Insulating material and method of making same

An insulating material is provided comprising an air tight enclosure maintained under vacuum and containing a plurality of alternate, successive layers of thin reflective foil and thin insulating sheeting. These layers are tightly packed together and are arranged generally perpendicular to the heat flow. The enclosure walls are thin and sufficiently ductile or resilient so that when air is pumped from inside the enclosure, the walls deform in response to atmospheric pressure and transmit this load to the layers of foil and sheeting thereby compressing them tightly together. A method of making such an insulating material is also provided, by wrapping multiple layers of laminated insulating components around a thin inner wall of the enclosure, covering and enclosing the layers and inner wall with thin outer walls of the enclosure, and subjecting the enclosure to a vacuum causing the thin enclosure walls to deform under atmospheric pressure and compress the insulating components together to thereby form a consolidated, rigid structural member having high performance insulating qualities.

BACKGROUND AND OBJECTS OF THE INVENTION 
This invention relates generally to insulating materials maintained under 
vacuum, and more particularly, to such materials which are light in weight 
and yet have high compressive strength, and possess superlative insulating 
qualities at extreme high or low temperatures. 
It is an object of the invention to provide a light weight, high 
performance insulating material maintained under vacuum, for use under 
extreme high or low temperature service conditions. An allied object is to 
provide such an insulating material which itself has such high compressive 
strength that it has load bearing capabilities, thereby enhancing its use 
in many applications where use of heavy load bearing members is 
prohibited. 
Still another object is to provide a method of making such an insulating 
material in which multiple layers of laminated insulating components are 
enclosed and subjected to a vacuum to thus compress the components and 
enclosure together to form a rigid, strong structural member having highly 
desirable insulating qualities. 
SUMMARY OF THE INVENTION 
We provide an insulating material comprising an air tight enclosure 
maintained under vacuum and containing a plurality of alternate, 
successive layers of thin reflective foil and thin fiberglass sheeting. 
These layers are tightly packed together and are arranged generally so as 
to be perpendicular to the direction of heat flow. The walls of our 
insulating enclosure are thin and sufficiently ductile or yielding so that 
when air is pumped from inside the enclosure, the walls deform in response 
to atmospheric pressure and transmit this load to the layers of foil and 
sheeting thereby compressing them tightly together. In this way, the 
layers retain their insulating quality while supporting atmospheric load. 
We also provide a method of making such an insulating material by wrapping 
multiple layers of laminated insulating components around an inner wall of 
the enclosure, covering and enclosing the layers and inner wall with outer 
walls of the enclosure, and subjecting the enclosure to a vacuum causing 
the enclosure walls to deform and compress the insulating components and 
enclosure walls together to thereby form a rigid, strong structural member 
having highly desirable insulating qualities.

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to the drawings, the illustrative insulating material takes the 
form of an insulating enclosure or container 20 for a battery 22 or like 
device which is required to be insulated so that it will function at 
extremely high or extremely low temperatures. The illustrated enclosure 20 
is an insulated box open at one end (upper end) for insertion therein of 
the battery 22. Typically, the enclosure 20 is sized and shaped so that 
the battery or the like closely fits within it in order to maximize the 
effectiveness of the insulation provided by the box. In this instance, the 
insulating box 20 has thin inner and outer walls or shells 24, 26, 
respectively, spaced apart to create a space therebetween for the 
specially constructed insulation components, as described below. A cover 
27 may be provided for the battery. 
The main steps in constructing the illustrative insulating container or box 
20 are as follows. An inner shell 24 is constructed of thin stainless 
steel sheet, for example on the order of 0.020" thick, with the inside 
dimensions of this shell being selected to fit the particular battery or 
other object to be held in the container. The inner shell 24 is tested for 
leaks to make sure that all joints are gas tight. Next, layers of special 
insulation components (see 28) are wrapped around the outer periphery of 
the shell as described below. Then the wrapped inner shell 24 is subjected 
to a vacuum, causing its outer periphery to shrink to a predetermined 
point so that the outer shell 26 will fit over the wrapped inner shell. 
Finally, the outer shell 26 is formed over the wrapped inner shell 24, 
utility connections 30 for the battery (e.g., electrical terminals, heat 
input) are supplied, and the insulating enclosure is ready for service. 
A detailed description of the construction of the illustrative insulating 
container 20 follows. To facilitate this construction, we have found it 
useful to employ a winding machine (not shown) having means for rotating 
the mandrel upon which the inner shell components are mounted. The mandrel 
is preferably slightly expandable/contractible radially to facilitate its 
removal from the completed container. In this instance, the mandrel has 
the shape of a rectangular solid, i.e., conforming generally to the shape 
of the battery to be encased. Two sheets of thin stainless steel are 
wrapped around the mandrel, being sized so that together they completely 
enclose the mandrel and form the walls 24a, 24b, of an open-ended 
enclosure. One such sheet 24a is clamped or banded around the upper half 
of the mandrel, which is radially contracted to its collapsed or smaller 
size. This sheet is then tack welded to a pair of longitudinal rails 32 
located on the inside of the shell 24 and which also form a channel for 
reception of the battery 22 therein. Preferably, the tack welding is begun 
at the longitudinal center of the sheets, and then worked outwardly to the 
ends. After the first sheet is tacked in place to the rails 32, the second 
sheet 24b is pulled tightly around the lower half of the mandrel. It is 
also tack welded to the rails 32, taking care to minimize heat distortion 
of the sheets during welding by alternate welding of spaced tack sites. 
After the tack welding of both sheets or walls 24a, 24b to the rails 32 is 
complete, the mandrel is radially expanded slightly thereby tensioning the 
walls and expanding them to the final dimensions of the inner shell 24. 
Then, a bottom cover or wall 24c (see FIG. 4), formed of the same or 
similar thin stainless steel sheet, is cut to size to cover the bottom of 
the open ended shell 24, is fit onto the mandrel and is tack welded to the 
walls 24a, 24b. Again, tack welding of the bottom 24c to the side walls 
24a, 24b, is preferably accomplished by starting at the longitudinal 
center of the bottom, and working outwardly toward its ends in order to 
minimize heat distortion of the thin steel sheets. A front flange 24d is 
welded to the walls 24a, 24b, at the upper end of the enclosure (see FIG. 
4). When all seal welding is complete, the inner shell 24 is leak tested 
with the mandrel in place within it, using conventional helium mass 
spectrometer leak testing techniques. All welds are bagged (for example, 
enclosed with polyethylene sheeting) and flooded with helium gas to be 
certain that the welds are leak tight. This is important at this point in 
the procedure because leak testing and repair of the inner shell 24 is 
difficult to accomplish once the inner shell has been enclosed within the 
outer shell 26. 
Layers of special insulating material 28 are then wound around the inner 
shell 24 with the mandrel still in place through use of the winding bench. 
In the illustrative insulating enclosure, two successive wraps around the 
shell 24 are made with stainless steel foil of 0.003" thickness. Heavy 
tension is applied to these wraps of foil using an air motor (not shown) 
as a braking system in order to assure that the foils tightly adhere to 
the shell 24. The two touching bare layers of metal foil advantageously 
enhance the tension applied through friction. Then, a series of 
alternating layers of insulating fiberglass paper or other inert 
insulating material such as mica (for example of about 0.00025" thickness) 
28a, and reflective thin metal foil (for example, stainless steel of 
0.0003" thickness) 28b are successively wound around the inner shell 24, 
followed by several (for example, seven) alternate combination layers of 
such fiberglass paper and metal foil. At this point the marginal ends of 
the stainless steel or other appropriate metal foils are tack welded to 
each other. Then, two successive wraps of thin aluminum foil, for example 
0.0003" thick, are wound around the previous windings, followed by another 
series of perhaps 20 to 40 alternate layers of insulating fiberglass paper 
and reflective thin metal (aluminum) foil. The number of such layers may 
be selected depending upon the expected operating temperature and the heat 
loss or gain limitations of the particular application. 
At this point, the fiberglass paper/metal foil alternate layers 28a, b are 
interrupted, and one full wrap of aluminum foil is wound around the shell 
24. Then, stainless steel screens 34 are inserted in each of the corners 
of the shell. These screens create a gas path or channel within the 
insulation to facilitate later pumping out or removing of air from between 
the inner (24) and outer (26) shells. Two additional successive aluminum 
foil wraps are then added to the built-up shell 24. Now, this first 
blanket of insulation is complete. 
We now add several additional similar blankets of insulating paper and 
reflective foil components. Each succeeding blanket is preferably started 
approximately 3/4 out of circumferential alignment with the preceding 
blanket, to effectively block the path of heat flow. We use insulation 
board 35 to fill in any gaps. A second blanket is wound over the first, 
the second one having approximately 40 successive alternate layers of 
fiberglass paper and aluminum foil. Then the procedure previously 
described for inserting the stainless steel screens 34 is repeated. After 
completion of this second blanket, the procedure is repeated for two 
additional (third and fourth) blankets. A final blanket wrap is applied 
having, in this instance, 96 successive alternate layers of fiberglass and 
metal foil. 
Following winding of the insulation blankets over the inner shell 24, the 
outer shell 26 is placed around the periphery of the wrapped insulation, 
and clamped or banded thereto using suitable bands. In this instance, the 
outer shell 26 is formed of stainless steel sheet of approximately 0.020" 
thickness. A vacuum bag, such as a plastic bag, is then placed around the 
entire wrapped outer 26 and inner 24 shell structure, and a vacuum (for 
example, 28" mercury) is drawn to deform the shells and thereby compress 
the insulation. In the process, the void spaces between the two shells 24, 
26 are eliminated as air is withdrawn thereby forcing the layers of metal 
foil and fiberglass paper tightly together, rigidifying them such that the 
final composite insulated material has load bearing capabilities in 
addition to presenting superlative insulating qualities. With the vacuum 
maintained, the outer shell 26 is drawn onto the outer insulating blanket. 
Then, by mechanically maintaining the load, the marginal ends of the shell 
26 are tack welded to a strip 36 (see FIG. 5) provided at the center. The 
abutting edges of the shell 26 are then welded to the strip 36. 
Next, a getter basket 38 (FIG. 4) is placed against the surface of the 
bottom cover 24c (see FIG. 4), following which the four layers of 
insulating blankets are folded over the getter basket. One layer of 
insulation board 35 is added, and the insulation blankets are folded 
against the bottom cover 24c. The front flange 24d and bottom cover 24c 
are tack welded to the outer shell 26. Utility connections 40 for the 
enclosure are provided in the shell bottom 26a (for air pump out port, 
vacuum gauge, and auxiliary port). After completion of the welding, the 
insulating enclosure 20 is evacuated and leak tested in a conventional 
manner. Finally, the mandrel is removed from the completed box 20, and a 
final leak check is performed. Preferably, this final leak testing is 
completed using a helium mass spectrometer. We found the box 20 to be leak 
free in accordance with industry standards. 
In the practice of our invention, the utilization of thin inner and outer 
enclosure walls, 24a, 24b, 24c, 26, 26a, made of ductile or yielding 
materials, makes it possible to bring about the desired deformity of the 
walls in response to atmospheric pressure resulting from application of 
vacuum to the inside of the enclosure. This deformity or inward collapse 
of the enclosure walls forces the layers of insulating components into a 
tightly consolidated insulating mass. 
When the illustrative battery 22 is in place within the box 20, the 
insulated walls of the box are perpendicular to the heat flow emanating 
from the battery. We find through use of our invention that it is possible 
to achieve minimal heat leaks on the order of only about 100 watts at 400 
degrees C. with an enclosure of approximately 15 square feet of surface 
area. 
In practicing our invention, we find that heat transmission through our 
insulating components by radiation is minimized through provision of the 
reflective foils which function as a radiation shield, transmission of 
heat by conduction is prevented by carefully avoiding direct metal to 
metal contacts, and transmission of heat by convection is minimized by 
application of the vacuum to the insulating components. 
In carrying out the invention, we have successfully used fiberglass paper 
having a basis weight of 16.1 grams per square meter, a thickness of 3.3 
mils, an air permeability of 60 liters per minute per 100 square 
centimeters, and a tensile dry strength of 240 grams per 25 millimeters 
machine direction. Other insulators may be used as well, for example mica 
or woven or weblike cloth materials such as bridal veil. 
It will also be recognized that reflective foil materials other than 
stainless steel and aluminum may be used in practicing our invention. In 
keeping with the need to reduce losses by radiation of heat, we find it 
advantageous to employ materials which are made of reflective metal or 
which have a reflective metal coating. For example, with low temperature 
systems, a thin plastic film (for example, on the order of 50 Angstroms) 
such as Mylar may be used if provided with a surface layer of a metal 
(such as aluminum, silver, gold). For very high temperature applications, 
metal alloys such as Inconel may be useful in practicing our invention. 
Stainless steel foils are useful for high and medium temperature 
applications, and inert synthetic films with metal coatings having 
reflective qualities are useful for lower temperatures or for cryogenic 
applications. 
In selecting materials for use as the reflective metal foils and insulating 
sheeting, it is essential to choose materials having low outgassing 
properties and low vapor pressures so as not to impair maintenance of the 
vacuum. In the case of the battery enclosure described herein, the 
insulating materials and getter give assurance that the vacuum system for 
the enclosure will operate maintenance free for five years or more. 
The features of our invention render it advantageous for use in many 
different applications. As explained above, our invention is useful for 
storage of batteries for electric vehicles. Another application is 
insulation of hot gas exhaust pipes or manifolds, for example automobile 
exhaust pipes, and insulation of turbine inlets. Our invention is 
especially useful in such applications because our insulating material has 
such light weight and takes up so little space. Other applications for our 
invention include cryogenic transporters, especially in situations where 
the weight of the cryogenic vessel is of overriding importance. It is, of 
course well known that it is difficult to construct a cryogenic vessel 
that is light in weight; our invention makes this possible.