Isoconversional analysis

Therefore, for particular values of the conversion of functional groups and temperature, the rate of chainwise polymerizations depends on the concentration of active species which, in turn, depends on the particular thermal history. Thus, phenomenological equations derived from Eq. (5.1), or isoconversional methods of kinetic analysis, should not be applied for this case. 

Vyazovkin and Lesnikovich [93] proposed that the type of complex process encountered in non-isothermal experiments could be identified by analysis of the shape of the curve of the dependence of the apparent on a, foimd by isoconversional methods. Concurrent competitive reactions are characterized by an increasing dependence of the apparent value of E on a, but detailed shapes are dependent on the ratios of the contributing rates. A decreasing dependence of on a, was found for intermediate reversible processes [93]. 

Methods of kinetic analysis that involve fitting of experimental data to assumed forms of the reaction model (first-order, second order, etc.) normally result in highly uncertain Arrhenius parameters. This is because errors in the form of the assumed reaction model can be masked by compensating errors in the values of E and A. The isoconversional technique eliminates the shortcomings associated with model-fitting methods. It assumes the unknown integrated form of the reaction model, g(a), as shown in Eq. (4), to be the same for all experiments. 

Lignin, phenol-fotmaldehyde resin, accelerator, isoconversional kinetic analysis, model-fiee kinetic analysis, laminates, paper impregnation, differential scanning calorimeter... 

The thermal properties of freshly prepared pure and additive-modified PF and KLPF resins were studied with DSC. In an attempt to accelerate the curing of KLPF, the additives zinc borate, potassium carbonate, sodium carbonate, and propylene carbonate were added to the resin and the curing behaviour was compared to the curing of unmodified KLPF and PF resins. Based on dynamic DSC experiments, isoconversional kinetic analysis of resin curing was performed. 

Analysis of tga data for thermal decomposition reactions has been the subject of a major comparative review (the ITACT project) of the application of various methods to both experimental and simulated data (34 7). The final conclusion is that isoconversional analyses tend to work fairly well and that kinetic analysis using single heating rate methods should no longer be considered acceptable (37). 

The isoconversional methods are also known as model-free methods. Therefore, the kinetic analysis using these methods is more deterministic and gives reliable values of activation energy E, which depends on degree of transformation, a. However, only activation energy... 

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]. 

Isoconversional methods rely on several TGA or DSC datasets for kinetic analysis. When performing non-isothermal experiments, care must be taken to ensure that each run is conducted under the same experimental conditions (i.e., sample weight, purge rate, sample size, particle morphology, etc.) so that only the heating rate varies for each run. For example, sample mass varying from one run to another may cause ... 

Friedman s isoconversional method [14] involves an Arrhenius analysis at constant levels of conversion, and we determined the apparent first-order frequency factor and activation energy at 1 % intervals using both LLNL and AKTS kinetics analysis programs. 

---