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  Method of forming multi-layered coating filmUSPTO Application #: 20070023288Title: Method of forming multi-layered coating film Abstract: This invention provides a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) onto cured coating film of a specific electrodeposition paint (A) of low weight loss under heating, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously. (end of abstract) Agent: Wenderoth, Lind & Ponack, L.L.P. - Washington, DC, US Inventors: Eiji Kuwano, Koji Kamikado, Tadayoshi Hiraki USPTO Applicaton #: 20070023288 - Class: 204486000 (USPTO) Related Patent Categories: Chemistry: Electrical And Wave Energy, Non-distilling Bottoms Treatment, Electrophoresis Or Electro-osmosis Processes And Electrolyte Compositions Therefor When Not Provided For Elsewhere, Coating Or Forming Of Object, Plural Coating Operations, Including Nonelectrophoretic Coating The Patent Description & Claims data below is from USPTO Patent Application 20070023288. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] This invention relates to a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint, second coloring paint and clear paint onto cured coating film of an electrodeposition paint, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously. BACKGROUND ART [0002] Coating of metallic shaped articles such as automobile bodies, metallic parts of two-wheeled vehicles, house electric appliances, steel furnitures and the like is normally conducted by the steps of applying an electrodeposition paint, baking the resulting coating film, applying an intermediate coat thereon and baking the same, and then applying a top coat and baking the same to form a multi-layered coating film. However, when baking is performed after application of each of those paints, not only high energy costs are incurred for the baking but also great labor and costs are required for operation and maintenance of the baking facilities. Hence development of coating film-forming method capable of reducing use of volatile organic compounds and achieving energy-saving and step-reduction has been in demand. [0003] For example, JP Sho 61(1986)-141969A disclosed a metallic finishing method comprising applying onto a substrate an organic solvent-based or non-aqueous dispersion type thermosetting paint, a thermosetting, water-based metallic paint and a transparent thermosetting paint, by the order stated, and heating the three-layered coating film to cure the multiple layers simultaneously. This method, however, is subject to a problem that once finishing quality of the electrocoated film which is the undercoat applied before the three layers are formed thereon degrades, that of the multi-layered coating film surface also degrades as influenced thereby. [0004] JP Hei 3(1991)-181369A disclosed a method for forming high quality multi-layered coating film by 3C1B process, comprising applying an intermediate coat, top base coat and top clear coat, by the order stated, onto a laser-treated dull steel plate onto which an undercoat has been applied by cationic electrocoating and subsequently baked, at least one of the three paints being a non-aqueous dispersion type paint; and baking the multi-layered coating film. This method, however, is subject to a problem that costly laser-treated dull steel plate-made shaped articles must be used to secure good finishing quality of perpendicular and horizontal parts, for forming a high quality three-layered coating film by single baking. [0005] Furthermore, JP 2002-273322A disclosed a method for forming multi-layered coating film having high decorative effect and visual appearance, comprising successively applying a first water-borne base paint, second water-borne base paint and clear paint onto an object to be coated and simultaneously heat-curing all the uncured coating films, the first water-borne base paint and/or the second water-borne base paint containing effect pigment and the solid content of uncured first base paint coating film being 40-95% by weight. While this method can provide multi-layered coating film of good appearance and free of foaming or pinholing, there remains a problem that the multi-layered coating film surface becomes uneven, as affected by unevenness on the electrocoated film surface. [0006] JP 2002-282773A disclosed a method of forming laminated coating film, comprising a step of forming an uncured intermediate coat by forming a cationic electrocoated film and applying onto the cured electrocoated film a water-borne intermediate paint; step of forming an uncured base coating film by applying a water-borne base paint onto the uncured intermediate coating film; a step of applying a clear paint on the uncured base coating film; and simultaneously heat-curing those uncured intermediate coat, base coat and clear coat; said cationic electrodeposition paint containing a binder resin which contains cationic epoxy resin and blocked isocyanate curing agent, neutralizing acid, organic solvent and metallic catalyst. However, because the method as described in the Official Patent Gazette uses, as the cationic electrodeposition paint, a paint containing cationic epoxy resin and blocked isocyanate curing agent, undulation or unevenness result on the electrocoated surface due to volatilization of the blocking agent during the baking for curing the electrocoated film, and when the multi-layered coating film is formed on such an electrocoated film surface by 3-coat-1-bake system of the first coloring paint, second coloring paint and clear paint, the finally formed multi-layered coating film shows degraded appearance, as affected by the unevenness of the electrocoated film. DISCLOSURE OF THE INVENTION [0007] The object of the present invention is to provide a method of forming multi-layered coating film excelling in appearance, corrosion resistance, chipping resistance and so on, while coping with environ mental problems by reducing generation of volatile organic compound in coating metallic objects. [0008] We have engaged in concentrative studies to now discover that the above object can be accomplished by successively applying a first coloring paint, second coloring paint and clear paint onto cured coating film surface of specific electrodeposition paint of little weight loss under heating, and curing the three-layered coating film by single baking (which procedures may hereafter be referred to as 3-coat-1-bake or 3C1B) and completed the present invention. [0009] Thus, the invention provides a method of forming multi-layered coating film which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) successively wet-on-wet, onto a cured coating film of an electrodeposition paint (A) showing a heat loss (X) of not more than 5% by weight, said heat loss being calculated according to the following equation. Heat loss (X)=[(Y-Z)/Y].times.100 [0010] [wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film, which is obtained by electrocoating the electrodeposition paint (A), by heating at 105.degree. C. for 3 hours; and [0011] Z is the weight of the cured coating film after heating the dry coating film at 170.degree. C. for 20 minutes]. [0012] According to the method of the present invention, coated articles with little generation of volatile organic compound and exhibiting excellent appearance, corrosion resistance, chipping resistance and so on can be obtained. Still in addition, because the method of the invention adopts 3C1B system, it can contribute to energy saving and space reduction. BRIEF EXPLANATION OF DRAWINGS [0013] FIG. 1 is a model diagram showing frequency characteristics of a power spectrum. [0014] In FIG. 1, 1 shows short wavelength region, 2 shows middle wavelength region and 3, long wavelength region. [0015] FIG. 2 shows frequency characteristics of power spectrum of an electrodeposited coating film according to the present invention. [0016] FIG. 3 shows frequency characteristics of power spectrum of a conventional electrodeposition coating film. [0017] Hereinafter the method of forming multi-layered coating film of the present invention is explained in further details. Substrate [0018] The substrate or object to which the method of the present invention is applicable is subject to no particular limitation so long as it is of an electrocoatable material. As such materials, for example, metals such as stainless steel, iron, steel, copper, zinc, tin, aluminium, alumite and the like; alloys of such metals, sheets which are plated with such metals and laminated sheets of such metals can be named, which can be given surface treatment, primer treatment or the like where necessary, to be imparted with improved corrosion resistance and adherability. For example, stainless steel can be given a chromium surface treatment. As the coating object, automobile bodies are preferred, and the steel sheet to serve as the substrate can be advancedly zinc phosphate-treated as customarily practiced. Electrodeposition Paint (A) [0019] The above-described substrate is electrocoated with said electrodeposition paint (A). In the present invention, as the electrodeposition paint (A), one having a heat loss (X).sup.(note 1) of its electrocoating film not more than 5 wt %, preferably not more than 4 wt %, inter alia, not more than 3.5 wt %, is used. When an electrodeposition paint forming an electrocoated film having a heat loss (X) more than 5 wt % is used, degradation in surface finish of 3C1B multi-layered coating film composed of the electrocoated film, first colored coating film, second colored coating film and clear coating film may take place, which is undesirable. [0020] (Note 1) Heat loss (X) is calculated according to the following equation: Heat loss (X)=[(Y-Z)/Y].times.100 [0021] [wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film, which is obtained by electrocoating the electrodeposition paint (A) by heating at 105.degree. C. for 3 hours; and [0022] Z is the weight of the cured film after heating the dry coating film at 170.degree. C. for 20 minutes]. [0023] Furthermore, the electrodeposition paint (A) to be used in the present invention is preferably such that it forms cured electrocoated film having an average power spectral value (note 2) within a wavelength region of 0.02-1 mm of generally not higher than 70, in particular, 20-49, inter alia, 33-45, and/or an integration value of the power spectral values within the wavelength region of 0.02-1 mm of generally not higher than 1.7.times.10.sup.5, in particular, 5.times.10.sup.4-1.2.times.10.sup.5, inter alia, 8.times.10.sup.4-1.1.times.10.sup.5, said values being obtained by power spectrum frequency analyses which comprise measuring surface roughness of an electrocoated film which has been cured at 170.degree. C. for 20 minutes, over a measuring length of 50 mm at 10 .mu.m intervals with a surface roughness meter and then Fourier transforming the so obtained measurement data. Continue reading... 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