Double tube

Abstract

To provide a double pipe that can prevent a fluid flowing through an inner pipe from being heated.SOLUTION: A double pipe 2 includes: an outer pipe 10; and an inner pipe 20 disposed inside the outer pipe 10 at an interval to the outer pipe 10. A heat insulation layer is formed between the outer pipe 10 and the inner pipe 20. A resin is coated over an outermost layer of the outer pipe 10 as a corrosion resistant layer. The resin contains a white material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a double pipe including an outer pipe and an inner pipe disposed inside the outer pipe with a gap therebetween. [Background technology]

[0002] When transporting a cryogenic fluid such as liquid nitrogen, a double pipe utilizing vacuum insulation is used to prevent the temperature of the cryogenic fluid inside the pipe from rising. Such a double pipe includes an outer pipe and an inner pipe disposed inside the outer pipe at a distance from the outer pipe, and the cryogenic fluid flows inside the inner pipe. In addition, insulation between the outer pipe and the inner pipe is achieved by forming an insulating layer such as a vacuum space between the outer pipe and the inner pipe. For example, a double pipe like this is disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 021509 (WO2018 / 021509A1) Summary of the Invention [Problem to be solved by the invention]

[0004] When a conventional double-walled pipe is placed outdoors where it is exposed to direct sunlight, the temperature of the surface of the outer pipe rises. If the double-walled pipe is bent or becomes misaligned due to sagging under its own weight, the outer pipe and the inner pipe may come into contact with each other, and the heat on the surface of the outer pipe may be transferred to the inner pipe, heating the cryogenic fluid flowing inside the inner pipe.

[0005] The present invention has been made in consideration of these points, and has an object to provide a double pipe that can prevent the fluid flowing inside the inner pipe from being heated. [Means for solving the problem]

[0006] The double pipe of the present invention is an outer tube and an inner tube disposed inside the outer tube with a gap therebetween; A heat insulating layer is formed between the outer pipe and the inner pipe, The outermost layer of the outer tube is coated with a resin as an anticorrosive layer, and the resin contains a white material. Effect of the Invention

[0007] The double pipe of the present invention can suppress the temperature rise on the surface of the outer pipe due to direct sunlight, and can prevent the fluid flowing inside the inner pipe from being heated by heat from the surface of the outer pipe being transferred to the inner pipe. [Brief description of the drawings]

[0008] [Figure 1] 1 is a perspective view showing a double pipe according to an embodiment of the present invention; [Diagram 2] 2 is a cross-sectional view of the double pipe shown in FIG. 1 taken along the line AA. [Diagram 3] 3 is a cross-sectional view of the double pipe shown in FIG. 2 taken along the arrow BB. [Figure 4] 1 is a graph showing the change in temperature of the outer surface and the inner surface over time when the outermost layer of an outer tube is coated with a white anticorrosion layer and a black anticorrosion layer, respectively. [Diagram 5] 1 is a graph showing the relationship between the wavelength of visible light and reflectance when the titanium oxide content in a sheet formed by compression molding a resin used in a corrosion protection layer that coats the outermost layer of an outer tube is 0 mass%, 0.25 mass%, 0.5 mass%, or 2 mass%. [Figure 6] 1 is a graph showing the relationship between elongation and stress in a tensile test when the titanium oxide content in a sheet obtained by compression molding a resin used in a corrosion protection layer that coats the outermost layer of an outer tube is 5 mass%, 10 mass%, or 25 mass%. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Figures 1 to 3 are a perspective view and a cross-sectional view showing a double pipe according to the present embodiment.

[0010] As shown in Figs. 1 to 3, the double pipe 2 according to this embodiment includes an outer pipe 10 and an inner pipe 20, and a vacuum space 30 is formed between the outer pipe 10 and the inner pipe 20. The inner pipe 20 is disposed inside the outer pipe 10 with a gap therebetween. The outermost layer of the outer pipe 10 is coated with resin as a corrosion protection layer 12. The outermost layer of the inner pipe 20 is coated with a heat insulating member 22. The heat insulating member 22 and the vacuum space 30 form a heat insulating layer. A fluid (specifically, a cryogenic fluid such as liquid nitrogen) flows inside the inner pipe 20. Each component of the double pipe 2 will be described below.

[0011] The material of the outer tube 10 is a metal, and is selected from the group consisting of aluminum, aluminum alloy, iron, steel, stainless steel, Ni-based alloy, and Co-based alloy. It is more preferable to use stainless steel as the material of the outer tube 10. As shown in Figures 1 and 3, the outer tube 10 is wavy in the longitudinal direction and is flexible.

[0012] As shown in Figs. 1 to 3, the outermost layer of the outer pipe 10 is coated with a resin as a corrosion protection layer 12. There is no particular limitation on the material of such a resin, and any resin can be used. Here, the resin is preferably selected from the group consisting of polyethylene resin, urethane resin, epoxy resin, fluorine-based resin, and mixtures thereof, and it is particularly preferable to use a low-density polyethylene resin of polyethylene resin. The thickness of such a resin is within a range of 10% to 100% of the wave depth on the inner surface of the outer pipe 10, preferably within a range of 30% to 70%, and particularly preferably within a range of 40% to 60%. If the thickness of the resin as the corrosion protection layer 12 is less than 10% of the wave depth on the inner surface of the outer pipe 10, the corrosion protection layer 12 is too thin and easily torn, which causes a problem that the outer pipe 10 is easily worn and corroded. On the other hand, if the thickness of the resin as the corrosion protection layer 12 is greater than 100% of the wave depth on the inner surface of the outer pipe 10, the corrosion protection layer 12 is too thick and easily breaks, which causes a problem that the outer pipe 10 is easily worn and corroded. The outer peripheral surface of the anticorrosion layer 12 may have projections and recesses corresponding to the corrugated shape of the outer pipe 10, or may be a circular surface. Such an anticorrosion layer 12 can protect the outer pipe 10 from external factors such as damage, wear, corrosion, and ultraviolet degradation, and can also identify the application of the double pipe 2 and harmonize with the surrounding environment.

[0013] In this embodiment, the resin as the anticorrosive layer 12 contains a white material. The white material contains a pigment, and the pigment contains a material selected from the group consisting of CI pigment white, sulfates of alkaline earth metals such as barium sulfate, carbonates of alkaline earth metals such as calcium carbonate, silicas such as fine powdered silicic acid and synthetic silicates, calcium silicate, alumina, alumina hydrate, metal compounds such as titanium oxide and zinc oxide, talc, clay, and the like. Here, it is particularly preferable that the pigment contains titanium oxide. Titanium oxide has characteristics such as a high refractive index, chemical stability, and high safety, and is therefore particularly suitable as a pigment as a white material. More specifically, titanium oxide has a small particle diameter of 0.2 to 0.3 μm and can increase the scattering of visible light, so that the reflectance of light on the outer surface of the anticorrosive layer 12 can be increased. In addition, titanium oxide is widely distributed and easy to obtain. In this way, titanium oxide has various advantages as a pigment for a white material.

[0014] In this embodiment, the anticorrosive layer 12 contains a resin containing a white material, so that the outer peripheral surface of the anticorrosive layer 12 can be made white. Therefore, even if the double pipe 2 is placed outdoors where it is exposed to direct sunlight, the outer surface of the anticorrosive layer 12 is white and has a high light reflectance, so that the temperature rise of the surface of the outer pipe 10 is suppressed. Therefore, even if the double pipe 2 is bent or the double pipe 2 is misaligned due to sagging due to its own weight, the temperature rise of the outer pipe 10 is suppressed even if the outer pipe 10 and the inner pipe 20 come into contact with each other, so that the cryogenic fluid flowing inside the inner pipe 20 can be prevented from being significantly heated.

[0015] The pigment content in the resin is 25% by mass or less. The pigment content in the resin is preferably within the range of 0.25% by mass to 25% by mass, more preferably within the range of 0.25% by mass to 10% by mass, and particularly preferably within the range of 0.5% by mass to 10% by mass. If the pigment content in the resin is greater than 25% by mass, the dispersion of the pigment in the resin varies, and therefore, when the external temperature is low, the elongation of the anticorrosive layer 12 varies, and therefore, when the outer pipe 10 is bent, the anticorrosive layer 12 may break. If the pigment content in the resin is less than 0.25% by mass, the visible light irradiated to the anticorrosive layer 12 cannot be sufficiently scattered due to the small pigment content, and the light reflectance on the surface of the anticorrosive layer 12 decreases, so that the surface of the outer pipe 10 may heat up when the double pipe 2 is exposed to direct sunlight.

[0016] The material of the inner pipe 20 is a metal, and the material of the inner pipe 20 is selected from the group consisting of aluminum, aluminum alloy, iron, steel, stainless steel, stainless steel, Ni-based alloy, and Co-based alloy. It is more preferable to use stainless steel as the material of the inner pipe 20. The material of the inner pipe 20 may be the same as the material of the outer pipe 10, or the material of the inner pipe 20 may be different from the material of the outer pipe 10. Also, as shown in Figures 1 and 3, the inner pipe 20 is corrugated along the longitudinal direction and has flexibility. In this way, when the outer pipe 10 and the inner pipe 20 are corrugated along the longitudinal direction and have flexibility, the double pipe 2 can be laid according to the external environment, and for example, the double pipe 2 can be bent to avoid pillars and beams of a building or to lay along corners.

[0017] 1 to 3, the outermost layer of the inner pipe 20 is covered with a heat insulating member 22. More specifically, the heat insulating member 22 is a super insulation (specifically, a metal foil or a metal-plated plastic film, etc.) that cuts heat transfer by radiation, or a spacer (specifically, a rod-shaped resin, etc.) with low thermal conductivity that reduces the amount of heat transfer when the outer pipe 10 and the inner pipe 20 come into contact with each other, which is wound around the outermost layer of the inner pipe 20.

[0018] As described above, a vacuum space 30 is formed between the outer pipe 10 and the insulating member 22 of the inner pipe 20. The vacuum space 30 does not mean a complete vacuum state, but by discharging most of the air present in the vacuum space 30 to the outside, heat transfer via the air is prevented together with the insulating member 22, which cuts off heat transfer by radiation. In this way, the insulating layer consisting of the insulating member 22 and the vacuum space 30 provides insulation between the outer pipe 10 and the inner pipe 20, and suppresses the rise in temperature of the fluid flowing through the inner pipe 20 caused by the rise in temperature of the outer pipe 10.

[0019] According to the double pipe 2 of the present embodiment having the above-mentioned configuration, a heat insulating layer is formed between the outer pipe 10 and the inner pipe 20, and the outermost layer of the outer pipe 10 is coated with a resin as the anticorrosive layer 12, and the resin contains a white material. In this case, since the resin as the anticorrosive layer 12 coated on the outermost layer of the outer pipe 10 contains a white material, even if the double pipe 2 is placed outdoors where it is exposed to direct sunlight, the light reflectance of the outer surface of the anticorrosive layer 12 is high. Therefore, the rise in temperature of the surface of the outer pipe 10 is suppressed. Therefore, even if the double pipe 2 is bent or the double pipe 2 is misaligned due to sagging due to its own weight, even if the outer pipe 10 and the inner pipe 20 come into contact with each other, the rise in temperature of the outer pipe 10 is suppressed, so that it is possible to prevent the cryogenic fluid flowing inside the inner pipe 20 from being significantly heated.

[0020] In the double pipe 2 of this embodiment, the outer pipe 10 and the inner pipe 20 are each wavy along the longitudinal direction. In this case, the double pipe 2 can be laid in accordance with the external environment, for example, the double pipe 2 can be bent to avoid pillars and beams of a building or to lay along a corner. Even if the outer pipe 10 and the inner pipe 20 come into contact with each other by bending the double pipe 2, the resin as the anticorrosive layer 12 contains a white material, so that the temperature rise of the outer pipe 10 is suppressed, and therefore the cryogenic fluid flowing inside the inner pipe 20 can be prevented from being significantly heated.

[0021] In the double pipe 2 of the present embodiment, when the outer pipe 10 is corrugated, the thickness of the resin is within a range of 10% to 100% of the wave depth of the outer pipe 10. In this case, since the thickness of the resin is 10% or more of the wave depth on the inner surface of the outer pipe 10, it is possible to suppress the occurrence of wear and corrosion of the outer pipe 10, and since the thickness of the resin is 100% or less of the wave depth on the inner surface of the outer pipe 10, it is possible to suppress the outer pipe 10 from becoming difficult to bend.

[0022] In the double pipe 2 of this embodiment, the white material contains a pigment. The pigment contains titanium oxide. As described above, titanium oxide has a small particle diameter of 0.2 to 0.3 μm and can increase the scattering of visible light, so that the reflectance of light on the outer surface of the anticorrosive layer 12 can be increased. Titanium oxide is also widely distributed and easy to obtain. Thus, titanium oxide has various advantages as a pigment for a white material. The pigment content is 25% by mass or less. Since the pigment content in the resin is 25% by mass or less, the occurrence of variation in the elongation of the anticorrosive layer 12 is suppressed even when the external temperature is low, so that the anticorrosive layer 12 can be prevented from breaking.

[0023] The double pipe according to the present invention is not limited to the above-mentioned embodiment, and various modifications can be made.

[0024] For example, in the double pipe according to the present invention, the outer pipe and the inner pipe are not limited to those having a corrugated shape along the longitudinal direction and flexibility. In another embodiment, the outer pipe and the inner pipe may each be a straight pipe. When the outer pipe and the inner pipe are corrugated, they have flexibility and can be laid according to the external environment, but the present invention can be applied even when the outer pipe and the inner pipe are each straight. Specifically, even when the outer pipe and the inner pipe are each straight, the outermost layer of the outer pipe is coated with a resin as a corrosion prevention layer, and when the resin contains a white material, it is possible to prevent the temperature of the outer pipe from rising even when the double pipe is placed outdoors where it is exposed to direct sunlight. In particular, when the outer pipe and the inner pipe are straight, there is a risk that the outer pipe and the inner pipe come into contact with each other due to misalignment caused by sagging due to their own weight, and the heat on the surface of the outer pipe is transferred to the inner pipe. In addition, when a spacer is provided between the inner surface of the outer pipe and the outer surface of the inner pipe to support the inner pipe, there is a risk that the heat on the surface of the outer pipe is transferred to the inner pipe through this spacer. To address these problems, the temperature of the outer tube can be prevented from rising by including a white material in the resin that coats the outermost layer of the outer tube. If the outer tube and the inner tube are both straight, the outer tube does not have flexibility but has a small surface area, which makes it possible to further prevent the temperature of the outer tube from rising.

[0025] In addition, when the outer tube has a straight shape, the thickness of the resin as the anticorrosive layer is preferably within a range of 0.05% to 15% of the outer diameter of the outer tube. If the thickness of the resin is less than 0.05% of the outer diameter of the outer tube, the anticorrosive layer is too thin and easily torn, which causes a problem that the outer tube is easily worn or corroded. On the other hand, if the thickness of the resin is more than 15% of the outer diameter of the outer tube, the weight of the outer tube increases, which makes it inconvenient to carry. In addition, in this case, there is a problem that the resin material is used excessively. On the other hand, if the thickness of the resin as the anticorrosive layer is within a range of 0.05% to 15% of the outer diameter of the outer tube, the above-mentioned problems can be suppressed.

[0026] Furthermore, the pigment contained in the anticorrosion layer is not limited to titanium oxide, and any pigment other than titanium oxide may be used as the pigment contained in the anticorrosion layer as long as it can make the outer surface of the anticorrosion layer white. EXAMPLES

[0027] Example 1 An outer pipe (1000 mm long, 150 mm outer diameter, 0.7 mm thick, 5.8 mm wave depth) was produced, in which the metal material was cold-rolled steel (SPCD) and the corrugated shape was formed along the longitudinal direction. In addition, the outer pipe was covered with 2.9 mm (50% of the corrugation depth) of resin containing a white material as a corrosion protection layer. Polyethylene resin (NUCG-7641) manufactured by ENEOS NUC Corporation was used as the resin. In addition, a master batch (PE-M 20N3127C WH) manufactured by Dainichi Seikagaku Kogyo Co., Ltd., which contains titanium oxide as a pigment, was used as the resin, and the titanium oxide concentration of the molded product was mixed to 1.56 to 2.19 mass%, and extrusion molding was performed. Such an outer pipe was placed in a place exposed to direct sunlight, and the temperature of the outer surface and the temperature of the inner surface of the outer pipe 10 were measured over time from 11:00 a.m. to 3:00 p.m. The average outside air temperature was 28.4°C. The line designated by reference numeral 50 in FIG. 4 indicates the change in temperature of the outer surface of the outer bulb, and the line designated by reference numeral 52 in FIG. 4 indicates the change in temperature of the inner surface of the outer bulb.

[0028] Comparative Example 1 An outer pipe (1000 mm long, 150 mm outer diameter, 0.7 mm thick, 5.8 mm wave depth) was produced, in which the metal material was cold-rolled steel (SPCD) and the corrugated shape was formed along the longitudinal direction. The outer pipe was coated with 2.9 mm (50% of the corrugation depth) of resin containing a black material as an anticorrosive layer. Polyethylene resin (NUCG-7641) was used as the resin. A master batch (M-1535) manufactured by ENEOS NUC Corporation, which contains carbon as a pigment, was used as the resin, and the titanium oxide concentration of the molded product was mixed to 0.8 to 2.5 mass %, and extrusion molding was performed. Such an outer pipe was placed in a place exposed to direct sunlight, and the temperature of the outer surface and the temperature of the inner surface of the outer pipe 10 were measured over time from 11:00 a.m. to 3:00 p.m. The average outside air temperature was 28.4°C. In the graph of FIG. 4, the line indicated by reference numeral 54 indicates the change in temperature of the outer surface of the outer pipe, and the line indicated by reference numeral 56 indicates the change in temperature of the inner surface of the outer pipe.

[0029] In the outer tube according to Comparative Example 1, the temperature of the outer peripheral surface and the inner peripheral surface of the outer tube rose to 45°C to 60°C due to direct sunlight hitting the outer surface. In this case, when the outer tube and the inner tube come into contact with each other, the inner tube is heated, which may heat the fluid flowing inside the inner tube. In contrast, in the outer tube according to Example 1, even when the outer surface is hit by direct sunlight, the temperature of the outer peripheral surface and the inner peripheral surface of the outer tube is maintained within a range of 30°C to 40°C. In particular, the temperature of the inner peripheral surface of the outer tube is maintained within a range of 30°C to 35°C, and does not rise significantly from 28.4°C, which is the average of the outside air temperature. In this case, it is considered that even if the outer tube comes into contact with the inner tube, a large rise in the temperature of the fluid flowing inside the inner tube is suppressed.

[0030] [Examples 2 to 4] A sheet (length 168 mm, width 187 mm, thickness 2 mm) was produced by compression molding a resin containing a white material into a plate shape. A polyethylene resin (NUCG-7641) manufactured by ENEOS NUC Corporation was used as the resin. Titanium oxide contained in a master batch (PE-M 20N3127C WH) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd. was used as a pigment in the resin, and was mixed to 0.25 mass% (Example 2), 0.5 mass% (Example 3), and 2 mass% (Example 4), respectively. The reflectance was measured when visible light with a wavelength of 400 nm to 700 nm was irradiated onto the surface of such a sheet using a UV-visible-near infrared spectrometer (SolidSpec-3700) manufactured by Shimadzu Corporation. In the graph of Figure 5, the line indicated by reference numeral 62 shows the relationship between wavelength and reflectance when the titanium oxide content is 0.25 mass%, the line indicated by reference numeral 64 shows the relationship between wavelength and reflectance when the titanium oxide content is 0.5 mass%, and the line indicated by reference numeral 66 shows the relationship between wavelength and reflectance when the titanium oxide content is 2 mass%.

[0031] Comparative Example 2 A sheet (length 168 mm, width 187 mm, thickness 2 mm) was produced by compression molding a resin that did not contain a white material into a plate shape. A polyethylene resin (NUCG-7641) manufactured by ENEOS NUC Corporation was used as the resin. The titanium oxide content in the resin was 0 mass%. Using a Shimadzu Corporation ultraviolet-visible-near infrared spectrometer (SolidSpec-3700), the reflectance was measured when visible light with a wavelength of 400 nm to 700 nm was irradiated onto the surface of such a sheet. In the graph of FIG. 5, the line indicated by reference number 60 indicates the relationship between the wavelength and the reflectance when the titanium oxide content is 0 mass%.

[0032] In the sheet according to Comparative Example 2, the reflectance was within a range of 15% to 30% when the surface of the sheet was irradiated with visible light having a wavelength of 400 nm to 700 nm. Therefore, when the outer surface of the sheet according to Comparative Example 2 is exposed to direct sunlight, the temperature of the sheet may rise due to the low reflectance. In contrast, in the sheets according to Examples 2 to 4, the reflectance was within a range of 70% to 100% when the surface of the sheet was irradiated with visible light having a wavelength of 400 nm to 700 nm. Therefore, even if the outer surface of the sheets according to Examples 2 to 4 is exposed to direct sunlight, it is considered that a large rise in the temperature of the sheet is suppressed.

[0033] [Examples 5 to 7] Resin containing a white material was compression molded into a plate-like sheet (length 168 mm, width 187 mm, thickness 2 mm). Polyethylene resin (NUCG-7641) manufactured by ENEOS NUC Corporation was used as the resin. Titanium oxide contained in a master batch (PE-M 20N3127C WH) manufactured by Dainichiseika Chemicals Mfg. Co., Ltd. was used as a pigment in the resin, and was mixed at 5 mass% (Example 5), 10 mass% (Example 6), and 25 mass% (Example 7). From such a sheet, a dumbbell-shaped tensile test piece based on "JIS K 7161-2 Plastics - Determination of tensile properties - Part 2, Appendix A, Small test piece, Figure A.1 and Table A.1" was prepared, and three tests were performed for each example at a tensile speed of 500 mm / min and a test temperature of 0°C based on the test method of "JIS K 7161-1 Plastics - Determination of tensile properties - Part 1". In the graph of Figure 6, the lines indicated by reference numbers 71, 72, and 73 show the relationship between elongation and stress when the titanium oxide content is 5 mass%, the lines indicated by reference numbers 74, 75, and 76 show the relationship between elongation and stress when the titanium oxide content is 10 mass%, and the lines indicated by reference numbers 77, 78, and 79 show the relationship between elongation and stress when the titanium oxide content is 25 mass%.

[0034] As shown in the graph of FIG. 6, when the titanium oxide content was 5 mass%, 10 mass%, and 25 mass%, the results of the three tensile tests did not show any significant variation. When the titanium oxide content was too high, the dispersion of titanium oxide in the resin varied, which caused the elongation of the anticorrosion layer to vary, especially when the external temperature was low, and there was a problem that the anticorrosion layer may break when the sheet was bent. However, when the titanium oxide content was 5 mass%, 10 mass%, and 25 mass%, the results of the three tensile tests did not show any significant variation. It is therefore considered that there is no significant variation in the dispersion of titanium oxide in the resin at these contents, and therefore it is considered that there is a low possibility that the anticorrosion layer will break when the sheet is bent. [Explanation of symbols]

[0035] 2 double tube 10 outer tube 12 Anticorrosive Layer 20 inner tube 22 Thermal insulation materials 30 Vacuum space

Claims

1. an outer tube and an inner tube disposed inside the outer tube with a gap therebetween; A heat insulating layer is formed between the outer pipe and the inner pipe, an outermost layer of the outer pipe is coated with a resin as a corrosion protection layer, the resin containing a white material makes an outer surface of the corrosion protection layer white; The outer tube and the inner tube are made of metal.

2. an outer tube and an inner tube disposed inside the outer tube with a gap therebetween; A heat insulating layer is formed between the outer pipe and the inner pipe, an outermost layer of the outer pipe is coated with a resin as a corrosion protection layer, the resin containing a white material makes an outer surface of the corrosion protection layer white; The outer pipe and the inner pipe each have a corrugated shape along a longitudinal direction.

3. 3. The double pipe according to claim 1, wherein when the outer pipe is corrugated, the thickness of the resin is within a range of 10% to 100% of the depth of the waves of the outer pipe.

4. 2. The double pipe according to claim 1, wherein the thickness of the resin when the outer pipe has a straight shape is within a range of 0.05% to 15% of the outer diameter of the outer pipe.

5. The double-walled pipe according to claim 1 , wherein the white material includes a pigment.

6. The double-walled pipe according to claim 5 , wherein the pigment comprises titanium oxide.

7. 7. The double-walled pipe according to claim 5, wherein the pigment content is 25% by mass or less.

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