Vulcanization by the Action of Metal Oxides

Chlorobutadiene or chloroprene rubbers (CR), also called neoprene rubbers, are generally vulcanized by the action of metal oxides. CR can be represented by the structure  

The crosslinking agent is usually zinc oxide, which is used along with magnesium oxide. CR can be vulcanized in the presence of zinc oxide alone however, magnesium oxide is necessary to give scorch resistance. The reaction is thought to involve the allylic chlorine atom, which is the result of the small amount of 1,2-polymerization  

A mechanism that has been written for the vulcanization of CR by the action of zinc oxide and magnesium oxide is as follows [3]  

Examples of recipes for metal oxide vulcanization are given in Table VI. It should be noted that in one case, calcium stearate was used instead of magnesium oxide to obtain better aging characteristics [81], 

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A small amount of the bis-azo compound—sufficient to react with only 1 or 2 percent of the isoprene units—converts the specimen to a material having all of the physical properties characteristic of vulcanized rubber. Polychloroprene may be vulcanized by the action of metal oxides... 

CR compounds are vulcanized by metal oxides, i.e., ZnO. MgO is also present in the compound to somewhat retard the action of ZnO. Thioureas are used as accelerators. For low-water-absorption vulcanizates, PbO or Pb204 can be used as the vulcanizing agent. 

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