Analysis Of Isothermal Data Research Articles

The glass transition temperature (tg) as a parameter for monitoring the cure of an amine/epoxy system at constant heating rates

AbstractA continuous heating transformation (CHT) cure diagram displays the time and temperature of events that a material encounters during the course of continuous heating at different heating rates. For thermosetting systems, these events include gelation, vitrification, and devitrification. Analysis of isothermal data for a diglycidyl‐type epoxy/tetrafunctional aliphatic amine system, which is initially liquid at room temperature, indicated that kinetic control applied prior to vitrification; i.e., for Tg<Tcure (temperature of reaction). The parameters so obtained (i.e., order of reaction, Arrhenius pre‐exponential term, and activation energy) together with an experimental relationship between Tg and extent of reaction were used as a basis for modeling the onset of vitrification and devitrification in the CHT diagram. For heating scans from below Tgo (glass transition temperature of the reactants), initial devitrification occurs when Tcure first passes through the Tg of the initial reacting material, except for very low heating rates for which inital devitrification is not encountered. Vitrification is identified as the point at which Tg becomes equal to the increasing curing temperature. Diffusion control retards the reaction rate beyond vitrification. In the limiting case of complete retardation of the rate after vitrification, the reaction can proceed along the Tg = Tcure path as long as the additional reaction with increasing temperature is sufficient to increase Tg at least at the same rate as the heating temperature; devitrification occurs thereafter. The critical heating rates at which the material does not devitrify in the vicinity of Tgo, and that at which no vitrification is encountered, are calculated.

Kinetic analysis of thermal decomposition reactions: I. Thermal decomposition of potassium bromate

The kinetics of the thermal decomposition of potassium bromate were studied using isothermal and dynamic thermogravimetric techniques. Kinetic analysis of isothermal data in view of various solid-state reaction models showed that the reaction is best described by phase-boundary controlled and random nucleation models. Kinetic analysis of dynamic TG curves were discussed and a critical comparison was made of three methods: the direct differential method and two integral methods, one of Coats and Redfern and the other of Ozawa. The results showed that the Ozawa method gives better correlation and the results are in good agreement with those obtained under isothermal thermogravimetric conditions.

Changes in the kinetics of the vaterite-calcite transformation with temperature and sample crystallinity

The effect of temperature and crystallinity on the kinetics of the vaterite-calcite thermal transformation has been examined by analysis of isothermal data evaluated by X-ray diffraction techniques and differential scanning calorimetry. These results were compared with dynamic differential scanning calorimetry results and complemented by X-ray line profile analysis and scanning electron microscopy studies. In the 445 to 485°C temperature range, the kinetic data can be fitted reasonably well to a phase-boundary equation. At higher temperatures (510 to 530°C) by using differential scanning calorimetry techniques, an Avrami equation describes more accurately the conversion curves and a decrease in activation energy is observed. This change in the formal mechanism is supoported by the external morphology of the particles, revealed by scanning electron microscopy. Prior mechanical treatment of vaterite causes an increase in activation energy, due to the recrystallization process that accompanies the reaction.

Influence of crystallinity on the kinetics of the litharge-massicot phase transition

The kinetics of the thermal transformation of litharge to massicot were studied by the analysis of isothermal data. A diffusion mechanism, formally three-dimensional, seems to be adequate to describe the kinetic data as deduced from reduced time and log-log plots. Comparison of two samples differing in particle size and crystallinity indicated the influence of preliminary grinding on the values of activation energy calculated with the Arrhenius equation. This value changed from 543.4 to 681.3 kJ mol −1 for a three-dimensional diffusion model as the erystallinity decreases. The determination of crystallite size and microstrains for the original and partially converted samples of litharge showed that a recrystallization process occurs simultaneously with the phase transition. This process is responsible for the change in activation energy and diminution of the peak area in the differential thermal analysis curves as the crystallinity decreases.

Stoichiometric and heating-rate effects on DSC-evaluated kinetics for a high-performance epoxy system

Dynamic DSC experiments on a high-performance amine-cured epoxy system have shown that the kinetic parameters determined assuming an overall reaction for the system are dependent on the heating rate of the scan. The high-performance system examined in this investigation is the TGDDM—DDS epoxy system. Changes in the number, location, and magnitude of the transitions in the thermograms which occur with changes in the heating rate were interpreted through the definition of two reaction regimes: one in which the primary-amine addition reaction predominates; and the other in which the secondary-amine addition, hydroxyl—epoxide, and homopolymerization reactions become significant. To elucidate these regimes, three compositions were examined: epoxy alone; excess epoxy; and excess amine. The activation energies for the two reaction regimes were determined to be 16.7 and 42.3 kcal mol −, respectively. A model proposed by Prime for the heating rate dependence of the kinetic parameters as determined by DSC is also analyzed for its applicability for the kinetic analysis of this reaction system. Further, analysis of isothermal data is considered as an integral part of the kinetic analysis for this system.

Kinetic study of the thermal decomposition of cobalt(III) oxyhydroxide

The kinetics of thermal decomposition of CoOOH have been studied by analysis of isothermal weight loss data under vacuum. The comparison of linear correlation coefficients of different kinetic expressions applied to these data does not allow an understanding of the mechanism, even when significance tests are performed (t test). A single value of the activation energy (193 kJ mol−1) is obtained from the Arrhenius plots, and is relatively independent of the choice of rate law. On the other hand, a change in the mechanism of formation of Co3O4 with temperature cannot be inferred from analysis of isothermal data. Thus, the statement of some authors that from formal kinetics it is possible to distinguish the proton and electron transfers involved in the transformation appears unacceptable.

Remarks on the relationship between isothermal and non-isothermal kinetic results. A new method for the kinetic analysis of isothermal data

Remarks on the relationship between isothermal and non-isothermal kinetic results. A new method for the kinetic analysis of isothermal data

The analysis of isotherm data to describe polyfunctional exchangers and soils

The analysis of isotherms of cation ion-exchange adsorption in heterogeneous polyfunctional ion-exchangers and soils containing groups of adsorption centres non-equivalent in their preference for exchanging cations permits calculation on the basis of experimental data of the selectivity coefficients ( K i ) and maximum adsorption of cations ( S i ) for each individual group of heterogeneous centres and thus provides an expression for the exchange isotherm with stable values of K i over a wide range of concentrations. A method based on a step-by-step breakdown of the experimental isotherm of ionexchange adsorption was used to describe Ca 2+ and Pb 2+ exchange in samples of a leached chernozemic soil. The data obtained demonstrated existence of two groups of non-equivalent adsorption centres. Theoretical and experimental data were found to be in sufficient agreement.

On the theoretical basis of the hancock and sharp “ln.ln method” of kinetic analysis of isothermal data

On the theoretical basis of the hancock and sharp “ln.ln method” of kinetic analysis of isothermal data

Photoemission method for the study of surface kinetics: CO on Mo(100)

Using a photoelectron spectrometer, a technique has been developed to continuously monitor changes that may occur in an adlayer when heated. After identifying adsorbate-induced features in UV photoemission (UPS) spectra from a chemisorption system, the intensities of individual features are recorded as a function of temperature and/or time. Kinetic parameters can be extracted from the rates of these intensity changes. The thermally activated decomposition of CO on Mo(100) has been used to demonstrate the technique here. Preliminary results show that this process does not obey simple first-order kinetics. Analysis of isothermal data gives a coverage-dependent dissociation activation energy with Edis?0.60 eV for 0?0.5, and a frequency factor near 1013 s−1.

Nonlinear least squares analysis of catalytic rate models

AbstractConventional data generation and analysis procedures used to select a descriptive Hougen‐Watson reaction rate model have been reviewed and some statistical objections to these conventional procedures have been presented. In an attempt to assess the practical importance of these objections, a recently published example was analyzed with both conventional techniques and nonlinear least squares procedures, which more closely conform to theoretical weighting requirements. Comparisons were made between the results of the conventional linear least squares analysis of isothermal data and those of nonlinear least squares analyses of both isothermal and nonisothermal data. It was found that the nonlinear least squares procedures were useful for a rational selection of an acceptable model and estimation of its parameters.General observations were made concerning good regions for further experimentation if a discrimination among rival models is desired. Additional well‐designed experiments were found to be necessary to allow a reduction of the confidence region of the parameters of the Hougen‐Watson models to an acceptable size.

Some aspects of the behavior of charcoal with respect to chlorine

The feasibility of adsorption and desorption behavior of nanochitosan(NCS)/sodium alginate(SA)/microcrystalline cellulose (MC) bead prepared in 2:8:1 ratio for Pb(II) removal has been investigated through batch studies. The proof of adsorption of Pb(II) ions onto NCS/SA/MC beads was identified from FT-IR and EDX-SEM Studies. Studies of the effect of pH, adsorbent dose, metal ion concentration and temperature reveals that the optimum conditions for adsorption was found to be pH:6; adsorbent dose:4 g; initial metal concentration: 62.5 mg/L and temperature:50 °C. Various equilibrium adsorption isotherm models namely Langmuir, Freundlich, Temkin and D-R applied for the analysis of isotherm data indicate that the Freundlich adsorption isotherm model was found to be followed. On the basis of kinetic studies, specific rate constants involved in the processes were calculated and the observed result shows that the pseudo second order kinetics was found to be a better fit. The desorption studies reveals that the recovery of Pb(II) from NCS/SA/MC bead was found to be effective by using 0.1 M HCl solution. From the results it was evident that the NCS/SA/MC bead showed better Pb(II) uptake performance and regeneration for further use and hence it was found to be an efficient biosorbent for treating industrial effluent.