Polyolefins extrapolation limits

In terms of time, for high temperature steels it is conventional to restrict extrapolation to a time factor of 3, although years of practical experience are allowing this to be extended as far as 10. ISO 2578 [4] recommends no more than a factor of 4, from 5,000 to 20,000 h. A maximum value of 100 (two orders of magnitude) is permitted for pipes with the restriction that the temperature difference is limited to 60 °C for polyolefins and to 25 °C for PVC. General advice is no more than a factor of 10. Figures 9.1 and 9.2 show instances of somewhat extreme prediction far into the future of the human race. 

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for PE hot-water pipes and cables insulated with plasticized PVC. Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system. 

Some of the growth in the use of TREF can be predicted by a simple extrapolation of the research activities noted in this review. However, the technique does not have to be limited to the analysis of polyolefins as seems to have been the case so far. There are certainly many other crystalline polymers and it would be surprising if the TREF technique did not find further apphcation in these polymer systems. The growing availability of TREF will certainly encourage this trend and is hoped that this review will also stimulate the broader use of this relatively new technique. 

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