Process for coating electrically conductive substrates, aqueous varnishes and crosslinking agents containing blocked NCO groups

摘要

The invention relates to a cathodic electro-dipping process wherein the electro-dipping primer employed contains a crosslinking agent which can be obtained by (I) reacting, in a first stage, (a) diphenylmethane diisocyanate and (b) a monoalcohol or a mixture of monoalcohols to give an intermediate product (A) containing isocyanate groups, and (II) reacting this intermediate product (A) in a second stage with (c) a polyol or a mixture of polyols to give an intermediate product (B) containing hydroxyl groups which (III) is reacted in a third stage either with (d) a cycloaliphatic, aliphatic or araliphatic polyisocyanate or a mixture of such polyisocyanates and (e) a monofunctional masking agent or a mixture of such masking agents or with (f) a partially-masked polyisocyanate prepared from (d) and (e) to give a crosslinking agent which is free from isocyanate groups.