Sulfur bloom

Insoluble Sulfur. In natural mbber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of mbber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

Bloom - A discoloration or change in appearance of the surface of a rubber product caused by the migration of a liquid or solid to the surface. (Examples sulfur bloom, wax bloom.) Not to be confused with dust on a surface from external sources. 

Figure 10. (left) Photograph of a flowery exudation of sulfur on concrete immersed in distilled water for 10 months. The basal mound of sulfur crystals is surmounted by a sulfur bloom grown at the tip of a calcite tubule (Magnification X8). (right) Liquid extrusions on dried concrete surface adjacent to earlier sulfur exudation. Same scale. 

Crystex . [Akzo] Sulfur and insoluble sulfur prevents sulfur bloom on uncured rubber surfaces detackifio retards bin scorch minimizes sulfur migration. 

Rubber compounders also learned to live with a certain number of failures. Accelerators and other additives which reduce curing time and prevent sulfur blooming were developed. The possible effects of the various allotropic forms of sulfur in rubber have been reported (18, 19, 20). Although too costly to have gained wide commercial acceptance, organic sulfur compounds make excellent vulcanizing agents for rubber (21). Related compounds cure bituminous mixtures (22). 

Three vulcanizing agents And extensive use in the rubber industry sulfur, insoluble sulfur, and peroxides. The chemistry of peroxides has been reviewed in Chapter 7. Rhombic sulfur is the most common form of sulfur used in the rubber industry and, other than normal factory hygiene and operational procedures, does not require any special handling or storage. Sulfur is soluble in natural rubber at levels up to 2.0 phr. Above this concentration, insoluble sulfur must be used to prevent migration of sulfur to the compound surface that is, sulfur bloom. 

Even though stabilized insoluble sulfur costs significantly more than rubber maker s sulfur, many rubber fabricators feel it is worth it because the insoluble sulfur normally does not bloom, especially for compounds where high levels of sulfur are beneficial for rubber-to-metal bonding, but sulfur bloom might be detrimental. 

In tire production, insoluble sulfur is important in order to avoid sulfur bloom and appearance problems. Also, bloom from uncured rubber might sometimes interfere with rubber-to-rubber adhesion. 

For increased solubiHty to prevent bloom, shorter-chain carboxyHc acids or zinc carboxylates can be substituted. The use of chain-branched carboxyHc acids reduces the tendency for the formulations to lose sulfur cross-links or revert upon prolonged heating (7). Translucent articles such as crepe soles can use a zinc carboxylate or employ zinc carbonate as a transparent zinc oxide. 

Some polysiloxanes are curable with lead monoxide, with a consequent reduction in both curing time and temperature. High-frequency electrical energy vulcanizes in one case at least. Zirconium naphthenate imparts improved resistance to high temperatures. Barium salts are said to prevent blooming. Sulfur dichloride is also used. Some resins are solidified by pressure vulcanization, using di-f-butyl peroxide. Improvements are to be found in lower condensation temperatures and shorter times of treatment... 

Most EPDM applications require crosslinking except when used as an impact modifier for PP, polystyrene (PS) and polyamides or as an oil additive, e.g., as viscosity index improver or dispersant. Most commonly, accelerated sulfur vulcanisation is used for the crosslinking of EPDM. As a result of the low amount of unsaturation in EPDM (< 1 mole/ kg versus NR -15 mole/kg), sulfur vulcanisation of EPDM is rather slow and a relatively large amount of accelerators is needed. Because of the low polarity of EPDM the solubility of polar accelerators is limited, often resulting in low effectivity and/or blooming. Typically, up to 5 different accelerators are used in EPDM formulations. As for other rubbers environmental issues, such as nitrosamine formation and may be in the future the presence of zinc, are prompting the development of new accelerator systems. 

Rozan, T.F., Taillefert, M., Trouwborst, R.E., Glazer, B.T., Ma, S., Herszage, J., Valdes, L.M., Price, K.S., and Luther III., GW. (2002) Iron-sulfur-phosphorus cycling in the sediments of a shallow coastal bay implications for sediment nutrient release and benthic macroalgal blooms. Limnol. Oceanogr. 47, 1346-1354. 

Adding more sulfur atoms to simple molecules like these can intensify the smell even more. The infamous lily titan arum (Amor-phophallus titanum)23] blooms only rarely but when it does it may even make national news, as it did in California in June 2003, when it attracted more than a thousand visitors, and in the UK in April 2005... 

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