Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-03-22
2002-05-21
Short, Patricia A. (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S386000, C427S387000, C525S108000, C525S110000, C525S111000, C525S117000, C525S123000, C525S124000, C525S125000, C525S131000, C525S133000, C525S142000, C525S155000, C525S161000
Reexamination Certificate
active
06391390
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns curable coating compositions, especially compositions for topcoats, particularly for clearcoats of color-plus-clear composite coatings.
BACKGROUND OF THE INVENTION
Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings provide topcoats with exceptional gloss, depth of color, distinctness of image, and special metallic effects. The automotive industry has made extensive use of these coatings for automotive body panels.
A topcoat coating should be durable to maintain its appearance and provide protection under service conditions during the lifetime of the coated article. Topcoat coatings for automotive vehicles, for example, are typically exposed to all kinds of weather, ultraviolet rays from the sun, abrasions from gravel thrown up during driving or from items set on the car when parked, and other conditions that can degrade the coating. A variety of additives can be added to a coating formulation to prevent or retard degradation, such as UV-light absorbers, hindered amine light stabilizers, anti-marring or slip additives, and antioxidants.
Because some additives tend to be removed from the coating over a period of time, due to leaching, volatilization, or other actions, techniques have been developed to covalently bond hindered amine light stabilizers and other ultraviolet light absorbers to the polymer binder of the coating composition so that the additive is covalently attached to the polymer network of the cured coating and cannot volatilize or be leached from the coating. These techniques are described in the article Peter A. Callais et al., “Synthesis of Acrylic HSC Resins with Peroxides Containing Hindered Amine Light Stabilizers,”
Modern Paint and Coatings
, September 1988, pages 41 to 47, in Oberg et al., U.S. Pat. No. 5,872,165, and in Johnson, U.S. Patent No. 4,504,628, each of which are incorporated herein by reference. In the Callais article, a hindered amine light stabilizer is attached to an acrylic polymer by preparing the acrylic polymer using a peroxide having a hindered amine light stabilizer substituent. The acrylic resin with attached hindered amine light stabilizer is combined with a melamine crosslinker resin, a catalyst, and solvent to provide a clearcoat composition. In the Oberg patent, a benzotriazole or triazine ultraviolet light absorber is covalently bound to a polymer such as an acrylic polymer. The polymer is combined with a crosslinker, catalyst, and other additives to prepare a coating composition. In the '628 patent, a UV stabilizer is reacted onto a methacrylate monomer that is then addition polymerized to form an acrylic polymer having UV stabilizer groups. The '628 patent describes preparing a homopolymer of the UV stabilizer monomer that can be added to a different polymer.
In order to prevent marring, scratching, degradation from UV light, and other problems that occur at the surface of the coating, surface protecting additives are most effective in that upper layer, e.g., perhaps the upper 0.5 micron of a 50 micron layer of clearcoat, of the coating in a high enough concentration to be effective. The polymer-bound surface protecting additives just discussed are incorporated in a uniform concentration throughout the coating layer to achieve the desired concentration at the surface. As a result, much more of the polymer-bound surface protecting polymer must be included to obtain an effective concentration at the surface.
It would be advantageous, both for performance and cost reasons, to produce a polymeric additive having bound surface protecting groups and a coating composition containing the additive that provides a cured coating layer having a higher concentration of the surface protecting additive at the surface of the coating relative to the rest of the coating thickness.
SUMMARY OF THE INVENTION
The present invention provides a curable coating composition having a resin component that includes a film-forming polymer, a polymeric additive having bound surface protecting groups/and a crosslinker that is reactive with the film-forming polymer. During curing of the applied coating composition on a substrate, the polymeric additive having bound surface protecting groups migrates toward the surface of the coating layer being formed. In the cured coating, the concentration of the polymeric additive having bound surface protecting groups is higher in the surface layer than its concentration in the rest of the film thickness. In particular, the concentration of the polymer-bound surface protecting additive in the upper, up to about 50% of the coating layer thickness is at least about 10% higher relative to its concentration in the rest of the coating.
In a preferred embodiment, the polymeric additive having bound surface protecting groups is an acrylic polymer. The acrylic polymer, at least before the coating composition is being cured, should have no group that is reactive toward the crosslinker, and more preferably the acrylic polymer having the bound surface protecting groups has a group that, during curing of the coating and preferably after the polymeric additive has moved toward the surface of the curing coating layer, is converted to a functionality that is reactive toward the crosslinker. Thus, the acrylic polymeric additive has monomeric units bearing the surface protecting groups and; optionally, monomeric units that are not reactive with the crosslinker but that can be converted during cure to functionality reactive with the crosslinker. The remainder of the monomeric units of the acrylic polymeric additive are free of crosslinkable groups and of groups with active hydrogens, and the remainder of the monomeric units are derived from vinyl monomers with aryl groups or alkyl groups that on average have at least about 4 carbon atoms. The vinyl monomers may optionally contain fluorine or silicon atoms.
The invention further provides an article having a substrate upon which substrate is a cured coating derived from a coating composition according to the invention and a method of producing such a coating on a substrate.
DETAILED DESCRIPTION
The present invention provides a curable coating composition-having a resin component that includes a film-forming polymer, a polymeric additive having bound surface protecting groups, and a crosslinker that is reactive with the film-forming polymer. During curing of an applied coating layer, the polymeric. additive having bound surface protecting groups is designed-to migrate toward the surface of the coating layer being formed. To allow the polymeric additive to migrate toward the surface of the coating layer during curing, the polymeric additive is substantially free from active hydrogen groups, other than those that might be part of the bound surface protecting groups. In addition, the polymeric additive has substantial aliphatic character, preferably in the form of pendant fatty groups having, on average, four or more carbon atoms and preferably having, on average, eight or more carbon atoms. The polymeric additive may also include fluorine or silicon atoms. In the cured coating, the concentration of the polymeric additive having bound surface protecting groups is higher in the surface layer than its concentration in the rest of the film thickness. In particular, the concentration of the polymer-bound surface protecting additive in the upper, up to about 50% of the coating layer thickness is at least about 10% higher relative to its concentration in the rest of the coating.
The weight average molecular weight of the polymeric additive is preferably at least about 7000, more preferably from about 7000 to about 5 100,000, and even more preferably from about 10,000 to about 50,000. In a preferred embodiment, the polymeric additive having bound surface protecting groups is an acrylic polymer.
The polymeric additive having bound surface protecting groups, at least before the coating composition is be
Boisseau John E.
Ohrbom Walter H
St. Aubin Donald L.
BASF Corporation
Budde Anna M.
Short Patricia A.
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