Water-compatible coating resin

Claim 1

A process for coating a metallic substrate with a cathodic electrophoretically depositable structural adhesive bonding primer, comprising the steps of: (A) placing said substrate in fluid contact with an anode and a coating composition comprising (i) water-compatible resin having a substantially linear structure of the formula: A.sup.- + C--E--(L--E).sub.y --C.sup.+ A.sup.- wherein: each E is independently a residue of a diepoxide; each L is independently a residue of an aryl or cycloalkyl linking compound having an average of two epoxide-reactive groups and bulky structure that is sufficiently T g -enhancing to raise by more than 20° C. the T g of a first cured epoxy resin that has been (a) made from the reaction product obtained by combining DGEBA and said linking compound at a 2.1:1 ratio of moles of said DGEBA to moles of said linking compound, (b) cured using piperidine at a 9.08:1 ratio of moles of epoxy groups in said reaction product to moles of said piperidine, and (c) analyzed using dynamic mechanical analysis, when compared to the T g of a second cured epoxy resin made and cured similarly but using bisphenol A in place of said linking compound; each C is independently a cationic water-compatible ammonium, sulfonium or phosphonium group; each A is independently the conjugate base of a water-soluble acid HA having a pK a for use under cathodic electrodeposition conditions; and y has an average value of about 1 to about 20; (ii) crosslinking agent; (iii) water; and (v) resin solvent or coalescing solvent; (B) passing electrical current between said substrata and said anode for a time period sufficiently long to attain a continuous coating of said primer on said substrate; (C) removing excess primer from said substrate; and (D) heating said primer at a curing temperature and for a curing time sufficient to crosslink said primer to a solvent-resistant state.