Radiative cooling structure

Abstract

To provide a radiative cooling structure that exhibits stable anti-glare properties even when used under high-temperature conditions.SOLUTION: A radiative cooling structure comprises a resin layer 12 composed of a thermoplastic resin containing ethylene vinyl acetate, and a metal reflective layer 13 laminated thereon. The interface between the resin layer 12 and the metal reflective layer 13 has irregularities, and the metal reflective layer 13 is formed to follow the irregularities at the interface with the resin layer 12.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a radiative cooling structure. [Background technology]

[0002] There are known radiative cooling structures having a configuration in which a light reflecting layer is provided on a resin layer. For example, Patent Document 1 discloses a radiative cooling structure having a configuration in which a silver light reflecting layer is provided on an ethylene vinyl acetate resin layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2023-002558 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a radiative cooling structure that has stable anti-glare properties even when used at high temperatures. [Means for solving the problem]

[0005] The present invention provides a radiative cooling structure comprising a resin layer formed of a thermoplastic resin containing ethylene vinyl acetate and a metal reflective layer laminated on the resin layer, wherein the interface between the resin layer and the metal reflective layer has irregularities, and the metal reflective layer is formed to conform to the irregularities of the interface with the resin layer. Effect of the Invention

[0006] According to the present invention, the interface between the resin layer formed from a thermoplastic resin containing ethylene vinyl acetate and the metal reflective layer has irregularities, and the metal reflective layer is formed so as to conform to the irregularities at the interface with the resin layer, thereby providing stable antiglare properties even when used at high temperatures. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a cross-sectional view of a radiative cooling sheet according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The embodiments will be described in detail below.

[0009] Figure 1 shows a radiative cooling sheet 10 according to an embodiment. This radiative cooling sheet 10 is attached to the surface of an object to be cooled, and blocks heat caused by solar radiation and cools the object inside. In the following description, the side irradiated by solar radiation is referred to as "upper" and the side of the object to be cooled is referred to as "lower."

[0010] Examples of objects to be cooled include building materials, containers, automobile-related parts, etc. Examples of building materials include building materials for housing, factory and warehouse building materials, etc. Examples of containers include installation containers, transport containers, portable containers, etc. Examples of automobile-related parts include interior parts, air conditioning systems, fuel systems, battery systems, etc.

[0011] The radiative cooling sheet 10 according to the embodiment includes a lower base material layer 11, a lower adhesive resin layer 12 laminated directly thereon, a metal reflective layer 13 laminated directly thereon, an upper adhesive resin layer 14 (second resin layer) laminated directly thereon, an upper base material layer 15 laminated directly thereon, and a surface protection layer 16 laminated directly thereon. In the radiative cooling sheet 10, the radiative cooling structure is configured by these laminates.

[0012] The lower base layer 11 is made of, for example, a resin sheet. Examples of the resin material forming the lower base layer 11 include polyester resin, polycarbonate resin, etc. The thickness of the lower base layer 11 is preferably 12 μm or more from the viewpoint of lamination processability, and is preferably 50 μm or less from the viewpoint of reducing heat generation due to heat absorption.

[0013] The lower adhesive resin layer 12 is composed of a coating layer of a thermoplastic resin containing ethylene vinyl acetate (hereinafter referred to as "EVA"). The content of EVA in the thermoplastic resin forming the lower adhesive resin layer 12 is preferably 80% by mass or more, more preferably 100% by mass, from the viewpoint of obtaining stable antiglare properties even when used under high temperatures as described below. From the same viewpoint, the vinyl acetate content of EVA (hereinafter referred to as "VA content") is preferably 10% by mass or more and 30% by mass or less, more preferably 12% by mass or more and 25% by mass or less, and even more preferably 14% by mass or more and 20% by mass or less. From the same viewpoint, the melting point of EVA is preferably 60°C or more, more preferably 65°C or more, even more preferably 70°C or more, and even more preferably 75°C or more. Here, the VA content of EVA is determined by Fourier transform infrared spectroscopy (FT-IR analysis) or the like. The melting point of EVA is determined by differential scanning calorimetry (DSC).

[0014] The interface between the lower adhesive resin layer 12 and the metal reflective layer 13 has irregularities. From the viewpoint of forming a metal reflective layer 13 that can obtain stable antiglare properties even when used at high temperatures, the lower adhesive resin layer 12 preferably has irregularities on its surface before lamination with the metal reflective layer 13. From the same viewpoint, the irregularities are preferably matte. From the same viewpoint, the arithmetic mean height Sa of the irregularities is preferably 1 μm or more and 3 μm or less, more preferably 1.5 μm or more and 2.5 μm or less. The arithmetic mean height Sa of the irregularities is measured using a laser microscope (for example, VK-X150 / KV-X100 manufactured by KEYENCE) based on ISO25178. The arithmetic mean height Sa of the irregularities can be easily controlled by heat treatment of the lower adhesive resin layer 12.

[0015] The maximum thickness of the lower adhesive resin layer 12 is preferably 10 μm or more from the viewpoint of obtaining sufficient adhesiveness, and is preferably 50 μm or less from the viewpoint of reducing heat generation due to heat absorption.

[0016] The metal reflective layer 13 is formed so as to follow the unevenness of the interface with the lower adhesive resin layer 12. The metal reflective layer 13 is, for example, composed of a metal film. Examples of metals forming the metal reflective layer 13 include silver, silver alloys, aluminum, and aluminum alloys. Among these, silver and silver alloys are preferable, and silver is more preferable, from the viewpoint of obtaining stable anti-glare properties even when used at high temperatures and having a high reflectance for light in the visible light to near infrared region. Examples of the film formation method of the metal reflective layer 13 include a dry film formation method and a wet film formation method. Examples of the dry film formation method include deposition, sputtering, and ion plating. Examples of the wet film formation method include a method using a silver mirror reaction. The thickness of the metal reflective layer 13 is preferably 50 nm or more, more preferably 80 nm or more, and even more preferably 100 nm or more, and is preferably 200 nm or less, from the viewpoint of regulating the transmission of heat rays and obtaining stable anti-glare properties even when used at high temperatures.

[0017] The upper adhesive resin layer 14 is, for example, a coating layer of a thermoplastic resin. The thermoplastic resin forming the upper adhesive resin layer 14 preferably contains EVA, similar to the lower adhesive resin layer 12, from the viewpoint of obtaining stable antiglare properties even when used under high temperatures. In this case, the content of EVA in the thermoplastic resin forming the upper adhesive resin layer 14 is preferably 80% by mass or more, more preferably 100% by mass. From the same viewpoint, the VA content in the EVA is preferably 10% by mass or more and 30% by mass or less, more preferably 12% by mass or more and 25% by mass or less, and even more preferably 14% by mass or more and 20% by mass or less. From the same viewpoint, the melting point of EVA is preferably 60° C. or more, more preferably 65° C. or more, even more preferably 70° C. or more, and even more preferably 75° C. or more. From the same viewpoint, the thermoplastic resin forming the upper adhesive resin layer 14 is preferably the same as the thermoplastic resin forming the lower adhesive resin layer 12.

[0018] The upper adhesive resin layer 14 is formed to fill and flatten the unevenness of the interface with the metal reflective layer 13, so that the surface is configured as a flat surface. The upper adhesive resin layer 14 may be formed to follow the unevenness of the interface with the metal reflective layer 13, so that the surface is configured as an uneven surface. In order to form a metal reflective layer 13 that can obtain stable antiglare properties even when used at high temperatures, when the upper adhesive resin layer 14 is in a film or sheet form before lamination with the metal reflective layer 13, it is preferable that the surface on the metal reflective layer 13 side has unevenness. From the same viewpoint, the unevenness is preferably matte. From the same viewpoint, the arithmetic mean height Sa of the unevenness is preferably 1 μm or more and 3 μm or less, more preferably 1.5 μm or more and 2.5 μm or less. When the surfaces of the upper adhesive resin layer 14 and the lower adhesive resin layer 12 before lamination with the metal reflective layer 13 both have unevenness, it is preferable that the unevenness of the upper adhesive resin layer 14 and the lower adhesive resin layer 12 is the same from the same viewpoint. When the upper adhesive resin layer 14 is in a film or sheet form before being laminated with the metal reflective layer 13, the surface facing the metal reflective layer 13 may be a flat surface.

[0019] The maximum thickness of the upper adhesive resin layer 14 is preferably 10 μm or more from the viewpoint of obtaining sufficient adhesiveness, and is preferably 50 μm or less from the viewpoint of reducing heat generation due to heat absorption. The maximum thickness of the upper adhesive resin layer 14 is preferably the same as the maximum thickness of the lower adhesive resin layer 12 from the viewpoint of obtaining stable antiglare properties even when used under high temperatures.

[0020] In the three-layer laminated structure in which the metal reflective layer 13 is sandwiched between the lower adhesive resin layer 12 and the upper adhesive resin layer 14, the metal reflective layer 13 is formed so as to follow the unevenness of the interface with the lower adhesive resin layer 12, and the upper adhesive resin layer 14 is formed so as to fill and flatten the unevenness of the interface with the metal reflective layer 13, thereby obtaining a high interlayer adhesive strength. This is considered to be due to the physical effect of the unevenness of the interfaces of the lower adhesive resin layer 12 and the upper adhesive resin layer 14 with the metal reflective layer 13, in addition to the chemical action of the adhesiveness due to the EVA that forms the lower adhesive resin layer 12. Here, from the viewpoint of enhancing the chemical action, the thermoplastic resin that forms the upper adhesive resin layer 14 preferably contains EVA, and from the viewpoint of the balance of the three-layer laminated structure, it is more preferable that it is the same as the thermoplastic resin that forms the lower adhesive resin layer 12.

[0021] The upper base material layer 15 is composed of, for example, a resin sheet. Examples of the resin material forming the upper base material layer 15 include polyester resin and polycarbonate resin. The resin material forming the upper base material layer 15 may be the same as the resin material forming the lower base material layer 11. The thickness of the upper base material layer 15 is preferably 12 μm or more from the viewpoint of lamination processability, and is preferably 50 μm or less from the viewpoint of reducing heat generation due to heat absorption. The thickness of the upper base material layer 15 may be the same as the thickness of the lower base material layer 11.

[0022] The surface protective layer 16 is composed of, for example, a resin coating layer. Examples of the resin material forming the surface protective layer 16 include urethane acrylic resin, urethane resin, acrylic resin, etc. The total light transmittance of the resin material forming the surface protective layer 16 measured based on JIS K7361-1:1997 is preferably 85% or more, more preferably 90% or more. The thickness of the surface protective layer 16 is preferably 1 μm or more from the viewpoint of protective ability, and is preferably 10 μm or less from the viewpoint of suppressing a decrease in heat shielding ability due to heat absorption, etc.

[0023] The radiative cooling sheet 10 according to the embodiment includes an adhesive layer 17 laminated directly below the lower base material layer 11, and a release layer 18 laminated directly below the adhesive layer 17.

[0024] The adhesive layer 17 is made of, for example, a resin adhesive. Examples of the resin adhesive constituting the adhesive layer 17 include an acrylic resin adhesive. The thickness of the adhesive layer 17 is preferably 10 μm or more from the viewpoint of improving the conformability to the lower base material layer 11, and is preferably 40 μm or less from the viewpoint of suppressing adhesive residue when peeled off.

[0025] The release layer 18 is made of, for example, release-treated release paper or polyester resin sheet. The thickness of the release layer 18 is, for example, 100 μm or more and 200 μm or less in the case of release paper, and, for example, 25 μm or more and 100 μm or less in the case of a polyester resin sheet.

[0026] The radiative cooling sheet 10 according to the embodiment is used by peeling off the release layer 18 and attaching it to the surface of an object to be cooled via the adhesive layer 17.

[0027] When sunlight is irradiated onto the radiative cooling sheet 10 according to this embodiment, the light enters through the surface protective layer 16, passes through the transparent upper substrate and the upper adhesive resin layer 14, and then reaches the metal reflective layer 13 and is reflected. The light reflected by the metal reflective layer 13 passes through the upper adhesive resin layer 14 and the upper substrate again, and then exits through the surface protective layer 16. In this process, when the wavelength of the light is concentrated in a wavelength band (7 to 14 μm) called the "atmospheric window" where electromagnetic radiation is hardly absorbed by the atmosphere, the object to be cooled is cooled by efficiently releasing heat into the atmosphere through the radiative cooling sheet 10, and the surface temperature decreases.

[0028] According to the radiative cooling sheet 10 according to the embodiment of the above configuration, the interface between the lower adhesive resin layer 12 formed of the thermoplastic resin containing EVA and the metal reflective layer 13 has unevenness, and the metal reflective layer 13 is formed to follow the unevenness of the interface with the lower adhesive resin layer 12, so that a stable antiglare property can be obtained even when used at high temperatures. This is considered to be because the thermoplastic resin containing EVA forming the lower adhesive resin layer 12 firmly adheres to the metal reflective layer 13, and the metal reflective layer 13 retains the shape of the unevenness of the interface with the lower adhesive resin layer 12, even when used at high temperatures. The structure in which the metal reflective layer 13 is formed to follow the unevenness of the interface with the lower adhesive resin layer 12 is thermally stable in shape retention, so that a wide range of processing methods can be selected when laminating the upper adhesive resin layer 14 on the metal reflective layer 13.

[0029] Moreover, according to the radiative cooling sheet 10 of the embodiment, the metal reflective layer 13 is formed to follow the unevenness of the interface with the lower adhesive resin layer 12, so that high anti-glare properties can be obtained by diffuse reflection of light at the unevenness of the interface of the metal reflective layer 13 with the upper adhesive resin layer 14.

[0030] Regarding the antiglare property of the radiative cooling sheet 10 according to the embodiment, the 20-degree specular gloss Gs (20 °) is preferably 300% or less, more preferably 200% or less, and even more preferably 150% or less. The 60-degree specular gloss Gs (60 °) is preferably 200% or less, more preferably 150% or less, and even more preferably 130% or less. The 85-degree specular gloss Gs (85 °) is preferably 120% or less, more preferably 100% or less, and even more preferably 90% or less. These specular gloss values ​​are measured based on JIS Z8741:1997.

[0031] The solar reflectance of the radiative cooling sheet 10 according to the embodiment in the entire wavelength range (300 nm to 2500 nm) is preferably 82% or more, more preferably 90% or more. This solar reflectance in the entire wavelength range is determined based on JIS K5602:2008.

[0032] The radiative cooling sheet 10 according to the embodiment having the above-mentioned configuration can be easily and cheaply manufactured by a combination of metal deposition processing, lamination processing of a film material, and coating processing of a coating agent.

[0033] In the above embodiment, the radiative cooling sheet 10 is attached to the object to be cooled, but the present invention is not limited to this. The radiative cooling structure of the radiative cooling sheet 10 according to the embodiment can be applied to clothing such as hats and clothes using a cloth material as a base material. EXAMPLES

[0034] [Test evaluation 1] A resin laminate sheet was prepared by laminating an EVA layer (VA content: 14.1 mass%, melting point: 88.3°C) with a maximum thickness of 25 μm and a matte textured surface on a PET base layer of polyester resin with a thickness of 25 μm. A silver layer with a thickness of 100 nm was formed on the EVA layer of some of the resin laminate sheets by a method using the silver mirror reaction so as to follow the surface irregularities.

[0035] The test pieces of Example 1 having a silver layer and Comparative Example 1 not having a silver layer were subjected to heat treatment in an electric oven with the temperature inside the oven set to 90° C. for 30 seconds, 60 seconds, 90 seconds, 120 seconds, and 180 seconds. The arithmetic mean height Sa of the surface of the heat-treated test pieces was measured using a laser microscope (VK-X150 / KV-X100 manufactured by KEYENCE) based on ISO25178. The arithmetic mean height Sa of the surface of the test pieces not subjected to heat treatment was also measured.

[0036] The measurement results are shown in Table 1. Table 1 shows that in Example 1, the arithmetic mean height Sa is almost constant regardless of the heat treatment time. Therefore, in Example 1, stable antiglare properties can be expected even when used at high temperatures. In contrast, in Comparative Example 1, the arithmetic mean height Sa varies greatly depending on the heat treatment time, and it is clear that the arithmetic mean height Sa tends to decrease as the heat treatment time increases.

[0037] [Table 1]

[0038] [Test evaluation 2] For one of the pair of resin laminate sheets used in Test Evaluation 1, a silver layer similar to that in Test Evaluation 1 was formed on the EVA layer of one of the sheets, and the other sheet was laminated so that the silver layer was sandwiched between the EVA layers on both sides, and the sheet was then thermally laminated by passing it through a laminator.

[0039] A 25 mm wide rectangular test piece of Example 2, in which the silver layer was sandwiched between EVA layers on both sides, was tested using a tensile tester to peel one resin laminate sheet from the other resin laminate sheet at a speed of 300 mm / min in a direction of 180° in the length direction, and the maximum peel strength per 25 mm width measured during this was taken as the peel adhesion strength. The peel test was performed on both test pieces whose length direction was the machine direction (MD) in the thermal lamination processing and test pieces whose length direction was the transverse direction (TD).

[0040] The measurement results are shown in Table 2. It was confirmed from Table 2 that the peel adhesion strength of Example 2 was sufficiently higher than the peel adhesion strength (10 to 15 N / 25 mm) of commercially available wallpaper in both MD and TD.

[0041] [Table 2]

[0042] In Example 2, the surface of the other resin laminate sheet side was used as the light irradiation surface and the surface of one resin laminate sheet side was used as the back surface, and a thermocouple was attached to each of the light irradiation surface and the back surface. The radiative cooling sheet of Example 2 was placed at a position 280 mm away from a 500 W halogen lamp so that the light irradiation surface faced the halogen lamp. The light from the halogen lamp was irradiated onto the radiative cooling sheet of Example 2. The temperatures of the light irradiation surface and the back surface were determined between 15 and 20 minutes after the start of light irradiation, and were 39.6 ° C and 32.2 ° C, respectively. From this, it was confirmed that the radiative cooling sheet of Example 2 has excellent radiative cooling performance. [Industrial Applicability]

[0043] The present invention is useful in the technical field of radiative cooling structures. [Explanation of symbols]

[0044] 10 Radiative cooling sheet 11 Lower base material layer 12 Lower adhesive resin layer 13 Metal reflective layer 14 Upper adhesive resin layer (second resin layer) 15 Upper base material layer 16 Surface protective layer 17 Adhesive layer 18 Release layer

Claims

1. A radiative cooling structure comprising a resin layer formed of a thermoplastic resin containing ethylene vinyl acetate and a metal reflective layer laminated on the resin layer, The radiative cooling structure, wherein the interface between the resin layer and the metal reflective layer has irregularities, and the metal reflective layer is formed so as to follow the irregularities of the interface with the resin layer.

2. The radiative cooling structure according to claim 1, The resin layer has an uneven surface before being laminated with the metal reflective layer.

3. The radiative cooling structure according to claim 2, The radiative cooling structure, wherein the arithmetic mean height Sa of the unevenness is 1 μm or more and 3 μm or less.

4. The radiative cooling structure according to claim 2, The radiative cooling structure, wherein the surface unevenness of the resin layer before lamination with the metal reflective layer is matte.

5. The radiative cooling structure according to claim 1, The radiative cooling structure, wherein the vinyl acetate content of the ethylene vinyl acetate is 10% by mass or more and 30% by mass or less.

6. The radiative cooling structure according to claim 1, The radiative cooling structure, wherein the melting point of the ethylene vinyl acetate is 60°C or higher.

7. The radiative cooling structure according to claim 1, The radiative cooling structure, wherein the metallic reflective layer is formed of silver or a silver alloy.

8. The radiative cooling structure according to claim 1, The radiative cooling structure, wherein the metal reflective layer has a thickness of 50 nm or more.

9. The radiative cooling structure according to claim 1, The radiative cooling structure further comprises a second resin layer laminated on the metal reflective layer, the second resin layer being formed so as to fill and flatten unevenness at the interface with the metal reflective layer.

10. The radiative cooling structure according to claim 9, The radiative cooling structure, wherein the thermoplastic resin forming the second resin layer is the same as the thermoplastic resin forming the resin layer.

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