A review of experimental studies of the role of free-radicals in polyethylene oxidation

This paper reviews current understanding of the role of free radicals in the oxidation of polyethylene induced by high-energy irradiation (gamma or e-beam). To evaluate the reactivity, stability and mobility of different macroradicals, their reactions after gamma irradiation of different polyethylenes (LDPE, LLDPE, VLLDPE, HDPE, UHMWPE) are considered. Macroradicals are formed in all phases of PE (crystalline, amorphous and interphase). Their overall determination is possible only if both irradiation and detection are carried out at 100 K, or below. At this temperature, the most abundant macroradical formed is the secondary alkyl macroradical (R’’°). On gradually raising the temperature, the macroradicals decay with formation of vinylene double bonds and molecular hydrogen. At room temperature, the macroradical concentration is about 4% of the original quantity at 100 K for LDPE and around 15% for HDPE. In HDPE and UHMWPE; the macroradicals are mainly present in the crystalline phase and in short times (hours) they migrate to the amorphous phase. In LDPE and LLDPE macroradicals are mainly allylic, present in minimal amounts at the crystalline-amorphous interphase. In the nearly fully amorphous VLDPE no residual macroradicals can be detected at room temperature. The mobility of R’’° is variable and is a function of the mobility of the polymer backbone, slow in the crystalline phase, relatively fast in the amorphous phase. Kinetic stability, or persistence, is often more important than thermodynamic stability in determining radical lifetimes, in particular for radical processes in the solid phase. Alkyl macroradicals react with chain imperfections, additives and oxygen, in a cyclic process involving initiation, propagation and termination reactions. The initiation reactions form macroradicals by cleavage of the C-H bonds of PE, induced by irradiation or by photo- or thermal-scission of peroxides formed during processing of the polymer. Propagation involves the reaction of R’’° with vinyl or vinylidene double bonds, with the formation of a new radical, in competition with their reaction with oxygen to form various oxidation products (ketones, hydroperoxides acids, alcohols and esters). It is notable that the formation of ketones does not necessarily require decomposition of the hydroperoxides. In the presence of stabilizing additives, radicals react with the additive (ADH), with the formation of a more kinetically stable radical (AD°), which considerably decreases the propagation rate, but a termination reaction between R’’° and AD° may also be envisaged. It is observed that radiation-induced oxidation has a constant rate during irradiation. Post-irradiation, the oxidation occurs via transfer of the macroradicals from the crystalline phase and the interphase to the amorphous phase, where oxygen is available, and the rate decreases, approaching zero. The occurrence of termination is apparent. Termination must occur through the reaction of two macroradicals. The reaction between two R’’° is difficult, due to steric hindrance. The most probable reaction is that between peroxy macroradicals (ROO°), fixed in position on the polymer chain and R’’°, which migrate through the polymer bulk. This reaction is difficult to confirm experimentally because of the lack of reliable analytical methods for ROOR species in the presence of ROOH. The formation reactions of the different oxidation products are reported and the branching reactions occurring in thermal and photo-oxidation are also discussed.