J. Málek

Computational aspects of kinetic analysis. Part A: The ICTAC kinetics project-data, methods and results

Abstract Part A of this series of papers (Parts B to E follow) presents the data and methods used, as well as the results obtained by participants in the ICTAC Kinetics Project. The isothermal and non-isothermal data sets provided were based on a hypothetical simulated process as well as on some actual experimental results for the thermal decompositions of ammonium perchlorate and calcium carbonate. The participants applied a variety of computational methods. Isoconversional and multi-heating rate methods were particularly successful in correctly describing the multi-step kinetics used in the simulated data. Reasonably consistent kinetic results were obtained for isothermal and non-isothermal data. There is, of course, no ‘true’ answer for the kinetic parameters of the real data, so the findings of the participants are compared. An attempt has been made to forecast the tendencies for the future development of solid state kinetics.

The effect of the CO2 pressure on the thermal decomposition kinetics of calcium carbonate

Abstract The influence of the pressure of carbon dioxide on the thermal decomposition kinetics of calcium carbonate is studied. It is shown that it is possible to obtain meaningful and reliable kinetic parameters from isothermal and non-isothermal data provided that the pressure correction term is included in the kinetic equation.

Possibilities of two non-isothermal procedures (temperature- or rate-controlled) for kinetical studies

The applicability of both conventional Thermal Analysis (TA) and Controlled Rate Thermal Analysis (CRTA) for kinetic analysis is discussed. It is shown that TA method can give a reliable kinetic information and meaningful kinetic parameters especially for solid state transformation. On the other hand the CRTA method is more suitable for decomposition process where one or more gasses are evolved.A consistent and reliable method of kinetic analysis is proposed for both techniques. This method is illustrated to analyze the crystallization process of chalcogenide glass and the decomposition of dolomite.ZusammenfassungEs wird die Anwendbarkeit von herkömmlicher Thermoanalyse (TA) und geschwindigkeitsgesteuerter Thermoanalyse (CRTA) bei kinetischen Untersuchungen diskutiert. Die TA Technik kann eine zuverlässige kinetische Information und sinnvolle kinetische Parameter besonders bei Feststoffumsetzungen liefern. Die CRTA Technik ist andererseits mehr für Zersetzungsprozesse geeignet, bei denen ein oder mehrere Gase freigesetzt werden.Für beide Techniken wird eine einheitliche und geeignete Methode zu kinetischen Analyse vorgeschlagen. Als Beispiel wird diese Methode zur Analyse des Kristallisationsprozesses von Chalkogenidgläsern sowie der Zersetzung von Dolomit angewendet.

Influence of the accelerator concentration on the curing reaction of an epoxy-anhydride system

The effect of accelerator content on the curing reaction of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with methyl-tetrahydrophthalic anhydride was studied by DSC. The uncatalysed curing occurs at high temperature (between 190 and 310°C) with thermal degradation. The addition of accelerator which is a tertiary amine catalyst agent, namely dimethylbenzylamine (DMBA), causes two exothermic peaks. The cure extent and the position of the peaks depend on the accelerator content. The first peak, which is sharp and well defined, appears between 80 and 200°C and may be attributed to the catalysed curing. The second peak, which is broad, only appears for low accelerator content (lower than 1 pbw) in the zone of uncatalysed curing (between 200 and 320°C), and may be attributed to the uncatalysed curing. The activation energy corresponding to the first exothermic peak in the catalysed curing, calculated by the Kissinger method, decreases with the accelerator content. Kinetic analysis performed by Maleks method shows that the autocatalytic model (two-parameter Sestak-Berggren equation) can describe satisfactorily the kinetics of the catalysed and uncatalysed curing. In the catalysed system, the parameters m and n increase slightly with the accelerator content, and the pre-exponential factor, ln A, undergoes a slight decrease. The maximum Tg of the fully cured epoxy obtained by isothermal curing at 110°C in catalysed systems shows no significant changes. These results mean that the differences observed in the kinetics of curing between catalysed systems do not imply significant changes in the structure of the network of the epoxy resin.

Addition of a reactive diluent to a catalyzed epoxy-anhydride system. I. Influence on the cure kinetics

The effect of a reactive diluent (RD) on the kinetics of the curing of an epoxy resin, based on diglycidyl ether of bisphenol A (DGEBA), with a carboxylic anhvdride derived from methyl-tetrahydrophthalic anhydride (MTHPA) catalyzed by a tertiary amine has been studied. The reactive diluent was a low-viscosity aliphatic diglycidyl ether, and the compositions per 100 parts by weight (pbw) of DGEBA were 10, 30, and 50 pbw of RD with the stoichiometric quantity of MTHPA and 1 pbw of catalyst. The curing kinetics was monitored by differential scanning calorimetry (DSC), and the kinetic parameters were determined from the nonisothermal DSC curves by the method described by Malek. The kinetic analysis suggests that the two-parameter autocatalytic model is the more appropriate to describe the kinetics of the curing reaction of this epoxy-anhydride system. The kinetic parameters thus derived satisfactorily simulate both the nonisothermal DSC curves and the isothermal conversion-time plots. Increasing the RD content leads to a small increase in both the nonisothermal and the isothermal heats of curing and has a slight effect on the kinetic parameters E, ln A, m, and n, and, consequently, on the overall reactivity of the system. On the other hand, the increase of the RD content significantly affects the structure of the crosslinked epoxy. It is confirmed that the introduction of aliphatic chains in the structure of the epoxy increases the mobility of the segmental chains in the glass transition region. The consequence of this chemical modification is a decrease of the glass transition temperature, Tg.

Accommodation of the actual solid-state process in the kinetic model function. Part 2. Applicability of the empirical kinetic model function to diffusion-controlled reactions

Abstract The significance of the empirical kinetic model function for diffusion-controlled reactions is discussed in connection with the fractal nature of the reaction geometry and the macroscopic character of the thermoanalytical curves for solid-state reactions. The mathematical properties of such empirical kinetic model functions based on the geometrical fractal were investigated numerically in order to evaluate their practical usefulness as a possible diagnostic tool for distinguishing the most appropriate kinetic model function. The procedure of distinguishing the kinetic model function is extended, including such empirical kinetic model functions based on the geometrical fractal, and applied to the kinetic analysis of the thermal dehydroxylation of synthetic brochantite Cu 4 (OH) 6 SO 4 .

Crystallization kinetics of Ge0.3Sb1.4S2.7 glass

Abstract The influence of the sample specific surface on the crystallization kinetics of the Sb 2 S 3 in Ge 0.3 Sb 1.4 S 2.7 glass has been studied. It was found that the DSC crystallization data for the bulk sample can be described by the Johnson-Mehl-Avrami (JMA) model corresponding to three-dimensional crystal growth. The same description for powder samples is not possible and the empirical Sestak-Berggren equation had to be used. The condition of the validity for the JMA model is discussed.

Crystallization kinetics of zirconia-yttria gels☆

Abstract The crystallization kinetics of ZrO2Y2O3 (3 mol%) dried gels prepared by four different methods was studied by differential scanning calorimetry. It was found that the Johnson-Mehl-Avrami model cannot be used for the description of crystallization of tetragonal zirconia crystals formed in the gel. It was established that this crystallization process can be described by the empirical Sestak-Berggren equation. The influence of the method of preparation is discussed.

Sol-gel Preparation of Rutile Type Solid Solution in TiO2-RuO2 System

The preparation of rutile type solid solutions in (TiO2)x -(RuO2)1−x system in the 0≤x ≤0.7 concentration range is described. The single phase solid solutions are formed by controlled nanocrystallization of amorphous gels prepared by the sol-gel method. The kinetics of this crystallization process has been analyzed. It was found that the crystallization does not correspond to the Johnson-Mehl-Avrami model and it can be described by the two-parameter Šesták-Berggren kinetic model.

STRUCTURAL RELAXATION IN AMORPHOUS SOLIDS STUDIED BY THERMAL ANALYSIS METHODS

Structural relaxation for simple and more complex thermal histories is described by a phenomenological model based on a non-exponential relaxation function, the reduced-time concept and the nonlinear structural contribution to the relaxation time. The history, development of experimental techniques and data analysis is described. It is shown that the volume and enthalpy relaxation response can conveniently be compared on the basis of a fictive relaxation rate, Rf. A simple equation relating Rf and the parameters of the phenomenological model is given. The calculated data for moderate departures from equilibrium are in good agreement with our experiments and data previously reported in the literature.