Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 162

In addition to their tendency to involve multiple steps, the thermally stimulated
reactions tend to involve multiple phases. For instance, the thermal decomposition
of solid calcium carbonate produces solid- and gas-phase products:

CaCO →+ CaO CO .

3(s) (s) 2(g)

Ammonium nitrate can be decomposed in the solid phase:

NH NO →+ NH HNO

() , 4 3 () s 3 () g 3 g

when heated below its meting temperature, and in the liquid phase:

NH NO →+ NH HNO

() , 4 3 () l 3 () g 3 g

when heated above the melting temperature. During the thermal liquid-state polym-
erization, the forming polymer product can undergo a phase transition from liquid
to glass (when the process temperature is below the glass transition temperature)
or from liquid to crystal (when the process temperature is below the melting tem-
perature).
These are just a few examples that demonstrate the wealth of the phase and reac-
tion situations in which thermally stimulated reactions can occur. This complicates
the reaction kinetics to the degree when each chemical reaction has some unique
kinetic features. Nevertheless, even in this situation, one can use isoconversional
methods to identify some general features and, therefore, get a hint at the reaction
mechanisms. In the following sections of this chapter, we discuss the application of
isoconversional methods to the three largest classes of chemical reactions: polym-
erization and cross-linking, degradation of polymers, and decomposition of solids.
Unlike in the case of phase transitions, the application of the isoconversional
methods to chemical reactions is several decades old. As already mentioned in
Chap. 2, the history of isoconversional methods starts by Kujirai and Akahira's [ 2 ]
application of the isothermal isoconversional method to the thermal degradation of
natural fibers. It was successfully continued by the inventors of nonisothermal iso-
conversional methods. Friedman [ 3 ] applied his method to the thermal degradation
of a phenolic plastic, and Ozawa [ 4 ] used his method on the thermal degradation of
nylon 6 and the thermal decomposition of calcium oxalate. In the area of polymer-
ization and cross-linking, the first application appears to date back to the work of
Barton [ 5 ], who proposed a simplified version of the Friedman method and applied
it to the reaction of epoxy-amine curing.