Electrostatic powder coating method

Abstract

An electrostatic powder coating method of forming an undercoating film having a volume specific resistivity of not more than 10.sup.13 .OMEGA..multidot.cm and a thickness of not more than 200 .mu.m on a metal surface and forming an overcoating layer on the undercoating film by electrostatic powder coating. The undercoating film is composed of a plurality of layers with only the top layer containing a conductive material.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of electrostatically powder-coating an undercoating film which is formed on a metal surface, and more particularly, it relates to an electrostatic powder coating method which can improve the amount of transfer by electrostatic powder coating.

2. Description of the Background Art

In general, electrostatic coating of a metal surface is generally carried out by spraying a solvent type coating. While the metal surface may be directly electrostatically coated, an undercoating film is generally formed on the metal surface so that an overcoating layer is formed on this film by electrostatic coating, in order to improve corrosion resistance, smoothness and the like.

However, employment of such a solvent type coating is undesirable in consideration of sanitation and environmental conservation, due to volatilization of the solvent. To this end, electrostatic powder coating by electrostatically coating a target with a powder coating has been studied.

When an undercoating film which is formed on a metal surface is electrostatically coated with a powder coating, however, the transfer efficiency is extremely reduced as compared with a case of electrostatically coating with a solvent type coating. The transfer efficiency, which is not much reduced when the metal surface is directly coated with the powder coating, is extremely reduced when an undercoating film is formed on the metal surface and electrostatically coated with the powder coating.

It is also known from U.S. Pat. No. 3,832,226 to Kondo, et al. to apply a conductive base primer layer onto a metal substrate prior to applying an electrostatic powder coating. The primer layer is applied as a solution of a resin containing electroconductive particles which includes powder of, for example, aluminum and carbon black. The dried primer layer has a thickness of preferably from 10-100 microns and a volume resistivity of 10.sup.9 -10.sup.14 ohm-cm. The Kondo, et al. patent does not disclose the use of multiple layers as making up the conductive base primer.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrostatic powder coating method of electrostatically coating an undercoating film formed on a metal surface, which can improve the transfer efficiency.

The electrostatic powder coating method according to the present invention comprises the steps of forming an undercoating film having a volume specific resistivity of not more than 10.sup.13 .OMEGA..multidot.cm. and a thickness of not more than 200 .mu.m on a metal surface, and forming an overcoating layer on the undercoating film by electrostatic powder coating.

According to the present invention, it is not necessary to drastically reduce the volume specific resistivity of the undercoating film, which is not more than 10.sup.13 .OMEGA..multidot.cm. In consideration of economy and practicality, the volume specific resistivity is preferably in the range of 10.sup.8 to 10.sup.13 .OMEGA..multidot.cm. It is possible to reduce the volume specific resistivity of the undercoating film to not more than 10.sup.13 .OMEGA..multidot.cm by a method of introducing conductive particles into the undercoating film. The conductive particles may be prepared from carbon black or a conductive metal oxide such as zinc oxide or tin oxide.

The undercoating film may alternatively contain an organic conductive material such as a cationic, anionic or nonionic surface active agent which is known as an antistatic agent, for example.

When the undercoating film is formed by a water-borne coating, the coating may be dried to leave moisture in the undercoating film, thereby providing the film with conductivity.

If the thickness of the undercoating film exceeds 200 .mu.m, improvement of the transfer efficiency is insufficient due to insufficient conductivity. The undercoating film is preferably formed of a plurality of layers, with only a top layer being provided with conductivity. Preferably, the base layer and intermediate layer in the undercoating film are nonconductive and do not contain a conductive material.

According to the present invention, the undercoating film has a volume specific resistivity of not more than 10.sup.13 .OMEGA..multidot.cm and a thickness of not more than 200 .mu.m. Therefore, charges of the powder coating adhering to the undercoating film are not stored in but discharged through this film. Thus, no back ionization is caused by charges of the same polarity which are stored in the undercoating film, and it is possible to stick a charged powder coating onto the undercoating film in an excellent state, thereby improving the transfer efficiency.

Japanese Patent Laid-Open Nos. 58-64164 (1983), 61-74682 (1986) and 3-80966 (1991) disclose methods of forming conductive coating films on nonconductive materials such as plastic through conductive primers or the like and electrostatically coating the films. However, every one of these conductive coating films is formed as an earth electrode which is employed for electrostatically coating the nonconductive material such as plastic, and must have a considerably low volume specific resistivity.

On the other hand, it is not necessary to drastically reduce the volume specific resistivity of the undercoating film which is employed in the present invention, since its surface potential is only slightly increased due to the presence of the metal surface provided under the same.

It is possible to remarkably improve the transfer efficiency in electrostatic powder coating by reducing the volume specific resistivity of the undercoating film to not more than 10.sup.13 .OMEGA..multidot.cm according to the present invention.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an undercoating film and an overcoating layer exemplary of the present invention; and

FIG. 2 is a block diagram for illustrating a method of measuring a volume specific resistivity exemplary of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is now described with reference to Examples.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

Steel plates were coated with undercoats A and B according to Example 1 and comparative example 1, respectively, and the undercoating films as formed were coated with a polyester-based powder coating (by Nippon Paint Co., Ltd.), respectively, to be subjected to comparison of transfer efficiencies.

Solid contents in the undercoats A and B were adjusted as shown in Table 1.

TABLE 1______________________________________ Undercoat A Undercoat B______________________________________Polyester Resin 41.0 41.0Melamine Resin 24.1 24.1Titanium Oxide 9.2 24.1Carbon Black 12.0 0.1Barium Sulfate 13.7 13.7Total 100.0 100.0______________________________________

The undercoats A and B contained polyester resin (by Nippon Paint Co., Ltd.) and curing agents of melamine resin (by Nippon Paint Co., Ltd.).

Table 1 shows the results of the transfer efficiencies as measured in Example 1 and comparative example 1, employing the undercoats A and B, respectively.

TABLE 2______________________________________ Volume Specific Rela- Resis- tive tivity Trans- Baking Film in fer Tempera- Baking thick Powder Effi- ture Time ness Coating ciency (.degree.C.) (min.) (.mu.m) (.OMEGA. .multidot. cm) (%)______________________________________Example 1 Under 140 20 50 2.3 .times. 10.sup.9 100 Coat AComparative Under 140 20 50 3.2 .times. 10.sup.15 78Example 1 Coat B______________________________________

As understood from Table 2, the undercoating film according to Example 1, having a volume specific resistivity of not more than 10.sup.13 .OMEGA..multidot.cm, exhibited an excellent transfer efficiency which was similar to that in the case of directly powder-coating a steel plate.

EXAMPLES 2 TO 4 AND COMPARATIVE EXAMPLES 2 AND 3

As shown in FIG. 1, a steel plate 1 was coated with an undercoating film 5 consisting of an electrodeposition base layer 2, an intermediate layer 3, and a top layer 4, and this undercoating film 5 was powder-coated with an overcoating layer 6, for evaluation of the transfer efficiency of the undercoating film 5.

The electrodeposition layer 2 was prepared from a cationic electrodeposition coating (Powertop U-80 (trade name) by Nippon Paint Co., Ltd.) and the intermediate layer 3 was prepared from a polyester intermediate coating (Orga P-2 Gray (trade name) by Nippon Paint Co., Ltd.), and these layers 2 and 3 were coated on the steel plate 1 under baking conditions shown in Table 3, and to have the thicknesses shown therein.

TABLE 3______________________________________ Baking Baking Film Temperature Time Thickness (.degree.C.) (min.) (.mu.m)______________________________________Electodeposition/Intermediate Coating Conditions forExamples 2 to 4 and Comparative Example 2Electrodeposition Layer 170 20 30Intermediate Layer 140 20 40Electodeposition/Intermediate Coating Conditions forComparative Example 3Electrodeposition Layer 170 20 30Intermediate Layer 140 40 180______________________________________

Top coats A (Example) and B (Comparative example) were prepared as shown in Table 4, and coated on intermediate layers which were prepared in the aforementioned manner. The top coats A and B contained resin Nos. 1 and 2 which were prepared from acrylic resin (by Nippon Paint Co., Ltd.), curing agents (Cymel 303 (trade name) by Mitsui Toatsu Chemicals, Ltd.), and aluminum powder (Alupaste 7160N (trade name) by Toyo Aluminum K.K.), respectively. The amounts of the resin Nos. 1 and 2 shown in Table 4 are those of solids.

TABLE 4______________________________________ (parts by weight) Top Coat A Top Coat B______________________________________Resin No. 1 38 60Resin No. 2 29 40Curing Agent 16 40Carbon Black 12 --Aluminum Powder 0.1 16.1Barium Sulfate 6 --Total 101.1 156.1______________________________________

Top coating layers were formed under the baking conditions and in the thicknesses shown in Table 3, to prepare undercoating films according to Examples 2 to 4 and comparative examples 2 and 3.

The thickness of each undercoating film was measured with a film thickness gauge to obtain the volume specific resistivity Rv by the following equation:

Rv=E.multidot.S/i.multidot.t where E represents 100 V, S represents the area of an electrode 10, i represents the current quantity, and t represents the thickness of the undercoating film.

The undercoating films of Examples 2 to 4 and comparative examples 2 and 3 were powder-coated with an acrylic powder coating (by Nippon Paint Co., Ltd.), respectively, for evaluation of transfer efficiencies. Each of the transfer efficiencies was relatively evaluated with reference to a transfer quantity (100%) in a case of directly powder-coating the steel plate (substrate). Table 5 shows the volume specific resistivities and the transfer efficiencies of the undercoating films according to Examples 2 and 4 and comparative examples 2 and 3.

TABLE 5__________________________________________________________________________Top Coating Layer Electrodeposition/ Test Result Baking Film Intermediate/ Volume Relative Tempera- Baking Thick- Base Coating Specific Transfer ture Time ness Film Thickness Resistivity EfficiencyType (.degree.C.) (min.) (.mu.m) (.mu.m) (.OMEGA. .multidot. cm) (%)__________________________________________________________________________Example 2 A 140 20 15 85 1.9 .times. 10.sup.10 100Example 3 A 80 3 15 85 8.3 .times. 10.sup.12 99Example 4 A 80 3 20 90 5.0 .times. 10.sup.12 100Comparative B 140 20 15 85 8.7 .times. 10.sup.15 70Example 2Comparative A 140 20 15 225 7.5 .times. 10.sup.14 78Example 3__________________________________________________________________________

As clearly understood from Table 5, the undercoating films according to Examples 2 to 4, having volume specific resistivities of not more than 10.sup.13 .OMEGA..multidot.cm, exhibited excellent transfer efficiencies, similarly to the case of directly powder-coating the steel plates.

Thus, it is understood that it is possible to extremely improve the transfer efficiency in electrostatic powder coating by reducing the volume specific resistivity of the undercoating film to not more than 10.sup.13 .OMEGA..multidot.cm.

EXAMPLE 5 AND COMPARATIVE EXAMPLES 4 AND 5

A) Preparation of Coatings Electrodeposition Coating A

A cationic electrodeposition coating (POWER TOP U-50 (trade name) by Nippon Paint Co., Ltd.) was employed.

Electrodeposition Coating B

12.0 parts by weight of carbon black was mixed with and dispersed in 88.0 parts by weight of cationic electrodeposition coating (POWER TOP U-50 (trade name) by Nippon Paint Co., Ltd.) by a Disper mixer (2,000 rpm, 5 minutes).

Electrodeposition Coating C

3.6 parts by weight of carbon black was mixed with and dispersed in 96.4 parts by weight of cationic electrodeposition coating (POWER TOP U-50 (trade name) by Nippon Paint Co., Ltd.) by a Disper mixer (2,500 rpm, 5 minutes).

Intermediate Coating D

A polyester intermediate coating (ORGA SELECT P-2 (trade name) by Nippon Paint Co., Ltd.) was employed.

Intermediate Coating E

4.8 parts by weight of carbon black was mixed with and dispersed in 95.2 parts by weight of polyester intermediate coating (ORGA SELECT P-2 (trade name) by Nippon Paint Co., Ltd.) by a Disper mixer (2,500 rpm, 5 minutes).

Base Coating F

A base coating F was prepared in accordance with the following formulation.

______________________________________Polyester Resin 54 parts by weightMelamine Resin 26 parts by weightTitanium Oxide 16 parts by weightBarium Sulfate 4 parts by weightTotal 100 parts by weight______________________________________ Base Coating G

A base coating G was prepared in accordance with the following formulation.

______________________________________Polyester resin 47.5 parts by weightMelamine Resin 23.0 parts by weightTitanium Oxide 14.1 parts by weightBarium Sulfate 3.5 parts by weightCarbon Black 12.0 parts by weightTotal 100.1 parts by weight______________________________________ Base Coating H

A base coating H was prepared in accordance with the following formulation.

______________________________________Polyester resin 52.1 parts by weightMelamine Resin 25.1 parts by weightTitanium Oxide 15.4 parts by weightBarium Sulfate 3.9 parts by weightCarbon Black 3.6 parts by weightTotal 100.1 parts by weight______________________________________ B) Preparation of Coated Plate in Example 5 1. Electrodeposition coating A was applied to a phosphate-treated steel plate (170 mm.times.70 mm) of 0.8 mm in thickness and baked at 170.degree. C. for 20 minutes to be a coating thickness of 22 .mu.m. 2. Intermediate coating D was applied to the coated plate obtained in the above step 1 hour and baked at 140.degree. C. for 20 minutes to be a coating thickness of 28 .mu.m. 3. Base coating G was applied to the coated plate obtained in the above step 2 and baked at 140.degree. C. for 20 minutes to be a coating thickness of 22 .mu.m. C) Preparation of Coated Plate In Comparative Example 4 (carbon black is contained only in electrodeposition layer directly on steel plate). 1. Electrodeposition coating B was applied to a phosphate-treated steel plate (170 mm.times.70 mm) of 0.8 mm in thickness and baked at 170.degree. C. for 20 minutes to be a coating thickness of 21 .mu.m. 2. Intermediate coating D was applied to on the coated plate obtained in the above step 1, and baked at 140.degree. C. for 20 minutes to be a thickness of 28 .mu.m. 3. Base coating F was applied to the coated plate obtained in the above step 2 and baked at 140.degree. C. for 20 minutes to be a thickness of 21 .mu.m. D) Preparation of Coated Plate in Comparative Example 5 (carbon black contained in each layer of electrodeposition coating, intermediate coating, and base coating layers in amounts corresponding to their thickness so that a total amount of carbon black in these layers is equal to that in Example 5 above). 1. Electrodeposition coating C was applied to a phosphate-treated steel plate (170 mm.times.70 mm) of 0.8 mm in thickness and baked at 170.degree. C. for 20 minutes to be a thickness of 22 .mu.m. 2. Intermediate coating E was applied to the coated plate obtained in the above step 1 and baked at 140.degree. C. for 20 minutes to be a thickness of 28 .mu.m. 3. Base coated H was applied to the coated plate obtained in the above step 2 and baked at 140.degree. C. for 20 minutes to be a thickness of 22 .mu.m. E) Evaluation Method Volume Specific Resistivity

Volume specific resistivity of each sample was evaluated by UNIVERSAL ELECTROMETER MMA II-17 (by Kawaguchi Denki Co., Ltd.) having concentric parallel plate electrodes.

Relative Transfer Efficiency

Relative transfer efficiency of each sample was evaluated by powder-coating with an acrylic powder coating (by Nippon Paint Co., Ltd.) employing a corona-electrical-charging type of electrostatic spraying machine.

The results of these measurements are set forth in Table 6 below.

TABLE 6__________________________________________________________________________Film Thickness (.mu.m) Volume Weight of RelativeElectrode Intermedi- Specific Transferred Transferposition ate Top Total Resistivity Powder EfficiencyLayer Layer Layer Thickness (.OMEGA. .multidot. cm) (g) (%)__________________________________________________________________________Inventive 22 28 21 71 1.65 .times. 10.sup.12 1.23 100Example 5Comparative 21 28 21 70 1.1 .times. 10.sup.15 0.85 69Example 4Comparative 22 28 21 71 4.2 .times. 10.sup.15 0.86 70Example 5__________________________________________________________________________

As seen from the above Table 6, the Inventive Example 5 in accordance with the present invention exhibited lower volume specific resistivity and excellent transfer efficiency compared to the samples prepared in Comparative Examples 4 and 5.

While the above examples have been described with reference to a single-layer undercoating film and three-layer films consisting of electrodeposition, intermediate, and top coating layers with only the top layer containing conductive material, the undercoating film according to the present invention is not restricted to such structures, but may have any structure so long as the same can serve as a substrate for electrostatic powder coating.

The volume specific resistivity of each undercoating film obtained in the aforementioned examples and comparative example was measured by the method as described herein. As shown in FIG. 2, an electrode 10 of conductive rubber having a diameter of 50 mm was placed on an undercoating film 5 as shown in FIG. 2 and a ring-shaped guard electrode 11 was placed around the electrode 10 while another electrode 12 was provided under the undercoating film 5, and the volume specific resistivity was measured with application of a voltage of 100 v.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims

1. An electrostatic powder coating method comprising the steps of:

forming an undercoating film having a volume specific resistivity of not more than about 10.sup.13 .OMEGA..multidot.cm and a thickness of not more than 200 .mu.m on a metal substrate, said undercoating film being formed of a plurality of layers wherein only a top layer of said plurality of layers includes a conductive material, the layers of said undercoating film between said metal substrate and said top layer being nonconductive; and

forming an overcoating layer on said top layer of said undercoating film by electrostatic powder coating.

2. The electrostatic powder coating method in accordance with claim 1, wherein said volume specific resistivity of said undercoating film is from about 10.sup.8 to 10.sup.13 .OMEGA..multidot.cm.

3. The electrostatic powder coating method in accordance with claim 1, wherein said conductive material is carbon black.

4. The electrostatic powder coating method in accordance with claim 1, wherein said conductive material is zinc oxide or tin oxide.

5. The electrostatic powder coating method in accordance with claim 1, wherein said undercoating film is a multilayer coating film consisting of an electrodeposition base layer on the metal substrate, an intermediate layer on the electrodeposition base layer, and the top layer on the intermediate layer, with only said top layer containing a conductive material.

6. The electrostatic powder coating method in accordance with claim 5, wherein said conductive material is carbon black.

7. The electrostatic powder coating method in accordance with claim 5, wherein said conductive material is zinc oxide or tin oxide.

8. The electrostatic powder coating method in accordance with claim 5, wherein said base layer of the undercoating film is a cationic electrodeposition coating.

9. The electrostatic powder coating method in accordance with claim 5, wherein said intermediate layer and top layer of the undercoating film comprise a synthetic resin.

10. The electrostatic powder coating method of claim 5, wherein said intermediate layer comprises a polyester resin.

11. The electrostatic powder coating method of claim 5, wherein said top layer comprises a polyester resin and melamine resin.

12. A metal substrate coated according to the method of claim 1.

13. A metal substrate coated according to the method of claim 5.