Hygroelastic properties of wood fibres for composite applications

Abstract: Wood pulp fibres are gaining increased use for other applications than paper and board. Wood fibres can also be used as reinforcement in plastics for load carrying purposes. Some advantages compared with conventional man-made fibres are that wood fibres come from a renewable resource, have high specific stiffness and strength, are generally less hazardous to health, biodegradable, and can be manufactured at low cost and high volumes. A clear disadvantage with cellulose-based materials for structural use is their dimensional instability in humid environments.The hygroelastic properties are of high importance in materials development of improved wood-fibre composites. This work deals with the stiffness and hygroexpansion of composite materials. Mechanistic models have been used to link the fibrous microstructure with these macroscopic engineering properties. The properties have been characterised experimentally for various wood-fibre composites and their fibre-mat preforms, by means of curvature measurements at various levels of relative humidity, as well as tensile and compressive tests. From these test results, the contribution of the fibres to the stiffness and hygroexpansion of composites were determined. These approaches can be used to rank different candidate fibres according to their potential as reinforcement in dimensionally stable and stiff composites. With straightforward macroscopic test methods and microstructural characterization, the longitudinal Young’s modulus and transverse coefficient of hygroexpansion of wood fibres were identified by inverse modelling. Some effects of various pulp processes and fibre modifications on the elastic properties of the fibre were observed, which illustrate how the mixed experimental-modelling approaches can be used in quantitative and more efficient materials development.