Curing, rubber sulfur vulcanization

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 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). 

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 rubbers. 

In dissimilar rubber/rubber blends, vulcanization ingredients also preferentially end up in only one of the phases, resulting in overcuring of this phase and under-curing of the other one. An example of this effect is the solubility of insoluble sulfur, which is much lower for NBR than for SBR and EPDM. 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

Vulcanization by the traditional curing methods utilized in the rubber industry is based on sulfur or sulphur-containing compounds and is also the most widely applied technique for curing liquid rubbers containing macromolecules with double bonds. Chinons, peroxides, phenol formaldehyde resins, and other compounds are often used for curing liquid rubbers as an alternative sulfur vulcanization. 

In manufacture, the abrasive grain is mixed with crude rubber, sulfur, and other ingredients for curing, then passed through calender rolls to produce a sheet of desired thickness. The wheels are stamped from this sheet and heated under pressure to vulcanize the rubber. 

Although the sulfur vulcanization of the rubber was a complex chemical process "Equation 3", we found that the overall rate of cure was given by a single first-order reaction ... 

Devulcanization in a sulfur cured rubber is defined as the cleavage of the mono-, di-, and polysulfidic cross-links formed during vulcanization (Fig. 1). The vulcanization process is irreversible and additional heating induces changes in the network with a shift toward shorter cross-links but does not devulcanize the compound. Other methods are therefore needed to induce devulcanization. 

Lubrication of the test pieces is now standard practice in order to eliminate one obvious source of variation. The more uniform flattening of the test piece also eases measurement of thickness after release from compression. However, there remain specifications in which set is determined in the absence of lubricants. It has also become common practice with general-purpose rubbers to measure compression. set after just one day at 70°C, which for sulfur-vulcanized elastomers can be a. sensitive measure of the state of cure. Higher test temperatures are specified for special-purpose and speciality synthetic rubbers, but the one-day test has remained popular, not least as a cla.ssification criterion and grade requirement in such specifications as ASTM D2000 and the British Standard series of material specifications for individual rubber types. Tests seldom last more than seven days, and recovery is usually confined to the standard. 30 minutes after release, during which time the test piece cools to standard laboratory temperature if taken from an oven. The short-term nature of the test and the absence of isothermal conditions during recovery has been questioned by Birley and other workers [43]. 

A comparison of polychloroprene and natural rubber or polyisoprene molecular structures shows close similarities. However, while the methyl groups activates the double bond in the polyisoprene molecule, the chlorine atom has the opposite effect in polychloroprene. Thus polychloroprene is less prone to oxygen and ozone attack than natural rubber is. At the same time accelerated sulfur vulcanization is also not a feasible proposition, and alternative vulcanization or curing systems are necessary. 

FIGURE 3.11 Rheometer ta-que normalized by the fully cured plateau value versus the gel content during sulfur vulcanization of three rubbers (Zhang et al., 2010). 

Since those early days, there has been continued progress toward the improvement of the process and in the resulting vulcanized rubber articles. In addition to natural rubber, over the years, many synthetic rubbers have been introduced. Also, in addition to sulfur, other substances have been introduced as components of curing (vulcanization) systems. This chapter is an overview of the science and technology of vulcanization. Emphasis is placed on general-purpose high-diene rubbers for example, natural mbber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR), vulcanized by sulfur in the presence of organic accelerators. 

There are obvious differences between accelerated vulcanization and unaccelerated vulcanization. (Greater crosslinking efficiencies and greater crosslinking rates are obtained with accelerated vulcanization.) But there are more subtle differences. Results from model reactions with curing ingredients indicate that sulfur becomes attached to the rubber hydrocarbon almost exclusively at allylic positions (Skinner, 1972). This is not the case with unaccelerated-sulfur vulcanization, thus ... 

Over the years, much of the research on accelerated-sulfur vulcanization was done by using natural rubber as a model substrate. Natural rubber was the first elastomer and therefore the search for the understanding of vulcanization originated with work on natural rubber. Most of the work cited in the previous sections is related to natural rubber. However, some rather early studies have been directed to the vulcanization of butadiene 1,4-polymers (Skinner and Watson, 1969 Wolfe et al, 1329 Gregg and Katrenick, 1970). More recent is the work of Pellicioli and coworkers. Early basic work on the vulcanization of ethylene-propylene-diene-monomer rubber (EPDM) has been carried out (van den Berg et al., 1984a,b). Recently, Kuno and coworkers did basic work on EPDM networks. They found that, essentially, the vulcanizate properties depend only on the crosslink density, not on the type of curing system (Dijkhuis et al., 2009). 

Natural rubber (NR)/SBR blends exhibit improved oxidative stability compared to pure component and their mechanical properties could be improved by vulcanization. Manshaie et al. compared NR/SBR cured blends either by electron beam irradiation or by sulfur vulcanization. The irradiated blends have better mechanical properties and better heat stability than those cmed by a sulfur system. The irradiated blends exhibited higher tensile strength, hardness, and abrasion resistance than nonirradiated ones. However, cross-linking provokes the decrease in elongation at break and resilience. 

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. 

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