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.