Photo-oxidation of thick isotactic polypropylene films I. Characterisation of the heterogeneous degradation kinetics

Photo-oxidation kinetics of thick isotactic polypropylene films have been compared to thermal-oxidation kinetics of thin films, and noticeable differences have been found. The non-classical kinetic trend of the former can be described as a three step evolution: a typical induction/autoacceleration POOH build-up at the beginning, an intermediate slower POOH content increase and, finally, a gentler POOH increase, which can be better described by quadratic functions of the oxidation time than by a linear dependence. In addition, a series of oscillations appearing from the beginning of the photo-oxidation are found. This kinetic heterogeneity is suggested to be related to the progression of the oxidation to the inside of the strips. However, the FTIR analysis of the evolution of the POOH band position in both photo and thermal oxidation, enables the observed shape changes to be associated to kinetic stages.     

“Close loop” mechanistic schemes for hydrocarbon polymer oxidation

Mechanistic schemes of radical oxidation of hydrocarbon polymers in which initiation is only due to unimolecular or bimolecular hydroperoxide decomposition have been studied. The results of their kinetic analysis have been compared with literature data relative to the thermal oxidation of polypropylene in solid state (60-160°C). These data are in remarkably good agreement with the “unimolecular” scheme whose main characteristics are: (1) the quasi-independence of the kinetic behavior with initial conditions (for low initial content of thermolabile structures), and (2) the fact that an arbitrarily defined induction period depends only on the rate constant of unimolecular hydroperoxide decomposition. © 1995 John Wiley & Sons, Inc.     

Semi-global mechanisms for the oxidation of diammonium phosphate impregnated wood

Thermogravimetric measurements in air of fir wood impregnated with diammonium phosphate in the concentration range 2-20% and heating rates between 5 and 20 K/min show the existence of three sequential stages: wood decomposition, induction and char oxidation. The additive causes an anticipation in the decomposition stage up to about 100 K, a reduction in the maximum rate up to a factor of three and an increase in the charred solid residue from about 30 to 60%. The total duration of the induction and oxidation stages is also increased. The experimental curves are interpreted by either a three-step mechanism (one reaction for each stage) or a seven-step mechanism (three reactions for decomposition and two reactions for the induction and oxidation stages). The estimated kinetic parameters are independent from the heating rate but vary with the diammonium phosphate concentration. However, for both mechanisms, the activation energies of the various steps remain practically constant except for the decomposition of the cellulose component or the decomposition step, depending on the complexity of the mechanism.     

Hydroperoxide build-up in the thermal oxidation of polypropylene - A kinetic study

The thermal oxidation of unstabilised polypropylene (PP) was studied at 80 °C under various oxygen pressures: 0.02, 0.5 and 5.0 MPa, and, under 5.0 MPa oxygen pressure at various temperatures: 60, 80, 100 and 120 °C. Hydroperoxides were titrated using a chemical titration method and modulated DSC (taking an enthalpy of −325 kJ mol⁻¹). Starting from a previous kinetic analysis of carbonyl growth in same exposure conditions, we have tried to simulate experimental results by a model based on the classical mechanistic scheme in which initiation results from POOH (mainly bimolecular) decomposition. The model, which takes into account substrate consumption and does not rely on usual simplifying assumptions (steady state for radicals, long kinetic chains, interrelations between termination rate constants), generates kinetic curves with the same shape as experimental ones and predicts well the effect of O₂ pressure and temperature on hydroperoxide and carbonyl concentrations.     

The kinetic compensation effect

On the basis of theoretical TG curves it has been shown that the kinetic compensation effect observed in thermal decomposition reactions is not due to the special form of the Arrhenius equation. Formally, the validity of a linear kinetic compensation law implies the existence of a characteristic temperature at which the rate constants of all reactions have the same value, but this temperature can be higher or lower than the temperatures at which the decomposition takes place.Thermal decomposition processes can be better characterized by means of the compensation parameters than by means of the common kinetic parameters or decomposition temperatures, since the shape and the position of the TG curves and the kinetic parameters derived can be very much influenced by procedural variables, but the compensation parameters are independent of them.     

Kinetic studies on the thermal decomposition of phosphate-doped sodium oxalate

The thermal decomposition kinetics of sodium oxalate (Na₂C₂O₄) has been studied as a function of concentration of dopant, phosphate, at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry (TG). The TG data were subjected to both model-fitting and model-free kinetic methods of analysis. The model-fitting analysis of the TG data of all the samples shows that no single kinetic model describes the whole α versus t curve with a single rate constant throughout the decomposition reaction. Separate kinetic analysis shows that Prout–Tompkins model best describes the acceleratory stage of the decomposition, while the decay region is best fitted with the contracting cylinder model. Activation energy values were evaluated by both model-fitting and model-free kinetic methods. The observed results favour a diffusion-controlled mechanism for the thermal decomposition of sodium oxalate.     

Reaction kinetics at linearly increased temperature. IV. Relationships between dta curves,rate curves and adiabatic calorimetry

A general concept is presented for the kinetic interpretation of DTA curves. This is based on the limiting conditions of a DTA measurement: either the kinetic cell constant is zero (adiabatic conditions), or infinite (rate curve). On the other hand, the self-heating effect (thermal feedback), based on the product of the reaction enthalpy with the reactant feed, may be absent (“ideal” kinetic DTA curve) or infinite (impulse reaction). Our recent formulae for the correction of the kinetic classification parameters, shape index and reaction type index, as well as other relationships and their utility, are successfully tested by application to ca. 2000 experimental DTA curves obtained in stirred solutions.The expressions reveal the influence of the activation parameters, heating rate, maximum signal height and cell constant and, therefore, allow a general discussion of the kinetics, independent of the experimental conditions.     

Kinetics of thermal decomposition of aseptic packages

Kinetics of thermal decomposition of aseptic packages (e.g. Tetrapak cartons) and pyrolysis of this waste in a laboratory flow reactor was studied. Three different models for the calculation of the reaction rate and the determination of apparent kinetic parameters of thermal decomposition were used. The first method assumes a two stage thermal decomposition and the kinetic parameters were determined by fitting a derivative thermogravimetric (DTG) curve to experimentally determined thermogravimetric data of whole aseptic cartons. The second method uses kinetic parameters determined by fitting DTG curves to thermogravimetric data of individual components of aseptic packages. The last method was a multi-curve isoconversion method assuming a change of kinetic parameters with the increasing conversion. All types of the determined kinetic parameters were used in a mathematical model for thermal decomposition of mini briquettes made from aseptic packages at the temperature of 650°C. The model calculated also the heat conduction in the particles and it was verified by an independent set of experiments conducted in a laboratory screw type flow reactor.     

The Decomposition of Hydrogen Peroxide. A non-linear dynamic model

All thermal systems are subject to problems of thermal regulation. These can be understood through the use of thermochemical systems, in particular for those in the liquid phase. A dynamic linear model was earlier applied to obtain both the reaction enthalpy and the rate constant at constant temperature for the catalytic decomposition of hydrogen peroxide. This first model did not yield a good fitting between the calculated and experimental data. The hypothesis that the rate constant was independent of temperature was too strong. In the present study, a more elaborate, non-linear model was developed, which takes into account the rate constant variations as a function of temperature (Arrhenius law). This model allowed the activation energy to be determined. The calculated data then successfully fitted the experimental data. The literature indicates that the first-order rate law is not valid for a certain range of concentrations; the present model verified this. The results of dynamic modelling confirm and increase the precision of results obtained in different ways. The developed model is validated through these comparisons. This revised version was published online in July 2006 with corrections to the Cover Date.     

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

Thermal 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     

Effect of Oxygen on the Polymerization of Vinyl Chloride

The effects of oxygen on the liquid-phase polymerization of vinyl chloride at 55°C in the presence of an added initiator, bis(4-tert-butylcyclohexy1) peroxydicarbonate, (Perkadox 16), have been studied by tumbled dilatometry. A conventional kinetic scheme involving a predominant cross-termination reaction is proposed to explaine the dependence of the induction period on initial oxygen concentration and initiator concentration. The degree of conversion of the initial oxygen to peroxidic compounds did not exceed 30% by weight under any experimental conditions employed, and the existence of other oxidation products such as formaldehyde, carbon monoxide, and methanol has been demonstrated. Radical decomposition reactions may produce some of the oxidation products. At 55°C, the average velocity constant for decomposition of vinyl chloride polyperoxides in dichloromethane solution was 8 × 10^?5 sec^?1 compared with 6.6 × 10^?5 sec^?1 for Perkadox 16. Perkadox 16 has been used as an initiator in a dilatometric study of the homogeneous polymerization of styrene at 60°c. Molecular weights of the polymers were determined viscometrically or by the use of gel-permeation chromatography. The results indicate that no transfer to initiator occurs in this systems.     

Kinetics of chemical ionization in shock waves: III. Inert diluent gas effect on the chemical ionization rate

An experimental study of ionization in hydrogen azide decomposition and methane oxidation in shock waves is reported. In both cases, the kinetics of chemical ionization changes significantly upon the replacement of argon with helium as the inert diluent gas. The diluent effect is explained in terms of the hypothesis that the reactions responsible for chemical ionization proceed via the formation of an excited intermediate complex. Calculations using kinetic schemes of the ionization processes are carried out.     

A kinetic evaluation of the thermal oxidation of a phenol stabilised polybutadiene

The thermal oxidation of hydroxy telechelic polybutadiene stabilised with 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) was studied at 100 °C using weight changes and stabiliser quantification by liquid chromatography. The extended induction time relative to the unstabilised sample and the initial rate of stabiliser depletion were found to be proportional to the initial stabiliser concentration.Previously published kinetic models, based on the hypothesis that the stabiliser is only consumed by reaction with peroxy radicals and that the stationary state assumption is appropriate, were examined and found insufficient to explain the observations. An improved model was suggested assuming the contribution of a phenol oxygen reaction that results in a competing oxidation of the stabiliser itself. Experimental and theoretical arguments in favour of this model refinement are proposed.     

Non-isothermal Kinetic Study of the Heterogeneous Thermal Decomposition of a Mannich Compound

The authors present data concerning the evaluation of kinetic parameters of the decomposition of a Mannich compound by using the classical method of constant heating rate thermal analysis and the new one of controlled rate thermal analysis (CRTA). The data processed using the CRTA method allow to obtain more reliable kinetic parameters according to the proposed reaction mechanism. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetic study of thermal decomposition of dolomite by controlled transformation rate thermal analysis (CRTA) and TG

The mechanism of the thermal decomposition of dolomite was studied. It was shown that a comparison of the kinetic data obtained from the kinetic analysis of a single TG trace and a single curve recorded using the constant rate thermal analysis (CRTA) method allows discrimination of the actual kinetic model obeyed by the reaction and also determination of its corresponding kinetic parameters.

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Zusammenfassung Es wurde der Mechanismus der thermischen Zersetzung von Dolomit untersucht. Es wurde gezeigt, daß ein Vergleich der kinetischen Angaben aus einer kinetischen Analyse eines einfachen TG-Durchlaufes und einer mittels CRTA einmal registrierten Kurve die Unterscheidung desjenigen kinetischen Modelles ermöglicht, dem die Reaktion gerade unterliegt und außerdem die Bestimmung der zugehörigen kinetischen Parameter ermöglicht.

     

Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene

The thermal oxidation of an unvulcanized, unstabilized polyisoprene rubber (IR) has been studied in the 40-140 °C temperature range. Ageing was monitored by FTIR determination of double bonds and carbonyl groups, mass uptake measurement, and weight average molar mass determination. A mechanistic scheme based on the standard scheme for radical chain oxidation, but taking into account the diversity of initiation processes and the existence of inter- and intramolecular radical additions to double bonds, was built. The kinetic model derived from this scheme is composed of seven differential equations to be solved in discrete thickness layers to take into account the kinetic control by oxygen diffusion. This system was numerically solved using a Matlab program dedicated to stiff systems of differential equations. The elementary rate constants and other kinetic parameters were then determined from experimental data, using an inverse approach. A set of physically reasonable parameter values was obtained, thus allowing us to envisage lifetime predictions at low temperature (long term). The results led to observations difficult to make from classical analytical studies, for instance the predominance of bimolecular hydroperoxide decomposition among other initiation modes or the competition between intermolecular hydrogen abstraction and intramolecular addition of peroxy radicals to double bonds.     

Pulsed laser powered homogeneous pyrolysis: A computational analysis

The use of the pulsed laser powered homogeneous pyrolysis technique for measuring unimolecular decomposition rate constants under unambiguously homogeneous conditions is investigated by numerical simulation of the experiment. The coupled partial differential equations which govern the gas dynamics and chemical kinetics are solved numerically and the results analyzed. Conditions under which rate constants can be extracted from the experimental data using a simplified analysis are determined. The effects of five sources of error in the simplified analysis are computed. A correlation is presented which may be used to correct for overestimation of the rate constant which is inherent in the simple analysis. Conditions under which the other four sources of error become negligible are presented. Overall, it is expected that this technique will be capable of routinely measuring rate constants within a factor of 2, and will do much better when a high power laser with a uniform beam profile is used and/or a well characterized thermal monitor molecule is available which decomposes with kinetic parameters close to that of the reactant being investigated. © 1994 John Wiley & Sons, Inc.     

Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene

The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.     

胶体泡沫(CGA)排液动力学研究

分别对三种表面活性剂, 十二烷基硫酸钠(SDS)、十六烷基三甲基溴化铵(HTAB)以及吐温80 (Tween 80)形成的胶体泡沫(CGA)的排液过程动力学进行了研究. 讨论了表面活性剂种类和浓度, 体系温度等对CGA排液过程的影响. 运用非线性最小二乘法拟合, 推出了表征CGA排液过程的动力学模型: V_t＝V_maxtⁿ/(Kⁿ＋tⁿ), 式中, V_t表示时间t时的排液量(mL), t表示排液时间(s), V_max表示CGA最大排液量(mL)的模型参数, K为模型参数, 表征CGA排液过程的半衰期t_1/2, n为排液曲线反曲特征指数. 通过该模型计算出了表征CGA排液过程的速率常数k_d和半衰期t_1/2等参数. 并通过lnk_d～T^－1的线性关系, 证明了该模型符合Arrhenius方程. 最后, 通过对CGA排液速率随时间的变化分析, 解析了CGA排液过程的“两段论”机理.