1993 Volume 2

Some recent advances in understanding the dynamic interactions of liquids with solid surfaces are reviewed. Specifically, acid-base theory is applied to wetting and adhesion, the meaning of precursor films for spreading is examined, the role of pore morphology in capillary penetration is explored theoretically and experimentally, adsorption and spreading of surfactant solutions is compared to that of aqueous alcohol solutions, three dimensional pressure penetration models are described along with new methods for characterizing penetration and pore size distribution, and means of reducing fiber swelling and the hygroreactivity of paper and board is discussed.

The tortuosity factor is given as the ratio of the theoretical absorption coefficient calculated from the cylindrical model to the observed absorption coefficient by the Bristow method. A new formula to give mean pore radius of paper is derived by taking account of the liquid absorption into every pore size according to the Lucas-Washburn equation.

The liquid absorption model is proposed to allow calculation of transferred liquid volume to paper in an arbitrary time from initial absorption through saturation. The results of the transfer tests of ethyl alcohol to four kinds of papers by the Bristow method show good agreement between the observed and calculated liquid volumes transferred to the papers.

For other non-aqueous liquids which show the same V(t) vs. (7L tlrqY/2 relationship in the liquid transfer tests to a particular paper as ethyl alcohol, it is assumed that the liquid volume transferred to the paper in an arbitrary time can be estimated by means of the model.

Relative flow porosity is defined as the fraction of the void volume in a porous medium through which fluid can flow under a macroscopic pressure gradient. In saturated paper, much of the void volume is occupied by water that cannot flow due to chemical or physical absorption and mechanical obstruction (isolated or dead-end pores). In partially saturated paper, surface tension effects further hinder fluid flow through the sheet. In this paper, we discuss various methods for examining relative flow porosity and present results of new experimental techniques based on in-plane flow measurements. The experimental approach involves radially injecting known volumes of aqueous, non-absorbing dye into the center of a compressed, saturated sheet restrained by solid surfaces. The volume of the sheet occupied by the dye is measured, as is the total porosity of the sheet. The ratio of injected dye volume to pore volume within the dyed region is an estimate of effective porosity.

We show that in unrefined, filler-free paper, effective porosity values are on the order of 40% or more. The relative porosity may be as high as 90% of the extra fiber pore volume. Data for both initially dry and initially saturated sheets are presented. A geometric theory exists to predict relative porosity in fibrous structures, but we find that this model predicts values for relative porosity much lower than we observe here. Using simple measures of the volume occupied by the swollen-fibers in a compressed mat, we find that most(on the order of 90%) of the extra fiber pores pace is available to flow.

The surface force technique, whereby the forces acting between two solid surfaces immersed in liquids or in adhesive contact are directly measured, represents a novel approach for both fundamental and application-oriented studies of the surface and colloid science of papermaking. The nature and measurement of surface forces are briefly discussed, and some results reported for mica surfaces are reviewed in order to illustrate the surface chemical information obtainable using a conventional Israelachvili-type surface force apparatus. In the case of cellulose surfaces immersed in water and aqueous electrolyte solutions the measured force vs. distance profile is characterized by three regimes. Significantly, conventional DLVO theory cannot explain the interaction forces measured between cellulose surfaces. Electrostatic double-layer forces, as anticipated, dominate the long-range interactions. However, as the two cellulose surfaces begin to “contact” each other, there is an interplay of steric and electrostatic forces due to dangling tails of cellulose chains. The observed force curves, therefore, are interpreted in terms of a new model — the “dangling tail” model — of the cellulose surface, namely, the water-swollen cellulose surface has long and weakly charged cellulose chains or “molecularfibrils” which extend into the aqueous solution. In addition, the application of the surface force technique to basic problems in the adsorption of polymers, both cationic polyelectrolytes and hemicelluloses, and the colloidal stability of kaolin suspensions is illustrated. The advantages of using a new type of surface force apparatus in future studies of surface and physicochemical phenomena relevant to paper manufacturing, coating and recycling are also briefly discussed.

Samples of two types of carton board, a coated white line chip (WLC) and a folding boxboard (FBB), were perforated using an experimental cutting forme. Variations such as depth of cut and rule condition were introduced. These samples were then torn using an Elmendorf tear tester and assessed for their mode of failure. The results indicate that the detrimental effects of worn rules and poor cutting depth can be magnified by the size and relationship of certain board properties, particularly tear and plybond strength. The test results and micrographs suggest an unaided visual examination of the perforated line is insufficient to guarantee clean tearing, particularly when worn rules have been used.

The effect of the pronounced directionality in the WLC has been demonstrated with regards to the perforation performance. The results have also indicated that plybond/tear ratio is significant in the mode of failure. The usefulness of transmitted light coupled with magnification has been shown with respect to the examination of perforations for quality control purposes. This holds possibilities within QCasa method of assessment.

Among the most extensively used chemicals in the paper industry are wet strength resins. They enhance the performance of paper products ranging from tissue to board. Environmental concerns are questioning the use of several of these additives and a strong effort is being made to replace these chemicals with benign ones. This paper reports one such study. Highly water soluble polyfunctional carboxylic acids succinic acid, citric acid, tricarballylic acid, and 1,2,3,4-butanetetracarboxylic acid (BTCA) when applied in a 1% solution to paper, dried, and heated (120 – 150 °C) with a catalyst develop wet strength as high as 55%. The effectiveness of these acids is in the order tetrafunctional>trifunctional >difunctional. This sequence is a reflection of their ability to form multiple anhydrides. Additional experiments indicate that the wet strength is the result of crosslinking of the hydroxyl groups in cellulose. The wet reinforced papers are easily dispersed under alkaline conditions. The crosslinking of the papers results in an increased dimensional stability.

Most wet strength chemicals are added at the wet-end where they are substantively adsorbed on the pulp and cured during the drying process. In contrast, the chemicals used in this study are water soluble and therefore are applied to the paper by a saturation technique. Additional research will focus on the optimization of their delivery.

Embossing processes are often used to increase the surface area of low basis weight papers giving them better absorbency properties. They also serve to add surface texture and increase ply bonding in multi-layer papers. The mechanical properties (e.g., stiffness and strength)of paper sheets are significantly effected in a detrimental manner by embossing. In this work, an initial study of the paper embossing process has been performed using finite element analysis. The paper was modeled as a nonlinear inelastic orthotropic solid and the rubber was taken to be an incompressible hyperelastic material. Interface elements were introduced to simulate the behavior of the contact surfaces. Since the measured mechanical properties of the paper under consideration were highly dependent on moisture content, analyses were performed at relative humidity levels of 50% and 90%. Permanent deformations, and residual stresses and strains in embossed paper sheets have been calculated. Also, the changes in the mechanical behavior of paper sheets due to embossing have been predicted.

This paper examines the effects of the natural ability of the fibres to bond and the introduction of moieties able to provide balanced ionic interactions, on the dry and wet mechanical properties of paper. Thus, the effects of the introduction of zwitterionic moieties (i.e. amino acids coupled with s-triazines) onto the surface of the fibers and their interaction are interpreted in the light of accepted theories of dry and wet paper strength and the topology of the location of the zwitterionic sites. The tensile strength and other mechanical properties of paper made with the zwitterionic fibers are reported. Paper made from zwitterionic fibers retain more than 35% of their dry strength after being immersed in water for weeks. The importance of these results is considerable when it is realized that no wet strength resins are being used. The benefits of zwitterionic bonding in papermaking are briefly discussed.

The current emphasis on the environment and its impact on recycling is placed into context with an introduction of how life cycle analysis and assessment techniques will play an increasing role in determining the direction of industrial development and legislation.

The review is based around the subjects identified by European Paper industry experts on recycling and recycling research as being the most critical to the long term development of the industry. These are the characterisation of the changes which occur to fibres and their surfaces on recycling; the influence of chemicals on water quality and production efficiency; the fundamentals of flotation deinking and recycling models. In addition a section on alternative uses for waste paper is included.

Recent reviews on the effects of recycling on paper properties are summarised and an explanation for the apparent anomalies and differences given. Results on post paper manufacturing treatment on recycle potential are also included.

The information on the effect of chemicals on recycling is sparse and tends to be empirical. The greatest effects are on paper strength and brightness where bleaches or brightening agents are used. The chemicals which dissolve also have a detrimental effect on process additive efficiency and add to effluent treatment and discharge costs.

The deinking section is subdivided into – a brief review of the chemistries and drying methods of inks and toners and their influence on particle size on repulping; the chemistry of the repulping process; the flotation process. The influences of chemicals on flotation and the fundamental phenomena which occur during the process are described.

Models for assessing age distribution and product properties at various levels of recycling are explored. In addition the build up of materials within a recycling loop is examined along with the energy balance of recycling compared with manufacture from virgin materials.

Alternative uses of wastepaper considered include energy generation, composting and as a raw material for a variety of applications. None of these are likely to become major uses, competitive with wastepaper used for paper and board production in the near future.

It is concluded that there are various studies of a fundamental nature which need to be completed to advance the technology of recycling. Greater emphasis on the supply side of wastepaper would also yield industrial benefit.

Hornification is the loss of swelling of the fibre wall resulting from a drying-and-rewetting cycle: it is associated with a stiffening of the fibres which reduces their ability to form inter-fibre bonds. In this paper,factors affecting the extent of hornification are examined using the solute exclusion technique to measure swelling. Two major observations are that hornification is a feature of low-yield pulps and that it is primarily brought about by the removal of water from fibre walls rather than anyassociated heattreatments. The mechanism is proposed to be an increase in the degree of cross-linking between microfibrils due to additional hydrogen bonds formed during drying and not broken during rewetting. In high-yieldpulps, the presence of lignin and hemicellulose between the microfibrils prevents this bonding. In low-yield pulps, the absence of these materials permits hornification to occur. However, hornification can be prevented by interfering with hydrogen bond formation either by i), partially substituting the cellulose hydroxyls by groups that do not hydrogen bond, or ii), drying pulps in the presence of additives that”bulk” the fibre wall and so effectively replace the ligno-hemicellulose gel.