The curing kinetics of thermosets based on unsaturated polyester resin crosslinked with styrene was studied by differential
scanning calorimetry (DSC). The isoconversional kinetic analysis was applied to non-isothermal data. The results obtained
show a dependence of the activation energy (Eα) on conversion (α) that proves the existence of a multistep process and a complex kinetic scheme that must be interpreted
in terms of chemical and physical mechanisms. The interrelationship of the Arrhenius parameters obtained from the isoconversional
kinetic data has been used as a tool to investigate the production of free radicals by the action of a promoter (cobalt octoate)
and the temperature on the initiator (methyl ethyl ketone peroxide). An optimum promoter/initiator ratio has been found.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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 occurring. 相似文献
There are many methods for analyzing solid-state kinetic data. They are generally grouped into two categories, model-fitting and isoconversional (model-free) methods. Historically, model-fitting methods were widely used because of their ability to directly determine the kinetic triplet (i.e., frequency factor [A], activation energy [E(a)], and model). However, these methods suffer from several problems among which is their inability to uniquely determine the reaction model. This has led to the decline of these methods in favor of isoconversional methods that evaluate kinetics without modelistic assumptions. This work proposes an approach that combines the power of isoconversional methods with model-fitting methods. It is based on using isoconversional methods instead of traditional statistical fitting methods to select the reaction model. Once a reaction model has been selected, the activation energy and frequency factor can be determined for that model. This approach was investigated for simulated and real experimental data for desolvation reactions of sulfameter solvates. 相似文献
In this paper, fundamentals of the isoconversional methods are summarized and the methods are analyzed from the point of view
of their correctness, their applications and the physical meaning of the activation parameters obtained. It is concluded that
the activation parameters represent apparent quantities without a mechanistic interpretation. The parameters cannot be used
for any theoretical considerations; however, they enable modelling the processes for other temperature regimes than those
applied in the measurements. The differential and incremental isoconversional methods give the actual values of activation
parameters whereas the isothermal and integral isoconversional methods provide the averaged values. For the modelling, the
activation parameters obtained from the treatment of non-transformed kinetic data should be used.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
The thermal behavior of Cd2+ and Co2+ phenyl-vinyl-phosphonates was studied using two different experimental strategies: the coupled TG-EGA (FTIR) technique by
decomposition in nitrogen respectively air, and the kinetic analysis of TG data obtained in dynamic air atmosphere at four
heating rates. In nitrogen two decomposition steps were observed: the loss of crystallization water, respectively the decomposition
of the phenyl-vinyl radical. In air, the same dehydration was observed as the first step, but the second one is a thermooxidation
of the organic radical with formation of the pyrophosphoric anion.
The kinetic analysis of the TG non-isothermal data was performed by the isoconversional methods suggested by Friedman and
Flynn, Wall and Ozawa, as well as by the non-parametric (Sempere-Nomen) method. All processes put in evidence in TG curves
exhibit strong changes of the activation energy values with the conversion degree, which mean that these processes are complex
ones. Assuming that each of these processes consists in two steps, the application of non-parametric method leads to average
values of the activation energy close to the average values of this parameter obtained by isoconversional methods. 相似文献
The curing of a thermosetting powder coating was studied by means of differential scanning calorimetry (DSC). The isothermal
cure was simulated by non-isothermal experiments. The results of the simulation were compared with experimental isothermal
data. From non-isothermal isoconversional procedures (free model), it was concluded that these permit simulation of the isothermal
cure but do not enable us to determine the complete kinetic triplet (A preexponential factor, E activation energy, f(a) and/or g(a) function of conversion). Non-isothermal procedures based on a single heating rate or on master curves present difficulties
for determination of all the kinetic parameters, due to the compensation effect between preexponential factor and activation
energy. The kinetic triplet can be determined by a combination of various non-isothermal methods or by using experimental
isothermal data in addition to non-isothermal data.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
The study of crystallization kinetics of amorphous alloys has been a matter of great interest for material researchers for past few decades, since it provides information about the kinetic parameters i.e., activation energy of crystallization and the frequency factor. These kinetic parameters can be calculated by model-free isoconversional methods. Isoconversional methods allow calculating the activation energy as a function of degree of conversion, α. Hence, these methods provide accurate results for multistep processes like crystallization. Model-free methods are categorized as linear and non-linear isoconversional methods. Linear methods are further classified as linear differential and linear integral isoconversional methods. In present work, we have used these isoconversional methods to study the effect of non-linear heating rate, employed by modulated differential scanning calorimetry (MDSC), on the non-isothermal crystallization kinetics of Ti20Zr20Cu60 metallic glass. For Ti20Zr20Cu60, MDSC curves clearly indicate a two-step crystallization process. Both crystallization peaks were studied based on the modified expressions for isoconversional methods by non-linear heating rate. The term corresponding to non-linearity comes out to be (ATω/2β)2. The effect of non-linear heating rate on measurement of kinetic parameters by isoconversional methods is studied. The activation energy of crystallization is calculated for Ti20Zr20Cu60 metallic glass for various degrees of conversion by linear integral isoconversional methods i.e., Ozawa–Flynn–Wall, Kissinger–Akahira–Sunose, and also with Friedman method which is a linear differential isoconversional method. 相似文献
A variety of isoconversional and model fitting approaches, all of which use multiple heating schedules, are used to analyze
selected data from the ICTAC kinetics and lifetime projects as well as additional simulated data sets created for this work.
The objective is to compare the accuracy and suitability of various approaches for various types of chemical reactions. The
various simulated data sets show that model fitting and isoconversional methods have comparable reliability for extrapolation
outside the range of calibration. First, there is as much variability in prediction for various isoconversional methods as
there is between isoconversional methods as a group and different plausible explicit models. Of the three isoconversional
models investigated, the Friedman method is usually the most accurate. This is particularly true for energetic materials that
have a drop in apparent activation energy in the latter stages of reaction, which leads to a delayed onset of rapid autocatalysis
at lower temperatures. It is difficult to determine a priori whether isoconversional or model fitting approaches will give
more accurate predictions. The greatest reliability is attained by using both the isoconversional and model fitting approaches
on a combination of isothermal and constant heating rate data. 相似文献
A nonlinear algorithm has been suggested to increase the accuracy of evaluating the activation energy by the integral isoconversional method. A minor modification of the algorithm has made it possible to adapt the isoconversional method for an arbitrary variation of the temperature. This advanced isoconversional method allows for trustworthy estimates of the activation energy when the thermal effect of a reaction makes the temperature of a sample deviate from a prescribed heating program. 相似文献
The paper presents a non-isothermal kinetic study of the decomposition of Zn acetate-based gel precursors for ZnO thin films,
based on the thermogravimetric (TG) data. The evaluation of the dependence of the activation energy (E) on the mass loss (Δm) using the isoconversional methods (Friedman (FR), Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS)) has been presented
in a previous paper. It was obtained that the sample dried at 125°C for 8 h exhibits the activation energy independent on
the heating rate for the second decomposition step. In this paper the invariant kinetic parameter (IKP) method is used for
evaluating the invariant activation parameters, which were used for numerically evaluation of the function of conversion.
The value of the invariant activation energy is in a good agreement with those determined by isoconversional methods. In order
to determine the kinetic model, IKP method was associated with the criterion of coincidence of the kinetic parameters for
all heating rates. Finally, the following kinetic triplet was obtained: E=91.7 (±0.1) kJ mol−1, lnA(s−1)=16.174 (±0.020) and F1 kinetic model. 相似文献
The thermal polymerization kinetics of dimethacrylate monomers was studied by differential calorimetry using non-isothermal
experiments. The kinetic analysis compared the following procedures: isoconversional method (model-free method), reduced master
curves, the isokinetic relationship (IKR), the invariant kinetic parameters (IKP) method, the Coats-Redfern method and composite
integral method I. Although the study focused on the integral methods, we compared them to differential methods. We saw that
even relatively complex processes (in which the variations in the kinetic parameters were only slight) can be described reasonably
well using a single kinetic model, so long as the mean value of the activation energy is known (E). It is also shown the usefulness of isoconversional kinetic methods, which provide with reliable kinetic information suitable
for adequately choosing the kinetic model which best describes the curing process. For the system studied, we obtained the
following kinetic triplet: f(α)=α0.6(1−α)2.4, E=120.9 kJ mol−1 and lnA=38.28 min−1. 相似文献
The crystallization kinetics of some glass-ceramics obtained from Romanian (Şanoviţa) basalt has been studied in non-isothermal
conditions using DTA technique. The activation energies of the crystallization processes were calculated using the isoconversional
methods Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall. The results obtained show a dependence of the activation energy (Eα) on the crystallized fraction (α) that proves the complex mechanism of the glass-ceramics crystallization process. It has
been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process. The
effect of 2% TiO2 as nucleating agent upon the crystallization kinetics and upon the microstructure of the studied glass-ceramics was analyzed. 相似文献
Summary: Isoconversional kinetic analysis involves evaluating a dependence of the effective activation energy on conversion or temperature and using this dependence for making kinetic predictions and for exploring the mechanisms of thermally stimulated processes. The paper discusses major results obtained by the authors in the area of the isoconversional analysis of polymer kinetics over the past decade. It provides a brief introduction to isoconversional methods and surveys the impact made by isoconversional analysis in several application areas that include kinetic predictions, thermal degradation, crosslinking (curing), glass transition, and glass and melt crystallization. It is concluded that isoconversional analysis has been used broadly and fruitfully because it presents a fortunate compromise between the single‐step Arrhenius kinetic treatments and the prevalent occurrence of processes whose kinetics are multi‐step and/or non‐Arrhenius.
An isoconversional method applies the Arrhenius equation to a narrow temperature region, ΔT related to a given extent of conversion. 相似文献
An analysis is presented of the consequences of the use of a one term equation containing apparent activation parameters,
instead of the true rate equation to describe two successive decomposition reactions undergone by a solid compound. It is
demonstrated that the apparent activation energy, obtained by means of isoconversional differential and integral methods,
varies with the conversion degree for a relatively narrow temperature range and with temperature at a given value of the conversion
degree. The activation energy values obtained with the isoconversional differential method are higher than the corresponding
values obtained with the isoconversional integral method.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献