JP2909881B2 - Powder paint - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder coating suitable for thin film coating.

[0002]

2. Description of the Related Art It is well known that powder coatings have less volatile components and odors than solvent coatings and are very useful in terms of pollution control and environmental regulations. Conventionally, powder coatings that are commercially available for general use have an average particle size of about 30 μm and are not strictly classified, so that the particle size distribution is very broad. By the way, in order to obtain good leveling properties, it is essential to form a uniform coating film, but for that purpose, it is necessary to form a powder adhesion layer having a thickness of two to three times the particle diameter. . When a conventional powder coating is used, the thickness of the powder adhesion layer must be 60 μm or more in order to obtain a coating film having good leveling properties. On the other hand, as market needs, improvement of leveling property, improvement of work efficiency and reduction of total cost by thinning the coating film are demanded. That is, 3
It has become desirable to form a coating film having a thickness of about 0 to 60 μm.

When a coating film is formed using a powder coating,
As a powder coating method commonly used in the past,
There is a method using a corona charging type spray gun. In this method, a powder coating charged by corona ions generated from a corona electrode provided at the tip of the spray gun is formed between an object to be coated, which is a conductor, and an electrode, and follows the electric field and air flow. Flies and adheres to the substrate. It has been found that this corona charging method has two major problems. One of them is called the Faraday cage effect, in which an electric field (line of electric force) is not formed in a concave portion of an object to be coated, so that a small amount of powder paint adheres to the concave portion, and conversely, the line of electric force concentrates. This is a phenomenon that a large amount adheres to the edge portion. The other is called reverse ionization, a phenomenon in which the accumulated charge of the powder paint and free corona ions deposited on the object to be coated becomes excessively large, causing spark discharge and causing a crater-like defective portion on the painted surface. It is.

In order to solve these problems, a system using a tribo-charging type spray gun has recently been used. In this method, the powder paint conveyed by the air flow is charged by friction with the inner wall of the spray gun, and flies between the objects to be coated only by the air flow. In this method, since no electric field is formed, the powder coating adheres well to the concave portions, and free ions are not generated, so that the reverse ionization phenomenon hardly occurs. However, since the charge of the powder coating is only due to friction, the absolute charge amount is lower than that of the corona charging method, and it is clear that there is a problem that the state of adhesion to the workpiece depends on the humidity environment of the coating line. It has become.

In the case where the powder coating is to be applied under the condition that the powder coating is left open and in a high-humidity environment, a spray gun of a corona charging type is used instead of a spray gun of a tribo charging type. Even in such a case, there is a problem that the fluidity of the powder coating material is deteriorated due to the adsorption of water to the surface of the powder particles, and the supply property of the coating material to the spray gun is deteriorated.

[0006] By the way, by improving the fluidity of the powder coating,
It is also known to attach a hydrophobic silica fine powder to the surface of a powder coating in order to improve the transport of the coating to a spray gun. However, the hydrophobic silica fine powder conventionally used is insufficiently subjected to the hydrophobic treatment with the surface treatment agent, and has a high equilibrium moisture adsorption amount per unit surface area. When such a silica fine powder having insufficient hydrophobizing treatment is used, when applied to a tribo-charging type spray gun, the frictional charging of the powder coating becomes poor, especially in a high humidity environment, and the substrate to be coated is Many paint particles that do not adhere to the surface are generated. In addition, since the silica fine particles on the surface of the powder coating easily adsorb moisture in the air, the transfer of the coating to the spray gun is not smooth, and the surface properties after spraying on the object to be coated become poor. there were.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems in the prior art. In other words, the object of the present invention is to use powder particles having a small particle size that enables thinning of a coating film, and achieve improvement in leveling property and improvement in working efficiency by thinning the coating film. Another object of the present invention is to provide a powder coating that can be used in a wide humidity environment.

[0008]

The powder coating of the present invention contains at least a binder resin and a curing agent, and equilibrium adsorption per unit surface area on the surface of powder particles having an average particle diameter of 5 to 20 μm. Hydrophobic silica fine powder having a water content of 2 × 10 ⁻⁵ g / m ² or less is added in an amount of 0.1 to 100 parts by weight of the powder particles.
It is characterized in that it is attached in an amount of from 0.01 to 5 parts by weight.

Hereinafter, the present invention will be described in detail. The powder coating used in the present invention comprises powder particles containing a binder resin and a curing agent as main components. As the binder resin,
Polyester resin, epoxy resin, acrylic resin, phenol resin, xylene resin, urea resin, melamine resin and the like can be used. As the curing agent, isocyanates, amines, polyamides, acid anhydrides, polysulfides,
Boron trifluoride, acid dihydrazide, imidazole and the like are used. The amount of these curing agents can be appropriately set according to the type of the binder resin.

The powder particles may contain other known additives. For example, fillers such as barium sulfate, calcium carbonate, aluminum oxide and calcium silicate, acrylic oligomers, spreading agents such as silicone,
Colorants such as titanium oxide, chromium oxide, iron oxide, and carbon black, and foaming inhibitors may be added as appropriate. The above-mentioned powder particles used in the present invention can be obtained by dry-mixing the above components, hot-melt kneading, pulverizing and classifying. It can also be prepared by blending a desired component with a monomer for forming a resin and polymerizing the mixture by a polymerization method such as a suspension polymerization method or an emulsion polymerization method.

In the present invention, the powder particles obtained as described above have an average particle diameter, that is, a 50% volume diameter measured by a Coulter Counter TAII type of 5 to 5.
It must be in the range of 20 μm. Average particle size is 5μ
In the case of powder particles having a particle diameter of less than m, the interparticle force due to van der Waals force or the like becomes large, the powder particles tend to aggregate, and the fluidity of the powder coating deteriorates, which is not practical. Furthermore, in order to produce such small-sized powder particles by a general melt-kneading, pulverizing, classification method,
Since a large amount of energy is required in the pulverization and classification process, the production cost is considerably increased. On the other hand, the average particle diameter is 20 μ
If it exceeds m, a thin and uniform powder adhering layer cannot be obtained on the surface to be coated, and therefore a good thin film cannot be obtained.

The powder coating composition of the present invention is characterized in that the above-mentioned powder particles are coated with hydrophobic silica fine powder having an equilibrium adsorbed water content per unit surface area of 2 × 10 ⁻⁵ g / m ² or less. It is characterized in that 0.01 to 5 parts by weight are attached to parts by weight. The equilibrium adsorption water content of the hydrophobic silica fine powder used in the present invention needs to be 2 × 10 ⁻⁵ g / m ² or less, but in order to improve the characteristics under a high humidity environment well. Is more preferably in the range of 1.5 × 10 ⁻⁵ g / m ² or less. By attaching the hydrophobic silica fine powder of the above equilibrium adsorbed water amount to the powder particle surface,
The fluidity as a powder coating is improved, and the handling of the powder as a powder is improved, for example, the transfer of the coating to a spray gun is improved. On the other hand, the equilibrium adsorbed water amount is 2 × 10
^When it is larger than ^-5 g / m ² , when applied to a tribo-charging type spray gun, the frictional charging of the powder coating becomes poor, especially in a high humidity environment, and many coating particles do not adhere to the object to be coated. When the powder coating is left in a high humidity environment, the silica fine particles on the surface of the powder coating adsorb moisture in the air, and the fluidity of the powder coating deteriorates. Since the transfer of the coating material is not smooth, the surface properties after spraying the coating object become poor.

The amount of equilibrium adsorbed water per unit surface area is measured by the following method. 1. The specific surface area A (m ² / g) of the silica fine powder is measured by the BET method. 2. The silica fine powder is allowed to stand at 25 ° C./80% RH for 45 days, and the equilibrium adsorbed water content B (% by weight) after the standing is measured by the Karl Fischer method. 3. From the following formula, the equilibrium adsorbed water amount per unit surface area C
(G / m ² ). C = B / (A × 100)

The specific surface area of the hydrophobic silica fine powder used in the present invention is preferably 100 m ² / g or more.
This is because hydrophobic silica fine powder having a specific surface area of 100 m ² / g or more has a high effect of imparting fluidity to powder particles, and a desired fluidity can be obtained with a small amount of addition.

The hydrophobic silica fine powder having the above equilibrium adsorbed water content used in the present invention can be obtained by highly surface-treating with a known surface treating agent such as a silane coupling agent and a polysiloxane. it can. Specifically, for example, hexamethyldisilazane is dissolved in ethanol, and an appropriate amount is sprayed on a fluidized untreated fine silica powder, followed by heating to volatilize the ethanol. By repeating this operation appropriately or adjusting the amount before the treatment, a highly surface-treated silica fine powder can be obtained.

The above-mentioned hydrophobic silica fine powder comprises powder particles 1
0.01 to 5 parts by weight, preferably 0.1 to 1 part by weight, is added to 00 parts by weight. When the addition amount is less than 0.01 part by weight, the fluidity is not sufficiently imparted to the powder particles. On the other hand, when the addition amount is more than 5 parts by weight, the hydrophobic silica fine powder is removed from the powder particle surface. It is easily released, and the free silica fine powder deteriorates the surface properties of the coating film.

In order to adhere the above-mentioned hydrophobic silica fine powder to the surface of the powder particles, for example, dry mixing is performed by a high-speed mixer such as a Henschel mixer manufactured by Mitsui Miike Co., or a super mixer manufactured by Kawada Seisakusho Co., Ltd. do it.

[0018]

The present invention will be described below with reference to examples. Example 1 Production of powder coating material 55.8 parts by weight of polyester resin (ER-6680, manufactured by Nippon Ester Co., Ltd.) 10.2 parts by weight of blocked isocyanate (BF-1540, manufactured by Daicel Huls Co., Ltd.) 33.0 parts by weight of titanium dioxide (CR-90, manufactured by Ishihara Sangyo Co., Ltd.) Dispersant 0.66 parts by weight (Acronal 4F, manufactured by BASF) Antifoaming agent 0.34 parts by weight (benzoin, manufactured by Midori Kagaku Co., Ltd.) The mixture was mixed by a super mixer, hot melt kneaded at 120 ° C. by a pressure kneader, pulverized by a jet mill, and then classified by a dry air classifier so that the average particle diameter (50% diameter by volume) became 13 μm. Hydrophobic silica fine powder (BET specific surface area: 210 m ² / g, equilibrium moisture content per unit surface area: 1.1 × 10 ⁻⁵ g / m ² ) was added to 100 parts by weight of the obtained powder particles. 4 parts by weight were stirred and mixed with a Henschel mixer to obtain a powder coating.

Example 2 A hydrophobic silica fine powder having a BET specific surface area of 125 m ² / g and an equilibrium adsorbed water content per unit surface area of 1.4 × 10 ⁻⁵ g / m ² was used, except that A powder coating was obtained in the same manner as in Example 1.

Comparative Example 1 Raw materials having the same compounding ratio as in Example 1 were mixed by a super mixer, hot-melt kneaded at 150 ° C. by a pressure kneader, pulverized by a jet mill, and then averaged by a dry air classifier. The particles were classified so that the particle diameter (50% volume diameter) was 4.8 μm. To 100 parts by weight of the obtained powder particles, 0.4 part by weight of the same hydrophobic silica fine powder as in Example 1 was stirred and mixed with a Henschel mixer to obtain a powder coating. Comparative Example 2 Raw materials having the same compounding ratio as in Example 1 were mixed with a super mixer, hot-melt kneaded at 120 ° C with a pressure kneader, pulverized with a jet mill, and then average particle size with a dry air classifier ( (50% diameter by volume) was 26.0 μm. To 100 parts by weight of the obtained powder particles, 0.4 part by weight of the same hydrophobic silica fine powder as in Example 1 was stirred and mixed with a Henschel mixer to obtain a powder coating. Comparative Example 3 Example except that a hydrophobic silica fine powder having a BET specific surface area of 102 m ² / g and an equilibrium moisture content per unit surface area of 2.9 × 10 ⁻⁵ g / m ² was used. In the same manner as in Example 1, a powder coating was obtained. Comparative Example 4 A powder coating was obtained in the same manner as in Example 1, except that the hydrophobic silica fine powder was not added. Comparative Example 5 A powder coating was obtained in the same manner as in Example 1 except that the amount of hydrophobic silica was changed to 6 parts by weight.

Examples 1 and 2, Comparative Examples 1 and 5
Powder paint applied to a tribo-charging type spray gun,
Bright-finished zinc phosphate treated steel sheet (SPCC-
(SB board) so that the thickness after baking was 30 μm, and baking was performed at 20 ° C. Table 1 shows the adhesion state after spraying and the evaluation result of the coating surface after baking. Table 1 also shows changes in the fluidity of the powder coating when each powder coating was left in a high humidity environment.

[0022]

[Table 1]

As is clear from Table 1, the powder coating of the present invention is not affected by the coating environment, has a good coating efficiency on the coating, and has a good fluidity even when left in a high humidity environment. It is possible to obtain a powder coating material having no deterioration and having a good coating surface after baking.

[0024]

The powder coating of the present invention has an average particle size of 5 to 5.
Since 20 μm powder particles are used, the coating film can be made thinner, and the equilibrium adsorbed water content per unit surface area per 100 parts by weight of powder particles is 2 × 10 ⁻⁵ g / m ² or less. Because 0.01 to 5 parts by weight of the hydrophobic silica fine powder is adhered, most of the sprayed powder coating adheres to the object to be coated even in a high humidity environment, and even when left in a high humidity environment. Fluidity does not decrease and a good coating film can be formed even after baking.

Claims (2)

(57) [Claims] 1. An equilibrium adsorbed water content per unit surface area of 2 × 10 ⁻⁵ g / powder containing at least a binder resin and a curing agent and having an average particle diameter of 5 to 20 μm on the surface of powder particles.
m ² or less of the hydrophobic silica fine powder
A powder coating characterized in that 0.01 to 5 parts by weight is attached to parts by weight. 2. The powder coating material according to claim 1, wherein the hydrophobic silica fine powder has a specific surface area of at least 100 m ² / g by a BET method.