General formula for hygroexpansion of paper
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A general formula for the hygroexpansion of paper has been derived. The hygroexpansion of paper is determined by two factors: one is the hygroexpansion of a single fibre, including the hygroexpansions in the fibre axis and the transverse directions; the other is the efficiency of the stress transfer from the network to the fibres when paper is subjected to uniaxial stress. The latter factor is dependent on mechanical properties of the fibre and the network. Because of its general nature and its simple physical significance, the formula can be used to interpret various experimental results in a qualitative or semi-quantitative manner. Some specialized cases are presented to illustrate unique hygroexpansion characteristics of bonded fibre networks. Effects of fibre orientation and the degree of fibre-to-fibre bonding are discussed based on experimental data and the derived formula. It is shown that the hygroexpansion of paper in the machine direction is almost entirely controlled by the hygroexpansion of fibre in the fibre-axis direction, whereas the hygroexpansion in the cross-machine direction is inherently sensitive to changes in the degree of fibre-to-fibre bonding and fibre orientation, because of the larger contribution of the hygroexpansion of fibre in the transverse direction.
KeywordsPolymer Experimental Data Mechanical Property Material Processing Transverse Direction
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