Influence of wood-fibre hygroexpansion on the dimensional instability of fibre mats and composites

Wood fibres are gaining increased use as reinforcement in plastics. One of the disadvantages of cellulose-based fibres is their propensity to absorb water and swell. This inevitably leads to undesired dimensional instability of the composite and its fibre-mat preform. A measure of the hygroexpansion behaviour of the fibres could serve to rank the suitability of different kinds of cellulosic fibres with regard to dimensionally stable composites. A method has been developed to determine the hygroexpansion coefficient of wood fibres. Since fibre mats manufactured with conventional techniques generally have a thickness gradient of fibre orientation, fibre mats and composites will curl if the moisture content varies. Models based on laminate mechanics and micromechanics of hygroexpansion have been used to estimate the transverse hygroexpansion coefficient of the fibres from experimental results of curvature and thickness changes, and tensile stiffness of fibre mats based on unbleached sulphate cooked softwood. The fibre orientation distribution through the fibre-mat thickness was characterized with a tape-splitting technique and image analysis. The transverse hygroexpansion coefficient of the wood fibres was found to depend on the assumed ratios of the elastic parameters of the transversely isotropic fibres. Values in the range of 0.10-0.15 strain per relative change in moisture content were found for plausible degrees of elastic anisotropy of the fibres. Means of how to suppress dimensional instability of cellulose based composites and fibre mats are addressed.