JPH01219041A - Composite powder for powder coating - Google Patents

Abstract

PURPOSE:To improve the electrostatic coating property, deposition efficiency on a metal to be coated, and retainability after deposition of the subject powder without deteriorating the characteristic of a glazed surface by coating the surface of inorg. powder with a thermally decomposable org. material. CONSTITUTION:100pts.wt. of the inorg. powder, appropriately frit for producing enamel, other ceramic powder for producing ceramics, and metal powder in certain circumstances is coated with 1-30pts.wt., or preferably 3-25pts.wt., thermally decomposable org. material to obtain composite powder for powder coating. A thermally decomposable org. material, which can be completely thermally decomposed and dissipated without leaving any carbon in the course of calcination at a temp. preferably more than 100 deg.C lower than the calcination temp., is used, and polymethyl methacrylate, etc., can be exemplified. The org. material is advantageously added to coat the inorg. powder, when inorg. mate rial is milled to a specified particle size. The calcination after coating is carried out at 700-920 deg.C in case of an enameled steel sheet and at 500-600 deg.C in case of an aluminum sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention provides a composite powder for powder coating, which has good adhesion efficiency to objects and good retention after adhesion, especially a composite powder for the production of enamel. It's about the body.

The conventional technology for making enamel products is to create a slip-like enamel glaze composition by pulverizing a ceramic raw material called frit with a mill-added emulsifier using a ball mill, etc., and use this slip as a metal base prepared in advance for glazing. It is manufactured by applying glaze to the surface or a surface that has already been underglazed by dipping or spraying, drying, and then firing.

This method has been widely used since ancient times, but
Recently, as an alternative to this traditional method, a powder coating method in which frit powder is electrostatically attached to a metal surface has been attracting attention.

That is, JP-A-51-7005 discloses a technique in which ceramic powder having organopolysiloxane on the surface is deposited on a metal panel using an electrostatic spray gun, and then fired.

Japanese Unexamined Patent Publication No. 52-98721 discloses a technique in which a waxy frit is reacted with an alkoxysilane such as methyltrimethoxysilane, and then the reacted frit is electrostatically sprayed onto a substrate.

JP-A-61-178440, JP-A-61-178
No. 441 and Japanese Unexamined Patent Publication No. 178442/1987 disclose a frit for powder coating with an improved composition of the frit, and a technique for pulverizing this frit and electrostatically attaching it to the surface of a metal base is disclosed. has been done.

Problems to be Solved by the Invention However, the conventional method of dipping or spraying glaze onto the surface of a metal substrate using a water-suspended thread slip requires a complicated slip preparation process and requires preliminary drying before firing. This has the disadvantage that the process is multi-step. In addition, there is a risk of secondary agglomeration of particles due to evaporation of water and solvent during drying, and it is difficult to obtain a smooth surface after firing unless the film thickness is above a certain level, and the thicker the film, the heavier it becomes. I can't escape it. Furthermore, it is necessary to solve the problem of pollution caused by spray mist and dipping waste liquid.

On the other hand, the powder coating method, in which frit powder is electrostatically attached to the metal surface, meets the requirements for shortening the process, thinning the film, and making it non-polluting, but it still has the following problems. , fundamental improvements are desired.

(1) Since ceramic material has a large specific gravity and a small amount of charge, it is necessary to apply a high voltage. However, the application of a high voltage means that the repulsive force between the adhered particles due to Coulomb force increases, and as a result, the retention time of the powder after adhesion becomes shorter.

(2) Particularly in the case of powder coating with powder of small particle size, the repulsive force after adhesion is extremely large, so that a so-called pack-emission phenomenon occurs on the adhesion surface, which tends to cause abnormalities in the coating film.

(3) When a simple mixed system of two or more components is used, each component behaves separately, making it impossible to add pigments or additives with different specific gravities later (post-blending). Therefore, in order to have a uniform composition,
It is essential to melt the mixed system into a homogeneous phase and then pulverize it.

(4) Lowering the melting point of the frit is advantageous in order to increase its adhesion to metal. However, when adding an alkaline component to make the frit highly alkaline in order to lower its melting point, the greater the amount added, the greater the amount of current applied, and if a certain limit is exceeded, electrostatic coating itself becomes impossible. For example, the total amount of NazOlK, O and Liz O in the oxide composition is about 15% by weight, at most 20% by weight.
Electrostatic painting is possible up to about 30% by weight.
As such, it is not possible to make the product highly alkaline, and even if electrostatic coating is possible, increasing the amount of alkaline components causes the problem of poor storage stability.

Among the powder coating methods, the above-mentioned JP-A-51-70
The ones described in JP-A No. 05 and JP-A-52-98721 are designed to have high resistivity because the frit powder is coated with a silicon compound, but during firing, crystals different from those inside are formed. Because silicon oxide precipitates on the surface, properties such as chemical resistance, hot water resistance, or transparency of the glazed surface may deteriorate.

In view of these circumstances, the present invention was made for the purpose of providing a composite powder for powder coating that has excellent electrostatic coating properties and adhesion efficiency without impairing the characteristics of the glazed surface. .

Means for Solving the Problems The composite powder for powder coating of the present invention is made by coating the surface of an inorganic powder with a thermally decomposable organic substance.

As the inorganic powder, a frit for manufacturing wax is particularly suitable, and a heat-resistant pigment and other secondary ingredients may be added in advance.

Raw materials for such frits include flammable materials such as silica and feldspar; boric acid, boric acid, lithium carbonate, soda ash, chili saltpeter, saltpeter, potassium carbonate, sodium fluoride, sodium silicofluoride, and cryolite. Low-flammability raw materials such as aluminum fufluoride, rubble, limestone, slaked lime, bone ash, magnesium carbonate, magnesia, zinc white, zinc carbonate, barium carbonate, lead red, lead white, Mitsudasho; antimony metal, antimony oxide, Milky white raw materials such as sodium antimonate, tin oxide, zirconium oxide, titanium dioxide, cerium oxide, molybdenum oxide, arsenite; AlzO, ? ,5nO1,Zr0Z
, oxide pigments such as TiO2 and Crz03, complex oxide pigments such as spinel type and pyrochlore type, ZrSiO
4,3CaOCrz033SiO2,Ca03n04S
iOz, 22nO5i01.2Mg0Si02. Examples include silicate-based pigments such as, and various ceramic pigments including these co-fired pigments.

The oxide composition of the frit is, for example, if the steel plate is opaque, is it a bottom glaze (black bottom glaze, white bottom glaze) or a top glaze (tin oxide, fluoride, antimony top glaze, titanium top glaze)? Or, depending on whether the glaze is colored or spotted, cast iron can have various compositions depending on whether it is underglazed or overglazed. A feature of the present invention is that any oxide composition can be coated with a thermally decomposable organic substance to significantly improve its electrostatic coating properties and coating efficiency.

Examples of the inorganic powder include frit powder for manufacturing waxes, ceramic powder for manufacturing ceramic substrates, and various other ceramic powders. The heat-resistant pigment can also be used as an inorganic powder, and in some cases, a non-ceramic powder such as a metal powder can also be used.

The pyrolyzable organic substance for coating the surface of the above-mentioned inorganic powder is one that pyrolyzes and dissipates substantially completely without leaving any carbon at a temperature up to the sintering temperature when the composite powder is sintered. Any organic substance can be used.

More preferably, one is used that completely thermally decomposes and dissipates at a temperature 100° C. or more lower than the firing temperature. The thermal decomposition behavior of a thermally decomposable organic substance can be easily determined by heating it at a constant temperature increase rate (for example, 10° C./5 inches) using a differential thermal analyzer.

Examples of such thermally decomposable organic substances include thermoplastic polymers, thermosetting polymers, low molecular weight polymers (including prepolymers and oligomers), and non-polymer organic substances. The most suitable thermally decomposable organic substances include polymethacrylic acid alkyl esters in which the alkyl group has 1 to 4 carbon atoms, especially polymethyl methacrylate. Other examples of thermally decomposable organic substances include polyvinyl alcohol,
Examples include polyvinyl butyral, polyacetal, nitrified cotton, sucrose ester, sucrose, and paraffin.

Various methods are employed to coat the surface of inorganic powder with a pyrolyzable organic substance.

The most advantageous method is to coexist with a pyrolyzable organic material in the form of powder when the inorganic material is ground to a predetermined particle size. When such a method is adopted, when the inorganic material is triturated, a new surface (
During grinding, pyrolyzable organic substances are also cut and radicals are generated. Therefore, grafting occurs between the inorganic powder and the organic substance, the pyrolytic organic substance powder fuses to the surface of the inorganic powder, and the inorganic powder is encapsulated with the pyrolytic organic substance powder. It becomes like this. In this case, if the system is closed and the required particle size is taken out without taking out large or small particles, the large particles will be coarsely ground, and the small particles will be granulated. Therefore, particles with a narrow particle size distribution can be obtained.

In addition, it is also effective to add a thermally decomposable organic substance powder dispersed in a poor solvent to an inorganic powder and grind it appropriately.

The ratio of the thermally decomposable organic substance to the inorganic powder is 1 to 30 parts by weight, particularly 3 to 25 parts by weight, per 100 parts by weight of the inorganic powder.
It is desirable to set it as weight%.

If the amount of thermally decomposable organic material used is too small, the improvement effect will be insufficient, while if it is too large, undecomposed organic material may remain during firing or the properties of the fired product may deteriorate.

The composite powder obtained as described above is coated onto the surface of an object using appropriate electrostatic means. Typical examples of objects include metal surfaces; - after coating with the above composite powder, another type of composite powder may be further coated; in addition to metal surfaces, ceramic substrates, tile surfaces, Coating can also be applied to glass surfaces, concrete surfaces, wood surfaces, etc.

Since the coated composite powder is only electrostatically attached to the surface of the object, it is then subjected to a firing process to fix it.

Firing is performed under an oxidizing gas atmosphere (such as air) or an inert gas atmosphere (such as nitrogen gas).

Moisture and carbon dioxide in the gas also affect the quality of the product, so they should be managed according to the purpose. The firing temperature is about 700 to 920°C in the case of a steel plate, and generally the firing temperature is set as follows: lower glaze, upper glaze, and painting. In the case of an aluminum plate, it is often fired at about 500 to 600°C.

Function: The resistivity of ordinary enamel frit that is not coated with a pyrolyzable organic substance is on the order of 106 to 10 Ωcm, and is not a perfect insulator. Since the surface of the inorganic powder is coated with a thermally decomposable organic substance, its resistivity is, for example, 1O'' to 1
It is on the order of 0''Ω・cm and has high insulation properties.

When an object coated with the composite powder of the present invention is subjected to firing, the pyrolyzable organic substances on the surface of the composite powder that have adhered to it are thermally decomposed before the firing temperature is reached, leaving no carbon behind. It will completely dissipate.

Example Next, the present invention will be further explained with reference to Examples.

Hereinafter, "1 part" and "1%" are expressed on a weight basis.

Example 1, Comparative Example 1 A frit having the composition shown in Table 1 was ground in a ball mill to obtain a frit powder having a particle size of 10 to 20 square meters. The resistivity of the obtained powders was 10' to 10? It was within the order of Ωcm.

In Table 1, No. 1 to No. 4 are the upper glaze compositions for steel plates, and No. 5 to No. 18 are upper glaze compositions for aluminum.

In addition, 10 parts of polymethyl methacrylate powder with a molecular weight of about 100,000 was added to 100 parts of frit of the same composition, and particles were designed for encapsulation by surface fusion using a special stirring surface fusion device (manufactured by Hosokawa Micron Corporation). A composite powder coated with polymethyl methacrylate fine powder and having a particle size of 10 to 20 mm was obtained. The resistivities of the resulting composite powders were all within the range of the order of lO'2 to 10'' Ω·cm.

In addition, when the above polymethyl methacrylate is heated at a temperature increase rate of 10°C/win using a differential thermal analyzer, the temperature rises to 40°C.
It completely thermally decomposes and dissipates without leaving any carbon behind by 0°C.

Table 1 Using an electrostatic powder coating gun, the above-mentioned frit powder and composite powder were applied to a grounded enameled steel plate that had been subjected to a predetermined preparation for compositions of No. 001 to No. 004.
Those with compositions 5 to No. 8 are applied to an aluminum plate at a voltage of 6.
Deposition was done electrostatically at 0 KV.

At this time, the adhesion efficiency of all frit powders was less than 50% and the holding time was less than 10 minutes, whereas the adhesion efficiency of all the composite powders was more than 80% and the holding time was more than 6 hours. Ta.

Then, using an electric furnace, if the compositions of No. 11 to No. 004 were used, they were heated at 800° C. for 5 minutes.
When the composition No. 8 was used, firing was performed at 550° C. for 10 minutes.

When the composite powder was used, the coating layer was mirror-like and had a thickness of 100-150 JL11.

On the other hand, when frit powder was used, falling off of the powder was observed, and a uniform enamel appearance could not be obtained. In addition, N
Powder coating itself was impossible when using a composition having a composition of o and El.

Example 2 To 100 parts of the composition of NO31 in Example 1, Ti
ozlO part, Co-A1 blue pigment 3 parts, molecular weight lO
10 parts of polymethyl methacrylate powder was added thereto, and pulverization was carried out in the coexistence of these powders using a special stirring cloth surface fusion device to obtain a composite powder having a particle size of 10 to 20 JL.

Using an electrostatic powder coating gun, the above composite powder was electrostatically applied to a grounded enamel aluminum plate at a voltage of 60 KV.

Then, it was fired in an electric furnace at 550° C. for 10 minutes to obtain sky blue enamel with a mirror-like appearance.

Furthermore, when the spray meest was collected and subjected to a second coating, favorable results similar to those of the first coating were obtained.

This revealed that the frit and pigment were firmly encapsulated in a uniform ratio.

Example 3 The frit of composition N006 in Example 1 is a high specific gravity, high alkaline type frit, and is generally difficult to powder coat.

To 100 parts of this frit, add 10 parts of Co-An blue pigment.
and 10 parts of polymethyl methacrylate powder with a molecular weight of 100,000 were added, and pulverization was performed using a special stirring surface fusion device in the coexistence of these powders to obtain a composite powder with a particle size of 10 to 20 gra.

Using an electrostatic powder coating gun, the above composite powder was electrostatically applied to a grounded enameled steel plate that had been pretreated at a voltage of 60 KV.

Then, when it was fired for 5 minutes at 800° C. in an electric furnace, a sky blue enamel with a mirror-like appearance was obtained.

Effects of the Invention In addition to the advantages of the powder coating method, which has been attracting attention recently (shortening of the process, thinning of the film, pollution-free, etc.), the composite powder of the present invention also has the following excellent effects: play.

1. Since it has high insulation properties, it can be handled in the same way as electrostatic powder coating, for which the technology has already been established.

2. Since coating is possible even at a relatively low voltage, the repulsive force between particles attached to the surface of the object is small.
Even if a composite powder with a small particle size is used, it will have good adhesion. Also, the amount of coating can be set in a wide range from thick coating to thin coating.

3. Pigments and additives with different specific gravity can be added later (post-compounding), and homogeneous melting is no longer essential.

4. Even if the inorganic powder is one that receives a large amount of current, such as a highly alkaline frit, it can be electrostatically sprayed and the charge will not escape even after it adheres to the object. Therefore, it can be applied to inorganic powder of any oxide composition.

5. Pyrolytic organic substances dissipate during firing, so they do not have any negative effect on the properties of the fired product.

Claims (1)

[Claims] 1. A composite powder for powder coating, which is formed by coating the surface of an inorganic powder with a thermally decomposable organic substance. 2. The pyrolyzable organic substance coating the surface of the inorganic powder in the composite powder is virtually completely pyrolyzed without leaving any carbon at temperatures up to the firing temperature when the composite powder is fired. The composite powder according to claim 1, wherein the composite powder is dissipated as a result of dissipation. 3. Claim 2, wherein the thermally decomposable organic substance is one that thermally decomposes and dissipates substantially completely without leaving any carbon at a temperature 100°C or more lower than the firing temperature during firing of the composite powder.
Composite powder as described. 4. The composite powder according to claim 1, wherein the ratio of the thermally decomposable organic substance to 100 parts by weight of the inorganic powder is 1 to 30 parts by weight. 5. The composite powder according to claim 1, wherein the inorganic powder is a frit powder for producing enamel.