Challenges for Wood–Plastic Composites: Increasing Wood Content and Internal Compatibility

Wood–plastic composites (WPCs) are interesting materials as the biobased content is determined by the inclusion of wood particles regenerated from residual wood sources or biomass products. At present, the aim is to increase the wood content in WPCs above 60%, while it is currently limited to around 40%. The rationale behind this is based on the need for an increase in the performance of WPCs, the relatively cheap price of wood and the aim to augment the biobased content. Most studies are presently carried out with a maximum of 50% wood particles (preferably ranging from around 30 to 40%), while there are only very few sources where the wood concentration is increased to 70%. The formulations are not yet optimized and there are problems in interface compatibility, leading to weak mechanical properties. Problems in the augmentation of the wood content have to be further controlled, e.g., aggregation, dimensional stability and water absorption. Alternative approaches for the treatment of wood chips before (or during) compounding with the polymer matrix should therefore be developed. As the water resistance is mainly related to the control of the surface properties of the hydroscopic wood particles, possible solutions should consider the better protection of the individual wood particles’ surfaces against water ingress, the better development of the wood–polymer interface and the prevention of the formation of a continuous network with contacting wood particles. Therefore, this overview suggests various processing routes together with their industrial potential based on various sources from the literature, including the effects of compatibilizers and additives, the spray coating of wood particles, chemical pretreatment, physical modifications and the thermal treatment of wood fillers.