GB2327894A - Method of multi-layer coating formation - Google Patents

Abstract

The present invention can provide a method for forming a composite coating on a substrate, which comprises: i) forming successively a primer coating and an intermediate coating on a substrate, ii) forming a color base coating on the intermediate coating, and iii) coating a color clear powder paint on the color base coating to form a color clear coating. The primer coating is preferably a cation electrodeposition paint. The colour base coating and colour clear powder paint coating are cured in a single step.

Description

METHOD FOR FORMING COMPOSITE COATING ON A SUBSTRATE FIELD OF THE INVENTION The present invention is related to a method for forming a composite coating on a substrate, such as an automobile body or automobile part. Particularly, it relates to a method for forming a composite coating, which does not necessitate repairing coating after removing surface defects caused by dust and the like.

BACKGROUND OF THE INVENTION A method for coating automobile bodies or parts generally comprises: (1) successively forming a primer coating and an intermediate coating on a metal substrate, (2) forming thereon a color base coating which has high hiding power properties of the color of the underlayers; (3) applying a clear top paint on the color base coating; and then (4) curing the coatings to form a composite coating.

In this method, the coating formed from the clear top paint is generally colorless and transparent.

In order to improve the appearance of the coated artide, Japanese Kokai Publication Hei 8 (1996)-134385 proposes that a small amount of pigment is formulated into the clear paint When the color clear paint is employed, the color of the clear top coating is compatible with the color of the color base coating to give the coated article with high chromatic color.

On the other hand, when forming a composite coating by the process mentioned above, dust or another object may be attached to or placed on the surface of coatings during the process for forming a composite coating, which creates coating defect. The defect can generally be removed by polishing the surface of the coatings. However, as the clear top coating of the composite coating usually has a thickness as thin as up to 40 m, when the clear top coating is polished, the polished portion usually changes its color so that the hue of the polished portion may decrease as compared with that of the unpolished portion. In order to unify the color throughout the coated article, recoating must be required for repairing the polished portion. The repair coating is difficult in color adjustment between the polished portion and the unpolished portion and burdensome in view of coating operation.

SUMMARY OF THE INVENTION Thus, the object of the present invention is to eliminate the disadvantage and to provide a method for forming a composite coating having a high chromatic color on the metal surface without recoating the composite coating after polishing.

That is, the present invention provides a method for forming a composite coating on a substrate, which comprises: i) forming successively a primer coating and an intermediate coating on a substrate, ii) forming a color base coating on the intermediate coating, and iii) coating a color clear powder paint on the color base coating to form a color clear coating.

In the present invention, the color clear powder paint contains a color pigment in an amount of 0.05 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, based on 100 parts by weight of the total solid content of the color clear powder paint. The color clear coating formed from the color clear powder paint may preferably have a thickness of 40 to 100 clam, more preferably 50 to 80 rm.

The present invention further provides an article having the composite coating formed by the method as above mentioned.

The composite coating of the present invention has high chromatic color, because, although the color base coating contains considerable amount of color pigment, the color clear powder coating additionally contains color pigment in a small amount. In addition, the color clear top coating formed by the method of the present invention has enough thickness not to need another clear coating for adjusting the color difference between the polished portion and the unpolished portion after eliminating the dust by polishing. According to the method of the present invention, the process for recoating the film is not necessary and number of steps for forming a composite coating reduces so that its production efficiency can be remarkably improved.

BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 schematically shows a cross sectional view of the composite coating formed onto a substrate (represented by (1)), such as a dall steel plate, by the method according to the present invention. In Figure 1, the composite coating (10) comprises coatings, which preferably consisted of a primer (2) (such as a cation electrodeposition coating), an intermediate coating (3), a color base coating (4), and a color clear coating (5).

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail as follows.

The substrate used in the present invention includes a metal plate which is generally employed for assembling a body of automobile including a motorcycle and a bus, etc. Examples of the metals are iron, steel, aluminum, tin, zinc, and the like; or an alloy thereof. The metal plate may also be one which is metal-plated or metal-deposited. The substrate may be defatted or chemically treated with phosphoric acid or chromic acid, etc., before use.

The substrate is generally coated with a primer and then coated with an intermediate paint, for imparting weather resistance and flatness.

Preferably, in view of corrosion resistance, the primer coating can be formed by cation electrocoating technique. On the other hand, the intermediate coating may be formed from a conventional intermediate paint which is colored with dark-gray, gray, white or a colored intermediate paint and has weather resistance.

A blackout coating for sash-window, if necessary, can be formed on the surface of the substrate after forming the primer coating and intermediate coating.

In the step (ii) of the present invention, the color base coating is formed on the intermediate-coated substrate by using a color base paint.

The color base paint may be any one which has been generally used in the art including a solution type paint (such as a solvent-borne paint and a water borne paint) and a powder paint. The solvent borne paint and water bome paint is more preferred in view of coating operability.

The solution type color base paint contains a thermosetting resin and a curing agent, which are both dissolved or dispersed in water or organic solvent as film forming component. Examples of the suitable thermosetting resins are thermosetting acrylic resins, polyester resins, epoxy resins and urethane resins. Preferred are thermosetting acrylic resins and polyester resins in view of weather resistance. Examples of the suitable curing agents are amino resins (especially melamine resins), blocked isocyanate, and the like. Preferred is alkoxymethyl melamine resin.

In the present invention, a combination of the thermosetting resin and the curing agent which are suitable for the solution type color base paint may be a combination of thermosetting acrylic resins and alkoxymethyl melamine resin.

In the color base paint used in the method of the present invention, a weight ratio of the thermosetting resins: the curing agent resin may preferably be within the range of 90:10 to 40: 60, preferably 80 : 20 to 60: 40. When the content of the curing agent is less than the above lower limit, the crosslinking density of the color base coating decreases and, therefore, the hardness of the coating is low. When the content of the curing agent is above the upper limit, the resulting coating has high internal stress and, therefore, defaults such as crack, etc., may occur.

The solution type color base paint is required to contain a pigment.

Examples of the suitable pigments are a color pigment, such as titanium oxide, carbon black, phthalocyanine blue, phthalocyanine green, carbazole violet, anthrapyridine, azo-orange, azo-yellow, flavanthrone yellow, isoindoline yellow, indanthrone blue, dibromo anthanthrone red, perylene red, azo-red, anthraquinone red, quinacridone red, quinacridone violet, and the like; an extender pigment, such as baryta powder, precipitated barium sulfate, barium carbonate, gypsum, clay, silica, white carbon, diatomaceous earth, talc, magnesium carbonate, alumina white, gloss white, satijn white; a glitter pigment, such as mica, aluminum, stainless, iron, copper, nickel, tin, graphite, flake colored pigment and the like. The above mentioned pigment may be employed alone or in combination.

In addition to the pigment, coloring dye may be formulated into the solution type color base paint if necessary. Preferably, dye having weatherability and good solubility in water and/or organic solvent, and the like, can be used. Specific examples of dye are 1: 2 chromium complex black, 1: 2 chromium complex yellow, 1: 2 cobalt complex yellow. Dye may be employed alone or in combination.

An amount of the pigment and, if necessary, dye of the solution type color base paint may be within the range of 1 to 50 parts by weight, based on 100 parts by weight of the total solid content of the color base paint.

The solution type color base paint may also contain additives known in the art, for example, hardening catalyst, such as dodecylbenzene sulfonic acid, etc.; surface modifier, such as silicone or an acrylic oligomer, etc., typically dimethylsilicone, methyl silicone, etc; antioxidant, such as benzophenone, etc; plasticizer; ultraviolet absorbent; benzoins; anti-sagging agent; thixotropic agent; and the like.

A typical example of the solvent borne color base paint for use in the present invention is SUPER-LAC,M-155 commercially available from NIPPON PAINT Co., LTD, which is a solvent borne color base paint containing acrylic resin, or the like.

Water borne color base paint is also commercially available, for example SUPER-NICKS NWB-230 available from NIPPON PAINT Co., LTD, which is a water borne color base paint containing acrylic resin; and the like.

The color base paint can be diluted with a suitable diluent, such as organic solvent or water, to a suitable viscosity for coating, for example, 10 to 30 second/No.4 Ford cup at 20 "C before use.

The above color base paint can be applied by art-known methods, such as spray coating, dipping and so on.

After coating, the color base coating may be subjected to keep standing for 1 to 10 minutes or drying, for example at 60 to 120 "C for 1 to 10 minutes. It may be subjected to curing at a temperature of 120 to 200 OC for 5 to 60 minutes.

The resulting color base coating may typically have a thickness of 10 to 50 zm, preferably 13 to 25 clam, when dried.

In the step (iii) of the present invention, the color clear powder paint is applied on the color base coating.

The color clear powder paint employed in the present invention may contain a combination of a thermosetting resin and a curing agent as a film forming component. The coating composition may further contain a pigment, a surface modifier and other additives.

The thermosetting resin suitably used for the present invention can be one which is solid in room temperature, for example acrylic resins, polyester resins including an alkyd resin, and the like. The thermosetting resin, if desired, may be employed alone or in combination.

The acrylic resin can be one which is obtained by polymerizing an acrylic monomer, such as (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, tert-butyl (meth)acrylate, glycidyl acrylate, glycidyl methacrylate, or 2-methylglycidyl methacrylate, etc., in a conventional technique. In this context, the term "(meth)acrylic" or "(meth)acrylate" includes both acrylic and methacrylic or acrylate and methacrylate. In the present invention, the acrylic resin also includes a copolymer obtained by copolymerizing the acrylic monomer with styrene.

Examples of polyester resins are one which is obtained by reacting a polyhydric alcohol (e.g. ethyleneglycol, propanediol, pentanediol, hexanediol, neopentylglycol, trimethylolpropane, pentaerythritol, etc.) with a carboxylic acid (e.g. terephthalic acid, isophthalic acid, phthalic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, -oxypropionic acid, etc.) in a conventional method; or an alkyd resin.

The thermosetting resin preferably has a glass transition temperature (Tg) of 25 to 80 "C, more preferably 40 to 70 "C. When the Tg is lower than 25 "C, it is difficult to prepare the clear color powder paint because the resin particles melts to adhere to a wall of a grinder by an exothermic heat generated when grinding the components. When Tg is above 80 "C, it is difficult to achieve flatness of the resulting composite coating.

A curing agent used in the color clear powder paint of the present invention can be selected by the type of functional groups contained in the thermosetting resin. Specific examples of the curing agents are blocked isocyanate; aliphatic polycarboxylic acid such as sebacic acid, etc.; aminoplast resins; aliphatic acid anhydride; triglycidyl isocyanate; amine curing agent; dicyandiamide; phenol resins; epoxy resin; alkylmethyl melamine resins; or benzoguanamine resins, etc.

In the combination of the acrylic resin or the polyester resin as the thermosetting resin with the melamine resin as the curing agent, a weight ratio of the thermosetting resin: the curing agent is preferably within the range of 9:1 to 5: 5, more preferably 8: 2 to 6: 4. When the content of the thermosetting resin is less than the lower limit, the resulting coating does not has enough strength. On the other hand, when the content of the curing agent is less than the lower limit, the resulting coating shows defects, such as crack etc. If the film-forming component is a combination of the thermosetting resin having hydroxyl group with polyisocyanate as the curing agent, a equivalent ratio of the thermosetting resin to polyisocyanate may preferably be 0.3:1 to 1.5: 1, more preferably 0.7:1 to 1.5: 1. When the content of the thermosetting resin is above the upper limit, water resistance is poor. When the content of the curing agent is less than the lower limit, the resulting paint can not be cured sufficiently.

The color clear powder paint preferably contains color pigment or extender pigment. Examples of the color pigments are titanium oxide, carbon black, phthalocyanine blue, phthalocyanine green, carbazole violet, anthrapyridine, azo-orange, azo-yellow, flavanthrone yellow, isoindoline yellow, indanthrone blue, dibromo anthanthrone red, perylene red, azo-red, anthraquinone red, quinacridone red, quinacridone violet, and the like.

Examples of the extender pigments are baryta powder, precipitated barium sulfate, barium carbonate, gypsum, clay, silica, white carbon, diatomaceous earth, talc, magnesium carbonate, alumina white, gloss white, satijn white and the like. The pigment may be employed alone or in combination.

The color clear powder paint may contain the color pigment or extender pigment in an amount of 0.05 to 5 parts by weight, preferably 0.3 to 3 parts by weight, based on 100 parts by weight of the total solid content of the paint.

The color clear powder paint may also contain additives known in the art, for example, hardening catalyst, such as dodecylbenzene sulfonic acid, etc.; surface modifier, such as silicone or an acrylic oligomer, etc., typically dimethylsilicone, methyl silicone, etc; antioxidant, such as benzophenone, etc; plasticizer; ultraviolet absorbent; benzoins; anti-sagging agent; thixotropic agent; and the like.

The color clear powder paint can be prepared by art-known process. For example, all ingredients are mechanically mixed and then mixed at a molten condition. The mixture is then cooled and ground and classified for adjusting particle size distribution. Preferably, the resulting powder coating may be subjected to further grinding and classifying until a desired average particle size of 5 to 40 lim is achieved.

In the method of the present invention, in order to render the evaporation of the volatile component in the coating composition easier when forming the coating and to improve the appearance of the film, it is preferable that an average particle size of the powder coating composition is controlled to less than 15 clam, and that a standard deviation of the particle size distribution is less than 20 pm. More preferably, for the sake of the inhibition of blocking between the powder particles on storage, it is also preferred that the powder paint particles have, at least on the surface of the particles1 other fine resin particles having an average particle size between 0.001 and 10 A m, a glass transition temperature of 50 to 150 "C and a solubility parameter (SP) of 9 to 15. Examples of the fine resin particles are those which are obtained by spray-drying a emulsion containing resin particles polymerized in a conventional emulsion polymerization technique.

Although the other fine resin particles may preferably be arranged on the surface of the powder paint particles as mentioned above, other fine silica particles can be formulated into the powder paint as an external additive.

The examples of the fine silica particle are HDK H2000, HDK H30 and HDK H20 all commercially available from Hoechst A.G., which are silica particles covered with trimethylsilyl groups; and AEROSIL R972 available from NIHON AEROSIL Co., which is obtained by reacting silanol groups present on the surface of silica particles with dimetyldichlorosilan to provide hydrophobic property therewith. In the present invention, it is desired to employ the hydrophobic fine silica particle.

In the method of the present invention, the color clear powder paint may typically be applied by electrostatic coating, preferably by electrostatic spray coating (such as, corona charge coating or frictional charge coating), etc. If necessary, it may be then subjected to drying at 80 to 120 "C for 5 to 30 minutes to form a uniform powder coating. In order to obtain desired hue and color depth, it is general that the above process, that is, applying the powder paint and drying, may be repeated. The resulting color clear coating preferably has a minimum thickness for hiding the underlayers of at least 200 m.

The color clear clear coating preferably has a dry thickness of 40 to 100 clam, more preferably 50 to 80 clam. When the thickness is lower than 40 clam, it is too thin, and therefore, the recoating is necessary after the dust present on the surface of the powder coating is removed. When the thickness is above 100 Clam, the coating may be sagged.

The composite coating formed by the method of the present invention can be cured at 120 to 200 t for 5 to 60 minutes. The condition for curing can be varied by the type of the paint used for preparing the film.

EXAMPLES The following Examples, Comparative Examples and Production Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the invention. In Examples, Comparative Examples and Production Examples, "part" is based on weight unless otherwise stated.

Production Examples 1 The color clear powder paint was prepared by the following procedures.

Two-liter flask was charged with 63 parts by weight of xylene and heated to 130 "C. To the flask, a mixture of 55 parts of glycidylmethacrylate, 25 parts of styrene, 20 parts of methyl methacrylate and 8 parts of tbutylperoxy-2-ethylhexanoate was added dropwise over 3 hours under an atmosphere of nitrogen gas. After 30 minutes pass from the completion of the addition, 0.5 parts of t-butylperoxy-2-ethylhexanoate was added dropwise to the mixture over 30 minutes. The mixture was aged for 1 hour and then the solvent was evaporated at a pressure of 5 mmHg and at a temperature of 130 "C to obtain a solid resin. It was then kept at the same condition for one hour and the resulting resin had a glass transition temperature of 50 "C and a number average molecular weight of 2,500.

Seventy parts of the resulting binder resin, 0.5 parts of color pigment (IRGADINE DPP Red BO available from CIBA SPECIALTY CHEMICALS Co.), 23.4 parts of 1,10 < lecane dicarboxylic acid (DDA), 1.0 part of benzoin and 0.3 parts of polysiloxane surface modifier (YF-3919 available from TOSHIBA SILICONE Co., Ltd.) were mixed, and then charged into a BUSS-COKNEADER manufactured by BUSS Co. In the Cokneader, the mixture was melted and kneaded. It was ground and classified by a sieve of 150 mesh. It was then classified by an air classification system (DS-2 manufactured by NIPPON PNEUMATIC MFG. CO. LTD.) to obtain a thermosetting color clear powder paint (I). In the paint (I), an equivalent ratio of COOH group to epoxy functional group was 0.75 and the paint had an average particle size of 10 pLm and a standard deviation of particle size distribution of not more than 20 clam. The particle size was determined by a MICROTRACK MK-2 system manufactured by NIKKISO Co, LTD. The thermosetting color clear powder paint (I) had a minimum thickness for hiding underlayers of at least 200 11 m.

Example I A dall steel plate having a size of 0.8 mm x 10 cm x 30 cm, which has been treated with zinc phosphate, was elecrocoated with a cation electrodeposition paint (POWER-TOP V-20 available from NIPPON PAINT Co., LTD.) until a dry film thickness of about 20 pm was obtained. Thereafter, it was cured at 160 "C for 30 minuets to obtain a cation electrodeposition coating. On the cation electrodepostion coating, gray color intermediate paint (ORGA P-2 GRAY available form NIPPON PAINT Co., LTD.) was spray-coated to obtain a dry film having a thickness of about 35 to 40 m and then, cured at 140 "C for 30 minuets. Hereinafter, the resulting coating composed of the cation electrodeposition coating and intermediate coating formed onto the surface of the steel plate was referred to as underlayers.

Onto the underlayers, red color base paint containing acrylic resin (SUPER-LAC M-155 from NIPPON PAINT Co., LTD.) was coated to form a film having a dry thickness of 20 pm. The red color base paint, before coating, was reduced with a diluent of a mixture of ethyl acetatelxylol = 1/1 to a coating viscosity of 14 secondlNo.4 Ford cup at the temperature of 20 t.

The color base coating was cured at 140 t for 20 minutes. The cured coating was covered with the thermosetting color clear powder paint (I) obtained in Production Examples 1 to form a dry film having a thickness of 80 clam. It was then cured at 140 t for 30 minutes to form a composite coating.

Example 2 A composite coating was formed as generally described in Example 1, with the exception that the color base coating was held at 25 t (room temperature) for 8 minutes without drying.

Example 3 A composite coating was formed as generally described in Example 1, with the exception that the color base coating was dried at the temperature of 80 "C for 5 minutes without curing.

Comparative Example 1 A composite coating was formed as generally described in Example 1, with the exception that a solvent-bome color clear paint (SUPER LAC 0-150 from NIPPON PAINT Co., LTD.) having a minimum thickness for hiding underlayers of at least 200 pm was used instead of the thermosetting color clear powder paint (I) obtained in Production Example 1, and that a dry thickness of the clear coating was 40 m. The solvent-bome color clear paint (SUPER-LAC 0-150) contained a color pigment (IRGADINE DPP Red BO) in an amount of PWC of 1%.

The brief constitutions of the composite coatings formed by Examples 1 to 3 and Comparative Examples 1 are shown in Table 1.

Table 1.

Type of Examples | Comparative Coating 1 2 3 Example 1 Color base SUPER-LAC SUPER-LAC SUPER-LAC SUPER-LAC coating M-155 " M-155 ') M-155 ') M-155 1) (thickness) (20 M m) (20 m) (20 m) (20 m) Curing or Cured at Not cured nor Dried at 80 C Cured at drying 140 C for 20 dried 2) for 5 minutes 1409C for 20 condition minutes minutes Color clear Color clear Color clear Color clear Solvent-borne coating powder powder powder color clear (thickness) coating 3) coating 3) coating 3) coating 4) (80 m) (80 m) (80 m) (80 m) (40 A m) Notes in Table 1 represent the following meanings: 1): Available from NIPPON PAINT Co., LTD; a solvent-borne color base paint containing an acrylic resin.

2): It was kept at 25 C (room temperature) for 8 minutes.

3): The thermosetting color clear powder paint obtained in Production Example 1.

4): SUPER-LAC 0-150 available from NIPPON PAINT Co., LTD: a solvent-borne color clear powder paint.

Evaluation method Each top layer of the composite coatings prepared from the above Examples 1 to 3 and Comparative Examples 1, i.e. the color clear coating, was polished by water proof polishing paper and polishing compound, with a certain polishing thickness of 5,10,15 or 20 clam. Then, evaluation was conducted in color difference (A E), appearance of surface and accelerated weather resistance between the polished portion and the unpolished portion as follows.

1) Color difference (A E) Each lightness value for the polished portion and the unpolished portion of the top layers of the composite coatings was determined by using a color difference meter (SM COLOR COMPUTER SM-4 manufactured by SUGA TEST INSTRUMENTS CO. LTD. The difference of the lightness between the polished portion and the unpolished portion is defined as a color difference (aE), wherein the color difference is represented in an absolute value. The results are shown in Table 2.

2) Surface appearance Each surface appearance of the polished and the unpolished portion of the top layers of the composite coatings was observed by visually evaluation by the following criteria. The results are shown in Table 2.

Criteria: O: No difference was observed between the polished and the unpolished portion.

x: The polished portion was distinguished from the unpolished portion 3) Accelerated weather resistance test Each of the polished composite coatings was subjected to an accelerated weathering test using an accelerated weathering system (SUNSHINE-WEATHEROMETER manufactured by SUGA TEST INSTRUMENTS CO. LTD. for 2,000 hours. Each color difference (A E) between the polished and the unpolished portion of the top layers of the coatings was determined as described above (1). The results are shown in Table 2.

Table 2.

Evaluation items Examples Comparative 1 2 3 Example 1 (1) Color difference (AE) Polished 5 pm 0.0 0.0 0.0 0.2 thickness 10 pm 0.1 0.1 0.1 0.4 15 m 0.3 0.3 0.3 0.7 20 m 0.5 0.5 0.5 1.1 (2) Surface appearance Polished 5 pm 0 0 0 O thickness 10 m O 0 0 0 15 m

Claims (8)

1. A method for forming a composite coating on a substrate, comprising: i) forming successively a primer coating and an intermediate coating on a substrate; ii) forming a color base coating on the intermediate coating; and iii) coating a color clear powder paint on the color base coating to form a color clear coating.

2. A method according to claim 1, which further comprises: iv) simultaneously curing the color base coating and the color clear coating to form the composite coating.

3. A method according to claim 2, wherein the color base coating formed in the step (ii) is not cured but dried before curing in the step (iv).

4. A method according to claim 1, wherein the color base coating formed in the step (ii) is subjected to neither curing nor drying, but kept at room temperature for 1 to 10 minutes.

5. A method according to any preceding claim, wherein the color clear powder paint contains a color pigment in an amount of 0.05 to 5 parts by weight based on 100 parts by weight of the total solid content thereof.

6. A method according to any preceding claim, wherein the color clear coating formed in the step (ii) has a thickness of 40 to 100 pm.

7. A method according to claim 1 substantially as described in any one of Examples 1 to 3.

8. An article having a composite coating formed by the method according to any preceding claim.