US3166511A

US3166511A - Thermal insulation

Info

Publication number: US3166511A
Application number: US73028A
Authority: US
Inventors: Ladislas C Matsch; Arthur W Francis
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1960-12-01
Filing date: 1960-12-01
Publication date: 1965-01-19
Anticipated expiration: 1982-01-19

Description

1965 c. MATSCH ETAL THERMAL INSULATION Filed Dec. 1. 1960 2 Sheets-Sheet 1 INVENTORS LADISLAS C. MATSCH ARTHUR W. FRANCS BY WWW;

ATTORNEY 1965' c. MATSCH ETAL 3,166,511

THERMAL. INSULATION Filed Dec. 1, 1960 2 Sheet s-Sheet 2 THERMAL PERFORMANCE OF MlCRO-CEL T-4 WITH VARIOUS METAL REFLECTING MATERIALS FOR BOUNDARY TEMPERATURES 0F 290K AND 90 K AND AT A PRESSURE or LESS THAN 01 MICRON HG.

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Ka- (BTU/HR. FT F) XIO'3 o 5 I5 20 25 3o '35 10 PERCENT METAL REFLECTING MATER|AL(BY WEIGHT) INVHVTORS LADISLAS C MATSCH ARTHUR W. FRANCIS By mm fw AT TOPNE Y United States Patent Ofitice 3,166,511 THERMAL INSULATION Ladislas C. Matsch, Kenmore, and Arthur W. Francis,

New Qity, N.l[., assignors to Union Carbidetcorporation, a corporation of New York Filed Dec. l, 1969, Ser. No. 73,28 (Ilairns. (Cl. 25262) This invention relates to an improved insulating material having a high resistance to all modes of heat transfer, and particularly concerns a low temperature insulating material for use in vacuum jackets of containers.

In the conservation and conveying of low-temperature commercial products, for example, perishable commodities which must be held at low temperatures for substantial periods of time,'and of volatile materials such as liquefied gases having boiling points at atmospheric pressure below 233 K., for example, liquid oxygen or nitro-' gen, a major problem encountered is the control of heat leak to the material which, in the caseof liquefied gases, results in' loss due to evaporation. In the conventional double-Walled liquid oxygen containers, the space between the walls is suitably insulated to limit this evaporation loss. Up to now, straight vacuum-polished metal or powder-in-vacuum insulation of the type disclosed in US. Patent No. 2,396,459, has been used to insulate the space between the walls. However, a general disadvantage of straight vacuum-polished metal insulation is the necessity of maintaining an extremely. high vacuum. Powder-in-vacuum heat insulation isless sensitive to the presence of small traces of air in the insulation space,

however, significant quantities of heat from the atmosphere are, nevertheless, transmitted from the external shell of the container to the inner vessel.

There is a great commercial need for efficient insulating materials capable of meeting more rigid and exacting requirements, and which will provide even lower thermal transmission than those afforded by either of the'above described insulating systems. Provisions of such materials would permit study and development of important new and improved control techniques for many processes and products.

The present invention is based on the discovery of important behavior characteristics of finely divided insulating powders used as heat insulation. The geometry of the insulating material has the greatest effect on solid heat conduction, the rate of heat transfer by conduction varying directly with the cross sectional area and inversely with the length of the heat path. The contacts between powder particles are usually of relatively small cross sectional area. Consequently, in powdc-rs, the area available for conductive heat transfer is an infinitesimal fraction of the insulated area. It would seem, therefore, that by reducing the powder size, the resistance to the flow of heat by conduction and the permissible temperature gradient would be correspondingly increased. However, our investigations have shown that as the particle size is reduced, the faculty of the particles to reflect the radiant rays of heat undergoes changes. Powders with extremely small particle sizes are more transparent to infra-red radiation, and such radiant heat transparency increases as the ratio of the particle size to the wave length of radiation decreases. As a consequence, radiation can become the predominant mode of heat transmission through very fine powders even at low temperature.

In order to achieve acceptable combinations of the various modes of heat transfer in an insulating material, means must be provided for reducing the infra-red radiation transparency effects accompanying small particle sizes without incurring an appreciable increase in heat transfer by solid conduction.

- aisasii Patented Jan. 1%, 1955 It is, therefore, an object of the present invention to provide in an insulation system, improved means for reducing the undesirable radiativeedects in low conductive powders of relatively small particle sizes.

Another object of the present invention is to provide in a vacuum-solid insulation system, an improved and efficient insulating material having a relatively high resistance to the passage of heat by conduction and by radiation. t

Still another object of the present invention is to pro vide in a vacuum-powder insulation system, additive material for minimizing the passage of heat by radiation without increasing the passage of heat by conduction to any significant degree. 7

Yet another object of the present invention is to provide a novel improvement in insulation material for use in an insulation system Where radiation would otherwise be the predominant mode of heat transfer.

Other objects, features and advantages of the present invention will be apparent from the following detailed description.

In the drawings:

FIG. lis a view of a double-walled container for liquelied gas embodying the principles of the invention;

FIG. 2'is a view of an enlarged section taken along line Z-2 in FIG. 1, showing the insulating material of the-present invention; and

FIG. 3 is a graph showing the thermal conductivity of the present insulation as a function of the quantity of opacifier.

In copending application Serial No. 580,897, filed April 26, 1956, in the names of L. C. Matsch et al., and issued as U.S. Patent No. 2,967,152, it was disclosed that the undesirable radiative effects accompanying the use of certain finely divided low heat conductive powders as insulating materials may be substantially reduced and miniferred because of its low thermal conductivity, relative inexpensiveness, and general availability. It was-found that finely divided metal flakes, e.g., aluminum and cop.- per, were admirably suited as the reflecting bodies,-and that such flakes should be present in quantities so as to constitute between about 1% and by weight of the novel insulating material. Furthermore, it was found that the optimum mixtures comprised between about 40% and 60% by weight of the reflective component. This represented the best balance between increased quantities of aluminum or copper flakes which reduce radiation heat transfer but increase heat transfer by conduction.

It has now been discovered that high insulating'quality may be obtained by employing finely divided particles of calcium silicate as the low conductive component, in combination with the finely divided radiant heat reflecting bodies. More specifically, the present invention relates to an insulating material characterizedby a low rate of heat transfer by conduction and radiation. This insulating materialconsists essentially of finely divided low heat conductive particles of calcium silicate having agglomerate sizes les than about 420 microns, and finely divided radiant heat reflecting bodies of sizes less than about 500 microns and having metallic surfaces, such bodies constituting between about 1% and 30% by weight of said insulating material.

For purposes of the present invention as disclosed and claimed herein, it is desirable to define clearly the difference between ultimate and agglomerate particle sizes. Ultimate sized particles are the smallest which exist as discrete, solid masses in a particular powder. The bodies which compose ultimate sized particles are molecules, and the forces which bind the ultimate particle together are inter molecular in nature. Agglomerate particles consist of a number of ultimate particles which adhere or coalesce together, for example by electrostatic forces. For example, an ultimate particle of only 0.02 micron is only 200 Angstrom units in dimension and is composed of only a very few molecules. It is generally accepted that it is impossible to reduce such particles by grinding or by other mechanical means. Agglomerate particles, for example, 10 microns in size, would obviously be composed of a very large number of 0.02 micron size ultimate particles.

The term settled density as used herein may be defined as the essentially constant density reached after continued vibration of the powder under -7 times the normal gravitational force.

The term vacuum as used hereinafter is intended to apply to ubatrnospheric pressure conditions not substantially greater than 5,000 microns of mercury, and preferably below 500 microns of mercury absolute.

A practical illustration of an apparatus embodying the invention shown in FIG. 1 may comprise a double-walled insulating vessel having spaced parallel walls defining an evaculable insulating space 11 therebetween for the reception of a solid, powder-type insulation mixture 12 embodying the principles of the invention. The insulation mixture 12 may comprise a finely divided agglomerate of low heat conductive calcium silicate material 13 in which particles or bodies 14 of radiant heat reflecting bodies are intermingled.

The low heat conductive calcium silicate powder 13 used in the insulation of the invention should be a material which may be produced in fine particle sizes, or can be readily reduced to a fine powder. It should be strong and rigid enough to fill the insulation space, and not pack down excessively during normal service. The calcium silicate powder should have agglomerate sizes of less than about 420 microns and preferably less than about 75 microns agglomerate size. Also, the ultimate particle size of suitable calcium silicate should be less than about 0.1 micron. The settled density of the low conductive calcium silicate powder is preferably between about 3 and 10 lbs. per cu. ft.

A preferred calcium silicate powder for practicing this invention is a synthetic material prepared by a hydrothermal reaction of diatomaceous earth with a source of calcium. This material is manufactured by the Johns- Manville Products Company and sold under the trade name Micro-Gel. In particular Micro-Col T-4 has been found especially suitable, and has the following physical properties:

Specific gravity 2.46 Refractive index 1.55 Moisture, free, percent by weight 4.0

a The chemical analysis of the preferred calcium silicate material is as follows:

TABLE II Typical chemical analysis of Micro-Ccl T 4 Percent by weight Ignition loss, 1800 F. 18.0 CaO 25.3 SiO 51.7 A1 0 1.8 Fe O 0.9 Nap-K 0 0.5

The radiant heat reflecting material 14 of the present invention may be either a metal or a metal coated material such as copper coated mica, which when mixed with the low conductive calcium silicate powder, will provide a discontinuous series of multiple radiation barriers for decreasing and minimizing the passage of heat by radiation through the insulation. The shape of the barrier particles should provide a large surface area per unit volume, thin flakes of relatively fine particle size being preferred. For example, the radiant heat barrier material may comprise aluminum or copper in powder or flake form, the latter being preferred. In addition to the materials already specified as radiant heat reflecting bodies in the insulation mixture of the invention, other reflectors such as copper paint pigments, aluminum paint pigments, and Copper coated mica flakes, either alone or in combination with each other, have been found to satisfactorily reduce the transmission of infra-red radiation, and to complement the desirably low conductivity characteristics of the powder insulation. The radiant heat reflecting bodies should be less than about 500 microns in size, and more suitably below 250 microns in size. Best results are obtained with particle sizes of less than 50 microns and flake thicknesses of less than 0.5 micron.

It is an essential feature of this invention that the calcium silicate insulating powder 13 and the radiant heat reflecting material 14 to be used in the insulation system be thoroughly mixed prior to their introduction into the insulating space 11. Only in this fashion is it possible to maintain a random dispersion of radiant heat reflecting material throughout the insulation powder, and realize maximum reduction in radiative heat transmission.

While we do not wish to be bound by any particular theory, we believe the reason for the far superior insulating results of the present invention resides primarily in the random physical dispersion of radiant heat reflecting particles in the insulation mixture. This permits the use of appreciable weight percentages of radiant heat reflecting particles to bring about a marked reduction in radiative heat transmission with only a very slight increase in heat transfer by conduction.

The striking superiority of the insulation mixture of the present invention is believed to be partially attributed to the employment of small particle sizes of low heat conductive calcium silicate insulating powder 13 and radiation reflecting flakes 14. This results in an insulation mixture in which the radiative flakes 14 are not in close surface contact with the calcium silicate insulating powder particles 13, but rather are in contact over a reduced surface area somewhat approaching point contact. The effect of this relationship between radiative flakes 14 and low conductive calcium silicate particles 13 is to prevent close contact between, and to separate insulating particles from each other, and to reduce the tendency for conductive heat to flow between particles by direct contact over a large contact area. This also restricts the passage of conductive heat flow across the insulation space to heat leak paths containing an indefinitely large number of exceedingly small contact areas, which offer considerable resistance to the flow of heat therethrough. As a consequence, heat entering the insulation space 11 may be aieasii further minimized by any combination of radiation refiection by the radiant heat barrier flakes, the relatively high contact resistances between like and unlike particles, as well as the relatively low conductivity of the calcium silicate insulating powder.

Thus the mechanisrn'of heat transfer which occurs in a typical combination of a particle of finely divided calcium silicate and a particle of aluminum flake might be as follows: i

Heat will reach the particles by the modes of radiation and conduction. Of the radiant heat, part will be reflected, part will be absorbed by the aluminum flake, and the remainder will be transmitted through and around the particle of finely divided calcium silicate. Through the mode of solid conduction, heat will pass from particle to particle and from flake to particle across the relatively small area of point contact. Thereafter such heat will travel by solid conduction across the low conductive particle of finely divided calcium silicate. By analogy it will be seen that an ordinary insulation layer having heat reflecting particles and an indefinitely large number of contact resistances between like and unlike particles is particularly eflicient in preventing heat losses by radiation as well as by conductiom Since the radiant heat reflecting particles used in the insulation mixture of the invention have excellent heat conductive characteristics, it would seem that an increase in the reflective material contained in the insulation mixture would impair the conductive insulation etliciency. Contrary to this concept, we have found that substantial increases in the amount of radiant heat reflective material continue to be beneficial to the overall insulating properties of the insulation mixture. In the present invention, increasing the amount of aluminum in a mixture of alumi mum and calcium silicate particles reduces the radiative heat transfer, and only slightly increases the heat transfer by conduction. Optimum thermal resistance is obtained when the sum of these two modes of heat transfer is at a minimum.

A principal advantage residing in the use of the insulating mixture of the present invention is that it is possible to employ a decreased insulation thickness without sacrificing the benefits of small exchange of heat by radiation or conduction. The greater efiiciency obtained from the use of a smaller insulation thickness arises from the multiple layers of reflective particles available in the particle arrangement of the present invention. In this respect it is to be noted that the low value of heat transfer rate is only attained when the metal or metal coated flakes are sufficiently separated by the particles of finely divided calcium silicate. If the metal flakes contact each other frequently enough, they will form a solid conductive path.

Tests indicated that about 57% of the heat transfer through Micro-Col T-4 is by radiation so that the addition of a finely divided reflective material should reduce its transparency and improve its overall thermal conductivity considerably. A series of tests were conducted using aluminum and copper flakes similar to those disclosed in previously discussed Serial No. 580,897 in order to de termine the optimum compositions and performance. The thermal conductivity testing was done with a cooldown type of tester which allows the apparent conductivity to be determined between a cold side temperature of 90 K. (liquid oxygen) and a warm side temperature of from 50 C. to +50 C. In these tests an insulation space of a double walled container was maintained at an absolute air pressure of less than 0.1 micron of mercury (0.0001 mm. Hg). The copper flakes were between about 7 and microns particle size, less than 0.5 micron thick and 95% passed through 325 mesh screen. The aluminum flakes were of two types, one type between about 3 and 60 microns size and 90% passing through 325 mesh screen. The other type of aluminum flake was between about 5 and 25 microns particle size, and 98% passed through 325 mesh screen. Both types of tested aluminum flakes were less than 0.5 micron thick.

The results of thesetests are illustrated graphically in FIG. 3. It will be apparent fronran inspection of this graph that mixtures of finely divided calcium silicate particles and finely divided radiant heat reflect-ing metal flakes provide substantially lower thermal conductivity values than unopacified particles when the metal flakes are present in quantities between about 1% and 30% by weight of the mixture. In the case of aluminum flakes, the thermal conductivity reaches a minimum at about 10% aluminum by weight, and the preferred range according to the invention is between about 6% and 20% by weight aluminum.

When copper flakes are employed as the radiant heat reflecting component, minimum thermal conductivity is reached at a value of about 18% copper by weight of the mixture, and the preferred range is between about 10% and 24% copper by weight. With larger amounts of the reflecting component the heat transfer by solid conduction increases rather rapidly, and with smaller percentages of this component, the heat transfer by radiation becomes appreciably greater.

Table III below compares the thermal. conductivity of the 10% aluminum finely divided calcium silicate mixture with perlite and a 50% finely divided copper flake- 50% finely divided silica mixture. The latter is in accordance with the inventive concept of the previously discussed copending application Serial No. 580,897.

it will be apparent from a study of Table III that the most attractive use of the presentinvention lies in its replacement of single component, low conductive powderous insulations such as perlite. If the lowest possible thermal conductivity value is needed, other low conductive components such as finely divided silica are more attractive when combined with radiant heat reflecting materials.

Since both finely divided calcium silicate and silica have ultimate particle sizes on the order of 0.02 micron, one may wonder why the two powders differ so widely in transparency (radiation=57% and greater than respectively) and in their optimum opacified conductivities (036x l0- and 022x10 B.t.u./hr. ft. F./ft. respectively). An important reason is believed to reside in the relative densities of the two powders, which are 9.3 and 6.0 lbs/ft. for calcium silicate and silica, respectively. The low mass of the latter coupled with its low intrinsic conductivity mean that the solid conductance is extremely small; radiation probably contributes far more than 80% of the total. Therefore, the addition of a suitable reflecting material to silica achieves a more marked improvement, and the low conductive powder can tolerate greater amounts of opacifier beforereaching the point at which a further reduction in radiation is equaled or exceeded by an increase in solid conduction due to adding more metal. However, it should be appreciated that the cost of the two component insulation rises with greater percentages of radiant reflecting material, so that the insulation of the present invention is preferable for some systems.

To further appreciate the advantages of the present invention, it should be noted that in U.S. Patent No. 2,396,459 is was pointed out that for containers up to two feet in diameter, vacuum-polished metal surfaces are more emcient and preferable to powder-vacuum insulation. For

larger sizes, the powder-vacuum insulation is advantageous. As a result of the significantly low rate of heat transfer possessed by the insulation mixture of the present invention, insulation layers of extraordinarily reduced thicknesses may now be advantageously employed, thus reducing the overall dimensions of low temperature storage containers for the entire size range of containers, including those under two feet in diameter.

The possibilities in the resultant reduced insulation thickness with insulation mixtures of the present invention indicate the scope and importance of this product. For instance, if it is desired to insulate a liter spherical container (diameter 10.5 inches), so that its holding time for liquid nitrogen is approximately four weeks (evaporation loss of 3.57% per day), it is to be expected that the necessary insulation thickness using the best known prior art insulating material will be several times the diameter, and many times the volume of the uninsulated container. Theoretical calculations shows the required thickness of prior art vacuum-powder insulation having silica powder as the low heat conductive powder to be at least three feet. Expressing this figure in terms of volume, the resultant quantity of insulating material would be almost 500 times the volume of the uninsulated spherical container. In contrast, an insulation mixture containing 16% copper (7-25 micron flakes) and the remainder finely divided calcium silicate, permits the use of a singularly unusual insulating thickness of about 2.8 inches.

From the foregoing it will be seen that the thermal heat transfer rate of powder-vacuum insulating material may be materially decreased by uniformly incorporating finely divided radiant heat reflecting bodies in a finely divided low heat conductivity powder. The heat reflecting bodies provide a series of heat reflective surfaces for minimizing the transmission of heat radiation through the insulation space. At the same time the small area contact between like and unlike particles provided maximum thermal resistance to the passage of heat by conduction. Increasing the proportion of radiation reflecting bodies, substantially reduces the radiative heat transfer and slightly increases the heat transfer by conduction. Through the use of the subject highly efiicient powder insulation mixture, the required thickness of insulation layer may be substantially reduced and the overall container dimensions minimized.

It will be understood that although the thermal insulation of the present invention has been described in connection with powder-in-vacuum insulating systems for the storage of liquefied gases, the insulation is also susceptible of use in the preservation of quick frozen biological specimens, living tissues and other perishable commodi ties, and may be applied as a thermal insulation at higher temperature levels, at which conditions the pressure in the insulation space will not be as critical or sensitive as at lower temperatures, without departing from the spirit and scope of the novel concepts of the present invention.

This is a cont-inuation-in-part of copending application Serial No. 580,897, filed April 26, 1956, by L. C. Matsch and A. W. Francis and issued as U.S. Patent No, 2,967,152.

What is claimed is:

1. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided low heat conductive particles of calcium silicate having a chemical analysi essentially the same as shown in Table II, a surface area of 95 sq. meters/gm. agglomerate sizes less than about 420 microns, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided radiant heat reflecting bodies of sizes less than about 500 microns and having metallic surfaces, such bodies constituting between about 1% and 30% by weight of said insulating material.

2. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided low heat conductive particles of calcium silicate having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, agglomerate sizes less than about 420 microns, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided radiant heat reflecting bodies of sizes less than about 500 microns and constituting between about 1% and 30% by Weight of said insulating material, said heat reflecting bodies consisting of at least one member selected from the group consisting of aluminum, copper, aluminum paint pigments, copper paint pigments and copper coated mica.

3. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided low heat conductive particles of calcium silicate having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gin, agglomerate sizes less than about 420 microns, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided radiant heat reflecting bodies of less than about 500 microns size, said radiant heat reflecting bodies consisting of aluminum flakes in an amount between about 1% and 30% by weight of said insulating material.

4. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in table ii, a surface area of 95 sq. meters/gm, of less than microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided radiant heat reflecting aluminum flakes of less than about 50 microns size in an amount between about 1% and 30% by weight of said material.

5. An insulating material characterized by a low rate of heat transfer by conduction .and radiation, consisting essentially of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of sq. meters/gin, of less than about 75 microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided radiant heat reflecting copper flakes of less than about 50 microns size in an amount between about 1% and 30% by weight of said insulation material.

6. In combination with a vacuum insulating system, a mixture of finely divided low heat conductive calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, said particles being so reduced in agglomerate size to less than about 420 microns, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft. as to substantially impede heat inleak by conduction and yield to the predominant passage therethrough of heat inleak by radiation, and finely divided radiant heat reflecting bodies of less than about 500 microns size and having metallic surfaces, such radiant heat reflecting bodies constituting between about 1% and 30% by weight of said mixture, whereby said system affords a high resistance to heat inleak by all modes of heat transfer.

7. In combination with a vacuum insulating system, a mixture of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, of less than about 75 microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft.; and finely divided aluminum flakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said aluminum flakes being present in an amount between about 1% and 30% by weight of said mixture.

8. In combination with a vacuum insulating system, a mixture of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, of less than about 75 microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and lbs. per cu. ft; and finely divided copper fiakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said copper flakes being present in an amount between about 1% and 30% by weight of said mixture.

9. In combination with a vacuum insulating system, a mixture of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, of less than about 0.02 micron ultimate particles size and less than about 75 microns agglomerate size, such particles having a settled density of between about 3 and 10 lbs. per cubic ft.; and

finely divided metal flakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said metal flakes being present in an amount between about 1% and. by Weight of said mixture.

10. In combination with a vacutun insulating system, a mixture of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, of less than about 0.02 micron ultimate particle size and less than about 75 microns agglomerate size, such particles having a settled density of between about 3 and 10 lbs. 7 per cubic it; and finely divided radiant heat reflectingaluminum flakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said aluminum flakes being present in an amount between about 6% and 20% by weight of said mixture.

ll. In combination with a vacuum insulating system, a mixture of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table ILa surface area of 95 sq. meters/gm, of less than about 0.02 micron ultimate particle size and less than about 75 microns agglomerate size, such particles having a settled density of between about 3 and 10 lbs. per cubic ft; and finely divided radiant heatreflecting copper flakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said copper flakes being present in an amount between about 10% and 24% by weight of said mixture.

12. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided low heat conductive particles of calcium silicate having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, agglomerate sizes less than about 420 microns, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft; and finely divided radiant heat reflecting bodies of less than about 500 microns size, said radiant heat reflecting bodies conlb sisting of copper flakes in an amount between about 1% and 30% by Weight of said insulating material.

13. An insulating material characterized by a low rate of heat transfer by conduction and radiation, consisting essentially of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm, of less than about microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft; and finely divided radiant heat reflecting bodies of less than about 250 microns size having metallic surfaces and constituting between about 1% and 30% by weight of said insulating material being uniformly interspersed between said calcium silicate particles.

14. In a vacuum insulated, double-Walled container for low-boiling liquefied gases wherein the liquid is stored in the inner vessel and an outer shell surrounds said inner vessel with an evacua'ble space therebetween, the cornbination therewith of an insulating mixture characterized by a low rate of heat transfer by conduction and radiation and substantially filling said evacuable space, said insulating mixture consisting of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of sq. meters/gm., of less than about 420 microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu. ft; and finely divided aluminum flakes of less than about 50 microns size and having thicknesses of less than 0.5 micron, said aluminum flakes being present in an amount between about 1% and 30% by weight of said mixture.

15. In a vacuum insulated, double-Walled container for low-boiling liquefied gases wherein the liquid is stored in the inner vessel and an outer shell surrounds said inner vessel with an evacuable space the-rebetween, the combination therewith of an insulating mixture characterized bya low rate of heat transfer by conduction and radiation and substantially filling said evacuable space, said insulating mixture consisting of finely divided calcium silicate particles having a chemical analysis essentially the same as shown in Table II, a surface area of 95 sq. meters/gm., of less than about 420 microns agglomerate size, ultimate size less than about 0.1 micron, and settled density between about 3 and 10 lbs. per cu.

ft; and finely divided copper flakes of less than about 50 microns size and having thicknesses of les than 0.5 micron, said copper flakes being present in an amount between about 1% and 30% by weight of said mixture.

References Qited in the file of this patent UNITED STATES PATENTS Matsch et a1. Jan. 3, 1961 OTHER REFERENCES Wilson: Industrial Thermal Insulation, McGraw-Hill, New York, 1959, pp. 3, 52, 53, 68, 97, 183-186.

Claims

1. AN INSULATING MATERIAL CHARACTERIZED BY A LOW RATE OF HEAT TRANSFER BY CONDUCTION AND RADIATION, CONSISTING ESSENTIALLY OF FINELY DIVIDED LOW HEAT CONDUCTIVE PARTICLES OF CALCIUM SILICATE HAVING A CHEMICAL ANALYSIS ESSENTIALLY THE SAME AS SHOWN IN TABLE II, A SURFACE AREA OF 95 SQ. METERS/GM. AGGLOMERATE SIZES LESS THAN ABOUT 420 MICRONS, ULTIMATE SIZE LESS THAN ABOUT 0.1 MICRON, AND SETTLED DENSITY BETWEEN ABOUT 3 AND 10 LBS. PER CU. FT.; AND FINELY DIVIDED RADIANT HEAT REFLECTING BODIES OF SIZES LESS THAN ABOUT 500 MICRONS AND HAVING METALLIC SURFACES, SUCH BODIES CONSTITUTING BETWEEN ABOUT 1% AND 30% BY WEIGHT OF SAID INSULATING MATERIAL.