O-rings made of PTFE are used in all applications in which the chemical resistance of conventional elasomer O-rings is not adequate. PTFE O-rings can only be used as static seals. PTFE O-rings are manufactured by machining and can be produced in all sizes. Back up rings for use with a seal will always have a special part no. allocated by Moontown. Back-up rings are used in combination with axial and radial seals when the extrusion gap is too big for the required pressure and temperature.