Natural rubber accelerated sulfur vulcanization

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

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. 

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 7.10 Improvements in the accelerated-sulfur vulcanization of natural rubber. 

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). 

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... 

TTie chemistry of the accelerated vulcanization of BR, SBR, and EPDM appears to have much in common with the vulcanization of natural rubber Before the formation of crosshnks, the rubber is first sulfurated by accelerator-derived polysulfides (Ac-S -Ac) to give macromolecular, polysul-fidic intermediates (rubber-Sx-Ac). However, whereas in the case of MBTS-or benzothiazolesulfenamide-accelerated sulfur vulcanization of natural rubber, MBT is given off during the formation of rubber-Sx-BT from the attack of rubber by BT-S -BT, in the case of BR and SBR, MBT is not eliminated and remains unextractable presumably because it becomes bound as the macromolecular thioether rubber-S-BT. (BT is a 2-benzothiazolyl group.) As in the case of natural rubber, the average length of a crosslink (its sulfidic rank, the value of x in the crosslink, rubber-Sx-rubber) increases with the ratio of sulfur concentration to accelerator concentration (S/Ac) used in the... 

Accelerated-Sulfur Vulcanization of Various Unsaturated Rubbers. Over the years, much of the research on accelerated-sulfur vulcanization was done by using natural... 

Manik, S. R, Baneijee S. (1970). Sulfenamide Accelerated Sulfur Vulcanization of Natural Rubber in Presence and Absence of Dicumyl Peroxide. Rubber Chem. Technol, 43(6), 1311-1326. 

The now generally accepted reaction scheme for the course of accelerated sulfur vulcanization which originally was proposed for natural rubber [116], is an interplay of the active rubber sites, i.e., methylene groups adjacent to main chain carbon-carbon double bonds, sulfur, the accelerator, and, usually, zinc oxide over different stages. Evidence for some rubber-bound intermediates has been obtained [123-125]. However, there is as yet still a debate on the chemical nature of the active sulfurating agent. 

The accelerated sulfur vulcanization of m-polyisoprene and natural rubber [66] has also been studied. Three different accelerators were used tetramethylthiruam disulfide (TMTD), A/ -oxydiethylene-2-benzothiazole sulfenamide (MOR), and N-cy-clohexyl-benzothiazole-2-sulfenamide (CBS). The NMR peaks that appeared with the 3 different accelerators were found to give similar peaks as in unaccelerated sulfur cured samples. The differences in network structure were reflected in differences in the relative peak intensities between the sulfur and accelerated sulfur cures as well as differences between the 3 accelerator cured samples. Varying the accelerator-sulfur ratio also produced changes in peak intensities. Examination of vulcanizations with and... 

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

These conclusions have been confirmed by Wood and Roth, who carried out measurements at both constant lengths and at constant elongations using natural rubber vulcanized with sulfur and an accelerator. Their results at constant elongation, to be considered later in connection with the thermodynamics of rubber elasticity at higher elongations, are summarized in Fig. 89. 

Rubber. The rubber industry consumes finely ground metallic selenium and Selenac (selenium diethyl dithiocarbamate, R. T. Vanderbilt). Both are used with natural rubber and styrene—butadiene mbber (SBR) to increase the rate of vulcanization and improve the aging and mechanical properties of sulfurless and low sulfur stocks. Selenac is also used as an accelerator in butyl mbber and as an activator for other types of accelerators, eg, thiazoles (see Rubber chemicals). Selenium compounds are useful as antioxidants (qv), uv stabilizers, (qv), bonding agents, carbon black activators, and polymerization additives. Selenac improves the adhesion of polyester fibers to mbber. 

A good elastomer should not undergo plastic flow in either the stretched or relaxed state, and when stretched should have a memory of its relaxed state. These conditions are best achieved with natural rubber (ds-poIy-2-methyl-1,3-butadiene, ds-polyisoprene Section 13-4) by curing (vulcanizing) with sulfur. Natural rubber is tacky and undergoes plastic flow rather readily, but when it is heated with 1-8% by weight of elemental sulfur in the presence of an accelerator, sulfur cross-links are introduced between the chains. These cross-links reduce plastic flow and provide a reference framework for the stretched polymer to return to when it is allowed to relax. Too much sulfur completely destroys the elastic properties and produces hard rubber of the kind used in cases for storage batteries. 

The reactions of alkenes with elemental sulfur is of great importance because of analogies to the vulcanization of natural or synthetic rubber by sulfur in the presence of various catalysts (accelerators). This technical process, and the related model reactions of simple alkenes with sulfur, will be described in Section 7. 

A larger amount of sulfur added to natural rubber (20-30%) generates a different product, vulcanite. Besides sulfur, during the vulcanization process other chemical compounds are commonly added to rubber. One group of such compounds consists of vulcanization accelerators (A in the previous scheme). Substances such as diphenylguanidine, mercaptobenzothiazole, tetramethythiuram disulfide, N-oxydiethylene-2-benzothiazolylsulfenamide, and cyclohexylbenzothiazolylsulfenamide are utilized as accelerators. 

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