Peroxide cross-linking compounding with peroxides

Polyesters. Unsaturated polyester resins based on DCPD, maleic anhydride, and glycols have been manufactured for many years. At least four ways of incorporating DCPD into these resins have been described (45). The resins are mixed with a cross-linking compound, usually styrene, and final polymerization is accompHshed via a free-radical initiator such as methyl ethyl ketone peroxide. 

Dimethyl silicone rubbers show a high compression set. (For example, normal cured compounds have a compression set of 20-50% after 24 h at 150°C.) Substantially reduced compression set values may be obtained by using a polymer containing small amounts of methylvinylsiloxane. Rubbers containing vinyl groups can be cross-linked by weaker peroxide catalysts. Where there is a high vinyl content (4-5% molar), it is also possible to vulcanize with sulfur. 

An exhaustive study on the reaction of mechanically degraded vulcanizates has been carried out [40]. Six different inhibitors were added in various percentages (5-25%) and combinations to rubber vulcanizates gelled in benzene. The solutions were degraded at 80°K in glass ampoules. The effectivenes of each stabilizer was determined by examining the ESR spectra for the degraded vulcanizates with each of the inhibitors as a function of temperature. Phenols were found to be the best inhibitors for radicals of the peroxy type. The most universal acceptor compound was found to be trichlorothio-phenol. Its effectiveness was questionable only in the case of alkyl sulfide radicals in this study of both sulfur and peroxide cross-linked vulcanizates. 

Antioxidants. The 1,2-dihydroquinolines have been used in a variety of ways as antioxidants (qv). For example, l,2-dihydro-2,2,4-trimethylquinoline along with its 6-decyl [81045-48-9] and 6-ethoxy [91-53-2] derivatives have been used as antio2onants (qv) and stabilizers (68). A polymer [26780-96-1] of l,2-dihydro-2,2,4-trimethylquinoline is used in resins, copolymers, lubricant oils, and synthetic fibers (69). These same compounds react with aldehydes and the products are useful as food antioxidants (70). A cross-linked polyethylene prepared with peroxides and other monomers in the presence of l,2-dihydro-6-ethoxyquinoline produces polymers with a chemically bonded antioxidant (71). 

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa. 

The Hquid polymer is then compounded with metal oxides or peroxides, as weU as fillers (carbon black) and can undergo cold vulcanization, ie, chain extension and cross-linking iato a soHd matrix. It is largely used as a sealant and gasket material for wiadows, automobile wiadshields, etc. 

The third process for cross-linking is the Sioplas process developed by Dow. The first stage of this involves the grafting of an easily hydrolysable trialkoxyvinylsilane onto the polyethylene chain, the site activation having been achieved with the aid of a small amount of peroxide. The compound is then extruded onto the wire, which is collected on a drum. The drum is then exposed to hot water, or, more commonly, low-pressure steam. The water hydrolyses the alkoxy groups, which then condense to form a siloxane cross-link. ... 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

The most commonly reported physical properties of radiation cross-linked natural rubber and compounds made from it are modulus and tensile strength, obtained from stress-strain measurements. Figure 5.6 illustrates some of the results obtained from gum rubber and from a natural rubber compound reinforced by HAF carbon black.143 In Figure 5.7, the tensile strength of radiation-cured gum is compared with vulca-nizates cured by sulfur and by peroxide.144 Selected tensile strength data from radiation-cured NR compounds are in Table 5.4. 

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