Under the terms of DIN ISO 815, the compression set is measured with a constant deformation. The compression set indicates the deformation component of the test material. A large number of test methods for elastomers, such as those for determining tensile strength, for example, provide (a greater or lesser degree of) information on the quality and properties of the material. The compression set, by contrast, is a key factor which needs to be taken into account before the material is employed for a specific purpose. In the case of seals and shims in particular, the remaining deformation – or compression set – is an important characteristic parameter.
To determine the compression set, a cylindrical test specimen is compressed by a percentage that is determined on the basis of its hardness and is then stored at a given temperature for a certain period of time in the compressed state. The temperature for the compression set test is a function of the material to be tested, its intended purpose and the test setup (e.g. 24 h at 70 °C for seals). 30 minutes after the pressure has been removed, the height of the specimen is measured again and, from this, the remaining deformation established.
A compression set of 0 % means that the specimen has attained its full original thickness again (something that is impossible in reality), while a compression set of 100 % means that the specimen underwent complete deformation during the test and does not show any recovery.
The calculation is performed on the basis of the following formula: CS (%) = ( L0 - L2 ) / ( L0 - L1 ) x 100 %, where:
- CS = compression set in %
- L0 = height of specimen prior to the test
- L1 = height of the specimen during the test (spacer)
- L2 = height of the specimen after the test
Significance: When a seal is fitted, for example, it is compressed to a certain thickness and exerts compression on the sealing surfaces. This compression falls with time, since the seal undergoes plastic deformation. If this plastic component – i.e. the compression set – is too high, the compressive force and hence the sealing effect will be reduced to such an extent that leaks will occur (the leak can be either at individual points or over the full surface of the seal), particularly if the seal has been insufficiently dimensioned. The compression set for flat seals should not exceed around 40 % at the continuous service temperature.
The following diagram shows the compression set that can be attained on average when using different types of chain extender. This is one of the means by which it is possible to influence the crystallinity of the rigid segment and the phase separation – as the most important parameters. While the Desmopan® grades in the standard ester series (100 – 300 series) generally have a compression set of between 45 and 55 % (24h/70°C), considerably more favorable compression sets of between 30 and 35 % (24h/70°C) can be achieved through the use of special raw materials in the special ester series (400 series). The first Desmopan® products with a technically serviceable compression set at 100 °C or 120 °C are currently under development.