Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Reexamination Certificate
1999-11-23
2002-10-29
Sergent, Rabon (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
C525S123000, C525S131000, C525S440030, C528S053000, C528S058000, C528S067000, C528S073000, C528S075000, C528S080000, C528S083000, C528S054000
Reexamination Certificate
active
06472493
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to solvent based polyurethane coating compositions having a low VOC (volatile organic content) and in particular to a clear coating composition for refinishing clearcoat/colorcoat finishes of vehicles such as automobiles or trucks.
Clearcoat/colorcoat finishes for automobiles and trucks have been used in recent years and are very popular. Nowadays, such finishes are produced by a wet-on-wet method, in which the colorcoat or basecoat which is pigmented is applied and dried for a short period of time but not cured and then the clearcoat, which provides protection for the colorcoat and improves the appearance of the overall finish such as gloss and distinctness of image, is applied thereover and both are cured together.
Repair of such clearcoat/colorcoat finishes that have been damaged, e.g., in a collision, has been difficult in that the low VOC clearcoat refinish compositions in current use, for example, as taught in Corcoran et al. U.S. Pat. No. 5,279,862, issued Jan. 18, 1994, Lamb et al. U.S. Pat. No. 5,286,782, issued Feb. 15, 1994, and Anderson et al. U.S. Pat. No. 5,354,797, issued Oct. 11, 1994, have short dust drying times but nonetheless take many hours to cure to a sufficiently hard and water resistant state at ambient or slightly elevated temperatures suitable for automotive refinishing, and the vehicle cannot be moved outside to free up work space in the autobody repair shop without risk of water spotting nor can the clearcoat be sanded (wet or dry) or buffed to a high gloss finish on the same day of application. In a typical refinish operation, after the colorcoat is applied, the clearcoat is then applied to the vehicle and the resulting finish is allowed to dry to at least a dust free state before the vehicle is moved out of the paint booth so that another vehicle can be painted. Before any further work can be done to the finish or before the vehicle can be stored outside to free up additional floor space, not only must the finish be dust free so that dust and dirt will not stick to the finish, it must also be sufficiently hard to sand or buff to improve the gloss or to remove minor imperfections as well as water resistant. Conventional finishes are unable to cure to a sufficiently hard and water resistant state in a relatively short period of time, and thus the productivity of a refinish operation is still lacking, since the vehicles cannot be stored outside or worked on quickly after application of the finish.
One approach used to improve the initial hardness and water resistance of a clearcoat composition on curing involves replacing a portion of the conventional polyisocyanate crosslinking agent (like hexamethylene diisocyanate (HDI) trimer) with a relatively hard or rigid material, such as isophorone diisocyanate (IPDI) trimer. Unfortunately, IPDI has a much slower curing rate than that of HDI. Consequently, these coatings must rely on significantly high baking temperatures and/or high levels of conventional tin catalysts to achieve the hardness offered by the IPDI trimer in addition to water resistance in a relatively short period of time. However, in the automotive refinishing industry, high baking temperatures are undesirable, as they will permanently damage a vehicle's upholstery, wiring, stereo, plastic bumpers, etc. High tin catalyst levels, on the other hand, produce certain unwanted side effects such as reduced pot life and increased “die-back”. Die-back mainly occurs as the film is formed before all solvents are evaporated. The solvents that are trapped create a stress on the film as they eventually flash away, which distorts or wrinkles the film and converts it almost overnight from an attractive high gloss mirror-like finish into a dull fuzzy appearance having poor gloss and distinctness of image.
Thus, a continuing need still exists for a low VOC coating composition suited for use as a clearcoat in automotive refinishing that offers high film hardness and water resistance in a very short period of time when cured at ambient or slightly elevated temperatures, with little or no pot life reductions and die-back consequences, so that a vehicle can be moved or worked on quickly after application.
SUMMARY OF THE INVENTION
The invention provides a low VOC solvent based polyurethane coating composition having improved early hardness and water resistance, containing a film forming binder and a volatile organic liquid carrier for the binder, wherein the binder contains
(A) a hydroxyl component comprising at least one hydroxyl-containing acrylic polymer and at least one hydroxyl-terminated polyester oligomer; and
(B) a polyisocyanate component, at least a portion of which comprises a trimer of isophorone diisocyanate;
wherein the ratio of equivalents of isocyanate per equivalent of hydroxyl groups in the binder is about 0.5/1 to 3.0/1;
wherein the composition further contains
(C) a catalyst system for the binder comprising at least one ogranotin compound, at least one tertiary amine, and at least one organic acid;
wherein the coating composition on curing at ambient temperatures is in a water spot free and sand or buff state within 4 hours after application and has a Persoz hardness of at least 35 counts, while at the same time exhibits little or no pot life reductions and little or no die-back in the attractive high gloss finish formed therefrom.
The present invention also provides an improved process for repairing a clearcoat/colorcoat finish of a vehicle using the aforesaid 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, which greatly improves the efficiency 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 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 of about 25-90% by weight and an organic liquid carrier content of about 10-75% by weight, preferably about 35-55% by weight binder and 45-65% by weight carrier.
The binder contains two components, a hydroxyl and an organic polyisocyanate crosslinking component, which are capable of reacting with each other to form urethane linkages. In the present invention, the hydroxyl component contains about 50-99% by weight of a hydroxyl functional acrylic polymer or a blend of such polymers and about 1-50% by weight of a hydroxyl-terminated polyester oligomer or blend of such oligomers. The total percentage of hydroxyl-containing materials in the hydroxyl component is herein considered to equal 100%. The polyisocyanate component, on the other hand, contains about 3-50% by weight of a trimer of isophorone diisocyanate and about 50-97% by weight of a second organic polyisocyanate or blend of such polyisocyanates, with the second polyisocyanate preferably being a timer of hexamethylene diisocyanate. The total percentage of polyisocyanates in the crosslinking component is herein considered to equal 100%. The hydroxyl and polyisocyanate components are generally.employed in an equivalent ratio of isocyanate groups to hydroxyl groups of about 0.5/1 to 3.0/1, preferably in the range of about 0.8/1 to 1.5/1.
The hydroxyl functional acrylic polymer used in the hydroxyl component of the
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Sergent Rabon
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