COMPOSITION AND PROCESS FOR COATING METAL SURFACES

摘要

#CMT# #/CMT# Composition for coating metal surfaces, based on a crosslinkable polyester resin binder, comprises 4-20 wt.% aluminum flakes and no more than 0.1 wt.% isocyanates. #CMT# : #/CMT# Independent claims are also included for: (1) coating steel strip by contacting the strip with an acid treatment solution that forms a conversion layer containing no more than 1 mg/m 2> chromium on the steel surface, coating the strip with a composition as above, and curing the coating by infrared irradiation or heating to 120-280[deg] C to form a 3-30 mu m thick coating; (2) steel strip coated as above, or a steel sheet or part made from it. #CMT#USE : #/CMT# Steel strip coated with the composition is useful for manufacturing motor vehicles, furniture, household appliances or architectural parts, especially walls, roofs or exterior cladding (claimed). #CMT#ADVANTAGE : #/CMT# Steel coated with the composition has a similar appearance and similar anticorrosion properties to galvanized steel. #CMT#METALLURGY : #/CMT# Preferred Composition: The aluminum flakes have an average diameter of 20-60 mu m and a thickness of 0.3-1 mu m. The composition contains 2-10 wt.% corrosion inhibitors or anticorrosion pigments. The composition comprises 20-50 wt.% binder and 30-70 wt.% solvent, to a total of no more than 90 wt.%. The binder comprises a hydroxy-functional polyester resin with an average molecular weight of 2000-15000 and a crosslinker, especially a melamine-formaldehyde crosslinker. The composition comprises 20-35 wt.% resin, 3-8 wt.% crosslinker and 0.1-0.5 wt.% of a blocked acid catalyst. Preferred Process: The coating and curing steps are performed twice, giving a coating thickness of 3-15 mu m and 10-27 mu m respectively. Preferred Steel Strip: The coating thickness is 3-10 mu m and the coating is overcoated with a lacquer other than a clear lacquer. The coating thickness is 3-15 mu m and the coating is not overcoated. The total coating thickness is 20-30 mu m and the coating is not overcoated or is overcoated with a clear lacquer. #CMT#EXAMPLE : #/CMT# The exemplified composition comprises (wt.%): hydroxy-functional linear saturated polyester resin (28.75), methylated melamine-formaldehyde crosslinker (5.4), calcium silicate anticorrosion pigment (5), mica, quartz and chlorite fillers (7), blocked acid catalyst (0.25), silicone-free flow improver (0.5), aluminum powder (6.4), solvent naphtha (13.2), diethylene glycol monobutyl ether acetate (5), propylene glycol methyl ether (2.8) and solvent (25.7).