Electric insulation is the containment of electric currents or voltage by materials (electrical insulators) that offer a high resistance to current flow, will withstand high voltages without breaking down, and will not deteriorate with age. Resistance to sunlight, rain, flame, or abrasion may also be important. The electrical resistance of insulators usually falls with temperature (paper and asbestos being exceptions) and if chemical impurities are present. The mechanical properties desired vary with the application: cables require flexible coatings, such as polyvinyl chloride, while glass or porcelain are used for rigid mountings, such as the insulators used to support power cables. In general, good thermal insulators are also good electrical ones.