Study of epoxy compounds. Part II. The gelation point of the epoxy resin–acid anhydride system

Equations are derived by means of Flory's original method for the gelation point of epoxy resin/acid anhydride/polyhydric alcohol systems containing various epoxy resins, acid anhydrides, and polyhydric alcohols. The resultant equations take no account of the reactivities of the various carboxyl, epoxy, and hydroxyl groups and are applied satisfactorily to epoxy resin/acid anhydride/polyhydric alcohol systems. The difference between the observed and calculated gelation point is about 2–3% when the acid anhydride used in HHPA and 4–5% when the acid anhydride used is tri‐ or tetrafunctional, such as TMA and PMDA. There is a little difference between resins E‐828 and E‐1001 for the gelation point. The discrepancy is beleived to be due to the reactivity difference between epoxide and hydroxyl to carboxyl groups and to the occurrence of intramolecular condensation, and it is contended that the degree of intramolecular condensation in these branching systems obtained is a function of the branch unit functionality of reactant, in this case, acid anhydride or epoxide.