Page 227 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.19

which uses the Clash-Berg Tester, and the ASTM D 1053, which uses the Gehman Tor-
sional Stiffness Tester. In each of these tests, a sample is chilled to a preset temperature
well below the transition temperature then twisted with a known constant force. The
amount of twist is measured as a function of the increasing temperature. The temperature
that produces a stiffness of 69 MPa is sometimes taken as the stiffness temperature.
In the temperature retraction test (ASTM D 1329), the test specimen is stretched to a
specified elongation and then frozen at –70°C. Then it is gradually warmed until it begins
to retract toward its original unstretched dimensions. The temperature at which the speci-
men retracts by 10 percent correlates with the brittleness temperature.
Dynamic Mechanical Properties. In an above section dynamic mechanical proper-
ties were discussed in connection with the differences between elastomers and plastics vis
à vis the glass transition temperature, indicated by the position of the peak in the tan δ in
its plot against temperature.
Dynamic mechanical properties of elastomers are important for other reasons. Rubber
is often used in applications where it undergoes rapid cyclic deformations over a range of
frequencies: engine mounts, tire sidewalls and treads, and so forth. In such uses, flexibility,
friction, cushioning, and damping properties of the rubber are important. Such properties
relate to frequency and temperature in some of the same ways as do dynamic mechanical
properties. They vary as a function of the difference between the ambient temperature and
the glass transition temperature, i.e., as a function of (T – T ). Also, the measurement of
g
tan δ at a particular condition is an estimate of hysteresis, which correlates with heat
buildup, and damping, this being important for such applications as engine mounts. There
are even other relationships (though complex) between dynamic mechanical properties
and friction (traction) and resistance to skidding on ice.
Compilation of ASTM Tests. Table 4.4 gives a compilation of selected ASTM desig-
nated tests.

4.4.1 Rubber Elasticity
Rubber elasticity is the reason for the use of elastomers. The origin of rubbery behavior
has been considered and pondered for quite a long time. On the molecular level, vulca-
nized and thermoplastic elastomers have something in common: their molecules have seg-
ments that are flexible at least at service or use temperatures (above T ). The flexible
g
segments have ends that are not free but, instead, are immobilized, either in cross-links
(e.g., in vulcanized rubber) or in glassy or crystalline domains [e.g., in segmented TPEs,
such as styrene-butadiene-styrene or segmented poly(butylene terephthalate) copolymers].
The rubbery molecular segments continuously change their conformations or configura-
tions as a result of Brownian motion, which increases with temperature.
Rubber-like behavior (rubber elasticity) has been described in terms entropy (a funda-
mental measure of disorder). As an elastomeric material is stretched, the ends of the elas-
tomeric molecular segments become increasingly separated from one another. The
disorganized molecular chain becomes more nearly straight; that is, they become less dis-
ordered. This is an unnatural condition, which is resisted, and when the stretching stress is
released (stopped), the entropy increases as the material returns to its original state of ease.
Thus, a supporting molecular chain (e.g., a flexible chain between cross-links) can be
viewed as an entropic spring. This idea is shown in Fig. 4.11. The arrows at the ends of
some of the chain segments simply indicate that they are part of a continuous network of
flexible mobile segments that terminate in junctures or cross-links. The effect of strain on
the entropy of a network chain is idealized by Fig. 4.12.
The molecular origin of the recovery from large deformations, which is the essence of
rubber elasticity, was not recognized until the early 1930s. Evidence for this was the fact

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