Vulcanization of elastomers

Cross-linking of natural mbber 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 aU temperatures, but industrially they are carried out from 50 to 75°C and above. At lower temperatures, however, the process may take days to complete. At temperatures of 135-155°C, 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, 2,2 -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 aUyUc hydrogen. This is a concerted reaction that results in formation of sulfur containing adducts of the 

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Fig. 1. Stmctures formed during sulfur vulcanization of elastomers.

There are a number of advantages, listed below, associated with the peroxide vulcanization of elastomers ... 

The various possible roles of MFAs in the vulcanization of elastomers have been covered by Heideman et al. in a recent publication [37]. 

B-64MI11500 Vulcanization of Elastomers , ed. G. Alliger and I. J. Sjothun Reinhold, New... 

Figure 8.4 The use of bis(3-trimethoxysilyl)tetrasulfane) modified silica in the vulcanization of elastomers (idealized a+b=4, l3Ax3A8) taken from ref (55).

Linear polymers are usually characterized in solution. However, if enough crosslinking has occurred, such as in the vulcanization of elastomers, the polymer will swell but cannot dissolve in typical solvents. Then, it is impossible to derive molar mass (molecular weight) information from solution behavior. However, the average molar mass of the chains in the polymer network can be determined from swelling (1,2). 

The cure of vulcanization of elastomers is a process whereby chemical crosslinks are introduced between the elastomer chains, resulting in the formation of a three-dimensional network. Overall, the chemistry accompanying snlfnr vulcanization is extremely complex and althongh several techniqnes have been developed to monitor the process and to analyze the prodncts formed, most are time-consnming and tedious. 

In the vulcanization of elastomers, metal oxides in combination with fatty acids activate the sulfur accelerator systems. 

Alliger, G Sjothun, U. Vulcanization of Elastomers, 1964, p.l31 Rheinhold Publishing Corporation, New York. 

G. Alliger and I.J.C Sjothun, "Vulcanization of Elastomers," Reinhold, New York, 1964. 

Metal deactivators may have side effects. For example, they act additionally as adhesion promoters in the vulcanization of elastomers (3,11). 

Peroxide induced vulcanization of elastomers requires three steps 1) radical formation, 2) hydrogen atom abstraction from polymer chain by a radical, and 3) radical carbon-radical carbon bonding as shown in Scheme 4.46. 

Dluzneski, P.R., Peroxide Vulcanization of Elastomers. Rubber Chemistry and Technology, 2001. 74(3) 451. 

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