Maleic anhydride with peroxide crosslinking

Polyethylene can be crosslinked with peroxides to make it more suitable for coatings for electrical power cables [18]. Peroxides are also used to crosslink (also called cure) thermoset polyesters. Thermoset polyesters have a reactive alkene and are used to make fiberglass compositions such as used for boat hulls. Variations are possible, but commonly they are copolymers of fumaric acid or maleic anhydride with phthalic acid or isophthalic acid and a diol. The oligomer is typically dissolved in styrene. When a peroxide is mixed with the solution, crosslinking occurs and the polyester cures. The styrene solvent takes part in the reaction and the solution hardens. 

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 ]. 

The catalytic or initiated reaction involves heating the poly(diene) in an aromatic solvent to temperatures between 120-150 °C in the presence of free radical initiators such as peroxides, hydroperoxides and azo compounds. The ensuing reaction involves addition of maleic anhydride to a polymeric radical which was formed by abstraction of an allylic hydrogen by initiator radicals. Four modes of addition are possible leading to partial structures such as (175)-(178) illustrated with poly(isoprene). It can readily be seen that some crosslinking is an inherent problem because of structures (177) and (178). The amount of gel formed, however, is found to be largely dependent on the initiator employed and can be minimized, especially with hydroperoxide initiators. 

In general, polyester resins are synthesized by the reaction between carboxylic acids and alcohols, with three or more reactive groups. Recently, unsaturated polyesters were incorporated in various ways to produce terminal, pendant, and internal double bonds [57-59]. In the case of unsaturated polyesters, maleic anhydride is most commonly used to produce internal unsaturation. The double bond present on unsaturated polyester reacts with a vinyl monomer, mainly styrene, resulting in a 3D crosslinked structure. This structure acts as a thermoset. The crosslinking is initiated through an exothermic reaction involving an organic peroxide, such as methyl ethyl ketone peroxide or benzoyl peroxide (Fig. 3.18). 

The crosslinking of polyethylene resulting from its reaction with peroxides is more profound if maleic anhydride is present in the system [113]. The crosslinked fraction contains a higher proportion of carbonyl groups from grafted maleic anhydride than... 

Free-radical initiated reactions can also be used, besides crosslinking, for otha- modifications. The thermal decomposition of benzoyl peroxide was successfully used to initiate the grafting of maleic anhydride on PHB which was reported to produce a material with increased thermal stability and biodegradability, the latter attributed to the appended maleic moieties [ 168,169]. 

Peroxides can be used to promote crosslinking of a rubber as is done in dynamic vulcanization. Dynamic vulcanization is usually done in an extruder and is the process of selectively crosslinking rubber during its melt mixing with a molten thermoplastic such as polypropylene. This produces thermoplastic vulcanates, often referred to as TPVs [16]. Peroxides are also used to provide grafting reactions. For example, polypropylene, maleic anhydride, and a peroxide initiator in a twin-screw extruder give a graft polypropylene [17]. These polymers can have improved properties such as improved adhesion. 

Unsaturated polyester resins are mainly made by condensing a dibasic acid (1,2-propanediol) with an anhydride (maleic or phthalic anhydrides), by forming ester linkages between the dibasic acid (or their anhydrides) and glycols. Then a reactive monomer (mostly styrene or vinyl toluene, MMA or diallyl phthalate) is used to crosslink the system when needed. Unsaturated denotes the uncompleted chemical activity (double bond) in the original structure, which are used for crosslinking afterwards. In this context, an excess of styrene as the crosslinker (10 to 50 %) is usually added to have it ready in the system, as well as to reduce the viscosity. There are also certain accelerators used (such as, cobalt naphthenate or tertiary amines like dimethyl aniline) to facilitate the cure at ambient temperatures. In addition, there may be pigments, fillers, various inhibitors, accelerators, stabilisers and flame retardants, added to the system. Polymerisation is activated whenever a catalyst (i.e., benzoyl or methyl-ethyl-ketone peroxide) is added. 

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