JPH08295827A - Powder coating - Google Patents

Abstract

PURPOSE: To obtain a powder coating enabling formation of thin coating film, improved in contact probability with a frictional electrification member and capable of providing sufficient electrification property even when applied to triboelectrification type spray gun by specifying average particle diameter and perfect circle degree of powder coating material. CONSTITUTION: This powder coating contains (A) a binder resin (e.g. polyester resin) and (B) a curing agent (e.g. isocyanate) and has 5-20μm average particle diameter and >=0.70 perfect circle degree. For example, the composition is subjected to dry mixing and thermally melted and kneaded to afford elementary grains and the grains are pulverized and classified and heat, impact force or frictional force is applied to the resultant particles in order to keep the perfect circle degree to >=0.70, and a filler such as barium sulfate, a spreader such as an acrylic oligomer, a coloring agent such as titanium oxide and foam preventing agent such as benzoin are preferably used as additives for adding to the composition.

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 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 not been strictly classified, and thus have a very broad particle size distribution.
The average particle size was around 30 μm.

[0003]

In order to obtain a good leveling property, it is essential to form a uniform powder adhesion layer, and for that purpose, an adhesion layer having a particle diameter of 2 to 3 times is required. . Therefore, with the conventional powder coating material, a good coating surface could not be obtained unless the thickness of the coating film was more than 60 μm. By making the coating film thinner, it can be expected to improve work efficiency, leveling property, and total cost reduction.
It is desired to be about m.

Further, the conventionally known powder coating material is obtained by mixing raw materials, hot-melt kneading and then pulverizing, so that the shape is indefinite. The roundness is about 0.5. For this reason, when it is applied to the object to be coated, air is easily contained therein, and minute bubbles are generated in the coating film after baking. The presence of a large number of such bubbles not only deteriorates the gloss of the coating film, but also reduces the strength of the coating film.

As a powder coating method which has been generally used conventionally, there is a corona charging type spray gun. 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 has been known that such a corona charging system has two major problems. One is called the Faraday cage effect. Since the electric field (lines of electric force) is not formed in the concave portions of the object to be coated, the powder coating adheres only a small amount to the concave portions, and conversely the lines of electric force concentrate. This is a phenomenon in which a large amount adheres to the edge portion. The other is called reverse ionization phenomenon, in which the accumulated charge of the powder coating and free corona ions deposited on the object becomes too large, causing spark discharge,
This is a phenomenon that causes crater-like defects on the painted surface.

In order to solve these problems, a tribo-charging type spray gun has been used in recent years. In this method, the powder coating material carried by the air flow is charged by friction with the inner wall of the spray gun, and fly and adhere to the object to be coated only by the air flow. According to this tribo-charging method, since no electric field is formed, the powder coating material adheres well to the recesses. In addition, since free ions are not generated, the reverse ionization phenomenon is unlikely to occur. That is, the above problem can be solved. However, since the powder paint is charged only by friction, the absolute charge amount becomes lower than that in the corona charging method, and the problem that the coating efficiency on the object to be coated is insufficient has been clarified.

The present invention has been made in view of the above circumstances, and has a small particle size so as to improve the above-mentioned problems and to make a coating film thin, and used in a tribo-charging type spray gun. In some cases, it is intended to provide a powder coating material which exhibits good coating efficiency and which can obtain excellent coating strength and uniform coating property after baking.

[0008]

The present invention comprises at least a binder resin and a curing agent and has an average particle size of 5 to 20 .mu.m.
m, and the roundness thereof is 0.70 or more.

The powder coating material of the present invention will be described in detail below. The powder coating material of the present invention contains 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, boron trifluoride, acid dihydrazide, imidazole and the like. In addition, a filler, a leveling agent, a coloring agent or the like can be used as an additive. Examples of the filler include barium sulfate, calcium carbonate, aluminum oxide, calcium silicate and the like. Examples of the leveling agent include acrylic oligomer and silicone, and examples of the coloring agent include titanium oxide, chromium oxide, iron oxide, carbon black and the like. Benzoin or the like can be used as the antifoaming agent.

The average particle diameter of the powder coating material of the present invention is a 50% volume diameter measured by Coulter Counter TAII type, and must be in the range of 5 to 20 μm. The powder particles having an average particle diameter of less than 5 μm have a large interparticle force due to Van der Waalsca,
It is not practical as a powder coating because it easily aggregates and the fluidity of the powder deteriorates. Further, if 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. On the other hand, if the average particle diameter exceeds 20 μm, a thin and uniform powder adhesion layer, which is one of the objects of the present invention, cannot be obtained on the coated surface.

The roundness of the powder coating material of the present invention is 0.7.
It is necessary to be above. When the roundness is 0.7 or more, the powder density when applied on the object to be coated is improved, and the generation of minute bubbles due to the inclusion of air can be reduced. That is, the density of the coating film can be improved and the coating film strength can be improved by reducing the minute bubbles. Further, when the roundness is 0.7 or more, the contact efficiency with the triboelectric charging member is improved when applied to the tribo-charging type spray gun, so that the charging efficiency is improved, so that the coating on the object to be coated is improved. Wearing efficiency is improved.

In the present specification, "roundness" is defined by the following general formula (1).

The circularity is determined by, for example, photographing a particle with a transmission electron microscope to obtain a projected image, and analyzing the image using an image analyzer (for example, product name: EXECLII manufactured by Nippon Avionics Co., Ltd.). It can be calculated from the obtained A and B. As is clear from the above equation, the roundness is close to 1 when the particles are close to the true sphere, and is smaller in the case of the irregular shape.

In the powder coating material used in the present invention, a method for achieving a circularity of 0.7 or more is to dry-mix the composition, heat-melt and knead the mixture to obtain elementary particles, and then pulverize and classify. A method of applying heat, impact force, or frictional force to the formed particles can be exemplified. In addition to the melt kneading method, a spray dry method,
A suspension polymerization method, an emulsion polymerization method, etc. can also be adopted.

As a method for applying heat to the particles, a heat fluidized bed for dispersing and flowing the particles in a hot air stream, a surfing system of Nippon Pneumatic Co., Ltd., and the like are used. Further, as a method for imparting an impact force to the particles, a surface modification machine having a rotating rotor, such as Nara Hybridization System manufactured by Nara Machinery Co., Ltd., can be used. Further, as a method for applying a frictional force to particles, a mechanofusion system of Hosokawa Micron Co., which is a surface reformer having a structure of compressing and rubbing particles between a rotor and a stator, can be used.

In the powder coating material of the present invention, a fine powder of hydrophobic silica or alumina may be adhered to the surface for the purpose of improving fluidity. In order to adhere the fine powder to the surface of the powder particles, both may be 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 Seisakusho.

[0017]

EXAMPLES Hereinafter, the powder coating material of the present invention will be described in more detail based on Examples and Comparative Examples. <Example 1> Production of powder coating polyester resin 55.8% by weight (Nippon Ester Co., Ltd. product name: ER-6680) Block isocyanate 10.2% by weight (Daicel Huls product name: BF-1540) Dioxide Titanium 33.0% by weight (Ishihara Sangyo's trade name: CR-90) Flowing agent 0.66% by weight (BASF's trade name: Acronal 4F) Antifoaming agent 0.34% by weight (Midori Kagaku's product (Name: benzoin) The raw materials having the above blending ratios are mixed in a supermixer, melted and kneaded by a pressure kneader at 120 ° C., pulverized by a jet mill, and then an average particle diameter is 1 by a dry air stream classifier.
The size was classified to 3 μm. This powder was put into a Nara Hybridization System (manufactured by Nara Machinery Co., Ltd., trade name: NHS-1 type) and treated at 6400 rpm for 3 minutes to obtain a powder having a roundness of 0.75. At this time, the material temperature was 57 ° C. 0.4 parts by weight of hydrophobic silica fine powder was mixed with 100 parts by weight of the treated powder by stirring with a Henschel mixer to obtain a powder coating material of Example 1.

<Embodiment 2> The powder after classification was treated with a mechanofusion system (trade name: A, manufactured by Hosokawa Micron Co., Ltd.).
(M-35F type) and treated for 20 minutes while adjusting the rotation number of the rotor so that the material temperature becomes 55 ° C., and processed into powder having a roundness of 0.72 in the same manner as in Example 1. A powder coating material of this example was obtained.

<Comparative Example 1> The average particle size after classification is 4.5.
Roundness was 0.70 in the same manner as in Example 1 except that the thickness was set to μm.
A powder coating composition of Comparative Example 1 was obtained. In this example, much energy was consumed in the crushing process and the yield was poor. <Comparative Example 2> A powder coating material of Comparative Example 2 having a roundness of 0.76 was obtained in the same manner as in Example 1 except that the average particle size after classification was 26.0 µm. <Comparative Example 3> A powder coating material of Comparative Example 3 having a roundness of 0.58 was obtained in the same manner as in Example 1 except that the treatment with the oak hybridization system was not performed.

Using the powder coating materials obtained in Examples 1 and 2 and Comparative Examples 1 to 3, the following evaluations and confirmations were carried out. 1. Average particle size The powder coatings obtained by classification in Examples and Comparative Examples were measured for 50% volume size by using Coulter Counter TAII type, and it was confirmed that desired particle sizes were obtained.

2. Coating efficiency Use a tribo-charging spray gun (Matsuo),
Powder coating obtained by temporarily adhering the 300 mm square steel plate suspended vertically in the central part of a 1000 mm square bright finished zinc phosphate treated steel plate (SPCC-SB plate) under the following coating conditions Was applied. Conveyor speed 2.0 (m / min) Reciprocator stroke 1000 (mm) Speed 20 (m / min) Gun distance 200 (mm) Discharge rate 80, 100, 120, and 150 (g / m ² ) Next, each 30 mm from the adherend that is applied according to the discharge amount
Peel off the steel plates on all sides, and attach the powder coating on the steel plates (X)
Then, the coating efficiency was derived from the theoretical total deposition amount obtained from the discharge amount. Coating efficiency (%) = (X / Y) × 100

3. Coating strength Use a tribo charging spray gun (Matsuo),
300mm square bright finished zinc phosphate treated steel plate (SPCC-SB plate) with a film thickness of 30μm after baking
And was baked at 200 ° C. The Erichsen value, impact resistance, and pencil hardness according to JIS-K5400 were measured on the obtained coated surface.

4. State of coated surface after baking The surface of the coating film strength measurement sample was evaluated by visual judgment. 5. Roundness The powder coating particles obtained were photographed with a transmission electron microscope to obtain a projected image, which was image-analyzed using an image analyzer (manufactured by Nippon Avionics Co., Ltd., trade name: EXECLII). The roundness was calculated from the value by the above equation (1). The evaluation results of the above characteristics are shown in Table 1 below.

[0024]

[Table 1]

As is clear from Table 1, the powder coating material of this example has a roundness of 0.70 or more, and therefore has a good coating efficiency on a coating material and a coating after baking. The film state and coating film strength were also excellent. On the other hand, since the powder coating material obtained in Comparative Example 1 had an average particle diameter of 4.5 μm, the coating efficiency was low and there was a problem in practical use. It is considered that this is because the average particle diameter was too small and particles were easily aggregated with each other to deteriorate the fluidity. In addition, Comparative Example 2
Since the average particle diameter of the powder coating material obtained in (2) was 26.0 μm, voids were easily formed between the particles, and the coating surface state was not reached to the extent that strength measurement could be performed. Furthermore, since the roundness of the powder coating material obtained in Comparative Example 3 was 0.58, there was a problem in coating strength. It is considered that this is because the powder coating material has an irregular shape and thus it is easy to hold air.

[0026]

The powder coating material of the present invention is characterized by containing a binder resin and a curing agent, having an average particle diameter of 5 to 20 μm and a circularity of 0.70 or more. It is powder paint. Since the average particle diameter of the powder coating material is 5 to 20 μm, the coating film can be thinned. Further, since the roundness is 0.70 or more, the contact probability with the triboelectric charging member is improved. Therefore, even when applied to a tribo-charging type spray gun, sufficient chargeability can be obtained. Therefore, according to the powder coating material of the present invention, most of the sprayed coating material adheres to the article to be coated, and good coating state and coating strength can be obtained even after baking.

Claims (1)

[Claims] 1. A powder coating material comprising a binder resin and a curing agent, having an average particle diameter of 5 to 20 μm and having a roundness of 0.70 or more.