Creep reduction using blended matrices in wood-filled polymer composites

摘要

Creep, the slow deformation over time of a material under stress, is one of the characteristics of wood-filled polymer composites that has resulted in poor performance in certain applications. The objective of this study was to examine the feasibility of reducing the creep of polyethylene (PE)-based products by blending the PE with a lower creep plastic (in this case, polystyrene (PS)) and also by incorporating wood flour filler (WF) in the composite. Various blends of PE, PS, and WF were prepared in either a laboratory extruder or a bowl mixer and then compression-molded. The mechanical properties, creep behavior, morphology, and thermal properties of extruded and compression-molded samples were determined. The modulus of elasticity of the extruded blends could be estimated by a weighted average of PS and PE even in the absence of a compatibilizer. Increasing the ratio of WF increased modulus of elasticity in all composites, except between 30 percent and 40 percent WF, while the effect of WF on ultimate stress was variable, depending on the composite. Scanning electron microscope images and thermal analysis indicated that the wood particles interacted with the PS phase, although the interactions were weak. The PS appeared to have better compatibility with the WF than the PE phase did. Processing strongly affected the morphology and mechanical properties of the blends. Flexural strength was 50 percent higher in the extruded samples than in compression-molding samples. The creep experiments were performed in three-point bending. Creep speed was calculated by using a three-point bending geometry with a load of 50 percent of the ultimate stress. Creep decreased only slightly with increasing WF content but more significantly with increasing PS content, except with pure PS. The WF-75% PS-25% PE blend showed the least creep. For a 0% WF-(75% PS-25% PE) blend, co-continuous phases were observed in the machine direction. A ribbon-like PS dispersed phase was observed in the 0% WF-(25% PS-75% PE) and 0% WF-(50% PS-50% PE) samples. The ribbon-like phase was not observed in filled samples. Blending low creep-speed PS with high creep-speed PE appeared to improve the performance of the final composite successfully.