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

Abstract

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[degrees] to 300[degrees]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 ([alpha]-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.

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Extruded wood-plastic composites (WPC) have experienced tremendous market growth in North America, primarily for application as deckboard (Wolcott and Englund 1999, Clemons 2002). Wheat straw (Triticum aestivum L.) could potentially be used instead of wood as filler in formulations resulting in the manufacture of straw-plastic composites (SPC). The use of straw in the manufacturing of various fiber-based composite materials has been previously suggested and evaluated (White and Ansell 1983, Sauter 1996, Simonsen 1996, Zhang et al. 2003, Boquillon et al. 2004). Simonsen (1996) suggested the use of rye grass straw as filler in polyethylene and polystyrene; however, composites were produced by compression-moulding on a small scale whereas in the present investigation, wheat straw- high-density polyethylene (HDPE) composites were manufactured using a commercial-scale extrusion process.

Wheat straw is an annually renewable agricultural byproduct. In the United States alone, 60 million metric tons of wheat straw are currently produced every year, 99 percent of which are either returned to the field by burning, tilling into the soil, or used as an on-farm fuel source (Cheng et al. 2004). The use of wheat straw in the production of natural fiber-reinforced thermoplastic composites potentially generates a new revenue stream for wheat producers, provides an incentive to reduce air pollution caused by field-burning, and accomplishes soil conservation goals in arid climates.

The present investigation succeeds a study by Houghton et al. (2004) in which non-sterile wheat straw stems were treated for 12 weeks with the white-rot fungus Pleurotus ostreatus (Jacq. ex Fr. Kummer) to estimate the variation in chemical composition of the straw with variations in initial moisture content (MC) and inoculum. The fungal straw treatment was intended for application in thermoplastic extrusion. Treatment of wheat straw with P. ostreatus was expected to cause a limited degradation of the waxy cuticle, lignin, and hemicelluloses in the straw without much cellulose removal (Valmaseda et al. 1991, Moyson and Verachtert 1991, Gamble et al. 1996). Fungal degradation of these specific straw components was expected to 1) improve adhesion between straw and HDPE and thus mechanical properties of SPC; 2) increase the internal pore volume and surface area within the straw particles through selective removal of hemicelluloses and lignin; and 3) reduce the amount of HDPE required in the formulations for SPC and therefore, the cost of the SPC. However, fungal treatment may potentially affect the thermal stability of the straw due to the selective degradation of lignin and hemicelluloses, and hence restrict the polymer class of potential thermoplastics to polyolefins (Wolcott and Englund 1999). The primary criterion used in the selection of a thermoplastic for production of a wood- or straw-plastic composite is that the melting or softening temperature of the thermoplastic is less than the thermal degradation temperature of the wood or straw filler (ca. 210[degrees]C for wood and undegraded wheat straw).

The overall goal of the study reported herein was to evaluate the feasibility of generating cost-effective large-quantities of P. ostreatus-degraded wheat straw for use in the commercial production of SPC. The specific objectives were to 1) evaluate the impact of fungal degradation on selected mechanical, physical, and thermal properties of extruded SPC; and 2) compare fungal-treated and untreated SPC to ...