Elastomer Properties - Stress Relaxation/Retained Sealing Force
Elastomers are viscoelastic in nature. When
deformed, energy storage is always accompanied by some energy dissipation. The
entanglements of the long elastomer chains act as obstructions to the movement
of the polymer chains. These obstructions enable the elastomer to store
energy—an elastic property. The rearrangements of the polymer chains are
dependent on the specific chemical structure, time, temperature and
deformation rate. Since elastomers are viscoelastic, the stored energy
decreases over time. This decrease of the stored energy (seen as contact
sealing force) over time is known as stress relaxation. In other words, stress
relaxation is the change in stress with time when the elastomer is held under
constant strain.
Common instruments for measuring stress
relaxation are Lucas and Wykeham Farrance. There are three standard methods:
Method A—compression is applied at test
temperature and all
force measurements are made at test temperature.
Method B—compression and force measurements are
made at ambient temperature.
Method C—compression is applied at ambient
temperature and all force measurements are made at test temperature.
The three methods do not give the same values
of stress relaxation. The resulting force measurements in all methods can be
normalized to the initial counterforce, and expressed as a Retained Sealing
Force percent.
Test Methods:
- ISO 8013—Creep Strain Relaxation
- ASTM D412—Creep Strain Relaxation—Tensile
Properties
- ISO 3384—Stress Relaxation
