, is the degree to which the process is able to minimize physical damage. In principle, the chemical degradation and solubilization of the lignin that binds fibers together makes it possible to liberate the fibers from each other without extensive generation of defects. However it has been shown that even higher strength can be achieved by use of laboratory processing methods that further minimize mechanical damage to fibers during pulping (Hakanen and Hartler 1995).

Selectivity has been a key criterion used by researchers when comparing the value of different pulping and bleaching treatment conditions. Ideally one would prefer that lignin could be removed by pulping and bleaching without causing damage or depletion of the polysaccharide components of the source material. Atalla (1977) showed, for instance, that superior pulp properties could be achieved when using laboratory pulping conditions selected to have an unusually gentle effect on the cellulose and hemicellulose components. Though the conditions employed were not economically attractive, they helped to reveal the extent of unintended damage to fibers that is brought about during conventional kraft pulping. To place matters into perspective, the relatively high strength of kraft pulp, compared to “soda pulping”, for instance, has been attributed to somewhat greater selectivity (Francis et al. 2006; Mollabashi et al.2011). Further well-known measures to increase the selectivity of kraft pulping, thus removing lignin with less decrease in yield and/or less degradation of cellulose degree of polymerization, have included polysulfide pulping and the use of anthraquinone, either separately or in combination (Luthe et al. 2003; Potucek et al. 2005).

Another approach to retaining a greater proportion of the strength inherent in the source fibers has been to increase the yield. In other words, one does not attempt to remove as much of the lignin. As explained by Weise (1998), low-yield cellulosic fibers, from which lignin has been removed, tend to be susceptible to embrittlement and a loss of swellability during drying. Leaving aside the difficulties that a high-yield approach might have relative to the brightness of the resulting pulp, it is reasonable to expect that higher-yield pulp fibers ought to have higher resistance to tensile failure and crushing – especially if those properties are evaluated at a fiber level. Relatively high yield targets are typically used in production of corrugating medium, for which high stiffness and resistance to crushing are critical (Nahrath 2004). In the past, one of the greatest barriers to increasing the yield of kraft pulping processes has been the fact that “under-cooked” chips fail to come apart into fibers when they are blown from the base of a digester at the end of a cooking cycle. Andrews and Hart (2013) showed that such fibers may be released by refining the semi-cooked chips, though the extent of damage to fiber quality can be significant if conditions are not in a suitable range. Sometimes kraft cooking to a relatively high yield is followed by more highly selective bleaching treatments as a means of achieving superior pulp performance and good yield values (Luo et al. 2012). As one application of this principle, Nguyen et al. (1993) showed that relatively high-yield unbleached kraft fibers obtained from corrugated containers can be subsequently bleached to obtain high-quality pulp suitable for printing grades of paper.

Thermomechanical pulps (TMP) represent another solution that papermakers have developed in past years as a means of preserving fiber length while minimizing the loss of yield. Reme et al. (1998) found that stone groundwood pulping has a much greater tendency to break and split fibers, in contrast to the more intact TMP fibers obtained at the same freeness level. On the other hand, when pulping softwood fibers, the TMP process typically yields fiber lengths in the range 1.2 to 1.6 mm, in comparison to about 2.4 mm for the corresponding kraft pulps. The yield of the TMP process is typically well above 90%. Preheating of wood chips with steam, under pressure, softens the lignin component, which has a very high concentration in the middle lamella regions that connect adjacent fibers within wood. Though not achieving as high bonding strength as kraft fibers, TMP fibers are quite resilient with respect to recycling. As shown by various researchers, the fibers can be formed into sheets and reslushed multiple times without loss of swelling characteristics of the fibers or tensile strength of the paper (Howard and Bichard 1992; Law 1996). Göttsching and Stürmer (1978a,b) reported similar findings in the case of groundwood pulp. As noted by Wegner (1982), it is possible to achieve pulp qualities intermediate between conventional TMP and kraft fibers by sulfonation during the mechanical pulping, i.e. chemithermomechanical pulping (CTMP). More recently it has become popular to employ alkaline peroxide mechanical pulping (APMP) of hardwoods for production of printings papers (Francis et al. 2001; Latibari et al. 2012; Muguet et al. 2012). Another alternative is to use old newsprint (ONP) and/or old magazine paper (OMP) in applications such as newsprint and light-weight coated products, for which TMP and other mechanical pulps have been customarily used.

Possible interference of lignin with inter-fiber bonding has been an ongoing concern when using high-yield lignocellulosic pulps. Shao and Li (2006) found that lignin at the fiber surface generally tends to reduce the bond strength per unit of bonded area. Wang et al. (2011) observed that the surfaces of chemithermomechanical pulp (CTMP) fibers are covered by granules of lignin, whereas the kraft fibers are covered by fibrils composed mainly of the polysaccharide components of wood. Though the lignin appeared to decrease the strength, the latter authors concluded that relative bonded area had a greater effect in determining bonding strength. The relatively poor bonding between polysaccharides and lignin, at least in the absence of naturally formed lignin-polysaccharide complexes (Lawoko et al.2005), is consistent with the great differences in solubility characteristics of the two kinds of materials (Hansen and Bjorkman 1998), which inhibits their three-dimensional interaction when the phases are in contact. However, there has been a scarcity of research aimed at improving the bondability of lignin-covered surfaces in papermaking applications.

With respect to swelling characteristics, Lindström (1986a) showed a strong relationship with pulping yield. The fiber saturation point (FSP) is a measure of how much water can enter the cell walls of fibers. The FSP of wood is typically about 0.4 mL/g (Lindström 1986a). The relatively low value is consistent with the rigidity of lignin in the structure, thus inhibiting the porous structure of cell walls in wood from expanding beyond a certain point. In the case of spruce wood, when some of the lignin and extractives were removed, achieving a pulp yield of 89.3%, the FSP value was increased from an initial 0.5 to 0.8 mL/g. When full delignification was carried out, as would ordinarily happen in the preparation of pulp intended for the production of bleached paper grades, a much higher FSP value of 1.45 mL/g was achieved. These differences help to explain the greater compliance of kraft pulp fibers – especially after refining – so that strong inter-fiber bonding can be achieved.

In principle, one of the most selective ways to carry out specific chemical reactions in lignocellulosic materials is by using purified enzymes. As an example, Liu et al. (2013) showed that the strength of paperboard made from unbleached softwood kraft pulps could be increased if the fibers had been functionalized by means of laccase together with either p-hydroxybenzoic acid (HBA), gallic acid (GA), or syringaldehyde (SyA). Such functionalization increased the anionic charge of the fiber surfaces. Lund and Felby (2001) found that laccase treatment improved the strength of unbleached kraft paper only when carried out in the presence of an oxidation mediator. Wong et al. (1999, 2000) reported related findings in the cases of high-yield kraft pulp and thermo-mechanical pulp. Widsten and Kandelbauer (2008) presented a thorough review on the subject, in which they came to the general conclusion that laccase treatment can improve strength by enzymatic oxidation, followed by crosslinking of the lignin compounds, especially if the product is heat-cured. Cure temperatures in the range 170 to 200 ^oC have been used to achieve the enzyme-induced strength benefits, and it is worth noting that the core temperature of a paper web usually is below the boiling point of water during most of conventional drying on a paper machine (Kiiskinen et al. 2000).

Treatment with cellulase can be used to change the properties of refined pulp. If the treatment is done before the pulp is refined, then the main benefit may entail a reduction in energy required to refine to a given level of strength improvement (Seo et al. 2000; Heitmann and Wallace 2004; Laothanachareon et al. 2011). For instance, Laothanachareon et al. (2011) treated old corrugated container (OCC) pulp with a cellulase-rich enzyme mixture. The treatment led to reductions in the energy required for refining, while maintaining compression strength and tensile strength. A more common approach, in the published literature, has been to treat already-refined pulp fibers with cellulase or xylanase enzymes; one of the effects of such treatment has been to increase rates of drainage (Bhat et al. 1991; Taleb and Maximino 2007). Pommier et al. (1989) showed that it is possible to somewhat over-refine the pulp; then, by carefully selecting the cellulase treatment conditions, it was possible to achieve higher bonding strength at a target freeness level of the pulp. Though Taleb and Maximino (2007) observed an increase in tensile strength following such treatment, there is a danger that over-treatment, due to too high a dosage of cellulase or too long time of contact, can seriously weaken the fibers (Pala et al. 2001). Mansfield and Saddler (1999) were able to achieve increases in burst and tensile strength by cellulase treatment, especially if only the long-fiber fraction of a Douglas fir kraft pulp was treated; however, there was clear evidence of fiber damage due to the enzymatic treatment, since the zero-span breaking length values fell strongly upon treatment with cellulase.

Another possible approach that seems interesting, but which is not well enough understood, entails treatment of fibers with deactivated enzyme products. For instance, Pala et al. (2001) found that treatment of pulp with enzymatic binding domains, lacking in hydrolytic actifivity, was able to increase both drainage rates and dry strength. Logic would suggest that the increased strength was due to proteinaceous material becoming firmly attached to fiber surfaces and then acting as a bonding agent. Yokota et al. (2009) demonstrated the effectiveness of a more ambitious approach, grafting acrylamide polymer chains onto cellulose binding domains and using the resulting compound as a self-retaining dry-strength additive.

Bleaching

Paper technologists do not usually associate the bleaching of pulp with its strength performance. However, the two topics often are related to each other (Annergren 1996). The first thing to be concerned about is degradation of the molecular mass of cellulose chains within the fibers in the course of bleaching. Reduced molecular mass of the cellulose, as indicated by a lower viscosity of its solution, has been correlated with lower tensile strength and tear strength (Annergren 1996). Westermark and Gustafsson (1994) found that whereas the cellulose molecular mass in unbleached birch kraft pulp was relatively uniform, there was a broad distribution and lower mean value of cellulose molecular mass in the same pulp after conventional bleaching.

Laine and Stenius (1997) showed that the selection of a bleaching sequence can affect the surface charge as well as the bonding ability of fibers. In general it was found that a higher negative charge on the bleached pulp resulted in more rapid refining and development of higher bonding strength. Higher charge can be achieved by such strategies as using a bleaching sequence that leaves a high content of hemicellulose in the fibers. As a consequence, bleaching sequences involving a strong oxidizing agent such as chlorine dioxide or ozone, followed by alkaline extraction, tend to remove more anionic charged groups than they generate, thus leaving the fiber in a less charged, less swollen, and less bondable conditions (Laine 1997; Laine and Stenius 1997).

The details of bleaching also can affect how much of the bond-promoting hemicellulose is retained within the pulp (Ratliff 1949). The importance of hemicellulose to inter-fiber bonding is also evident from the fact noted earlier, that excessive pre-extraction of hemicellulose from wood chips can hurt the strength potential of the resulting kraft fibers, even after bleaching (Oksanen et al. 1997; Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008). Helmerius et al. (2010) showed that hot-water extraction was particularly effective for extracting hemicellulose form birch wood chips, but pronounced decreases in strength properties resulted. Hamzeh et al. (2013) showed that while pre-extraction of hemicellulose from sugarcane bagasse fibers decreased their bonding ability, the strength problem could be overcome by treatment with dry-strength additives before formation of the paper.

Optimization of refining

In view of the very strong relationship between pulp refining and the resulting strength properties of paper, one can expect there to have been many advances in refining technology to achieve better overall results (Bhat et al. 1991; Baker 1995; 2000; Batchelor et al. 1999; Nazhad 2004; Ghosh 2006; Le Ny and Messmer 2007; Li et al. 2011).

As was noted earlier, fibers recovered from used paper products tend to be susceptible to damage, especially if harsh refining conditions are used (Bawden and Kibblewhite 1997; Ghosh 2006). So, as a general rule, such secondary pulps need to be treated gently. One aspect of this is to employ relatively low intensity levels during refining (Wild 1998; Nazhad and Awadel-Karim 2001; Ghosh and Vanderhoek 2001). In other words, one employs a relatively fine pattern of raised bars on the refiner plates, such that the ratio of energy input over the number of crossings of bar edges is relatively low. One should note, however, that intensity alone may be insufficient to characterize the optimum refining treatment (Nazhad 2004). Lundberg and de Ruvo (1978b) showed that high-consistency refining can achieve better drainage characteristics of such pulps after re-refining, consistent with lesser production of cellulosic fines.

Wang et al. (2003) showed that refining is not able to completely reverse the effect of loss of swelling due to the drying of pulp fibers. In particular, the crystalline domains in the recycled and re-refined fibers tended to be larger, compared to the crystallite sizes present when the fibers were being formed into the first generation of paper. Interestingly, Wang et al. (2003) found that increased refining rendered the fibers somewhat more resistant to pore closure on drying. Related results were obtained by Zhang et al. 2004a), who evaluated changes in the flexibility of fibers after they had been dried and rewetted. Refining of the never-dried softwood kraft fibers not only made them more flexible, but also the relative decrease in flexibility upon drying and rewetting was less in comparison to unrefined fibers. Welf et al. (2005) found that regardless of the degree of refining of bleached kraft pulps, the water retention value (Jayme 1944) decreased to approximately the same level upon drying. Though the reduction in swelling increased with increasing severity of drying, no memory of the differences in refining were apparent in the WRV data after drying and reslurrying.

Surface sizing

After a paper sheet has been fully formed and is still hot from the drying operation, many paper machine systems – especially when the goal is to produce a suitable product for printing – include a size press operation (Klass 1991; Grön and Rantanen 2000; Maurer 2001). Size press equipment is designed to apply a solution of starch (or other water-soluble polymer or mixture with starch) to the paper surface ahead of a nip between two smooth rolls. The primary purpose of surface sizing, in most cases, is to increase the paper’s strength. For example, Grön and Rantanen (2000) reported increases in the range of 1.4 to 2 in the crush resistance of corrugating medium after surface sizing with starch solution, depending on the basis weight. In addition, depending on the degree to which the paper has been hydrophobically sized to hold the solution out near to the paper surface, the surface strength tends to be increased and the dustiness of the paper is generally decreased. Because the amount of starch applied at the surface can be about 5% of the weight of the paper, for some commodity grades of printing papers, the impacts on strength can be large, sometimes dwarfing the effect of cationic starch added at the wet end of a paper machine.

An area of innovation, with potential to increase the strength benefits of surface sizing, has involved supplementary use of various synthetic polymer additives to the size press. For instance, a water-borne epoxy resin added at the size press with starch has shown promise for increasing the strength and stiffness of low basis weight paper (Li et al. 2012b). Alternatively, promising strength improvements have been achieved by the addition of latex products at the size press (Yan et al. 2013).

Given such promising results that have been achieved with size-press addition of strengthening agents, one might ask why don’t all paper machines have size presses. For many grades of paper, the answer comes down to money and limitations on the production rate. Use of a size press entails at least some re-wetting of the paper, so the sheet needs to be dried for a second time. In a typical size press operation, the dry content of the web falls from about 95-98% down to about 65-75% (Kuhasalo et al. 2000). The need to redry the paper after the size press usually implies that the overall rate of production is much lower than what could have been achieved if the size press operation had been bypassed. In addition, any nonuniformities in rewetting of the sheet at the size press can be expected to adversely affect the flatness and the uniformity of tension within the paper after the second cycle of drying.

Another question that can be asked is, “As long as the paper web becomes rewetted at a conventional size press, why not completely saturate the paper with a solution of strength-enhancing additives?” Such a question was considered by Jopson (1993). Based on test results it was suggested to saturate paperboard for corrugated containers with use of moisture-resistant, thermoplastic binders in combination with starch products. A pressure saturating system, based on wetting one side of the board as the web follows the circumference of a roll, was found to be more effective than conventional size-press systems for applying a lot of solution at high basis weights.

In cases where a way can be found to eliminate the need for and to bypass size press, such a change has the potential to increase the rate of production on a dryer-limited paper machine. Such an approach was suggested by Brouwer (1997), who showed that a certain combination of wet-end additives was able to achieve strength benefits equivalent to those of size-press starch addition in certain cases. Another strategy to minimize the need to re-dry the paper involves adding less water during surface sizing. Thus, Lipponen et al. (2004) showed that a size press could be run with a starch concentration as high as 18% solids, and that it was still possible to keep the degree of polymerization high enough to achieve strength benefits.

Ply construction of paper products

Increased stiffness, as well as various other desired attributes, can be achieved when using paper machine systems that are set up to form multiple plies. Though this is not a new technology, there is still considerable potential for more widespread implementation. The most commonly employed strategy employs strong, well-refined fibers in the outer plies, while cheap, bulky fibers, often with lower bonding ability, are placed in an inner ply or plies (Navaee-Ardeh and Nazhad 2008). Bristow and Pauler (1983) described relatively early efforts to extend multi-ply technology to the basis weight range of conventional paper products. Notably, it was shown that the greatest tensile strength and resistance to tearing could be achieved if the usual arrangement of fibers were reversed, placing the highly bondable, long fibers in the center of the structure, and placing the bulky, less bondable fibers in the outside layers. However, in printing papers it is more common to engineer the sheet so as to maximize filler content while achieving the specified stiffness and smoothness at a specified caliper and basis weight.

Corrugated construction of paper and paperboard

Corrugated construction might be regarded as a way to “cheat” and to achieve strength attributes beyond what can be achieved in an ordinary sheet of paper – even a sheet composed of multiple plies (Bi 2012). The basic product is assembled from two linerboard layers, a corrugated layer of “fluting medium”, and some means to adhere the components together, e.g. starch adhesive. The corrugated board product takes advantage of an “I-beam” type of construction, in which the high-modulus linerboard layers are held apart from each other. Such a system can provide a high resistance to bending. Corrugated paperboard has enjoyed a dominant position for many years in the production of single-use, recyclable shipping containers (Twede 2008).

One trend that merits watching is the degree to which corrugated construction may increasingly compete with lower-weight products (Brunauer 2004). Such products, which have been used especially in the packaging of some fast-food items, involve the use of much finer ply construction and finer corrugations (Brunauer 2004). Notably, the kind of test procedures that have been used in the past to characterize the strength of corrugated board products do not appear to work as well in testing of these finer corrugated products (Wilson and Frank 2009). Localized buckling, which depends on the flatness of the liners, becomes a major mode of failure as the liner weight is decreased (Popil 2012).

Optimization of the Nanostructure

There is considerable evidence that paper strength depends on structures formed at the nanometer scale during the ordinary papermaking processes. High resolution imaging has revealed the importance of nano-scale structures, including very narrow fibrils and fines, in the bonding zones of paper (Nanko and Ohsawa 1989; Nanko et al. 1989; Stratton and Colson 1993; Kang et al. 2004). Various imaging studies have revealed nano-sized cellulosic fibrils that apparently assist in the bonding between adjacent fibers in a typical sheet of paper (Mayhood et al. 1962; Clark 1978a,b; Moss and Retulainen 1997; Schmied et al. 2013).

Mihara et al. (2008) showed that paper strength properties can differ depending on the details of where strength additives are located, including at the surfaces of fibers or concentrated at the edges of bonded areas. Conventional wet-end addition was found to favor accumulation of an acrylamide-type dry-strength polymer generally over the surfaces of kraft fibers, and to some extent permeated within the cell walls of the fibers. Due to the bonding agent being present between fibers at each point, one might expect wet-end addition to favor increasing strength of bonding within the bonded areas. By contrast, addition of the strength-enhancing polymer by external application, i.e. by means of a size press, allowed the dissolved polymeric material to become concentrated during the evaporation of water. Because a meniscus of aqueous fluid between fibers tends to recede towards crevices such as bonded areas during evaporation, one can expect a reinforcement of the edges of such bonds (Pelton et al.2003; Pelton 2004). Since either one of these models can be expected to increase the strength of paper, it is not clear which mechanism might be judged to be more effective in a given circumstance.

The location of bonding agents on fibers vs. cellulosic fines also has been shown to make a big difference relative to paper strength (Strazdins 1980; Stratton 1989). Each of these studies showed that placement of the bonding agents on the surfaces of fibers rather than on cellulosic fines tended to yield higher strength. However, to achieve such placement, each of the cited studies employed fractionation of the furnish, followed by selective treatments and then recombination of the fines and fibres. Because fractionation would entail additional equipment and operating expenses, such procedures are almost never carried out at a commercial scale. Fortunately, there is a practical way to at least partly achieve some of the expected benefits. That is, the ratio of fine matter to fibers tends to be somewhat lower if the pulp stream is treated prior to its dilution at a fan pump. Fines tend to be enriched in the “thin stock loop” of a conventional paper machine due to the fact that some of the fines are not retained on the forming fabric and get recirculated in the “white water” returning to the fan pump. For this reason it is a common practice to add cationic starch and other dry-strength agents to thick stock so that it can become adsorbed onto the fibers shortly before the stock is diluted at a fan pump.

Nano-scale aspects of filler use and paper properties

The mineral products that are widely used in papermaking tend to have overall diameters in the range of about 1 to 5 µm, which is too large to justify use of the term “nano-scale”. However, it turns out that some of the most widely used filler products, i.e. the scalenohedral or “rosette” form of precipitated calcium carbonate (PCC), are actually fused-together structures composed of yet smaller parts (Gill and Scott 1987; Fairchild 1992; Hubbe 2004). The reason that rosette-type PCC has become very popular can be traced to its low packing density and bulky structure (Gill and Scott 1987). Because mineral products, i.e. “fillers” used in papermaking, typically have a much higher intrinsic density than the cellulosic fibers, and also due to their strong debonding effect, as noted earlier, it is important to consider ways in which papermakers have dealt with these issues. Though papermakers have always had a strong incentive to replace some of the fibers in paper’s composition with something cheaper, it is still necessary to meet customer requirements for the expected thickness (i.e. “caliper”) and strength properties. Even though filler particles are much smaller than the fibers in a typical piece of paper, a bulky filler structure can tend to brace sections of the fibers farther part from each other, thus increasing the caliper of the paper. Then the paper typically has to pass through calendering nips to achieve the required smoothness, depending on the expectations of the customers. When using a filler product that has a bulky composite structure, it can be easier to maintain the caliper at least as high as the required minimum value. Although, in general, a bulky type of filler tends to have a greater debonding effect on paper (Gill 1989), such effects can be at least partly compensated by use of cationic starch and other bonding agents at the wet end (Lindström and Florén 1984; Lindström et al. 1985; Alince et al. 1990; Tanaka et al. 2001; Sang et al. 2011), as well as application of size-press starch to the paper’s surface (Gerli et al. 2011).

Another type of structuring occurs when mineral particles are agglomerated together, usually as a result of treatment with a solution of cationic acrylamide copolymer or cationic starch (Riddell et al. 1976; Brooks and Meagher 1982; Stark et al. 1987; Shen et al. 2009; Sang et al. 2012; Seo et al. 2012). Such agglomeration leads to an effective increase in the size – and a concurrent decrease in the effective number of filler particles, such that a greater proportion of the fiber surfaces can be expected to remain uncovered and available for strong inter-fiber bonding. In addition, the somewhat flexible nature of the agglomerates held together by flocculating polymers means that the fibers are not braced apart nearly to the same degree as it would be in an equivalent case with highly rigid agglomerates, as might be created by the same nominal size of agglomerates formed by a calcination or crystallization process.

Filler also can be agglomerated together with cellulosic fines. Subramanian et al. (2008) and Lin et al. (2010) showed that such an approach can be used to achieve higher filler contents while maintaining strength properties. One of the side-benefits of such an approach is that the system becomes less dependent on subsequent addition of a retention aid to retain filler. Backing off on the amount of retention aid can make the paper somewhat more uniform, which also tends to contribute to higher strength.

The role of fines in helping to hold paper together, overcoming a strong debonding effect by fillers, was demonstrated in some carefully designed experiments by Xu et al. (2005). Pairs of filler-free, damp handsheets were prepared, and a sparse layer of filler particles was placed between them. Bonding agent was applied either to the handsheet surfaces or to the filler particles. After drying, the resistance to delamination was measured. Treatment of one or both plies with the bonding agent was found to increase bonding between the sheets, but that approach was effective only below a critical level of filler addition between the plies. Treatment of the PCC helped bonding to a modest degree, without as sharp a fall-off in properties when the filler loading between the plies was increased. But the highest strength in systems containing a lot of PCC between the plies was observed when cellulosic fines were present together with the PCC between the plies. The fibrillar fines created by refining were found to be much more effective in terms of bonding in such situations compared to the primary fines present in the pulp before refining (Xu and Pelton 2005).

Approaches involving treatment of the surfaces of filler particles with a bonding agent also might be considered in the “nano” category, given the very small thickness of such adsorbed layers. For instance, it has been shown that the cooking of starch in the presence of mineral particles can be a practical way to cover each particle with a thin layer (Yan et al. 2005; Zhao et al. 2005; Deng et al. 2010). Increased paper strength appears to be a key benefit of such an approach. In practice there is likely to be an overlap between treatments aimed at enhancing the bondability of filler particle surfaces and those aimed at agglomerating them together, as was discussed earlier.

One of the most effective, but difficult to achieve means of minimizing the adverse effect of mineral particles on paper strength properties involves loading them either into the lumens or cell walls of fibers, ideally leaving the fiber surfaces uncovered and available for bonding with adjacent fibers (Green et al. 1982; Middleton and Scallan 1985, 1989). Such approaches were reviewed recently by Kumar et al. (2011). Though lab results have shown potential to achieve superior strength properties at a given mineral content, there is little indication of successful implementation of such an approach at a commercial scale.

In a related approach, calcium carbonate particles can be precipitated in the presence of cellulosic fibers. Ciobanu and Bobu (2009) carried out such work and showed that the resulting optical properties of the paper exceeded what was achieved when conventional PCC was used. In addition, the filler remained firmly fixed on or in the fibers during a simulated recycling operation, thus minimizing the loss of materials. However, lower strength was observed compared to use of conventional fillers; such results suggest that the in-situ procedure, at least in the reported case, had a greater debonding effect.

If one’s goal is to replace fibrous cellulosic material with something else, while maintaining a suitable structure in a paper product, it makes logical sense to consider fibrous mineral particles. For instance, certain calcium silicate minerals, composed of clusters of long fibrous elements, have shown promise for achieving high bulking, high optical characteristics, and favorable strength properties of the resulting paper (Mathur 2004). Some key challenges facing such approaches include health hazards associated with certain highly insoluble fibrous minerals (Sturm 2010), the huge difference in size between typical cellulosic fibers and typical mineral fillers (Bown 1983), and the high levels of abrasivity that would be expected if the size of typical mineral particles were to be increased greatly (Rapp and Laufmann 1995). Consistent with these constraints, Hu et al. (2009) and Chen et al. (2011) showed that favorable strength results can be obtained with the use of whisker-like CaCO₃ particles, which are sufficiently soluble so that significant health concerns do not seem to arise. Chen et al. (2011) suggested that the whiskers were partly embedded in the fiber walls, thus increasing a frictional contribution to paper strength.

Nanocellulose wet-end addition and paper strength

Much progress has been achieved in recent years to identify various promising ways of preparing nano-sized cellulosic materials, such as nanofibrillated cellulose and nanocrystalline cellulose. Such approaches have been reviewed (Klemm et al. 2011; Peng et al. 2011; Ramires and Dufresne 2011; Chauhan and Chakrabarti 2012). The cellulose nanomaterials themselves can be exceedingly strong (Saito et al. 2013). In addition, paper-like sheets formed from various 100% nanocellulose suspensions have achieved outstanding strength levels (Klemm et al. 2011), in addition to some unique features such as translucency (Iwamoto et al. 2005; Yano et al. 2005). Two challenging issues seem to be standing in the way of greater implementation of such approaches. The first issue is the relatively low yields of certain nanocellulose products. For instance, when cellulose nanocrystals are isolated from cotton, a relatively pure cellulose source, yields can be about 61% of the starting material (Pirani and Hashaikeh 2013). Notably, the cited authors were able to recover much of the material solubilized during preparation of nanocrystalline cellulose and reprecipitate it as potentially useful cellulose II. The second issue is energy. If cellulosic nanomaterials are prepared from kraft fibers, for instance, mainly by application of high levels of mechanical shearing, the energy input can be in the range 12,000 to 70,000 kWh/ton (Eriksen et al. 2008; Klemm et al.2011). Based on these considerations, the present review article will not further consider the possibility of making paper from high proportions of cellulosic nanomaterials.

The question of whether nanocellulose could contribute to paper strength when used as a wet-end additive has been considered by several groups. Ahola et al. (2008) used nanofibrillated cellulose (NFC) together with a cationic wet-strength polymer, poly(amidoamine epichlorohydrin). By adding the polyelectrolyte first, a dense layer of NFC could then be layered onto the fibers. By contrast, pre-mixing of the polyelectrolyte and NFC gave rise to agglomerates, which tended to form less uniform and more rigid layers when added to the fiber slurry. Increases in both wet- and dry-strength properties were observed, especially when the additives were applied sequentially to the fiber slurry.

Related work was reported more recently by Joseleau et al. (2012), who compared the effects of once-dried cellulosic fines (i.e. “dead fines”) in comparison to what they called microfibrillar cellulose, freshly created by refining. The freshly prepared fines were found to generate 30% more hydrogen bonding within the sheet, according to a test using 6 M urea solution. The authors assumed that the urea would selectively detach those cellulosic fines that were attached to fibers by hydrogen bonding. Eriksen et al. (2008) showed that nanofibrillated cellulose could increase the tensile index of sheets made from thermomechanical pulp. Guimond et al. (2010) observed increased tensile strength when cellulose nanofibers produced by TEMPO oxidation were added to a newsprint grade furnish. Gao et al. (2011) showed that bacterial cellulose, added in small proportions, was able to increase the strength of sheets made from softwood fibers. González et al. (2012) showed that nanofibrillated cellulose (NFC) can be used as a kind of substitute for refining of the fibers themselves; combining the NFC with unrefined Eucalyptus kraft fibers gave about the same effect as refining of the fibers. Dai and Fan (2013) modified the surface of hemp fibers by high-pH treatment with cationic surfactant and nanocrystalline cellulose. As confirmed by various analyses, such treatment increased the strength of the fibers themselves, filling in the gaps in the cell wall and between fibrils.

Taipale et al. (2010) found that several different forms of nanocellulose were able to increase paper strength, and most such systems tended to slow the rate of drainage. Fortunately, it appears that the adverse effects on dewatering can be overcome by careful selection of materials and process conditions. There is a critical need for research aimed at maximizing both dry strength and dewatering when adding nanocellulose products at the wet end. Presumably it might be beneficial to precipitate the nanocellulose onto fiber surfaces, where it might play a role analogous to that of surface-bound cellulosic fibrils rather than as detached cellulosic fines.

Nanomaterials use in paper coatings

Nanomaterials having a high aspect ratio, such as fibrillar nanocellulose or highly platy clay-like particles, have potential to increase the elastic modulus of coating layers, thus increasing the stiffness of paper products. In such applications the reinforcing role of the nanocellulose can be understood in terms of nanocomposite technology, which has been discussed in earlier review articles (Hubbe et al. 2008b; Siro and Plackett 2010; Kalia et al. 2011; Moon et al. 2011; Johansson et al. 2012; Khalil et al. 2012). Cengiz (2001) observed increased strength upon introduction of nanoparticles to a mixed resin applied to paper. Husband et al. (2009) were able to achieve higher stiffness by incorporating a highly platy fine clay into coating layers on paper. Despite such evidence of potential contributions to stiffness, such approaches face inherent challenges. In particular, many coated grades of paper have challenging targets of gloss and print quality, which are likely to be more difficult to meet when incorporating high-aspect-ratio nanomaterials. Also, the high-shear flow characteristics of the coating formulation can be adversely affected when high aspect ratio materials are used (Lohmander and Rigdahl 2000; Willoughby and Sharma 2004; Kugge et al. 2011). Paunonen (2013) has reviewed progress related to the use of nanomaterials in laminate films on paper products, for the enhancement of both strength and barrier properties.

Chemical Strategies for Stronger Bonding

Though some fibers break during the tensile failure of a paper structure, most of the damage often involves failure of inter-fiber bonds (Van den Akker et al. 1958; Page 1969a). This section will consider chemical strategies that could be used to make such bonds stronger.

Alkaline processing

One of the most remarkable features of papermaking technology is the fact that quite strong bonding can be achieved among the fibers even without adding any type of glue. Thus, it is appropriate first to consider what can be done to strengthen paper just by more effective swelling of the fibers. Several studies have shown that one of the most effective ways of accomplishing this objective is by use of alkaline pH conditions (Young 1994). For instance, the following studies showed that alkaline conditions used in the repulping of fibers then can promote easier swelling during refining, thus leading to higher strength (Szwarcsztajn and Pryzybysz 1976; Klungness and Caulfield 1982; Guest and Voss 1983; Bhat 1991; Gurnagul 1995; Ogden 1999). Pönni et al. (2013) found evidence that alkaline conditions can re-open pore surfaces within fibers that previously had been closed due to drying. Gurnagul observed benefits from alkaline processing of recycled fibers in the case of thermomechanical pulp (TMP); no such benefits were observed when similar treatments were applied to low-yield recycled fibers. Such results are consistent with the effect of carboxyl groups on the swelling of pulps under different pH conditions (Lindström and Carlsson 1982; Lindström and Kolman 1982). Other studies have shown a similar positive effect associated with weakly alkaline conditions employed during the formation of paper (Lindström and Kolman 1982; Forester 1985). An added benefit of such processing conditions is that the fibers within paper formed under neutral or weakly alkaline pH conditions are then less subject to acid hydrolysis during storage of the paper (Williams 1980; McComb and Williams 1981; Scallan 1998; Weise 1998).

Application strategies for dry-strength additives

Gruber et al. (1996) carried out an interesting thought experiment by considering all of the different kinds of additives, as well as fibers, cellulosic fines, and mineral particles, in a typical papermaking operation. On this basis they estimated that about 100 different pair-wise interactions can be expected to be important during papermaking. To begin to make sense of the complexity, it was recommended to focus on processes of adsorption, noting that the sequences of addition can affect how the additives perform in a paper machine system. Rojas and Neuman (1999) observed unique adsorption phenomena in systems that contained both charged and uncharged hydrophilic species; it appeared that adsorption of polysaccharides onto fibers resulted in the complexation of some other species. To explain their findings the cited authors needed to go beyond the usual explanations based on electrostatic charges. Factors such as hydrogen bonding, hydrophobic effects, and the entropy changes that occur upon adsorption in different situations appear to be important. These various levels of complexity, even in a highly conventional paper machine system, can help to explain the patient trial-and-error approach that paper machine operators face in their day-to-day work.

Conventional wet-end strategies are often classified using such terms as single- or dual-additive programs. Many single-polymer wet-end treatments have shown promise for dry-strength development. For instance, Retulainen and Nieminen (1996) compared a range of different polymeric additives on a one-at-a-time basis and found that the cationic starch products had the greatest beneficial effects on paper strength. Decreases in strength were observed in the cited study when using polyelectrolytes having a sufficiently high molecular weight to cause strong flocculation of the fibers. In line with this observation, various studies have suggested that there can be an optimum range of molecular weight of dry-strength agents, depending also on such factors as linear vs. branched molecular structure (Reynolds and Wasser 1980; Hubbe 2006). In addition, some studies have provided evidence that dry-strength agents based on natural gums (Swanson 1950; Leech 1954) or acrylamide (Carlsson et al. 1977) can outperform typical starch-based products due to favorable properties of the chemical structure.

Cationically charged dry-strength additives often have been found to have greater effectiveness than their uncharged or anionic counterparts when added to papermaking furnish in the absence of other cationic materials. Such results often can be attributed to more efficient retention on fibers during the papermaking process (Roberts et al. 1986, 1987). Horvath et al. (2008) showed that cationic polyelectrolytes having a sufficiently high density of change tend to remain immobilized near to their first points of contact with a negatively charged material. Such behavior can, in principle, have a favorable effect of keeping a cationic additive at the fiber surface and thereby able to contribute to bonding.

Still focusing on approaches that can be considered as well-established in the industry, certain “dual additive” systems can be used in efforts to push bonding strength somewhat beyond what can be achieved by cationic starch addition to the same furnish. For instance, sequential addition of a relatively high-charge cationic polymer, together with a carboxylated (anionic) polymer, i.e. a dual system, has been shown to be highly effective for increasing dry strength (Linke 1968; Hubbe et al. 2003; Wielema and Brouwer 2003; Esser et al. 2005; Galván et al. 2012). As shown by Hubbe et al. (2003), it can be an effective strategy to add just enough of the cationic additive, in the first step, so that the fiber surfaces are saturated; subsequent addition of the anionic component of the dry-strength system was found to be especially effective. This strategy is illustrated in Fig. 9. Somewhat higher dry strength levels, compared with single-polymer dry-strength strategies, were attributed to higher amounts of retained bonding agent, as well as a more three-dimensional extension of polymer segments from the fiber surfaces prior to bond formation.

Fig. 9. Illustration of dual-polymer dry-strength additive strategy based on saturation of adsorption capacity of the fibers, thus achieving a more extended conformation and higher levels of adsorbed polymers

Initiatives to supplement the hemicellulose content of fibers

As was noted earlier, vigorous pre-extraction of wood chips has been suggested as a way to reduce costs associated with pulping and bleaching, but the resulting pulp can be deficient with respect to bonding ability (Oksanen et al. 1997; Al-Dajani and Tschirner 2008; Yoon and van Heiningen 2008). Thus, it makes sense to consider what would happen if hemicellulose, either in its extracted form or after modifications, were added to kraft pulp fibers – whether or not those fibers had been subjected to pre-extraction. In general, studies have shown that addition of hemicellulose products to the fiber slurry tends to increase the strength of the resulting paper (Mobarak et al.1973; Bhaduri et al. 1995; Denis et al. 2003; Lima et al. 2003; Suurnäkki et al. 2003; Ahrenstedt et al. 2008; Zhang and Hu 2010; Han et al. 2012; Bigand et al. 2013; Lindqvist et al. 2013; Song and Hubbe 2013, 2014). Lima et al. (2003) reported steeply rising dry strength upon addition of unmodified seed-derived xyloglucan to eucalyptus kraft fibers as the dosage was increased in the range zero to about 1% in the absence of any retention aid. Higher amounts provided little additional effect, suggesting that 1% was the adsorption capacity of the pulp. The fact that the resistance to air penetration also rose sharply indicated that the hemicellulose promoted densification of the paper. Bai et al. (2012) compared wet-end addition of three different hemicellulose types and found that galactoglucomannan had the largest beneficial effect on strength, followed by xylan and arabinogalactan.

Derivatizing hemicellulose to provide it with a positive charge has been shown to increase its effectiveness as a wet-end dry-strength additive (Ren et al. 2009, 2011; Liu et al. 2011a; Schwikal et al.2011; Kataja-aho et al. 2012; Bigand et al. 2013), presumably due to the better retention on the fiber surfaces. Schwikal et al. (2011) observed an optimum in strength performance at an intermediate level of cationic charge density. Lindqvist et al. (2013) found that the effects of galactoglucomannans as dry-strength agents could be improved in various aspects by carboxymethylation, cationization, or partial hydrophobation. Likewise, Song and Hubbe (2013, 2014) showed that increasing the amount of carboxyl groups in glucan obtained from oats by TEMPO-mediated oxidation or by carboxymethylation greatly increased its effectiveness as a dry-strength agent when used in combination with aluminum sulphate and a cationic retention aid. The higher strength results were attributed to higher solubility of the modified glucans, leading to their better distribution in the furnish.

It has sometimes been assumed in the past that addition of hemicellulose during the cooking of kraft pulps would be a waste of resources due to the likelihood of molecular breakdown or dissolution during subsequent bleaching and washing operations. However, Danielsson and Lindström (2005) observed a significant increase in tensile strength upon addition of a high-mass xylan to a softwood kraft cook. The central importance of the bonding contribution of the xylan was supported by the fact that high-mass xylan had a much greater beneficial effect than low-mass xylan. The ability of hemicellulose to remain on fiber surfaces and to withstand pulping and bleaching operations is not well understood.

Another option to consider is the addition of hemicellulose to fiber stock before refining of the fibers (Lima et al. 2003; Han et al. 2012). Such practices are consistent with the traditional use of “beater adhesives,” especially in the years before development of cationic wet-strength agents capable of adsorbing efficiently on the fiber surfaces (Swanson 1950). Addition of uncharged polysaccharides and other additives before the refiners not only gave the additives more time to become adsorbed on the fibers, but also such treatment was found to accelerate the reduction in freeness values during refining (Walecka 1956; Centola and Borruso 1958; Becka 1979; Allan et al.1985; Blomstedt et al. 2007). In each case the mechanism appears to involve an increase in the negative charge density of the surface, following adsorption of the respective additive. Accordingly, Walecka (1956) observed more rapid decreases of freeness during refining with increasing degree of substitution of carboxymethylcellulose (CMC). The higher charge of the cellulosic material, after adsorption of the negatively charged additive, results in a higher degree of swelling, leading to more effective development of bonded area and a higher density of hydrogen bonds connecting the fibers after drying. In the work of Lima et al. (2003), though addition of seed-derived hemicellulose to eucalyptus kraft pulp at the 1% level before refining appeared to increase bonding strength, somewhat greater increases were observed when the addition was made after refining.

Treatment of papermaking fibers with carboxymethylcellulose (CMC)

Despite the fact that it has a strong negative charge, which is the same sign of charge as untreated cellulosic fibers, a series of studies have shown that CMC can be successfully deposited onto fiber surface, and strong increases in dry strength have been observed in the resulting paper (Walecka 1956; Laine et al. 2002, 2003a,c; Watanabe et al. 2004; Duker and Lindström 2008; Aarne et al. 2012a). The cited studies have considered, among other things, why the reported treatments of fibers with CMC appear to be quite effective, even when no cationic additive has been employed. By contrast, earlier users employed cationic additives such as alum to retain CMC to develop dry strength (Horsey 1947). Subsequently it has been shown that addition of calcium chloride or other salts can facilitate the adsorption of the anionic CMC onto generally anionic cellulosic surfaces (Laine et al. 2002; Duker and Lindström 2008; Liu et al.2011b). Evidence of a special affinity between CMC and pure cellulose surfaces was shown by Kargl et al. (2012), who attributed the favorable adsorption to chemical similarity. It was suggested by Laine et al. (2000) that the effect can be attributed to a form of co-crystallization between the flexible CMC macromolecules and the partially crystalline cellulose surfaces to which they are adsorbed. Such an explanation is consistent with the relatively long treatment times that have been found necessary in the cited studies to achieve the best results; such a conclusion follows from the idea that substantial time may be required for fairly long CMC molecules to become well lined up with the adjacent cellulose chains, rather than remaining adsorbed in a disorganized and presumably less stable fashion.

The adsorption of CMC will provide the cellulosic surface with a much greater negative charge density, thus leading to greater swelling. An unfavorable aspect of such procedures, in addition to the cost of the CMC, is the requirement that salts ordinarily need to be added to suppress electrostatic repulsions sufficiently to allow the adsorption of CMC to proceed (Laine et al. 2000). Such salts, upon release of effluent from the process, would be expected to adversely affect fresh water organisms (Canedo-Arguelles et al. 2013). To some extent this problem can be minimized by using CMC having a relatively low degree of substitution, thus lowering its solubility in water and minimizing the concentration of salt needed to encourage its adsorption from solution onto the fibers (Watanabe et al. 2004).

Carboxylation of fiber surfaces

As a possible alternative to the adsorption of CMC, as just described, some researchers have considered oxidation or derivatization of fiber surfaces to increase their negative charge density. Such chemical derivatization of fiber surfaces can be viewed as a relatively aggressive strategy, compared to relying on adsorption. Carboxylation, leading to higher bonding strength in paper, has been achieved by periodate oxidation (Alince 1975), by carboxymethylation (Ruffini 1966; Lindström and Carlsson 1982; Laine et al. 2003; Torgnysdøtter and Wågberg 2003), by ozone treatment (Minor et al. 1993), by TEMPO-mediated oxidation (Kitaoka et al. 1999; Le Roux et al. 2006; Song and Law 2010; Aracri et al. 2012; Ma et al. 2012), or by enzyme-catalyzed derivatization with various carboxylic acid or aldehyde-containing species (Liu et al. 2013). Most of the cited studies showed increased strength of paper prepared from the modified fibers.

In the dry state, fibers can be oxidized by corona treatment (Goring and Suranyi 1969; Cramm and Bibee 1982) or by exposure to the vapors of maleic anhydride (Hubbe et al. 1999), with similar positive effects on paper strength after reslurrying those fibers. All of these effects are consistent with increased swelling of the fibers with increasing amounts of anionic charge. The more swollen fibers can be expected to undergo more effective integration of cellulosic surfaces at the nano-scale during the drying process. The effect also is consistent with a model in which hydrogels, when dried in contact with each other, become fused together due to some mixing of polymeric chains in a three-dimensional transition zone at what used to be the interface (Pelton 2004).

Three-dimensional extension of adsorbed dry-strength agents

Many studies considered in an earlier review article (Hubbe 2006), as well as some more recent studies (Fatehi et al. 2010; Bai et al. 2012; Kataja-aho et al. 2012) have shown that the performance of various dry-strength enhancing polymers increases with their molecular mass. There are at least two explanations for this trend. On the one hand, a higher-mass polymer in solution will require more time to diffuse into the mesopores of fiber and thus lose its function as a bonding agent (Wu et al. 2009). On the other hand, a higher-mass polyelectrolyte adsorbed at a surface can be expected to spend more time in an extended, three-dimensional conformation before it is able to flatten itself out on the surface (Wågberg et al. 1988, 1996). During the seconds or minutes that the adsorbed hydrophilic polymers remain in an extended conformation, one can expect there to be more effective development of bonding with a second fiber, as paper is being formed. Such a mechanism is supported by observations that, in very clean pulp systems, low-density cationic polymer strength additives often out-perform higher charge-density additives (Fatehi and Xiao 2008). A loopy structure is expected in such cases because uncharged, hydrophilic segments of a polymer chain can be expected to extend away from the surface.

Layer-by-layer (polyelectrolyte multilayer) technology

Some remarkable and sometimes idiosyncratic increases in paper strength have been achieved in various recent studies that have employed “layer-by-layer” or “polyelectrolyte multilayer” (PEM) technology. In principle, PEMs are built up by first dipping the substrate into a solution containing a polyelectrolyte having an opposite sign of charge from that of the substrate. Then the substrate is dipped into a rinsing solution, often followed by a drying step. Then the process is repeated, starting with a bath containing polyelectrolyte of the opposite charge. The process is continued back and forth, building up as many layers as the investigators have patience to apply. The majority of studies aiming to understand the science underlying PEM effects have involved the use of very smooth surfaces, such as mica or quartz crystals (Decher 1997). Such surfaces have made it possible to precisely quantify the changes in layer thickness and new adsorbed amount with each additional cycle of layering. Best results, in terms of layer regularity and adsorbed amounts, typically have been achieved in systems containing a salt concentration that has been optimized so as to suppress the range of electrostatic forces and allow the polyelectrolytes to adsorb in a coiled, rather dense conformation.

Though it is often not practical to replicate all aspects of the usual PEM approach when using cellulosic fibers (such as drying the substrate after each application of a layer or precisely measuring the layer thicknesses), several such studies have been carried out (Wågberg et al. 2002; Eriksson et al. 2005a,b, 2006; Torgnysdøtter and Wågberg 2006; Brännvall et al. 2007; Lingström and Wågberg 2008; Ankerfors et al. 2009, 2012; Zhou et al. 2010; Chin et al. 2012; Li et al. 2012a). Polyelectrolyte bilayer application based on the same principles, and its effect on paper strength, also has been reported (Myllytie et al. 2009; Aarne et al. 2012a). To summarize what is most often observed in such studies, paper’s dry strength has been found to increase with increasing numbers of polyelectrolyte bilayers. In certain cases, higher strength has been observed when a certain one of a pair of polyelectrolytes is outermost (Wågberg et al. 2002; Eriksson et al. 2005b). Eriksson et al. (2005a) showed that accelerated deposition of increasing amounts of polyelectrolyte during successive cycles could be achieved by switching the pH conditions back and forth such that the charge of the outermost already-adsorbed layer was much greater than an equal amount of the next-to-be adsorbed layer.

Aarne et al. (2012a) found that they were able to achieve a favorable combination of relatively low water retention value (WRV) and strong positive contribution to dry strength when adding enough high-charge cationic polymer to approximately neutralize the net charge of fibers that had been coated with an initial layer of the anionic CMC. Such a result was unexpected, since increased swelling of papermaking fibers is usually associated with development of higher bonding strength. The strengthening effect also was able to be carried over efficiently to recycled sheets, obtained under relatively gentle laboratory conditions.

Cranston and Gray (2009) have reviewed studies in which nanocellulose was applied as one of the layers in PEM strategies. For instance, Ahola et al. (2008) employed a system in which cellulose nanofibrils constituted one of the layers, added after a layer of cationic polyamidoamine-epichlorohydrin wet-strength polymer; a bilayer addition strategy was found to increase both dry-strength and wet-strength. Aulin et al. (2010) incorporated the alternating use of two types of treated nanocellulose into a layer-by-layer strategy. Microfibrillated cellulose batches were prepared with either cationic or anionic surface charge.

Polyelectrolyte complexes for dry strength

If one mixes together solutions of polyelectrolytes having opposite charges, the mixed polymeric material is likely to become decreased in solubility, often precipitating out of solution (Michaels 1965). Several studies have employed the principles of polyelectrolyte complexation as a means of treating cellulosic fiber surfaces with relatively high amounts of hydrophilic polyelectrolyte material, thus enhancing paper’s dry strength (Carr et al. 1974, 1977; Heath et al.1974; Nagata 1991; Gärdlund et al. 2003, 2005; Gernandt et al. 2003; Koljonen et al. 2003; Hubbe 2005a; Hubbe et al. 2005; Lofton et al.2005; Maximova et al. 2005; Heermann et al. 2006; Lvov et al. 2006; Torgnysdøtter and Wågberg 2006; Truppner et al. 2006; Vainio et al.2006; Larsson and Wågberg 2008; Ankerfors et al. 2009, 2012; Fatehhi et al. 2009; Sang et al. 2010). As noted by Rojas and Neuman (1999), even in papermaking systems where there has been no explicit intention to form polyelectrolyte complexes (PECs), such effects still can play a role due to interaction between a cationic polyelectrolyte and various anionic polyelectrolytes already present in the suspension, such as dissolved hemicellulose species.

Effective complexation of polyelectrolytes, leading to increases in paper strength, do not necessarily require that both additives are polyelectrolytes. Thus, highly promising results have been achieved by sequential addition of anionic wet-end starch with an aluminum-based coagulant (Brouwer 1997; Wielema and Brouwer 2003). Also it is likely that some reports of favorable dry-strength improvements obtained with mixtures of native starches and cationic polymers might be due to formation of PECs (Wu et al. 2004), especially if the native starch has some anionic character. Similarly, Bhardwaj et al.(1997) achieved promising improvements in dry strength upon addition of a combination of amphoteric starch and cationic acrylamide copolymer.

Though most of the studies cited in the preceding paragraph have involved pre-mixing of pairs of polyelectrolyte solutions before their addition to papermaking furnish, such a strategy had been shown to give less favorable effects on paper’s strength in comparison to in-situ addition, i.e. allowing the two polyelectrolyte solutions to come into contact in the presence of a stirred fiber suspension. For instance, Hubbe (2005a) reported that in-situ formation of PECs was able to achieve roughly twice the tensile strength compared to addition of pre-formed PECs of the same composition to a suspension of otherwise non-bonding glass microfibers. As shown in Fig. 10, the best results were achieved when the ratio of additives was within a range of 40:60 to 60:40 relative to the charge stoichiometry of the two polyelectrolytes employed. Tensile breaking force continued to rise to higher and higher levels even when the level of polymer addition was pushed to startlingly high levels, up to 10% of the total mass of solids in the resulting paper sheet. The lowest strength was observed if, after premixing the two polyelectrolytes with each other, the addition of the PEC mixture to the fiber suspension was delayed by several minutes.

Fig. 10. Replotting of data from Hubbe (2005a) in linear form to highlight strong advantage of in-situ PEC formation to achieve high tensile strength when using glass microfibers that do not bond together on their own or even with single polyelectrolytes

In follow-up work involving typical kraft pulp fibers, Heerman et al.(2006) showed that it was possible to push net polyelectrolyte addition levels up to unheard-of levels as high as 40% of the weight of fiber, again with sharply rising dry strength with increasing polymer dosage. At such high levels of polyelectrolyte addition, the product might reasonably be classified as a composite rather than as ordinary paper. By employing the in-situ addition method it was possible to delay the onset of serious problems related to stickiness by a factor of about ten in the net amount of added polymer. Again it was shown that much higher tensile strength values could be achieved by the in-situ addition scheme, in comparison to pre-mixing of the polyelectrolytes. Such high levels of treatment seem unlikely to be implemented commercially, partly due to the generally higher costs of polyelectrolytes in comparison to the fibers. However, it is clear that the use of PECs offers a way to achieve dry strength levels that greatly exceed what can be achieved by adsorption of a single polyelectrolyte.

The ratio between the positively and negatively charged polyelectrolytes was found to play a critical role in the PEC systems just described (Hubbe 2005a,b). In general, when the fibers in suspension had a negative surface charge, somewhat higher adsorbed amounts of polyelectrolyte, as well as higher dry strength, were obtained when the PEC mixture was either charge-balanced or as a 55:45 ratio of cationic to anionic polymer-bound ionic groups.

To attempt to explain the much greater effectiveness of in-situ formation of PECs, by balanced sequential addition of polyelectrolytes of opposite charge to a stirred suspension of fibers under moderately low electrical conductivity conditions, it is apparent that kinetic effects are important. It is proposed, therefore, that the freshly-formed PECs are in some sense “active” and able to interact with the fibrils and other surfaces of cellulosic fibers in ways that are no longer effective if the PECs are formed separately and added later. For instance, it is reasonable to expect progressive dehydration and densification of PECs with the passage of time after their preparation. The word “self-assembly” has sometimes been used to denote processes in which events occur at the nano scale, based on the nature of the materials and the interactive forces. Further research work is needed, perhaps with molecular dynamics simulation or quartz crystal microbalance (QCM) measurements, to shed more light on such mechanisms. Another issue that might be explored in future work is whether polyelectrolyte complexes might contribute to more flexible and resilient attachments between fibers in the wet web; such flexibility could lead to more effective sharing of the tensile loads once the paper has been dried.

Microparticle systems as an example of polyelectrolyte complexes

A type of polyelectrolyte complexation also appears to underly the widespread application, within the paper industry, of cationic polyelectrolytes in sequential addition with negatively charged colloidal solids such as colloidal silica, montmorillonite (bentonite), and certain hydrogel products (Swerin et al. 1995; Wågberg et al. 1996; Hubbe 2005b; Khosravani et al. 2010; Kim and Hubbe 2010). Some similarities with conventional polyelectrolyte complexation can be noted. For instance, in both cases a balance between positive and negative charges tends to minimize the amount of water held within the structure and to favor precipitation of the material out of solution and onto the surfaces of fibers. For that reason, microparticle systems can be viewed not only as a strategy to optimize retention and dewatering, but also as a way to more effectively retain a dry-strength additive, such as cationic starch. Indeed, various studies have shown that combinations of cationic starch and colloidal silica can be beneficial for increasing paper’s dry-strength properties (Moberg 1985, 1986; Nilsson and Carlson 1993; Sunley 1997; Khosravani and Rahmaninia 2013).

As shown in Fig. 11, it is possible to envision microparticle systems as being a type of nanocomposite, in which fused chains of nano-sized silica particles serve as reinforcement within a matrix composed of cationic starch. The colloidal silica can be expected to play two key roles. First, by having a large surface area and strongly negative surface charge, it neutralizes the positive charge of the starch, thus promoting the deposition of the starch on the fiber surfaces. Secondly, one can expect that the silica renders the combined adhesive material more rigid once the starch-silica mixture has been dried.

Fig. 11. Illustration of the role of chain-like colloidal silica particles within a matrix of cationic starch, serving as reinforcement in a resulting “nanocomposite” adhesive between fibers in paper

Silva et al. (2013) reported another promising approach in which cellulose nanocrystals were employed, together with cationic starch, as the anionic component of a PEC system. The greatest increases in paper dry strength were obtained under conditions of balanced charges between the starch and the nanocellulose. A unique and promising aspect of this kind of approach is that nanocellulose particles typically are highly fibrillar, and thus in principle able to provide structural reinforcement to bonded areas. More research is needed in order to figure out how both the macro-scale structures and the nano-scale structures within paper can be optimized simultaneously by control of additives and process conditions during papermaking. Otherwise, due to such factors as shrinkage during drying, one can imagine the paper structure developing gaps and non-uniformities, leading to lower than the anticipated levels of strength.

Polyampholytes as dry-strength agents

The terms “polyampholyte” and “amphoteric polymer” refer to polyelectrolytes that have both cationic and anionic substituent groups. Several studies have shown that polyampholytes sometimes outperform otherwise similar dry-strength additives having only one sign of charge (Fukunaga 1999; Song et al. 2006, 2010; Wang et al. 2006, 2007b; Hubbe et al. 2007a,b; Silva et al. 2009). Relative to polyelectrolytes bearing a single sign of charge, polyampholytes offer two important potential advantages: On the one hand, they are not expected to cause strong flocculation of a fiber suspension. This expectation was borne out in tests measuring the state of flocculation in suspensions of bleached kraft fibers (Hubbe et al. 2007a). On the other hand, because polyampholytes already can contain a balanced proportion of positive and negative charges, there is little risk that their addition to a paper machine system – even at rather high levels – will cause disturbances in the balance of charges in the system.

A series of tests were carried out to determine the effects of different charge-related quantities on the performance of polyampholytes as dry-strength agents. Terpolymers of acrylamide, itaconic acid (a monomer with two carboxylic acid groups and acrylamide), and a cationic monomer dimethylaminepropylacrylamide were prepared by random free-radical polymerization (Sezaki et al. 2006a; Hubbe et al.2007a). Best results, in terms of dry strength, were obtained when the pH was adjusted so as to decrease the solubility of the polymer, favoring the precipitation of bonding agent onto the fibers (Fig. 12).

Fig. 12. Tensile strength resulting from addition of acrylamide-based polyampholytes having 5:4 charge ratio (cationic to anionic) at the 1% level to bleached kraft pulp at pH=5, followed by addition of the indicated amounts of poly-(aluminum chloride) (PAC) based on solids. The last two sets of three bars correspond to polyelectrolytes having a single sign of charge. Figure redrawn from Song et al. 2006.

Further work by Sezaki et al. (2006b) showed that adsorption of the polyampholytes onto cellulosic fibers tended to be maximized under conditions where the net charge was somewhat to the positive side of neutralization. Such a maximum was explained by the fact that it allows the polyampholyte to have a net charge opposite to that of the fiber surfaces, while at the same time to be sufficiently charge-neutralized so as to promote its relative insolubility in water and deposition onto the fibers. Follow-up tests with polyampholytes having a fixed charge ratio (5 parts cationic charge and 4 parts of anionic charge) showed high adsorption efficiency onto cellulosic fibers within a pH range of about 4 to 9 (Wang et al. 2006). High adsorption efficiency was observed over a wide range of the content of charged groups at the same fixed ratio, though there was notably lower adsorption efficiency at the lowest net charge considered (2.5% cationic charge and 2% anionic charge groups). As shown in Fig. 12, polyampholytes greatly outperformed the corresponding polyelectrolytes having only a single sign of charge (Hubbe et al.2007a).

To explain the sometimes higher performance of polyampholyte systems, it might be assumed that the many ionic groups contained within such molecular chains have the ability to hold onto water, thus providing the molecules with a three-dimensional structure and the capability of participating in effective bonding between the rough surfaces of adjacent fibers. Such a model was supported by the results of water retention value (WRV) tests (Hubbe et al. 2007b). Related effects were also found by tests with the quartz crystal microbalance (QCM) method (Song et al. 2010). The QCM results showed that adsorption of polyampholytes resulted in a relatively large uptake of mass onto a model substrate, consistent with binding of water, along with the polymer adsorption. In addition, the observed changes in energy dissipation during the adsorption process revealed a non-rigid structure of the adsorbed material, which is consistent with a swollen, three-dimensional state of adsorption as a hydrogel within the time scale of the experiments.

Ionic bonding

A common feature of most of the dry-strength strategies discussed above is that ionic charges often play a central role. It is reasonable then to consider whether or not ionic bonds might make a significant contribution to adhesion between fibers in the paper after it has been dried. Though ionic bonds generally lack the water-resistance that is characteristic of most covalent bonds, their energy content can be large, in the range of about 90 to 180 kcal/mole to separate various pairs of oppositely charged univalent ions (Huheey 1972). This can be compared to a range of about 50 to 125 kcal/mole for typical covalent single bonds (Nebergall et al. 1980). This is much greater also than the energy associated with hydrogen bonds, which have been estimated as up to 20 kcal/mole in the case of cellulose (Nissan et al. 1985). Accordingly, it has been proposed that ionic bonds can play a significant role in the bonding of paper, depending on the chemical treatments (Allan and Reif 1971; Reif 1972).

Experimental evidence supporting ionic bonding has been hard to come by, possibly due to the possibility of multiple contributions to bond strength in typical cases. For example, zwitterionic groups in an acrylamide resin used to impregnate paper contributed a wet-strengthening effect to the paper (Delgado et al. 2004); however it is likely that much of the strength benefit was due to a more thoroughly entangled polymer structure brought about by the charge interactions – rather than an effect due to the ionic charges themselves. Likewise, one might argue that the demonstrated strength of adhesion within a polyelectrolyte multilayer (PEM) in the wet condition (Aulin et al.2010) constitutes evidence of the importance of ionic bonding.

A problem with such evidence is that opposite charges can induce changes in flocculation, causing significant differences in the uniformity of paper (Delgado-Fornue et al. 2011). Torgnysdøtter and Wågberg (2004) measured the strength of adhesive joints formed between single fibers that were optionally treated to make the surface have a positive or negative charge. Increased strength was observed with increasing charge density of the fibers, an effect that might be related to ionic bonding; however, the results also could be explained by enhanced dewatering in a charge-balanced interaction, leading to a denser paper structure upon formation of the sheet. Allan and Reif (1971) avoided flocculation by preparing paper from fibers treated with strongly anionic CMC. Subsequent impregnation of the pre-formed sheets with the strongly cationic polymer polyethyleneimine (PEI) yielded very strong paper. One can envision the cationic PEI forming bridges between the negatively charged fiber surfaces already fully assembled in the sheet. Rojas et al. (2005) were able to avoid such complexities by measuring the adhesion between highly anionic mica sheets, one or both of which had been treated with cationic polymer. The strongest force, under wet conditions, was achieved in cases where just one surface had been treated with a high-charge-density cationic polymer – a situation consistent with a charge contribution to bond strength. However the observed bonding showed a highly reversible character – something more akin to adhesive tape, rather than the kind of adhesion that typically holds paper together. In addition to demonstrating the importance of ionic interactions in the cases considered, the study also demonstrated the importance of the viscoelastic nature of the cationic polyelectrolytes.

Aluminum as a strength booster

Since the early 1800s, aluminum-containing chemicals have been widely used by papermakers for such purposes as promoting retention of fine particles, increasing rates of drainage, and the setting of rosin size (Strazdins 1989; Kitaoka and Tanaka 2002). Investigations of the molecular mechanisms underlying such interactions have revealed a complex interplay between different soluble ionic species of hydroxylated aluminum species, along with a strong tendency towards the formation of oligomeric or polymerized species (Bottéro et al. 1980; Bi et al. 2004; Zhao et al. 2009). In that context it is perhaps not surprising that soluble aluminum species appear capable of boosting the dry-strength performance of polyelectrolytes in some hard-to-explain situations, such as when polyampholytes are used as dry-strength agents (Fukunaga 1999; Song et al. 2006). One possible way to explain such beneficial effects is to assume that complexation occurs between the high positive charge associated with oligomeric aluminum compounds and the negatively charged segments that can be expected to exist due to the random nature of synthesis of this type of amphoteric polymers. Then, upon the drying of the paper, one can expect the processes of polymerization and reorganization to proceed, such that an inorganic polymer structure at least partially interpenetrates the organic polymer. Such an interaction would be analogous to what happens when aluminum sulfate is added to the paper machine wet end in sequence with anionic dry-strength additives, serving to bond the latter to the negatively charged surfaces of fibers (Linke 1968; Wielema and Brouwer 2003).

Wet-end application of hydrogels, including uncooked starch

A hydrogel can be defined as a water-loving polymeric material that swells in aqueous solution but does not dissolve (or at least has not yet dissolved). The criterion that a promising candidate from dry-strength applications ought to be three-dimensional and highly swollen with water thus appears to be well fulfilled by materials in the hydrogel category. Some of the systems already discussed, such as polyelectrolyte multilayers and complexes, might be considered as being kinds of hydrogels.

Water-swollen starch granules can be considered not only as hydrogel particles, but also as potentially cost-effective dry-strength additives. Although the spray application of uncooked starch onto the wet web of paper, just before the wet-pressing and drying stages, has been practiced for many years, it still represents a game-changing option when there is an urgent need to greatly increase the bonding strength of paper (Proctor and Reid 1974; Davies 1977). The application technology can be quite simple – such as a spray-boom extended across the width of a Fourdrinier paper machine forming table. Alternatively, the starch granules can be applied in a falling curtain of water (Fougler and Parisian 1999). As another option, the uncooked starch can be added at the wet end of the paper machine, using retention aids to retain the particles (Greenwood 1999). In any such system, Wielema and Brouwer (2003) reasoned that the drying process of a conventional paper machine does not offer a sufficiently intense combination of temperature, time, and moisture content to allow more than a partial swelling and cooking of the starch granules before the completion of drying. But on the positive side, uncooked starch is relatively inexpensive, and the amount added to the system does not have to depend on the adsorption capacity of the fiber surfaces.

Further promising results with hydrogels as dry-strength agents were obtained by Pelton and co-authors (Miao et al. 2007, 2008; Wen and Pelton 2012). Miao et al. (2007) found that 1.5 µm hydrogel particles gave substantially higher wet tensile strength as well as dry strength of softwood kraft sheets compared to a similar mass of linear polyelectrolyte of the same type added to the fiber suspension. The difference was attributed to the ability to retain a thicker layer of polymer at the fiber surfaces, and also to the fact that the hydrogels were much too large to permeate into the cell walls of fibers. When comparing different sizes of hydrogel and different degrees of cross-linking, the most promising results were with the smallest hydrogels and the lowest levels of crosslinking (Miao et al. 2008). In follow-up work involving the adhesion of cellulose films, Wen and Pelton (2012) showed that acrylamide-based hydrogel particles in the diameter range of 0.3 to 4.5 µm were advantageous for achieving strengthening effects at low polymer dosage, consistent with more efficient coverage of surfaces. But larger hydrogel particles were advantageous for achieving high increases in dry strength at relatively high addition levels, consistent with an ability to get more polymer into the bonding areas. Notably, the results did not depend on the size of the cationic vinylamine polyelectrolytes that were used to coat the negatively charged hydrogels and make them adhere to the fibers – only the size of the gel particles themselves. However, practical concerns such as stickiness and costs can be expected to place constraints on both the size and amount of hydrogel material that is used in wet-end applications (Heermann et al. 2006). Another concern is that hydrogels, by definition, contain a lot of water. It follows that an increased need to evaporate water might slow the production rate or raise the cost of production when using a hydrogel-based strategy for increasing paper’s dry strength.

Bonding Strategies for High Humidity Applications

Due to the hydrophilic nature of cellulosic fibers, and in particular the high dependency of paper structures on hydrogen bonding as a major contribution to dry strength, conditions of high humidity and contact with aqueous liquids represent particular challenges for papermakers. The subsections that follow consider publications dealing with a wide range of strategies that papermakers have employed to maintain paper strength as the moisture content of the paper increases. The hydrophobic sizing of paper, which has been reviewed elsewhere (Hubbe 2007a; Lindström and Larsson 2008), will not be considered here. In an effort to achieve humidity-resistant bonding between cellulosic fibers, it makes sense to try to supplement or replace hydrogen bonding as the main contribution to bonding within paper.

Wet-strength treatments and paper performance at high humidity

High humidity is not the same thing as soaking. Yet it is logical to assume that a treatment providing strength to paper when it is thoroughly wet might also bolster its strength under humid conditions. The idea is to supplement hydrogen bonding in paper with moisture-resistant kinds of bonding. Some such treatments fall into the category of “wet strength” strategies, about much has been written (Espy 1995; Lindström et al. 2005; Obokata and Isogai 2009; Ozaki 2011; Su et al. 2012). Strong wet-strength gains in addition to dry-strength gains have been reported in cases where conventional wet-strength agents were added to suspensions of fibers that had been treated in some way to increase their carboxylic acid content (Ahola et al. 2008). It is generally agreed that wet-strength treatments involve three-dimensional crosslinking of the resin with itself, a process that can be expected to envelope and hold onto fine cellulosic fibrils and polymer chains extending into the resin from the two adjacent fiber surfaces (Espy 1995; Hagiopol and Johnston 2012). Such a mechanism is illustrated in Fig. 13. The cationic nature of typical poly-amidoamine-epichlorohydrin wet-strength resins can be expected to encourage such three-dimensional incorporation of cellulosic polymer segments in the resin during paper formation and drying. As shown in the figure, one can envision a cross-linked mass of polymer chains that incorporate fibrils extending from the adjacent fiber surfaces. It is also likely that the amidoamine-epichlorohydrin resins covalently bond to carboxyl groups on the fiber surfaces as part of the curing mechanism (Espy 1995; Hagiopol and Johnston 2012).

Fig. 13. Three-dimensional covalent crosslinking during the “curing” of a conventional wet-strength resin, resulting in a hardened mass that incorporates cellulosic chains and nanofibrils from the adjacent two fibers. Filled (green) circles represent crosslink point. Squares (red) represent ester links between the resin and carboxylic groups associated with the fiber.

Boonyasarn et al. (1992) showed that although wet-strength treatment of paperboard, in combination with some other measures, can significantly increase the dry strength of paper, essentially all of the dry-strength gains are lost relative to ordinary paperboard when the testing is carried out under high humidity conditions. In other words, although wet-strength agents can keep the strength of paper from falling to zero in the soaked condition, they fail to protect the pervasively hydrogen-bonded structure of paper from the effects of high humidity. Other measures employed by the cited authors, in an attempt to produce humidity-resistant premium paperboard, were increased refining, increased cross-machine fiber orientation, an increased proportion of virgin kraft fibers, and extended-nip pressing. While all of these listed measures can be expected to contribute to the manufacture of a premium-quality paperboard product, none of them deals effectively with problems related to the decreasing modulus of elasticity of cellulosic materials as the humidity and moisture content are increased.

Cross-linking has been considered in some recent studies involving innovative dry-strength treatments. Horvath et al. (2010) observed improved performance of cationic dextran as a dry-strength additive under low humidity conditions when it contained acetal groups capable of crosslinking. Surprisingly, however, the crosslinking yielded no benefit when the paper was tested at high humidity; the authors suggested that a stiff “joint” between two bonded fibers cannot be effective if the surrounding material has been softened by moisture. This is a shame, since the hemiacetal bonds formed by aldehyde groups during the drying of paper are reversible after thorough rewetting, allowing excellent recyclability and restoration of temporary wet-strength properties in the recycled paper (Farley 1994; Chen et al. 2002; Hagiopol and Johnston 2012). The formation of covalent amide linkages is possibly significant during the thermal curing of paper that has been treated with polyelectrolytes bearing both amine groups and carboxyl groups (Eriksson et al. 2006). Whether or not such covalent bonding reaches significant levels during ordinary papermaking has not been established, and again there is no assurance that such bonds would positively affect paper’s performance at high humidity.

Polyelectrolyte complexes and tolerance to high humidity

Papermakers have observed for many years that treatment of the fiber furnish with certain highly-charged cationic polyelectrolytes can lead to measurable increases in residual strength after the paper has been fully wetted (Trout 1951; Espy 1995). Aarne et al. (2012a) observed especially strong bonding when highly charged linear polyelectrolytes with amine groups in the main chain were added to suspensions of fibers that had been treated with the highly anionic polymer CMC. Marais and Wågberg (2012) observed similar effects when polyallylamine or polyvinylamine were added to unbleached kraft stock, followed by heat-curing of the paper. Gärdlund et al. (2005) observed that wet-strength effects could be achieved by addition of polyelectrolyte complexes (PECs), but only if the paper was subjected to heat treatment. One possible explanation of the beneficial effect of thermal treatment is that components such as lignin are temporarily softened, and that those components, possibly with interaction with other additives, engage in a curing effect when the temperature falls below their glass transition point (Salmén 1986). Linhart (2005) suggested that the wet-strengthening effects induced by highly cationic polymers in mixtures of negatively charged fibers are due to a de-swelling effect. This is basically the same mechanism by which polyelectrolyte complexes generally show a minimum of swelling in the charge-balanced condition (Michaels 1965). At the charge-neutral condition of a PEC, there is a minimization of osmotic effects that might swell the fibers. In view of the progress that has been achieved in the studies just cited, in-situ complexation between oppositely charged polyelectrolytes deserves increased study relative to increasing the humidity-tolerance of paper products.

Aarne et al. (2013) recently explored the use of amphiphilic diblock copolymers as a way to hydrophobize paper. A strong hydrophobic sizing effect was found with triblock copolymers having relatively short cationic end-chains and a relatively long hydrophobic central block. The cationic functionality ensured efficient retention of the additive during papermaking. The large central block was found to provide strong hydrophobization when the paper was dried with sufficient heat to melt the additive. It is not yet known whether such additives might improve the humidity-tolerance of paper products.

Impregnation with silicate

Another approach that has been used to strengthen paperboard products and to improve their performance under humid conditions has involved impregnation with a solution of sodium silicate (Walthy 1987). The cited author describes equipment that can be used, as well as presenting some test results. Impregnation with sufficiently high levels of sodium silicate made it possible, for instance, to achieve the compression strength expected for a 69-lb (337 g/m²) linerboard at a much lower basis weight of 42-lb per ream (196 g/m²). The author suggested that the solubility of sodium silicate is a major concern, presumably making such systems vulnerable to failure upon wetting. Given the promising results shown in the cited work, and also noting the widespread success of cellulose-reinforced cement products (Jorge et al. 2004; Brandt 2008; Karade 2010), it is recommended that more research be carried out related to the development of mineral-reinforced paperboard. It appears that there is a critical need for industrial and academic research to make further progress in this important branch of technology.

Cellulose nanocrystal coalescence as a dry-strength strategy

Co-crystallization at the interface between cellulosic surfaces (Nazhad and Pazner 1994; Wang et al. 2003; Newman 2004; Pönni et al. 2012) might be viewed as a possible strategy to achieve moisture-resistant bonding. The concept can be regarded as a proposed mechanism, for which the evidence has not been well developed. The approach can take its inspiration from the insolubility and dimensional stability of natural wood, in which the majority of the cellulose component is expected to be associated with crystalline nano-domains. Lindström (1986b) proposed that fibrillar aggregation and the coalescence of adjacent nanocrystals could account for contraction of the honeycomb nanostructure of kraft fibers upon drying, thus decreasing their ability to swell. This is basically the same mechanism as was proposed by Laine et al. (2006) to explain the essentially irreversible attachment of CMC to fibers under suitable conditions of high temperature, salt concentration, and time. Yuan et al. (2013) presented intriguing evidence that the state of crystallinity may also be affected by the refining of pulp, followed by drying under TAPPI standard conditions. All of these findings prompt the question as to whether it is feasible to put the same mechanism to work in creating durable bonds between fibers, rather than just within them.

In theory, a strong and moisture-resistant contribution to bonding would result if cellulosic crystallites were able to grow at the interface between two fibers in paper, resulting in coalescence of cellulosic chain segments. However, it is not certain whether or not the conditions present during the drying of paper are suitable for the type and extent of microfibrillar coalescence that would contribute to inter-fiber bonding. The predominant temperature of paper during the drying of paper, especially while the web still contains a substantial amount of water, is bounded by the boiling point of water. By contrast, Hattula (1986) observed that temperatures in the range 100 to 150 ^oC were needed in order to significantly increase the degree of crystallinity of cellulose within thermomechanical pulp or dissolving pulp fibers. On the other hand, NMR-based studies by Newman and Hemmingson (1997) and Newman (2004) detected co-crystallization, i.e. the coalescence of adjacent crystallites, during the first drying cycle of freshly pulped kraft fibers. Sheikhi et al. (2010) found similar results in the case of sugarcane bagasse fibers. It is important to note that the cited findings can be explained by closing up of pores within rather than between cellulosic fibers. Because of the manner of biosynthesis of cellulose within fibers, many of the cellulose chains within the cell wall of a kraft-pulped fiber already can be expected to be approximately in the ideal state of alignment with each other so as to allow healing between adjacent cellulose crystallites. In any case, conventional drying of paper can be expected to yield paper having ordinary properties, including substantially reversible hydrogen bonding between the fibers, allowing non-wet-strengthened paper to be readily repulped after placement into water.

Elements of a possible strategy to induce cellulose nanofibrillar coalescence at fiber junctions during the formation of paper can be drawn from published work. The term “zipper” can be used to describe the manner in which hydrogen bonding could be expected to take place between adjacent cellulosic surfaces, starting at one end and then progressing forward to achieve neatly repeating registration and efficient hydrogen bonding between adjacent cellulose chains (Hubbe 2010). For example, Akagane et al. (1979) proposed a concept of multicenter hydrogen bonding to account for the bonding contribution of a DNA-inspired imidazolic polymer that was sprayed onto handsheets as a solution. Although the relatively low energy associated with each individual hydrogen bond is not sufficient to create a permanent attachment, the multiple and regularly repeating hydrogen bonds in a cellulosic structure can be very hard to disrupt. In order for this to happen at the interfaces between fibers during the drying of paper, the cellulose needs to be in a suitable form. One approach may involve the use of low-mass cellulose chains in some fashion. Kato and Cameron (1999) noted that the hydrolytic breaking down of long cellulose chains might promote more rapid crystallization. The idea is that short polymeric chains can more easily change their molecular conformation, including their lying down flat to be incorporated into a crystalline arrangement.

Based on such findings, there appears to be a need for research in which a suitably derivatized form of cellulose becomes regenerated in-situ only when the paper web is strongly heated in the course of drying. The first step would be to prepare a meta-stable, water-soluble derivative of cellulose that reverts to cellulose form under the conditions of drying of paper. In other words, the substituent group should be susceptible to rapid hydrolysis under hot conditions. Presumably such nacent cellulose chains would tend to quickly engage in crystal-forming interactions with all nearby surfaces. An illustration of such a mechanism is given in Fig. 14, which is based on the hydrogen bonding arrangement shown by Dinwoodie (1975). If such crystallization is able to bridge between adjacent fiber surfaces, then one could expect a welding together of adjacent fibers. Effects on wet-strength development, as well as questions about recyclability, would need to be resolved.

Fig. 14. Suggested mechanism whereby a suitably designed water-soluble derivative of cellulose becomes regenerated back to its crystallizable cellulose form in situ after the paper sheet has been formed only when subjected to the intense heating associated with the drying of paper

Covalent bonding to prevent unraveling of an extensively hydrogen-bonded system

It is possible that covalent bonds, such as ester bonds within a cellulosic material might act to prevent unraveling of a hydrogen-bonded structure, acting something like the “stop” at the end of a zipper. As noted by Ruffini (1966) and Laine et al. (2000), if sufficient heat is applied to paper that has been treated with CMC, then very high paper strength can be achieved. The effect is consistent with ester formation between the carboxylic acid groups and –OH groups at the fiber surfaces. The conditions required for significant ester formation tend to be somewhat more intense than what paper experiences during conventional drying procedures.

As mentioned earlier, one type of bond that is much more likely to become fully cured during the drying of paper is the hemi-acetal bond – a type of bonding that is employed in so-called temporary wet-strength agents (Chen et al. 2002). The cited authors found that wet-strength development was favored by low pH, as might be expected for the formation of hemi-acetal bonds. Aracri et al (2012) suggested that such bonds might account for observed wet-strength effects when paper was prepared after TEMPO-mediated oxidation of the fibers. Depending on the conditions of oxidation, such procedures may lead to different proportions of aldehyde and carboxyl functions on the treated material (Isogai and Kato 1998; Isogai et al. 2010). Laleg and Pikulik (1993a,b) described use of cationic starches having sufficient aldehyde group content to provide wet strength. Xu et al. (2006) described the treatment of papermaking furnish with polycarboxylic acids and glutaraldehyde/poly(vinyl alcohol) to create cross-linking and wet strength development. There has been a need for research to demonstrate whether or not such covalent bonds might be able to block the unraveling of an adjacent sequence, planar assemblage, or three-dimensional structure in which multiple hydrogen bonds keep two fibers from peeling apart.

A more aggressive approach can be based on the use of so-called “permanent” wet-strength agents such as poly-(amidoamine-epichlorohydrin) resins. Thus, Kitaoka et al. (1999) and Laine et al.(2003b) observed very strong bonding when such a resin was added to fibers that had been carboxylated by TEMPO-mediated oxidation. Su et al. (2012) were among many researchers who have observed development of increased dry strength in addition to wet strength when permanent wet-strength agents are used. However, as already noted at the start of this section, conventional wet-strength treatments have not been noted as being promising for paper’s dry strength in highly humid situations.

“Enzymatic” bonds

A particularly creative way to achieve dry-strength improvements among fibers in paper involves the use of cellulase binding domains (Kitaoka and Tanaka 2001; Pala et al. 2001). The bonding agent employed in the cited studies consisted of a non-catalytic fragment of a cellulase protein complex. Though enzymes, like other molecules, ultimately must interact by means of electrostatic forces, van der Waals forces, and hydrogen bonding, etc., what makes them somewhat unique is their specific molecular topography that can bind with a “lock-and-key” precision to certain substrates, with high specificity. Yokota et al. (2009) reported related work in which cellulose binding domains fixed onto an acrylamide-type polymer chain were able to act as an effective dry-strength aid, achieving high retention efficiency on the fibers. Pala et al. (2001), in their study employing deactivated enzymes, observed not only increased dry strength, but also faster dewatering. Since none of these system involved active enzymes, the effects have to be attributed to the proteins at the fiber surfaces acting as bonding agents. Whether or not such approaches can improve paper’s performance under high-humidity conditions has not been established.

Chemical reaction after formation of a paper sheet

Chemical treatment of an already-formed sheet of paper has not often been considered in the literature. This is possibly due to inconvenient aspects of such procedures. As an exception, such a treatment scheme for a web of paper has been widely used for many years in the preparation of vulcanized or parchmentized paper (Van Ommen 1969; van der Reyden et al. 1993; Cartier et al. 1994; Toyo 2013). Such processing involves passing several layers of the paper together through a bath of concentrated acid (gelatinization), followed by extensive rinsing, then drying on steam-heated cylinders. The resulting sheets have been shown to have substantial resistance to delamination between the plies, in addition to high bonding strength and water resistance. Substantial wet strength is observed, even though no wet strength agent is used. These attributes appear to be consistent with the earlier discussion of microfibrillar coalescence. In other words, it appears that the treatment somehow activates the cellulose such that it is then able to join adjacent fiber surfaces together more effectively upon redrying. Notably, Cartier et al.(1994) found evidence that the fibers within parchmentized sheets are held together by poorly crystallized cellulose II. Some key barriers to implementation of this type of technology include relatively slow production speeds and brittle and translucent nature of the product, relative to typical uses of paper. Soltani et al. (2013) showed that some related properties can be achieved through the acetylation of paper after it has been formed. Water absorption was strongly suppressed, whereas fold and burst resistances were increased.

INCREASING DRY STRENGTH BY CHANGING HOW PAPER IS MADE

This section of the article will consider the unit operations of papermaking. Emphasis will be placed on how such operations might be reconfigured in ways that may achieve substantial improvements in paper strength. Presumably if the paper strength properties can be increased, then less virgin fiber would be needed to meet the requirements for different grades of paper. In general, an efficient system with a minimization of additive levels will provide a basis for low production costs. The task of finding the most efficient solution may be complex, as it requires that the conditions through the whole paper process are chosen in coordination with the different fiber alternatives and additives. Minimization of variations in paper properties needs to be a key focus, since it is often the lowest strength values, rather than the averages, that determine the amounts and qualities of fibers that are needed to meet strength specifications. For those readers who want additional background, the following sources give basic information about processing equipment and its usage for the manufacture of pulp and paper (Thorp 1991; Smook 1992; Biermann 1996; Gavalin 1998).

Advances in Refining of Cellulosic Fibers

Once choices have been made regarding the types of cellulosic fibers and the pulping procedures, the next critical issue affecting paper strength involves how the fibers are mechanically treated before papermaking, i.e. the refining process. Background information about modern refining methods have been reviewed (Baker 1995, 2000; Paulapuro 2000). To a large extent the effects of refining can be understood based on the concepts of fiber conformability, as it affects relative bonded area (Page 1969a). When applying these concepts, Batchelor et al. (1999) found that certain assumptions were needed in order to achieve good agreement when using Page’s equation to predict paper strength. In particular, it was necessary to assume that fiber coarseness was not changed during conventional refining, even though some measurements may suggest otherwise.

There appears to be a consensus among paper technologists that dry-strength properties can be maximized by selecting an advantageous level of intensity of refining (Baker 2000). In other words, the ratio of applied force to the length of crossings of bars in a refiner should be appropriate for the type of fibers to be refined. Relatively high intensity, achieved my means of using a coarse plate pattern with fewer bars, can be expected to give suitable results for virgin softwood kraft fibers, which have relatively thick cell walls. By contrast, a lower intensity, achieved with a relatively fine plate pattern, would be needed for thin-walled fibers or for recycled kraft fibers. Excessively high intensity leads to substantial “fiber cutting,” i.e. the reduction of fiber length. Too low intensity (which may be achieved in the case of softwood fibers) may fail to develop the fibers, probably because they experience only elastic strain (Croney et al. 1999).

Strategies for the refining of recycled fibers have been considered by various authors (Baker 2000; Zhang et al. 2004a; Wan et al. 2011; Olejnik et al. 2012). Oljenik found that removal of fines, followed by refining of the long-fiber portion yielded the most promising results. Notably, these results parallel those obtained much earlier by Brecht (1947), who warned not to over-refine recycled kraft fibers. As noted by McKee (1971), higher levels of refining may imply less potential for strength gains to be achieved in subsequent cycles by further refining of the fibers Osovskaya and Poltoratskii (1999) found surprisingly that freezing of fiber suspensions can yield a refining effect, leading to increased strength properties for the once-frozen pulp fibers. Such results suggest a process in which the growth of ice crystals can provide an alternative means of bringing about delamination within the cell walls of fibers, thus providing a gentle refining effect. Higher consistency refining has been shown to deliver better outcomes in certain cases (Manfredi and Claudio-Da-Silva 1986; Lundin et al. 2008). Also it should be noted that bar sharpness, which can be affected by wear and pulp cleanliness, can affect the outcome, with sharper bars generally yielding stronger refined pulp (Berger 1997).

Two particularly interesting options for future development have arisen out of a focus on the surface-specificity of refining. On the one hand, it is possible to employ low-intensity treatment that relies mainly on repeated compression of fibers; such processing can be expected to internally delaminate papermaking fibers with a minimization of external fibrillation of the fiber surfaces (Wild 1998; Seo et al. 2000; Ghosh and Vanderhoek 2001; Wang et al. 2003; Le Ny and Messmer 2007). For instance, Seo et al. (2000) showed that “impact refining,” carried out by striking a slurry of fibers repeatedly with a rod, was able to promote bonding without loss of fiber length; the described method appears to be essentially the same as the traditional method most widely used around the world for the manual preparation of fibers for papermaking before the invention of modern refining technology (Hubbe and Bowden 2009). On the other hand, refining can be carried out by passing the fibers between a rotor and stator having abrasive surfaces; such treatment can be expected to promote fibrillation of the surfaces, and it is possible to minimize the internal delamination (Kang and Paulapuro 2006; Wang et al. 2007a; Somboon and Pynnonen 2008; Somboon et al. 2009a,b). The external fibrillation was found to increase not only tensile strength and delamination resistance of paper, but it also tended to increase paper density somewhat, consistent with tighter bonding. A potential advantage of imparting mainly external fibrillation is that one can minimize damage to the internal parts of the fiber. In that way, one can preserve the fiber’s inherent strength not only for the paper product being made, but also for later generations of recycled paper products.

Cellulosic fines removal or addition

Given the limited ability of most recycled kraft fibers to accept further refining, it makes sense to reconsider an approach described over 65 years ago by Brecht (1947). It was found that highly fibrillated cellulosic fines, prepared by refining, can be used as an alternative bonding agent. Brecht and others have shown that the cellulosic fines present in recovered kraft furnish may contribute rather little to the inter-fiber bonding within the resulting paper (Brecht 1947; Laivins and Scallan 1995; Selder et al. 2002). By contrast, Brecht found that paper strength equal to that achieved during the first cycle of papermaking could be produced if one removed all of the recycled cellulosic fines and replaced them with freshly-prepared fibrillar fines. In principle, only a minor quantity of fresh cellulosic material would be needed in order to reach strength levels of commodity paper products, while avoiding over-refining and therefore preserving what is left of the inherent strength of the recycled fibers. Because results of such a strategy can be expected to depend strongly on the manner and extent of preparation of the fines, there is a critical need for related research.

It is likely that in the past papermakers have been discouraged from adding fines to papermaking furnish due to slower release of water from the wet web, which often translates into a slower rate of production. However, by use of modern chemical-based dewatering technologies (Hubbe 2005b; Hubbe and Heitmann 2007), as well as greater dewatering capabilities of modern paper forming equipment (Thorp 1991; Gavelin 1998; Lindström et al. 2009), such considerations may not be as much of a barrier as before. In addition, the recent advances in technologies related to nanocellulose, as mentioned earlier in this article (Peng et al. 2011; Ramires and Dufresne 2011; Chauhan and Chakrabarti 2012), can contribute either together with or as an alternative to the use of freshly-prepared cellulosic fines. Promising results, in term of dry strength, have been reported, especially when adding microfibrillated cellulose – a class of nanocellulose that retains high flexibility due to its content of amorphous cellulose (Ahola et al. 2008; Eriksen et al. 2008; Guimond et al. 2010; Joseleau et al. 2012; González et al. 2012).

Sheet forming and paper strength

The manner of forming of the paper web is known to have a large effect on paper’s strength. Research related to the uniformity of formation of paper, concentrating on chemical factors, was considered in an earlier review article (Hubbe 2007b). But advances in this area also will depend on the associated equipment. State-of-the-art forming equipment for paper machines, as well as their effects on paper structure have been described (Swerin and Ödberg 1996; Norman 2000; Paulapuro 2000). Theoretical advances also have been achieved relative to the forming process (Aidun 1998a,b). Because machine-made paper is typically anisotropic, usually with a higher MD strength and modulus compared to the CD (Nordström and Norman 1995a,b; Nordström 2003c,e; Nordström 2006), it has been suggested to make general comparison of strength based on the geometric mean of MD and CD values (Htun and Fellers 1986).

Niskanen et al. (1998) noted that some of the most important advances in paper machine design from the standpoint of uniformity are gap formers and the use of flexible foils. The term “gap former” implies a forming section in which the jet of fiber suspension emerges from a hydraulic headbox and lands within the closing gap between a pair of forming fabrics (Thorp 1991; Gavelin 1998). Dewatering elements such as foils and forming blades adjacent to the forming fabrics in such systems have been shown to play a key role relative to the uniformity of the resulting paper (Swerin and Ödberg 1996). Lindström et al. (2009) developed a numerical model to simulate and explain the effects of dewatering elements on the paper structure as it passes through such a twin-wire system. The concentration gradient of the paper web was predicted to be relatively uniform across the thickness of the wet web, except at very low consistencies, throughout the dewatering process, which is consistent with a thickening mechanism of dewatering (Kershaw 1980; Niskanen et al.1998). It should be noted, however, that a pure thickening mechanism is inconsistent with other observations, such as strongly uneven distributions of filler relative to the thickness dimension of paper (Zeilinger and Klein 1995).

Evidence from various studies suggests that it can be advantageous to form a paper sheet in a relatively gentle manner, teasing the water out gradually (Kershaw 1980; Manson 1991; Mitchell and Johnston 2003). In a typical Fourdrinier papermaking operation, water is removed by passing the forming fabric over a series of hydrofoils (Manson 1991). Figure 15 provides a schematic view of a wet web of paper resting on a forming fabric as it passes over a hydrofoil of a Fourdrinier paper machine. The vertical deflection of the forming fabric is greatly exaggerated in the diagram in order to emphasize the effect of a vacuum effect between the trailing edge of the hydrofoil and the wet web of paper. During this passage the wet web is kept somewhat fluidized, and the web experiences thickening to some degree, rather than undergoing pure filtration (Kershaw 1980). In other words, the paper web increases in consistency throughout its whole thickness during the dewatering process. The action of the hydrofoils, depending on their design, spacing, and angular settings, can induce a localized repeated eruption and contraction of thickness within the wet paper. When well adjusted, such localized flow events can make the paper more uniform. According to Radvan et al. (1966), although filtration is the primary mechanism by which papermaking fibers arrange themselves during a typical papermaking operation, there can be some fluidization and rearrangement within the sheet as it is acted upon by dewatering elements such as foils. As a consequence, the resulting paper has a “mixed mode structure” in which some fibers are not lined up with the plane of the sheet. However, if the action of hydrofoils is too aggressive, causing droplets of water and fiber to be flung into the air for instance, then the integrity of the sheet is lost, and much lower strength results.

Fig. 15. Schematic illustration of hydrofoil action, promoting dewatering of a wet web during papermaking on a Fourdrinier machine. Fluidization and wave action, if not too severe, can promote paper uniformity.

Circumstantial evidence of the importance of nano-scale uniformity of formation can be drawn from studies in which paper strength properties were not well correlated with the results of formation tests, such as those based on image analysis. For instance Norman (1986) reported that sometimes a stronger sheet of paper is obtained when the aggressiveness of hydrofoil-induced action is reduced in intensity, even when the change results in a less uniform visual appearance of the paper. These results were confirmed and extended in later work (Nordström and Norman 1996; Nordström 2003a,b, 2006). In general, the highest strength performance has been associated with a twin-wire roll forming type of paper machine (Nordström and Norman 1995b); in such a design no forming blades are used so there is a minimum of disruption of the initially formed fiber mat. To account for such cases, it can be hypothesized that overly aggressive efforts to break up fiber flocs within a wet web of paper can have the unintended effect of disentangling associations between sub-microscopic fibrils on the adjacent fibers. Demonstration of such effects and the possibility of overcoming such effects can be expected to remain as great challenges for the coming generations of researchers and papermakers.

Advances in ply construction

Multi-ply paper forming equipment has been in use for many decades (Attwood 1991). For instance, there is widespread use of multi-ply “cylinder formers”, which are used in the production of relatively heavy-weight paperboard products. Likewise, Fourdrinier paper machines can be set up with secondary headboxes to produce a two-ply paperboard sheet. Alternatively, damp sheets produced simultaneously on two or three forming sections can be brought together and combined during wet-pressing. All of these options give papermakers potential opportunities to place brighter or stronger materials as the outside plies of the combined sheet to maximize, for instance, brightness and stiffness (Ranger 1967; Yang 1981; Haggblom-Ahnger 1998, 1999; Navaee-Ardeh and Nazhad 2008). In addition, inherently bulky fibers, such as chemithermomechanical pulp fibers, may be placed in inner plies to increase the caliper of the product; this can be a further means to enhance stiffness (Anon. 1976). In addition, if the material within or between plies has low resistance to tensile stress perpendicular to the plane of the sheet (so-called Z-directional strength), such construction can be optimized for retaining permanent folds, as in the case of folding boxboard grades. The mechanism of such folding was discussed in a review of handmade paper attributes (Hubbe and Bowden 2009). In some other paperboard grades it can be important that internal delamination be avoided. Different shrinkage of different plies within a multi-ply board can be the cause of undesired delamination or weakness.

Some more recent developments, which still have not received widespread implementation on a worldwide scale, involve ply construction in lower basis weight grades. Specialized headboxes are being used to deliver as many as three plies, most often with two different fiber compositions, into the same dewatering zone (Bristow and Pauler 1983; Gavelin 1998). As shown by Bristow and Pauler (1983), higher stiffness can be obtained by use of ply construction, despite the relatively low basis weights of the paper produced. Such results, along with other analysis of ply purity, indicate that the plies remain sufficiently well defined, even in the product, so that the desired effects can be achieved. Haggblom-Ahnger (1999) showed that xerographic copy paper can be made with higher stiffness, formation uniformity, and smoothness by placing the long fibers (needed for tensile strength) in a central ply and hardwood pulp in the outer plies. The same author (1998) found that other advantages could be achieved by selection of the filler and other additives for different plies. Such approaches, though well proven, have been implemented at a commercial scale only to a limited extent due to capital costs and the increased complexity of multi-ply operations.

Wet-pressing advances affecting paper strength

Without wet-pressing, most paper products would be unacceptably bulky and weak; however it is important to keep in mind that one of the prime motivations for the wet-pressing of paper is economic: If all the water remaining in the sheet after the forming section needed to be evaporated, then the production rate of most papermaking operations would be drastically lower, and the amount of energy needed to evaporate the water in the paper would far exceed the heat content of steam generated by incineration of pulping liquors, bark, and other materials in a typical pulp and paper operation. The equipment and procedures that can be used to press water out of the paper web have been reviewed (Reese 1991; Paulapuro 2000).

Effects of wet-pressing on paper strength are profound (Back 1987; Vainio and Paulapuro 2007a; Wan et al. 2011). As noted by Back (1987), pressing of dry paper, as in a calendering operation, usually has little beneficial effect on strength, if at all. By contrast, higher density achieved through wet-pressing has a high correlation with elastic modulus and related strength properties. Back (1987) recommended the application of steam to heat up the web of paper entering the press nip; such heating lowers the viscosity of the water, thus decreasing the resistance to flow from the paper into the press felts. To minimize densification, while still achieving favorable strength properties of paper grades requiring maintenance of relatively high caliper, Back (1987) recommended the use of double-felted nips or extended nips, which apply a somewhat lower nip pressure for a longer time period. All of the forementioned approaches allow more of the applied load in the press nip to be borne by the fibers, since less of the load is supported by hydraulic forces. In principle, the more effective pressing of the fibers, one against the next, can be expected to lead to greater bonded area.

Increased production rates when using extended nip presses have motivated developmental efforts, as well as some key publications (Pikulik 1999; Schlegel and Kaipila 2002; Lange and Meitner 2006; Hii et al. 2012). Danielsson (2002), among others, has reported that the structure of the press felt can have a major impact on the results achieved in an extended nip press. Hii et al. (2012) showed that there can be an advantage achieved by designing a press nip such that the sheet experiences increasing pressure almost to the point of exit. Such a profile of applied pressure was found to achieve not only higher solids of the emerging web, but also higher bulk in the resulting paper. By contrast, Pikulik (1999) observed lower strength following extended-nip pressing, compared with what could be achieved with conventional pressing. A less expensive, and therefore more widely implemented approach to extending the effective length of the nip has been through the use of relatively large press rolls (Kinnunen and Kiviranta 1992; Gavelin 1998). Lacasse (2012) described advances in the rubber covers used on such rolls to further increase the effective length of the nip.

Mathur et al. (1987) showed that to some extent it is possible to substitute more effective pressing in place of more refining to enhance paper strength. The combination can be economically attractive to papermakers, since both reduced refining and higher wet-pressing conditions tend to promote dewatering. Higher rates of production then can be achieved in dryer-limited paper machine systems.

Based on analysis of sheet structure following wet-pressing, it appears that there may be some potential to optimize the density profile within paper. Studies have shown that the surface of paper tends to become densified as it is pressed against an absorbent felt during its passage through a wet-press nip (MacGregor 1983a,b; Szikla and Paulapuro 1989). As illustrated in Fig. 16, the mechanism appears to involve the same balance of hydraulic and mechanical forces resisting compaction, as mentioned earlier. Flow of water out from the paper web and into the adjacent felt results in a localized decrease in the hydraulic pressure, such that the fibers are squashed more tightly against each other. In the center of the damp web, or adjacent to a non-felted roll surface, the hydraulic component remains high, so those parts of the sheet experience less compaction.

Probably due to challenging experimental aspects, not much research has been reported about effects of wet-end chemical conditions on the effects of wet-pressing. Torgnysdøtter and Wågberg (2004) observed that higher joint strength was achieved between fibers when pressing was carried out at higher salt concentration. The effects might be explained in terms of a less swollen condition of the fibers in the presence of salt, leading to a denser structure in the pressed zone. The authors noted that increasing salt concentration will tend to decrease electrostatic forces that would tend to resist compaction. Wegner (1987) attempted to find a relationship between treatment of furnish with a cationic acrylamide-based retention aid polymer and dewatering at a wet press. Though the polymer treatment was found to promote dewatering in the forming section of the paper machine, including a significant effect on vacuum dewatering, no significant effect on wet-pressing results was observed.

Fig. 16. Conceptual illustration of densification of paper’s surface adjacent to a press felt during conventional wet-pressing in a single-felted press nip

Drying technology that can affect paper strength

If one walks along the length of a typical paper machine at a steady pace, the majority of time during that walk will have been spent adjacent to steam-heated cylinders devoted to the drying of paper. Another indication of the importance of the dryer section is the fact that paper’s tensile strength increases by a factor of about 10 as it travels from the last of the wet-presses to the reel of the paper machine (Lyne and Gallay 1954). Figure 7, earlier in the article, illustrated an exponential increase of paper strength with increasing solids content in the range between about 44 and 93% (Kuhasalo et al. 2000). Background related to the drying of paper is available from various sources (Hill 1991; Smook 1992).

Regarding strength properties, one of the most important considerations during drying is the degree to which the paper is held in tension (Green 2006). One extreme is represented by “flash drying,” a process in which fibers suspended in a current of air are allowed to shrink without constraint as they dry. Such fibers are good for producing bulky products such as fluff pulp for absorbent products, but the fibers are considered inferior for paper or paperboard products requiring high strength (de Ruvo and Htun 1983). Likewise, pulp sheets dried without restraint were found to yield lower-strength paper after they were reslurried (Mocchiutti et al. 2006). At the other extreme are typical paper machine systems in which the machine direction of paper is typically stretched by several percentage points in relative length (Barnet and Harvey 1980). The effects of drying tension also result in strong contrasts in the strength of laboratory sheets, depending on whether they are dried in a clamped position while adhering to a smooth surface, or if the sheets are allowed to shrink naturally (Higgins 2002; Green 2006). In each case, holding the paper in tension or slightly stretching it in the course of drying can be used as a strategy to increase the straightness of fiber segments in the resulting sheet (Page and Tydeman 1962; Vainio et al. 2007a,b; Retulainen et al. 1998). Htun (1986b) proposed that straining of the wet web also causes some machine-directional ordering of the fibers. By such means the tensile modulus and sheet stiffness can be increased, at least in the direction that was held in tension (Htun 1986b; Niskanen and Kärenlampi 1998; Retulainen et al. 1998). Meanwhile, properties requiring extensibility, such as resistance to impact and burst strength, can be decreased (Salminen et al. 1996). It is worth noting that the paper is completely restrained from stretching or shrinking in the plane for the sheet during the drying of many kinds of handmade papers (Hubbe and Bowden 2009) and when preparing test sheets to evaluate the strength potential of pulp at different refining levels (TAPPI Method 205). Such restraint is achieved by placing the damp sheet firmly against a smooth surface and not removing it until it is dry.

Restrained drying can be used as a strategy to improve paper’s dimensional stability in the presence of changing humidity conditions (Uesaka and Qi 1994; Vainio et al. 2007a,b). In an effort to further improve dimensional stability, Larsson and Wågberg (2008) carried out tests with a highly effective dry-strength chemical system. A polyelectrolyte multilayer (PEM) system was applied, as discussed earlier. In the case of sheets that were prevented from shrinking during the PEM bonding system made the sheets stronger but had no effect on dimensional stability. In the case of unconstrained drying, the resulting sheets were highly sensitive to humidity, even to a greater extent than the untreated sheets. Myllytie et al. (2009) found that different polymer treatments had different effects on the development of tension during drying.

Hot-pressing strategies and paper strength

An inherent problem associated with industrial drying on a paper machine is that there is relatively little restraint on the cross-directional contraction. At the same time the high temperatures associated with drying have been considered as a way to promote more effective removal of water from the sheet by application of pressure (Seth et al. 1985). These two ideas – applying heat and applying pressure – can be combined in a system called hot-pressing. Back (1987) showed that the combination can be expected to achieve higher sheet density, thus giving higher modulus. Related technologies can be called press-drying or impulse drying, depending on how the process is implemented.

Perhaps the best example of press drying involved laboratory conditions in which paperboard was dried between a smooth metal surface and blotter paper (Seth et al. 1985). For a wide range of cellulosic pulp types, higher tensile strength, modulus of elasticity, and edgewise compression strength were achieved under heated press-dried conditions in comparison to control tests in the same format but without heating applied during drying. The effects were partly attributed to a lower equilibrium moisture content of the press-dried paper. It was proposed that the lower moisture was due either to some kind of covalent crosslinking or to irreversible hydrogen bonding. In addition, in the case of fibers having relatively high content of lignin, the authors proposed that the results would be affected by flow of the hemicellulose-lignin matrix within the fibers. The latter proposal is consistent with the authors’ observation of greater collapse of the lumen structure within the fibers of press-dried paper. The greatest advantage of press drying, according to the cited paper, is that higher sheet density can be achieved, thus reducing the amount of energy needed to refine the fibers to achieve bonding. Also it was proposed that some resistance to humid conditions might be provided either by the flow of the matrix materials during wet-pressing or by auto-crosslinking reactions. Such reactions might include ester formation (see Laine et al. 2000) or the coalescence of crystalline domains, as discussed earlier (see Pönni et al. 2012).

The term “impulse drying” has been applied to systems in which paper is passed through a nip in which one surface is hot and the other side of the sheet is against a porous felt. Such an arrangement provides volume for the escape and possible condensation of both water and steam from the paper web (Back 1991; Boerner and Orloff 1994; Larsson et al. 2001; Larsson and Orloff 2001; Lucisano et al.2003). In principle, rapid evaporation of water at the heated surface can be expected to create superheated steam, which then tends to push water and steam through the sheet and out the other side. It appears that the main limiting factor discouraging implementation of the technology has been a tendency of the sheet to delaminate as it leaves the nip. Boerner and Orloff (1994) and Larsson and Stenstrom (1998) found that such delamination could be minimized by keeping the basis weight as well as the refining degree low. Unfortunately, those criteria rule out many products of greatest interest to papermakers. Larsson and Orloff (2001) found more promising results when using dwell times corresponding to extended nip pressing. Both paper dryness after the nip and the resulting paper strength were judged superior to what was achieved by conventional pressing, then drying. Mendes et al. (2002) found that though the use of cationic starch with impulse drying was able to enhance paper strength, it also worsened problems associated with sticking of the sheet to the hot surface.

Condebelt drying

In view of the interesting, but in the end problematic results that have been obtained with impulse drying, there has been sufficient motivation for researchers to consider using a radically different type of equipment. In particular, a key limitation of impulse drying, as described above, has been the relatively short time during which the web passes through a press nip. A longer exposure to heat and pressure would allow more opportunity for moisture to escape from the sheet without causing delamination. Such treatment has been effectively achieved at a commercial scale in the form of two production-scale installed drying systems having the trade name Condebelt (Kunnas et al. 1993; Retulainen 2003; Kiviranta et al.2000). As diagrammed in Fig. 17, these systems work by drying the sheet in a sandwich between two solid steel belts, one heated and the other cooled. On the cooled side the sheet is backed by a course fabric, offering sufficient void volume within which condensate can collect.

Superior levels of elastic modulus have been routinely achieved by means of Condebelt drying, in particular when using relatively high-yield fibers. Kiviranta et al. (2000) estimated that the increases in compression strength can be sufficient to enable papermakers to reduce basis weights by about 20 to 30%. Such strength improvements can be important in such products as linerboard, which need to be able to support heavy loades in the stacking of corrugated boxes. Some of the effects that occur in the course of impulse drying or Condebelt drying appear to result from a combination of pressure and the thermal softening of lignin and hemicellulose in the presence of sufficient moisture (Back et al. 1967; Back and Salmén 1982; Irvine 1984). Kiviranta et al. (2000) estimated that the side of the sheet adjacent to the heated steel belt remains near to 120 ^oC for an extended period. By contrast, during conventional drying the paper web is expected to remain within the range of only about 65 to 75 ^oC for most of its passage through the dryers (Kiiskinen et al. 2000).

Fig. 17. Schematic drawing of Condebelt drying system (redrawn based on concept shown by Retulainen (2003)

Pynnönen et al. (2002) showed that even inherently very stiff high-temperature thermomechanical pulp (HTMP) fibers, which usually are quite deficient in bonding ability, are able to soften sufficiently in a Condebelt system so as to achieve good bonding. Microscopic analysis has revealed a tendency for greater deformation of individual fibers during Condebelt drying, with an intense densification at fiber crossing points (Kunnas et al. 1993).

Another factor that can be expected to contribute to the superior strength of Condebelt sheets is the fact that the web is prevented from shrinking at all in the plane of the sheet – either in the lengthwise or the crosswise direction – throughout the drying process (Kiviranta et al 2000). As a consequence, all of the fibers tend to become stretched, straightened, and to use a term from Giertz (1964), activated. In other words, an applied tensile stress will tend to be shared simultaneously by multiple fibers and bonded areas, rather than the stress being concentrated, thus causing early failure at points of stress concentration.

The intensive nature of the Condebelt system, as well has the high levels of bonding that are achieved, can lead to concern about the quality of the fibers after they are recycled (e.g. see Pycraft and Howarth 1980). Initial evaluations have not revealed greater loss of strength potential in comparison to the recycling of kraft paperboard that has been conventionally dried (Kiviranta et al. 2000). A likely explanation is that the tendency of the Condebelt drying system to hold the fibers straight and minimize kinks probably would compensate, to some degree, for the expected loss of bondability when kraft fibers are intensively dried (Weise 1998). Any overdrying effects may be mainly experienced by the upper layer of the sheet facing the heated steel belt; it is likely that the saturated steam temperature during Condebelt is mainly controlled by the temperature of the cooled belt and that over-drying is avoided for most of the sheet thickness. It has been shown that some reslushed Condebelt-dried paper can be stronger than cylinder-dried paper after recycling (Retulainen 2003). There was indication that the fibers were flattened by the Condebelt drying and that they remained flat after being dispersed in water. The consequence of such flattening can be a greater area of bonding between fibers in the recycled sheets. A potentially negative feature of Condebelt paper, depending on the proposed application, is that one side of the sheet will have a very pronounced pattern, i.e. very severe “wire mark”.

LIFE CYCLE ISSUES

As stated in the Introduction, the initial goal of this article has been to examine published information concerning how to achieve paper strength requirements with less use of freshly pulped cellulosic fibers, i.e. with lower basis weight, by using minerals in place of some of the fiber mass, or by using higher amounts of recycled fibers. As has been shown in the preceding pages, the literature gives evidence of a great deal of progress in achieving such goals. The present section deals with another kind of question – whether reducing the amount of virgin fiber used in papermaking is necessarily a net benefit to the environment. As will be shown, the evidence is mixed and there is a need for detailed life cycle assessments (LCAs) that take into account the unique features of different strength-enhancing strategies.

When attempting to judge environmental impacts, a primary question to address is whether or not cellulosic resources are likely to be utilized at a rate that exceeds their photosynthetic replacement. According to a report by the Pöyry company (2011), the proportion of fresh wood pulp in the fiber mix has been decreasing for many years, and by 2011 it had fallen to 41% of the materials used worldwide for papermaking. If one were to take into account the filler content of many paper grades, then the estimated proportion of fresh wood fibers would be yet lower. Furthermore, the absolute consumption of fresh wood pulp has been relatively unchanged since 1995. In Western Europe and North America the amount of wood harvested has decreased in recent years. As of 2010 the potential wood harvest is said to be greater than the actual wood harvest.

Even if one makes the assumption that the world will not soon reach its capacity to replace forest resources that are being used for pulp manufacture, there still are various other reasons that could justify efforts to reduce reliance on fresh fiber resources. These include possible reductions in fresh water consumption, reduced emissions to the air, reduced electrical energy consumption (partly by less need to refine the pulp), and reduced thermal energy consumption (partly due to higher dry solids content after wet pressing).

Hekkert et al. (2002) obtained LCA results showing that the production and consumption of paper products contribute to the emission of greenhouse gases. Various approaches to reducing such usage of resources were considered, including the use of thinner paper, more efficient printing methods, duplex copying, and printing on demand. The authors estimated that a 37% reduction in paper usage would have been theoretically possible over the course of the next dozen years by use of a combined approach. Lowering of basis weights was judged to be the most important opportunity. Print-on-demand technology was judged to be second in importance relative to its potential to conserve resources and avoid the production of greenhouse gases.

LCA studies invariably deal with choices. For instance, the recycling of paper to recover the fibers can be compared with other alternatives such as incineration or the substitution of reusable products in place of single-use products (Hubbe 2007c, 2008). Also there are various details to consider regarding how the processing of fibers during the initial pulping and papermaking may affect their ability to be recycled (Hubbe 2010). For example, as noted earlier, loss of hemicellulose during processing, especially when the paper is subjected to acidic conditions during papermaking, will tend to hurt the subsequent quality of the fibers after their recovery and repulping.

A combined optimization and simulation model by Bystrom and Lonnstedt (1997a) showed that a mix of production of paper from primary resources and from recycled paper made the best sense in the cases considered, though the results depended strongly on location. In southern Europe, where forest resources are limited, the model showed a minimization of environmental impact when 53% of the fiber is obtained by recycling. By contrast, in the Nordic countries the model predicted that a utilization rate of about 30% fibers from recycling would maximize environmental benefits. Another study by the same authors (Bystrom and Lonnstedt 1997b) showed that fiber recycling generally can be expected to have a negative impact on the environment due to its greater dependency on electrical energy from the grid. This is because, unlike the kraft pulping process, the operations cannot benefit from the fuel value obtained by the combustion of lignin and other organics in spent pulping liquor. The study results suggested that waste paper ought to be incinerated, with the production of electricity, thus displacing the use of fossil fuels used in power plants. Various other LCA studies have similarly concluded that incineration of used paper products, combined with recovery of their energy content, will yield a net environmental benefit compared to recycling (Gains and Stodolsky 1996). The cited authors made an exception in the case of used newspapers and corrugated containers, since in those cases the fibers are relatively easily recycled. Old newspaper (ONP) pulp can be readily made into recycled newsprint paper, and old corrugated container (OCC) pulp can be made into containerboard with relatively simple processing. As an alternative to incinerating the used paper, the cited authors also advocated the use of wastepaper resources for production of cellulosic insulation. Pajula and Kärnä (1995) predicted only marginal differences in sustainability outcomes based on LCA when comparing the use of virgin fibers, full recycling, and full use of incineration to recover the energy content of used paper. Remarkably, the incineration option was predicted to release lower net amounts of CO₂ and SO₂ than the recycling option. Both options were regarded as being far superior to landfilling, however, since landfilling was predicted to result in the release of substantial amounts of methane, nitrogen oxides, and chemical oxygen demand in typical installations.

Counsell and Allwood (2007) took a more imaginative approach to envisioning possible alternatives to current practices. Their analysis considered opportunities to bypass different stages in the conventional paper-based publishing cycle. Recycling of waste paper was compared relative to its incineration, landfilling, the use of freshly harvested fiber resources, and a hypothetical process that the authors called “un-printing”. Of all of these options, un-printing was projected to have the highest potential benefit in terms of the environment. Unfortunately, it is not clear how one would actually implement a process of removing and then re-applying print to existing pieces of paper, which then would need to be reassembled into the proper format for distribution to the readers.

In view of the fact that many of the cited life-cycle studies cited above cast doubt on environmental benefits of paper recycling and related technologies, what then can be the justification for attempting to use less virgin fiber when making paper to meet various requirements? It turns out that there are several strong reasons:

Life cycle assessment studies as cited above typically are based on an assumption that adequate supplies of harvestable wood or other plant material are presently available and that the current growth rates are sufficient to keep up with demand. The reality is that if all recycling operations were immediately converted to use of freshly pulped fibers, then there would be quite visible reductions in the amounts of standing timber in the world’s forested areas. Though there have been some claims that the amount of harvestable timber has been stable in North America (AFPA 2013; Twosides 2013), throughout the world there has been a strong and persistent trend toward the loss of forested area and its conversion to agricultural land or for other human needs (Mather 2005; Humphreys 2006; Douglas and Simula 2010).
The European Union has established a target to move to a sustainable, low-carbon bioeconomy by 2050 (CEPI 2011). Such efforts can be expected to result in scarcity and increased prices of virgin cellulosic materials.
Though, in theory, yet higher levels of freshly pulped fiber can be sustainably produced from plantation forests and from other planted fiber sources, such as grasses, there are environmental costs to be considered in each such case. Plantation forestry typically requires substantial use of fertilizers and pesticides in order to meet the expectations of the growers (da Silva 2013; Menezes et al. 2012). Also there are concerns that the widespread planting of monoculture crops will fail to support the natural diversity of other plant and animal species that make up the world’s natural environment (Bremer and Farley 2010; Hartmann et al. 2010).
The chemical-based pulping facilities that would be needed to satisfy all of the world’s present usage of cellulosic fiber based on fresh raw materials do not presently exist. A state-of-the-art pulping facility tends to be very large and can cost billions of dollars (Mitchell 1994; Anon. 2000; RISI 2012). Economy-of-scale issues are favoring the construction of very large pulping facilities, which then require raw materials to be transported longer distances (Gonzalez-Garcia et al. 2009). Though paper recycling facilities also entail high costs and often higher relative amounts of electricity (Merrild et al. 2009; Iosip et al.2012), such facilities can be smaller (Koepenick 1995; Shearin 1995), a factor that can minimize transportation distances of collected materials.
Finally, there can be wisdom in widely-held opinions. The public has learned, often as a result of personal experience that authoritative-sounding statements can later turn out to be unsupportable. Methods of life cycles assessment have not yet reached a level of reliability and uniform application that has been achieved in more mature fields of technology, such as engineering or chemistry. The jury of public opinion still favors a frugal usage of the world’s cellulosic fiber resources, and in the end we can expect the jury or public opinion to decide the case.

SUMMARY STATEMENTS

As illustrated in Fig. 18, publications considered in this review article have documented numerous promising steps that can be implemented either singly or in combination in order to reduce the amounts of freshly pulped cellulosic fibers that need to be used in the manufacture of particular paper products. As shown, various contributing measures can be implemented at many different stages of the process. Though the technologies to carry out many of these steps are already well known, others merit further research.

Because paper’s strength depends on many different inter-dependent factors, one needs to be cautious when projecting benefits from a given improvement; strengthening a link in a chain may merely cause the chain to break elsewhere. The inherent physical properties of the fibers themselves, and fundamental aspects such as hydrogen bonding within cellulosic materials, imply that there will be “ceilings” beyond which it is not feasible to increase paper’s strength. For instance, a recent study has shown that the maximum lengthwise strength of individual cellulosic crystals from wood is about 3 GPa (Saito et al.2013). If a higher value is sought, then perhaps one needs to consider using something other than cellulose. Another danger is that exotic measures to enhance paper’s strength may harm other attributes, such as ordinary paper’s relatively low cost, its ability to be recycled, and its generally non-toxic nature.

Fig. 18. Summary of promising contributions to increased fiber-efficiency of paper products and their broad distribution across the entire cycle of production

Numerical values given in Table A of the appendix can provide a general idea of the extent to which the main dry-strength properties (such as tensile breaking load) can be increased by various strategies that have been demonstrated in the literature. Commonly used measures such as refining are clearly very important. Also, it is clear that further strength gains by factors in the range between 1 and 2 can be achieved by use of a wide range of technologies. Some cautionary statements deserve to be repeated:

The default conditions used in different studies cover a very wide range. In principle it is easier to achieve large gains in paper strength when using fibers that are initially deficient in inter-fiber bonding ability.
Many of the individual strategies listed in Table A are either overlapping or mutually inconsistent. For instance, if inter-fiber bonding is improved by one means, then there may be limited additional benefits that can be achieved by simultaneously applying a different strategy to do the same thing.
The nature of cellulosic materials imposes some inherent limits on the maximum strength of paper. Even if inter-fiber bonding is increased greatly, the strength of paper cannot exceed the combined strength of the component fibers.
Certain strength-enhancing strategies may entail use of exotic chemistry that is not environmentally friendly.

A variety of potential profit-making opportunities are apparent from the present review, though each potential opportunity comes with its own set of costs and risks. To give just three examples, there are opportunities for chemical companies to produce the next generations of dry-strength agents. There are opportunities for other companies to improve such equipment as the fabrics used to transport paper during the drying of paper; higher and more uniform tension controls for such fabrics, as well as mechanisms to hold paper firmly against uni-run dryer fabrics, can be expected to decrease shrinkage of the paper, resulting in higher tensile modulus and other properties. And when dealing with discarded paper products, there are business opportunities related to the more efficient and effective sorting of mixed wastes, thus making it possible to direct each component of a mixed waste stream to its most valuable or its most environmentally justifiable usage (Iosip et al. 2010; Rahman et al. 2012; Ramasubramanian et al. 2012; Miranda et al. 2013).

Important advances in technology to reduce usage of fresh fibers in paper products are probably most likely to take place in grade segments that are relatively large and expanding. Examples include containerboard and carton board. The innovations most likely to see widespread adoption will be those that combine technological feasibility with high process efficiency and opportunities to save costs. Cost analysis could be considered as a focus of future studies and reviews.

One of the foremost challenges facing major initiatives to address the sustain-ability of papermaking operations may involve the communication of information in a form that evokes confidence and agreement – or at least acquiescence – from a public that has become somewhat jaded by conflicting statements about environmental issues. Because LCA studies are inherently complex and always include many simplifying assumptions, often not communicated to the general public, one can expect a natural distrust of findings that appear to support one scenario over another. In view of such public doubt, reductions in the amounts of virgin fiber that are used in meeting a given customer need for paper – especially fibers from natural forests – can serve as a practical proxy in place of deeper LCA work. Though reductions in the amounts of freshly-pulped fiber do not tell the full story, they at least are a concrete step from the viewpoint of a common citizen, who in the end will serve as the judge in a democratic system.

ACKNOWLEDGMENTS

The author is much indebted to the following individuals who volunteered their time to check a draft and to suggest corrections and additions: Eero Kontturi (Aalto University), Bengt Nordstöm (SCA R&D Centre), Anna Vainio (National Board of Patents and Registration of Finland), Elias Retulainen (VTT, Finland), and Warren Batchelor (Australian Pulp and Paper Institute; Dept. Chem. Eng., Monash University). Errors that remain in the article are entirely the fault of the author.

REFERENCES CITED

Aarne, N., Kontturi, E., and Laine, J. (2012a). “Carboxymethyl cellulose on a fiber substrate: The interactions with cationic polyelectrolytes,” Cellulose 19(6), 2217-2231.

Aarne, N., Kontturi, E., and Laine, J. (2012b). “Influence of adsorbed polyelectrolytes on the pore size distribution of water-swollen biomaterial,” Soft Matter 8, 4740-4749.

Aarne, N., Laine, J., Hänninen, T., Rantanen, V., Seitsonen, J., Ruokolainen, J., and Kontturi, E. (2013). “Controlled hydrophobic functionalization of natural fibers through self-assembly of amphiphilic diblock copolymer micelles,” ChemSusChem 6(7), 1203-1208.

AFPA (2013). “Forestry,” and “Forest facts,” American Forest and Paper Association, http://www.afandpa.org/issues/forestry, Accessed Aug. 28, 2013.

Ahola, S., Osterberg, M., and Laine, J. (2008). “Cellulose nanofibrils-adsorption with poly(amideamine) epichlorohydrin studied by QCM-D and application as a paper strength additive,” Cellulose 15(2), 303-314.

Ahrenstedt, L., Oksanen, A., Salminen, K., and Brumer, H. (2008). “Paper dry strength improvement by xyloglucan addition: Wet-end application, spray coating and synergism with borate,” Holzforschung62, 8-14.

Aidun, C. K. (1998a). “Quantitative evaluation of the forming jet delivered from four different hydraulic headboxes using high speed digital imaging,” TAPPI J. 81(4), 172-179.

Aidun, C. K. (1998b). “Growth and decay of streaks and waves on the forming table,” TAPPI J. 81(5), 159-162.

Akagane, K., Allan, G. G., Dutkeiwicz, J., Liu, T. M., and Palicka, J. (1979). “Fiber surface modification. 20. Multicenter hydrogen bonding, a new concept in adhesion,” Paperi Puu 61(10), 624-628, 634.

Alanko, K., Paulapuro, H., and Stenius, P. (1995). “Recyclability of thermomechanical pulp fibers,” Paperi Puu 77(5), 315-317, 319, 321, 323-325, 327-328.

Al-Dajani, W. W., and Tschirner, U. W. (2008). “Pre-extraction of hemicelluloses and subsequent kraft pulping. Part 1: Alkaline extraction,” TAPPI J. 7(6), 3-8.

Alince, B. (1975). “Development of paper tensile strength by periodate oxidation,” Svensk Papperstidning 78(7), 253-257.

Alince, B., Lebreton, R., and St.-Amour, S. (1990). “Using cationic starch in filled papers,” TAPPI J. 73(3), 191-193.

Alince, B., Vanerek, A., de Oliveira, M. H., and van de Ven, T. G. M. (2006). “The effect of polyelectrolytes on the wet-web strength of paper,” Nordic Pulp Paper Res. J. 21(5), 653-658.

Allan, G. G., Aravanuthan, R., Christien, C., and Reghuraman, M. K. (1985). “Chemical modification of pulp refining rates,” in Punton (ed.), Papermaking Raw Materials, Mechanical Eng. Publ. Ltd., UK, Vol. 2, 879-894.

Allan, G. G., and Reif, W. M. (1971). “Fiber surface modification. The steriotopo-chemistry of ionic bonding in paper,” Svensk Papperstidning 74(18), 563-570.

Andreasson, B., Forsström, J., and Wågberg, L. (2003). “The porous structure of pulp fibres with different yields and its influence on paper strength,” Cellulose 10(2), 111-123.

Andrews, J. D., and Hart, P. W. (2013). “Improving pulp yield for integrated southern hardwood kraft mills – Significance and impact on chemical recovery, steam and power generations, and bleaching,” TAPPI J. 12(2), 41-53.

Ankerfors, C., Lingström, R., Wågberg, L., and Ödberg, L. (2009). “A comparison of polyelectrolyte complexes and multilayers: Their adsorption behaviour and use for enhancing tensile strength of paper,” Nordic Pulp Paper Res. J. 24(1), 77-86.

Ankerfors, C., Pettersson, T., and Wågberg, L. (2012). “AFM adhesion imaging for the comparison of polyelectrolyte complexes and polyelectrolyte multilayers,” Soft Matter 8(32), 8298-8301.

Annergren, G. E. (1996). “Strength properties and characteristics of bleached chemical and (chemi)mechanical pulps,” in: Dence, C. W. (ed.), Pulp Bleaching. Principles and Practice, TAPPI Press, Atlanta, Sec. 7, Ch. 3, pp. 717-747.

Anon. (1976). “Folding boxboard grades (from Finland),” Papier Kunststoff Verarbeiter 11(9), 58, 60-61.

Anon. (2000). “Greenfield kraft pulp mill plans advancing,” Pulp and Paper Project Report, 20(1), 1.

Aracri, E., Valls, C., and Vidal, T. (2012). “Paper strength improvement by oxidative modification of sisal cellulose fibers with laccase-TEMPO system: Influence of the process variables,” Carbohydr. Polym. 88(3), 830-837.

Asakura, K. (2003). “Effects of internal addition of fatty acid diamide salts on sheet properties,” Nordic Pulp Paper Res. J. 18(2), 188-193.

Atalla, R. H. (1977). “The full potential of native cellulose fibers,” Southern Pulp Paper Manufacturer 40(8), 12-15.

Atkins, J. F. (ed.) (1979). The Paper Machine Wet Press Manual, TAPPI Press, Atlanta, 104 pp.

Attwood, B. W. (1991). “Multi-ply forming,” in: Thorp, B. A. (ed.), Paper Machine Operations, Pulp and Paper Manufacture series, Vol. 7, TAPPI, Atlanta, Ch. 10, pp. 237-259.

Aulin, C., Johansson, E., Wågberg, L., and Lindström, T. (2010). “Self-organized films from cellulose I nanofibrils using the layer-by-layer technique,” Biomacromolecules 11(4), 872-882.

Avolio, R., Bonadies A., I., Capitani, D., Errico, M. E., Gentile, G., and Avella, M. (2012). “A multitechnique approach to assess the effect of ball milling on cellulose,” Carbohydr. Polym. 87(1), 265-273.

Back, E. (1987). “Using the wet press to optimize paper properties,” Paper Technol. Ind. 8(3), 454-455, 458-459.

Back, E. L. (1991). “Press drying/ impulse drying, the author’s answer to Lindsay,” TAPPI J. 74(9), 239-241.

Back, E. L., Htun, M. T., Jackson, M., and Johanson, F. (1967). “Ultrasonic measurements of the thermal softening of paper products and the influence of thermal auto-cross-linking reactions,” TAPPI50(11), 542-547.

Back, E. L., and Salmén, N. L. (1982). “Glass transitions of wood components hold implications for molding and pulping processes,” TAPPI 65(7), 107-110.

Bai, L. K., Hu, H. R., and Xu, J. F. (2012). “Influences of configuration and molecular weight of hemicelluloses on their paper-strengthening effects,” Carbohydr. Polym. 88(4), 1258-1263.

Baker, C. F. (1995). “Good practice for refining the types of fiber found in modern paper furnishes,” TAPPI J. 78(2), 147-153.

Baker, C. (ed.) (2000). Refining Technology, Pira International Ltd., Leatherhead, Surrey, UK.

Barnet, A. J., and Harvey, D. M. (1980). “Wet web characteristics and relation to wet-end draws,” Pulp and Paper Canada 81(11), 60-69.

Barzyk, D., Page, D. H., and Ragauskas, A. (1997). “Acidic group topochemistry and fibre-to-fibre specific bond strength,” J. Pulp Paper Sci. 23(2), J59-J61.

Batchelor, W. J., Kure, K. A., and Ouellet, D. (1999). “Refining and the development of fibre properties,” Nordic Pulp Paper Res. J.14(4), 285-291.

Batchelor, W. J., and Kibblewhite, R. P. (2006). “Calculation of relative bonded area and scattering coefficient from sheet density and fibre shape,” Holzforschung 60(3), 253-258.

Batten, G. L., and Nissan, A. H. (1987). “Unified theory of the mechanical properties of paper and other H-bond-dominated solids. 1,” TAPPI J. 70(9), 119-123.

Battista, O. A., Coppick, S., Howsmon, J. A., Morehead, F. F., and Sisson, W. A. (1956). “Level-off degree of polymerization – Relation to polyphase structure of cellulose fibers,” Indus. Eng. Chem. 48(2), 333-335.

Baty, J. W., Maitland, C. L., Minter, W., Hubbe, M. A., and Jordan-Mowery, S. K. (2010). “Deacidification for the conservation and preservation of paper-based works: A review,” BioResources 5(3), 1955-2023.

Baum, G. A. (1991). “Sheet structure considerations – Paper as an engineered material,” in: Pulp and Paper Manufacture, Vol. 7, Paper Machine Operations, Thorp, B. A. (ed.), TAPPI Press, Atlanta, Ch. 3, 54-84.

Bawden, A. D., and Kibblewhite, R. P. (1997). “Effects of multiple drying treatments on kraft fibre walls,” J. Pulp Paper Sci. 23(7), J340-J346.

Beaudoin, R., Gratton, R., and Turcotte, R. (1995). “Performance of wet-end cationic starches in maintaining good sizing at high conductivity levels in alkaline fine paper,” J. Pulp Paper Sci. 21(7), J238-J243.

Becka, L. (1979). “Effect of chemical additives on energy usage,” Papir a Celuloza 34(5), 129-130.

Beghello, L., Long, L. Y., and Eklund, D. (1997). “Laboratory study on carboxymethyl-cellulose as a wet-end additive in paperboard making,” Paperi Puu 79(1), 55-57.

Berger, T. H. (1997). “The impact of plate material on pulp quality over time,” Proceedings 4^th International Refining Conference, PIRA International, Leatherhead, UK., paper 18.

Berthold, J., and Salmén, L. (1997). “Effects of mechanical and chemical treatments on the pore-size distribution in wood pulps examined by inverse size-exclusion chromatography,” J. Pulp Paper Sci. 23(6), J245-J253.

Bhaduri, S. K., Ghosh, I. N., and Deb Sarkar, N. L. (1995). “Ramie hemicellulose as beater additive in paper making from jute-stick kraft pulp,” Industrial Crops Prod. 4, 79-84.

Bhardwaj, N. K., Bajpai, P., and Bajpai, P. K. (1997). “Enhancement of strength and drainage of secondary fibers,” APPITA J. 50(3), 230-232.

Bhat, G. R., Heitmann, J. A., and Joyce, T. W. (1991). “Novel techniques for enhancing the strength of secondary fiber,” TAPPI J.74(9), 151-157.

Bi, L. J. (2012). “Research on corrugated cardboard and its application,” Advan. Mater. 535-537, 2171-2176.

Bi, S., Wang, C., Cao, Q, and Zhang, C. (2004). “Studies on the mechanism of hydrolysis and polymerization of aluminum salts in aqueous solution: Correlations between the “Core-links” model and “Cage-like” Keggin-Al₁₃ model,” Coord. Chem. Revs. 248, 441-455.

Biermann, C. J. (1996). Handbook of Pulping and Papermaking, 2^ndEd., Academic Press, San Diego.

Bigand, V., Pinel, C., Da Silva Perez, D., Rataboul, F., Petit-Conil, M., and Huber, P. (2013). “Influence of liquid or solid phase preparation of cationic hemicelluloses on physical properties of paper,” BioResources 8(2), 2118-2134.

Blomstedt, M., Kontturi, E., and Vuorinen, T. (2007). “Surface modification of eucalyptus pulp by carboxymethylcellulose: Effects on fiber properties and sheet strength,” O Papel 68(6), 51-63.

Bobalek, J. F., and Chaturvedi, M. (1989). “The effects of recycling on the physical properties of handsheets with respect to specific wood species,” TAPPI J. 72(6), 123-125.

Boerner, J., and Orloff, D. (1994). “Effects of basis weight and freeness on sheet permeability and critical impulse-drying temperature,” TAPPI J. 77(2), 163-168.

Boonyasarn, A., Harte, B. R., Twede, D., and Lee, J. L. (1992). “The effect of cyclic environments on the compression strength of boxes made from high-performance (fiber-efficient) corrugated fiberboard,” TAPPI J. 75(10), 79-85.

Bottéro, Y. T., Poireir, J. E., and Fiessinger, F. (1980). “Study of partially neutralized aqueous aluminum chloride solutions: Identification of aluminum species and relation between the composition of the solutions and their efficiency as coagulant,” Prog. Water Tech. 13(1), 601-612.

Bovin, A., Hartler, N., and Teder, A. (1973). “Changes in pulp quality due to repeated papermaking,” Paper Technol. 14(10), 261-264.

Bown, R. (1983). “Die Einflüsse der Teilchengröße und Teilchenform von Papierfüll-stoffen auf die Papiereigenschaftenn,” Wochenbl. Papierfabr. 111(20), 737-740.

Bown, R. (1985a). “Review of methods for increasing filler loadings,” Paper Technol. Ind. 26(10), 289-292.

Bown, R. (1985b). “The relationship between strength and light scattering coefficient for filled papers,” in: Papermaking Raw Materials. Their Interaction with the Production Process and their Effect on Paper Properties, Trans. 8^th Fund. Res. Symp., Oxford, Punton, V. (ed.), Mechanical Eng. Publ. Ltd., London, Vol. 2,543-576.

Bown, R. (1998). “Particle size, shape, and structure of paper fillers and their effect on paper properties,” Paper Technol. 39(2), 44-48.

Brandt, A. M. (2008). “Fibre reinforced cement-based (FRC) composites after over 40 years of development in building and civil engineering,” Composite Structures 86(1-3), 3-9.

Brännvall, E., Eriksson, M., Lindström, M. E., and Wågberg, L. (2007). “Fibre surface modifications of market pulp by consecutive treatments with cationic and anionic starch,” Nordic Pulp Paper Res. J. 22(2), 244-248.

Brecht, W. (1947). “Über die Eigenschaftsveränderungen eines Papiers bei seiner mechnischen Regenierung,” Das Papier 1(1/2), 16-21.

Bremer, L. L., and Farley, K. A. (2010). “Does plantation forestry restore biodiversity or create green deserts? A synthesis of the effects of land-use transitions on plant species richness,” Biodivers. Conserv.19(14), 3893-3915.

Bristow, J. A., and Pauler, N. (1983). “Multi-layer structures in printing papers,” Svensk Papperstidning 86(15), R164-R172.

Brooks, K., and Meagher, J. (1982). “The increasing role of calcium carbonate in the paper industry,” Paper (London) 198(5), 18-27.

Brouillette, F., Paradis, J., and Lafreniere, S. (2010). “Pilot paper machine production of newsprint using high filler loads and dry strength technologies,” Pulp Paper Can. 111(5), 38-42.

Brouwer, P. H. (1997). “Anionic wet-end starch in, size-press out?,” Wochenbl. Papierfabr. 125(9), 928-937.

Brunauer, E. (2004). “Trends in corrugated packaging,” Wochenbl. Papierfabr. 132(18), 1063.

Burnett, R., and l’Anson S. J. (2003). “The nature of strength reduction when newsprint is calendered,” TAPPI J. 2(3), 8-12.

Bystrom, S., and Lonnstedt, L. (1997a). “Paper recycling: Environmental and economic impact,” Resources Conserv. Recycling21(2), 109-127.

Bystrom, S., and Lonnstedt, L. (1997b). “The economic and environmental impact of paper recycling,” Critical Reviews Environ. Sci. Technol. 27, S193-S211.

Campbell, W. B. (1934). “Hydration and beating of cellulose pulps,” Ind. Eng. Chem. 26, 218-219.

Campbell, W. B. (1947). “Academic aspects of paper stock preparation,” Tech. Assoc. Papers 30(6), 177-180.

Campbell, W. B. (1959). “The mechanism of bonding,” TAPPI 42(12), 999-1001.

Canedo-Arguelles, M., Kefford, B. J., Piscart, C., Prat, N., Schafer, R. B., and Schulz, C. J. (2013). “Salinisation of rivers: An urgent ecological issue,” Environ. Pollut. 173, 157-167.

Cao, B., Tschirner, U., and Ramaswamy, S. (1998). “Impact of pulp chemical composition on recycling, TAPPI J. 81(12), 119-127.

Cao, B., Tschirner, J., and Ramaswamy, S. (1999). “Study of changes in wet-fiber flexibility and surface condition of recycled fibers,” Paperi ja Puu/Paper and Timber 81(2), 117-122.

Cardwell, R. D., and Alexander, S. D. (1977). “Effects of recycling on softwood kraft pulp properties,” APPITA 30(4), 327-333.

Carlsson, G., Lindström, T., and Söremark, C. (1977). “The effect of cationic polyacryl-amides on some dry strength properties of paper,” Svensk Papperstidn. 80(6), 173-177.

Carr, M. E., Hamerstrand, G. E., Hofreiter, B. T., and Russell, C. R. (1977). “A polysalt complex for wet-end addition,” TAPPI 60(12), 148-149.

Carr, M. E., Hofreiter, B. T., Hamerstrand, G. E., and Russell, C. R. (1974). “Interpolymer from starch xanthate and polyamide-polyamine-epichlorohydrin resin in linerboard and newsprint,” TAPPI57(10), 127-129.

Carson, C. G., and Popil, R. E. (2008). “Examining interrelationships between caliper, bending, and tensile stiffness of paper in testing validation,” TAPPI J. 7(12), 17-24.

Cartier, N., Eschaffre, P., Mathevet, F., Chanzy, H., and Vuong, R. (1994). “Structure and recycling of vegetable parchment,” TAPPI J.77(5), 95-101.

Caulfield, D. F. (1994). “Ester crosslinking to improve wet performance of paper using multifunctional carboxylic acids, butanetetracarboxylic and citric acid,” TAPPI J. 77(3), 205-212.

Cengiz, C. (2011). “Nano-hybrid surface coatings on uncoated papers from softwood fibers and woodfree fibers and printability,” Asian J. Chem. 23(8), 3747-3754.

Centola, G., and Burruso, D. (1958). “The influence of electrolytes and certain substantive azo-dyes on the rate of beating,” in Bolam, F. M. (ed.), Fundamentals of Papermaking Fibers, Trans. Symp. Cambridge, 1957, Tech. Sec. British Paper and Board Maker’s Assoc., London, UK, 349-375.

CEPI (2011). “Unfold the future. The forest fiber industry. 2050 roadmap to a low-carbon bioeconomy,” http://www.unfoldthefuture.eu/, downloaded Nov. 2013.

Chan, L. L. (1976). “Dry strength resins: Useful tools for paper making,” Pulp Paper Can. 77(6), T93-T95.

Chandra, S. (1997). “Effluent minimization – A little water goes a long way,” TAPPI J. 80(12), 37-42.

Chauhan, V. S., and Chakrabarti, S. K. (2012). “Use of nanotechnology for high performance cellulosic and papermaking products,” Cellulose Chem. Technol. 46(5-6), 389-400.

Chen, H., Park, A., Heitmann, J. A., and Hubbe, M. A. (2009). “Importance of cellulosic fines relative to the dewatering rates of fiber suspensions,” Ind. Eng. Chem. Res. 48(20), 9106-9112.

Chen, N., Hu, S. W., and Pelton, R. (2002). “Mechanisms of aldehyde-containing paper wet-strength resins,” Indust. Eng. Chem. Res. 41(22), 5366-5371.

Chen, T. Y., Chiu, M. S., and Weng, C.-N. (2006). “Derivation of the generalized Young-Laplace equation of curved interfaces in nanoscaled solids,” J. Appl. Physics 100(7), article. No. 074308.

Chen, X. Y., Qian, X. R., and An, X. H. (2011). “Using calcium carbonate whiskers as papermaking filler,” BioResources 6(3), 2435-2447.

Chiam, H.-H., Shen, W., and Batchelor, W. J. (2013). “Study of previously loaded cellulose fibre sheets to investigate bond degradation prior to fracture and local strength distribution,” Fundamental Research Symposium, Sampson, W. W. (ed.), Fundamental Research Society, Cambridge, 175-199.

Chin, S. M., Youn, H. J., Ryu, J., and Sim, K. (2012). “Polyelectrolyte multilayering of fibers with different polymer combinations and a comparison of its effect on paper strength with a conventional dry strength agent treatment,” Nordic Pulp Paper Res. J. 27(5), 915-922.

Cildir, H., and Howarth, P. (1972). “The effect of re-use on paper strength,” Paper Technology 13(10), T149-T151.

Ciobanu, M., and Bobu, E. (2009). “Improving the recycleability of printing paper by in-situ filler loading,” Environ. Eng. Manag. J.8(2), 307-313.

Clark, J. d’A. (1942). “The measurement and influence of bonding between paper fibers,” Technical Association Papers 26, 462-468.

Clark, J. d’A. (1973). “Components of the strength qualities of pulps,” TAPPI 56(7), 122-125.

Clark, J. d’A. (1978a). “Bonding of cellulose surfaces,” In: Pulp Technology and Treatment for Paper, Clark, J. d’A. (ed.), Miller Freeman, San Francisco, pp. 145-159.

Clark, J. d’A. (1978b). “Fibrillation and fiber bonding,” In: Pulp Technology and Treatment for Paper, Clark, J. d’A. (ed.), Miller Freeman, San Francisco, pp. 160-180.

Clark, J. d’A. (1978c). “The making of paper,” In: Pulp Technology and Treatment for Paper, Clark, J. d’A. (ed.), Miller Freeman, San Francisco, Ch. 10, pp. 201-228; see p. 222.

Clark, J. d’A. (1984). “New thoughts on cellulose bonding,” TAPPI J.67(12), 82-83.

Clupak Inc. (1967). “Process and apparatus for producing a modified web material,” Canada Pat. 769,851.

Considine, J. M., Vahey, D. W., Evans, J. W., Turner, K. T., and Rowlands, R. E. (2012). “Evaluation of strength-controlling defects in paper by stress concentration analyses,” J. Composite Mater.46(11), 1323-1334.

Counsell, T. A. M., and Allwood, J. M. (2007). “Reducing climate change gas emissions by cutting out stages in the life cycle of office paper,” Resour. Conserv. Recycling 49(4), 340-352.

Courchene, C. E., Peter, G. F., and Litvay, J. (2006). “Cellulose microfibril angle as a determinant of paper strength and hygroexpansivity in Pinus taeda L.,” Wood Fiber Sci. 38(1), 112-120.

Cox, H. L. (1952). “The elasticity and strength of paper and other fibrous materials,” Brit. J. Appl. Phys. 3(3), 72-79.

Cramm, R. H., and Bibee, D. V. (1982). “The theory and practice of corona treatment to improve adhesion,” TAPPI 65(8), 75-78.

Cranston, E. D., and Gray, D. G. (2009). “Polyelectrolyte multilayer films containing cellulose: A review,” In: Model Cellulosic Surfaces, Roman, M. (ed.), ACS Symp. Ser. 1019, pp. 95-114.

Croney, C., Ouellet, D., and Kerekes, R. J. (1999). “Characterizing refining intensity for tensile strength development,” Scientific and Technical Advances in Refining (5th International Conferences on Refining), Pira International, Leatherhead, UK.

Cushing, M. L., and Schuman, K. R. (1959). “Fiber attraction and interfiber bonding – The role of polysaccharide additives,” TAPPI 42(12), 1006-1016.

Dai, D. S., and Fan, M. Z. (2013). “Green modification of natural fibres with nanocellulose,” RSC Advan. 3(14), 4659-4665.

Danielsson, M. (2002). “Water flow capability measurements for shoe press belts and press felts – Flow capability is greatly controlled by the structure of the felt,” Pulp Paper Canada 103(1), 35-38.

Danielsson, S., and Lindström, M. E. (2005). “Influence of birch xylan adsorption during kraft cooking on softwood pulp strength,” Nordic Pulp Paper Res. J. 20(4), 436-441.

da Silva, P. H. M., Poggiani, F., Libardi, P. L., and Goncalves, A. N. (2013). “Fertilizer management of eucalypt plantations on sandy soil in Brazil: Initial growth and nutrient cycling,” Forest Ecology and Management 301, 67-78.

Davies, R. F. B. (1977). “Starch spraying,” Paper Technol. Ind. 18(6), 186-188.

Davison, R. W. (1980). “Theory of dry strength development,” in Reynolds, W. F. (ed.), Dry Strength Additives, TAPPI Press, Atlanta, Ch. 1, 1-31.

Decher, G. (1997). “Fuzzy nanoassemblies: Toward layered polymeric multicomposites,” Science 277, 1232-1237.

Delgado, E., Lopez-Dellamary, F., Allan, G. G., Andrade, H., Contreras, H., Regla, H., and Cresson, T. (2004). “Zwitterion modification of fibers: Effect of fibre flexibility on wet strength of paper,” J. Pulp Paper Sci. 30, 141-144.

Delgado-Fornue, E., Contreras, H. J., Toriz, G., and Allan, G. G. (2011). “Adhesion between cellulosic fibers in paper,” J. Adhes. Sci. Technol. 25(6-7), 597-614.

DeMaio, A., and Patterson, T. (2007). “Rheological modeling of the tensile creep behavior of paper,” J. Appl. Polymer Sci. 106(5), 3543-3554.

Demel, I., Balzer, R., Demharter, W., Kaindl, N., Cordes-Tolle, M., and Pfaff, D. (2012). “Sustainability in the paper industry: Review and meaning,” Wochenbl. Papierfabr. 140(11), 830-833.

Deng, M., and Dodson, C. T. J. (1994). Paper. An Engineered Stochastic Structure, TAPPI Press, Atlanta, 284 pp.

Deng, Y. L., Jones, P., McLain, L., and Ragauskas, A. J. (2010). “Starch-modified fillers for linerboard and paper grades: A perspective review,” TAPPI J. 9(4), 31-36.

Denis, U. L., Rubens, C. O., and Marcos, S. (2003). “Seed storage hemicelluloses as wet-end additives in papermaking,” Carbohydr. Polym. 52, 367-373.

de Ruvo, A., Fellers, C., and Kolseth, P. (1986). “Descriptive theories for the tensile strength of paper,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 13, pp. 267-279.

de Ruvo, A., and Htun, M. (1983). “Fundamental and practical aspects of papermaking with recycled fibers,” In: The Role of Fundamental Research in Papermaking, Brander, J. (ed.), Vol. 1, Mechanical Engineering Pub. Ltd., London.

Desharnais, L., Chabot, B., Daneault, C., Montplaisir, D., and Croteau, L. (2002). “Thermomechanical pulp washing effect on retention and drainage,” Pulp Paper Can. 103(4), 44-48.

de Souza Lima, M. M., and Borasli, R. (2004). “Rodlike cellulose microcrystals: Structure, properties, and applications,” Macromol. Rapid Commun. 25, 771-787.

Diaz-Balteiro, L., Voces, R., and Romero, C. (2011). “Making sustainability rankings using compromise programming. An application to European paper industry,” Silva Fennica 45(4), 761-773.

Diniz, J. M. B. F., Gil, M. H., and Castro, J. A.. A.. M. (2004). “Hornification – Its origin and interpretation in wood pulps,” Wood Sci. Technol. 37(6), 489-494.

Dinwoodie, J. M. (1975). “Timber – A review of the structure-mechanical property relationships,” Journal of Microscopy 104(1), 3-32.

Doelle, K., and Amaya, J. J. (2012). “Application of calcium carbonate for uncoated digital printing paper from 100% eucalyptus pulp,” TAPPI J. 11(1), 51-59.

Doshi, M. R. (1998). “Take care of your fines, please!!” Prog. Paper Recycling 7(1), 8.

Douglas, J., and Simula, M. (2010). The Future of the World’s Forests, Ideas vs. Ideologies, Springer, New York, see. Ch. 3.

Duker, E., Ankerfors, M., Lindstrom, T., and Nordmark, G. G. (2008). “The use of CMC as a dry strength agent – The interplay between CMC attachment and drying,” Nordic Pulp Paper Res. J.23(1), 65-71.

Duker, E., and Lindström, T. (2008). “On the mechanisms behind the ability of CMC to enhance paper strength,” Nordic Pulp Paper Res. J.23(1), 57-64.

Dulemba, M., Qi, D., and Aravamuthan, R. (1999). “The effect of repeated drying and wetting on single fiber flexibility,” Prog. Paper Recycling 9(1), 38-45.

Edwards, K. G., and Edmunds, L. (1968). “Minimize restraint to maximize toughness,” Pulp Paper 42(27), 29-32.

Ehrnrooth, E., Htun, M., and de Ruvo, A. (1977). “Esterification as a means of improving the properties of once-dried fibers,” Trans Symp. Fiber-Water Interactions in Papermaking, Oxford, British Paper and Board Makers Assoc., London, 899-915.

Eichinger, R. (1981). “Festigkeitspotentiale wiederverwendeter Faserstoffe,” Wochenblatt für Papierfabrikation 109(18), 682-685.

Ellis, R. L., and Sendlachek, K. M. (1993). “Recycled versus virgin-fiber characteristics: A comparison,” in Secondary Fiber Recycling, R. J. Spangenberg (ed.), TAPPI Press, Atlanta, GA, Ch. 2, 7-19.

Enarsson, L. E., and Wågberg, L. (2007). “Kinetics of sequential adsorption of polyelectrolyte multilayers on pulp fibres and their effect on paper strength,” Nordic Pulp Paper Res. J. 22(2), 258-266.

Eriksen, O., Syverud, K., and Gregersen, O. (2008). “The use of microfibrillated cellulose produced from kraft pulp as strength enhancer in TMP paper,” Nordic Pulp Paper Res. J. 23(3), 299-304.

Eriksson, M., Notley, S. M., and Wågberg, L. (2005a). “The influence on paper strength properties when building multilayers of weak polyelectrolytes onto wood fibers,” J. Colloid Interface Sci. 292(1), 38-45.

Eriksson, M., Pettersson, G., and Wågberg, L. (2005b). “Application of polymeric multilayers of starch onto wood fibers to enhance strength properties of paper,” Nordic Pulp Paper Res. J. 20(3), 270-276.Eriksson M., Torgnysdøtter A., and Wågberg, L. (2006). “Surface modification of wood fibers using the polyelectrolyte multilayer technique: Effects on fiber joint and paper strength properties,” Industrial & Engineering Chemistry Research 45 (15), 5279-5286.

Espy, H. H. (1995). “The mechanism of wet-strength development in paper – A review,” TAPPI J. 78(4), 90-99.

Esser, A., Rübenacker, M., Schall, N., and Haffke, R. (2005). “Synthetic dry strength agents on a polyvinylamine basis,” Intl. Paper World 2005(10), 25-28.

Fairchild, G. H. (1992). “Increasing the filler content of PCC-filled alkaline papers,” TAPPI J. 75(8), 85-90.

Fält, S., and Wågberg, L. (2003). “Influence of electrolytes on the swelling and strength of kraft-liner pulps,” Nordic Pulp Paper Res. J. 18(1), 69-73.

Farley, C. E. (1994). “Glyoxalated polyacrylamide resin,” in: Wet-Strength Resins and their Application, Chan, L. (ed.), TAPPI Press, Atlanta.

Fatehi, P., McArthur, T., Xiao, H. N., and Ni, Y. H. (2010). “Improving the strength of old corrugated carton pulp (OCC) using a dry strength additive,” APPITA J. 63(5), 364-369.

Fatehi, P., Qian, L. Y., Kititerakun, R., Rirksomboon, T., and Xiao, H. N. (2009a). “Complex formation of modified chitosan and carboxymethyl cellulose and its effect on paper properties,” TAPPI J.8(6), 29-35.

Fatehi, P., Ward, J. E., and Xiao, H. N. (2009b). “Treatment of fractionated fibers with various cationic-modified poly(vinyl alcohols) and its impact on paper properties,” Indust. Eng. Chem. Res. 48(23), 10485-10490.

Fatehi, P., and Xiao, H. N. (2008). “The influence of charge density and molecular weight of cationic poly (vinyl alcohol) on paper properties,” Nordic Pulp Paper Res. J. 23(3), 285-291.

Fellers, C. (1983). “Edgewise compression strength of paper,” in Mark and Murakami (eds.), Handbook of Physical and Mechanical Testing of Paper and Paperboard, Vol. 1., Dekker, New York, Ch. 8, 349-383.

Fellers, C. (1986). “The significance of structure for the compression behavior of paperboard,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 14, pp. 281-310.

Fellers, C., Melander, J., and Mohlin, U.-B. (2001). “Predicting th effeect of reinforcement pulp characterisitcs in TMP papers for web breaks using fracture mechanics,” Nordic Pulp Paper Res. J. 16(4), 257-260, 265.

Fineman, I., and Lindström, T. (1985). “Hochgefüllte Feinpapiere,” Wochenblatt für Papeirfabrikation 113(16), 585-589.

Fischer, W. J., Hirn, U., Bauer, W., and Schennach, R. (2012). “Testing of individual fiber-fiber joints under biaxial load and simultaneous analysis of deformation,” Nordic Pulp Paper Res. J.27(2), 237-244.

Forester, W. K. (1985). “Recycling of neutrally sized calcium carbonate filled paper,” TAPPI Pulping Conf. 141-144.

Forman, L. V. (1950). “Fiber-to-fiber bonding,” TAPPI 33(9), 444-450.

Formento, J. C., Maximino, M. G., Mina, L. R., Srayh, M. I., and Martinez, M. J. (1994). “Cationic starch in the wet end: Its contribution to interfiber bonding,” APPITA 47(4), 305-308.

Fougler, M., and Parisian, J. (1999). “New technology to apply starch and other additives,” Pulp Paper Can. 100(2), T42-T43.

Francis, R. C., Hausch, D. L., Xu, E. C., and Kamdem, D. P. (2001). “Hardwood chemimechanical pulps – Sulfonation versus hydrogen peroxide pretreatment,” APPITA J. 54(5), 439-443.

Francis, R. C., Shin, S. J., Amidon, T. E., and Blain, T. J. (2006). “Soda pulping of hardwoods catalyzed by anthraquinone and methyl substituted anthraquinones,” J. Wood Chem. Technol. 26(2), 141-152.

French, J., Conn, A., Batchelor, W., and Parker, I. (2000). “The effect of fibre fibril angle on some handsheet mechanical properties,” APPITA J. 53(3), 210-226.

Fukunaga, Y. (1999). “New application method of dry strength agents by effective utilization of amphoteric polyacrylamide and aluminum sulfate,” Proc. Japan TAPPI Ann. Mtg., 349-357.

Fuhrmann, A., Li, X. L., Rautonen, R., Toikkanen, L., Hausalo, T., and Sagfors, P. E. (1996). “Influence of TCF and ECF bleaching chemicals on softwood kraft pulp components,” Paperi Puu 78(4), 172-179.

Gains, L. L., and Stodolsky, F. (1996). “Is recycling the best policy option? Insights from life cycle analysis,” TAPPI Life Cycle Assessment Symposium, TAPPI Press, Atlanta.

Galván, M. V., Mocchiutti, P., Cornaglia, L. M., and Zanuttini, M. A. (2012). “Dual-polyelectrolyte adsorption of poly(allylamine hydrochloride) and xylan onto recycled unbleached kraft pulps,” BioResources 7(2), 2075-2089.

Gao, W.-H., Chen, K.-F., Yang, R.-D., Yang, F., and Han, W.-J. (2011). “Properties of bacterial cellulose and its influence on the physical properties of paper,” BioResources 6(1), 144-153.

Gärdlund, L., Forsström, J., Andreasson, B., and Wågberg, L. (2005). “Influence of polyelectrolyte complexes on the strength properties of papers from unbleached kraft pulps with different yields,” Nordic Pulp Paper Res. J. 20(1), 36-42.

Gärdlund, L., Wågbberg, L., and Gernandt, R. (2003). “Polyelectrolyte complexes for surface modification of wood fibers. 2. Influence of complexes on wet and dry strength of paper,” Colloids Surf. A 218(1-3), 137-149.

Garg, M., and Singh, S. P. (2006). “Reasons of strength loss in recycled pulp,” APPITA J. 59(4), 274-279.

Gaspar, L. A. (1982). “Intrinsic bonding of cationic starch and application of cationic starch with recycled fiber,” Proc. TAPPI Annual Meeting, TAPPI Press, Altanta.

Gavalin, G. (1998). Paper Machine Design and Operation. Descriptions and Explanations, Angus Wilde Publ., Vancouver, BC.

Gerli, A., Eigenbrood, L. C., and Nurmi, S. (2011). “Relationship of surface strength and bulk strength properties in uncoated woodfree paper,” TAPPI J. 10(2), 17-24.

Gernandt, R., Wågberg, L., Gärdlund, L., and Dauzenberg, H. (2003). “Polyelectrolyte complexes for surface modification of wood fibers. 1. Preparation and characteriza-tion of complexes for dry and wet strength improvement of paper,” Colloids Surf. A 213(1), 15-25.

Gerspach, W., Luo, C., and Göttsching, L. (1993). “Modeling of the age distribution of fibers in recycling systems,” Papier 47(6), 288-299.

Ghosh, A. K. (2006). “Refining optimisation of secondary fibre – Use of Bijective Diagram technique,” APPITA J. 59(1), 24-30.

Ghosh, A. K., and Vanderhoek, N. (2001). “Improving stock quality through refining and screening,” Proc. 55^th APPITA Ann. Conf., pp. 471-479.

Giertz, H. W. (1964). “Contribution to the theory of tensile strength,” EUCEPA/ European TAPPI Conf. on Beating, Venice, Italy, Sept. 1964 Proceedings, EUCEPA, pp. 39-47.

Gill, R. A. (1989). “The behavior of on-site synthesized precipitated calcium carbonates and other calcium carbonate fillers on paper properties,” Nordic Pulp Paper Res. J. 4(2), 120-127.

Gill, R., and Scott, W. (1987). “The relative effects of different calcium carbonate filler pigments on optical properties,” TAPPI J.70(1), 93-99.

Gimåker, M., Horvath, A., and Wågberg, L. (2007). “Influence of polymer additives on short-time creep of paper,” Nordic Pulp Paper Res. J. 22(2), 217-227.

Glittenberg, D. (1993). “Starch alternatives for improved strength, retention, and sizing,” TAPPI J. 76(11), 215-219.

González, I., Boufi, S., Pèlach, M. A., Alcalà, M., Vilaseca, F., and Mutjé, P. (2012). “Nanofibrillated cellulose as paper additive in Eucalyptus pulps,” BioResources 7(4), 5167-5180.

Gonzalez-Garcia, S., Berg, S., Feijoo, G., and Moreira, M. T. (2009). “Comparative environmental assessment of wood transport models A case study of a Swedish pulp mill,” Sci. Total Environ. 407(11), 3530-3539.

Goring, D. A. I., and Suranyi, G. (1969). “Improved bonding of cellulose and other polymers by surface treatment with a gas,” Pulp Paper Mag. Can. 70(20), 103-110.

Görres, J., Amiri, R., Wood, J. R., and Karnis, A. (1995). “The shear bond strength of mechanical pulp fibers,” J. Pulp Paper Sci. 22(5), J161-J164.

Göttsching, L., and Stürmer, L. (1978a). “Physical properties of secondary fibers under the influence of their previous history, Part 1: Secondary fiber pulp in the context of technology and research” Wochenbl. Papierfabr. 106(21), 801-808.

Göttsching, L., and Stürmer, L. (1978b). “Physical properties of secondary fibers under the influence of their previous history. Part 2: Influence of multiple recycling,” Wochenbl. Papierfabr. 106(23/24), 909-918.

Gradin, P., Nyström, S., Flink, P., Forsberg, S., and Stollmaier, F. (1997). “Acoustic emission monitoring of light-weight coated paper,” J. Pulp Paper Sci. 23(3), 113-118.

Grau, U., Schuhmacher, R., and Kleemann, S. (1996). “Effect of recycling and the performance of dry-strength agents,” Wochenbl. Papierfabr. 124(17), 729-735.

Green, C. (2000). “Curl in paper,” APPITA J. 53(4), 272-275.

Green, C. (2006). “Improved pulp evaluations using a combination of free and restrained drying of handsheets,” BioResources 1(2), 174-175.

Green, H. V., Fox, T. J., and Scallan, A. M. (1982). “Lumen-loaded paper pulp,” Pulp Paper Can. 83(7), T203-T207.

Greenwood, G. (1999). “Manufacture of paper,” US Pat. 5,876,563.

Grön, J., and Rantanen, R. (2000). “Surface sizing and film coating,” in: Lehtinen, E. (ed.), Pigment Coating and Surface Sizing of Paper, Papermaking Science and Technology series, Fapet Oy, Helsinki, Ch. 25487-541.

Gross, A. S., and Chu, J.-W. (2010). “On the molecular origins of biomass recalcitrance: The interaction network and solvation structures of cellulose microfibrils,” J. Phys. Chem. 114, 13333-13341.

Gruber, E., Grossmann, K., and Schempp, W. (1996). “Wechselwirkungen von synthetischen kationischen Polymeren mit Fasern und Füllstoffen. Tiel 1: Enfluß auf die Adsorption,” Wochenblatt fur Papierfabrikation 124(1), 4-6, 8, 10-11.

Gruber, E., and Weigert, J. (1998). “Chemical modification of pulp to reduce its hornification tendency,” Papier 52(Suppl. 10A), V20-V26.

Guest, D. A., and Voss, G. P. (1983). “Improving the quality of recycled fiber,” Paper Technol. Ind. 24(7), 256-260, 268.

Guest, D. A., and Weston, J. (1990). “Fiber-water interactions: Modified by recycling,” Or, “I dried my fibres yesterday, now I can’t do nothin’ with ‘em,” In: Recycling Paper, Vol. 1, TAPPI Press, 169-175.

Guimond, R., Chabot, B., Law, K. N., and Daneault, C. (2010). “The use of cellulose nanofibres in papermaking,” J. Pulp Paper Sci. 36(1-2), 55-61.

Gullichsen, J., and Fogelholm, C.-J. (1999). Chemical Pulping, Fapet Oy, Helsinki.

Gurnagul, N. (1995). “Sodium hydroxide addition during recycling: Effects on fiber swelling and sheet strength,” TAPPI J. 78(12), 119-124.

Gurnagul, N., Ju, S., and Page, D. H. (2001). “Fiber-fiber bond strength of once-dried pulps,” J. Pulp Paper Sci. 27(3), 88-91.

Gutmann, H., Nelson, W. J., and Yerke, J. F. (1993). “Rule 41 implications for linerboard producers,” TAPPI J. 76(1), 158-163.

Haggblom-Ahnger, U. (1998). “Layering of office paper,” Paperi Puu80(7), 508-513.

Haggblom-Ahnger, U. (1999). “Optimum location of softwood sulfate pulp in three-ply office paper,” TAPPI J. 82(6), 181-187.

Hagglund, R., and Isaksson, P. (2006). “Analysis of localized failure in low-basis-weight paper,” Intl. J. Solids Structures 43(18-19), 5581-5592.

Hagiopol, C., and Johnston, J. W. (2012). Chemistry of Modern Papermaking, CRC Press, Taylor & Francis, Boca Raton.

Hakanen, A., and Hartler, N. (1995). “Fiber deformations and strength potential of kraft pulps,” Paperi Puu 77(5), 339-344.

Hallgren, H., and Lindström, T. (1989). “Influence of stock prep on forming efficiency,” Paper Technol. Ind. 30(2), 35-39.

Halme, M. (1967). “Factors affecting strength anisotropy of paper. (2),” Paperi Puu 49(4a), 147-150.

Hamzeh, Y., Ashori, A., Khorasani, Z., Abdulkhani, A., and Abyaz, A. (2013). “Pre-extraction of hemicelluloses from bagasse fibers: Effects of dry-strength additives on paper properties,” Indust. Crops Prod. 43, 365-371.

Hamzeh, Y., Najafi, S. M. H., Hubbe, M. A., Salehi, K., and Firouzabadi, M. R. D. (2012). “Recycling potential of unbleached and bleached chemical pulps from juvenile and mature wood of Populus deltoids,” Holzforschung 66, 155-161.

Han, W. J., Zhao, C. S., Elder, T., Chen, K. F., Yang, R. D., Kim, D., Pu, Y. Q., Hsieh, J., and Ragauskas, A. J. (2012). “Study on the modification of bleached eucalyptus kraft pulp using birch xylan,” Carbohydr. Polym. 88(2), 719-725.

Hänninen, T., Kontturi, E., Leppänen, K., Serimaa, R., and Vuorinen, T. (2011). “Kraft pulping of Juniperus communis results in paper with unusually high elasticity,” BioResources 6(4), 3824-3835.

Hannuksela, T., and Holmbom, B. (2004). “Effect of sorbed galactoglucomannans and galactomannans on pulp and paper handsheet properties, especially strength properties,” Nordic Pulp Paper J. 19(2), 237-244.

Hansen, C. M., and Bjorkman, A. (1998). “The ultrastructure of wood from a solubility parameter point of view,” Holzforschung 52(4), 335-344.

Hansson, T., Fellers, C., and Htun, M. (1989). “Drying strategies and a new restraint technique to improve cross-directional properties of paper,” Fundamentals of Papermaking. Trans. 9^th Fund. Res. Symp., Cambridge, Baker, C. (ed.), Vol. 2, pp. 743-781.

Hartmann, H., Daoust, G., Bigue, B., and Messier, C. (2010). “Negative or positive effects of plantation and intensive forestry on biodiversity: A matter of scale and perspective,” Forestry Chronicle86(3), 354-364.

Haslach, H. W. (2000). “The moisture and rate-dependent mechanical properties of paper: A review,” Mechanics Time-Depend. Mater. 4(3), 169-210.

Haslach, H. W. (2009). “Time-dependent mechanisms in fracture of paper,” Mechanics Time-Depend. Mater. 13(1), 11-35.

Hattula, T. (1986). “Crystallization and disordering of wood cellulose during hydro-thermal treatment,” Paperi Puu 68(11), 847-850.

Heath, H. D., Ernst, A. J., Hofreiter, B. T., Phillips, B. S., and Russell, C. R. (1974). “Flocculating agent-starch blends of interfiber bonding and filler retention: Comparative performance with cationic starches,” TAPPI 57(11), 109-111.

Heermann, M. L., Welter, S. R., and Hubbe, M. A. (2006). “Effects of high treatment levels in a dry-strength additive program based on deposition of polyelectrolyte complexes: How much glue is too much?” TAPPI J. 5(6), 9-14.

Heikkilä, P., and Paltakari, J. (2000). “Fundamentals of paper drying,” in: Karlsson, M., Papermaking. Part 2, Drying, Paper Science and Technology ser., Book 9, Fapet Oy, Ch. 2, pp. 55-83.

Heitmann, J., and Wallace, C. S. (2004). “Use of enzymes to reduce refining energy,” Proc. 2^nd Intl. Symp. Technol. Pulping, Papermaking, Biotech. Fiber Plants (2^nd ISTPPBFP), Nanjing, China, Nanjing Forestry Univ., China.

Hekkert, M. P., van den Reek, J., Worrell, E., and Turkenburg, W. C. (2002). “The impact of material efficient end-use technologies on paper use and carbon emissions,” Resources Conserv. Recycling36(3), 241-266.

Helle, T. (1963). “Some aspects on fibre strength and fibre bondings in sulphate and sulphite paper,” Svensk Papperstidning 66(24), 1015-1030.

Helle, T. (1965). “Fiber strength – bonding strength – paper strength,” Norsk Skogind. 19(9), 359-363.

Helmerius, J., von Walter, J. V., Rova, U., Berglund, K. A., and Hodge, D. B. (2010). “Impact of hemicellulose pre-extraction for bioconversion on birch kraft pulp properties,” Bioresour. Technol. 101(15), 5996-6005.

Hestmo, R. H., Gregeren, O. W., and Lamvik, M. (2002). “Calendering of wood containing paper. A laboratory study of temperature, moisture and pressure effects on fibre wall damage,” Nordic Pulp Paper Res. J. 17(1), 9-13.

Higgins, H. (2002). “Sticking together – How interfibre cohesion works. The magic of hydrogen bonds,” APPITA 55(3), 187.

Higgins, H. G., and McKenzie, A. W. (1963). “The structure and properties of paper. XIV. Effects of drying on cellulose fibers and the problem of maintaining pulp strength,” APPITA 16(6), 145-164.

Hii, C., Gregersen, O. W., Chinga-Carrasco, G., Eriksen, O., and Toven, K. (2012). “The web structure in relation to the furnish composition and shoe press pulse profiles during wet pressing,” Nordic Pulp Paper Res. J. 27(4), 798-805.

Hill, K. C. (1991). “Paper drying,” in: Thorp, B. A. (ed.), Paper Machine Operations, Pulp and Paper Manufacture series, Vol. 7, TAPPI, Atlanta, Ch. 12, pp. 282-305.

Hipple, B. J. (1991). “Fine paper properties and the effects of wet-end starch when using deinked recycled fiber in an alkaline system,” TAPPI J. 74(5), 79-84.

Hon, D. N.-S. (2001). Wood and Cellulosic Chemistry, Marcel Dekker, New York.

Horn, R. A. (1975). “What are the effects of recycling on fiber and paper properties?” Paper Trade J. 159(7/8), 78-82.

Horn, R A. (1995). “What are the effects of recycling on fiber and paper properties?” Prog. Paper Recycling 4(2), 76-82.

Horsey, E. F. (1947). “Sodium carboxymethylcellulose for papermaking,” Tech. Assoc. Papers 30, 294-298.

Horvath, A. T., Horvath, A. E., Lindström, and Wågberg, L. (2008). “Diffusion of cationic polyelectrolytes into cellulose fibers,” Langmuir 24(19), 10797-10806.

Horvath, A. T., Pelton, R., Larsson, P. T., and Wågberg, L. (2010). “Effect of cross-linking fiber joints on the tensile and fracture behavior of paper,” Indust. Eng. Chem. Res. 49(14), 6422-6431.

Howard, R. C. (1990). “The effects of recycling on paper quality,” J. Pulp Paper Sci. 16(5), J143-J149.

Howard, R. C., and Bichard, W. (1992). “The basic effects of recycling on pulp properties,” J. Pulp Paper Sci. 18(4), J151-J159.

Howard, R. C., and Jowsey, C. J. (1989). “The effect of cationic starch on the tensile strength of paper,” J. Pulp Paper Sci. 15(6), J225-J229.

Hristopulos, D. T., and Uesaka, T. (2003). “Factors that control the tensile strength distributions of heterogeneous brittle materials with emphasis on fiber networks,” Physical Review B 70(6), article 064108.

Htun, M. (1986a). “Internal stress in paper,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 11, pp. 227-239.

Htun, M. (1986b). “The control of mechanical properties by drying restraint,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 15, pp. 311-326.

Htun, M., and Fellers, C. (1986). “The in-plane anisotry of paper in relation to fiber orientation and drying restrains,” in: Paper Structure and Properties, Bristow, J. A., and Kolseth, P. (eds.), Marcel Dekker, New York, 327.

Hu, Z. S., Shao, M. H., Li, H. Y., Cai, Q., Zhong, C. H., Zhang, X., and Yeng, Y. L. (2009). “Synthesis of needle-like aragonite crystals in the presence of magnesium chloride and their application in papermaking,” Adv. Composite Mater. 18(4), 315-326.

Hubbe, M. A. (2004). “Filler particle shape vs. paper properties – A review,” Proc. Spring Tech. Conf., TAPPI Press, Atlanta.

Hubbe, M. A. (2005a). “Dry-strength development by polyelectrolyte complex deposition onto non-bonding glass fibers,” J. Pulp Paper Sci. 31(4), 159-166.

Hubbe, M. A. (2005b). “Microparticle programs for drainage and retention,” in Rodriguez, J. M. (ed.), Micro and Nanoparticles in Papermaking, TAPPI Press, Atlanta, Chapter 1, 1-36.

Hubbe, M. A. (2006). “Bonding between cellulosic fibers in the absence and presence of dry-strength agents – A review,”BioResources 1(2), 281-318.

Hubbe, M. A. (2007a). “Paper’s resistance to wetting – A review of internal sizing chemicals and their effects,” BioResources 2(1), 106-145.

Hubbe, M. A. (2007b). “Flocculation and redispersion of cellulosic fiber suspensions: A review of effects of hydrodynamic shear and polyelectrolytes,” BioResources 2(2), 296-331.

Hubbe, M. A. (2007c). “Incinerate, recycle, or wash and reuse?” BioResources 2(1), 1-2.

Hubbe, M. A. (2008). “Minimizing the environmental impact of the papermaking process,” Proceedings of the 2^nd IPEC Conference, Tianjin, China, Book A, 37-40.

Hubbe, M. A. (2010). “Factors to consider for enhancing and extending the recyclability of paper,”O Papel 71(4), 40-60.

Hubbe, M. A., and Bowden, C. (2009). “Handmade paper: A review of its history, craft, and science,” BioResources 4(4), 1736-1792.

Hubbe, M. A., and Heitmann, J. A. (2007). “Review of factors affecting the release of water from cellulosic fibers during paper manufacture,” BioResources 2(3), 500-533.

Hubbe, M. A., Jackson, T. L., and Zhang, M. (2003). “Fiber surface saturation as a strategy to optimize dual-polymer dry strength treatment,” TAPPI J. 2(11), 7-12.

Hubbe, M. A., Moore, S. M., and Lee, S. Y. (2005). “Effects of charge ratios and cationic polymer nature on polyelectrolyte complex deposition onto cellulose,” Indus. Eng. Chem. Res. 44(9), 3068-3074.

Hubbe, M. A., Nazhad, M., and Sánchez, C. (2010). “Composting as a way to convert cellulosic biomass and organic waste into high-value soil amendments: A review,” BioResources 5(4), 2808-2854.

Hubbe, M. A., Pawlak, J. J., and Koukoulas, A. A. (2008a). “Paper’s appearance: A review,” BioResources 3(2), 627-665.

Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, Y., Song, J., Sulić, N., and Sezaki, T. (2007a). “Charge and the dry-strength performance of polyampholytes. Part 2. Colloidal effects,” Colloids Surf. A 301(1), 23-32.

Hubbe, M. A., Rojas, O. J., Lucia, L. A., and Sain, M. (2008b). “Cellulosic nanocomposites, A review,” BioResources 3(3), 929-980.

Hubbe, M. A., Rojas, O. J., Sulić, N., and Sezaki, T. (2007b). “Unique behavior of polyampholytes as dry-strength additives,” APPITA J. 60(2), 106-111.

Hubbe, M. A., Sundberg, A., Mocchiutti, P., Ni, Y., and Pelton, R. (2012). “Dissolved and colloidal substances (DCS) and the charge demand of papermaking process waters and suspensions: A review,” BioResources 7(4), 6109-6193.

Hubbe, M. A., Venditti, R. A., Barbour, R. L., and Zhang, M. (2003). “Changes to unbleached kraft fibers due to drying and recycling,” Progress in Paper Recycling 12(3), 11-20.

Hubbe, M. A., Venditti, R. A., and Rojas, O. J. (2007c). “What happens to cellulosic fibers during papermaking and recycling? A review,” BioResources 2(4), 739-788.

Hubbe, M. A., Wagle, D. G., and Ruckel, E. R. (1999). “Method for increasing the strength of a paper of paperboard product,” U.S. Patent 5,958,180, Sept. 28.

Hubbe, M. A., and Zhang, M. (2005). “Recovered kraft fibers and wet-end dry-strength polymers,” Proc. TAPPI Practical Papermakers Conf., TAPPI Press, Atlanta.

Huheey, J. E. (1972). Inorganic Chemistry: Principles of Structure and Reactivity, Harper & Row, New York.

Huhtamäki, M. (2003). “Closing the water cycles – How far can we go?” Paper Technol. 44(10), 27-32.

Humphreys, D. (2006). Logjam. Deforestation and the Crisis of Global Governance, Earthscan, Sterling, VA, USA.

Husband, J. C., Gate, L. F., Norouzi, N., and Blair, D. (2009). “The influence of kaolin shape factor on the stiffness of coated papers,” TAPPI J. 8(6), 12-17.

I’Anson, S. J., Karademir, A., and Sampson, W. W. (2006). “Specific contact area and the tensile strength of paper,” APPITA J. 59(4), 297-301.

Ingmanson, W. L., and Thode, E. F. (1959). “Factors contributing to the strength of a sheet of paper. II. Relative bonded area,” TAPPI 42(1), 83-93.

Iosip, A., Hortal, M., Dobon, A., and Bobu, E. (2012). “Assessing environmental impact of packaging paper production based on recycled fibre raw material,” Environ. Eng. Manag. J. 11(1), 99-108.

Iosip, A., Nicu, R., Ciolacu, F., and Bobu, E. (2010). “Influence of recovered paper quality on recycled pulp properties,” Cellulose Chem. Technol. 44(10), 513-519.

Isaksson, P., and Hägglund, R. (2007). “Evolution of bond fractures in a randomly distributed fiber netword,” Intl. J. Solids Struc. 44, 6135-6147.

Isogai, A., and Kato, Y. (1998). “Preparation of polyglucuronic acid from cellulose by TEMPO-mediated oxidation,” Cellulose 5(3), 153-164.

Isogai, T., Saito, T., and Isogai, A. (2010). “TEMPO electromediated oxidation of some polysaccharides including regenerated cellulose fiber,” Biomacromol. 11(6), 1593-1599.

Irvine, G. M. (1984). “The glass transitions of lignin and hemicellulose and their measurement by differential thermal analysis,” TAPPI J. 67(5), 118-121.

Iwamoto, S., Nakagaito, A. N., Yano, H., and Nogi, M. (2005). “Optically transparent composites reinforced with plant fiber-based nanofibers,” Appl. Phys. A. 81, 1109-1112.

Iyer, P. B., Sreenivasan, S., Chidambaraswaran, P. K., Patil, N. B., and Sundaram, V. (1991). “Induced crystallization of cellulose in never-dried cotton fibers,” J. Appl. Polymer Sci. 42(6), 1751-1757.

Jahan, M. S. (2003). “Changes of paper properties of nonwood pulp on recycling,” TAPPI J. 2(7), 9-12.

Jang, H. F., and Seth, R. S. (1998). “Using confocal microscopy to characterize the collapse behaviour of fibers,” TAPPI J. 81(5), 167-173.

Jayaraman, K., and Kortschot, M. T. (1998). “Closed-form network models for the tensile strength of paper – A critical discussion,” Nordic Pulp Paper Res. J. 13(3), 233-242.

Jayme, G. (1944). “Micro-swelling measurement in cellulosic pulp,” Papier-fabr. / Wochenbl. Papierfabr. 6, 187-194.

Jentzen, C. A. (1964). “The effect of stress applied during drying on some of the properties of individual pulp fibers,” TAPPI 47(7), 412-418.

Johansson, C., Bras, J., Mondragon, I., Nechita, P., Plackett, D., Simon, P., Svetec, D. G., Virtanen, S., Baschetti, M. G., Breen, C., Clegg, F., and Aucejo, S. (2012). “Renewable fibers and bio-based materials for packaging applications – A review of recent developments,” BioResources 7(2), 2506-2552.

Jopson, R. N. (1993). “Saturation technology for corrugated containers,” TAPPI J. 76(4), 207-214.

Jorge, F. C., Pereira, C., and Ferreira, J. M. F. (2004). “Wood-cement composites: A review,” Holz. Roh- Werkst. 62(5), 370-377.

Joseleau, J. P., Chevalier-Billosta, V., and Ruel, K. (2012). “Interaction between microfibrillar cellulose fines and fibers: Influence on pulp qualities and paper sheet properties,” Cellulose19(3), 769-777.

Joshi, K., Batchelor, W., and Rasid, K. A. (2011). “Investigation of the effect of drying and refining on the fiber-fiber shear bond strength measured using tensile fracture line analysis of sheets weakened by acid gas exposure,” Cellulose 18(6), 1407-1421.

Joubert, J. M., Kirge, G. J. R., and Borgin, K. (1959). “Evidence for a hydrate of cellulose from studies of its surface properties,” Nature184(4698), 1561-1562.

Kalia, S., Dufresne, A., Cherian, B. M., Kaith, B. S., Averous, L., Njuguna, J., and Nassiopoulos, E. (2011). “Cellulose-based bio- and nanocomposites: A review,” Intl. J. Polymer Sci., Article no. 837875, DOI: 10.1155/2011/837875.

Kang, T., and Paulapuro, H. (2006). “Effect of external fibrillation on paper strength,” Pulp Paper Canada 107(7-8), 51-54; (12), 10.

Kang, T., Paulapuro, H., and Hiltunen, E. (2004). “Fracture mechanism in interfiber bond failure – microscopic observations,” APPITA J. 57(3), 199-203.

Kappel, J. (1999). Mechanical Pulps: From Wood to Bleached Pulp, TAPPI Press, Atlanta, 396 pp.

Karade, S. R. (2010). “Cement-bonded composites from lignocellulosic wastes,” Construction and Building Materials 24(8), 1323-1330.

Kargl, R., Mohan, T., Bračič, M., Kulterer, M., Doliška, A., Stana-Kleinschek, K., and Ribitsch, V. (2012). “Adsorption of carboxymethyl cellulose on polymer surfaces: Evidence of a specific interaction with cellulose,” Langmuir 28(31), 11440-11447.

Karnis, A. (1993). “Latency in mechanical pulp fibers,” Paperi Puu75(7), 505-511.

Kataja-aho, J., Haavisto, S., Asikainen, J., Hyvarinen, S., and Vuoti, S. (2012). “The influence of cationized birch xylan on wet and dry strength of fine paper,” BioResources 7(2), 1713-1728.

Kato, K. L., and Cameron, R. E. (1999). “A review of the relationship between thermally-accelerated ageing of paper and hornification,” Cellulose 6(1), 23-40.

Kellogg, R. M., and Thykeson, E. (1975). “Predicting kraft mill paper strength from fiber properties,” TAPPI 58(4), 131-135.

Kerman, N., Wirth, B., and Welt, T. (2009). “First operational experience with the high efficiency synthetic dry strength agents,” Wochenblatt fur Papierfabrikation 137(15-16), 711-715.

Kershaw, T. N. (1980). “Sheet formation and drying,” in: Casey, J. P. (ed.), Pulp and Paper Chemistry and Chemical Technology, 3^rd Ed., Wiley-Interscience, Vol. 2, Ch. 8, pp. 965-1127.

Ketola, H., and Andersson, T. (1999). “Dry-strength additives,” in L. Neimo, ed., Paper-making Chemistry, Papermaking Sci. Tech. Ser. 4, Fapet Oy, Helsinki, 269-287.

Khakifirooz, A., Ravanbakhsh, F., Samariha, A., and Kiaei, M. (2013). “Investigating the possibility of chemi-mechanical pulping of bagasse,” BioResources 8(1), 21-30.

Khalil, H. P. S. A., Bhat, A. H., and Yusra, A. F. I. (2012). “Green composites from sustainable cellulose nanofibrils: A review,” Carbohyd. Poly. 87(2), 963-979.

Khosravani, A., Latibari, A. J., Mirshokraei, S. A., Rahmaninia, M., and Nazhad, M. (2010). “Studying the effect of cationic starch – anionic nanosilica system on retention and drainage,” BioResources5(2), 939-950.

Khosravani, A., and Rahmaninia, M. (2013). “The potential of nanosilica – cationic starch wet end system for applying higher filler content in fine paper,” BioResources 8(2), 2234-2245.

Kiiskinen, H., Paltakari, J., and Pakarinen, P. (2000). “Drying and paper quality,” in: Karlsson, M., Papermaking. Part 2, Drying, Paper Science and Technology ser., Book 9, Fapet Oy, Ch. 10, pp. 333-368.

Kim, G.-Y., and Hubbe, M. A. (2010). “Engineering of a wet-end additives program relative to process parameters and to the physical and optical properties of filled paper,” Indus. Eng. Chem. Res. 49(12), 5644-5653.

Kinnunen, J., and Kiviranta, A. (1992). “Effect of wide-nip pressing on linerboard,” Paperi Puu 74(4), 314-316.

Kitaoka, T., Isogai, A., and Onabe, F. (1999). “Chemical modification of pulp fibers by TEMPO-mediated oxidation,” Nordic Pulp Paper Res. J. 14(4), 279-284.

Kitaoka, T., and Tanaka, H. (2001). “Novel paper strength additive containing cellulose-binding domain of cellulase,” J. Wood Sci. 47(4), 322-324.

Kitaoka, T., and Tanaka, H. (2002). “Fiber charge characteristics of pulp suspension containing aluminum sulfate,” J. Wood Sci. 48(1), 38-45.

Kiviranta, A., Lehtinen, J., Talja, R., and Retulainen, E. (2000). “Drying of paper board and packaging grades,” in: Karlsson, M., Papermaking. Part 2, Drying, Paper Science and Technology ser., Book 9, Fapet Oy, Ch. 6, pp. 187-206.

Klass, C. P. (1991). “Surface sizing,” in: Pulp and Paper Manufacture, Vol. 7, Paper Machine Operations, Thorp, B. A. (ed.), TAPPI Press, Atlanta, Ch. 13, 306-322.

Klemm, D., Kramer, F., Moritz, S., Lindström, T., Ankerfors, M., Gray, D., and Dorris, A. (2011). “Nanocelluloses: A new family of nature-based materials,” Angew. Chem. Intl. Ed. 50(24), 5438-5466.

Klungness, J. H., and Caulfield, D. F. (1982). “Mechanisms affecting fiber bonding during drying and aging of pulps,” TAPPI J. 65(12), 94-97.

Koepenick, M. (1995). “Mini mill, major impact,” PIMA Mag. 77(1), 24-27.

Koljonen, K., Vainio, A., Hiltunen, E., Laine, J., and Stenius, P. (2003). “The effect of polyelectrolyte adsorption on the strength properties of paper made from mixtures of mechanical and chemical pulps,” Intl. Paper Coating Symp., Montreal, pp. 223-227.

Kolseth, P., and de Ruvo, A. (1986). “The cell wall components of wood pulp fibers,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 1, pp. 3-25.

Kolseth, P., and Ehrnrooth, E. M. L. (1986). “Mechanical softening of single wood pulp fibers,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 2, pp. 27-50.

Kontturi, E., Mitikka-Eklund, M., and Vuorinen, T. (2008). “Strength enhancement of a fiber network by carboxymethyl cellulose during oxygen delignification of kraft pulp,” BioResources 3(1), 34-45.

Kontturi, E., Suchy, M., Penttilä, P., Jean, B., Pirkkalainen, K., Torkkeli, M., and Serimaa, R. (2011). “Amorphous characteristics of an ultrathin cellulose film,” Biomacromol. 12(3), 770-777.

Kontturi, E., and Vuorinen, T. (2006). “Fibre surface and strength of a fibre network,” Holzforschung 60(6), 691-693.

Kontturi, E., and Vuorinen, T. (2009). “Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying,” Cellulose 16(1), 65-74.

Korteoja, M. J., Lukkarinen, A., Kaski, K., and Niskanen, K. J. (1997). “Computational study of formation effects on paper strength,” J. Pulp Paper Sci. 23(1), J18-J22.

Kotik, O., Ljungqvist, C. H., and Thuvander, F. (2005). “Effect of the strain-at-failure distribution of pulp fibres on tensile strength of paper,” APPITA J. 58(4), 288-292, 322.

Kugge, C., Vanderhoek, N., and Bousfield, D. W. (2011). “Oscillatory shear response of moisture barrier coatings containing clay of different shape factor,” J. Colloid Interface Sci. 358(1), 25-31.

Kuhasalo, A., Niskanen, J., Paltakari, J., and Karlsson, M. (2000). “Introduction to paper drying and principles and structure of a dryer section,” in: Karlsson, M., Papermaking. Part 2, Drying, Paper Science and Technology ser., Book 9, Fapet Oy, Ch. 1, pp. 16-53.

Kumar, P., Negi, Y. S., and Singh, S. P. (2011). “Filler loading in the lumen or/and cell wall of fibers – A literature review,” BioResources6(3), 3526-3546.

Kumar, S. (2012). “Composting of municipal solid waste,” Crit. Revs. Biotech. 31(2), 112-136.

Kunnas, L., Lehtinen, J., Paulapuro, H., and Kiviranta, A. (1993). “The effect of Condebelt drying on the structure of fiber bonds,” TAPPI J. 76(4), 95-104.

Kuys, K., and Zhu, Q. (1994). “Surface chemistry of recycled paper furnishes,” APPITA Ann. Gen. Conf., 48^th, 409-414.

Lacasse, M. (2012). “Jumbo presses: Increased performance with high tech roll covers,” APPITA J. 65(1), 32-37.

Laine, J. (1997). “Effect of ECF and TCF bleaching on the charge properties of kraft pulp,” Paperi Puu 79(8), 551-559.

Laine, J., Lindstrom, T., Bremberg, C., and Glad-Nordmark, G. (2003a). “Studies on topochemical modification of cellulosic fibres – Part 5. Comparison of the effects of surface and bulk chemical modification and beating of pulp on paper properties,” Nordic Pulp Paper Res. J. 18(3), 325-332.

Laine, J., Lindström, Glad Nordmark, G., and Risinger, G. (2000). “Studies on topochemical modification of cellulosic fibers. Part 1. Chemical conditions for the attachment of carboxymethyl cellulose onto fibers,” Nordic Pulp Paper Res. J. 15(5), 520-526.

Laine, J., Lindstrom, T., Nordmark, G. G., and Risinger, G. (2002). “Studies on topochemical modification of cellulosic fibres – Part 2. The effect of carboxymethyl cellulose attachment on fibre swelling and paper strength,” Nordic Pulp Paper Res. J. 17(1), 50-56.

Laine, J., Lindström, Glad Nordmark, G., and Risinger, G. (2003b). “Studies on topochemical modification of cellulosic fibers. Part 3. The effect of carboxymethyl cellulose attachment on wet-strength development by alkaline-during polyamide-amine epichlorohydrin resins,” Nordic Pulp Paper Res. J. 17(1), 57-60.

Laine, J., Lindström, Glad Nordmark, G., and Risinger, G. (2003c). “Studies on topochemical modification of cellulosic fibers. Part 5. Comparison of the effects of surface and bulk chemical modification and beating on pulp and paper properties,” Nordic Pulp Paper Res. J.18(3), 325-332.

Laine, J., and Stenius, P. (1997). “Effect of charge on the fibre and paper properties of bleached industrial kraft pulps,” Paperi Puu79(4), 257-266.

Laivins, G. V., and Scallan, A. M. (1993). “The mechanism of hornification of wood pulps,” In: Products of Papermaking, Baker, C. F. (ed.), Trans. 10^th Fundamental Res. Symp., Oxford, UK, Sept., Pira Intl., Leatherhead, Surrey, UK, Vol. 2, 1235-1259.

Laivins, G. V., and Scallan, A. M. (1995). “The influence of drying and beating on the swelling of fines,” J. Pulp Paper Sci. 22(5), J178-J184.

Laleg, M., and Pikulik, I. I. (1993a). “Modified starches for increasing paper strength,” J. Pulp Paper Sci. 19(6), J248-J255.

Laleg, M., and Pikulik, I. I. (1993b). “Unconventional strength additives,” Nordic Pulp Paper Res. J. 8(1), 41-47.

Lange, D., and Meitner, R. (2006). “Experiences with various shoe press applications,” Pulp Paper Canada 107(9), 22-24.

Laothanachareon, T., Khonzue, P., Rattanaphan, N., Tinnasulanon, P., Apawasin, S., Paemanee, A., Ruanglek, V., Tanapongpipat, S., Champreda, V., and Eurwilaichitr, L. (2011). “Production of multi-fiber modifying enzyme from Mamillisphaeria sp. for refining of recycled paper pulp,” Biosci. Biotech. Biochem. 75(12), 2297-2303.

Larsson, H., Nilsson, J., and Stenström, S. (2001). “Experiments in a pilot-scale impulse unit – Heat transfer and water removal rates,” Nordic Pulp Paper Res. J. 16(2), 156-161.

Larsson, H., and Orloff, D. I. (2001). “Lab-scale impulse drying experiments with extension to higher impulse,” Drying Technol.19(10), 2549-2558.

Larsson, H., and Stenstrom, S. (1998). “Critical pressure control of delamination in impulse drying,” TAPPI J. 81(7), 117-122.

Larsson, P. A., and Wågberg, L. (2008). “Influence of fibre-fibre joint properties on the dimensional stability of paper,” Cellulose 15(4), 515-525.

Latibari, A. J., Pourali, K., and Roghani, A. F. (2012). “Alkaline peroxide mechanical pulping of fast-growth Paulownia wood,” BioResources 7(1), 265-274.

Law, K. N. (1996). “Study on cyclic reslushing of mechanical pulps,” Prog. Paper Recycling 6(1), 32-38.

Lawoko, M., Henriksson, G., and Gellerstedt, G. (2005). “Structural differences between the lignin-carbohydrate complexes present in wood and in chemical pulps,” Biomacromol. 6(6), 3467-3473.

Lee, H. L., and Joo, S. B. (2000). “The effect of recycling procedures of papermaking fibres and fines on the adsorption of cationic polyacrylamide,” Nordic Pulp Paper Res. J. 15(5), 446-451.

Lee, H. L., Ham, C. H., and Lee, S. G. (2006). “Influence of papermaking system closure on paper properties,” TAPPI J. 5(5), 27-31.

Leech, H. J. (1954). “An investigation of the reasons for increase in paper strength when locust bean gum is used as a beater adhesive,” TAPPI 37(8), 343-349.

Le Ny, C., and Messmer, M. (2007). “Potential of refining and dispersing to develop recycled fibre properties,” Pulp Paper Canada108(2), 38-41.

Le Roux, J., Daneault, C., and Chabot, B. (2006). “Acidic groups in TMP oxidized fibres by TEMPO to improve paper strength properties,” Pulp Paper Canada 107(4), 39-41.

Li, B., Li, H., Zha, Q. Q., Bandekar, R., Alsaggaf, A., and Ni, Y. H. (2011). “Review: Effects of wood quality and refining process on TMP pulp and paper quality,” BioResources 6(3), 3569-3584.

Li, H., Fu, S. Y., Peng, L. C., and Zhan, H. Y. (2012a). “Surface modification of cellulose fibers with layer-by-layer self-assembly of lignosulfonate and polyelectrolyte: Effects on fibers wetting properties and paper strength,” Cellulose 19(2), 533-546.

Li, L., Collis, A., and Pelton, R. (2002). “A new analysis of filler effects on paper strength,” J. Pulp Paper Sci. 28(8), 267-273.

Li, Y. Y., Dai, H. Q., Wan, L., and Zhu, Z. J. (2012b). “Surface sizing application of waterborne epoxy resin on low basis weight paper,” BioResources 7(1), 5-14.

Lima, D. U., Oliveira, R. C., and Buckeridge, M. O. (2003). “Seed storage hemicelluloses as wet-end additives in papermaking,” Carbohyd. Polym. 52(4), 367-373.

Lin, T., Yin, X. F., Nazhad, M. M., and Lehtinen, E. (2010). “Increased filler content by co-flocculation of chemical pulp fines and filler,” APPITA J. 63(4), 276-280.

Lind, M. (2011). “De-bottlenecking of a washing constraint,” APPITA J. 64(2), 121-125.

Lindqvist, H., Homback, J., Rosling, A., Salminen, K., Holmbom, B., Auer, M., and Sundberg, A. (2013). “Galactoglucomannan derivatives and their application in papermaking,” BioResources 8(1), 994-1010.

Lindqvist, H., Salminen, K., Kataja-Aho, J., Sundberg, A., Holmbom, B., and Retulainen, E. (2009). “Effects of electrolytes, pH and surface tension on dewatering, dry and wet web properties,” J. Pulp Paper Sci. 35(3-4), 148-154.

Lindström, T. (1986a). “The concept and measurement of fiber swelling,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 4, pp. 75-97.

Lindström, T. (1986b). “The porous lamellar structure of the cell wall,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 5, pp. 99-120.

Lindström, S. B., Uesaka, T., and Hirn, U. (2009). “Evolution of the paper structure along the length of a twin-wire former,” Advances in Pulp and Paper Research, 14th Fundamental Research Symposium on Advances in Pulp and Paper Research Location: St Annes Coll, Oxford, Ianson, S. J. (ed.), 207-245.

Lindström, T., and Carlsson, G. (1982). “The effect of carboxyl groups and their ionic form during drying on the hornification of cellulose fibers,” Svensk Papperstidn. 85(15), R146-R151.

Lindström, T., and Florén, T. (1984). “The effects of cationic starch wet end addition on the properties of clay filled papers,” Svensk Pappersidn. 87(12), R99-R104.

Lindström, T., and Kolman, M. (1982). “The effect of pH and electrolyte concentration during beating and sheet forming on paper strength,” Svensk Papperstidn. 85(15), R140-R145.

Lindström, T., Kolseth, P., and Näslund, P. (1985). “The dry strengthening effect of cationic starch wet-end addition on filled papers,” in: Punton, V. (ed.), Papermaking Raw Materials, Trans 8^thFund. Res. Symp., Oxford, Mechanical Eng. Publ. Ltd., Vol. 2, 589-611.

Lindström, T., and Larsson, P. T. (2008). “Alkyl ketene dimer (AKD) sizing – A review,” Nordic Pulp Paper Res. J. 23(2), 202-209.

Lindström, T., Wågberg, L., and Larsson, T. (2005). “On the nature of joint strength in paper – A review of dry and wet strength resins used in paper manufacturing,” Proc. 13^th Fundamental Research Symp., Cambridge, Pira Intl., Leatherhead, Surrey, UK.

Lingström, R., and Wågberg, L. (2008). “Polyelectrolyte multilayers on wood fibers: Influence of molecular weight on layer properties and mechanical properties of papers from treated fibers,” J. Colloid Interface Sci. 328(2), 233-242.

Linhart, F., Horn, D., Eisenlauer, J., and Hemel, R. (1987). “Überwachung und Steuerung des Blattbildung mit Hilfe eines fasteroptischen Flockungssensors,” Wochenblatt für Papierfabrikation 115(8), 331-338.

Linhart, F. (2005). “Some thoughts on the mode of action of paper strength agents,” Wochenblatt für Papierfabrikation 133(11-12), 662-672.

Linke, W. F. (1968). “Retention and bonding of synthetic dry strength resins,” TAPPI 51(11), 59A-65A.

Linna, H. I., Parola, M. J., and Virtanen, J. O. (2002). “Improving productivity by measuring web tension profiles,” APPITA J. 55(4), 317-322.

Lipponen, J., Gron, J., Bruun, S. E., and Laine, T. (2004). “Surface sizing with starch solutions at solids contents up to 18%,” J. Pulp Paper Sci. 30(3), 82-90.

Liu, N., Qin, M., and Li, Z. (2013). “Laccase-catalyzed fiber functionalization with different phenolic compounds for enhancing pulp strength,” BioResources 8(1), 887-899.

Liu, Y. X., Sun, B., Hu, H. R., and Chen, K. L. (2011a). “Enhance characteristics of paper surface with cationic hemicelluloses,” Appl. Chem. Eng. 236-238, 1342-1346.

Liu, Z., Choi, H., Gatenholm, P., and Esker, A. R. (2011b). “Quartz crystal microbalance with dissipation monitoring and surface plasmon resonance studies of carboxymethyl cellulose adsorption onto regenerated cellulose surfaces,” Langmuir 27(14), 8718-8728.

Lobben, T. H. (1975). “The tensile stiffness of paper. Part 1. A model based on activation,” Norsk Skogindustri 29(12), 311-315.

Lofton, M. C., Moore, S. M., Hubbe, M. A., and Lee, S. Y. (2005). “Polyelectrolyte complex deposition as a mechanism of paper dry-strength development,” TAPPI J 4(9), 3-7.

Lohmander, S., and Rigdahl, M. (2000). “Influence of a shape factor of pigment particles on the rheological properties of coating colours,” Nordic Pulp Paper Res. J. 15(3), 231-236.

Lucisano, M. F. C., Petrini, J. B., and Martin, A. R. (2003). “The role of evaporative dewatering in impulse pressing,” TAPPI J. 2(3), 26-32.

Lund, M., and Felby, C. (2001). “Wet-strength improvement of unbleached kraft pulp through laccase-catalyzed oxidation,” Enzyme Microb. Tech. 28, 760-765.

Lundberg, R., and de Ruvo, A. (1978a). “Influence of drying conditions on recovery of swelling and strength of recycled fibers,” Svensk Papperstidn. 81(11), 355-358.

Lundberg, R., and de Ruvo, A. (1978b). “The influence of defibration and beating conditions on the paper making potential of recycled paper,” Svensk Papperstidn. 81(12), 383-386.

Lundin, T., Batchelor, W. J., and Fardim, P. (2008). “Fibre trapping in low consistency refining: New parameters to describe the refining process,” TAPPI J. 7(7), 15-21.Luo, X. L., Hu, H. C., Chai, X. S., Cao, S. L., Huang, L. L., and Chen, L. H. (2012). “Improving bleached pulp yield and paper strength properties of eucalyptus through integrating kraft pulping to high kappa number and oxygen delignification,” J. Biobased Mater. Bioenergy 6(5), 531-537.Luthe, C., Berry, R. M., Radiotis, T., and Nadeau, L. (2003). “Measuring softwood yield gain at kraft pulp mills,” J. Pulp Paper Sci. 29(11), 371-376.Lvov, Y. M., Grozdits, G. A., Eadula, S., Zheng, Z. G, and Lu, Z. H. (2006). “Layer nanocoating of mill broken fibers for improved paper,” Nordic Pulp Paper Res. J. 21(5), 552-557.

Lyne, L. M., and Gallay, W. (1950). “The effect of drying and heating on the swelling of cellulose fibers and paper strength,” TAPPI 33(9), 429-435.

Lyne, L. M., and Gallay, W. (1954). “Measurement of wet web strength,” TAPPI 37(12), 694-698; “Studies in the fundamentals of wet web strength,” TAPPI 37(12), 698-704.

Ma, P., Zhai, H., Law, K., and Daneault, C. (2012). “Influences of integrated TEMPO-mediated oxidation and recycling on the properties of TMP fibers,” BioResources 7(2), 2260-2271.

MacGregor, M. A. (1983a). “A description of sheet stratification caused by wet pressing,” TAPPI J. 66(6), 53-57.

MacGregor, M. A. (1983b). “Practical effects of sheet stratification caused by wet pressing,” TAPPI J. 66(7), 65-69.

Maier, G., Zipper, P., Stugicar, M., and Schurz, J. (2005). “Amorphization of different cellulose samples by ball milling,” Cellulose Chem. Technol. 39(3-4), 167-177.

Maloney, T. C., Li, T.-Q., Weise, U., and Paulapuro, H. (1997). “Intra- and inter-fiber pore closure in wet pressing,” APPITA J.50(40), 301-306.

Maloney, T. C., Johansson, T., and Paulapuro, H. (1998a). “Removal of water from the cell wall during drying,” Paper Technol. 39(6), 39-47.

Maloney T. C., Paulapuro, H., and Stenius P. (1998b), “Hydration and swelling of pulp fibers measured with differential scanning calorimetry,” Nordic Pulp & Paper Research Journal 13(1), 31-36.

Manfredi, V., and Claudio-Da-Silva, E. (1986). “Refining – Operational variables vs. raw materials,” Advances in Refining Technologies, Pira International, Leatherhead, Surrey, UK, Paper 12, 42 pp.

Mansfield, S. D., Parish, R., Di Lucca, C. M., Goudie, J., Kang, K. Y., and Ott, P. (2009). “Revisiting the transition between juvenile and mature wood: A comparison of fibre length, microfibril angle and relative wood density in lodgepole pine,” Holzforschung 63(4), 449-456.

Mansfield, S. D., and Saddler, J. N. (1999). “Sheet properties of Douglas fir kraft pulp after selective treatments of different fiber length fractions with cellulases,” J. Pulp Paper Sci. 25(3), 84-89.

Manson, D. W. (1991). “Fourdrinier papermaking,” in: Thorp, B. A. (ed.), Paper Machine Operations, Pulp and Paper Manufacture series, Vol. 7, TAPPI, Atlanta, Ch. 8, pp. 192-215.

Mao, Z., and Yang, C. Q. (2001). “IR spectroscopy study of cyclic anhydride as intermediate for ester crosslinking of cotton cellulose by polycarboxylic acids. V. Comparison of 1,2,4-butanetricarboxylic acid and 1,2,3-propanecarboxylic acid,” J. Appl. Polymer Sci. 81, 2142-2150.

Marais, A., and Wågberg, L. (2012). “The use of polymeric amines to enhance the mechanical properties of lignocellulosic fibrous networks,” Cellulose 19(4), 1437-1447.

Mark, R. E. (2002). Handbook of Physical Testing of Paper, Marcel Dekker, New York.

Mather, A. S. (2005). “Assessing the world’s forests,” Global Environmental Change-Human and Policy Dimensions 15(3), 267-280

Mathur, G. M., Mahajan, S., Unnikikshanan, K. P., and Sharma, Y. K. (1987). “Studies on sheet pressing. 1. Effect of intensity of pressure during wet pressing on physical properties of standard test sheets made from short fibers,” IPPTA 4(3 Suppl.), 23-33.

Mathur, V. K. (2004). “Novel silicate “fibrous fillers” and their application in paper,” Proc. TAPPI Spring Technical Conference.

Maurer, H. W. (2001). Starch and Starch Products in Surface Sizing and Paper Coating, TAPPI Press, Atlanta.

Maus, H. (1967). “More practical suggestions from a papermaker’s notebook,” Paper Trade J. 151(27), 34-36; (28), 53-54, 58.

Maximova, N., Laine, J., and Stenius, P. (2005). “Adsorption of lignin-cationic starch complexes on cellulose fibers and their effect on sheet properties,” Paperi Puu 87(3), 176-182.

Mayhood, C. H., Jr., Kallmes, O. J., and Cauley, M. M. (1962). “The mechanical properties of paper. Part II. Measured shear strength of individual fiber to fiber contacts,” TAPPI 45(1), 69-73.

McComb, R. E., and Williams, J. C. (1981). “The value of alkaline papers for recycling,” TAPPI 64(4), 93-96.

McKee, R. C. (1971). “Effect of repulping on sheet properties and fiber characteristics,” Paper Trade J. 155(21), 34-40.

McKenzie, A. W. (1978). “The structure and properties of paper. Part XX. The tensile properties of paper and papermaking fibres,” APPITA32(3), 207-212.

McKenzie, A. W. (1984). “The structure and properties of paper. Part XXI: The diffusion theory of adhesion applied to interfiber bonding,” APPITA 37(7), 580-583.

McKenzie, A. W. (1987). “The effect of esterification on papermaking fibers – A re-appraisal,” APPITA 40(4), 273-278.

McNabb, K. L., and Wadouski, L. H. (1999). “Multiple rotation yields for intensively managed plantations in the Amazon basin,” New Forests 18(1), 5-15.

Mendes, P., Aguilar, H., Ranita, J., Belgacem, M. N., Silvy, J., and Costa, C. A. V. (2002). “The influence of impulse drying on the performances of papers containing cationic starch and calcium carbonate,” TAPPI J. 1(9), 10-15.

Menezes, C. W. G., Soares, M. A., Santos, J. B., Assis, S. L., Fonseca, A. J., and Zanuncio, J. C. (2012). “Reproductive and toxicological impacts of herbicides used in Eucalyptus culture in Brazil on the parasitoid Palmistichus elaeisis (Hymenoptera: Eulophidae),” Weed Res. 52(6), 520-525.

Meng, Q., Wan, J., Ma, Y., and Wang, Y. (2013). “Effects of different deinking processes on fiber morphology, hydrogen bond models, and cellulose supramolecular structure,” BioResources 8(2), 2398-2416.

Merrild, H., Damgaard, A., and Christensen, T. H. (2009). “Recycling of paper: Accounting of greenhouse gases and global warming contributions,” Waste Manag. Res. 27(8), 746-753.

Miao, C., Leduc, M., and Pelton, R. (2008). “The influence of polyvinylamine microgels on paper strength,” J. Pulp Paper Sci.34(1), 69-75.

Miao, C. W., Pelton, R., Chen, X. N., and Leduc, M. (2007). “Microgels versus linear polymers for paper wet strength – Size does matter,” APPITA J. 60(6), 465-468.

Michaels, A. S. (1965). “Polyelectrolyte complexes,” Indust. Eng. Chem. 57(10), 32-40.

Middleton, S. R., and Scallan, A. M. (1985). “Lumen-loaded paper pulp: Mechanism of filler-to-fibre bonding,” Colloids and Surfaces16, 309-322.

Middleton, S. R., and Scallan, A. M. (1989). “Lumen loading of bleached pulps,” J. Pulp Paper Sci. 15(5), J229-J234.

Mihara, I., Sakaemura, T., and Yamauchi, T. (2008). “Mechanism of paper strength development by the addition of dry strength resin and its distribution within and around a fiber wall – Effect of application method,” Nordic Pulp Paper Res. J. 23(4), 382-388.

Minor, J. L., Atalla, R. H., and Harten, T. M. (1993). “Improving interfiber bonding of recycled fibers,” J. Pulp Paper Sci. 19(4), J152-J155.

Miranda, R., Monte, M. C., and Blanco, A. (2013). “Analysis of the quality of the recovered paper from commingled collection systems,” Resources Conserv. Recyc. 72, 60-66.

Mitchell, A. (1994). “Greenfield giant gears up for startup,” Pulp Paper Internat. 36(9), 22-29.

Mitchell, C., and Johnston, R. (2003). “Modeling water removal from wet fibre mats,” APPITA J. 56(6), 433-437.

Mobarak, F., El-Ashmawy, C. S., and Fahmy, Y. (1973). “Hemicelluloses as additive in papermaking. Part II. The role of added hemicellulose, and hemicellulose in situ on paper properties,” Cellulose Chem. Technol. 7, 325-335.

Moberg, K. (1985). “Highly filled papers: Obstacles and approaches,” TAPPI 1985 Papermakers Conf. 119-121.

Moberg, K. (1986). “Wet end chemicals for reducing board density,” Southern Pulp Paper 49(4), 29-31.

Mocchiutti, P., Galvan, M. V., Inalbon, M. C., and Zanuttini, M. A. (2011). “Improvement of paper properties of recycled unbleached softwood kraft pulps by poly(allylamine hydrochloride),” BioResources 6(1), 570-583.

Mocchiutti, P., Zanuttini, M., Citroni, M., and Inalbon, M. C. (2006). “Effects of drying conditions and of the existing cationic starch in liner paper recycling,” Cellulose Chem. Technol. 40(1-2), 87-91.

Molin, U., and Teder, A. (2002). “Importance of cellulose/hemicellulose-ratio for pulp strength,” Nordic Pulp Paper Res. J. 17(1), 14-19, 28.

Mollabashi, O. G., Saraeian, A. R., and Resalati, H. (2011). “The effect of surfactants application on soda pulping of wheat straw,” BioResources 6(3), 2711-2718.

Moon, R. J., Martini, A., Nairn, J., Simonsen, J., and Youngblood, J. (2011). “Cellulose nanomaterials review: Structure, properties and nanocomposites,” Chem. Soc. Revs. 40(7), 3941-3994.

Moss, P. A., and Retulainen, E. (1997). “The effects of fines on fibre bonding: Cross-sectional dimensions of TMP fibres at potential bonding sites,” J. Pulp Paper Sci. 23(8), J382-J388.

Muguet, M. C. D., Colodette, J. L., and Jääskeläinen, A. S. (2012). “Alkaline peroxide mechanical pulping of novel Brazilian eucalyptus hybrids,” BioResources 7(3), 3823-3836.

Myllytie, P., Yin, J. Y., Holappa, S., and Laine, J. (2009). “The effect of different polysaccharides on the development of paper strength during drying,” Nordic Pulp Paper Res. J. 24(4), 469-477.

Nagata, T. (1991). “Recent advances of paper strengthening agent: New mechanism of dry-strength development; Concept of anionic and cationic polyacrylamide mixing system,” Jpn. TAPPI J. 45(2), 245-249.

Nahrath, G. (2004). “Trends in the manufacturing of corrugated board and demands on the corrugating stock,” Wochenblatt fur Papierfabrikation 132(1-2), 18-23.

Nanko, H., Button, A., and Hillman, D. (2005). The World of Market Pulp, WOMP LLC.

Nanko, H., and Ohsawa, J. (1989). “Mechanisms of fiber bond formation,” in: Fundamentals of Papermaking, Proc. Fundamental Res. Symp, Vol. 2, Waveney Print Services, Suffolk, UK, 783-830.

Nanko, H., Ohsawa, J., and Okagawa, A. (1989). “How to see interfiber bonding in paper sheets,” J. Pulp Paper Sci. 15(1), J17-J23.

Navaee-Ardeh, S., and Nazhad, M. M. (2008). “A new model for maximizing the bending stiffness of a symmetric three-ply paper or board,” TAPPI J. 7(10), 28-31.

Nazhad, M. M. (2004). “The influence of refining energy and intensity on enhancing the bonding potential of an OCC pulp,” APPITA J. 57(3), 191-198.

Nazhad, M. M. (2005). “Recycled fiber quality – A review,” J. Ind. Eng. Chem. 11(3), 314-329.

Nazhad, M. M., and Awadel-Karim, S. (2001). “Possibilities for upgrading OCC pulp,” Proc. 55^th APPITA Annual Conf., 487-494.

Nazhad, M. M., and Pazner, L. (1994). “Fundamentals of strength loss in recycled paper,” TAPPI J. 77(9), 171-179.

Nealey, L., Hamada, M., and Kimura, T. (1989). “Some factors affecting strength enhancement by a polyacrylamide dual polymer system,” Proc. TAPPI 1989 Papermakers Conf., TAPPI Press, Atlanta, 261-268.

Nebergall, W. H., Holtzclaw, H. F., and Robinson, W. R. (1980). General Chemistry, Heath and Co., Lexington, MA, USA.

Nemeth, R., Abraham, J., and Molnar, S. (2008). “Juvenile wood of hardwoods – Differences to mature wood,” in: European Wood Processing Strategy: Future Resources Matching Products and Innovations, COST Conference on a European Wood Processing Strategy, Milan, May 30 – June 03, 109-118.

Newman, R. H. (2004). “Carbon-13 NMR evidence for cocrystallization of cellulose as a mechanism of hornification of bleached kraft pulp,” Cellulose 11, 45-52.

Newman, R. H., and Hemmingson, J. A. (1997). “Cellulose cocrystallization in hornification of kraft pulp,” Proc. 1997 ISWPC, Paper 01, 1-4.

Nguyen, X. T., Shariff, A., Earl, P. F., and Eamer, R. J. (1993). “Bleached pulps for printing & writing papers from old corrugated containers,” Prog. Paper Recycling 3(2), 25-32.

Nilsson, L., and Carlson, U. (1993). “Improvements in the production of boards with a silica microparticle system approach,” TAPPI 1993 Papermakers Conf., TAPPI Press, Atlanta, 181-186.

Nishiyama, Y., Kim, U.-J., Kim, D.-Y., Katsumata, K. S., May, R. P., and Langan, P. (2003). “Periodic disorder along ramie cellulose microfibrils,” Biomacromol. 4, 1013-1017.

Niskanen, K. (2012). Mechanics of Paper Products, De Gruyter, Berlin.

Niskanen, K., Kajanto, I., and Pakarinen, P. (1998). “Paper structure,” in: Paper Physics, Niskanen, K. (ed.), Papermaking Science and Technology series No. 16, Fapet Oy, Helsinki, Ch. 1, pp. 14-53.

Niskanen, K., and Kärenlampi, N. (1998). “In-place tensile properties,” in: Paper Physics, Niskanen, K. (ed.), Papermaking Science and Technology series No. 16, Fapet Oy, Helsinki, Ch. 5, pp. 139-191.

Niskanen, K., and Leskela, M. (1997). “Model simulations give new insights into paper strength,” Paperi Puu 79(5), 317-323.

Nissan, A. H., and Batten, G. L. (1990). “On the primacy of the hydrogen bond in paper mechanics,” TAPPI J. 73(2), 159-164.

Nissan, A. H., and Batten, G. L. (1997). “The link between the molecular and structural theories of paper elasticity,” TAPPI J. 80(4), 153-158.

Nissan, A. H., Byrd, V. L., Batten, G. L., and Ogden, R. W. (1985). “Paper as an H-bond dominated solid in the elastic and plastic regimes,” TAPPI J. 68(9), 118-124.

Nissan, A. H., and Higgins, H. G. (1959). “Molecular approach to the problem of viscoelasticity,” Nature 184, 1477-1478.

Nissan, A. H., and Sternstein, S. S. (1964). “Cellulose-fiber bonding,” TAPPI 47(1), 1-6.

Nordström, B. (2003a). “Twin-wire blade forming versus roll forming of a linerboard furnish – Effects on tensile strength and formation,” Nordic Pulp Paper Res. J. 18(3), 245-254.

Nordström, B. (2003b). “Effects of roll wrapping angle and headbox flow rate in twin-wire roll-blade forming of a linerboard furnish,” Nordic Pulp Paper Res. J. 18(3), 255-261.

Nordström, B. (2003c). “Fourdrinier versus twin-wire roll forming – Effects on fiber orientation anisotropy and other sheet properties,” Nordic Pulp Paper Res. J. 18(3), 262-269.

Nordström, B. (2003d). “Effects of grammage on sheet properties in one-sided and two-sided roll forming,” Nordic Pulp Paper Res. J.18(3), 280-287.

Nordström, B. (2003e). “Effects of pulp type and headbox design on anisotry and other sheet properties in twin-wire roll forming,” Nordic Pulp Paper Res. J. 18(3), 288-295.

Nordström, B. (2003f). “Influence of consistency in twin-wire roll forming of two recycled linerboard furnishes,” Nordic Pulp Paper Res. J. 18(3), 303-309.

Nordström, B. (2006). “Twin-wire roll forming of mechanical based paper from three furnishes – Effects on formation and mechanical properties,” Nordic Pulp Paper Res. J. 21(3), 349-3558.

Nordström, B., and Norman, B. (1994). “Influence on sheet anisotropy, formation, Z-toughness and tensile stiffness of reduced feed area to a headbox nozzle,” Nordic Pulp Paper Res. J. 9(1), 53-59.

Nordström, B., and Norman, B. (1995a). “Effects of headbox jet quality and blade pulse force on formation, retention and mechanical properties during roll-blade forming with curved blades,” Nordic Pulp Paper Res. J. 10(1), 33-40,45.

Nordström, B., and Norman, B. (1995b). “Effects of roll-blade and roll forming on formation, retention, mechanical properties and anisotropy – A preliminary comparison,” J. Pulp Paper Sci. 21(7), J223-J230.

Nordström, B., and Norman, B. (1995c). “Effects of grammage on paper properties for twin-wire roll forming of TMP,” J. Pulp Paper Sci. 21(12), J427-J431.

Nordström, B., and Norman, B. (1996). “Effects on paper properties and retention of the proportion of roll dewatering during twin-wire roll-blade forming of TMP,” J. Pulp Paper Sci. 22(8), J283-J289.

Norman, B. (1986). “The formation of paper sheets,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 6, pp. 123-150.

Norman, B. (1991). “Overview of the physics of forming,” Overview of the Fundamentals of Papermaking, Pulp and Paper Fundamental Research Society, pp. 39-115.

Norman, B. (2000). “Web forming,” in: Paulapuro, H. (ed.), Papermaking. Part 1. Stock Preparation and Wet End, Fapet Oy, Helsinki, Ch. 6, 193-250.

Norman, R. J. (1965). “Dependence of sheet properties on formation and forming variables,” in: Consolidation of the Paper Web. Transactions of the 3^rd Fundamental Research Symposium, Bolam, F. (ed.), Technical Section of the British Paper and Board Makers’ Association, Cambridge, p. 269.

Obermanns, H. E. (1936). “A study of the effect of hemicelluloses on the beating and strength of pulps,” Paper Trade J. TAPPI Sec. 103(8), T109-T117.

Obokata, T., and Isogai, A. (2009). “Wet-strength development of cellulose sheets prepared with polyamideamine-epichlorohydrin (PAE) resin by physical interactions,” Nordic Pulp Paper Res. J.24(2), 135-140.

O’Brian, H. (2002). “Pilot trials become basis for new SymPress BBI press configuration at SCA Muksund,” Fiber and Paper 4(3), 19-21.

Odell, M. H. (2000). “Paper structure engineering,” APPITA J. 53(5), 371-377.

Ogden, R. W. (1999). “Rehydration of once-dried fiber,” US Pat.5,825,218.

Oksanen, T., Buchert, J., and Viikari, L. (1997). “The role of hemicelluloses in the hornification of bleached kraft pulps,” Holzforschung 51(4), 355-360.

Olejnik, K., Stanislawska, A., Wysocka-Robak, A., and Przybysz, P. (2012). “Evaluation of the possibilities of upgrading the papermaking potential for different recycled pulp grades,” Fibers & Textiles in Eastern Europe 20(2), 102-106.

Omholt, I. (1999). “The effects of curl and microcompressions on the combination of sheet properties,” TAPPI Intl. Paper Physics Conf., Proceedings, TAPPI Press, Atlanta, pp. 499-515.

Osovskaya, I. I., and Poltoratskii, G. M. (1999). “Role of low-temperature cellulose treatment in paper sheet strengthening,” Russian J. Appl. Chem. 72(8), 1458-1461.

Ozaki, Y. (2011). “Application of confocal laser scanning microscopy (CLSM) for observing adhesives in paper,” J. Adhesion Sci. Technol.25(6-7), 723-741.

Paavilainen, L. (1993). “Conformability – flexibility and collapsibility – of sulfate pulp fibers,” Paperi Puu 75(9-10), 896-702.

Page, D. H. (1960). “Fibre-to-fibre bonds. Part 1 – A method for their direct observation,” Paper Technology 1(4), 407-411.

Page, D. H. (1969a). “A theory for the tensile strength of paper,” TAPPI 52(4), 674-681.

Page, D. H. (1969b). “The axial compression of fibers – A newly discovered beating action,” Pulp Paper Mag. Can. 67(1), T2-T12.

Page, D. H. (1983). “The origin of the differences between sulfite and kraft pulps,” J. Pulp Paper Sci. 84(1), TR15-TR20.

Page, D. H. (1985). “The mechanism of strength development of dried pulps by beating,” Svensk Papperstidning 88(3), R30-R35.

Page, D. (1993). “A quantitative theory of the strength of wet webs,” J. Pulp Paper Sci. 19(4), J175-J176.

Page, D. H., and Tydeman, P. A. (1962). “A new theory of the shrinkage, structure, and properties of paper,” in Formation and Structure of Paper, F. Bolam (ed.), Tech. Sect. British Paper and Board Makers’ Assoc., London, Vol. 1, 397-425.

Pajula, T., and Kärnä, A. (1995). “Life cycle scenarios of paper,” The First EcoPaperTech, An International Conf. Papermaking Paper Machine Technology, Helsinki, Finland, June 6-9, The Finnish Pulp and Paper Research Inst.

Pala, H., Lemos, M. A., Mota, M., and Gama, F. M. (2001). “Enzymatic upgrade of old paperboard containers,” Enzyme Microbial Technol. 29(4-5), 274-279.

Pang, L., and Gray, D. G. (1998). “Heterogeneous fibrillation of kraft pulp fibre surfaces observed by atomic force microscopy,” J. Pulp Paper Sci. 24(11), 369-372.

Park, S., Venditti, R. A., Jameel, H., and Pawlak, J. J. (2006). “Changes in pore size distribution during the drying of cellulose fibers as measured by differential scanning calorimetry,” Carbohydrate Polymers 66(1), 97-103.

Paulapuro, H. (1993). Developments in Wet Pressing: A Literature Review, Pira International, Leatherhead, Surrey, UK.

Paulapuro, H. (2000). Papermaking. Part 1, Stock Preparation and Wet End, Papermaking Science and Technology series, Fapet Oy, Helsinki.

Paulapuro, H. V., and Thorp, B. A. (1983). “Effect of pulp and papermaking variables on linerboard ring crush,” Pulp Paper 57(11), 142-145.

Paunonen, S. (2013). “Strength and barrier enhancements of cellophane and cellulose derivative films: A Review,” BioResources8(2), 3098-3121.

Peel, L. R. (1989). “Technical breakthrough in wet end additives,” Paper Technol. 30(Dec.), 20-23.

Pelton, R. (1993). “A model of the external surface of wood pulp fibers,” Nordic Pulp Paper Res. J. 8(1), 113-119.

Pelton, R. (2004). “On the design of polymer for increased paper dry strength – A review,” APPITA J. 57(3), 181-190.

Pelton, R., Zhang, J., Wågberg, L, and Rundlöf, M. (2000). “The role of surface polymer compatibility in the formation of fiber/fiber bonds in paper,” Nordic Pulp Paper Res. J. 15(5), 400-406.

Pelton, R., Zhang, J., Chen, N., and Moghaddamzadeh, A. (2003). “The influence of dextran molecular weight on the dry strength of dextran-impregnated paper,” TAPPI J. 2(4), 15-18.

Peng, B. L., Dhar, N., Liu, H. L., and Tam, K. C. (2011). “Chemistry and applications of nanocrystalline cellulose and its derivatives: A nanotechnology perspective,” Can. J. Chem. Eng. 89(5), 1191-1206.

Pfaler, E. (1933). Papier Ztg. 48(76), 282.

Pikulik, I. I. (1999). “Effects of shoe pressing on fine paper properties,” TAPPI J. 82(11), 88-92.

Pirani, S., and Hashaikeh, R. (2013). “Nanocrystalline cellulose extraction process and utilization of the byproduct for biofuels production,” Carbohydrate Polymers 93(1), 357-363.

Pommier, J. C., Fuentes, J. L., and Goma, G. (1989). “Using enzymes to improve the process and the product quality in the recycled paper industry. Part 1: The basic laboratory work,” TAPPI J. 72(6), 187-191.

Pönni, R., Kontturi, E., and Vuorinen, T. (2013), “Accessibility of cellulose: Structural changes and their reversibility in aqueous media,” Carbohydrate Polymers 93, 424-429.

Pönni, R., Vuorinen, T., and Kontturi, E. (2012). “Proposed nano-scale coalescence of cellulose in chemical pulp fibers during technical treatments,” BioResources 7(4), 6077-6108.

Popil, R. E. (2012). “Overview of recent studies at IPST on corrugated board edge compression strength: Testing methods and effects of interflute buckling,” BioResources 7(2), 2553-2581.

Potucek, F., Milichovsky, M., Chylkova, J., and Cesek, B. (2005). “Polysulphide kraft cooks and their influence on papermaking behavior,” Cellulose Chem. Technol. 39(5-6), 473-481.

Pöyry (2011). “Average papermaking fibre furnish the world 1990-2025,” Pulp & RP Consumption 1995-2025, World Fibre Outlook, Pöyry LLC.

Proctor, A. A., and Reid, J. G. (1974). “Developments in starch spraying techniques in paper and board making,” Paper Technol. 15, 283-287.

Przybysz, K., and Czechowski, J. (1985). “The effect of pulp fines on the drying process and paper strength properties,” Cellulose Chem. Technol. 19(2), 197-209.

Pycraft, C. J. H., and Howarth, P. (1980). “Does better paper mean worse waste paper?” Paper Technol. Ind. 21(12), 321-324.

Pynnönen, T., Paltakari, J., Hiltunenn, E., Laine, J. E., and Paulapuro, H. (2002). “Effect of press drying on sheet properties of high-temperature thermomechanical pulp (HTMP),” APPITA J. 55(3), 220-223.

Radvan, B., Dodson, C., and Skold, C. G. (1966). “Detection and cause of the layered structure of paper,” in: Consolidation of the Paper Web, Trans Sump. Oxford, Sept. 1965, Bolam, F. (ed.), Tech. Sec. British Paper and Board Makers’ Assoc., London, 189-215.

Rahman, M. O., Hussain, A., Basri, N. E. A., Scavino, E., Basri, H., and Hannan, M. A. (2012). “Chromaticity based waste paper grade identification,” Intl. Arab J. Inform. Technol. 9(5), 411-417.

Ramasubramanian, M. K., Venditti, R. A., and Gillella, P. K. (2012). “Sensor systems for high-speed intelligent sorting of waste paper in recycling,” TAPPI J. 11(2), 33-39.

Ramires, E. C., and Dufresne, A. (2011). “A review of cellulose nanocrystals and nanocomposites,” TAPPI J. 10(4), 9-16.

Ranger, A. E. (1967). “Flexural stiffness of multi-ply boxboard,” Paper Technol. 8(1), 51-56.

Rapp, H. U., and Laufmann, M. (1995). “Wire abrasion and its possible causes,” Wochenbl. Papierfabr. 123(18), 803-808.

Ratliff, F. T. (1949). “The possible correlation between hemicelluloses and the physical properties of bleached kraft pulps,” TAPPI 32(8), 357-367.

Rebuzzi, F., and Evtuguin, D. V. (2006). “Effect of glucuronoxylan on the hornification of Eucalyptus globulus bleached pulps,” Macromol. Symp. 232, 121-128.

Reese, R. A. (1991). “Pressing operations,” in: Thorp, B. A. (ed.), Paper Machine Operations, Pulp and Paper Manufacture series, Vol. 7, TAPPI, Atlanta, Ch. 11, pp. 260-281.

Reif, W. M. (1972). “The utilization of ionic bonds in the development of paper strength,” Proc. TAPPI 1972 Papermakers Conf., TAPPI Press, Atlanta, 47-61.

Reme, P. A., Helle, T., and Johnsen, P. O. (1998). “Fibre characteristics of some mechanical pulp grades,” Nordic Pulp Paper Res. J. 13(4), 263-268.

Ren, J. L., Peng, F., and Sun, R. C. (2009). “The effect of hemicellulosic derivatives on the strength properties of old corrugated container pulp fibres,” J. Biobased Mater. Bioenergy 3(1), 62-68.

Ren, J. L., Peng, X. W., Peng, F., and Sun, R. C. (2011). “The preparation and application of the cationic biopolymer based on xylan-rich hemicelluloses from agricultural biomass,” Advanced Materials 239-242, 463-467.

Retulainen, E. (2003). “Condebelt press drying and sustainable paper cycle,” Paperi Puu 85(6), 329-333 (2003).

Retulainen, E., and Nieminen, K. (1996). “Fiber properties as control variables in papermaking? Part 2. Strengthening interfiber bonds and reducing grammage,” Paperi Puu 78(5), 305-312.

Retulainen, E., Niskanen, K., and Nilsen, N. (1998). “Fibers and bonds,” in: Paper Physics, Niskanen, K. (ed.), Papermaking Science and Technology series No. 16, Fapet Oy, Helsinki, Ch. 2, pp. 55-87.

Reynolds, W. F., and Wasser, R. B. (1980). “Dry-strength resins,” in Pulp and Paper Chemistry and Chemical Technology, J. P. Casey, ed., 3^rd Ed., Wiley-Interscience, New York, Vol. 3, Ch. 13, 1447-1474.

Riddell, M. C., Jenkins, B., Rivers, A., and Waring, I. (1976). “Three developments at Wolvercote Paper Mill,” Paper Technol. Ind. 17(2), 76- 85; T47-T56.

Rigdahl, M., and Hollmark, H. (1986). “Network mechanics,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 12, pp. 241-266.

RISI (2012). “RISI Annual Review 2012,” accessed March 2013; http://www.risiinfo.com/

Roberts, J. C., Au, C. O., Clay, G. A., and Lough, C. (1986). “The effect of C¹⁴-labelled cationic and native starches on dry strength and formation,” TAPPI J. 69(10), 88-93.

Roberts, J. C., Au, C. O., Clay, G. A., and Lough, C. (1987). “A study of the effect of cationic starch on dry strength and formation using C14 labeling,” J. Pulp Paper Sci. 13(1), J1-J5.

Robertson, J. A., and Bailey, F. W. (1934). “Cause of fiber orientation in paper,” Paper Trade J. 99, 35-36.

Robinson, J. V., Jr. (1980). “Fiber bonding,” in Pulp and Paper Chemistry and Chemical Technology, J. P. Casey, ed., 3^rd Ed., Vol. II, Ch. 7, 915-961.Rojas, O. J., Dedinaite, A., Byrd, M. V., Hubbe, M. A., and Claesson, P. M. (2005). “On the origins of adhesion in papermaking systems,” Proc. XIII Fundamental Research Symposium, Cambridge. (S. J. I’Anson, ed.), 2: 1351-1378, Digital Books Logistics Ltd, Peterborough, UK.Rojas, O. J., and Neuman, R. D. (1999). “Adsorption of polysaccharide wet-end additives in papermaking systems,” Colloids Surf. A – Physicochem. Eng. Aspects155(2-3), 419-432.

Ruffini, G. (1966). “Improvement in bonding of wood pulps by the presence of acidic groups,” Svensk. Papperstidn. 69, 72-76.

Rutland, D. F. (1992). “Dimensional stability and curl,” in: Pulp and Paper Manufacture, Vol. 9, Mill Control and Control Systems: Quality and Test

Sachs, I. B., and Kuster, T. A. (1980). “Edgewise compression failure mechanism of linerboard observed in a dynamic mode,” TAPPI 63(10), 69-73.

Saito, T., Kuramae, R., Wohlert, J., Berglund, L. A., and Isogai, A. (2013). “An ultrastrong nanofibrillar biomaterial: The strength of single cellulose nanofibrils revealed by sonication-induced fragmentation,” Biomacromolecules 14(1), 248-253.

Sakaemura, T., and Yamauchi, T. (2011). “Strength properties of paper containing polyacrylamide-based dry strength resin – Effect of its Z-directional distribution,” APPITA J. 64(4), 331-337.

Salmén, L. (1986). “The cell wall as a composite structure,” in: Bristow, J. A., and Kolseth, P. (eds.), Paper. Structure and Properties, Marcel Dekker, New York, Ch. 3, pp. 51-73.

Salminen, L. I., Räisänen, V. I., Alava, M. J., and Niskanen, K. J. (1996). “Drying-induced stress state of inter-fibre bonds,” J. Pulp Paper Sci. 22(10), J402-J407.

Salminen, L. I., Tolvanen, A. I., and Alava, M. J. (2003). “Acoustic emission from paper fracture,” Phys. Rev. Lett. 89(18), 185503:1–4.

Samyn, P., Deconinck, M., Schoukens, G., Stanssens, D., Vonck, L., and Van den Abbeele, H. (2010). “Modifications of paper and paperboard surfaces with a nanostructured polymer coating,” Prog. Organic Coatings 69(4), 442-454.

Sang, Y., McQuaid, M., and Englezos, P. (2011). “Optimization of chemical use for highly filled mechanical grade papers with precipitated calcium carbonate,” BioResources 6(1), 656-671.

Sang, Y., McQuaid, M., and Englezos, P. (2012). “Pre-flocculation of precipitated calcium carbonate filler by cationic starch for highly filled mechanical grade paper,” BioResources 7(1), 354-373.

Sang, Y. Z., Qian, L. Y., He, B. H., and Xiao, H. N. (2010). “The adsorption of cationic PVA on cellulose fibres and its effect with polyelectrolyte complex on paper strength,” APPITA J. 63(4), 294-299, 307.

Santos, R. B., Jameel, H., Chang, H.-M., and Hart, P. W. (2013). “Impact of lignin and carbohydrate chemical structures on degradation reactions during hardwood kraft pulping,” BioResources8(1), 158-171.

Scallan, A. M. (1998). “Mechanisms of hornification,” In: Improvement of Recyclability and the Recycling Paper Industry of the Future,” Blanco, A., Negro, C., Gaspar, I, Monte, M. C., and Tijero, J. (eds.), Las Palmas de Gran Canaria, 24-26 Nov., 312-314.

Schlegel, J., and Kaipila, J. (2002). “Shoepress and specialty papers,” Wochenbl. Papierfabr. 130(11-12), 806-816.

Schmied, F. J., Teichert, C., Kappel, L., Hirn, U., Bauer, W., and Schennach, R. (2013). “What holds paper together: Nanometre scale exploration of bonding between paper fibers,” Sci. Reports 3, article no. 2432, DOI 10.1038/srep02432.

Schwikal, K., Heinze, T., Saake, B., Puls, J., Kaya, A., and Esker, A. R. (2011). “Properties of spruce sulfite pulp and birch kraft pulp after sorption of cationic birch xylan,” Cellulose 18(3), 727-737.

Scott, W. E. (1996). Principles of Wet End Chemistry, TAPPI Press, Atlanta, GA, USA.

Scott, W. E., and Abbot, J. C. (1995). Properties of Paper: An Introduction, TAPPI Press, Atlanta, 191 pp.

Selder, H., Dockal-Baur, J., and Borschke, D. (2002). “Upgrading by decrilling of recovered paper material for the production of packaging paper,” Wochenblatt für Papierfabrikation 130(5), 292-298.

Seo, D., Im, W. H., Youn, H. J., and Lee, H. L. (2012). “The effects of process variables for GCC pre-flocculation on floc and handsheet properties,” Nordic Pulp Paper Res. J. 27(2), 382-387.

Seo, Y. B., Shin, Y. C., and Joen, Y. (2000). “Enzymatic and mechanical treatment of chemical pulp,” TAPPI J. 83(11), 64 (digital document).

Seth, R. R., Michell, A. J., and Page, D. H. (1985). “The effect of press-drying on paper strength,” TAPPI J. 68(10), 102-107.

Seth, R. S., Soszynski, R. M., and Page, D. H. (1979). “Intrinsic edgewise compression strength of paper,” TAPPI 62(12), 97-99.

Severo, E. T. D., Sansígolo, C. A., Calonego, F. W., and Barreiros, R. M. (2013). “Kraft pulp from juvenile and mature woods of Corymbia citriodora,” BioResources 8(2), 1657-1664.

Sezaki, T., Hubbe, M. A., Hietmann, J. A., Argyropoulos, D. S., and Wang, X. (2006a). “Colloidal effects of acrylamide polyampholytes. Part 1. Electrokinetic behavior,” Colloids Surf. A 281(1-3), 74-81.

Sezaki, T., Argyropoulos, D. S., Heitmann, J. A., and Hubbe, M. A. (2006b). “Colloidal effects of acrylamide polyampholytes. 2. Adsorption onto cellulosic fibers,” Colloids Surf. A 289(1-3), 89-95 (2006).

Shao, Z., and Li, K. (2006). “The effect of fiber surface lignin on interfiber bonding,” J. Wood Chem. Technol. 26(3), 231-244.

Shearin, R. H. (1995). “Mini-mill built in 1878 operating with today’s principles in mind,” TAPPI J. 78(10), 240-17 to 240-18.

Sheikhi, P., Talaeipour, M., Hemasi, A. H., Eslam, H. K., and Gumuskaya, E. (2010). “Effect of drying and chemical treatment on bagasse soda pulp properties during recycling,” BioResources 5(3), 1702-1716.

Shen, J., Song, Z. Q., Qian, X. R., and Liu, W. X. (2009). “Modification of papermaking grade fillers: A brief review,” BioResources 4(3), 1190-1209.

Shen, J., Zhou, X., Wu, W., and Ma, Y. (2012). “Improving paper strength by gelation of native starch and borax in the presence of fibers,” BioResources 7(4), 5542-5551.

Silva, D. J., Aldeida, J. M., Oliveira, R. C., Silva, J. C., and Neto, A. B. M. (2013). “The use of polyelectrolyte with cellulose nanocrystals association as an alternative to improve paper strength,” O Papel74(3), 51-56.

Silva, D. J., Rojas, O. J., Hubbe, M. A., Park, S. W., Yamaguchi, T., and Song, J. (2009). “Polyampholytes: Their use in papermaking and their solution and adsorption behaviors,” O Papel 70(9), 40-50.

Singh, S., Dutt, D., and Tyagi, C. H. (2011). “Complete characterization of wheat straw (Triticum aestivum PBW-343 L. Emend. Fiori & Paol.) – A renewable source of fibres for pulp and paper making,” BioResources 6(1), 154-177.

Siro, I., and Plackett, D. (2010). “Microfibrillated cellulose and new nanocomposite materials: A review,” Cellulose 17(3), 459-494.

Sjöström, L., and Ödberg, L. (1997). “Influence of wet-end chemicals on the recyclability of paper,” Papier 51(6A), V69-V73.

Skowronski, J. (1991). “Fiber-to-fiber bonds in paper. 2. Measurement of the breaking energy of fiber-to-fiber bonds,” J. Pulp Paper Sci. 17(6), J217-J222.

Slavic, I., and Kucerova, M. (1967). “Formation of lactone bonds in cellulose during acidification and drying,” Faserforsch Techtiltech. 18, 396-397.

Smith, D. C. (1992). “Chemical additives for improved compression strength of unbleached board,” Proc. TAPPI 1992 Papermakers Conf., TAPPI Press, Atlanta, 393-404.

Smook, G. A. (1992). Handbook for Pulp & Paper Technologists, 2^ndEd., Angus Wilde Publ., Vancouver.

Soltani, M., Andalib, M., and Ravanbakhsh, M. (2013). “Some physical and mechanical properties of uncatalysed acetylated paper,” BioResources 8(1), 1274-1282.

Somboon, P., and Pynnonen, T. (2008). “Performance of abrasive segments in mechanical pulp refining. A review,” Paperi Puu 90(5), 41-45.

Somboon, P., Vuorela, J., Pynnönen, T., and Paulapuro, H. (2009a). “Grit segments in TMP refining. Part 1: Operating parameters and pulp quality,” APPITA J. 62(1), 37-41.

Somboon, P., Vuorela, J., Pynnönen, T., and Paulapuro, H. (2009b). “Grit segments in TMP refining. Part 2: Potential for energy reduction,” APPITA J. 62(1), 42-45, 59.

Song, J., Wang, Y., Hubbe, M. A., Rojas, O. J., Sulić, N., and Sezaki, T. (2006). “Charge and the dry-strength performance of polyampholytes. Part 1. Handsheet properties and polymer solution viscosity,” J. Pulp Paper Sci. 32(3) 156-162.

Song, J., Yamaguchi, T., Silva, D. J., Hubbe, M. A., and Rojas, O. J. (2010). “Effect of charge asymmetry on adsorption and phase separation of polyampholytes on silica and cellulose surfaces,” J. Phys. Chem. B 114(2), 719-727.

Song, X., and Hubbe, M. A. (2013). “Enhancement of paper dry strength by carboxymethylated -D-glucan from oat as additive,” Holzforschung, in print.

Song, X., and Hubbe, M. A. (2014). “TEMPO-mediated oxidation of oat -D-glucan and its influences on paper properties,” Carbohydrate Polymers 99, 617-623.

Song, X. L., and Law, K. N. (2010). “Kraft pulp oxidation and its influence on recycling characteristics of fibres,” Cellulose Chem. Technol. 44(7-8), 265-270.

Stark, H., Novak, J., and Eichinger, R. (1987). “Einfluß der Füllstoffvorgehandlung auf Papierfestigkeinen und Retention,” Das Papier 41(5), 209-215.

Steenberg, G. (1949). “Behaviour of paper under stress and strain,” Pulp Paper Mag. Can. 50(3), 207-214.

Stephens, C. H., Whitmore, P M., Morris, H. R., and Bier, M. E. (2008). “Hydrolysis of the amorphous cellulose in cotton-based paper,” Biomacromol. 6, 1093-1099.

Stone, J. E., and Scallan, A. M. (1966). “Influence of drying on the pore structures of the cell wall,” in Consolidation of the Paper Web, F. Bolam, ed., Tech. Sec. Brit. Paper and Board Makers Assoc., London, Vol. 1, 145-174.

Stone, J. E., and Scallan, A. M. (1968). “A structural model for the cell wall of water-swollen wood pulp fibers based on their accessibility to macromolecules,” Cellulose Chem. Technol. 2(3), 343-358.

Stratton, R. A. (1989). “Dependence of sheet properties on the location of adsorbed polymer,” Nordic Pulp Paper Res. J. 4(2), 104-112.

Stratton, R. A., and Colson, N. L. (1993). “Fiber wall damage during bond failure,” Nordic Pulp Paper Res. J. 8(2), 245-249, 257.

Strazdins, E. (1980). “New approaches towards optimizing dry strength of waste paper containing stock furnishes,” Papier 34(10A), V49-V55.

Strazdins, E. (1984). “Chemical aids can offset strength loss in secondary fiber furnish use,” Pulp Paper 58(3), 73-77.

Strazdins, E. (1989). “Theoretical and practical aspects of alum use in papermaking,” Nordic Pulp Paper Res. J. 4(2), 128-134.

Sturm, R. (2010). “Deposition and cellular interaction of cancer-inducing particles in the human respiratory tract: Theoretical approaches and experimental data,” Thoracic Cancer 1(4), 141-152.

Su, J. L., Mosse, W. K. J., Sharman, S., Batchelor, W., and Garnier, G. (2012). “Paper strength development and recyclability with polyamideamine-epichlorohydrin (PAE),” BioResources 7(1), 913-924.

Subramanian, R., Fordsmand, H., Paltakari, J., and Paulapuro, H. (2008). “A new composite fine paper with high filler loading and functional cellulosic microfines,” J. Pulp Paper Sci. 34(3), 146-152.

Suckling, I. E., Allison, R. W., Campion, S. H., McGrouther, K. G., and McDonald, A. G. (2001). “Monitoring cellulose degradation during conventional and modified kraft pulping,” J. Pulp Paper Sci.27(10), 336-341.

Sundholm, J. (ed.) (1998). Mechanical Pulping, Finnish Paper Engineers’ Assoc. and TAPPI.

Sunley, S. (1997). “Workington achieves dry strength improvements from efficient wet end chemistry,” Paper Technol. 38(8), 51-54.

Suurnäkki, A., Oksanen, T., Kettunen, H., and Buchert, J. (2003). “The effect of mannan on physical properties of ECF bleached softwood kraft ﬁber handsheets,” Nordic Pulp and Paper Research Journal 18, 429-435.

Swanson, J. W. (1950). “The effects of natural beater additives on papermaking fibers,” TAPPI 33(9), 451-462.

Swerin, A., Glad-Normark, G., and Sjödin, U. (1995). “Silica based microparticulate retention aid systems,” Paperi Puu 77(4), 215-221.

Swerin, A., and Ödberg, L. (1996). “Flocculation and floc strength. From the laboratory to the FEX paper machine,” Papier 50(10A), V45-V47; 51(2), 77.

Szikla, Z., and Paulapuro, H. (1989). “Changes in Z-direction density distribution of paper in wet pressing,” J. Pulp Paper Sci. 15(1), J11-J17.

Szwarcsztajn, E., and Przybysz, K. (1976). “Investigations on changes in the properties of recycled pulps fractions,” Cellulose Chem. Technol. 10, 737-749.

Taipale, T., Osterberg, M., Nykanen, A., Ruokolainen, J., and Laine, J. (2010). “Effect of microfibrillated cellulose and fines on the drainage of kraft pulp suspension and paper strength,” Cellulose17(5), 1005-1020.

Taleb, M. C., and Maximino, M. G. (2007). “Effects of enzymatic treatment on cellulosic fibres from recycled paper. Analysis using a response curve experimental design,” APPITA J. 60(4), 296-300.

Tanaka, A., Hiltunen, E., Kettunen, H., and Niskanen, K. (2001). “Inter-fiber bonding effects of beating, starch, and filler,” Nordic Pulp Paper Res. J. 16(4), 306-312.

Tanaka, H., Suzuki, K., and Senju, R. (1976). “Preparation of cationic polymers and their applications. Part 3. Effects of the partially aminated polyacrylamides on the improvement of paper strength,” Japan TAPPI 30(9), 492-500.

TAPPI (1996-1997). TAPPI Test Methods, TAPPI Press, Atlanta.

Tas, N. R., Escalante, M., van Hosnschoten, J. W., Jansen, H. V., and Elwenspoek, M. (2010). “Capillary negative pressure measured by nanochannel collapse,” Langmuir 26(3), 1474-1476.

Taylor, B. (1999). “The interaction of paper moisture and temperature and its effect on corrugated board quality,” Corrugating Intl. 1(3), 39-50.

Tejado, A., and van de Ven, T. G. M. (2010). “Why does paper get stronger as it dries?” Mater. Today 13(9), 42-49.

Thode, E. F., and Ingmanson, W. L. (1959). “Factors contributing to the strength of a sheet of paper. I. External specific surface and swollen specific volume,” TAPPI 42(1), 74-83.

Thomas, D., Henson, M., Joe, B., Boyton, S., and Dickson, R. (2009). “Review of growth and wood quality of plantation-grown Eucalyptus dunnii Maiden,” Australian Forestry 72(1), 3-11.

Thompson, J. O., Swanson, J. W., and Wise, L. E. (1953). “Hemicelluloses and arabogalactans as beater adhesives,” TAPPI36(12), 534-541.

Thorp, B. A. (1991). “Principles of twin-wire and multiple-wire formation and drainage,” in: Thorp, B. A. (ed.), Paper Machine Operations, Pulp and Paper Manufacture series, Vol. 7, TAPPI, Atlanta, Ch. 9, pp. 216-236.

Thorpe, J. L. and Choi, D. (1991). “Corrugated container failure – Creep strain measurements,” TAPPI J. 74(10), 172-178; 75(7), 155-161.

Torgnysdøtter, A., and Wågberg, L. (2003). “Study of the joint strength between regenerated cellulose fibres and its influence on the sheet strength,” Nordic Pulp Paper Res. J. 18(4), 455-459.

Torgnysdøtter, A., and Wågberg, L. (2004). “Influence of electrostatic interactions on fibre/fibre joint and paper strength,” Nordic Pulp Paper Res. J. 19(4), 440-447.

Torgnysdøtter, A., and Wågberg, L. (2006). “Tailoring of fiber/fiber joints in order to avoid the negative impacts of drying on paper properties,” Nordic Pulp Paper Res. J. 21(3), 411-418.

Touchette, R. V., and Jenness, L. C. (1960). “Effect of surface active agents on drainage and physical strength properties of sulfite pulp,” TAPPI 43(5), 484-489.

Toyo (2013). “What is vulcanized fibre?” URL: http://www.toyofibre.com/fibre.html, referenced July 28, 2013.

Trout, P. E. (1951). “The mechanism of the improvement of the wet strength of paper by polyethylenimine,” TAPPI 34(12), 539-544.

Truppner, O., Esser, A., Schall, N., and Haffke, R. (2006). “Report on the application of new dry-strength agents for packaging papers,” Wochenblatt für Papierfabrikation 134(9), 472.

Tryding, J., and Gustafsson, P. J. (2000). “Characterization of tensile fracture properties of paper,” TAPPI J. 83(2), 84-89.

Tschirner, U., Cao, B., and Ramaswamy, S. (1998). “Study of changes in wet-fiber flexibility and surface condition of recycled fibers,” Proc. TAPPI 1998 Recycling Symposium: Proceedings, March 8, New Orleans, LA, TAPPI Press, Atlanta, 149-162.

Twede, D. (2008). “The history of corrugated fiberboard shipping containers,” J. Macromarketing 28(1), 93 (abstract); from 13^th Conf. on Historical Analysis & Research in Marketing (CHARM): Marketing History at the Center, Branchik, B. J. (ed.), Quinnipiac Univ.

Twosides (2013). “Promoting sustainability of print and paper,” http://www.twosides.us

Uesaka, T. and Qi, D. (1994). “Hygroexpansivity of paper – Effects of fiber-to-fiber bonding,” J. Pulp Paper Sci. 20, 175-179.

Ulbrich, M., Radosta, S., Kiessler, B., and Vorwerg, W. (2012). “Interaction of cationic starch derivatives and cellulose fibres in the wet end and its correlation to paper strength with a statistical evaluation,” Starch-Starke 64(12), 972-983.

Vainio, A., Kangas, J., and Paulapuro, H. (2007). “The role of TMP fines in interfibre bonding and fiber segment activation,” J. Pulp Paper Sci. 33(1), 29-34.

Vainio, A., and Paulapuro, H. (2007a). “The effect of wet pressing and drying on bonding and activation in paper,” Nordic Pulp Paper Res. J. 22(4), 403-408.

Vainio, A. K., and Paulapuro, H. (2007b). “Interfiber bonding and fiber segment activation in paper,” BioResources 2(3), 442-458.

Vainio, A., Paulapuro, H., Koljonen, K., and Laine, J. (2006). “The effect of drying stress and polyelectrolyte complexes on the strength properties of paper,” J. Pulp Paper Sci. 32(1), 9-13.

Van den Akker, J. A. (1959). “Structural aspects of bonding,” TAPPI42(12), 940-947.

Van den Akker, J. A. (1969). “An analysis of the Nordman ‘bonding strength’,” TAPPI 52(12), 2386-2389.

Van den Akker, J. A., Lathrop, A. L., Voelker, M. H., and Dearth, L. R. (1958). “Importance of fiber strength to sheet strength,” TAPPI41(8), 416-425.

van der Reyden, D., Hofmann, C., and Baker, M. (1993). “Effects of aging and solvent treatments on some properties of contemporary tracing papers,” J. Amer. Inst. Conserv. 32, 177-206.

Van de Ven, T. G. M. (2008). “Capillary forces in wet paper,” Ind. Eng. Chem. Res. 47, 7250-7256.

Van Ommen, H. (1969). “The development of high grade vegetable parchment,” APPITA 23, 211-215.

Van Wyk, W., and Gerischer, G. (1982). “The influence of recycling on the strength properties of machine made paper,” Paperi Puu 64(9), 526-7, 529, 531-533.

Via, B. K., Stine, M., Shupe, T. F., So, C. L., and Groom, L. (2004). “Genetic improvement of fiber length and coarseness based on paper product performance and material variability – A review,” IAWA J. 25(4), 401-414.

Viljakainen, E. (2006). “High-quality mechanical pulps in paper and board manufacture,” Intl. Paperworld IPW 2006(9), 47-51.

Vishtal, A., and Retulainen, E. (2012). “Deep-drawing of paper and paperboard: The role of material properties,” BioResources 7(3), 4424-4450.

Wadehra, L., and Manley, R. St. J. (1965). “Recrystallization of amorphous cellulose,” J. Appl. Polym. Sci. 9(7), 2627-2630.

Wågberg, L., Björklund, M., Åsell, I., and Swerin, A. (1996). “On the mechanism of flocculation by microparticle retention-aid systems,” TAPPI J. 79(6), 157-164.

Wågberg, L., Forsberg, S., Johansson, A., and Juntti, P. (2002). “Engineering of fiber surface properties by application of polyelectrolyte multilayer concept. Part 1. Modification of paper strength,” J. Pulp Paper Sci. 28(7), 222-228.

Wågberg, L., Ödberg, L., Lindström, T., and Aksberg, R. (1988). “Kinetics of adsorption and ion-exchange reactions during adsorption of cationic polyelectrolytes onto cellulosic fibers,” J. Colloid Interface Sci. 123(1), 287-295.

Waleka, J. A. (1956). “An investigation of low degree of substitution carboxymethyl-celluloses,” TAPPI 39(7), 458-463

Walthy, G. L. (1987). “Paperboard chemical enhancement for strength and other benefits,” TAPPI J. 70(10), 35-39.

Wan, J. Q., Yang, J., Ma, Y. W., and Wang, Y. (2011). “Effects of pulp preparation and papermaking processes on the properties of OCC fibers,” BioResources 6(2), 1615-1630.

Wang, B., Li, R., He, B., and Li, J. (2011). “The impacts of lignin coverage, relative bonded area, and fiber properties on sheet strength,” BioResources 6(4), 4356-4369.

Wang, X., Maloney, T. C., and Paulapuro, H. (2003). “Internal fibrillation in never-dried and once-dried chemical pulps,” APPITA J.56(6), 455-459.

Wang, X., Maloney, T. C., and Paulapuro, H. (2007a). “Fibre fibrillation and its impact on sheet properties,” Paperi Puu 89(3), 148-151.

Wang, Y., Hubbe, M. A., Rojas, O. J., Argyropoulos, D. S., Wang, X., and Sezaki, T. (2007b). “Charge and dry-strength performance of polyampholytes. Part 3. Streaming potential analysis,” Colloids Surf. A 301(1), 33-40.

Wang, Y., Hubbe, M. A., Sezaki, T., Wang, X., Rojas, O. J., and Argyropoulos, D. S. (2006). “The role of polyampholyte charge density on its interactions with cellulose,” Nordic Pulp Paper Res. J. 21(5), 158-165.

Watanabe, M., Gondo, T., and Kitao, O. (2004). “Advanced wet-end system with carboxymethyl-cellulose,” TAPPI J. 3(5), 15-19.

Wegner, T. H. (1982). “Improve strength in high-yield pulps through chemical treatment,” TAPPI 65(8), 103-107.

Wegner, T. (1987). “The effects of polymeric additive on papermaking,” Tappi 70(7), 107-111.

Weise, U. (1998). “Hornification – Mechanisms and terminology,” Paperi Puu 80(2), 110-115.

Weise, U., and Paulapuro, H. (1996). “The relationship between fiber shrinkage and hornification,” Papier 50(6), 328-333; Reprinted as: Weise, U., and Paulapuro, H. (1998). “Relation between fiber shrinkage and hornification,” Prog. Paper Recycling 7(3), 14-21.

Weise, U. and Paulapuro, H. (1999). “Effect of drying and rewetting cycles on fiber swelling,” J. Pulp Paper Sci. 25(5), 163-166.

Welf, E. S., Venditti, R. A., Hubbe, M. A., and Pawlak, J. (2005). “The effects of heating without water removal and drying on the swelling as measured by water retention value and degradation as measured by intrinsic viscosity of cellulose papermaking fibers,” Prog. Paper Recycling 14(3), 1-9.

Wen, Q., and Pelton, R. (2012). “Design rules for microgel-supported adhesives,” Indust. Eng. Chem. Res. 51(28), 9564-9570.

Westermark, U., and Gustafsson, K. (1994). “Molecular-size distribution of wood polymers in birch kraft pulps,” Holzforschung48(supplement), 146-150.

Widsten, P., and Kandelbauer, A. (2008). “Adhesion improvement of lignocellulosic products by enzymatic pre-treatment,” Biotechnol. Adv. 26(4), 379-386.

Wielema, T. A., and Brouwer, P. H. (2003). “Paper performance and the increased use of fillers and pigments,” Paper Technol. 44(9), 27-40.

Wild, M. L. (1998). “Development of strength of mechanical and deinked pulp with low intensity levels,” Wochenbl. Papierfabr.126(23-24), 1218-1222.

Williams, J. C. (1980). “Retaining the strength of secondary fibers with alkaline calcium carbonate fillers,” Paper Trade J. 164(22), 33-34.

Willoughby, D. B., and Sharma, S. (2004). “Coating talc does not have to have poor rheology!” TAPPI J. 3(8), 25-31.

Wilson, C. J., and Frank, B. (2009). “An evaluation of ECT sample height for small flute board grades and box manufacturer’s certification compliance,” TAPPI J. 8(6), 24-28.

Wistara, N., and Young, R. A. (1999). “Properties and treatments of pulps from recycled paper. Part I. Physical and chemical properties of pulps,” Cellulose 6(4), 291-324.

Wistara, N., Zhang, X. J., and Young, R. A. (1999). “Properties and treatments of pulps from recycled paper. Part II. Surface properties and crystallinity of fibers and fines,” Cellulose 6(4), 325-348.

Wong, K. K. Y., Anderson, K. B., and Kibblewhite, R. P. (1999). “Effects of the laccase-mediator system on the handsheet properties of two high kappa kraft pulps,” Enzyme Microb.Tech. 25, 125-131.

Wong, K. K. Y., Richardson, J. D., and Mansfield, S. D. (2000). “Enzymatic treatment of mechanical pulp fibers for improving papermaking properties,” Biotechnol. Progr. 16(6), 1025-1029.

Wool, R. P. (2008). “Self-healing materials: A review,” Soft Matter4(3), 400-418.

Wright, R. S., Bond, B. H., and Chen, Z. (2013). “Steam bending of wood; Embellishments to an ancient technique,” BioResources 8(4), 4793-4796.

Wu, N., Hubbe, M. A., Rojas, O. J., and Park, S. (2009). “Permeation of polyelectrolytes and other solutes into the pore spaces of water-swollen cellulose: A review,” BioResources 4(3), 1222-1262.

Wu, Z., Chen, S., and Lu, Y. (2004). “Investigation on mechanisms of the complexes of starch and poly(diallyldimethylammonium chloride) as a dry strength agent,” Proc. 2^nd Intl. Symp. Technol. Pulping Papermaking Biotech Fiber Plants, Nanjing, pp. 378-380.7

Xu, G. G. Z., Yang, C. Q. X., and Den, Y. L. (2006). “Mechanism of paper wet strength development by polycarboxylic acids with different molecular weight and glutaraldehyde/poly(vinyl alcohol),” J. Appl. Polymer Sci. 101(1), 277-284.

Xu, Y. L., Chen, X. N., and Pelton, R. (2005). “How polymers strengthen filled papers,” TAPPI J. 4(11), 8-12.

Xu, Y., and Pelton, R. (2005). “A new look at how fines influence the strength of filled paper,” J. Pulp Paper Sci. 31(3), 147-152.

Yamauchi, T., and Hatanaka, T. (2002). “Mechanism of paper strength development by the addition of dry strength resin,” APPITA J. 55(3), 240-243.

Yamauchi, T., Okumura, S., and Noguchi, M. (1990). “Acoustic emission as an aid for investigating the deformation and fracture of paper,” J. Pulp Paper Sci. 16(2), 44-47.

Yamauchi, T., and Yamamoto, M. (2008). “Effects of repeated drying-and-rewetting and disintegration cycles on fundamental properties of kraft pulp fibres and paper made from them,” APPITA J. 61(5), 396-401.

Yan, N., and Kortschot, M. T. (1997). “Micromechanical modelling of tear strength in kraft and TMP papers,” In: Fundamentals of Papermaking Materials, Proc. 11^th Fundamental Research Symposium in the Fundamentals of Papermaking Materials, Cambridge, UK, pp. 1249-1271.

Yan, X. H., Ji, Y. X., and He, T. (2013). “Synthesis of fiber crosslinking cationic latex and its effect on surface properties of paper,” Prog. Organic Coatings 76(1), 11-16.

Yan, Z. G., Liu, Q. J., Deng, Y. L., and Ragauskas, A. (2005). “Improvement of paper strength with starch modified clay,” J. Appl. Polymer Sci. 97(1), 44-50.

Yang, C. F. (1981). “Predicting structural rigidity of multi-ply paperboard,” TAPPI J. 64(11), 73-76.

Yano, H., Sugiyama, J., Nakagaito, A. N., Nogi, M., Matsuura, T., Hikita, M., and Handa, K. (2005). “Optically transparent composites reinforced with networks of bacterial nanofibers,” Advan. Mater.17(2), 153-155.

Yokota, S., Matsuo, K., Kitaoka, T., and Wariishi, H. (2009). “Retention and paper-strength characteristics of anionic polyacrylamides conjugated with carbohydrate-binding modules,” BioResources 4(1), 234-244.

Yoon, S. H., and van Heiningen, A. (2008). “Kraft pulping and papermaking properties of hot-water pre-extracted loblolly pine in an integrated forest products biorefinery,” TAPPI J. 7(7), 22-27.

Young, R. A. (1994). “Comparison of the properties of chemical cellulose pulps,” Cellulose 1(2), 107-130.

Yuan, L., Wan, J., Ma, Y., Wang, Y., Huang, M., and Chen, Y. (2013). “The content of different hydrogen bond models and crystal structure of Eucalyptus fibers during beating,” BioResources 8(1), 717-734.

Zauscher, S., Caulfield, D. F., and Nissan, A. H. (1996). “The influence of water on the elastic modulus of paper. 1. Extension of the H-bond theory,” TAPPI J. 79(12), 178-182.

Zauscher, S., Caulfield, D. F., and Nissan, A. H. (1997). “Influence of water on the elastic modulus of paper. 2. Verification of predictions of the H-bond theory,” TAPPI J. 80(1), 214-223.

Zeilinger, H., and Klein, M. (1995). “Modern measuring methods for determination of the filler distribution of paper in the Z-direction,” Wochenbl. Papierfabr. 123(20), 903-910.

Zeng, X., Vishtal, A., Retulainen, E., Sivonen, E., and Fu, S. (2013). “The elongation potential of paper – How should fibres be deformed to make paper extensible?” BioResources 8(1), 472-486.

Zervos, S. (2010). “Natural and accelerated aging of cellulose and paper: A literature review,” in: Cellulose: Structure and Properties, Derivatives and Industrial Uses, Nova Science Publ., Inc., New York.

Zhang, J., Pelton, R., Wågberg, L., and Rundlöf, M. (2000). “The effect of charge density and hydrophobic modification on dextran-based paper strength enhancing polymers,” Nordic Pulp Paper Res. J. 15(5), 440-445.

Zhang, J., Pelton, R., Wågberg, L., and Rundlöf, M. (2001). “The effect of molecular weight on the performance of paper strength-enhancing polymers,” J. Pulp Paper Sci. 27(5), 145-151.

Zhang, J. Y., and Hu, H. R. (2010). “Modiﬁcation and application of hemicellulose separated from APMP spent liquor,” Transactions of China Pulp and Paper 25, 33-35.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2001b). “Effect of chemical pretreatments of never-dried pulp on the strength of recycled linerboard,” Proc. TAPPI Papermakers Conf.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2002). “Can recycled kraft fibers benefit from chemical addition before they are first dried?,” APPITA J., 55(2), 135-144.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004a). “Refining to overcome effects of drying unbleached kraft fibers in the presence or absence of sugar,” Progress in Paper Recycling 13(2), 5-12.

Zhang, M., Hubbe, M. A., Venditti, R. A., and Heitmann, J. A. (2004b). “Effects of sugar addition before drying on the wet-flexibility of redispersed kraft fibers,” J. Pulp Paper Sci. 30(1), 29-34.

Zhao, B. X., and Kwon, H. J. (2011). “Adhesion of polymers in paper products from the macroscopic to molecular level – An overview,” J. Adhes. Sci. Technol. 25(6-7), 557-579.

Zhao, H., Liu, H. J., and Qu, J. H. (2009). “Effect of pH on the aluminum salts hydrolysis during coagulation process: Formation and decomposition of polymeric aluminum species,” J. Colloid Interface Sci. 330(1), 105-112.

Zhao, Y., Hu, Z., Ragauskas, A., and Deng, Y. (2005). “Improvement of paper properties using starch-modified precipitated calcium carbonate filler,” TAPPI J. 4(2), 3-7.

Zhou, Y., Renneckar, S., Pillai, K. V., Li, Q. Q., Lin, Z. Y., and Church, W. T. (2010). “Layer-by-layer nanoscale bondlines for macroscale adhesion,” BioResources 5(3), 1530-1541.

Table A. Reported Relative Changes in Paper Strength Resulting from Various Treatments & Processes

Key to Table A

APAM Anionic copolymer of acrylamide

Bl. Bleached

Cat. Cationic (positive charge)

CD Cross-directional (not in manufacturing direction)

CEH Bleaching sequence: chlorine, NaOH, hypochlorite

CMC Carboxymethylcellulose (anionic polymer)

CPAM Cationic copolymer of acrylamide

CSF Canadian standard freeness

DADMAC A high-charge cationic polyelectrolyte

HTMP High-temperature mechanical pulp

HW Hardwood

MFC Microfibrillated cellulose

NSSC Neutral sulfite semichemical pulp

OCC Old corrugated container pulp

ODED Bleaching sequence with oxygen bleaching

ONP Old newspaper pulp

PAA Polyacrylic acid

PAC Polyaluminum chloride (oligomeric, cationic)

PAH Polyallylamine hydrochloride (cationic polymer)

PAM Polyacrylamide (family of water-soluble polymers)

PCC Precipitated calcium carbonate (filler)

PE Polyelectrolyte

PEC Polyelectrolyte complex

PEI Polyethyleneimine (highly cationic polymer)

PEM Polyelectrolyte multilayer

PFI Norwegian Forest Prod. Institute (refiner type)

PSS Polystyrene sulfonate

PVA Polyvinylamine (highly cationic polymer)

PVOH Polyvinyl alcohol

SMI Polystyrene-co-maleimide nanoparticles

SR Schopper Riegler degrees of freeness

SW Softwood

TEMPO Oxidation treatment for cellulose

TMP Thermomechanical pulp

Prospects for maintaining strength of paper and paperboard products while using less forest resources: A Review

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