Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1997-03-19
2001-12-11
Wu, David W. (Department: 1713)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S299000, C427S407200, C427S409000, C427S419100, C427S412300, C427S412500, C524S457000, C524S507000
Reexamination Certificate
active
06329020
ABSTRACT:
FIELD OF INVENTION
The invention relates to a method for the preparation of a multilayer coating and to suitable aqueous, non-yellowing coating materials or coating compositions suitable for its implementation. These can be used as water-dilutable plain base coats or metallic base coats and are usually oversprayed with a clear lacquer, which is water dilutable or dissolved in organic solvents. The vehicles employed can be used as industrial paints, especially, for example, in the automobile industry. They lead to coatings which, aside from a good optical effect and excellent mechanical properties, result in good intermediate adhesion, show little swelling in water and can be applied trouble-free by electrostatic spraying. In addition, they are suitable for repair purposes, since they lead to excellent properties already when cured at low temperatures, such as 80° C.
BACKGROUND INFORMATION AND PRIOR ART
It is well known that substrates may be provided with several superimposed coating layers in order to obtain coatings with a good decorative effect and, at the same time, good protective properties. For example, multilayer coats of lacquer are applied preferably according to the so-called “base coat/clear coat” method. After a short exposure to air without a stoving step, the initially applied pigmented base lacquer is painted over by the “wet-on-wet” method with a clear lacquer. Both layers are then stoved together. Metal-effect lacquers are preferably processed in the automobile industry by this method. Because of the composition of the base lacquer, high proportions of solvents are consumed in this procedure. The problem therefore existed of converting such a lacquer system to a water-dilutable basis.
Such a vehicle must
be applicable trouble-free by electrostatic spraying with, for example, high rotation bells
be oversprayable “wet-on-wet” with conventional or water-dilutable clear lacquers, that is, in spite of the shortest possible air exposure times and/or the shortest possible heating times, no interfering incipient solution phenomena should be seen through the (transparent) covering layer;
orient and fix metal-effect pigments spatially in order to achieve a good optical effect;
for repair purposes, lead to films with excellent mechanical properties already after being cured at low temperatures, such as 80° C.; together with the clear lacquer, these films should be highly resistant to weathering.
In the development of such water-dilutable systems, problems arise, which are difficult to solve and are attributable to the special properties of water and to the low heating temperatures, at which practically no cross linking reactions take place. Attempts have been made in the past to arrive at suitable water-dilutable systems. However, it was not possible to combine all the desired properties optimally in one vehicle.
In the German Offenlegungsschrift 2,860,661, water-dilutable vehicles based on acrylate are described, which contain polymer microparticles, which are insoluble in nonaqueous solvents and water. They are prepared with the help of steric dispersion stabilizers by polymerization in nonaqueous solvents and subsequently transferred into the aqueous medium. This conversion process is very costly and also very susceptible to failure, because the effectiveness of the dispersion stabilizer can be affected by variations in the synthesis process. In the European Patent A 38 127, these vehicles are used for the preparation of metallic base coats, the aluminum platelets or pigments being brought into the lacquer with the help of a melamine resin. At low stoving temperatures, this resin acts like a plasticizer and worsens the moisture susceptibility of the film.
In the German Patent 2,736,542, primers for metal spools are described, which are based on acrylate resin/polyurethane dispersions. Combinations of polyurethane dispersions with latex polymers are used here, optionally with addition of melamine resins. Special combinations, which are matched to the requirements of metallic base coats, are not named.
In the German Offenlegungsschrift 1,953,348, a free radical emulsion polymerization of vinyl monomers is carried out in the presence of aqueous dispersions of high molecular weight polyurethanes with anionic groups. In the examples, the polyurethane dispersions contain alkali salts of 1:1 adducts of diamines and sultones. Because of the non-volatile salt content in the film, adhesion problems arise in multilayer lacquer structure. There are no special references to the selection of suitable polyurethane resins or of unsaturated monomers, with which the requirements of metallic base coats are fulfilled.
In the German Offenlegungsschrift 2,363,307, vinyl monomers are reacted by free radical polymerization in the presence of a polyurethane latex. In the examples, only polyurethane dispersions based on polypropylene glycol are used, which moreover are reacted with a relatively low monomer content. In so doing, films are formed in multilayer structures, which are not sufficiently resistant to the effects of moisture. Here also, special selection conditions cannot be inferred for the preparation of metallic base coats.
The German Offenlegungsschrift 3,545,618 relates strictly to polyurethane dispersions for the preparations of water base coats for multilayer coatings. There is no reaction with unsaturated monomers. They are prepared by reaction of linear polyether and polyester glycols, to which triols have been added, with diisocyanates in inert organic solvents. The resulting polyurethanes must be readily soluble in the solvents used. Neutralization and dilution with water is technically controllable only if relatively low molecular weight resins and increased amounts of solvent are used. The film properties show a high solvent sensitivity, which leads to difficulties during the application of the clear lacquer on the base coat. Higher molecular weight polyurethanes can be emulsified only with difficulty in water and lead to emulsions with very coarse particles. Stable systems require a high salt group content.
In the German Offenlegungsschrift 3,210,051, water-dilutable polyurethane dispersions are used for the preparation of metallic base coats. The dispersions are obtained by dispersing a neutralized anionic NCO prepolymer in water and subsequently subjecting it a chain extension with polyamines. These coating materials create problems when used under practical conditions in batch lacquering processes, because the rapidly drying coating materials in the application devices used (for example, lacquer spray gun or electrostatically supported high rotation bells) coagulate at the walls and adhere there so well, that they can be removed again only with great difficulty. The danger of forming pin-holes and gel particles in the film surface is therefore very great and a quick shade change is made formidable by difficult cleaning operations. Additional large amounts of solvent are introduced into the mixture by the addition of acrylate resin solutions. The addition of melamine resins makes it difficult to produce suitable water-resistant films at low stoving temperatures, as required for repairs.
In U.S. Pat. No. 4,318,833, thermoplastic polymers are prepared by polymerization of unsaturated monomers in the presence of fully reacted, water-soluble polyurethanes. The polymers either have oxidative drying properties or, after addition of cross linking agents such as aminoplasts, phenoplasts or blocked polyisocyanates, can be stoved during 10 to 15 minutes at 125° to 175° C. as coating lacquers, which can be pigmented. The water-soluble polyurethane resin can be prepared from polyether or polyester glycols and contains anionic or cationic groups. The resin must be readily soluble in the inert solvents required for the preparation. The neutralization and dilution with water can be controlled on an industrial scale only if relatively low molecular weight resins and increased amounts of solvent are used. The examples of this US patent show only that a polymerization of unsaturated mo
Gobel Armin
Meier Hans Ulrich
Patzschke Hans-Peter
Herberts Gesellschaft mit beschrankter Haftung
Schweitzer Cornman Gross & Bondell LLP
Wu David W.
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