Mechanical properties and creep resistance in polystyrene/polyethylene blends

Abstract

Recycled plastics, predominantly high-density polyethylene (PE), are being processed in the shape of dimension lumber and marketed as “plastic lumber.” One drawback to these products is their low creep resistance or high creep speed. The objective of this study was to examine the feasibility of reducing the creep speed of PE-based products by blending the PE with a lower-creep plastic, in this case polystyrene (PS). Various blends of PE and PS 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. Processing strongly affected the morphology and mechanical properties of the blends. For 50% PS : 50% PE blends, the stress–strain curves of the extruded samples showed PE-like behavior, whereas those from compression-molded samples were brittle, PS-like curves. Flexural strength was 50% higher in the extruded samples than in those from compression molding. The creep experiments were performed in three-point bending. Creep speed was lower in 50% PS : 50% PE and 75% PS : 25% PE blends than in pure PS. Creep speed of 75% PS : 25% PE was lowest of all the extruded blends. PE formed the continuous phase even when the PS content was as high as 50 wt %. For a 75% PS : 25% PE blend, cocontinuous phases were observed in the machine direction. A ribbonlike PS-dispersed phase was observed in the 25% PS : 75% PE and 50% PS : 50% PE samples. Blending low-creep-speed PS with high-creep-speed PE appeared to successfully improve the performance of the final composite. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1100–1108, 2000