Thermal insulation material

Abstract

The present invention provides a thermal insulation material that is excellent in thermal insulation performance under high temperature, and has high strength and hardness. The thermal insulation material according to the present invention is obtained by molding a mixture containing 62 to 86 mass % of fumed silica and 10 to 30 mass % of an infrared-opaque material. The thermal insulation material has a mode diameter of 20 nm or less in a pore distribution range of 70 nm or less, a total number of pores of 3.5 × 1016 / g or more, and, at 800°C, a thermal conductivity of 0.06 W / (m·K) or less, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more.

Description

insulation

[0001] The present invention relates to a thermal insulation material.

[0002] As part of efforts to become carbon neutral, there is a demand for improved insulation performance, particularly at high temperatures, of insulation materials to reduce heat loss in steelmaking furnaces and improve the power generation efficiency of fuel cells. Meanwhile, high strength and hardness are also required for ease of handling during installation and assembly, resistance to compression caused by thermal expansion of contents when used at high temperatures, and vibration resistance during transportation. Generally, insulation performance, strength, and hardness are in a trade-off relationship, making it difficult to achieve both.

[0003] Conventionally, ultrafine silica powder such as fumed silica has been widely used as a raw material for heat insulating materials. For example, Patent Document 1 discloses a technology for forming ultrafine silica powder into a porous body by itself, by devising the powder characteristics, the microstructure of the formed body, the forming method, etc., to obtain a silica formed body with good heat insulating properties and formability.

[0004] Japanese Patent Application Laid-Open No. 2007-169158

[0005] In the technology of Patent Document 1, ultrafine silica powder is molded alone, so there is a concern that the thermal insulation performance may rapidly decrease at high temperatures (400°C or higher) where the influence of radiation is significant. That is, the silica molded body (thermal insulation material) obtained by the technology of Patent Document 1 has problems with thermal insulation performance, particularly at high temperatures. Furthermore, the strength and hardness of the silica molded body (thermal insulation material) obtained by the technology of Patent Document 1 are based on the assumption that silica is molded alone, and therefore may not be sufficient when molded as a mixture with other raw materials. Furthermore, the technology of Patent Document 1 does not anticipate the mixing of additives (fibers, infrared-opaque materials, etc.) with ultrafine silica powder and compression molding the mixture, and it is conceivable that the addition of additives would deteriorate moldability.

[0006] In view of the above, an object of the present invention is to provide a heat insulating material that has excellent heat insulating performance at high temperatures and also has high strength and hardness.

[0007] According to one aspect of the present invention, there is provided the following heat insulating material: The heat insulating material is obtained by molding a mixture containing 62 to 86 mass % of fumed silica and 10 to 30 mass % of an infrared opaque material, and has a mode diameter of 20 nm or less in the range of 70 nm or less in the pore distribution, and a total number of pores of 3.5 × 10 １６ and a thermal conductivity at 800°C of 0.06 W / (m·K) or less, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more.

[0008] According to the present invention, it is possible to obtain a heat insulating material that has excellent heat insulating performance at high temperatures and also has high strength and hardness.

[0009] Graph showing the pore distribution of the heat insulating material according to the present invention (Example 1). Graph showing the pore distribution of the heat insulating material in Comparative Example 1. Schematic diagram showing the manufacturing process of fumed silica. Schematic diagram showing the difference in structure of fumed silica.

[0010] Hereinafter, an embodiment of the present invention will be described. The mean free path of molecules in air at room temperature is approximately 70 nm. Therefore, it is known that heat transfer due to air convection and conduction is suppressed in a porous body having pores with a diameter of 70 nm or less, and such porous bodies exhibit excellent thermal insulation properties. In the present invention, in order to significantly reduce thermal conductivity and increase strength and hardness of thermal insulation materials compared to conventional materials, extensive research was conducted, focusing on the distribution of pores in the range of 70 nm or less in the porous body and the number of pores inside the thermal insulation material.

[0011] In the case of insulation materials made from ultrafine powder, the pore size distribution can be roughly classified into the following three types: (1) 100 nm or less: Pores surrounded by chain-like primary particles in primary aggregates (mode diameter A shown in Figures 1 and 2) (2) 100 nm to 10 μm: Pores formed in the gaps between primary aggregates in secondary aggregates or between fibers (mode diameter B shown in Figures 1 and 2) (3) 10 μm or more: Relatively large spaces due to gaps between secondary aggregates or defects or cracks in the molded body (none in the case of the samples in Figures 1 and 2)

[0012] The fumed silica used as the primary raw material for the thermal insulating material in this invention is a highly pure material with a high heat resistance of 800°C or higher. As shown schematically in Figure 3, it is obtained by burning silicon tetrachloride with oxygen and hydrogen in a flame (above 1000°C) to produce ultrafine silica particles (flame hydrolysis). Primary particles (diameter approximately 5-30 nm) bond with other particles in the flame to form primary aggregates (diameter approximately 100-400 nm) with a complex chain structure. It is difficult to break down primary aggregates into primary particles, and primary aggregates are essentially the smallest unit of powder. The inventors have discovered that by minimizing the pore diameter (mode diameter) of these primary aggregates in the range of 70 nm or less and increasing the number of pores formed by the entanglement of the chain structure of the aggregates and the gaps between the aggregates, it is possible to achieve lower thermal conductivity, higher strength, and higher hardness for the thermal insulating material than conventional materials.

[0013] The pore size of the primary aggregates can be minimized by, for example, increasing the specific surface area of ​​the fumed silica to reduce the size of the primary particles. To reduce the size of the primary particles, for example, manufacturing conditions such as the supply amount of silicon tetrachloride and the combustion time can be adjusted to bond the primary particles before they grow, forming primary aggregates. The pore size of primary aggregates with reduced primary particle size becomes smaller, resulting in a smaller mode diameter in the pore distribution. Furthermore, the complexity of the chain structure of the primary aggregates (the number of branches, the chain length (structure), the surface properties (the number of surface silanol groups, etc.)) is also thought to affect the pore size and number of pores, and these factors combined are thought to determine the thermal insulation performance, strength, and hardness of the insulation material.

[0014] The reason why minimizing the mode diameter in the 70 nm or less range of the pore distribution contributes to the low thermal conductivity of the insulating material is believed to be that, since the pores are loose spaces between chain-like primary particles in the primary aggregates, the size of the pores changes partially due to molecular vibration, rotation, translation, contraction, elongation, etc., making it impossible to completely suppress heat transfer by gas molecules. Therefore, the smaller the pore diameter (mode diameter) (especially if it is 20 nm or less), even if the pore size changes slightly, it is thought to contribute to suppressing heat transfer by gas molecules by being smaller than the mean free path of molecules in air. In addition, secondary aggregates (particle size approximately 10 to 50 μm) are formed by physical bonding of primary aggregates with other aggregates by van der Waals forces. The bonding force is relatively weak, and although they are dispersed into primary aggregates by applying strong shear force, etc., they are prone to re-agglomeration. Therefore, it is difficult to completely eliminate pores of 100 nm or more that are mainly formed within secondary aggregates. Therefore, it is effective to increase the number of pores in the primary or secondary aggregates by making the chain structure in the primary aggregates larger and more complex. This increases the number of spatial partitions created by the chain-like primary particles, thereby further suppressing heat transfer due to air convection and conduction, improving thermal insulation. Furthermore, increasing the number of partitions densifies the pores and increases the point contact between particles, thereby improving structural hardness and strength.

[0015] In the present invention, based on the above-mentioned technical considerations, low thermal conductivity and high strength and hardness of the heat insulating material have been realized by specifically configuring it as follows: That is, the heat insulating material of the present invention is obtained by molding a mixture containing 62 to 86 mass % of fumed silica for creating fine pores and 10 to 30 mass % of an infrared opaque material for reflecting radiant heat in the high temperature range, and has a mode diameter of 20 nm or less in the range of 70 nm or less in the pore distribution, and a total number of pores of 3.5 × 10 １６ This makes it possible to obtain a heat insulating material having a thermal conductivity of 0.06 W / (m·K) or less at 800°C, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more.

[0016] The content of each raw material in the mixture is more preferably 70 to 86 mass% of fumed silica and 10 to 25 mass% of infrared opaque material in order to reduce the thermal conductivity and increase the strength and hardness of the heat insulating material. Here, in the present invention, the mixture refers to a mixture of each raw material of the heat insulating material and a binder described below, and the content of each raw material in the mixture refers to the proportion of that material relative to the total amount of each raw material of the heat insulating material and the solid content of the binder, which is 100 mass%.

[0017] Furthermore, in the present invention, the mode diameter in the range of 70 nm or less of the pore distribution is 16 nm or less, and the total number of pores is 4.5 × 10 １６ By making the density of particles / g or more, it is possible to obtain a heat insulating material having a thermal conductivity of 0.055 W / (m·K) or less at 800° C. In the present invention, the thermal conductivity is measured by a hot wire method, specifically in accordance with JIS R 2251-1.

[0018] The fumed silica used as the main raw material for the heat insulating material in the present invention preferably has a DBP absorption of 370 mL / 100 g or more, more preferably 390 mL / 100 g or more. The magnitude of DBP absorption corresponds to the magnitude of the structure of the fumed silica. The structure is a chain-like or branched structure formed by the connection of multiple primary particles, and is related to the size of the primary aggregates, the extent of the structure, etc. When the structure is large (long), as shown in Figure 4, the chain structure in the primary aggregates becomes more entangled, which is thought to result in a smaller average pore diameter and an increased number of pores, which contributes to improving the heat insulating properties, strength, and hardness of the heat insulating material as mentioned above. Therefore, controlling the structure is thought to be effective in adjusting such an aggregate structure. In addition, the specific surface area of ​​fumed silica (BET method, N ２ Adsorption: BET specific surface area is 300-500m ２ / g, and 400 to 450m ２ / g is more preferable. As mentioned above, the larger the specific surface area, the lower the thermal conductivity and the higher the hardness and strength. In other words, if the specific surface area is small, the pore size becomes larger and the thermal conductivity becomes higher. However, if the specific surface area is too large, the agglomerates will be difficult to disperse due to the strong cohesion force, and the primary particles will be small, which may lead to sintering and a decrease in heat resistance. Note that the mixture may contain fumed alumina, precipitated alumina, etc. to improve heat resistance by suppressing sintering.

[0019] In thermal insulation at high temperatures (400°C or higher), heat transfer by radiation is significantly greater than heat conduction or convection, making radiation reduction an effective way to suppress heat transfer at high temperatures. Therefore, in the present invention, an infrared opaque material is included in the mixture as a material that reflects and absorbs infrared rays. Materials with high refractive indexes, such as silicon carbide, iron oxide, zirconium silicate, and titanium oxide, can be used as the infrared opaque material. To efficiently reflect radiant heat, the average particle size (D50) of the infrared opaque material is preferably 1 to 5 μm, more preferably 2 to 4 μm, taking into account the infrared wavelength. Furthermore, the content and dispersibility in the mixture are also important for blocking infrared rays (radiation). It is recommended that the content of the infrared opaque material be 10 to 30% by mass, and that the material be mixed with fumed silica in a mixer or the like, and crushed as much as possible by causing them to collide with each other, thereby dispersing and mixing the materials. Note that if the content of the infrared opaque material is less than 10% by mass, heat transfer by radiation increases, resulting in higher thermal conductivity. Furthermore, if the content of the infrared opaque material exceeds 30 mass %, the content of the fumed silica decreases, causing the pores to become coarse, resulting in not only a decrease in strength and hardness but also an increase in thermal conductivity.

[0020] In order to improve the shape retention of the heat insulating material, which is a molded body, inorganic fibers may be contained in the mixture. The inorganic fibers are fibers made of inorganic materials, and silica fibers, glass fibers, alumina fibers, etc. can be used. In consideration of heat resistance and cost, silica fibers (SiO ２It is preferable to use inorganic fibers having a content of 95% by mass or more. The content of the inorganic fibers in the mixture is preferably 1 to 10% by mass, more preferably 2 to 5% by mass, from the viewpoint of moldability and processability. Furthermore, the shape of the inorganic fibers is preferably such that the average fiber diameter is 4 to 12 μm and the average fiber length is 3 to 10 mm. More preferably, the average fiber diameter of the inorganic fibers is 5 to 9 μm and the average fiber length is 5 to 8 mm.

[0021] The higher the bulk density of the insulation material, the greater the hardness and strength that can be obtained. However, if it is too high, the insulation properties may deteriorate, and if it is too low, the strength may decrease. Therefore, the bulk density is set to 240 to 350 kg / m ３ Preferably, it is 260 to 310 kg / m ３ It is more preferable that:

[0022] The heat insulating material of the present invention can be obtained by a manufacturing method including the following steps: (1) Mixing step: The raw materials of the heat insulating material, i.e., fumed silica, infrared opaque material, inorganic fibers, etc., are dry mixed in a mixer, and then further dry mixed in the mixer while spraying an inorganic binder (e.g., a colloidal silica solution) to produce a powdery mixture. (2) Molding step: The mixture obtained in the mixing step is subjected to dry uniaxial pressure molding using a press molding machine having upper and lower press sections each provided with a large number of holes.

[0023] In the mixing process described above, fumed silica with a large specific surface area and structure is thoroughly crushed, dispersed, and mixed. This allows the pore size to be made smaller and the number of pores to be increased. As a result, thermal conductivity is reduced and strength and hardness are improved. In addition, an infrared opaque material with a high infrared shielding effect is also thoroughly crushed, dispersed, and mixed, resulting in high thermal insulation even at high temperatures. The mixer used in the mixing process is not particularly limited as long as it has the appropriate shear force, impact force, etc. to crush powder (especially agglomerates). Henschel mixers, super mixers, Spartan mixers, etc. can be used. The shape of the blades used in the mixer should be selected appropriately as long as it can exert a high shear force.

[0024] The heat insulating material of the present invention can be produced without a heating step of heating the molded body obtained in the above-mentioned molding step. Therefore, it is possible to produce it in a short tact time and since no heating equipment is required, it is possible to reduce both the initial cost and the running cost, and therefore it is possible to reduce the overall production cost. Furthermore, CO ２ As explained above, the method for producing a thermal insulating material of the present invention does not require post-treatment such as heat treatment, and therefore not only is it low cost, but it is also possible to obtain a thermal insulating material that has high heat resistance, low thermal conductivity, and high strength, despite the simple production method and short takt time.

[0025] The uses of the thermal insulating material of the present invention are not particularly limited, and can be used, for example, for insulating high-temperature parts in steelmaking furnaces such as ladles, torpedo cars, and tundishes; rolling furnaces such as soaking furnaces and heating furnaces; heat treatment furnaces such as carburizing furnaces, metal sintering furnaces, and induction treatment furnaces; non-ferrous metal furnaces such as melting furnaces and fuel heating furnaces; ceramic furnaces such as glass melting furnaces, cement firing furnaces, and refractory firing furnaces; between cells or modules of secondary batteries; and between high-temperature parts such as reformers and cell stacks of fuel cells.

[0026] Table 1 shows the raw material blends and the physical properties of the resulting heat insulating materials for the examples of the present invention and the comparative examples.

[0027]

[0028] The measurement methods for each physical property shown in Table 1 are as follows. Unless otherwise specified, measurements were taken at room temperature (approximately 20 to 25°C). [DBP absorption (oil absorption) of fumed silica] The DBP (dibutyl phthalate) absorption (oil absorption) of fumed silica, the raw material of the thermal insulation material, was measured using an absorption measuring device (S410E manufactured by Asahi Research Institute, compliant with JIS K 6217-4). The amount of DBP dripped (mL) at 70% of the maximum torque with an oil drip rate of 4 mL / min was calculated, and the value converted to DBP absorption per 100 g of sample (mL / 100 g) was used. [Pore distribution] The pore distribution of the thermal insulation material was measured using a mercury porosimeter (Autopore IV 9520 manufactured by Micromeritics) in accordance with JIS R1655. The sample was processed to a size that could be inserted into a holder (length, width, and thickness each 10 mm or less), and the measurement conditions were a surface tension of 0.480 N / m and a contact angle of 140°. The measurement range was 0.004 to 500 μm, and the pore diameter at the peak observed in the range of 70 nm or less in the log differential pore volume distribution (dV / d (logD)) was defined as the "mode diameter in the range of 70 nm or less in the pore distribution" (hereinafter referred to as "mode diameter (≦70 nm)"). The total number of pores (number / g) was calculated using the formula for the volume of a sphere (4 / 3 × π × (radius)) based on the average pore diameter (4V / A) obtained by measurement with a mercury porosimeter, representing the shape of the pores. ３ ) to determine the volume per pore (mL / pore), and then divide the total pore volume (mL / g) by the volume per pore. [Thermal Conductivity] The thermal conductivity (W / (m·K)) of the insulating material was measured using a thermal conductivity measuring device (HWM-15 manufactured by Spain Labo Co., Ltd., compliant with JIS R 2251-1) using the hot wire method (orthogonal method). The measurement atmosphere was air, and the temperature setting was room temperature (25°C) or 800°C. The sample size was 230 mm in length, 114 mm in width, and 20 mm in thickness, and two samples were used. [Compressive Strength] The compressive strength of the insulating material was measured using an autograph manufactured by Shimadzu Corporation. The sample size was 50 mm in length, 50 mm in width, and 20 mm in thickness, and the compression speed was 0.5 mm / min. At this time, the test force (N) at the time of 10% compression from the initial thickness was calculated as the area of ​​the compressed surface (mm ２The value obtained by dividing the hardness by the total hardness (kg / m) was taken as the compressive strength (MPa). [Hardness] The hardness of the heat insulating material was measured using a durometer (type C, in accordance with JIS K 7312 / JIS S 6050). The sample size was 50 mm in length, 50 mm in width, and 20 mm in thickness, and measurements were taken at any five points, and the average value was calculated. [Bulk density] The bulk density (kg / m) of the heat insulating material was ３ ) was calculated by measuring the dimensions and mass of the sample for measuring thermal conductivity and dividing the mass by the volume determined from the dimensions.

[0029] In Example 1, a thermal insulating material was produced as follows. Fumed silica (BET specific surface area 400 m) was used as a raw material for the thermal insulating material. ２ / g, DBP absorption capacity 437 mL / 100 g) 2.88 kg (72 mass%), fumed alumina (BET specific surface area 100 m ２ 0.16 kg (4 mass%) of silica (1 / g), 0.8 kg (20 mass%) of an infrared opaque material (silicon carbide (average particle size (D50) of approximately 3 μm)), and 0.12 kg (3 mass%) of inorganic fibers (silica fibers (average fiber diameter of approximately 7 μm, average fiber length of approximately 6 mm)). Also, 0.133 kg (1 mass% (solids)) of colloidal silica (silica sol) solution (solids content 30 mass%) was prepared as a binder (4 kg in total of each raw material and binder solids). First, the fumed silica, fumed alumina, infrared opaque material, and inorganic fibers were all dry mixed in a Henschel mixer (rotation speed approximately 3000 rpm) for 14 minutes. Next, the entire amount of the binder was added by spraying with a spray nozzle while dry mixing for 6 minutes in a mixer (same conditions as above) to obtain a powder mixture (total mixing time 20 minutes). A mesh was then placed on the bottom side of the press surface of the metal frame in the press molding machine, about 0.92 kg of the mixture was added, and a mesh was also placed on the top side, and the mixture was dry uniaxially press-molded for 1 minute while degassing at a molding pressure of about 1 MPa using upper and lower press sections each provided with a large number of holes, to obtain a base plate for the insulation material sample (length 400 mm, width 300 mm, thickness 26 mm). Next, the surface of the base plate was smoothed by milling using a machining center, the thickness was reduced to 20 mm, and then it was cut to the specified dimensions to obtain each test piece.

[0030] For Examples 2 to 7 and Comparative Examples 1 to 3, thermal insulating materials were produced as prototypes in the same manner as in Example 1, and test pieces were obtained. The differences between Examples 2 to 7 and Comparative Examples 1 to 3 and Example 1 are as follows. In Example 2, fumed silica having a BET specific surface area of ​​300 m was used. ２ In Example 3, fumed silica having a BET specific surface area of ​​450 m ２ / g and DBP absorption of 449 mL / 100 g were used. In Example 4, the contents of fumed silica and infrared opaque material were 62 mass% and 30 mass%, respectively. In Example 5, the contents of fumed silica and infrared opaque material were 82 mass% and 10 mass%, respectively. In Example 6, the contents of fumed silica, fumed alumina, and infrared opaque material were 86 mass%, 0 mass%, and 10 mass%, respectively. In Example 7, fumed silica having a BET specific surface area of ​​380 m was used. ２ In Comparative Example 1, fumed silica having a BET specific surface area of ​​200 m ２ / g and DBP absorption of 354 mL / 100 g were used. In Comparative Example 2, the contents of the fumed silica and the infrared opaque material were 92 mass% and 0 mass%, respectively. In Comparative Example 3, the contents of the fumed silica and the infrared opaque material were 52 mass% and 40 mass%, respectively.

[0031] On the other hand, in Comparative Example 4, an original plate (length 500 mm, width 500 mm, thickness 25 mm) of Roslim Board GH ("Roslim" is a registered trademark) manufactured by Nichias, a commercially available silica-based heat insulating material, was smoothed by milling using a machining center, the thickness was reduced to 20 mm, and then cut to the specified dimensions to obtain each test piece.

[0032] Table 1 shows the measurement results of each physical property value in Examples 1 to 7 and Comparative Examples 1 to 4. Table 2 also shows the reduction rate of thermal conductivity in Examples 1 to 7 relative to Comparative Examples 1 to 4 (the rate of the value obtained by dividing the thermal conductivity of an Example by the thermal conductivity of a Comparative Example and subtracting 1).

[0033]

[0034] As shown in Table 1, in Example 1, the mode diameter (≦70 nm) was 11.2 nm, and the total number of pores was 6.0×10 １６ The resulting insulating material had a particle size of 0.015 / g, a thermal conductivity (800°C) of 0.047 W / (m·K), a compressive strength of 0.58 MPa, and a hardness of 81. Furthermore, compared to Comparative Examples 1 to 4, as shown in Table 2, a significant reduction in thermal conductivity (800°C) of 23 to 83% was observed. Furthermore, despite the high insulating performance, the compressive strength and hardness were also high, and the handling was better than that of the insulating materials of Comparative Examples 1 and 3. Note that the insulating materials of Comparative Examples 1 and 3 suffered from raggedness and chipping during molding and handling of the insulating material samples. It is presumed that the fumed silica in Example 1, which has a high specific surface area and a large structure, resulted in a smaller pore size and a larger number of pores in the primary aggregates. As a result, the mode diameter (≦70 nm) was further reduced compared to conventional technology and commercially available products, and further, the total number of pores was 7.5 times that of Comparative Example 1. This is thought to have improved the insulation performance, compressive strength, and hardness, even though the total pore volume and the pore volume in the range of 70 nm or less in the pore distribution (pore volume (≦70 nm)) were similar to those of Comparative Example 1.

[0035] In Examples 2 to 7, the mode diameter (≦70 nm) was 8.9 to 15.8 nm, and the total number of pores was 4.4 to 12.1 × 10 １６ The resulting heat insulating material had a thermal conductivity (800°C) of 0.046 to 0.057 W / (m·K), a compressive strength of 0.44 to 0.92 MPa, and a hardness of 77 to 85. As in Example 1, a significant difference was obtained compared to Comparative Examples 1 to 4, and it was possible to achieve both high heat insulating performance and high compressive strength and hardness.

Claims

1. A heat insulating material obtained by molding a mixture containing 62 to 86% by mass of fumed silica and 10 to 30% by mass of an infrared opaque material, the mode diameter in the range of 70 nm or less of the pore distribution is 20 nm or less, and the total number of pores is 3.5 x 10 １６ and a thermal conductivity at 800°C of 0.06 W / (m·K) or less, a compressive strength of 0.4 MPa or more, and a hardness of 75 or more.

2. The insulation material according to claim 1, wherein the DBP absorption of the fumed silica is 370 mL / 100 g or more.

3. The BET specific surface area of ​​the fumed silica is 300 to 500 m ２ The heat insulating material according to claim 1 or 2, wherein the viscosity is 100 / g.

4. The mode diameter in the range of 70 nm or less of the pore distribution is 16 nm or less, and the total number of pores is 4.5 x 10 １６ 2. The heat insulating material according to claim 1, wherein the fumed silica has a DBP absorption of 390 mL / 100 g or more and a thermal conductivity at 800° C. of 0.055 W / (m·K) or less.

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