1985 Volume 1

Some fundamental aspects of mechanical actions on wood pulp fibers have been studied. Specifically, the objectives of the study were to produce internal fibrillation in pulp fibers, evaluate its influence on paper properties, and establish its importance relative to external fibrillation and fines in terms of paper property development.

An apparatus was constructed that subjected pulp fibers (in the form of a wet handsheet) to repeated compressive loading cycles. Pulp was also treated in an experimental apparatus designed to promote external fibrillation, and in a Valley beater. Several fiber and paper properties were measured.

The results showed that internal fibrillation could be produced with the repeated compressive action of the apparatus. The effect of internal fibrillation on paper properties caused a threefold increase in breaking length, from 2 km to 6 km. This level was 75% of the highest breaking length achieved with the Valley beater, which was 8 km. The reason for the difference in breaking length between the samples is due to the (calculated) differences in fiber-fiber bond shear strength. By adding fibrillation to internally fibrillated fibers and forming the sheet, no change in breaking length was achieved. However, adding fines to a suspension of internally fibrillated fibers increased the sheet breaking length, almost to the level produced by Valley beaten fibers.

From these results, it was concluded that internal fibrillation in pulp fibers plays the largest part in improving sheet breaking length. Also, fines are a necessary supplement in the fiber network to improve interfiber bonding. The effect of external fibrillation had no apparent influence on improving breaking length, but its importance may lie elsewhere in the sheet forming process. Internal fibrillation and fines can be produced in separate steps, indicating an additive approach to developing sheet properties through refining is possible.

A new individual wet fibre flexibility test has been devised, and used to investigate the relationship between fines-free sheet apparent density and the wet flexibility of the constituent fibres. Linear relationships were found for a number of pulps, ranging from bleached chemical to thermomechanical, and for several different pulp treatments. It was concluded that changes in the average wet fibre flexibility of a particular pulp, caused by mechanical and/or chemical treatment, can be evaluated by measuring the apparent density of the fines-free sheet, at least over the investigated range (240-760 kg/m³).

The deformation of paper and its fibre bonds was studied by straining thin paper structures inside the specimen chamber of a scanning electron microscope. The average bond strength values of different sheet structures were characterized by several methods. The handsheet structures were varied by refining and by addition of bond strength chemicals.

The preliminary results obtained show that the structure of the sheet has a pronounced effect on the elongation properties of paper and of its fibres. The coarseness of the fibres had a distinct effect on the loading capacity of fibres and on the simultaneous straining behavior. The thin wall springwood fibres often became inactive only after the final rupture of the structure. The summerwood fibres tended to become inactive earlier through breakage of the fibre bonds. A blinking light phenomenon was observed during the SEM straining of some paper specimens. The light blinks were interpreted as complete breakages of fibre-to-fibre bonds.

The structural features of the studied handsheets had different effects on the bond strength values obtained by the various methods. These results seemed to indicate that refining produced a sheet structure which could be loaded in a more homogeneous manner. This was also reflected as higher values of bond strength. All bond strength methods used showed that starch increases the bond strength and that the debonding chemical decreases it.

The curliness of fibres and he degree of microcompression in the fibre wall strongly influence the properties of pulp suspensions, wet-webs, and dry sheets. In mill operation, curl and microcompression can be induced accidentally or intentionally, by shearing at high consistency. Some pulps are highly susceptible to curling; others are more resistant. Curl is not necessarily stable; it is readily removed from some pulps but not from others. Curl can be stabilized by certain treatments, notably by heat treatment at high consistency. This can be deliberate, or it can occur accidentally during mill operation, when a pulp is stored at an elevated temperature. Both curl and microcompression are often disregarded because they cannot be easily measured. Yet in practice their effects often dominate the properties of pulp suspensions, wet webs, and dry sheets. Ignoring these effects has led to costly surprises both in research and mill operation.

In this paper the literature is reviewed and new data are introduced, illustrating the importance of curl and microcompression for mechanical, chemi-mechanical, and chemical pulps.

Charge/pH isotherms were determined for various cellulose fibres: cotton linters, bleached sulphate, and unbleached sulphate pulp. The chare was determined as a function of pH in 1.0, 10⁻¹, 10⁻², and 10⁻³ mol dm⁻³ NaCl. The effect of various cations on the charge was also investigated. The surface areas of the fibres were determined by BET nitrogen adsorption; the pulps were initially in a ‘never-dried’ state and for the BET work they were specially prepared using solvent-exchange techniques whereby all the water was replaced by dry pentane. The surface areas of the fibres were also obtained using the method of negative adsorption: corrections for low surface potential were applied using Gouy-Chapman theory and the charge/pH isotherms. Surface areas obtained by these two entirely different methods are compared. Once drying the ‘never-dried’ pulps halved the surface areas.

Zeta potentials for cotton linters and bleached sulphate pulp were calculated from measurements of the streaming potential. These measurements were made at the same electrolyte and pH conditions as the charge/pH isotherms and the zeta potentials compared with the surface potentials calculated from Gouy-Chapman theory. It was found that the zeta potential is not a good relative measure of the surface charge and cannot be used for qualitative comparison between such similar materials as bleached sulphate pulp and cotton linters.

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.