Using micro- and nanofibrillated cellulose as a means to reduce weight of paper products: A review

Zambrano, F., Starkey, H., Wang, Y., Abbati de Assis, C., Venditti, R., Pal, L., Jameel, H., Hubbe, M. A., Rojas, O. J., and Gonzalez, R. (2020). "Using micro- and nanofibrillated cellulose as a means to reduce weight of paper products: A review," BioRes. 15(2), 4553-4590.

Abstract

Based on publications related to the use of micro- and nanofibrillated cellulose (MNFC) in papermaking applications, three sets of parameters (intrinsic and extrinsic variables, furnish composition, and degree of dispersion) were proposed. This holistic approach intends to facilitate understanding and manipulation of the main factors describing the colloidal behavior in systems comprising of MNFC, pulp fibers, and additives, which directly impact paper product performance. A preliminary techno-economic assessment showed that cost reductions driven by the addition of MNFC in paper furnishes could be as high as USD 149 per ton of fiber (up to 20% fiber reduction without adverse effects on paper’s strength) depending on the cost of papermaking fibers. It was also determined that better performance in terms of strength development associated with a higher degree of MNFC fibrillation offset its high manufacturing cost. However, there is a limit from which additional fibrillation does not seem to contribute to further strength gains that can justify the increasing production cost. Further research is needed regarding raw materials, degree of fibrillation, and combination with polyelectrolytes to further explore the potential of MNFC for the reduction of weight of paper products.

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Using Micro- and Nanofibrillated Cellulose as a Means to Reduce Weight of Paper Products: A Review

Franklin Zambrano,^a Heather Starkey,^a Yuhan Wang,^a Camilla Abbati de Assis,^aRichard Venditti,^a Lokendra Pal,^a Hasan Jameel,^a Martin A. Hubbe,^a Orlando J. Rojas,^b and Ronalds Gonzalez ^a,*

Keywords: Micro- and nanofibrillated cellulose (MNFC); Microfibrillated cellulose (MFC); Nanofibrillated cellulose (NFC); CNF; CMF; Tensile strength; Fiber reduction; Light-weight paper; Paper products; Retention aids; Cellulose fibers

Contact information: a: Department of Forest Biomaterials, Science and Engineering, P. O. Box 8005, North Carolina State University, Raleigh, NC 27695-8005 USA; b: Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, Espoo, 02150 Finland;

* Corresponding author: rwgonzal@ncsu.edu

INTRODUCTION

The global trend toward digitalization has caused a decline in the consumption and production of printing and writing paper grades. Such reduction has been reported to be approximately 15% across the last 10 years with a forecasted drop of 4% over the next five years. Recycled fibers, more specifically “mixed office waste (MOW)” and “white office ledger (WOL)”, are the most used recycled fibers in the hygiene tissue industry (De Assis et al. 2018b). As digitalization continues to force a reduction in production of printing and writing papers, less MOW and WOL are available to produce recycled paper grades. This disruption in fiber supply has resulted in huge increases and fluctuations in fiber prices (Fig. 1).

Not only has the availability of fiber been decreasing, but the quality of the fiber that is available has been continuously decreasing as well, resulting in weaker paper. Decrease in paper strength is a major concern since quality standards are rising (RISI 2017). Papermakers tend to redress this situation by using expensive fibers that are better quality than the low-cost alternatives. Some even resort to the use of synthetic additives, which results in increased costs per ton of finished product. To meet market expectations regarding paper strength, mechanical refining of recycled and virgin fibers is a common practice in the industry (Hubbe 2007a). However, in the case of tissue products, even though refining helps to develop fiber and web strength, at the same time it makes the sheet denser and more rigid, which negatively affects water absorbency, bulk, and softness of the tissue sheet, which are key properties of the final product (Kullander et al. 2012).

Fig. 1. Historic fiber cost data for major grades of recycled and virgin fibers: BEK: bleached Eucalyptus kraft; SBSK: southern bleached softwood kraft; SBHK: southern bleached hardwood kraft; DIP: deinked pulp; graph generated with data collected from Fastmarkets RISI (2017)

There is a pressing need to develop new technologies to face current and future market challenges related to fiber supply, quality, and cost while meeting changes in consumption patterns. Micro- and nanofibrillated (MNFC) has emerged as a promising candidate to generate either high-value applications or low-cost alternatives. Thus far, available reports have been focused on the improvement in tensile strength by addition of MNFC in paper furnishes (Eriksen et al. 2008; Taipale et al. 2010; He et al. 2017). This might be mainly beneficial for poor quality furnishes composed of recycled fibers, where strength properties of such fibers can be insufficient to meet specifications of a paper grade. However, for paper products where strength is not an issue, consumers are not willing to pay a premium for a product that has a superior strength (De Assis et al. 2018a). Therefore, in such cases it makes more sense to consider the gains in strength obtained by MNFC to reduce the fiber content of the paper product instead of merely developing excess strength. This strategy could potentially allow the production of a lighter-weight version of commercially available papers with properties that are consistent to those available in the market but at a lower manufacturing cost. Moreover, a more rapid adoption of the nanomaterial by the industry can be stimulated given the possible overall economic gain offsetting the high perceived cost of MNFC.

Acknowledging this opportunity, the main goal of this work is to review what is known about factors that affect the ability of highly fibrillated cellulosic materials, such as MNFC, to provide strength and possibly to allow for reductions in the basis weight of various paper products. To accomplish that goal, this review will begin by examining background information concerning nanocellulosic materials and their application in papermaking. To this end, a holistic approach will be used to provide readers with an effective means to rationalize the main variables affecting the performance of MNFC in paper furnishes. Identification of knowledge gaps as potential areas for further research will be emphasized. When considering the factors affecting paper strength – with attention to how the usage of MNFC can augment paper strength – it will be argued that some of the key challenges in research, up to this point, have involved uncertainties concerning the retention of MNFC. Another key challenge, especially when attempting to compare results of different studies, is that chemical aids intended to retain MNFC in the paper may also affect fiber network formation, and therefore the strength of the sheet. After reviewing these factors, two case studies will be considered to highlight economic considerations that may be important relative to commercialization of MNFC as an additive for fiber reduction in papermaking.

DIGGING INTO THE CELLULOSE STRUCTURE: NANOCELLULOSE

To enable a better understanding of the potential roles of MNFC as an additive in paper grade applications, this section provides background about MNFC, including its types, some aspects of its chemistry and morphology, and production.

Cellulose and Nanocellulose

Cellulose is one of the most important renewable natural biopolymers and is almost inexhaustible as a raw material (Siró and Plackett 2010; González et al. 2014). Wood is the major source of cellulose, but other important natural sources where cellulose is likewise widely distributed are plant fibers (cotton, hemp, flax, etc.), marine animals (tunicates), and to a lesser degree algae, fungi, invertebrates, and bacteria (Lavoine et al. 2012). Irrespective of its source, cellulose is a high molecular weight homopolymer whose repeating unit is glucose (French 2017). Cellulose consists of a linear homopolysaccharide composed of β-D-glucopyranose units linked together by β-1-4-linkages (Habibi et al. 2010).

In nature, cellulose is found as assemblies of individual cellulose chains that are formed into fibers. This structure is the result of a hierarchical organization (Fig. 2). Approximately 36 individual cellulose molecular chains are biologically assembled within biomass into larger units known as elementary fibrils. These elementary fibrils, which are commonly considered as the smallest morphological units in the fibers, are packed into a bundle of larger units called cellulose microfibrils; these are in turn assembled to constitute the original cellulosic fiber (Habibi et al. 2010). In this configuration, each microfibril can be seen as a flexible hair strand made of crystalline cellulose regions linked along the microfibril axis by amorphous domains. The diameter of elementary fibrils is approximately 3 nm (Isogai 2013), whereas cellulose microfibrils have diameters ranging between 20 and 50 nm (Lavoine et al. 2012). Cellulose particles that exhibit at least one dimension in the nanometer range (1 to 100 nm) are known as nanocellulose (Abdul Khalil et al. 2014).

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Fig. 2. Hierarchical organization of cellulose fiber showing molecular structure of cellulose polymer; Figure reinterpreted from Lavoine et al. 2012

Types of Nanocellulose

The manufacturing conditions used to convert macro-scale cellulose into its nano-scale form have a critical influence on the dimensions, composition, and properties of the resulting product. According to the type of treatment applied, two main classes of nanocellulose are distinguished: (i) cellulose nanocrystals (CNC) or cellulose nanowhiskers, which are obtained by acid treatment, and (ii) CNF, also known as nanofibrillated cellulose (NFC), microfibrillated cellulose (MFC), or cellulose nanofibril, which are mainly produced by mechanical disintegration (Nechyporchuk et al. 2014). Table 2 summarizes the different nomenclatures found in literature to refer to cellulose nanostructures, as well as typical dimensions and raw materials used for their manufacture. The third type of nanocellulose formed by aerobic bacteria is discussed elsewhere (Nakagaito et al. 2005; Klemm et al. 2011; Ilyas et al. 2018).

Table 1. Family of Cellulose Nanostructures (Adapted from Siró and Plackett 2010; Klemm et al. 2011; Ilyas et al. 2018)

Cellulose nanocrystals consist of rod-like crystals produced through the acid hydrolysis of cellulose fibers (Jonoobi et al. 2015). The acid treatment degrades the amorphous regions of cellulose, leaving the crystalline regions intact (Lavoine et al. 2012). The morphology, dimensions, and degree of crystallinity highly depend on the source of cellulosic material used, as well as on the conditions applied for the nanocellulose production (Habibi et al. 2010). As a general trend, CNC particles exhibit a typical width of 2 to 20 nm, with a length ranging between 100 nm and 250 nm when produced from cellulose fibers, and a crystallinity index that varies between 54 and 88% (Moon et al. 2011). CNC produced from tunicates can reach lengths of several micrometers but they are rarely used in practical systems.

Cellulose nanofibrils consist of a bundle of stretched cellulose chain molecules moderately degraded and with a greatly expanded surface area (Klemm et al. 2011). Unlike CNC, these nanofibrils are comprised of strongly entangled networks that contain both crystalline and amorphous domains. Depending on the production pathway, CNF has dimensions of 5 to 50 nm in width and a length of several micrometers. This range considers the blend of single elementary fibrils and their bundles. As a general estimation, if elementary fibrils have between 2- and 10-nm-thick fibrous cellulose structure, CNFs are composed of approximately 10 to 50 units of elementary fibrils (Siró and Plackett 2010; Lavoine et al. 2012).

It is worth noting that the many terminologies considered to describe these cellulosic nanomaterials have led to some misunderstanding. Consequently, several technical committees and organizations have initiated standards, e.g., ISO/TC6-TG1 (1947) and ISO/TC 229 (2005), TAPPI WI 3021 (2012), and CSA Z5100 (2014), for defining the different types of nanocellulose (Nechyporchuk et al. 2016). The irregularity inherent to the mechanical process used to produce cellulose nanofibrils makes standardization a challenging task, as the produced material may consist of a blend of different structures. Chinga-Carrasco (2011) concluded that microfibrillated cellulose obtained by homogenization might be composed of (1) nanofibrils, (2) fibrillary fines, (3) fiber fragments, and (4) fibers. For properly produced MFC materials, nanostructures represent the main component. Other authors claim that CNF can only be obtained from cellulose fibers pretreated using TEMPO-mediated oxidation (Isogai 2013). To avoid possible ambiguities, the authors of this review prefer the term MNFC for considering it broad enough to include the various structures derived from the smallest morphological units of the cellulosic fibers that can have sizes ranging between micrometers and nanometers. However, any reference to external study will consider the terminology used by the corresponding authors.

Production Pathways

The most common pathway to produce MNFC is through delamination of wood pulp via an intensive mechanical process after chemical or enzymatic treatment (Klemm et al. 2011). According to the nature of the raw material and degree of processing desired, the feedstock can be submitted to chemical treatment before mechanical processing, e.g., TEMPO-oxidation or carboxymethylation, to produce MNFC at higher fibrillation and lower energy consumption (Islam et al. 2014). Once the purified cellulose pulp is prepared, several methods can be applied for its conversion into highly purified nanofibrils. Typical mechanical procedures used are refining, homogenization (homogenizers and microfluidizers), and grinding. These technologies, which are suitable for upscaling, have been demonstrated to be highly efficient tools used in delamination of the fiber cell wall and subsequent MNFC isolation, despite requiring high amounts of energy (Nechyporchuk et al. 2016).

Depending on the disintegration process, the cellulosic raw material and its pre- and post-treatment (if applied), MNFC with different fibril dimensions and amount of residual microscopic fiber fragments are obtained. Other important changes in features, such as surface chemistry, crystallinity, and degree of polymerization are also influenced by those factors (Abdul Khalil and Bhat 2012; Nechyporchuk et al. 2014). Therefore, the production pathway should be selected based on a techno-economic assessment and the desired features of the final product (Spence et al. 2010a,b, 2011). Figure 3 shows conventional strategies and other alternative paths available for each stage of the manufacturing process of MNFC.

Fig. 3. MNFC production tree showing general stages and available processing operations (Copyright Elsevier; Nechyporchuk et al. 2016)

From an operational point of view, direct treatment of dry cellulose pulp using mechanical methods alone leads to segments of MNFC having a low degree of polymerization, crystallinity, and aspect ratio, which is a consequence of fiber shredding, rather than elementary fibril delamination. These features can result in poor performance of MNFC when used to improve the mechanical properties of materials. To overcome this situation, production of MNFC can be completed in aqueous dispersions of cellulose with a low concentration (< 5 wt%), which eases the delamination of nanofibrils due to a decrease in the interfibrillar hydrogen bonding energy. At the same time, these operating conditions minimize the potential cutting of fibrils (Nechyporchuk et al. 2016). It is important to note that the high-water absorption capacity exhibited by cellulose nanostructures produces highly viscous dispersions even at low concentrations. Such dispersions can be thought of to have a gel-like structure, which can be difficult to process. For this reason, the dependence of the viscosity on the MNFC concentration is a key factor to consider when evaluating practical yields.

STATE-OF-THE-ART APPLICATIONS FOR THE USE OF MICRO- AND NANOFIBRILLATED CELLULOSE IN PAPERMAKING

Before considering evidence that MNFC can help to address some of the challenges introduced above, this section provides a patent perspective regarding the evolution of the applications for MNFC and includes review papers that have discussed the use of MNFC as a papermaking additive.

The study of nanomaterials represents an emerging field that is finding an increasing number of applications in daily consumer commodities (Wijnhoven et al. 2009). Micro- and nanoscale fibrillated cellulose can be introduced to improve the performance of paper products, one of the most promising areas where these bio-nanomaterials can find a commercial niche in a short term (Osong et al. 2016). This arises as a result of nanocellulose’s set of features, such as high abundance, high stiffness, low density, and environmentally friendly nature, all of which can serve as a starting point to provide a final product with exceptional characteristics (Siró and Plackett 2010; Dufresne 2013).

Increasing interest in nanocellulose technology is reflected in the large number of patents available on the topic. Charreau et al. (2012) provided a comprehensive review on the number of patents published every year on cellulose nanoparticles, which included cellulose nanocrystals, microfibrillated cellulose, and bacterial cellulose. Numerous patents regarding micro- and nanofibrillated cellulose have been issued since 2012. A selection of patents specifically looking at MNFC applications in papermaking is presented in Table 1 to highlight specific areas of growing interest: coated paper and tissue and towel. For each publication number, the title, current assignee, status, publication year, and application field are indicated. Table 1 shows a trend between the application field and the publication year for the group of patents. Coated paper applications correspond to early patents, published between 1994 and 2012, dealing with methods for preparing aqueous suspensions comprising MNFC to be used as coating layers in different fiber-based substrates. A brief patent overview published by Brodin et al. (2014) elaborates on the use of MNFC in the coating of paper.

Table 1. Patents Issued on Micro- and Nanofibrillated Applications in Papermaking

Beginning in 2012, the application of MNFC expanded into broader categories, such as consumer products, more specifically tissue and towel grades. Sumnicht, and Sumnicht and Kokko from Consumer Products LP at Georgia-Pacific, submitted several patent applications on the hygiene consumer segment. The first two patents related to a method of making cellulose microfibers by splitting larger fibers of regenerated cellulose in high yield using low-intensity refining and incorporating such microfibers into absorbent sheets to provide strength, softness, bulk, and absorbency to tissue, towel, and personal care products (Sumnicht 2012; Sumnicht and Kokko 2012). A third patent provided more insights into the benefits that can be obtained by using microfibers. This latter invention related to an absorbent sheet made from papermaking fibers (e.g., softwood and hardwood cellulosic pulps) including regenerated cellulose microfibers. When comparing an equivalent sheet prepared without fibrillated cellulose microfiber, the resulting product was claimed to have higher absorbency (+15%), wet tensile (+40%), and a specific bulk (+5%), making it an ideal candidate for applications in tissue papers (Sumnicht and Miller 2016). Goto et al. (2014) at Nippon Paper Group, Inc. filed a patent on fibrous materials with an assembly of microfibrils with a width of 3 µm or more for obtaining sheets with low density and high surface quality in addition to high strength. The product of the invention was claimed for use in different paper grades, including facial tissue, toilet tissue, and paper towels (Goto et al. 2014). A recent patent filed by Stora Enso relates to a wet-laid sheet of a microfibrillated material composition intended for hygiene tissue applications (Heiskanen et al. 2016).

As pointed out by Charreau et al. (2012), and based on this brief patent review, worldwide corporations owning most of the patents have kept a consistent focus for the last five years, namely, finding high-value applications for MNFC to push value creation. Within this segment, MNFC is meant to improve water absorbency and tensile strength without affecting other key properties of interest in consumer products such as softness and bulkiness.

In academia, numerous authors have published recent reviews dealing with the use of MNFC as an additive in papermaking. A review was presented by Brodin et al. (2014), who discussed different strategies for incorporating cellulose nanofibrils (CNF) in pulp furnishes and results regarding drainage and paper properties that included density, permeability, strength, and light scattering coefficient. Osong et al. (2016) discussed the critical variables to consider before adding MNFC to pulp furnishes, i.e., production pathways, energy consumption, chemical and enzymatic pre-treatments, and characterization techniques. Meanwhile, Boufi et al. (2016) published a review that highlighted the progress in the field of cellulose nanofibers in papermaking applications and analyzed the effect of CNF according to the type of papermaking furnish.

MICRO- AND NANOFIBRILLATED CELLULOSE AS A PAPER STRENGTH ADDITIVE IN PAPERMAKING APPLICATIONS

Micro- and nanofibrillated cellulose products have been shown to be high-performance strength additives in paper and paperboard products (Eriksen et al. 2008; Taipale et al. 2010; He et al. 2017; Kasmani et al. 2019; Konstantinova el al. 2019). Improvements in the strength of wet web of base paper after the addition of MNFC have been also reported (Lu et al. 2019, 2020), despite the decrease in the web solid content observed after pressing of the paper sheet containing MNFC (Lu et al. 2019). There are two main features that might explain the MNFC strengthening capacity. First, the surface area expanded by the nanoscale dimensions allows MNFC to act as an effective adhesion promoter. By filling the interstices within the fiber network, fibers can come closer together, increasing the fiber-fiber bonding and thus the total bonded area. Secondly, the tendency of MNFC to form entangled networks enhances the mechanical properties of the paper. The outstanding intrinsic strength of these nano-networks embedded along larger fibers provides the macroscopic network with points of high resistance, which improves the overall tensile strength (González et al. 2012). Additionally, the similarity found in the chemical structure of both MNFC and cellulosic fibers reduces chances of incompatibility when combining the biomaterials (Balea et al. 2016).

Several studies highlight how MNFC decreases porosity and air permeability when added into the sheet (Eriksen et al. 2008; Taipale et al. 2010; González et al. 2012; Sehaqui et al. 2013; Brodin et al. 2014; He et al. 2017; Balea et al. 2019; Kasmani et al. 2019). This decrease in porosity is caused by the MNFC bonding with the fibers in the sheet network, which closes off the porous structure (Brodin et al. 2014; He et al. 2017). Pore blockage increases when the content and fibrillation degree of MNFC used increases (Balea et al. 2019). Taipale et al. (2010) proposed that air permeability indicates the complexity of the resulting network.

The reduction in porosity with the addition of MNFC also correlates with an increase in paper density (Sehaqui et al. 2011; He et al. 2011). Brodin et al. (2014) suggests that MNFC behaves similarly to fines in regard to their ability to close pores in the sheet structure which increases the number of hydrogen bonds. Other studies also report a significant increases in sheet density (Eriksen et al. 2008; Manninen et al. 2011; Charani et al. 2013; Su et al. 2013).

Factors Affecting the Usage of MNFC as a Paper Strength Additive

The goal of this section is to review the most important factors affecting paper strength when MNFC is added to papermaking furnishes. This must be considered with caution, not only because of very different pulp slurry conditions utilized in different published studies, but also because the efficiency of retention of the MNFC is rarely known or reported in such studies. Furthermore, in cases where the investigators have employed chemical-based strategies (retention aids or fixatives) to achieve relatively high retention efficiency in the course of their work, there can be profound changes in the uniformity of formation, and such differences can greatly affect the paper’s strength and other characteristics.

In light of such uncertainties, results of studies in the absence of chemical additives will be regarded as a good source of information about the direction, but not the extent of resulting changes in paper properties, because in many cases it is not possible to estimate the MNFC content of the paper. By contrast, studies conducted with the participation of cationic polymers will be used as evidence of what magnitude of quantitative changes are possible, with the caveat that large differences in formation uniformity might reduce one’s confidence in generalizing the published findings.

Though other reviews have discussed general aspects related to applications of MNFC in papermaking, there are still limitations regarding an integrated comprehension of the colloidal behavior of systems containing MNFC. To address such gap, this review will systematically discuss and analyze the latest studies on applications of MNFC in papermaking. For a better understanding, three sets of main parameters describing the colloidal behavior of systems comprised of MNFC, pulp fibers, and retention aids (or any other additive) are defined. These parameters are (1) intrinsic and extrinsic variables, (2) furnish composition, and (3) degree of dispersion. Any element included in these categories can be expected to affect the paper performance. This approach will give papermakers a clear overview of how to manipulate the MNFC application to tailor the final properties of the paper product.

The intrinsic variables describe the physicochemical nature of each of the components comprising the colloidal system, whereas extrinsic variables refer to the effect of outside parameters, such as temperature. This set can be further divided as follows:

Properties of MNFC, affected by (i) morphology (a function of the production pathway, the fiber source used for manufacturing, and the intensity of the mechanical treatment applied), and (ii) chemistry (a function of the fiber source used for manufacturing, and the biological/chemical pre- and post-treatment applied, which will dictate the chemical composition).
Properties of pulp fibers used as the paper matrix, affected by (i) pulp source, (ii) pulping method, (iii) lignin content, and (iv) degree of beating.
Properties of additives, affected by (i) nature of the additive and (ii) addition strategy, i.e., the sequence of addition used to mix the MNFC, pulp fibers, and additive in the paper furnish.
Bulk conditions, affected by (i) pH and (ii) salinity.

The furnish composition defines the relative amount of each of the species in the colloidal system, whereas the degree of dispersion relates to the mechanical protocol applied to disperse the species in the bulk of the paper furnish. Table 3 shows a breakdown of the sets of parameters previously defined.

Table 3. Sets of Main Parameters Describing the Colloidal Behavior of Systems Comprising MNFC – Pulp Fiber – Retention Aid (Or Any Other Additive)

Intrinsic and Extrinsic Variables

Fiber source

The type of fiber used for the production of the MNFC has an important influence on the fibrillation development, fines generation, and subsequent performance of the nanocellulosic material (Stelte and Sanadi 2009; Lahtinen et al. 2014; Johnson et al. 2016). At similar levels of mechanical treatment, hardwood cellulose nanofibrils will produce a comparable but slightly weaker film, i.e., lower tensile strength, than softwood cellulose nanofibrils (Spence et al. 2010a,b). Thus, if the tensile strength of the resulting film is used as an indication of the fibrillation degree induced by the treatment, hardwood cellulosic fibers are harder to fibrillate than softwood fibers; i.e., they will require a higher level of pre-treatment and mechanical treatment (Stelte and Sanadi 2009; Vartiainen et al. 2015; Zhao et al. 2017).

Figure 4 shows scanning electron microscopy (SEM) images comparing the progression of hardwood fibrillation to the fibrillation of softwood fibers after a given number of passes through a refiner. Similarly, when using CNF as an additive to hardwood-based pulp handsheets, hardwood CNF produces lower tensile and internal bond values compared to softwood CNF at a given fines content (< 86% fines). However, for fines content above 90%, the change in the handsheets properties is independent of the source used for the CNF production (Johnson et al. 2016).