Sulfur Without Accelerator

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

The chemistry of unaccelerated vulcanization is controversial. Many slow reactions occur over the long period of vulcanization. Some investigators have felt that the mechanisms involved free radicals [7-9]  

Other investigators have promoted ionic mechanisms [10] R—S —S —R----------- RS/ + RSy 

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Accelerators, e.g. zinc oxide and fatty acids, increase the rate of vulcanisation of rubber by sulfur and they reduce the amount of sulfur required from 10% to <3%. Certain sulfur-donating accelerators, like thiuram disulfides (1) and mercaptobenzothiazole (2), will effect vulcanisation without added sulfur to yield products with greatly enhanced ageing properties.1... 

The efficacy of polyurethane and styrene butadiene rubber (SBR) as binders for ground rubber prepared from waste tires was compared to a formulation of a compound developed without binder. Without binder, the effect of both sulfur and accelerator content on tensile properties are studied, as well as the effect of ageing on these properties [29]. The suggested uses of the unbound product include rubber blocks, and ballast mats for railway applications. 

While free radical attack in step (i) is by no means confined to carbon atom 4, the products obtained in the reactions involving the lower polyisoprenes indicate that this process is the dominant one. Likewise in step (ii) sulfur may frequently add at carbon atom 4 rather than at atom 2. Addition in the manner shown is indicated, however, by infrared spectra, which reveal the formation of —CH=CH— groups during vulcanization. The scheme accounts also for the observed constancy of the C/H ratio during vulcanization and for the relatively low efficiency of utilization of sulfur in the formation of cross-linkages in the absence of accelerators. A preponderance of the sulfur is involved in addition without formation of cross-linkages a considerable fraction of the thus-combined sulfur may occur in five- and six-membered heterocyclic rings formed by the mechanisms indicated. 

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]. 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

Use Vulcanization agent without added sulfur, or as a primary or secondary accelerator with sulfur. 

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