Chapter 7 - Polymer Seals and Sealants

Abstract

Seals and sealants are an often forgotten part of containment technology but are critical parts of the systems. They have to retain flexibility under a wide range of environmental conditions, such as temperature and exposure to active fluids. A brake seal failed by fatigue through wrong material selection and poor dimensional tolerance. Fatigue likely started at a surface scratch and a crack grew until sudden failure and loss of brake power. The Challenger disaster of 1986 was caused when Viton fluoroelastomer O-rings failed to respond during takeoff, and exploding gas escaped. A diaphragm valve in a semi-conductor plant fractured and brought manufacture to a halt. The failure was caused by ozone cracking of nitrile rubber, in turn cause by change of a compressor at the front of the pneumatic line. Ozonolysis occurs at very low ozone gas concentration (parts per billion) and cuts polymer chains stoichiometrically. Thermoplastic elastomer radiator central heating system washers failed by thermal fatigue and chain degradation. The rubber was a polyester elastomer and sensitive to hydrolysis at high temperatures. Mastic is widely used as builder's putty in window glazing, the linseed oil slowly hardening by oxidation, but modern attempts to use polybutene oligomer have experienced problems of oxidative degradation and leaching. Poor use of silicone and other sealants in metal ducting in a fire brigade training building led to a fire owing to absorption and swelling of the non-standard mastics by hydrocarbon oil used to generate smoke. Low density polyethylene seals in a pneumatic system failed fatigue testing, and birefringence analysis in a polarizing microscope showed that a weld line from the moulding process was the cause of the problem. An irrigation system in New Zealand failed according to one side because of faulty rubber Z seals in a PVC pipe system, but the evidence was very weak and failed to mention poor installation.