Curing, rubber diene rubbers

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

The level of accelerator used varies from polymer to polymer. Some typical curing systems for the diene rubbers NR, SBR and NBR and for two olefin rubbers (discussed in Section 11.9) are given in Table... 

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. 

Diene rubbers can be vulcanized by the action of phenolic compounds like phenol-formaldehyde resin (5-10 phr). Resin-cured NR offers good set properties and low hysteresis [54]. 

Comparison of Properties of Sulfur- and Peroxide-Cured Ethylene-Propylene-Diene Rubber (EPDM)... 

Chlorobutyl (CIIR) and bromobutyl (BIIR) are modified types containing 1.2% wt of chlorine or bromine, the isoprene unit being the site of halogenation. Introduction of the halogen gives greater cure flexibility, and enhanced cure compatibility in blends with other diene rubbers. It also confers increased adhesion to other rubbers and metals. 

The simplest diene that satisfies this requirement is 1,4-hexadiene, and indeed it has been adopted as the cure site monomer in commercial ethylene-propylene-diene rubber. Because 1,4-hexadiene exists in both trans and cis configurations, significant amounts of work have been devoted to find ways to control the selectivity of the catalysts for one of the isomers over the other. 

These two rubbers are prepared by halogenation of the butyl. Halogenation gives increased cure reactivity. As a result, improvement occurs in vulcanization rates resulting in improved properties over butyl. The state of cure and reversion resistance and covulcanization with other diene rubbers can also be suitably monitored. CIIR and BIIR vulcanizates have lower gas permeability,... 

Cure-rate mismatch is extreme when the blends constitute high unsaturated diene mbbers like NR, and low unsaturation rubbers like EPDM. It involves the migration of polar curatives from the low unsaturation phase to a more polar high unsaturation phase, further undercuring the low unsaturation phase (11-13,34) and it has been shown that unvulcanized EPDM exists in the vulcanized blend with NR (35). Several approaches have been made to obtain a cocured blend vulcanizate of NR-EPDM without sacrificing the physical properties by (1) increasing the unsaturation of EPDM elastomer so that the cure rate becomes at par with NR or other diene rubbers... 

Sengupta A. and B.B. Konar. 1997. Cure characteristics of ethylene propylene diene rubber-polypropylene blends. J. Appl. Polym. Sci. 66 1231-36. 

Since those early days, there has been continued progress toward the improvement of the process and in the resulting vulcanized rubber articles. In addition to natural rubber, over the years, many synthetic rubbers have been introduced. Also, in addition to sulfur, other substances have been introduced as components of curing (vulcanization) systems. This chapter is an overview of the science and technology of vulcanization. Emphasis is placed on general-purpose high-diene rubbers for example, natural mbber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR), vulcanized by sulfur in the presence of organic accelerators. 

The inhibitors act as ligands, blocking the hydrosilylation reaction by coordination to the metal center, but vmder conditions employed for cure, they release the active catalyst. The numerous vmsaturated organic compounds, such as esters, alcohols, ketones, sulfoxides, phosphines, phosphites, nitriles, azodicarbonyl compounds, triazoline, dienes, amine N-oxides, hydroperoxides, and others, are known and have been reported as platinum catalyst inhibitors during the cure of silicon rubber through addition processes (3,4). The unsaturated diesters (e.g., maleates and fumarates) are the most important and commonly used platinum inhibitors in industrial curing processes and have been described in many articles and patents. 

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