Structure and Reactivity in Oxidation of Elastomers

Abstract

Oxidation of hydrocarbons is a much studied and much reviewed subject, yet much remains to be understood about complexities of competing reactions that take place as hydrocarbons react with oxygen. The generally accepted free radical mechanism for low temperature reaction (below 200° C) stems from kinetic studies by Semenov, Hinshelwood, Farmer, Gee, Bolland, and Bateman of hydrocarbon and olefin oxidation in the gas and liquid phase: Such a chain mechanism appears to be straightforward, but studies of autoxidations are complicated by sensitivity to trace metals, which decompose hydroperoxides, and to the nature of reaction products, which may function as inhibitors or accelerators. The character of the reaction may vary considerably with time as these reaction products accumulate. Hydroperoxides may decompose by unimolecular or bimolecular pathways to produce alkoxy, peroxy, or hydroxyl radicals. These are known to have various selectivities and hence will produce different propagating species with differing rates of termination. Alternately hydroperoxides may react with sulfur, nitrogen, aldehydic, basic, or acidic functions producing no radicals at all. Such reactions vary drastically from one system to another. The study of autoxidations thus becomes the challenging problem of trying to separate one reaction from another. One method of avoiding such difficult problems is to work under simplifying conditions. One approach is to study oxidations with an external initiator such as azobisisobutyronitrile or benzoyl peroxide under conditions such that the hydroperoxides formed during oxidation are relatively stable products. This has been most successfully applied to kinetic studies on liquid hydrocarbons containing benzylic or allylic hydrogens. This review is concerned with some recent work on initiated oxidations of hydrocarbon polymers in the solid phase and attempts to show how the results may be related to the more complicated autoxidation process.

Oxidation behavior of isoprene polymers and butadiene polymers

“…Figure 4 shows the relationship between l / M n ( t ) and t. It is apparent that the points in Figure 4 fit straight lines, as expected from eq. (2). The values of k thus obtained for oxidation by heat aging in solution are also shown in Figure 4.…”

“…It is apparent from Table IV that the polymers exhibit a sharp decrease in number-average molecular weight, which means that the polymers predominantly undergo chain scission, bcause it is believed that the polymers do not appreciably undergo crosslinking in such dilute solution^.^ In this case, the relationship between the rate of chain scission, K , and number-average molecular weight of the polymers can be described by the following equationgJO on the assumption that the rate of chain scission is independent of the time of heat aging: (2) where t is the time of heat aging, M,, of the polymers at time t. Consequently, the rate of chain scission k can be determined as the slope of the plot of l/M,(t) against t . Figure 4 shows the relationship between l / M n ( t ) and t. It is apparent that the points in Figure 4 fit straight lines, as expected from eq.…”

Thermal oxidation of polybutadiene. Part 1: Effect of temperature, oxygen pressure and sample thickness on the thermal oxidation of hydroxyl-terminated polybutadiene

“…This can be stored for several years, but HTPB is known to be the most easily oxidizable polymers among others owing to the unsaturated character of the repeated unit [2]. The technological interest of polybutadiene, in propellants as in many other applications, especially in block copolymers of the SBR or ABS type, has led to a relatively abundant literature on its oxidation mechanisms and kinetics [3e6].…”

Functionalization of Liquid Natural Rubber via Oxidative Degradation of Natural Rubber

“…The OH radical attacks the labile proton bonded to the alpha carbon, (abstracting the proton or OH insertion) and causing cleavage of the chain to form macro-radicals of the NR chain [42]. The chain breaks at the CαH2-CαH2 (C1-C4) bond linking the isoprene units.…”

“…The process was reported to introduce reactive terminal groups, depending on the type of system employed [42]. Bac et al [6] degraded epoxidized NR latex by using sodium nitrite.…”