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Polypropylene/coir fiber composites were prepared according to an experimental statistical design, in which the independent variables, coir fiber, and compatibilizer content, were varied. The compatibilizer used was maleic anhydride grafted polypropylene (PP-g-MA). Compatibilizer free composites were also prepared. Composites were processed in a corotating twin-screw extruder and submitted to mechanical and morphological analyses. The effects of the independent variables on the mechanical properties were assessed through tensile strength, elongation at break, flexural modulus, and impact strength. The morphological properties were assessed by scanning electron microscopy (SEM). The results indicated the need for using compatibilizers in the composites due to the incompatibility of PP and coir fiber. The variable with the strongest effect on the properties was coir content, whose increase caused increase in tensile strength, impact strength and elastic modulus, and decrease in elongation at break. The presence of PP-g-MA was fundamental to achieving the aforementioned results. The effect of increasing compatibilizer content was only observed for the elastic modulus.

Polypropylene/sawdust composites were investigated to assess the effect of sawdust surface treatment and compatibilizer addition on polymer/fiber adhesion. Two silane coupling agents were used for sawdust surface treatment: vinyl-tris (2-methoxyethoxy) silane and (3aminopropyl)triethoxysilane. Maleic anhydride grafted polypropylene was used as compatibilizer. Composites were prepared in a corotating twin-screw extruder coupled to a Haake torque rheometer and submitted to tensile and bending tests as well as scanning electron microscopy (SEM). Moisture uptake tests were also performed. Results showed that incorporation of untreated sawdust to polypropylene (PP) caused reduction in composite tensile strength and increase in stiffness. When the only treatment used was surface modification with silane coupling agents, no significant changes were observed in mechanical properties. However, when compatibilizer was added to the composites, tensile strength was increased, and % elongation at break reduced, indicating improved system compatibility. The composite presenting the highest increase in tensile strength was that containing sawdust treated with amino silane in addition to the compatibilizer. SEM analyses corroborated the mechanical property results. POLYM. ENG. SCI., 50:978-985, 2010.

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