Other Vulcanizations

Polydiene rubbers can also be crosslinked by heating with p-dinitrosobenzene, phenolic resins, or maleimides [Coran, 1978 Gan and Chew, 1979 Gan et al., 1977, 1978 Sullivan, 1966]. The crosslinking mechanism is similar to that for accelerated sulfur vulcanization, for example, for vulcanization by p-dinitrosobenzene 

These products find use in specialty applications requiring better thermal stability than available in the sulfur vulcanized elastomers. Other processes are also used to crosslink polydiene rubbers (Secs. 9-2c and 9-2d). 

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The anodic cleavage of disulfides was used on the laboratory scale for the synthesis of other vulcanization enhancers 330 331), for the production of phenyl sulfinates 332), and for the synthesis of intermediates for penicillins and cephalosporins 333). 

Rubbers are usually subjected to a vulcanization or curing process to improve their properties. Vulcanization is carried out commonly by reaction with sulfur, which leads to the formation of a three-dimensional structure through the formation of sulfur bridges between the polymer chains. Other vulcanizing agents include peroxides, metal oxides, amines, etc. As in the case of plastics, rubber goods also incorporate a number of additives 7... 

Worn-out tires and scraps and trimmings of other vulcanized products constitute the raw material for reclaimed rubber. Therefore a good reclaiming process must not only turn the rubber soft and plastic but also must remove reinforcing cords and fabrics that may be present. There are a number of commercial processes [43] for rubber regeneration (1) alkali digestion process, (2) neutral or zinc chloride digestion process, (3) heater or pan process, and (4) reclaimator process. 

The accelerated-sulfur vulcanization of these rubbers along with the vulcanization of other rubbers, which are vulcanized by closely related technology, comprises more than 90% of all vulcanization. These rubbers include ethylene-propylene-diene-monomer rubber (EPDM), butyl rubber (HR), halobutyl rubbers, and nitrile rubber (NBR). Nevertheless, we give some consideration to vulcanization by the action of other vulcanization agents such as organic peroxides, phenolic curatives, and quinoid curatives. 

V-Nitrosamines, some of which are classified as carcinogenic, can form in a chemical reaction of certain amines and nitroso compormds. The formation of stable nitrosamines in mbber and other vulcanized materials requires the presence of ... 

Sulphur-curing grades are usually preferred by the rubber industry as they make use of established technology, but two other vulcanization techniques are also well established as they produce elastomers with certain superior properties these are crosslinking with peroxide or diisocyanate. Peroxide curing can be universally used as it has a free radical-type... 

The other vulcanization of liquid silicones is a radical reaction with UV irradiation at room temperatures. As silicone rubbers are anti-thrombotic, non-toxic, hydrophobic, not reactive with living tissue and chemically stable compared with the other polymers, they are used for parts dealing with blood and other medical materials. The silicone rubbers are used in various parts having a high heat resistance and a high fatigue resistance by adding electrically conductive materials such as carbon black. The uses of silicone rubbers are shown in Table 8.4. 

Rubber maker s sulfur is by far the most commonly used vulcanizing agent for curing rubber. Elemental sulfur cures impart advantages in flex fatigue, scorch safety time, and cost versus other vulcanizing agents such as peroxides. 

A wide variety of vulcanization agents are used in the rubber industry. For specialty elastomers, peroxide cross-linking is probably the most widely used and fastest growing system. The various peroxides available for cross-linking and newer developments in this area will be reviewed. Other vulcanization agents for specialty elastomers will also be presented. 

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