Coating material for prodn. of transparent top coats

摘要

Coating material suitable for the prodn. of transparent topcoats is claimed (I), contg. binder(s) based on OH-functional (meth)acrylic copolymer (II), crosslinker(s) based on (cyclo)aliphatic polyisocyanates (III), solvent(s) and normal paint additives. The binders contains mixts. of (A) 40-60 wt.% copolymer (IIA) and 60-40 wt.% copolymer (IIB). (IIA) is obtd. by copolymerisation of (al) 50-70 wt.% acrylate ester(s) mixed with sufficient hydroxyalkyl acrylate(s) to give a mixt. with OH no. 40-70 mg KOH/g and (a2) 30-50 wt.% methacrylate ester(s) mixed with sufficient hydroxyalkyl methacrylate to give OH no. 180-450. (IIB) is obtd. by copolymerisation of (b1) 27-40 wt.% alkyl acrylate(s), (b2) 49-55 wt.% methacrylate ester(s) mixed with hydroxyalkyl methacrylate to give OH no. 240-300, (b3) 1-3 wt.% acrylic acid and (b4) 10-15 wt.% vinyl ester(s) of unsatd. aliphatic monocarboxylic acid(s) with the COOH gp. on a tert. C atom.Also claimed are (1) prepn. of (I), by mixing 40-60 wt.% (A) with 60-40 wt.% (B) and with solvent(s) and additives, then adding (III) immediately before use, in amts. such that the mixt. contains 0.5-2.0 NCO gps. per OH gp. in (A + B); and (2) prepn. of a multilayer coating on a substrate which may be precoated with primer, filler and/or other intermediate layers, by applying a basecoat (pref. a water-based basecoat), briefly drying or hardening, and then applying (I) as the topcoat, pref. by spraying at a solids content of above 55 wt.%.Pref. amt. of polyisocyanate (III) is as given above. Component (A) has OH no. 120-170, Mw 3000-5000 and Tg 10-30 deg.C; component (B) has acid no. 25-50, OH no. 120-160, Mw 5000-8000 and Tg 20-50 deg.C..