Isothermal Crystallization Kinetics of Highly Filled Wood Plastic Composites: Effect of Wood Particles Content and Compatibilizer

The isothermal crystallization behavior and crystal structure of the polypropylene (PP) component in wood plastic composites (WPC) with respect to wood particle content and maleic anhydride-grafted polypropylene (MAHPP) compatibilizer were studied by means of polarized optical microscopy, scanning electron microscopy, x-ray diffraction, and differential scanning calorimetry. It was found that under the experimental conditions of this research, the speed of crystallization of PP was faster in WPC with MAHPP than in composites without MAHPP. This is ascribed to the difference in undercooling due to the change in the equilibrium melting temperatures (T(m)(0)) of the PP component in WPC due to the addition of wood flour and MAHPP compatibilizer. T(m)(0) decreased with the increase of wood particle content, and it decreased more severely with the addition of wood flour than the addition of compatibilizer. The half-crystallization time was the smallest in PP/wood composites, intermediate in PP/wood/compatibilizer system, and the largest in pure PP under the same undercooling. The fast crystallization in PP/wood composites is ascribed to the heterogeneous nucleation effects of wood particles, which could be hindered by the MAHPP compatibilizers; this was verified by the higher fold surface free energy in WPC with compatibilizer than in WPC without compatibilizer.