The term ???hydrolysable polymers??? here is taken to mean thermoplastic polyesters, polyamides or polycarbonates, such as polyethylene terephthalate or naphthalate, polypropylene terephthalate or naphthalate, polybutylene terephthalate or naphthalate, polyamide 6 or 6.6, poly(bisphenol A carbonate) or copolymers thereof.