Page 232 - Handbook of Plastics Technologies

ELASTOMERS

4.24 CHAPTER 4

Vulcanization chemically produces network junctures by the insertion of cross-links
between polymer chains. A cross-link may be a group of sulfur atoms in a short chain, a
single sulfur atom, a carbon to carbon bond, a polyvalent organic radical, an ionic cluster,
or a polyvalent metal ion. The process is usually carried out by heating the rubber, mixed
with vulcanizing agents.

4.5.1.1 Effect of Vulcanization on Elastomer Properties. Vulcanization causes signifi-
cant changes at the molecular level. The long rubber molecules (molecular weight usually
between 100,000 and 500,000 daltons) become linked together with junctures (cross-
links) spaced along the polymeric chains. The average distance between junctures corre-
sponds to a molecular weight between cross-links of about 4,000 to 10,000 daltons. Be-
cause of network formation, the rubber becomes almost entirely insoluble in any solvent,
and it cannot be processed by any means that requires it to flow—e.g., in a mixer, in an ex-
truder, on a mill, on a calender, or during shaping, forming, or molding. Thus, vulcaniza-
tion must occur only after the rubber article is in its final shape or form.
Major effects of vulcanization on use-related properties are indicated by Fig. 4.14. It
should be noted that static modulus increases with vulcanization to a greater extent than
does the dynamic modulus. (Here, static modulus is more correctly the equilibrium modu-
lus, approximated by a low strain, slow-strain-rate modulus. Dynamic modulus is gener-
ally measured with the imposition of a sinusoidal, small strain at a frequency of 1 to
100 Hz.)

FIGURE 4.14 Vulcanizate properties as functions of the extent of vulca-
nization.

Hysteresis is reduced with increasing cross-link formation. Hysteresis is the ratio of the
rate-dependent or viscous component to the elastic component of deformation resistance.
It is also a measure of deformation energy that is not stored (or borne by the elastic net-
work) but is converted to heat. Vulcanization then causes a trade-off of elasticity for vis-
cous or plastic behavior. Tear strength, fatigue life, and toughness are related to the

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