Page 247 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.39

FIGURE 4.24 Cross-link types and chain modifications.

The contours for flex fatigue life are complex. The test is run such that the specimens
are about equally strained; however, there is some question as to whether the tests should
be run at equal strain or at equal strain energy. For some cases, where strain is restricted by
fabric reinforcement, fatigue test data should be compared at equal strain amplitude. For
other applications, where the strain is not limited, the tests should be run at equal strain en-
ergy. The contours as presented here can be interpreted in terms of either constant strain or
constant strain energy. All points on the chart can be compared at an approximately equal
strain per cycle; however, if we interpolate between the flex-life contours but only along a
constant modulus contour, we can extract values corresponding to approximately equal
strain energy per cycle. By choosing higher modulus contours, we are considering higher
strain energies.
The low values for fatigue life at low levels of sulfur, but high levels of accelerator,
have been attributed to high concentrations of accelerator-terminated appended groups and
high concentrations of monosulfidic cross-links. Monosulfidic cross-links are not able to
exchange, rearrange, or break to relieve stresses without the breakage of main chains.
On the other hand, polysulfidic cross-links are able to rearrange under stress. The rear-
rangement of a cross-link occurs in two steps: (1) breaking and (2) reforming. Recent data
indicate that only the breaking of the weak polysulfide cross-links is required for the
strengthening of the vulcanizate network. It is better to relieve the stress by the breaking of
a cross-link than by the breaking of a polymer chain.
When even higher concentrations of sulfur are used (with the maintenance of constant
modulus), flex life decreases. It is possible that this is due to the large amount of cyclic
chain modification associated with high levels of sulfur. As always, there are compro-
mises.

4.5.1.7 Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers. Over the
years, much of the research on accelerated-sulfur vulcanization was done by using natural

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