16 августа, 2015 
Malyar Developing alongside the move to higher solids systems was the trend towards clear over base automotive topcoat systems(6). They consist of a 12 to 15 um dry film, highly pigmented base coat and an overlacquer of approximately 40 |im dry thickness. The base coats are applied at around 12 to 15 % nvc by weight, less by volume. In metallic coatings(7), where the method of flake orientation for non-leafing aluminiums is mainly by film shrinkage, the COB type base coats shrink to about 10 % of their volume giving much better orientation than the one coat systems which only shrink to 20 — 25 % of their volume. COB systems are therefore used in metallic and pearlescent finishes. In addition to the improvement in appearance and weathering resistance (gloss retention), the COB system results in increased resistance to film spotting and colour change due to accidental attack of the pigment by chemicals.
The pigmented basecoat is formulated on either a highly durable solvent based polyester, or an aqueous acrylic dispersion.
Critical to formulating a successful metallic basecoat is the selection of aluminium paste. In general the larger the particle size of the flake, the better the lustre. A polyethylene wax dispersion is also incorporated into the formulation to aid metal control and lustre. The binder in solvent based systems is a durable saturated polyester cured with a melamine formaldehyde resin and modified with CAB, to assist in film setting and hardness.
Solvents used in basecoats must be fast evaporating, e. g. ethyl, butyl acetate or MEK. This is essential so that the metal particles set randomly and do not have time to flow out or orientate. Slow evaporating solvents result in dull lustre effects.
Coloured metallic basecoats are formulated using strong transparent light fast pigments predispersed in the polyester.
The basecoat is thinned to a spray viscosity of 15-20 secs Ford No. 4 Cup, with fast evaporating solvents and this is applied in 2 thin coats, and allowed to dry for at least 15 minutes prior to application of the thermosetting clear overlacquer.
The clear lacquer is based on an hydroxy thermosetting acrylic resin as described earlier, and it is cured with a melamine formaldehyde resin. The binder is modified with a UV absorber system comprising:
1 % hydroxy phenyl benzotriazole UV absorber, which converts the harmful UV radiation to heat.
1 % hindered amine light stabiliser (HALS) which functions by quenching highly reactive singlet oxygen to the ground state.
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