U.S. patent application number 12/365234 was filed with the patent office on 2009-09-17 for method of manufacturing of aerogel composites.
This patent application is currently assigned to Aspen Aerogels, Inc.. Invention is credited to Redouane Begag, George L. Gould, Christopher J. Stepanian.
Application Number | 20090229032 12/365234 |
Document ID | / |
Family ID | 22976306 |
Filed Date | 2009-09-17 |
United States Patent
Application |
20090229032 |
Kind Code |
A1 |
Stepanian; Christopher J. ;
et al. |
September 17, 2009 |
Method of Manufacturing of Aerogel Composites
Abstract
Aerogel composite materials having a lofty fibrous batting
reinforcement preferably in combination with one or both of
individual short randomly oriented microfibers and conductive
layers exhibit improved performance in one or all of flexibility,
drape, durability, resistance to sintering, x-y thermal
conductivity, x-y electrical conductivity, RFI-EMI attenuation,
and/or burn-through resistance.
Inventors: |
Stepanian; Christopher J.;
(Somerville, MA) ; Gould; George L.; (Mendon,
MA) ; Begag; Redouane; (Hudson, MA) |
Correspondence
Address: |
ASPEN AEROGELS INC.;IP DEPARTMENT
30 FORBES ROAD, BLDG. B
NORTHBOROUGH
MA
01532
US
|
Assignee: |
Aspen Aerogels, Inc.
Northborough
MA
|
Family ID: |
22976306 |
Appl. No.: |
12/365234 |
Filed: |
February 4, 2009 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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11419175 |
May 18, 2006 |
7504346 |
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12365234 |
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10034296 |
Dec 21, 2001 |
7078359 |
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11419175 |
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60257437 |
Dec 22, 2000 |
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Current U.S.
Class: |
2/69 ; 2/243.1;
428/99; 442/136 |
Current CPC
Class: |
Y10T 428/249965
20150401; C04B 30/02 20130101; B32B 15/18 20130101; B32B 15/14
20130101; E04B 1/7662 20130101; C04B 2111/503 20130101; D04H 13/00
20130101; B32B 2305/38 20130101; C04B 2111/00258 20130101; B32B
2250/40 20130101; E04B 2001/7687 20130101; B01J 13/0091 20130101;
Y02B 80/14 20130101; C03C 13/00 20130101; Y10T 442/60 20150401;
Y10T 442/622 20150401; Y10T 442/624 20150401; B32B 15/20 20130101;
Y10T 428/24008 20150115; Y10T 442/2475 20150401; Y10T 442/2869
20150401; E04B 9/045 20130101; Y10T 442/2984 20150401; Y02B 80/10
20130101; Y10T 442/20 20150401; B32B 2250/20 20130101; C04B
2111/00413 20130101; H05K 9/009 20130101; Y10T 428/249962 20150401;
D04H 1/413 20130101; Y10T 428/249964 20150401; E04B 1/78 20130101;
Y10T 442/291 20150401; Y02A 30/24 20180101; Y10T 442/2631 20150401;
Y10T 442/2926 20150401; Y10T 442/2861 20150401; Y10T 442/2918
20150401; B32B 15/02 20130101; B32B 2307/718 20130101; Y10T
428/249924 20150401; Y10T 442/2902 20150401; B32B 5/26 20130101;
C04B 14/064 20130101; Y02A 30/243 20180101; C04B 30/02 20130101;
C04B 14/064 20130101; C04B 14/34 20130101; C04B 14/386 20130101;
C04B 32/02 20130101; C04B 30/02 20130101; C04B 14/064 20130101;
C04B 14/34 20130101; C04B 20/006 20130101; C04B 32/02 20130101 |
Class at
Publication: |
2/69 ; 442/136;
428/99; 2/243.1 |
International
Class: |
A41D 1/00 20060101
A41D001/00; B32B 27/12 20060101 B32B027/12; B32B 3/06 20060101
B32B003/06; A41D 27/00 20060101 A41D027/00 |
Claims
1-10. (canceled)
11. A method of preparing a composite article, which comprises:
mixing a gel precursor to a lofty fibrous batting to form a
mixture; transitioning the mixture from sol state to gel state to
obtain a gel; and drying the gel to obtain the article wherein a
continuous aerogel is through the batting.
12. The method of claim 1 wherein the gel precursor is obtained by
acidifying a water soluble, basic metal oxide precursor in water
and optionally removing any salt by-products.
13. The method of claim 1 wherein the gel precursor is obtained
from the group consisting of oxides of silicon, aluminum, titanium,
zirconium, hafnium, yttrium, vanadium and a combination
thereof.
14. The method of claim 1 wherein the gel precursor is obtained
from the group consisting of tetraethoxysilane, tetramethoxysilane,
tetra-n-propoxy-silane, aluminum isopropoxide, aluminum
sec-butoxide, cerium isopropoxide, hafnium tert-butoxide, magnesium
aluminum isopropoxide, yttrium isopropoxide, titanium isopropoxide
and zirconium isopropoxide.
15. The method of claim 1 wherein the gel precursor is a hydrolyzed
or partially hydrolyzed silica precursor.
16. The method of claim 1 wherein the gel precursor is obtained
from the group consisting of polyacrylates, polystyrenes,
polyacrylonitriles, polyurethanes, polyimides, polyfurfural
alcohol, phenol furfural alcohol, melamine formaldehyde, resorcinol
formaldehyde, cresol formaldehyde, phenol formaldehyde, polyvinyl
alcohol dialdehyde, polycyanurates, polyacrylamides, epoxies, agar,
agarose and a combination thereof.
17. The method of claim 1 wherein the density of the batting is
between 0.001 to 0.26 g/cc.
18. The method of claim 1 further comprising the step of mixing
microfibers with the gel precursor.
19. The method of claim 1 wherein the batting may be formed from
the group consisting of fiberglass, quartz, polyester,
polyethylene, polypropylene, polybenzimidazole,
polyphenylenebenzo-bisoxasole, polyetheretherketone, polyarylate,
polyacrylate, polytetrafluoroethylene, poly-metaphenylene diamine,
poly-paraphenylene terephthalamide, ultra high molecular weight
polyethylene, novoloid resins, polyacrylonitrile,
polyacrylonitrile/carbon and carbon.
20. The method of claim 1 further comprising the step of
modification of the surface of the gels.
21. The method of claim 10 wherein surface hydroxyl groups of the
gel are converted to tri-methylsilylethers.
22. The method of claim 2 wherein the water is exchanged with a
polar solvent.
23. The method of claim 1 wherein the gel is dried by supercritical
extraction.
24. The method of claim 12 wherein the gel is dried below the
critical point of the solvent.
25. The method of claim 10 wherein the gel is dried below the
critical point of the solvent.
26. The method of claim 1 wherein the batting comprising a
conductive layer.
27. The method of claim 1 wherein the batting comprising a
metal.
28. The method of claim 1 further comprising the step of adding
dopants to the mixture.
29. The method of claim 18 wherein the dopants are selected from
the group consisting of carbon black, titania, iron oxides, silicon
carbide, molybdenum silicide, manganese oxides and
polydialkylsiloxanes.
30. The method of claim 1 wherein the mixing is performed in a
solvent selected from the group consisting of lower alcohols with 1
to 6 carbons, ethyl acetate, ethyl acetoacetate, acetone,
dichloromethane and the like.
Description
BACKGROUND OF THE INVENTION
[0001] The field of the present invention is aerogel composite
materials. More particularly, this invention is directed to aerogel
composites wherein the resulting composite exhibits improved
performance as compared to prior aerogel composite products in one
or more of the following qualities: reduced aerogel sintering;
higher temperature performance; improved flexibility and
drapeability; improved durability; decreased aerogel particle
shedding; enhanced x-y plane thermal conductivity; enhanced x-y
plane electrical conductivity; enhanced radio frequency
interference (RFI) and/or electromagnetic interference (EMI)
attenuation, enhanced infrared radiation (IR) suppression; and/or
enhanced burn-through resistance. The fiber reinforcement is
preferably a combination of a lofty fibrous structure (batting),
individual randomly oriented short microfibers, and conductive
layers. More particularly both fiber reinforcements are based upon
either organic (e.g. thermoplastic polyester) or refractory (e.g.
silica) fibers.
[0002] Insulating materials have been developed to solve a number
of physical problems. Stiff polymeric foam and fiberglass
insulating boards are well known as insulators for low and high
temperature applications in fields such as refrigeration, building
construction, and heating systems. Flexible battings such as those
made from fiberglass have been used in applications that required
flexibility, low density, and the ability to expand to fill a void
space such as building construction. Aerogels, more specifically
aerogel composites, were developed seeking to combine the strengths
of both classes of materials.
[0003] Aerogels describe a class of material based upon their
structure, namely low density, open cell structures, large surface
areas (often 900 m.sup.2/g or higher) and sub-nanometer scale pore
sizes. Supercritical and subcritical fluid extraction technologies
are commonly used to extract the fluid from the fragile cells of
the material. A variety of different aerogel compositions are known
and may be inorganic or organic. Inorganic aerogels are generally
based upon metal alkoxides and include materials such as silica,
carbides, and alumina. Organic aerogels include carbon aerogels and
polymeric aerogels such as polyimides.
[0004] Low density aerogels (0.02-0.2 g/cc) based upon silica are
excellent insulators, better than the best rigid foams with thermal
conductivities of 10 mW/m-K and below at 100.degree. F. and
atmospheric pressure. Aerogels function as thermal insulators
primarily by minimizing conduction (low density, tortuous path for
heat transfer through the nanostructures), convection (very small
pore sizes minimize convection), and radiation (IR suppressing
dopants may easily be dispersed throughout the aerogel matrix).
Depending on the formulation, they can function well at
temperatures of 550.degree. C. and above. However, in a monolithic
state they tend to be fragile and brittle and are thus not well
suited for most applications outside of the laboratory.
[0005] U.S. Pat. No. 5,306,555 (Ramamurthi et al.) discloses an
aerogel matrix composite of a bulk aerogel with fibers dispersed
within the bulk aerogel and a method for preparing the aerogel
matrix composite. The fibers may be long or short fibers of varying
thicknesses, whiskers, mineral wool, glass wool, and even
particles. The composition of the reinforcing material is an oxide
such as SiO.sub.2 and Al.sub.2O.sub.3 (fibers, whiskers, and wools)
and carbon, metals, and a variety of oxides (particles). Preferred
fibers are glass wool and rock wool. The fibers may be randomly
distributed or oriented. They may also be in the form of individual
fibers, bundles of fibers, mats or sheets, woven or unwoven. The
aerogel matrix composite is substantially crack-free with
substantially no volume shrinkage. The composites are formed by
infiltrating fibrous pre-forms, either woven or non-woven, with gel
precursors, followed by drying of the wet gel under supercritical
conditions. The products can be obtained on the scale of about 3-7
hours, but suffer a major drawback of having a high elastic
modulus, making the products quite stiff as manufactured. The
Ramamurthi et al. articles improve in flexibility as they are
utilized because they form cracks in the aerogel matrix domains. A
second drawback is that the thermal conductivities of the aerogel
matrix composites are also relatively high (18 to 21 mW/m-K at
ambient conditions) compared to the preferred embodiments of this
invention 8.6 to 14 mW/m-K at ambient conditions).
[0006] U.S. Pat. No. 5,789,075 (Frank et al.) appears to describe
the same structure as Ramamurthi et al. after the Ramamurthi et al.
structure is removed from its mold, except that the Frank et al.
composite is intentionally cracked in a controlled manner. The
controlled cracking is said to give additional flexibility to the
resulting composite. Suitable fibers are individual fibers randomly
or ordered, preferably at least 1 cm in length. The fibers may also
be used in the form of a web or mat. A plurality of webs or mats
can be superposed upon one another. In the case of a layered
arrangement of mats, a change in the direction from one layer to
the next is deemed advantageous. Although the Description and
Claims disclose a manufacturing process which includes step (b)
"adding fibers to the sol," the Examples only show the addition of
a non-fiber-containing sol to a polyester or glass fiber web.
Individual randomly distributed fibers are not used in combination
with a fibrous web.
[0007] U.S. Pat. No. 5,972,254 (Sander) is directed to ultra-thin
pre-stressed fiber reinforced aerogel honeycomb catalyst monoliths.
Thin panels or monoliths of aerogels, xerogels, zeolites, and other
low density material are reinforced with pre-stressed fibers in two
of three dimensions. A mixture of metal alkoxides, water, and a
catalyst are poured into a gas permeable mold containing
pre-tensioned reinforcing fibers running perpendicular to each
other at defined intervals, followed by polymerization and
supercritical drying.
[0008] U.S. Pat. Nos. 5,973,015 and 6,087,407 (Coronado, et al.)
describe aerogel composites made from organic precursors, e.g.
formaldehyde, which infiltrate a fiber pre-form. The resultant
composite is said to have mechanical stability. The re-inforcing
fibers described in the figures run lengthwise and are shown to be
planar structures in the figures. The products suffer from
relatively low thermal stability in air under high heat loads as
well as insufficient flexibility for many uses.
[0009] U.S. Pat. No. 6,068,882 (Ryu et al.) disclose aerogel
composite materials previously manufactured and sold by Aspen
Systems, Inc. The aerogel contents of the product were an aerogel
powder rather than an aerogel monolith. Thus flexure of the product
resulted in the shedding of significant quantities of the powder.
The thermal performance was significantly degraded as compared to
aerogel monolith alone. The prior products were stiff and readily
fractured or fragmented.
[0010] Thus the prior aerogel composite materials have not been
suitable for many uses due to one or more of: low flexibility, low
durability, excessive aerogel sintering when exposed to heat, less
than ideal thermal conductivity, insufficient x-y thermal and/or
electrical conductivity, poor RFI-EMI attenuation, and/or
insufficient burn-through resistance.
[0011] The present invention arose from research directed to
resolving these problems. Accordingly, it is an object of the
present invention to produce an improved aerogel composite
structure which exhibits one or more of the following qualities:
low sintering/higher temperature performance; improved flexibility,
exceptionally low thermal conductivity, drapeability, or
conformability; enhanced x-y thermal and/or electrical
conductivity; enhanced RFI-EMI attenuation; and/or enhanced
burn-through resistance.
SUMMARY OF THE INVENTION
[0012] This invention is directed to an aerogel composite which
exhibits improved performance over prior aerogel composites in one
or more of the areas of flexibility, durability, aerogel sintering,
x-y thermal and/or electrical conductivity, RFI and EMI
attenuation, and burn-through resistance.
[0013] More specifically, the invention is directed to a composite
having two parts, namely reinforcing fibers and an aerogel matrix
wherein the reinforcing fibers are in the form of a lofty fibrous
structure (i.e. batting), preferably based upon either
thermoplastic polyester or silica fibers, and more preferably in
combination with individual randomly distributed short fibers
(microfibers). The use of the lofty batting reinforcement minimizes
the volume of unsupported aerogel while generally improving the
thermal performance of the aerogel, rather than degrading it as in
the prior art. Moreover, when an aerogel matrix is reinforced by a
lofty batting material, particularly a continuous non-woven batting
comprised of very low denier fibers, the resulting composite
material at least maintains the thermal properties of a monolithic
aerogel in highly flexible, drapeable form, making the composite
suitable, for instance, for clothing applications.
[0014] Under very high heat loads, such as those generated by
direct surface impingement of a gas/oxygen torch flame, monolithic
aerogels can rapidly sinter and shrink within seconds. When the
aerogels are reinforced by the combination of the lofty fibrous
batting and microfibers, as in one embodiment of this invention,
the rate of shrinkage, sintering, and ultimate failure of the
insulation structure can be delayed by one or more orders of
magnitude time, i.e. increasing burn through from seconds to
hours.
[0015] Still more specifically, an aerogel composite further
including a thermally conductive layer has been found helpful in
improving the thermal performance of the composite. For example,
carbon fiber cloth or two orthogonal plies of unidirectional carbon
fiber placed at the center of a composite provide a thermal
breakthrough barrier under a high heat load, a high degree of IR
opacification, and a thermally dissipative layer structure that
will spread the heat out in the x-y plane of the composite. More
specifically, the thermally conductive layer in the middle, through
the thickness, of the aerogel composite can be selected to have a
minimal effect on the stiffness of the composite. Moreover, if
desired the layer can have malleability or intrinsic conformability
so that the resulting aerogel composite will be conformable, e.g. a
copper wire mesh placed at the interlayer of the aerogel composite
article confers conformability and deformability when the composite
is bent. In addition, the conductive mesh also provides RFI and EMI
resistance.
[0016] These and still further embodiments of the present invention
are described in greater detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 illustrates a general fabrication process of the
present invention.
[0018] FIG. 2 is a view of an aerogel composite of the present
invention.
[0019] FIG. 3 is an exploded view of a 3 layer laminate useful as a
reinforcement material in the present invention.
[0020] FIG. 4 is an exploded view of an alternative 3 layer
laminate useful as a reinforcement material in the present
invention.
[0021] FIG. 5 is an exploded partial view of an aerogel composite
showing the composite reinforced both on a macro level with a fiber
batting and on a micro level with individual filaments.
[0022] FIG. 6 is an exploded view of an alternative 5 layer
laminate useful in the present invention.
[0023] FIG. 7 is a graph of the thermal conductivity of five
manufactured aerogel composites of this invention through a range
of temperatures.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Aerogels are a class of materials formed by removing a
mobile interstitial solvent phase from the pores of a gel structure
supported by an open-celled polymeric material at a temperature and
pressure above the solvent critical point. By keeping the solvent
phase above the critical pressure and temperature during the entire
solvent extraction process, strong capillary forces generated by
liquid evaporation from very small pores that cause shrinkage and
pore collapse are not realized. Aerogels typically have low bulk
densities (about 0.15 g/cc or less, preferably about 0.03 to 0.3
g/cc), very high surface areas (generally from about 400 to 1,000
m.sup.2/g and higher, preferably about 700 to 1000 m.sup.2/g), high
porosity (about 95% and greater, preferably greater than about
97%), and relatively large pore volume (more than about 3.8 mL/g,
preferably about 3.9 mL/g and higher). The combination of these
properties in an amorphous structure gives the lowest thermal
conductivity values (9 to 16 mW/m-K at 37.degree. C. and 1
atmosphere of pressure) for any coherent solid material.
[0025] One of the most attractive uses for aerogels is for passive
insulation bodies to maintain either a constant temperature or a
significant delta temperature between an object and its
surroundings at the lowest possible energy cost. Monolithic aerogel
structures normally have minimal flexibility before failure (e.g.
flexural modulus of 0.5 MPa at a density of 0.1 g/ce for silica
aerogel monolith).
[0026] The aerogel composite material of the present invention
comprises two phases. The first is a low-density aerogel matrix and
the second is a reinforcing phase. This reinforcing phase consists
primarily of a lofty fibrous material, preferably a combination of
the lofty batting and one or more fibrous materials of
significantly different thickness, length, and/or aspect ratio. A
preferred combination of a two fibrous material system is produced
when a short, high aspect ratio microfiber (one fibrous material)
dispersed throughout an aerogel matrix that penetrates a continuous
lofty fiber batting (the second fibrous material).
[0027] The present invention can be seen via FIGS. 1-6. FIG. 1
illustrates the fabrication process of the present invention
wherein a gel precursor 11 is added to a reinforcing batting 12 in
some constraining mold type structure 10. FIG. 2 shows an aerogel
composite 20 of the present invention formed with an inorganic or
organic batting 21 and an aerogel matrix. FIG. 3 shows a gel
precursor mixed with microfiber material being cast into a
continuous lofty fiber batting material to generate the composite
illustrated in FIG. 4.
[0028] The aerogel matrix of the present invention may be organic,
inorganic, or a mixture thereof. The wet gels used to prepare the
aerogels may be prepared by any of the gel-forming techniques that
are well-known to those trained in the art: examples include
adjusting the pH and/or temperature of a dilute metal oxide sol to
a point where gelation occurs (R. K. Iler, Colloid Chemistry of
Silica and Silicates, 1954, chapter 6; R. K. Iler, The Chemistry of
Silica, 1979, chapter 5, C. J. Brinker and G. W. Scherer, Sol-Gel
Science, 1990, chapters 2 and 3). Suitable materials for forming
inorganic aerogels are oxides of most of the metals that can form
oxides, such as silicon, aluminum, titanium, zirconium, hafnium,
yttrium, vanadium, and the like. Particularly preferred are gels
formed primarily from alcohol solutions of hydrolyzed silicate
esters due to their ready availability and low cost (alcogel).
[0029] It is also well known to those trained in the art that
organic aerogels can be made from polyacrylates, polystyrenes,
polyacrylonitriles, polyurethanes, polyimides, polyfurfural
alcohol, phenol furfuryl alcohol, melamine formaldehydes,
resorcinol formaldehydes, cresol formaldehyde, phenol formaldehyde,
polyvinyl alcohol dialdehyde, polycyanurates, polyacrylamides,
various epoxies, agar, agarose, and the like (see for instance C.
S. Ashley, C. J. Brinker and D. M. Smith, Journal of
Non-Crystalline Solids, volume 285, 2001). However, as insulating
articles at high temperatures in oxygen-containing atmospheres,
these materials can burn away and are thus not preferred for this
invention.
[0030] For the sake of convenience the alcogel route of forming
inorganic aerogels is used below to illustrate the invention,
though this is not intended to limit the present invention to any
specific type of aerogel and/or method of preparation. The
invention is applicable to other aerogels and preparation
methods.
[0031] Generally the principal synthetic route for the formation of
an inorganic aerogel is the hydrolysis and condensation of an
appropriate metal alkoxide. The most suitable metal alkoxides are
those having about 1 to 6 carbon atoms, preferably from 1-4 carbon
atoms, in each alkyl group. Specific examples of such compounds
include tetraethoxysilane (TEOS), tetramethoxysilane (TMOS),
tetra-n-propoxysilane, aluminum isopropoxide, aluminum
sec-butoxide, cerium isopropoxide, hafnium tert-butoxide, magnesium
aluminum isopropoxide, yttrium isopropoxide, titanium isopropoxide,
zirconium isopropoxide, and the like. In the case of silica
precursors, these materials can be partially hydrolyzed and
stabilized at low pH as polymers of polysilicic acid esters such as
polydiethoxysiloxane (PDEOS). These materials are commercially
available in alcohol solution (for example Silbond.RTM. 40, 40%
silica content, Silbond Corporation). Pre-polymerized silica
precursors are especially preferred for the aerogel composite
articles of this invention.
[0032] Suitable materials for use in forming the aerogels to be
used at low temperatures are the non-refractory metal alkoxides
based on oxide-forming metals. Preferred such metals are silicon
and magnesium as well as mixtures thereof. For higher temperature
applications, suitable alkoxides are generally refractory metal
alkoxides that will form oxides, e.g. such as zirconia, yttria,
hafnia, alumina, titania, ceria, and the like, as well as mixtures
thereof such as zirconia and yttria. Mixtures of non-refractory
metals with refractory metals, such as silicon and/or magnesium
with aluminum, may also be used. An advantage of using more than
one metal oxide matrix material for the aerogel structure is an
enhancement of IR opacification, achieved by providing chemical
functional groups that absorb radiation at a wider range of
wavelengths.
[0033] Finely dispersed dopants, such as carbon black, titania,
iron oxides, silicon carbide, molybdenum silicide, manganese
oxides, polydialkylsiloxanes wherein the alkyl groups contain 1 to
4 carbon atoms, and the like, may be added to improve thermal
performance at higher temperatures by increasing the opacity of the
article to IR transmission. Suitable amounts of such dopants
generally range from about 1 to 20% by weight of the finished
composite, preferably about 2 to 10%.
[0034] Major variables in the inorganic aerogel formation process
include the type of alkoxide, solution pH, and
alkoxide/alcohol/water ratio. Control of the variables can permit
control of the growth and aggregation of the matrix species
throughout the transition from the "sol" state to the "gel" state.
While properties of the resulting aerogels are strongly affected by
the pH of the precursor solution and the molar ratio of the
reactants, any pH and any molar ratio that permits the formation of
gels may be used in the present invention.
[0035] Generally, the solvent will be a lower alcohol, i.e. an
alcohol having 1 to 6 carbon atoms, preferably 2 to 4, although
other liquids can be used as is known in the art. Examples of other
useful liquids include but are not limited to: ethyl acetate, ethyl
acetoacetate, acetone, dichloromethane, and the like.
[0036] Alternatively, any of the following methods can be utilized
to make an aerogel composite article of this invention, but the
methods that allow for obtaining the lowest density and/or best
thermally insulating articles are preferred. For example, in a
first alternative implementation of gel making, a water soluble,
basic metal oxide precursor can be gelled by acidification in water
to make a hydrogel. Sodium silicate has been widely used for this
purpose. Salt by-products may be removed from the silicic acid
precursor by ion-exchange and/or by washing subsequently formed
gels with water. Removing the water from the pores of the gel can
be performed via exchange with a polar organic solvent such as
ethanol, methanol, or acetone. The resulting dried aerogel has a
structure similar to that directly formed by supercritical
extraction of gels made in the same organic solvent. A second
alternative method entails reducing the damaging capillary pressure
forces at the solvent/pore interface by chemical modification of
the matrix materials in their wet gel state via conversion of
surface hydroxyl groups to tri-methylsilylethers (see U.S. Pat. No.
5,877,100 for example) to allow for drying of the aerogel materials
at temperatures and pressures below the critical point of the
solvent.
[0037] For silica aerogel containing low temperature insulation,
the currently preferred ingredients are tetraethoxysilane (TEOS),
water, and ethanol (EtOH). The preferred ratio of TEOS to water is
about 0.2-0.5:1, the preferred ratio of TEOS to EtOH is about
0.02-0.5:1, and the preferred pH is about 2 to 9. The natural pH of
a solution of the ingredients is about 5. While any acid may be
used to obtain a lower pH solution, HCl, H.sub.2SO.sub.4 or HF are
currently the preferred acids. To generate a higher pH, NH.sub.4OH
is the preferred base.
[0038] For the purposes of this patent, a lofty batting is defined
as a fibrous material that shows the properties of bulk and some
resilience (with or without full bulk recovery). The preferred form
is a soft web of this material. The use of a lofty batting
reinforcement material minimizes the volume of unsupported aerogel
while avoiding substantial degradation of the thermal performance
of the aerogel. Batting preferably refers to layers or sheets of a
fibrous material, commonly used for lining quilts or for stuffing
or packaging or as a blanket of thermal insulation.
[0039] The reinforcing fibrous material used in the present
invention is one or more layers of a lofty fibrous batting. The use
of a lofty batting reinforcement minimizes the volume of
unsupported aerogel while avoiding substantial degradation of the
thermal performance of the aerogel. While generally a "batting" is
a product resulting from carding or Garnetting fiber to form a soft
web of fiber in sheet form, for purposes of this invention
"batting" also includes webs in non-sheet form, e.g. the
Primaloft.RTM. products from Albany International, provided that
they are sufficiently open to be "lofty." Batting commonly refers
to a fibrous material commonly used for lining quilts or for
stuffing or packaging or as a blanket of thermal insulation.
Suitable fibers for producing the batting are relatively fine,
generally having deniers of 15 and below, preferably 10 and below.
The softness of the web is a byproduct of the relatively fine,
multi-directionally oriented fibers that are used to make the fiber
web.
[0040] A batting is "lofty" for purposes of this invention if it
contains sufficiently few individual filaments (or fibers) that it
does not significantly alter the thermal properties of the
reinforced composite as compared to a non-reinforced aerogel body
of the same material. Generally this will mean that upon looking at
a cross-section of a final aerogel composite, the cross-sectional
area of the fibers is less than 10% of the total surface area of
that cross section, preferably less than 8%, and most preferably
less than 5%. The lofty batting preferably has a thermal
conductivity of 50 mW/m-K, or less at room temperature and pressure
to facilitate the formation of low thermal conductivity aerogel
composites.
[0041] Another way of determining if a batting is sufficiently
lofty to be within the scope of this invention is to evaluate its
compressibility and resilience. In this case a lofty batting is one
that (i) is compressible by at least 50% of its natural thickness,
preferably at least 65%, and most preferably at least 80%, and (ii)
is sufficiently resilient that after compression for a few seconds
it will return to at least 70% of its original thickness,
preferably at least 75%, and most preferably at least 80%. By this
definition a lofty batting is one that can be compressed to remove
the air (bulk) yet spring back to substantially its original size
and shape. For example a Holofil.TM. batting may be compressed from
its original 1.5'' thickness to a minimum of about 0.2'' and spring
back to its original thickness once the load is removed. This
batting can be considered to contain 1.3'' of air (bulk) and 0.2''
of fiber. It is compressible by 87% and returns to essentially 100%
of its original thickness. Fiberglass batting used for home
insulation may be compressed to a similar extent and springs back
to about 80% of its original thickness, but does that quite
slowly.
[0042] The batting useful herein is substantially different from a
fibrous mat. A fibrous mat is "a densely woven or thickly tangled
mass," i.e. dense and relatively stiff fibrous structures with
minimal open space between adjacent fibers, if any. While a mat
generally has a density of greater than 25 lbs/ft.sup.3 (0.41
g/cc), a lofty batting useful herein has a much lower density, i.e.
in the range of about 0.1 to 16 lbs/ft.sup.3 (0.001-0.26 g/cc),
preferably about 2.4 to 6.1 lbs/ft.sup.3 (0.04 to 0.1 g/cc).
Generally, mats are compressible by less than about 20% and show
little to no resilience. In an aerogel composite prepared with a
mat reinforcement, the cross sectional surface area of the mat
fibers is about 30 to 50% of the total surface area of the cross
section.
[0043] Preferably the batting retains at least 50% of its thickness
after the gel forming liquid is poured in.
[0044] A way of understanding the need for openness in the fiber
reinforcing material used herein is to recognize that fiber
reinforcements that tend to run along the z axis, (in the direction
of the heat flow) will significantly increase the thermal
conductivity of the resulting composite by acting as thermal
conduits. A batting that has highly aligned (straight) fibers,
particularly in the x-y horizontal plane is stiffer than a typical
lofty batting of the same density with bent or crimped fibers
running in all three axes. In order to minimize heat flow in the z
direction (as is the desire with most insulating materials) the
batting should have low heat flow along the z axis (in the
direction of the heat flow). Thus a suitable batting has a high
enough quantity of fibers oriented along the z axis to maintain
loft, yet not so great a quantity that the insulating properties of
the resulting composite are compromised by these fibers. The fibers
along the z axis may be of a different material (preferably one
with lower thermal conductivity) than those in the x and y axes.
The z axis fibers may also be made more circuitous, so that they
present a more tortuous path for heat conduction than do the fibers
in the x-y direction. The same fiber materials and methods may be
used throughout the batting in an attempt to minimize thermal
conduction in all axes, but in many insulating applications,
however, it is heat flow in a specific direction that is being
addressed, and using such materials and methods may compromise the
flexibility of the resulting composite. The ideal lofty batting is
one with fine, crimped fibers, evenly dispersed throughout the
composite.
[0045] While the composite produced with a lofty batting is
flexible, durable, has a low thermal conductivity and has a good
resistance to sintering, the performance of the aerogel composite
may be substantially enhanced by incorporating randomly distributer
microfibers into the composite, particularly microfibers that will
help resist sintering while increasing durability and decreasing
dusting. The effect of short fiber reinforcement (microfiber) on
the performance of a composite will depend on a number of
variables, such as fiber alignment, diameter, length, aspect ratio
(fiber length/fiber diameter), strength, modulus, strain to
failure, coefficient of thermal expansion, and the strength of the
interface between the fiber and the matrix. The microfibers are
incorporated into the composite by dispersing them in the gel
precursor liquid and then using that liquid to infiltrate the lofty
batting.
[0046] Suitable microfibers useful herein typically range from 0.1
to 100 .mu.m in diameter, have high aspect ratios (L/d>5,
preferably L/d>100), and are relatively uniformly distributed
throughout the composite. Since higher aspect ratios improve
composite performance, the longest microfibers possible are
desired. However, the length of the fibers used herein is
constrained to avoid (or at least minimize) any filtration by the
chosen lofty batting has when a microfiber-containing gel precursor
is infused into the batting. The microfibers should be short enough
to minimize filtration by the lofty batting and long enough to have
the maximum possible effect on the thermal and mechanical
performance of the resulting composite. The micro-fibers preferably
have a thermal conductivity of 200 mW/m-K or less to facilitate the
formation of low thermal conductivity aerogel composites.
[0047] When the microfibers are dispersed in a sol, they often will
rapidly settle. To overcome this problem, a suspension or
dispersion agent that will not deleteriously effect the gel
formation should be added to the sol. Suitable
suspension/dispersion agents include solutions of high molecular
weight block copolymers with pigment affinic groups (Disperbyk-184
and 192 from BYK-Chemie), and the like. The agents need to be
effective during at least the period of time between the dispersion
of the microfiber in the gel precursor and the gelation of the
sol.
[0048] The quantity, type, and/or size and aspect ratio of the
microfibers used within a specific aerogel composite may be varied
to meet specific tasks. For example, an application may involve
insulating regions of different temperatures using a continuous
aerogel composite; the composite may be made such that more
microfibers will be present in the areas of the composite that will
contact the higher temperature regions. Similarly, different
microfibers (e.g. different material, different aspect ratio, size)
may be incorporated in such areas for best insulation performance.
Such microfiber modification may be accomplished by using a variety
of suspension agents and/or microfibers to cause the microfibers to
settle into the composite at different rates and thus in different
locations.
[0049] Suitable fibrous materials for forming both the lofty
batting and the micro-fibers include any fiber-forming material.
Particularly suitable materials include: fiberglass, quartz,
polyester (PET), polyethylene, polypropylene, polybenzimidazole
(PBI), polyphenylenebenzo-bisoxasole (PBO), polyetherether ketone
(PEEK), polyarylate, polyacrylate, polytetrafluoroethylene (PTFE),
poly-metaphenylene diamine (Nomex), poly-paraphenylene
terephthalamide (Kevlar), ultra high molecular weight polyethylene
(UHMWPE) e.g. Spectra.TM., novoloid resins (Kynol),
polyacrylonitrile (PAN), PAN/carbon, and carbon fibers.
[0050] While the same fibrous material may be used in both the
batting and the microfibers, a combination of different materials
may be utilized. One such combination is a lofty fiberglass batting
with carbon microfibers distributed throughout.
[0051] As indicated the combination of batting and microfiber
reinforcement has been found to enhance sintering resistance. This
may be accomplished by incorporating microfibers of a suitable
material, e.g. carbon filaments, within the gel precursor
(generally in combination with a suitable non-reactive dispersing
agent) prior to pouring the gel precursor onto the fibrous batting.
FIG. 5 is an exploded view of such an aerogel composite where the
composite is reinforced on both a macro level with a fibrous
batting 51 and on a micro level with carbon fiber filaments 52.
When dispersed in a silica matrix, carbon microfibers provide a
combination of IR opacification and microscale strengthening that
give a non-refractory metal oxide such as silica greatly improved
thermal and mechanical performance at higher temperatures relative
to non-strengthened and opacified silica.
[0052] In another embodiment of this invention, the lofty
reinforcing fibrous batting is used in the form of a multi-layer
laminate as shown in FIGS. 3, 4, and 6. In addition to including
fibrous material batting, the laminates may include layers of
materials which will help provide specific characteristics to the
final composite structure. For example, the inclusion of a metal
layer in the x-y plane, such as a copper mesh, can improve x-y
thermal and/or electrical conductivity, RFI-EMI attenuation, the
ability to anchor the composite to a support structure, and/or
provide additional physical strength. While any metal can be used
to produce the metal mesh, copper and stainless steel are currently
preferred. Suitable meshes will be made from wires having diameters
ranging from about 0.001 to 0.1 inches, preferably about 0.01 to
0.05, and the wire spacing may range from as tight as a window
screen to 0.3 inches.
[0053] When the additional layer is of a high (>1 W/m-K) thermal
conductivity material such as a carbon fiber, silicon carbide, or a
metal, the resulting composite has been found to exhibit a
significantly enhanced ability to rapidly dissipate heat throughout
the x-y plane of a multilayer composite, further improving the
durability of the composite under a high heat load.
[0054] FIG. 3 shows a 3 layer laminate consisting of a layer of
lofty fiber batting 32, a fine copper mesh 31, and a second layer
of lofty fiber batting 32. FIG. 4 shows another 3 layer laminate of
a layer of lofty fiber batting 42, a woven carbon fiber textile 41,
and a second layer of fiber batting 42. While these laminates are
shown to be symmetric, this is preferred and not mandatory.
[0055] When a metal mesh is used as one or more of the central
layers, it also offers the benefit of producing an aerogel
composite material which is not only drapeable or flexible, but is
also conformable, i.e. it can retain its shape after bending.
[0056] Other approaches to couple the high conductivity layer into
the composite include metal sheet where portions of the sheet are
cut and bent out of plane. These bent portions serve as an anchor
between the conductive layer and the rest of the composite. Metal
foil strips may be similarly utilized, as may a combination of such
materials.
[0057] The conductive layer has a number of secondary benefits.
Aerogel composites containing metal conductive layers, may be
deformed to conform to a shape and hold that shape. The composite
of FIG. 3 can be deformed to both simple and complex curvatures. It
can spring back to a limited extent but also effectively
plastically deformed to hold a shape. Another benefit of the
conductive layer is that it commonly consists of structural
fibers--strong and continuous. This conductive layer can serve as
an anchoring material through which mechanical fasteners may be
driven. The fasteners would grip onto composite or the conductive
layer itself.
[0058] Mechanical loads experienced by the composite can be
transmitted through a metal conductive layer to the fastener and
into other structures. An example of this would be fastening the
aerogel composite onto a vehicle chassis to serve as a heat
barrier. If the reinforcement is adequately magnetized, it may be
attached to a ferrous or magnetic structure without the need of
mechanical fasteners. The heat transmitted by the conductive layer
can be either dumped to the environment and/or a heat sink
(radiation, convection) or used to run secondary processes. For
example, excess heat may be used directly (heating water, etc.) or
converted into electrical energy through a Peltier element or the
like. The design of the aerogel composite can be such that the hot
side of the blanket has a conductive layer near the surface that
directs heat flux to the cold side of the blanket only at points
where Peltier elements are placed. Examples of high thermal
conductivity fibers include graphite, silicon carbide, copper,
stainless steel, aluminum, and the like.
[0059] FIG. 6 is an exploded view of a laminate consisting of a
layer of fiber batting 61, a layer of silicon carbide felt 62, a
fine copper mesh 63, a layer of silicon carbide felt 62, and a
layer of fiber batting 61.
[0060] After identification of the aerogel to be prepared, a
suitable metal alkoxide-alcohol solution is prepared. The
preparation of aerogel-forming solutions is well known in the art.
See, for example, S. J. Teichner et al, Inorganic Oxide Aerogel,
Advances in Colloid and Interface Science, Vol. 5, 1976, pp
245-273, and L. D. LeMay, et al., Low-Density Microcellular
Materials, MRS Bulletin, Vol. 15, 1990, p 19.
[0061] While a single alkoxide-alcohol solution is generally used,
a combination of two or more alkoxide-alcohol solutions may be used
to fabricate mixed oxide aerogels. After formation of the
alkoxide-alcohol solution, water is added to cause hydrolysis so
that a metal hydroxide in a "sol" state is present. The hydrolysis
reaction, using tetra-ethoxysilane as an example, is:
Si(OC.sub.2H.sub.5).sub.4+4 H.sub.2O.fwdarw.Si(OH).sub.4+4
(C.sub.2H.sub.5OH) (1)
To form an aerogel monolith, this sol state alkoxide solution is
then aged for a sufficiently long period (commonly overnight) so
that a condensation reaction (as shown in Eq. 2) occurs and forms
precursors which after supercritical drying become aerogels.
Si(OH).sub.4.fwdarw.SiO.sub.2+2 H.sub.2O (2)
[0062] Further details and explanation of the present invention may
be found in the following specific examples, which describe the
manufacture of aerogel composites in accordance with the present
invention and test results generated there from. All parts and
percents are by weight unless otherwise specified.
EXAMPLE 1
[0063] A section (2'.times.3'.times.1/4'') of polyester
Thinsulate.RTM. Lite Loft insulation from 3M Company was placed in
a walled container. 1300 ml of a commercially available
pre-hydrolyzed silica precursor (Silbond H-5) is mixed with 1700 ml
of denatured alcohol, 95%. The solution is stirred for 15 min. The
solution is then gelled by the slow addition of HF (2% by volume of
the total solution) with stirring. The resulting solution is poured
on the blanket previously placed in a container. Gelation occurs
within a few minutes. The fresh "blanket-gel" is aged overnight in
a sealed bath of ethanol at 50.degree. C. The alcohol trapped in
the gel is removed by subcritical and supercritical CO.sub.2
extraction over the span of four days.
[0064] The resulting aerogel composite has a density of
approximately 0.1 g/cc. The thermal conductivity of the aerogel
composite was determined by a Thin Heater Thermal Conductivity Test
(ASTM C1114-98) to be 16.6 mW/m.degree.K. A pure aerogel monolith
prepared using the same source materials and manufacturing process
had a thermal conductivity of 17.4 mW/m.degree.K.
[0065] The aerogel composite is extremely flexible. It can be
draped over a person's arm without macroscopic failure. This "drape
test" is the equivalent of approximately a 2'' radius of curvature,
180.degree. bend.
[0066] The blanket shows a significant resistance to heat transfer
and thermal degradation/sintering when subjected to propane, a
combination of liquefied petroleum and methylacetylene-propadiene
(MAPP) gas, and oxyacetylene torch flames. If the blanket is
subjected to the heat from the flame on one side, the other side
can be touched by bare skin without damage. The poly-ester batting
alone degrades rapidly when exposed to direct flame. The polyester
batting reinforced aerogel composite resists thermal degradation of
the polyester to a much greater degree (instantaneous vs.
approximately 40 seconds for burn-through for a 1/4'' thick sample
subjected to a propane torch flame). As long as the polyester
remains in the aerogel matrix, the composite remains flexible and
has a low degree of thermal conductivity. Burn-through will occur
if the flame is placed too close to the aerogel composite.
EXAMPLE 2
[0067] The procedure of Example 1 is repeated except that the
polyester fiber batting was replaced by a lofty silica fiber
structure (Quartzel from Saint-Gobain Quartz) having a density of
65 g/m.sup.2 with a polyvinylalcohol binder.
[0068] The resulting silica batting/silica aerogel composite has a
thermal conductivity of 15.0 mW/m-K as tested on a guarded hotplate
(ASTM C-177). The flexibility of the composite is less than that of
the aerogel-polyester blanket of Example 1, but still significant.
It is quite flexible but does not drape to the same extent. The
density of the aerogel composite was 0.113 g/cc. The thickness of
the composite was approximately 3 mm. This composite resists
thermal degradation when exposed to open flame much better than the
product of Example 1.
[0069] Aerogel sintering appears to be minimized by the presence of
the fiber. An oxyacetylene torch is placed 5-6'' below the sample
with the top of the flame impinging on the bottom of the blanket.
Minimal sintering is seen on the bottom surface of the sample after
5 hours of exposure. The top of the sample could be touched with a
bare hand during the test. The temperature on the top of the
aerogel composite varies from 150-230.degree. C. based upon the
distance between the blanket and the flame source. The bottom of
the blanket glows orange-yellow. A single pyrometer reading taken
from the yellow section of the blanket bottom yields a temperature
of 1100.degree. C.
EXAMPLE 3
[0070] The procedure of Examples 1 and 2 is repeated except that
the reinforcement fiber batting is replaced by a 5 layer fiber
laminate of polyester/silicon carbide/-16 copper mesh/silicon
carbide/polyester).
[0071] The thermal conductivity (tested per ASTM C-177) is 12.5
mW/m-K (average). The composite is not very flexible. The thickness
of the laminate is 10.3 mm. The copper mesh improves the x-y
thermal conductivity by spreading out point loads over a larger
area. The copper mesh also provides an EMI-RFI shield. Aerogel
sintering appears to be minimized by the presence of the
reinforcing polyester and silicon carbide fibers.
EXAMPLE 4
[0072] The procedure of Example 3 is repeated except that the
reinforcement consists of a 4 layer laminate of polyester batting,
uni-directional carbon fiber with a polymeric binder, light copper
mesh, and a lofty polyester batting.
[0073] The thermal conductivity (tested per ASTM C-177) is 14.1
mW/m-K (average). The composite has little flexibility. The
thickness of the laminate is 8.0 mm. Aerogel sintering is minimized
by the presence of the reinforcing fibers.
EXAMPLE 5
[0074] The procedure of Example 3 is repeated except that the
reinforcement consists of a 3 layer laminate of silica felt,
stainless steel mesh, and silica felt, about 6'' on a side. A
second reinforced aerogel composite where a copper mesh is
substituted for the stainless steel is also prepared.
[0075] The thermal conductivity (tested per ASTM C-177) is 12.4
mW/m-K (average). The composite is somewhat flexible and is
conformable in that it retains the shape into which it is bent. The
thickness of the laminate is 5.3 mm. Aerogel sintering appears to
be minimized by the presence of the reinforcing fibers in a flame
test using an oxyacetylene torch set at a 6'' distance from the
bottom of the composite and giving approximately a 2 inch diameter
impingement area (glowing red-orange). The temperature is
120.degree. C. at the edge of the composite (thermocouple attached
to the steel mesh through the top), while more than two inches away
from the center of the impingement (though still directly over the
flame) the temperature measures 60.degree. C. at steady state
conditions.
[0076] The aerogel composite with the copper mesh substituted for
the stainless steel mesh shows the same effect.
EXAMPLE 6
[0077] The procedure of Example 2 is repeated except that two
additional ingredients are added to the silica sol. The first is a
low denier carbon fiber (Pyrograf III, Grade PR-11-AG from Pyrograf
Products, Zenia, Ohio. The second is a dispersion agent (Disperbyk
184 from BYK-Chemie). Two grams of carbon fiber and six grams of
dispersion agent are added to 750 ml of ethanol in a 1000 ml
beaker. The beaker is placed in an ice bath and sonicated at full
power by a Misonix 2020 sonicator for one hour to break up fiber
clumps and form a suspension that is visibly stable for at least an
hour. When a drop of the suspension is placed on a glass slide and
allowed to sheet the fibers do not rapidly clump.
[0078] The resulting silica batting/carbon fiber/silica aerogel
composite has a thermal conductivity of 14.8 mW/m-K (ASTM C-177).
The flexibility of the composite is slightly less than that of the
aerogel blanket of Example 2 (Blanket # 2), but still significant.
The aerogel matrix tends to crack in a macroscopic fashion when
strained. The density of the aerogel composite is approximately
0.12 g/cc. The thickness of the composite is approximately 3
mm.
[0079] This composite material resists thermal degradation when
exposed to open flame much better than the aerogel blanket of
Example 2.
[0080] A MAPP gas torch is used as the flame source. The torch,
when applied to the quartz batting alone, would pucker and finally
melt the batting. The MAPP gas torch is used to similar effect with
Blanket #2. If the torch nozzle is placed close to Blanket #2,
eventual degradation/sintering and burn-through occurs. The short
carbon fiber augmented aerogel batting of this Example shows no
degradation other than on the very bottom surface. It can not be
burned through using the MAPP gas torch. The top of the sample can
be touched with a bare hand during the test. The bottom of the
blanket glows orange-yellow-white depending on how far away the
torch is placed. Aerogel sintering appears minimal. The combination
of macro and micro fiber reinforcement works far better than the
macro fiber reinforcement alone.
EXAMPLE 7
[0081] To evaluate the effects of various reinforcing systems on
aerogel composites of one or more embodiments of the present
invention, a series of composites was prepared in accordance with
the procedure of Example 1 but varying the reinforcements. The
aerogel composites are prepared by infiltrating a lofty
reinforcement with an appropriate sol followed by supercritical
drying. FIG. 7 shows the results of thermal performance vs.
temperature for the following samples:
[0082] Sample A used a less than 2 denier lofty polyester batting
where the cross-sectional area of fibers was less than 15% of the
total cross-sectional area of the aerogel composite, and where
after compression the lofty batting returned to 75% of its original
thickness.
[0083] Sample B used quartz wool prepared from 9 .mu.m fibers, with
a batting density of 0.005 g/cc, and where after compression the
lofty batting returned to 75% of its original thickness.
[0084] Sample C used the batting of Sample B, in combination with
5% by weight based upon the total weight of the dried composite
carbon black dopant and 3% (same basis) carbon microfiber. The
carbon black was Cabot Vulcan carbon black. The carbon microfibers
were 0.1 to 100 .mu.m diameter and approximately 1-2 mm in length.
Disperbyk 184 dispersant was used.
[0085] Sample D used the batting of Sample B, in combination with
6% by weight based upon the total weight of the dried composite
carbon black dopant and 4% (same basis) carbon microfiber. The
carbon black was Cabot Vulcan carbon black. The carbon microfibers
were 0.1 to 100 .mu.m diameter and approximately 1-2 mm in length.
Disperbyk 184 dispersant was used.
[0086] Sample E used the batting of Sample B, in combination with
6% by weight based upon the total weight of the dried composite
carbon black dopant, 4% (same basis) carbon microfiber, and 10% by
weight polydimethylsiloxane dopant. The carbon black was Cabot
Vulcan carbon black. The carbon microfibers were 0.1 to 100 .mu.m
diameter and approximately 1-2 mm in length. Disperbyk 184
dispersant was used.
[0087] Sample E has survived more than 100,000 flexure cycles
wherein the material was doubled over upon itself without loss of
thermal performance.
* * * * *