J. Pospisil

Chemical and photochemical behaviour of phenolic antioxidants in polymer stabilization: A state of the art report, part II

Abstract In practical stabilization of polymers, phenols are mostly applied in combination with secondary antioxidants and light stabilizers. Mechanisms of interaction with these additives, as well as specific features of the reactivity of phenolic moieties creating a part of UV absorbers and bifunctional stabilizers, are presented together with the chemistry of the undesired reactivity of phenols with environmental pollutants and metallic impurities in polymers. The chemistry of α-tocopherol is mentioned as an example of an environmentally safe stabilizer.

Degradation and aging of polymer blends I. Thermomechanical and thermal degradation

The commercial importance of polymer blends implies interest in knowledge of their degradation behaviour under environmental stresses. Processes accompanying thermomechanical and thermal degradation of blends of commodity polymers are reviewed. Thermomechanical degradation, characteristic of melt processing, includes reactions between component macromolecules and relevant macroradicals. Cross-reactions giving grafted copolymers occur in some blends. Thermal degradation in an inert environment includes cross-reactions between macromolecules and low-molecular weight molecules or free radicals migrating over phase boundaries. As a consequence, thermal stability of the component polymers is either reduced or increased, according to the reactivity of involved species.

Influence of testing conditions on the performance and durability of polymer stabilisers in thermal oxidation

Information on the efficiency of stabilisers in protecting polymers against thermal oxidation under service conditions is essential for selection and development of appropriate stabilizing systems. Accelerated tests are necessary to get data in an acceptable time and need carefully selected failure criteria. Appropriate testing severity, avoiding misuse of accelerated methods and understanding of the limitations of the methods are essential if reliable data are to be obtained. The potential and limitations of oven ageing tests, oxygen uptake measurements, differential scanning calorimetry (DSC) and chemiluminescence (CL) in stabiliser testing are reviewed.

Chain-breaking stabilizers in polymers: the current status

The present knowledge of the trapping abilities of C-and O-centered free radicals by phenols, aromatic, nonhindered and hindered heterocyclic amines is outlined. In spite of their sacrificial character in polymer stabilization, additives which scavenge free radicals and break oxidation chains are not likely to be displaced in the near future by any other kind of stabilizers. The similarity between oxidative processes in vitro and in vivo and between activities of synthetic stabilizers and those of natural origin or their replicas enables a more efficient exploitation of mechanistic knowledge in protection of commercial polymers, human nutrients and biological systems.

Upgrading of recycled plastics by restabilization—an overview

Material properties of recycled plastics are diminished because of chemical degradation of the polymer matrix and partial or even complete depletion of the stabilizing system during their previous service time. This accounts for a decreased stability of the reclaimed plastics in reprocessing and subsequent long-term heat aging and/or weathering. The upgrading of the recycled post-consumer plastics by restabilization is mandatory for high-value products. Upgrading of re-used mixed plastics by compatibilizers is mentioned in this context with the restabilization concept.

Discoloration of polymers by phenolic antioxidants

Part of colour development in aged polyolefins stabilized with phenolic antioxidants can be attributed to the formation of conjugated diene compounds, arising as a consequence of sacrificial trapping of alkylperoxy radicals by phenolics. The discoloration depends on the structure and concentration of the phenol transformation products. Peroxycyclohexadienones are formed transiently in low concentrations and themselves do not discolour the polymer matrix. However, some products of their thermolysis are discolouring and accumulate slowly in the polymer matrix. The principal contribution to polymer discoloration is due to formation of quinone methides. Their discolouring effect is reduced in propionate-type phenolics, as a consequence of intramolecular rearrangement of a part of the primarily formed quinone methide, and is due to oxidative dimerization accounting for nonconjugated dimeric quinone methides. Attention was paid to transformations of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate and related quinone methides and 4-hydroxycinnamates.

The role of quinone methides in thermostabilization of hydrocarbon polymers-I. Formation and reactivity of quinone methides

Abstract The structures and properties of quinone methides(QM) formed by monoelectron oxidation from two commercially important classes of phenolic antioxidants are reviewed. Dimerisation of QM derived from the benzyl-type phenols is controlled by the steric hindrance on the exocyclic methylene group. Isomerisation of QM derived from the propionate-type phenols results in regeneration of the phenolic function.

The role of quinone methides in thermostabilization of hydrocarbon polymers —II. Properties and activity mechanisms

Retardation of thermal oxidation by quinone methides (QM) was observed in model experiments with liquid hydrocarbons. The retardative effect decreases with increasing temperature. Long-term heat ageing of polypropylene is more efficiently reduced by phenolic antioxidants prone to form the corresponding QM in situ. Potential mechanistic features involving reactivity of QM with free radicals, hydroperoxide decomposing antioxidants and hindered amines are reviewed from the point of view of thermal processes in polyolefins.

Physically persistent stabilizers by functionalization of polymers

The durability of polymers exposed to harsh application conditions is reduced by physical losses of stabilizers. Their physical persistence can be improved by increasing their molecular weight, without changing the action mechanism. Funtionalization of conventional polymers by grafting with monomers bearing stabilizing moieties during reactive processing, grafting during mastication, and phototriggered grafting are particularly suitable methods for synthesis of polymer-bound stabilizers. Properties and examples of application of stabilizer-functionalized polymers are given.

Lifetime prediction of ABS polymers based on thermoanalytical data

The service life of ABS polymer, stabilized by 2-(3,5-di-tert-butyl-4-hydroxyanilino)-4,6-bis(octylthio)-1,3,5-triazine and containing 50% of a modifying rubber component, was estimated from oxidative induction times measured by DSC in isothermal mode in the temperature interval 140–170°C. The lifetime of ABS powder at the actual temperature of drying was predicted by linear extrapolation according to Arrhenius. However, the extrapolated value was much longer than the real lifetime determined from the long-term oven aging tests at 70 and 90°C, simulating the industrial drying process. The effect of changes in the apparent activation energy of oxidation due to antioxidant consumption during polymer aging is discussed.