Sulfur vulcanization

Other Uses. Other uses include intermediate chemical products. Overall, these uses account for 15—20% of sulfur consumption, largely in the form of sulfuric acid but also some elemental sulfur that is used directly, as in mbber vulcanization. Sulfur is also converted to sulfur trioxide and thiosulfate for use in improving the efficiency of electrostatic precipitators and limestone/lime wet flue-gas desulfurization systems at power stations (68). These miscellaneous uses, especially those involving sulfuric acid, are intimately associated with practically all elements of the industrial and chemical complexes worldwide. 

The "smelly shoe" of the elements. The oxidation product S02 has an acrid, burning smell, the reduction product H2S stinks like rotten eggs and is very toxic. Sulfur is, nevertheless, a most useful element. It occurs in elemental form and has therefore been known for a long time is mentioned in the Old Testament. Its main application today is in the production of fertilizers. Considerable amounts of sulfur are used in tire production for vulcanization. Sulfur is also a component of gunpowder. Physiologically indispensable as thioacetic acid and especially the S-S bridges that fix proteins in their shapes (e.g. insulin, but also in perms). A 70-kg human being contains 140 g of sulfur. 

The double bonds in isoprene and chloroprene allow these compounds to be vulcanized. In vulcanization, sulfur attaches to the doubly bonded carbon to produce cross-linked chains ... 

Sulfur compounds are used to make rubber strong enough to be used in tires, in a process called vulcanization. Sulfur is also used to make detergent, paper, fertilizers, and medicines. Several sulfur compounds have a strong odor to them. Onions, garlic, skunk spray, and even bad breath get their unique odor from sulfur compounds. Sulfur by itself has no smell. 

The cost from extrusion is due to the capital cost incurred for the machine and dies, and the energy cost to run the ram. Machines can cost upwards of 100,000, and dies can exceed 5,000. When a rubber is vulcanized, sulfur bridges connect individual polymer units, making the overall compound harder and more resistant to chemical attack. This is an irreversible process that creates a thermoset material. For automotive applications, tires are vulcanized and compression molded more than any other component. Figure 5.3 shows this process with the addition of heat. 

Ethylenebis (oxyethylene) bis (3-t-butyl-4-hydroxy-5-methylhydrocinnamate) 1,3,5-Trimethyl-2,4,6-tris (3,5-di-t-butyl-4-hydroxybenzyl) benzene Tris (3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate stabilizer, post vulcanization sulfur cures/IR Disodium hexamethylene bisthiosulfate stabilizer, post vulcanization sulfur cures/NBR Disodium hexamethylene bisthiosulfate stabilizer, post vulcanization sulfur cures/NR Disodium hexamethylene bisthiosulfate stabilizer, post vulcanization sulfur cures/SBR Disodium hexamethylene bisthiosulfate stabilizer, post latex Dicyclohexyl sodium sulfosuccinate Octoxynol-25 Octoxynol-30 Sodium octoxynol-2 ethane sulfonate stabilizer, post-ad synthetic latex Laureth-23... 

The properties of synthetic rubbers can be greatly enhanced by the incorporation of additives like carbon black. Refer to Compounding, Mill Mixing, Carbon Black, Vulcanization, Sulfur Vulcanization, Peroxide Cure, and Half Life. 

Figure 10.9 shows the SEM micrographs of control, dynamically vulcanized (sulfur and HVA-2) HDPE/NR/TPS blends and the blends containing 10 wt%... 

Most EPDM applications involve some sort of crosslinidng [15], about 80% via sulfur vulcanization. Sulfur vulcanizates have a relatively low thermal stability, which explains the slow transition to peroxide cure in critical applications. The tensile and dynamic properties of sulfur-vulcanized EPDM are superior, whereas the elastic recovery of peroxide-cured EPDM is superior. 

Only about 1.2% of sulfur consumption is used in rubber vulcanization. Sulfur is also used in many nonrubber applications, ft is used in agriculture to make fungicides and fertilizers. It is also used to make fumigants for dried fruit and wood pulping. Also, sulfur is basically essential to life itself. In addition, about 90% of the sulfur used today is in the production of sulfuric acid, the largest volume industrial chemical in today s commerce. 

Sulfur is a component of black gunpowder, and is used in the vulcanization of natural rubber and a fungicide. It is also used extensively in making phosphatic fertilizers. A tremendous tonnage is used to produce sulfuric acid, the most important manufactured chemical. 

Originally, vulcanization implied heating natural rubber with sulfur, but the term is now also employed for curing polymers. When sulfur is employed, sulfide and disulfide cross-links form between polymer chains. This provides sufficient rigidity to prevent plastic flow. Plastic flow is a process in which coiled polymers slip past each other under an external deforming force when the force is released, the polymer chains do not completely return to their original positions. 

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. 

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

Historically, the development of the acrylates proceeded slowly they first received serious attention from Otto Rohm. AcryUc acid (propenoic acid) was first prepared by the air oxidation of acrolein in 1843 (1,2). Methyl and ethyl acrylate were prepared in 1873, but were not observed to polymerize at that time (3). In 1880 poly(methyl acrylate) was reported by G. W. A. Kahlbaum, who noted that on dry distillation up to 320°C the polymer did not depolymerize (4). Rohm observed the remarkable properties of acryUc polymers while preparing for his doctoral dissertation in 1901 however, a quarter of a century elapsed before he was able to translate his observations into commercial reaUty. He obtained a U.S. patent on the sulfur vulcanization of acrylates in 1912 (5). Based on the continuing work in Rohm s laboratory, the first limited production of acrylates began in 1927 by the Rohm and Haas Company in Darmstadt, Germany (6). Use of this class of compounds has grown from that time to a total U.S. consumption in 1989 of approximately 400,000 metric tons. Total worldwide consumption is probably twice that. 

As vulcanizing agents, amino acids with or without sulfur are used for nipple mbber of babies botdes and mbbers used in medical appHcations... 

Vulcanization was first reported in 1839 with the discovery that heating natural mbber with sulfur and basic lead carbonate produced an improvement in physical properties (2). In 1906, aniline was the first organic compound found to have the abiUty to accelerate the reaction of sulfur with natural mbber (3). Various derivatives of aniline were soon developed which were less toxic and possessed increased acceleration activity. 

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

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. 

Other Accelerators. Amine isophthalate and thiazolidine thione, which are used as alternatives to thioureas for cross-linking polychloroprene (Neoprene) and other chlorine-containing polymers, are also used as accelerators. A few free amines are used as accelerators of sulfur vulcanization these have high molecular weight to minimize volatility and workplace exposure. Several amines and amine salts are used to speed up the dimercapto thiadiazole cure of chlorinated polyethylene and polyacrylates. Phosphonium salts are used as accelerators for the bisphenol cure of fluorocarbon mbbers. 

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. 

Fig. 1. Stmctures formed during sulfur vulcanization of elastomers.

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

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. 

Activators. Activators are chemicals that increase the rate of vulcanization by reacting first with the accelerators to form mbber soluble complexes. These complexes then react with the sulfur to achieve vulcanization. The most common activators are combinations of zinc oxide and stearic acid. Other metal oxides have been used for specific purposes, ie, lead, cadmium, etc, and other fatty acids used include lauric, oleic, and propionic acids. Soluble zinc salts of fatty acid such as zinc 2-ethyIhexanoate are also used, and these mbber-soluble activators are effective in natural mbber to produce low set, low creep compounds used in load-bearing appHcations. Weak amines and amino alcohols have also been used as activators in combination with the metal oxides. 

The role of activators in the mechanism of vulcanization is as follows. The soluble zinc salt forms a complex with the accelerator and sulfur. This complex then reacts with a diene elastomer to form a mbber—sulfur—accelerator cross-link cursor while also Hberating the zinc ion. The final step involves completion of the sulfur cross-link to another mbber diene segment (18). 

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). 

There are three generally recognized classifications for sulfur vulcanization conventional, efficient (EV) cures, and semiefficient (semi-EV) cures. These differ primarily ki the type of sulfur cross-links that form, which ki turn significantly influences the vulcanizate properties (Eig. 8) (21). The term efficient refers to the number of sulfur atoms per cross-link an efficiency factor (E) has been proposed (20). 

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