A low-cost, low-fiber-breakage, injection molding process for long sisal fiber reinforced polypropylene

Composites of polypropylene (PP) and non-treated sisal fiber (SF) were prepared in a non-conventional two-step process that offers significant advantages. Maleic anhydride-grafted polypropylene (MA-g-PP) was used as a coupling agent, to improve adhesion between the polar sisal fiber and the non-polar polypropylene continuous matrix. At a first step, SF/MA-g-PP pellets with large aspect ratio and very high fiber content are prepared by extrusion impregnation and coating of a continuous SF yarn, followed by cooling and cutting. The composite pellets are thus dry blended with regular PP pellets in the injection machine hopper, and injected to obtain composite tensile specimens with a minimum quantity of expensive MA-g-PP, minimum fiber breakage and thermal degradation, and excellent mechanical properties. The SF/MA-g-PP pellets have a fiber content of 70% (w/w). The composite tensile specimens have final fiber contents ranging from about 3.5% to 24.5% (w/w). The PP tensile strength rises by about 44%. The tensile modulus increases by 126%, and the heat distortion temperature (HDT) is raised by about 35 K. FT-AR spectroscopy and SEM micrographic observation show that the MA-g-PP is covalently bonded to SF through esterification. Besides the improvement in mechanical and thermal properties, costs are reduced because of the lower content of very expensive MA-g-PP, and the use of a single-screw extruder at high production rates. (C) 2004 Society of Plastics Engineers.