Vulcanization, accelerated-sulfur chemistry

G. Heideman, R.N. Datta, J.W.M. Noordermeer, and B. van Barle, Activators in accelerated sulfur vulcanization. Rubber Chemistry and Technology, 77(3), 512-541, 2004. 

TTie chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears to have much in common with the vulcanization of natural rubber Before the formation of crosshnks, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-S -Ac) to give macromolecular, polysul-fidic intermediates (rubber-Sx-Ac). However, whereas in the case of MBTS-or benzothiazolesulfenamide-accelerated sulfur vulcanization of natural rubber, MBT is given off during the formation of rubber-Sx-BT from the attack of rubber by BT-S -BT, in the case of BR and SBR, MBT is not eliminated and remains unextractable presumably because it becomes bound as the macromolecular thioether rubber-S-BT. (BT is a 2-benzothiazolyl group.) As in the case of natural rubber, the average length of a crosslink (its sulfidic rank, the value of x in the crosslink, rubber-Sx-rubber) increases with the ratio of sulfur concentration to accelerator concentration (S/Ac) used in the... 

Rubber Chemistry and Technology 70, No.3, July/Aug.1997, p.368-429 THHJRAM-AND DITHIOCARBAMATE-ACCELERATED SULFUR VULCANIZATION FROM THE CHEMIST S PERSPECTIVE METHODS, MATERIALS AND MECHANISMS REVIEWED... 

A. B. Sullivan, C. J. Hann, and G. H. Kuhls, "Vulcanization Chemistry—Fate of Elemental Sulfur and Accelerator during Scorch Delay as Studied by Modem HPLC", Paper No. 9, presented at th eMCS Rubber Division Meeting, Toronto, Canada, May 21 —24, 1991, American Chemical Society, Washington, D.C., 1991. 

Initially, vulcanization was accomplished by using elemental sulfur at a concentration of 8 parts per 100 parts of rubber (phr). It required 5 h at 140°C. The addition of zinc oxide reduced the time to 3 h. The use of accelerators, in concentrations as low as 0.5 phr, has since reduced the time to as short as 1-3 min. As a result, elastomer vulcanization by sulfur without accelerator is no longer of much commercial interest. (An exception to this is the use of about 30 or more phr of sulfur, with little or no accelerator to produce molded products of hard mbber or ebonite. ) Even though unaccelerated-sulfur vulcanization is not of commercial significance, its chemistry has been the object of much of the early research and study. 

The process that makes the chemistry, properties, and applications of elastomers so different from other polymers is cross-linking with sulfur, commonly called vulcanization. The modem method of cross-linking elastomers involves using a mixture of sulfur and some vulcanization accelerator. Those derived from benzothiazole account for a large part of the market today. Temperatures of 100-160°C are typical. 

The chemistry of the vulcanization of rubber is complex. The reaction of rubber with sulfur is markedly expedited by substances called accelerators, of which those commonly known as mercaptobenzothiazole and tetramethyl-thiuram disulfide are examples ... 

Vulcanization, named after Vulcan, the Roman God of Fire, describes the process by which physically soft, compounded rubber materials are converted into high-quality engineering products. The vulcanization system constitutes the fourth component in an elastomeric formulation and functions by inserting crosslinks between adjacent polymer chains in the compound. A typical vulcanization system in a compound consists of three components (1) activators (2) vulcanizing agents, typically sulfur and (3) accelerators. The chemistry of vulcanization has been reviewed elsewhere in this text. It is appropriate, however, to review each of these components within the context of developing a compound for a defined service application. 

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