L. Taimr

The action of anthraquinone sensitizers in the photo-oxidative degradation of low density polyethylene: Mechanical evidence of dark processes

Abstract Low density polyethylene (LDPE) films containing three quinoid sensitizers, anthraquinone (I), 2-butylanthraquinone (II) and 2-octanoyloxyanthraquinone (III), have been exposed to the uv light of a fluorescent tube. The kinetics of the photo-oxidative degradation have been monitored by measuring infra-red spectra and three mechanical characteristics derived from stress-strain traces, viz. strain at break, Youngs modulus and yield stress. During the first stage of the process, the greater solubility of (II) and (III) in LDPE as compared to (I) was clearly manifested in both the mechanical and spectral data. The sensitizers are consumed at the very beginning of the exposure, but once the degradation starts it continues at a higher rate around introduced or intrinsic centers. The increase of yield stress values after a long-term dark storage which followed the irradiation period is ascribed to a slow continuous crystallization of the degraded polymer. No dark storage effect has been found with samples not previously exposed to uv light.

Antioxidants and stabilizers: Part XCV—A co-operative effect between antioxidants N-iso-propyl-N′-phenyl-1,4-phenylene diamine and 2,6-di-tert-butylphenol

Abstract In the process of stabilization against the autooxidation of hydrocarbons, the more effective of the two title antioxidants, i.e. N - iso -propyl- N ′-phenyl-1,4-phenylenediamine (Ia), is transformed into N - iso -propyl- N ′-phenyl-1,4-benzoquinonediimine (IIa). The stability of (IIa) is reduced in an acid medium. In an inert atmosphere and in the presence of weak acids (IIa) reacts quickly with 2,6-di-tert-butylphenol, giving rise to 3,3′,5,5′-tetra-tert-butyl-4,4′-biphenyldiol (VI) and 2,6-di-tert-butyl-4-(4-phenyl-aminophenylamino)-2,5-cyclohexadiene-1-one (Va). In the presence of a relatively low amount of oxygen (Ia), 3,3′,5,5′-tetra-tert-butyldiphenoquinone (IV), (Va) and 2,6-di-tert-butyl-1,4-benzoquinone (VII) are formed. In this way, conditions for co-operative action between the two antioxidants are provided. In the absence of acids the reaction proceeds much more slowly and (Va) is not formed. The mechanism of formation of all the compounds is discussed. There is no pronounced synergism between (Ia) and 2,6-di-tert-butylphenol during the autooxidation of squalene (natural rubber model) in pure oxygen and in a neutral medium. In the presence of 1 vol. % of acetic acid the effectiveness of antioxidant (Ia) is strongly reduced, particularly at elevated concentrations. In a mixture of (Ia) with 2,6-di-tert-butylphenol, a pronounced co-operative effect can be observed.

Antioxidants and stabilizers: Part CXII—Influence of 1-phenylamino-4-(sec-alkylamino)-3,6-bis(4-phenylaminophenylimino)-1,4-cyclohexadiene on the vulcanization and ageing of natural rubber☆

Abstract Commercial antidegradants having the structure of N-sec-alkyl-N′-phenyl-1,4-phenylenediamine (I) are contaminated with dark coloured compounds. Quinone imines having the structure of 1-phenylamino-4-(sec-alkylamino)-3,6-bis(4-phenylaminophenylimino)-1,4-cyclohexadiene (III) were isolated from 1,4-phenylenediamines (Ia, b) and their influence on the vulcanization of natural rubber and ageing of vulcanizates was tested. It was found that compound (III) has no harmful effect on ageing characteristics. This finding was confirmed by studies on autoxidized squalene. The processing safety of rubber compounds containing a sulphenamide accelerator is slightly reduced by (III) but the rate of crosslinking, the temperature dependence of the latter and physical properties of the vulcanizate are not influenced. The contamination of commercial (I) with quinone imine (III) has no harmful effect on the properties of the diamine (I). (III) contributes to the discoloration of rubber doped with (I).

Antioxidants and stabilizers: Part CI—A typical impurity contained in an industrial antiozonant, N-(1,3-dimethylbutyl)-N′-phenyl-1,4-phenylenediamine

Abstract A typical coloured impurity contaminating commercial N -(1,3-dimethylbutyl)- N ′-phenyl-1,4-phenylenediamine (HPPD) was isolated and its structure has been determined. The pathway of its formation during the production and/or storage of HPPD has been proposed.

Antioxidants and stabilizers: Part XCVI—Antioxidant behaviour of 2,6-di-tert-butyl-4-(4-phenylamino-phenylimino)2,5-cyclohexadiene-1-one

Abstract In the autoxidation of squalene (rubber model) the title compound (I) first behaves as an effective antioxidant which, however, gradually loses its efficiency in the course of autoxidation without being destroyed. This behaviour is caused by a reaction in which radical (III), derived from antioxidant (I), reacts with the hydrogen atom in the oxidation products of squalene. The hydrogen atom is not that of the hydroperoxide group. In the reaction, (I) is regenerated and an active radical is formed from the substrate which continues the oxidation. Oxidation of (I) with lead dioxide gave rise to the dimer of the radical (III).