Peter P. Klemchuk

Stabilization mechanisms of hindered amines

Abstract As a result of studying the interaction of hindered amine stabilizers (2,2,6,6-tetramethylpiperidines) with simple hydroperoxides, peroxy radicals, and acylperoxy radicals, the last two in AIBN-initiated oxidation experiments in chlorobenzene, the following conclusions have been reached: 1. 1. Hindered amines have multiple mechanisms of functioning as photo-stabilizers of polymers. 2. 2. Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization. 3. 3. Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures are very likely too slow and too inefficient for these to be important polymer photo-stabilizing reactions. 4. 4. Hydrocarbon polymer photo-oxidation proceeds by two major paths—the usually accepted alkyl radical/alkylperoxy radical/hydroperoxide route and the usually neglected aldehyde/acyl radical/acylperoxy radical/peracid route. 5. 5. Hindered amine stabilizers are able to participate in inhibiting both photo-oxidation reactions—they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitroxyl radicals in the process; the nitroxyl radicals trap alkyl radicals and the hindered amines trap alkylperoxy radicals to inhibit the other oxidation pathway. 6. 6. Nitroxyls are regenerated from N-alkyloxy hindered amines in a fast, efficient reaction with acylperoxy radicals and in a slow, inefficient reaction with alkylperoxy radicals. We postulate that neither reaction yields peroxides: carboxylic acids and oxidized alkyloxy substituents are obtained from the first reaction; alcohols and oxidized alkyloxy substituents are obtained from the second reaction.

Transformation products of hindered phenolic antioxidants and colour development in polyolefins

Abstract Several quinoidal oxidation products of 2,6-di-t-butyl-phenol-based antioxidants have been evaluated for UV/visible absorption characteristics in methylene chloride solutions and for contributions to Yellowness Index (YI) values in polyolefins. The YI values were determined in polypropylene (PP), high density polyethylene (HDPE) and linear low density polyethylene (LLDPE). The quinoidal compounds with conjugated π-electrons exhibited the highest extinction coefficients in the visible spectrum and also contributed the most colour to polymer formulations. 3,3′,5,5′-Tetra- tert -butyl-stilbene-4,4′-quinone had the highest extinction coefficient in the visible spectrum and caused the highest YI values in the polyolefins—its presence at 5 ppm in the polyolefins was already discernible visually. Colour development during use of polymeric articles containing hindered phenolic antioxidants is attributed to the formation of conjugated quinoidal products as a consequence of radical trapping by the hindered phenols. It is a natural consequence of the stabilization process which can be ameliorated through appropriate choice of antioxidants and through the use of trivalent phosphorus coadditives.

Degradable plastics: A critical review

Abstract A review of degradable plastics technology has revealed that several effective copolymer and additive methods are used commercially for photodegradable plastics. However, it was found that all commercial packaging plastics are not biodegradable; all of them because their molecular weights are too high and their structures are too rigid for assimilation by organisms, and most of them also because they have substituents which prevent biodegradation via the enzymatic fatty acid oxidation mechanism. Linear polyethylene is the only commercial packaging plastic with potential for biodegradation when its molecular weight has been reduced drastically by photodegradation. Degradable plastics are not a satisfactory solution to the problems of municipal solid waste. For those problems, multiple approaches need to be used, including especially recycling and incineration.

Hindered Amine Mechanisms: Part III—Investigations using isotopic labelling☆

Abstract The literature ‘peroxide’ mechanism and our proposed mechanism for the regeneration of hindered amine nitroxyls have been tested with isotopically labelled reactants. Both mechanisms deal with the reaction of alkylperoxy radicals with N-alkyloxy hindered amines, which are products of the reaction of nitroxyl radicals with alkyl radicals. The ‘peroxide’ mechanism predicts the regeneration of nitroxyl and formation of dialkyl peroxide. Our mechanism predicts the regeneration of nitroxyl and formation of ketones, alcohols and carboxylic acids, depending on the oxidizing substrate. The results of this work confirm the new proposed regeneration mechanism but not the previous ‘peroxide’ mechanism. The features of the confirmed mechanism are This basic mechanism is also proposed for acylperoxy radicals

Perspectives on the stabilization of hydrocarbon polymers against thermo-oxidative degradation

Abstract Hydrocarbon polymers are sensitive to thermo-oxidative degradation at all stages of their existence. Measurable oxygen consumption by unstabilized, powdered high density polyethylene was observed at as low a temperature as 40°C in this work. We also found relatively small degrees of oxidation and small degrees of chain scissioning of high density polyethylene resulted in drastic reduction of tensile elongation. Small amounts of antioxidants and other stabilizing additives are shown to be effective in protecting hydrocarbon polymers against thermooxidative degradation. Stabilizers protect polymers through the severe conditions of processing and fabrication and also in end use. Stabilized polypropylene samples which had been stored under ambient conditions for up to 20 years were found to retain high levels of original oven life. Hydrocarbon polymers usually can be stabilized through proper choice and concentration of stabilizing additives to meet most performance requirements.

Influence of pigments on the light stability of polymers: A critical review

Abstract Pigments can have significant influences, both favourable and unfavourable, on the light stability of polymer formulations. This paper reviews information on the manifestations of pigment influences on polymer stability and on their underlying causes. It aims to provide some understanding of the complex phenomena, identify possible causes of unfavourable pigment effects on polymer light stability, and provide some direction for improving the light stability of polymer-pigment-stabiliser systems of unfavourable performance. Favourable pigment influences on polymer light stability may be due to screening of harmful radiation, selective absorption of harmful radiation, and deactivation of polymer photoexcited species. Negative influences may be due to initiation of polymer photo-oxidation by photoexcited pigment chromophores, adsorption or chemical transformation of stabilising additives, influence on polymer morphology, or sensitisation of singlet oxygen formation. Pigment-polymer-stabiliser interactions are complex. Much work is needed to understand them better. A special challenge is a better understanding of why some orange, yellow, and red organic pigments exhibit negative influences on pigment light stability.

Protecting polymers against damage from gamma radiation

Abstract The sensitivity of polymers to oxidation and chain scissioning during exposure to high energy radiation is mitigated by the use of radical trapping stabilizers of the hindered phenol and hindered amine classes. The stabilizers undergo oxidative transformations in the process and, in the case of hindered phenols, yield colored products with conjugated π-electrons, which contribute to the discoloration of the polymers. Hindered amine stabilizers, in combination with trivalent phosphorus melt processing stabilizers, are the stabilizers of choice to protect polymers against gamma radiation since they are effective in protecting polymer properties and since they are usually much less discoloring than hindered phenolic stabilizers.

Flash photolysis and time-resolved electron spin resonance studies of triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers as electron and hydrogen atom donors

Abstract Transient absorption spectroscopy and time-resolved electron spin resonance (TR-ESR) were employed to understand the electron and hydrogen atom transfer quenching of triplet benzophenone ( 3 BP) in acetonitrile and benzene by amine and aminoether derivatives of 2,2,6,6-tetramethylpiperidine (hindered amine light stabilizers, HALS). The amines ( I and II ) are efficient quenchers of 3 BP. The observed quenching rate constants were found to be in the range 6 × 10 8 –6 × 10 9 M −1 s −1 and depend on the structure of the amine and the solvent. On the other hand, the aminoethers ( III and IV ) do not exhibit any quenching at all. Reactions of 3 BP with amines I and II in benzene yield ketyl radicals (detected by transient absorption spectroscopy) and the corresponding aminyl or aminomethyl radicals (detected by TR-ESR). Neither experimental method provided any evidence for products of the reactions of 3 BP with aminoethers III and IV . The experimental results are discussed from the viewpoint of the mechanism of polymer stabilization by HALS.