Peroxide curing process concentration

The curing process of VER (Scheme 33) using methylethylketone peroxide (MEKP) as the initiator and cobalt hexanoate (CoHx) as the promoter was studied by means of thermal scanning rheometry (TSR) and DMTA under isothermal conditions. Some kinetic parameters, i.e. gel time, apparent activation energy ( a) for this process were calculated [201]. The value of Ea obtained from the TSR measurements is independent of both the initiator and promoter concentration. 

It was reported [231] that phenolic and aromatic aminic AO are able to form bound-in species in EPM cured with peroxides. This principle was exploited in NR doped with 2,6-di-terr-butyl-4-methylphenol and tert-butyl peroxide. Other phenolic AO bearing methyl groups may take place in a similar process too [232]. The extent of the coupling of radicals derived from phenolic antioxidants with macroalkyls is influenced by the concentration of phenols. A competitive process, autocoupling of phenol derived radicals, increases with increasing concentration of the phenolic antioxidant [17]. 

Other factors that are Important to this process are the amount of gel (crossllnked polymer) formed, the effect of peroxide concentration on the reaction rate and the temperature at which the reaction Is carried out. An example of the relationship among cure temperature (150-180 C), gel content (100% - % extractables) and a solvent swelling ratio (xylene or toluene) for polyethylene containing 2% dlcumyl peroxide Is shown In Figure 11. 

A longer pot-life of several hours can be obtained if the finish contains peroxide but no accelerator. In this process, such a finish is applied as the first coat, and is immediately followed by a second coat of finish which contains no peroxide, but has a high dose of accelerator. Free radicals are produced in high concentration at the inter-coat boundary, from whence polymer chains grow outwards into the two coats. The process is particularly suited to curtain-coating. Curtains of the two finishes fall continuously from slits in two heads placed one behind the other and the object passes through one curtain, then the other, in one pass. One drawback of the process is that the two coats have to be carefully matched and the correct film thicknesses applied or serious defects in appearance and curing can occur. Another is that the pot-life problem is not avoided it is made a little more tolerable. 

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