The methods are intended to measure the ability of rubbers of hardness within the range 10 IRHD to 95 IRHD
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
80 IRHD and above, a lower compression strain is used: 15 % for a nominal hardness from 80 IRHD to 89 IRHD
and 10 % for a nominal hardness from 90 IRHD to 95 IRHD.
NOTE 1 When rubber is held under compression, physical or chemical changes can occur that prevent the rubber
returning to its original dimensions after release of the deforming force. The result is a set, the magnitude of which depends
on the time and temperature of compression as well as on the time and temperature of recovery. At elevated temperatures,
chemical changes become increasingly more important and lead to a permanent set.