JPH08209033A - Powdery coating material for electrostatic coating - Google Patents

Abstract

PURPOSE: To obtain the subject powdery coating material having excellent smoothness and appearance, capable of keeping high efficiency of electrostatic coating by comprising granule groups simultaneously satisfying an average granular diameter of each granule group and conditions of cumulative undersize. CONSTITUTION: This powdery coating material is composed of a granule group simultaneously satisfying the conditions such as (A) <=20μm average granular diameter of granule group, (B) cumulative undersize 25% granular diameter (D25 )/cumulative undersize 75% granular diameter (D75 ) of >=0.6 and, further preferably, (C) an average value of (4π×projected area/(peripheral length)<2> )>=0.72. An average granular diameter of the powdery coating material is preferably 1-10μm, more preferably 4-7μm. The powdery coating material is obtained by, e.g. mixing raw powders, making pellets by melting and blending, pulverizing with an atomizer, etc., and preferably classifying in an air flow-type classifier.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder coating used for electrostatic powder coating.

[0002]

2. Description of the Related Art Electrostatic coating is a method of spraying electrostatically charged powder coating on an object to be electrostatically coated and then heating and melting to form a coating film. In contrast to the fluid coating method used for coating small electronic parts, relatively large objects such as home appliances, automobile parts,
It is often used for painting steel furniture.

Conventional paints used in electrostatic coating are:
Generally, a binder resin, a pigment, a curing agent, a mixture of raw material particles such as a surface modifier is kneaded and melted to produce pellets,
It is configured as a powder particle group having a particle diameter of about 30 to 35 μm prepared by crushing this.

Since this electrostatic coating does not use a solvent, the working environment is good and the safety is high, there is no problem of pollution to the surrounding environment, and there are few appearance defects such as dripping and running of the paint. However, since the coating powder applied to the object to be coated is melted to form a continuous coating film as described above, the coating powder particle size is about 30 to 35 μm particle size as described above. Due to its relatively large size, small irregularities (usually called "orange peel") are easily formed on the coated surface, and there is the problem that the smoothness is inferior compared to solvent-based coating methods, and that thin film coating is difficult. There is also.

In order to solve such a problem, it is usually considered to reduce the average particle size of the coating powder (fine pulverization). From such an idea, for example, the average particle size of the coating powder is 5 to 20 μm. The proposal to
98193).

[0006]

However, reducing the average particle size of the paint particles requires a fine pulverization cost and, as will be described later, the tendency to scatter when adhering to a coated object is significant. In terms of efficiency, it is not always an advantageous direction. Therefore, conventionally, it has been generally desired that the state of the coated surface be excellent in smoothness without making the average particle size too small.

The following are problems in the above work efficiency. That is, in the method of applying the powder coating material to the object to be coated in the electrostatic coating, unlike the fluid coating method in which the coating powder is simply suspended in the fluidized tank, for example, it is charged by a corona charging gun or a tribo charging gun. Since the coating powder is flown by an air stream to be electrostatically applied to the surface of the object to be coated, the state of the coating film formed is greatly affected by the charging state of the coating powder. When the coating powder is pulverized as described above (for example, the average particle diameter of the coating powder is 10 μm or less), the coating powder is not sufficiently charged by the corona charging gun or the like and is applied to the object to be coated. The wear efficiency tends to decrease.

This uncoated powder coating material can be recovered and reused. However, since particle size adjustment and the like are required for reuse, charged particles can be used to reduce the reuse ratio. It is not only desirable to make the coating efficiency of the coating as high as possible, but it is also necessary to reduce the coating unevenness as much as possible in order to form a coating film with a beautiful appearance. From these things, the degree of pulverization is up to about 10 μm. Was traditionally considered to be the limit.

The present invention provides a powder coating material having a particle size close to the conventional particle size of about 20 μm and having a smoothness of a coated surface of about 10 μm, and, as described above, it has been difficult to obtain fine powder. The purpose of the present invention is to provide a new powder coating material that enables electrostatic coating using powder coating material.

Another object of the present invention is to provide a powder coating material capable of forming a coating film having excellent smoothness and excellent appearance.

Still another object of the present invention is to provide a powder coating which improves the charging property when powder having a small particle size is used and has a high coating efficiency.

[0012]

The feature of the powder coating material for electrostatic coating of the present invention which achieves the above object is that the powder coating material is constituted by a group of particles which simultaneously satisfy the following conditions. .

The average particle diameter of the particle group is 20 μm or less, and the particle group's (25% particle diameter under cumulative sieve (D ₂₅ ) / 75% particle diameter under cumulative sieve (D ₇₅ )) is 0.6 or more.

In the present invention, the powder coating material can also be constituted by a group of particles satisfying the above conditions and the following conditions at the same time.

The average value of (4π × (projected area) / (peripheral length) ² ) of the particles is 0.72 or more. Further, in the present invention, it is more preferable for the particle group to simultaneously satisfy the above three conditions. A powder coating can be constructed.

The above-mentioned powder coating material according to the present invention can use thermosetting resins such as epoxy resin, acrylic resin, polyester resin, etc., which have been conventionally used as binders, as constituent components without particular limitation. , Pigments, surface conditioners, plasticizers, ultraviolet absorbers, antioxidants, anti-armpitting agents, pigment dispersants, and other additives that have been conventionally used may be used without particular limitation. it can.

A known method can be used for producing the powder coating material. For example, raw material powders selected from the above-mentioned various constituents are mixed and then melt-kneaded to form pellets, which are then atomized or jet milled. It can be pulverized by a method such as pulverization, spheronized if necessary, and then preferably classified by an airflow classifier to obtain the intended powder coating material for electrostatic coating.
In addition to such a manufacturing method, the existing commercially available powder paint for electrostatic coating having a large average particle size (for example, powder paint having an average particle size D ₅₀ = 35 μm) is crushed, classified, and if necessary. It can also be re-prepared by each sphering treatment to obtain the intended powder coating material for electrostatic coating. The sphering treatment is a method of rounding the edge portion of the particles to be treated by using a mechanical pulverizer or an air stream type pulverizer, or by floating the particles to be treated in a heated air stream to melt the surface of the edge portion. It can be performed by a method such as rolling.

The powder coating material of the present invention is required to be fine so that the average particle diameter is within the above-mentioned range, and it is necessary that the particles are uniform. That is, of the particle group (25% particle size under cumulative sieve (D ₂₅ ) /
The 75% particle size (D ₇₅ ) under cumulative sieving is 0.6 or more, preferably 0.7 or more. This (D ₂₅ ) /
When (D ₇₅ ) is less than 0.6, the surface roughness of the formed coating film becomes large, the smoothness is lowered, and the appearance of the coating film is deteriorated. Here, D ₂₅ is a particle size value corresponding to a cumulative amount of 25% in the particle size distribution cumulative curve of the sample powder, and D ₇₅ is a particle size value corresponding to a cumulative amount of 75%. Ie D ₂₅ /
D ₇₅ indicates the spread of the particle size range, and the closer the value is to 1, the more uniform the particle size. That is, the closer the value is to 1, the narrower the distribution width of the coating powder and the more uniform the particle diameter.

The average particle size (average particle size D
₅₀ ) needs to be 20 μm or less. If the average particle size exceeds 20 μm, even if the particle size is uniform, the surface roughness of the formed coating film is remarkably increased, the smoothness is deteriorated, and the coating film appearance is deteriorated. The average particle size can be made small as long as the coating state due to the charging property improved by the sphering treatment is not impaired, but preferably 1 to
The thickness is preferably 10 μm, more preferably 4 to 7 μm.

In the powder coating material of the present invention, the average value of (4π × (projected area) / (perimeter) ² ) of particles (hereinafter referred to as “sphericity”) is 0.72 or more, preferably 0. It is preferably 75 or more, but the coating efficiency does not become so high even when it is 0.75 or more. When the average value of the particle group showing the sphericity is less than 0.72, the coating efficiency is remarkably reduced. The sphericity can be measured by subjecting an electron micrograph to image analysis processing. Although the smoothness of the coated surface can be sufficiently maintained by reducing the particle size of the paint powder and arranging the particles evenly, such a paint will not adhere well to the coated object and the work efficiency of the coating will be poor. Tends to decline. It is thought that this is because the sprayed paint is scattered and dispersed, and the number of particles that reach the vicinity of the surface of the coated object is reduced.
Therefore, work efficiency mainly depends on the degree of charge of particles, and the degree of charge retention and the amount of charge itself contribute to the adhesion depending on the shape of the particles.Therefore, by making the paint particles spherical, the amount of charge is improved. It was done.

In order for the coating powder to satisfy the above-mentioned sphericity, if the coating powder obtained in the above-mentioned powder manufacturing process is within this numerical range, no particular treatment for spheroidization is required, but in general, In general, the powder obtained by the crushing treatment is usually non-circular and has an angular edge portion, and therefore it is often desirable to perform the spheroidizing treatment.

The powder coating material of the present invention can be coated with various kinds of objects such as automobiles, home electric appliances, building materials, miscellaneous goods, etc. by using the conventional electrostatic coating apparatus including the above-mentioned corona charging gun or tribo charging gun. It can be applied to the surface of a metal plate or the like constituting the above, and a coating film can be formed through melting treatment.

[0023]

By the powder coating material for electrostatic coating of the present invention, a coating film having excellent smoothness and good appearance can be formed from powder having an average particle diameter of 20 μm or less and having a spherical shape. Therefore, the efficiency of electrostatic coating can be kept high.

[0024]

EXAMPLE FIG. 1 shows an example of the outline of the configuration of an apparatus for producing the powder coating material of the present invention. This apparatus of this example is a commercially available electrostatic coating powder having a large particle size. Body paint (average particle size D ₅₀ = 35 μm) (hereinafter referred to as “raw powder paint”)
1 shows a manufacturing apparatus for re-preparing a powder coating (hereinafter referred to as "sample powder coating") which has been pulverized and classified into fine powder.

Hereinafter, a specific apparatus configuration of FIG. 1 for producing the sample powder coating material described below will be described.

In FIG. 1, reference numeral 1 denotes a storage tank, which is an acrylic powder paint A100 used as a raw material powder paint in the following examples.
(Average particle size D ₅₀ = 35 μm: manufactured by Nippon Paint Co., Ltd.) is stored and continuously fed in a fixed amount by a screw feeder 2 to a supply port of a classifier 3 (air flow type classifier TC-25 (manufactured by Nisshin Engineering Co., Ltd.)). Was set up.

This classifier 3 conveys fine powder classified to 10 μm or less to a cyclone 5 in the next stage by air flow, and coarse powder of more than 10 μm is crushed by a crusher 4 (jet mill CJ-25 (manufactured by Nisshin Engineering Co., Ltd.). ) And pulverized, and then returned to the supply port of the classifier 3 for circulation.

The cyclone 5 is provided in order to further remove submicron of 1 μm or less which may be contained in the fine powder conveyed from the classifier 3, and the removed static electricity satisfying the above conditions of the present invention. The powder coating material for coating was collected in the container 8 below the cyclone. Reference numeral 7 is a blower for giving a required negative pressure suction action to the cyclone 5 and the classifier 3.

Using the above equipment, acrylic powder paint A
100 (average particle diameter D ₅₀ = 35 μm: manufactured by Nippon Paint Co., Ltd.)
A sample powder coating material is manufactured as follows using as a raw material powder coating material, and electrostatic coating is performed by an electrostatic powder coating device GX3600S (manufactured by Onoda Cement Co.) to obtain smoothness of the coating film and electrostatic coating efficiency. Was measured. The average particle size of the produced powder coating, (D ₂₅ ) / (D ₇₅ ), the smoothness of the coating film formed on the surface of the object to be coated, and the coating efficiency during electrostatic coating are as follows. Measured.

[0030] The average particle diameter size measuring apparatus "Microtrac FRA" (manufactured by Nikkiso Co., Ltd.), was measured by weight 50% particle size.

(D ₂₅ ) / (D ₇₅ ) The 25% particle diameter and the 75% particle diameter on a weight basis were measured and calculated by a particle size measuring device “Microtrac FRA” (described above).

Smoothness of coating film (surface roughness: difference between irregularities) It was measured by a surface roughness meter (manufactured by Mitutoyo Corporation).

Electrostatic coating efficiency It was calculated from the ratio of the amount of powder discharged from the coating gun to the amount of powder attached to the object to be coated.

It was performed by subjecting the sphericity electron micrograph to image analysis processing.

Test Example 1 Using the apparatus shown in FIG. 1, the above raw material powder coating material was pulverized and pulverized into 3 μm, 5 μm, 8 μm, 10 μm, 14 μm,
The average particle size is 20 μm and the particle size distribution is (D ₂₅ ) /
(D ₇₅ ) = 0.4 and 0.7 sample powder paints were produced, and average particle size was applied to the surface of the iron plate, which is the object to be coated, by using the above electrostatic powder coating device for each of these sample powder paints. A coating film having a thickness about twice the diameter was formed. After forming the coating film, the surface roughness of the coating film was measured and the results are shown in FIG.

For comparison, a raw material powder coating having (D ₂₅ ) / (D ₇₅ ) of 0.4 and 0.7 (D ₅₀ = 35 μm)
In the case of the steel plate, a coating film having a thickness of about 70 μm was formed on the surface of the iron plate, and the surface roughness of the coating film after the coating film was formed was measured.
Are also shown.

As can be seen from these results, the roughness of the coating film surface of 4.5 μm is the limit of the smoothness, and (D ₂₅ ) / (D ₇₅ ) having a relatively uniform particle size is within the range of the present invention. In the powder of No. 7, the average particle size of 20 μm sufficiently satisfies this limit, but the powder of (D ₂₅ ) / (D ₇₅ ) outside the range of the present invention has to be finely ground to about 8 μm or less. can not satisfy the above limits, also be the same particle size _{(D 25) / (D 75} )
The unevenness was about 2-3 times larger than that of the powder of 0.7. In addition, the raw material powder coating (average particle size 35 μm) showed a large roughness with a surface roughness of 9.4 μm.

Test Example 2 By using the apparatus shown in FIG. 1, the powder coating material obtained by pulverizing the raw material powder coating material so as to have an average particle size of 5 μm was adjusted to classify (D ₂₅ ). / (D ₇₅ ) is 0.29
7, 0.415, 0.522, 0.673, 40.78
Sample powder paints having particle size distributions of 4, 0.900 were produced, and the average particle size of each of these sample powder paints was measured on the iron plate surface by the electrostatic powder coating device in the same manner as in Test Example 1 above. A coating film having a thickness twice the diameter was formed, and the surface roughness of the coating film after the formation of the coating film was measured. The results are shown in FIG. Furthermore, powders having average particle diameters of 10 μm and 15 μm were measured in the same manner, and the results are also shown in FIG.

As can be seen from these results, even if the average particle size is the same, if (D ₂₅ ) / (D ₇₅ ) is less than 0.6, the surface roughness of the coating film is large, for example, D ₅₀ = At 5 μm (D
_{In the case of 25} ) / (D ₇₅ ) = 0.784 sample powder coating, the surface roughness of the coating film is about 1.1 μm, whereas (D ₂₅ ) /
In the sample powder coating material having (D ₇₅ ) = 0.522, the surface roughness was about 1.5 μm, and large irregularities appeared. This tendency is D ₅₀ =
The same was shown in the case of powders of 10 and 15 μm. In addition,
It can also be seen that the smaller the powder particle size, the smaller the surface roughness.

Further, from these results, even when the particle diameter (average diameter) of the coating particles is different, the roughness of the coating film surface is (D ₂₅ ).
It can be seen that an inflection point appears near / (D ₇₅ ) = 0.6 and the change is small.

Test Example 3 D _{50 of} Test Example 2 = 5 μm and (D ₂₅ ) / (D ₇₅ ) =
When producing a sample powder coating having a particle size distribution of 0.784, the temperature of the air stream for pulverizing the powder while conveying the air stream by the pulverizer 4 was set to various values from room temperature to 80 ° C., and the sphericity was set to 0.
67, 0.69, 0.71, 0.73, 0.76, 0.
The sample powder paints showing 78 sphericity were manufactured, and the sample powder paints were applied to the surface of the iron plate using the electrostatic powder coating device, and the coating efficiency was measured to obtain the results. Is shown in FIG. Also, in addition, when D ₅₀ = 10 μm, (D ₂₅ ) /
(D ₇₅ ) = 0.801, D ₅₀ = 15 μm, (D ₂₅ ) /
(D ₇₅ ) = 0.770, D ₅₀ = 35 μm (D ₂₅ ) /
The measured values of the results of testing the powder having (D ₇₅ ) = 0.792 are also shown in FIG.

As can be seen from these results, D ₅₀ = 5
In the case of μm, the coating efficiency is remarkably reduced in the case of the sample powder coating material having a sphericity of less than 0.72. For example, the sphericity of the sample powder coating material produced at an airflow temperature of 50 ° C. is 0.78 and the powder coating While the efficiency was 89%, the sphericity of the sample powder coating material manufactured at room temperature (20 ° C) was 0.69, and the coating efficiency of the powder was low at 61%. Also shown together D ₅₀ = 10μ
From the results of the powders of m, 15 μm, and 35 μm, it can be seen that if the sphericity is 0.72 or more, good coating efficiency can be obtained regardless of the average particle size. The average particle size is 2
When the average particle size of the powder is small, the effect of the sphericity on the adhesion efficiency is remarkable when the average particle size is 2 μm or less.
It is understood that the sphericity is an important factor in the powder coating material for electrostatic coating of the present invention which uses powder of 0 μm or less.

[0043]

According to the powder coating material for electrostatic coating of the present invention,
It has become possible to realize electrostatic coating using a powder coating of fine powder with an average particle size of 20 μm or less, which has been difficult to provide in the past, and which has not been provided. As a result, a coating film that is superior in smoothness and appearance to the conventional one can be obtained. The effect that it can be formed is obtained.

Further, according to the powder coating material of the present invention, when the powder having a small particle size is used, the charging property is improved and the good coating efficiency can be secured, and the ratio of the uncoated powder is low. This also has the effect of reducing the burden of reuse.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a schematic configuration of an apparatus for producing a powder coating material for electrostatic coating according to the present invention.

FIG. 2 is a diagram showing the results of Test Example 1.

FIG. 3 is a diagram showing the results of Test Example 2;

FIG. 4 is a diagram showing the results of Test Example 3;

[Explanation of symbols]

1 ... Storage tank, 2 ... Screw feeder, 3 ...
Classifier, 4 crusher, 5 cyclone, 6 ...
-Dust collector, 7 ... blower.

Claims (4)

[Claims] 1. The average particle size of the particle group is 20 μm or less, and the particle group has an average particle size of (25% particle size under cumulative sieving (D ₂₅ ) / 75% particle size under cumulative sieving (D ₇₅ )) of 0.6. Powder coating material for electrostatic coating characterized by comprising a group of particles satisfying the above conditions 2. The average particle size of the particle group is 20 μm or less, and (4π × (projected area) / (perimeter) ² ) of the particles
Powder coating material for electrostatic coating, characterized in that it consists of a particle group satisfying the condition that the average value of 3. Particles having an average particle size of 20 μm or less, particle size of (particle diameter under cumulative sieving 25% (D ₂₅ ) / 75% particle size under cumulative sieving (D ₇₅ )) of 0.6 or more, (4π × (projected area) / (peripheral length) ² ) has an average value of 0.72 or more. 4. The method according to claim 1, wherein
A powder coating material for electrostatic coating, wherein the average particle diameter of the particle group is 4 to 7 μm.