Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR PRODUCING A SOLID OXIDIC MATERIAL
The present invention relates to a process for producing a porous solid oxidic
material from a
hydrogel of the oxidic material and to the porous solid oxidic material as
such.
Porous oxidic materials are of interest for numerous applications, for example
as adsorbents,
fillers, release agents, thickeners, dispersing aids, free-flow aids,
defoamers, matting additives,
active ingredient carriers and/or catalyst supports. Among the porous solid
oxidic materials, the
class of aerogels is of particular significance. Aerogels are porous solid
oxidic materials
generally consisting of silicon oxides, i.e. silica, or metal oxides.
Aerogels, especially aerogels of
silica, are of excellent suitability as thermal insulation material because
their thermal
conductivity is only low, or as support material for catalysts because their
specific surface area
is high. Further fields of use of aerogels are in the fields of plastics, for
example natural and
synthetic rubbers, adhesives, paints, coatings, pharmaceuticals, cosmetics,
the paper, textile,
mineral oil and fiber industry, and glass technology, pyrotechnology and
foundry technology,
where the aerogels find various uses as dispersing aids, reinforcers, free-
flow aids, antisettling
agents, fillers, defoamers, active ingredient carriers, matting additives
and/or absorbents.
The production of porous solid oxidic materials, for example aerogels, is
generally possible by
dewatering hydrated forms of the oxidic materials, called hydrogels. However,
this dewatering
operation is associated with a number of problems. The removal of the water
from the hydrogel
by simply heating can lead to the collapse of the hydrogel or to the
crystallization of the oxidic
material, such that the resulting oxidic material is compact and has only low
porosity, if any. In
order to avoid these problems, the hydrogel can be generated and immediately
dried in situ, for
example by spraying waterglass and mineral acid in a spray drying apparatus.
It is known that the water present in the hydrogel can be displaced by
treatment with a lower-
boiling water-soluble liquid, for example volatile alkanols such as methanol,
ethanol or
isopropanol, and that the dewatered material obtained (called an alcogel when
alcohols are
used) can be dried under supercritical conditions (see, for example, US
2249767). EP 171722
discloses performing such a supercritical drying operation in CO2.
For many applications, especially in the case of use as thermal insulation
material, the
absorption of water into the porous solid oxidic material is undesirable,
since the material ages
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in the process and its advantageous properties are lost. The drying of the
organogel in the
presence of alcohols does lead to a certain hydrophobization, since the
alcohol molecules,
through their OH groups, can enter into a chemical bond with the surface of
the oxidic material.
However, the hydrophobization achieved is low and is not stable in the long
term.
Known hydrophobizing reagents include further compounds, for example
organosilicon
compounds, with which the dried hydrogel is treated in the gas phase or which
may also already
be present in the course of precipitation, further intermediate process steps
or supercritical
drying. The coverage of the surface with hydrophobic compounds is supposed to
prevent the
porous solid oxidic material from absorbing water again. However, the reagents
used for
hydrophobization are costly, and the long-term stability of the
hydrophobization achieved is
likewise unsatisfactory.
WO 95/06617 describes a process for producing hydrophobized silica aerogels
having improved
properties, which comprises the reaction of a waterglass solution with an
acid, washing the
hydrogel formed with water to remove ionic constituents, treatment of the
hydrogel with an
alcohol, especially isopropanol, and supercritical drying of the resulting
alcogel in the presence
of the alcohol. However, the hydrophobization achieved, more particularly the
long-term stability
thereof, is likewise unsatisfactory.
The porous solid oxidic materials produced in accordance with the prior art
thus have the
disadvantage that, in spite of a hydrophobized surface, they have a tendency
to absorb water
and therefore do not have long-term stability. It was therefore an object of
the invention to
provide a process which overcomes these disadvantages of the prior art.
It has now been found that, surprisingly, these disadvantages can be overcome
if the removal of
the water is accomplished by treating a hydrogel of an oxidic material with a
water-miscible
organic liquid and then drying the organogel obtained under supercritical
conditions in the
presence of at least one polyfunctional compound C having at least two
reactive functionalities
F which can react to form a bond with the atoms of the solid oxidic material,
the at least one
polyfunctional compound C being used in the supercritical drying operation as
a solution of the
compound C in at least one organic solvent S' having 0 or 1 reactive
functionality F, and/or as a
mixture with CO2, the reactive functionalities F being selected from hydroxyl
groups, especially
carbon-bonded hydroxyl groups, carboxyl groups, carbonate groups, and oxygen
atoms bonded
to phosphorus atoms.
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The invention therefore relates to a process for producing porous solid oxidic
materials,
comprising the provision of a hydrogel of the oxidic material, removal of the
water by treatment
of the hydrogel with a water-miscible liquid and drying of the organogel
obtained under
supercritical conditions in the presence of at least one polyfunctional
compound C having at
least two, for example 2, 3, 4, 5 or 6, especially 2 or 3, reactive
functionalities F which can react
to form a bond with the atoms of the solid oxidic material and which are
selected from hydroxyl
groups, especially carbon-bonded hydroxyl groups, carboxyl groups, carbonate
groups, and
oxygen atoms bonded to phosphorus atoms, which gives the porous solid oxidic
material, the at
least one polyfunctional compound C being used in the supercritical drying
operation as a
solution of the compound C in at least one organic solvent S' having 0 or 1
reactive functionality
F, and/or as a mixture with CO2.
The porous solid oxidic materials obtainable in accordance with the invention
have the
advantages of only low water absorption, high water resistance and high long-
term stability. The
polyfunctional compound C used in the process according to the invention may
particularly be
inexpensive compounds, for example polyhydric alcohols, hydroxycarboxylic
acids, phosphates,
polyphosphates and/or polycarboxylic acids.
The invention is based on the observation that polyfunctional compounds C
which have at least
two reactive functionalities F selected from hydroxyl groups, especially
carbon-bonded hydroxyl
groups, carboxyl groups, carbonate groups, and oxygen atoms bonded to
phosphorus atoms,
and which are present at least during the supercritical drying operation
stabilize the resulting
porous solid oxidic material by bond formation with the surface thereof, i.e.
prevent water
absorption into the porous solid oxidic material.
Preferably in accordance with the invention, the starting materials used for
the production of the
inventive materials are preferably inorganic hydrogels, i.e. hydrogels based
on semimetal or
metal oxides, particularly hydrogels based on silicon dioxide, zinc oxide,
tin(IV) oxide,
titanium(IV) oxide, cerium(IV) oxide and aluminum oxide, especially based on
silicon dioxide.
The proportion of hydrogels which are based on semimetal or metal oxides and
are used with
preference is generally at least 90% by weight, especially at least 95% by
weight, based on the
total amount of the hydrogels used.
Processes for producing hydrogels which give rise to the porous solid oxidic
materials are
known in principle, for example from the prior art cited at the outset. In
general, the hydrogels
are produced by hydrolysis of suitable metal oxide precursors, for example
metal salts or
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covalent metal compounds or semimetal compounds such as (semi)metal halides or
(semi)metal alkoxides, optionally followed by a partial condensation of the
(semi)metal
hydroxides or (semi)metal oxide hydroxides formed in the hydrolysis.
For example, hydrogels based on silicon dioxide are generally produced by
condensation of
alkali metal waterglass, especially sodium waterglass. This is typically done
by mixing a
waterglass solution, for example a 10 to 30 percent by weight, preferably 12
to 20 percent by
weight, waterglass solution, with a dilute aqueous acid, for example a 1 to 50
percent by weight,
especially 5 to 40 percent by weight, acid, especially an aqueous mineral
acid, preferably
sulfuric acid. Preference is given to using a sufficient amount of acid that a
pH of 7.5 to 11,
especially 8 to 11, more preferably 8.5 to 10, most preferably 8.5 to 9.5, is
established in the
mixed product. Especially suitable for this process is the use of a mixing
nozzle from which the
mixture of waterglass solution and dilute mineral acid is sprayed, and where
the sol formed in
the course of mixing solidifies in the air during the aerial phase to form
hydrogel droplets. It is of
course also possible, for example, to produce hydrogel moldings by combining
waterglass and
dilute acid in suitable form and then to allow gelation.
Prior to removal of the water, preference is given to freeing the hydrogel of
ionic constituents by
washing with water or dilute aqueous solutions of inorganic bases, preference
being given to
proceeding in such a way that the pH of the hydrogel barely changes, i.e. less
than 2 pH units,
especially less than 1 pH unit, and corresponds virtually to the value
established in the mixed
product. The inorganic bases used may, for example, be aqueous solutions of
alkali metal
hydroxides such as sodium hydroxide solution or aqueous ammonia. The procedure
here will
preferably be such that the hydrogel, even after the washing operation, has a
pH within the
range mentioned of 7.5 to 11, preferably 8.5 to 10, more preferably 9 to 10.
The washing
operation is preferably conducted until the conductivity of the washing water
flowing away is
about 20 to 300 pS/cm, preferably 50 to 150 pS/cm. This corresponds to an
alkali metal
(sodium) content of the hydrogel of generally 0.1 to 1.7% by weight,
preferably 0.4 to 1.3% by
weight, determined on a sample dried at 80 C in a water jet vacuum.
The hydrogels produced in accordance with the invention may also, as described
in DE
3914850, contain pigments, in which case suitable pigments are especially
those which scatter,
absorb or reflect infrared radiation of wavelength 3 to 10 pm. Such pigments
are generally
added to the hydrogel at an early stage, in the course of production thereof.
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According to the invention, the water is removed from the hydrogel by
treatment with a water-
miscible organic liquid. The water-miscible organic liquid used for removal of
the water is
essentially anhydrous, i.e. it generally has a water content of not more than
5% by weight,
particularly 0 to 2% by weight and especially 0 to 1% by weight, based on the
overall water-
miscible liquid.
The treatment of the hydrogel with the water-miscible organic liquid
substantially or especially
virtually completely replaces the aqueous phase present in the hydrogel with
the substantially or
essentially anhydrous water-miscible organic liquid. For treatment of the
hydrogel with the
water-miscible organic liquid, the hydrogel is contacted with the liquid, and
then the treated
product is separated from the liquid. For example, the hydrogel can be
suspended in the water-
miscible organic liquid and then the solid or gel constituents can be
separated from the liquid
phase, for example by filtration or centrifugation. Advantageously, the
treatment is undertaken
with the aid of a flow apparatus. For this purpose, the hydrogel is introduced
into a suitable
vessel having an inlet for the water-miscible organic liquid and an outlet,
the inlet and outlet
being arranged such that the water-miscible organic liquid flows through the
hydrogel. The
water-miscible organic liquid is fed in through the inlet, and the mixture of
the water-miscible
organic liquid and water is drawn off via the outlet. The treatment is
preferably conducted until
the water content of the organic phase flowing away is less than 2% by volume,
preferably less
than 1% by volume.
The temperature at which the treatment is undertaken is typically in the range
from 0 to 60 C,
preferably in the range from 10 to 50 C, for example 20 to 30 C. The treatment
of the hydrogel
with the anhydrous water-miscible organic liquid can, however, also be
conducted at elevated
temperature.
The removal of the water by treatment with the water-miscible organic liquid
is preferably
effected under subcritical conditions. Preference is given to removing the
water under ambient
pressure. Another possibility is exchange under reduced pressure or under
elevated pressure.
Typically, the treatment of the hydrogel with the water-miscible organic
liquid is effected at
ambient pressure.
As a result of the water exchange in the hydrogel, what is called an organogel
is obtained, in
which the physically bound water has substantially been exchanged for the
constituents of the
water-miscible organic liquid.
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According to the invention, the organic liquid used for treatment of the
hydrogel is water-
miscible, i.e. the liquid at 20 C has no miscibility gap with water.
Preference is given to liquids
which have a boiling point at standard pressure in the range from 10 to 100 C,
especially in the
range from 10 to 90 C. The water-miscible organic liquid is preferably an
organic solvent S or a
mixture of organic solvents S consisting to an extent of at least 70% by
weight, based on the
total amount of the water-miscible organic liquid, of one or more organic
solvents S which at
20 C have no miscibility gap with water. As well as the organic solvent S, the
water-miscible
organic liquid may also comprise one or more organic solvents which are
immiscible or
incompletely miscible with water, for example C2-C8-alkanes such as ethane,
propane, butane,
isobutane, pentane, isopentane, n-hexane and its isomers, n-heptane and its
isomers, and n-
octane and its isomers. As well as the organic solvent S, the water-miscible
organic liquid may
also comprise the compound C.
Preference is given to organic solvents S which have a boiling point at
standard pressure in the
range from 30 to 120 C, especially in the range from 30 to 100 C. The organic
solvent S is
preferably selected from Ci-C4-alkanols, for example methanol, ethanol, n-
propanol,
isopropanol, n-butanol, isobutanol, 2-butanol and tert-butanol, C1-C4-alkanals
such as
formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde and
isobutyraldehyde, and C3-C4-
ketones such as acetone or methyl ethyl ketone, and mixtures thereof. The
organic solvent S is
more preferably a C1-C4-alkanol. It is most preferably isopropanol.
The water-miscible organic liquid which is used in the removal of the water
from the hydrogel
may already comprise the polyfunctional compound C as defined below.
Preferably, when the
water is removed from the hydrogel, the organic solvent S is used without
addition of the
polyfunctional compound C
The organogel is dried under supercritical conditions in the presence of the
polyfunctional
compound C. The polyfunctional compound C can be used either as a mixture with
CO2 or as a
solution in an organic solvent S' which, unlike the compound C, has at most
one or no
functionality F. It is equally possible to use the polyfunctional compound C
in a mixture of CO2
and solvent S. Preferably, either a mixture of the compound C with CO2 or a
solution of the
compound C in an organic solvent S' is used in the supercritical drying
operation.
The concentration of the polyfunctional compound C in the organic solvent S'
and/or CO2 is
generally selected such that the resulting mixture can be converted readily to
the supercritical
state.
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In general, the concentration of the polyfunctional compound C in the solution
in the solvent S'
and/or in CO2 is therefore in the range from 0.01 to 50% by weight, especially
in the range from
0.1 to 20% by weight, based on the overall solution and/or CO2. Accordingly,
the polyfunctional
compound C is used generally in an amount in the range from 0.01 to 50% by
weight, especially
in the range from 0.1 to 20% by weight, based on the total amount of solvent
S' and/or CO2 and
polyfunctional compound C.
According to the invention, the polyfunctional compound C has at least two,
for example 2 to 10
or 2 to 5, reactive functionalities F. The compound C preferably has two or
three reactive
functionalities F. Reactive functionalities F are understood in the context of
the invention to
mean atoms and/or atom groups which can react with the atoms of the solid
oxidic material to
form a chemical bond, preferably a covalent chemical bond.
According to the invention, the reactive functionalities F are selected from
hydroxyl groups,
carboxyl groups, carbonate groups, and oxygen atoms bonded to phosphorus
atoms. More
particularly, the reactive functionalities F are selected from carbon-bonded
hydroxyl groups,
carboxyl groups and carbonate groups. More preferably, the reactive
functionalities F are
selected from carbon-bonded hydroxyl groups and carbonate groups.
Examples of suitable compounds C are
C2-C6-alkanepolycarboxylic acids, i.e. polybasic, e.g. di- or tribasic, linear
or branched
alkanecarboxylic acid having two to six carbon atoms. Examples are oxalic
acid, malonic
acid, succinic acid, glutaric acid, adipic acid and maleic acid;
hydroxy-C2-C6-alkanemono- and -polycarboxylic acids, i.e. mono- or polybasic,
e.g. mono-
di- or tribasic, linear or branched alkanecarboxylic acid having two to six
carbon atoms,
which have at least one hydroxyl group in addition to at least one carboxyl
group.
Examples are lactic acid, 2-hydroxybutanoic acid and citric acid;
C2-C6-alkanepolyols, e.g. di- or trihydric, linear or branched aliphatic
alcohols having two
to six carbon atoms. Examples are ethylene glycol, 1,2-propanediol, 1,3-
propanediol,
neopentyl glycol and glycerol;
C3-C6-cycloalkanepolyols, i.e. polyhydric, e.g. di- or trihydric,
cycloaliphatic alcohols
having three to six carbon atoms, such as 1,2-cyclopropanediol, 1,2-
cyclopentanediol and
1,2-cyclohexanediol;
2-hydroxyphenol (catechol) and mono- and di-C1-C4-alkyl-2-hydroxyphenols,
especially
mono- and dimethy1-2-hydroxyphenols;
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C2-C4-alkylene carbonates, i.e. cyclic esters of carbonic acid with C2-C4-
alkanediols, e.g.
ethylene carbonate (1,3-dioxolan-2-one) and propylene carbonate (4-methyl-1,3-
dioxolan-
2-one);
phosphates, polyphosphates, Ci-C8-alkyl mono- and polyphosphates;
and mixtures thereof.
Preferred compounds C are 2-hydroxyphenol, C1-C4-alkyl-2-hydroxyphenols, C2-C6-
alkanepolyols, especially ethylene glycol, 1,3-propanediol or 1,2-propanediol,
hydroxy-C2-C6-
alkanemono- and -polycarboxylic acids, especially lactic acid and citric acid,
C2-C4-alkylene
carbonates, especially ethylene carbonate or propylene carbonate and C2-C6-
alkanepolycarboxylic acids, especially malonic acid or oxalic acid.
Particularly preferred compounds C are ethylene glycol, 1,2-propanediol, 1,3-
propanediol, 2-
hydroxyphenyl, ethylene carbonate, propylene carbonate and mixtures thereof.
Suitable solvents S' are the aforementioned water-miscible solvents S, and
also C2-C8-alkanes
and mixtures thereof. Preferred solvents S' are the aforementioned Ci-C4-
alkanols, especially
isopropanol. Preferred solvents S' are additionally mixtures of the
aforementioned C1-C4-
alkanols, especially isopropanol, with C2-C8-alkanes.
Supercritical drying can be undertaken in a customary manner, for example in
analogy to the
prior art cited at the outset.
In general, the organogel and the solution of the compound C in the solvent S'
and/or CO2 is
heated to a temperature above the critical temperature under pressure.
The drying of the organogel under supercritical conditions is effected
preferably at a
temperature of not more than 40 K, especially not more than 20 K, above the
critical
temperature of the solvent S', or not more than 50 K, particularly not more
than 30 K, especially
not more than 20 K, above the critical temperature of 002.
In general, the temperature is in the range from 100 to 300 C, preferably 150
to 280 C. The
pressure required for this is typically in the range from 30 to 90 bar,
preferably 40 to 70 bar.
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If the supercritical drying takes place, for example, with isopropanol as the
solvent S', a
temperature of about 240 to 270 C and a pressure of about 50 to 70 bar are
generally
established.
According to the invention, the drying under supercritical conditions is
effected in the presence
of at least one polyfunctional compound C present either as a solution in the
organic solvent S',
as defined above, and/or in CO2. If the compound C is not already present in a
sufficient amount
in the essentially anhydrous water-miscible organic liquid used in the water
exchange, the
compound C is added to the organogel, preferably as a solution in the solvent
S'.
For drying, the mixture of organogel, the compound C and the solvent S' or CO2
is typically
introduced into a pressure vessel and the mixture is brought under
supercritical conditions. For
this purpose, the closed pressure vessel, for example an autoclave, is
preferably heated with
limitation of the pressure to a supercritical temperature. The mixture is
preferably kept under
supercritical conditions for 1 min to 8 h, especially 1 min to 4 h.
The solvent S' and/or CO2 is then removed from the pressure vessel by
decompression,
preferably isothermal decompression, preferably gradually by gently opening
the pressure
valve. Preference is given to conducting the decompression at a decompression
rate in the
range from 0.1 to 5 bar/min.
During the supercritical drying operation, the formation of any great volumes
of gas through
uncontrolled vaporization or outgassing will preferably be prevented by means
of
decompression, i.e. said removal of the gas mixture via the pressure valve.
The supercritical drying step may be followed by further purification and
workup steps. These
may, for example, be the purging of the pressure vessel with compressed air or
gaseous
nitrogen, in order particularly to remove residues of the solvent S still
present. The supercritical
drying step may also be followed by a subcritical, conventional drying
operation at slightly
elevated temperature, optionally while purging with compressed air or gaseous
nitrogen.
The process product obtained from the process according to the invention is a
porous solid
oxidic material which, owing to the treatment with the compound C, has
improved properties,
especially a hydrophobized surface and lower water absorption, even in the
case of prolonged
water contact.
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Owing to the high porosity, the material has only low bulk densities of about
25 to 300 g/L,
especially 50 to 250 g/L. The proportion of pores in the total volume of the
material is about 50
to 97% by volume.
In preferred embodiments of the invention, the porous solid oxidic material
obtainable in
accordance with the invention comprises, as the main component, i.e. in an
amount of 90 to
100% by weight, based on the total weight of the oxidic material, at least one
oxide from the
group of silicon dioxide, zinc oxide, tin(IV) oxide, titanium(IV) oxide,
cerium(IV) oxide and
aluminum oxide. More particularly, the porous solid oxidic material obtainable
in accordance
with the invention comprises, as the main component, i.e. in an amount of 90
to 100% by
weight, based on the total weight of the oxidic material, at least one oxide
from the group of
silicon dioxide, titanium(IV) oxide and aluminum oxide or a mixture of these
oxides with at least
one further oxide from the group of zinc oxide, tin(IV) oxide and cerium(IV)
oxide. Specifically,
the solid oxidic material comprises, as the main component, i.e. in an amount
of 90 to 100% by
weight, based on the total weight of the oxidic material, at least one oxide
from the group of
silicon dioxide.
The porous solid oxidic material is preferably an aerogel. The porous solid
oxidic material is
more preferably an aerogel based on silicon dioxide.
The porous solid oxidic material obtained in accordance with the invention can
be used either in
the form of granules (typical particle sizes from 1 to 8 mm) or after prior
grinding or the like as
powder (particle sizes of less than 1 mm) for different purposes, for example
as described in the
introduction.
The porous solid oxidic material obtainable by the process according to the
invention generally
has a density in the range from 0.025 to 0.25 g/cm3.
The inventive materials are suitable for a multitude of applications.
The examples which follow serve to illustrate the invention.
Examples:
Chemicals used:
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isopropanol (99.9%, from BCD Chemie)
ethylene glycol (99.8%, from Sigma-Aldrich)
1,2-propanediol (99%, from Sigma-Aldrich)
1,3-propanediol (99%, from Sigma-Aldrich)
2-hydroxyphenol (99%, from Sigma-Aldrich)
ethylene carbonate (99%, from Sigma-Aldrich)
propylene carbonate (99%, from Sigma-Aldrich)
1-propanol (99%, from Sigma-Aldrich)
ethanol (anhydrous, 99%, from Sigma-Aldrich)
Analysis:
Bulk density based on ISO 3944
Specific surface area by adsorption of nitrogen according to BET at a
temperature of -196 C to
DIN ISO 9277
Elemental analysis (determination of the carbon content of the samples as a
measure of the
success of the surface reaction): vario MICRO cube (from Elementar, CHN
operating mode at
1000 C)
Contact angle measurements to DIN 55660
Water absorption: measurement of the increase in weight of the samples after
storage at 23 C
and > 90% relative humidity for 24 h
Water resistance: The material to be examined was ground in a mortar to a
powder. About 5 mL
of water were introduced into a closable 10 mL glass vessel and a sufficient
amount of powder
was added to the glass vessel that the powder covered the entire surface of
the water. Because
of its low density, the powder floated on the surface of the water. The powder
volume required
for the full surface coverage of the water in the glass vessel was about 1 mL.
The glass vessel
was closed and stored at room temperature, and the time until the floating
powder started to fall
to the base of the glass vessel was measured. The longer the powder remained
completely on
the surface of the water, the more hydrolysis-resistant was the hydrophobic
surface modification
of the material examined.
Preparation examples:
Preparation of a hydrogel based on silica:
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A 13% by weight waterglass solution was prepared by diluting a technical
waterglass solution
comprising 27% by weight of silicon dioxide and 8% by weight of sodium oxide
with water.
In a mixing nozzle, at 20 C and 2.5 bar, 45.7 L/h of the 13% by weight
waterglass solution
prepared were combined with 6 L/h of a 23% by weight aqueous sulfuric acid
solution. The
unstable hydrosol which formed as a result of progressive neutralization of
the waterglass
solution in the mixing chamber had a pH of 8.1 0.1 and, after a residence
time of 0.1 s, was
sprayed through the nozzle mouth (diameter 2 mm). As it flew through the air,
the liquid jet
separated into individual droplets, which solidified to give transparent,
mechanically stable
hydrogel spheres before hitting the water basin.
The hydrogel obtained in this way was washed with demineralized water until
the wash liquid
flowing away had an electrical conductivity of less than 110 pS/cm and a pH of
9.8 0.1. The
sodium content of a sample of the hydrogel dried at 80 C in a water jet vacuum
was 1.1% by
weight.
Preparation of the alcogel (isopropanol):
2000 g of the hydrogel based on silica were introduced into a 5 L vessel,
which was filled
completely with isopropanol. At 25 C, anhydrous isopropanol was pumped through
the vessel
until the water content of the isopropanol flowing away was less than 0.1% by
volume. This
required about 8 L of isopropanol.
Supercritical drying of the alcogel based on isopropanol (general method):
2 L of the alcogel together with 4 L of isopropanol and 40 mL of the compound
C were
introduced into a heatable stainless steel (RA4) pressure vessel having a
capacity of 20 L and
the mixture was heated to 270 C within 5 h, in the course of which the
pressure in the pressure
vessel was limited to 70 bar. This was followed by isothermal decompression
within 60 min. The
cooled reaction product was withdrawn and subjected to further drying at 80 C
and 200 mbar for
approx. 2 h.
Example 1 (ethylene glycol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, the compound C used was ethylene glycol.
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The bulk density of the resulting aerogel was 110 g/L. The specific surface
area was 300 m2/g.
The carbon content was 5.8% by weight. The surface was hydrophobic and had a
contact angle
with respect to water of 155 . The water absorption was 1.1% by weight. The
water resistance
was about 12 to 14 days.
The surface of the aerogel was examined by means of solid state 13C NMR. This
gave two
different carbon signals which are attributable firstly to the two chemically
equivalent carbon
atoms of ethylene glycol, and secondly to the oxygen-bonded carbon atom of
isopropanol. The
intensity ratio of the signals between doubly bonded ethylene glycol molecules
and singly
bonded isopropanol molecules was about 3 : 1.
Example 2 (1,2-propanediol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, the compound C used was 1,2-propanediol.
The bulk density of the resulting aerogel was 112 g/L. The specific surface
area was 310 m2/g.
The carbon content was 5.9% by weight. The surface was hydrophobic and had a
contact angle
with respect to water of approx. 140 . The water absorption was 1.8% by
weight. The water
resistance was about 8 days.
Example 3 (1,3-propanediol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, the compound C used was 1,3-propanediol.
The bulk density of the resulting aerogel was 120 g/L. The specific surface
area was 310 m2/g.
The carbon content was 6.2% by weight. The surface was hydrophobic and had a
contact angle
with respect to water of approx. 145 . The water absorption was 1.9% by
weight. The water
resistance was about 6 days.
Example 4 (2-hydroxyphenol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, the compound C used was 2-hydroxyphenol.
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The bulk density of the resulting aerogel was 110 g/L. The specific surface
area was 340 m2/g.
The carbon content was 6.5% by weight. The surface was hydrophobic and had a
contact angle
with respect to water of approx. 155 . The water absorption was 1.0% by
weight. The water
resistance was more than 14 days.
Example 5 (ethylene carbonate):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, the compound C used was ethylene carbonate.
The bulk density of the resulting aerogel was 120 g/L. The specific surface
area was 320 m2/g.
The carbon content was 6% by weight. The surface was hydrophobic and had a
contact angle
with respect to water of approx. 150 . The water absorption was 1.2% by
weight. The water
resistance was about 10 days.
Comparative example 1 (isopropanol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, instead of a mixture of isopropanol and compound C, exclusively
isopropanol was used.
The bulk density of the resulting aerogel was 115 g/L. The specific surface
area was 330 m2/g.
The carbon content was 6.0% by weight. The surface was hydrophobic and had a
contact angle
with respect to water between 120 and 140 . The water absorption was 2.1% by
weight. The
water resistance was about 12 to 24 h.
Comparative example 2 (1-propanol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, rather than a mixture of isopropanol and compound C, exclusively 1-
propanol was used.
The bulk density of the resulting aerogel was 125 g/L. The specific surface
area was 320 m2/g.
The carbon content was 5.8% by weight. The surface was hydrophobic and had a
contact angle
with respect to water between 120 and 140 . The water absorption was 2.0% by
weight. The
water resistance was about 18 h.
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Comparative example 3 (ethanol):
The starting material was the above-described alcogel based on isopropanol.
For supercritical
drying, rather than a mixture of isopropanol and compound C, exclusively
ethanol was used.
The bulk density of the resulting aerogel was 125 g/L. The specific surface
area was 300 m2/g.
The carbon content was 5% by weight. The surface was hydrophobic and had a
contact angle
with respect to water between 120 and 1300. The water absorption was 2.2% by
weight. The
water resistance was about 8 to 12 h.
