Rubber Washers
Flat rubber washers can be manufactured using a variety of materials. Some of the most common compounds include silicone, fluorosilicone, viton (fluoro-elastomer), neoprene, nitrile (Buna-N), hypalon, EPDM, SBR (red rubber), FDA white nitrile, polyurethane and natural rubber. Rubber washers come in hardnesses between very soft and very hard, indicated by numbers between 30 (softest) and 90 (hardest). The thicknesses of rubber washers range from about 0.031" to 0.375".
The size of rubber washers is determined by outside diameter (the outer-most edge) and inside diameter (the center hole). Outside diameters range from around 0.125" to 2.984" and inside diameters from approximately .094" to 2.859". Manufacturers offer flat rubber washers or the option of rubber pads, disc-shaped rounds without a center hole.