Heat and Mass Transfer in Wood Composite Panels During Hot Pressing. Part II. Modeling Void Formation and Mat Permeability

Theoretical models have been developed to predict the porosity and permeability of wood strand mats during consolidation. Based on the Poisson distribution of mat formation, the porosity model predicts the formation of both inside- and between-strand void volumes. It is proposed and predicted that the between-strand voids consist of voids between non-contact strand faces and voids around strand edges, with the former dominating in the early stage of consolidation and the latter dominating in the latter stage of consolidation. The permeability model is developed based on the Carman-Kozeny theory for porous materials. The model is compared and agrees with experimental results obtained from this study and from the literature. The results show mat permeability is mainly controlled by voids between strands instead of those inside strands. Mat density has a primary effect and strand size has a secondary but very important effect on mat porosity and permeability especially in the later stage of consolidation. Strand thickness has a stronger impact than strand width and length. Strand dimensions and mat permeability are shown to have significant effects on internal environmental conditions in wood composites during hot-pressing