Initiation Of Photo-oxidation Research Articles

New synergists for hindered amine light stabilizers

The development of hindered amine light stabilizers (HALS) in the seventies led to a tremendous increase in the outdoor use of polyolefins. Although since that time a great deal of insight has been gained into the UV-degradation mechanism of polyolefins and the mechanism of action of HALS, no new UV-stabilization chemistry has been discovered. In this paper, the most recent thoughts about the UV-degradation mechanism of polyolefins (initiation of photo-oxidation by polymer oxygen charge transfer complexes (CTCs)) and new insights into the mechanism of action of HALS stabilizers (quenching of these CTCs) are explored as a basis for designing new UV-stabilizers. A mechanism is proposed that explains the action of HALS as a quencher. Based on this mechanism several other possible quenchers (bridged amines) have been suggested. It is shown that these bridged amines are active as UV-stabilizers in films and plaques and that they show a synergism with HALS stabilizers. These new stabilizers are the most effective when they are present at concentrations higher than 0.1% and at higher concentrations of HALS. In such cases the addition of these synergists to a HALS stabilized system can lead to an improvement of a factor of 2–3.

Mechanisms and kinetics of photostabilization of polyolefins with hals

AbstractThe HALS stabilization mechanisms usually envisaged do not explain the experimental facts. New mechanisms are proposed instead. They include quenching of excited charge‐transfer complexes (CTC) polymer‐oxygen by CTC HALS‐oxygen. In addition, the formation of CTC between peroxy radicals and NH‐HALS is of prime importance. These CTC HALS‐peroxy radicals can either react photolytically after absorption of UV radiation or with a second peroxy radical, thus promoting termination between two peroxy radicals.Initiation of photooxidation in HALS stabilized polyethylene involves usually CTC HALS‐oxygen throughout the useful lifetime of the polymer. In contrast to that, initiation of photooxidation in HALS stabilized polypropylene is classical and due to photolysis of tertiary hydroperoxides.The formal kinetics based on the mechanisms envisaged explain the experimental results observed with HALS stabilized polyethylene and polypropylene. On the one hand, they concern, the performance of HALS: in polyethylene it is a linear function of the square root of HALS concentration whereas in polypropylene it increases linearly with HALS concentration as long as the last does not become too high. On the other hand, the experimental facts explained, concern the decrease of the HALS concentration during W exposure of HALS‐stabilized polyethylene and polypropylene. The decrease is first order in both polymers but is independent of the initial HALS concentration in polyethylene, whereas it decreases with increasing initial HALS concentration in polypropylene. This is in agreement with the corresponding initiation mechanisms envisaged for polyethylene and polypropylene.

Thermal and photo-chemical degradation of nylon 6,6 polymer: Part 1—Influence of amine-carboxyl end group balance on luminescent species

Nylon 6,6 polymers of different degrees of polymerisation have been characterised by ultra-violet (uv) absorption, fluorescence and phosphorescence emission spectroscopy, viscosity number and amine end-group balance and the data related to their subsequent thermal and photo-chemical stabilities. The latter, determined by the percentage change in viscosity number, clearly shows that in both cases polymer stability is markedly influenced by the final stages of polymerisation under both steam and nitrogen atmospheres, the latter being the most detrimental. During both of these processes the molecular weight increases, as indicated by an increase in viscosity number, as does the concentration of fluorescent and phosphorescent impurities and products absorbing uv at 294 nm. Extraction of some of these species by 2-propanol results in a marked improvement in polymer photo-stability, implicating the involvement of these chromophores in initiation of photo-oxidation. Thermal degradation at 120°C confirms the presence of two types of fluorescent species. The first is reactive, rapidly diffuses out of the polymer and is associated with cyclic α, β-unsaturated carbonyl compounds; the second is non-volatile in-chain in type, increases in concentration and is associated with α-ketoimide structures. However, whilst correlations between the presence of light absorbing/luminescent carbonyl species is evident to some extent another, more important, factor based on the concentration of amine end-groups appears to be dominant. In this case a higher concentration of amine end-groups at lower degrees of polymerisation results in an increase in polymer stability, both thermally and photo-chemically and is associated with the radical/oxygen scavenging ability of the amine end-group, not unlike that of the well-known hindered piperidine light stabilisers. This is confirmed by the observation that treatment of nylon 6,6 films with sulphur dioxide gas, which reacts with the amine groups, thus generating sulphonamides, sensitises the photo-chemical oxidation of the polymer, an effect which contrasts with that found earlier for polyolefins.

Co-photolysis of poly(styrene) and poly(vinylacetophenone)—I. Vacuum photolysis

Films made from blends of poly(styrene) (PS) and poly(vinylacetophenone) (PVAP) (up to 12% mol PVAP) were exposed under high vacuum to long wave u.v. radiation ( λ > 300nm). Spectral changes (phosphorescence, NMR and i.r. indicated that PVAP was photolysed: the nature of gaseous products (in order of abundance, methane, carbon monoxide, ethane and acetaldehyde) confirmed that PVAP was undergoing a Norrish Type I decomposition. There was also evidence of photoreduction, OH absorptions appearing in the i.r. The blended films undergo cross-linking but the insoluble material formed after about 10hr contains little carbonyl. The effects of PVPA concentration and molecular weight on the rate of product formation were also investigated. Experiments with poly(α-D-styrene) confirmed that methyl radicals were abstracting H atoms at the α-position in PS. The implications of such reactions in the initiation of photo-oxidation by polymeric ketonic species are discussed.

On the Possible Initiation of Photooxidation by Charge-Transfer Excitation

On the Possible Initiation of Photooxidation by Charge-Transfer Excitation