Transformations of phenolic antioxidants during the inhibited oxidation of polymers

Abstract

During ageing and oxidative degradation of stabilized polymers, transformations of antioxidants take place as a result of reactions with radicals Ro· and Roo·, alkyl hydroperoxides and/or oxygen. Knowledge of these processes as well as of the properties of products thus forrned is necessary for a complex evaluation of antioxidants. Data are given dealing with typical products of oxidative transforrnations of phenolic antioxidants prepared by independent syntheses, with model transformations under conditions simulating interactions in a stabilized organic substrate, with products identified under real conditions of oxidation of a stabi1ized substrate, and with influence of sorne transformation products on the course of oxidation of tetralin and polypropylene.The oxidative radical degradation of polymers which takes place under most varied accompanying conditions during the atmospheric ageing or processing of polymers is limited by stabilization. The importance of the stabilization process and the theoretical interest devoted to it are an unequivocal consequence of the technical and commercial interests. The majority of the antioxidants technically used have an empirical basis. Complex requirements dealing with the properties of the stabilization systems have been gradually formulated. The data acquired so far about the stabilization processes and the chemical structure of the stabilizers have been summarized in a nurober ofmonographs 1 -7 • Every year suggestionsarealso made concerning the use of new stabilization systems.When choosing the antioxidants, one concentrates on attaining the maximum activity while preserving at the sametime the optimum technical properties of the polymer. In the initiationstage of the degradation process, free radicals are formed from the oxidized polymer, RHAccording to the generally accepted scheme of autoxidation, alkyl peroxyls and alkyl hydroperoxides are formed in the propagation step ofthe reaction; they are a source of further radicals which play their role in the degradation process

Loss and transformation products of the aromatic antioxidants in MDPE film under long‐term exposure to biotic and abiotic conditions

“…A linear relationship between the concentration of the antioxidant and the OIT has been found for combinations of phenolic and phosphite stabilizers. 26 The effects of the residual amounts of Irganox 1010 and Irgafos 168 on the Tox and OIT of the MDPE film are shown in Figures 3 and 4, respectively.…”

“…This product caused a certain light yellowing of all the aged MDPE films, and is a common product of the oxidation of such hindered phenols. 26,29,30 The stabilizing function of the organic phosphites is based on the presence of the phosphite moiety. The two basic conversion products of Irgafos 168 are the phosphate that forms as a result of oxidation and the phosphonate that forms as a result of hydrolysis.…”

Spin Trapping of Radical Intermediates Generated by the Oxidation of Substituted 4‐Methylphenols

“…-It is generally known that the oxidation of monohydric phenols with different agents, such as PbO 2 , MnO 2 , or RO . 2 leads to the formation of corresponding phenoxy radicals. The large number of EPR parameters continuously published in catalogues during the last decades [1] contributed to the detailed knowledge of this class of radicals, and seemingly, any new principal information cannot be expected in this field.…”

“…High stability is the typical feature of the phenoxy radicals derived from 2,6-dialkyl-substituted phenols, especially those with bulky t Bu substituents (sterically hindered phenols). For this reason, 2,6-di(tert-butyl)-4R-phenols are frequently utilized as very efficient antioxidants [2]. Monohydric phenols with 2,6-dialkyl substituents containing an a-CH bond, as well as phenols with unsubstituted or partially substituted ortho-position (sterically unhindered phenols) provide on oxidation unstable phenoxy radicals, which can be detected only by using special EPR techniques, e.g., flow or spin-trapping methods [3] [4].…”

Reactions of stabilizer compounds. III. Oxidative reactions of some 2′‐hydroxy‐2‐phenylbenzotriazole light stabilizers in the AIBN‐initiated autoxidation of cumene

“…used14J5 as a model system for the investigation of the inhibition of autoxidation reactions by hindered phenols and other antioxidant compounds. During inhibited autoxidation, under sufficient partial pressures of oxygen, phenolic antioxidants react with the intermediate cumylperoxy radical (IV); the nature of the final products from this reaction depends on the combined effects of reaction conditions and the structure and stability of the intermediate phenoxy radical (V) 16. Typically, the products developed from the reaction of hindered phenols in autoxidizing media are dimeric and polymeric phenols, quinones and quinonoid compounds, and peroxycyclohexadienones.PhCMe200' t PhCMe2H + PhCMe200H + PhCMe,' PhCMe,OO* + ArOH -+ PhCMe200H + ArO' non-radtcol products 2' SCHEME I The essential features of the initiated autoxidation of cumene inhibited by a chain-breaking antioxidant are shown in scheme 1.…”