Page 233 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.25

breaking energy. Values of these properties increase with small amounts of cross-linking,
but they are reduced by further cross-link formation. Properties related to the energy to
break increase with increases in both the number of network chains and hysteresis. Since
hysteresis decreases as more network chains are developed, the energy-to-break related
properties are maximized at some intermediate cross-link density.
Reversion. Reversion is usually the loss of network structures by nonoxidative ther-
mal aging. It is generally associated with isoprene rubbers vulcanized by sulfur. It can be
the result a long vulcanization time (overcure) or of hot aging of thick sections. It is most
severe at temperatures above about 155°C. It occurs in vulcanizates containing large pro-
portions of polysulfidic cross-links. Sometimes the term “reversion” is applied to other
types of nonoxidative degradation, especially with respect to rubbers not based on iso-
prene. For example, thermal aging of SBR (styrene-butadiene rubber), which can cause in-
creased cross-link density and hardening, has been called reversion since it can be the
result of overcure.
4.5.1.2 Characterization of the Vulcanization Process. Important characteristics of the
vulcanization process are the time period before cross-linking starts, the rate of cross-link
formation once it starts, and the extent of cross-linking when the process ends. There must
be sufficient delay or scorch resistance (resistance to premature vulcanization or induction
period) to permit mixing, shaping, forming, and flowing in the mold, autoclave, or curing
oven before vulcanization begins. Then cross-link formation should be rapid and con-
trolled. The importance of heat history with respect to the vulcanization process is illus-
trated by Figs. 4.15 and 4.16.
Scorch resistance is frequently measured by the time at a given temperature required
for the onset of cross-link formation as indicated by an abrupt increase in viscosity. The
Mooney viscometer is usually used. Fully compounded, unvulcanized rubber is contained
in a heated cavity. Embedded in the rubber is a rotating disc. Viscosity is continuously
measured (by the torque required to keep the rotor rotating at a constant rate) as a function
of time. The temperature is selected to be characteristic of rather severe processing (extru-
sion, calendering, and so on).

FIGURE 4.15 The effect of processing on heat history.

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