U.S. patent application number 11/960278 was filed with the patent office on 2008-07-10 for electrothermal heater made from thermally conducting electrically insulating polymer material.
This patent application is currently assigned to Goodrich Corporation. Invention is credited to David Louis Brittingham, Daniel Paul Christy, Stanley Gerald Prybyla.
Application Number | 20080166563 11/960278 |
Document ID | / |
Family ID | 39048638 |
Filed Date | 2008-07-10 |
United States Patent
Application |
20080166563 |
Kind Code |
A1 |
Brittingham; David Louis ;
et al. |
July 10, 2008 |
ELECTROTHERMAL HEATER MADE FROM THERMALLY CONDUCTING ELECTRICALLY
INSULATING POLYMER MATERIAL
Abstract
Thermally conductive films, composite materials including the
films, and electrothermal heaters including the films, are
disclosed. The films include a polymer and a sufficient
concentration of hexagonal boron nitride to provide adequate heat
transfer properties, and have high thermal conductivity, peel
strength, and shear strength. The films can include thermoset
polymers, thermoplastic polymers, or blends thereof, and can also
include electrically conductive materials, reinforcing materials
such as fiberglass, carbon fiber, metal mesh, and the like, and
thermally conductive fillers, such as aluminum oxide, aluminum
nitride, and the like. The films can be included in composite
materials. The films can be used as part of a layered structure,
and used in virtually any application, for example, various
locations in aircraft, where heating is desirable, including
nacelle skins, airplane wings, heated floor panels, and the like.
The electrothermal heaters provide a more even heat, and a more
rapid heat, than current resistive heaters formed from metal foils
adhered to an adhesive film.
Inventors: |
Brittingham; David Louis;
(Canton, OH) ; Prybyla; Stanley Gerald;
(Brecksville, OH) ; Christy; Daniel Paul; (Akron,
OH) |
Correspondence
Address: |
WOMBLE CARLYLE SANDRIDGE & RICE, PLLC;GOODRICH CORP.
ATTN: PATENT DOCKETING 32ND FLOOR, P.O. BOX 7037
ATLANTA
GA
30357-0037
US
|
Assignee: |
Goodrich Corporation
Charlotte
NC
|
Family ID: |
39048638 |
Appl. No.: |
11/960278 |
Filed: |
December 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60878629 |
Jan 4, 2007 |
|
|
|
Current U.S.
Class: |
428/411.1 ;
219/202; 528/394 |
Current CPC
Class: |
Y10T 29/49087 20150115;
H05B 2214/04 20130101; Y10T 29/49083 20150115; Y10T 428/31504
20150401; C08J 3/18 20130101; Y10T 29/49099 20150115; C08L 75/04
20130101 |
Class at
Publication: |
428/411.1 ;
528/394; 219/202 |
International
Class: |
B32B 27/06 20060101
B32B027/06; C08G 79/08 20060101 C08G079/08; B60L 1/02 20060101
B60L001/02 |
Claims
1. A film formed from a blend of hexagonal boron nitride and a
polymerizable monomer, wherein the hexagonal boron nitride is
present in a concentration of from about 12 percent to about 40
percent weight, based on total weight of the film.
2. The film of claim 1, wherein the hexagonal boron nitride is
present in a concentration of from about 27 percent to about 33
percent weight, based on total weight of the film.
3. The film of claim 1, wherein the monomer is a thermoset resin
selected from the group consisting of epoxy based resin systems,
matrices of bismaleimide (BMI), phenolic, polyester, PMR15
polyimide, acetylene terminated resins, acrylics; polyurethanes,
and free-radically induced thermosetting resins.
4. The film of claim 1, wherein the monomer is an epoxy resin.
5. The film of claim 4, wherein the epoxy resin is selected from
diglycidyl ethers of bisphenol A (2,2-bis(4-hydroxyphenyl)propane)
or sym-tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)methane,
bisphenol, F, tetrabromobisphenol A, their polyepoxide condensation
products, cycloaliphatic epoxides, epoxy-modified novolacs
(phenoli-formaldehyde resins) and the epoxides derived from the
reaction of epichlorohydrin with analine, o-, m- or p-aminophenol,
and methylene dianaline.
6. The film of claim 1, further comprising an electrically
conductive material.
7. The film of claim 6, wherein the electrically conductive
material is selected from the group consisting of metal powders,
metal-coated microspheres, metal-coated carbon-nanotubes, carbon
nanofibers, carbon nanotubes, graphite nanoplatelets, copper
screen, and aluminum screen.
8. The film of claim 6, wherein the electrically conductive
material is present in a concentration of from about 10 percent to
about 60 percent by weight of the hexagonal boron nitride.
9. The film of claim 6, wherein the electrically conductive
material does not provide electrical conductivity.
10. The film of claim 1, wherein the polymerizable monomer forms a
thermoplastic polymer.
11. The film of claim 10, wherein the thermoplastic polymer is
selected from polyetheretherketone (PEEK), polyetherketone (PEK),
polyphenylene sulfide (PPS), polyethylene sulfide (PES),
polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone
(PS), polycarbonate (PC), polyphenylene ether/oxide, nylons,
aromatic thermoplastic polyesters, aromatic polysulfones,
thermoplastic polyimides, liquid crystal polymers, and
thermoplastic elastomers.
12. The film of claim 10, wherein the hexagonal boron nitride is
present in concentrations of from about 5 percent to about 50
percent by weight of the thermoplastic polymer.
13. A composite material comprising a film and a thermally
insulating layer, wherein the film is formed from a blend of
hexagonal boron nitride and a polymerizable monomer, and wherein
the hexagonal boron nitride is present in a concentration of from
about 12 percent to about 40 percent weight, based on total weight
of the film.
14. The composite material of claim 13, further comprising an
electrically conductive layer between the thermally insulating
layer and the film.
15. The composite material of claim 13, further comprising an
additional layer overlying the film, wherein the additional layer
is capable of dissipating energy from a lightning strike.
16. A method for forming an thermally conductive film, comprising
the steps of blending hexagonal boron nitride and a polymerizable
monomer wherein the hexagonal boron nitride is present in a
concentration of between about 12 and about 40 percent weight;
forming a layer of the resulting blend; and polymerizing the
polymerizable monomer to form the thermally conductive film.
17. A method for forming an thermally conductive film, comprising
the steps of blending hexagonal boron nitride and a melted
thennoplastic polymer wherein the hexagonal boron nitride is
present in a concentration of from about 12 to about 40 percent
weight, based on total weight of the film; forming a layer of the
resulting blend; and cooling the melted polymer to form the
thermally conductive film.
18. A method of electrothermally heating an airplane component,
comprising the steps of providing a thermally conductive film to
the airplane component, wherein the thermally conductive film
comprises hexagonal boron nitride and a polymer; attaching at least
one conductor to the thermally conductive film, wherein the
conductor is further attached to a converter; and supplying
electrical energy to the film via the converter and conductor so as
to heat the airplane component.
19. The method of claim18, wherein the thermally conductive film is
rectangular in shape with a length substantially greater than the
width for covering at least a portion of a leading edge of an
airfoil.
20. The method of claim 18, wherein the airplane component is
selected from the group consisting of at least one nacelle,
fuselage, wing and stabilizer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is generally in the field of polymeric
films or composite materials including hexagonal boron nitride,
composite materials formed from the films, and electrothermal
heaters and/or heat sinks formed from the films or composite
materials.
[0003] 2. Description of the Related Art
[0004] There are currently various electrothermal de-icing or
anti-icing products for leading edge ice protection of aero
surfaces. These typically use heating elements or electrodes
disposed at a leading edge surface of the aero structure in the
form of a serpentine or interdigitated finger area grid to deliver
heat to any ice formed on the surface. Typical electrothermal
de-icing or anti-icing products utilize an embedded heating
element, which is located below the external surface. Heat
generated by electrothermal element must pass through a thick layer
of thermally insulating polymer matrix composite prior to transfer
of heat to the surface-ice interface.
[0005] It would be advantageous to provide new electrothermal ice
protection systems which facilitate the rapid transfer of thermal
energy to the surface-ice interface, providing energy savings and
enhanced functionality. It would further be advantageous to provide
electrothermal ice protection systems rugged enough to withstand
exposure to an aero structure operational environment, and capable
of shedding ice from an aero surface at safe voltages and power
levels, ideally before the ice accretes to produce any appreciable
thickness. The present invention provides such systems.
SUMMARY OF THE INVENTION
[0006] Films comprising a polymer and a sufficient concentration of
hexagonal boron nitride to provide adequate heat transfer
properties for use in electrothermal heating applications are
disclosed. Composite materials including these films, and
electrothermal heaters formed from these composite materials, are
also disclosed.
[0007] The films comprise a polymer and hexagonal boron nitride.
The polymer can be a thermoset polymer, a thermoplastic polymer, or
a blend thereof. The film can also include other components, for
example, electrically conductive materials, including
nanoparticulate materials such as carbon nanotubes, carbon
nanofibers, metal nanowires, metal-coated glass microbubbles,
graphene sheets, and the like. These materials are typically
present in a range of from about 10 percent to about 60 percent by
weight of the hexagonal boron nitride. Also, the films can be
reinforced with various materials, such as fiberglass, carbon
fiber, metal mesh, and the like, and thermally conductive fillers,
such as aluminum oxide, aluminum nitride, and the like, can
optionally be added.
[0008] The films can be formed by blending polymerizable monomers
and hexagonal boron nitride, spreading the blend into a film
thickness, and curing the monomers. In some embodiments, the blend
is fairly viscous at room temperature, for example, the viscosity
of paste or putty, and can be warmed to temperatures above room
temperature to thin the blend to make it easier to spread. The
films can also be formed by blending thermoplastic polymers and
hexagonal boron nitride, and casting the blend into a film. In one
aspect, the thermoplastic polymers are melted or softened before
blending, or the blend is heated to an elevated temperature to
facilitate spreading. The melted polymers can be cooled to form the
resulting film.
[0009] The films ideally have suitable physical properties for
inclusion in laminates subjected to a wide range of environmental
conditions. These properties include high thermal conductivity,
peel strength, and shear strength. The thermal conductivity must be
suitably high to permit rapid heating. The peel strength of the
film must be suitably high to prevent delamination. The shear
strength must also be relatively high for the film to have
desirable properties.
[0010] In one aspect, the thermally conductive films are included
in a composite material that includes an insulating layer, an
electrically conductive film layer, and thermally conductive film
layer which includes hexagonal boron nitride. As current is passed
through the electrically conductive film layer, and the layer heats
up, the heat passes through the electrically conductive layer but
not, at least to a significant extent, through the insulating
layer. Thus, this composite material can be used in electrothermal
heating applications.
[0011] In another aspect, the thermally conductive films include,
in addition to the hexagonal boron nitride, sufficient electrically
conductive materials, such as electrically conductive
nanoparticulate materials, such that the films can both produce
heat and conduct the heat. When a composite material including this
type of film and a thermally insulating film is used, it can
perform substantially the same function including the thermally
insulating layer, the electrically conductive layer, and the
thermally conductive layer, but with one less layer.
[0012] In a third aspect, the thermally conductive film includes,
in addition to the hexagonal boron nitride, electrically conductive
materials, in an amount sufficient to increase the thermal
conductivity of the layer, but insufficient to cause the layer to
be electrically conductive.
[0013] Thus, depending on the desired use, the films can act as
resistors, and not conduct electricity, or they can conduct
electricity, where the degree of conductance of the film can be
modulated by including, in addition to the hexagonal boron nitride,
various amounts of electrically conductive nanoparticulate
materials.
[0014] The films, and composite materials including the films, can
be included as part of the composite material used to form a wing,
nacelle, or other outer surface of an aeroplane and serve to de-ice
the wing, nacelle, or other outer surface. Alternatively, the films
and composites can be used in other heating applications, for
example, floor panels, water tanks, pipes, and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 illustrates one embodiment of a composite material
comprising a thermally conductive film of the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0016] In accordance with the present invention, thermally
conductive films, composite materials formed from the films,
composite materials including the films, and electrothermal heaters
including the composite materials, are all described herein. The
present invention will be better understood with reference to the
following detailed description.
[0017] In one embodiment, the composition that can be formed into a
thermally conductive film comprises a polymerizable monomer and
hexagonal boron nitride. In another embodiment, the composition
comprises a meltable thermoplastic resin and hexagonal boron
nitride.
[0018] The boron nitride powder used to prepare the films described
herein is not limited by any particular type of crystalline system,
shape and size of powder grain, cohesion degree of powder particle
or particle distribution. With respect to the crystalline form,
boron nitride powder of hexagonal, cubic, wurtzite, rhombohedral,
or any other crystal forms can be used. Among them, hexagonal boron
nitride powder of the commercially available hexagonal form, which
provides heat conductivity of roughly 10 to 100 W/mK or so, or of
cubic structure presenting an extremely high heat conductivity of
1300 W/mK maximum, can be used.
[0019] The particle shape of hexagonal boron nitride powder is not
limited to scaled, or flat shape, but hexagonal boron nitride
powder of various particle forms such as granular, lump, spheric,
fiber, whisker-shaped hexagonal boron nitride powder, or ground
products of these can be used. The particle diameter of hexagonal
boron nitride powder can vary, however, the individual average
primary diameter in the range of 0.01 to 100 .mu.m, or more
preferably, in the range of 0.1 to 20 .mu.m can be used. No
practical limit is found with regard to the minimum hexagonal boron
nitride particle size, while a hexagonal boron nitride powder of
larger than 100 .mu.m is difficult to produce and is not easily
entrained into thin polymer films. When scaled hexagonal boron
nitride powder is used, a range of 0.5 to 50 .mu.m as maximum
diameter can be easily blended into the film, and may be oriented
using a magnetic field, if desired. Additionally, hexagonal boron
nitride powder in the form of cohered primary particles can be
used.
[0020] In some embodiments, the hexagonal boron nitride has a
bi-modal particle size, whereby the bimodal particle size allows
for more intimate contact of particles, and enhanced thermal
conductivities, even at lower overall particle content.
[0021] The concentration of hexagonal boron nitride powder in the
heat conductive film is typically in the range of from about 12
percent to about 40 percent by weight, based on the total weight of
the film. In one embodiment, the film comprises nanoparticulate
materials, in which case a range of from about 0.2 percent to about
2.0 percent by weight of hexagonal boron nitride powder is
preferred. If more than these preferred amounts are used, the
viscosity of the composition increases and the fluidity decreases,
making the handling difficult. Also, air bubbles can enter making
it difficult to form a film while the shear strength and other
physical properties can suffer.
[0022] The hexagonal boron nitride can be, but need not be,
aligned. The alignment of the particles can be performed, for
example, using a magnetic field using means known to those of skill
in the art. Also, the hexagonal boron nitride particles can be
surface treated prior to use using means known to those of skill in
the art.
[0023] The films typically include thermoset monomers capable of
forming thermoset polymers. Conventional thermoset resin systems
which can be used to form the films include, for example, epoxy
based resin systems, matrices of bismaleimide (BMI), phenolic,
polyester, PMR-15 polyimide, acetylene terminated resins, acrylics,
polyurethanes, free-radically induced thermosetting resins, and the
like. As a result of such considerable choices in thermosetting
resins, the primers, paints and/or films of the invention can be
tailored as desired.
[0024] Suitable epoxy resins include those used in established
thermoset epoxy/fiber reinforced prepregs used in manufacturing
aircraft components. They are frequently based, inter alia, on one
or more of diglycidyl ethers of bisphenol A
(2,2-bis(4-hydroxyphenyl)propane) or sym-5
tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)methane,
bispheniol F, tetrabromobisphenol A, their polyepoxide condensation
products, cycloaliphatic epoxides, epoxy-modified novolacs
(phenol-formaldehyde resins) and the epoxides derived from the
reaction of epichlorohydrin with analine, o-, m- or p-aminophenol,
and methylene dianaline.
[0025] The epoxy resin systems contain epoxy curing agents which
cure the resin to a solid, infusible product. For this purpose,
epoxy curing agents which are acidic, neutral or alkaline may be
used. Examples include, among others, amine hardeners, phenols,
acid anhydrides, polyamides and Lewis acids and bases. Accelerators
may also be used to decrease the cure time and include imidazoles
and substituted ureas.
[0026] The amount of the hardener employed is usually
stoichiometrically equivalent on the basis of one amine group per
epoxy group in the resin, Some adjustment of the stoichiometry may
be required with the addition of the nanoreinforcement.
[0027] The hexagonal boron nitride and, optionally, other
components, can be added to thermoset monomer, hardener, or mixed
resin. The method of dispersion will depend on when the hexagonal
boron nitride is added. For example, if the hexagonal, boron
nitride is added to a B-staged resin, the high viscosity may
require heating and ultrasonic dispersion or high shear mixing. The
hexagonal boron nitride can also be deposited onto the film surface
using heat or adhesive to hold it in place during composite
processing.
[0028] The hexagonal boron nitride can also be added to a
thermoplastic polymer formed from thermoset monomers for
consolidation with a thermoplastic structure, or bonding with a
thermoset structure. In a preferred embodiment, the hexagonal boron
nitride is present in concentrations of from about 5 percent to
about 50 percent by weight based on the weight of the thermoplastic
polymer. Conventional thermoplastic systems which can be used
include, for example, polyetheretherketone (PEEK), polyetherketone
(PEK), polyphenylenel sulfide (PPS), polyethylene sulfide (PES),
polyetherimide (PEI), polyvinylidene fluoride (PVDF), polysulfone
(PS), polycarbonate (PC), polyphenylene ether/oxide, nylons,
aromatic thennoplastic polyesters, aromatic polysulfones,
thennoplastic polyimides, liquid crystal polymers, thermoplastic
elastomers, and the like.
[0029] The hexagonal boron nitride can be added to the
thermoplastic, which can then be mixed with a thermoset, before
cure or vice versa. The hexagonal boron nitride can be added to one
thermoplastic, which is then mixed with another thermoplastic
(e.g., pellets made and then extruded simultaneously).
[0030] In addition to the polymer and hexagonal boron nitride, the
composition used to form the film can include additional
components, for example, those which enhance the strength of the
film and those which enhance the electrical conductivity of the
film.
[0031] Examples of materials that enhance the strength of the film
include materials commonly found in pre-pregs, such as carbon
fibers, fiberglass, metal wires or mesh, and the like. Examples of
materials used to enhance the conductivity of the film include
nanoparticulate materials, which are described in more detail
below.
[0032] Electrically conductive materials added to the hexagonal
boron nitride containing film may allow the film to double as both
the heat source and as a means for delivering heat quickly to the
ice interface. In this configuration it is desirable to provide
rapid heat transfer to the ice interface by placing an electrically
insulating, thermally conducting layer between the heat source and
the external skin, while placing an electrically and thermally
insulating layer on the backside to prevent heat losses away from
the ice interface. In total, this configuration directs thermal
energy to the ice interface.
[0033] An embodiment of this configuration would be the use of
hexagonal boron nitride particles in combination with a carbon
fiber or woven carbon fabric containing pre-preg. The combination
of hexagonal boron nitride and carbon fabric will allow for rapid
and efficient transfer of energy away from the carbon fabric heat
source. The ability to rapidly transfer heat and equally rapidly
stop the transfer of heat to the ice interface provides the
simultaneous benefits of efficient ice shedding and limiting of
runback and refreezing of shed ice.
[0034] In one aspect, the films include powders, such as copper
powder, carbon nanotubes or nanofibers which are also known as a
type of multi-walled carbon nanotubes (collectively, carbon
nanotubes), and which are distributed throughout the carrier,
ideally in a substantially homogenous fashion.
[0035] In this aspect, the powders typically have a particle size
in the range of 4 nm to 100 .mu.m. The particles can be irregular
in shape or, smooth and round, or have texture. One example of a
suitable textured particle is a "spiky" copper powder where the
carbon nanotubes are embedded into the copper. While not wishing to
be bound by a particular theory, it is believed that the presence
of the carbon nanotubes brings the CTE of the polymer closer to
that of the metal powder, such that the material can conduct
sufficient electrical energy to provide sufficient heat energy.
[0036] In one embodiment, a heating element having the capacity to
carry up to 10 amperes, leading to watt densities at the ice
interface of up to 30 watt/in.sup.2 for anti-icing operation is
provided.
[0037] In one embodiment, the heating element is separate from the
thermally conducting, electrically insulating layer and the
thermally conducting, electrically insulating layer lies between
the heating element and the surface skin to enable rapid conduction
of heat to the ice/surface interface.
[0038] In a second embodiment, the heating element consists of a
combination of electrically conductive and thermally conductive
materials. In this case, the heating element may be thermally
conductive and optionally electrically conductive. Alternatively,
the heating element may be electrically conductive, and a second
material present along with the heater may be thermally conductive,
electrically conductive, or a combination of both. In this second
embodiment, a thermally conducting, electrically insulating layer
separates the heater from the surface skin and the ice
interface.
[0039] In a third embodiment, a mixture of thermally and
electrically conductive nanoparticles is mixed with hexagonal boron
nitride in such a manner that the conductive nanoparticles do not
reach their percolation limit, i.e. they do not touch each other
and therefore form a discontinuous network. However, the thermal
conductivity of the nanoparticles provides a synergistic effect of
enhancing the overall thermal conductivity of the mixture beyond
that attainable without the presence of the thermally and
electrically conductive nanoparticles.
[0040] The film can be reinforced with carbon nanotubes (CNT),
carbon nanofibers (CNF) or graphite nanoplatelets. The carbon
nanotubes can be present in as little as from about 0.1 percent to
about 5.0 percent by weight of the surface film. The CNT weight can
be optimized to match the CTE of the hexagonal boron nitride film,
to minimize microcracking and allow the use of a lighter surface
film.
[0041] In another aspect of the invention, the film comprises metal
coated particles, for example, silver-coated, hollow glass
microspheres and/or metal-coated carbon nanotubes (collectively,
silver coated particles). While not wishing to be bound by a
particular theory, it is believed that the metal coating helps with
the dispersion of the particles within the film. In one embodiment,
a silver coating is applied onto carbon nanotubes by electroless
plating, which is believed to improve the interfacial adhesion of
the composites to which the material is applied. The metal-coated
particles can be subjected to pretreatments such as oxidation,
sensitizing treatment and activation treatment, which can introduce
various functional groups on the particles. These functional groups
can improve the dispersion of the particles into the film, increase
the number of activated sites, and lower the deposition rate.
[0042] In another aspect, carbon nanotubes only can be added to the
polymer film. The 10,10 armchair configuration carbon nanotube has
a resistivity close to copper and it is six times lighter than
copper, and accordingly may be a preferred nanotube. The nanotubes
may be aligned through various methods, including mechanical,
chemical, and magnetic methods. For example, the nanotubes can be
mixed with the polymerizable monomer and extruded into a film
coating. The feed screw can be vibrated to improve the alignment of
fibers in the flow direction (similar to vibration injection
molding used with recycled thermoplastics). The nanotubes can be
functionalized to react with the tail or head of each nanofiber
such that it will self-assemble (similar to lipid bi-layer
assembly). This would require optimizing the nanotube loading so
that the nanotubes attract each ether, while also ensuring that the
epoxy does not interfere with the process. Finally, the nanotubes
can be made such that a nickel particle is attached to one end.
Ferrous alloy nanoparticles and carbon nanotubes (with the nickel
particle) can be added to the adhesive, primer, or paint and
subjected to a magnetic field to align the nanotubes.
[0043] Metal nanorods/nanowires/nanostrands (collectively called
nanowires) can also be used. Carbon nanotubes or others
nanoparticles that have been modified to decrease the resistivity
can be added to the film. These modified nanotubes can be oriented
in-plane to replace or reduce the metal screen.
[0044] The use of carbon nanotubes, the metal powder/carbon
nanotube blends, low density metal screens reinforced with carbon
nanotubes, metal-coated particles and/or aligned graphite
nanoplatelets can provide electrical conductivity to the thermally
conductive film.
[0045] The compositions described above can be formed into
thermally conductive films. The manner in which the films are
formed depends, in part, on the nature of the compositions, i.e.,
whether they include thermoplastic materials or curable thermoset
resins.
[0046] In one aspect, the films are formed by blending
polymerizable monomers and hexagonal boron nitride, spreading the
blend into a film thickness, and curing the monomers. In some
embodiments, the blend is fairly viscous at room temperature, for
example, the consistency of paste of putty, and can be warmed to
temperatures above room temperature to thin the blend to make it
easier to spread.
[0047] In another aspect, the films are formed by blending
thermoplastic polymers and hexagonal boron nitride, and casting the
blend into a film. In one aspect, the thermoplastic polymers are
melted or softened before blending, or the blend is heated to an
elevated temperature to facilitate spreading.
[0048] The pre-cured composition comprising the hexagonal boron
nitride, and, optionally, nanoparticulate conductive materials, can
be placed on a surface of a composite structure via conventional
means, such as knife coating or doctor blading. In one embodiment,
the hexagonal boron nitride and polymerizable monomer blend is
spread into a thin layer and the monomers are polymerized.
[0049] If the film includes too much hexagonal boron nitride, the
film can suffer from relatively low shear strength. Alternatively,
if the film does not include sufficient hexagonal boron nitride,
there is not sufficient thermal conductivity. For this reason, the
amount of hexagonal boron nitride in the film is ideally in the
range of from about 12 percent to about 40 percent by weight, based
on the total weight of the film. Preferably, the concentration of
hexagonal boron nitride in the film is from about 27 percent to
about 33 percent by weight, based on the total weight of the film.
The hexagonal boron nitride provides the film with electrical
conductance, which advantageously is between 10.sup.+10 and
10.sup.+15 ohm-cm.
[0050] The rate at which the film provides heating and/or cooling
can be important. By uniformly mixing the hexagonal boron nitride
in the polymer material, there is a high degree of uniformity and a
relatively high heat-up rate. In addition to having a relatively
high thermal conductivity, the material also has a relatively high
"heat-up" rate. In one embodiment the material has a thermal
conductivity of 4.33 W/mK, which is very high. The heat-up rate,
which is proportional to thermal conductivity, is rapid.
[0051] Although virtually any polymer can be used to form the
films, there are some situations where the films will be subjected
to extreme heat variations. For example, when used in aerospace
applications, temperature variations from 120.degree. F. to
-140.degree. F. might be observed. The film must be capable of
avoiding delamination under this type of temperature variation. In
these embodiments, epoxy resins are preferred for forming such
films. Representative epoxy resins include, but are not limited to,
diglycidyl ethers of bisphenol A
(2,2-bis(4-hydroxyphenyl)propane)orsym-tris(4-hydroxkyyphenyl)propane,
tris(4-hydroxyphenyl)methane, bisphenol F, tetrabromobisphenol A,
their polyepoxide condensation products, cycloaliphatic epoxides,
epoxy-modified novolacs (phenol formaldehyderesins) and the
epoxides derived from the reaction of epichlorohydrin with analine,
o-, m- or p-aminophenol, and methylene dianaline
[0052] In one embodiment, the thermally conductive films of the
present invention can form, in part, composite materials. The
composite materials described herein include various layers
laminated to the thermally conductive film layer. These composite
materials typically include superposed sheets, layers and plies. As
illustrated in FIG. 1, the composite material 100 includes an
insulating layer 110, an electrically conductive layer 120, and an
thermally conductive layer 130 in one embodiment. Additional
materials can be adhered over or under these layers, and in some
embodiments, the electrically conductive layer 120 is also the
thermally conductive layer 120. When used in aircraft applications,
the composite can be covered, for example, with one or more metal
layers used to form the "skin" of the aircraft, or, alternatively,
can include a further layer of a composite material, such as a
carbon fiber layer.
[0053] As shown in the embodiment illustrated in FIG. 1, a
composite layer with no electrical conductivity and low thermal
conductivity (an insulating layer 110) typically lies beneath the
electrically conductive layer 120. Insulating layers 110 provide
insulation with respect to electricity and, also, ideally, with
respect to heat.
[0054] Electrically conductive layers 120 provide the heat that the
thermally conductive layer 130 transfers to the outer surface.
These layers typically lie beneath, in some cases, directly
beneath, the thermally conductive layer 130, except in those
embodiments where the thermally conductive layer 130 is also an
electrically conductive layer 120. The electrically conductive
layer 120 can include components commonly present in typical
electrothermal heaters and their constructions, including metal
wires, foils, and mesh, which can be buried in fiberglass or other
thermally nonconducting materials.
[0055] The thermally conductive layer 130 has two faces, one of
which is in contact with a source of heat, and another face which
is or is in contact with a surface to be heated. The thermally
conducting films described herein can enhance the performance of
these heaters, by increasing the speed in which the heat is
transferred. The composite materials described herein can be used
to prepare aircraft fuselage or an aircraft component.
[0056] Additional layers can include various prepregs, fabrics,
honeycomb core, foam core, resin and adhesive layers. The structure
may be fabricated using dry fabrics which are infused with resin
using resin film infusion or resin transfer molding. The
electrically conductively layer may also be laid up dry and infused
with the polymer during composite fabrication. One or more of the
layers in the composite material can be woven materials including
fiberglass, aramid, carbon prepreg, or other fibers, and/or can
include non-woven layers. In one embodiment, a layer capable of
dissipating energy from a lightning strike (i.e., a lightning
strike protection layer) is placed over the thermally conductive
layer.
[0057] The films, or composite materials including the films, can
be used as components of electrothermal heaters, which can be used
for example, as an electro-thermal ice protection system for an
airfoil. The films can be used as part of a layered structure, and
applied in virtually any location in an aircraft where heating is
desirable. For example, the film can be one layer of a nacelle
skin, or on a wing. The film can be used to provide heated floor
panels.
[0058] Current resistive heaters formed from metal foils adhered to
an adhesive film, with a pattern etched out to form ribbons,
provide uneven heating. In contrast, electrothermal heaters formed
from the films described herein provide even heat. Indeed, by
having the hexagonal boron nitride spread out evenly, the heat is
also spread out evenly.
[0059] When the films are included in electrothermal heaters, the
heaters can include an integral parting strip, with the film
configurable to cover at least a portion of a leading edge of the
airfoil with the integral parting strip disposed along an
air-stagnation zone of the leading edge, and a controller coupled
electrically to the film for controlling electrical energy from a
power source to the film in accordance with a pulse duty-cycle and
for controlling power to the parting strip of the heater to
maintain the air-stagnation zone virtually free of ice
formation.
[0060] The electrothermal heaters can be prepared by preparing the
films, as described above, in a rectangular shape having a length
substantially greater than the width, suitable for covering at
least a portion of a leading edge of the airfoil. Conductor wires
can be attached to the film, or to a metal layer, such as a copper
layer, adjacent to the film layer. The wires can be attached, for
example, at each edge by using bus bars.
[0061] Ideally, the heater includes a converter, powered by a
suitable power source, for supplying electrical heating energy to
the heater over source and return lines which are electrically
isolated from the power source. The converter ideally prevents the
electrical heating energy from being conducted through the
conductive structure of the airfoil. An airplane can include a
plurality of these electrothermal heaters, each heater covering a
segment of a leading edge of the airfoil, with a controller coupled
electrically to each of the heaters for multiplexing electrical
energy from a power source among the plurality of heaters in
accordance with a pulse duty-cycle. The plurality of heaters can
include parting strip areas comprising a multiplicity of
differently shaped island areas disposed on the heater surfaces
within the parting strip area, where each island area can be
separated from the other island areas by the surface of the
electrothermal heater.
[0062] In one embodiment, the electrothermal heater is coupled to a
conductive structure of the airfoil for distributing or dissipating
electrical energy of a lightning strike from the region through the
conductor to the conductive structure.
[0063] The electrothermal heaters and composite materials described
herein can be incorporated in or used to replace some or all of the
composite materials in aircraft components such as nacelles,
fuselage, wings, stabilizers, and other surfaces in need of
de-icing.
[0064] The heaters and materials can also be present in water
heaters, air heaters, heated floor panels, electrothermal ice and
erosion protection, potable water systems, hoses, pipes, ducting,
walls, ceilings, heated seats, heating pads, aid other articles
that require protection from freezing or where the presence of heat
would provide comfort and/or safety. Such articles are well known
to those skilled in the art.
[0065] Methods for manufacturing a composite material including the
thermally conducting films and/or electrothermal heaters described
herein are also disclosed. In one aspect of the invention, the
methods involve forming a composite material without a thermally
conductive layer, forming the thermally conductive films, and
adhering the film to the remainder of the composite material, for
example, using an adhesive layer or in-situ cure.
[0066] The film thickness can range from 0.003 in to 0.010 in, and
the thickness can be controlled using known methods for forming
polymer films, such as calendaring, using a doctor blade, and the
like. In those embodiments where a UV-polymerizable material is
used, the polymerization can be effected using ultraviolet light,
and in other embodiments, the polymerization reaction can be
facilitated by exposing the forming film to heat. The thermoset
film can be polymerized in-situ with the thermoset composite layers
or adhesively bonded secondarily. A thermoplastic film can be
heated and pressed with thermoplastic composite layers, or bonded
using resistance or ultrasonic welding and the like. The film can
also be adhesively bonded to thermoset or thermoplastic layers. The
film can be laid up with dry fabric layers or prepreg for
subsequent infusion of the polymer using resin transfer molding or
resin infusion.
[0067] While the present invention has been described herein above
in connection with a plurality of aspects and embodiments, it is
understood that these aspects and embodiments were presented by way
of example with no intention of limiting the invention.
Accordingly, the present invention should not limited to any
specific embodiment or aspect, but rather construed in breadth and
broad scope in accordance with the recitation of the claims
appended hereto.
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