Kinetic analysis, Ozawa method

Ozawa, T. Kinetic analysis by repeated temperature scanning. Part 1. Theory and methods. Thermocim. Acta 2000, 356, 173-180. 

PEN/CNT nanocomposites induced by the incorporation of the CNT may be explained by the function of the CNT as an effective physieal barrier to retard the thermal degradation of volatile components and to prevent the diffusion out of the deeomposed polymeric molecules in the PEN/CNT nanocomposites [145, 146], Based on the Flynn-Wall-Ozawa analysis, it can be deduced that the Ea values of the PEN/CNT nanocomposites calculated from Flyim-Wall-Ozawa method exhibited good reliance on describing the thermal degradation kinetics of their nanocomposites, which was confirmed by the fact that the values of the correlation coefficient (r2) were greater than 0.99. 

Similar to studies reported by Litwinienko and co-workers discussed above, a recent report (Dunn, 2006b) demonstrated that non-isothermal (conventional) DSC, static mode P-DSC and dynamic mode P-DSC may be employed to study kinetics of the oxidation of SME. OT results obtained at ambient pressure for DSC and P = 2000kPa for P-DSC and with varying p = 1-20 °C/ min were analyzed by the Ozawa-Flynn-Wall method to calculate activation energies and rate constants. This work concluded that rates of the oxidation reaction could be calculated at any temperature based on accurate measurement of kinetic parameters from analysis of non-isothermal dynamic mode P-DSC scans. 

The experimental study of kinetics, as mentioned earlier, has as its basis the identification of the rate of reaction with instrument signal and the extent of reaction with fractional area of the peak. Analysis of experimental data often makes use of named approaches which exploit the advantages of dynamic experiments to achieve results without recourse to protracted experimental effort. Two popular but very different methods are those of Borchardt and Daniels and Ozawa and both appear in ASTM methods. Both are supported by commercial software. The concern here is with the Borchardt and Daniels method [ASTM E 2041 (1999)] which had a considerable impact on kinetic evaluation both by DTA and subsequently DSC. The analysis was devised originally for DTA experiments in which the thermocouples were in large volumes of stirred liquids. This is in direct contrast to the current application of the method to DSC studies of solids. A number of assumptions were made which were met more readily in stirred liquid systems than with solids. As a result there are a number of caveats associated with its application to solids. In particular the analysis assumed the absence of temperature... 

To evaluate the apparent activation energy, the isoconversional methods are use as suitable analysis procedures. These methods are based on the assumption that at a constant extent of conversion degree (a), the decomposition rate da/dt is a function only of the temperature. In methods developed by Friedman and Flynn-Wall-Ozawa, linear functions are obtained from which slopes the apparent activation energy at constant conversion a is achieved. In the free kinetic method set by Kissinger is calculated from the slope of the linear function takes into consideration the relationship between the heating rate and peak temperature of the first-derivative thermogravimetric curve [97]. 

Reaction kinetics from DSC, DTA or TGA, have been used to examine the stability of a limited number of pharmaceutical materials. Various models have been used including the Power Law, Avarami-Erofeev and Prout-Tomkins models [72]. These methods are also based on the Kissinger [73], ASTME 698 [74] or Ozawa [75] methods [8]. Most frequently, they have been applied to the dehydration of various materials such as theophylline monohydrate [76], phenobarbitone monohydrate or hemihydrate [77], phenylbutazone [78], oxazepam [23] and trazodone tetrahydrate [79]. The uses are limited for pharmaceutical systems, not least because dehydration is particle size dependent. Thermal analysis, especially DSC, DTA and TG, has been used outside the pharmaceutical area in the prediction of reaction kinetics as described elsewhere in this handbook. Methods used include those by Borchart and Daniels [80], Kissinger [73], Freeman and Carroll [81] and Flynn and Wall [82]. Although these techniques are well established and, if used properly, can give pertinent information, their use in pharmaceutical arenas is restricted to dehydration and decomposition. 

The kinetics of non-isothermal crystallization using differential scanning calorimeUy (DSC) has been a known approach to study the crystallization behavior of polymers. The analysis was commonly performed through the use of modified Avrami [1 ], Ozawa [5] and other methods [6-7]. Kissinger method [8] was often used to obtain the activation energy of crystallization. These methods have been helpful to differentiate the crystallization behavior between materials. 

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