In part 1 blog, I mentioned that the “model-free” approach, which is based on the differential isoconversional method, is one of the recent approaches to determine chemical reaction kinetics. This approach is better suited for commercial formulations than prior approaches that rely on mathematical equations for the chemical conversion expression. This is because the “model-free” approach can be viewed as a curve fitting exercise and is more adaptable to commercial formulations that typically contain additives in addition to curing agents.
With this approach, one finds that the activation energy stays relatively a constant during conversion. However, one also notices that in most cases the activation energy varies significantly at the onset (conversion a < ~0.1) and at the end of reaction (conversion a > ~0.8). This is one main reason where specified mathematical models, which assume constant activation energy and pre-exponential factors do not predict reactivity well, especially at temperatures where reactivity is slow.
One potential downside to the “model-free” approach is the difficulties of applying the said kinetic parameters to further complete our quest in answering questions such as time-to-maximum-reactivity and safe process window of time and temperature. This is because instead of constant values, the parameters are now a row of values based on the degree of cure. Therefore, heat transfer models would require to be re-written to incorporate the row of values instead. Fortunately with abundant computing power available nowadays, this is usually not a major difficulty.