JPH08253711A - Powder coating material - Google Patents

Abstract

PURPOSE: To obtain a powder coating material which has a particle size small enough to form a thin coating film, can moderate the inverse ionization when applied with a corona-charge spray gun, and can be used for a wide range of film thickness. CONSTITUTION: This powder coating material is formed by attaching or fixing a fine zinc oxide or titanium oxide powder having a vol. resistivity of 1×10<4> ΘWcm or lower to the surfaces of particles which at least contain a binder resin and a curative and have a vol. average particle size of 5-20μm.

Description

Detailed Description of the Invention

[0001]

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 coating materials have less volatile components and odors than solvent coating materials and are very useful in terms of pollution control and environmental regulations. The powder coatings that have hitherto been put on the market for general use have an average particle diameter of about 30 μm and have not been strictly classified, so that the particle diameter distribution is very broad. In order to obtain a uniform coating surface after powder coating, it is essential to form a uniform powder adhesion layer, but for that purpose, a coating film thickness of 2 to 3 times the particle diameter is required. To do. Therefore, in the conventional powder coating material, a good coating surface could not be obtained unless the thickness of the coating film was 60 μm or more. On the other hand, as market needs, there is a demand for improvement of coating surface uniformity, improvement of work efficiency by thinning coating film, and total cost reduction.
A film thickness of about μm is desired.

A corona charging type spray gun has been conventionally used as a coating method for powder coating. In this method, the powder coating material charged by corona ions generated from the corona electrode provided at the tip of the spray gun is applied to the electric field and air flow formed between the object to be coated which is a conductor and the electrode. It flies along and adheres to the object to be coated. It is known that such a corona charging method has a problem called a reverse ionization phenomenon (or electrostatic repulsion). The reverse ionization phenomenon is a phenomenon in which the accumulated charge of the powder coating material and free corona ions deposited on the object to be coated becomes too large to cause spark discharge, resulting in a crater-like defective portion on the coated surface. This reverse ionization phenomenon becomes a major obstacle when it is attempted to form a multi-layered paint layer on an object to be coated.

Further, in the case of coating with a corona charging type spray gun, when the object to be coated has unevenness, the electric field concentrates on the convexes and an effective electric field is not formed on the concaves, so that powder coating adheres to the concaves. It will be difficult. This phenomenon is called the Faraday cage effect, but because of this phenomenon, the target coating film thickness is currently obtained by performing correction coating near the recess.

Even if such correction coating is performed, the film thickness difference between the concave portion and the convex portion is large, and even if the film thickness is 30 μm, the edge portion which is the convex portion has a thick film close to 100 μm. In some cases, the reverse ionization phenomenon at the edge is often observed.

[0006]

The object of the present invention is to solve the above problems, to enable thinning of the coating film, and to suppress the occurrence of the reverse ionization phenomenon even when coating with a corona charging spray gun. It is to provide a powder coating material that can be obtained.

[0007]

The present invention comprises at least a binder resin and a curing agent and has a volume average particle size of 5 to 20.
The volume resistivity is 1 × 10 on the surface of powder particles of μm.
^The powder coating material is characterized in that conductive zinc oxide fine powder or conductive titanium oxide fine powder of ⁴ Ω · cm or less is adhered or fixed.

The present invention will be described in detail below. The powder coating material of the present invention comprises powder particles containing at least a binder resin and a curing agent. As the binder resin, polyester resin, epoxy resin, acrylic resin, phenol resin, xylene resin, urea resin, melamine resin and the like can be used. Examples of the curing agent include isocyanate, amine, polyamide, acid anhydride, polysulfide, trifluoroboric acid, acid dihydrazide, and imidazole. In addition, a leveling agent such as an acrylic oligomer or silicone, or a foaming inhibitor may be appropriately added to the powder particles.

The powder coating material used in the present invention is obtained by dry-mixing the composition of the above-mentioned powder particles, hot-melt kneading, and then pulverizing and classifying, or by subjecting the composition to a suspension polymerization method, It may be obtained by a polymerization method such as an emulsion polymerization method. In this case, the particle diameter of the obtained powder particles must be such that the volume average particle diameter measured by Coulter Counter TAII type, that is, the volume 50% diameter is in the range of 5 to 20 μm. Powder particles having a volume 50% diameter of less than 5 μm have a large inter-particle force due to van der Waals forces and the like, and are apt to agglomerate, which deteriorates the fluidity of the powder and is not practical as a powder coating material. Further, when it is attempted to manufacture such powder particles having a small particle diameter by a general melt-kneading and pulverizing and classifying method, a large amount of energy is required in the pulverizing and classifying step, so that the manufacturing cost becomes considerably high. If the volume 50% diameter is larger than 20 μm, a thin and uniform powder adhesion layer cannot be formed on the surface to be coated, and a good thin film cannot be obtained.

Further, in the present invention, in order to alleviate the reverse ionization phenomenon, conductive zinc oxide fine powder or conductive titanium oxide fine powder having a volume resistivity of 1 × 10 ⁴ Ω · cm or less is applied to the surface of the powder particles. It is characterized in that it adheres to or sticks to. In this case, the zinc oxide fine powder or titanium oxide fine powder generally used for pigments has a volume resistivity of more than 1 × 10 ⁴ Ω · cm. Even if it adheres or sticks, it cannot be expected to reduce the reverse ionization phenomenon.

The volume resistivity of conductive titanium oxide and conductive zinc oxide used in the present invention is measured as follows. First, 10g for a cylinder type electrode with an inner diameter of 25mm
The sample is put and a pressure of 100 kg / cm ² is applied. Then, the thickness and electric resistance value of the sample under pressure are measured, and the volume resistivity value is calculated by the following formula. Volume resistivity (Ω · cm) = S · R / L where L: sample thickness (cm) S: cylinder cross-sectional area (cm ² ) R: electrical resistance (Ω)

The conductive zinc oxide fine powder used in the present invention is obtained by surface-treating the zinc oxide fine powder with antimony-doped tin oxide. The conductive titanium oxide fine powder can be obtained by surface-treating the titanium oxide fine powder with antimony-doped tin oxide. By attaching or fixing conductive zinc oxide fine powder or conductive titanium oxide fine powder on the surface of the powder particles, excess electric charge on the powder coating applied to the coating object leaks to the coating object side. , The accumulation of charges on the object to be coated is mitigated. The amount of the conductive zinc oxide fine powder or conductive titanium oxide fine powder adhering or adhering to the powder particles was 0.1.
˜5.0 wt% is preferred. In this case, if it is less than 0.1% by weight, the accumulation of electric charge on the article to be coated cannot be relaxed,
If it is more than 5% by weight, the chargeability of the powder coating material is deteriorated and the coating efficiency on the object is deteriorated.

The conductive zinc oxide fine powder or conductive titanium oxide fine powder used in the present invention has a particle size of 0.1 μm.
The following is desirable. If it is larger than 0.1 μm, the adhesion or sticking to the powder coating particles becomes insufficient,
It becomes easy to fall off from powder coating particles. If the conductive zinc oxide fine powder or the conductive titanium oxide fine powder falls off from the powder coating particles, it is not possible to expect the intended relaxation of the reverse ionization phenomenon.

The conductive zinc oxide fine powder or conductive titanium oxide fine powder used in the present invention is white and has a small particle size as described above, and therefore does not affect the color of the coated surface after baking. Incidentally, the adhesion of the fine powder in the present invention means a state in which the fine powder is sprinkled and adhered to the surface of the powder particles, while the fixing means the fine particles and the powder particles generated by the device described below. At least a part of the fine particles are embedded and fixed on the surface of the powder particles due to heat generation, mechanical frictional force, or compressive force during the mixing.

To deposit the conductive zinc oxide fine powder or the conductive titanium oxide fine powder used in the present invention on the surface of the powder coating material, a Henschel mixer manufactured by Mitsui Miike Co., a super mixer manufactured by Kawada Seisakusho, or the like can be used. It is performed by dry-mixing both with a high-speed mixer. Further, the fixation is performed by using a Nara hybridization system manufactured by Nara Machinery Co., Ltd., a mechanofusion system manufactured by Hosokawa Micron Co., or a surfusing system manufactured by Nippon Pneumatic Co., Ltd.

Inorganic fine particles such as hydrophobic silica and hydrophobic alumina may be attached to the surface of the powder particles of the present invention for the purpose of improving fluidity. To adhere the inorganic fine particles to the surface of the powder particles, they are dry-mixed with a high speed mixer such as a Henschel mixer manufactured by Mitsui Miike Co., Ltd. or a super mixer manufactured by Kawata Manufacturing Co., Ltd.

[0017]

EXAMPLES The present invention will be described below based on examples. <Example 1> The raw materials having the above mixing ratios are mixed in a super mixer, melted and kneaded at 120 ° C. in a pressure kneader, pulverized in a jet mill, and then classified by a dry air classifier to have a volume of 50% and a diameter of 13 μm. Powder particles were created. With respect to 100 parts by weight of the powder particles, 0.4 parts by weight of hydrophobic silica and conductive zinc oxide fine powder (volume resistivity 1 × 10 ³
0.4 parts by weight (Ω · cm, primary particle diameter 0.08 μm) was mixed by stirring with a Henschel mixer to obtain a powder coating material according to the present invention. The powder coating material was applied to a corona charging type spray gun (manufactured by Lansberg Co.), and an applied voltage of -30 kV,
After spraying the zinc phosphate-treated steel plate (SPCC-SB plate) bright-finished under the conditions of a discharge amount of 70 g / min and a distance between the coating object and the spray gun of 200 mm,
Baking was performed at 200 ° C.

<Example 2> The same paint particles 1 as in Example 1
0.4 parts by weight of hydrophobic silica and conductive titanium oxide fine powder (volume resistivity 2 × 10 ² Ω · c)
m, primary particle diameter 0.05 μm) 0.4 part by weight was mixed by stirring with a Henschel mixer to obtain a powder coating material of the present invention. The powder coating composition was sprayed on a zinc phosphate-treated steel plate (SPCC-SB plate) bright-finished under the same conditions as in Example 1, and then baked at 200 ° C.

<Example 3> The same powder particles 1 as in Example 1
For 100 parts by weight, conductive zinc oxide fine powder (volume resistivity 1 × 10 ³ Ω · cm, primary particle diameter 0.08 μm).
4 parts by weight are mixed with a Henschel mixer, put into a Nara hybridization system (NHS-1 type), and treated for 2 minutes at a rotor rotation speed of 6000 rpm to fix conductive zinc oxide fine powder to the surface of the powder particles. Let Furthermore, for 100 parts by weight of this powder, 0.4 parts of hydrophobic silica was used.
By mixing parts by weight with a Henschel mixer, the powder coating material of the present invention was obtained. A zinc phosphate-treated steel sheet (SPCC) obtained by brightly finishing the above powder coating under the same conditions as in Example 1.
-SB plate) was sprayed and then baked at 200 ° C.

<Comparative Example 1> Raw materials having the same blending ratio as in Example 1 were mixed in a supermixer, melted and kneaded at 120 ° C. in a pressure kneader, pulverized in a jet mill, and then in a dry airflow classifier. The powder was classified so as to have a volume 50% diameter of 4.8 μm to obtain powder particles for comparison. This powder particle 10
0.4 parts by weight of hydrophobic silica and 0.4 parts by weight of conductive zinc oxide fine powder (volume specific resistance 1 × 10 ³ Ω · cm, primary particle size 0.08 μm) were stirred with a Henschel mixer, relative to 0 parts by weight. A powder coating for comparison was obtained by mixing. The above powder coating material was sprayed on a zinc phosphate-treated steel sheet (SPCC-SB board) that had been bright-finished under the same conditions as in Example 1, and then baked at 200 ° C.

<Comparative Example 2> Raw materials having the same blending ratio as in Example 1 were mixed in a super mixer, hot melt kneaded at 120 ° C. in a pressure kneader, pulverized in a jet mill, and then in a dry air stream classifier. The powder was classified so that the 50% volume diameter was 26.0 μm to obtain powder particles for comparison. This powder particle 1
0.4 parts by weight of hydrophobic silica and conductive zinc oxide fine powder (volume specific resistance 1 × 10 ³ Ω · cm,
0.4 parts by weight of primary particle diameter (0.08 μm) was mixed by stirring with a Henschel mixer to obtain a powder coating material for comparison. The above powder coating material was sprayed on a zinc phosphate-treated steel sheet (SPCC-SB board) that had been bright-finished under the same conditions as in Example 1, and then baked at 200 ° C.

Comparative Example 3 A powder coating for comparison was obtained in the same manner as in Example 1 except that the conductive zinc oxide fine powder was not used. The above powder coating was bright-finished under the same conditions as in Example 1 and treated with zinc phosphate-treated steel sheet (SPCC-SB).
After spraying on the plate, baking was performed at 200 ° C.

Comparative Example 4 The same procedure as in Example 1 was carried out except that zinc oxide fine powder having a high volume resistivity (volume resistivity 3.5 × 10 ⁹ Ω · cm, primary particle diameter 0.08 μm) was used. A powder coating for comparison was obtained. After spraying the powder coating material on a zinc phosphate-treated steel plate (SPCC-SB plate) that has been bright-finished under the same conditions as in Example 1, 2
Baking was performed at 00 ° C.

Evaluation of the state of the coating surface before baking of the coating surface of the powder coating material obtained in Examples 1 to 3 and Comparative Examples 1 to 4, that is, the occurrence of the reverse ionization phenomenon The results are shown in Table 1, and the evaluation results of the coating film state after baking are shown in Table 2. As is clear from Tables 1 and 2, the use of the powder coating material of the present invention alleviates the reverse ionization phenomenon and enables setting in a wide coating thickness range. It was also confirmed that the powder coating material of the present invention had a good coating surface after baking.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

As described above, according to the present invention, since the average particle diameter of the powder coating material is 5 to 20 μm, the coating film can be made thin and the surface of the powder particles has a volume resistivity of 1 ×. 1
By applying or fixing conductive zinc oxide fine powder or conductive titanium oxide fine powder of 0 ⁴ Ω · cm or less, it is possible to reduce the reverse ionization phenomenon when applied to a corona charging type spray gun, and it is wide. It is possible to obtain a powder coating material that can be used within the coating thickness range.

Claims (3)

[Claims] 1. A conductive zinc oxide fine powder having a volume resistivity of 1 × 10 ⁴ Ω · cm or less on the surface of powder particles having a volume average particle diameter of 5 to 20 μm and comprising at least a binder resin and a curing agent. Alternatively, a powder coating material in which conductive titanium oxide fine powder is adhered or fixed. 2. The powder coating composition according to claim 1, wherein the content of the conductive zinc oxide fine powder or the conductive titanium oxide fine powder is 0.1 to 5.0% by weight. 3. The powder coating composition according to claim 1, wherein the conductive zinc oxide fine powder or the conductive titanium oxide fine powder has a primary particle diameter of 1.0 μm or less.