Vyazovkin kinetic methods

Vyazovkin and Liimert [56] argue that kinetic data, A and E, values obtained on the assumption of a one-step reaction may be incorrect because the possibility that thermal decompositions proceed by multistep processes has been ignored. This potential error can be avoided by using isoconversional methods to calculate Arrhenius parameters as a fimction of a. A real isokinetic relationship in a multistep process can be identified fi om the dependence of and its confidence limits, on a. The contribution fi om the second reaction step is negligible at the start of chemical change and thereafter rises as ar increases. 

Vyazovkin and Lesnikovich [42] emphasize the need for careful statistical testing of the significance of the calculated parameters. Such tests may, at least, decrease the number of kinetic models which need to be considered. They specifically warn against the practice of forcing the model to be of the reaction order (RO) type where the value of n may not possess physical significance. Coincidence of the parameters calculated by alternative methods confirms only the equivalence of the methods of calculation and not the validity of the parameters obtained. 

Kinetic analysis according to the model-free approach by Kissinger-Akahira-Sunose [25, 26] was performed with the computer program Excel. For kinetic analysis according to the advanced Vyazovkin method [27], the STAR software... 

Vyazovkin (1997, 2001) developed an enhanced isoconversional method that allows evaluation of an effective activation energy ( ) as a function of the extent of reaction (a). This methodology, often referred to as model-free kinetics (MFK), is described in Section 3.5. The MFK software allows calculations such as conversion-time plots at selected temperatures that can be compared with actual measured data. It also allows the calculation of DSC curves that can be compared with actual measured DSC curves to help validate the analyses. The same curves analyzed by ASTM kinetics may be evaluated by MFK kinetics, and the same guidance is given. MFK kinetics is very comprehensive in that it is applicable to the simplest as well as to the most complex cure reactions, provided that a baseline can be drawn between a clear beginning and a clear end of the reaction. But it should be pointed out that the software is provided only by Mettler Toledo and Perkin-Elmer. For other users it is possible to measure versus conversion by the ASTM method and generate a spreadsheet with the appropriate MFK equation [e.g., Eq. (3.31) in Chapter 3], to calculate conversion-time plots. 

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