Recycled thermoplastic polyurethane as a coupling agent in automotive polypropylene compounds with coconut fibers

This study evaluates the effect of recycled thermoplastic polyurethane (RTPU) on the Izod impact resistance of polypropylene (PP) compounds with 50% coconut fibers arranged randomly. The Izod impact resistance was compared to the performance of a commercial coupling agent (CCA) of maleic anhydride grafted PP copolymer (MAPP). The PP compounds were made with 0, 2.5, and 5% by weight of RTPU or CCA, and with 50% of short or long coconut fibers, in the form of plates molded by thermo-compression, where the plastic components were previously extruded in the form of filaments. The morphology of the resulting materials was studied by scanning electron microscopy (SEM). The functional groups and their possible interactions present were determined by Fourier transform infrared spectroscopy (FTIR). Furthermore, the interfacial shear strength between the polymeric matrix and the coconut fiber was determined. The results showed a higher Izod impact resistance and a higher adhesion between long coconut fiber and PP when they contain 5% RTPU. This is explained by the higher interfacial shear strength between them given by the chemical and/or physical interaction between functional groups such as the lignin of coconut fibers and the isocyanate groups of RTPU, which in turn determined the shortest critical length of coconut fiber. The impact resistance values ranged from 19.8 to 24.4 J/m for mixtures of PP with short coconut fibers and 39.9-71.7 J/m for PP with long coconut fibers.