U.S. patent application number 10/408051 was filed with the patent office on 2005-01-13 for method for heat absorption using polyoxymethylene polymer compositions.
This patent application is currently assigned to AlliedSignal, Inc.. Invention is credited to Olzak, Richard A., Osuch, Christopher E., Ren, Jane.
Application Number | 20050006621 10/408051 |
Document ID | / |
Family ID | 25507933 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006621 |
Kind Code |
A1 |
Ren, Jane ; et al. |
January 13, 2005 |
METHOD FOR HEAT ABSORPTION USING POLYOXYMETHYLENE POLYMER
COMPOSITIONS
Abstract
The present invention provides methods for heat absorption and
thermally protected enclosures for containing heat sensitive
devices, which include a heat absorption composition including
polyoxymethylene polymer.
Inventors: |
Ren, Jane; (Morris Plains,
NJ) ; Osuch, Christopher E.; (Mine Hill, NJ) ;
Olzak, Richard A.; (Kirkland, WA) |
Correspondence
Address: |
HONEYWELL INTERNATIONAL INC.
101 COLUMBIA ROAD
P O BOX 2245
MORRISTOWN
NJ
07962-2245
US
|
Assignee: |
AlliedSignal, Inc.
Morristown
NJ
|
Family ID: |
25507933 |
Appl. No.: |
10/408051 |
Filed: |
April 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10408051 |
Apr 3, 2003 |
|
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|
09329005 |
Jun 9, 1999 |
|
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Current U.S.
Class: |
252/70 |
Current CPC
Class: |
Y10T 428/1352 20150115;
H05K 5/0213 20130101; Y10T 428/13 20150115; G11B 33/14 20130101;
C09K 5/16 20130101; Y10T 428/1334 20150115 |
Class at
Publication: |
252/070 |
International
Class: |
C09K 003/18 |
Claims
1. A method for dissipating heat away from a heat sensitive device
exposed to a high temperature environment, the method comprising:
surrounding the heat sensitive device with a solid heat absorption
composition comprising polyoxymethylene polymer and a catalyst,
wherein said polyoxymethylene polymer undergoes endothermic
decomposition directly from solid to gas when exposed to the high
temperature environment, and wherein said composition absorbs heat
from the high temperature environment during the endothermic
decomposition of the polyoxymethylene polymer while remaining in a
solid state and thereby dissipates heat away from the heat
sensitive device.
2. The method of claim 1, wherein said catalyst establishes an
initiation temperature of said endothermic decomposition.
3. The method of claim 1, wherein said catalyst is selected from
the group consisting of benzoic acid, diphenyliodonium triflate and
triphenylsulfonium triflate.
4. The method of claim 1, wherein said heat absorption composition
further comprises a binder.
5. The method of claim 1, wherein said catalyst is applied to the
surface of the heat absorption composition.
6. The method of claim 5, wherein said catalyst is applied as a dry
power to the surface of the heat absorption composition.
7. The method of claim 1, wherein said catalyst is dispersed
throughout the heat absorption composition.
8. The method of claim 1, wherein said catalyst further comprises a
catalyst that generates acid upon exposure to heat.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to methods and
compositions useful for heat absorption, particularly in crash
survivable protective enclosures for flight recorders used in
aircraft and other vehicles operating over land and water. More
particularly, the present invention relates to heat absorption
compositions for use in a crash survivable enclosure which is
capable of withstanding high impact, shock and mechanical
penetration forces that commonly occur in a crash and the high
temperatures, flames, and heat that often develop after a crash.
The enclosure is designed to resist damage to solid state memory
devices contained therein, even if the enclosure is subjected to
immersion in fresh water or salt water for a prolonged period.
SUMMARY OF THE INVENTION
[0002] As a first aspect, the present invention provides a method
for dissipating heat away from a heat sensitive device exposed to a
high temperature environment. The method comprises surrounding the
heat sensitive device with a composition comprising a
polyoxymethylene (POM) polymer and optionally also a binder. The
polyoxymethylene polymer exists in a solid state at normal
temperatures that undergoes endothermic decomposition when exposed
to the high temperature environment, but remains in a solid state
during endothermic decomposition. The composition absorbs heat from
the high temperature environment during the endothermic
decomposition of the polyoxymethylene polymer and dissipates heat
away from the heat sensitive device.
[0003] As a second aspect, the present invention provides an
enclosure for thermally protecting one or more heat sensitive
devices from a high temperature environment. The enclosure
includes: a) an outer housing having interior surfaces defining an
interior cavity for containing one or more heat sensitive devices;
and b) a heat absorption composition comprising a solid
polyoxymethylene polymer that remains in a solid state while
undergoing endothermic decomposition. The composition occupies at
least a portion of the interior cavity and substantially surrounds
the heat sensitive devices. The polymer exhibits endothermic
decomposition when the enclosure is subjected to the high
temperature environment, and the composition absorbs heat from the
high temperature environment during the endothermic decomposition
of the polymer.
[0004] These and other aspects of the present invention are
described further in the detailed description of the preferred
embodiments of the invention which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a graphical representation of the time versus
temperature curve for the polyoxymethylene polymer compositions of
the present invention as well as conventional heat absorption
compositions employed in thermally protected enclosures such as
flight data recorders.
[0006] FIG. 2 is a cross-section of a thermally protected enclosure
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0007] As used herein, all terms have their ordinary meaning in the
art unless otherwise indicated. All percentages are percent by
weight unless otherwise indicated.
[0008] The methods of the present invention involve the use of
certain polymer compositions which are characterized in that they
absorb heat from a high temperature environment and thereby
dissipate heat away from heat sensitive devices which are exposed
to the high temperature, environment. Such compositions are
particularly useful in thermally protected enclosures such as
flight data recorders. The compositions of the present invention
are designed to possess greater heat absorption, and thus heat
dissipating, characteristics than conventional heat absorption
compositions which are currently employed in flight data recorders
to absorb and dissipate heat away from heat sensitive devices. The
heat absorption characteristics of the composition are useful for
providing thermal insulation to heat sensitive devices within the
enclosure in a fire or other high temperature environment. In
accordance with the present invention, the composition absorbs heat
from the high temperature environment during endothermic
decomposition of a polyoxymethylene (POM) polymer having the novel
characteristic of being a solid that undergoes endothermic
decomposition at a temperature below the melting temperature of the
material. The polymer thereby undergoes endothermic decomposition
while remaining in a solid state. The absorption of heat from the
high temperature environment effectively dissipates the heat away
from the heat sensitive devices by controlling the temperature rise
within the interior compartment of the flight data recorder. As a
result, the temperature to which the heat sensitive devices are
exposed does not rise to a level at which the heat sensitive
devices will be damaged even though the temperature to which the
flight data recorder is exposed is significantly higher than the
temperature at which damage to the heat sensitive device will
occur.
[0009] The temperature at which the endothermic decomposition
occurs is selected to be: (a) above the peak temperature
encountered under normal operating conditions; (b) at or below the
peak temperature objective for the heat sensitive devices within
the enclosure which are desirously protected from the high
temperature environment; and (c) below the melting temperature of
the POM polymer. Accordingly, the temperature at which the
endothermic decomposition occurs at about 140 degrees C.
Furthermore, the melting temperature of the POM polymer composition
exceeds 172 degrees C. The melting temperature thus exceeds the
decomposition temperature by at least 32 degrees C., so that
decomposition occurs before melting. Therefore, each of the POM
compositions employed in the methods and enclosures of the present
invention absorb heat from the high temperature environment during
the endothermic decomposition of the POM polymer while remaining in
a solid state and thereby dissipate heat away from the heat
sensitive device.
[0010] The POM compositions employed in the methods and enclosures
of the present invention include a polyoxymethylene polymer having
the general formula:
H--(--O--CH.sub.2--).sub.n--OH
[0011] where n is a number from 8 to 100 inclusive.
[0012] POM polymers having the forgoing general formula are
generally well-known and commercially available in the art and are
also known as "paraformaldehyde," and several versions of the
polymer having the foregoing structural formula are commercially
available under the trade name DELRIN .COPYRGT. from E.I. du Pont
de Nemours and Company, as are other suitable POM polymer materials
from other sources. Those skilled in the polymer art may readily
identify other suitable commercially available polymers having the
foregoing general formula by reference to any of several chemical
reagent supplier catalogues.
[0013] Certain commercially available polyoxymethylene polymers may
be stabilized to prevent unzipping. Catalytic stabilization of POM
compositions is well known to those of ordinary skill in the art.
Such stabilization increases the decomposition temperature to about
300 degrees C. The melting temperature remains in the range of
about 172-180 degrees C. When the melting point and decomposition
temperatures differ significantly composition melts before it
decomposes. Accordingly, when such polymers are employed in the
methods and compositions of the present invention, it may be
desirable to include, in the composition, an acid catalyst capable
of initiating the degradation or unzipping of the stabilized
polyoxymethylene polymer. Acid catalysts are used to initiate and
control unzipping so that the composition first decomposes and then
melts, the control of unzipping is thus used in the invention to
control a heat transfer rate.
[0014] The required decomposition rate, which is a function of the
heat flow rate into the interior of the enclosure, will determine
whether a catalyst is needed. In addition, the heat-flow rate will
also determine how the catalyst should be included in the
composition. If only a little catalytic activity is required, the
catalyst may be applied as a dry power to the surface of the heat
absorbing composition. If greater catalytic activity is required,
it may be beneficial to disperse the acid catalyst throughout the
polymer composition. Suitable acid catalysts which may be employed
to catalyze the decomposition of the polyoxymethylene polymer in
the high temperature environment include, catalysts which generate
acids upon exposure to heat. Specific examples of acid catalysts
include benzoic acid, diphenyliodonium triflate and
triphenylsulfonium triflate. Typically, the acid catalyst is
provided in an amount of from about 0 to about 5 percent by weight
of the composition.
[0015] If desired, the heat absorption compositions of the present
invention may include a binder, although a binder is not necessary.
The polymers employed in the present invention are typically in the
form of a powder, flake or other particulate form, which can be
molded extruded, or machined into a complex shape. However, a
binder may be admixed with the particulate polymer to assist in
holding the polymeric particles together during the molding
process. For example it is often desirable for compositions for use
in thermally protected enclosures to be molded into the general
configuration of the interior cavity of the enclosure or smaller
geometrically shaped units such as blocks and cubes. The binder
holds the aggregated powder in the desired configuration. As a
further advantage, the binder also provides shock absorption
properties to the composition.
[0016] The binder is typically a wax binder. Examples of suitable
wax binders include but are not limited to polyethylene wax,
polypropylene wax, oxidized polyethylene, copolymers of ethylene
and comonomer, and copolymers of propylene and comonomer. Suitable
comonomers include those monomers which are copolymerizable with
ethylene or propylene, such as linear, branched, or cyclic alkene
monomers including butene, hexene, pentene, octene, and higher
homologs; acrylic acid; and vinyl acetate. These binders, including
the copolymers, are commercially available. Preferred binders
include oxidized polyethylene wax, paraffin wax, and
poly(ethylene-vinyl acetate) oxidized polyethylene wax is currently
the most preferred binder for use in the compositions of the
present invention.
[0017] Typically, the composition includes a) between about 90
percent and about 100 percent by weight inclusive of the
polyoxymethylene polymer; and b) between about 10 percent and about
0 percent by weight of the binder. More preferably the composition
comprises about 95 percent by weight of a polymer and about 5
percent by weight of a binder. One preferred composition includes
about 99 percent by weight of the polymer and about 1 percent by
weight of the binder. The physical characteristics of a combination
of POM polymer, acid and a binder, in the proportions described in
the invention, are well known to those of ordinary skill in the
art. Such combinations are found in a solid form at room
temperature.
[0018] In another preferred embodiment, the composition does not
include binder, and includes 100 percent by weight of the
polyoxymethylene polymer.
[0019] In those embodiments-wherein a binder is included in the
composition, the composition may be prepared by manually combining
the polymer and binder in a manner that provides a relatively
uniform composition. The composition may be prepared by manually
combining the two components in a manner that provides a relatively
uniform composition. The composition may be prepared by
mechanically mixing, stirring or otherwise combining the components
so as to provide a blend of uniform consistency. For example, the
composition may be prepared by blending the binder into the
particulate polymer component. Once a composition of uniform
consistency is achieved, the composition may be molded into any
suitable shape by using relatively low pressure. The blocks which
are often desirable for use in flight data recorders may be
prepared using compression molding techniques. For example blocks
may be prepared by grinding the wax binder below room temperature
into a fine powder, mixing the powdered wax binder with the polymer
at a temperature of from between about 50 and about 95 at a
pressure between about 5,625 psi and about 22,500 psi. Typically,
the higher temperatures and pressures are preferred in the
compression molding of the compositions of the present
invention.
[0020] In a fire or other high temperature environment, the
compositions of the present invention act as a heat shield
initially, by exhibiting thermal inertia. When the composition
reaches the reactivity temperature, i.e., the temperature at which
decomposition of the polymer begins, the composition acts as a heat
sink by utilizing the heat energy of the high temperature
environment to initiate and maintain the endothermic decomposition
reaction of the polymer. The reactivity temperature at which
decomposition of the composition begins is about 140 degrees C. In
contrast, the melting temperature of the POM polymer of the
invention as described herein is at least 172 degrees C. The
reactivity temperature is thus adjusted to be much lower than the
melting temperature so that the polymer decomposes directly from
its solid state into its gaseous components, without passing
through an intermediate liquid state.
[0021] Once the compositions of the present invention reach the
reactivity temperature, the endothermic decomposition reaction
begins. At temperatures equal to or greater than the reactivity
temperature, the polyoxymethylene polymer undergoes a polymer
"unzipping" reaction as follows.
H--(--O--CH.sub.2--).sub.n--OH_nH.sub.2C.dbd.O+H.sub.2O.
[0022] In other words, the polymer decomposes back to its original
monomers, namely formaldehyde, without passing through an
intermediate liquid state. The polymer unzipping proceeds by an
endothermic reaction mechanism. The term "endothermic" is well
defined in the art and refers to reactions which require the input
of heat energy to maintain the reacting mass at a constant
temperature. Thus, the heat absorption composition absorbs heat
from the high temperature environment to initiate and drive the
endothermic decomposition of the polyoxymethylene polymer. The use
of the heat energy to initiate and drive this endothermic reaction
has the effect of maintaining the maximum temperature environment
within the enclosure at an acceptable level which does not cause
significant heat damage to the heat sensitive devices within the
enclosure.
[0023] The formaldehyde product which results from the endothermic
decomposition reaction is flammable. The amount of formaldehyde gas
released from the composition is often significant. Without
venting, the pressure could rise excessively and the heat
dissipation properties would also be lower. To avoid the potential
pressure damage to the heat sensitive devices within the enclosure
as the pressure from the formaldehyde gas increases, a vent is
provided in the enclosure to allow the escape of the generated
formaldehyde gas.
[0024] The expected gas evolution rate can be predicted from the
heat flow anticipated in the final design. The heat flow can be
estimated from the wall type and thickness, the insulation type and
thickness (or thermal conduction coefficient) and the approximate
physical size of the unit. Given the heat flow, the temperature
profile and gas evolution rates can be estimated from each
materials known thermochemical behavior.
[0025] FIG. 1 is a graphical representation of the time versus
temperature characteristics of the compositions of the present
invention as compared to sodium bicarbonate, another new heat
absorption composition, which is described in copending application
Ser. No. 09/052,913, Attorney Docket No. 543-97-004, filed
concurrently herewith. As can be seen from the graph, the
polyoxymethylene polymer compositions of the present invention
absorb more heat than the sodium bicarbonate compositions, which in
turn are more efficient than conventional heat absorption
compositions. The time required for the polyoxymethylene polymer
composition to reach a given temperature in a high temperature
environment is as long or longer than the time required for the
bicarbonate composition to reach the same temperature. This result
demonstrates that the polyoxymethylene polymer compositions are as
effective or more effective than the sodium bicarbonate at
absorbing heat from the high temperature environment.
[0026] A POM polymer composition of the invention begins to
decompose directly into its gaseous components when exposed to a
thermal environment as shown in FIG. 1. The temperature at which
endothermic decomposition begins is about 140 degrees C. The
compositions of the present invention thus have an added advantage
over conventional silica compositions in that they will not loose
water in an unconfined space because the compositions of the
present invention do not melt before endothermic decomposition
reaction of the polymer is complete. The compositions of the
present invention thus normally exist in a solid state and remain
in a solid state during endothermic decomposition. An additional
advantage over hydrated silica composition is ease of formability
and subsequent handling during the manufacturing process.
[0027] The improved heat absorption properties observed with the
polyoxymethylene polymer compositions of the present invention as
compared to conventional heat absorption compositions is a very
important advantage in applications such as flight data recorders.
In the event of a crash, the flight data recorder may be subjected
to temperatures in the range of 1,100 degrees C. for up to an hour.
Thus it is clearly advantageous to use a heat absorption
composition which absorbs more heat energy and thereby provides
greater protection to the heat sensitive devices contained
therein.
[0028] In addition to the foregoing, the compositions of the
present invention provide a number of advantages. The components of
the composition are relatively easy to handle, relatively low in
cost, formable, and more effective at heat absorption as compared
to conventional heat absorption materials. Also, because the
compositions of the present invention absorb heat more efficiently
than conventional heat absorption agents, flight data recorders
including the compositions of the present invention may be smaller
than conventional flight data recorders without sacrificing heat
absorption protection for the heat sensitive devices contained
therein.
[0029] The compositions of the present invention are particularly
useful in enclosures for thermally protecting one or more heat
sensitive devices from a high temperature environment, such as
aircraft flight data recorders which include heat sensitive devices
for recording and storing data. Flight data recorders are known in
the art and are configured to provide a record of various important
aircraft performance parameters over a predetermined time interval.
Flight data recorders record performance parameters continuously
from the time the electrical system of the aircraft is activated.
Because they have a limited memory they can only hold information
relating to such parameters for the predetermined time interval.
Hence, as new data is continuously collected, it is continuously
written over old data. In the event that the flight data recorder
is deactivated, which occurs if the aircraft crashes, the flight
data recorder will retain in memory the performance parameters
recorded over the predetermined time interval immediately prior to
deactivation. In operation, the information stored within a memory
unit is continually supplied by additional components of the flight
data recorder system such as a data acquisition unit that receives
input signals from various aircraft sensors and systems, and
processes those signals to produce signals compatible with the
recording or storage medium employed by the flight data recorder
memory unit.
[0030] The compositions of the present invention may be employed in
any conventional thermally protected flight data recorder,
including but not limited to the flight data recorders described in
U.S. Pat. Nos. 4,694,119 and 4,944,401 both to Gruenwegan, U.S.
Pat. No. 4,600,449 to White et al., and U.S. Pat. No. 5,438,162 to
Thompson et al., the disclosures of which are hereby incorporated
by reference in their entirety. Generally, the thermally protected
flight data recorders are one type of thermally protected
enclosures which include an outer housing, the heat sensitive
devices for recording data, and the heat absorption (i.e., thermal
protecting) composition of the present invention.
[0031] Referring to FIG. 2, the outer housing 15 of the enclosure
10 is typically constructed of a metal that exhibits a high thermal
conductivity and high resistance to crushing and piercing. An
intumescent coating or paint is generally applied to the exterior
surfaces of the outer housing 15 for additional thermal insulation.
The enclosure 10 may be provided in any suitable shape, and is
typically substantially rectangular in cross-section when viewed
perpendicular to each of its major axes. An insulating layer of
solid material that exhibits a relatively low thermal conductivity
adjoins each interior surface of the outer housing 15 to form a
rectangular interior cavity that is centrally located within the
outer housing 15. As noted above, the housing 15 may be designed to
permit the escape of gaseous carbon dioxide which is produced in
the endothermic decomposition of the bicarbonate compound.
[0032] The one or more heat sensitive devices 25 employed in the
enclosure 10 are located within and contained by the interior
cavity of the housing 15. In a preferred embodiment of the
invention, the heat sensitive devices 25 are solid state electronic
memory devices for storing data which is to be recovered from each
of the solid state electronic memory devices following exposure of
the enclosure 10 to a high temperature environment. Examples of
such devices include semiconductor electronically erasable
programmable read-only memory circuits. A data acquisition unit
periodically supplies digital signals which are sequentially
written into the semiconductor memory circuits so that the memory
circuits store a sequence of digital words that is a time sampled
data representation of the history for each parameter being
monitored. Typically, with respect to currently employed
techniques, data compression is generally employed to allow storing
digital signals representative of a 15-30 minute time history for
each monitored parameter.
[0033] To provide the heat shielding necessary to protect these
heat sensitive devices 25 from a high temperature environment to
which the enclosure 10 is subjected, the composition 20 of the
present invention is provided in the open spaces of the interior
cavity of the housing 15, substantially surrounding the heat
sensitive devices 25. The composition 20 occupies at least a
portion of the interior cavity. The compositions 20 of the present
invention are formable under low pressure and at temperatures below
the melting point of either component, as discussed above.
Accordingly, the composition 20 may be molded to both conform to
the shape of the interior cavity and to surround the heat sensitive
devices therein.
[0034] The compositions of the present invention do not combust
within the enclosure because the small amount of oxygen within the
enclosure will be rapidly depleted or expelled. The compositions of
the present invention also absorb radiant heat from any residual
insulation and from the interior surfaces of the enclosure. This
heat is subsequently carried outside the container and disposed of
in the flame front rather than building up inside the small cavity
containing the heat sensitive devices. Furthermore, because the POM
composition does not melt into liquid form before or during
endothermic decomposition, the protective shell shape is retained
until decomposition is complete.
[0035] Polyoxymethylene (POM) polymers as disclosed herein are
well-known and commercially available, for example, under the trade
name DELRIN .COPYRGT. manufactured by du Pont de Nemours and
Company. The chemical and physical characteristics of these POM
polymers are readily known to one of ordinary skill in the art.
[0036] Additionally, the following characteristics of the POM
composition as described herein were known to one of ordinary skill
in the art on or before the filing date of the present
application.
[0037] Catalytic stabilization of POM polymer compositions is
well-known to those of ordinary skill in the art. Such
stabilization increases the decomposition temperature to about 300
degrees C. It is also well-known in the art that the presence of
acids, such as benzoic acid, reduces the decomposition temperature
of POM compositions. In addition, the physical characteristics of a
combination of POM polymer, acid and a binder, in the proportions
described herein, are well known to those of ordinary skill in the
art. Such combinations are found in a solid form at room
temperature.
[0038] Furthermore, a POM composition such as that as described
herein begins to decompose directly into its gaseous components,
without passing through an intermediate liquid state, when exposed
to a thermal environment as shown in FIG. 1. It is known that the
temperature at which endothermic decomposition begins is about 140
degrees C. and that the POM composition as described herein has
melting temperature exceeding 172 degrees C.
[0039] It is also known that a POM composition as described herein,
when exposed to a thermal environment as shown in FIG. 1, retains
its physical form as a solid mass until heated to its reaction
temperature of endothermic decomposition at about 140 degrees C. At
its reaction temperature of endothermic decomposition, the POM
composition begins to decompose directly into its gaseous
components without an intermediate state change from solid to
liquid.
[0040] In addition to their utility as heat absorbing materials,
the compositions of the present invention, particularly in those
embodiments including a binder, are also useful in enclosures such
as flight data recorders as shock absorbing materials. For example,
the enclosure may be subjected to intense shock in the area of
3,400 Gs on impact during crash conditions. Under such physical
impact force, the compositions of the present invention absorb a
portion of the shock by deforming enough to divert the impact shock
wave away from the memory device, but not enough to penetrate any
surrounding insulation creating voids that might become heat flux
paths to the heat sensitive devices.
[0041] Although the essential elements of the enclosure include
only the housing, the heat sensitive devices, and the composition
of the present invention, one skilled in the art will understand
that a number of modifications to the enclosure are possible which
are based upon conventional technology in the art of flight data
recorders, and are therefore contemplated by the instant invention.
For example, it may be desirable to include as another element of
the enclosure, a thermal insulating layer between the compositions
of the present invention and the interior surfaces of the interior
cavity. Suitable thermal insulating liners will be readily
determinable by those skilled in the art and include a shell-like
thermal liner positioned within the interior cavity adjacent to the
interior surfaces of the interior cavity, which conforms to the
geometric shape of the interior cavity. Preferably, the thermal
liner is a unitary structure formed of a solid material that is a
good thermal insulator (i.e., has a low thermal conductivity, K)
and relatively low density. Examples of such materials include but
are not limited to thermal insulators sold under the trade names
MIN-K20000, from Johns-Manville Co., of Denver Colo., and
MICROTHERM .COPYRGT., from Upton-Wirral Merseyside, England.
[0042] The foregoing is illustrative of the present invention and
is not to be construed as limiting thereof. The invention is
defined by the following claims, with equivalents of the claims to
be included therein.
* * * * *