Vulcanization unaccelerated

Organic chemical accelerators were not used until 1906, 65 years after the Goodyear-Hancook development of unaccelerated vulcanization (Figure 14.1), when the effect of aniline on sulfur vulcanization was discovered by Oenslayer [3]. 

The chemistry of unaccelerated vulcanization is controversial. Many slow reactions occur over the long period of vulcanization. Some investigators have felt that the mechanisms involved free radicals (Farmer and Shipley, 1946 Farmer, 1947a,b) ... 

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 ... 

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-... 

Accelerated vulcanization by this mechanism gives greater crosslinking efficiencies and rates than unaccelerated vulcanization. Here sulfur becomes attached to the rubber hydrocarbon exclusively at aUyHc positions [99 ]. The presence of fatty acids or zinc ions causes an increased early reaction and leads to the formation of rubber-Sj -Ac [100, 101]. This is because the chelated form of the accelerator is more reactive than the free accelerator during the early reactions. The introduction of a premature vulcanization inhibitor such as N-(cyclohexylthi o)phthalamide(CTP) made it possible to control the rate of crosslink formation [ 102]. 

The vulcanization of polybutadiene rubbers deviates substantially from this reaction pattern, evidently because the bulk of the accelerator becomes irreversibly bound to the rubber at an early stage. This denudes the vulcanizing system of zinc accelerator-thiolate complexes and, therefore, prevents desulfuration from occurring. The crosslinks thus remain di- and poly-sulfidic and are apparently less prone to decomposition than in the case of the polyisoprene rubbers, since cyclic sulfides and conjugated hydrocarbon groupings seem not to be prominent products. The removal of accelerator leaves the system unresponsive to zinc and gives it the characteristics of unaccelerated vulcanization with the result that vicinal crosslinking and crosslinks with saturated chain junctions become important products. 

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. 

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). 

Mukhopadhyay R. and S.K. De. 1978. Effect of elevated temperature on the unaccelerated and accelerated sulphur vulcanization of natural rubber. Rubber Chem. Technol. 51 704-17. 

Initially, vulcanization was accomplished by using elemental sulfur at a concentration of 8 parts per 100 parts of rubber (phr). It required 5 h at 140°C. The addition of zinc oxide reduced the time to 3 h. The use of accelerators, in concentrations as low as 0.5 phr, has since reduced the time to as short as 1-3 min. As a result, elastomer vulcanization by sulfur without accelerator is no longer of much commercial interest. (An exception to this is the use of about 30 or more phr of sulfur, with little or no accelerator to produce molded products of hard mbber or ebonite. ) Even though unaccelerated-sulfur vulcanization is not of commercial significance, its chemistry has been the object of much of the early research and study. 

The sulfur attacks the a position to the double bond in unaccelerated hot vulcanization with rubber and inter and intramolecular cross-linking occurs ... 

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... 

Poly(diene) vulcanization can be carried out hot or cold. In the unaccelerated hot vulcanization at 120-160°C, sulfur attacks the a positions to double bonds and forms inter- and intramolecular cross-links, whereby some of the cis double bonds convert to trans double bonds (see also Section 23.5.3) ... 

For reasons associated with these useless or labile crosslinks, the unaccelerated sulfur vulcanization is commercially inattractive. 

More attractive, however, is the question of what the chemical mechanism of unaccelerated sulfur vulcanization is. A free radical mechanism as first assumed [119-121] had to be abandoned because no evidence was found that free radicals are envolved [116,117] these sulfur-olefin reactions are insensitive to free-radical initiators and do not respond to free-radical retarders or inhibitors. 

The irreversible binding of accelerator moieties by desulfuration or by some other reaction limits the formation of zinc accelerator complexes and subsequently hinders desulfuration of both pendent groups and crosslinks. Although direct evidence for this pathway being responsible is lacking, it does appear that with NR at high temperatures, or with butadiene-based rubbers at all temperatures, removal of the accelerator by combination with the polymer gives the system many of the characteristics of an unaccelerated sulfur vulcanization" (see Section 4.7). 

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... 

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