Epoxidized natural rubber bonding

Epoxidized natural rubber (ENR) is an important modified form of natural rubber whose properties are close to those of special purpose synthetic rubbers. ENR is prepared by reacting natural rubber in latex form with performic acid formed in situ by the reaction of formic acid and hydrogen peroxide under controlled conditions. [45,46]. The double bonds of natural rubber react readily with peracids to yield epoxide groups. Though any level of epoxidation can be achieved, only up to 50 mole percent is used in practice. 50, 25, and 10 mole% epoxidized products are termed ENR-50, ENR-25, and ENR-10. Studies using NMR have shown that the epoxide groups are randomly distributed along the natural rubber backbone [47]. 

Carbon-centered radicals and peroxyl radicals were also observed in the e-beam-irradiated blends of PP with epoxidized natural rubber (ENR, 5-30wt%) [93], The former decayed within a few hours after exposure, while the latter survived for more than 20 days. The post-irradiation decrease in total radical concentration proceeded according to a two-step second-order kinetics, with the lifetimes almost independent of the ENR percentage in the blend. Interestingly, the dependence of the initial radical yield versus %ENR was nonmonotonic (Figure 23.22). Hence, the authors suggested that such behavior reflects two effects (i) the presence of epoxy rings and double bonds of ENR favors radical formation and (ii) the ENR as an amorphous polymer facilitates recombination processes. 

A.K. Manna, P.P. De, D.K. Tripathy, S.K. De, M.K. Chatterjee. Effect of surface oxidation of carbon black on its bonding with epoxidized natural rubber in the presence of silane coupling agent. Rubb. Chem. TechnoL, 72, 398-409, 1999. 

In a sustainable future, natural rubbers will be used more and synthetic rubbers less. The epoxidation of natural rubber can be used to extend the applications or rubber and improve bonding.145... 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

In these materials, chains are linked together by covalent bonds. This forbids any transition to a fluid state. Examples range from elastomers (e.g., silicones, natural and synthetic rubbers) to thermosetting resins like the epoxides (e.g., Araldite). The relevant quantity here is the molecular weight between crosslink points. 

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