JPH1076218A - Method for electrostatic coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the formation of a coating film of a uniform thickness by electrostatic atomization coating in which after the surface of a substrate having a nonconductive surface is charged positively, coating is applied negatively. SOLUTION: A substrate having a nonconductive surface is a nonconductive material, a backing in which a conductive material and a nonconductive material are mixed, a surface-nonconductive material, etc. Before electrostatic coating, the surface of the substrate is charged positively. The charge potential immediately before the electrostatic coating is made 1-20kV to prevent the nonuniformity of film thickness due to the excessively low potential and the danger of fire due to the excessively high potential. To charge the surface of a substrate 10, for example, it is made to face the positive electrode 16 of a high voltage generator 15. The substrate charged positively is coated electrostatically with coating applied at minus 20-90kV. In this way, the negative charge of coating particles stuck to the substrate 10 is neutralized, and defective coating caused by the electrostatic repulsion of negative charge between the coating particles stuck at the beginning of the coating and the particles which are going to be stuck can be solved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic coating method capable of applying a coating film having a uniform film thickness on an object having a non-conductive surface.

[0002]

2. Description of the Related Art
When applying the paint to the object by electrostatic atomization, the paint is applied negatively and the sprayed paint particles are negatively charged without electrically pre-treating the object to be coated. The painting is performed with the grounding. However, when an object having a non-conductive surface is applied by a conventional electrostatic atomization coating method, the negative charge of the applied paint particles is not removed by the ground, and the negatively-charged paint particles having a negative charge come later. It is thought to be due to the initial electrostatic repulsion that hinders coating, but the part to be painted at the beginning of the object to be painted (the part to be painted at the beginning of the object to be painted, usually about 3 to 10 cm from the end) In some cases, the film thickness becomes thin, and the film thickness becomes uneven, resulting in poor appearance of the coated surface such as gloss and smoothness of the coated surface and deterioration of coating film performance. When an object having a non-conductive surface is continuously moved by a conveyor or the like, unevenness in film thickness is likely to occur.However, even when a stopped object is subjected to electrostatic atomization coating, initial electrostatic repulsion occurs. This causes unevenness in film thickness.

Therefore, the present inventors have proposed that even when an object having a non-conductive surface is stopped or moved continuously, a uniform film is formed on the object by electrostatic atomization coating. We conducted intensive research on the method of forming a thick coating film. As a result, they found that a coating film having a uniform thickness can be formed by applying electrostatic atomization coating after positively charging an object to be coated before coating, and completed the present invention.

[0004]

That is, in the present invention, when a coating material is applied negatively to perform electrostatic coating on an object having a non-conductive surface, the surface of the object is positively charged before coating. Then, an electrostatic coating method characterized by performing electrostatic atomization coating is provided.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, the objects to be coated having a non-conductive surface include plastics, glass,
Non-conductive base materials such as wood and paper; base materials with mixed conductive and non-conductive materials such as printed circuit boards; non-conductive coatings formed on non-conductive or conductive base materials Any material may be used as long as the surface of the object to be coated is non-conductive, such as a material whose surface is made non-conductive.

[0006] In the method of the present invention, the form of the coating material applied to the object to be coated may be a liquid or a powder. Or water-based.

In the method of the present invention, the surface of the object is positively charged before the electrostatic coating is performed. The charging potential on the surface of the object to be coated is 1 to 20 k before the electrostatic coating.
V, more preferably in the range of 3 to 15 kV. If the charging potential is too low, the effect of improving the film thickness unevenness is not sufficient, while if the charging potential is too high, a danger such as fire due to static electricity occurs. The surface of the workpiece to be charged needs to be charged at least at a site where initial electrostatic repulsion is likely to occur, and only this site may be charged, or the entire surface of the workpiece to be coated may be charged. .

In order to positively charge the surface of the object to be coated, for example, a method of charging the surface of the object to be coated by facing the positive electrode of the high voltage generator can be mentioned. In this method, a positive electrode is applied to a high voltage generator to ionize air positively by corona discharge, and the surface of the object facing the electrode is positively charged by positively ionized air. In this case, the applied voltage of the high voltage generator is usually 1 to 30 kV,
The range of 20 kV is preferable, and the object to be coated and the electrode of the high-voltage generator face each other at a distance of 25 to 100 mm. By applying a voltage for about 0.5 to 10 seconds, the surface of the object to be coated can be charged to a predetermined positive charging potential.

In the method of the present invention, the positively charged object is then electrostatically coated. Since the positively charged object gradually discharges and the charged potential decreases,
It is preferable to perform electrostatic atomization coating immediately after charging.
Usually, it is preferable that the coating be performed within 5 minutes, preferably within 2 minutes after charging.

In the method of the present invention, the coating device used for the electrostatic atomizing coating includes an air atomizing electrostatic coating device to which static electricity is added, an airless atomizing electrostatic coating device; a bell type electrostatic coating device, A rotary atomizing type electrostatic coating device such as a disk type electrostatic coating device can be used. Any coating device capable of atomizing paint particles and adding static electricity can be used without any particular limitation.

In the method of the present invention, at the time of electrostatic coating, a coating material is applied in the negative, usually in the range of -20 to -90 kV, preferably -20 to -40 kV, and the object to be coated is grounded. It can be performed. Even if the object to be coated is grounded, the surface of the object to be coated is non-conductive at the beginning of coating, so that the positive charge charged on the object remains, and the object is applied to the object by electrostatic coating. Neutralizes with the negative charge of paint particles attached. Due to this, the negative charge of paint particles attached to the object to be coated in the early stage of coating, which was a problem in the past,
The problem of poor coating due to electrostatic repulsion of the negatively charged paint particles to be coated is solved.

When a conductive paint such as a paint containing a hydrophilic solvent such as water, an alcohol-based solvent, or an ether-based solvent is used as the paint, as the coating progresses, the coated paint layer is applied. It is preferable to use a conductive paint because the surface of the material becomes conductive, and the negative charge of the paint particles applied to the material to be coated is quickly removed by grounding to smoothly perform electrostatic coating.

In the method of the present invention, a conventionally known decoy plate may be used for electrostatically coating a non-conductive object to be coated, for example, to improve coatability. The decoy board is used in non-contact with the surface of the object to be coated opposite to the surface to be coated, in face-to-face contact with the object to be coated. The decoy plate has a function of preventing the paint particles from flowing to the back side of the object to be coated, further improving the coating efficiency and preventing the adhesion of the paint particles to the booth wall.

As the material of the decoy board, for example, cloth,
Paper; stainless steel, aluminum, iron, galvanized steel sheet,
Metals such as tinplate; polyethylene, polypropylene,
Examples include plastics such as vinyl chloride, vinylidene chloride, polyamide, and Teflon. In the present invention in which coating is performed by electrostatic coating, a decoy plate having conductivity is preferably used and is electrically grounded. Even when the paint adheres to materials such as cloth and paper, the material becomes conductive. The size of the decoy board is
Although not particularly limited, usually, the area of 4
Those having an area twice or more are preferably used.

The distance between the decoy plate and the object to be coated may be any distance as long as the effect as the decoy plate can be exerted, and is usually about 2 to 20 cm, preferably about 5 to 10 cm. If the distance between the decoy plate and the object to be coated is too small, there is a risk that the decoy plate and the object to be coated come into contact with each other due to shaking of the object to be coated, and if the distance is too wide, the effect as the decoy plate described above is sufficient. You can not demonstrate.

An example in which a paint, for example, a conductive solder resist is applied by electrostatic atomization to a continuously moving article to be coated, for example, a flat printed wiring board by the method of the present invention, will be described below. .

A plurality of printed wiring boards to be coated are
The interval between objects to be coated on a continuously moving conveyor is 10 to 5
Hang at equal intervals with a hanger so that it is about 0 mm. The track of the conveyor passes through the object facing the electrode of the high-voltage generator and charges the printed circuit board with a positive charge. Paint. After the passage, the object is dried by heating or the like as necessary.

The hanger is connected through the conveyor so as to be grounded. However, since a part of the surface of the printed circuit board charged with the positive charge by facing the electrode of the high voltage generator is a non-conductive material, the hanger is positively charged. The charge is grounded and does not disappear instantaneously.

The printed wiring board charged with a positive charge is coated with solder resist sprayed from the coating head when passing through the front of the coating head of the electrostatic coating machine. The solder resist generally has a viscosity (20 rpm) at a rotation speed of 60 rpm by a Brookfield viscometer (BM type).
C.) is adjusted to 2 to 5 poise, and the film is usually coated so as to have a thickness of 5 to 100 μm, preferably 20 to 60 μm. Although the sprayed solder resist particles are negatively charged, they do not cause initial electrostatic repulsion because they neutralize with the positive charges of the printed wiring board when applied to the printed wiring board. As the solder resist particles are applied to the printed wiring board, the surface of the substrate becomes conductive, and the charge on the resist surface is grounded and removed, so that a good resist film without coating unevenness can be formed. .

In the case where a coating film is formed on both sides of a printed wiring board which is an object to be coated without interposing a drying step, one side of the object to be coated is coated as described above, and the other side is similarly coated. do it. When the moving line of the conveyor is linear, a plurality of coating machines can be used, and after one-side coating, the coating can be similarly performed from the side opposite to the first coating (the uncoated surface). After one side is coated, an object to be coated can be inverted by providing a reversing device in the middle of the conveyor line, and the uncoated surface can be similarly coated from the same side as the first coating.

Further, as shown in FIG. 1 to be described later, on the conveyor moving line, the conveying direction is opposite, and the conveyer device 12 conveys the object 10 to be coated and is sent from one straight line portion B. A reversing device 14 for receiving the object to be coated, inverting the object to be coated, and supplying it to the other linear portion D, and a rotary atomizing coating in which the rotation axis is located at the center of the two linear portions of the conveyor device; Using an electrostatic coating apparatus equipped with the apparatus 18, both sides can be coated with one coating machine.
Here, the rotary atomizing head of the coating apparatus 18 can move up and down, and can simultaneously supply the paint to both the linear portions B and D while moving up and down as needed. The moving line of the object to be coated can be, for example, U-shaped, and the object to be coated 10 whose surface is positively charged facing the electrode 16 of the high voltage generator 15 is coated on one side with a linear portion B. Then, it is inverted by the inverting device 14 and supplied to the other linear portion D. In the straight portion D, the unpainted surface of the object faces the coating device, and the straight portion D is similarly coated.

[0022]

The present invention will be described more specifically with reference to the following examples.

Example 1 A single-side coating was performed using the electrostatic coating apparatus shown in FIG. Width 40 with circuit on glass-epoxy substrate surface
The printed circuit board having a size of 6 × 340 mm was faced with the high voltage generator at a set voltage of 10 kV and the distance between the substrate and the electrode of the high voltage generator was set to 30 mm for about 5 seconds to positively charge the substrate. One minute after the charging, a "Sonnenal Solder PW-2000" (a conductive solder resist containing an ether solvent, manufactured by Kansai Paint Co., Ltd.) was applied to the charged surface of the substrate with a Brookfield viscometer (B).
(M type) The viscosity is adjusted so that the viscosity (20 ° C.) becomes 3 poise. Electrostatic coating is performed with the target of a dry film thickness of about 22 μm, removed before the reversing device, and baked at 80 ° C. for 20 minutes. A resist film was formed. The charged potential on the substrate surface immediately before coating was 7 kV. The applied voltage of the paint at the time of electrostatic coating was -30 kV.

Example 2 Example 1 except that the set voltage of the high voltage generator was 5 kV.
The same was done. The charged potential of the substrate surface immediately before painting is 3k
V.

Example 3 The operation was performed in the same manner as in Example 1 except that the set voltage of the high voltage generator was set to 20 kV. The charged potential on the substrate surface immediately before painting is 1
It was 3 kV.

Comparative Example 1 The operation was performed in the same manner as in Example 1 except that the charging by the high voltage generator was not performed.

With respect to the coated plates obtained in the above Examples and Comparative Examples, the thickness of the resist film and the state of the coated surface were examined. The thickness of the resist film is as follows: a portion A where coating is first started (a portion approximately 20 mm from the end), a portion B where the coating is applied in the middle (a portion approximately 203 mm from the end), and a portion C where the coating is finally performed (the rear portion). About 20 mm from the end of the coating plate), the upper (around 20 mm from the top), the middle (around 170 mm from the top), and the lower (around 20 mm from the bottom) coating plates
The measurement was performed at nine sites. The measurement was performed three times for each part, and the average value was defined as the measured film thickness and is shown in Table 1 below. When there was no abnormality in the coated surface state, it was indicated as ○ (good).

[0028]

[Table 1]

[0029]

According to the method of the present invention, since the surface of the object to be coated is charged with a positive charge before the object to be coated having the non-conductive surface is electrostatically coated, a negative charge is applied during the electrostatic coating. When the negatively charged paint particles are applied to the object to be coated, the negative charge of the paint particles neutralizes with the positive charge on the surface of the object to be coated. Therefore, in the initial stage of coating, there is no initial electrostatic repulsion between the negative charge of the paint particles on the surface of the object to be coated and the negative charge of the paint particles to be applied. Electropainting can be performed.

[Brief description of the drawings]

FIG. 1 is a simplified plan view of an electrostatic coating apparatus used as an example of a method for performing electrostatic coating using the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Coated object 12 ... Conveyor apparatus 14 ... Reversing device 15 ... High voltage generator 16 ... Electrode of high voltage generator 18 ... Rotary atomizing coating device A ... Coated object charging part B ... 1st linear part (coating) Part) C: Inverted part of coated object D: Second straight line part (painted part)

Claims (3)

[Claims] When applying a coating material negatively to perform electrostatic coating on an object to be coated, the surface of the object having a non-conductive surface is positively charged before coating, and then electrostatic atomization coating is performed. An electrostatic coating method, characterized in that: 2. The electrostatic coating method according to claim 1, wherein the object to be coated is a continuously moving flat object to be coated. 3. The electrostatic coating method according to claim 1, wherein the charged potential of the surface of the object to be coated immediately before the electrostatic coating is within a range of 1 kV to 20 kV.