Hindered Piperidine Stabilisers Research Articles

Impact of stabilisers on the thermal catalytic activity of micro- and nano-particulate titanium dioxide in oxidizing condensed mediums

This investigation was undertaken to elucidate an interplay between stabilisers and titanium dioxide nano-particles in oxidizing condensed mediums comprising of the initiatied (initiator is 2,2′-azobisisobutyronitrile, AIBN) oxidation of cumene and ethylbezene and an intercomparison with the thermal oven ageing of high-density polyethylene (HDPE). As a preliminary study, the catalytic influence of different nano and micro-particle sized anatase and rutile titanium dioxide pigments on the uninitiated oxidation of cumene at moderate temperature (60 °C) has been established. The oxidation rates depend linearly on the square root of the concentration of the titanium dioxides over a sufficiently wide range and thereby follow the mechanism of oxidation with recombination of peroxy radicals. Kinetic measurements of oxidation rates of the model reactions of initiated oxidation of cumene and ethylbenzene in the absence and presence of a hindered piperidine stabiliser, Chimassorb 119 FL, Cyasorb 3529 and sterically hindered Irganox 1010 stabilisers clearly showed that the stabilisers promote over-additional initiating activity of titanium dioxide pigments. This effect was found to be very large, i.e. the values of afforded additional initiation rates are 2–9 times more than those compared with the value of the initial initiation rate induced by titanium dioxides in the system without stabilisers. In the case of hindered amine stabilisers this phenomenon is more pronounced than that for the Irganox phenolic antioxidants. On the basis of susceptibility towards all the examined stabilisers, resulting in a sharp increase of the initial initiating activity, the titanium dioxides can be ordered as nano-rutile untreated > nano-anatase treated hydroxyapatite ≥ nano-anatase untreated ≥ (nano-anatase (75%) + nano-rutile (25%)) untreated > micro-anatase untreated > micro-rutile treated. These additional initiation rates of titanium dioxides which are affected by the stabilisers can be explained in terms of physical absorption of the stabilisers onto the surface of the titanium dioxide particles. The stabilisers being adsorbed onto the surface of titanium dioxides block titania surface-OH groups and thereby prevent the formation of water in the oxidizing system to promote the additional catalytic capacity of the titanium dioxide pigments. Basic amine functionalities of Chimassorb 119 FL and Cyasorb 3529 are energetically more protonated by the titanium dioxides' surface hydroxyls during oxidation than when compared with phenolic hydroxyl and ester groups of Irganox 1010 and thereby the amine functionalities stimulate a relatively higher level of additional initiating activity of the pigments. This explanation is in good accordance with the data of work elsewhere [Allen NS, Edge M, Sandoval G, Ortega A, Liauw CM, Stratton J, et al. Factors affecting the interfacial adsorption of stabilizers on to titanium dioxide particles (flow microcalorimetry, modeling, oxidation and FTIR studies): nano versus pigmentary grades. Dyes Pigments 2006; 70(3):192–203]. Nano-anatase pigment C is shown to exhibit a sensitizing effect at a concentration of 1 and 2% w/w in the model reaction of cumene initiated oxidation whereas it exhibits a different effect in the thermal sensitizing oxidation at 1% w/w to the stabilising at 2% w/w during thermooxidative ageing in HDPE film.

Degradation and stabilisation of polymers and coatings: nano versus pigmentary titania particles

A series of nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in a metallocene polyethylene with a monomodal molar mass distribution and alkyd/acrylic based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, colour, gloss change and weight loss following artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photooxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photoactive with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer-pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photoactivity of the pigments in a polymer matrix. Photooxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. Here nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. The photosensitivity of titanium dioxide is considered to arise from localised sites on the crystal surface, and occupation of these sites by surface treatments inhibits photo-reduction of the pigment by ultraviolet radiation and hence the destructive oxidation of the binder is inhibited. Coatings containing 2–5% w/w alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% w/w. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminium phosphates of other metals. For example, the presence of hydrous aluminium oxide particles lowers Van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminium oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilisation in various water and oil based paint media in comparison with conventional organic stabilisers. Hindered piperidine stabilisers are shown to provide no additional benefits in this regard often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated.

A kinetic investigation into the effect of stabilisers on the photo-oxidation of water based silica acrylic-based coatings

Benzotriazole, triazine and hindered piperidine stabilisers have been incorporated into a copoly-methyl methacrylate/butylacrylate film that contains silica as a matting agent in order to investigate the effect of the stabilisers on photo-oxidation. The mode of addition of the stabilisers have also been examined by comparing their effects when directly added into the acrylic lacquer formulation and when first pre-adsorbed onto the silica before incorporation into the lacquer system. The photo-oxidation process was accelerated by irradiating the films in a Microscal unit (500 watt high pressure Hg/W source; λ>300 nm) and the rate of this photo-oxidation was followed using reflectance FT–IR spectroscopy to monitor changes in the concentration of key functional groups. Scanning electron microscopy was also used to examine changes in the physical appearance of the films. This study showed that the acrylic films with silica containing preadsorbed stabilisers are stabilised more effectively than those silica films containing directly added stabiliser. They also highlight the fact that the mode of addition of the stabiliser plays a significant role in the degree of stabilisation and that this, in turn, depends upon the type of functional group produced during photooxidation. Hydroxyl growth, de-esterification and vinyl group transformations were impaired by the preadsorbed silica based stabilisers. From the SEM analysis stabiliser preadsorption appeared to prevent oxidation of the acrylic matrix at the silica particle surface.

Aspects of the thermal oxidation, yellowing and stabilisation of ethylene vinyl acetate copolymer

The thermal oxidation of ethylene-vinyl acetate copolymer [EVA-17 and 28% w/w VA (vinyl acetate) units] has been examined by thermo-gravimetric and hydroperoxide analysis, FTIR (Fourier transform infra-red), fluorescence spectroscopy and yellowness index. Thermal analysis indicates the initial loss of acetic acid followed by oxidation and breakdown of the main chain. The degradation rate is greater in an oxygen atmosphere as is the formation of coloured products. FTIR spectroscopic analysis of the oxidised EVA shows evidence for de-acetylation followed by the concurrent formation of hydroxyl/hydroperoxide species, ketone groups, α, β-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydroperoxide evolution follows typical auto-oxidation kinetics forming ketonic species. In severely oxidised EVA, evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spectra of EVA is not unlike that reported for polyolefins confirming the presence of low levels of unsaturated carbonyl species. There are, however, significant differences in a long wavelength component in the fluorescence emission indicating the presence of other active chromophores. These long wavelength emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on PVC these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated carbonyl species of a limited length confined to the EVA blocks. During oxidation of EVA the original unsaturated carbonyl species remain as distinct emitting chromophores. This suggests that the growth and decay of these chromphores is virtually constant indicating that they could be an integral part of the EVA polymer that are responsible for inducing degradation. Degradation is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. The EVA degradation is therefore, different from that of PVC where in the latter case polyconjugated vinyl groups are evident through conjugated absorption bands in the UV spectrum. In the case of degraded EVA no such bands are observed. Also, degraded coloured EVA is not bleached by treatment with bromine, maleic anhydride or peracetic acid. Primary phenolic antioxidants exhibit variable activity in inhibiting the yellowing of EVA while combinations with phosphites and hindered piperidine stabilisers display powerful synergism confirming the importance of hydroperoxides as precursors. Thermal oxidation was also was displayed through the inhibition of lactone, carboxylic acid and alkene groups illustrating the fact that oxidation and yellowing are synonymous reactions.

Migration and release profile of Chimassorb 944 from low-density polyethylene film (LDPE) in simulated landfills

A low-density polyethylene film (LDPE) with a commercial polymeric hindered piperidine stabiliser was exposed to simulated landfill environments, viz. aqueous media at pH 5 and 7, temperatures of 25, 50, 80 and 105°C and solid-to-liquid ratios of 1:10 and 1:5. The migration of the stabiliser, including diffusion and desorption, and the resultant degradation of the polymeric matrix were studied. For this purpose, an ultrasonic extraction technique using chloroform as extraction solvent was explored to recover the non-migrated stabiliser from the polymeric matrix, followed by quantitative analysis of the extracted stabiliser by UV spectroscopy. It was found that the release of Chimassorb 944 from the degrading matrix increases at higher temperature, longer time, lower pH and higher solid-to-liquid ratio. In general, the kinetics of this total migration process are well described by equations of the first order and an Arrhenius-type relationship applicable for the temperature-dependence of the constants. General constants calculated for the total migration process allowed the complete release of the stabiliser at different conditions in landfill to be predicted. Surface morphology of the LDPE film before and after treatment was studied by scanning electron microscopy (SEM). Degradation of the polymeric matrix was shown by Fourier transform infrared spectroscopy (FTIR) and size-exclusion chromatography (SEC).

Stabiliser interactions in the thermal and photooxidation of titanium dioxide pigmented polypropylene films

The thermoxidative (oven ageing) and photooxidative degradation of polypropylene films (200 μm thick) containing a range of antioxidants and light stabilisers, together with anatase and rutile titanium dioxide pigments are studied by Fourier Transform infra-red (FTIR) spectroscopy. Rates of thermal and photooxidative degradation are determined by measuring the formation of non-volatile carbonyl and hydroperoxide oxidation products which absorb in the infra-red region of the spectrum with maxima at 1710 and 3410cm −1, respectively. During photooxidation the rutile pigment is synergistic in stabilisation with phenolic antioxidants and hindered piperidine stabilisers (HAS) but antagonistic with benzotriazole and benzophenone absorbers. In the case of anatase marked autocatalytic oxidation is observed giving strong antagonistic effects. With mixed antioxidant/stabiliser combinations, synergistic stabilisation is not significantly influenced by rutile but markedly antagonised by the presence of the anatase pigment. The most effective light stabiliser in retarding the catalytic oxidative effect of anatase is the polymeric HAS, Chimassorb 944. During thermal oxidation the polymeric HAS exhibit strong thermal antioxidant activity compared with the non-polymeric HAS. Both antioxidants and the benzophenone absorber are strongly antagonistic with rutile while the HAS and benzotriazole stabilisers display weak synergism. The anatase strongly accelerated and catalysed the thermal stabilisation effects with all the stabilisers and antioxidants. For the stabiliser combinations, the HAS are more effective at inhibiting oxidative degradation than the absorbers in the presence of both pigment types. The thermal catalytic effects of anatase are strongly suppressed by antioxidant/stabiliser combinations.

The photooxidation and stabilisation of water-borne acrylic emulsions

The influence of light exposure on the photooxidative stability of aqueous acrylic based latices is examined by Fourier transform-infrared spectroscopy (FT-IR) and hydroperoxide analysis. These are based on emulsion polymerised formulations of methyl methacrylate and butyl acrylate. Here acrylic latices based on different formulations are chosen as homo-polymers and copolymers. The nature of impurities and oxidation products generated during their manufacture are characterised and inter-related to their influence on subsequent photooxidative degradation. This involves the use of reflectance FT-IR spectroscopy to show functional group changes together with colorimetric UV analysis to determine photochemical generation of hydroperoxides. The early chemical changes and their subsequent influence on the physical and chemical properties of the latices during the later stages of photooxidation are found to exhibit a close inter-relationship. In terms of stabilisation co-reactive hindered piperidine stabilisers are found to be effective and nono-extractable. However, the incorporation of simple terminal dialkyl acrylamide/methacrylamide groups are equally as effective.

The photooxidative degradation and stabilisation of water‐borne acrylic coating systems

AbstractThe influence of manufacturing and processing parameters on the photooxidative stability of aqueous acrylic based latices is examined by FTIR and hydroperoxide analysis. These are based on emulsion polymerised formulations of methyl methacrylate and butyl acrylate. Here acrylic latices based on different formulations are chosen as homo‐polymers and copolymers. The nature of impurities and oxidation products generated during their manufacture are characterised and inter‐related to their influence on subsequent photooxidative degradation. This involves the use of reflectance FT‐IR spectroscopy to show functional group changes together with colourmetric U.V. analysis to determine the photochemical generation of hydroperoxides. The early chemical changes and their subsequent influence on the physical and chemical properties of the latices during the later stages of photooxidation are found to exhibit a close inter‐relationship. To date the nature of the surfactant and the composition of the latex in terms of end group modification are important parameters. De‐esterification and hydroperoxide formation are found to be important processes during latex photooxidation while the addition of low levels of co‐monomer to the emulsion give latices with improved photostability. While co‐reactive hindered piperidine stabilisers are found to be effective the incorporation of simple terminal dialkyl acrylamide/methacrylamide groups are equally as effective. Oxygen scavenging via the formation of alkylamino radicals is disussed.

The photostabilisation of water-borne acrylic coating systems

Emulsions based on a 50:50 polymerised formulation of methyl methacrylate and butyl acrylate have been prepared containing various reactive and non-reactive hindered piperidine light stabilisers and co-reactive acrylamides. The oxidation products generated during photooxidation are characterised and related to their influence on subsequent photooxidative degradation. This involves the use of reflectance FT-1R spectroscopy to show functional group changes together with colorimetric UV analysis to determine photochemical generation of hydroperoxides. De-esterification and hydroperoxide formation are found to be important processes during latex photooxidation while the addition of low levels of co-monomer to the emulsion, give latices with improved photostability. Thus, while co-reactive hindered piperidine stabilisers are found to be effective, the incorporation of simple terminal dialkyl acrylamide/methacrylamide groups is equally as effective. The efficiency of diaklyamide groups operating through a sacrifical oxidation mechanism is discussed.

Interactions in the thermal and light stabilising action of novel aromatic phosphites with a 2-hydroxybenzophenone and hindered piperidine stabiliser in polyolefin films

The interactions of two novel phosphite stabilisers with a commercial polymeric hindered piperidine stabiliser, Chimassorb 944, and a 2-hydroxy-4-n-ctoxybenzophenone stabiliser, Cyasorb UV 531, have been examined in the thermal and photooxidation of polypropylene and high density polyethylene films using FTIR spectroscopy. The two structures are based on a bis-hindered phenolic moiety coupled to a 2-hydroxybenzophenone and a 4-amino-tetramethylpiperidine (3) chromophore. Thermal oven ageing and photooxidation of the polyolefin films showed that the hindered piperidine phosphite (phosphoramide) is synergistic over a wide concentration ratio with both commercial stabilisers. The only exception is in polyethylene with the Chimassorb 944, wherein the latter shows poor solubility in polyethylene. Generally, the phosphoramide is found to be more effective as a thermal antioxidant than as a UV antioxidant. The 2-hydroxybenzophenone phosphite was generally ineffective as a light stabiliser and exhibited unusual S-shaped profiles in combination with both the commercial stabilisers, suggesting the involvement of two distinct processes. With Cyasorb UV 531 the antagonistic/synergistic patterns were reversed from polypropylene to polyethylene. With Chimassorb 944 strong synergism is observed at high phosphite ratios while antagonism is prevalent at low ratios. In general, one interesting feature of the results is that the thermal and UV effects often exhibit a similar behaviour.

Aspects of the inhibition of post-cure photo-yellowing of novel amine-diacrylate-terminated ultraviolet- and electron-beam-cured TMPTA-based coatings

The post-ultraviolet (UV) and -electron-beam (EB) cure photo-yellowing of four novel amine cosynergist terminated diacrylate monomers has been compared with that of a standard commercial diethylamine diacrylate monomer based on trimethylol propane triacrylate (TMPTA) using second-order derivative UV spectroscopy. Inhibition of photo-yellowing was also studied using four acrylated hindered piperidine stabilisers as well as an acrylated hindered phenol. Of the amine co-synergists the diethylamine diacrylate system gave the greatest degree of photo-yellowing in both UV- and EB-cured coatings whilst those based on piperazine gave the least yellowing. The presence of readily abstractable hydrogen atoms alpha to the nitrogen atom and subsequent ease of amine oxidation appears to be a key feature in the yellowing mechanism. The incorporation of acrylated and methacrylated hindered piperidine stabilisers as well as that of a hindered phenol are effective in impairing the photo-yellowing of both the UV- and EB-cured diethylamine diacrylate system. Of the stabilisers studied the acrylated hindered phenol was the most effective in both. UV- and EB-cured coatings. Inhibition of hydroperoxidation of the tertiary amine functionality in the amine diacrylate coating is considered to be of primary importance in the overall mechanism. Significantly lower levels of photo-yellowing are observed in the non-amine TMPTA. Here UV curing of the stabilised coatings gave rise to an initial yellowing which was photodecomposed on subsequent irradiation, the rate depending upon the stabiliser structure. The EB-cured coatings however, exhibited no initial yellowing followed by only a minor growth in photo-yellowing on subsequent irradiation, again with a rate dependent upon the stabiliser structure.

Thermal and photooxidative stabilisation behaviour of 4-naphthyl esters of 2-hydroxybenzophenone and 3,5-di- tert-butylphenol in polyolefin films

Two novel naphthyl ester derivatives have been synthesised and characterised based on a 2-hydroxybenzophenone and a 3,5-di- tert-butylphenol. Thermal and photochemical ageing studies have been carried out to determine their potential as stabilisers in polypropylene (PP). Whilst both compounds exhibit no oven ageing antioxidant properties alone they are somewhat effective melt stabilisers. Differential thermal analysis and mass spectrometry show both compounds fragment at the carbon-oxygen bond (CO) of the ester group. Whereas both compounds exhibit good oven ageing stability for PP with a thermal hindered phenolic antioxidant, Irganox 1010, antagonism is observed when combined with a hindered piperidine light stabilizer, Chimassorb 944. In the latter case the antagonism is significant in the presence of the benzophenone derivative. The benzophenone derivative is not an effective light stabiliser alone in PP despite its high absorptivity in the near-UV region 300–350 nm and its ability to act as an excited state quencher. However, it does exhibit strong synergism with both the hindered phenolic antioxidant and the hindered piperidine stabilisers. The naphthyl benzoate ester on the other hand is an extremely effective light stabiliser alone in PP, has weak absorptivity in the near-UV region and is unable to act as an excited state quencher. Strong synergism is observed with a hindered piperidine light stabiliser, Chimassorb 944 but not with a hindered phenolic antioxidant. Spectroscopic evidence indicates that the naphthylbenzoate ester undergoes a rapid initial photo-Fries rearrangement to produce a more strongly absorbing hydroxynaphthylphenyl methanone product.

Comparison of the thermal and light stabilising action of novel imine and piperazine based hindered piperidine stabilisers in polyolefins

A novel imine stabiliser based on 2-hydroxybenzophenone and a 4-amino-tetramethylpiperidine structure has been synthesised and characterised, as have two novel mono and bis-alkylated esters of tetramethylpiperidine with piperazine. Using FTIR spectroscopy, rates of polymer oxidation were measured by carbonyl index. Hydroperoxide formation was also measured. All three structures are shown to be highly effective thermal and light stabilisers for polyethylene and polypropylene. In all cases their thermal and light stabilising activities are consistent and follow their ability to inhibit hydroperoxide formation during thermal aging. The ability of the stabilisers to operate as effective radical scavengers is discussed and related to the effects of some commercial thermal and light stabilisers.

Comparison of the thermal and light stabilising action of novel aromatic phosphites substituted with 2-hydroxybenzophenone and hindered piperidine moieties in polyolefins

Three novel phosphite stabilisers have been synthesised and characterised and their thermal and photochemical effectiveness examined in polyethylene and polypropylene films. Two structures are based on a bis-hindered phenolic moiety coupled to a 2-hydroxybenzophenone (2) and a 4-amino-tetramethylpiperidine (3) chromophore. The third is a tris-2-hydroxybenzophenone based phosphite (4). Thermal oven aging and thermal analysis on the polymer films showed that the hindered piperidine phosphite is an effective primary and secondary antioxidant compared with the 2-hydroxybenzophenone derivatives. These results are consistent with its ability to inhibit the formation of hydroperoxides during thermal aging. The 4-aminopiperidine phosphite compound is also an effective light stabiliser for polypropylene. The 2-hydroxybenzophenone derivative (2) was ineffective as a light stabiliser in all cases and antagonised the effectiveness of a commercial polymeric hindered piperidine stabiliser (Chimassorb 944). The tris-2-hydroxybenzophenone phosphite compound (4), however, was more effective as a light stabiliser in polypropylene than in polyethylene. Both mass spectrometry and thermal analysis indicated initial fragmentation resulting in the loss of the 2-hydroxybenzophenone and 4-aminopiperidine stabiliser moieties from the phosphorus atom. the importance of combining different stabiliser moieties within the same structure in terms of performance is discussed.

Effect of processing on the thermal and photooxidative behaviour of 2-hydroxybenzophenone stabilisers in polyolefin films: Influence of 2,2,6,6-tetramethylpiperidine substitution

The effect of processing history has been examined on the thermal and photostabilising action of two types of 2-hydroxybenzophenone light stabilisers in polypropylene (PP) and Philips grade high density polyethylene (HDPE). Both differ in terms of the nature of the 4-substitution, one with a butoxy-4-amino-2,2,6,6-tetramethylpiperidinyl group and the other with an n-octoxy group. Using infrared absorption spectroscopy (FTIR), hydroperoxide analysis and differential scanning calorimetry, the former stabiliser is shown to exhibit both effective thermal (oven ageing and melt) and light stabilisation in the two polyolefins. Carbonyl growths are consistent with initial hydroperoxide concentrations in the polymers as well as the polymer type and its processing history. In both polymers, processing history reduces the stability of the stabilisers and their stabilising effectiveness in the polymers due to the formation of increased levels of hydroperoxides. The hindered piperidine stabiliser exhibits strong autoretardation on both thermal and photoageing, illustrating its behaviour as an effective photothermal antioxidant, the effect being dependent upon the processing history of the polymer. This is in marked contrast to the 4- n-octoxy stabiliser which exhibits no thermal antioxidant behaviour. Differences in the behaviour of the two types of stabiliser and the complex interplay of processing history are discussed.

Thermal and photooxidative behaviour of 2-hydroxybenzophenone stabilisers in polyolefin films: Effect of 4-butoxy-4-amino-tetramethylpiperidine substitution

The effects of 4-substitution of a butoxy-4-amino-2,2,6,6-tetramethylpiperidinyl group on the thermal and photostabilising action of 2-hydroxybenzophenone have been studied in polypropylene (PP) and Zeigler and Philips grade high density polyethylenes (HDPE), and these effects have been compared with the corresponding activities of 2-hydroxy-4- n-octoxybenzophenone (Cyasorb UV 531). Using infra-red absorption absorption spectroscopy (FTIR), hydroperoxide analysis, differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) the former stabiliser is shown to exhibit both effective thermal (oven ageing and melt) and light stabilisation in the three polyolefin systems. Carbonyl and hydroperoxide growths in the polymers are consistent, with the degree of thermal and photostabilisation being dependent upon the initial hydroperoxide concentrations in the polymers as well as the polymer type and its processing histroy. Both the oven ageing and thermal analysis data correlate well, showing a relationship with the rates of thermal decomposition of the stabilizers themselves. In both cases processing history reduces the stability of the stabilisers and their stabilising effectiveness in the polymers due to the consequent formation of increased levels of hydroperoxides. The hindered piperidine stabiliser exhibits strong autoretardation on both thermal and photoageing, illustrating its behaviour as an effective photothermal antioxidant. Fourier transform infra-red analysis and weight loss from TGA indicate that the hindered piperidine stabiliser undergoes a primary mechanism of hydrogen atom abstraction at the methylene-amine link by active hydroxyl radicals produced from the homolysis of hydroperoxides and subsequent splitting of the carbon-nitrogen bond into alkyl and amino radicals. The latter then effectively scavenge oxygen, preventing further oxidative breakdown of the polymer. The ability of the hindered piperidine stabiliser to complex and react with oxygen is also implicated in the overall scheme.

Thermal and photo-chemical degradation of nylon 6,6 polymer: Part II—Influence of hindered piperidine light stabilisers

The photo-stabilisation of nylon 6,6 polymer by various hindered piperidine stabilisers has been examined by viscosity number, ultra-violet absorption and luminescence spectroscopy. Photo-oxidation of the polymer is inhibited to varying degrees depending upon the structure of the molecule and its ability to react with terminal carboxylic acid groups. Thus, the monomeric stabilisers impart more stability than the larger polymeric types and this is associated with improved solubility and compatibility of the former types in the polar and highly crystalline nylon 6,6 polymer. Reaction of terminal carboxylic acid groups with 2,2,6,6-tetramethylpiperidin-4-ol (TMP) markedly improves the photo-stability of the polymer and this is confirmed by extraction experiments with 2-propanol. In all cases changes in viscosity number of the polymer films correlate closely with changes in their ultra-violet absorption at 294 nm during irradiation. Analysis of the polymers initially by fluorescence and phosphorescence spectroscopy indicates some degree of correlation between polymer stability and the concentration of cyclic α,β-unsaturated carbonyl oxidation products absorbing and emitting light with wavelength maxima at 297 and 330 nm, respectively. These results are compared and discussed with regard to earlier work on the stability of nylon 6,6 polymers with high concentrations of terminal amine groups.

Interaction between coloured pigments and hindered piperidine/antioxidant combinations in the photo-stabilisation of polypropylene

The complex interaction between coloured pigments, a primary antioxidant and a hindered piperidine compound in the photo-stabilisation of polypropylene has been examined using infra-red absorption spectros-copy. Alone, the pigments offered little photo-protection, their order of efficiency being copper phthalocyananine > chromic oxide > cadmium yellow. In the presence of an antioxidant only an additive effect was observed whereas, in the presence of a hindered piperidine stabiliser, there was powerful synergism. However, in the presence of both antioxidant and hindered piperidine compound the synergistic effects were considerably reduced with two of the pigments, cadmium yellow and chrome green, whereas, with copper phthalocyanine, the synergism was enhanced even further. The results indicate that additive adsorption is important with the first two inorganic pigments whereas, with the latter, the bulky organic ligands inhibit any such interaction. In fact, copper phthalocyanine and a hindered piperidine stabiliser exhibit a highly favourable interaction for photo-stabilisation.

Inhibition of the anthraquinone photosensitised oxidation of polypropylene by a hindered piperidine stabiliser: A spectroscopic study

A hindered piperidine stabiliser, bis{2,2,6,6-tetramethyl-4-piperidinyl}-sebacate, has been found to inhibit the photo-sensitising effect of anthraquinone in polypropylene. On flash photolysis in n-hexane solution transient formation due to the semi-quinone radical (AH·) at 370 nm and subsequent formation of the hydroquinone (9,10-dihydroxyanthracene (AH 2) were found to be effectively inhibited by the corresponding nitroxyl radical but not by the amine. The absence of the hydroquinone photoproduct was confirmed using fluorescence spectroscopy. With the aid of phosphorescence spectroscopy it is concluded that the nitroxyl radical is the effective stabilising species and operates by a radical scavenging mechanism to form the hydroxylamine, and not by excited state quenching.

Photo-stabilisation of commercial polypropylene by piperidine compounds: The rôle of stable free radicals

The photo-stabilisation of commercial polypropylene by a hindered piperidine stabiliser, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate, a model piperidine compound, 2,2,6,6-tetramethyl-4-piperidinol, and its corresponding N-oxy derivative is examined using luminescence and ESR spectroscopy. All three compounds inhibit the photolysis of the luminescent α,β-unsaturated carbonyl impurity groups present in the polymer. ESR spectroscopy shows that the hindered piperidine stabiliser operates by the same mechanism as that of the stable hindered piperidino- N-oxy radical compound. Possible mechanisms whereby the piperidine compounds inhibit the photolysis of the α,β-unsaturated carbonyl groups are also discussed.