Agents vulcanizing

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. 

Accelerators are products that increase both the rate of sulfur crosslinking in a rubber compound and crosslink density. Secondary accelerators, when added to primary accelerators, increase the rate of vulcanization and degree of crosslinking, with the terms primary and secondary being essentially arbitrary. A feature of such binary acceleration systems is the phenomenon of synergism. Where a combination of accelerators is synergistic, its effect is always more powerful than the added effects of the individual components. 

Rate of vulcanization Ultra-accelerators include dithiocarbamates and xanthates. Semiultra-accelerators include thiurams and amines. Fast accelerators are thiazoles and sulfonamides. A medium-rate system is diphenylguanidine. A slow accelerator is thiocarbanilide. 

Chemical classifications Most accelerators fall into one of eight groups aldehydeamines, sulfonamides, thioureas, dithiocarbamates, guanidines, thiurams, thiazoles, and xanthates. 

Factors involved in the selection of vulcanization systems must include the type of elastomer, type and quantity of zinc oxide and fatty acid, rate of vulcanization, required resistance to fatigue, and service conditions. It is also recommended that use of nitrosamine-generating accelerators be avoided. 

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Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

Gut Rubber. To produce cut mbber thread, smoked mbber sheet or crepe mbber is milled with vulcanizing agents, stabilizers, and pigments. This milled stock is calendered into sheets 0.3—1.3 mm thickness, depending on the final size of the mbber thread desired. Multiple sheets are layered, heat-treated to vulcanize, then sHt into threads for textile uses (Fig. 2). Individual threads have either square or rectangular cross-sections. 

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

Sulfur is the almost universal, primary vulcanizing agent for all elastomers, whether in latex or dry form. Less sulfur is normally required for latex than for dry mbber compounds. By increasing sulfur concentration to 30—50 parts by weight (30—50 parts/100 parts dry mbber content (DRC)) in latex compound, satisfactory hard mbber articles, particularly mbber—metal coatings, are obtained. 

The vulcanizing agent, which supplies the bridge between the polymer chains, is stiU furnished predominantly by the sulfur molecule in commercial... 

The reaction with morpholine is used to make a commercial vulcanizing agent. 

Reaction with anilines yields the Herz compounds, which are intermediates for certain sulfur dyes (qv) as well as vulcanizing agents for mbber (149). 

A similar reaction with phenols is employed to make commercial vulcanizing agents and antioxidants (see Antioxidants Antiozonants). 

AH of the polyether elastomers, like other vulcanizable elastomers, can be compounded with processing aids, fillers, plasticizers, stabilizers, and vulcanizing agents to make useful mbber products. A typical compounding recipe for epichlorohydrin elastomer is as follows ... 

Curing Systems. The most commonly used vulcanizing agent for the polyethers not containing AGE, that is, ECH and ECH—EO, is 2-mercaptoimidazoline, also called ethylenethiourea [96-45-7]. Other commercially appHed curing agents include derivatives of 2,5-dimercapto-l,3,4-thiadiazole, trithiocyanuric acid and derivatives, bisphenols, diamines, and other substituted thioureas. 

Red lead (Pb304) is manufactured on the 20000-tonne scale annually and is used primarily as a surface coating to prevent corrosion of iron and steel (check oxidation-reduction potentials). It is also used in the production of leaded glasses and ceramic glazes and. very substantially, as an activator, vulcanizing agent and pigment in natural and artificial rubbers and plastics. 

Vulkanisatjons-mittel, n. vulcanizing agent, -probe, /. vulcanizing test or sample, -ver-zdgerung, /. retardation vulcanization. 

Compatibilization along with dynamic vulcanization techniques have been used in thermoplastic elastomer blends of poly(butylene terephthalate) and ethylene propylene diene rubber by Moffett and Dekkers [28]. In situ formation of graft copolymer can be obtained by the use of suitably functionalized rubbers. By the usage of conventional vulcanizing agents for EPDM, the dynamic vulcanization of the blend can be achieved. The optimum effect of compatibilization along with dynamic vulcanization can be obtained only when the compatibilization is done before the rubber phase is dispersed. 

Ha [29] has shown that in PP-HDPE (of high-den-sity PE) dynamically cured EPDM, the cured EPDM act as a compatibilizer to the HDPE-PP system. Blending was done in two ways. EPDM was cured first and then blended with PP and HDPE. In the second case, EPDM was cured in the presence of PP and HDPE using dicu-myl peroxide (DCP) as the vulcanizing agent. In EPDM-rich composition, mechanical properties were increased by increasing the concentration of DCP, whereas in PP-rich composition, the reverse was the case. 

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

When two polymers interact or react with each other, they are likely to provide a compatible, even a miscible, blend. Epoxidized natural rubber (ENR) interacts with chloro-sulfonated polyethylene (Hypalon) and polyvinyl chloride (PVC) forming partially miscible and miscible blends, respectively, due to the reaction between chlorosulfonic acid group and chlorine with epoxy group of ENR. Chiu et al. have studied the blends of chlorinated polyethylene (CR) with ENR at blend ratios of 75 25, 50 50, and 25 75, as well as pure rubbers using sulfur (Sg), 2-mercapto-benzothiazole, and 2-benzothiazole disulfide as vulcanizing agents [32]. They have studied Mooney viscosity, scorch... 

Aside from the sulfur, the sulfur bearing compounds that can liberate sulfur at the vulcanization temperature can be used as vulcanizing agents. A few sulfur donors are given in Table 14.5, which include some compounds like dithiodimorpholine (DTDM), which can directly substitute sulfur. Others, like tetramethylthiuramdisulhde (TMTD), can act simultaneously as vulcanization accelerators. The amount of active sulfur, as shown in Table 14.5, is also different for each compound. Sulfur donors may be used when high amount of sulfur is not tolerated in the... 

Some special vulcanizing agents can vulcanize diene rubbers such as NR, SBR, and BR. They are described in the following sections. 

A high diene rubber can also be vulcanized by the action of a dinitrosobenzene, made in situ by the oxidation of a quinonediooxime (Figure 14.13) [56-60] incorporated into the rubber together with the vulcanizing agent lead peroxide. 

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