Increasing the Impact Strength of Grass Fiber Polypropylene Composites with Virgin and Recycled Polypropylene

Natural fibers from different plants are widely used as reinforcing fillers in polymers due to their ecological, economic and technical benefits, with applications in all major industries. Since a typical drawback of incorporating natural fibers into polymers is a reduction in impact strength, this work investigates ways to enhance impact strength. Grass fibers of 30 wt. % from permanent pasture grasslands are compounded with polypropylene and varying amounts of polypropylene-graft-maleic anhydride with three different degrees of grafting as coupling agents and ethylene propylene diene rubber as the impact modifier. The impact strength and the melt flow index are measured for each compound while the adhesion between the fibers and matrix is investigated by scanning electron microscopy. It can be shown that around 5 wt. % of coupling agents are sufficient to increase impact strength to a saturation level, while the impact modifier has no significant influence. Through multiple extrusion cycles of a pure grass fiber polypropylene compound, it can be shown that the melt flow index increases constantly with the number of extrusion cycles, while the impact strength remains constant after an initial increase, indicating matrix degradation but enhanced fiber matrix coupling.