1985 Volume 1

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.

High resolution ¹³C NMR of crystalline celluloses is both complementary and supplementary to x-ray diffraction analysis because it is effective both for crystalline an non-crystalline materials. Good spectral quality has been achieved for a range of cellulose samples and spectral elements related to lateral order are observed but some interpretational details are still evolving.

The spectrum of a complex material such as wood, shows morphological and conformational features for each chemical component. The resolution achieved is sufficient to allow identification of carbohydrate resonances, methoxyl, and aromatic resonances and methyl and carbonyl resonances of hemicellulose acetyls. The effect of solid state chemical treatments such as acetylation and prehydrolysis are readily detected. The use of interrupted decoupling allows one to separate the lignin and cellulose components of the spectrum.

The potential of the technique for rapid ¹³C NMR analysis of paper debris, coated sheets and insoluble resins is now becoming well-established. More complex biosubstances such as grasses, bark, and plant cell wall are being molecularly examined in their true nascent state for the first time. In this paper, a series of spectra are presented covering the various physical states of Esparto grass: native, holocellulose, alkali extracted, pulp; these spectra are compared to Esparto xylan. The line broadening effect of the latter on the C-1 resonance of cellulose demonstrates the difficulty in interpreting effects of fine structure vs. heterocomposition.

Our studies of cellulose structure based on X-ray diffractometry, Raman spectroscopy, and Solid State ¹³C-NMR have led us to a model which addresses questions of structure at two levels. The first is that of the organization of individual chains. Two stable ordered states of cellulose chains are postulated, together with a disordered state in which there is less coherence between the orientations of adjacent anhydroglucose rings. The ordered states are identified as kᵢ and kᵢᵢ based on their predominance in celluloses I and II, respectively; both conformations are based on the dimeric anhydrocellobiose as the basic repeat until in the ordered chain. The disordered state is identified as kₒ.

The second level of organization is that of aggregation of chains into three-dimensionally ordered crystalline domains. At this level our model recognizes two crystalline forms within the native state. These are identified as Iα and Iß, the first found to be dominant in bacterial and algal celluloses, the second dominant in celluloses from higher plants. These two forms are found to contain chains possessing the same molecular conformation ᵏI, but the patterns of hydrogen bonding are found to be different. Cellulose II, which is derived from the native state by mercerization or regeneration at low temperatures, is found to consist predominantly of chains in the ᵏII conformation in yet a third distinct crystalline lattice.

An analysis of published work, together with new data, has clarified the effect of chemical composition on the strength of wood pulp fibres. Among fibres of low fibril angle, and in nondegrading pulping processes, strength (expressed as breaking stress) is directly proportional to α-cellulose content over a wide yield range. This implies that the cellulose fibrils are the sole tensile-load-bearing elements; hemicellulose and lignin only serve as a matrix that transfers the stress under shear from fibril to fibril. However, pulping to a yield corresponding to an α-cellulose content higher than 80% tends to reduce fibre strength apparently because of the elimination of this stress-equalizing matrix. In cellulose-degrading processes, fibre strength falls below this expectation to an extent dependent upon the degradation. Thus a tool is provided that permits the degradative effect of new pulping or bleaching processes on fibre strength to be assessed. The value of zero-span strength as an index of fibre strength is confirmed.

This review examines the extent to which wood property variation in New Zealand’s radiata pine resource determines pulp quality. The qualities of radiata pine papermaking fibres are very dependent on their original position within a tree (growth rings from pith and/or height in tree), as well as the geographic altitude and latitude of sites on which the trees are grown. Two categories of radiata pine pulpwood are recognised in New Zealand: slabwood of high basic density from the outside of sawlogs; and corewood of relatively low basic density from the smaller logs (non-sawlogs) of the upper part of a tree and from whole-trees less than 20 years old.

The kraft pulp fibres from corewood are shorter and have thinner walls than corresponding fibres from slabwood, but the diameters of these two fibre populations are essentially identical. The handsheet properties (apparent density, and burst, tear, and tensile indices) are strongly correlated with, and can be predicted from, the wall thickness:diameter ratio of pulp fibres or the basic density of the wood sample pulped. These trends hold for whole trees of different age, for parts of trees, and for commercial pulpwood and slabwood material obtained from throughout New Zealand.

Mechanical pulps can be correlated with wood properties to a lesser extent than are kraft pulps. In thermomechanical (TMP) pulp production, slabwood consumes more energy to a given freeness and produces pulps of higher strength than corewood. Pulps from corewood, however, have excellent optical properties whereas those from slabwood are of slightly lower quality. These differences are partly explained by the very different qualities of slabwood and corewood fibres and fines. Slabwood TMP pulps are rich in fibrillar fines, which have a strong consolidating influence on the long and relatively stiff fibres of this furnish. Alternatively, corewood fines are of a more heterogeneous arid coarse quality (and have a lesser consolidating effect on fibres which are shorter and more collapsible than corresponding slabwood fibres) and therefore pack more tightly within handsheets. The handsheets of corewood pulps have excellent optical properties since the fibres and fines of this furnish give more air-to-fibre and air-to-fibre element interfaces than do those of corresponding slabwood fibres and fines.

The forming of the web in a paper machine is a highly dynamic process in which the dynamic mechanical properties of the web in the x, y, and z directions are each of central importance for both the process operation and the properties of the final product. The properties in the x and y directions, such as the dynamic tensile stiffness and stress relaxation, affect control of the draw in open draws, web flutter, and stress variation. The dynamic mechanical properties in the z direction affect the behavior of the web in the press section and the post-press dry solids content. All these features are also related to the properties of the final product.

Laboratory research on the dynamic mechanical behavior of wet webs has been carried out with special equipment designed to simulate the paper production process, particularly its dynamic characteristics. The draw and press simulators have been built to monitor and handle properties related to the dynamic mechanical behavior of paper. The draw simulator has been used to study dynamic tensile stiffness and visco-elastic component of the web during drying. The press simulator has been used to monitor the compressibility and pressure in the web as a function of different press impulses and wet pressing temperature.

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.

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.

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.