JPH07144955A - Silica compact for heat insulating board and vacuum heat insulating board - Google Patents

Abstract

PURPOSE:To obtain a vacuum heat insulating board with high shape retentivity by increasing the amt. of fibrous polytetrafluoroethylene to a specified value in a compact contg. fine powdery silica as well as the polytetrafluoroethylene. CONSTITUTION:Polytetrafluoroethylene is added as a binder to fine powdery silica by 0.5-10 pts.wt. per 100 pts.wt. of the silica, they are vigorously mixed under high shear to disperse the polytetrafluoroethylene in a fibrous state in the fine powdery silica and the resulting mixture is compacted to obtain the objective compact. Since the amt. of fibrous polytetrafluoroethylene added is larger than the conventional amt., shape retentivity can be improved while maintaining low density. The fine powdery silica is preferably silica produced by a wet method and having 100-700m<2>/g BET specific surface area and 0.5-10mum average particle diameter. The pref. bulk density of the compact is 0.15-0.25g/ml.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molded product used for a vacuum heat insulating plate for an electric refrigerator, and more particularly to a silica molded product having improved shape retention. Furthermore, the present invention relates to a method for manufacturing the above-mentioned molded body and a vacuum heat insulating plate using the above-mentioned molded body.

[0002]

2. Description of the Related Art It is known to use a molded product for a heat insulating plate having a silica powder as a base material in a heat insulating plate for an industrial high temperature wall or a household electric refrigerator. These molded products are molded after filling an air-permeable container such as paper or non-woven fabric with an inorganic powder such as pearlite powder or silica powder, and the resulting molded product is used as it is as a heat insulating plate. Alternatively, after this molded product is dried, it is filled in a synthetic resin container or the like laminated with metallic aluminum foil so that it is substantially impermeable to air, and then the air in the molded product is exhausted by means such as vacuum exhaust. After that, the container may be used as a vacuum-molded heat insulating plate by closing the opening of the container with a means such as welding.

In particular, in a vacuum-formed heat insulating plate used for an electric refrigerator or the like, recently, in order to prevent environmental damage due to CFC gas, a part of urethane foam used as a heat insulating plate for a refrigerator or the like is a non-polluting inorganic type. There is a growing movement to replace them with heat insulating plates. Also, from the viewpoint of energy saving, the thermal conductivity of urethane foam using the current specific CFC is about 0.014 kcal / m · hr · ° C, whereas the vacuum heat insulating plate using silica has thermal conductivity. Since the rate can be reduced to about 1/2 or less of that of urethane foam, it has attracted great attention.

As described above, the silica molded body is dried, inserted into a container or the like, evacuated, and then the exhaust port of the container or the like is welded to obtain a vacuum heat insulating plate. However, the silica molded product obtained by compression molding only the silica powder is not sufficiently strong, and cracks, chips, or breakage may occur during commercialization. If a molded product lacking in shape retention is used, the final vacuum heat insulating plate will have poor flatness and poor attachment to the heat insulating portion. as a result,
If the performance of the silica vacuum heat insulating plate with low thermal conductivity is not sufficiently exhibited, and if the shape retention after molding is extremely poor, it may not be usable as a heat insulating plate. Furthermore, there is a problem that it is difficult to automate the manufacturing process because the shape retention of the silica molded body is not good. Therefore, it is desired to provide a durable silica molded body that does not lose its shape in the process of obtaining a vacuum heat insulating plate.

[0005]

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a silica molded article having improved shape retention and a method for producing the same. A further object of the present invention is to provide a vacuum heat insulating plate using the above-mentioned silica molded body having improved shape retention. That is, the present invention eliminates the shape collapse in the manufacturing process of the silica vacuum forming heat insulating plate, facilitates automation of the manufacturing process, and makes it possible to sufficiently exhibit the performance, a silica molded body for a vacuum heat insulating plate, It is intended to provide a manufacturing method and a vacuum heat insulating plate.

[0006]

The present inventors have proposed a method for improving the shape retention of a silica compact while maintaining a low thermal conductivity when using the silica fine powder as a silica compact for a vacuum heat insulating plate. Earnestly researched about. Originally, in order to realize the low thermal conductivity which is the function as the silica vacuum heat insulating plate, it is important to reduce the three factors of the thermal conductivity due to the residual gas, the radiant thermal conductivity and the thermal conductivity due to the solid. . In the case of a silica vacuum insulation plate, the thermal conductivity due to residual gas can be substantially ignored by keeping the vacuum degree at about 0.5 Torr or less. Also, when using fine silica powder, the radiant heat conductivity has a small contribution to the heat conductivity because the voids between particles are kept as small as approximately submicron. Therefore, in order to reduce the thermal conductivity of the silica vacuum heat insulating plate, it is sufficient to reduce the thermal conductivity of the solid.

That is, it is necessary to reduce the solid thermal conductivity of the silica compact used for the silica vacuum heat insulating plate. This is to keep the density of the silica compact low and to make the contact between the silica particles be point contact. It is important to. However, the point contact between the particles while keeping the density of the silica molded product low and the increase in the strength of the silica molded product to improve the shape retention are contradictory factors. For example, if the compression pressure is simply increased to perform the molding, the strength of the silica molded product can be improved and the shape retention is improved, but it is difficult to maintain the low thermal conductivity. Attempts have been made to improve the shape-retaining property by using various binders, but although the cause is not clear, the effect of improving the shape-retaining property by the binder is low. As a result, the amount of the binder is increased, and the density of the molded body must be increased. Therefore, the inventors conducted repeated studies on how to contradict this, that is, how to improve the shape retention while keeping the density of the silica molded product low.

As a result, surprisingly, by using fibrous polytetrafluoroethylene as a binder and adding the amount of fibrous polytetrafluoroethylene more than the conventionally known amount, the density was kept low. The inventors have found that the shape-retaining property can be improved, and have completed the present invention. That is, the present invention relates to a molded article having good shape retention, to which 0.5 to 10 parts by weight, preferably 1 to 5 parts by weight of fibrous polytetrafluoroethylene is added to 100 parts by weight of fine powder silica. .

Polytetrafluoroethylene is a dust-proof additive Teflon K (trademark of Mitsui DuPont Fluorochemicals).
Is marketed as. It has a unique property of becoming fibrous when a shearing force is applied. Usually, an addition amount of 0.01 to 0.1% is recommended for dustproof treatment of powder. However, for the purpose of the present invention to improve the shape-retaining property of the silica molded product, a good result is not obtained with an addition amount of 0.1% or less, and as described above, it is based on 100 parts by weight of fine powder silica. It is suitable to add 0.5 parts by weight or more. If it is less than 0.5 part by weight, the shape retention of the molded product is insufficient. Further, even if a certain amount or more is added, the effect reaches the ceiling, and practically, an addition amount of 10 parts by weight or less to 100 parts by weight of fine powder silica is sufficient.

The fine powder silica to be molded is as follows:
Examples thereof include dry process silica obtained by hydrolyzing silane such as silicon tetrachloride at a high temperature, and wet process silica obtained by reacting an aqueous sodium silicate solution with a mineral acid. However, wet process silica is preferable in order to obtain lower thermal conductivity. The physical properties of silica are 100 BET specific surface area.
~ 700 m ² / g, more preferably 150-400 m ² / g
And the average particle size is 0.5 to 10 μm, and more preferably 0.5 to 5 μm. When the BET specific surface area is within the above range, the bulk density of silica is small, and low thermal conductivity can be maintained while maintaining moldability, which is appropriate. In general, a smaller average particle size can maintain a lower bulk density, but within the above range, there is no great difference.

Further, the bulk density of the silica molding is 0.1.
The range of 5 to 0.25 g / ml is preferable from the viewpoint of obtaining a heat insulator having a low thermal conductivity. The bulk density in the above range is obtained by using the silica fine powder having the BET specific surface area and the average particle diameter described above. More preferably, the bulk density is 0.15 to 0.2 g / ml. However, even if the bulk density is made too small, molding may become impossible, or the vacuum heat insulating plate may not withstand external pressure and may collapse. for that reason,
It is suitable that the bulk density is 0.15 g / ml or more.

In the molded article of the present invention, finely divided silica and polytetrafluoroethylene are mixed in a fibrous state of polytetrafluoroethylene to disperse the fibrous polytetrafluoroethylene in the finely divided silica. Then, it is obtained by molding. The mixing of polytetrafluoroethylene in a fibrous state can be obtained, for example, by strongly mixing fine powder silica and polytetrafluoroethylene under high shear. Finely powdered silica and polytetrafluoroethylene are strongly mixed under high shear,
Polytetrafluoroethylene becomes fibrous and can be satisfactorily dispersed in finely divided silica. Polytetrafluoroethylene is usually a powder or an aqueous suspension, and becomes a fiber when a shearing force is applied. The polytetrafluoroethylene cannot be made into a fibrous state simply by mixing the both. In such a state, both can be obtained by dispersing using a high speed fluid mixer such as a Henschel mixer, loosening by hand, or dispersing using a ball mill. When using a Henschel mixer, for example, the peripheral speed is mixed at about 20 m / sec. If the state of dispersion is good, polytetrafluoroethylene becomes moxa-like, and dust generation of silica powder is not seen,
Fine particles of polytetrafluoroethylene and finely divided silica are well dispersed, the entanglement between silica particles and fibers becomes strong, and by molding this mixture, it is possible to obtain a molded article having high shape retention. .

The obtained mixture of finely powdered silica and fibrous polytetrafluoroethylene can be formed into a molded product as it is, or by filling it into a container or bag having air permeability and then molding by press molding or the like. The press molding can be performed at a pressure in the range of 0.03 to 0.3 kg / cm ² G, for example. The obtained silica molded product can be used as it is as a heat insulating plate without vacuum evacuation in applications other than electric refrigerators. Further, the obtained silica molded body is further inserted into a container having substantially no air permeability and filled, and then the inside of the container is evacuated, the container is sealed, and a silica vacuum heat insulating plate suitable for an electric refrigerator is provided. Obtainable. Here, the substantially air-impermeable container may be a synthetic resin bag, a metal bag, a metal-deposited synthetic resin bag, a metal foil laminated synthetic resin bag, or the like. it can. Also, the vacuum inside the container after evacuation is suitably in the range of about 0.01 to 0.5 torr.

[0014]

EFFECTS OF THE INVENTION Since the silica molded article for heat insulating plate of the present invention is excellent in shape retention, when it is used as a silica vacuum heat insulating plate, there is no fear of chipping, cracking or breaking, and troubles in the manufacturing process. Can be eliminated. Furthermore, it is possible to eliminate the generation of silica dust in the manufacturing process. Further, since the generation of dust is reduced, there is also an advantage that when the electric refrigerator or the like using the molded product of the present invention becomes unusable and is discarded or dismantled, there is no fear of scattering silica powder.

[0015]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples. The measurement of each physical property value was performed by the following methods. 1) BET specific surface area Canter soap (manufactured by Quantachrome, USA)
Was measured by the one-point method. 2) Average particle size of secondary particles Coulter counter-TA-II (Coulter El)
(manufactured by ectonics), 30 μm and 100 μm
It was measured with an aperture tube of m. 3) Thermal conductivity It was measured using Auto Λ HC-072 type (manufactured by Eiko Seiki Co., Ltd.). 4) Shape-retaining property Judgment was made visually and by touch, and those having good shape-retaining property were indicated by ◯, and those having poor shape-retaining property were indicated by x.

Example 1 To 1 kg of precipitated silica having a BET specific surface area of 200 m ² / g and an average particle size of 2 μm, 30 g of polytetrafluoroethylene powder was added and well kneaded in a high-speed fluid mixer to form fibrous polytetrafluoro. A mixture was obtained in which ethylene was dispersed in silica. A portion of the obtained mixed powder is hermetically filled in a kraft paper bag, and then press-molded using a mold (pressure: 3
0 kg / cm ² G) and the length x width x thickness is 200 each
A silica molded body having a size of mm × 200 mm × 12 mm was prepared. At this time, the size of the mold was set so that the bulk density of the silica molded body was 0.2 g / ml. After observing the shape-retaining property of the obtained silica molded product, it was dried at 120 ° C. for 12 hours, inserted into a bag made of synthetic resin having substantially no air permeability, and filled, and then 0.4 Torr in a vacuum chamber. It was sealed by heat sealing under vacuum to obtain a silica vacuum heat insulating plate. The shape retention of the obtained silica vacuum heat insulating plate was observed again and the thermal conductivity was measured. The results are shown in Table 1.

Examples 2 to 6 and Comparative Example 1 A silica vacuum heat insulating plate was prepared and evaluated by the same method as in Example 1 while changing the type of silica, the physical properties and the amount of polytetrafluoroethylene added. The degree of vacuum was 0.4 Torr in each example. The results are shown in Table 1.

[0018]

[Table 1]

Claims (6)

[Claims] 1. To 100 parts by weight of finely divided silica, 0.
A silica molded product for a heat insulating plate, which contains 5 to 10 parts by weight of fibrous polytetrafluoroethylene. 2. Finely divided silica is 100 to 700 m ² / g.
The silica molded body according to claim 1, which is a wet process silica having a BET specific surface area of 5 and an average particle diameter of 0.5 to 10 μm. 3. The silica molded product according to claim 1, which has a bulk density of 0.15 to 0.25 g / ml. 4. Finely powdered silica and polytetrafluoroethylene are mixed in a fibrous state of polytetrafluoroethylene to disperse the fibrous polytetrafluoroethylene in the finely powdered silica and then molded. The method for producing a silica molded body according to claim 1, which is a molded body. 5. The method according to claim 4, wherein the fine powder silica and polytetrafluoroethylene are vigorously mixed under high shear to disperse the fibrous polytetrafluoroethylene in the fine powder silica. 6. A vacuum heat insulating plate, wherein the silica molded product according to claim 1 is filled in a vacuum container having substantially no air permeability.