Rubber Seals : Use of Polyurethane elastomers

Another large field of application for polyurethane is that of rubber seals .The rubber seals can be static , reciprocating or rotary and can be used in pneumatic or hydraulic system. Since the rubber seals can be in the form of O-ring ,lip seals or simple square section seals.

The economic advantages of polyurethane as a rubber seals are associated with its improved wear resistance and lower friction.Up to pressures of about 1500 psi nitrile rubber sealing are generally quite satisfactory but above this pressure the use of plastic or fabric reinforcement is required to avoid extrusion of the seal. The hard grades of polyurethane are able to seal quite successfully without reinforcement up to pressure of aproximately 6000 psi.

Since the seals are generally required to have the highest possible physical properties the cast polyurethane have been widely used in the past.

When hydraulic fluids are present the main limitation on the use of polyurethane is that of restricted temperature range. Under dry condition , whether pneumatic or hydraulic , an upper temperature limit of around 70 *C for continuous operation should not be exceeded. When the oil is contaminated with water or a water-base hydraulic fluid is used the upper temperature limit is reduced to 40 *C. Certain synthetic oils also attack polyurethane.

Although the abrasion resistance of polyurethane is exceptional at low abrasive speeds, high speeds can cause premature breakdown. For this reason the use of polyurethanes for rotary seals is generally restricted to maximum surface speeds 1 – 2 ft/sec.