Activators for Sulfur Vulcanization
Typical rubber vulcanization systems consist of rubber, sulfur accelerator, metal oxide and fatty acid, where the last two ingredients represent the activator. They are important rubber processing additives that not only activate cure but also improve the efficiency of sulfur based cure systems. In fact, almost all organic accelerators require the addition of an organic activator to achieve the desired cure and end-use properties.
The most common activator is zinc fatty acid ester which is often formed in-situ by reaction of fatty acid with zinc oxide. The most common fatty acids include stearic, lauric, palmitic, oleic and naphthenic acid. The fatty acid solubilizes the zinc and forms the actual catalyst.1 The zinc oxide can also act as a filler or white colorant in rubber products whereas the fatty acid improves filler incorporation and dispersion by wetting the oxide particles and reducing interfacial tension (wetting agent). The addition of activators in combination with secondary alkaline accelerators also allows for a more controlled onset of cure.
The mechanism of zinc catalyzed sulfur vulcanization is very complex and is often not fully understood.2 The main catalyst is zinc whereas the fatty acid functions as a solubilizing agent for the zinc which forms a complex with sulfur in the accelerator-polysulfide or is covalently bonded to sulfur atoms in the accelerator molecules as shown below. Both the solubility and reactivity increases if the zinc coordinates with an amine or amide, for example with sulfenamide. The chelated amines increase the nucleophilicity of the sulfur in the polysulfide complex and thereby increase the reaction rate of precursor formation. The exact position where zinc complexes in the accelerator complex is often unknown.2 Three possible structures of a Zn - polythiobis(benzothiazole) complex are shown below. The position of the zinc in the accelerator complex affects both the reaction path and the product distribution.
The accelerator complex plays an important role in both the insertion of sulfur atoms into the complex and in the formation of initial polysulfidic crosslinks.2 As it is the case with other catalysts, remarkable small quantities of solubilized zinc are needed to initiate and speed up the vulcanization process.
In the case 2-2’-dithiobis(benzothiazole) (MBTS), the zinc is assumed to complex with the nitrogen atom of a benzothiazole ring4 as shown below.
The zinc catalyst lowers the energy of dissociation of the sulfur bonds and allows for faster insertion of sulfur molecules (cyclic S8) into the polysulfide sulfurating agent which then reacts with rubber to form a crosslink precursor and 2-mercapto-benzothiazole (MBT). The later reacts with another MBT molecule in the presence of zinc to form another MBTS-zinc complex. Assuming a radical mechanism6, the cross-link precursor cleaves homolytically into a rubber-polysulfide and polysulfidic benzothiazole radical. The later reacts with rubber to form a new cross-link precursor whereas the polymeric persulfenyl radicals either combine or react with other rubber molecules to form sulfur bridges.
The crosslinks that are formed initially are predominantly polysulfides. During post cure or service life, these polysulfidic crosslinks can degrade (desulfurate) to more stable mono or disulfidic crosslinks which changes the original rubber properties markedly, and/or the polysulfidic crosslinks degrade to elastically ineffective cyclic sulfides or pendant groups. The reaction rate of these post-vulcanization reactions is higher for longer sulfur bridges, since the S-S bonds are weaker when the crosslinks are longer.
References and Notes
- A.M. Joseph, B. George, K.N. Madhusoosaban, and R. Alex, Rubber Science, 28(1), 82-121 (2015)
- P.J. Nieuwenhuizen, A.W. Ehlers, J.W. Hofstraat, S.R. Janse, M.W.F., Nielen, J. Reedijk, and E.-J. Baerends, Chem. Eur. J., Vol. 4, No. 9, pp. 1816-1821 (1998)
- P. Ghosh, S. Katare, P. Patkar, J.M. Caruthers, V. Venkatasubramanian, and K.A. Walker, Rubber Chem. Technol., Vol. 76, No. 3, 592-693 (2003)
- A. Y. Coran, Rubber Chem. Technol. 38, 1 (1965)
In the absence of the zinc catalyst, the sulfur bond at or near the center of the polysulfide precursor is more likely to split because the S-S bond strength decreases with increasing distance from the terminal organic groups (benzothiazole)3
Coran suggested a non-radical mechanism for crosslink-precursor formation that involves a six-membered transition state with MBT as a byproduct.3,7
- A. Y. Coran, in The Science and Technology of Rubber, Chapter 7 - Vulcanization, pp 321-366 (2005)