Pilot-scale production and material properties of extruded straw-plastic composites based on untreated and fungal-treated wheat straw

The predominant filler used in the commercial extrusion of natural fiber reinforced thermoplastic composites in North America is wood flour. Fibers such as wheat straw (Triticum aestivum L.) represent a promising filler alternative. In this investigation, untreated and fungal-treated wheat straw was employed as filler for extruded high-density polyethylene (HDPE) based composites. Fungal treatment of straw was accomplished with the white-rot fungus Pleurotus oetreatus (Jacq. ex Fr. Kummer) to improve adhesion between straw and HDPE and thus mechanical properties of straw-plastic composites (SPC). The straw used in this research was not sterilized prior to fungal treatment for 6 and 12 weeks to achieve maximum cost-efficiency of large-scale SPC production. Our results indicate that the mechanical properties of SPC produced with untreated straw are comparable to those of a wood-plastic composite based on pine flour. In the temperature range with the most relevance to the extrusion process (100° to 300°C), fungal-degraded straw appeared thermally less stable than untreated straw, but this did not negatively affect composite manufacture. Complete dominance of P. ostreatus on the straw was not achieved under the non-sterile conditions applied in this study. Furthermore, treatment did not have a statistically significant (α-value of 0.05) influence on either modulus of rupture or modulus of elasticity of SPC. Hence, under the conditions applied in this study, degraded straw offered no advantages compared to untreated straw. At the same time, it was demonstrated that untreated wheat straw offers potential as a substitute for wood fillers in the extrusion of thermoplastic composites.