Hindered Amine Stabilizers Research Articles

The determination of nitroxide radical distributions in polymers by EPR imaging

The concentration of nitroxide radicals formed by photooxidation of one of four different hindered amine stabilisers (HAS) in polypropylene plaques 2 mm thick subjected to continuous irradiation for periods ranging from 1 to 6 months, has been measured by electron paramagnetic resonance (EPR) spectroscopy. The spatial distribution of these radicals across the polymeric plaques has also been determined either by cutting thin slices of the plaque and then measuring the nitroxide concentration within each slice, or by using an X-band EPR imaging technique (EPRI) which provides the radical concentration profile along the magnetic field direction. Experiments on samples containing two UV absorbers, beside the HAS stabilisers, are also reported. It is shown that the changes of the nitroxide distribution inside the polypropylene plaques observed by increasing the irradiation time, follow some interesting trends peculiar to the additives contained in the polymer. It seems that the EPRI technique may provide useful information on the mechanism of stabilisation by HAS of polymers subjected to photo-degradation.

A Comparative Study of the Efficiency of a Typical Hindered Amine Stabilizer in Different Hydrocarbon Polymers

Abstract Inhibition of the photo-oxidation of three different polymers by the same HALS has been studied: an ethylene-propylene random copolymer (EPM), a styrene-butadiene-styrene block copolymer (SBS) and the same block copolymer previously hydroperoxidized by reaction with singlet oxygen. Two techniques have been used and compared: oxygen absorption measurements and infra-red spectroscopy in the carbonyl region. The results indicate that HALS inhibits more efficiently the propagation of the photo-oxidation in the unsaturated polymers studied. However, it also catalyses a new type of termination that in EPM delays and in SBS promotes the production of ketones. It is suggested that the practical efficiency of HALS could be mainly dependent of that last effect.

Mechanisms of thermooxidative stabilisation with HAS

Hindered amine stabilisers (HAS) stabilise polyolefins against thermooxidative degradation. The effects are well documented, especially with polypropylene. However, the mechanisms responsible for these effects have been studied much less than the mechanisms of protection from photooxidation. Stabilisation of PP against thermooxidation with HAS is compared with the corresponding stabilisation with phenolic antioxidants. The analogies and differences of both stabilisation modes are discussed. The possible stabilisation mechanisms of HAS under thermooxidative conditions are examined in detail. The kinetics of stabilisation are developed for tertiary HAS. These kinetics are applied to a quantitative interpretation of long-term heat ageing data obtained with PP. It is found that there is good agreement between experimental and theoretical dependence of the lifetime of HAS stabilised PP on HAS concentration.

The mechanism of action of hindered amine stabilizers (HAS) as long-term heat stabilizers

Hindered amine stabilizers (HAS) are promoted as long-term heat stabilizers. A difficulty for this application is that at low temperatures (below 100°C) the effectiveness of HAS is more pronounced than at high temperatures (150°C). In order to measure the effectiveness of HAS, prolonged experiments are necessary. Another difficulty is that for HAS-containing polypropylene (PP) the mechanical properties drop gradually, while for phenolic antioxidants a fast decline of the mechanical properties is observed after a long period of no change. These two difficulties are due to different mechanisms of action for phenolic antioxidants and HAS and to a change of the degradation mechanism of PP with temperature. At high temperatures the degradation of PP is mainly due to the oxidation of the polymer itself, while at low temperatures the oxidation of oxidation products is more important. This secondary oxidation results in peracids, via aldehydes, which determines the oxidation rate at low temperatures. Model experiments show that HAS are capable of preventing this aldehyde oxidation, thereby giving good performance at low temperatures. These experiments also show that HAS are not able to prevent the oxidation of hydrocarbons, which takes place at high temperatures. The slow decline of the mechanical properties of HAS-containing PP is due to the oxidation of the polymer itself, which is not stopped by the HAS.

Photostabilization of photocrosslinked unsaturated polyesters

The photo-oxidative stability of photocrosslinked unsaturated polyesters endcapped with glycidyl acrylate has been investigated by measuring UV, IR and ESCA spectra of thin films and tensile properties of photocrosslinked glass fiber laminates of the polyesters without and with added stabilizers. When the resulting films and laminates are aged by irradiation in a UV-CON irradiator for various lengths of time, surface oxidation occurs to a certain level. The addition of 0·15–0·30% hindered amine stabilizer based on 2,2,6,6-tetramethylpiperidine, TINUVIN-770, TINUVIN-292 and TINUVIN-144 to the resins before crosslinking decreases the rate of surface oxidation and prevents the decrease in tensile strength that occurs due to aging without the addition of the stabilizers. TINUVIN-770 with NH groups is found to provide higher levels of stabilization in the UV crosslinked unsaturated polyester system than the other two stabilizers with NCH 3 groups, especially during the early stages of irradiation.

Semiempirical prediction of the thermochemistry of intermediates involved in the cyclic mechanism of hindered amine stabilizers

The suitability of the AM1 computational method for predicting the thermochemistry of amine derivatives related to the functioning of hindered amine light stabilizers (HALS) was established by comparing computed data with representative published experimental data for HALS-related intermediates. Thus, AM1 accurately predicts the relative energies of 2,2,6,6-tetramethyl-4-oxopiperidine, 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl and 1-hydroxy-2,2,6,6-tetramethyl-4-oxopiperidine. The AM1 method was then utilized to estimate key bond dissociation energies for which experimental values do not exist. For the 2,2,6,6-tetramethyl-4-oxopiperidine series the following bond strengths are predicted: NO • = 366 kJ/mol; NOH = 234 kJ/mol; NOH = 296 kJ/mol; NOCH 3 = 176 kJ/mol; NOCH 3 = 185 kJ/mol; NOCH 2H = 359 kJ/mol; NOC 2H 5 = 164 kJ/mol; NOC 2H 5 = 172 kJ/mol; NOCH 2HCH 3 = 344 kJ/mol; NOCH(CH 3) 2 = 143 kJ/mol; NOCH(CH 3) 2 = 154 kJ/mol; NOCH(CH 3) 2 = 333 kJ/mol. The data should be of general value for the thermodynamic evaluation of proposed HALS mechanisms.

Protecting polymers against damage from gamma radiation

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.

The influence of substituents on the photo-stabilizing efficiency of hindered amine stabilizers in polypropylene

The following series of 4-benzoyloxy and 4-stearoyloxy 2,2,6,6-tetramethyl-piperidine derivatives substituted at the nitrogen atom were prepared: H, OH, OCOCH 3, COCH 3 and O •. The photo-stabilizing efficiency in these series depends on the method of incorporation of stabilizer into isotactic polypropylene. The N—O • and N—OH derivatives were usually the most efficient. ESR spectroscopy revealed that during processing in a Brabender, the concentration of the NO •-radicals decreased. On the other hand, an increasing NO •-radical concentration was observed and a maximum was reached in the first stages of irradiation. Some mechanistic aspects of these observations are discussed. 4-Stearoyloxy-derivatives were found to be substantially more effective than 4-benzoyloxy derivatives.

The Mechanism of Action of Hindered Amine Stabilizers. Production of Nitroxy Radicals and Inhibition of the Photo‐Oxidation in Different Polymers

AbstractOxidation of bis[2,2,6,6‐tetramethyl‐4‐piperidinyl] sebacate (HALS) to itroxy radicals has been monitored by ESR spectroscopy in three polymers: an ethylene‐propylene random copolymer (EPM), a styrene‐butadiene‐styrene block copolymer (SBS) and the same block copolymer previously hydroperoxidized by reaction with singlet oxygen. Nitroxy radicals are produced in all three polymers on irradiation in the presence of oxygen and, in the last one also in the dark, at room temperature. The mechanism of the reaction is discussed: it involves direct interaction of a peroxy radical with the piperidine nitrogen. The role of clustered hydroperoxides as a source of peroxy radicals is emphasized.Inhibition of the photo‐oxidation of all three polymers by HALS has been studied by oxygen absorption measurements and by infra‐red spectroscopy in the carbonyl region. The results indicate that HALS not only inhibits the propagation of the photo‐oxidation but also catalyses a new type of termination that, in some polymers, delays the production of ketones. It is suggested that the practical efficiency of HALS in keeping the mechanical properties of polyolefins exposed to sunlight is mainly due to that last effect.

Stabilization mechanisms of hindered amines

AbstractAs 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: Hindered amines have multiple mechanisms of functioning as photostabilizers of polymers. Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization. Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures occur at varying rates with varying effectiveness for stabilization. With favorable alignment among reaction rates for oxidation propagation and termination, reactions between tetramethylpiperidines and alkylperoxy radicals can play a significant role in oxidation inhibition. Hydrocarbon polymer photooxidation 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. Hindered amine stabilizers are able to participate in inhibiting both photooxidation 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. Nitroxyls are regenerated from N‐alkyloxy hindered amines in a fast, efficient reactions with acylperoxy radicals and in slow reactions with alkylperoxy radicals. We postulate 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.

Wavelength sensitivity of acrylonitrile–butadiene–styrene

AbstractThe wavelength sensitivity of unpigmented 100 mil thick ABS exposed to sunlight and filtered xenon are radiation was determined by the sharp cut filter technique based on three types of photochemical changes: bleaching, yellowing and loss in impact strength. Bleaching of the yellow‐colored species formed in the processed material is caused by wavelengths between 380 and 525 nm with maximum color change by the 475–485 nm region. Photochemical yellowing is due to wavelengths between 300 and 380 nm with all wavelengths being almost equally effective. The spectral sensitivity based on change in impact strength shifts from the UV to the visible region as photochemical yellowing progresses. Addition of two stabilizers, a benzotriazole ultraviolet absorber and a hindered amine stabilizer, shifts the wavelength sensitivity based on yellowing to wavelengths shorter than 330 nm, but has no influence on the spectral effects based on impact strength. It is postulated that the rate of yellowing is reduced mainly by the ultraviolet absorber and stabilization against loss in impact strength is due largely to the hindered amine. Differences in rates and spectral response of the three types of photochemical changes indicate that they are due to different initiating mechanisms and thus require different types of stabilization. The significance to stability testing is discussed.

Stabilization mechanisms of hindered amines

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.

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

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.

Photo-oxidation poly(butadiene); quenching of excited states by nitroxides

The phosphorescence from three model bicyclic enones and those formed in thermally oxidized poly(butadiene) has been shown to be quenched efficiently by a nitroxide [3,3,4,4-tetramethyldiazetine N, N′-dioxide] in CH 2Cl 2:tetrahydrofuran glass at 77K. The Perrin model for static quenching was applied, and the interchromophoric distance for half-quenching found to be ∼ 12 A. At room temperature the nitroxide additive inhibited the formation of volatile products from thermally oxidized poly(butadiene) films: results also obeyed the Perrin relationship, pointing to a quenching mechanism of photostabilization. By contrast, a commercial hindered amine stabilizer was found not to act as a quencher of phosphorescence at 77K, and to have a concentration dependence for the reduction in volatile product formation at 298K in PBD films different from the nitroxide, indicating different mechanisms of action.

Photostabilization of commercial polypropylene by a hindered amine stabilizer

AbstractThe photostabilization of commercial polypropylene by a hindered amine stabilizer is examined by luminescence and ESR spectroscopy. The presence of the amine stabilizer inhibits the photolysis of the α,β unsaturated carbonyl impurity groups present in the polymer. Examination by ESR, of the stabilized polymer before and after irradiation shows the development of a highly stabilized free radical indicating that the stabilizer can act as an ultraviolet stable antioxidant. Possible mechanisms whereby the hindered amine inhibits the photolysis of the α,β unsaturated carbonyl groups are discussed.