Sulfur vulcanized rubber

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. 

Anderson patented the reclaiming of sulfur vulcanized rubber in the presence of oil, water vapor, and a mixture of aryl disulfides (diphenyl disulfide, dicresyl disulfide, and dixylyl disulfide) at elevated temperature and pressure. " ... 

Saville, B. Watson, A.A. Structural characterization of sulfur-vulcanized rubber networks. Rubber Chem. Technol. 1967, 40, 100. 

The presence of sulfur in polysulfides, polysulfones, and in sulfur-vulcanized rubber can be demonstrated by the following somewhat uncertain test. The sample is heated in dry air (pyrolysis) and the gases formed during this process are bubbled through a dilute barium chloride solution. The presence of sulfur is indicated by a white precipitate of barium sulfate. 

The use of mono- and dimaleimides has enabled rubbers to be cross-linked at appreciably lower doses while at the same time retaining the physical properties of sulfur-vulcanized rubbers. Vale and his co-workers (77, 18) have studied the effect of a range of monomaleimides in vulcanization, the -phenyl maleimide being found to be most efficient. The G value in the presence of 6.28 % of the monomaleimide was 70, and 140 in the presence of 7 % of the />-bromo- -phenyl maleimide. Carbon black was found to enhance the cross-linking of the rubbers. 

Fig. 11. Plot of the energetic (/ e) and entropic (fs) contributions to the stress at 20 C for sulfur-vulcanized rubber (99). N/mm = MPa to convert MPa to psi, multiply by 145. Courtesy of the American Chemical Society.

Saville, B., Watson, A. A. (1967). Stractural Characterization of Sulfur-Vulcanized Rubber Networks. 7 iifcfcerC/ie i. Technol, 40(1), 100-148. 

Chloropropyl, vinyl, and methacrylate-functional silanes on glass were only slightly better than untreated surface for adhesion to most of the rubbers. The mercaptan and amine-functional silanes have been the preferred silanes on fillers in sulfur-vulcanized rubbers. The amine gave better adhesion to natural, nitrile, and Hypalon rubbers, while the mercaptan was better with SBR, Neoprene and EPDM. The iso-thiuronium-functional silane appears to be fairly effective in bonding all rubbers—but especially SBR. Anionic or cationic styryl coupling agents were the best unsaturated silanes and contributed true adhesion to EPDM and Hypalon. 

In contrast to nitrile rubber, fully hydrogenated NBR is not attacked by ozone. Even under extreme conditions (e.g., 2 ppm ozone, 40 °C, 50% rel. humidity, 60% strain, 168 h) no cracks appear. Even under cyclic load these rubbers exhibit excellent ozone crack resistance. Elastomer parts based on HNBR are highly ozone-resistant without the addition of antiozonants. Partially-hydrogenated grades behave differently, depending on the vulcanization system. Ozone resistance quickly decreases with increasing double bond content. However, peroxide vulcanized rubbers behave more favorably than sulfur vulcanized rubbers [697]. 

In general, covalent bonds govern the thermal and photochemical stability of polymers. Bond strength can be used as a clue to degradation mechanisms. For example, sulfur-vulcanized rubber is more likely to degrade at the comparatively weak S—S bonds than at the strong C—C bonds, both of which occur in the structure. 

Ethylene-propylene-diene rubber is polymerized from 60 parts ethylene, 40 parts propylene, and a small amount of nonconjugated diene. The nonconjugated diene permits sulfur vulcanization of the polymer instead of using peroxide. 

Accelerators. During sulfur vulcanization of rubber, accelerators serve to control time to onset of vulcanization, rate of vulcanization, and number and type of sulfur cross-links that form. These factors in turn play a significant role in determining the performance properties of the vulcanizate. 

Ethylene—Propylene (Diene) Rubber. The age-resistant elastomers are based on polymer chains having a very low unsaturation, sufficient for sulfur vulcanization but low enough to reduce oxidative degradation. EPDM can be depicted by the following chain stmcture ... 

Butyl Rubber. Butyl mbber was the first low unsaturation elastomer, and was developed ia the United States before World War II by the Standard Oil Co. (now Exxon Chemical). It is a copolymer of isobutylene and isoprene, with just enough of the latter to provide cross-linking sites for sulfur vulcanization. Its molecular stmcture is depicted ia Table 1. 

Rubber used in practical applications is crosslinked through disulfide (-S-S-) bonds, and is known as vulcanized rubber. Can you name another important class of polymers which are crosslinked through disulfide bonds Examine vulcanized rubber. How many individual strands does it comprise Are these strands of natural rubber or of gutta-percha What is the percentage (by weight) of sulfur incorporated into the polymer (The molecular weight of the sample is 1701 amu.) Does this classify as a low-sulfur polymer (<3%), a high-sulfur polymer (>10%) or in between ... 

Sulfur is of major industrial importance. Most of the sulfur that is produced is used to make sulfuric acid, but an appreciable amount is used to vulcanize rubber (Section 19.12). 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

FIGURE 19.17 The gray cylinders in the small inset represent polyisoprene molecules, and the beaded yellow strings represent disulfide (—S—S—) links that are introduced when the rubber is vulcanized, or heated with sulfur. These cross-links increase the resilience of the rubber and make it more useful than natural rubber. Automobile tires are made of vulcanized rubber and a number of additives, including carbon. 

The accelerated sulfur vulcanization of general-purpose diene rubbers (e.g., NR, styrene-butadiene rubber [SBR], and butadiene rubber [BR]) by sulfur in the presence of organic accelerators and other rubbers, which are vulcanized by closely related technology (e.g., ethylene-propylene-diene monomer [EPDM] mbber, butyl rubber [HR], halobutyl mbber [XIIR], nitrile rubber [NBR]) comprises more than 90% of all vulcanizations. 

FIGURE 14.3 Development of accelerated sulfur vulcanization of natural rubber (NR). (From A.Y. Coran, Chem. Tech., 23, 106, 1983.)... 

Most accelerators used in the accelerated sulfur vulcanization of other high diene rubbers are not applicable to the metal oxide vulcanization of CR. An exception is the use of so-called mixed-curing system for CR, in which metal oxide and accelerated sulfur vulcanization are combined. Along with the metal oxides, TMTD, DOTG, and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

C.S.L. Baker, Non-Sulfur Vulcanization, in Natural Rubber Science and Technology, A. Rober, Eds., Oxford Science, London, 1988. 

Cepeda-Jimenez C.M., Pastor-Bias M.M., Ferrandiz-Gomez T.P., and Martm-Martmez J.M., 2000, Surface characterization of vulcanized rubber treated with sulfuric acid and its adhesion to polyurethane adhesive, J. Adhes., 73, 135-160. 

In 1839, Charles Goodyear discovered that sulfur could cross-link polymer chains and patented the process in 1844 [1]. Since then rubber became a widely usable material. By the year 1853, natural rubber (NR) was in short supply. So attempts were made to undo what Goodyear had accomplished. Goodyear himself was involved in trying to reclaim vulcanized rubber to overcome the shortage of NR. Later, as a consequence of World War I, Germany introduced synthetic rubbers, namely the Buna rubbers, which raised the curiosity of polymer chemists all over the world. Subsequently, synthetic rubbers with tailor-made properties were born. This was followed by the discovery of new methods and chemicals for vulcanization and processing. It is obvious... 

Ic. Cross-Linking of Polymer Chains.—Formation of chemical bonds between linear polymer molecules, commonly referred to as cross-linking, also may lead to the formation of infinite networks. Vulcanization of rubber is the most prominent example of a process of this sort. Through the action of sulfur, accelerators, and other ingredients present in the vulcanization recipe, sulfide cross-linkages are created by a mechanism not fully understood (see Chap. XI). Vulcanized rubbers, being typical network structures, are insoluble in all solvents which do not disrupt the chemical structure, and they do not undergo appreciable plastic, or viscous, flow. 

Various other chemical agents which by their nature are capable of producing cross-linkages between polymer chains effect the same changes in physical properties that are observed in sulfur vulcanization. One of the best known of these agents is sulfur monochloride, which readily combines with two molecules of an olefin (the mustard gas reaction). Applied to rubber, it induces vulcanization even at moderate temperatures, the probable structure of the cross-linkage being... 

Unreacted free sulfur can be determined to ISO 7269 1995 — Rubber — Determination of free sulfur. Three methods for the determination of free sulfur in vulcanized rubber are detailed two versions of the copper spiral method and the sodium sulfite method. The copper spiral methods are also applicable, subject to limitations, to unvulcanised rubber. The technique of DSC can detect non-reacted rubber curatives such as residual peroxides. 

Polychloroprene rubbers are not efficiently vulcanized by sulfur. The chlorine atoms deactivate the double bonds toward reaction with sulfur. Vulcanization is achieved by heating with zinc and magnesium oxides. Crosslinking involves the loss of... 

The materials selected for evaluation included three materials currently being used in these applications Biomer (Thoratec Laboratories Corporation, Emeryville, CA), representative of segmented ether-type polyurethanes Avcothane-51 (Avco Everett Research Laboratory, Inc., Everett, MA), a block copolymer of 10% silicone rubber and 90% polyurethane and Hexsyn (Goodyear Tire and Rubber Company, Akron, OH), a sulfur vulcanized hydrocarbon rubber that is essentially a polyhexene. Also selected, because of their easy availability, were Pellethane (Upjohn Company, North Haven, CT), an ether-type of polyurethane capable of being extruded in sheet form, and a butyl rubber formulation, compounded and molded at the National Bureau of Standards. The material thickness varied, but the sheets were generally about 1 mm thick. 

Used industrially as a chemical intermediate in the production of rayon, carbon tetrachloride, xanthogenates, flotation agents, and pesticides used in the cold vulcanization of vulcanized rubber, in adhesive compositions for food packaging as a solvent for phosphorus, sulfur, selenium, bromine, iodine, fats, resins, rubbers, waxes, lacquers, camphor, resins and in the production of optical glass, paints, enamels, varnishes, paint removers, tallow, putty preservatives, rubber cement, soil disinfectants, explosives, rocket fuel, and electronic vacuum tubes. 

Peachy A process for vulcanizing rubber by successive exposure to hydrogen sulfide and sulfur dioxide. Not commercialized. 

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