1997 Volume 1

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.

Effects of fibre morphology on in-plane hygroexpansivity of paper have been studied using a general micromechanics formula and experimental results obtained for different fibre fractions. It was deduced from the micro-mechanics analysis that the major fibre geometry factors affecting paper hygroexpansivity are fibre width, wall thickness, and fibre length,all of which control the stress transfer ratio parameter defined in the formula. The microfibril angle was found to bean other major factor affecting the paper hygroexpansivity through the hygroexpansivity of a single fibre.

The results from the cut-fibre experiment showed that decreasing fibre length increased the hygroexpansivity, but the effect was small within the range of fibre length more than 1mm. The results from Bauer-McNett classification, on the other hand, showed considerable increases of hygroexpansivity as the fraction became shorter. This large variation was attributed to the cell wall structure, in particular microfibril angle and cell wall thickness, both of which directly affect the key parameters in the formula. Fibre fractionation was found to be a very effective tool to consistently change all the fibre morphology factors for controlling dimensional stability of paper.