The Composites Of Wood Flour/Maleic Anhydride Grafted Polyolefin Blends

Wood plastic composites (WPC), usually made from wood fiber and thermoplastic, are environmentally friendly materials with good performance and high added value. The wide application of composites provides an alternative for effective utilization of waste plastic and natural fiber resources. However, the waste plastics are usually mixtures of various nonpolar polymers bearing the different properties. The compatibility between the compositions in the resulting WPC is weak thereby causing poor mechanical properties.The mixture of virgin PP and PE with varying proportions was compounded to simulate the mixed plastic wastes. Grafting maleic anhydride (MAH) onto 80%PP-20%HDPE and 80%HDPE-20%PP blends were performed in a twin screw extruder at the melt state in the presence of dicumyl peroxide (DCP) as initiator. The effects of extruding parameters and MAH content on melt flow index and grafting degree were investigaged and the optimized grafting conditions for each system were obtained. (1) Both the tensile strength and impact strength of grafted blends increased at the optimum MAH content; (2) Characterizing the grafted blends using FTIR and SEM techniques revealed that MAH can be grafted onto the PP/HDPE blends and the compatibility between PP and HDPE was improved. (3) The rheological analysis of PP/HDPE blends using the rotational rheometer showed that the melt structure of 80%PP-20%HDPE blends were easier to fail due to grafting and the blends exhibited a broader molecular distribution with increasing MAH content. A "shoulder shape tendency", increased elasticity, and trail phenomenon were observed at the low frequencies for 80%PP-20%HDPE blends. The elasticity of 80%HDPE-20%PP was obvious increased with the MAH content. (4) Rheological study and extrusion processes showed that the processibility of grafted blends was improved compared with the individually grafted polyolefin.The MAH grafted PP/HDPE blends were used as a polymer matrix to prepare the WPC by two-step extrusion. The effects of MAH and wood flour content, and particle size of wood flour on the mechanical properties and dimensional stability of WPC were respectively investigated. The results are summarized as below:(1) Mechanical properties of the resulting WPC were improved due to grafting. The optimum MAH contents were 1.0%and 0.1%for the 80%PP-20%HDPE based and 80%HDPE-20%PP based composites, respectively. (2) Addition of 70%wood flour provides the resulting WPC the good mechanical properties. The WPC obtained a highest tensile and flexural properties at the wood flour size of 30-50 meshes; however, is the size of 60-80 meshes was optimum for the impact strength of resulting WPC. (3) The water uptake and swelling of composites were significantly reduced due to grafting of matrix. For the 80%PP-20%HDPE based composites, the water uptake and swelling reach the lowest value after 24h immersion at 1.0%MAH level, while for 80%HDPE-20%PP based composites, the MAH content is 0.1%. The water uptake and swelling of composites increased with wood flour content and reduced with increase wood flour mesh. (4) The fractured WPC surfaces observed using SEM indicated an improved interfacial bonding between wood flour and matrix due to matrix grafting. (5) As using the mixed plastic wastes instead of origin PP/HDPE blends, the resulting WPCs exhibited a comparable improvement in the physical and mechanical properties.The rheological and creep behavior of WPC were investigated using the rotational rheometer at varying MAH content, wood flour content, and wood flour size. The results showed that, (1) Grafting the PP/HDPE blends eliminated the uPayne effect" of WPC, which confirmed the improved interfacial bonding. The "Payne effect" of WPC became observable with increasing wood flour content and wood flour mesh. (2) The WPC exhibited a solid-like behavior, which did not occur in the case of blends alone. The storage modulus, loss modulus and complex viscosity of WPC decreased due to MAH grafting. With increasing frequency, the storage modulus and loss modulus increased and the composites therefore exhibited a shear thinning behavior. (3) The storage modulus and complex viscosity of WPC elevated as increasing wood flour content suggesting that the "soild-like behavior" was more significant. The storage modulus and complex viscosity of WPC reduced with decreased wood particle size. (4) Increasing the environmental temperature resulted in an increased creep deformation of WPC. Grafting modification of PP/HDPE blends apparently reduced the creep of the resulting WPC. The creep deformation was generally reduced with increasing wood flour content and particle size of wood flour, except at the high wood concentrations due to the weak interfacial bonding.Summarily, incorporation of MAH into PP/HDPE blends via grafting reaction in the extruder faciliated the compatibility between the plastic compositions in the blends. The interfacial bonding between the grafted plastic blends and wood flour was also improved thereby. These results demonstrate that grafting blends with anhydride is a feasible approach to improve the properties of wood/plastic blend composites and provide an alternative strategy for utilizing the recycled waste plastics.