JPH0146540B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat collecting material that is coated on the surface of a solar heat collector, mainly made of metal such as copper or aluminum, to impart selective absorption of solar heat to the surface. Due to the energy situation in recent years, solar heat utilization technology has attracted attention, and it is already being put into practical use for household purposes such as hot water heating. A method of imparting selective absorption properties to the surface of a solar collector that absorbs as much solar energy as possible as a desirable property, and minimizes the radiant energy radiated from the surface whose temperature has increased as a result of absorption. There is. Most of the radiant energy of sunlight passing through the atmosphere is in the short wavelength range of 0.2 to 2.5 μm, and on the other hand, this energy is absorbed and converted into heat.
For example, the radiant energy emitted from a surface whose temperature has increased to 80 to 100°C is in the infrared region with long wavelengths of 2.5 μm or more. Utilizing this wavelength shift between input and output,
The closer the absorption rate is to 1 on the short wavelength side of 2.5 μm or less, and the closer the emissivity is to 0 on the long wavelength side of 2.5 μm or more, the more ideal the selective absorption surface becomes. In order to form a paint film on a metal surface and make it a surface that selectively absorbs solar heat, the absorption rate can be reduced by using a black pigment that has excellent absorption of light with a wavelength of 2.5 μm or less. It can be increased relatively easily. However, since ordinary coating films also absorb wavelengths of 2.5 μm or more, the emissivity of wavelengths of 2.5 μm or more increases as a result, and the challenge is how to lower this emissivity. The general idea to achieve this is to take advantage of the fact that metal surfaces reflect infrared rays well, and by making the coating film as thin as possible to a few micrometers or less, the absorption of infrared rays of 2.5 micrometers or more is reduced. There is a method to keep the emissivity low, that is, to keep the emissivity low. In this method, by selecting paint components to a certain extent,
Even if selective absorption can be achieved,
Depending on the type of metal used as the support, it has been extremely difficult to obtain a stable support due to poor adhesion with the coating film. When stainless steel is used as a support,
A proposal by the present inventors has already achieved a practical level of achieving both selective absorption and coating film properties. According to this, a fluororesin is added to a paint whose main components are an inorganic black pigment and an acrylic resin to improve heat resistance and corrosion resistance, thereby improving the physical properties of the paint film. The present inventors applied this system to copper and aluminum metal surfaces and conducted various coating film physical property tests similar to those for stainless steel, and found that the coating film peeled off due to corrosion from pinholes. Admitted. Stainless steel itself has excellent corrosion resistance, so even if there are pinholes in the coating, corrosion progresses extremely slowly. The fluororesin present in the paint film exhibits water repellency and repels water, which ultimately suppresses corrosion. This is thought to be the reason for the good adhesion between stainless steel and the paint film. On the other hand, copper and aluminum have poor corrosion resistance compared to stainless steel, and in highly corrosive environments, pinholes exist in the paint film, and corrosion progresses from there, eventually resulting in peeling of the paint film. In this case, although the fluororesin can suppress corrosion to some extent, it cannot withstand severe tests using salt spray or boiling water. In view of the above, the present invention is applied to heat collecting surfaces made of copper, aluminum, etc., which have poor corrosion resistance compared to stainless steel, and is intended to collect solar heat in order to achieve a good balance between selective absorption and coating film properties. It provides heat materials. The heat collecting material of the present invention is obtained by mixing an inorganic black pigment, an acrylic resin and an epoxy resin as binders, together with a solvent as a binder, and dispersing the mixture using a dispersing machine such as a ball mill. Note that auxiliary agents such as surfactants or fillers may be added to the coating material as necessary. In order to achieve selective absorption, the black pigment
In particular, it is inappropriate to apply organic pigments that absorb infrared rays of 2.5 μm or more, so inorganic pigments are used. Particularly good examples include oxides or composite oxides of at least one metal selected from the group of Fe, Mn, Cu, Cr, Co, and Ni. Among them, cobalt oxide-based pigments are excellent, but pigments such as Fe ₂ O ₃ , MnO ₂ , and CuO are most suitable when considering practicality including cost. The particle size of these inorganic black pigments is important in order to obtain stable hiding power for thin film formation, low scattering and absorption of infrared rays, and low emissivity. The particle size is 1.5 μm, but in the case of the present invention, it is finer than that.
An average particle size of 0.01-0.5 μm is desirable. Resins are used as binders for forming coating films, and most resins absorb in the infrared region, and acrylic resins are examples of resins that have relatively low absorption. Thermosetting acrylic resins are examples of acrylic resins that are advantageous in coating film properties such as weather resistance and adhesion, and in selective absorption ability. A paint can be obtained from a thermosetting acrylic resin and an inorganic black pigment, but in this case, physical properties such as adhesion, heat resistance, and corrosion resistance as a paint film are not satisfactory if good selective absorption is desired. In order to improve these drawbacks, the present inventors investigated various resins such as epoxy resins, melamine resins, and silicone resins, and found that the addition of epoxy resins was extremely effective. The addition of this epoxy resin slightly increases the infrared emissivity, but since the infrared reflection of the copper and aluminum used as supports is extremely good, it is on the same level as the selective absorption of the coating film on stainless steel. become. The present invention will be explained below using examples. Copper and aluminum degreased with a solvent were used as base materials to form the coating film. The infrared emissivity of these base materials is ε = 0.02 for copper and ε = 0.02 for aluminum.
It was 0.03. In evaluating selective absorption, the solar absorption rate is determined by measuring the spectral reflectance using a Shimadzu spectrophotometer MPS-5000, and evaluating it from the ratio to the black body radioactivity of 6000〓, and calculating the infrared emissivity. DEVICES &
It was measured using an emissivity meter manufactured by SERVICES COMPANY. Example 1 Using a commercially available Fe ₂ O ₃・MnO ₂・CuO-based black pigment,
Six types were prepared by classifying the particle size. As an acrylic resin, Mitsubishi Rayon Co., Ltd.'s thermosetting silicone-modified acrylic resin "Dyanal"
SE-653" (solvent content: 50% by weight), 100 parts by weight of it, 26 parts by weight of the above-mentioned pigment, and epoxy resin "Epicoat 834-x-90" from Yuka Ciel Epoxy Co., Ltd.
5 parts by weight and 500 parts by weight of a solvent were added and mixed and dispersed in a ball mill for 24 hours to prepare a paint. The solvent was composed of 29% by weight of n-butanol, 21% by weight of xylene, and 50% by weight of "Solbetsuso #100" manufactured by Esso Standard Oil Co., Ltd. Using the paint prepared as above, spray paint to a film thickness of approximately 3μm,
Baked at ℃ for 25 minutes. The coatings on copper and aluminum both showed good coating properties. The following table shows the relationship between pigment particle size and selective absorption. Note that there was no difference between the supports, copper and aluminum.

[Table] As shown in the table, the particle size of the pigment is preferably in the range of 0.01 to 0.5 μm. This is thought to be related to light scattering of particles, and in the particle size range of 0.01 to 0.50 μm,
Does not scatter or absorb much light on the long wavelength side of 2.0μm or more,
It is presumed that this is because particles with a diameter of 2.0 μm or less exhibit properties of good scattering and absorption. It was also confirmed that as the thickness of the coating film increases, the infrared emissivity (infrared absorption rate) increases due to the acrylic resin component that is the binder. From this, in order to make the infrared emissivity 0.5 or less, the thickness of the coating film is required to be 5 μm or less. Example 2 Same as Example 1, but the particle size of the pigment was changed from 0.02 to
0.50μm, adjust the paint by changing the ratio of pigment to acrylic resin, and after painting by spray,
A coating film was created by firing, and its physical properties were evaluated. As a result, the following points were clarified. (1) When the ratio of pigment/acrylic resin is 45/100 or less, the surface light selectivity increases and the adhesion of the coating film becomes extremely good, but the absorption rate of sunlight decreases and the infrared emissivity increases. come. (2) When the ratio of pigment/acrylic resin is 65/100 or more, there is almost no light selection on the surface. Although the sunlight absorption rate is high, the infrared emissivity is low, and although the selective absorption is good, the physical properties of the coating film are significantly worse. From the above, the pigment/acrylic resin ratio is 45/100.
Although a certain level can be obtained within the range of ~65/100, a particularly good range is between 50/100 and 55/100. Example 3 Same as Example 1, but the particle size of the pigment was changed from 0.02 to
0.50, and the paint was adjusted by changing the amount of epoxy resin, painted by spraying, and the paint film was evaluated after baking. (1) When the ratio of epoxy resin solid content/acrylic resin solid content is 5/100 or less. Although the selective absorbency was at the same level as that without the addition of epoxy resin, the adhesion and corrosion resistance were poor, and in particular, when a tape peel test was performed after 500 hours of salt water spraying, peeling of the paint film was observed. (2) When the ratio of epoxy resin solid content/acrylic resin solid content is 5/100 to 15/100, the infrared emissivity is approximately 10% higher than when no epoxy resin is added. This is because the epoxy resin itself has high infrared absorption.
The physical properties of the coating film are good, and the salt spray 500
No peeling was observed even when a tape peeling test was performed after a certain period of time. (3) When the ratio of epoxy resin solid content/acrylic resin solid content is 15/100 or more Corrosion resistance is good, but infrared emissivity becomes extremely high, making it difficult to achieve ε = 0.50 or less. From the above, it is desirable that the ratio of epoxy resin to acrylic resin is 5/100 to 15/100. In this case, the infrared emissivity increases by about 10%, but since the infrared emissivity of the copper and aluminum supports is low, it is possible to stably achieve ε=0.50 or less. Epoxy resin has strong adhesion to metals due to its molecular structure. Therefore, even if corrosion occurs due to pinholes, the paint film will not deteriorate, so it is thought that peeling of the paint film will not occur. Furthermore, it is believed that the addition of the epoxy resin promotes the crosslinking reaction of the acrylic resin, improves the leveling of the resin solid content, and reduces the number of pinholes in the coating film.
Note that the molecular weight of the epoxy resin is preferably 1000 or less in view of compatibility with the acrylic resin. As described above, the present invention can be applied to the particle size of the pigment constituting the coating film, the composition of the pigment and binder, the type of binder,
Only when the necessary conditions for the thickness of the coating film are met can both excellent light selective absorption and physical properties of the coating film (erodibility, adhesion) be achieved. By realizing this, a highly reliable solar heat collecting material can be obtained.

Claims (1)

[Claims] 1 45 to 65 parts by weight of a black pigment made of a metal oxide with a particle size of 0.01 to 0.5 μm and a thermosetting acrylic resin on a support made of copper or aluminum.
100 parts by weight (resin solid content) and 5 to 5 parts of epoxy resin
A solar heat collecting material coated with a coating composition consisting of 15 parts by weight (resin solid content) and a solvent to a film thickness of 5 μm or less.