Modeling the hygroexpansion of aligned wood fiber composites

The effect of wood fiber ultrastructure and cell wall hygroelastic properties on wood fiber composite hygroexpansion has been analyzed. An analytical concentric cylinder model extended to include also free hygroexpansion of orthotropic phase materials has been used on several length scales. Using properties of the three main wood polymers, cellulose, hemicellulose and lignin the longitudinal and transverse hygroexpansion coefficients for the microfibril unit cell were obtained and the volume fraction change of the wood polymers in the microfibril unit cell depending on relative humidity was calculated. The fiber cell wall was modeled regarding each individual S1, S2 and S3 layer and the cell wall longitudinal hygroexpansion coefficient was determined depending on microfibril angle in the S2 layer. A homogenization procedure replacing the S1, S2 and S3 layers with one single layer was found not to influence the results significantly for low microfibril angles. Finally the hygroexpansion coefficients of an aligned softwood fiber composite were calculated.