BS ISO 815-1:2019

This document specifies methods for the determination of the compression set characteristics of vulcanized and thermoplastic rubbers at ambient (one method) or elevated temperatures (three methods, A, B, and C, depending on the way the test piece is released at the end of the test).The methods are intended to measure the ability of rubbers of hardness within the range 10IRHD to 95IRHD to retain their elastic properties at specified temperatures after prolonged compression at constant strain (normally 25%) under one of the alternative sets of conditions described. For rubber of nominal hardness 80IRHD and above, a lower compression strain is used: 15% for a nominal hardness from 80IRHD to 89IRHD and 10% for a nominal hardness from 90IRHD to 95IRHD. NOTE1 When rubber is held under compression, physical or chemical changes that prevent the rubber returning to its original dimensions after release of the deforming force can occur. The result is a set, the magnitude of which depends on the time and temperature of compression as well as on the time, temperature, and conditions of recovery. At elevated temperatures, chemical changes become increasingly more important and lead to a permanent set. NOTE2 Short-time compression set tests, typically for 24h, at elevated temperatures are commonly used as a measure of the state of cure, a means of material classification, and a specification to ensure the quality of a compound. Longer tests, typically for 1000h, at elevated temperatures take account of the effect of ageing and are often used to predict service performance, including that of sealing materials. Short-time tests at ambient temperature show mainly the effect of physical changes (re-orientation of the molecular chains and the fillers).