JP2652188B2 - Composite powder for powder coating - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a composite powder for powder coating, which has good adhesion efficiency to an object and good retention after adhesion,
In particular, it relates to a composite powder for producing an enamel.

2. Description of the Related Art Enamel products are prepared by grinding a ceramic raw material called a frit with a mill-added emulsifier and a ball mill to form a slip-shaped enamel glaze composition. It is manufactured by glazing the surface that has already been underglazed by dipping or spraying, drying and firing.

This method has been widely practiced for a long time, but recently, instead of this traditional method, a powder coating method of electrostatically attaching a frit powder to a metal surface has attracted attention.

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

Japanese Patent Application Laid-Open No. 52-098721 discloses a technique in which an enamel frit is reacted with an alkoxysilane such as methyltrimethoxysilane, and then the reaction frit is electrostatically sprayed on a substrate.

JP-A-61-178440, JP-A-61-178441 and JP-A-61-178442 disclose powder coating frits in which the composition of the frit is devised. A technique for electrostatically adhering to a metal substrate surface is disclosed.

Problems to be Solved by the Invention However, the conventional method of dipping or spraying a glaze on a metal ground using a water-suspended slip is complicated in the slip preparation process and requires preliminary drying before firing. However, there is a disadvantage that the process is multi-stage. In addition, there is a possibility that particles may undergo secondary aggregation due to evaporation of water and solvent during drying. If the thickness is not a certain thickness or more, it is difficult to obtain a smooth surface after firing, and if the thickness is large, the weight becomes large. I cannot escape it. Further, 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 of electrostatically adhering frit powder to a metal surface is a method that meets the requirements for shortening the process, making the film thinner, and eliminating pollution, but still has the following problems. The fundamental improvement is desired.

(1) Since the ceramic has a large specific gravity and a small charge amount, 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 the Coulomb force increases, and as a result, the retention time of the powder after the adhesion becomes short.

(2) In particular, in the case of powder coating with a powder having a small particle diameter, since the repulsive force after adhesion is extremely large, so-called back emission occurs on the adhesion surface, and the coating film tends to be deformed.

(3) When a simple mixed system of two or more components is used, each component behaves separately, so that it is not possible to insert (post-mix) pigments or additives having different specific gravities. Therefore, in order to obtain a uniform composition,
After melting the mixed system to a uniform phase, it is indispensable to pulverize.

(4) Lowering the melting point of the frit is advantageous for increasing the adhesion to the metal. However, when the alkali content is increased by adding an alkali component to lower the melting point of the frit, the amount of electricity increases as the amount of addition increases, so that if a certain limit is exceeded, the electrostatic coating itself becomes impossible. For example, Na ₂ O, K ₂ O and Li ₂
Electrostatic coating is possible up to a total amount of O of about 15% by weight, at most about 20% by weight, but it cannot be made highly alkaline, such as 30% by weight. Further, even in the range where electrostatic coating is possible, there is a problem that the storage stability is deteriorated as the amount of the alkali component is increased.

Among the powder coating methods, the above-mentioned JP-A-51-7005
In JP-A-52-98721 and JP-A-52-98721, the frit powder is coated with a silicon compound, so that it is devised to have a high resistivity. As a result, silicon oxide precipitates on the surface, and properties such as chemical resistance, hot water resistance and transparency of the glazed surface may decrease.

It has been proposed to coat the frit powder with a resin (for example, Japanese Patent Application Laid-Open Nos. 56-35770 and
4-25484, JP-A-50-59425, JP-A-48-5
No. 5916), since the frit powder is simply covered with the resin, the bonding strength of the resin to the frit powder is small, and the expected improvement cannot be obtained.

In view of such circumstances, an object of the present invention is to provide a composite powder for powder coating that does not impair the characteristics of a glazed surface and has excellent electrostatic coating properties and adhesion efficiency. .

Means for Solving the Problems The composite powder for powder coating of the present invention is a composite powder used for applying a powder coating to an object, wherein the composite powder grinds an inorganic substance to a predetermined particle size. The method is characterized by comprising a surface-fused composite powder obtained by coexisting a thermally decomposable organic substance as a powder and fusing the thermally decomposable organic substance powder on the surface of the inorganic powder.

As the inorganic powder, a frit for producing enamel is particularly suitable, and a heat-resistant pigment and other secondary components may be previously blended.

Raw materials for such frit include high-flammable raw materials such as quartzite and feldspar; boric acid, borax, lithium carbonate, soda ash, chile saltpeter, saltpeter, potassium carbonate, sodium fluoride,
Sodium silicate, cryolite, aluminum fluoride, fluorite, limestone, slaked lime, bone ash, magnesium carbonate, magnesia, zinc white, zinc carbonate, barium carbonate, lead red, lead white,
Low-flammable raw materials such as Midaka; anti-metallic raw materials such as metal antimony, antimony oxide, sodium antimonate, tin oxide, zirconium oxide, titanium dioxide, cerium oxide, molybdenum oxide, and arsenous acid; Al ₂ O ₃ , SnO ₂ , ZrO ₂ , TiO ₂ , Cr ₂ O ₃
Such as oxide pigments such as spinel type and pyrochlore type, ZrSiO ₄ , 3CaOCr ₂ O ₃ 3SiO ₂ , CaOSnO ₂ SiO
_2, 2ZnOSiO _2, silicate-based pigments such as 2MgOSiO _2, or various ceramic pigments, including those co-firing pigments; and others.

The oxide composition of the frit can be, for example, a steel plate enamel with a lower glaze (black lower glaze, white lower glaze) or an upper glaze (tin oxide,
Various compositions can be obtained depending on whether the cast iron enamel is a lower glaze or an upper glaze depending on whether it is fluoride, antimony over white glaze, titanium over white background), colored glaze, or spot glaze. It is a feature of the present invention that any oxide composition can be coated with a thermally decomposable organic substance to significantly improve its electrostatic coating property and coating efficiency.

Examples of the inorganic powder include frit powder for producing an enamel, ceramic powder for producing a ceramic substrate, and various other ceramic powders. A heat-resistant pigment can be used as the inorganic powder, and in some cases, a non-ceramic powder such as a metal powder can be used.

As the thermally decomposable organic substance for coating the surface of the above-mentioned inorganic powder, when the composite powder is fired, it is substantially completely thermally decomposed and dissipated without leaving carbon at a temperature up to the firing temperature. Any organic substance can be used. More preferably, one that completely decomposes and dissipates at a temperature lower by 100 ° C. or more than the firing temperature is used. The thermal decomposition behavior of the thermally decomposable organic substance can be easily known by heating at a constant heating rate (for example, 10 ° C./min) using a differential thermal analyzer.

Such thermally decomposable organic materials include thermoplastic polymers, thermosetting polymers, low molecular weight polymers (prepolymers,
Oligomers) and non-polymeric organic substances. As the most suitable heat dispersible organic substance, polyalkyl methacrylate having 1 to 4 carbon atoms in the alkyl group,
Among them is polymethyl methacrylate. Other examples of the thermally decomposable organic material include polyvinyl alcohol, polyvinyl butyral, polyacetal, nitrified cotton,
Sucrose ester, sucrose, paraffin and the like can be mentioned.

In order to coat the surface of the inorganic powder with the thermally decomposable organic substance, the following specific method is employed in the present invention.

That is, when the inorganic substance is ground to a predetermined particle size, a method in which the thermally decomposable organic substance coexists in the form of powder is used.
When such a method is employed, a new surface (crushed surface) in which active bare ions and radicals are present at the time of grinding of an inorganic substance is developed, and radicals are generated by cutting the thermally decomposable organic material at the time of grinding. Therefore, grafting occurs between the inorganic powder and the organic substance, and the thermally decomposable organic substance powder is fused to the surface of the inorganic powder, and the organic powder is encapsulated in the thermally decomposable organic substance powder. Become like In this case, if the system is closed and the required particle size is taken out without taking out the large or small particle size, the large particle size is further milled and the small particle size is granulated. Therefore, a material having a narrow particle size distribution can be obtained.

The ratio of the thermally decomposable organic substance to the inorganic powder is desirably 1 to 30 parts by weight, particularly 3 to 25% by weight, based on 100 parts by weight of the inorganic powder. When the amount of the thermally decomposable organic substance is too small, the improvement effect is insufficient. On the other hand, when the amount is too large, undecomposed organic substances may remain at the time of firing or the properties of the fired product may be deteriorated.

The composite powder obtained as described above is subjected to coating on the surface of the object using a suitable electrostatic means.
A typical example of the object is a metal surface. Once the composite powder is coated, another type of composite powder may be coated. In addition to metal surfaces, coating can be performed on ceramic substrates, tile surfaces, glass surfaces, concrete surfaces, wood surfaces, and the like.

Since the composite powder after coating is only electrostatically attached to the surface of the object, the composite powder is then subjected to a baking step to fix it.

The firing is performed in an oxidizing gas atmosphere (such as air) or an inert gas atmosphere (such as nitrogen gas). Since the moisture and carbon dioxide in the gas also affect the quality of the product, control them according to the purpose. The firing temperature is 700 for enameled steel sheet.
The firing temperature is generally set as follows: lower glaze> upper glaze> painting. 50 composite aluminum sheets
It is often fired at about 0 to 600 ° C.

Action The resistivity of a normal enamel frit that is not coated with a thermally decomposable organic substance is on the order of 10 ⁶ to 10 ⁸ Ω · cm and is not a perfect insulator. On the other hand, in the composite powder of the present invention, since the surface of the inorganic powder is coated with the thermally decomposable organic substance, its resistivity is, for example, 10 ¹² to 10 ¹⁶ Ωcm.
And high insulation.

When the object coated with the composite powder of the present invention is subjected to firing, the thermally decomposable organic substance on the surface of the attached composite powder is thermally decomposed before reaching the firing temperature without leaving carbon. Will completely dissipate.

EXAMPLES Next, the present invention will be further described with reference to examples. Less than,
“Parts” and “%” are expressed on a weight basis.

Example 1, Comparative Example 1 A frit having the composition shown in Table 1 was pulverized with a ball mill to obtain a frit powder having a particle size of 10 to 20 μm. The resistivity of each of the obtained powders was in the range of 10 ⁶ to 10 ⁸ Ω · cm.

In Table 1, No.1 to No.4 are the composition of steel plate enamel top glaze, No.5
No. 6 is the upper glaze composition for aluminum enamel.

Also, add 100 parts of frit of the same composition to 10 parts of polymethyl methacrylate powder having a molecular weight of about 100,000, and use a special agitation surface fusion device (manufactured by Hosokawa Micron Corporation) to design particles for encapsulation by surface fusion, A composite powder having a particle diameter of 10 to 20 μm coated with polymethyl methacrylate fine powder was obtained. The resistivity of each of the obtained composite powders is 10 ¹² to 10 ¹⁶
It was in the range of the order of Ω · cm.

When the above-mentioned polymethyl methacrylate is heated at a heating rate of 10 ° C./min using a differential thermal analyzer, it is completely thermally decomposed and dissipated without leaving carbon up to 400 ° C.

Using an electrostatic powder coating gun, the above frit powder and composite powder, those of the compositions of No. 1 to No. 4 are grounded to a grounded enameled steel plate with a predetermined preparation, The compositions Nos. 5 to 6 were electrostatically attached to an aluminum plate at a voltage of 60 KV.

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

Then, using an electric furnace, when using the composition of No. 1 to No. 4 for 5 minutes at 800 ℃, when using the composition of No. 5 to No. 6 for 10 minutes at 550 ℃ The firing was performed.

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

On the other hand, when the frit powder was used, powder fell off and the like, and a uniform enamel appearance was not obtained. In addition,
When the composition of No. 6 was used, powder coating itself was impossible.

Example 2 TiO ₂ 10 was added to 100 parts of the frit having the composition No. 1 of Example 1.
Parts, 3 parts of Co-Al-based blue pigment and 10 minutes of polymethyl methacrylate powder having a molecular weight of 100,000, and pulverized using a special stirring surface fusion device in the presence of these to obtain a particle size of 10 to 20 μm.
Was obtained.

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

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

Furthermore, when the spray mist was collected and subjected to the second coating, the same favorable results as in the first coating were obtained.

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

Example 3 The frit having the composition of No. 6 in Example 1 is a high specific gravity, high alkali type frit, and generally powder coating is difficult.

To 100 parts of this frit, 10 parts of a poly-methyl methacrylate powder having a molecular weight of 100,000 and 10 parts of a Co-Al-based blue pigment were added,
In the coexistence of these, pulverization was performed using a special stirring surface fusion device to obtain a composite powder having a particle size of 10 to 20 μm.

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

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

Effect of the Invention The composite powder of the present invention has the following advantages, in addition to the advantages of the powder coating method that has recently attracted attention (shortening of the process, thinning, non-polluting, etc.). Play.

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

2.Since coating is possible even at a relatively low voltage, the repulsion between particles adhering to the object surface is small, and even if a composite powder having a small particle diameter is used, it has good adhesion. . Further, the amount of adhesion can be set in a wide range from thick coating to thin coating.

3. It is possible to add (post-mix) pigments and additives having different specific gravities, so that uniform melting is not essential.

4. Even if the inorganic powder has a large amount of electricity, such as a high alkali frit, electrostatic spraying is possible and the charge does not escape even after it adheres to the target. Therefore, it can be applied to inorganic powders having any oxide composition.

5. Since the thermally decomposable organic substances dissipate during firing, there is no adverse effect on the characteristics of the fired product.

Claims (5)

(57) [Claims] 1. A composite powder used for applying powder coating to an object, wherein the composite powder coexists with a thermally decomposable organic substance when the inorganic substance is ground to a predetermined particle size. A composite powder for powder coating, comprising a surface-fused composite powder obtained by fusing a thermally decomposable organic substance powder onto the surface of an inorganic powder. 2. The heat-decomposable organic substance covering the surface of the inorganic powder in the surface-fused composite powder is capable of leaving no carbon at a temperature up to the firing temperature when the surface-fused composite powder is fired. The composite powder for powder coating according to claim 1, which is substantially completely thermally decomposed and dissipated. 3. The method according to claim 1, wherein the thermally decomposable organic substance is substantially completely thermally decomposed and dissipated at the temperature lower than the firing temperature by 100 ° C. or more without firing carbon at the time of firing the surface-fused composite powder. The composite powder for powder coating according to claim 2. 4. The composite powder for powder coating 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 for powder coating according to claim 1, wherein the inorganic powder is a frit powder for producing an enamel.