Elastomer Properties - Compression Set/Creep
Elastomer
compression set is a measurement of the ratio of elastic to viscous components
of an elastomer’s response to a given deformation. Longer polymer chains tend
to give better “set resistance” because of the improved ability to store
energy (elasticity).
Elastomer compression set measurement standards
call for a 25% compression for a given time and temperature. The cross-section
is measured after the load is removed. Elastomer compression set is the
percentage of the original compression (25%) that is not recovered. This test
may be conducted on cylindrical disks or O-rings. At the end of the test, the
samples are removed and allowed to cool at room temperature for 30 minutes
before measuring. After a load is released from an elastomer, the difference
between the final dimensions and the original dimensions is considered the
compression set.
The use of elastomer compression set
measurements is most beneficial for production quality control, indicating the
degree of curing. Elastomers with high compression set values may require
special considerations for gland design and handling. Elastomer compression
set is a relatively simple test to perform, and as such, may not yield the
type of predictive information desired for custom sealing applications.
When a constant load is placed on an elastomer,
the deformation is not constant, but rather it increases gradually with time.
Terms used to describe this behavior are relaxation or creep. These
properties, including compression set, are a result of physical (viscoelastic)
and chemical (molecular structure) changes in an elastomer.
Test Methods:
- ISO 815 (Ambient & High Temp.)
- ISO 1653 (Low Temp.)
- ASTM D395 (Ambient & High Temp.)
- D1229 (Low Temp.)
