Low-viscosity epoxy resin, phenolic chain extender, catalyst and boron inhibitor

1. A curable, low volatile organic solvent, epoxy resin-containing formulation comprising:

(1) a low-viscosity epoxy resin, being either a liquid at 20° C. or having an average formula weight per epoxy equivalent of no more than 350 for all non-halogen atoms in the molecule;

(2) a phenolic chain extender which contains an average 1.8-2.1 phenolic hydroxyl groups per molecule and whose concentration is from 0.1 to less than 0.6 equivalents of phenolic hydroxyl group per epoxide equivalent of the low-viscosity epoxy resin;

(3) a catalyst containing amine, phosphine, heterocyclic nitrogen ammonium, phosphonium, arsonium sulfonium moieties that promotes self-curing reactions between epoxy groups containing on average no more than about 1 active hydrogen moiety per molecule;

(4) an inhibitor which is a Lewis acid in an inhibiting amount of from at least 0.3 mole to 3 moles of inhibitor per mole of catalyst which inhibits the activity of the catalyst during B-staging so as to retard the curing reaction of epoxy resin with epoxy resin at B-stage and which is selected from the group consisting of boric acid, metaboric acid, boroxine, and boron oxide;

(5) from at least 5 weight percent to no more than 20 weight percent of a volatile organic solvent; and

(6) a multifunctional cross-linking agent containing on average more than two active hydrogen moieties per molecule; wherein the formulation contains at least 0.01 equivalents of multifunctional cross-linking agent per epoxide equivalent;

wherein the viscosity of the resultant curable, low volatile organic solvent, epoxy resin-containing formulation for making composites or laminates at 20° C. is no more than 800 mP·s; and wherein the formulation contains no more than 0.5 equivalents of phenolic chain extender and multifunctional cross-linking agent per epoxide equivalent.

2. A formulation as described in claim 1 wherein the chain extender is a dihydric phenol or a halogenated variation thereof having a melting point higher than 100° C.

3. A formulation as described in claim 1 wherein the multifunctional cross-linking agent is a polyamine, polyamide, polyanhydride, polyphenol, or polyacid compound that contains more than two reactive sites per molecule on average, and the concentration of multifunctional cross-linking agent is at least 0.05 equivalents per epoxide equivalent.

4. A formulation as described in claim 1 wherein the formulation contains from 0.2 to 0.5 equivalents of chain extender (based on phenolic hydroxyl groups) per epoxy equivalent.

5. A formulation as described in claim 1 wherein the catalyst has a concentration of from 0.05 to less than 3 parts per 100 parts resin, by weight.

6. A process to make an electrical laminate, said process containing the steps of:

(1) applying the formulation of claim 1 to form an impregnated substrate;

(2) heating the impregnated substrate at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, whereby a prepreg is formed;

(3) stacking one or more sheets of prepreg in alternating layers with one or more sheets of a conductive material; and

(4) pressing the laid-up sheets at high temperature and pressure for a time sufficient to cure the resin and form a laminate.