JP2007161861A - High performance heat insulation material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To supply a high performance heat insulation material with a low density, a low thermal conductivity, and a high strength, and to provide a high performance heat insulation structure applicable to a liquified natural gas storage tank etc. <P>SOLUTION: The high performance heat insulation material is prepared by adding an infrared blocking substance to a foam material and by foaming the foam material so that its density becomes lower than a density causing the thermal conductivity in the foam of the foam material itself to be lowest; and thus prepared high performance heat insulation material is used as a high temperature heat insulation member 25. A heat insulation material comprising fine bubbles with sizes of about the average free path of gas molecules is used as a low temperature heat insulation member 23. An air-tight sheet 24 is inserted into between the high temperature heat insulation member 25 and the low temperature heat insulation member 23, and the side of the low temperature heat insulation member is evacuated with a cryopump 22, thus giving the high performance heat insulation structure 20. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-performance heat insulating material that improves the heat insulating performance of a porous heat insulating material, and more particularly to a high-performance heat insulating material that can be optimally used in a low-temperature storage container for liquefied natural gas or liquefied petroleum gas.

  A porous heat insulating material such as a foaming urethane resin is in a state where a large amount of bubbles are present in the resin solid, and the total heat transfer amount Q is the heat Qs conducted in the resin solid and the gas convection in the foam cell. It can be expressed by the sum of the heat Qg conducted through and the radiant heat transfer Qr passing through the heat insulating material. Therefore, a general method for improving the heat insulating performance of the heat insulating material is to suppress the gas convection heat transfer action by suppressing the pore diameter to 1 mm or less, and to reduce the resin solid conduction heat Qs having a relatively high conductivity. It was.

When the density of the porous heat insulating material is lowered, the resin solid portion decreases and the bubble portion increases. For this reason, the conduction heat Qs in the resin solid having a high conductivity is lowered, and the contribution of the gas conduction heat Qg having a low thermal conductivity is increased. Therefore, the apparent thermal conductivity decreases as the density decreases, and the heat insulating performance can be maintained even if the heat insulating material is thin.
In particular, in the heat insulation material applied to storage tanks for cryogenic liquefied natural gas and liquefied petroleum gas, if the heat insulation material can be made thin and lightweight, it will not only save the raw material of the heat insulation material but also operate when the tank is installed in a carrier ship. The effect is great because it leads to cost savings.

  However, as seen in FIG. 7, for example, when the density is less than a certain density, the thermal conductivity increases conversely and the heat insulation performance deteriorates. This is because at a certain density or less, the resin film forming the bubbles becomes thin and the amount of infrared rays transmitted through the heat insulating material increases, so that the radiant heat transfer Qr cannot be ignored. In addition, FIG. 7 is a graph which shows the example of a measurement about a rigid urethane foam, and it cannot be overemphasized that the relationship of the heat conductivity with respect to a density changes with resin materials, foaming gas, a bubble diameter, etc. largely.

As described above, since the amount of infrared transmission increases in the low density region, there is a limit to reducing the thickness and weight of the heat insulating material using the decrease in thermal conductivity due to the reduction in density.
As a method for reducing the infrared transmission of the heat insulating material, it is known that mixing an infrared attenuating agent material in the foam is effective. The infrared attenuating agent is an infrared reflecting material or an infrared absorbing material, and for example, granular flakes of metal such as aluminum, carbon black, graphite, titanium dioxide and the like are used.

Patent Document 1 discloses a heat insulating foam structure in which carbon black having a quality called a thermal grade having a particle size larger than 150 nm is mixed. Thermal grade carbon black performs Rayleigh scattering with respect to normal infrared rays, so that infrared scattering is increased and the thermal conductivity of the foam structure is reduced. Moreover, since thermal grade carbon black has the characteristic of being easily deposited on the foam wall of the foam, the solid heat transfer thermal conductivity and the radiant heat conductivity are lowered, and the effective thermal conductivity of the foam structure is lowered.
Patent Document 2 discloses that the thermal conductivity is remarkably lowered by mixing an infrared attenuating agent in the foam and setting the bubble diameter to 70 μm or less. In particular, when the average bubble diameter is 1.0 μm or less and when the bubbles are exhausted to an absolute pressure of 0.1 Torr (about 13 Pa) or less, the greatest decrease in thermal conductivity is observed.

However, since carbon black has a high infrared absorption rate and a high thermal conductivity, the low density heat insulating material mixed with carbon black increases the heat conduction heat of the solid portion, and the heat conductivity of the heat insulating material does not sufficiently decrease. In addition, when a foam structure with a large cell ratio and a very low density is formed with a foam containing an infrared attenuating agent, the brittleness of the solid portion containing the granular attenuating agent in a state in which it is difficult to be fused increases, and a particularly thin cell membrane The part becomes brittle and the foam structure is easily damaged.
Japanese National Patent Publication No. 8-504856 Japanese National Patent Publication No. 8-504859

  Therefore, the problem to be solved by the present invention is to supply a high-performance heat insulating material with low density, low thermal conductivity and high strength, and a high-performance heat insulating structure applicable to a liquefied natural gas storage tank or the like. Is to provide.

In order to solve the above-mentioned problems, the high-performance heat insulating material of the present invention adds a needle-like single crystal to the foam material so that the density of the foam material itself is lower than the density at which the thermal conductivity in the foam is minimized. It is characterized by being foamed.
Foam materials include various organic and inorganic materials such as thermoplastic polymers, thermosetting polymers, aerogels, ceramics, and glass. In addition, a well-known foaming agent is added to these foam materials as needed.

  The acicular single crystal is also called a whisker, and for example, carbon whisker, zinc oxide whisker, silicon carbide whisker and the like can be obtained relatively easily. The whisker has an elongated shape like a very rigid needle. For example, carbon whiskers are elongated with a diameter of 0.3 μm to 0.6 μm and a length of 5 μm to 15 μm. By adding whiskers to the foam to strengthen the cell walls, the strength of the heat insulating material is significantly increased. In addition, it is preferable that the whisker is completely embedded in the bubble wall and does not protrude into the bubble.

  As for the radiation ray to the heat insulating material, the remainder partially reflected on the surface of the heat insulating material is absorbed, scattered or transmitted by the heat insulating material. When the radiation that passes through the heat insulating material increases, the apparent thermal conductivity increases. Among the radiation rays, infrared rays of 0.8 μm to 2.4 μm, particularly called heat rays, have a great influence on heat transfer. Therefore, the high-performance heat insulating material of the present invention is strengthened by adding 1 to 5% by weight of whisker, which is a rigid fiber, to the foam material, and at the same time, the infrared transmission amount is reduced to reduce the thermal conductivity. Let

Addition of a needle-like single crystal is particularly effective in a low density region where the infrared transmission rate of the foam is large and in a region where the thermal conductivity is increased.
For example, in the case of rigid urethane foam, the thermal conductivity becomes the lowest when the density as the foam is about 35 kg / m ³ , and the thermal conductivity increases rapidly when the density is reduced below that.
Therefore, for example, when using with a density of 10 to 30 kg / m ³ , particularly about 20 kg / m ³ , it is highly effective to add a needle-like single crystal. If it is 10 kg / m ³ or less, the solid ratio becomes too low and breaks easily, and if it is 30 kg / m ³ or more, the infrared transmittance is not large, so that the effect unique to the present invention cannot be expected.

The additive for blocking infrared conduction has a different effect on the thermal conductivity depending on the ratio of the added amount to the foam material and the size of the additive. In general, when the diameter of the additive is smaller than the wavelength of light, the dispersibility is improved by the Rayleigh scattering effect, but there is an appropriate region because the reflection of light is reduced. From the experimental results, it is known that the effect is particularly great when the weight ratio is about 1% to 5%, and from the Rayleigh scattering effect, the particle diameter is around 1 μm.
Since the diameter of the whisker used in the present invention is close to 1 μm, it meets this condition.

  As a result of measuring the apparent thermal conductivity of the high-performance heat insulating material of the present invention, a decrease of 16% in the case of adding carbon whisker and 19% in the case of adding zinc oxide whisker was observed as compared with the unadded foam. It was. Compared to the reduction rate of 9.4% for carbon black, it can be said that the improvement in thermal conductivity of the present invention is extremely remarkable.

Carbon black has a high infrared absorptivity and lowers the transmittance until the infrared rays are almost blocked. However, due to the high thermal conductivity of carbon black itself, the thermal conductivity of the entire heat insulating material cannot be sufficiently reduced.
On the other hand, whiskers have a moderate reflectivity and absorptivity, and reduce infrared transmittance, and since heat conductivity is not high, the heat conductivity of the heat insulating material can be substantially reduced. .

  The high-performance heat insulating structure of the present invention has a cryopump system cryosurface arranged on the wall surface of a container to be kept cold, and covers a low temperature heat insulating member made of a foam heat insulating material formed of fine open cells from above the cryosurface. Further, it is characterized in that an air-tight sheet is further coated thereon, a high-temperature heat insulating member composed of a high-performance heat insulating material is further disposed, and a waterproof / moisture-proof sheet is coated on the surface thereof.

  The high-performance heat insulation structure of the present invention can be applied to a cryogenic liquefied natural gas container. In this case, the outer surface temperature of the heat insulating structure is the outside air temperature (for example, 300K), and the inner surface temperature is the liquefied natural gas boiling point is close to the methane boiling point (111K). An airtight sheet such as a heat insulating sheet is interposed and divided into a low temperature heat insulating member and a high temperature heat insulating member.

The cryopump system is a well-known pump system in which a cryogenic cryosurface is installed in a vacuum vessel, and gas molecules in the vessel are condensed or adsorbed to trap and exhaust, and the low-temperature insulation member is evacuated. It has a function to maintain.
The low-temperature heat insulating member is made of a heat insulating material that is entirely formed of bubbles formed by foaming a foam. It is preferable that the air bubbles are very fine and are mostly open air bubbles communicating with each other. When the atmosphere is evacuated, the open bubbles communicate with the atmosphere and the inside is also evacuated, so heat transfer due to gas convection is eliminated and the thermal conductivity is sufficiently low. In the region of free molecular heat conduction without convection, the amount of heat transfer is proportional to the pressure and is zero in a complete vacuum.

Furthermore, when the average bubble diameter is set to the average free path level of gas of 1 μm or less, the apparent thermal conductivity is reduced to about ３ even when compared with the gas static thermal conductivity, which is effective. The mean free path of gas molecules is about 0.1 μm at atmospheric pressure, but becomes very large at an extremely low pressure, and is about 1 mm at 10 Pa.
If it is operated at 1000 Pa as a level that can be easily reached without applying a load to the cryopump, the mean free path is about 7 μm, and if it is operated at 100 Pa, it is about 70 μm.

The high-performance heat insulating material used for the high-temperature heat insulating member is made by adding an infrared shielding material to the foam material so that the foam material itself has a lower density than the density at which the thermal conductivity of the foam is lowest. It is.
Preferred examples of the infrared blocking material include needle-shaped single crystals such as carbon whiskers and zinc oxide whiskers. In place of the needle-like single crystal, an elongated hard fiber object such as carbon fiber may be used. In addition, when the high temperature heat insulating member is not required to have high strength, such as when protection by a waterproof / moisture proof sheet is sufficient, the needle-like single crystal may be replaced with iron oxide powder.
Since such a high-performance heat insulating material has a good heat insulating efficiency even though its weight per unit volume is small, the amount of use can be suppressed, and the weight of the high temperature heat insulating member can be reduced.

  Solar radiation incident on the outer surface of the high-performance heat insulation structure is mostly reflected and partially absorbed by a waterproof / moisture-proof sheet such as an aluminum sheet. The waterproof / moisture-proof sheet is heated and radiates heat rays corresponding to the temperature inside. The high-temperature heat insulating member effectively blocks radiation from the waterproof / moisture-proof sheet and keeps the intermediate graphite sheet cool. Since the graphite sheet is maintained at about 200K, long wavelength infrared rays corresponding to the temperature are radiated inward. The low temperature heat insulating member blocks infrared rays radiated from the graphite sheet to keep the container wall cool.

Hereinafter, the present invention will be described in detail based on examples with reference to the drawings.
FIG. 1 is a conceptual configuration diagram showing an embodiment of the high-performance heat insulating material of the present invention, FIG. 2 is a comparative table of infrared shielding materials used in this embodiment, and FIG. 3 is a graph showing the infrared transmittance of the high-performance heat insulating material. FIG. 4 is a graph showing a measurement example of infrared reflectance of a high performance heat insulating material, FIG. 5 is a table explaining the performance of the high performance heat insulating material, and FIG. 6 is a second embodiment of the present invention. It is sectional drawing which illustrates a high performance heat insulation structure notionally. FIG. 7 is a diagram showing the relationship between the density of the rigid urethane foam and the thermal conductivity.

The high-performance heat insulating material 10 of the present embodiment is formed by foaming a foam material to which acicular single crystals are added, and a large number of bubbles 12 are formed in the foam matrix 11 in which the acicular single crystals 13 are scattered. Has been.
Foam materials are various organic and inorganic materials such as thermoplastic polymers, thermosetting polymers, aerogels, ceramics, and glass, and suitable foaming agents are added to these foam materials. Foaming agents include carbon dioxide, nitrogen, argon, water, air, inorganic foaming agents such as helium, aliphatic hydrocarbons such as methane, propane, and butane, organic foaming agents such as halogen or hydrocarbon, azodicarbonamide, etc. There is a foaming agent by action.

The bubble 12 is a space surrounded by a thin film formed of the foam material 11 including the needle-like single crystal 13. The size of the bubbles can be controlled by adjusting a foaming agent, foaming conditions, a nucleating agent composed of atrium bicarbonate or the like.
When the bubble size is adjusted to have a diameter that is about the mean free path of the internal gas, the gas conduction heat Qg becomes, for example, 1/3 or less of the gas static conduction heat, and the thermal conductivity of the heat insulating material 10 is effectively reduced. Reduce.
In addition, the bubbles may be formed as independent spaces, or may be formed so as to be continuous with each other. Open cells can be easily evacuated by evacuating the atmosphere. The lower the gas molecule density in the bubbles, the lower the thermal conductivity. Therefore, it is preferable to make the atmosphere vacuum by using open cells.

The acicular single crystal 13 is an elongated single crystal having extremely high rigidity, and is also called a whisker. Examples of whiskers that are relatively easy to use include carbon whiskers, zinc oxide whiskers, and silicon carbide whiskers.
The heat transfer amount Q of the heat insulating material 10 is a solid conductive heat Qs conducted through a matrix formed of resin or the like, a gas conductive heat Qg conducted by a gas in a bubble, and a radiant heat transfer Qr by infrared rays transmitted through the heat insulating material. Become sum. That is,
Q = Qs + Qg + Qr
Can be expressed as

Normally, the solid conduction heat Qs having the highest contribution ratio is determined by the abundance ratio of the foam material and the foam material to the heat insulating material, and the solid conduction heat Qs decreases as the apparent density of the heat insulating material 10 decreases. Apparent thermal conductivity is also reduced. However, where the apparent density is very small, the abundance ratio of bubbles is increased and the wall of the bubbles is extremely thin, so that infrared rays are easily transmitted and the contribution of the radiant heat transfer Qr is increased, and the apparent thermal conductivity is increased. Becomes larger.
Therefore, in order to reduce the apparent thermal conductivity with the low-density heat insulating material 10, it is necessary to reduce the radiant heat transfer Qr or the gas conduction heat Qg.

The high-performance heat insulating material of this example has a needle-like single-piece material for the foam material 11 in order to reduce the radiant heat transfer Qr with respect to the heat insulating material whose apparent density is extremely small and the thermal conductivity of the foam material itself is large. A crystal 13 is added and dispersed in a matrix.
Conventionally, there has been a heat insulating material which has a large infrared absorption capacity and carbon black or the like which blocks the infrared light almost completely to reduce the radiant heat transfer Qr and reduce the thermal conductivity. Then, rather, it has succeeded in further effectively reducing the thermal conductivity of the heat insulating material by causing appropriate scattering and absorption.

  Further, if particles are mixed into the bubble wall thin film made of the foam material, it becomes brittle and the heat insulating material is easily damaged. Therefore, in this embodiment, whisker effective as a plastic reinforcing material is used instead of a simple granular material. It was rather strengthened.

Infrared transmission properties, reflection properties, and apparent thermal conductivity were measured for samples in which various infrared blocking substances were added as additives to the foam insulation, and the infrared blocking effect was confirmed.
Urethane foam was used as the heat insulating material, and the sample material was prepared by a two-liquid mixing method of P liquid (polyol) and I liquid (isocyanate liquid). The additive was added during mixing of the two liquids and foamed using Freon HFC14b as the foaming gas. The bulk density of the sample material was about 20 kg / m ³ , and the bubble rate was about 98%. The bubble rate is
Bubble ratio (%) = (1− (apparent density) / true density) × 100 (%)
Determined by

FIG. 2 is a list of infrared shielding substances to be measured. Carbon black is the same measurement for comparison. In addition, although iron oxide is a granular material, it is listed in order to confirm that the effect of reducing the thermal conductivity is large. A granular material can also be used in a site where structural strength is not required.
The carbon black to be compared is a particle having a diameter of 0.04 μm and has a strong infrared absorption function.
The iron oxide particles may have a particle size larger than that of carbon black and have a large infrared scattering effect.
The carbon whisker is a needle-like particle having a diameter of 0.3 to 0.6 μm and a length of 5 to 15 μm, and also has a function as a reinforcing material. The zinc oxide whisker is a needle-like granule having a diameter of 0.2 to 0.3 μm and a length of 2 to 50 μm, and has a reinforcing function.

The infrared transmittance and infrared reflectance of the heat insulating material sample to which each additive is added are shown in FIGS. 3 and 4, respectively. Transmittance and reflectance were measured with a spectrophotometer. The measurement wavelength range was from 300 nm to 2500 nm of visible light to near infrared light. In the figure, wavelength characteristics are shown with the horizontal axis representing wavelength and the vertical axis representing transmittance or reflectance. For comparison, the measurement results were also shown for an additive-free sample material to which no infrared blocking substance was added.
The sample had a size of 30 mm × 30 mm × 5 mm, and each additive was added in an amount of 5% by weight with respect to the foam material to facilitate comparison.

Looking at the transmittance in the near-infrared region in the range of 0.8 μm to 2.5 μm, the additive-free sample material (blank) is the largest, followed by zinc oxide whiskers, which drop considerably and the iron oxide powder and carbon whiskers are almost the same. On the other hand, the transmittance is zero in the sample added with carbon black. It is observed that the transmittance decreases on the long wavelength side and the difference due to the additive also decreases.
On the other hand, looking at the infrared reflectance in the same range, the sample with zinc oxide whisker added to the blank sample showed a high reflectance, and the iron oxide powder and the carbon whisker decreased to about half of the blank sample, and the carbon The sample added with black shows the smallest reflectance.
From these measurement results, the absorptance can be obtained using the following equation.
Absorptivity = 1-Transmittance-Reflectance

Furthermore, the apparent thermal conductivity was measured for the 30 mm × 50 mm × 10 mm sample by the same manufacturing method using the unsteady hot wire method. Note that the measurement was performed in a measurement environment at about room temperature.
Using the measurement results of transmittance, reflectance, and absorptance of these light rays, an average value was calculated in the near infrared region of 0.8 to 2.5 μm, and FIG. 5 shows these average values and the measured apparent values. The thermal conductivity and the reduction rate of the thermal conductivity are shown.

As can be seen from the figure, the heat insulating material to which iron oxide is added and the heat insulating material to which carbon whiskers are added are similar in infrared transmittance, reflectance, and absorptance, and all have high absorptance. However, the contribution to the reduction in thermal conductivity is greater for iron oxide, which is 22% lower than for heat-insulating materials that do not contain additives, whereas carbon whiskers are only reduced by 16%.
In addition, the zinc oxide whisker-added heat insulating material is similar to the additive-free heat insulating material (blank) in that the infrared transmittance and reflectance are large, but the transmittance is greatly reduced, and the thermal conductivity is lower than that of the additive-free heat insulating material. It has decreased by 19%.
In addition, it can be seen that the heat insulating material to which carbon black is added substantially blocks infrared rays, but the thermal conductivity is only reduced by 9.4%, and the contribution is small compared to other additives. This is probably because carbon black itself has a high solid thermal conductivity.

From these results, the thermal conductivity reduction rate of the heat insulating material added with iron oxide is the largest among the comparative samples, followed by the heat insulating material added with zinc oxide whisker and the heat insulating material added with carbon whisker. It can be seen that the carbon black-added heat insulating material has a thermal conductivity nearly 10% lower than that of the non-added heat insulating material, but does not reach the whisker-added heat insulating material.
In the case of carbon black-added heat insulating material, when the density is low, granular materials may be mixed into the thin film partition walls, which may lead to brittleness, but the whisker-added heat insulating material can be expected to reinforce the whisker, greatly increasing the apparent density of the heat insulating material. The strength can be maintained even if it is lowered to a low level.
On the other hand, the iron oxide-added heat insulating material has a large effect of reducing the thermal conductivity, but since the additive is granular, the brittleness increases and the heat insulating material is easily damaged. Therefore, the iron oxide-added heat insulating material can be expected to have a great effect when used in a portion where the strength is not a problem.

Next, a method for suppressing gas conduction heat will be described.
The smaller the pressure inside the bubble, the smaller the gas heat transfer, so the pressure inside the bubble is made as low as possible. In order to reduce the pressure inside the bubbles, there is a method in which foaming is performed under vacuum. In addition, there is a method in which continuous bubbles are formed and communicated with the outside, and the inside of the bubbles is evacuated by evacuating the outside atmosphere.
Moreover, if the apparent density of the high-performance heat insulating material is the same, the smaller the bubbles, the lower the thermal conductivity. This is because gas convection in the bubbles is suppressed and heat transfer is suppressed. Therefore, it is preferable to make the bubble diameter as small as possible. Note that when the bubble diameter is reduced with the same apparent density, the partition wall of the bubble becomes thin and weak, but it can be reinforced by adding whiskers.

The high-performance heat insulating material of this example is a heat insulating material formed to a low density so that the foam material is foamed and infrared rays are transmitted and the thermal conductivity is increased. It reduces permeation, reduces thermal conductivity and enhances structural strength. Furthermore, it is preferable to reduce the thermal conductivity by reducing the average diameter of the bubbles.
The high-performance heat insulating material of the present embodiment can be used for various applications such as for homes and home appliances, but can be optimally used as a heat insulating material for a low temperature storage container or as a component thereof.

FIG. 6 is a sectional view conceptually illustrating the high performance heat insulating structure of the present invention.
The high-performance heat insulation structure of this embodiment is a heat insulation structure that covers and cools the surface of a cryogenic structure such as a liquefied natural gas storage tank or a liquefied petroleum gas storage tank. The surface temperature of the liquefied natural gas storage tank may be considered as 111K.
The high-performance heat insulating structure 20 of the present embodiment includes a cryopump 22 of a cryopump system disposed on the surface of a container wall 21 of a storage tank to be kept cold, and a foam heat insulating material formed from fine open cells from above. A low-temperature heat insulating member 23, and an air-tight sheet 24 is further sealed on the low-temperature heat insulating member 23, and a high-temperature heat insulating member 25 composed of a high-performance heat insulating material is further disposed. Covered and configured.

The waterproof / moisture-proof sheet 26 using aluminum sheets has the effect of reflecting sunlight well and preventing the heat rays from directly entering the high-temperature heat insulating member 25, but the infrared rays generated from the warm waterproof / moisture-proof sheet 26 are hot. The heat insulating member 25 is irradiated.
The high-performance heat insulating material used for the high-temperature heat insulating member 25 is foamed so as to have a density lower than the density at which the thermal conductivity in the foam of the foam material itself is lowest by adding an infrared shielding substance to the foam material. Is. Needless to say, the high-performance heat insulating material according to the first embodiment can be used.
In addition, since the heat insulating structure of the present embodiment can be used even if the strength of the heat insulating material is somewhat reduced by taking sufficient measures, as the infrared shielding material, needle-like single crystals such as carbon whisker and zinc oxide whisker, carbon In addition to elongated hard fiber objects such as fibers, iron oxide powder and the like that have a great effect on reducing thermal conductivity can be used.

The cryopump system has a function of removing the gas molecules condensed on the cryosurface 22 and maintaining the low-temperature heat insulating member 23 in a vacuum. In order to maintain the vacuum, the airtight sheet 24 is precisely applied so that the airtight sheet 24 can be kept sufficiently airtight. A graphite sheet etc. are used for an airtight sheet. Since the cryopump system is powerful, the pressure in the low-temperature heat insulating member 23 can be maintained at 100 to 1000 Pa relatively easily.
The low-temperature heat insulating member 23 is made of a foam heat insulating material and is filled between the container wall 21 and the airtight sheet 24. The bubbles formed inside are very fine open cells. If the atmosphere is evacuated, the inside of the open cells is also evacuated, and the thermal conductivity is sufficiently low. In addition, the lower the absolute pressure, the smaller the gas heat transfer and the lower the thermal conductivity of the heat insulating material.

Further, when the average diameter of the bubbles is set to the average free path level of the bubble gas, the collision between the gases is reduced, so that the apparent thermal conductivity is drastically reduced and is reduced to about 1/3 compared with the gas static thermal conductivity. . Therefore, it is preferable to set the bubble diameter to be equal to or less than the mean free path length of the gas.
Note that the mean free path L (m) of gas molecules can be determined by the following equation using the temperature T (K), the pressure P (Pa), and the molecular mean diameter σ (m).
L = 3.11 × 10 ⁻²⁴ T / (Pσ ² )

The mean free path of gas molecules is about 0.1 μm at room temperature and atmospheric pressure, but is inversely proportional to the absolute pressure, so it becomes large in the low pressure region and becomes about 1 mm at 10 Pa. Therefore, it is possible to select a combination of bubble internal pressure and bubble diameter that can be realized.
The airtight sheet 24 is installed in the vicinity of 200K, which is intermediate between the surface temperature of the heat insulating structure (300K) and the surface temperature of the liquefied natural gas storage tank (111K). Therefore, since the low-temperature heat insulating member 23 is used in a vacuum under a temperature condition of 150 K on average, the combination of the diameter of the formed bubbles and the absolute pressure / absolute temperature at the time of use is appropriately selected to determine the average diameter. And the mean free path relationship can be matched.

In this embodiment, since the low temperature heat insulating member 23 is used at around 150 K and 1000 Pa, the mean free path of gas molecules in the air is estimated to be approximately 7 μm. Furthermore, when operating at 100 Pa, the mean free path is about 70 μm.
Note that the foam heat insulating material of the low-temperature heat insulating member 23 is not directly irradiated with external infrared rays, and it is only necessary to consider re-radiation of the airtight sheet 24 at about 200K. For use under vacuum, it is preferable to add whiskers or fibrous reinforcing materials for the purpose of reinforcing the matrix.

The high-performance heat insulating material used for the high-temperature heat insulating member 25 is formed at a very low density, so that the weight per unit volume is small, but since an infrared shielding material is added, the heat insulating efficiency is good, so that a thinner material is used than before. be able to. Since the improvement rate of thermal conductivity is about 20%, the amount of use can also be saved by 20%.
Moreover, since the heat conductivity of the heat insulating material used for the low temperature heat insulating member 23 is about 1/3 as compared with the conventional mechanism, 2/3 can be saved as a material.

It is a notional block diagram showing one Example of the high performance heat insulating material of this invention. It is a comparison table | surface of the infrared shielding material used for a present Example. It is a graph which shows the example of a measurement of the infrared rays transmittance | permeability regarding the high performance heat insulating material of a present Example. It is a graph which shows the example of a measurement of the infrared reflectance regarding the high performance heat insulating material of a present Example. It is a table | surface explaining the performance of the high performance heat insulating material of a present Example. It is sectional drawing which illustrates one Example of the high performance heat insulation structure which concerns on this invention notionally. It is a figure which shows the relationship between the density of rigid urethane foam, and thermal conductivity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 High-performance heat insulating material 11 Foam matrix 12 Bubble 13 Needle-like single crystal 20 High-performance heat insulating structure 21 Container wall 22 Cryopump system 23 Low temperature heat insulating member 24 Airtight sheet 25 High temperature heat insulating member 26 Waterproof / moisture-proof sheet

Claims (8)

A high-performance heat insulating material characterized by adding a needle-like single crystal to a foam material and foaming the foam material so that the density of the foam material itself is lower than the density at which the thermal conductivity in the foam is minimized. The needle-like single crystal is a carbon whisker having a diameter of 0.3 to 0.6 μm and a length of 5 to 15 μm, or a zinc oxide whisker having a diameter of 0.2 to 0.3 μm and a length of 2 to 50 μm. The high performance heat insulating material according to claim 1. The high-performance heat insulating material according to claim 1 or 2, wherein the acicular single crystal is added in an amount of 1 to 5% by weight with respect to the foam material. The foam material is a rigid polyurethane foam, high-performance thermal insulation material according to any one of claims 1 to 3, characterized in that the apparent density by bubbling was set to 10 to 30 kg / m ^3. The cryosurface of the cryopump system is placed on the wall surface of the container to be kept cold, and a low temperature heat insulating member made of a foam heat insulating material in which fine open cells are formed is coated on the wall surface, and the low temperature heat insulating member is covered on the low temperature heat insulating member. High performance that covers an air-tight sheet and foams the foam material by adding an infrared shielding material from above the air-tight sheet so that the density of the foam material itself is lower than the density at which the thermal conductivity of the foam is lowest. A high-performance heat insulating structure characterized in that a high-temperature heat insulating member made of a heat insulating material is coated, and a waterproof / moisture-proof sheet is coated on the surface of the high-temperature heat insulating member. The infrared blocking material is an iron oxide granule, a carbon whisker having a diameter of 0.3 to 0.6 μm and a length of 5 to 15 μm, or a zinc oxide whisker having a diameter of 0.2 to 0.3 μm and a length of 2 to 50 μm. The high-performance heat insulating structure according to claim 1. The high-performance heat insulating material according to claim 5 or 6, wherein the foam material of the high-temperature heat insulating member is a hard urethane foam, and an apparent density is 10 to ³⁰ kg / m ³ by foaming. The high-performance heat insulating structure according to any one of claims 5 to 7, wherein the open-cells of the low-temperature heat insulating member have an average diameter of 1 to 70 µm.

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