The most debatable and discrepant viewpoints of non-isothermal kinetics are discussed in the form of twelve questions and answers. The reputation of non-isothermal kinetics when carried out by thermoanalysts; the consequences of simplified concepts transferred from the kinetics of homogeneous reactions; the physical meaning of basic kinetic parameters in solid-state processes; the kinetic compensative effect and interdependence of kinetic parameters using the Arrhenius rate constant; the mutual usefulness of differential and integral methods of kinetic data evaluation; their accuracy and correctness; the reliability of DTA measurements; non-isothermal versus isothermal investigations; equilibrium and kinetic data and their mutual effect; the extended discussion initiated by MacCallum and Tanner; non-isothermal data publication policy; and finally the use of computers.
Authors:Eda Elmas, Kenan Yildiz, Nil Toplan, and H. Özkan Toplan
The effects of mechanical activation on the thermal behavior of kaolinite – alumina ceramic system and the non-isothermalkinetics of mullite formation were investigated and the following results were obtained.
In the kaolinite
Theoretical consideration has been made of the non-isothermal kinetics of consecutive reactions based on the superposition
principle. In the model the first reaction product reacts to form the final product and the two reactions proceed independently.
The amount of the first reaction product and the production rate of the final product have been obtained as a function of
time for isothermal cases and as a function of the reduced times for non-isothermal cases.
Authors:B. Janković, B. Adnađević, and J. Jovanović
An isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel in the temperature range from 306 to 361 K was
investigated. The specific parameters connected with shape of the conversion curves were defined. The activation parameters
(E, lnA) of the isothermal dehydration of equilibrium swollen poly(acrylic acid) hydrogel were calculated, using Johnson-Mehl-Avrami
(JMA), ‘initial rate’ and ’stationary point’ methods. The reaction models for the investigated dehydration are determined
using the ‘model-fitting’ method. It was established that both, the reaction model and activation parameters of the hydrogel
dehydration were completely different for the isothermal process than for the non-isothermal one. It was found that the increase
in dehydration temperature lead to the changes in isothermal kinetic model for the investigated hydrogel dehydration. It was
established that the apparent activation energy (E) of hydrogel dehydration is similar to the value of the molar enthalpy
of water evaporation.
Authors:J. Morancho, A. Cadenato, X. Fernández-Francos, J. Salla, and X. Ramis
The purpose of this work is to analyse certain kinetic features related to thermoinduced and photoinduced isothermal curing
in the 25/75 mass% bis-GMA/TEGDMA system. The kinetic parameters associated with photo and thermal curing were determined and compared using an
isoconversional procedure and the kinetic model was obtained by means of a reduced master plot. In photocuring, the kinetic
results obtained by means of this phenomenological methodology were compared with those obtained on the basis of mechanistic
considerations. In this case, we estimated the propagation and termination constants associated with photocuring at different
conversions. When the phenomenological procedure is performed, the rate constant decreases slightly during the curing process
and the autoacceleration effect of the process is demonstrated in the kinetic model, which is autocatalytic. However, in the
mechanistic model, this same effect is noted through an increase in the rate constants, while it is assumed that the kinetic
model is in the order of n with n=1.
It is shown that the total differential of the function of the amount of conversion versus temperature and time (α=f(T, t)) is equal to zero non-isothermal kinetics at constant heating rate. Hence, the mathematical expression used in the literature for the rate of the non-isothermal transformation,
, is not valid.
The authors continue their considerations concerning the validity of the steady-state approximation in non-isothermal kinetics.
A sequence of two first-order consecutive reactions with an active intermediate was subjected to kinetic analysis by numerical
solution of the corresponding differential kinetic equations for a number of particular cases. The results demonstrated that
the rate of change of concentration of the active intermediate is negligibly small if the assumption made in the isothermal
case is also accepted for the non-isothermal case, i.e. k2(T(t))>>