The term polyvinyl chloride is understood as meaning crystallisable or non-crystallisable vinyl chloride homopolymers, or their copolymers containing at least 80% by weight of vinyl chloride units and up to 20% of other vinyl compounds which can be polymerised with vinyl chloride (for example vinylidene chloride, acrylonitrile and vinyl esters, such as vinyl acetate, acrylates, methacrylates and