One approach to the purification of recycled thermoplastic mixtures is selective grinding to induce differences in sizes and shapes between polymers with different; compositions. These mixtures can then be separated using one of several technologies including conventional sieving or hydrocyclones. Recycled poly(vinyl chloride) and poly(ethylene terephthalate) often are cross-contaminated with each other since they have overlapping density ranges and are very difficult to separate using methods such as flotation. Selective grinding followed by physical separation might be a preferred method for separating such a polymer pair if processing "windows" for inducing differences in failure mechanisms can be found. There is a temperature range over which PET fails in a ductile mode while PVC fails in a brittle mode for impact grinding experiments. This range is not accurately predicted by failure mechanism and beta-transition temperature diagrams.