JP7743345B2 - exhaust pipe - Google Patents

Description

This specification discloses technology related to exhaust pipes.

Patent Document 1 discloses a composite member in which an insulating layer is provided on the inner surface of a metal pipe as a heat insulating material. In the composite member of Patent Document 1, the insulating layer contains ceramic fibers. As a result, the insulating layer deforms in response to the deformation (thermal expansion and contraction) of the metal, preventing the insulating layer from peeling off from the metal. Due to its high insulating properties, the composite material of Patent Document 1 is expected to be used as an exhaust pipe for internal combustion engines.

International Publication No. WO2020/145366

In the composite member of Patent Document 1, the insulating layer deforms in response to the deformation of the metal, preventing delamination due to differences in the thermal expansion coefficients of the metal and the insulating layer. However, in the case of an exhaust pipe for an internal combustion engine, exhaust gases come into direct contact with the insulating layer and apply force to it. Vibrations from the internal combustion engine are also transmitted to the exhaust pipe. As a result, there is concern that the adhesion between the metal pipe and the insulating layer may decrease over long-term use, leading to delamination of the insulating layer from the metal pipe. Therefore, further improvements are needed for exhaust pipes in which an insulating layer is provided on the inner surface of a metal pipe. The purpose of this specification is to provide a novel exhaust pipe in which an insulating layer is provided on the inner surface of a metal pipe.

The exhaust pipe disclosed in this specification may comprise a metal pipe and an insulating layer provided on the inner surface of the metal pipe. In this exhaust pipe, the porosity of the insulating layer at a position 5% from the interface with the metal relative to the total thickness of the insulating layer may be lower than the porosity at the center of the thickness direction.

FIG. An enlarged view of a portion of the exhaust pipe is shown. An enlarged cross-sectional view of the heat insulating layer is shown. The results of the experimental example are shown below.

The exhaust pipe disclosed in this specification comprises a metal pipe and an insulating layer provided on the inner surface of the metal pipe. The insulating layer comes into contact with exhaust gas passing through the inside of the exhaust pipe. The insulating layer is formed from an inorganic porous material and may contain ceramic fibers. The ceramic fibers can absorb the effects of the difference in thermal expansion coefficient between the metal pipe and the insulating layer. Specifically, the insulating layer can deform in response to deformation (thermal expansion and thermal contraction) of the metal pipe, thereby preventing the insulating layer from peeling off from the metal pipe. The insulating layer prevents exhaust gas from coming into contact with the metal pipe, preventing deterioration of the metal pipe.

The thickness of the insulating layer may be 0.5 mm or more and 5 mm or less. If the thickness of the insulating layer is 0.5 mm or more, sufficient strength can be obtained and sufficient insulating properties can be exhibited. Furthermore, if the thickness of the insulating layer is 5 mm or less, a sufficient exhaust gas flow path can be secured and the force applied by the exhaust gas at the upstream end of the exhaust pipe (the portion upstream of the exhaust gas flow path where the side of the insulating layer is exposed) can be reduced. This prevents the insulating layer from peeling off from the metal pipe. The thickness of the insulating layer may be 1 mm or more, 1.5 mm or more, or 2 mm or more. Furthermore, the thickness of the insulating layer may be 4 mm or less, 3 mm or less, or 2 mm or less.

The porosity of the insulation layer is not constant in the thickness direction. In the exhaust pipe disclosed in this specification, the porosity of the insulation layer at a position 5% from the interface between the metal pipe and the insulation layer (hereinafter referred to as the 5% interface position) is lower than the porosity at the center of the thickness direction, relative to the total thickness of the insulation layer. By making the porosity at the 5% interface position lower than the porosity at the center of the thickness direction, the contact area between the metal pipe and the insulation layer near the interface between the metal pipe and the insulation layer is increased while maintaining the insulating properties of the insulation layer, thereby increasing the adhesion of the insulation layer to the metal pipe.

Specific examples of the insulating layer include a porosity of 5% or more at 5% to 45%. A porosity of 5% or more at 5% to 45% prevents the Young's modulus from becoming excessive, and reduces the thermal shock resistance. As a result, the occurrence of fractures or cracks in the insulating layer can be suppressed. Furthermore, a porosity of 45% or less at 5% to 45% ensures sufficient adhesion of the insulating layer to the metal pipe, and the insulating layer itself has sufficient strength. The porosity of the insulating layer at 5% to 45% of the interface can be 10% or more, 15% or more, 25% or more, or 30% or more. Furthermore, the porosity of the insulating layer at 5% to 45% of the interface can be 30% or less, 25% or less, 20% or less, or 15% or less.

Furthermore, the range of the porosity of the insulation layer on the interface side between the metal pipe and the insulation layer (hereinafter referred to as the interface-side low porosity range) between 5% and 45% may be between 2% and 20% of the total thickness of the insulation layer from the interface between the metal pipe and the insulation layer. If the interface-side low porosity range is 2% or more from the interface between the metal pipe and the insulation layer, sufficient adhesion to the metal pipe is obtained, and peeling of the insulation layer from the metal pipe is suppressed. Furthermore, if the interface-side low porosity range is 2% or more from the interface between the metal pipe and the insulation layer, the insulation layer obtains sufficient strength and can suppress defects such as cracking. Furthermore, if the interface-side low porosity range is 20% or less from the interface between the metal pipe and the insulation layer, an excessive Young's modulus is suppressed, and a decrease in thermal shock resistance can be suppressed. The interface-side low porosity range may be 5% or more, 10% or more, or 15% or more from the interface between the metal pipe and the insulation layer. The interface-side low-porosity area may be 15% or less, 10% or less, or 5% or less from the interface between the metal pipe and the insulating layer. The interface-side low-porosity area may be provided over the entire surface of the metal pipe, or may be provided over only a portion of the surface of the metal pipe. In other words, the interface-side low-porosity area may be provided over only a portion of the surface of the metal pipe, and the high-porosity area described below may be provided over the remaining portion.

The porosity of the insulation layer at the thickness center may be higher than the porosity of the insulation layer on the interface side between the metal pipe and the insulation layer (5% of the interface, low porosity range on the interface side). The porosity of the insulation layer at the thickness center may be 45% or more and 90% or less. If the porosity of the insulation layer at the thickness center is 45% or more, insulation properties are ensured and the transfer of heat from the exhaust gas to the metal pipe can be sufficiently suppressed. Furthermore, if the porosity of the insulation layer at the thickness center is 90% or less, the strength of the insulation layer is ensured and peeling of the insulation layer from the metal pipe is suppressed. The porosity of the insulation layer at the thickness center may be 50% or more, 60% or more, or 70% or more. Furthermore, the porosity of the insulation layer at the thickness center may be 80% or less, 70% or less, or 60% or less.

In addition, the exhaust pipe disclosed in this specification may have a lower porosity of the insulation layer at a position 5% from the surface of the insulation layer relative to the total thickness of the insulation layer (hereinafter referred to as the "surface 5% position") than at the center of the thickness direction. By making the porosity at the surface 5% position of the insulation layer lower than the porosity at the center of the thickness direction, the strength of the insulation layer surface is increased, and durability against forces from the exhaust gas flow and vibrations when the internal combustion engine is operating is improved. As a result, cracks in the insulation layer and peeling of the insulation layer from the metal pipe can be suppressed.

The porosity of the insulating layer at the surface 5% position may be 5% or more and 45% or less. If the porosity of the insulating layer at the surface 5% position is 5% or more, the Young's modulus is prevented from becoming excessive, and a decrease in thermal shock resistance can be suppressed. As a result, the occurrence of fractures or cracks in the insulating layer can be suppressed. Furthermore, if the porosity of the insulating layer at the surface 5% position is 45% or less, sufficient strength can be ensured. The porosity of the insulating layer at the surface 5% position may be 10% or more, 15% or more, 25% or more, or 30% or more. Furthermore, the porosity of the insulating layer at the surface 5% position may be 30% or less, 25% or less, 20% or less, or 15% or less.

Furthermore, on the surface side of the insulation layer, the range in which the porosity of the insulation layer is 5% to 45% (hereinafter referred to as the surface-side low porosity range) may be 2% to 20% of the total thickness of the insulation layer from the surface of the insulation layer. If the surface-side low porosity range is 2% or more from the surface of the insulation layer, the insulation layer will have sufficient strength and defects such as cracking can be suppressed. Furthermore, if the surface-side low porosity range is 20% or less from the surface of the insulation layer, an excessive Young's modulus can be suppressed, and a decrease in thermal shock resistance can be suppressed. The surface-side low porosity range may be 5% or more, 10% or more, or 15% or more from the interface between the metal pipe and the insulation layer. Furthermore, the surface-side low porosity range may be 15% or less, 10% or less, or 5% or less from the interface between the metal pipe and the insulation layer.

The heat insulating layer may be composed of 15% by mass or more of an alumina component and 45% by mass or more of a titania component. An heat insulating layer with this composition has a high melting point and can prevent its shape from changing due to the heat of the exhaust gas.

The titania component constituting the thermal insulation layer may contain a rutile crystalline phase. The titania component may also contain an anatase crystalline phase, a brookite crystalline phase, or an amorphous phase in addition to the rutile crystalline phase. Rutile titania is more stable in high-temperature environments than other crystalline and amorphous phases. Specifically, rutile titania does not undergo phase transition even when exposed to high temperatures. Therefore, an insulating layer containing rutile titania is less likely to undergo volumetric changes associated with phase transitions and is less likely to develop internal stress. As a result, deterioration of the insulating layer is suppressed even when high-temperature exhaust gas directly contacts the insulating layer. In other words, the inclusion of a rutile crystalline phase in the titania component constituting the insulating layer improves the thermal shock resistance of the insulating layer. The anatase crystalline phase and the brookite crystalline phase undergo a phase transition to the rutile crystalline phase in high-temperature environments.

The titania component may be primarily composed of the rutile crystalline phase. That is, the rutile crystalline phase may account for 50% or more by mass of the titania component. As described above, the rutile crystalline phase is stable in high-temperature environments. Therefore, if the rutile crystalline phase accounts for 50% or more by mass of the titania component, the thermal shock resistance of the insulating layer is significantly improved. The proportion of the rutile crystalline phase in the titania component may be 60% or more by mass, 70% or more by mass, 80% or more by mass, or 90% or more by mass. The entire titania component may be the rutile crystalline phase.

The thermal insulation layer may contain flat, plate-shaped ceramic particles. By using plate-shaped ceramic particles, some of the ceramic fibers can be replaced with plate-shaped ceramic particles. Typically, the length (longitudinal size) of the plate-shaped ceramic particles is shorter than the length of the ceramic fibers. Therefore, by using plate-shaped ceramic particles, the heat transfer path within the thermal insulation layer is interrupted, making it more difficult for heat to be transferred within the thermal insulation layer. As a result, the thermal insulation performance of the thermal insulation layer is further improved.

The insulating layer may contain granular particles of 0.1 μm or more and 10 μm or less. When the insulating layer is formed (fired), the ceramic fibers are bonded together via the granular particles, resulting in a high-strength insulating layer. The insulating layer may also be 1 mm or more thick. This allows the exhaust pipe to fully utilize its insulating properties. Since the insulating layer of the exhaust pipe contains ceramic fibers, an insulating layer of 0.5 mm or more can be achieved. That is, since the insulating layer contains ceramic fibers that are less likely to shrink during the forming process (e.g., the firing process), the insulating layer can be formed to a thickness of 0.5 mm or more. For example, if the insulating layer does not contain ceramic fibers, the insulating layer will shrink during the forming process, causing cracks and other problems. Therefore, if the insulating layer does not contain ceramic fibers, it is difficult to form an insulating layer thicker than 0.5 mm.

It is preferable that the metal pipe and the insulating layer have a large difference in thermal conductivity. Specifically, the thermal conductivity of the metal pipe may be 100 times or more that of the insulating layer. The thermal conductivity of the metal pipe may be 150 times or more that of the insulating layer, 200 times or more that of the insulating layer, 250 times or more that of the insulating layer, or 300 times or more that of the insulating layer.

The thermal conductivity of the metal pipe may be 10 W/mK or more and 400 W/mK or less. The thermal conductivity of the metal pipe may be 25 W/mK or more, 50 W/mK or more, 100 W/mK or more, 150 W/mK or more, 200 W/mK or more, 250 W/mK or more, 300 W/mK or more, or 380 W/mK or more. The thermal conductivity of the metal pipe may be 350 W/mK or less, 300 W/mK or less, 250 W/mK or less, 200 W/mK or less, or 150 W/mK or less.

The thermal conductivity of the insulating layer may be 0.05 W/mK or more and 3 W/mK or less. The thermal conductivity of the insulating layer may be 0.1 W/mK or more, 0.2 W/mK or more, 0.3 W/mK or more, 0.5 W/mK or more, 0.7 W/mK or more, 1 W/mK or more, 1.5 W/mK or more, or 2 W/mK or more. The thermal conductivity of the insulating layer may be 2.5 W/mK or less, 2.0 W/mK or less, 1.5 W/mK or less, 1 W/mK or less, 0.5 W/mK or less, 0.3 W/mK or less, or 0.25 W/mK or less.

The metal pipe may be a single pipe or a multiple pipe (e.g., a double pipe). The metal pipe may be straight, entirely (or partially) curved, tapered in the middle, or branched. The insulating layer may be provided on the inner surface of the metal pipe in the case of a single pipe, or on the inner surface of the innermost metal pipe in the case of a multiple pipe. The insulating layer may cover the entire inner surface of the metal pipe, or may cover only a portion of the inner surface of the metal pipe. For example, the insulating layer may cover the entire metal pipe except for one or both ends.

The insulating layer may be composed of a uniform material in the thickness direction. That is, the insulating layer may be a single layer. The insulating layer may also be composed of multiple layers with different compositions in the thickness direction. That is, the insulating layer may have a multilayer structure in which multiple layers are stacked. Alternatively, the insulating layer may have a gradient structure in which the composition gradually changes in the thickness direction. When the insulating layer is a single layer, the exhaust pipe can be easily manufactured (the process of forming the insulating layer on the inner surface of the metal pipe). When the insulating layer has a multilayer or gradient structure, the properties of the insulating layer can be changed in the thickness direction. For example, the proportion of rutile-type crystalline phase in the titania component can be made higher in the surface layer of the insulating layer than in other parts. The structure of the insulating layer (single layer, multilayer, gradient structure) can be selected appropriately depending on the intended use of the exhaust pipe.

The heat insulating layer is composed of one or more of the following materials: ceramic particles (granular particles), plate-like ceramic particles, and ceramic fibers. The ceramic particles, plate-like ceramic particles, and ceramic fibers may contain alumina and/or titania as constituent components. In other words, the ceramic particles, plate-like ceramic particles, and ceramic fibers may be formed from alumina and/or titania.

The ceramic particles may be used as a bonding material for bonding aggregates that form the skeleton of the thermal insulation layer, such as plate-shaped ceramic particles and ceramic fibers. The ceramic particles may be granular particles with an average particle size of 0.1 μm to 10 μm. The ceramic particles may have a larger particle size during the manufacturing process (e.g., the firing process) due to sintering or the like. That is, as a raw material for manufacturing the thermal insulation layer, the ceramic particles may be granular particles with an average particle size of 0.1 μm to 10 μm (before firing). The ceramic particles may be 0.2 μm or more, 0.5 μm or more, or 5 μm or less. For example, a metal oxide may be used as the material for the ceramic particles. Examples of metal oxides include alumina ( _Al2O3 ), spinel ( _MgAl2O4 ), _titania ( _TiO2 ), _zirconia ( _ZrO2 ), magnesia ( _MgO ), mullite ( _Al6O13Si2 ), _cordierite ( _{MgO.Al2O3.SiO2} ), yttria ( _Y2O3 ), steatite ( _MgO.SiO2 ), _forsterite ( _2MgO.SiO2 ), _lanthanum aluminate ( _LaAlO3 ), strontium _titanate ( _SrTiO3 ), etc. These metal oxides have high corrosion resistance and can be suitably used as protective layers for exhaust pipes. When the ceramic particles contain titania particles, the titania particles contain a rutile crystal phase, which improves the thermal shock resistance of the bonding material that bonds the aggregates together, and suppresses deterioration of the heat insulating layer.

The plate-shaped ceramic can function as an aggregate and reinforcing material within the thermal insulation layer. In other words, like ceramic fibers, the plate-shaped ceramic improves the strength of the thermal insulation layer and also prevents the thermal insulation layer from shrinking during the manufacturing process. Furthermore, by using plate-shaped ceramic particles, it is possible to interrupt the heat transfer path within the thermal insulation layer. Therefore, thermal insulation can be improved compared to configurations that use only ceramic fibers as aggregate.

The surface shape (shape observed from the thickness direction) of the flat plate-like ceramic particles is not particularly limited, and may be, for example, a polygon such as a rectangle, a substantially circular shape, or an irregular shape surrounded by curves and/or straight lines. The longitudinal size when observed in cross section may be 5 μm to 100 μm. A longitudinal size of 5 μm or more can suppress excessive sintering of the ceramic particles. A longitudinal size of 100 μm or less can achieve the effect of disrupting the heat transfer path within the thermal insulation layer, as described above, making the particles suitable for use in exhaust pipes used in high-temperature environments. Furthermore, the plate-like ceramic particles contained in the thermal insulation layer may have an aspect ratio of 10 to 60 when observed in cross section with an SEM. For example, by using plate-like ceramic particles with a cross-sectional aspect ratio of 60 to 100 as raw materials, the aspect ratio becomes smaller during the manufacturing process of the thermal insulation layer, and as a result, the plate-like ceramic particles remain in the inorganic porous material. When the cross-sectional aspect ratio is 10 or more, sintering of the ceramic particles can be effectively suppressed. Furthermore, when the cross-sectional aspect ratio is 10 or more, the heat insulating layer can be prevented from becoming too hard (the Young's modulus can be prevented from becoming too high) after production (after firing). As a result, damage to the heat insulating layer (occurrence of cracks, etc.) due to thermal shock (contact of high-temperature exhaust gas with a low-temperature inorganic porous material) immediately after starting the internal combustion engine can be prevented. Furthermore, when the cross-sectional aspect ratio is 60 or less, a decrease in the strength of the plate-shaped ceramic particles themselves can be suppressed, and damage to the heat insulating layer due to exhaust pipe vibration or exhaust gas flow can be suppressed. In addition to the metal oxides used as materials for ceramic particles described above, minerals/clays such as talc ( _Mg3Si4O10 (OH) ₂ ), mica, kaolin _, and _the like, glass, etc. can also be used as materials for the plate-shaped ceramic particles.

Ceramic fibers can function as aggregates and reinforcing materials within the thermal insulation layer. In other words, ceramic fibers improve the strength of the thermal insulation layer and further suppress shrinkage of the thermal insulation layer during the manufacturing process. The length (average fiber length) of the ceramic fibers may be 50 μm or more and 200 μm or less. The diameter (average diameter) of the ceramic fibers may be 1 to 20 μm. The volume fraction of ceramic fibers in the raw materials used to form the thermal insulation layer (the volume fraction of ceramic fibers in the materials constituting the thermal insulation layer) may be 5 vol% or more and 25 vol% or less. By including 5 vol% or more of ceramic fibers in the raw materials for the thermal insulation layer, shrinkage of ceramic particles within the thermal insulation layer during the manufacturing process (firing process) of the thermal insulation layer can be sufficiently suppressed. Furthermore, by limiting the volume fraction of ceramic fibers in the raw materials to 25 vol% or less (i.e., the volume fraction of ceramic fibers within the thermal insulation layer to 25 vol% or less), heat transfer paths within the thermal insulation layer can be disrupted, effectively suppressing heat transfer to the metal pipe. Ceramic fibers can also be confirmed by SEM observation of the cross section of the thermal insulation layer. Ceramic fibers appear approximately circular in SEM images. In other words, the radial cross section of the ceramic fibers appears in the SEM image. Furthermore, if the ceramic fiber material is different from the other materials that make up the thermal insulation layer, the ceramic fibers can be identified (confirmed) by performing EDS analysis. Furthermore, if the ceramic fiber material is different from the other materials that make up the thermal insulation layer, the proportion (volume fraction) of ceramic fibers in the thermal insulation layer can be measured by image processing the results of EDS analysis.

The ceramic fiber material may be _the same as the _above -mentioned plate-like _{ceramic particle} material, such as alumina ( _Al2O3 ), spinel ( _MgAl2O4 ), titania ( _TiO2 ), zirconia ( _ZrO2 ), magnesia ( _MgO ), mullite ( _Al6O13Si2 ), cordierite _{( MgO.Al2O3.SiO2} ), yttria ( _{Y2O3 )} , steatite ( _MgO.SiO2 ), forsterite ( _2MgO.SiO2 ), lanthanum aluminate ( _LaAlO3 ), strontium titanate ( _SrTiO3 ), etc. The heat insulating layer may contain one or more types of ceramic fibers made of the above-mentioned materials.

The content of aggregate and reinforcing material (ceramic fibers, plate-shaped ceramic particles, etc.; hereinafter simply referred to as aggregate) in the raw materials used to form the insulating layer may be 15% by mass or more and 55% by mass or less. If the aggregate content in the raw materials is 15% by mass or more, shrinkage of the insulating layer during the firing process can be sufficiently suppressed. If the aggregate content in the raw materials is 55% by mass or less, the ceramic particles will effectively bond the aggregate together. The aggregate content in the raw materials may be 20% by mass or more, 30% by mass or more, 50% by mass or more, or 53% by mass or more. The aggregate content in the raw materials may be 53% by mass or less, 50% by mass or less, 30% by mass or less, or 20% by mass or less.

As described above, both ceramic fibers and plate-shaped ceramic particles can function as aggregates and reinforcing materials within the thermal insulation layer. However, in order to reliably prevent the thermal insulation layer from shrinking after the exhaust pipe is manufactured (fired), even when both ceramic fibers and plate-shaped ceramic particles are used as aggregates, the ceramic fiber content of the raw materials used to form the thermal insulation layer may be at least 5% by mass. The ceramic fiber content of the raw materials may be 10% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more. The ceramic fiber content of the raw materials may be 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

When both ceramic fibers and plate-shaped ceramic particles are used as aggregate, the proportion of plate-shaped ceramic particles in the total aggregate may be 70% by mass or less. That is, at least 30% by mass of the aggregate may be ceramic fibers. The proportion of plate-shaped ceramic particles in the total aggregate may be 67% by mass or less, 64% by mass or less, 63% by mass or less, 60% by mass or less, or 50% by mass or less. Note that plate-shaped ceramic particles are not necessarily required as aggregate. Furthermore, the proportion of plate-shaped ceramic particles in the total aggregate may be 40% by mass or more, 50% by mass or more, 60% by mass or more, 62% by mass or more, 63% by mass or more, or 65% by mass or more.

The content of plate-shaped ceramic particles in the raw materials used to form the thermal insulation layer may be 5% by mass or more and 35% by mass or less. By including 5% by mass or more of plate-shaped ceramic particles in the raw materials for the thermal insulation layer, shrinkage of the ceramic particles in the thermal insulation layer during the manufacturing process (firing step) of the thermal insulation layer can be sufficiently suppressed. Furthermore, by limiting the content of plate-shaped ceramic particles in the raw materials to 35% by mass or less (i.e., the proportion of plate-shaped ceramic particles in the thermal insulation layer is 35% by mass or less), the heat transfer path within the thermal insulation layer can be disrupted, and heat transfer to the metal pipe can be suitably suppressed. The content of plate-shaped ceramic particles in the raw materials may be 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, or 33% by mass or more. The content of plate-shaped ceramic particles in the raw materials may be 35% by mass or less, 33% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

The _SiO2 content in the heat insulating layer may be 25 mass % or less. This suppresses the formation of an amorphous layer in the heat insulating layer, improving the heat resistance (durability) of the heat insulating layer.

When forming the insulating layer, raw materials containing ceramic particles, plate-shaped ceramic particles, and ceramic fibers, as well as a binder, pore-forming material, and solvent, may be used. Inorganic binders may be used. Examples of inorganic binders include alumina sol, silica sol, titania sol, and zirconia sol. These inorganic binders can improve the strength of the insulating layer after firing. Pore-forming materials may include polymeric pore-forming materials and carbon-based powders. Specific examples include acrylic resin, melamine resin, polyethylene particles, polystyrene particles, carbon black powder, and graphite powder. The pore-forming material may have various shapes depending on the purpose, such as spherical, plate-like, or fibrous. The porosity and pore size of the insulating layer can be adjusted by selecting the amount, size, and shape of the pore-forming material. The solvent may be any solvent that can adjust the viscosity of the raw materials without affecting the other raw materials. Examples of solvents that can be used include water, ethanol, and isopropyl alcohol (IPA).

The inorganic binder mentioned above is also a constituent material of the heat insulating layer. Therefore, when alumina sol, titania sol, etc. are used to form the heat insulating layer, the heat insulating layer should contain 15% by mass or more of an alumina component and 45% by mass or more of a titania component in all constituent materials, including the inorganic binder.

The composition and raw materials of the insulating layer are adjusted depending on the type of metal pipe. The exhaust pipe disclosed in this specification is not particularly limited, but examples of the metal pipe that can be used include stainless steels such as SUS430, SUS429, and SUS444, iron, cast iron, copper, Hastelloy, Inconel, Kovar, and nickel alloys. The composition and raw materials of the insulating layer may be adjusted depending on the thermal expansion coefficient of the metal pipe used. Specifically, when the thermal expansion coefficient of the insulating layer is α1 and the thermal expansion coefficient of the metal pipe is α2, the composition and raw materials may be adjusted to satisfy the following formula 1. For example, when the metal pipe is SUS430, the composition and raw materials of the insulating layer may be adjusted so that the thermal expansion coefficient α1 satisfies the relationship 6×10 ⁻⁶ /K<α1<14× ^{10 −6} /K, more preferably 6×10 ⁻⁶ /K<α1<11×10 −6 /K. Furthermore, when the metal pipe is copper, the composition and raw materials of the thermal insulation layer may be adjusted so that the thermal expansion coefficient α1 satisfies the relationship 8.5× ^{10 −6} /K<α1<20×10 ⁻⁶ /K, more preferably 8.5×10 −6 /K<α1<18×10 ⁻⁶ /K. The value of "α1/α2" may be 0.55 or more, 0.6 or more, 0.65 or more, 0.75 or more, or 0.8 or more. The value of "α1/α2" may be 1.15 or less, 1.1 or less, 1.05 or less, or 1.0 or less.
Formula 1: 0.5<α1/α2<1.2

In the exhaust pipe disclosed in this specification, the above-mentioned raw material may be applied to the inner surface of the metal pipe, followed by drying and firing to form an insulating layer on the inner surface of the metal pipe. Methods for applying the raw material include dip coating, spin coating, aerosol deposition (AD), brush coating, trowel coating, and mold casting. The porosity can be adjusted by adjusting the amount of pore-forming material added in the low porosity region on the interface side, the low porosity region on the surface side, and the central portion of the insulating layer in the thickness direction (the high porosity region between the low porosity region on the interface side and the low porosity region on the surface side).

The exhaust pipe 10 will be described with reference to Figures 1 to 3. The exhaust pipe 10 has an insulating layer 4 on the inner surface of a metal pipe 2 made of SUS430. The insulating layer 4 is bonded to the inner surface of the metal pipe 2 (see Figures 1 and 2). Figure 3 shows an enlarged view of the insulating layer 4. Note that in Figure 3, the inner surface of the metal pipe 2 is shown in plan view for clarity. The insulating layer 4 has, in order from the surface layer 4s side of the insulating layer 4, a surface-side low-porosity region 4a, a high-porosity region 4b, and an interface-side low-porosity region 4c. The back surface 4r of the interface-side low-porosity region 4c is bonded to the metal pipe 2. The thickness 4T of the insulating layer 4 is 2 mm, and the thicknesses 4Ta and 4Tb of the surface-side low-porosity region 4a and the interface-side low-porosity region 4c are both 100 μm. The porosity of the surface-side low-porosity region 4a and the interface-side low-porosity region 4c are both 15%, while the porosity of the high-porosity region 4b is 60%.

The exhaust pipe 10 was manufactured by immersing the metal pipe 2 in a raw material slurry while masking its outer surface, followed by drying and firing. Specifically, the raw material for the interface-side low-porosity region 4c was applied to the inner surface of the metal pipe 2 and dried. The raw material for the high-porosity region 4b was then repeatedly applied and dried. Finally, the raw material for the surface-side low-porosity region 4a (the same raw material as the interface-side low-porosity region 4c) was applied and fired. The raw material slurry was prepared by mixing alumina fibers (average fiber length 140 μm), plate-shaped alumina particles (average particle diameter 6 μm, longitudinal size 25 μm, aspect ratio 85), rutile-type titania particles (average particle diameter 0.25 μm), alumina sol (alumina content 1.1% by mass), acrylic resin (average particle diameter 8 μm), and ethanol. The raw material slurry for the interface-side low-porosity region 4c and the surface-side low-porosity region 4a was adjusted to a viscosity of 50 mPa·s, and the raw material slurry for the high-porosity region 4b was adjusted to a viscosity of 200 mPa·s. The drying process was carried out in an air atmosphere at 200°C for 1 hour. The firing process was carried out in an air atmosphere at 800°C for 3 hours. The plate-shaped alumina particles had an aspect ratio of 40 when the cross section of the heat insulating layer was observed with an SEM.

(Experimental Example)
As described above, the heat insulating layer was prepared by mixing alumina fibers, plate-shaped alumina particles, rutile-type titania particles, alumina sol, acrylic resin, and ethanol to prepare a raw material slurry. The metal was then immersed in the raw material slurry, followed by drying and firing. In this experimental example, the amount of pore-forming material (amount of acrylic resin) and the number of times the raw material was applied were varied, and the state of the heat insulating layer after a thermal vibration test (number of cracks, presence or absence of peeling) was confirmed. Specifically, the porosity of the surface-side low-porosity region 4a (surface side), the interface-side low-porosity region 4c (interface side), and the high-porosity region 4b (center portion), as well as the thickness of the heat insulating layer 4, were varied as shown in Figure 4, and the resulting samples 1 to 18 were evaluated.

First, the porosity of samples 1 to 18 was measured. A bulk body was formed using the raw material slurry described above, and the porosity was evaluated. The porosity was measured at the 5% interface position (interface side), the 5% surface position (surface side), and the central portion. The measurement at the 5% interface position was performed at a position 5% of the thickness 4T of the thermal insulation layer 4 from the back surface 4r (within the interface-side low porosity range 4c). The measurement at the 5% surface position was performed at a position 5% of the thickness 4T of the thermal insulation layer 4 from the surface layer 4s (within the interface-side low porosity range 4c). The measurement at the central portion was performed at the center 4d of the thickness direction of the thermal insulation layer 4. Measurement was carried out using a mercury porosimeter in accordance with JIS R1655 (Method for testing pore size distribution in molded bodies by mercury intrusion method for fine ceramics), and the total pore volume (unit: ^cm3 /g) was calculated from the apparent density (unit: g/ ^cm3 ) measured with a gas displacement density meter (Micromeritics, Accupyc 1330) using the following formula (2). The results are shown in Figure 4.
Equation 2: Porosity [%] = total pore volume / {(1/apparent density) + total pore volume} × 100

A thermal vibration test was performed on samples 1 to 18. The thermal vibration test was performed on samples with an insulating layer formed on the inner surface of a metal pipe. Specifically, a pipe (SUS430, copper) with an inner diameter of φ55 mm, an outer diameter of φ62 mm (thickness 3.5 mm), and a length of 80 mm was immersed in the raw material slurry while the outer surface was masked, and an insulating layer was applied to the inner wall of the pipe. Each sample was then dried at 200°C and fired at 800°C. For the thermal vibration test, the sample was attached to a thermal vibration tester, and propane combustion gas was passed through the pipe from the thermal vibration tester for 5 minutes, followed by room temperature air for 5 minutes. The combustion gas was adjusted so that the maximum gas temperature at the inlet end of the pipe was 900°C and the gas flow rate was 2.0 Nm ³ /min. Next, while the combustion gas was continuously supplied into the pipe, longitudinal vibrations were applied to the pipe. The vibration conditions were 100 Hz and 30 G, and vibration was applied for 50 hours. The test was carried out under these conditions, and the appearance of the heat insulating layer (presence or absence of cracks) and the rate of peeling (%) were measured after the test.

Samples that met both the criteria of fewer than 20 cracks and a peeling rate of less than 20% were given an "A" rating. Samples that met either the criteria of fewer than 20 cracks and a peeling rate of less than 20% and the criteria of fewer than 30 cracks and a peeling rate of less than 30% were given a "B" rating. Samples that met either the criteria of 30 or more cracks and a peeling rate of 30 or more were given a "C" rating. Samples that met both the criteria of 30 or more cracks and a peeling rate of 30 or more were given a "D" rating. Figure 4 shows the results. "A" or "B" is an acceptable level for an exhaust pipe, while "C" or "D" is an unacceptable level for an exhaust pipe.

As shown in Figure 4, all samples (Samples 1-16) in which the porosity at the 5% interface was less than the porosity at the center were confirmed to be acceptable for use as exhaust pipes (rated A or B). Focusing on Samples 1-5, it was confirmed that the acceptable level was achieved even when the thickness of the insulation layer was changed (0.3 mm to 4 mm). In particular, samples with an insulation layer thickness of 0.5 mm to 3 mm (Samples 2-4) were confirmed to have extremely good results (rated A). In particular, Samples 2-4 were confirmed to have improved peelability compared to Samples 1 and 5. These results suggest that adjusting the thickness of the insulation layer to 0.5 mm to 5 mm is preferable in order to reduce the force applied by exhaust gas while preventing the Young's modulus at the interface between the metal pipe and the insulation layer from becoming excessive. In particular, adjusting the thickness of the insulation layer to 3 mm or less reduces the force applied by exhaust gas and improves peelability.

Focusing on samples 6 to 9, it was confirmed that samples with a porosity of 5% to 45% at the surface 5% (samples 7 and 8) achieved particularly good results (rating A). These results show that by adjusting the porosity at the surface 5% to 5% to 45%, the Young's modulus of the insulation layer surface can be prevented from becoming excessive and sufficient strength can be obtained.

Focusing on samples 10 to 12, it was confirmed that samples with a porosity of 5% to 45% at the 5% interface position (samples 11 and 12) achieved particularly good results (rating A). These results show that by adjusting the porosity at the 5% interface position to 5% to 45%, the Young's modulus at the insulation layer interface is prevented from becoming excessive, ensuring sufficient adhesion of the insulation layer to the metal pipe and achieving sufficient strength.

Focusing on samples 3, 13 to 16, it was confirmed that samples with a central porosity of 45% or more and 90% or less (samples 14 and 15) achieved particularly good results (rating A). In particular, samples 14 and 15 were confirmed to have improved peelability compared to samples 13 and 16. These results confirm that by adjusting the central porosity to 45% or more and 90% or less, a heat insulating layer with sufficient strength can be obtained.

Although the embodiments of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. Furthermore, the technical elements described in this specification or drawings exhibit technical utility either alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings simultaneously achieves multiple objectives, and achieving one of those objectives is itself technically useful.

2: Metal pipe 4: Heat insulating layer 10: Exhaust pipe

Claims (7)

An exhaust pipe having a metal pipe and a heat insulating layer provided on the inner surface of the metal pipe,
The thermal insulation layer is a single layer structure made of a uniform material.
An exhaust pipe in which the porosity of the insulation layer at a position 5% from the interface between the metal pipe and the insulation layer relative to the total thickness of the insulation layer and the porosity of the insulation layer at a position 5% from the surface of the insulation layer relative to the total thickness of the insulation layer are lower than the porosity at the center of the thickness direction. 2. The exhaust pipe according to claim 1 , wherein the porosity of the heat insulating layer at a position 5% from the interface between the metal pipe and the heat insulating layer relative to the total thickness of the heat insulating layer is 5% to 45%. 3. The exhaust pipe according to claim 2 , wherein the range in which the porosity of the heat insulating layer is 5% to 45% is 2% to 20% of the total thickness of the heat insulating layer from the interface between the metal pipe and the heat insulating layer. 4. The exhaust pipe according to claim 1 , wherein the porosity of the heat insulating layer at a position 5% from the surface of the heat insulating layer relative to the total thickness of the heat insulating layer is 5% or more and 45% or less. 5. The exhaust pipe according to claim 4 , wherein the range in which the porosity of the heat insulating layer is 5% to 45% is 2% to 20% of the total thickness of the heat insulating layer from the surface of the heat insulating layer. 6. The exhaust pipe according to claim 1 , wherein the heat insulating layer has a porosity of 45% or more and 90% or less at the center in the thickness direction. 7. The exhaust pipe according to claim 1, wherein the thickness of the heat insulating layer is 0.5 mm or more and 5 mm or less.