Peroxide Crosslinking

Crosslinking systems based on peroxides are usually simple. Unlike with sulphur vulcanisation systems, the addition of ZnO and stearic acid is not necessary, so crosslinking is achieved in many cases simply using a crosslinking peroxide. However, the addition of a coagent may be useful (see below). In some other cases, a combination of various types of crosslinking peroxides may be suitable. 

If such fillers are to be used, they should have a neutral or slightly alkaline pH, otherwise additives such as ethylene glycol and triethanolamine, which are preferentially adsorbed on the surface of the filler, should be used, preventing any undesirable interference reactions between the filler and the crosslinking peroxide. These additives must, however, always be added to the mix before the peroxide. With some mineral fillers, such as some types of clay, the polymer may be bound to the filler by means of silane treatment, and the surface of the filler becomes completely non-polar. Consequently, the interaction with the polymer matrix increases, while the adsorption of the crosslinking peroxide by the filler is severely suppressed. 

Extending oils for compounds crosslinked with peroxides have to be carefully selected. Synthetic ester plasticisers such as phthalates, sebacates and oleates may be used in combination with crosslinking peroxides without affecting the crosslinking reaction. Some derivatives of alkylated benzenes are also known for their very low consumption of free radicals, which is clearly desirable. Mineral oil with double bonds, tertiary carbon atoms or containing heterocyclic aromatic structure may react with radicals paraffinic mineral oils are more effective than naphthenic types, which usually require extra treatment in order to guarantee optimum results when used in peroxide crosslinked blends. 

Peroxide, crosslinking Peroxide, decomposer Peroxide, degradation Peroxy free radicals PES (Poly-ether sulfone) PES blends... 

Tris (dimethylaminomethyl) phenol activator, crosslinking peroxide vulcanization... 

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

It is important to store crosslinking peroxides for longer times than one week at temperatures above 30°C, although after several months at T < 30°C most peroxides exhibit no reduction in their activity. 

Table 31 Typical crosslinking temperatures of crosslinking peroxides based on their half-life...

Coacrete can also be made water-repeUent by the polymerisation of vinyl monomers on the surface (85). Polymerisation can be iaitiated with peroxides, and polyfunctional methacrjiates can be used as crosslinking agents. These treatments have a tendency to produce changes ia color and gloss. 

The original compound, maleimide (2,5-dioxo-A -pyrroline), is synthesized by the cyclo-condensation of ammonia and maleic acid. Similarly, primary amine is added to maleic anhydride, followed by cyclocondensation, to form N-substituted maleimide (Fig. 2). This reaction is applied to the preparation of bis-maleimides (BMl) [1]. At first, BMI was used as a crosslinking agent for natural rubber (NR). An o-dichlorobenzene solution of NR was crosslinked by BMI at I08-150°C in the presence of peroxides. The radicals generated from peroxides react with the double bonds of both BMI and NR [ 1 ]. 

Conventional crosslinking agents, such as sulphur, accelerators, and peroxide, etc., used in dynamic vulcanization and melt-mixing of two polymers is the first step in the preparation of an EA. The dynamic vulcanization is done by dynamic shear at a high temperature to activate the process. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts... 

In order to bring about crosslinking of polyesters with styrene one of two types of initiator systems is used, which differ in the temperature at which they are effective. For curing at elevated temperatures, peroxides are used which decompose thermally to yield free radicals. Among those peroxides employed are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-butyl peroxide, and dodecyl peroxide. Mixtures of polyester prepolymer, styrene, and such initiators are reasonably stable at room temperatures but undergo fairly rapid crosslinking at temperatures between 70 °C and 150 °C, depending on which particular peroxide is used. 

W.C. Endstra and C.T.J. Wressmann, Peroxide Crosslinking of EPDM Rubbers, in Elastomer Technology Handbook, N.P. Cheremisinoff and Pn. Cheremisinoff, Eds., CRC Press, New Jersey, 1993. 

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