Temperature Dependence of Kinetic Friction of Thermoplastics: A Handle for Plastics Sorting?

A crucial step in post-consumer plastics recycling (PCR) is sortation whereby properties such as density or spectral signature are used to sort plastics. However it is difficult to sort polyolefin flakes/pellets at high throughput by these properties. Here we examine friction near the melting point for four common PCRs as an alternative property for sortation. Specifically, we measure the temperature dependence of kinetic friction for three common polyolefins (high and low density polyethylene and polypropylene) and also a polyethylene terephthalate (PET). For the three polyolefins, we find strong increases in the coefficients of kinetic friction during temperature ramps as we approach the pre-melting regime. For the PET, we find a strong peak in crystallization that we associate with cold crystallization. We discuss the enhanced friction in the context of rubber friction, which exhibits comparable coefficients of kinetic frictions.