Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1999-11-17
2001-04-03
Cameron, Erma (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S412100
Reexamination Certificate
active
06210758
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to composite primer coatings that provide chip resistance and to aqueous primer compositions that provide such composite coatings.
BACKGROUND OF THE INVENTION
Coating finishes, particularly exterior coating finishes in the automotive industry, are generally applied in two or more distinct layers. One or more layers of primer coating composition may be applied to the unpainted substrate first, followed by one or more topcoat layers. Each of the layers supplies important properties toward the durability and appearance of the composite coating finish. The primer coating layers may serve a number of purposes. First, the primer coating may be applied in order to promote adhesion between the substrate and the coating. Secondly, the primer coating may be applied in order to improve physical properties of the coating system, such as corrosion resistance or impact strength, especially for improving resistance to gravel chipping. Third, the primer coating may be applied in order to improve the appearance of the coating by providing a smooth layer upon which the topcoat layers may be applied. The topcoat layer or layers contribute other properties, such as color, appearance, and light stabilization.
In the process of finishing the exterior of automotive vehicles today, metal substrates are usually first coated with an electrocoat primer. While the electrocoat primer provides excellent surface adhesion and corrosion protection, it is often desirable to apply a second primer layer. The second primer layer provides additional properties not available from the electrocoat primer. Resistance to gravel chipping is one of the critical properties provided by the second primer layer. The second primer layer may also enhance the corrosion protection of the finish and provide a smoother surface than the electrocoat primer. The second primer also serves to provide a barrier layer between the electrocoat primer layer, which usually contains aromatic moieties and other materials that can cause yellowing on exposure to sunlight, and the topcoat.
Mitsuji et al, U.S. Pat. Nos. 5,281,655, 5,227,422, and 4,948,829, all of which are incorporated herein by reference, disclose automotive basecoat coating compositions containing polyurethane resin emulsion, a second resin emulsion than can be an acrylic resin, and a crosslinking agent. In Mitsuji '829, the polyurethane resin is prepared by dispersing an isocyanate-functional prepolymer and having the water react with the isocyanate groups to chain-extend the prepolymer. The prepolymer is prepared using an aliphatic diisocyanate, a polyether or polyester diol, a low molecular weight polyol, and a dimethylolalkanoic acid. In Mitsuji '655 and '422, the polyurethane resin is prepared by reacting an aliphatic polyisocyanate, a high molecular weight polyol, a dimethylolalkanoic acid, and, optionally, a chain extender or terminator. Because the Mitsuji patents are directed to basecoat coatings, these patents provide no direction for preparing compositions that have the chip resistance and other properties required for primer coating layers.
Hatch et al., U.S. Pat. No. 5,817,735, incorporated herein by reference, discloses an aqueous primer composition for golf balls that includes a polyurethane dispersion and an acrylic dispersion. The primer has a very low content of volatile organic solvent, which is important for minimizing regulated emissions from the coating process. The Hatch patent, however, does not disclose a curable (thermosetting) composition. More importantly, the golf ball primers of the Hatch patent do not provide the properties, such as resistance to stone chipping and corrosion protection, that are required of an automotive primer.
While the primer composition may be formulated to provide good resistance to gravel chipping for a vehicle body, some areas of the vehicle are particularly prone to gravel chipping. These areas include the A pillars (pillars on either side of the windshield), the front edge of the roof, the leading edge of the hood, and rocker panels. In these areas, it is advantageous to provide an additional layer of a chip-resistant primer before the primer that is applied to the rest of the vehicle body to obtain increased protection against stone chipping. In general, primer compositions applied for this purpose are solventborne, thermosetting compositions. While these chip-resistant layers have worked well with solventborne primer compositions, there remains a need for a chip-resistant primer composition compatible with aqueous primer compositions. Further improvements in chip resistance of the primer are also necessary.
It would be desirable, therefore, to have a composite primer coating that includes an upper layer of an aqueous body primer composition that provides improved resistance to stone chipping and other properties that are important for an automotive primer and an under layer of a chip-resistant primer layer, compatible with the upper primer layer, particularly for wet-on-wet applications of the upper primer layer over the chip resistant primer layer, that provides additional chip resistance in particular areas of the vehicle body. In addition, for environmental and regulatory considerations, it would be desirable to produce both the upper primer layer and the lower layer of chip resistant primer from compositions having a very low content of volatile organic solvent.
SUMMARY OF THE INVENTION
The present invention provides a method of applying a composite coating to an automotive vehicle. In the method, a layer of a chip resistant primer composition is applied to at least one area of the vehicle and the applied primer composition forms a chip resistant primer layer. The chip resistant primer composition includes as the resinous portion a polyurethane polymer having a glass transition temperature of 0° C. or less and, optionally, a second component that has reactive functionality. Then, a thermosetting primer composition is applied to the vehicle.
The reactive functionality is reactive with either the polyurethane polymer of the chip resistant primer composition or with one of the components of the thermosetting primer composition. The thermosetting primer composition includes a polyurethane polymer, an acrylic polymer, and a crosslinking component that is reactive with at least one of the polyurethane polymer and the acrylic polymer. The polyurethane polymer has a glass transition temperature of 0° C. or less. The acrylic polymer has a glass transition temperature that is at least about 20° C. higher than the glass transition temperature of polyurethane resin. The polyurethane polymer of both primers and acrylic polymer are preferably dispersed or emulsified in an aqueous medium. As used herein, “emulsion” or “dispersion” will each be used to refer both to dispersions and emulsions.
The invention further provides a composite coating having a first layer of a chip resistant primer, a second primer layer over the first layer of chip resistant primer, and a topcoat layer over the second primer layer. The first layer of chip resistant primer is formed from a composition including as the resinous portion a polyurethane polymer having a glass transition temperature of 0° C. or less and, optionally, a second component that has reactive functionality. The reactive functionality is reactive with either the polyurethane polymer of the chip resistant primer composition or with one of the components of the primer composition forming the second primer layer. The second primer layer is the product of a primer composition including a polyurethane polymer has a glass transition temperature of 0° C. or less, an acrylic polymer has a glass transition temperature that is at least about 20° C. higher than the glass transition temperature of polyurethane resin, and a crosslinking component.
DETAILED DESCRIPTION OF THE INVENTION
A layer of the chip resistant primer composition is applied to at least one area of the vehicle. In a preferred embodiment, the chip resistant primer
Gilbert John
McNeil Rock S.
BASF Corporation
Budde Anna M.
Cameron Erma
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