Page 231 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.23

sulfur, many other substances have been used in curing (vulcanization) systems. In this
section, emphasis is placed on the vulcanization of general-purpose “high-diene” rubbers
[natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR)] by sul-
fur in the presence of organic accelerators.
The accelerated-sulfur vulcanization of these rubbers, along with the vulcanization of
other rubbers that are vulcanized by closely related technology [e.g., ethylene-propylene-
diene monomer rubber (EPDM), butyl rubber (IIR), halobutyl rubbers, and nitrile rubber
(NBR)] comprises more than 90 percent of all vulcanization. Nevertheless, we give some
consideration to vulcanization by the action of other vulcanization agents such as organic
peroxides, phenolic curatives, and quinoid curatives.
Definition of Vulcanization. Vulcanization is a process generally applied to rubbery
or elastomeric materials, which forcibly retract to their approximately original shape after
a rather large mechanically imposed deformation. Vulcanization can be defined as a pro-
cess that increases the retractile force and reduces the amount of permanent deformation
remaining after removal of the deforming force. Vulcanization increases elasticity while it
decreases plasticity. It is accomplished by the formation of a cross-linked molecular net-
work (Fig. 4.13).

FIGURE 4.13 Network formation.

The theory of rubber elasticity (above) proposes that the retractile force to resist a de-
formation is proportional to the number of network supporting polymer chains per unit
volume of elastomer. A supporting polymer chain is a linear polymer molecular segment
between network junctures. An increase in the number of junctures (i.e., cross-links) gives
an increase in the number of supporting chains. In an unvulcanized linear high polymer
(above its melting point), only molecular chain entanglements act as junctures.

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