MAPP optimization through elucidation of wood plastic composite microstructural behaviour in melt and solid state

Abstract The compatibility of wood flour (WF) and thermoplastic matrix in wood plastic composites (WPC) has over the years been improved by means of coupling agents, surfactants and filler or matrix surface modifications. However, understanding the composite microstructure and the effects of coupling agents on microstructural behavior is pertinent for process optimizations. In the current work, the effect of varying maleic anhydride grafted polypropylene (MAPP) amount at 10 and 30% WF contents on composite microstructure was investigated by monitoring changes in their rheo-mechanical properties. The study revealed that increase in the WF content increases the composite melt’s complex viscosity, dynamic moduli, stress, and creep flow resistance. In the absence of MAPP, the tensile strength, impact strength, and the elongation at break were reduced with WF content due to prevalent WF agglomerations. Incorporating MAPP in 1, 3, and 5% amounts showed a corresponding decrease in the abovementioned rheological properties while the tensile strength, impact strength, and elongation at break increased. Composites containing 10% WF content exhibited a levelling effect at 1% MAPP, while those containing 30% WF levelled off at 3% MAPP. This was indicative of a possible competitive process upon reaching the MAPP optima, resulting in zero or insignificant property changes upon further MAPP additions. Evidently, at 30% WF content, 5% MAPP exhibited discrepant behaviour characterized by higher complex viscosity, dynamic moduli, stress, creep flow resistance and levelling of mechanical properties compared to 3% MAPP. This was attributed to excess MAPP clustering at interphases, resulting in ineffective wetting and consequently higher internal friction within the composite microstructure.