Page 256 - Handbook of Plastics Technologies

ELASTOMERS

4.48 CHAPTER 4

polymeric radicals to form a cross-link or by unproductive processes. A polymeric radi-
cal can unite with a radical derived from the peroxide. Also, if a polymeric radical de-
composes to give a vinyl group and a new polymeric radical, a scission of the polymer
chain is the result.
Few monomeric radicals are lost by coupling with polymeric radicals when dialkyl
peroxides are used as the curative. Also, if the elastomer is properly chosen, the scission
reaction is not excessive. For dicumyl peroxide in natural rubber, the cross-linking effi-
ciency has been estimated at about 1.0. One “mole” of cross-links is formed for each mole
of peroxide; cross-linking is mainly by the coupling of two polymeric radicals. One perox-
ide moiety gives two monomeric free radicals that react with rubber to give two polymeric
radicals, which couple to form one cross-link.
In the case of BR or SBR, the efficiency can be much greater than 1.0, especially if all
antioxidant materials are removed. A chain reaction is indicated here. One might expect
that nitrile rubber would also be vulcanized with efficiencies greater than 1.0; however,
though the double bonds in nitrile rubber are highly accessible, the cross-linking effi-
ciency is somewhat less than 1.0.
Peroxide Vulcanization of Saturated Hydrocarbon Elastomers. Saturated hydro-
carbon polymers are also cross-linked by the action of organic peroxides, though the effi-
ciency is reduced by branching. Polyethylene is cross-linked by dicumyl peroxide at an
efficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubber
cannot be cured at all. For polyethylene, the reaction scheme is similar to that of the unsat-
urated elastomers. However, branched polymers undergo other reactions. Though the per-
oxide is depleted, no cross-links may be formed between polymer chains, and the average
molecular weight of the polymer can even been reduced by scission. Sulfur or the so-
called coagents can be used to suppress scission. Examples of coagents are m-phenyleneb-
ismaleimide, high-1,2 (high-vinyl) polybutadiene, triallyl cyanurate, diallyl phthalate, eth-
ylene diacrylate, and others.
Peroxide Vulcanization of Silicone Rubbers. Silicone rubbers (high-molecular-
weigh polydimethylsiloxanes) can be represented by

where R can be methyl, phenyl, vinyl, trifluoropropyl, or 2-cyanoethyl. Silicone rubbers
that contain vinyl groups can be cured by dialkyl peroxides such as dicumyl peroxide. Sat-
urated silicone rubbers require diacyl peroxides such as bis-(2,4-dichlorobenzoyl)perox-
ide. In the case of saturated siloxane rubbers, the mechanism is hydrogen atom abstraction
followed by polymeric radical coupling to give cross-links. The incorporation of vinyl
groups in the rubber molecule improves the cross-linking efficiency.
Vulcanization is frequently done in two steps. After a preliminary vulcanization in a
mold, a high-temperature (e.g., 180°C) postcure is carried out in air. The high-temperature
postcure removes acidic materials that can catalyze hydrolytic decomposition of the vulca-
nizate. Also, the high temperature enables the formation of additional cross-links of the
following type:
Peroxide Vulcanization of Urethane Elastomers. Urethane elastomers suitable for
peroxide vulcanization are typically prepared from an hydroxyl-group-terminated oligo-

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