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Abstract  

An incremental integral isoconversional method for the determination of activation energy as a function of the extent of conversion is presented. The method is based on the treatment of experimental data without their transformation so that the resulting values of activation parameters should not be biased. The method was tested for recovering the activation energies from simulated data and employed for the treatment of experimental data of the NiS recrystallisation.

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Abstract  

Solid-gas phase transition processes of some triazines were studied from kinetic and thermodynamic viewpoint. DSC measurements and Clausius-Clapeyron equation were used to determine enthalpy values related to these processes. Model-fitting methods (based on Arrhenius, Šatava equations and Šestk-Berggren equations) and model-free methods (based on Ozawa-Flynn-Wall and Kissinger equations) allow to hypothesis R2 mechanism. An attempt to determine the activation parameters (ΔH #, ΔG #, ΔS #) related to these processes was carried out. Accordance between the activation enthalpy values with those of activation energy obtained by means of kinetic methods and with the experimental (DSC) and calculated (Clausius-Clapeyron) enthalpy values was found.

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Journal of Thermal Analysis and Calorimetry
Authors: J. C. O. Santos, I. M. G. Santos, F. S. M. Sinfrônio, M. A. Silva, E. V. Sobrinho, M. M. Conceiçăo, V. J. Fernandes Jr., and A. G. Souza
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Abstract  

The kinetic and thermodynamic study of synthetic lubricant oils was accomplished in this work, using isothermal and non-isothermal thermogravimetry based on mass loss as a function of time and temperature. The thermodynamic and kinetic behavior of the synthetic lubricant oils depends on atmosphere and heating rates used in TG analysis. The kinetic and thermodynamic results were satisfactory, presenting good correlation.

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Abstract  

The possibility to determine the kinetic parameters for temperature programmed reduction of Cu/Fe3O4 using only one TPR profile is analyzed. The same data are analyzed both by Friedman’s iso-conversional method and another one previously derived and published by the authors. One shows that taking into account the experimental restrictions of Monti and Baiker, the Friedman’s method, although gives values of the activation energy smaller than the real values, indicates a very similar dependence of these on the reduction degree. On the basis of some synthetic data one shows that the errors are very large when these recommendations are neglected, being possible to determine a false dependence on the degree of reduction.

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Abstract  

Thermal analysis has a long and prominent role in the characterization of materials, including polymeric materials. Kinetic studies in one form or another have often been employed in an attempt to assess stability, predict lifetime, establish degradation pathway, or project suitable processing conditions. The results of such studies have often formed the basis for the proposal of the ‘mechanism’ of reaction. This despite the fact that the reaction being observed is often unknown or is not a single process but rather several parallel or consecutive events. This latter is particularly true for ‘variable temperature kinetics’. The utility/value of such exercises is marginal at best and contributes nothing to an understanding of the mechanism of any of the reactions involved.

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Abstract  

Several dynamic methods for estimating activation energies have been developed. This development has arisen largely as a matter of convenience and the desire to minimize analysis time. While these methods generally afford values which are somewhat similar, the agreement among values from various methods is never outstanding. Further, the values obtained are often, at best, only approximations of the values obtained by the traditional isothermal approach. To better ascertain the utility of dynamic methods for the determination of activation energies, the activation energy for the thermal degradation of a standard vinylidene chloride/methyl acrylate (five-mole percent) copolymer has been generated by a variety of methods. The degradation of this polymer is an ideal reaction for evaluation of the various methods. At modest temperatures (<200C), the only reaction that contributes to mass loss is the first order evolution of hydrogen chloride, i.e., there is only one significant reaction occurring and it is not impacted by competing processes. The best values (most reproducible; best correspondence to values obtained by titrimetry and other methods) are those obtained by plotting the natural logarithm of rate constants obtained at various temperatures vs. the reciprocal of the Kelvin temperature. Various dynamic methods yield values which are less reproducible and which approximate these values to a greater or lesser degree. In no case is the agreement good.

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Abstract  

The thermal polymerization of styrene is a long-known and well-practiced phenomena. While the mechanism of the thermal initiation event has been the subject of several investigations, it is not yet well understood. In an attempt to gain further insight as to the details of possible initiation from styrene dimer, analogous stable cycloadducts (maleic anhydride, tetracyanoethylene) of 1- and 2-vinylnaphthalene have been synthesized, fully characterized spectroscopically, and subjected to thermal decomposition. In the main, the major thermal event observed for these styrene dimer mimics is retro cycloaddition. This process is characterized by an activation enthalpy of approximately 30 kcal mol–1. Aminor process which accompanies the major reaction is the homolysis of a carbon–hydrogen bond to generate a carbon radical which may be trapped as a stable adduct of the 2,2,6,6-tetramethylpiperinyloxy (TEMPO) radical.

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Abstract  

Vinylidene chloride copolymers containing a predominance of vinylidene chloride (85-90%) have long been important barrier polymers widely used in the plastics packaging industry. These materials display excellent barrier to the ingress of oxygen and other small molecules (to prevent food spoilage) and to the loss of food flavor and aroma constituents (to prevent flavor scalping on the supermarket shelf). While these polymers have many outstanding characteristics, which have made them commercial successes, they tend to undergo thermally-induced degradative dehydrohalogenation at process temperatures. The dehydrochlorination occurs at moderate temperatures (120-200C) and is a typical chain process involving initiation, propagation and termination phases. Defect structures, namely internal unsaturation (allylic dichloromethylene groups), serve as initiation sites for the degradation. These may be introduced during polymerization or during subsequent isolation and drying procedures. If uncontrolled, sequential dehydrohalogenation can lead to the formation of conjugated polyene sequences along the polymer mainchain. If sufficiently large, these polyenes absorb in the visible portion of the electromagnetic spectrum, and give rise to discoloration of the polymer. The dehydrochlorination process may be conveniently monitored by thermogravimetric techniques. Both initiation and propagation rate constants may be readily obtained.

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activation parameters for both the HCHO-dependent and -independent reactions were obtained by variation of [HCHO] at various temperatures between 22 and 35 °C ( Table 2 ). The parameters Δ H # and Δ S ∗ were obtained from the Eyring equation

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