The gelation point of alkyd resins

Using Flory's basic method, an equation is derived for the gelation point of oil‐modified alkyd resins in which account is taken of the different reactivities of the various carboxyl groups. The resultant modified equation is useful when phthalic anhydride is used as the dibasic acid because of the widely differing carboxyl group reactivities which occur in this case. If adipic or isophthalic acid is used as the dibasic acid, the gel point equation deduced by Flory's original method, which takes no account of reactivity differences, may be used. In such a case the difference between the observed and calculated gel point is about 5% when the polyhydric alcohol used is trifunctional and 8–9% when the polyhydric alcohol used is tetrafunctional. The modified gel point equation, applicable when phthalic anhydride is used as the dibasic acid, is shown to predict gel points with similar degrees of accuracy, discrepancies being also greater for alkyds using tetrahydric alcohols than for alkyds using trihydric alcohols. The discrepancy is believed to be due to the occurrence of intramolecular condensation, and it is contended that the degree of such intramolecular condensation in branching systems is proportional to some function of the branch unit functionality.