Structure development in thermoset recyclate-filled polypropylene composites

Polypropylene containing comminuted fiber reinforced thermoset recyclate has been shown to exhibit enhanced mechanical properties relative to particulate-filled materials. Optimum mechanical performance in these recyclate-filled materials is achieved in compositions made from rubber-modified polypropylene containing maleic anhydride-modified PP in conjunction with silane coupling agent. Although matrix crosslinking was found to enhance properties in both filled and unfilled systems, composite properties are dominated by the development of strong interfacial bonding between polypropylene and recyclate reinforcement. A mechanism for the formation of interfacial bonding is proposed involving reaction between maleic anhydride functionalized PP, formation of trisilanol groups and their subsequent condensation with hydroxyl groups on the recyclate surfaces, together with molecular entanglement and co-crystallization of the grafted and ungrafted polypropylene molecules. Furthermore, in the absence of treatment there is evidence that the elastomer particles encapsulate the filler particles. However, this effect is strongly hindered when functionalized-PP is added, either in isolation or in combination with the silane co-treatment. The crystalline nucleation of PP by thermoset recyclate and treatment is also considered. The treatment system investigated was found to promote interfacial bonding to both the polyester (DMC) and woven glass-reinforced phenolic recyclates investigated, suggesting it may be suitable for treating mixed composite scrap.