The aqueous dissociation constants for a number of substituted picric acids and related compounds have been determined spectrophotometrically, and the values obtained correlated with the mid-equivalence potentials obtained by half-neutralization in acetone solution. Analysis of these results together with those obtained similarly for substituted benzoic acids shows that while the mid-equivalence potential method affords a rapid and convenient means of making a good estimate of the acid strength of a compound, it has its limitations and is only strictly applicable within the confines of a series of closely similar compounds. The observed pK, values of a large series of polysubstituted phenols were correlated with the predicted values obtained from the Hammett relation, and good agreement obtained, even with substituted picric acids; styphnic acids appear to behave anomalously.

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SynopsisThe hydrolytic stability of triglycidyl(4-aminophenol) (ERL 0500), tetraglycidyl(methy1ene-dianiline) (MY720), and a number of model compounds has been investigated. I t has been shown that at 6OoC and 96% relative humidity undiluted N-glycidyl aromatic amine-based epoxy resins readily undergo hydrolysis followed by homopolymerization. . Impurities in the resin may significantly accelerate the rate of reaction. The effect of the consequent loss of epoxide on the performance of an adhesive based on ERL 0500 has been studied. The high-temperature tensile-shear strength of joints was markedly reduced when the adhesive was aged, before cure, for 24 hr a t 60°C and 96% relative humidity. Some implications of these findings for aerospace applications of epoxy compounds are discussed.

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