Phenolic curatives, vulcanization

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

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. 

This is another example of what has variously been called a pseudo-Diels-Alder, ene, or no-mechanism reaction (Hoffmann, 1969). It is similar to the reaction written for the attack of rubber molecules by phenolic curatives or the in situ formed nitroso derivative of the quinoid (e.g., benzoquinonedioxime) vulcanization system. It is also closely related to the sulfurization scheme written for accelerated-sulfur vulcanization. Comparisons between accelerated sulfur, phenolic, quinoid, and maleimide vulcanization can then be visualized as follows ... 

Table 7.5 gives selected recipes for vulcanization by phenolic curatives, benzoquinone-dioxime, or m-phenylenebismaleimide. Vulcanizates based on these types of curatives are particularly useful in cases where thermal stability is required. 

TABLE 7.5 Recipes for Vulcanization by Phenolic Curatives, Quinone ... 

VII. Vulcanization by Phenolic Curatives, Benzoquinone Derivatives, or Bismaleimides... 

VII. VULCANIZATION BY PHENOLIC CURATIVES, BENZOQUINONE DERIVATIVES, OR BISMALEIMIDES... 

This is an explanation for the fact that this type of vulcanization is not enabled by double bonds per se, without aUyic hydrogens in the elastomer molecules [4,74]. (It should be pointed out that the phenolic curative can also act by a slightly different mechanism to give crosslinks which contain chromane structural moieties [75], the allylic hydrogens stUl being required.)... 

Two important types of elastomeric polyolefin blends are reactor-made iPP/ EPR blends and postreactor blend iPP/EPDM. The latter is called thermoplastic vulcanizates (TPVs), produced by dynamic vulcanization of blends containing a thermoplastic and an elastomer. To make iPP/EPDM TPV, the two polymers PP and EPDM are mixed with curatives, such as peroxides, phenolic resins, or sulfur with accelerators, and dynamically cured in an extmder resulting in a blend consisting of micrometer-sized elastomer particles dispersed in the PP matrix (20-24). Paraffinic oils are added in the melt mixing process for viscosity control and cost. In iPP/ EPDM TPV, the crystalline iPP resin is normally the minor phase. Recently, polyolefin plastomers have been added to the class of elastomeric polyolefin blends. Polyolefin plastomers are ultralow molecular weight linear low density polyethylenes (ULMW-LLDPE). Nonelastomeric polyolefin blends are blends of polyolefins with mostly nonpolyolefin (other thermoplastic) matrices as mentioned earlier. 

Excellent elastomeric NBR-nylon compositions have also been prepared by dynamic vulcanization during the melt-mixing of intimate blends of NBR with various nylons. In this case, the effect of curatives was complicated by the fact that some nitrile rubbers tend to self-cure at temperatures of mixing. Sulfur, phenolic, maleimide, or peroxide curatives can be used. The thermoplastic elastomeric compositions prepared by the dynamic vulcanization of nylon-NBR blends are highly resistant to hot oil. As in the case of the EPDM-polyolefin blends, increases in the amount of rubber in the composition reduce stiffness but increase resistance to permanent set. 

With EPDM/PP, the rubber is EPDM and the thermoplastic resin is polypropylene. Even though early patents show the vulcanizing agent to be sulfur with an accelerator such as 2-bis(benzothiazolyl)disulfide (MBTS) or tetramethyl thiuram disulfide (TMTD), these sulfur-based curatives are not used today for TPVs. Likely a methylol-terminated phenol-formaldehyde resin curative with a tin chloride activator is used. For a lighter color, sometimes a peroxide crosslinker is used instead. Also, paraffinic... 

Methylol-terminated para-alkyl-substituted phenol formaldehyde resin is used as the vulcanizing agent for compounds based on butyl and EPDM rubber. The alkyl group is usually octyl. It is commonly used to cure butyl rubber where superior heat resistance is needed. Therefore, this vulcanizing agent is commonly used as the curative to make butyl bladders for repetitive curing of tires. It is also sometimes used in dynamic vulcanization with a tin chloride activator to make thermoplastic vulcanizates (TPVs). 

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