PTFE forms seal of approval
15 Jan 2000
The phase-out of chlorofluorocarbon (CFC) refrigerants has forced engineers to find alternative materials. An older refrigerant gas, ammonia, is once again becoming popular. However, this has been giving rise to a problem: safety systems sometimes detect ammonia leakage when neoprene rubber seals on the heat exchangers appear to be intact. It appears, according to John Toynbee of Technical Rubber Products (TRP), that the ammonia permeates through the neoprene.
The obvious answer would seem to be finding a new material for the gaskets, but this proved difficult. PTFE, for example, has better sealing properties than neoprene and other elastomers, because it has a semi-crystalline structure, but it tends to creep.
Toynbee's team solved the problem by combining the properties of the two materials: they lined the wetted edge of the neoprene gaskets with PTFE. At 50 degrees C, a 3.6mm-thick PTFE/neoprene gasket was over 100 times less permeable to ammonia than a 2mm-thick neoprene one, they found. The gasket shows no signs of ammonia permeation until 26 days after installation, compared with less than two days for a neoprene seal.
