Constant rate thermal analysis for thermal stability studies of polymers

This paper explores the relationship between the shapes of temperature-time curves obtained from experimental data recorded by means of constant rate thermal analysis (CRTA) and the kinetic model followed by the thermal degradation reaction. A detailed shape analysis of CRTA curves has been performed as a function of the most common kinetic models. The analysis has been validated with simulated data, and with experimental data recorded from the thermal degradation of polytetrafluoroethylene (PTFE), poly(1,4-butylene terephthalate) (PBT), polyethylene (PE) and poly(vinyl chloride) (PVC). The resulting temperature-time profiles indicate that the studied polymers decompose through phase boundary, random scission, diffusion and nucleation mechanisms respectively. The results here presented demonstrate that the strong dependence of the temperature-time profile on the reaction mechanism would allow the real kinetic model obeyed by a reaction to be discerned from a single CRTA curve.Junta de Andalucía TEP-03002Ministerio de Ciencia e Innovación MAT 2008-06619/MA

Critical study of the isoconversional methods of kinetic analysis

A critical study of the use of isoconversional methods for the kinetic analysis of non-isothermal data corresponding to processes with either a real or an apparent variation of the activation energy, E, with the reacted fraction, α, has been carried out using for the first time simulated curves. It has been shown that the activation energies obtained from model-free methods are independent of the heating rate. However, the activation energy shows a very strong dependence of the range of heating rates used for simulating the curves if the apparent change of E with α is caused by overlapping processes with different individual activation energies. This criterion perhaps could be used for determining if a real dependence between E and α is really occurringEspaña Ministerio de Educación y Ciencia No. MAT2004-0264

Kissinger kinetic analysis of data obtained under different heating schedules

The dynamic heating rate method developed by TA Instruments ( Hi-Res ™ ) is a kind of sample controlled thermal analysis in which a linear relati onship between the logarith m of the heating rate and the rate of weight change is imposed. It is s hown in this paper that the reacted fraction at the maximum reaction rate strongly depends on the parameters selected for the Hi-Res heating algorithm, what invalidates the use of the Kissinger method for analysing Hi-Res data unless that the reaction fits a first order kinetic law. Only in this latter case, it has been demonstrated that it is not required that a constant value of the reacted fr action at the maximum reaction rate is fulfilled for determining the activation energy from the Kissi nger method. In such a case the Kissinger plot gives the real activation energy, independently of both the heating schedule used and the value of the reacted fraction, α m , at the maximum

Kinetic analysis of solid-state reactions: Precision of the activation energy calculated by integral methods

The integral methods are extensively used for the kinetic analysis of solid-state reactions. As the Arrhenius integral function [p(χ)] does not have an exact analytical solution, different approximated equations have been proposed in the literature for performing the kinetic analysis of experimental integral data. Since the first approximation of Van Krevelen, a large number of equations have been proposed with the objective of increasing the precision in the determination of the Arrhenius integral, as checked from the standard deviation of the approximated function with regard to the real exact value of the integral. However, the main application of these equations is the determination of the kinetic parameters, in particular activation energies, and not the computation of the Arrhenius integral. A systematic analysis of the errors involved in the determination of the activation energy from these integral methods is still missing. A comparative study of the precision of the activation energy as a function of χ and T computed from the different integral methods has been carried out

Influence of Al2O3 reinforcement on precipitation kinetic of Cu-Cr nanocomposite

In this paper, the kinetic of precipitation process in mechanically alloyed Cu-1 wt.% Cr and Cu-1 wt.% Cr/3 wt.% Al2O3 solid solution was compared using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The ageing kinetics in Cu¿Cr and Cu¿Cr/Al2O3 can be described using Johnson¿Mehl¿Avrami (JMA) and Sestak¿Berggren (SB) models, respectively. These different behaviors have been discussed in details. It was found that in presence of Al2O3 reinforcement, the ageing activation energy is decreased and the overall ageing process is accelerated. This behavior is probably due to higher dislocation density previously obtained during ball milling and Al2O3¿Cu interface. TEM observations confirm that Al2O3¿Cu interface and structural defects act as a primary and secondary nucleation sites, respectively.Gobierno de España ENE 2007-67926-C02-0

Influencia de la adición de estaño en el proceso de precipitación en una aleación de Cu-Ni-Zn

Mediante calorimetría diferencial de barrido (DSC), medidas de microdureza y microscopía electrónica de transmisión de alta resolución (HRTEM) se estudió la influencia de la adición de 1,1% en peso de estaño en el endurecimiento por precipitación en una aleación de Cu-11Ni-20Zn (% en peso). Las curvas calorimétricas de la aleación ternaria, en el intervalo de temperaturas analizado, muestran la presencia de dos reacciones exotérmicas asociadas al desarrollo de orden de corto alcance, favorecidas por la migración de vacantes. En cambio, en la aleación cuaternaria se observa una reacción exotérmica y una endotérmica, asociadas a la formación y disolución de precipitados de Cu2NiZn, respectivamente. Se ha demostrado que una adición de 1,1% de estaño juega un papel importante en la formación de los precipitados de Cu2NiZn, responsables del endurecimiento por precipitación de la aleación ternaria.Influence of tin additions on the precipitation processes in a Cu-Ni-Zn alloys. The influence of 1.1 wt% tin additions on the precipitation hardening of Cu-11 wt% Ni-20 wt% Zn alloy was studied by Differential Scanning Calorimetry (DSC), microhardeness measurements and High Resolution Transmission Electron Microscopy (HRTEM). The calorimetric curves, in the range of temperatures analyzed, show the presence of two exothermic reactions in the ternary alloy, associated to the short-range-order development assisted by migration of excess vacancies. On the other hand, one exothermic and one endothermic reaction are observed in the quaternary alloy, associated to the formation and dissolution of Cu2NiZn precipitates, respectively. It has been show that an addition of 1.1% tin plays an important role in the formation of Cu2NiZn precipitates, responsible for the precipitation hardening of the ternary alloyFondo Nacional de Desarrollo Científico y Tecnológico FONDECYT 114078

Limitations of model-fitting methods for kinetic analysis: Polystyrene thermal degradation

In this paper, some clarifications regarding the use of model-fitting methods of kinetic analysis are provided in response to the lack of plot linearity and dispersion in the activation energy values for the thermal degradation of polystyrene found in the literature and some results proposing an nth order model as the most suitable one. In the present work, two model-fitting methods based on the differential and integral forms of the general kinetic equation are evaluated using both simulated and experimental data, showing that the differential method is recommended due to its higher discrimination power. Moreover, the intrinsic limitations of model-fitting methods are highlighted: the use of a limited set of kinetic models to fit experimental data and the ideal nature of such models. Finally, it is concluded that a chain scission model is more appropriate than first order.Junta de Andalucía TEP‐03002Ministerio de Economía y Competitividad CTQ2011‐2762

Magnesium hydride for energy storage applications: The kinetics of dehydrogenation under different working conditions

A new approach to the kinetics of magnesium hydride dehydrogenation is considered. A model able to predict the dehydrogenation under different experimental conditions has been proposed. A new combined kinetic analysis method, which considers the thermodynamic of the process according to the microreversibility principle, has been used for performing the kinetic analysis of data obtained under different thermal schedules at hydrogen pressures ranging from high vacuum up to 20 bar. The kinetic analysis shows that the dehydrogenation mechanism of magnesium hydride depends on the experimental conditions. Thus, the reaction follows a first order kinetics, equivalent to an Avarmi-Erofeev kinetic model with an Avrami coefficient equal to 1, when carried out under high vacuum, while a mechanism of tridimensional growth of nuclei previously formed (A3) is followed under hydrogen pressure. An explanation of the change of mechanism is given. It has been shown that the activation energy is closed to the Mg-H bond breaking energy independently of the hydrogen pressure surrounding the sample, which suggests that the breaking of this bond would be the rate limiting step of the process. The reliability of the calculated kinetic parameters is tested by comparing simulated and experimental curves.Ministerio de Economía y Competitividad CTQ2014-52763-C2-1-RJunta de Andalucía TEP-7858, TEP-190

Combined kinetic analysis of thermal degradation of polymeric materials under any thermal pathway

Combined kinetic analysis has been applied for the first time to the thermal degradation of polymeric materials. The combined kinetic analysis allows the determination of the kinetic parameter from the simultaneous analysis of a set of experimental curves recorded under any thermal schedule. Besides, the method does not make any assumption about the kinetic model or activation energy and allows the analysis even when the process does not follow one of the ideal kinetic models already proposed in literature. In the present paper the kinetics of the thermal degradation of both polytetrafluoroethylene (PTFE) and polyethylene (PE) have been performed. It has been concluded, without previous assumptions on the kinetic model, that the thermal degradation of PTFE obeys a first order kinetic law, while the thermal degradation of PE follows a diffusion-controlled kinetic model

Development of a universal sample controlled thermal analysis (SCTA) system for being used with any thermoanalytical instrument

The SCTA method implies to control the te mperature in such a way that the reaction rate changes with the time according to a function previously defined by the user. Constant Rate Thermal Analysis (CRTA) is one of the most commonly used SCTA methods and implies achieving a temperature profile at whic h the reaction rate remains constant all over the process at a value previously selected by the user. This method permits to minimize the influence of heat and mass transfer phenomena on the forward reaction. The scope of this work is to develop a universal CRTA temperat ure controller that could be adapted to any thermoanalytical device. The thermoanalytical signal is programmed to follow a preset linear trend by means of a conventiona l controller that at the time controls a second conventional temperature programmer that forces the temp erature to change for achieving the trend programmed for the thermoanalytical signal. Ex amples of the performa nce of this control system with a Thermobalance and a Thermomechanical Analyser (TMA) are given