Note: Descriptions are shown in the official language in which they were submitted.
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The object of the invention is a method of producing substaatiaLly globular
lyogels and
a~ogels.
Aerogets, particularly those with a porosity above 60°!e and a density
of teas than
0.6 g/eu.c~m, display exrretuely tow thsrraai conductivity and are therefore
used as a heat
imaterial, ae dfor example in rP-~s G :."I X22. Furthermore, by virtue
of their very low refraction inde~c far solid substances, it is lrnowr~ to u3e
them for
~erere~ow detectors. Furtnernwre, by reason of their particular acoustic
impedance, the
literature describes a poss?'hla use as sn icupedance adaptation means, for
vcample in the
alpha ~ouud range. It is also possible for them to be used as carriers for
effective
3ubatattces iu pharmacy or agriculture.
Aerogels in the broader sense, e. g. in the sense of "gel with air as the
dispersion agent"
are produced by the drying of a suitable gel_ The term"aerogel" in this sense
embraces
aerogels in the narrower sense, xarogels and cryogels. In this respect, a
dried gel i9
wed sa serogel in the narrower sense when the liquid of the gel is eliminated
at above
critical temperature and starting from pressures above critical pressure_ Oa
chi other
head, if the liquid is eliatiaated frtun the gel under sub-critical
conditions, fnr example with
the formation oCa liquid/favour interpnase, tfcn tbc resulting gd is
ftequeruly referred to
also as a xerogel.
When the term aerogels is used in the present invention, these are aarogels in
the broader
sense, e.g. in the sense of "gel with air as the digpe~tiori medium."
The term dots not include aerogels itnown from earlier literature and which
are obtained
for example by precipitation of silicic acid (e.g. DE 3025437, Dip 29G 898) or
which
occur as pymgen siliac sad, e_g. Aerosil~. fn chase cases, during manufacture,
no three-
dimcnaional gd lattice develops which is homogeneous ovEr relatively great
distances_
Where acrogels are concerned, it is possible basically to differentiate
between inorganic
and orgamic aerogels.
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Iuotgauic aeroge>s love already beets knows since 1931 (S_ S. Ki3tler, Na=use
1931, 127,
741). Saooc them, aerogCls h$ve boeu f~thwming from various darting
a~aterials_ In this
respect, for acarnple SiO~" TiOz-, ZrO~-, SnO2-, LizO-, CeO~-, Y~O"aerogels
arid
mixtures of these were produced (H. D. Gasser, p. C_ Goswami, Chsiu. Rev.
1989, 89,
765 et seq.).
For some years, organic serogels have also been known which are derivrd from
the most
wide~y diverse star'4iqg materials, e.g. melamine forttraldehyde ~ W. Pnkals,
~. lSater,
Sci. 1989, 24, 3221).
Inorgasiic aerogels can thereby be produced in diffeteut ways.
On the one hand, SiO~ aerogeis can for acample be produced by acid hydrolysis
and'
~ ~~yl y~e ~ . During rbis process, a gel is produced
whadr can be dried by super critical dryit~ while its stavc:ure is
mainta'sned. Production
methods based on this drying technique are known for example from EP-A-0 39G
67G,
WO g?J0337$ or Vb0 95/06617.
i he high prcsswe technique involved in the super-critical drying of asrogels
is however
an expensive process sad involves a high security risk. In addition, however,
super-
critical drying of aerogels is a very cost-intensive production method.
In principle, an alternative to super-critical drying is 8ffotded by a method
for the sub-
critical drying of SiOs gels. The costs involved in sub-crisi~:al drying are
substantially less
by reason of the simpler technology, the lower energy costs and tIK Lesser
security risk.
i'1te SiOx gels can for extunple be obtained by and hydrolysis of tetra-alkoxy
silanes in a
suitable organic solvent by means of water. Otrce the golver~t has beets
exchanged for a
suitable organic sulv~t, tta' gel obtained is in a fuitlter step reacted with
a silylating agent.
Tt~ SiO~ gel resulting from this e~an they frog an organic solvent, be dried
is air. Thus,
aerogelx with densities of leas tbaa 0 4 glcu.cne and porosities above 60'.~o
ten be achieved.
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The production method based on this drying technique is described in detail in
WO 94125149.
Furthermore, the above-described gels can, prior io drying and is the alcohol-
aqueous
sohuion, be rbixed with revs-alkoxy siIanes and aged, is order to iucre8se tho
gel lattice
streqgtl~ as disclosed in WO 92!20623.
The tetra alkoxy silat»s used as starting materials in the above-described
processes do,
however, likewise represent an extremely high cost factor.
A not inconsiderable cost reduction can be achieved by using water-glass as a
st.atting
maieraal for the production of SiOi gels_ To this end, it is possibly for
example to produce
a sdicic acid (rout an aqueous water-glass solution with t:he help of err ion
excbsage rrsin,
the silicic acid they bei~ poiycondensed by the addition of a base to produce
an SiOi gd.
After exchange of the aqueous medium for a suitable organic solvent, it is
then possible
in a fiut>>er step to react the resulting gel with a silylatirtg agent
containiu,g chlorine. The
SiOZ geI which is s~mfa~ce modified for example with methyl s~7y1 groups can
then sad
likewise from an organic solvent, be dried in air. Tbc production method based
oa this
technique is known from DE-A-43 42 54S
Alseraative methods with r~rd to the production of an SiOa aerogel on a basis
of water-
glass with subsequent suh.etiticai drying are described m German Patent
Application
19S 4 i 71 S.5 and 195 41 992.8.
Disclosed in German Pateirt Application No. 196 48 798.6 is a tuethod of
producing
aerogells in which hydtogels arc rnadiffed without prior solvent exchange,
i.c. with
essentially water in the pores, after which they are dried
From DE-PS 896 189, it is known that spherical silicic acid hydroget3 can be
produced
by producing f ~om a taw m8tetiel, ag, water glesS, which contains siliCie
acid, sad by
reaction with an acid, for e~cample sulphuric acid, a gehfortnirtg siliac acid
hydrosol which
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is ti»er~ in the Eosin of individual drops, passed through a gaseous or liquid
tned'sura, e.g.
a mineral oil, which is not miscible with water and the hydrosol. The hydrosol
drops
thereby scquirC a more ~ less spherical foam and remain in the oil layer
su>lrciea~tly long
for the conversion from the sol to the solid hydrogel to takc place. ~'he
hydrogel base
produced by the indicated method do however contain mineral oil contaminazioas
which
cannot be cottipletely eliminated even by very expensive washing processes.
~a the case of thi3 method, if the mixture is injected irno a gaseous ntodium,
then the
procedure adopted is one whereby firstly hydrogol Qropletat are produced by a
mixing jet
from water-glass, sulphuric acid and aluminium sulphate, these droplets rhea
beia~
iajectad into an air-filled vessel. IJader the conditioas~ applied;
tor~ver3ion of the hydrosol
into a hydrogel occurs within about 1 second so that the tiny hydrogel
droplets can be
trapped in a layer of water in the bottom of the vessel aQd furthzr proceed _
DE-C-21 03 243 desv.'ribes a ruethod of and an apparatus for producing
substantially
spherical hydrogels which contain silicic acid, the silicic acid hydrosol
being fortaed in a
special mixing jet from a raw materiel containing silicic acid and an acid
solution. T'fe
hydrosol thus formed is, for ;he purpose of drop formation, sprayed into a
gaseous
medium which does not noticeably dissolve in the hydrosol, for example air.
By reason of the necessary fall time as a reaction time for gel formation,
however,
dependent upon the particle size, so the overall heisht for the appliance
i.ato which the
hydrogol is injected is unfonvnately considerable.
Common to all the aforesaid metDods is that in order to initiate gcl
formation, two or
more sEardug coznpona~u, e.g. water glass solution and mineral acid, have to
be brought
together. In this respe~x, it is apparently favourable for the properties
o>t'the gel panicles,
particularly for their subsequent stability, if the form and size of the
particles can be
adjusted already prior to the gel forniatioa process. It is in particttlar
edvatuagcous for
the subsequent stages of the process CoDowiag gel formation and shaping, such
as for
example for washing, for possibly subsequent reactions acrd for the later
drying process,
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if last panicles are present in as easily-handled form, in other words for
cxa>rnple as batlS
Spherical particles are in terms of stability, supezior to all otbGr Shapes.
By vircuc of the
regular geometry and the lack of edges and corners, it is possible
Subs~ntialty to avoid
undesirable abrasion during the following stages of rise process.
5ubstaatially spherical
lyogels have the advantage that the particle site disttibutioa of the end
product produced
from the lyogcl can be adjusted particularly easily by the Shaping p>;ocess_
Tbetefore, else pse,8at invention is based oa the problem of providi>~ a
method of
producing substantially spherical lyogeis ire which the disadvantages of the
prior art
mecnods are avoidcd_
This problem is resolved iu that the gcI-fornu~ng compotrents are mixed
together to form
a lyosoi, in order to fmm the lyogei, the iyogel (Translator s note. evidently
a typo-
graphical error for "lyoaol"j is introd>iccd into a medium which dissolves
perceptibly in
the lyosol. 1n the present Application, the term lyosol or lyogel must be
understood to
mxxn a aol or a gel in which the sol or gd imerstices are filled with fluid.
If the said cou-
>~sts essentially of water, then one speaks of 8 hydrosol or hydrogei, as the
case gray be.
Ideally, the medium is a vapour atmosphere, whereby further substances can be
added to
the lyosol before ii is irnroduced into ~he vapour atmosphere. The air can
thereby also
contain other ga.~eous u~aiia. Asiy apparatuses known to a man skiUad is the
art for this
putpose can be used for mixing the get-fo:znizig componcm and for
iruorpotating the
lyosol.
~xpedicotly, the lyosol i3 introduced dropwise into the vapour auuosphere,
preferably in
the direction of gravity_
A prefetred embodiment provides for the lyosol to be sprayed or added dropwise
to a
vapow atmosphere without any forced flaw of tin vapour in this case, at least
such an
amount ofvapour is advantageously fed to the apparatus a~ is applied by
condensation on
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the Iyoso~/lyogel drops and oa the walls of the apparatus and which ig
required in order
to adjust the process tesuperature.
A furtiuer etnbodimeat resides irwDE lyosol being incorporated imo a flow of
vapour.
Acoordissg to a prefetrod embodiment, the lyosol is added to a $ow of vapour
which $ows
substantially agairet the n of gravity. T'he vapour $ow can also contain
otherwise
directed velocity aomponenu.
A ~rthet development of thi9 embodiment provides for thv lyosol to be
introduced into
a vapour $ow, the velocry of which diminishes is the direction of flow.
Suitable starting substances for the method according to the invention $re
baBic~Ily arty
which are usable for the prior art ways of synthesising lyogels, for exataple
as s pre-stage
for as aerogel (nee for example J. $ritrker, G. W. Schac, Sol-Cel Science; The
Physics
and Chemistry of SoUGe1 Processing, Acadecaic Press L;d., London 1990;
DE-A-43 42 548; US-A-S 081 1G3; US-A-4 $73 218).
PreFerted thereby are the pre-sieges of SiOZ hydrosols, e.g. silicic acid and
mir~ral acid.
Sodium watergla$s solutions and hydrochloric acid ate particularly preferred
A further trnbodimeat of the method resides in theta after being converted to
the lyoge),
ttrese lyosol droplets are trapped in a layer of water.
It is advamageous for the method sowrding to the invemioti for the vapour
atmosphere
preferably to be at a tar~paature in the range from 60°C to 130aC,
particularly preferably
of 80°C to 120'~C apd in particular 95"C to 105~C.
The invent,ioa is based on the stuprisiag e$ect that the condensate forming ou
the lyosol
droplets from tire vapour phase does not have a disadvantageous in$uencx oa
the
composition and homogeneity of the droplets, although the condensers is
completely
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miscible with the lyosol. pus to the very rapid beetles-up of the lyosol
droplets in the
vapour atmosphere and the resulting considerably accelerated gel~fortniag
reaction,
dt~utioa of the lyosot with the condet>sate from the vapour phase is
surprisingly prevented.
Furchetruore, by reason of the caa9iderabIy accclerateai gel-forming raartion
according to
the raethnd of;bc inveacion, nu<rkedly lower heights of spray comainer are
necessary than
with methods it which one sprays into a gas, such ss for example air, which
does not
perceptibly dissolve .n zhe lyoeo~
Furd~e~re, as the reset of a vapour flow direvted in opposition to the force
of gavity,
the dwell tithe of the particles is the Vapour can be incrd under control,
resulting irr
a fiuther saving on the overall height ofthe appliance into which the lyogol
is introduced,
A further easel Qf the vapour !lowing ire oppositioa to the dirtx;tion of fall
arises from the
reduced fall~g rate of the Galls; this lras the effect of a more berrtiC
ptungireg of the lyogel
balls into a layer of water for eaatnplc.
The vapour flow directed in opposition to the force of ,gravity can
futthermort be usod
to grade of screen the drops or panicles duiutg gel Form4tion Particles with a
diameter
below the limit stale diameter which corresponds to the rate of flow are
delivered
upwardly, whereas larger particle are delivered downwardly.
A furthet problem on which the present invention is based resides io Providing
a method
of producing substaatiatiy spherical aerogels.
Ttus problestt is resolved by a method irm wlaicl~ a substantially sphcrioal
lyogel, as cxn be
produced is aocordance with the presser invention, i' converted to an acrogei
'lhe method of converting the tyogei to an serugel is is ao way limited. Any
alternative
methods known to a man skilled in the an can be applied.
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Ia a prefsrre~d embodiment, tlue substantially globular iyogel is reaued with
a silylatiag
agent. Any silylating agents such as for examplo ttimatbyl chloroeilane, which
are known
to a man skilled in the art, may be used. Prior to ailylatior~ the lyogels can
be washed
and/or the solvent ofthe lyogel may be exchanged for another organic solvent.
Washing
of tire lyogel or hydrogel and the solvent rxchangre can also be carried out
by any methods
described in the state of the art.
Drying can Lkewise be carried out by any methods known to a man skilled in the
a<t. In
this re.,pea, super-~xitical as well a9 sub-critical drying processes la~own
for aerogeis are
preferred, sub-critically drying bang particularly preferral.
The method according to the invention is desctitseed in greater detail
hereinafter with
rcferct~ce to an example of embodiment.
Et sodium ware-glass solution is producod by dilution of 53.5 kg comrnercialiy
available
sod>,Wa water-glass 3dution with 25. 5°~ Si(.!~ and 7.6°~6 ha20
with 31.7 kg of deioni3ed
water. A dilute hydrochloric acid is produced by dilution of I9.3 kg
cotnraercially
av 'sa;ab~ 25% hydrochloric acid with 65.8 kg deionised water. In eaC~h case
30 kg/hr Of
the dilute hydrochloric acid sad of the dilate water-glass solution are fed to
a mincing and
spraying apparatus in pr~cisety ateasured amounts. The outlet of the mixing
nozzle is
situated at cha top end of a spray to~rar, the bottom third of which is
cortstructcd as a
cone_ 1fie cone is filled with water and is so scavenged by vapow that tht
tempetaaue
in'ide the ~ower is 140°C. The hydragel spheres ate captured is the
water Layer, scale
through the layer of water and are delivered from the spray tower io a stream
of water.
'i'he swell hydrogel balls ate counbnuously washed with 4.1 mol hydrochloric
acid and then
with dtionised water. Subsequently, the Iyogel balls ate washed ~ovith acrtone
is several
stage until tire wattr ecmtet~t in the gel is IEa3 than 1%. The acetawe-moist
get is exposZd
to a r~ture of acetone and 5% trimabyl chlotnstlsnc for 10 hours. Then, again
in several
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~ 8~ ~ ~ . The a~c~tone--rttoi~c gd balls are dried in a liuidised
bed with niuogen ai 180"C fos 5 mina. The aerogel balls ob~t~iutd have a
dettaity
of 130 kglcu_m and their heat condoctiviry is O.OI W/mK
