Preparation of wood powder/polypropylene composites in presence of subcritical fluids

A new method has been applied to prepare the wood powder/polypropylene (PP) composites. The key procedure is to inject fluids (ethanol and propanol) during extrusion process and to make the fluid in the subcritical state at a certain temperature. Furthermore, the structures of wood powder/polypropylene composites were analyzed and observed by FTIR, SEM. Due to the hydrophilic nature, there are a large number of hydroxyls in wood powder, which can esterify with maleic anhydride grafted polypropylene (PP-g-MAH). The results show that the subcritical fluid as a swelling agent and reaction medium promotes the interfacial reaction between wood powder and PP-g-MAH, which leads to the improvement of the interface bond strength. When ethanol is at subcritical state (190℃, (2.4±0.1)MPa, 120 r/min), ethanol as a reaction media is helpful to swelling and liquefaction of wood powder. Subcritical ethanol can further promote the esterification between wood powder and PP-g-MAH as well as the dispersibility of wood powder in PP matrix. In addition, wood powder can be permeated by PP-g-MAH under the high temperature and pressure, the interfacial reaction is strengthened. The tensile strength, flexural strength, flexural modulus and non-notched impact strength of the wood powder/polypropylene composites at the reaction condition in subcritical ethanol reach 23.5 MPa, 52.5 MPa, 4.8 GPa, and 5.9 kJ·m^-2, respectively, comparing with wood powder/PP composites without fluids during extrusion procession.