Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2000-03-16
2002-03-12
Lovering, Richard D. (Department: 1712)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S388200, C427S512000, C427S559000, C525S127000, C525S128000, C526S937000
Reexamination Certificate
active
06355307
ABSTRACT:
This application is a 371 of PCT/EP98/03463 filed Jun. 9, 1998.
This invention relates to clear stoving lacquer coating media, to the use thereof for the production of multi-layer coatings, particularly multi-layer coatings of the base lacquer/clear lacquer type, and to a method of producing multi-layer coatings.
Current mass-production lacquer coatings for automobiles mostly consist of a base lacquer/covering lacquer coating which is applied to a pre-coated automobile body. The clear stoving lacquers which have been used in particular in practice are those which comprise a binder vehicle-crosslinking agent system based on a combination of hydroxy-functional binder vehicles and aminoplastic resins. In clear stoving lacquers such as these, the hydroxy-functional binder vehicles and the aminoplastic crosslinking agents are generally present in a solids content ratio of 50:50 to 90:10. Clear lacquer coats produced from clear lacquers such as these do not exhibit satisfactory resistance to acids. When there are increased demands on the acid-resistance of clear lacquer coats, the use of clear lacquer systems which have other crosslinking mechanisms is preferred, for example the use of clear lacquer systems such as these which crosslink with the formation of urethane groups or ester groups or with the formation of a C—C bond.
The visual and aesthetic impression of base lacquer/clear lacquer two-layer coatings is significantly influenced by the quality of the clear lacquer coat. In the ideal situation, the structure of the clear lacquer coat is identical on horizontal and vertical surfaces of a three-dimensional substrate, of an automobile body for example. In practice, however, the ideal situation described above cannot be achieved without difficulty. For example, differences may occur in the surface structure or in the flow of the outer clear lacquer coat, since after their application, and particularly during the heat-up phase of the stoving process, clear lacquers tend to run off or sag on surfaces which depart from the horizontal as a result of a decrease in viscosity due to temperature. This risk is greater, the greater is the clear lacquer coat thickness.
It is known from DE-C 27 51 761 and EP-A-0 198 519 that certain urea compounds can be used as additives in clear stoving lacquers which contain aminoplastic resins as crosslinking agents, in order to counteract unwanted run-off phenomena of the clear lacquer. In addition, it is known from EP-A-0 192 304 that urea compounds can be used as anti-runoff agents in clear lacquers which contain hydroxy-functional binder vehicles and crosslinking agents which are different from aminoplastic resins. Within the scope of the present invention, it has been shown that the use of urea compounds in clear lacquers results in reduced brilliance and in dullness (milkiness) of stoved clear lacquer coats. The dullness phenomenon becomes particularly pronounced when clear lacquer coats are formed on dark backgrounds, for example on base lacquer coats of a dark hue.
The object of the present, invention is to provide clear lacquer coating media which are substantially free from aldehyde condensation resin crosslinking agents, which exhibit a reduced tendency to run-off, and which can be hardened to form non-dulled, brilliant, acid-resistant clear lacquer coats.
This object is achieved by clear lacquer coating media containing one or more self-crosslinking or externally crosslinkable binder vehicles, one or more organic solvents, in the case of externally-crosslinkable binder vehicles containing one or more crosslinking agents which are different from aldehyde condensation resins, optionally also containing one or more reactive thinners, and optionally also containing customary additives for clear lacquers, which are characterised by an additional content of
A) 0.1 to 3% by weight one or more urea compounds, and
B) 0.05 to 10% by weight one or more substances which supply formaldehyde,
with respect in each case to the resin solids content of the clear lacquer coating medium which is formed by the binder vehicles and by the reactive thinners which are optionally present and by the crosslinking agents which are optionally present.
The clear lacquer coating media according to the invention are thermally hardenable, and are what are termed clear stoving lacquer coating media, which harden at temperatures of 80° C. and above, e.g. up to 160° C.
In the clear lacquer coating media according to the invention, the resin solids content is composed of a binder vehicle which can be thermally hardened by addition or condensation reactions, optionally plus a reactive thinner system, optionally plus a crosslinking agent system, which crosslinks chemically during stoving with the formation of covalent bonds. Binder vehicle systems which are thermally hardenable by addition and/or condensation reactions are binder vehicles which are optionally admixed with reactive thinners (compounds which are chemically incorporated in the lacquer film during stoving) and are optionally admixed with crosslinking agents for clear lacquer coating media, preferably for clear lacquer coating media such as those which can be used for the production of base lacquer/clear lacquer two-layer coatings, and which are known from a multiplicity of patent documents, for example.
The clear lacquer coating media according to the invention are liquid. They contain organic solvents and one or more customary film-forming binder vehicles; they optionally contain one or more reactive thinners in addition, as well as one or more crosslinking agents if the binder vehicles are not self-crosslinking. In principle, neither the binder vehicles nor the crosslinking agents are subject to any restriction, although systems in which a significant proportion of the chemical crosslinking occurs due to aldehyde condensation resins are excluded, i.e. the clear lacquer coating media according to the invention are substantially or completely free from aldehyde condensation resins. In particular, the clear lacquer coating media according to the invention contain, with respect to their resin solids content, less than 5, preferably less than 3, most preferably less than 1, for example 0 to 1% by weight of aldehyde condensation resins. Particular examples of aldehyde condensation resins such as these include the partially or completely alkylated aldehyde condensation resins which are customary in lacquer technology as crosslinking agents for hydroxy-functional binder vehicles, for example phenol-(form)aldehyde condensation resins or aminoplastic resins such as urea-(form)aldehyde or amine-(form)aldehyde condensation resins (e.g. melamine resins, benzoguanamine resins).
The clear lacquer coating media according to the invention are preferably externally-crosslinkable systems, in which binder vehicles are present which optionally contain reactive thinners and crosslinking agents, generally in a stoichiometric ratio, which is matched to the sought-after degree of crosslinking, of 50 to 90% by weight binder vehicles, 0 to 20% by weight reactive thinners and 50 to 10% by weight crosslinking agents, wherein the sum amounts to 100%. In principle, neither the binder vehicles nor the reactive thinners are subject to any restriction.
There is also no restriction on the selection of the crosslinking agents which are optionally contained. This depends on the functionality of the binder vehicles, i.e. the crosslinking agents are selected so that they have a reactive functionality which is complementary to the functionality of the binder vehicles. In this respect, the use of aldehyde condensation resins as crosslinking agents is substantially or completely avoided, as stated above. Polyester, polyurethane and/or (meth)acrylic copolymer resins can be used as film-forming binder vehicles, for example. The binder vehicles, particularly polyester and polyurethane resins, are preferably substantially free from aromatic structural units and are based on aliphatic and/or cycloaliphatic components. The (meth)acrylic copolymer resins may conta
Brenke Stephan
Dössel Karl-Friedrich
Herrmann Friedrich
Reifferscheidt Heinz-Walter
Dupont Performance Coatings GmbH & Co.
Lovering Richard D.
Pillsbury & Winthrop LLP
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