Abstract: Disclosed is an urethane acrylate oligomer to be obtained by reacting an isocyanate group-having urethane prepolymer and hydroxyl group-having acrylate and/or methacrylate, in which the urethane prepolymer is one having isocyanurate rings and the hydroxyl group-having acrylate and/or methacrylate comprises 4-hydroxybutyl acrylate and/or 4-hydroxybutyl methacrylate. The oligomer has a much lower viscosity of itself and a much higher ultraviolet-hardening speed, than the conventional urethane acrylate oligomers, and may be hardened to give a film having excellent physical properties of flexibility, heat resistance, weather resistance of hot-cold repetition cycles and scratch resistance.

Abstract: The present invention relates to a process for surface treatment for cathodic electrodeposit coating of a zinc-plated or zinc alloy-plated steel sheet comprising coating and drying a protective coating material on said steel sheet chromate-treated beforehand, to form a protective film before subjecting the sheet to the cathodic electrodeposit coating, which comprises coating a protective coating material having as a principal constituent of film-forming organic component a block isocyanate-containing prepolymer containing in one and the same molecule respectively at least one blocked isocyanate group, hydroxyl group, and tertiary amono group and containing 10-100 parts by weight of silica sol relative to 100 parts by weight of said prepolymer, in an amount of 0.3 g-4 g/m.sup.2, and then heat-drying the same.