Cure of epoxy novolacs with aromatic diamines. I. Vitrification, gelation, and reaction kinetics

The cure reaction of a commercial epoxidized novolac with 4,4' diaminodiphenylsulfone (DDS) was studied at constant cure temperatures in the range 120–270°C, as well as at constant heating rates (differential scanning calorimetry, DSC). Stoichiometric formulations did not attain complete conversion due to the presence of topological restrictions. The limiting conversion was xmax = 0.8. Samples containing an amine excess (≥ 20%) could be completely reacted, whereas this was not possible for formulations containing an epoxy excess. Samples containing a 20% amine excess showed the maximum value of the glass transition temperature (Tg230°C). Cure took place by epoxy‐amine hydrogen reactions catalyzed by (OH) groups. A reactivity ratio of secondary to primary amine hydrogens equal to 0.2 was found. The activation energy was E = 61 kJ/mol, as arising from Tg versus time shift factors and time to gel measurements. A unique relationship between Tg and x could be obtained. Gelation took place at xgel = 0.45 and the maximum Tg for the stoichiometric system was Tgmax = 215°C for x = 0.8. A conversion versus temperature transformation diagram was used to represent conditions where gelation, vitrification, degradation, and topological limitations took place.