, synthetic rubbers such as Hycars.RTM. or neoprene, silicone polymers, Teflon.RTM.-silicone laminates, Teflon.RTM.-rubber laminates and other elastomers known in the art to be useful in, for example, closingvials and like vessels containing pharmaceuticals, diagnostic agents etc.Plastics materials used in accordance with the invention may if desired incorporate absorbants, e.g. inorganic material