Study On Properties And Central Influencing Factors Of Polypropylene/Wood Flour Composites

Polypropylene/wood flour composites (PP-WPC) mainly composed of wood flour (or plant fiber) and polypropylene (PP) are an excellent alternative to natural wood. In comparison to other WPCs, such as polyethylene/wood flour composites (PE-WPC), PP-WPC shows better performances in hardness, elasticity modulus, wear-resistence and thermal deformation temperature, and so on. However, there is rare reports on the preparation and industrialization of PP-WPC by extrusion-molded in China, now. In this thesis, the mechanical properties, water absorption, dimensional stability and ageing properties of PP-WPC were extensively investigated. The objective of this study is to establish a new method to prepare PP-WPC with high mechanical strength and low cost.(1) Study on mechanical properties of PP-WPCPP-WPC was prepared by extrusion through using PP as a plastic substrate and wood flour as a filler. The influences of wood flour, toughening agent, compatibilizer, coupling agent and pigment on the mechanical properties of PP-WPC were investigated. The increase of amount of toughening agents, polyethylene octene (POE) and ethylene-propylene-diene monomer (EPDM) could significantly improve the impact strength of PP-WPC, however, a negative influence on the flexural strength and flexural modulus were also observed. In comparison to EPDM, POE showed a better performance on toughening of PP-WPC. The mechanical properties of PP-WPC could be improved by using a suiTab. amount of the compatibilizer (maleated polypropylene, MAPP) or the coupling agent (y-methacryl oxypropyltrim ethoxysilane, KH570or vinyltrimethoxysilane, KH171). It was found that the mechanical properties were more dependent on the compatibilizer than the coupling agent. The increase of amount of wood flour in this report, a negative influences on the flexural strength and impact strength. The optimum amounts of the compatibilizer and coupling agent were8%and1%relative to, respectively. The mechanical strength was reduced by the addition of pigments, in the order of titanium dioxide> iron oxide> carbon black.(2) Study on water absorption and dimensional stability of PP-WPCThe effects of the thermal expansion characteristics of the wood flour, addictives and plastic substrate on the linear thermal expansion coefficient of WPC were studied. The water absorption of PP-WPC was also studied in terms of the content of compatibilizer (maleated polypropylene), silane coupling agent and plastic. The linear thermal expansion coefficients of the main raw materials in WPC decreased in the order of wood flour<PP<toughening agent, and the linear thermal expansion coefficient of WPC increased with increasing the plastic content due to the higher linear thermal expansion coefficient of PP compared to wood flour. The water absorption of WPC increased significantly within the first30days and remained almost constant afterwards. The water absorption of WPC was reduced by the introduction of the silane coupling agent and the compatibilizer. The silane coupling agent mainly reduced the initial absorption percentage, while the compatibilizer reduced both the initial absorption percentage and the saturated absorption percentage. The water absorption of WPC was reduced rapidly with increasing plastic content. The swell rate of soaking increased with increasing the water absorption of WPC.(3) Study on stability of PP-WPCThermal properties and oxidation induction time (OIT) of the WPC, PP and wood flour were studied by differential scanning calorimetry (DSC) and thermo-gravimetric (TG). The effects of wood flour and antioxidant on OIT of WPC were investigated. Influences of carbon black, titanium dioxide, iron oxide as pigments on aging properties of WPC were studied based on thermal-oxidative aging in oven and xenon lamp aging.The results of DSC-TG thermal analysis showed that no significant oxidation was observed in PP or WPC with antioxidant tetra-[methylene-β-(3,5-ditert-butyl-4-hydro-xyphenyl)-propionate]methane (1010). An exothermic peak in DSC curve and a rapid weightlessness in TG curve, which means an obvious thermal oxidation decomposition of PP or WPC, appeared after exhausting antioxidant1010(after OIT). The slowly pyrolysis of wood flour is the main reason of the continued gently weightlessness in TG curve of WPC, and no significant effect of antioxidant on improving thermal stability of wood flour was observed. The thermal stability of WPC was improved obviously by adding antioxidant dilauryl thiodipropionate (DLTP), which is without remarkable effect on improving the thermal stability of PP matrix.The results of thermal-oxidative aging in oven and xenon lamp aging analysis showed that with increasing aging time of WPC, the tensile strength and oxidation induction time (OIT) decreased, and melt index and color change increased. Before and after aging in the oven, the tensile strength and OIT were lower for the WPC with titanium dioxide or iron oxide as pigment, and the melt index of WPC with the pigments was larger. In xenon lamp aging condition, the change rates of tensile strength, melt index and OIT of the WPC with the pigments was lower than that of the WPC without adding the pigments.