Vulcanization of elastomers.

Vulcanizing agents

As previously described, vulcanizing agents are chemicals which produce a three dimensional structure by formation of crosslinks between the long chain segments of the rubber molecules This provides the elastic properties observed

Sulfur and sulfur donors

Sulfur is the most common vulcanizing agent used for the widely used diene containing elastomers such as natural rubber, SBR and polybutadiene. Rubbermakers sulfur is a rhombic form existing as a cyclic or eight member ring structure. Insoluble sulfur is an amorphous polymeric form with molecular weights of 100,000-300,000. Because this material is insoluble in rubber it resists migration to the surface prior to cure and this bloom free attribute contributes to maintaining better building tack and better interply adhesion (refs. 10-12).

Sulfur containing chemicals such as dimorpholinyl disulfide (DTDM) and tetramethylthiuram disulfide (TMTD) are not only effective accelerators, but they can also be used as "sulfur donors." As such, they are effective in controlling sulfur crosslink length to form primarily mono and di sulfide crosslinks. These short crosslinks are more thermally stable than conventional sulfur curing and thereby provide better heat and set resistance. Vulcanization mechanisms for sulfur vulcanization will be discussed in more depth later.

Non-sulfur vulcanizing agents

Many high performance specialty elastomers do not contain diene moieties in their molecular structure and, therefore, cannot be sulfur cured. These elastomers require crosslinking agents capable of reacting with the specific functional group(s) contained by the specific elastomer. Some common, non-sulfur curatives include peroxides, difunctional resins and metal oxides. Following is a brief description of crosslinking agents used in some of the more technologically important, non-sulfur curing elastomers.

Peroxides

Peroxides are probably the most common materials used after sulfur because of their ability to:

* Crosslink a variety of diene and non-diene-containing elastomers; and

* their ability to produce thermally stable carbon-carbon crosslinks.

This stability is derived from the inherently stronger linkages formed compared to the carbon-sulfur bonds developed with sulfur vulcanization (table 2, ref. 13).

                     Dissociation
                        energy
Bond                  (Kcal/mole)
- C - C                    80
- C - S - C -              74
- C - S - S - C            74
- C - S - S - S - C -      54
- C-  S - S - S - S - C-   34

Peroxides decompose when heated to produce active free radicals which, in turn, react with the rubber to produce crosslinks. This three step mechanism includes: initiation, propagation and termination which will be discussed later.

The rate of peroxide cure is controlled by temperature and by selection of the specific peroxide (based upon half life considerations). Although some chemicals such as bis-maleimides, triallyl isocyanurate and diallyl phthalate act as coagents in peroxide cures, they are not vulcanization accelerators. Instead, they act to improve crosslink efficiency (crosslinking vs. scission), but not rate of crosslink formation.

Metal oxides

Halogen containing elastomers such as polychloroprene and chlorosulfonated polyethylene are crosslinked by their reaction with metal oxides, typically zinc oxide. The metal oxide reacts with halogen groups in the polymer to produce an active intermediate which then reacts further to produce carbon-carbon crosslinks. Zinc chloride is liberated as a byproduct, and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used to scavenge excess Zn[Cl.sub.2] to control the cure rate and minimize premature crosslinking (scorch).

Amine crosslinking

Two commercially important, high performance elastomers which are not normally sulfur cured are the fluoroelastomers (FKM) and the polyacrylates (ACM).

Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diaminecarbamate (Diak #1). Since the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM in question. Sulfur and sulfur bearing materials can be used as cure retarders, and they also serve as age resistors (ref. 14).

Fluoroelastomer cure systems typically contain amines as the primary crosslinking agent and metal oxides as acid acceptors.

Other difunctional compounds

The diamine class of crosslinkers for FKM and ACM materials were discussed above. Other examples of difunctional compounds are: epoxy resins for nitrile rubber, quinone sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (ref. 16). In this study, initial primary accelerator levels were chosen to produce nearly equivalent modulus or state of cure to the natural rubber.

Activators

Activators are chemicals which increase the rate of vulcanization by reacting first with the accelerators to form rubber soluble complexes. These complexes then activate the sulfur to effect vulcanization. The most common activators are combinations of zinc oxide and stearic acid. Other metal oxides have been used for specific purposes ie. lead, cadmium etc., and other fatty acids used include lauric and proprionic acids. Soluble zinc salts of fatty acid such as zinc 2-ethyl hexanoate are also available, and these "rubber soluble" activators are effective in natural rubber to produce low set, low creep compounds used in load bearing applications. Also, weak amines and amino alcohols have also been used as activators in combinations with the metal oxides.

Natural rubber usually contains sufficient fatty acid to solubilize the zinc salt. However, if the fatty acids are first extracted by acetone, the resultant "clean" natural rubber exhibits a much lower state of cure. Therefore, to insure consistent cure rate, fatty acids are usually added for insurance. Synthetic rubbers, especially the solution polymers, do not contain fatty acids and require their addition to the cure system.

Sulfenamide accelerators generally require less fatty acid because they release an amine during the vulcanization process which acts to solubilize the zinc. Guanidines as similar amine accelerators also serve to both activate and accelerate vulcanization.

Paris has systematically studied the effect of stearic acid and zinc oxide on a sulfenamide accelerated, sulfur cured natural rubber compound (ref. 16). Figure 7 dramatically shows the need for both the zinc and fatty acid activators.

Layer has published a schematic mechanism describing the role of activators in the mechanism of vulcanization. In his scheme, the soluble zinc salt forms a complex with the accelerator and sulfur. This complex then reacts with a diene elastomer to form a rubber-sulfur-accelerator crosslink precursor while also liberating the zinc ion. The final step involves completion of the sulfur crosslink to another rubber diene segment (ref. 9).

References

[9.] Layer, R.W, Elastomerics, May, 1988. [10.] Fath, M.A. and Lederer, D.A., "Precautions in using insoluble sulfur, " Rubber World, 3, 1979. [11.] Helt, W.F., To, B.H. and Paris, W.W., "Post vulcanization stabilization of NR," Rubber World, 8, 1991. [12.] Brydson, J.A., "Rubber Chemistry," Applied Science Publishers, Ltd., London, 1978. [13.] Paris, W.W., "Vulcanization, its activation and acceleration," Education Symposium Rubber Division, ACS, Fall 1982. [14.] Studabaker, M., "Vulcanization of hydrocarbon rubbers," Phillips Chemical Company, Circa 1970. [15.] "Vulkalent E Retarder," Product information Bulletin, Miles, Inc., 1987. [16.] "General catalog for peroxides and specialty chemicals," Pennwalt Chemicals Company, Buffalo, N. Y.