Vulcanization, accelerated-sulfur

Vulcanization by heating with sulfur alone is a very inefficient process with approximately 40-50 sulfur atoms incorporated into the polymer per crosslink. Sulfur is wasted by the formation of long polysulfide crosslinks (i.e., high values of m in XHI), vicinal crosslinks (XIV), and intramolecular cyclic sulfide structures (XV). (Structures XIV and XV do not contribute significantly to the physical properties of the polymer.) 

Commercial sulfur vulcanizations are carried out in the presence of various additives, referred to as accelerators, which greatly increase the rate and efficiency of the process 

Studies with model alkenes indicate that the action of accelerators is to increase the extent of sulfur substitution (crosslinking) at the allylic positions of the diene polymer [Skinner, 

The mechanism of vulcanization by XVI involves the initial formation of 2,2 -dithio-bisbenzothiazole (XVII) via cleavage of XVI to 2-mercaptobenzothiazole followed by oxidative coupling. 2,2 -Dithiobisbenzothiazole reacts with sulfur to form the accelerator 

Crosslinking occurs by the corresponding reaction between XIX and the polydiene  

Organic chemical accelerators were not used until 1906 (65 years after the Goodyear-Hancock development of unaccelerated vulcanization [Fig. 8]), when the effect of aniline on sulfur vulcanization was discovered by Oenslager [11]. This could have been, at least partially, in response to the development of pneumatic tires and automobiles near the turn of the century. Aniline, however, is too toxic for use in rubber products. Its less toxic reaction product with carbon disulfide, thiocarbanilide, was introduced as an accelerator in 1907. Further developments lead to guanidine accelerators [12]. Reaction products formed between carbon disulfide and aliphatic amines (dithiocarba-mates) were first used as accelerators in 1919 [13]. These were and are still the most active accelerators with respect to both crosslinking rate and extent of crosslink formation. However, most of the dithiocarbamate accelerators give little or no scorch resistance and their use is impossible in many factoryprocessing situations. The first delayed-action accelerators were introduced in 1925 with the development of 2-mercaptobenzothiazole (MET) and 2- 

FIGURE 10 Improvements in the accelerated-sulfur vulcanization of natural rubber. 

Accelerated-sulfur vulcanization is the most widely used method. For many applications, it is the only rapid crossUnking technique that can, in a practical manner, give the delayed action required for processing, shaping, and forming before the formation of the intractable vulcanized network. It is used to vulcanize natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber (HR), chlorobutyl rubber (ClIR), bromobutyl rubber (BUR), and ethylene-propylene-diene-monomer rubber (EPDM). The reactive moiety for all of these elastomers can be represented by 

Typically a recipe for the vulcanization system for one of the above elastomers contains 2-lOphr of zinc oxide, 1-4phr of fatty acid (e. g., stearic), 0.5-4phr 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 that can form complexes with accelerators and reaction products formed between accelerators and sulfur. (Accelerators are classified and illustrated in Table I.) 

Frequently, mixtures of accelerators are used. Typically, a benzothiazole type is used with smaller amounts of a dithiocarbamate (thiuram) or an amine type. An effect of using a mixture of two different types of accelerator can be that each activates the other and better-than-expected crosslinking rates can be obtained. Mixing accelerators of the same type gives intermediate or average results. 

---

The accelerated sulfur vulcanization of general-purpose diene rubbers (e.g., NR, styrene-butadiene rubber [SBR], and butadiene rubber [BR]) by sulfur in the presence of organic accelerators and other rubbers, which are vulcanized by closely related technology (e.g., ethylene-propylene-diene monomer [EPDM] mbber, butyl rubber [HR], halobutyl mbber [XIIR], nitrile rubber [NBR]) comprises more than 90% of all vulcanizations. 

Accelerated sulfur vulcanization is the most widely used method. This method is useful to vulcanize NR, SBR, BR, HR, NBR, chloroprene mbber (CR), XIIR, and EPDM mbber. The reactive moiety present in aU these mbbers is... 

FIGURE 14.3 Development of accelerated sulfur vulcanization of natural rubber (NR). (From A.Y. Coran, Chem. Tech., 23, 106, 1983.)... 

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. 

G. Heideman, R.N. Datta, J.W.M. Noordermeer, and B. van Barle, Activators in accelerated sulfur vulcanization. Rubber Chemistry and Technology, 77(3), 512-541, 2004. 

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene... 

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. 

By using this method, the chemical shifts of the resonances in the spectra of a sulfur vulcanized natural rubber (Fig. 32 expanded aliphatic region in shown in Fig. 33 [top]) are assigned to various units of the polymer network, which arise from structural modifications induced by the vulcanization 194,196 200). Different sulfidic structures are found for unaccelerated and accelerated sulfur vulcanizations, respectively. With increasing amount of accelerator (as compared to the sulfur), the network structure exhibits less crosslinking, fewer main chain structural modifications, and fewer cyclic sulfide structures 197). 

Manik, S.P. Banerjee, S. Sulfenamide accelerated sulfur vulcanization of natural rubber in presence and absence of dicumyl peroxide. Rubber Chem. Technol. 1970, 40, 1311. 

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. 

In order to explain the rather long delay period frequently enconntered in accelerated sulfur vulcanization, especially wherein thiazole sulfonamides or other scorch delay accelerators are nsed, the following scheme has been proposed ... 

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. 

Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers... 

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). 

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 ... 

PREPARATION The polymer obtained by ring-opening polymerization of norbomene. Both ds and trans structures may result. Polymer is typically free of oligomers and macrocycles. Cross-linking Ccm occur by conventional accelerated sulfur vulcanization. ... 

Hazardous Decomp. Prods. Heated to decomp., emits toxic vapors of NOx and SOx Uses Prevulcanization inhibitor, retarding agent delays onset of accelerated sulfur vulcanization... 

Common Accelerators Used for Accelerated Sulfur Vulcanization... 

Sulfur vulcanization can be divided into two main categories unaccelerated and accelerated sulfur vulcanization. Unaccelerated formulations typically consist of sulfur, zinc oxide, and a fatty acid such as stearic acid, while accelerated formulations include an accelerator in the system. A subcategory of accelerated... 

---