DE963387C - Thermal insulation body - Google Patents

Description

ISSUED MAY 9, 1957

/ 6131 XII147 a

The invention relates to thermal insulation bodies and in particular to vacuum insulators, in which the walls of a vacuum vessel are supported against one another by a filler material.

It is already known that with the help of vacuum spaces a very effective thermal Isolation can be achieved. However, thermal insulators are made using vacuum so far practically only carried out as a cylindrical glass container or glass bottles, z. B. as Dewar flasks in which the curved walls have sufficient strength against the atmospheric Had pressure. Such vessels also suffer from the disadvantage that the internal pressure must be kept very low, for example to ι microns of mercury or less, if the thermal insulation should be good.

Attempts have also been made to use plate-shaped insulation bodies Establish vacuum, in which the vacuum space is limited by flat walls. However, since these designs, the external atmospheric pressure can lead to a sagging of the walls, must the flat walls against each other by a relatively large number of spacers are supported, which act as a heat conductor between the walls and the Undo the advantages of the vacuum as an insulating medium to a considerable extent. the

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direct heat conduction in the spacers between the walls thus reduces the insulation effect, even if the spacers are made of relatively poorly thermally conductive Fabrics are made.

There are also already evacuated insulation bodies known whose flexible walls against the external air pressure by a powder or. fibrous filling material are supported. Due to the thermal conduction of the filling, which of course has to be packed relatively tightly and therefore results in short thermal conduction paths between the cold and warm surface of the wall, the advantage of evacuation is largely reversed.
The fiber boards, which are also known and which are produced by applying layers of glass fibers and a binding agent, for example asphalt, are practically unsuitable for thermal insulation because they do not contain any insulating spaces.

According to the invention, a thermal insulation body is intended using a vacuum between two walls can be used with a filler material to support the walls, which has a low thermal conductivity.

In addition, the invention aims to provide a thermal insulator which allows the construction of such Objects such as refrigerator containers, cooking kettles, etc. allow only a fraction of the walls the usual thickness, so that the space inside the refrigerator container, the boiler etc. is increased significantly.

The thermal insulation body according to the invention consists of a closed evacuated one Container with two parallel flexible walls between which a filling material made of fiber material lies. It is characterized in that the fiber material consists of threads of a hard material less There is thermal conductivity that is randomly arranged in planes parallel to the walls are.

Due to the random position of the supporting fibers according to the invention in parallel to the walls lying planes, heat conduction is achieved by reducing the contact surfaces between the Fiber layers and significantly reduced by lengthening the heat conduction paths, as in detail should be explained with reference to the drawings.

Fig. Ι illustrates part of such an arrangement with two spaced apart Walls, the space being filled in the manner according to the invention;

Figures 2-4 illustrate other embodiments of the isolation assembly;

Fig. 5 shows on a greatly enlarged scale approximate arrangement and mutual position of the individual threads within the vacuum space;

Figs. 6, 7 and 8 are graphs showing the properties of the thermal isolator under different conditions.

Before the heat insulators according to the invention are described in more detail, let us briefly consider how the heat transfer takes place through vacuum plates of the type dealt with here. The heat transfer takes place partly through thermal conduction in the gas, partly through radiation and partly through thermal conduction in solid bodies. The heat conduction in gases consists in the heat transfer between the individual gas molecules. When the gas pressure decreases, the heat transfer between the gas molecules remains approximately constant until the free path of the gas molecules approximately assumes the pore size of the filler material. The heat conduction in the gas then decreases approximately linearly with the pressure until it becomes negligible, so that the heat transfer then takes place practically exclusively through radiation and through the heat conduction in the filler material itself. The general relationship between the coefficient of thermal conductivity (K) and the pressure in the vacuum space is illustrated in FIG. 6. This variable will be discussed in more detail below in connection with the thermal insulation capacity of the arrangements according to the invention.

The heat transfer through radiation is greatest when the vacuum space does not have any filler material contains. In the case of Dewar vessels, the radiation is generated by applying reflective coatings on the walls, e.g. B. by silver or aluminum coatings, reduced. But if a filler material is used, as is to be done according to the invention, so the filler material is reduced even with a suitable selection, the heat transfer through radiation is considerable.

The physical conduction of heat consists in the heat transfer between the walls through the solid material contained in the vacuum space.

According to the invention, the insulating body can be constructed according to FIG. 1, for example. This insulating plate consists of a wall 1 made of thin flexible sheet of relatively high thermal conductivity, for example from low carbon steel and also from a second Wall 2 made of a thin flexible sheet of relatively low conductivity, for example made of stainless steel. The walls 1 and 2 are connected to one another at their edge 3, for example by welding. The space between sheets 1 and 2 is 4 xio with a filler material filled in, which supports these walls against the external atmospheric pressure, and is otherwise in evacuated in a known manner. The filling material 4 can either before being introduced into the Interior can be pressed or it will after -? An installation between the walls 1 and 2 under the effect of the outside. Compressed to atmospheric pressure. The evacuation process includes also a bakeout, e.g. B. at 450 ° C, which is in the walls and the filling material Pumping off occluded gas is permitted.

To support the walls 1 and 2 against the external atmospheric pressure and at the same time the heat transfer through the filling material 4, in particular the physical heat conduction through the Filler material through and the thermal radiation to

Zoom out becomes a special type and arrangement. of the filling material used. The filler material 4 consists of long thin glass thread or threads of a similar material, being practical all threads lie in mutually parallel planes and are completely disordered within this plane are. The long threads are therefore in planes that run parallel to the walls ι and 2, see above that essentially each thread is completely perpendicular to the direction of heat transfer between the walls, and the projected length of each thread runs perpendicular to the wall direction trending plane is very small. In each of these parallel planes, however, the threads are perfect disordered. With this arrangement of the touching threads, between the thread directions If there is a sizeable angle, there is physical conduction of heat through the threads very small through it, as will be explained below.

Some 'commercially available insulation materials contain glass threads in the orientation described and can, if they are about to submit certain. Foreign matter in the still explained below Wise treated, readily as filler material for an arrangement according to the invention to be used. So have certain insulating materials, e.g. B. those, which in the V. St. v. America are sold under the name Fiberglas, TWF-Fiber or B-Fiber, even after they have been manufactured a suitable thread orientation. These isolating substances can be used to construct the inventive Vacuum plates can be used if you take care to install the insulation material in such a way that that the layers of thread are approximately parallel to the walls, i.e. H. substantially perpendicular to the Direction of heat transfer between the walls. Below that for simplicity in the The expression "glass threads" used in the description should also include all thread-like substances with similar Properties like glass in terms of hardness, low thermal conductivity and chemical properties Persistence, d. H. Substances can be understood without a significant gas release for long periods of time. So z. B. Quartz threads, stone threads or a similar material are used, provided, of course, that the threads are oriented in the manner noted above.

Another form of insulator, i. H.

a plate is shown in FIG. There the two walls 6 and 7 have a relative high conductivity and consist e.g. B. made of low carbon steel. On the edges of the walls 6 and 7 the plate is sealed by means of a strip 8 of low thermal conductivity, for example by means of a stainless steel strip. The strip 8 can be grooved to to extend the heat conduction path.

In the embodiments according to FIGS. 1 and 2, the two walls run parallel to one another. For insulation bodies according to the invention, however, arrangements can also be used in which one or both walls do not run flat, or in which both walls do not run parallel to one another. As an example, an arrangement is shown in FIG. 3 which contains a flat wall 9 and a corrugated wall 10. The space between the two walls is largely filled with a filler material 4 'of the type described above. The rest of the space, namely the space within the grooves of the wall 10, is filled with a thermal insulation material 11, which can be of the same type as the material 4 or can consist of a different thermal insulation material. As a further example, the arrangement shown in FIG. 4 will be discussed, in which the walls 12 and 13 extend at an acute angle to one another. At the edges, the walls 12 and 13 are connected by means of a strip 14 of low thermal conductivity. At least the majority of the interior space is filled with a filler material 4 of the type described. For example, pieces of a filler material 4 delimited by parallel surfaces can be used and the remaining triangular cross-sectional spaces can be filled with thermal insulation material of the type of material 4 or other thermal insulation material 15.

The orientation of the threads is shown on a greatly enlarged scale in FIG. In the this figure with 16 to 19 designated glass thread are oriented completely randomly within one and the same plane, i. H. these threads lie at completely different angles to each other, but are all roughly in the same plane or in successive parallel planes. In other words, there is no such thing as an irregular one three-dimensional orientation of the fibers. If you build a latticework of this type, so parts of individual threads can of course lie in different planes, but that is on one The projected length of the individual threads is very small in the plane perpendicular to the wall direction Compared to the length projected onto the walls. When arranging the filler material inside of the vacuum space in Fig. 1, for example, are "the planes in which the threads are randomly oriented, parallel to walls 1 and 2, i.e. H. perpendicular to the direction of heat transfer from a wall to the other. A whole series of such planes lies parallel to one another in FIG. 1 walls 1 and 2.

From Fig. 5 it can be seen that in the described Arrangement of the glass threads between a relatively long physical heat conduction path the walls 1 and 2 is created. The heat flow in FIG. 5 thus flows between points 20 and 21 along the dotted line 22, i.e. longitudinally a long and often winding path, and the physical conduction of heat is very much smaller, as if the heat were to pass immediately between point 20 and 21. The long and The tortuous heat transfer path is in reality because of the numerous layers of thread between the two walls are much more complicated and longer, so that the physical conduction of heat

is considerably less than when using fixed spacers between the walls. aside from that one can see that the random orientation of the threads leads to very small contact cross-sections leads between the individual threads. If, on the other hand, in contrast to the described irregular Orientation would arrange the threads all parallel next to each other, so would a touch lengthways between two adjacent threads

ίο the entire length of the thread come about and therefore a relatively large contact area is created.

By using threads made of glass and similar hard materials with low heat

conductivity, another benefit is achieved. Glass, for example, is relatively hard and incompressible and has a Young's number of, for example, 492 · 10 ⁶ to 843 · 10 ^e g per cm ² . Because of the hardness and incompressibility, the contact area of the individual threads increases only very little under the external atmospheric pressure against the evacuated insulating plate. Therefore, the physical thermal conductivity of the filler material changes very little between the uncompressed and the highly compressed state. This is of course a very great advantage, since an evacuated plate of the type described is necessarily under high pressure. Thus, by using a filler material made of individual threads with the properties and arrangement described, an increase in the physical thermal conductivity as a result of the strong pressure exerted on the filler material is kept very small. It should also be noted that in addition to the fact that the use of hard and incompressible filler material does not significantly increase the contact area between the individual threads with the existing pressure difference, a small increase in the contact area also has only a small influence on the whole Has thermal resistance, since most of this thermal resistance lies within the individual threads between two points of contact with the adjacent threads and the length between two points of contact is not influenced by the pressure prevailing at the points of contact.

As stated above, the filling material according to the invention is necessarily highly compressed, to support the walls against the considerable external atmospheric pressure, and from this resulting small increase in physical thermal conductivity is pronounced In contrast to the very strong increase in conductivity of other pressurized materials, such as finely divided powder materials or ordinary fibers. The reason for this different behavior lies in the fact that in the compressed state the small powder grains " touch practically on their entire surface. The powder therefore grows under strong pressure Contact surface very strongly and thus also the physical thermal conductivity, since the heat flow practically on the shortest path between the Walls over the touching powder grains. The path of the heat flow runs hence as good as within a pressurized powder or ordinary pulp along the temperature gradient, while the path through the randomly oriented threads according to FIG Invention runs essentially in the direction of the threads, i. H. perpendicular to the temperature gradient. The filler material according to the invention, consisting of randomly oriented threads, is therefore a finely divided powder or ordinary fiber used on a vacuum insulation body, where the filler material is necessarily under high pressure, significantly superior.

A comparison of the change in the value K, ie the thermal conductivity coefficient as a function of the external pressure for powdery or granular substances on the one hand and the filler material according to the invention on the other hand, is possible with reference to FIG. There, A denotes the coefficient of thermal conductivity of an 80-mesh sand for various values of the external pressure, while B relates to diatomaceous earth or kieselguhr and C to the filler material according to the invention. It can be seen that the coefficients of the thermal conductivity of these three substances deviate relatively little from one another without any compressive stress. The 80-mesh sand has ο when printing a conductivity coefficient of about 9 x io ~ ⁶ cal / cm ⁰ C sec, atoms earth di- or diatomaceous see a coefficient of about 12 × 10 ^-6 cal / cm ⁰ C and filling material according to the invention a coefficient of about 9 x 10 "" ° cal / cm ⁰ C lake. Under the pressure of one atmosphere, as must occur in the inventive panels, of course, the coefficient of sand absorbs about 83 · 10 ^-6 cal / cm ⁰ C lake, ie to the nine-fold value. The coefficient of kieselguhr increases to about 52 · 10 ~ ⁶ cal / cm ⁰ C see, ie to more than four times the amount. In contrast to these very strong changes in the coefficient for the inventive filler material refers only to 10,5 · 10 ⁶ cal / cm ⁰ C to see, that is only one-sixth compared to the unpressurized value. It can be seen that the filling material according to the invention has great advantages over powdery or granular material for use in vacuum insulators.

The difference between powdery or granular materials on the one hand and those according to the invention Filling materials on the other hand for vacuum insulators can also be based on the formula for thermal conductivity are explained.

K =

Q-d A-AT

Here, Q is the number of calories per second which penetrates an area A with a thickness d at a temperature gradient ΔT. When the filler material according to the invention is pressurized

963

becomes, d decreases without any appreciable increase in Q- When a granular o-ier powdery! Material decreases d little, but Q increases several times. The filler material according to the invention thus delays the transfer of heat, even if its thickness d decreases considerably and is rather small.

Because, filling material for the insulating body according to the invention has certain advantages over powdered !, ίο core-shaped or the usual fibrous filling materials with regard to easier evacuation. Powder ponds tend to adhere to one another and block the escape of gas from the interior of the powder, so that evacuation becomes difficult and requires longer pumping times. The lattice structure of the threads used as filler material according to the invention, on the other hand, leaves numerous pump paths free for the gas outlet and facilitates the evacuation of the fill space.

With regard to the physical conduction of heat, it also plays a role that the threads are practically in parallel planes or layers and are oriented randomly within these planes and that these planes run approximately parallel to the walls and perpendicular to the direction of heat transfer between the walls. It can be seen that a three-dimensional random orientation of the threads, in which many threads would be perpendicular to the walls, would result in a shorter heat conduction path between the walls than is given by the long and often winding path in the direction of the threads, which are approximately parallel to the. Walls run. Natures borrowed int in a filler material, which contains many thousands of threads and is arranged in the manner described, also now and then a thread is present, which partly runs obliquely to a plane or layer and pass through several such .j.0 planes or layers can. However, the length of such a yarn in the projection onto a plane standing perpendicular to the walls is very small compared to the total length of the thread will lie with an above-r ying part of its length within a plane. As an example, it should be mentioned that when using glass thread with a conductivity of 2.8 · o- ³ cal / cm ° C see in parallel planes and in random orientation a package with a density of 0.23 g per cm ³ between 25 ⁰ C and 140 ⁰ C a total conductivity coefficient in vacuum of 8.8 x 10- ⁶ cal / cm ⁰ C showed lake. If the same threads were randomly oriented and a considerable fraction was present parallel to the direction of heat transfer (instead of perpendicular to it) in a test specimen of the same density, the thermal conductivity increased to ⁶ g · 10-6 cal / cm ⁰ C see. It is not only the filling material in the insulating body according to the invention that is superior to a finely divided or powdered material, but the advantages also lie in the arrangement according to the invention for threads over a thread-like, but

filler material not arranged in the manner according to the invention. In addition, since the hardness S _{5 of} the glass or the material used elsewhere is important for the slight increase in the contact area between the threads under external pressure, it is also clear that the filler material used according to the invention is also advantageous over other and more compressible materials than glass and the like .like. is. It should also be noted that in a conventional air-filled insulating body with a glass pane insert, as is used, for example, in today's refrigerators at atmospheric pressure, the random orientation of the threads within parallel planes has a relatively small influence on the thermal conductivity. So z. B. the increase in thermal conductivity with three-dimensional orientation of the threads (as opposed to random orientation in parallel planes) amount to approximately 5%. In contrast, in the case of evacuated insulation bodies, the difference in the case of random orientation in parallel planes, as shown in the example above, can be approximately 800%.

It is of interest to compare the thermal conductivities of different materials that are used today in thermal insulation arrangements under atrnospheric pressure, ie under air on the one hand and under vacuum on the other hand. One might expect that if known insulation materials were used in a vacuum insulator, the properties of all materials would change in approximately the same proportion, since the evacuation reduces the thermal conductivity in the gas. However, it has been found that the decrease in thermal conductivity for the various materials does not occur in nearly the same proportion, and it has also been found that the decrease in thermal conductivity for the filler material used according to the invention is much greater than for other insulating materials used today. A comparison of the different materials is possible using the table below. The numbers in the table are the values of K (thermal conductivity) multiplied by 10 ⁶ cal / cm ⁰ C see.

in air in a vacuum Glass threads
(in parallel planes
randomly oriented). ..
Filaments of rock ......... 90
■ 82 6 to 10
20 to 50 Silicon airgel
Sil-O-Cel 55-
100 to 130
110 32
50
110 Vermiculite

From this table it can be seen that the decrease in thermal conductivity for materials such as Filaments of rock, silicon airgel, etc. is relatively small when used in a vacuum. in the

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In contrast, a filler made of many glass threads shows in parallel planes and more randomly Orientation within these levels a significantly greater decrease in thermal conductivity, namely a decrease to a tenth of the value in air.

Another factor which influences the effectiveness of the filler material as it is to be used according to the invention is the material density. As shown in FIG. 8, the coefficient of thermal conductivity decreases with the decrease in density to a minimum value, while it increases again with a further decrease in density. It is seen from Fig. 8, that a see a minimum value of conductivity of slightly less than g · io ~ ⁶ cal / cm ⁰ C of glass filaments having an average diameter of i, 5 x io- ³ cm existing filling material in \ ^r acuum has when the material has a density of approximately 0.25 g per cm ³ . When the density to about 1.2 g per cm ³ is increased, the coefficient of thermal conductivity decreases to 20 · 10 ^-6 cal / cm ⁰ C to see and also increases, namely to about 25 · Kr ^"6 cal / cm ⁰ C see if the density is ^{lowered to 0.05 g per cm 3.} Of course, a certain range of density can be allowed, depending on the deviation from the minimum value in the thermal conductivity allowed, but curve 8 gives a relationship, from the optimal point for two different filler materials Similar curves can be learned according to the invention to be used in kind with the use of glass fiber having an average diameter of 0.325 · 10- ³ cm or 1.5 x 10- ³ cm of. can be It can be seen that the point of lowest thermal conductivity depends on the density and also on the diameter of the threads used. an average diameter of 0.325 · 10- ³ cm have, to reach the minimum value of the coefficient of thermal conductivity of 6 x io ~ ⁶ cal / cm ° C lake, when the density is about 0.175 g per cm ^3. J: The larger the diameter of the threads used, the higher the density at which the minimum of the coefficient of thermal conductivity occurs. As a practical rule, it follows that a density of 0.15 to 0.2 g per cm ^{3 is} necessary in order to provide support against atmospheric pressure, so that this number is a practical, lower limit with regard to the density used. Material represents. The upper limit depends entirely on what deviation from the possible minimum value can be permitted. In any event, Fig. 8 gives a relationship between the coefficient of thermal conductivity and the density of the filler material so that for each type of thread of a given average diameter the minimum value of the coefficient of thermal conductivity and the optimum density can be determined.

As mentioned above, vacuum isolators, e.g. B. Dewar bottles, without filler material, the cross-sections were generally circular so that the walls could withstand the external pressure. Apart from the fact that such arrangements are only possible when restricted to certain vessel shapes, there are also other restrictions with regard to the practical use of such arrangements in domestic refrigerators and the like will. In the mass production of relatively large items, such as household refrigerators and the like. Where vacuum insulators could be useful to increase the capacity, the production and maintenance of this very low pressure is difficult because very careful evacuation is necessary and these high degrees of vacuum from gas bursts from the metal of the walls can later deteriorate. The insulating material to be used according to the invention avoids this difficulty, as can be seen from FIG. 6, in which the effective coefficient of thermal conductivity of filler materials with an average thread diameter of 0.325 × ³ and 1.5 × 10 ^-3 cm is plotted against the internal pressure is. "Fig. 6 shows that, at 0.325 · io ~~ ³ cm thread diameter of the coefficient of thermal conductivity increases very little, until a pressure of 1000 microns of mercury is reached. at 1.5 · 10- ³ thread diameter increases to over 100 Microns the coefficient of thermal conductivity to very little. Therefore, with the arrangement according to the invention, pressures of about 100 microns instead of 1 micron can be allowed without the thermal insulation deteriorating. As noted above, this is of particular importance when the insulation body is in Mass production process is to be fabricated and if the walls of metal dewars are best in contrast to glass walls hen that give off certain amounts of gas over long periods of time and can therefore increase the pressure in the vacuum space.

It can be seen from FIG. 8 that in thermal insulation arrangements of the type according to the invention the coefficient of thermal conductivity for a given density increases with the diameter of the filaments. The effectiveness of the thread material in terms of thermal insulation decreases with decreasing. Thread diameter up to the smallest thread available today. As a specific example. be mentioned that heat insulators of the invention with low thermal conductivity using threads of iao glass with a diameter of 2.5 x 10- ³ cm or less can be constructed in accordance with. If threads with very different diameters and the same insulating filling are used, this leads to a bridging of some threads, so that there then no contact for the physical

Thermal conduction comes about. In order to keep the increase in contact area that takes place under the external pressure as low as possible, the threads are made of hard material, e.g. B. made of glass. As an example it should be mentioned that glass threads with a Young value of 492 · ⁶ g per cm ² or more can be used. Of course, the physical conductivity of the filler material changes with the conductivity of the material from which the threads are made. As a further numerical value it should be mentioned that the thermal conductivity of the thread material should preferably be less than 0.0035 cal / cm ° C see. The tensile strength of the threads must be so high that only a few threads break under atmospheric stress. Many thread breaks would lead to the transformation of the thread material into a material which has approximately the properties of a powder or a spherical filler material.

The tensile strength should, for example, be at least 105 · 10 ² g per cm ² .

The length of the threads can vary within a wide range, but should 'die The minimum amount must still be large enough to bridge several threads lying next to one another, so that the threads do not shift and when handling before compression the Maintain the desired orientation.

As noted above, certain commercially available insulating materials can be used as fillers when placed between walls in the manner of the invention. It should be noted that such commercially available materials contain certain binders and certain lubricants. It has been found that, in order to achieve the best insulating effect of the thread-like filler material, binders and lubricants have to be avoided, that is to say that the surfaces of the threads have to be clean. The filling materials according to the invention have at a load through an atmosphere external pressure a coefficient of thermal conductivity of 10.6 ■ 10- ⁶ cal / cm ° C lake. With the addition of a lubricant, as is commonly used in today's refrigerators, this value has risen ^{to 17.8 · 10 "" 6.} Also, a filler material having a conductivity of about 10 x io ~~ ⁶ adopted by addition of a conventional binder, a conductivity of about 20 ■ io ~. ⁶

Furthermore, lubricants and binders have the property to give off gases, so that the pressure in the insulating body is significantly increased and the insulating effect that comes from maintaining it a relatively low pressure is dependent, decreases.

In the case of the insulating body described, the heat conduction in the gases is achieved by evacuation to a pressure at which the mean free path of the gas molecules is large compared to the pore size of the filler is kept to a minimum. The physical conductivity of the Filler material is brought to a minimum value by using a filler material which consists of many thin threads of glass or a similar material, which are 6g number of practically parallel planes and are randomly oriented within these planes. These planes run roughly in the direction of the walls and thus perpendicular to the direction of the Heat transfer between the walls. For the physical transfer of heat therefore arise long and winding paths. The heat transfer through radiation is also controlled by the used filler material is kept to a minimum. Each of the parallel planes forms one Area that reduces the heat transfer through radiation. Because the heat transfer through Radiation changes with the fourth power of the temperature difference, the heat transfer drops due to radiation actually looks very small if one has a multitude of parallel planes or layers of randomly oriented threads and thus the space between the walls broken down into many small parts in which the temperature difference is small compared to the whole temperature difference lying on the plate.

Claims (3)

PATENT CLAIMS: 1. Thermal insulation body, consisting of a closed evacuated container, which has two parallel flexible walls between which a fibrous filler material lies, characterized in that the fibrous material (4) consists of threads of a hard material of low thermal conductivity, which are randomly arranged in planes parallel to the walls (1, 2). 2. Insulation body according to claim 1, characterized in that the threads (4) consist of glass, have a minimum tensile strength of 105 · ι o ² kg / cm ² and less than 2.5 · io ~ ³ cm in diameter. 3. Insulation body according to claim 1 and 2, characterized in that the filler material body has a density between 0.15 g / cm ³ and 0.4 g / cm ³ . Considered publications:
German Patent Nos. 592047, 665319; French patent specification No. 758 370. 1 sheet of drawings © 609 707/189 11.56 709 513/117; 5.57