JPS5926352B2 - Electrostatic powder coating method - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for electrostatic powder coating on metal substrates.

Paint finishes that have a metallic appearance, such as metallic finishes, are often used for automobile bodies, metal building materials, and the like.

In order to obtain this kind of paint finish, conventionally, a so-called two-coat method is used, for example, to apply an undercoat with a solvent-based thermosetting acrylic resin metallic base paint, and then apply and bake a solvent-based thermosetting acrylic resin-based transparent paint. A one-bake method has been used. In recent years, as the demand for pollution-free painting operations has increased, metallic finishing techniques have been developed in the field of powder coating, and some of them have reached the stage of practical application. For a metallic finish in powder coating, a base paint containing a metallic foil pigment such as aluminum powder is first applied as an undercoat, the paint film is cured by baking under specified baking conditions, and then a clear paint is applied, followed by further baking. A so-called two-coat, two-bake method was used.

This two-coat, two-bake method cannot be said to be more streamlined in terms of not only painting and baking equipment, but also the painting process and number of man-hours, compared to conventional metallic painting using solvent-based paints. Furthermore, on mass production lines, it is unavoidable that dust and foreign matter adhere to the paint surface during, after, or during painting, which impairs the aesthetics of the finished paint film and poses a major hindrance to the painting process. ing. Establishment of a two-coat one-bake metallic finish electrostatic coating method using a powder coating has been expected for a long time as it can eliminate the above-mentioned defects.

However, since the coating film thickness is usually over 50 microns, it is likely to cause electrostatic repulsion (reverse ionization phenomenon), which may deteriorate the smoothness of the coating surface and cause craters on the coating surface.
Additionally, it is currently difficult to obtain a uniform metallic finish, as the powder paint applied while applying high voltage scatters from the painted surface, making it difficult to form a film, and the technology is basically unresolved. has been done. An object of the present invention is to provide a two-coat one-bake method using an electrostatic coating method in which the pigment in the base coating does not diffuse or transfer to the clear coating film.
The purpose of this is to use resins that are difficult to mix with each other in the binder resin of the base paint and the binder resin of the clear paint, and then electrostatically apply high voltages of opposite polarity to these paints. will be achieved.

That is, in the present invention, a base paint is applied directly onto a metal base material or on a metal base coated with an undercoat, and then a base paint is applied that is incompatible with the binder resin in the base paint or has a light transmittance of 70%. In the powder coating method, the base paint and the clear paint have opposite polarities, respectively, by applying a clear paint with the following compatible resin as a binder and then baking it to simultaneously cure the coating film of both paints. The gist is an electrostatic powder coating method characterized by electrostatic coating by applying a high voltage of negative polarity to the base paint. Painting is done with a spray paint machine (hereinafter simply referred to as a paint machine), and then the clear paint is applied with a paint machine that applies a high voltage of positive polarity, or the base paint is applied with a high voltage of positive polarity. This is an embodiment in which the clear paint is coated with a machine, and then the clear paint is applied with a paint machine that applies a high voltage of negative polarity, and then the coatings of both paints are simultaneously cured by baking.

In other words, this is a new coating method that enables a two-coat, one-bake method for metallic finishing using powder coating. In the present invention, the resin in the clear paint is not compatible with the resin in the base paint, or the light transmittance is 70%.
A resin exhibiting the following compatibility refers to the following.

That is, the resin powders constituting the base paint and clear paint are mixed in equal parts by weight, and then dissolved in an organic solvent or the like.
The Teflon plate was air-sprayed uniformly to a film thickness of about 70 microns, then baked at a constant temperature of 160 to 220°C and for a constant time of 15 to 60 minutes, and left to cool. When measuring the light transmittance of the isolated film, the transmittance for ultraviolet light (wavelength of about 300 millimicrons) and visible light (wavelength of about 500 millimicrons) shows values of 0 to 70%, respectively. It means that. By selecting a base paint and a clear paint that have resins as binders that have compatibility as indicated by a light transmittance within the range of 0 to 70%, the base paint can be applied and then the clear paint can be applied again. When baked under the specified baking conditions, there is no phenomenon in which some of the pigments in the base paint diffuse or transfer to the clear paint film, and high voltages of opposite polarity are applied to the base paint and clear paint. By electrostatic coating by applying , it is possible to form a coating film with a uniform finish and smoothness that is equal to or better than that of the conventional two-coat, two-bake metallic finish.

If the light transmittance of the above-mentioned isolated film exceeds 70% for both ultraviolet and visible light, apply a base paint and clear paint made using these resins. When the base paint is baked under predetermined baking conditions, some of the pigments in the base paint diffuse and transfer to the clear paint film, making it impossible to obtain the desired uniform metallic finish. In the light transmittance of the above-mentioned isolated film, the compatibility between resins is expressed as a value within the range of 0 to 70% for ultraviolet rays and visible light rays, respectively, which means that the isolated film is opaque in appearance. It exhibits a semitransparent state, and this means that when the above-mentioned equal weight mixed resin is melted and hardened by baking, both resins do not fuse with each other, or only partially fuse. do.

In other words, these conditions basically prevent the resin contained in the base paint and some of the pigments enclosed in it from diffusing and migrating into the clear paint film, allowing the desired uniform metallic finish to be formed. It constitutes a requirement for
In addition, combinations of resins that have incompatible properties as shown by light transmittance within the range of 0 to 70% are those that have the same chemical structure, such as combinations of acrylic resins, polyester resins, etc. It may be a combination of resins, or it may be a combination of different chemical structures, such as a combination of a polyester resin and an acrylic resin, or a combination of an epoxy resin and an acrylic resin.

Furthermore, the metallic finish in the present invention includes a paint finish that forms a coating film with a metallic appearance, and a paint finish that forms a coating film that has a non-metallic appearance.

Paint finishes that form coatings with a metallic appearance include the following: In other words, a paint finish in which a continuous layer of fine metal powder or fine metal particles is incorporated in the lower layer of the finish coating (so-called metallic finish), a foil pigment or non-foil pigment, and a patterned pigment in the form of metal flakes or flakes. A paint finish built into the lower layer of the paint film (so-called yellow tone finish), a continuous layer containing a mixture of metallic or non-metallic foil pigments and having a non-metallic appearance. It includes a paint finish that is built into the lower layer of the finish coating, or a paint finish that has a continuous layer mixed with non-metallic fine particulate pigments and has a metallic appearance built into the bottom layer of the finish coating. Also, a paint finish with a non-metallic appearance is one that contains no or a small amount of metallic or non-metallic foil-like pigments and that has a continuous layer based on the non-metallic appearance as the underlying layer of the finish coating. Refers to the built-in paint finish.

For paint finishes with a metallic appearance, the desired metallic appearance can be achieved by containing metallic or non-metallic foil pigments or both foil pigments in the base paint, that is, in the lower layer of the finished coating. The base paint contains only a foil pigment or a mixed composition of a foil pigment and a non-foil pigment, in which the former is blended in a relatively high proportion.

When carrying out the present invention, a metal base material (which may be subjected to a chemical conversion treatment if necessary) is used.

The same applies hereafter) Electrostatic painting is performed by applying a high voltage of negative or positive polarity to the metal substrate coated with top or undercoat paint,
Next, without baking, a clear paint is applied electrostatically by applying a high voltage of the opposite polarity to that of the base paint, and then baking is performed. The base paint and clear paint used in this case are resins that are incompatible with the binder resin in the base paint or have a light transmittance of 70% or less as the binder resin in the clear paint. For the rest, conventionally known formulations and proportions may be used.

Therefore, in the present invention, as the binder resin used for the base paint and the clear paint, any resin that has been conventionally used for this type of metallic finish can be effectively used, such as acrylic resin, polyester resin, etc. , epoxy resin, etc. are used.

When selecting each of these resins, it is sufficient to appropriately select resins in which these resins are difficult to be compatible with each other. The base paint also contains pigments. Various pigments are used in this case, and in terms of shape, not only foil-like pigments but also granular pigments are used. In terms of materials, inorganic pigments and organic pigments are used, and metallic pigments are also used as the inorganic pigments. Specifically, aluminum powder, aluminum foil strips, copper powder, copper foil strips, mica powder, processed mica powder (for example, mica powder with a thin film of titanium oxide welded to it), titanium white, carbon black, and phthalocyanine green. etc. are used. Further, in the clear paint used in the present invention, transparent colorants such as oil-soluble dyes, transparent pigments (eg, phthalocyanine blue), etc. are used as necessary. Various known additives may be added to these base paints and clear paints as necessary. When carrying out the method of the present invention, the X-base paint and the clear paint may be applied directly onto the metal substrate, or an undercoat may be applied to the metal substrate in advance. As the undercoating paint in this case, normal so-called primer paints are used, such as electrodeposition primers such as epoxy ester, polybutadiene, or aminoalkyd, and solvent-based primers such as aminoalkyd, epoxy ester, or phenol alkyd. is used. In order to carry out the coating method according to the present invention using the above base paint and clear paint, a metal base material, optionally coated with an undercoat, is used as the object to be coated, and a counter electrode is connected to this metal base material. and after grounding,
A coating machine that applies the base paint with negative high voltage (for example, Studio JR5O manufactured by Tamsames in France,
Generator 990, polarity: negative, output voltage 30 to 90, etc.), and then apply clear paint with a high voltage of positive polarity, which is the opposite polarity to the base paint (for example, Studio JR5O, etc.). generator 9
60, polarity: positive, output voltage 30 to 60, etc.) and then baking under specified baking conditions, or use a coating machine that applies the base paint with a positive high voltage (e.g. Studio JR5O, Generator). 960, etc.), and then the clear paint is applied with a high voltage of negative polarity, which is the opposite polarity to that of the base paint. Baking is performed under baking conditions to simultaneously cure the base paint and clear paint films.

Regarding the selection of polarity and applied voltage in the above-mentioned electrostatic coating of base paint and clear paint, the presence or absence of electrostatic repulsion (especially the presence or absence of electrostatic repulsion that occurs on the painted surface) is evaluated by observing the paint film after applying both paints. Depending on the coating efficiency and the content and distribution of pigments (particularly metallic pigments) in the applied base paint, the polarity can be either negative-positive or positive-negative. What is necessary is to determine the combination and appropriate applied voltage for each. In this case, the coating thicknesses of the base paint and the clear paint are usually selected to be approximately 15 to 60 microns and 20 to 80 microns, respectively, but are not necessarily limited to these thickness ranges.

Furthermore, baking after applying the base paint and clear paint is usually carried out at 160 to 220°C for about 15 minutes depending on the type and properties of each paint, but it is not necessarily limited to the baking temperature and baking time mentioned above. It is not something that will be done. In the painting method according to the present invention, after baking, the paint film surface is polished using an abrasive such as sandpaper and lapping compound as necessary, thereby modifying the paint film surface without damaging the metallic finish surface in any way. It is possible to improve the aesthetic appearance.

In contrast, in the conventional two-coat, two-bake method, polishing the base paint film causes damage to the metallic finished surface, necessitating further repair painting. In the two-coat, one-bake electrostatic powder coating method according to the present invention, the colorless or colored clear paint is applied onto the base paint film using the base paint film as the formation source. This is to protect the desired metallic appearance and further emphasize and beautify this appearance.

Furthermore, the present invention is applicable not only to paint finishes having a metallic appearance, but also to cases in which the gloss of powder coating films containing pigments is generally increased and the aesthetic appearance thereof is improved. As described above, according to the present invention, in a powder coating method for obtaining a coating film with a uniform metallic finish on a metal substrate, a base paint and a clear paint each containing a binder resin that is difficult to be compatible with each other are used. By applying high voltages of opposite polarity and applying electrostatic coating, the coated powder particles become saturated charged, which prevents the particles from falling off the coated surface due to electrostatic repulsion. A smooth, glossy, and beautiful finished coating surface can be obtained without causing any craters.

Therefore, not only the coating efficiency is improved, but also the repair work on the coated surface becomes extremely easy, contributing to the rationalization of the coating process, and exhibiting the effect of improving coating productivity and economic efficiency. Production Examples, Examples, and Comparative Examples will be shown below to clarify the features of the present invention.

However, in each example below, parts or % indicate parts by weight or % by weight. Production Example After dry blending an acrylic copolymer AlOO part copolymerized with the following monomer composition (weight ratio) and 9 parts of adipic acid using a pen shell mixer (manufactured by Mitsui Miike Seisakusho, FMlOL type), a Busco kneader was used. (P manufactured by Buss, Switzerland)
The mixture was melted and kneaded using a cutter mill (manufactured by Horai Tekkosho), then finely pulverized with a mini-kettle grinding mill (manufactured by Ketsuku Co., Ltd.), and then mixed with a 100-mesh gyro shifter (model R-46). Thermosetting acrylic resin A was prepared by sieving with a sieve (manufactured by Tokuju Kosho).

In addition, an acrylic copolymer BlOO part and adipic acid 9 copolymerized with the following monomer composition by a similar manufacturing method.
Thermosetting acrylic resin B was prepared from the above. Further, 100 parts of a polyester condensate obtained by condensing 47.2 parts of dimethyl terephthalate, 27.2 parts of neopentyl glycol, 10.5 parts of glycerin, and 15.1 parts of isophthalic acid and an adduct BLO65 (manufactured by Fueva, West Germany) ) A thermosetting polyester resin was manufactured from 30 parts by the same manufacturing method as the above thermosetting acrylic resin A.

Thermosetting acrylic resin A, thermosetting acrylic resin B
and a thermosetting polyester resin were mixed in the proportions shown below, and then dissolved with methyl isobutyl ketone to a viscosity suitable for air spraying to prepare resin liquids A-L, resin liquid A-2, and resin liquid B. Resin liquid A-1, A-2 and B
Each was coated uniformly on a Teflon plate with a film thickness of approximately 70 microns using an ordinary air spray gun, and
After baking at ℃ for 30 minutes and letting it cool, the coating was peeled off and the area was approximately 10 x 30 m7! Samples A-1, A-2, and B for measuring light transmittance were prepared by cutting into L size.

The light transmittance of specimens A-1, A-2, and B was measured by the method described below, and the values shown in Table 1 were obtained. Method for measuring light transmittance> Using a Hitachi EPU-2A spectrophotometer, manufactured by Hitachi, Ltd., ultraviolet rays (wavelength approximately 300 millimicrons) and visible light (
The transmittance at a wavelength of approximately 500 millimicrons was measured.

The coating sample was inserted into a cell holder attached to a photometer and measured using an EPU-2A photometer. Example 1 and Comparative Example 1 Aluminum powder 5 was added to the thermosetting acrylic resin AlOO part.
part, hydrophobic silica powder (Erosil R- manufactured by Degussa, Germany)
972, the same applies to the following examples) and 0.5 parts of a coating surface conditioner (ModaFlo, manufactured by Monsanto Chemical Company, the same applies to the following examples), and dry blended using a pen shell mixer. Base paint A was prepared.

In addition, there is no paint surface conditioner in the thermosetting acrylic resin BLOO part.
．． Clear paint A was prepared by adding 5 parts and using the same manufacturing method as base paint A. 300 treated with zinc phosphate treatment (Bonderite 37, manufactured by Nippon Parkerizing Company)
Polybutadiene-based electrodeposition primer (manufactured by Kansai Paint Co., Ltd., Elecron 72
00) to a film thickness of approximately 20 microns,
After baking at 170°C for 30 minutes, the coated surface was lightly sanded with #360 sandpaper.

After connecting this steel plate to the opposite electrode and grounding it, apply base paint A to approximately 40 microns each using a coating machine (Studio JR5O) with the polarity and applied voltage shown in Table 1, and use Generator 990 or Generator 960 depending on the polarity. Ta. The same applies to the clear paint application below), and then electrostatically coated with clear paint to a thickness of about 60 microns using the polarity and applied voltage shown in Table 1, and then heated at 180°C. It was baked for 30 minutes to simultaneously cure both coatings and finish. Table 1 shows the results of an investigation into the condition of these finishing coats.

The coated surfaces in Examples 1a and 1b had no electrostatic repulsion and had good metallic feel, smoothness, and gloss. As a result, by applying a high voltage of positive polarity to the base paint A and painting the clear paint by applying a high voltage of negative polarity, which is the opposite polarity, the state of the metallic finish can be stabilized. It turned out to be a thing. That is, the finish was significantly improved compared to Comparative Examples 1a to 1d. Example 2 and Comparative Example 2 To 100 parts of the thermosetting polyester resin prepared in the production example, 3 parts of cyanine blue ES (manufactured by Sanyo Shiki Kaisha), 5 parts of aluminum powder, 0.1 part of hydrophobic silica powder, and coating surface adjustment were added. 0.5 part of the agent was added thereto, and a paint was prepared according to the manufacturing method of base paint A in Example 1 and Comparative Example 1 to obtain base paint B.

Base paint B and clear paint (same as Example 1 and Comparative Example 1) were electrostatically coated according to the coating method of Example 1 and Comparative Example 1 using the polarity and applied voltage shown in Table 2. It was cured by baking at 180°C for 30 minutes*.

The results of the investigation into the condition of these finishing coatings are summarized in the second section.
Shown in the table.

The coated surfaces in Examples 2a and 2b had no electrostatic repulsion and had good metallic feel, smoothness, and gloss. As a result, by applying a high voltage of negative polarity to the base paint B and applying a high voltage of positive polarity, which is the opposite polarity, to the clear paint, the state of the metallic finish is stabilized. It turned out to be a thing. That is, the finish was significantly improved compared to Comparative Examples 2a to 2d. Example 3 and Comparative Example 3 30 parts of rutile titanium white, 0.1 part of hydrophobic silica, and 0.5 part of a coating surface conditioner were added to the thermosetting acrylic resin AlOO part prepared in the production example, and then melted and kneaded. - It was made into a powder coating by a known manufacturing method that involves the steps of pulverization and classification, and was used as base coating C.

After electrostatically coating base paint C and clear paint (same as in Example 1 and Comparative Example 1) according to the coating method of Example 1 and Comparative Example 1, using the polarity and applied voltage shown in Table 3. It was cured by baking at 180°C for 30 minutes. Table 3 shows the results of an investigation regarding the condition of these finished coatings.

The coated surfaces in Examples 3a and 3b had no electrostatic repulsion and had good smoothness and gloss. By applying a high voltage of negative polarity to the base paint C and painting the clear paint by applying a high voltage of positive polarity, which is the opposite polarity, or applying a positive voltage to the base paint C. It has been found that by applying a positive voltage to the clear paint and applying a negative high voltage to the clear paint, it is possible to stabilize the painted surface and obtain an excellent finish. Comparative Example 4 5 parts of aluminum powder, 0.1 part of hydrophobic silica powder and 0.5 part of coating surface conditioner were added to the thermosetting acrylic resin BlOO part prepared in the manufacturing example, and the same manufacturing method as base paint A was applied. This was made into a paint and used as base paint D.

Pace paint D and clear paint (same as in Example 1 and Comparative Example 1) were applied to base paint D at positive polarity +60 to V according to the coating method of Example 1 and Comparative Example 1, and clear paint After electrostatic coating was applied by applying a negative polarity of -90 V, it was baked at 180° C. for 30 minutes to harden it.

Table 4 shows the results of an investigation regarding the condition of this coating film.

Comparative Example 5 Base paint D (Comparative Example 4) was applied to a 300 x 100 x 0.8 mm dull steel plate coated with the same electrodeposited primer as in Example 1 and Comparative Example 1 using a regular coating machine (manufactured by Gema, Switzerland). G
After applying electrostatic coating to a film thickness of approximately 40 microns using EMA72L and baking at 180°C for 15 minutes, clear paint (same as in Example 1 and Comparative Example 1) was applied to a film thickness of approximately 60 microns. Apply electrostatic coating in the same way so that
It was cured by baking at 80°C for 30 minutes.

Table 4 shows the results of an investigation regarding the condition of this coating film.

In Example 1 and Comparative Example 1, the light transmittance of the film obtained from the equal weight mixture of each binder resin constituting the base paint and the clear paint was 59.5% for a wavelength of approximately 300 mm, and 59.5% for a wavelength of approximately 500 mm. About micron 6
It is 5.3%.

Furthermore, the light transmittance of the coatings in Example 2 and Comparative Example 2 was 10.5% and 12.0%, respectively. As shown in Tables 1 and 2, the transfer of aluminum powder in the base paint to the clear paint film is prevented, and although the degree of finish varies depending on the painting conditions, the entire surface of the paint film is maintained. The blackness increased and the metallic feel was not significantly impaired. On the other hand, in Comparative Example 4, the light transmittance of the binder resin was 80.7% and 89.0%, respectively, and during baking, the base paint and clear paint films merged with each other from the interface, and the light transmittance of the binder resin was 80.7% and 89.0%. The aluminum powder migrated to the clear paint film, resulting in a finished film with slightly low gloss, a blackish appearance, and poor metallic luster.

Comparative Example 5 is a conventional two-coat, two-bake coating finish, and the finished state of the metallic coating is the same as that of Example 1.
Examples 1a and 1b were almost the same as Examples 1a and 1b, but were slightly inferior in smoothness, and overall, Examples 1a and 1b were superior.

Claims (1)

[Claims] 1. A powder coating containing a pigment (hereinafter referred to as base coating) is applied directly onto a metal substrate or on a metal substrate coated with an undercoat, and then the binder resin in the base coating is applied. After applying a powder transparent paint (hereinafter referred to as clear paint) whose binder is a resin that is either incompatible with the resin or has compatibility with the light transmittance of 70% or less, baking is performed to remove the coating of both powder paints. An electrostatic powder coating method in which a film is simultaneously cured, characterized in that high voltages of opposite polarity are applied to a base paint and a clear paint to perform electrostatic coating. 2 The high voltage applied to the base paint and clear paint is
Claim 1 having negative polarity and positive polarity, respectively.
Electrostatic powder coating method described in section. 3 The high voltage applied to the base paint and clear paint
Claim 1 having positive polarity and negative polarity, respectively.
Electrostatic powder coating method described in section.