Ring-shaped seals and gaskets, having an inner diameter of 400 mm or less, made of any of the following materials— a copolymers of vinylidene fluoride having 75 % or more beta crystalline structure without stretching; b fluorinated polyimides containing 10 % by weight or more of combined fluorine; c fluorinated phosphazene elastomers containing 30 % by weight or more of combined fluorine; d polychlorotrifluoroethylene (PCTFE, e.g. Kel-F ®); e fluoro-elastomers (e.g. Viton ®, Tecnoflon ®); f polytetrafluoroethylene (PTFE).