The rigid vinyl chloride resins are preferably of the kind known as Type I, i.e. polyvinyl chloride and copolymers of vinyl chloride with a minor amount, e.g. up to 15% of a copolymerizable monomer such as vinyl acetate, vinylidene chloride, alkyl maleates and fumarates, e.g. dibutyl fumarate and diethyl maleate, vinyl butyrate, butyl acrylate, methyl acrylate and other alkyl acrylates, methyl met