Initial Hydroperoxide Concentration Research Articles

A unifying approach to lipid oxidation in emulsions: Modelling and experimental validation

Lipid oxidation is a longstanding topic within the field of food technology, and is strongly related to loss of product quality and consumer acceptance. Both for bulk oils and emulsions, the chemical phenomena involved in lipid oxidation have been extensively researched, and various reaction pathways have been identified. They are different in bulk oil compared to oil-in-water (O/W) emulsions in which the oil–water interface plays a prominent role. Most probably because of the complexity of the reaction scheme in combination with mass transfer effects, there is no model that describes lipid oxidation in emulsions in a unified fashion, and that is the aim that we have set ourselves to achieve.We use lipid oxidation data previously obtained in O/W emulsions made with 5 different emulsifiers (2 surfactants, and 3 proteins), in well-mixed systems where the oxygen-to-oxidizable lipid ratio is strictly controlled. We use data pertaining to headspace oxygen concentration, and to primary and secondary lipid oxidation products to develop a model based on reaction kinetics, including not only the classical reaction scheme (starting from an unsaturated lipid, LH) but also radical initiation from hydroperoxides, which is thought to be an effect that is overlooked in the classical description of the initiation step.We were able to describe the course of the reactions in these emulsions using the same reaction rate constants for all emulsions, with the exception of the two related to radical-based initiation. In Tween 20- and Tween 80-stabilized emulsions, initiation stems most probably solely from decomposition of hydroperoxides; this implies that lipid oxidation in these emulsions is co-determined by the initial (“pre-existing”) hydroperoxide concentration. In protein-stabilized emulsions, on the other hand, lipid radical initiation is probably linked to reactions involving proteins (co-oxidation reactions), whereas initiation through decomposition of hydroperoxides seems less important, if at all. From this, we can conclude that the difference between both types of emulsions with regard to lipid oxidation mechanisms is related to differences in radical initiation.The developed model can serve as a unified basis for understanding lipid oxidation in emulsions, through which additional effects beyond the bare reaction kinetics, such as mass transfer effects, can be identified and used to e.g., quantify antioxidant effects, which is part of follow-up research.

Sterol carrier protein-2 (SCP-2) involvement in cholesterol hydroperoxide cytotoxicity as revealed by SCP-2 inhibitor effects

Sterol carrier protein-2 (SCP-2) plays an important role in cholesterol trafficking and metabolism in mammalian cells. The purpose of this study was to determine whether SCP-2, under oxidative stress conditions, might also traffic hydroperoxides of cholesterol, thereby disseminating their cytotoxic effects. Two inhibitors, SCPI-1 and SCPI-3, known to block cholesterol binding by an insect SCP-2, were used to investigate this. A mouse fibroblast transfectant clone (SC2F) overexpressing SCP-2 was found to be substantially more sensitive to apoptotic killing induced by liposomal 7α-hydroperoxycholesterol (7α-OOH) than a wild-type control. 7α-OOH uptake by SC2F cells and resulting apoptosis were both inhibited by SCPI-1 or SCPI-3 at a subtoxic concentration. Preceding cell death, reactive oxidant accumulation and loss of mitochondrial membrane potential were also strongly inhibited. Similar SCPI protection against 7α-OOH was observed with two other types of SCP-2-expressing mammalian cells. In striking contrast, neither inhibitor had any effect on H(2)O(2)-induced cell killing. To learn whether 7α-OOH cytotoxicity is due to uptake/transport by SCP-2, we used a fluorescence-based competitive binding assay involving recombinant SCP-2, NBD-cholesterol, and SCPI-1/SCPI-3 or 7α-OOH. The results clearly showed that 7α-OOH binds to SCP-2 in SCPI-inhibitable fashion. Our findings suggest that cellular SCP-2 not only binds and translocates cholesterol but also cholesterol hydroperoxides, thus expanding their redox toxicity and signaling ranges under oxidative stress conditions.

Induction period in the low‐temperature thermal oxidation of saturated hydrocarbons: Example of polyethylene

AbstractThermal oxidation of hydrocarbon substrates at low‐to‐moderate temperature, typically T ≤ 150°C, results from a radical chain process initiated by hydroperoxide decomposition and displays an induction period. A reliable model exists to simulate oxidation kinetics, but an incertitude remains on initial steps because they are out of reach of all available analytical methods. This work is aimed to have a kinetic approach of the problem, by comparing various mechanisms, i.e., (A) bimolecular decomposition of initially present hydroperoxides; (B) combined uni‐ and bimolecular decomposition of hydroperoxides; (C) the presence of radicals at the beginning of the exposure; and (D) radicals generation at (low) constant rate from irradiation, for instance by ionizing radiation linked to natural radioactivity or from a direct oxygen–substrate reaction. Scheme A is not realistic at low initial hydroperoxide concentrations. All the other mechanisms generate similar behaviors: the induction time tends toward a constant value almost independent of the nature of initial steps, when the concentration of precursors (initially present hydroperoxides or radicals) or the rate of their initial production (from species other than hydroperoxides) tends toward zero. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 769–777, 2008

Thermolysis of polyethylene hydroperoxides in the melt, Part 12: Mechanisms and kinetics of thermolysis

The thermolysis of polyethylene hydroperoxides is attributed to the reaction of two hydroperoxide groups. This bimolecular reaction appears as a first-order reaction with the mean values of the hydroperoxide concentrations that can be used for the experimental verification of the kinetics. In low molecular mass liquids and solutions these findings would be irreconcilable. However, in polymer melts, this contradiction is more apparent than real. It is a consequence of the heterogeneous kinetics valid in polymer melts. The bimolecular reaction involves the decomposition of pairs of hydroperoxide groups that are relatively close in the elementary oxidation volumes. By diffusion these hydroperoxide groups can come close enough for reaction. From the chemical point of view the decomposition is a bimolecular reaction. However, from the kinetic point of view it is a first-order reaction of the hydroperoxide pairs. The dependency of the first-order rate on the initial hydroperoxide concentration is explained by the heterogeneous kinetics. The activation energy of the overall process can be related to the sum of the activation energies pertaining to the chemical reaction and to the diffusion process.

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.

Substrate Specificity, Localization, and Essential Role of the Glutathione Peroxidase-type Tryparedoxin Peroxidases in Trypanosoma brucei

Trypanosoma brucei, the causative agent of African sleeping sickness, encodes three nearly identical cysteine homologues of the classical selenocysteine-containing glutathione peroxidases. Although one of the sequences, peroxidase III, carries both putative mitochondrial and glycosomal targeting signals, the proteins are detectable only in the cytosol and mitochondrion of mammalian bloodstream and insect procyclic T. brucei. The enzyme is a trypanothione/tryparedoxin peroxidase as are the 2 Cys-peroxiredoxins of the parasite. Hydrogen peroxide, thymine hydroperoxide, and linoleic acid hydroperoxide are reduced with second order rate constants of 8.7 x 10(4), 7.6 x 10(4), and 4 x 10(4) m(-1) s(-1), respectively, and represent probable physiological substrates. Phosphatidylcholine hydroperoxide is a very weak substrate and, in the absence of Triton X-100, even an inhibitor of the enzyme. The substrate preference clearly contrasts with that of the closely related T. cruzi enzyme, which reduces phosphatidylcholine hydroperoxides but not H(2)O(2). RNA interference causes severe growth defects in bloodstream and procyclic cells in accordance with the peroxidases being essential in both developmental stages. Thus, the cellular functions of the glutathione peroxidase-type enzymes cannot be taken over by the 2 Cys-peroxiredoxins that also occur in the cytosol and mitochondrion of the parasite.

Hydroperoxide-lyase activity in mint leaves: Volatile C6-aldehyde production from hydroperoxy-fatty acids

The extraction of 13-hydroperoxide-lyase activity from mint leaves as well as its use for C6-aldehyde production was studied in this work. The enzyme cleaves 13( S)-hydroperoxy-C18 fatty acids into C6-aldehyde and C12-oxo-acid. Two mint species were tested: Mentha veridis and Mentha pulegium. The headspace injection method coupled to gas chromatography was used for volatile compound analysis. The optimal conditions for temperature and pH were, respectively, 15 and 7 °C. We also studied the specific synthesis of hexanal and hexenals respectively from 13( S)-hydroperoxy-linoleic acid and 13( S)-hydroperoxy-linolenic acid. Considerable quantities of aldehyde (up to 2.58 μmol) were produced after 15 min of cleavage reaction in 2 ml stirred at 100 rpm, especially in presence of extract of M. veridis. The conversion yields decreased from 52.5% as maximum to 3.3% when using initial hydroperoxide concentrations between 0.2 and 15 mM. An unsaturated aldehyde, the 3( Z)-hexenal was produced from 13( S)-hydroperoxy-linolenic acid. The 3( Z)-isomer was unstable and isomerized in part to 2( E)-hexenal. In this work, we observed a very limited isomerization of 3( Z)-hexenal to 2( E)-hexenal, since the reaction and the volatile purge were carried out successively in the same flask without delay or any contact with the atmosphere. These aldehydes contribute to the fresh green odor in plants and are widely used in perfumes and in food technology. Their importance increases especially when the starting materials are of natural biological origin as used in this work. GC–MS analysis allowed the identification of the products.

Thermolysis of polyethylene hydroperoxides in the melt 4. Effect of phenolic antioxidants and temperature on oxidation product formation

Phenolic antioxidants mostly lead to reduction of the oxidation product formation rate. This is similar to the effect of the phenols on the rate of thermolysis of polyethylene hydroperoxides in the melt. The most distinctive feature between the different products of hydroperoxide decomposition pertains to the maximum amount formed. For alcohol groups this maximum amount is lower in the presence of phenolic antioxidants than in their absence. For carbonyl groups it is the opposite. Formation of γ-lactones seems least affected by the presence of the antioxidants. Formation of trans-vinylene groups is most affected as the maximum amount formed is approximately doubled in the presence of phenolic antioxidants. Vinyl groups can be measured only in the presence of phenolic antioxidants. The effect of temperature on product formation has been determined mainly with the “initial linear” rates, the rates calculated according to the first-order rate equation showing too much scattering. The activation energy for alcohol group formation is practically the same in the absence and presence of phenolic antioxidants. It is comparable to the activation energy of pseudo-monomolecular hydroperoxide decomposition in polyethylene melts. The situation is different for carbonyl groups. In fact, the activation energy is depending heavily on the initial hydroperoxide concentration, much more than on the presence of phenolic antioxidants. It increases from 21 to 27 kcal/mol (88 to 113 kJ/mol) if polyethylene processing in air at 150 °C has been performed for 60 min instead of only 30 min. The activation energy for γ-lactone formation does not depend on the presence of phenols. For trans-vinylene groups, the activation energy seems to increase with the initial hydroperoxide concentration in the absence of phenols. This effect is much less pronounced in the presence of phenols. The most reliable value for the activation energy of vinyl group formation is close to 26 kcal/mol (109 kJ/mol).

Thermolysis of polyethylene hydroperoxides in the melt: 1. Experimental kinetics of hydroperoxide decomposition

Polyethylene hydroperoxides have been generated in an open mixer at 150°C. Their thermolysis has been examined in the temperature range 150 to 200°C. The overall kinetics of decomposition could be easily explained by primary thermolysis reactions combined with auto-accelerated decomposition involving the alcohol groups resulting from hydroperoxide decomposition. However, these kinetics do not allow distinguishing between primary first-order and second-order decomposition. Both mechanisms yield the same good fit for the experimental data. The difficulties encountered with the integral approach showed the necessity for different attempts to thermolysis data analysis. The methods consisted in the determination of the rate constants through use of the data for the first stages only of hydroperoxide decomposition. One of these methods consisted in plotting the reciprocal polyethylene hydroperoxide concentration as a function of thermolysis time. Another method consisted in the determination of the first-order decomposition rate constant for the initial stages of hydroperoxide decomposition. These procedures allowed eliminating to a large extent the contribution from auto-accelerated hydroperoxide decomposition. Nonetheless, even with these precautions, the calculated first-order rate constant was increasing with the initial hydroperoxide concentration. Furthermore, a similar increase with the initial hydroperoxide concentration, although on a lower level, occurred if the experiments were carried out in the presence of phenolic antioxidants. This behavior is attributed to hydroperoxide decomposition induced by the free radicals arising from the primary hydroperoxide decomposition reaction. Linear extrapolation of the initial first-order rate constants to zero initial hydroperoxide concentration yields the true first-order rate constant for hydroperoxide thermolysis. The activation energy deduced from the variation of the extrapolated rate constants with temperature is close to 109 kJ mol −1. This value is much too small for monomolecular hydroperoxide decomposition. Hence, primary thermolysis of polyethylene hydroperoxides in the polymer melt is a pseudo-first-order reaction involving the interaction of the hydroperoxide group with a segment of the polymer. Although the data have been generated in the temperature range 150 to 200°C, the reaction is estimated to be dominant over true first-order decomposition up to 300°C. The rate constants for monomolecular decomposition of polyethylene hydroperoxide found in the literature are discussed.

Thermolysis of polyethylene hydroperoxides in the melt: 2. Formal kinetics of hydroperoxide decomposition

Thermolysis of polyethylene hydroperoxides in the melt results from different reactions. The first-order rate of hydroperoxide decomposition in the initial stages of thermolysis increases linearly with the initial hydroperoxide concentration. This seems to indicate that in addition to primary hydroperoxide decomposition there is hydroperoxide decomposition induced by the free radicals arising from the primary reaction. The mathematical models deduced from formal kinetics have been compared to these experimental results. It has been found that conventional kinetics based on chain recombination between randomly distributed free radicals could not explain linear increase of the first-order rate with initial hydroperoxide concentration. Conventional kinetics could explain merely linear increase with the square root of this hydroperoxide concentration. The conclusion was the same in the presence and in the absence of phenolic antioxidants. The best explanation of the experimental data involves geminate chain recombination, i.e. termination between two chains originating both from the same initiation reaction yielding a pair of free radicals. In the absence of phenolic antioxidants, geminate chain termination results from the interaction of two alkyl radicals. In the presence of phenols, this geminate chain termination seems to involve essentially an alkyl radical and a phenoxy radical. Besides polymer solutions, polymer melts certainly are the most homogeneous form possible for polymers. Since geminate chain recombination seems to be dominant in polymer melts, it can be assumed that it will be at least as important with polymer states for which diffusion is even more restricted, e.g. polymers in the solid phase.

Kinetic study of the thermal oxidation of polypropylene

The thermal oxidation of non-stabilized isotactic polypropylene was studied in air at 80, 90, 100 and 150 °C using gravimetric and IR spectrophotometric determinations (hydroxyls and carbonyls). A kinetic model in which radical production results only from unimolecular hydroperoxide decomposition was developed. This model predicts that: • • the induction time depends only on the rate constant of hydroperoxide decomposition. It must be almost independent of the polypropylene source (for low content of catalytic impurities and low initial hydroperoxide concentration) and its apparent activation energy must be about 100 kJ mol −1; • • the induction times relative to, respectively, weight, carbonyl and hydroxyl gain must be in the ratio 2:3:3; • • ‘the steady state’ oxidation rate must be independent of initiation rate constant. The experimental data on oxidation kinetics in the temperature range under study confirm all these predictions.

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.

Synthesis and thermal and photooxidative behaviour of 1, n-bis(3-hydroxy-4-benzoylphenoxy) alkanes in polyolefin films

A series of light stabilizers based on the 2,4-dihydroxybenzophenone unit linked at each end of an alkane chain have been prepared via the Williamson ether synthesis and characterized. Six homologues were prepared in this way from propane to octane. The thermal and photooxidative stabilization activities of all six compounds have been examined and compared with those of 2,4-dihydroxybenzophenone (DHBZ) and 2-hydroxy-4- n-octoxybenzophenone (HOBZ) in medium density polyethylene and polypropylene films at 0·1% w/w concentration using IR absorption spectroscopy. In medium density polyethylene all the bis-benzophenone homologues are weak antioxidants, the propane derivative being the most effective. The commercial light stabilizers 2,4-dihydroxybenzophenone (DHBZ) and 2-hydroxy-4- n-octoxybenzophenone (HOBZ) were also weak antioxidants but less effective than the bisbenzophenone alkane homologues. In polypropylene, similar results were obtained but in this case the DHBZ was the most effective antioxidant and HOBZ was the least effective. In the presence of a hindered phenolic antioxidant, Irganox 1010, all the stabilizers except the HOBZ were synergistic in the thermal oxidation of polyethylene whereas in polypropylene the effects were antagonistic. In both polymers, with Irganox 1010, the BHB-alkane structures with odd numbered alkane links were superior to those with even numbered alkane links. On photooxidation, the effects were reversed with antagonism being observed in the case of polyethylene and synergism in polypropylene. Surprisingly, all the bis-benzophenone homologues were less effective as light stabilizers than the two simple commercial benzophenone stabilizers HOBZ and DHBZ despite a marked increase in their extinction coefficients. In polypropylene the bis-benzophenone stabilizers with an even numbered alkane chain were the most effective whereas the reverse was the case in polyethylene. In the presence of Irganox 1010, however, the effect was reversed in polypropylene but not in the case of polyethylene. Initial hydroperoxide concentrations in the polypropylene films with and without Irganox 1010 were found to match the trends in light stabilization, confirming earlier theories on the radical scavenging behaviour of these compounds as chain-breaking donors. The thermal and light stability data are discussed in terms of possible cis and trans conformational arrangements of the benzophenone terminal groups on the alkane chains. In the former case this would allow some degree of self quenching as well as the possibility of association with the hindered phenolic hydroxyl groups on Irganox 1010 to give improved stabilization efficiency.

Photostabilising action of Ni(II) bis(1-phenyl-3-methyl-4-decanoyl-5-pyrazolate), sanduvor NPU, in a polypropylene matrix

The photostabilising action of the metal chelate nickel(II) bis(1-phenyl-3-methyl-4-decanoyl-5-pyrazolate) in polypropylene is examined using normal and second-order derivative ultraviolet, infrared and phosphorescence spectroscopic techniques and hydroperoxide analysis. Although the chelate quenched the phosphorescence emission of a powerful sensitiser, benzophenone, there was no photoprotective action when the samples containing the chelate and the sensitiser were irradiated. In the case of anthraquinone there was no quenching and no photoprotection. Processing history plays a major role in controlling the photostabilising performance of the chelate. The effect of photosensitised oxidation with 365 nm light is examined. While the initial hydroperoxide concentration appears to control the onset of carbonyl growth in the polymer, the rate of photodecomposition of the chelate shows no dependence on hydroperoxide concentration. Furthermore, the rates of photooxidation under polychromatic irradiation follow the same trends as those obtained under monochromatic irradiation. The results are explained in terms of hydroperoxide inhibition during processing, stabiliser consumption during photooxidation and macroalkyl radical (P·) scavenging.

Mechanistic action of a new p‐hydroxybenzoate light stabiliser in polypropylene film: A spectroscopic study

AbstractThe photostabilising action of a new aliphatic p‐hydroxybenzoate light stabiliser has been examined in polypropylene film together with the aid of 3,5‐di‐tert‐butyl‐4‐hydroxybenzoic acid by both normal and derivative UV absorption and IR spectroscopy, and hydroperoxide analysis. The stabiliser has also been examined in solution together with a number of related compounds using micro‐second flash photolysis. During processing the stabiliser operates as an effective chain breaking donor by terminating macroalkyl radicals and inhibiting the formation of hydroperoxides. Under both monochromatic (365 nm) and polychromatic (λ > 320 nm) irradiation conditions the decomposition of the stabiliser shows a direct dependence on initial hydroperoxide concentration in the film, indicating that it operates as an effective light stable alkoxy and hydroxy radical scavenger. The protective effect of a peroxide decomposer confirms this mechanism. Evidence is also presented to show that under photosensitised radical attack the presence of the long aliphatic hydrocarbon chain has a powerful protective effect on the molecule. On flash photolysis of all the compounds studied end‐of‐pulse transient absorption spectra were assigned to phenoxy radicals. The radicals decayed by a second order process to give dimeric coupling products. The nature of these products are discussed and identified for the compounds studied. The technique of flash photolysis is concluded to be a useful method for assessing anti‐oxidant activity.

Photo-stabilising action of metal chelates in polypropylene—Part II: Photolysis versus photo-sensitised oxidation under monochromatic irradiation

The photo-stabilising action of three metal chelates in unprocessed and processed polypropylene is examined using normal and second order derivative ultraviolet and infra-red spectroscopic techniques and hydroperoxide analysis. The effects of photolysis with 254 nm light versus photo-sensitised oxidation with 365 nm light are compared. For each exposure condition the rate of carbonyl formation in the polymer is compared with the rate of decomposition of the metal complex. On photolysis, carbonyl growth commences well before the complete destruction of the complexes and none offers protection to the polymer. In fact, all three chelates behave as photo-sensitisers, indicating that stabiliser photolysis products are photo-active. On photo-sensitised oxidation, while the initial hydroperoxide concentration appears to control the onset of carbonyl growth in the polymer, the rate of decomposition of the complexes shows no dependence on hydroperoxide concentration. Solution experiments indicate that there are no dark reactions with hydroperoxides apart from one of the nickel chelates (Cyasorb UV 1084) at high concentrations (∼ 10 −2 m) only. Essentially, the metal chelates operate by scavenging macroalkyl radical species (P·) and not alkoxy (PO·) and hydroxy radicals (·OH) during photo-oxidation. They also inhibit hydroperoxide formation during processing and one of the nickel chelates (UV 1084) gives products during the early stages of photo-oxidation which appear to operate as effective stabilisers.

Photo-stabilising action of a p-hydroxybenzoate compound in polyolefins—Part I: Thermal and photo-chemical behaviour in polypropylene film

The photo-stabilising action of a new aliphatic p-hydroxybenzoate light stabiliser, Cyasorb® UV 2908 (American Cyanamid Company), has been examined in polypropylene film, with the aid of a number of related compounds, by both normal and derivative uv absorption, infra-red techniques and hydroperoxide analysis. During processing and oven ageing the stabiliser operates as an effective chain breaking donor, terminating macroalkyl radicals and inhibiting the formation of hydroperoxides. Under both monochromatic (365 nm) and polychromatic ( λ′s > 300 nm) irradiation conditions the decomposition of the stabiliser shows a direct dependence on initial hydroperoxide concentration in the film, indicating that it operates as an effective light stable alkoxy and hydroxy radical scavenger. Under ‘direct photolysis’ conditions (254 nm light) the stabiliser does not undergo unfavourable dimerisation reactions like other related p-alkyl substituted phenols. Evidence is also presented to show that the presence of the long aliphatic hydrocarbon chain has a powerful protective effect on the molecule and this is associated with a radical recombination process due to the polymer cage.

Photo-stabilising action of metal chelates in polypropylene—Part III: Thermal antioxidant action and its relationship to photo-stabilisation

The thermal antioxidative behaviour of three metal chelates in polypropylene film has been examined using hydroperoxide analysis and infra-red spectroscopy. All three chelates inhibit both hydroperoxide and carbonyl group formation during oxidation. Their thermal antioxidant efficiencies follow closely their photo-stabilising effects, suggesting that similar mechanisms of protection are involved. The rates of photo-decomposition of the metal chelates measured by second order derivative absorption spectroscopy were found to be independent of initial hydroperoxide concentration, indicating that they are ineffective macroalkoxy and hydroxy radical traps. The results suggest that metal chelates compete effectively with oxygen for macroalkyl radicals thermally and photochemically.

A computational analysis of a chemical switch mechanism. Catalysis-inhibition effects in a copper surface-catalyzed oxidation

A reaction scheme was designed for the tert.-butyl hydroperoxide-initiated autoxidation of hexadecane at 100/sup o/C, which was inhibited by small amounts of copper and accelerated by copper in excess of a critical ratio of initial hydroperoxide concentration to copper surface area. The model consisted of a homogeneous reaction set of 53 steps based on well-known free-radical oxidation, and a surface reaction set of 34 steps based on a proposed competition for trapping of hydroperoxide at inhibition and catalysis surface sites. A computational procedure for numerical solution of the mass-action differential equations by the Gear integration technique was used. The homogeneous mechanism correctly predicted the observed oxidation rates and insensitivity to initial hydroperoxide concentrations in the absence of copper. The surface mechanism, tested with various initial conditions and rate parameters, gave results in agreement with experiment for the following phenomna: location of the switch point of the mechanism, the shape of the reaction curve, and the insensitivity of the rate to the hydroperoxide concentration above the switch point. Graphs, tables, and 15 references.