Mechanisms of antioxidant action: Synergism between tin stabilizers and polymer-bound antioxidants in the u.v. stabilization of PVC

It is shown that a conventional tin stabilizer (dibutyltin maleate) (DBTM) forms an effective synergistic u.v. stabilizing combination when used in conjunction with a bound phenolic antioxidant (BHBM) and a bound u.v. stabilizer (EBHPT). These additives become bound to the polymer through sulphur during processing in the presence of DBTM and it is suggested that the sulphide of the adduct makes its own contribution to u.v. stabilization by acting as a peroxide decomposing (PD-C) antioxidant during photo-oxidation. However, the 2-hydroxybenzophenone function appears to be an essential part of the system and is probably involved in quenching of excited states of the intermediate sulphoxides which precede the formation of the PD-C antioxidant.     

The mechanism of u.v. stabilization of polypropylene films by 2-hydroxy-4-octyloxybenzophenone

The rates of oxygen absorption by unstabilized and u.v.-stabilized polypropylene (PP) films irradiated with a high-pressure mercury lamp have been determined as a function of the light intensity (I). The induction time is nearly proportional to I^?0.5 for unstabilized PP and for PP stabilized with Tinuvin P, and proportional to about I^?0.9 for PP stabilized with 2-hydroxy-4-octyloxybenzophenone (HOBP). The change in concentration of HOBP in PP during the photo-oxidation is determined with radioactive HOBP. During the induction time, the observed decrease obeys zero-order kinetics even when the starting concentration of HOBP is as low as 0.008%. The decrease is caused by reaction of HOBP with the polymer and by fragmentation into gaseous products. These results are interpreted on the basis of the same reaction scheme as that worked out for the uninhibited and inhibited autoxidation of PP. It is concluded that HOBP acts not only as a u.v. absorber or as an excited-state quencher, but also as a scavenger of free radicals.     

Atomic absorption spectroscopy in the v.u.v.-range: determination of bromine

The conditions for the determination of elements by atomic absorption spectroscopy (AAS) in the vacuum-ultraviolet range are discussed. The requirements for the absorption vessel are considered and the equilibrium concentrations of possible dissociation products of different chemical compounds are calculated. In this way it is possible to estimate the detection limit attained with such an AAS arrangement. When losses by diffusion of sample vapour are disregarded a theoretical value of 5 × 10^?10 g Br results. AAS measurements are described using a special graphite oven in conjunction with a vacuum-ultraviolet monochromator. For KBr as a representative substance we obtained the experimental detection limit of 1.5 × 10^?9 g Br with Br I 148.86 nm.     

Metal complexes with ligand-organic phosphites as polyolefin antioxidants. II. Kinetics and mechanism of synergism at oxidation inhibition by metal acetylacetonates and organic phosphites

The widely known synergism of oxidation inhibitors with metals of transient valency is shown in the new process with organic phosphite participation. The effects of inadditive strengthening phosphite action by acetylacetonates of transient metals (Me) have been investigated in different oxidation systems (solid polymer, melt, and liquid phase). The kinetic interpretation of effects, their quantitative analysis, and mathematical modeling are given, and the general regularities governing the efficiency of Me + organic phosphite compositions in any oxidation mode in the presence of initiator and at polymer autoxidation are revealed.     

The effect of processing conditions on the u.v. stability of polyolefins

It is shown that the lifetime of polyolefins in the presence of u.v. light is markedly dependent on the degree of oxidation occurring during the prior processing operation. This is influenced strongly by the presence of pro-oxidants and anti-oxidants, and their effect on subsequent u.v. life can be rationalized on the basis of their functions during processing.     

Determination of the ionization quantum yield and thermalization distance of photoelectrons created by v.u.v. photoionization in pure liquid alkanes and tetramethylsilane

The photocurrents, in liquid n-hexane, n-pentane, cyclohexane, cyclopentane, 2-2-4 trimethylpentane, 2-2 dimethylbutane, neopentane and tetramethylsilane, produced by v.u.v. irradiation, are measured at electric fields from 2 to 35 kV cm^-1. Assuming the one- and three-dimensional Onsager probabilities are respected, the values of the attenuation parameter of the photon flow B₁, the quantum yield for the production of an ion-pair θ_i, and the attenuation parameter of the photoelectron flow B₃ can be deduced. These values are compared with those provided by other injection methods. Both the validity of these values and the theoretical models are discussed. The discussion is extended to other parameters: V₀, μ₀ and K_J, all correlated to emission and propagation mechanisms in non-polar liquids.     

Electronic structure of xanthine and its biological methyl derivatives by u.v. photoelectron spectroscopy

He(I) excited photoelectron spectra of xanthine, theophylline, theobromine and caffeine are presented and discussed. The spectra are unequivocally assigned using CNDO calculations and comparison arguments to some related molecules. Methyl substitution effects proved to be a powerful tool for reliable assignments of the spectra.     

Mechanisms of antioxidant action: Antioxidant behaviour of nickel complex u.v. stabilisers

The antioxidant mechanisms involved in the u.v. stabilising activity of two nickel complexes, nickel dibutyl dithiocarbamate (NiDBC) and nickel acetophenone oxime (NiOx), have been investigated both by oxygen absorption measurements and in peroxide decomposition studies. NiDBC gives rise to a powerful catalyst for non-radical hydroperoxide decomposition; the rate of the reaction is faster in the presence than in the absence of light although the overall mechanisms appear to be very similar. NiOx is decomposed by hydroperoxides in a series of stoichiometric reactions which involve the consumption of at least six molecules of hydroperoxide per molecule of NiOx. Again the reaction is catalysed by light but the mechanism, which does not involve free-radical formation, is the same in the presence and absence of light. NiOx also appears to have weak radical trapping properties. Attempts to show that NiDBC and NiOx might interact sacrificially with photo-excited states of molecules (triplet carbonyl and singlet oxygen) were unsuccessful; the rates of photo-destruction of both complexes were unaffected by the presence of ketones both in the presence and absence of oxygen. This work confirms earlier conclusions that the nickel complex u.v. stabilisers function by auto-synergistic mechanisms involving u.v. screening and conventional chain-breaking and preventive antioxidant processes which operate during thermal processing operations as well as during environmental exposure. Although excited state quenching processes may occur, they appear to be less important in the overall scheme than the well established antioxidant mechanism.     

Fourier transform u.v.-visible molecular luminescence spectrometry: Fluorescence excitation spectra

The analytical utility of Fourier transform spectrometry in the u.v.-vis region was investigated. Molecular fluorescence excitation spectra were acquired for four polycyclic aromatic hydrocarbons frozen in Shpol'skii solvents. Analytical figures of merit were determined for coronene, including a limit of detection of 2 ppb and a precision better than 13% RSD. The major sources of noise in FT/u.v. molecular fluorescence spectrometry are discussed, and results are compared to those reported earlier from wavelength dispersive instruments.     

Mechanisms of antioxidant action: Metal complexes as u.v. stabilisers in polyethylene

In order to evaluate the importance of u.v. screening, peroxide decomposition and excited state quenching in the mechanism of action of a variety of metal complex stabilisers, screening and additive studies were carried out in high density polyethylene and in model hydrocarbons. All the metal complexes studied were found to be u.v. screening agents when contained in a separate phase between the u.v. source and the polymer and all were rather more effective than a typical commercial u.v. absorber (2-hydroxy-4-octyl-oxybenzophenone, HOBP). However, when used as additives, all the stabilisers had an additional function. In the case of the transition metal dibutyl dithiocarbamates (NiDBC and FeDBC), the additional stabilisation mechanism is consistent with their known peroxide decomposing behaviour. In the case of the 2-hydroxy-acetophenone oxime complexes (NiOx and CuOx), the initial function is a stoichiometric reaction with hydroperoxides to give non-radical products. An amine complex of nickel bis-octylphenol sulphide (NiBOPS) appeared to behave primarily as a screen. Only one of the metal complexes (FeOx) appeared to be capable of functioning as an excited state quencher and this complex is effectively sensitised by triplet carbonyl to give a photo-pro-oxidant effect in cumene. It is concluded that the concept of excited state quenching, although it may occur with some metal complexes, is not a sufficient criterion of u.v. stabilising activity.     

Changes in the electrochemical characteristics and u.v.-spectra of polydienyl-lithium chains on storage in polar media

The parallel changes in the electric conductivity and u.v. spectra of polybutadienyl- and polyisoprenyl-lithium (M_nLi) have been investigated in tetrahydrofuran and dimethoxyethane solutions during prolonged storage at 0 and 25°. A considerable dependence of the rate of these changes on the nature of the medium and M_n Li concentration was found. The decrease in the initiating activity during storage has been estimated for the PBLi-THF system.     

Solvent effects in u.v. absorption spectra. IV. Substituted phenols,anisole and phenetole

The u.v. absorption spectra of some substituted phenols, mostly with the substituents in the ortho positions, have been measured in mixtures of cyclohexane and ethanol and, in some cases, in mixtures of cyclohexane and alternative hydrogen bonding solvents. The loss of the vibrational fine structure has been used as a measure of the ability of the solute molecules to form hydrogen bonds. The behaviour of o-isopropyl phenol does not differ significantly from that of phenol; hence the isopropyl group does not produce hindering to hydrogen bonding. A detectable degree of hindering occurs with 2,6-xylenol and 2,6-dichlorophenol. The vibrational structure of the latter does not disappear completely in ethanol and this is the result of intramolecular bonding between the chlorine atoms and the hydroxyl group. Although 3,5-dichlorophenol is comparable in bonding behaviour to phenol some residual band structure is present even in very strongly associative solvents; possible explanations are discussed. Anisole and phenetole associate rather weakly with ethanol and this is confirmed by the measurement of the shift in the hydroxyl stretching frequency in the i.r. spectrum. This result is interpreted in terms of a marked delocalisation of the ether oxygen lone-pair electrons.     

Protection of poly(2,6-dimethyl-1,4-phenylene oxide) against u.v. irradiation

The photo-oxidative degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) films have been examined in the temperature range 263–313 K in air with u.v. light of 253.7 nm. The changes in weight-average molecular weight, quantum efficiencies, carbonyl index, hydroperoxide concentration and i.r. spectra have been followed in the absence and presence of the stabilizer. The heats of activation of the systems have been calculated and the mechanism of stabilization has been postulated. A saturation limit in photostabilization of the polymer was achieved beyond 0.6 wt% of zinc di-n-hexyl dithiophosphate.     

Solvent effects in u.v. absorption spectra. I. Phenol in cyclohexane ethanol mixtures

The u.v. and i.r. absorption spectra of phenol in a series of solvent systems have been studied and the changes in the spectra, particularly the loss of fine structure in the u.v. spectra, have been qualitatively interpreted in terms of hydrogen bonding effects. The systems studied were chosen to favour either self bonding of the phenol or bonding between the phenol and the minor component of the solvent system. In the latter case the solvent systems were such that the phenol could act either as a proton donor (with an aliphatic ether) or as a donor and acceptor (with ethanol).     

Electronic and molecular structure of some substituted azomethines: A gas-phase u.v. photoelectron spectroscopic investigation

Four azomethinic molecules, three of which present a seven-member ring with two carbon atoms shared with an aromatic system, have been studied by gas-phase u.v. photoelectron spectroscopy with the aim of correlating their electronic and molecular structure. The analysis of the experimental findings, supported by quantum-mechanical calculation (EHT and HAM/3) on model molecules, showed that no appreciable interactions occur between the systems of the aromatic and of the seven-member ring, and that the electronic structure of these molecules depends critically on the geometry. The comparison with smaller parent molecules pointed out that the formation of the seven-member ring induces drastic changes in the electronic structure of the valence levels.     

Influence of additives on some physical properties of high density polyethylene-II. commercial u.v. stabilizers

The changes in mechanical properties of processed high density polyethylene, when “u.v. stabilizers” are added, are examined using tensile and melt flow measurements. The results show that, at low additive concentrations, antioxidant effects predominate; at high concentrations, a reinforcement effect is observed. Degradation measurements show that the u.v. stabilizers behave as excellent oxidation inhibitors in the dark; this finding has implications regarding their mechanistic action.     

Photooxidative stability of electron-beam and u.v.-cured di- and triacrylate resins. Influence of resin/initiator structure

The post-cured photooxidative stabilities of electron-beam and u.v.-cured di- and triacrylate based resins have been examined using i.r. and second order derivative u.v. spectroscopic techniques. Using hydroxyl index at 3450 cm¹ as a measure of the rate of photooxidation of the resin films, their stabilities were found to be strongly dependent on the curing process and structure of the residual photoinitiator. The structure of the resin also played a major role in influencing the development of potentially active intermediate chromophores. Resins containing a terminal secondary amine group developed a strong absorption band at 250–300 nm during photooxidation, which paralleled the production of i.r. bands at 1680 and 1605 cm¹. The development of this chromophore is strongly influenced by the nature of the curing process and residual photoinitiator and is shown to be produced through an oxidative mechanism. The chromophore is associated with an α,β-unsaturated carbonyl species and is considered to be a key intermediate in the process of photoyellowing. Resins with no terminal secondary amine groups exhibited no such characteristic spectral changes. Possible mechanistic schemes are proposed to account for the observations.     

Degradation of poly(vinyl chloride) by u.v. radiation—II: Mechanism

The mechanism of the light-induced degradation of solid poly(vinyl chloride) (PVC) has been investigated, and an overall reaction scheme has been developed, based on values of the quantum yields for the primary photoproducts. Only a very small fraction (0.2%) of the excited polyenes induces the degradation of PVC, primarily by photocleavage of the allylic CCl bond. The high instability of β-chloroalkyl radicals is responsible for the chain dehydrochlorination that leads to formation of polyenes. In the absence of O₂, chain scissions and crosslinking are postulated to originate mainly from α-chloroalkyl radicals through β-cleavage of CC bonds and radical coupling, respectively. In the presence of O₂, the chain dehydrochlorination still proceeds, together with an oxidative chain process which yields, via peroxy and alkoxy radicals, hydroperoxides, ketones and peroxide crosslinks. Cleavage of the polymer backbone results most probably from the decomposition of tertiary alkoxy radicals by a carbon-carbon β-scission process.     

Determination of bromide in suspended particulate matter by high-performance liquid chromatography with u.v. detection

High-performance liquid chromatography on a Zipax SAX column with u.v. detection at 200 nm gives precise results for bromide. Chloride, bromate, nitrate, nitrate, iodide, thiocyanate and various cations do not interfere; prior treatment with activated charcoal removes interfering organic compounds. Calibration graphs are linear over the range 0–50 mg 1 ^?1 bromide in aqueous extracts of the particulates; the limit of detection is 10 ng of bromide. The relative standard deviations at the 0.5 and 1.0 mg 1^?1 levels are 2.3 and 1.0%, respectively.     

Determination of nitrate in suspended particulate matter by high-performance liquid chromatography with u.v. detection

For the determination of nitrate in suspended particulate matter (s.p.m.), high-performance liquid chromatography (h.p.l.c.) with a u.v. detector at 210 mm gives precise and accurate results. Chloride, bromate, iodide, nitrite, thiocyanate and various cations do not interfere. Calibration graphs are linear over the range 0–20 ppm of nitrat-enitrogen, and the limit of detection is 0.25 ng of nitrate-nitrogen. The coefficients of variation at 5.0- and 10.0-ppm levels are 3.4 and 2.9%, respectively. Results obtained by the h.p.l.c. method and by two 2,4-xylenol spectrophotometric methods for aqueous extracts of s.p.m. are compared. Agreement is generally good, particularly when choride is removed in the 2,4-xylenol method, but the spectrophotometric methods are much more prone to interference.