Vulcanization of Elastomers
Cross-linking of natural rubber was discovered by Goodyear back in 1839. Sulfur, which was the original cross-linking agent, is still utilized today in many processes. Early studies demonstrated that the cross-links are mainly polysulfides:
The reactions take place at all temperatures, but industrially they are carried out from 50 to 75C and above. At lower temperatures, however, the process may take days to complete. At temperatures of 135–155C, approximately 8% of sulfur (by weight of rubber) reacts [272]. Also, sulfur dissolves in unvulcanized rubber even at room temperature. The overall mechanism of the reaction is still being studied. Most evidence points to an ionic mechanism and a sulfonium ion intermediate [272]. It was shown [273] that a straightforward reaction of sulfur with rubber is insufficient. Somehow, between 40 and 100 atoms of sulfur must be combined in order to obtain one cross-link. Out of 40–100 atoms, only 6–10 are actually engaged in the formation of the cross-links. The rest of the atoms form cyclic sulfide units that become spread along the main chain [273]. To improve the efficiency of the vulcanization reaction, various accelerators were developed. Among them are zinc oxide combined with fatty acids, and/or amines. Zinc oxide forms zinc mercaptides like (XS)2ZnL2 where X is an electron withdrawing substituent and L is a ligand from a carboxyl or an amine group. The function of the ligand is to render the complex soluble. The mercaptide complexes are believed to react with additional sulfur to form zinc perthiomercaptides. The accelerators that are most commonly used are derivatives of 2-mercaptobenzothiazole. They are very effective when used in combinations of metal oxides with fatty acids (referred to as activators). The favorite activators are zinc oxide combined with stearic acid. The combinations permit rapid vulcanizations that take minutes compared to hours when sulfur is used alone. In the process of vulcanization, 2,20-dithiobisbenzthiazole forms initially and then reacts with sulfur to form polysulfides [273]:
The products from reactions with sulfur in turn react with natural or synthetic rubber at any allylic hydrogen. This is a concerted reaction that results in formation of sulfur containing adducts of the polymers:
Once the cross-links are formed, further transformations take place. Some of them consist of reactions that shorten the polysulfide links:
Also, some cross-links are lost through elimination reactions:
In addition, some cyclic sulfur compounds form in the process [273]:
