Crosslinking peroxide-curing

Free radical crosslinking. Peroxide curing is through the methylene bridge in a diphenylmethane diisocyanate urethane prepolymer. 

Some terpolymers contain an additional cure site monomer, for example, bromotetrafluorobutene, to permit crosslinking with peroxides. Peroxide curing gives vulcanisates more resistance to amine stabilisers in motor oils, more resistance to methanol containing motor fluids. Resistance to acids, aqueous media and steam is also improved. Compression set and heat resistance are slightly inferior to bisphenol A cure systems. 

Crosslinking time is directly dependent on the rate of decomposition of the peroxide. The effectiveness of the overall crosslinking reaction is thus dependent on the type of peroxide and polymer radicals produced during the process. Cure time and temperature can, in a peroxide cure system, be determined solely from knowledge of the rate of peroxide thermal decomposition. 

In more recent studies from Gonzalez and co-workers [88-90] it was concluded from dynamic mechanical analysis of peroxide-cured NR that a non-uniform crosslinked network results if a large amount of peroxide is used. This result seems to be in line with the optical spectroscopy studies discussed. 

Co-agents are multi-unsaturated compounds, which are used in the peroxide-curing of elastomers. When classical co-agents, such as triallylcyanurate (TAC), trimethylolpropanetrimethacrylate (TRIM) or diallylterephthalate (DATP), are added, the crosslinking efficiency is enhanced [98-102]. Various mechanisms for the increase of the crosslinking efficiency have been proposed. In all cases a fast reaction between the... 

Apart from the effect on the crosslinking efficiency, the use of co-agents in peroxidecuring also imparts the molecular structure of crosslinks. It has been reported that coagents with two or more unsaturated moieties can be incorporated as individual molecules between two elastomer strands to form crosslinks [103-109]. In this way the crosslink structure of peroxide-cured elastomers can be altered. Thus, apart from the expected benefits, such as improved crosslinking efficiency, decreased compound viscosity and faster cure, the use of co-agents may also provide a tool for manipulating mechanical properties. 

The effect of co-agents in the peroxide-curing of EPDM is very similar to the effect of third monomers. It was concluded that the pendent unsaturation of the third monomer acts as a co-agent, i.e., the amount of third monomer governs the amount of chemical crosslinks formed by macroradical addition reactions via the unsaturated moiety of the third monomer, whereas the amount of peroxide governs the amount of crosslinks formed by macroradical combination reactions. 

In a second paper Brown and Tinker [101] examined the effects of a number of parameters, such as the accelerator used in the sulfur curing, peroxide versus sulfur curing, and swelling ratio at constant crosslink density. The results for c/s-polyisoprene show that the value of H% is independent of accelerator. However, lower values of H% were seen for peroxide-cured materials. The results for BR were independent of curant the reasons... 

Vinyl Free radical cure systems crosslinked polyethylene, peroxide cured elastomers, polyesters. Polyethylene. Polypropylene. 

Although epoxies dominate the thermoset fracture literature, work has been reported on other systems, e.g., polyester resins, phenol-formaldehyde compounds, peroxide cured polystyrene, and highly crosslinked polyurethanes. In general, these materials exhibit fracture behaviors similar to epoxies, and suggest that thermosets, as a class of materials, display characteristic crack growth properties. 

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