Vulcanization system accelerators

Novor. [Akrochem] Urethane vulcanizing system accelerator. 

Novmr. [Akrohem] UreAane vulcanizing system accelerator. 

New efficient vulcanization systems have been introduced in the market based on quaternary ammonium salts initially developed in Italy (29—33) and later adopted in Japan (34) to vulcanize epoxy/carboxyl cure sites. They have been found effective in chlorine containing ACM dual cure site with carboxyl monomer (43). This accelerator system together with a retarder (or scorch inhibitor) based on stearic acid (43) and/or guanidine (29—33) can eliminate post-curing. More recently (47,48), in the United States a proprietary vulcanization package based on zinc diethyldithiocarbamate [14324-55-1]... 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

Sulfur vulcanization leads to a variety of cross-link structures as shown in Figure 1. All the sulfur does not result in cross-links some of it remains as pendent accelerator polysulfide groups and internal cyclic polysulfides. These alternative structures do not contribute to load bearing or strength properties and are more prevalent in unaccelerated or weakly accelerated vulcanization systems. Additional heating can also reduce the polysulfide rank of the cross-links. In some elastomers, this leads to a larger number of cross-links. However, in natural mbber or its synthetic polyisoprene equivalent, the overall result is a loss of cross-links, especially at temperatures over 160°C. 

At the present time, an accelerated sulfur vulcanization system is used for RubCon curing. This system consists of sulfurs as the structuring agent of vulcanization, tetramethylthiuram disulfide and 2-mercaptobenzothiazole as accelerators, and zinc oxide as the activator of this process. 

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. 

Typically a recipe for the vulcanization system for one of these elastomers contains 2-10 phr of zinc oxide, l phr of fatty acid (e.g., stearic), 0.5-4 phr of sulfur, and 0.5-2 phr of accelerator. Zinc oxide and the fatty acid are vulcanization-system activators. The fatty acid with zinc oxide forms a salt, which can form complexes with accelerators and reaction products, formed between accelerators and sulfur. Accelerators are classified and illustrated in Table 7.1. 

This type of vulcanization-system design was reported by McCall (1969). He found that by judiciously balancing the levels of accelerator, sulfur, and DTDM, he could obtain good vulcanization characteristics, good thermal stability, good flex life, and superior retention of flex life. Others have reported on more recent work on the effects of crosslink type on reversion (Datta et al., 2007 Fan et al., 2001). 

The attack upon rubber molecules by the vulcanization system Can be visualized in a way similar to that which was postulated for the sulfurization of the rubber molecules by the action of accelerated-sulfur vulcanization systems. Reaction schemes for these two types of vulcanization can be written as follows ... 

TABLE 7.4 Recipes for Accelerated-Sulfur Vulcanization Systems ... 

This is another example of what has variously been called a pseudo-Diels-Alder, ene, or no-mechanism reaction (Hoffmann, 1969). It is similar to the reaction written for the attack of rubber molecules by phenolic curatives or the in situ formed nitroso derivative of the quinoid (e.g., benzoquinonedioxime) vulcanization system. It is also closely related to the sulfurization scheme written for accelerated-sulfur vulcanization. Comparisons between accelerated sulfur, phenolic, quinoid, and maleimide vulcanization can then be visualized as follows ... 

Vulcanization system components Sulfur, accelerators, activators. 

High-tensile-strength butyl compounds generally use FEF- or GPF-grade carbon blacks. Vulcanization systems tend to be based on thiazole accelerators such as mercaptobenzothiazole disulfide (MBTS) and thiuram accelerators such as tetramethylthiuram disulfide (TMTD). Low-tensile-sfrengfh compounds will use a clay or silica reinforcing filler in place of carbon black. 

Thermo-oxidative stability is primarily a function of the vulcanization system. Peroxide vulcanization or cure systems tend to perform best for reversion resistance as a result of the absence of sulfur and use of carbon-carbon crosslinks. Efficient vulcanization (EV) systems that feature a low sulfur level (0.0-0.3 phr), a high acceleration level, and a sulfur donor similarly show good heat stability and oxidation resistance. Such systems do, however, have poor resistance to fatigue because of the presence of predominantly monosulfidic crosslinks. Conventional cure systems that feature a high sulfur level and low accelerator concentration show poor heat and oxidation resistance because the polysulfidic crosslinks are thermally unstable and readily oxidized. Such vulcanization systems do, however, have better fatigue resistance. Semi-EV cure systems, which are intermediate between EV and conventional systems, are a compromise between resistance to oxidation and required product fatigue performance. 

Vulcanization, named after Vulcan, the Roman God of Fire, describes the process by which physically soft, compounded rubber materials are converted into high-quality engineering products. The vulcanization system constitutes the fourth component in an elastomeric formulation and functions by inserting crosslinks between adjacent polymer chains in the compoimd. A typical vulcanization system in a compound consists of three components (1) activators (2) vulcanizing agents, typically sulfur and (3) accelerators. 

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. 

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. 

Design of the vulcanization system is probably the most challenging aspect in developing a compound formulation for application in a tire. Knowledge of accelerator activity, reaction kinetics, and nature of the resulting crosslink network is important in constructing such systems. 

For semi-efficient vulcanization systems, intermediate sulfur level of 1-2 phr and 2.5-1 phr of accelerator are often used. The vulcanizates have physical properties intermediate between those of conventional vulcanization and efficient vul-canizationvulcanizates. hi fact, they give some improvements in reversion, ageing resistance and compression set compared with conventional vulcanizationvulca-nizates, but resistance to fatigue and low temperature crystallization is impaired. However, they have higher scorch safely, particularly when sulfenamide accelerators are used in the system. 

TABLE IV Recipes for Accelerated Sultiir Vulcanization Systems"... 

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