D.M. Wiles

Polyolefin oxidation: Quantification of alcohol and hydroperoxide products by nitric oxide reactions☆

Abstract A new procedure has been developed for the identification and quantification of key products from the oxidation of solid polyolefins, based on reactions with gaseous nitric oxide. Alcohol and hydroperoxide groups are converted to nitrites and nitrates, respectively, and can be detected with high sensitivity by infrared spectroscopy. Furthermore, the nitrate and nitrite absorptions are influenced by the primary, secondary or tertiary nature of the alkyl substituents so that it is possible to identify differing sites of attack in branched polyolefins. Preliminary results from the application of nitric oxide reactions to the analysis of polyethylene and polypropylene oxidized by γ-irradiation are used to illustrate the potential of the technique and a comparison made with other current methods.

Polyethylene hydroperoxide decomposition products

Abstract The decomposition products from pre-oxidized, linear low-density polyethylene have been identified and quantified for films exposed in the absence of oxygen to ultra-violet irradiation, heat or γ-irradiation. Films were preoxidized under controlled conditions by γ-irradiation in air. Vacuum decomposition products were then analyzed by infrared derivatization techniques to allow the quantification of carboxylic acid, ketone, ester, γ-lactone, alcohol, hydroperoxide and unsaturation products. From the relative proportion of these products it is possible to compare the mechanisms of the three initiation methods. These mechanisms are compared and contrasted with previously proposed mechanisms for polyethylene degradation.

Polyolefin photo-stabilisation mechanisms. Reactions of tetramethylpiperidine derivatives in model systems☆

Abstract The relative rate of cyclohexyl radical scavenging by oxygen or 4-oxo-2,2,6,6-tetramethylpiperidino-N-oxyl has been measured at 25°C, together with the relative rates of cyclohexyl peroxy radical attack on cyclohexane or substituted hydroxylamines derived from 2,2,6,6-tetramethylpiperidine. These competitive processes are important in the stabilisation of polymers against photo-oxidative destruction by piperidine derivatives. Peroxy radical scavenging by the substituted hydroxylamine appears to be considerably more important than alkyl radical scavenging by the nitroxide, although both processes are essential for prolonged ultra violet (uv) stabilisation of a polymer. However, a comparison of experimental photo-protection with that predicted by the measured rate constant ratios shows that other processes are needed to account entirely for the observed stabilisation. Other factors which may be involved in piperidine photo-protection of polymers include thermal decomposition of the substituted hydroxylamine to reform nitroxide in polar(oxidised) zones and especially the association of nitroxides with hydroperoxide groups (the dominant photo-initiator in degrading polyolefins).

Photostabilisation mechanisms in polymers: A review☆☆☆

Abstract Key degradative processes for various polymers are itemised, together with photostabilisation mechanisms which can minimise these processes. Polymers can be conveniently classified as inherent absorbers, which absorb large amounts of solar uv, and as ‘non-absorbers’, that is polymers transparent in the near uv. The former group which includes aramids, polycarbonates and polyesters is predominantly photodegraded by primary processes causing direct bond scission and stabilisation is best effected by the use of uv absorbing additives or opaque pigments. The ‘non-absorbers’ include polyolefins and poly(vinylchloride) which are degraded as the result of oxidative chain processes initiated by chromophoric impurities. For polypropylene the dominant photooxidation product is hydroperoxide which photocleaves to initiate further oxidation. Various uv. stabilisers for polyolefins appear to owe their effectiveness to their ability to decompose hydroperoxide groups and/or to their ability to scavenge free radical intermediates in the oxidative process.

Polypropylene photo-stabilization by hindered secondary amines. A spectroscopic investigation

Abstract The conversion products from a commercially important photo-stabilizer based on 2,2,6,6-tetramethylpiperidine have been identified and quantified during the photo-oxidation of polypropylene. From a comparison with model compounds and model systems, direct evidence for the dominant involvement of grafted substituted hydroxylamine species was obtained by infra-red spectroscopy. Non-stabilizing conversion products, including N -acyloxy compounds, were also identified and the overall mechanism of photo-protection by hindered amine stabilizers is discussed.

Oxidation of solid polyethylene films: Effects of backbone branching

Abstract The oxidation rates and product distribution of a series of linear and branched polyethylene films have been studied when initiated by γ-irradiation. Branch groups in laboratory-synthesized samples ranged from ethyl to C 16 H 33 —and were at levels comparable with those found in commercial linear-low and low-density polyethylenes. In the complete absence of stabilizers and at the controlled rates of radical formation possible with γ-irradiation, all of the polyethylene samples oxidized at essentially the same rate to give the same mixture and distribution of products. The differences in oxidation rates previously attributed to the presence of tert. CH groups at branch sites more probably originate from the effects of impurities, especially on photo- and thermal initiation processes.

Photo-stabilizing efficiency of N-substituted hindered amines in polypropylene: Effects of processing conditions and exposure to a protonic acid☆

Abstract The effects of exposure to processing conditions and strongly acidic gases on the photo-stability of polypropylene films stabilized with hindered amine light stabilizers (HALS) have been studied. A series of monofunctional hindered amines based on tetramethylpiperidine containing and substituents in the piperidyl ring was compared with a bifunctional secondary amine. Exposure to heat and shear in a Brabender Plastograph caused loss by volatilization and decomposition of the additive. All of the HALS derivatives formed salts upon exposure to gaseous HCl and their photo-stabilizing ability was greatly impaired although the monofunctional secondary amine was superior to the bifunctional amine under these conditions. The additive was extensively decomposed by the HCl exposure.

The oxidative degradation of ethylene/vinyl alcohol copolymers☆

Abstract The oxidative behavior of ethylene/vinyl alcohol copolymers has been examined after initiation by γ-irradiation. Oxidation products were identified and quantified by infrared spectroscopy after the use of selective derivatization reactions which allowed the resolution of the various alcohol, hydroperoxide and carbonyl species produced. Extensive preferential attack at the sites was observed to give backbone ketone and probably hydrogen peroxide. The latter was not unambiguously identified or quantified because of its volatility. The derivatization techniques failed at vinyl alcohol levels above 20 wt%, probably because of the low gas permeability of these films. Implications of the observed products to the oxidation of polyethylene are discussed.

A spectroscopic study of the carbonisation of poly(acrylonitrile) and poly(acrylonitrile-co-acrylic acid)

Abstract The advantages and limitations of modern infra-red (ir) and solid state 13 C nuclear magnetic resonance (NMR) spectroscopic techniques are described for the characterisation of poly(acrylontrile) (PAN) and poly(acrylonitrile-co-acrylic acid) after various thermal treatments. The thermal treatments were chosen to model the pre-oxidation and carbonisation steps in the production of carbon fibres from PAN. Infra-red is particularly useful in following reactions of nitrile groups and the production of carbonyl species. Nuclear magnetic resonance spectroscopy is particularly valuable in following aromatisation processes. Both techniques are more limited by problems in spectral assignments than by difficulties in obtaining spectra. At high carbonisation temperatures (>500°C) 13 C NMR is restricted by the reduction in the amount of hydrogen available for the effective use of cross polarisation techniques.

Oxidation Behavior of High Strength Chain-Extended Polyethylene Fibers1

The resistance to oxidation of polyethylene fiber intended for engineering applications has been compared with that of conventional polyethylene films. The highly ordered fiber was appreciably more resistant to γ-initiated oxidation and somewhat more resistant to photo-oxidation. Oxidation products from the γ-initiated oxidation were quite highly ordered with respect to the fiber axis as shown by polarized infrared spectroscopy. Photo-oxidation products showed random orientation, possibly as a result of the more extensive chain scission that must accompany the photo-process. Despite the higher resistance of γ-radiation damage of the chain-extended fiber structure as compared to unoriented film, the irradiated fiber did show a slow post-irradiation oxidation.