2.3. Thermal Characterization

The study of the curing was performed by differential scanning calorimetry (DSC) in a Mettler DSC3 + 700/970 (Columbus, OH, USA) calorimeter calibrated using an indium standard (heat flow calibration) and an indium–lead–zinc standard (temperature calibration). A flow of N₂ at 100 mL/min was used and the weight of the samples for the analysis was 10 mg. The curing process was studied by DSC in the non-isothermal mode at 1, 2, 5, 10 and 20 °C/min from 20 to 275 °C. The glass transition temperatures (T_gs) of the samples once cured were determined in dynamic scans at 20 °C/min from −50 to 120 °C in the case of formulations with Jeffamine^® D-400 and from −50 to 200 °C in the case of formulations with IPDA.

The isoconversional activation energy at different degrees of conversion x was determined from multiple heating rate experiments using the KAS method [32]:

where β is the heating rate, A is the pre-exponential factor, E is the activation energy and g(x) is an integral function corresponding to the kinetic model. The representation of ln(β/T²) in front of 1/RT for the experimental results should produce a straight line, in which the slope of the line is related to the activation energy, E, and the ordinate in the origin is related to the pre-exponential factor A.

The time needed to reach a given conversion in an isothermal experiment can be determined from the results of the isoconversional analysis of nonisothermal experiments using the following expression:

The thermal stability of cured samples was studied by thermogravimetric analysis (TGA), using a Mettler TGA/SDTA 851e thermobalance (Columbus, OH, USA). All experiments were performed under inert atmosphere (N₂ at 100 mL/min). Pieces of the cured samples with an approximate mass of 8 mg were degraded between 30 and 600 °C at a heating rate of 10 °C/min.