These polymers include but are not limited to the polymers of styrene, substituted styrene, acrylonitrile, methacrylonitrile, u-methylstyrene, and copolymers of these; acrylic polymers such as alkyl methacrylate polymers and copolymers, in particular methyl methacrylate polymers and copolymers with minor quantities of alkyl acrylates; engineering plastics such as polycarbonates, polyacetals, polys