17 Natural rubber, gutta-percha, gum raw or partly processed, asbestos, mica; packing, stopping and insulating flexible gaskets for lining, sealing or repairing a damaged pipe, made wholly or predominantly of plastics material, elastomeric material, nitrile rubber or polyvinyl chloride, and parts and fittings therefor; flexible pipes, not of metal; pipes, pipe couplings, pipe adapters including adaptor couplings for coupling pipes of different sizes, made wholly or predominantly of plastics material, elastomeric material, nitrile rubber or polyvinyl chloride, and parts and fittings therefor; bushes being non-metal fittings for pipes for use in the joining of pipes; sealant compounds for joints; artificial resins being semi-processed; synthetic resins being semi-processed; air admittance valves, wholly or principally of plastic material, for soil, waste and ventilation pipes