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E xtending the life of existing plant and equipment together with an improved prediction of performance appear to be topics that have if anything been given a boost in interest by the current economic situation. This can be seen by the number of meetings being organised on such topics and a continuing interest in qualifying seals and materials. Critical to the life and reliability of practically any equipment is the performance of the seals. A lot of thought and engineering development has been devoted to this cause across all types of seals. Evidence of this includes the large amount of data generated for gasket materials and the various devices for monitoring mechanical seals. A huge amount of effort has been expended on attempting to predict the performance of elastomers, and some may ask how far has it got us? Perhaps some of the lack of progress can be attributed to entrenched attitudes and a tendency to stick to old ideas. A good example of this is what appears to be continuing debate on the relative merits of stress relaxation and compression set. Opposing camps seem to argue endlessly about the virtues of one and the relative uselessness of the other. In reality neither gives a complete picture, and I can provide examples of results where one or other if used in isolation can give very misleading results. A further problem with this debate is the method used to measure compression set. The long established standard methods were developed for material processing QA, not for predicting seal performance. The hot release used can give results that are not necessarily particularly useful. Methods using a cold release followed by a re-heat can give a very useful comparison of recoverable and permanent set, also often referred to as physical and chemical set. The use of this method provides a great deal more information about the performance of the material at a range of temperatures and hence much of the information provided by stress relaxation but with rather less expensive equipment. And, with the ever wider application of high fluorine elastomers which can have rather doubtful low temperature properties such an approach can be especially useful. A more flexible and broad minded approach to some areas of performance prediction could have important benefits.
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