Curing system sulfur-based

Vulcanizing or Curing Systems. The function of these is to crosslink the polymer. The most common curing systems are based on sulfur. While sulfur... 

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). 

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. 

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)... 

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...

The curing system based on magnesia would be used to provide resistance against concentrated sulfuric acid and other drying agents. If there is a possibility where the lining is exposed to 30% hydrochloric acid, magnesia should not be used. 

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... 

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... 

Blends of NR/EVA are conventionally cured after intermix mixing. The blends are cured with a mixed cure system of S + DCP (sulfur + dicumyl peroxide). While sulfur cures only the NR phase of the blend, DCP is expected to cure both the phases. The possible network structure is represented in Figure 1.15. The morphology of the blend is predicted based on uncured blend SEM studies and is schematically represented in Figure 1.16. 

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. 

Sulfur cures do not always use elemental sulfur. Sometimes with special EV (efficient vulcanization) cure systems no elemental sulfur is used at all. Instead a sulfur donor chemical is used. The most common sulfur donor is dithiodimorpholine (DTDM), which donates two sulfur atoms from the center of its molecule to participate in the sulfur vulcanization process. These EV cures are more expensive than conventional sulfur cures based on elemental sulfur. This is because sulfur donors such as DTDM are more expensive per pound than sulfur itself. Elowever, the EV cure will usually impart better air aging resistance than that of a conventional sulfur cure system using a significant concentration of elemental sulfur. 

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

The original, and currently extensively used, cure-site group is chlorine based. A number of cure systems may be used, but the most common is the soap-sulfur one using sodium and potassium stearate with sulfur or a sulfur donor. Very good physical properties and heat resistance may be obtained with the chlorine cure sites, however compression set resistance is only fair. 

Balasubramanian [16] has described a devulcanisation process that uses a counter-rotating twin-screw extruder to devulcanise GTR. The DR was then blended with virgin NR in various proportions and the blends revulcanised using a sulfur cure system. The Mooney viscosity, cure characteristics and mechanical properties of the resulting vulcanisates were characterised and a four-parameter rheometric equation, based on the standard logistical model for the curing behaviour of extrusion processed blends, was derived and validated for the different levels of virgin NR. 

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