Collective term for a large group of synthetic polymer compounds containing polyorgano-siloxanes, e.g. (CH₃)₂-SiO)_n as primary molecules. Of all the elastomers, silicon rubbers have the widest temperature range of application (flexible at low temperatures down to -60° C, resistant to hot air up to +230 °C) and display great resistance to aging and weathering. They are proof against animal and vegetable oils, alcohols and solvents, but have only moderate to low resistance to mineral fats, oils, fuels, acids and lyes. Used in hot water, they are decomposed by hydrolysis at temperatures above 100°C. They are pervious to gases and so cannot be employed in vacuum seals. For material characteristics see materials for secondary seals.