Sulfur-based cure

Double-Bond Cure Sites. The effectiveness of this kind of reactive site is obvious. It allows vulcanization with conventional organic accelerators and sulfur-based curing systems, besides vulcanization by peroxides. Fast and controllable vulcanizations are expected so double-bond cure sites represent a chance to avoid post-curing. Furthermore, blending with other diene elastomers, such as nitrile mbber [9003-18-3] is gready faciUtated. 

Fig. 8. Sulfur-based cure system designs where conventional systems are polysulfidic, EV systems are mono-/disulfidic, and semi-EV systems are clean polysulfidic. A shows pendent sulfide groups terminated by accelerator B, monosulfide cross-links C, disulfide cross-links D, polysulfide cross-links...

It is known that OC promote a faster sulfur-based cure of unsaturated polymer chains. The activation energy of the crosslinking reaction is decreased and the crosslinking density in enhanced. To explain these effects by OC, the formation of tertiary amines from the thermal degradation of ammonium cations and the enhanced mobility of sulfur-accelerating anionic species were proposed. ... 

EPDM is a terpolymer of ethylene, propylene, and a small amount of an unsaturated diene as a third monomer to provide a cure site. Unlike the elastomers previously discussed, the unsaturation in EPDM is not in the main chain, but it is pendent to the chain. Peroxide-based cure systems afford better aging resistance and low compression set. A comparison of a sulfur-based cure to two different peroxides in EPDM is shown in Table 21 (4). Initial properties for these three compounds are reasonably close. However, after air aging, the advantages of peroxide curing are apparent. Most dramatic is the improved compression set... 

Special Purpose Elastomers, Sulfur-Based Cure Systems ... 

Guzman and co-workers [27] investigated whether it is possible to use waste tyre crumb as a replacement for zinc oxide as an activator in the sulfur vulcanisation of natural rubber (NR). They used the unsaturated organic compound squalene as a model compound for NR in their work, and followed the course of the vulcanisation reaction using the analytical technique high-performance liquid chromatography. The results confirmed that waste rubber crumb was an alternative to zinc oxide as an activator in the curing of NR compounds by sulfur-based cure systems. 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

Three different covalent cure systems are commonly used sulfur-based or sulfur donor, peroxide, and maleimide. These systems rely on a cross-linking agent and one or more accelerators to develop high cross-link density. 

Cure system (species) Sulfur-based phthalimide (PVI)... 

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. 

The problem of cross-linking or grafting low unsaturated rubbers (e. g. EPDMs or EPTMs) or thermoplastics (e. g. ethylene-butadiene copolymer is fundamentally the same. Radical mechanisms are believed to be operative when peroxides or sulfur-based formulations are used, even though in the latter case ionic mechanisms also seem to contribute to the curing process ... 

The cure system most widely employed for CSM is based on sulfur and its derivatives, sometimes referred to as the sulfur or sulfur donor cure. Unlike... 

Poiyisobutylene rubber (IM) was a precursor to butyl rubber, which was first developed in Germany. However, this polymer had no unsaturation it could not be cured with a sulfur-based system. In 1937 Standard Oil (now ExxonMobil) developed the copolymer version that is used today in high volume. A small amount of isoprene provides the unsaturation that enables conventional curing with sulfur or other crosslinkers. 

Natural-mbber-based pressure-sensitive adhesives can be cured by standard mbber curatives, eg, sulfur plus an accelerator (see Rubber, natural) ... 

Another commercially available retarder for sulfur vulcanization is based on an aromatic sulfenamide. Like CTP, this product is most effective ki sulfenamide cure systems, but it also works well ki thiazole systems. Performance properties are generally not affected except for a slight modulus kicrease. In some cases this feature allows for the use of lower levels of accelerator to achieve the desked modulus with the added potential benefits of further scorch delay and lower cost cure system (23). 

One way to obtain enhanced heat and solvent resistance in SBR-based PSA is sulfur addition. It is difficult to obtain sufficient cure with sulfur during normal drying cycles (2-5 min at 150-180°C). Furthermore, cure via sulfur may continue after leaving the oven, thus degrading adhesion. When used in contact with copper, the sulfur may promote corrosion. The use of phenolic overcomes all of these problems despite the fact that its natural cure speed is also insufficient for the time frame available. This is overcome through addition of one of the accelerators... 

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