Note: Descriptions are shown in the official language in which they were submitted.
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HYDROPHOBIC AEROGELS
FIELD
The present disclosure relates to hydrophobic aerogels and methods of making
hydrophobic aerogels. In particular, the methods of the present disclosure
include
combining a hydrophobizing agent with an aerogel precursor prior to gelation
rather than
adding a hydrophobizing agent to an existing gel.
BACKGROUND
Aerogels are a unique class of ultra-low-density, highly porous materials. The
high porosity, intrinsic pore structure, and low density make aerogels
extremely valuable
materials for a variety of applications including insulation. Low density
aerogels based
upon silica are excellent insulators as the very small convoluted pores
minimize
conduction and convection. In addition, infrared radiation (IR) suppressing
dopants may
easily be dispersed throughout the aerogel matrix to reduce radiative heat
transfer.
Escalating energy costs and urbanization have lead to increased efforts in
exploring more effective thermal and acoustic insulation materials for
pipelines,
automobiles, aerospace, military, apparel, windows, houses as well as other
appliances and
equipment. Silica aerogels also have high visible light transmittance so they
are also
applicable for heat insulators for solar collector panels.
Aerogels tend to be very hygroscopic due to the presence of hydroxyl groups on
the surface. Unmodified aerogels absorb water and other organic solvents
adversely
affecting desired properties (e.g., surface area, porosity, and density)
thereby degrading
performance (e.g., thermal insulation). However, many applications of aerogels
require
exposure to water or atmospheric moisture. Therefore, methods are needed to
prepare
aerogels having hydrophobicity at ambient conditions as well as over a range
of
temperature and pressure conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an SEM image of the aerogel of Example 25.
FIG. 2 is an SEM image of the hydrophobic aerogel of Example 27.
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SUMMARY
In one aspect, the present disclosure provides methods of preparing a
hydrophobic aerogel. Such methods comprise forming a surface-modified metal
oxide gel
from a sol comprising a solvent, a metal oxide precursor, and a hydrophobic
surface
modifying agent; and drying the gel to form the hydrophobic aerogel.
In some embodiments, the methods further comprise solvent-exchanging the
hydrophobic, aerogel precursor with an alkyl alcohol to form a hydrophobic
alcogel. In
some embodiments, the methods further comprise supercritically drying the
alcogel to
form the hydrophobic aerogel.
In some embodiments, the solvent comprises water. In some embodiments, the
solvent comprises an alkyl alcohol.
In some embodiments, the metal oxide precursor comprises an organosilane. In
some embodiments, the organosilane comprises a tetraalkoxysilane, optionally
wherein the
tetraalkoxysilane is selected from the group consisting of tetraethoxysilane,
tetramethoxysilane, and combinations thereof. In some embodiments, the
organosilane
comprises an alkyl-substituted alkoxysilane, optionally wherein the alkyl-
substituted
alkoxysilane comprises methyltrimethoxysilane. In some embodiments, the
organosilane
comprises a pre-polymerized silicon alkoxide, optionally wherein the pre-
polymerized
silicon alkoxide comprises a polysilicate.
In some embodiments, the molar ratio of the hydrophobic surface modifying
agent to the metal oxide precursor is no greater than 1. In some embodiments,
the molar
ratio of the hydrophobic surface modifying agent to the metal oxide precursor
is at least
0.2.
In some embodiments, the sol comprises at least two moles of water per mole of
metal oxide precursor. In some embodiments, the sol further comprises an acid,
optionally
wherein the acid is hydrochloric acid.
In some embodiments, the methods further comprise applying the mixture to a
substrate prior to forming surface-modified metal oxide gel. In some
embodiments, the
substrate is non-woven substrate. In some embodiments, the substrate is a
bonded web.
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In another aspect, the present disclosure provides aerogel articles made
according to the methods of the present disclosure.
In yet another embodiment, the present disclosure provides hydrophobic
aerogels made by the methods of the present disclosure.
The above summary of the present disclosure is not intended to describe each
embodiment of the present invention. The details of one or more embodiments of
the
invention are also set forth in the description below. Other features,
objects, and
advantages of the invention will be apparent from the description and from the
claims.
DETAILED DESCRIPTION
There are two main processes for making aerogels. The first process involves
the hydrolysis and condensation of a metal oxide precursor (e.g., alkoxysilane
precursors)
followed by supercritical drying. This process typically yields monolithic
aerogels. The
second process is a waterglass-based synthesis route that typically yields
powders, beads,
or granules.
A typical method for making aerogels hydrophobic involves first making a gel.
Subsequently, this preformed gel is soaked in a bath containing a mixture of
solvent and
the desired hydrophobizing agent in a process often referred to as surface
derivatization.
For example, United States Patent No. 5,830,387 (Yokogawa et al.) describes a
process
whereby a gel having the skeleton structure of (SiO2)n was obtained by
hydrolyzing and
condensing an alkoxysilane. This gel was subsequently hydrophobized by soaking
it in a
solution of a hydrophobizing agent dissolved in solvent. Similarly, United
States Patent
No. 6,197,270 (Sonada et al.) describes a process of preparing a gel having
the skeleton
structure of (SiO2)m from a water glass solution, and subsequently reacting
the gel with a
hydrophobizing agent in a dispersion medium (e.g., a solvent or a
supercritical fluid).
Due in part to the very fine pore structure of the gel, mass transfer occurs
by
diffusion alone, and the rate of penetration of the hydrophobizing agent into
the gel is
slow. Therefore, it often takes many hours for the hydrophobizing agent to
penetrate
throughout the skeletal structure of the gel. In addition, large amounts of
solvent may be
required to complete the process.
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Commonly used hydrophobizing agents typically cannot be added prior to
gelation because such agents inhibit, alter, or completely prevent gelation.
However, the
methods of the present disclosure allow for the hydrophobizing agent to be
included prior
to gelation without significantly affecting gel times. In some embodiments,
the
hydrophobizing agent provides a catalyst for gelation.
Generally, the methods of the present disclosure begin with a sol comprising a
solvent, a metal oxide precursor, and a hydrophobic surface modifying agent.
In some
embodiments, the solvent comprises water. In some embodiments, one or more
organic
solvents such as an alkyl alcohol may be used. In some embodiments, the sol
may include
both water and one or more organic solvents, e.g., a water/alkyl alcohol
blend.
The methods of the present invention are not particularly limited to specific
metal oxide precursors. In some embodiments, the metal oxide precursor
comprises an
organosilane, e.g., a tetraalkoxysilane. Exemplary tetraalkoxysilanes include
tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). In some embodiments,
the
organosilane comprises an alkyl-substituted alkoxysilane, e.g.,
methyltrimethoxysilane
(MTMOS). In some embodiments, the organosilane comprises a pre-polymerized
silicon
alkoxide, e.g., a polysilicate such as ethyl polysilicate.
Generally, during the gel formation process, the hydrophobic surface modifying
agent combines with the skeletal structure formed by the metal oxide precursor
to provide
a hydrophobic surface. In some embodiments, the hydrophobic surface modifying
agent is
covalently bonded to the metal oxide skeleton. In some embodiments, the
hydrophobic
surface modifying agent may be ionically bonded to the metal oxide skeleton.
In some
embodiments, the hydrophobic surface modifying agent may be physically
adsorbed to the
metal oxide skeleton.
Generally, the hydrophobic surface modifying agent comprises two functional
elements. The first element reacts with (e.g., covalently or ionically) or
absorbs on to the
metal oxide skeleton. The second element is hydrophobic. Exemplary hydrophobic
surface modifying agents include organosilane, organotin, and organophosphorus
compounds. One exemplary organosilane is 1,1,1,3,3,3-hexamethyldisilazane
(HMDZ).
Following gel formation, the solvent is removed, drying the gel to form a
hydrophobic aerogel. Generally, any known gel drying technique may be used. In
some
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embodiments, the gel may be supercritically dried using, e.g., supercritical
carbon dioxide.
After solvent removal, the resulting material is typically referred to as an
aerogel.
In some embodiments, a solvent exchange step may precede the drying step.
For example, it may be desirable to replace water present in the initial sol
with other
organic solvents. Generally, any known method of solvent exchange may be used
with the
methods of the present disclosure. Generally, it may be desirable to replace
as much water
as possible with the alternate organic solvent. However, as is commonly
understood, it
may be difficult, impractical, or even impossible to remove all water from the
gel. In
some embodiments, the exchange solvent may be an alkyl alcohol, e.g., ethyl
alcohol.
After solvent exchange with an organic solvent, the resulting gel is often
referred to as an
organogel as opposed to a hydrogel, which refers to gel wherein the solvent is
primarily
water. When the exchange solvent is an alkyl alcohol, the resulting gel is
often referred to
as an alcogel.
In some embodiments, the molar ratio of the hydrophobic surface modifying
agent to the metal oxide precursor is no greater than 1, e.g., no greater than
0.8, or even no
greater than 0.6. In some embodiments, the molar ratio of the hydrophobic
surface
modifying agent to the metal oxide precursor is at least 0.2, e.g., at least
0.3.
In some embodiments, the sol comprises at least two moles of water per mole of
metal oxide precursor. In some embodiments, the sol comprises 2 to 5, e.g., 3
to 4, moles
of water per mole of metal oxide precursor.
In some embodiments, the sol further comprises an acid. In some embodiments,
the acid is an inorganic acid, e.g., hydrochloric acid. In some embodiments,
the sol
comprises between 0.0005 and 0.0010 moles of acid per mole or metal oxide
precursor. In
some embodiments, comprises between 0.0006 and 0.0008 moles of acid per mole
or
metal oxide precursor.
In addition to forming hydrophobic aerogels, the methods of the present
disclosure may be used to form aerogel articles, e.g., flexible aerogel
articles. For
example, in some embodiments, the sol may be applied to a substrate prior to
forming a
gel. Gelation, solvent exchange (if used), and drying may then occur on the
substrate.
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In some embodiments, the substrate may be porous, e.g., a woven or nonwoven
fabric. Exemplary substrates also include bonded web such as those described
in U.S.
Patent Application No. 11/781,635, filed July 23, 2007.
Examples
Test Methods
Brunauer, Emmett, and Teller (BET). Surface area was measured via BET
analysis, conducted using an AUTOSORB-1 model AS1 MP-LP instrument and
associated software (AS1Win version 1.53) available from Quantachrome
Instruments
(Boynton Beach, FL). Sample material was placed in a 9 mm sample tube with a
uniform
initial weight of approximately 0.0475 grams. The samples were degassed for at
least 24
hours at 80 C prior to analysis. Nitrogen was used as the analyte gas. The
BJH method
was applied to desorption data to determine pore volume and diameter.
Hydrophobicity. A small sample was placed in a 4.5 ounce jar containing
deionized water at room temperature (about 22 C). If the samples remained
floating after
30 minutes, it was judged to be hydrophobic. If the sample was not floating
after 30
minutes, it was judged to be non-hydrophobic.
Bulk Density. To enable measurement of bulk density, aerogel cylinders were
synthesized within plastic syringes with one end cut off. Once gelled, the
aerogel cylinder
was extracted from the syringe using the syringe plunger and dried. The
diameter and
length of the dried cylinders were measured and the volume calculated. The
weights of
the samples were measured on an analytical balance. The bulk density was then
calculated
from the ratio of weight to volume.
Skeletal Density. The skeletal density was determined using a Micromeritics
ACCUPYC 1330 helium gas pycnometer. The instrument uses Boyle's law of partial
pressures in its operation. The instrument contains a calibrated volume cell
internal to the
instrument. The sample was placed in a sample cup, weighed and inserted into
the
instrument. The sample was pressurized in the instrument to a known initial
pressure.
The pressure was bypassed into the calibrated cell of the instrument and a
second pressure
recorded. Using the initial pressure, the second pressure, and the volume of
the calibrated
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cell, the skeletal volume of the sample was determined. The skeletal density
was then
determined from the skeletal volume and the sample weight.
Porosity. The percent porosity was calculated from the measured bulk density
(Pbulk) and the and skeletal density (Pskeletal) using the following formula:
porosity(%) = 1- Pbulk x100
'skeletal
Thermal Conductivity. Thermal conductivity was measured at a mean
temperature of 12.5 C using a LASERCOMP "Fox200" instrument.
Supercritical Fluid Drying. The sample was weighed and placed in a permeable
cloth bag sealed with a draw string. The bag containing the sample was placed
inside a
stainless steel chamber. The bottom and top of this chamber were fitted with
metal frits
and O-rings. This chamber was inserted into a vessel rated to handle high
pressure (40
MPa (6000 psig)). The outside of this vessel was heated by a jacket.
Carbon dioxide was chilled to less than minus 10 degrees Celsius and pumped
with a piston pump at a nominal flow rate of one liter per minute through the
bottom of the
unit. After ten minutes, the temperature of the unit was raised to 40 C at a
pressure of
10.3 MPa (1500 psig). The carbon dioxide is supercritical at these conditions.
The drying
period was conducted for a minimum of seven hours. After the drying period,
the carbon
dioxide flow was ceased and the pressure was slowly decreased by venting the
carbon
dioxide. When the pressure was at 370 kPa (40 psig) or lower, the now dry
samples were
removed and weighed.
Examples 1-6: TEOS-based aerogels with pre-hydrolyzation and surface
treatment prior to gelation.
A stock solution was prepared by mixing 209.39 grams of tetraethoxysilane
(TEOS, 99+%) (Alfa Aesar) with 234.95 grams of ethanol (EtOH, 200 proof)
(Aaper
Alcohol), 54.09 grams of deionized water (H20) and 0.701 grams of 1 Molar
hydrochloric
acid (1M HC1) Q.T. Baker) in a round bottom flask fitted with water cooled
reflux
condenser. The mixture was heated to 70 C for 1 hour under constant stirring.
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1,1,1,3,3,3-hexamethyldisilazane (HMDZ) was used as a silylating/surface
modifying agent to render the silica gel hydrophobic. In principle, other
silylating agents
can also be used for this purpose. The silylating agent here performs the dual
role of
modifying the surface and providing ammonia upon reaction with water, which
acts as a
catalyst for the hydrolysis and condensation of the silica precursor.
For each synthesis, 49.57 grams of the stock solution was transferred to a
glass
jar. Under vigorous stirring using a stir bar, 1,1,1,3,3,3-
hexamethyldisilazane (HMDZ,
99+%) (Alfa Aesar) was added to the mixture. Gel time was defined as the
required after
mixing all reactants for the sol to substantially cease moving even when the
glass jar is
moved or inverted. In some cases, a stop watch was used to measure the gel
time. The
start time was the point at which all the reactants were mixed. The point at
which the stir
bar stopped moving was taken as the end time. If the gel continues to move
slowly then
the gel quality is weak, otherwise the gel was considered strong. Those
examples which
resulted in gels were solvent exchanged three times with 75 ml of EtOH. After
the final
solvent exchange, the samples were supercritically dried.
The molar ratios of the various reactants and the resulting gel times and gel
characteristics are shown in Table 1. Example 1 gelled but was not hydrophobic
indicating insufficient surface treatment. Examples 2-4 gelled in less than
one minute and
were hydrophobic. In the case of Example 5, even though gelation occurred in
less than
one minute, the gel quality was poor and hence the sample could not be
supercritically
dried. Example 6 did not gel.
Table 1: Formulations, gel times, and gel characteristics for Examples 1-6
TEOS Ratio of moles per mole TEOS Gel gel
Ex. (moles) H2O EtOH HC1 HMDZ time characteristic
1 1 3 5 0.0007 0.033 47 sec. (a) strop
2 1 3 5 0.0007 0.2 14 sec. strong
3 1 3 5 0.0007 0.33 12 sec. strong
4 1 3 5 0.0007 0.5 10 sec. strong
5 1 3 5 0.0007 1 11 sec. weak
6 1 3 5 0.0007 2 did not gel did not gel
(a) Seconds ("sec.").
The characteristics of Examples 1-5 are shown in Table 2. The surface areas
and densities of Examples 2-4 are typical of aerogels. These examples clearly
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demonstrate a process by which silica aerogels can be prepared in the presence
of surface
modifying agents in a time efficient manner when an appropriate amount of HMDZ
is
used for surface modification.
Table 2: Characteristics of the aerogels of Examples 1-5.
surface pore bulk skeletal
porosity
Ex. area volume density density (o) hydrophobic
m2/ (cc/g) (g/cc) (g/cc)
1 N/A N/A N/A N/A N/A No
2 684 3.0 0.36 1.51 76 Yes
3 569 2.1 0.27 1.59 83 Yes
4 554 1.8 0.35 1.73 80 Yes
N/A N/A N/A N/A N/A Yes
5 Examples 7-14: TEOS-based aerogels with pre-hydrolyzation and surface
treatment prior to gelation
Examples 7-14 were prepared in a manner similar to Examples 1-6 except that
the H20/TEOS and EtOH/TEOS molar ratios were varied. Table 3 shows that the
gel
time trends for Examples 7-14 are similar to those for Examples 1-6. Examples
8 and 12
did not gel. Example 9 was not hydrophobic due to insufficient surface
treatment, while
Examples 7, 10, 11, 13, and 14 were hydrophobic. As summarized in Table 4,
Examples
11 and 13 exhibited characteristic aerogel surface areas and densities.
Table 3: Formulations and gel times for Examples 7-14.
TEOS Moles per mole of TEOS gel
Ex. (moles) H2O EtOH HC1 HMDZ time
7 1 3.5 5 0.0007 0.5 > 15 min. (b)
8 1 3.5 5 0.0007 2 did not el
9 1 4 5 0.0007 0.033 25 sec.
10 1 4 5 0.0007 0.5 4 sec.
11 1 4 5 0.0007 0.33 3 sec.
12 1 4 5 0.0007 2 did not gel
13 1 3 6 0.0007 0.33 14 sec.
14 1 3 7 0.0007 0.33 21 sec.
(b) Minutes ("Min.").
Table 4: Characterization results for Examples 11 and 13.
surface area Pore volume bulk density skeletal porosity
Ex. m2/ (cc/g) (g/cc) density (g/cc)
11 612 2.5 0.22 1.50 86
13 596 2.4 0.32 1.44 77
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Examples 15-20: TEOS-based aerogels without pre-hydrolyzation but with
surface treatment prior to gelation.
Tetraethoxysilane (TEOS, 99+%) (Alfa Aesar) was mixed with 1,1,1,3,3,3-
hexamethyldisilazane (HMDZ, 99+%) (Alfa Aesar) in a glass beaker to prepare
solution
A. In another beaker, ethanol (EtOH, 200 proof) (Aaper Alcohol), deionized
water (H20)
and 1 molar hydrochloric acid (1M HC1) (J.T. Baker) were mixed to form
solution B.
Solution B was added instantaneously to Solution A under vigorous stirring,
such that the
vortex formed by stirring approached the bottom of the container. The molar
ratios of the
various reactants and the gelation times for these mixtures are listed in the
Table 5. For
Examples 15-20, the molar ratio of H20/TEOS and EtOH/TEOS was kept constant at
3
and 5, respectively. Those examples which resulted in gels were solvent
exchanged three
times with 75 ml of EtOH. After the final solvent exchange the samples were
supercritically dried.
Examples 15-17 did not gel within 15 minutes; however, gelation did occur
after
several days. Examples 18-20 did not gel even after several days (samples were
observed
for a period of two weeks). Higher HMDZ/TEOS ratios (> 0.5) resulted in no
gelation or
very long gel times.
Table 5: Formulations, gel times, and gel characteristics for Examples 15-20
TEOS Moles per mole of TEOS gel gel
Ex. (moles) H2O EtOH HC1 HMDZ time characteristics
15 1 3 5 0.0007 0.033 days strong, opaque
16 1 3 5 0.0007 0.2 days strong, translucent
17 1 3 5 0.0007 0.33 days strong, opaque
18 1 3 5 0.0007 0.5 did not el clear
19 1 3 5 0.0007 1 did not gel clear
1 3 5 0.0007 1.3 did not el clear
Example 16 was hydrophobic and had a surface area of 453 m2/g, a pore volume
20 of 2.5 cc/g, a bulk density of 0.26 g/cc, a skeletal density of 1.48 g/cc,
and a porosity of
83%.
Examples 21-24: TEOS-based aerogels without pre-hydrolyzation, but with
surface treatment prior to gelation.
Solutions A and B were prepared and mixed as described for Examples 15-20.
The molar ratios of the various reactants and the gelation times for these
mixtures are
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listed in the Table 6. For Examples 21-24, the molar ratios of H20/TEOS and
EtOH/TEOS were varied while the molar ratio of HMDZ/TEOS was held constant at
0.33.
Those examples which resulted in gels were solvent exchanged three times with
75 ml of
EtOH. After the final solvent exchange the samples were supercritically dried.
No gelation was observed within 15 minutes for Examples 21-24; however,
gelation occurred several days later. The resulting gels were hydrophobic.
Table 6: Formulations, gel times, and gel characteristics for Examples 21-24
Ex. TEOS Moles er mole of TEOS gel gel
(mol) H2O EtOH HC1 HMDZ time characteristics
21 1 4 5 0.0007 0.33 > 15 min. opaque
22 1 7 5 0.0007 0.33 > 15 min. opaque
23 1 3 6 0.0007 0.33 > 15 min. opaque
24 1 3 7 0.0007 0.33 > 15 min. opaque
Examples 25-29: TMOS-based aerogels without pre-hydrolyzation, but with
Surface Treatment prior to Gelation.
Tetramethoxysilane (TMOS, 98+%) (Alfa Aesar) was mixed with 1,1,1,3,3,3-
hexamethyldisilazane (HMDZ, 99+%) (Alfa Aesar) in a glass beaker to prepare
solution
C. In another beaker Methanol (MeOH, 99.8%) (J.T. Baker), deionized water
(H20) and
1 Molar Hydrochloric acid (1M HC1) (J.T. Baker) were mixed to form solution D.
These
solutions were cooled in a dry ice bath. Solution D was added instantaneously
to Solution
C under vigorous stirring, such that the vortex formed by stirring approached
the bottom
of the container. The molar ratios of the various reactants and the gelation
times for these
mixtures are listed in the Table 7. Those examples which resulted in gels were
solvent
exchanged three times with 75 ml of MeOH. After the final solvent exchange the
samples
were supercritically dried.
Except for Example 29, all examples gelled in 1 minute or less. Example 29
shows longer gelation time and had a higher HMDZ/TMOS molar ratio. This is
consistent
with examples shown above for TEOS-based gels where higher HMDZ/TEOS ratios
also
resulted in longer gel times and, in some cases, no gelation was observed.
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Table 7: Formulations, gel times, and gel characteristics for Examples 25-29.
Ex. TMOS moles per mole TMOS gel gel
(moles) H2O EtOH HCl HMDZ time characteristics
25 1 3 5 0.0007 0.033 1 min. clear, strong
26 1 3 5 0.0007 0.2 15 sec. clear, strong
27 1 3 5 0.0007 0.33 15 sec. translucent, strong
28 1 3 5 0.0007 0.5 15 sec. translucent, strong
29 1 3 5 0.0007 1 > 15 min. opaque, weak
Characteristics of the aerogels of Examples 25-29 are summarized in Table 8.
The surface areas and densities are characteristic of TMOS-based aerogels.
Example 25
was not hydrophobic and had a low HMDZ/TMOS molar ratio, which is consistent
with
results for TEOS-based aerogels. Examples 26-29 were all hydrophobic.
Table 8: Characteristics of the aerogels of Examples 25-29.
surface pore bulk skeletal
porosity
Ex. area volume density density (o) hydrophobic
m2/ (cc/g) (g/cc) (g/cc)
25 N/A N/A 0.22 1.88 89 No
26 N/A N/A 0.20 1.64 88 Yes
27 503 2.6 0.17 1.64 90 Yes
28 N/A N/A 0.14 1.78 92 Yes
29 N/A N/A N/A N/A N/A Yes
A scanning electron microscope was used to obtain images of the aerogels.
Figure 1 is an image of the aerogel of Example 25. Figure 2 is an image of the
hydrophobic aerogel of Example 27.
Examples 30-41: TEOS-based aerogels derived from a commercially-available
pre-hydrolyzed sol, surface treated prior to gelation.
Ethyl polysilicate containing 45-47 wt% Si02 (SILBOND 50 from Silbond
Corporation) was mixed with 1,1,1,3,3,3-hexamethyldisilazane (HMDZ, 99+%)
(Alfa
Aesar) in a glass beaker to prepare solution E. In another beaker ethanol
(EtOH, 200
proof) (Aaper Alcohol), deionized water (H20) and 1 molar hydrochloric acid
(1M HC1)
(J.T. Baker) were mixed to form solution F. Solution F was added
instantaneously to
Solution E under vigorous stirring, such that the vortex formed by stirring
approached the
bottom of the container. Those examples which resulted in gels were solvent
exchanged
three times with 75 ml of EtOH. After the final solvent exchange the samples
were
supercritically dried.
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All the compositions in Table 9 are listed in weight (grams). Generally, very
low concentrations of HMDZ led to long gel times (e.g., Examples 30 and 34).
In
contrast, samples with very high concentrations of HMDZ (e.g., Example 33)
showed no
gelation. For higher concentrations of water (e.g., Example 35), the gel time
is the
shortest (2 minutes). A trend similar to Examples 30-37 was observed in the
case of
Examples 38-41 where no hydrochloric acid was used in the syntheses.
Table 9: Formulations (in grams), gel times, and gel characteristics for
Examples 30-41
Ex. polysilicate time characteristics
30 20 3.8 16.4 0.073 0.35 15 rain, clear, strong
31 20 3.8 16.4 0.068 3.54 3 nmijio clear, strong
32 20 3.8 16.4 0.067 5.64 5 Inin. clear, strong
33 20 3.8 16.4 0.071 11.1 did not el N/A
34 20 3.8 16.4 0.072 0.36 20 nmiji. translucent strong
35 20 3.8 16.4 0.069 3.55 2 n:m_m . clear, strong
36 20 3.8 23.4 0.073 0.36 25 non. clear, weak
37 20 3.8 23.4 0.067 3.55 4 nmin. clear, strong
38 20 3.8 16.4 0 3.59 3 min. clear, strong
39 20 3.8 23.4 0 3.55 5 min. clear, strop
40 20 3.8 16.4 0 0.36 18 ini . clear, strong
41 20 3.8 16.4 0 5.63 5 inin. clear, strong
Generally, the aerogels were hydrophobic and had surface area and densities
characteristic of aerogels. For example, the aerogel of Example 31 was
hydrophobic and
had surface area of 690 m2/g, a pore volume of 1.9 cc/g, a bulk density of
0.14 g/cc, a
skeletal density of 1.48 g/cc, and a porosity of 71 %.
Examples 42-49: TEOS-based aerogels prepared from a pre-hydrolyzed sol
subject to further pre-hydrolysis and surface treatment prior to gelation.
Ethyl polysilicate containing 45-47 wt% Si02 (SILBOND 50 from Silbond
Corporation) was mixed with ethanol (EtOH, 200 proof) (Aaper Alcohol),
deionized water
(H20) and 1 molar hydrochloric acid (1M HC1) (J.T. Baker) in a glass jar. The
mixture
was heated at 50 C for 15 minutes under constant stirring. While vigorously
stirring,
different amounts of 1,1,1,3,3,3-hexamethyldisilazane (HMDZ, 99+%) (Alfa
Aesar) were
added to the mixture. Those samples which resulted in gels were solvent
exchanged three
times with 75 ml of EtOH. After solvent exchange the samples were
supercritically dried.
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All the compositions in Table 10 are listed in weight (grams). Comparing
Examples 30-41 with Examples 42-49 shows that further pre-hydrolysis (Examples
42-49)
resulted in shorter gel times. Examples 43, 44, 45, 47 and 49 were
hydrophobic.
Examples 42, 46, and 48 were not.
Table 10: Formulations (grams), gel times, and gel characteristics for
Examples 42-49.
Ex. o Ethyl
silicate H2O EtOH HCl HMDZ tgme characteristics
42 20.024 3.77 16.4 0.072 0.364 7 fl, translucent
43 19.994 3.77 16.4 0.069 3.545 1 Jilin. translucent
44 19.995 3.77 16.4 0.069 5.639 10 sec, opaque strong
45 19.993 3.77 16.4 0.068 11.104 days opaque slurry
46 19.993 7.2 16.4 0.069 0.357 10 in. translucent
47 20.01 7.2 16.4 0.074 3.554 30 sec. opaque
48 20.012 3.77 23.4 0.073 0.364 12 in. clear
49 20.033 3.77 23.4 0.074 3.576 3 min, translucent
Examples 50-60: TEOS/MTMOS-based aerogels with pre-hydrolyzation and
surface treatment prior to gelation.
Tetraethoxysilane (TEOS, 99+%) (Alfa Aesar) and methyltrimethoxysilane
(MTMOS, 95%) (Aldrich) were mixed with ethanol (EtOH, 200 proof) (Aaper
Alcohol),
deionized water (H20) and hydrochloric acid (HC1) (J.T. Baker) in a glass jar.
For
Examples 50-55, 0.005M HC1 was used and for Examples 56-60 1M HCl was used.
The
glass jar containing the mixture was heated at 50 C for 45 minutes under
constant stirring.
While vigorously stirring, HMDZ was added to the mixture. The molar ratios of
the
various reactants in the final mixture are listed in Table 11. The resulting
gels were
solvent exchanged three times with 75 ml of EtOH. After the final solvent
exchange the
samples were supercritically dried.
Table 11: Formulations, gel times, and gel characteristics for Examples 50-60.
Relative Mole% Moles per total moles
TEOS and MTMOS
Ex. TEOS MTMOS H2O EtOH HCl HMDZ gel gel
time characteristics
50 50 50 9 3 0.0007 0.33 5 min. clear
51 50 50 9 3 0.0007 0.2 N/A clear
52 50 50 9 3 0.0007 0.033 N/A clear
53 50 50 9 3 0.0007 0.5 N/A clear
54 90 10 9 3 0.0007 0.33 7 min. translucent
55 80 20 9 3 0.0007 0.33 6 min. clear
56 90 10 2.5 5 0.0007 0.33 8 min. clear
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Relative Mole% Moles per total moles
TEOS and MTMOS
Ex. TEOS MTMOS H2O EtOH HC1 HMDZ gel gel
time characteristics
57 90 10 2.5 5 0.0007 0.2 N/A clear
58 90 10 2.5 5 0.0007 0.033 N/A clear
59 80 20 3 5 0.0007 0.33 1 min. clear
60 70 30 3 5 0.0007 0.33 3 min. clear
Examples 50-60 were all hydrophobic. Generally, the inclusion of MTMOS
increased the gel time compared to the pure pre-hydrolyzed TEOS samples (e.g.,
Examples 59 and 60 showed an increase in gel time with increasing MTMOS
content
relative to Example 3, which did not contain MTMOS). Examples 50-60 also
showed that
surface modification prior to gelation can be used with other organosilanes
(containing Si-
C groups) and not just pure silica precursors like tetraalkoxysilanes (e.g.,
TEOS and
TMOS), pre-hydrolyzed TEOS, and pre-polymerized silicon alkoxides (e.g.,
SILBOND
50).
Examples 61 and 62: Flexible aerogels with surface treatment prior to gelation
on a nonwoven substrate.
The gel precursor of Example 3 was prepared. The pre-hydrolyzed TEOS and
HMDZ were cooled using dry ice prior to mixing in order to slow gelation. The
mixture
was coated onto a bonded fibrous flexible substrate. (A 75-25 blend of 3d
WELLMAN
PET fibers and 6d KOSA PET fibers at 30 gsm was carded, corrugated, and bonded
to 30
gsm of PP 7C05N strands wherein the corrugating pattern had 10 bonds per 2.54
cm (i.e.,
10 bonds per inch). The sample gelled in about 5 minutes. The substrate
containing the
gel was solvent exchanged using EtOH three times to remove residual water. The
substrate containing the gel was then supercritically dried. The thermal
conductivity of
the Example 61, measured at a mean temperature of 12.5 C, was 29.4 mW/m=K.
Example 62 was prepared in the same manner as Example 61, except that the gel
precursor
of Example 54 was used. The thermal conductivity of the Example 62, measured
at a
mean temperature of 12.5 C, was
25.9 mW/m-K.
Various modifications and alterations of this invention will become apparent
to
those skilled in the art without departing from the scope and spirit of this
invention.
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