| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for improving the exposure corrosion resistance of a lead-free cationic electrodeposition coating film, which process comprises using a composition capable of forming a cured coating film having a specified thermal shrinkage stress, a specified glass transition point, and a specified impedance. SOLUTION: This method comprises using a lead-free cationic electrodeposition composition capable of forming a cured coating film having a thermal shrinkage stress of 100-120 kg weight/cm2 (40 deg.C), a glass transition point of 70-90 deg.C, and an impedance of 108Ω.cm2 or above. It is desirable that the lead- free cationic electrodeposition coating composition comprises a base resin, a curing agent, and a lead-free curing catalyst. The base resin is desirably an amine-epoxy resin adduct or anε-caprolactone-modified amine/epoxy resin adduct. The curing agent is desirably the addition product of a polyisocyanate compound having at least three isocyanate groups in the molecule with an isocyanate-blocking agent. The lead-free catalyst is desirably a bismuth compound.
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