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
2001-11-09
2004-03-09
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S458000, C525S028000, C524S507000, C524S590000, C524S523000, C524S528000, C524S714000, C524S285000, C524S522000, C524S538000, C526S271000
Reexamination Certificate
active
06703452
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 volatile 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 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.).
The coating composition 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 polyhydroxyl-tertiary amine, at least two hydroxyl groups of said amine being indirectly bonded to a nitrogen atom of said amine through a chain containing at least two carbon atoms, and
(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.
In the broadest sense, the present invention comprises component (A) by itself, preferably also contained in a volatile organic liquid carrier, the combination of the hydroxyl-containing acrylic polymer and polyhydroxyl-tertiary amine 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 polyhydroxyl-tertiary amine of (A) to form urethane linkages, whereby the cured coating film is a polyurethane. The polyhydroxyl-tertiary amine 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 polyhydroxyl-tertiary amine 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 polyhydroxyl-tertiary amine is 1-60 wt %, based on the total weight of (A).
Film coatings formed from compositions 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 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 of component (C) in component (A). “Solids content” as used herein refers to the film-forming binder content of the composition, i.e. although the binder is in solution in the carrier, upon evaporation of the carrier, solid coating film of the binder remains.
The hydroxyl functional acrylic polymer used in the hydroxyl component of the binder is prepared by conventional solution polymerization techniques in which monomers, solvents and polymerization catalyst are charged into a conventional polymerization reactor and heated to about 60-200° C. for about 0.5-6 hours to form a polymer having a weight average molecular weight (Mw) of preferably about 2,000-13,000, more preferably about 3,000-11,000.
All molecular weights disclosed herein are determined by GPC (gel permeation chromatography) using polymethyl methacrylate standard, unless otherwise noted.
The acrylic polymer thus formed also has a glass transition temperature (Tg) generally of at least 20° C. and preferably about 40-80° C.
All glass transition temperatures disclosed herein are determined by DSC (differential scanning calorimetry).
Typically useful polymerization catalysts are azo type catalysts such as azo-bis-isobutyronitrile, 1,1′-azo-bis(cyanocylohexane), acetates such as t-butyl peracetate, peroxides such as di-t-butyl peroxide, benzoates such as t-butyl perbenzoate, octoates such as t-butyl peroctoate and the like.
Typical solvents that can be used are ketones such as methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone, aromatic hydrocarbons such as toluene, xylene, alkylene carbonates such as propylene carbonate, n-methyl pyrrolidone, ethers, ester, such as butyl acetate, and mixtures of any of the above.
The h
Benjamin Steven C.
Cheung William K
E. I. du Pont de Nemours and Company
Wu David W.
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