Process for preparing epoxy resins having improved physical properties when cured

Claim 1

In a process for preparing resin impregnated substrates for use in preparing electrical laminates by a process which comprises: (I) saturating said substrate with a resin forming mixture comprising; (A) an epoxy resin having an epoxide equivalent weight of from about 300 to about 600 which has been prepared by reacting (1) a glycidyl ether of a dihydric phenol having an average of more than one epoxide group per molecule and an epoxide equivalent weight (EEW) of from about 156 to about 400; with (2) a dihydric phenolic compound; in the presence of (3) a phosphonium catalyst for effecting the reaction between (1) and (2), wherein components (1) and (2) are employed in quantities which provide a theoretical percent epoxide of the reaction product of from about 8 to about 15; (B) a curing agent for said epoxy resin and (C) a solvent system for Components (A) and (B); (II) heating the resultant impregnated substrate to B-stage the resin and remove the solvent system; the improvement which comprises: employing as the catalyst, Compound (A-3), a phosphonium salt or complex thereof which has at least one aromatic ring attached to the phosphorus atom of the phosphonium group and at least one aliphatic hydrocarbon group attached to said phosphorus atom, said catalyst being employed in a quantity such that the epoxy resin which would result when subjecting a mixture of components (A-1), (A-2) and (A-3) to suitable reaction conditions produces an advanced epoxy resin wherein the difference obtained by subtracting the precent epoxide obtained by analysis from the theoretical percent epoxide is from about 0.5 to about 4.