Page 224 - Handbook of Plastics Technologies

ELASTOMERS

4.16 CHAPTER 4

Resilience is measured by using one of several methods including the rebound pendu-
lum test (for example, ASTM D 1054), the vertical rebound test (ASTM D 2632), or the
Yerzley oscillograph test (ASTM D 945).
For a pendulum test, the pendulum is set to strike the surface of a secured stationary
rubber specimen, the surface being place directly below the fulcrum of the pendulum. The
pendulum is then released from a certain angle of displacement from the surface of the
rubber. The percentage energy released is calculated as (1 – cos angle of rebound)/(1 – cos
angle of displacement).
In the vertical rebound test, a metal plunger is allowed to fall on the test specimen, and
the height to which the plunger rebounds is recorded as a percent of the release height.
Heat Buildup and Compression Fatigue. The temperature increase in an elasto-
meric material during repeated deformations is due to the accumulation of thermal energy
because of hysteresis or internal friction. The Goodrich Flexometer is used for heat
buildup testing in the Compression Fatigue Test (ASTM D 623). A cylindrical test speci-
men is alternately compressed or stressed and then released in rapid cycles for a specified
period of time or until it fails. The temperature rise within the test specimen is recorded. In
addition, permanent set after the test is measured. The test specimen is fatigued, or weak-
ened, by the rapidly cycling compression stress. The deterioration of the composition can
actually increase the hysteresis and further increase heat buildup until, at some tempera-
ture, the sample completely degrades almost explosively. This temperature and time can
also be noted.
Compression Set. Compression set is the residual deformation of a material after re-
moval of an applied compressive stress. For good performance in many applications, com-
pression set values should be low. A low compression set value indicates that a material
recovers much of its original height after compression and release of the compressive
force.
Compression set tests are run either by applying a specified force to the test specimen
(Method A of ASTM D 395) or by compressing the specimen to a specified deflection
(Method B of ASTM D 395). The specimen is held in the compressed state for a specified
period of time at a specified temperature, after which the compressive force is removed,
and the specimen is allowed to recover at room temperature for 30 min. In Method A,
compression set is the difference between the original and final thickness of specimen, ex-
pressed as a percentage of the original thickness. In Method B, compression set is ex-
pressed as the difference between the original and final thickness of the specimen as a
percentage of the deflection to which the specimen was subjected. The standard specimen
for a compression set test is a round pellet. O-rings of specified dimensions can also be
tested. Compression set is a property that is an important consideration with seals and gas-
kets where the rubber part must maintain sealing force under a compressive stress or strain
or in rubber hoses clamped onto nipples. Method A is a measure of compressive creep,
whereas method B is a measure of compressive stress relaxation.
Compression set tests are usually run at elevated temperatures to simulate conditions or
aging effects. Common test conditions are 70 hr at 70°C or 100°C, although heat-resistant
materials such as fluoroelastomers may be tested for longer periods of time at tempera-
tures up to 200°C or more. If the end product is expected to perform at low temperatures,
e.g., below 0°C, compression set is measured at the expected service temperature.
Heat Resistance. Heat resistance is the resistance to irreversible changes in proper-
ties due to exposure to elevated temperatures. Reversible changes due to elevated tempera-
tures include expansion and softening. Permanent changes include further cross-linking,
polymer chain degradation by oxidation, and stiffening due to loss of plasticizer by evapo-
ration. In the case of nonoxidative elevated temperature aging of natural rubber, permanent
softening due to a loss of cross-links (reversion) can occur.

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