Sulfur vulcanization with

Tipova, N. PhD Thesis Investigation of Zinc Stearate as Activator of the Accelerated Sulfur Vulcanization with a view to Zinc Reducing in the Rubber Compoimds , 2006, University of Chemical Technology and Metallurgy, Sofia (in Bulgarian). 

Vulcanization is the most important NR chemical reaction. Most applications require cross-linking via vulcanization to increase resiliency and strength. Exceptions are crepe rubber shoe soles and rubber cements. There are a number of methods for sulfur vulcanization, with certain methods producing polysulfidic cross-linking and other methods producing more monosulfidic cross-links. 

Dithiophosphates. These compounds (13) are made by reaction of an alcohol with phosphoms pentasulfide, then neutralization of the dithiophosphoric acid with a metal oxide. Like xanthates, dithiophosphates contain no nitrogen and do not generate nitrosamines during vulcanization. Dithiophosphates find use as high temperature accelerators for the sulfur vulcanization of ethylene—propylene—diene (EPDM) terpolymers. 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

Films deposited from compounded nitrile latices can be vulcanized with sulfur and accelerators, assisted by relatively high levels (ca 4.0—5.0 parts /100 DRC) of ziac oxide. For other uses, nitrile latices are sometimes used ia unvulcanized form. An appHcation of medium soHds nitrile latex, eg, Nitrex J-6849 and Polysar 845, has been ia preparation of oil-resistant foams for lubricants ia heavy-duty beariags, such as railroad-car journal boxes. 

Sulfurized olefins (S2CI2 plus isobutene) are further reacted with S and Na2S to give products useful as extreme pressure lubricant additives (144,145). The reaction of unsaturated natural oils with sulfur monochloride gives resinous products known as Factice, which are useful as art-gum erasers and mbber additives (146,147). The addition reaction of sulfur monochloride with unsaturated polymers, eg, natural mbber, produces cross-links and thus serves as a means for vulcanizing mbber at moderate temperatures. The photochemical cross-linking of polyethylene has also been reported (148). 

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. 

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

Cross-linking reactions for the polyisobutylene-type polymers depend on adding a reactive site, usually an aHyUc hydrogen or halogen. These reactive sites allow vulcanization with sulfur and accelerators or metal oxides (76,77). 

The product is a random polymer that is vulcanized with sulfur or with metal oxides (zinc oxide, magnesium oxide etc.). Vulcanization with sulfur is very slow, and an accelerator is usually required. 

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. 

The accelerator that has been widely used with metal oxide cures is ethylene thiurea (ETU) or 2-mercaptoimidazoline. Further extensive use of ETU in vulcanization of CR is restricted because of suspected carcinogen. The related compound, thiocarbanahde, used formerly as an accelerator for sulfur vulcanization, has been revived for CR vulcanization other substitute for ETU has been proposed [29,30]. 

Fig. 85.—Force-temperature curves at constant length obtained by Anthony, Gaston, and Guth for natural rubber vulcanized with sulfur for elongations from 3 percent to 38 percent (at 20°C), as indicated.

These conclusions have been confirmed by Wood and Roth, who carried out measurements at both constant lengths and at constant elongations using natural rubber vulcanized with sulfur and an accelerator. Their results at constant elongation, to be considered later in connection with the thermodynamics of rubber elasticity at higher elongations, are summarized in Fig. 89. 

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... 

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... 

Figure 4. Rheological properties of the modeling unfilled rubber mixes on the basis of isoprene rubber with a various type and contents of sulfur vulcanization activators...

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

Figur 5. Rheometer data of sulfur vulcanization process of modeling unfilled elastomeric compositions on the basis of isoprene mbber at 155°C with ZnCFO as the activator ...

It is possible to explain the decrease of ZnCFO efficiency as component of various vulcanization systems for rubbers of general and special assignment in the earlier submitted line (fig. 10) also by character of formed morphology of compositions. So, at use of ZnCFO as the activator of sulfur vulcanization the structure of rubbers with the minimal value of parameter r is formed, and at transition from sulfur to peroxide vulcanization of elastomeric compositions the particles size of heterophase is increased (fig. 11 b). 

The second type of EPM is the EPDM, which is made by polymerizing ethylene, propylene, and a small amount (3-10 mol%) of nonconjugated diolefine employing ZNCs. The side chains allow vulcanization with sulfur. They are employed in the production of appliance parts, wire and cable insulation, coated fabrics, gaskets, hoses, seals, and high-impact PP. 

The absence of strong intermolecular forces, the presence of pendant methyl groups discouraging close association, and crankshaft action associated with the cis isomer all contribute to the flexibility of NR. The introduction of a few cross-links by vulcanization with sulfur reduces slippage of chains but still permits good flexibility. 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene... 

The vulcanization of polychloroprene (Neoprene) is carried out in different ways. Vulcanization by sulfur, even with an accelerator, is not practiced to a large extent. Vulcanizations by metal oxides (without diamine), either alone or in combination with sulfur (sometimes together with an accelerator), give the best physical properties for the crosslinked product. Halogenated butyl rubber is crosslinked in a similar manner. The mechanism for crosslinking by metal oxide alone is not established [Stewart et al., 1985 Vukov, 1984]. 

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