JPS6329901B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a coating composition for selective absorption applied to the surface of a solar heat collector plate. Conventional configurations and their problems Conventionally, with the aim of improving the heat collection characteristics of solar heat collectors, we have developed selective absorption films on the surface of heat collection plates, selective permeation films on glass transparent plates, and convection prevention films inside the heat collectors. Various technologies have been introduced. Among these, the development of selective absorption membranes is being actively pursued. Desirable properties for a surface that collects solar energy include absorbing as much solar radiation energy as possible, most of which exists in the wavelength range of 0.3 to 2.0 μm, and absorbing solar energy to a temperature of 80 to 100 degrees Celsius.
The wavelength radiated from the surface of the heat collector plate whose temperature has increased to
One of its characteristics is to minimize the amount of radiant energy, most of which is present at 3 μm or larger. This characteristic is called selective absorption. Specific examples of heat collector plates that have been given selective absorption properties through plating or chemical conversion treatment include colored stainless steel, electrolytic black aluminum oxide, black chrome, and black cutter, all of which have wavelengths of 0.3 to 2.0. The solar energy absorption rate α (hereinafter simply abbreviated as α) in μm is 0.90 to 0.95, and the infrared emissivity ε (hereinafter simply abbreviated as ε) at a wavelength of 3 to 30 μm.
shows good selective absorption of 0.10 to 0.15. However, these methods require complicated management to control the composition and thickness of the black film formed on the metal surface that is the support, and the manufacturing equipment (including waste liquid treatment) is difficult to manage.
The problem was that the scale was large and the cost was high. Against this background, we have focused on coating, which has high productivity and is advantageous in terms of total cost.Recently, coatings that impart selective absorption have been developed, and heat collecting plates that have selective absorption due to coating have appeared. ing. The characteristics of a heat collector plate that has been given selective absorption by coating are that when the coating film thickness is approximately 3 μm and the support is stainless steel, α is 0.94 and ε is 0.45, and when the film thickness is the same and the support is aluminum, α is is 0.94 and ε is 0.40
It has the disadvantage that ε is significantly inferior to the above-mentioned colored stainless steel, black chrome, etc.
In order to lower ε below the above value, the thickness of the coating film can be made even thinner in order to reduce the infrared absorption of the binder resin. 0.8～2.0μm
Since the reflection in the near-infrared region of
In the case of aluminum, ε is 0.30, and α is 0.88.
In addition to having a characteristic of ε of 0.25, a thin film has the disadvantage that it becomes difficult to hide the support and the adhesion and corrosion resistance of the paint film are significantly deteriorated, and a film thickness of 1.5 μm or less is not practical. Ta. A low ε is important for improving the performance of a heat collector, but a high α is also important for low-temperature heat collectors intended for hot water supply. In fact, when comparing the heat collection performance of a heat collection plate with characteristics of α: 0.94, ε: 0.45 and a heat collection plate with characteristics of α: 0.90, ε: 0.30 in a water heating system, it is found that the two have almost the same heat collection performance. show,
Lowering ε is not the only way to improve the performance of a heat collector; the important issue is how to maintain a high α and achieve a low ε. Purpose of the Invention An object of the present invention is to provide a coating composition for selective absorption for solar heat collector plates that has excellent selective absorption properties, is low cost, and has excellent durability such as corrosion resistance and weather resistance. be. Structure of the Invention The selective absorption coating composition of the present invention contains a black pigment, a binder agent, a tetrafluoroethylene resin powder, an ultraviolet absorber, and a surfactant. The main purpose of the black pigment is to absorb solar energy, but in order to achieve good selective absorption, it is preferable to use an inorganic pigment that absorbs less infrared rays at wavelengths of 3 μm or more than organic pigments. . Furthermore, inorganic pigments also have excellent heat resistance.
Particularly preferred black inorganic pigments include oxides or composite oxides of one or more metals selected from the group consisting of iron, manganese, copper, chromium, cobalt, and nickel.
CoO, Cr ₂ O ₃ , MnO ₂ , Fe ₂ O ₃ , Fe ₂ O ₃ −MnO ₂ −
Examples include CuO and CuO−Cr ₂ O ₃ . Considering practicality including cost, Fe ₂ O ₃ −MnO ₂ −CuO pigment is optimal. In addition, the particle size of these pigments is preferably 1 μm or less in order to exhibit stable hiding power, have little scattering absorption at wavelengths of 3 μm or more, and obtain a low ε.
In order to obtain particularly high α, the average particle diameter should be 0.3~
Preferably, it is 0.5 μm. This is thought to be because the particle diameter suitable for scattering and absorbing near-infrared rays with a wavelength of 0.8 to 2.0 μm, where solar energy is distributed, is 0.3 to 0.5 μm. The binder used in this invention is preferably one that can form a thin coating film and has excellent heat resistance, weather resistance, and corrosion resistance. Examples of such a binder include acrylic resin, silicone resin, and epoxy resin. At least one or two or more selected from the group consisting of resin, fluororesin, urethane resin, and polyester resin,
Furthermore, modified versions of these resins can also be used. Considering practicality including cost, acrylic resin with epoxy resin added to improve adhesiveness can be cited. In addition, the ultraviolet absorber absorbs ultraviolet light from a wavelength of 0.3 μm, which is insufficiently absorbed by black inorganic pigments,
It is added to absorb as much light as possible in the wavelength range of 0.3 to 2.0 μm, where solar energy exists. Examples of ultraviolet absorbers include transparent iron oxide and ultrafine metal particles, which have a particle size of
A material with a diameter of 0.1 μm or less has particularly excellent ultraviolet absorption. This is thought to be because the particle size of 0.1 μm or less is suitable for scattering and absorbing ultraviolet light with a wavelength of about 0.3 μm. The tetrafluoroethylene resin powder (hereinafter referred to as PTFE powder) in this invention was originally used to improve the corrosion resistance, heat resistance, and abrasion resistance of coating films, but this powder has a wavelength of 0.8 to 2.0. It has become clear that it effectively contributes to the scattering and absorption of near-infrared rays in the μm range. This is thought to be due to the fact that the average particle diameter of the PTFE powder is about 0.3 μm, which has a strange effect on the scattering and absorption of the near-infrared rays. In order to achieve selective absorption through painting, the thickness of the paint film must be made extremely thin, and in order to obtain a uniformly stable paint film with a thinner film thickness, the degree of dilution of the paint to be applied must be increased to reduce the wet film thickness. It is necessary to increase the difference between the thickness and the dry film thickness after solvent evaporation. However, when highly diluted paint is applied at a low paint discharge pressure of 1 kg/ ^cm2 , convection occurs within the paint film containing a large amount of solvent as the solvent evaporates, causing the pigment and resin to evaporate. Separation occurs, resulting in problems such as spots and stripes on the paint film. The surfactant in this invention is added to paint to solve these problems, and after studying various additives, we found that amino-modified silicone resin,
The surfactant of the polyether-modified silicone resin was good. In particular, in order to reduce the thickness of the paint film to 1 μm or less, the dilution of the paint is quite high.
Addition of the above-mentioned surfactant can be said to be extremely effective. The coating composition of the present invention can be prepared by dispersing and mixing the above-mentioned black pigment, binder agent, and various additives together with an appropriate amount of a solvent using a dispersing machine such as a ball mill, roll mill, or attritor. As the support to which the coating composition of the present invention is applied, metals with low ε are advantageous, such as aluminum, copper, stainless steel, and the like. Furthermore, even if the support is not made of metal, it is also possible to use a material such as plastic whose surface has been treated to lower ε. Description of Examples The selective absorption coating composition of the present invention will be explained with reference to Examples. The test pieces used in each example were made of aluminum (JIS P 1100P standard) and had dimensions of 75.
It is mm x 150mm x 0.5mm. Example 1 (About pigment/resin) Thermosetting acrylic resin ("Dyanal SE-5661" manufactured by Mitsubishi Rayon Co., Ltd., resin solid content 50% by weight)
To 100 parts (parts by weight, the same applies hereinafter) of 40 parts of a commercially available Fe ₂ O ₃ -MnO ₂ -CuO black pigment with an average particle size of 0.3 μm was added, and further 58 parts of n-butanol was added.
1 part, 42 parts of xylene, and 100 parts of "Solbetsuso #100" (trade name) were added and dispersed and mixed in a ball mill for 24 hours to prepare a paint. After adjusting this paint to an appropriate viscosity, it was sprayed onto an aluminum test piece and baked at 200°C for 10 minutes to obtain a coating film (sample 1) with a thickness of 1 μm. Further, coating films with a thickness of 1 μm (Samples 2, 3, and 4) were obtained in the same manner as described above, except that the amount of pigment was changed to 60 parts, 80 parts, and 100 parts. For each sample 1 to 4, the spectral reflectance at a wavelength of 0.75 to 2.0 μm was measured using a self-recording spectrophotometer “MPS-5000 model” manufactured by Shimadzu Corporation with an integrating sphere reflector “ISR-2 model” added. FIG. 1 shows the spectral reflectance of each sample 1 to 4. As is clear from FIG. 1, as the ratio (pigment/resin) increases, the spectral reflectance decreases and α increases, but this is because the pigment density on the support increases. In addition, as a result of measuring ε of these samples at a wavelength of 3 to 30 μm using an "emissivity meter" manufactured by Devices & Services, the results showed that ε was 0.20 to 0.21 in all cases.
(pigment/resin) as it is not affected much.
The higher the (pigment/resin) content, the more advantageous the selective absorption becomes. However, when the (pigment/resin) content increases, the viscosity of the paint increases, which impairs coating workability, and has the disadvantage that the adhesion and hardness of the paint film itself deteriorate. When testing the adhesion of the paint film on these test pieces, it was found that those with a ratio of 100/100 (pigment/resin) had poorer adhesion than the others. Therefore, considering both the adhesion and selective absorption of the paint film, the blending ratio (pigment/resin) should be 60/100 to 80/100.
A range of is good. As mentioned above, since the solid content of the acrylic resin used is 50% by weight, the ratio (pigment/resin solid content) is in the range of 60/50 to 80/50. Example 2 (About PTFE powder) Using the same acrylic resin and black pigment as used in Example 1, 70 parts of black pigment was added to 100 parts of acrylic resin, and PTFE powder with an average particle diameter of about 0.3 μm was added.
PTFE powder (Daikin Industries, Ltd. "Ruburon L-"
2) was added at a ratio of 5 parts to 100 parts of the acrylic resin, a paint was prepared in the same manner as in Example 1, and this was sprayed onto an aluminum test piece,
A coating film (sample 5) with a thickness of 1 μm was obtained by baking at 200° C. for 10 minutes. In addition, the film thickness was 1 μm in the same manner as above except that the amount of PTFE powder was changed to 10 parts and 15 parts.
coating films (Samples 6 and 7) were obtained. Regarding each of these samples 5 to 7, the spectral reflectance was measured in the wavelength range of 0.75 to 2.0 μm in the same manner as in Example 1. The results are shown in FIG. Further, when ε was measured in the same manner as in Example 1, ε was 0.20 for all samples. From Figure 2, it can be seen that as the amount of the PTFE powder added increases, the spectral reflectance tends to decrease, and it contributes effectively to absorption in the near-infrared region of wavelengths 0.8 to 2.0 μm. It was revealed that. This is thought to be because the particle size of the PTFE powder is suitable for scattering and absorption in the near-infrared region. Furthermore, since all samples 5 to 7 have the same ε, the PTFE powder has a wavelength of 3μ.
It is thought that the absorption of infrared rays above m is very low. Moreover, when the adhesion of the coating film was tested for these samples 5 to 7, good results were obtained in all of them.
However, since the PTFE powder is white and exists in the coating film as particles, if the amount added is further increased, the coating surface becomes white and the wavelength
There was a tendency for absorption in the ultraviolet and visible regions of 0.3 to 8.0 μm to deteriorate. Therefore, the amount of PTFE powder added is preferably around 15 parts per 100 parts of the acrylic resin. Note that the amount is approximately 30 parts for 100 parts of the solid acrylic resin. Example 3 (About ultraviolet absorber) The coating composition of Example 2 had an average particle size of about 0.01μ.
A paint was prepared in the same manner as in Example 1 by adding 5 parts of transparent iron oxide ("Transoxide Red" manufactured by Dainichiseika Co., Ltd.), and this was sprayed onto an aluminum test piece. It was baked at ℃ for 10 minutes to obtain a coating film with a thickness of about 1 μm. In addition, instead of the transparent iron oxide, average particle diameter
A coating film was obtained in the same manner as described above, except that ultrafine metal particles made of copper of 0.05 μm were added. For each sample obtained in this way, the wavelength is 0.3~
The spectral reflectance at 2.0 μm was examined in the same manner as in Example 1. The results are shown in Table 1.

[Table] As shown in Table 1, coating films containing transparent iron oxide and coating films containing ultrafine metal particles made of copper have a higher wavelength than coating films containing no ultraviolet absorber.
Absorption of ultraviolet rays of 0.3 to 0.4 μm was observed, which can be said to contribute to the improvement of α. In particular, the coating film containing ultrafine metal particles made of copper has higher absorption up to a wavelength of about 0.6 μm than other coatings, and has achieved desirable results. Note that at wavelengths of 0.7 μm or more, all three exhibited approximately the same spectral reflectance. Furthermore, for these test pieces, ε was set to Example 1.
When measured using the same method as ε=0.20~
0.21, and it can be said that the addition of ultrafine metal particles made of transparent iron oxide and copper does not worsen ε.
In addition, since the transparent iron oxide and copper metal ultrafine particles absorb ultraviolet rays, deterioration of the resin as a binder agent is suppressed, and an improvement effect on weather resistance is also expected. In addition, almost the same results as the copper metal fine particles were obtained with ultrafine metal particles of iron and nickel-iron. Example 4 (About the surfactant) For 100 parts of a paint prepared in the same manner as in Example 1 using the paint composition of Example 3, a weight ratio of n-butanol: xylene: "Solbetsuso #100" was added.
Dilute by adding 200 parts of a mixed solvent of 29:21:50,
Various additives shown in Table 2 were added. Paints with various additives were applied to aluminum test pieces under coating conditions that tend to cause spots and stripes on the paint film, and the film thickness was approximately
It was coated to a thickness of 1 μm and baked at 200°C for 10 minutes. The surface of the coating film on the test piece thus obtained was observed to evaluate coating defects. The results are shown in Table 2. Note that 20 test pieces were prepared for each type of additive and evaluated. Moreover, the evaluation shown in Table 2 was performed based on the following criteria. ×: Less than half of the test pieces had noticeable spots on the coating surface.
Striped patterns occur △: Less than half of the test pieces have noticeable spots on the coating surface.
Striped patterns occur 〇: No abnormality in all cases

[Table] From the test results shown in Table 2, when paint additives are used for the purpose of stably forming a thin film when paint is highly diluted, polyether-modified silicone resins, amino-modified silicone resins, etc. should be used rather than dispersants. It can be said that addition of a surfactant is effective. The reason why these surfactants are effective is that they improve the leveling of the paint relative to the support during painting. In addition, when the additives listed in Table 2 were not included, more than half of the 20 test pieces had spots or striped patterns on the coating film surface. Example 5 Thermosetting acrylic resin “Dyanal SE-
5661” (mentioned above) 100 parts, with an average particle size of approximately 0.3 μm.
70 parts of Fe ₂ O ₃ -MnO ₂ -CuO black pigment, 15 parts of PTFE powder "Luburon L-2" (mentioned above), 5 parts of transparent iron oxide "Transoxide Red" (mentioned above), n
- A paint was prepared in the same manner as in Example 1 using 58 parts of butanol, 42 parts of xylene and 100 parts of "Sorbetso #100". n- per 100 parts of this paint
Butanol: Xylene: "Sorbetso #100"
After diluting by adding 200 parts of a mixed solvent with a ratio of 29:21:50 (weight ratio) and further adding 1.5 parts of a surfactant polyether-modified silicone resin, a film thickness of about 1 μm was formed on an aluminum test piece. It was spray painted and baked at 200℃ for 10 minutes.
For the sample prepared in this way, the wavelength was 0.3
Spectral reflectance of ~2.0 μm was measured in the same manner as in Example 1, α for solar energy at atmospheric mass M = 2 was calculated from the spectral reflectance, and ε was further measured in the same manner as in Example 1 to determine selective absorption. When the characteristics were evaluated, the characteristics were obtained that α was 0.93 and ε was 0.18 to 0.20. Example 6 Silicone resin (“KR-282” manufactured by Shin-Etsu Chemical Co., Ltd.) was used instead of the thermosetting acrylic resin in Example 5.
Example 5 except that a resin solid content of 50% by weight was used, and only xylene was used as a solvent and diluent.
A paint was prepared in the same manner as above, and it was sprayed onto an aluminum test piece to a film thickness of about 1 μm, and baked at 200°C for 30 minutes. When α and ε of the thus prepared test piece were evaluated in the same manner as in Example 5, α was 0.93 and ε was 0.18 to 0.20.
The following characteristics were obtained. Example 7 The reliability of the coating film, such as adhesion, heat resistance, corrosion resistance, and accelerated weathering resistance, was evaluated for each sample obtained in Examples 5 and 6, and the results were good as shown in Tables 3 and 4. The results were obtained.

【table】

【table】

【table】

[Table] Also, when epoxy resin, fluororesin, urethane resin, and polyester resin were used as the binder, almost the same results as in Examples 1 to 7 were obtained. Effects of the Invention The selective absorption coating composition of the present invention not only provides extremely excellent selective absorption properties, but also improves durability such as corrosion resistance and weather resistance, increases reliability, and can be manufactured at low cost. This has the effect that a film can be obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the spectral reflectance of each sample (coating film) obtained in Example 1, and Figure 2 is a graph showing the spectral reflectance of Example 2.
It is a graph showing the spectral reflectance of the sample obtained in .

Claims (1)

[Scope of Claims] 1. A coating composition for selective absorption containing a black pigment, a binder agent, a tetrafluoroethylene resin powder, an ultraviolet absorber, and a surfactant. 2 The black pigment is iron, manganese, copper, chromium,
The paint composition for selective absorption according to claim 1, which is an oxide or composite oxide of one or more metals selected from the group consisting of cobalt and nickel. 3. The selective absorption material according to claim 1, wherein the binder agent is one or more selected from the group consisting of acrylic resin, silicone resin, epoxy resin, fluororesin, urethane resin, and polyester resin. Paint composition. 4. The coating composition for selective absorption according to claim 1, wherein the ultraviolet absorber is fine metal particles or transparent iron oxide with an average particle size of 0.1 μm or less. 5. The coating composition for selective absorption according to claim 1, wherein the surfactant is a polyether-modified silicone resin or an amino-modified silicone resin.