Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-06-21
2002-08-13
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
active
06433085
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to coating compositions which are especially useful for clear coating over a colored base coat.
2. Description of Related Art
U.S. Pat. No. 5,279,862 discloses a clear coating composition which can be used as the clear coat of a motorized vehicle clear coat/color coat finish. The clear coating composition has a film-forming binder and volitile organic liquid carrier, the binder including hydroxy functional acrylic polymer and organic polyisocyanate and is characterized by rapid curing to form a finish that is tack free and can be buffed as soon as 3-4 hours after application, resulting in increased productivity of the paint shop.
Even more rapid curing of the clear coat is desired for further increase in paint shop productivity. A catalyst for the hydroxy-isocyanate crosslinking reaction is present in the clear coating composition, typically an organo tin compound. When the amount of catalyst is increased to speed-up the cure, other problems arise, including decreased potlife and reduced coating quality. In the latter case, the faster cure entraps liquid carrier within the dried clear coating, causing the coating to have poor gloss and distinctness of image.
There are other indicia of increased productivity, e.g. how soon after application the film coating dries sufficiently to be dust-free, so that the painted article (vehicle) can be moved from the paint booth, to make room for the next vehicle to be painted. The vehicle can be moved outside the paint shop, i.e. into the open air, only after the film coating has dried further so as to be free of water spotting damage.
SUMMARY OF THE INVENTION
The present invention provides (a) a coating composition which forms film coatings which provide improved productivity as determined by one or more of the indicia of rapidly becoming dust-free and water spot resistant, and/or rapid curing sufficiently to be buffed, all occuring at ambient temperature (20° C.), (b) a novel binder component for use in (a), and (c) a novel ingredient for use in (b). The coating composition (a) of the present invention contains a film forming binder and a volatile organic liquid carrier binder, wherein the binder contains
(A) hydroxyl-containing acrylic polymer and polyol prepared by reacting together polyhydroxyl-tertiary amine with organic polyisocyanate, the polyhydroxyl-tertiary amine having at least two hydroxyl groups being indirectly bonded to a nitrogen atom of said amine through a chain containing at least two carbon atoms, essentially all of the isocyanate groups of said organic polyisocyanate being reacted with hydroxyl groups of said amine,
(B) organic polyisocyanate, the ratio of equivalents of isocyanate in (B) per equivalent of hydroxyl groups in (A) being in the range of 0.5/1 to 3.0/1, and
(C) an effective amount of catalyst to cure said composition.
The novel binder component (b) of the present invention is the combination of the hydroxyl-containing acrylic polymer and polyol described as component (A) above, preferably also contained in a volatile organic liquid carrier, the combination of the hydroxyl-containing acrylic polymer and the polyol also constituting a film-forming binder. Components (A) and (B) are packaged separately and are combined just prior to application, because component (B) crosslinks the combined components. Component (C) speeds up the crosslinking reaction and can be provided to the crosslinking reaction either as part of (A), (B), or as a separate component. In any event, the pot-life of the combined components is sufficient to enable the combined components to be applied, typically by spraying, onto the substrate to be coated, typically an vehicle body part, including the entire vehicle body.
The curing of the composition after it is applied to form the film coating occurs by the isocyanate groups of (B) reacting with the hydroxyl groups of both the acrylic polymer and the polyol of (A) to form urethane linkages, whereby the cured coating film is a polyurethane. The polyol both speeds up the curing reaction, as does the catalyst (C), and becomes part of the crosslinked structure by the reaction of its hydroxyl groups with the isocyanate groups. Thus, the polyol is present in an effective amount to increase the crosslinking reaction rate during curing of the composition. Preferably, the amount of said acrylic polymer is 40-99 wt % and the amount of polyol is 1-60 wt %, based on the total weight of (A).
The novel ingredient (c) of the present invention is the polyol of component (A). It is desirable that the polyol is essentally free of isocyanate groups so that there will be no appreciable reaction between the polyol and the hydroxyl-containing acrylic polymer when these ingredients are combined into component (A), otherwise there would be premature crosslinking of component (A) that would interfere with the application of the coating composition such as by spraying and would give a defective coating, if sprayable at all. The reaction between the polyhydroxyl-tertiary amine and the polyisocyanate to form the polyol of component (A) is spontaneous, i.e. it occurs merely by mixing these ingredients together at ambient temperature and under atmospheric pressure. The polyol reaction product is a mixture of reaction products in which different number of molecules of the amine react with a single molecule of the poyisocyanate and in which one molecule of the amine may react with a plurality of molecules of the polyisocyanate. The fact that essentially all of the isocyanate groups of the polyisocyanate are used up in the reaction and that the reaction product is a polyol indicates that an excess of hydroxyl groups provided by the polyhydroxyl-tertiary amine is present in the reaction. These hydroxyl groups in the polyol bear the same relationship to the amine nitrogen as in the polyhydroxl-tertiary amine reactant.
Film coatings formed from compositions (a) of the present invention typically become dust-free within 10 min and even within 5 min, free of water spot damage within 30 min, and can be buffed in less than three hours and possibly as early as one hour after application, all with ambient temperature drying and cure, without sacrifice in either the ease of applying the coating composition or the ultimate quality of the clear coat. Of course, the film coating becomes tack-free prior to becoming buffable. Thus, the coating compositions of the present invention are highly useful for repairing a clearcoat/colorcoat finish of a vehicle using the coating composition as a refinish clearcoat, which process allows the vehicle to be moved outside and the finish to be sanded (wet or dry), buffed or polished, if necessary, to remove minor imperfections and enhance gloss within a short period of time after application. This greatly improves the productivity of a refinish operation by allowing more vehicles to be processed in the same or in less time.
DETAILED DESCRIPTION OF THE INVENTION
The coating composition (a) of this invention is a low VOC (volatile organic content) composition that is particularly suited for use as a clearcoat in automotive refinishing. The composition contains a film forming binder and an organic liquid carrier which is usually a solvent for the binder. Since the invention is directed to a low VOC composition, the amount of organic solvent used in the liquid carrier portion results in the composition having a VOC content of less than 0.6 kilograms per liter (5 pounds per gallon) and preferably in the range of about 0.25-0.53 kilograms (2.1-4.4 pounds per gallon) of organic solvent per liter of the composition, as determined under the procedure provided in ASTM D-3960.This usually translates to a film forming binder content (components (A)+(B)+(C)) of about 25-90% by weight and an organic liquid carrier content of about 10-75% by weight, preferably about 30-55% by weight binder and 45-70% by weight carrier. Component (A), by itself can have the same solids content in organic liquid carrier, with or without the presence
Benjamin Steven C.
Cain Edward J.
E.I. Du Pont de Nemours and Company
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