Proceeding Articles

A novel transmission electron microscopy (TEM) technique, developed to observe the nano-scale interactions of polymeric additives and cellulosic fibrils under idealized laboratory conditions, was applied for the first time in a comprehensive study of the colloidal interactions within a mill producing light-weight coated publication paper. The technique allows the observation of incremental changes in the nano-scale appearance of the paper-making slurry as successive additives are introduced to the system. Such changes include the coagulation of colloidal and dissolved substances present in thermomechanical (TMP) pulp after the addition of a low molecular weight, high charge density polymer, and the subsequent flocculation of the coagulated matter, hydrophobic materials, and fines following the introduction of talc, aluminum sulfate, a high mass cationic polyelectrolyte, and silica nanoparticles. The new results demonstrate that the TEM technique can be applied even in systems as complex as commercial papermaking, leading to a more accurate understanding of what happens on a macromolecular level.

Poly(ethylene oxide) (PEO), a widely known flocculation agent used primarily as a fines retention aid in mechanical grade papers, has its efficiency enhanced by various compounds, known as cofactors. These cofactors form a complex with PEO, which acts as an efficient bridging agent for fines flocculation. The nature of the PEO/cofactor complex is mainly unknown, and it was originally believed that the association was driven by hydrogen bonding. Therefore it was decided to investigate the complex formation in more detail. As a model system we studied PEO and a model cofactor, corilagin, a precursor of tannic acid, a known cofactor for PEO. We performed both Semi-Empirical Molecular Orbital (PM3) gas phase calculations and Molecular Dynamics (MD) calculations in the presence of water. Both methods lead to the same surprising conclusion: no association between PEO and corilagin occurs at room temperature. The reason is that the gain of association enthalpy is not large enough to overcome the loss in entropy. No correlation was found between the association enthalpy and the number of hydrogen bonds between PEO and corilagin. The absence of PEO/corilagin complexation was confirmed by NMR, isothermal titration calorimetry and the inability of PEO/corilagin to flocculate MCC (microcrystalline cellulose). In the presence of low concentrations of salt, complexation and MCC flocculation was observed.

Regenerated cellulose films were laminated with polyvinylamine, PVAm, and the wet peel delamination forces were used to explore the mechanism by which PVAm increases the wet strength of paper. Conventional wet strength resins contain highly reactive chemical groups which can crosslink the resin and graft it to fibre surfaces. By contrast, it is not obvious how PVAm provides wet strength.

The delamination experiments revealed that PVAm gives strong adhesion which was approximately independent of drying temperature (23 to 110°C), pH 3 to 9, PVAm molecular weight (34,000 to 1,500,000Da), and PVAm coverage (monolayer to 70mg/m2 ). By contrast the adhesion increased with the amine content of PVAm and with the degree of oxidation of the cellulose films. It is proposed that the PVAm adhesion is a combination of electrostatic and covalent bonding. The electrostatic bonding is between protonated amines, which are positively charged, and carboxyl groups on the cellulose. Whereas the covalent bonds, aminal and imine linkages, are formed between amines and aldehyde groups on the oxidized films.

Two studies were carried out in an effort to gain a better understanding of how ketene dimers develop sizing. In the first study, eight ketene dimers with a range of melting points, vapor pressures, and molecular weights were evaluated for rate of sizing development. Ketene dimer melting point had a clear effect on the rate of sizing development. High melting ketene dimers initiated sizing at higher sheet moistures than low melting dimers. High melting dimers also developed their ultimate level of sizing faster than low melting ketene dimers. These results suggest that solid and liquid ketene dimers have different mechanisms of sizing development. Ketene dimer vapor pressure and molecular weight had no consistent effects on the rate of sizing development.

Pseudo first order rate constants for sizing development were then measured for a high melting solid ketene dimer and a liquid ketene dimer over dryer temperatures ranging from 55 °C to 85 °C. The rate constants measured for the liquid ketene dimer increased steadily as dryer temperature increased. An Arrhenius plot of the rate constants obtained for the liquid ketene dimer yielded an activation energy of 11 kcal per mole for sizing development. These results are consistent with the hypothesis that the chemical reaction of the lactone ring is the rate determining step in sizing development for liquid ketene dimers. The high melting solid ketene dimer followed more complex kinetics and probably developed sizing by a combination of mechanisms. It is likely that the differing sizing responses measured for the solid and liquid ketene dimers are due to differing sizing contributions from the unreacted and hydrolyzed ketene dimers.

Using the colloidal probe technique, the interaction between carboxymethylated cellulose films and a cellulose sphere was studied. At low pH (pH 3.5), the interaction was dominated by dispersion forces. This was due to the low dissociation of carboxyl groups within the film at pH 3.5. However, at pH greater than 5, the interaction was dominated by an electrostatic repulsion. The increase in pH had the two-fold effect of completely dissociating the charged groups as well as causing appreciable swelling of the film leading to a decrease in the van der Waals component of the interaction. From these results it can be concluded that these cellulose films are suitable for a range of surface forces measurements including electrostatic, van der Waals’, steric and adhesion forces. Furthermore, the measurement of forces using the colloidal probe technique can be extended to inorganic particles as well as for interactions between surfaces in the presence of for example wet and dry strength agents and other materials relevant to the paper-making industry.

The purpose of this study was to investigate the effect of polyelectrolytes on the adhesion forces between polyelectrolyte-coated surfaces and to relate the observed phenomena to the performance of dry strength additives in papermaking systems.

The adhesive properties (pull-off forces) between polyelectrolyte-coated surfaces in air were determined as a function of the polymer charge density and the number of contacts. We found
strong adhesion energy for model systems coated with polymers of the highest (cationic) charge density, which suggests that electrostatic surface-polyelectrolyte-surface bridges are the main cause of the adhesion. At low charge density another force contribution due to entanglement of polymer chains gives rise to a significant contribution to the adhesion forces. The disruption of the contact leads to irreversible changes in the adsorbed layers when either the number of electrostatic bridges formed is large or when entanglement effects are important for polymers with a relatively low binding strength to the surface. These results are discussed in terms of our current understanding of development of dry strength in fiber systems.

The influence of surface roughness and surface hydrophobicity on ink detachment of water-based flexographic ink was studied. It was shown that increased surface roughness and an increased surface hydrophobicity both had a negative effect on ink detachment. The increased surface roughness was suggested to increase the molecular contact area between ink and cellulose and thereby also to decrease ink detachment. Ink cellulose interaction was evaluated from interfacial energies and contact angle measurements. A new technique in which the adhesion properties between ink and the model cellulose surface were directly measured using a Micro Adhesion Measurement Apparatus (MAMA) was also used. Upon increasing the hydrophobicity of the model cellulose surface it was shown that the work of adhesion between ink and model cellulose surfaces decreased. At the same time the interfacial energy between cellulose and ink increased, as did the interfacial energy between cellulose and water resulting in a lower degree of ink detachment.

NA

Emphasis on the tissue culture propagation of forest tress has increased dramatically. Tissue culture methods available to forestry and the pulp and paper industry are micropropagation, organogenesis, and somatic embryogenesis. Somatic embryogenesis, although more difficult to accomplish, seems to have the most promise for use with forest trees because (1) when appropriately employed it can be a true mass production procedure and (2) the approach can be used efficiently with several genetic engineering techniques. Major genetic gains in growth rate, wood quality, insect and disease resistance, and improved climatic adaptability are anticipated when tissue culture techniques are used in conjunction with genetic engineering. Emphasis in The Institute of Paper Chemistry’s tissue culture research is on the development of a somatic embryogenesis procedure for conifers.

A substantial amount of research has allowed the introduction of efficient techniques, for genetic improvement of trees from Eucalyptus species. The basic density of wood has been identified as one of the main hereditary characteristics, to be employed as an important selection parameter for propagation of new forests. An important part of this program is the evaluation of the effect of various chemical and anatomical properties, which may be associated with wood basic density, upon industrial aspects of the production of pulp and paper.

The influence of wood basic density on chemical and anatomical characteristics, on production parameters, and on paper properties was evaluated for twenty-five hybrid trees of Eucalyptus grandis. The trees were selected at random from the same plantation site, with basic density values varying from 418 to 666 kg/m³, at seven years of age.

Anatomically, it was observed that fibers presented smaller diameters and thicker walls as wood basic density increased. On the chemical side, denser trees showed a tendency to have more lignin and less pentosans, as compared to individuals of the same species and age, with lower density values. Fiber length and extractives content were not correlated to wood basic density, which suggests that these properties may be genetically controlled.

Pulping evaluations, performed under simulated mill conditions, indicated that pulp yield increased when basic density varied from 418 kg/m to approximately 470 kg/m³, but decreased from this point up to 666 kg/m³, accompanied by a steady increase in rejects content. Nevertheless, an estimate for digester yield showed that production capacity can be improved through utilization of denser woods. Hence, from a production viewpoint, other aspects, such as the observed increase in chemical consumption during pulping, as well as the higher viscosity and solids content of black liquor, may become more relevant when pulping
denser woods. The latter may translate into a limiting factor in production, if the recovery system is not dimensioned to handle extra loads.

Unbleached pulps of the same kappa number showed diminishing trends in viscosity and pentosan content, when wood density was increased in the range studied. The properties of paper were shown to be strongly correlated with variations in basic density. Sheet consolidation and fiber bonding decreased almost linearly with wood density, as indicated by lower apparent density and tensile strength values, which are mere consequences of lower fiber
flexibility in denser woods. The porosity of handsheets, which is also closely related to sheet structure, demonstrated an almost exponential increase with wood density.

Trends in tear resistance were shown to be dependent on the amount of beating. At low to medium beating levels the tear strength was lower for denser woods, and after extensive mechanical treatment no correlation was found. All apparent variations have led to the conclusion that selection of Eucalyptus grandis trees with wood densities beyond a certain level may result in undesirable combination of paper properties, for most end-uses.