The cure behavior and thermal degradation of high Tg epoxy systems have been investigated by comparing their isothermal time-temperature-transformation (TTT) diagrams. The formulations were prepared from di- and trifunctional epoxy resins, and their mixtures, with stoichiometric amounts of a tetrafunctional aromatic diamine. The maximum glass transition temperatures (Tg∞) were 229°C and > 324°C for the fully cured di- and trifunctional epoxy materials, respectively. Increasing functionality of the reactants decreases the times to gelation and to vitrification, and increases the difference between Tg after prolonged isothermal cure and the temperature of cure. At high temperatures, there is competition between cure and thermal degradation. The latter was characterized by two main processes which involved devitrification (decrease of modulus and Tg) and revitrification (char formation). The experimentally inaccessible Tg∞ (352°C) for the trifunctional epoxy material was obtained by extrapolation from the values of Tg∞ of the less highly crosslinked systems using a relationship between the glass transition temperature, crosslink density, and chemical structure.
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Chan et al. (1984) studied this question.