Note: Descriptions are shown in the official language in which they were submitted.
W092/20623 PCT/NO92/0~96
210971~ ~
Process for the preparation of a silica aerogel-like material.
This invention relates to a process for the preparation of a
material similar to silica aerogel. Silica aerogel is a porous
material of a very low density and it contains up to 99% of
air. The type of silica aerogel prepared according to the
present i~vention is normally calIed silica xerogel. Due its
high porosity, the material has excellent heat insulating
properties, and since the pore size of the material is smaller
than the wave length of visible light, it is also trans~arent.
Typical properties of silica aerogel are as follows: -
Density: 70 - 25Q kg/m3 ~;
Re~ractive index: 1.02 - 1.05
Thermal conductivity in air: 0.021 W/mK at 20C
Thermal conductivity in vacuum: 0.~08 W/mK at 20C
Particle size: 4 - 7 nm
Pore size: 10 - 20 nm
~ransmittance 88% for 10 mm thickness
. ~-
Silica aerogel has an insulating property which is
approximately twice as good as that of rockwool, and since it
is also transparent, it may be used as insulation in windo~s.
Due to its porosity silica xerogel/aerogel has also a very
high capacity for sucking up liquid and may also be used as
starting material for composite materials and carrier for
catalyst and liquids, e.g. electrolytes.
Originally, sodium silicate-hydrate was used as starting
material for the preparation of silica aerogel, and in a
hydrogen chlo~ide catalysed reaction between the silicate and
water an aquagel was formed. However, this preparation route
is very time-consuming since the a~uagel has to be washed with
alcohol before the drying step may take plare. The drying
process is necessary to remove preferably alI liguid, leaving
only the desired silica network.
.
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More recently tetramethoxysilane, Si(OCH3)4, TMOS, has also
been used as starting material for the preparation of silica
aerogel. TMOS is a suitable starting material since it is easy
to handle, easy to prepare in pure form and easy to hydrolyse.
However, it is toxic and expensive. The alcogel formation
takes place by a direct acid- and/or base-catalysed hydrolysis
of TMOS in a so-called sol-gel technology. The alcogel formed
is a silica skeleton surrounded by aqeous methanol. The
problem which then arises is to remove the aqueous methanol
from the silica skeleton to obtain the aerogel.
In order to prepare the desired aerogel, it is as mentioned
above, necessary to remove the liquid surrounding the silica
skeleton, and this may be done by eliminating or at least
reducing the capillary forces working in the alcogel when the
liquid front w}thdraws, or the network may be stengthenéd so
that the pores do not collapse when the liquid is removed.
`
Two drying procedures are possible where the capillary forces
are e~iminated:
a) Supercritical drying where the solvent in liquid phase is
transformed to the gaseous phase under elevated pressure in
an autoclave at a temperature~above the critical point of the
solvent. A modification of this drying method is to extract
the solvent with another compound such as CO2, which has lower
critical point. Supercritical drying is a somewhat dangerous
method because the large amounts of methanol which is expelled
may lead to an explosion if there is a leak.
b) Freeze dryingr where the alcogel is frozen to becom@
!, solid, whereafter the solvent is subjected to sublimation
under reduced pressure.
In the third possible method, viz.
c) Air drying and direct evaporation of the solvent where
the liquid phase is transformed to gaseous phase, the drying
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2~371~
process will normally lead to collapse of the pores and crack
formation in the gel due to capillary forces. In the
literature, the addition a DCCA (Drying Control Chemical
Additive) to the sol has been reported, to obtain a narrow
pore size distribution and hence a reduction in the
differential strain in the alcogel, whereby cracking is
considerably reduced. However, the resulting silica xerogel
will have a relatively high density, e.g. about 1,4 g/cm3.
Strictly speaking, only preparation method a) will result in
an aerogel. In the other two methods b) and c) there will be
obtained a gel which may be described as silica cryogel and
silica xerogel respectively, which may have approximately the
same properties as silica aerogel.
Drying method a) with supercritical drying results in an
aerogel with low density and relatively good optical
properties, but the method is expen~ive and dangerous due to
the high pressure and the high temperature during drying. In`
the production of tiles for use as insulation in windows it
will be necessary with an autoclave which will represent a
considerable element of risk during operation due to large
amounts of methanol which can be ignited. By using e.g. C02 as
medium during the supercritical drying, an extraction of the
solvent is necessary.
Drying method b) is cheap, but due to volume expansion of the
solvent during crystallisation (i.e. freezing), the result may
easily be cracking of the gel during the drying.
Drying method c) normally takes place with a large shrinkage
of the gel due to a collapse of the gel structure. In the
preparation of high density glass by sintering from these
xerogels which are not particularly reactive, it is
problematic to remove organic residues, i.e. DCCA.
The purpose of the present invention is to pro~ide a process
for preparing silica xerogel (aerogel) having a relatively low
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21()~7~
density (i.e. below 0.5 g/c~.3) and such that the material is
suitable as a heat insulating material.
According to present invention there is provided a new and
improved process for the preparation of silica xerogel of lo~
density by hydrolysis and polycondensation of a
tetraalkoxysilane, to form a an alcogel which is a silica
skelet~n surrounded by aqueous alcohol. The process is
characterised by the fact that the alcogel formed is contacted
with a solution of a tetraalkoxysilane. The contact is
maintained at or above room temperature to stabilise the
silica skeleton, whereafter the alcogel is slowly dryed at or
above room temperature and at above approximately atmospheric
pressure. Before the alcogel is contacted with the solution
of tetraalkoxysilane it may be suitable to contact the alcogel
with a liquid~to wash the gel (in order to change the
composition of the aqueous alcohol surrounding the silica
skeleton) and to replace functional groups on the surface.
This liquid may e.g. consist of a mixture of water and
alcohol, preferably about 40~ methanol. This liquid may also
be replaced several times. The liquid added for washing may
have a temperature up to the boiling point, preferably room
temperature.
Suitably tetrametoxysilane is used for preparing the alcogel,
whereafter the gel formed is contacted with a solution of
tetraethoxysilane, e.g. in a lower alcohol, particularly
methanol,
The contact between the alcogel and the later added
teraalkoxysilane is suitably maintained for a sufficiently
long time for a strengthening of the gel network to take
place, e.g. from 6 hours to 16 days, at a temperature which
does not represent any risk for the alcogel, preferably from
20 to 200C, particularly 40-80C. An increased ageing time
results in the largest pores in the product, i.e the lowest
density. The washing of the alcogel with liquid, e.g aqueous
Y~O 92/20623 PC~r/N 092/00096
21û~715
alcohol results in less shrinking of the gel and larger pores.
The alcohol used is normally a mono-, di- or trihydric
alkanol, particularly with 1-10 C-atoms, preferably methanol.
It is of essential importance that the drying takes place
under controlled conditions, since a to quick drying may lead
to collapse. Controlled drying may be accomplished by allowing
it to take place in an atmosphere which contains components
which are present in the liquid surrounding the silica
skeleton, or components formed from said liquid at higher
temperature. The drying may e.g. take place in a chamber with
such an atmosphere, or it may take place while the alcogel is
kept almost completely covered, e.g. up to 99~, whereby the
same effect is obtained~ The drying is suitably carried out at
a temperature in the range 40 - 200C. The drying temperature
may also be gradually varied from room temperature to the
boiling point of aqueous alcohol.
.~ .
The thereby formed xerogel has a low density (e.g. 0,5 g/cm3
or lower) and has also a homogeneous structure.
Exam~le 1
An alcogel was prepared by hydrolysis and polycondensation of
TMOS. The hydro~ysis took place with NH40H as catalyst, a
stoichiometric amount of water (TMOS:water, molar ratio 1:4),
and with methanol as a solvent. The gel formation took place
at approximately 0C. The alcogel was cast in PyrexJteflon
moulds with a removable bottom of Nescofilm. After up to
several hours standing in completely covered condition after
the gel formation, the mould with the alcogel was immersed in
- an ageing solution of tetraethoxysilane (TEOS~ and methanol.
The mould was~removed from the bath after approximately 24
hours, and the bath was then kept covered for additional 24
hours. During the entire periode of immersing and continued
storage, the temperature was kept at about 60C. The gel was
then covered to an extent of 97% and was dried at about 60C
W092/20623 PCT/NO92/00096
2109715
and a pressure of one atmosphere. The drying resulted in a
monolithic gel having a density in the range 0.42 - 0.73
g/cm3. Different molar ratios between TEOS and methanol in the
ageing solutions were used. The density decreases linearly,
and correspondingly the pore size increases with increasing
amount of TEOS in the ageing liquid and increased time in this
liquid. The shrinkage of the alcogel during the drying is in
the range 15 - 30% depending on the molar ratio and ageing
time. The surface area of the xerogel is in the range 600 -
650 m2/g. The thermal conductivity of the silica xerogelmaterial formed has been measured down to 0.024+0.004 W/mK at
45C.- -
Example 2
An alcogel was prepared as described in Example 1. After up toseveral hours of standing in covered condition the mouled with
the alcogel was immersed in liquid consisting of methanol and
water (e.g. 30 volume % of water). The liquid mixture was
changed three times with 24 hours intervals. The alcogel in
the liquid mixtures was kept at room temperatuxe during the
entire washing. After four days the washing solution was
replaced by a solution of tetraethoxysilane as in Example 1.
The rest is as in Example 1.
The treatment of the alcogel in the solution of water and
methanol causes less shrinkage of the alcogel during drying.
The shrinkage of the alcogel during drying is about 13~. The
thermal conductivity of the silica xerogel prepared has been
measured down to 0.028+0.04W/mK.
The surface area decreases and the pore size increases
linearly with increasing amount of water in the liquid mixture
used for washing the alcogel.
Example 3
The same procedure as in Examples 1 and 2, but methanol was
replaced by ethanol in the liquid mixture used during the
W092/20623 PCT/NO92/00096
- 7 2 10 g 7 1 ~
washing and as solvent for the tetraalkoxysilane used to
stengthen the gel network.
