Mechanical, thermal and flammability properties of hybrid rice husk/nanofillers filled polypropylene nanocomposites

Abstract

This study investigated the mechanical, thermal, and flammability properties performance of hybrid rice husk / nanofiller filled polypropylene (PP) nanocomposite. With rice husk (RH) content at 15 wt.%, and graphene nanoplatelets (GNP) at increasing contents of 0.5, 1, 1.5, 2, 2.5, and 3 parts per hundred (phr) of PP/RH composite, the hybrid blends were melt blended in a twin-screw extruder and injection molded for tensile, flexural, impact, thermal and flammability tests. Before this hybrid combination, a comparative study on RH and rice husk ash (RHA), and different compatibilizers of PP grafted maleic anhydride (MAPP) and ethylene-acrylic ester maleic anhydride (E-AE-MA) was studied. The result showed that RH performed better than RHA with MAPP better than E-AE-MA in enhancing mechanical and thermal properties. Further comparative study between the nanofillers of GNP, graphene oxide (GO) and halloysite nanotubes in PP nanocomposite systems also showed that GNP exhibited better mechanical and thermal stability property. Hence the hybrid of GNP-RH filled PP nanocomposite was chosen for a synergistic probe. Among the hybrid blends, results showed that GNP optimum content was attained at 1phr with enhanced tensile strength (25%), tensile modulus (72%), flexural strength (10%), flexural modulus (0.3%), impact strength (29%), thermal stability (25 °C) and flame retardancy-LOI (5%), compared to single-filled PP/RH composite system. Microstructural analysis of the tensile fractured surfaces of the hybrid blends revealed good nanofiller dispersion was attained with 0.5 and 1phr GNP contents demonstrating good filler-matrix interfacial bonding quality aided by MAPP compatibilizer and, responsible for excellent composite strength performance. Based on the PP/RH/GNP/MAPP hybrid nanocomposite field emission scanning electron microscopy micrograph, an interaction model of RH and GNP fillers with PP matrix was suggested in which GNP nanosheets are observed to attach to the layers of RH fibers and filled possible gaps between PP and RH necessitating effective stress transfer from the matrix to the fillers. The void filling mechanism, large surface area, and exfoliation of GNP nanosheet within the hybrid nanocomposite system are believed to be contributory to the enhanced properties of the hybrid nanocomposite compared to the single-filler filled PP/RH composites. It can be noted that formulations involving single nanofillers-PP nanocomposite systems attained high stiffness-toughness balance with PP/GNP/MAPP nanocomposite achieving increased tensile strength (8%), tensile modulus (96%), elongation at break (29%), flexural strength (20%), flexural modulus (18%), impact strength (104%) and thermal stability (12 °C) compared to pristine PP. The outcome of this study suggests that synergistic incorporation of GNP and RH is beneficial to the enhancement of overall hybrid PP nanocomposite properties.