Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Patent
1995-03-14
1997-05-06
Dudash, Diana
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
427410, B05D 136, B05D 716
Patent
active
056269170
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a process for the multi-layer lacquer coating of substrates, which process may in particular be used in industrial and automotive lacquer coating, preferably in automotive repair lacquer coating.
BACKGROUND OF THE INVENTION
DE-A-41 15 015 and DE-A-41 15 042 describe physically drying water-based coating compositions, in particular for the production of metallic coatings and for the production of plain coloured coatings. The metallic lacquers contain at least one binder based on polyurethanes containing carbonate groups and having a number average molecular weight (M.sub.n) of 70,000 to 500,000 and further polyurethanes having an M.sub.n of 20,000 to 60,000. Plain coloured coatings are produced by using binder mixtures based on polyurethanes containing carbonate groups and having a glass transition temperature (T.sub.g) of below 0.degree. C. and unsaturated reaction products of carboxy-functional polycondensation products with ethylenically unsaturated monomers having a glass transition temperature of above 0.degree. C.
Thanks to their ability to harden at room temperature or with slight heating, for example to temperatures of up to 60.degree. C., such physically drying coating compositions are particularly suitable for automotive repair lacquer coating. Once applied, they may be coated with transparent topcoat lacquers. Application may be performed wet-on-wet after briefly flashing off or once the base lacquer coat has dried. The clear lacquers used are customary transparent lacquers used in the automotive sector. These are in particular conventional formulations of acrylate/isocyanate-based two-component lacquers.
However, after overcoating with clear lacquer, the hardness of the base lacquer coat in such a multi-layer structure and consequently of the entire topcoat layer has been found to be inadequate. Particularly at elevated coat thicknesses (approx. 40 .mu.m), the base lacquer remains too soft. The gloss and water resistance achieved also remain unsatisfactory.
DE-A-37 31 652 describes a conventional two-component clear lacquer based on an acrylate resin containing hydroxyl, carboxyl and amino groups and a polyisocyanate crosslinking agent, which lacquer is in particular intended for application onto a metallic base coat lacquer. The acrylate resin is obtained by copolymerisation of hydroxyalkyl esters, .alpha., .beta.-unsaturated carboxylic acids with ethylenically unsaturated carboxylic acids and further olefinically unsaturated monomers. The resultant copolymer is further reacted with compounds containing one free isocyanate group and at least one tert.-amino group per molecule. Crosslinking is achieved at an NCO:OH ratio of 0.3:1 to 3.5:1. Only inadequate hardness in a multi-layer structure is obtained with these clear lacquers too.
SUMMARY OF THE INVENTION
The object of the present invention is thus to provide coating compositions which give rise to coatings with elevated gloss, high hardness and good weathering resistance and which in particular retain these properties when used as clear lacquers for overcoating per se known (plain and metallic) water-based base lacquers.
This object is achieved by a process for the multi-layer lacquer coating of a substrate by applying a base lacquer layer of an aqueous coating composition onto the optionally precoated substrate and applying a clear lacquer, which process is characterised in that a clear lacquer is used which contains in .alpha.-position with one or more epoxide groups or ethylenic double bonds in the alcohol portion, carboxylic acids and unsaturated carboxylic acids, per molecule in a quantity such that the equivalent ratio of OH groups in component A) to NCO groups of component B) is 1:1.6 to 1:2.2 and additives.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to a preferred embodiment, component A) may also optionally be produced also using one or more .alpha., .beta.-ethylenically unsaturated carboxylic acids as component d) and/or optionally also using one or more vin
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Brock Thomas
Wilczek Erwin
Dudash Diana
Herberts GmbH
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