Sulfur-cure accelerators

The peak at 33 ppm is assigned to the trans structure of 1,4-BR. An increasing intensity at 33 ppm peak with cure in both sulfur-cured and accelerated sulfur-cured BR postulates the occurrence of cis-to-trans chain isomerisation in these systems. The resonances at 38 and 50 ppm are assigned to cyclic monosulfide and polysulfidic crosslink structures. The expected monosulfidic junctions are not detected in this study possibly due to the low concentration of these species [33]. 

Cold E-SBRs (those produced at the lower temperatures) contain less long-chain branching than do the so-called hot rubbers. An effect of this is that the cold-process rubbers generally can be more easily processed than the hot-process rubbers. SBRs can be vulcanized by the same types of systems as used for NR. As with NR, accelerated sulfur curing systems are, by far, the most used. 

Since EPR rubber molecules do not contain unsaturation, they can be vulcanized only by organic peroxide curing systems. If a third monomer is added during the polymerization, i.e., a diene monomer (wherein only one of the two double bonds takes part in the polymerization), unsaturation can be introduced into the molecule, and it can then be vulcanized by accelerated sulfur curing systems. A chemical structure for ethylene-propylene-diene-monomer (EPDM) rubbers can be expressed as follows ... 

Sulfenamide accelerators generally require lower levels of fatty acid because they release an amine during the vulcanization process which acts to solubihze the zinc. Guanidines and similar amine accelerators also serve to both activate and accelerate vulcanization. A study of the effect of stearic acid and zinc oxide on a sulfenamide-accelerated, sulfur-cured natural rubber compoimd showed the need for both zinc and fatty acid activators is presented in Figure 11 (52). 

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... 

Sulfur-containing chemicals such as dimorpholinyl disulfide (DTDM) and tetraethylthiuram disulfide (TMTD) are not only effective accelerators, but they can also be used as sulfur donors. As such, they are effective ia controlling sulfur cross-link length to form primarily moao- and disulfide cross-links. These short cross-links are more thermally stable than conventional sulfur curing and thereby provide better heat and set resistance. 

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 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. 

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 mechanism of the accelerated sulfur vulcanisation of EPDM is probably similar to that of the highly unsaturated polydiene rubbers. The vulcanisation of EPDM has been studied with emphasis on the cure behaviour and mechanical and elastic properties of the crosslinked EPDM. Hardly any spectroscopic studies on the crosslinking chemistry of EPDM have been published, not only because of the problems discussed in Section 6.1.3 but also because of the low amount of unsaturation of EPDM relative to the sensitivity of the analytical techniques. For instance, high-temperature magic-angle spinning solid-state 13C NMR spectroscopy of crosslinked EPDM just allows the identification of the rubber type, but spectroscopic evidence for the presence of crosslinks is not found [72]. 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

Model compounds based on 2-methyl-2-pentene were studied to supplement the 13C chemical shift assignments of the products from accelerated sulfur vulcanisation of NR. It is observed in the model compound data that it may not be possible to distinguish between a 13C NMR resonance which is due to disulfidic crosslinks and a peak due to pendent accelerator groups, while a large chemical shift difference ( 3 ppm) is observed for the monosulfidic bonds. The MBS-accelerated sample shows similar new resonances as seen in the TMTD accelerated systems. In this comparison however, the quantitative aspects of the data might be obscured due to the differences in the state of cure among the different formulations. 

The 13C chemical shifts were assigned in more detail for monosulfidic and polysulfidic crosslinks occurring in the accelerated sulfur vulcanisation of NR [18]. The NR was cured with a pure thiuram formulation (TMTD alone) in order to predominantly prepare monosulfidic bridges in the network. The distortionless enhancement by polarisation transfer (DEPT) experiments, in which the carbons with different level of protonation can be distinguished [22-24], were performed for the NR cured with extended levels of sulfur. Based on the DEPT results and previously reported model compound results [20], the chemical shifts of the resonances occurring in the spectra were assigned. 

Microstructural changes of an accelerated sulfur vulcanisation of HR with TMTD/ZnO/ sulfur has been studied by solid-state 13C NMR spectroscopy [47]. The HR containing 2% isoprene and 98% isobutylene were formulated using EV and cured at 160 °C for several cure times. The resonances at 20.3 and 24.4 ppm, which are due to trans isoprene units in the HR, decrease with cure, while the resonances at 26.9 and 25.2 ppm which arise from cis isoprene units increase with cure times. The cis trans ratio increases up to a maximum ratio of approximately 4 1 at a cure time of 60 minutes. New resonances are observed at 15, 21, 23.6 and 49 ppm. The peak at 49 ppm is assigned to the mixture of the isoprene units in czs-IIR, polysulfidic Alt and polysulfidic Ale structures. The resonance peaks at 15, 21 and 23.6 ppm are assigned to the isoprene units in mono- and polysulfidic Bit, mono- and polysulfidic Blc and polysulfidic Alt structures, respectively. No reaction occurs in the isobutylene units. No migration of the double bond saturation, internal cyclisation or sulfurisation resulting in Clt and Clc structures is observed. 

In a second paper Brown and Tinker [101] examined the effects of a number of parameters, such as the accelerator used in the sulfur curing, peroxide versus sulfur curing, and swelling ratio at constant crosslink density. The results for c/s-polyisoprene show that the value of H% is independent of accelerator. However, lower values of H% were seen for peroxide-cured materials. The results for BR were independent of curant the reasons... 

Accelerators reduce the cure time considerably by increasing the cure rate. They are not catalysts because they are chemically altered and, in many cases, also react as curing agents. Common sulfur-cure accelerators are amines, dithiocarbamates, sulfenamides, thiazoles, and thiurams. Some accelerators, because of their reactivity, may form toxic compounds, such as 2-(2-hydroxyethylmercapto)benzothiazole from mer-captobenzothiazole (2-MCBT), residues of which may be extractable. Accelerators that are secondary amines may form toxic nitrosamines. 

The crosslinking reaction rate may be too slow for some commercial processes and the reaction may exceed the oxidation resistance time for the elastomer compound. In such cases, curing accelerators are used with the sulfur-curing process. Zinc oxide is a commonly used accelerator, however thioureas, hexamethylenetetramine, and others are effective. For organic peroxides, the cure rate can be greatly increased with an increase in applied temperature, though oxides of zinc,... 

As sulfur content increased, the change in sulfur bond ratios exhibit a transition from a semi-efficient , or lower sulfur/accelerator ratio, to a conventional ,or higher sulfur/accelerator ratio, cure system (4). Overall, the... 

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. 

A sulfur-curing system thus has basically four components a sulfur vulcanizing agent, an accelerator (sometimes combinations of accelerators), a metal oxide, and a fatty acid. In addition, in order to improve... 

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