Proceeding Articles

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Pinus radiata and eucalypts are fast-grown species, well suited to plantation forestry in New Zealand and elsewhere, and for the manufacture of a wide range of solid wood and reconstituted wood products, including pulp and paper.

This paper examines the variation and end-use potential of the individual-tree kraft fibre and handsheet properties of 25 trees of 13-year-old P. radiata and 29 trees of 15-year-old Eucalyptus nitens. Individual-tree fibre property differences are assessed with reference to the fibre quality requirements of a range of wood-free paper grades. Strategies and procedures are also described which will enable parent trees with desired fibre properties to be identified, propagated and mass produced.

In selecting fibre types for different paper and pulp grades, the apparent density of “unrefined” pulps(500 PFI mill rev) is the base against which other “unrefined” handsheet properties are compared. Apparent density is a direct measure of fibre packing density and arrangements in handsheets, and is determined by fibre length and cross-section dimensions, and the related morphological configurations of collapse and straightness. Although the individual-tree pulps of both species can normally be refined to the same tensile index, apparent density values can be very different depending on their fibre properties. Thus, minimal pulp refining is preferred for comparing individual-tree pulps for trees election.

Apparent density is best predicted by the kraft fibre property combination of the fibre width/thickness ratio and length. The combination of chip basic density and kraft fibre length is also a good predictor of handsheet apparent density but not necessarily of the best fibres for the manufacture of particular products. Wood density is a measure of the ratio of wood substances to void space in each individual-tree chip sample and is not indicative of the numbers of fibers which make up a unit volume.

Kraft handsheet propeties varied widely among trees for both species and were well-predicted by kraft-fibre dimensions. The high broad-sense heritabilities shown for these traits in P. radiata mean that clonal forestry could provide pulpwood of uniform and predictable pulping performance from monoclonal forest blocks. The high narrow-sense heritabilities shown so far for wood properties in P. rafiata (and for some wood properties in E. nitens) indicate that planting control-pollinated families of known characteristics could have a similar though less uniform result.

The transverse dimensions of pulp fibres influence strongly their response to the papermaking process, and most end-use properties of products. However, fibre transverse dimensions are difficult to measure. Confocal microscopy combined with image analysis has been used for rapid and accurate measurement of fibre wall cross-sectional area,perimeter, and thickness. Results on kraft pulp fibres obtained from a variety of wood species are presented. They demonstrate how fibre transverse dimensions are distributed within a species, and can be described analytically. Comparison between different species shows that species with coarse or thick-walled fibres are likely to be more heterogeneous. Implications for pulp quality and fibre selection for end-use requirements are discussed.

This paper addresses the application of Raman spectroscopic techniques to analyzing the deformation micromechanics of regenerated cellulosic fibres. In addition to information obtained on the macroscopic deformation, Raman spectroscopy enables the study of changes in internal strain associated with creep and recovery. The analysis of the mechanics of deformation of the fibres begins by following the response of the Raman-sensitive bands to external tensile loading. It is shown that the peak positions of the 895 cm-1 and 1095 cm-1 Raman bands shift to lower vibrational frequency under the action of tensile stress or strain due to the macroscopic deformation leading to direct stretching of the polymer molecules. Moreover, this approach makes possible the modelling of single fibre properties using simple viscoelastic dynamics; thereby relating the macromechanical properties of stress and strain to those obtained at the microscopic level via spectroscopy.

Phenomenological theories on the effect of pulp fiber properties on the fracture energy of paper are discussed. The effect of fiber length and strength is clarified experimentally. Fiber length appears to affect fiber failure probability only slightly. When fiber strength is changed, the fracture energy decreases greatly with only a small increment in fiber failure probability. This suggests that the fracture energy contribution of a fiber may be correlated between fibers. The effect of fiber length and strength on the cohesive stress – crack widening relationship is clarified.

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Development of earlywood and latewood fibres was investigated to find out how morphologically different fibres undergo delamination. Fibre fractions rich in earlywood and latewood, were separated from mechanical pulps using a hydrocyclone and refined further in awing defibrator. Changes in fibre structure due to defibration were studied using microscopy techniques that included measurement of fibre stiffness, fibre wall thickness and external fibre surface. Before refining, the latewood fibres were stiff and their external fibre wallswere poorly developed. Refining reduced the stiffness of both fibre types. The stiffness of latewood fibres decreased to around that of unrefined earlywood fibres, andthe external walls of latewood fibres became fibrillated. The wall thickness of both earlywood and latewood fibres was reduced only slightly. Although the tensile and tear indices of sheets made of late wood fibres were improved by refining, the tensile index of flexible latewood fibres was only half of that measured for unrefined earlywood fibres. This indicates that there are fibre properties other than stiffness which must be changed in order to get latewood fibres to bond and conform properly.

The swelling of fibres in water has a large impact on the consolidation of the web in papermaking and thus on the properties of the final paper. The water taken up by the fibre is held by many different mechanisms and it is not always clear from the techniques used which quantity of water is being measured. In this report, an attempt is made to obtain more knowledge regarding the water- holding mechanisms of fibres, by studying the amounts of bound water, of pore water and of the total water in the fibres. Effects of delignification, recycling, beating and ion exchange are examined. It is concluded that the amount of bound water is a reflection only of the wood polymer composition of the fibre, whereas the pore water is affected by physical changes of the fibre wall. The ionic charges mainly affect the surface water of the fibre.

The uptake of moisture by paper sheets was analyzed as an adsorption process occurring in a porous medium. Water vapor was assumed to diffuse into the pore space and was subsequently adsorbed onto the surfaces of the fibers constituting the paper sheet. The response of the sheets to variations in relative humidity was investigated.

Since the transient moisture profiles inside paper sheets depend upon the moisture sorption equilibria for paper, the equilibrium behavior was investigated with special emphasis on a description of sorption hysteresis. It is necessary to follow equilibrium trajectories inside the hysteresis loop for paper sheets. Sorption equilibria inside the hysteresis loop for paper sheets (bleached kraft linerboard, 290 gsm) were evaluated experimentally. Complete sets of desorption and adsorption scanning trajectories were obtained. Further higher order loops were obtained experimentally. The theory of independent domain complexions was applied to the hysteresis loop. By constructing a so called moisture distribution function for the hysteresis, arbitrary trajectories representing equilibrium sorption behavior under cyclic humidity changes could be predicted.

An investigation of transient sorption was also undertaken. A model for moisture uptake based upon diffusion inside the pore space and the fibers in the sheet was set up. Experimental data on transient moisture uptake was obtained under ramped changes in humidity. It was found that the model for moisture uptake incorporating a linearized isotherm could describe the sorption response of paper sheets to ramped changes in humidity adequately. From the experimental data, a value for the intro-fiber mass transfer coefficient representing moisture diffusion through fibers was determined.