Inhibiting effect of phenolic compounds in initiated oxidation of tridecane

Initiated oxidation of tridecane in chlorobenzene solution with molecular oxygen in the presence of a series of phenol derivatives was studied by chemiluminescence and gas-volumetric methods. The kinetic parameters of the antiradical and antioxidant activity of the compounds were determined.     

Fundamental aspects of oxidation of electroexplosive nickel nanopowder

Fundamental aspects of the process in which a nickel nanopowder produced by an electric explosion of conductors is oxidized under heating in air at a linearly increasing temperature and in the isothermal mode were studied. It is shown that the dispersion composition of the powder, structure of the metallic core of nickel particles, and characteristics of the oxide shell affect the kinetic parameters of the process. An explanation is suggested for specific features of the macrokinetic reaction mode caused by the joint influence exerted by the relative amounts of fractions of different-size particles in the powder and by the characteristic of the oxide layer. The reaction kinetics is simulated, with the size distribution function of nickel particles taken into account.     

Fundamental aspects of the behavior of various lignosulfonates in solutions

Fundamental aspects of associative-dissociative and acid-base transformations of technical-grade high-and low-molecular mass lignosulfonates were identified by carrying out a set of physicochemical studies.     

Sugar end-capped polyethylene: Ceric ammonium nitrate initiated oxidation and melt phase grafting of glucose onto polyethylene and its microbial degradation

Low density polyethylene (LDPE) was modified to introduce biodegradability by grafting highly hydrophilic monomers (which can act as nutrients for microorganisms) such as glucose by a novel melt phase reaction using Brabender plasti-corder in the presence of ceric ammonium nitrate (CAN) to obtain 4-O-hydroxymethyl d-arabinose (sugar) end-capped LDPE (Su-g-LDPE) at a maximum grafting of 16%. The grafted polymers were characterized by FTIR, thermal analysis, WAXD and mechanical property measurements. The biodegradability of Su-g-LDPE was carried out by soil-burial test and by optical density measurements in presence of an aerobic bacterium Pseudomonas sp. The degraded polymer shows changes in weight, crystallinity and inherent viscosity. Optical density of the medium registered an increase with degradation. FTIR spectra of the degraded samples showed 70% decrease in the ketone carbonyl index (ν₁₇₁₉/ν₁₄₆₅) of Su-g-LDPE indicating microbial degradation of LDPE matrix, which was further confirmed by SEM micrographs. The present data support a microbial oxidation process involving β-oxidation whereby the carbonyl is further oxidized to carboxylic acid and affects cleavage of the LDPE chain at the ends.     

Some particular aspects of the low-temperature oxidation of methane initiated by nitric acid

During methane oxidation initiated by nitric acid or nitrogen dioxide, it is possible to observe, under certain conditions of temperature, initiator content, and flow rates, the formation of an adiabatic “temperature peak.” Because the maximum temperature of the peak does not exceed the initial temperature of the reactor by more than 220°C, the recorded temperature-increase curves were used for a kinetic interpretation. The obtained kinetic parameters (overall n and E) agree with the values reported in the literature for isothermal methane oxidation and suggest that the overall mechanism does not change in the conditions of “temperature peak.”     

Air oxidation of organic compounds in aqueous solutions. Ecochemical and analytical aspects

The results of static and dynamic laboratory experiments confirm the oxidation of organic compounds in aquatic medium by dissolved oxygen of atmospheric air to be possible under conditions close to normal. Alkylaromatics containing hydrogen atoms in alkyl substituents in α-positions to the aromatic system, as well as simple ethers, are characterised by the highest chemical reactivity in relation to triplet (³Σ _g ^? ) which is consistent with the pattern of free-radical reactions. Principal environmental and analytical consequences of the obtained results are under discussion. Liquid-phase free-radical reactions of atmospheric oxygen may proceed with the participation of atmospheric aerosols, i.e water droplets within clouds. Another example relates to the prevention of accumulation in natural bodies of water stable nonylphenol (predominantly 4-tert-isomers), characterized by endocrine activity. Suggested the possibility of replacing them with secondary alkylphenols, identical in all chemical properties except for the ease of oxidation by air oxygen in aqueous solutions     

Fundamental aspects of film formation in electrochemical polishing of silver and silver-copper alloys in thiosulfate solutions

The anodic behavior of silver and its alloys with copper in thiosulfate solutions was studied by cycling voltammetry. A scheme of an electrochemical polishing process including the stage of formation of a resistive layer of silver thiosulfate and the chemical stage of its dissolution is suggested.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 8, 2004, pp. 1299–1302.Original Russian Text Copyright © 2004 by Grishina, Galanin, Ivanova.     

Fundamental aspects of electrocatalysis

A theory for electrochemical electron transfer is proposed which explicitly accounts for the electronic structure of the electrode; it applies both to simple and to bond-breaking reactions. Interactions with narrow d-bands lead to changes in the local density of states of the reactant as its electronic level fluctuates due to solvent reorganization. More importantly, it can significantly reduce activation barriers, and in extreme cases induce dissociative adsorption. The model gives some justification to recent suggestions that the center of the d-band is a key factor for electrocatalysis, and it offers a route for calculating reaction rates using results from ab initio calculations as input.     

Low-temperature oxidation of polyethylene

To increase the surface energy of polyethylene (PE), the surface is subjected to oxidative treatment, such as chemical oxidation or Bunsen flame treatment. It is shown that oxidation of the polymer surface below 100° leads to increase in the contents of double bonds and of hydroxyl, carbonyl and carboxyl groups and also to branching of the molecules. Increase in the concentration of the polar groups causes increase in the surface free energy.     

Profile of oxidation in irradiated polyethylene

Following gamma irradiation in air which causes bond scission and yields large concentrations of peroxy radicals, maximum oxidation and an increase in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals generate carbonyls, mostly ketones. On the polymer surface, peroxy radicals continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer abstract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of oxygen is available to react with radiolytic radicals, so that more efficient crosslinking and a low level of hydroperoxide chain reaction occur. After long periods of time a surface maximum in carbonyl concentration is produced. Heating polyethylene in high pressures of oxygen accelerates the oxidative process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 329–339, 1998     

Kinetic characteristics of initiated oxidation of limonene

The kinetics of oxygen absorption .n the process of limonen (4-isopropenyl- l-methulcyclohex-1-ene) oxidation was studied in chlorobenzene solution at 40–80 °C. The values of the kinetic parameters of oxidation and the activation energy (38.1 kJ mol^–1) have been determined. The nonlinear dependence of the rate of the oxidation of limonene on its concentration has been established, The rate constants of the chain termination reactions were estimated by chemiluminescence techniques.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1676–1681, July, 1996.     

Some aspects of polyethylene photooxidation

The hydroperoxides produced by thermal oxidation of LDPE films were used to study their photolysis. Product analysis, kinetics of hydroperoxide decomposition and product formation as well as experiments with model compounds point to new mechanisms of hydroperoxide photolysis. Intermolecular as well as intramolecular decomposition mechanisms are proposed. In polyethylene, these reactions are essentially non-initiating. In addition, it is confirmed that ketones such as those formed by oxidation of polyethylene do not have a significant initiating effect. Reactions of excited charge-transfer complexes polyethylene-oxygen are proposed to account for initiation of photo-oxidation. One of these reactions yields trans-vinylene groups and hydrogen peroxide whose direct decomposition or subsequent photolysis will generate hydroxyl radicals. It is found that this reaction is quenched very efficiently by small amounts of HALS (Hindered Amine Light Stabilizers) and by amines in general. It is postulated that quenching is due to energy transfer from the charge-tranfer complex polymer-oxygen to a charge-transfer complex HALS-oxygen or amine-oxygen. The data available so far support such a mechanism.     

Fundamental aspects of rubber-elasticity in real networks

Based on the van der Waals-equation of state a characterization of the elastic properties of molecular networks in the total range of strain is presented by the use of Mooney-Rivlin plots. This discussion elucidates the importance of finite chain extensibility as well as global interactions for a quantitative interpretation of the deformation properties of molecular networks. An interpretation of the so-called Mooney-Rivlin coefficients in terms of the van der Waals-parameters delivers at least a novel, yet very consistent understanding of simple extension of swollen networks.     

Rheology of chlorosulphonated polyethylene solutions

The viscosity behaviour of chlorosulphonated polyethylene was studied as a function of shear rate, concentration and temperature in three aromatic solvents viz. benzene, toluene and xylene. The Carreau model was used to correlate the data. The time constant t₁ arising from the Carreau model was compared with the Bueche time constant λ_H; there appears to be good agreement between them especially at lower temperatures and higher concentrations. A unified curve based on plots of η/η_O vs γλ_H correlated the data successfully. The temperaure dependence of the zero shear viscosity was studied. The apparent activation energy for viscous flow was linearly related to the polymer concentration.     

Initiated oxidation and auto-oxidation of polyethylene in trichlorobenzene solution

Thermal auto-oxidation and initiated oxidation (by dicumyl peroxide) of low density polyethylene were studied in trichlorobenzene solution. The rate dependence on the pressure of oxygen, temperature and concentrations of initiator and polymer have been investigated. Oxidation of the initiator itself has also been studied. The experimental results obtained are well interpreted by highly simplified mechanisms. Extrapolation from the rate data obtained in relatively dilute solutions at 160°C to the rate of oxidation in the molten phase gave surprisingly good agreement with the previously measured experimental value.     

Inhibited oxidation of a polyethylene melt

The oxidation of linear polyethylene inhibited by an effective antioxidant of the phenolic type, viz. 2,2′-methylene-bis (4-methyl-6-tert. butylphenol), has been studied over the temperature range 170–210. There is a critical concentration of antioxidant above which the rate of its consumption is directly proportional to its concentration. Deviations from this dependence are due to chain initiation by direct oxidation of the polymer and to reactions involving the products derived from the antioxidant: these products can also retard the oxidation. Similar results have been obtained using the amine type antioxidant, N-phenyl-N′-cyclohexyl-p-phenylenediamine. Some features of the mechanism of polyolefin oxidation are discussed.     

Nitric acid oxidation of irradiated polyethylene

Samples of high-density polyethylene were irradiated with x-rays and oxidized with concentrated nitric acid to determine the location of unsaturated groups. Vinyl groups initially present in the polymer were rapidly oxidized to the extent of 85% and are assumed to be excluded from the crystals. Vinylene groups show a rapid oxidation followed by slow oxidation. The initial oxidation is about 35%, which is slightly greater than the 25% amorphous content of the polymer prior to irradiation. Diene groups are rapidly oxidized by nitric acid but are formed at about the same rate in crystalline and amorphous samples. This is interpreted to indicate that diene groups are formed throughout the polymer but form large defects in crystalline regions and are accessible to oxidation. Defects formed in crystalline regions during irradiation lower the melting point to a greater extent than predicted by the mole fraction of noncrystallizable units.     

Fundamental aspects of HCOOH oxidation at platinum single crystal surfaces with basal orientations and modified by irreversibly adsorbed adatoms

Oxidation of formic acid at Pt(hkl) electrodes with basal orientations and modified by irreversibly adsorbed adatoms is revisited. It was shown that the adatoms of the nitrogen group enhance the electrocatalytic activity through long-range electronic effects in the case of Pt(111) substrates or through shorter third-body effects when adsorbed at Pt(100) electrodes. In both cases, it appears that free platinum sites were required. Special attention is given here to the reactivity at Pt(110) substrates. It is found that maximum electrocatalysis is observed at fully blocked surfaces. This result points out again structure sensitivity effects for this characteristic reaction. Unlike the more compact planes, in which the enhancement of the oxidation rate was explained through essentially rigid models for the adlayer, it is suggested that the resulting enhancement observed at Pt(110) substrates could be explained if some adatom mobility is considered.     

Thermal oxidation products and kinetics of polyethylene composites

The thermal oxidation behavior of high-density polyethylene (HDPE) composites was investigated at 60 °C, 90 °C and 110 °C, using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The inorganic fillers do not modify the thermal oxidation mechanisms of HDPE. However, they have great effects on the thermal oxidation kinetics—both the activation energy and the pre-exponential factor increase. That means that although the addition of inorganic fillers retards the onset of thermal oxidation of HDPE, once the oxidation begins, it proceeds much faster than that of HDPE. Stability evaluation of HDPE composites by TGA was not consistent with the result by FTIR. The inorganic fillers influence the oxidation products and their distribution greatly. HDPE/STC and HDPE/mica oxidations were delineated by large amount of carbonyl formation, especially esters and ketones, while HDPE/wollastonite and HDPE/diatomite showed minimum carbonyl formation. In HDPE composites, there is a good relation between the carboxylic formation and the carbonyl index.