Influence of chemical modification on the water uptake properties of wood plastic composites

Wood plastic composites (WPC) are materials which gained increasing interest from both industry as well as from academic researchers over the last decades. These materials, built up from wood residues like sawdust, a thermoplastic processable matrix polymer and some additives have found their way into applications like decking or siding. Nevertheless one of the main problems associated with WPC, which has not been solved until yet, is the water uptake and the corresponding problems in terms of dimensional stability and weathering influences. Therefore the aim of this work was to search possible ways to reduce this uptake through chemical modification of the wood particles. The different formulations, based of softwood particles (untreated as well as chemically modified) as well as polypropylene, were produced via compounding on a co-rotating twin screw extruder and subsequent injection molding processing to retrieve universal test specimens. These were tested for mechanical (tensile and impact) as well as water uptake properties. Evaluation of the latter was carried out by a simple model, to retrieve parameters like the maximum sorption constant and the equilibrium moisture. In conclusion, we found that with a chemical modification, e.g., with maleic anhydride, it is possible to reduce water uptake, as well as to influence the mechanical properties. Furthermore the type of chemical compound added to the wood particle surface also shows an influence on the performance of the different wood plastic composites.