Note: Descriptions are shown in the official language in which they were submitted.
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i fv
The object of the irmeotion is a method of producing substtsntially globular
lyogels and
eerogels.
AerogeIs, particularly those with a porosity above 60% sad a deagily of less
than
0.6 g/cu.at>, diap>Ay extremely, low thermal conductivity and are therefore
used as a hear
insulating material, as described for example in EP-A-0171 722. purtbe~nore,
by virtue
of their very low re&actian index faf solid substances, it is k»own to use
them for
Cerenkov detectors. Fl~r~ by reason of their particular acouspc impedance, the
literature describes a possible use as an innpedance adaptatiop means, for
example in the
alpha sound range. It is also possible for them to be ua4d as carriers for
e$ecTive
substances in phanrracy or apiculture
Aerogds in the broader sense, e.g. is the xnse of "gel with air as the
dispersion agent"
are produced by the drying of a suitable gel. The tee, "a,~og~" ~ this sense
embraces
aerogels is the narrows xase, xerogels and cryogels. In this respect, a dried
gel is
tamed as aerogd in the narro~rer sense whrn the liquid of the gel is
eliminated at above
critical temperature and starring from pressures above critical pressure_ On
the oti>er
hand, ifthe dquad is eliminated from the gel under sub-ail coaditiot~ for
exaxople with
the formation of a liquid/favwr interphaac, then the rcavlting gel is
frequently referred to
also as s xaogel.
W6ea the team asrogals is used iu the presrnt invention, these are aerogels in
the broader
sense, e.g. in the sense of "gel with air as the dispersion medium. °
1'he teem does not include aerogels known from earlier literaaue and which are
obtained
for example by precipitation of silicic said (~e.g. DE 3025437, DD 296 898) or
which
occur as sdiCic and, e.g. A'r''s In these cases, during lure, no Ihree-
dimerniional gel lattice develops which is homogeneous rnfet relatively great
distances.
~~ ~ogels at'e coaee~ed, it is ppssible basically to diffcreptiate beiwoen
inorganic
and organic aeragels.
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Inorganic aetog~ds have already been laoowa since 1931 (S_ S. Kistler, Nature
7 93 i, 127,
741 ). Since rhea, aerogds have beau ~ortbcomrog from various materials. Iu
this
Tespect, for example SiO~, TiO~-, ZrOx , Sr~2., LiaO-, Ce0=., V=O~serogels and
udxauea ofthese watt produced (H, D. Gesser, P. C. Goswatni, Cbem. Rev_ 1989,
89,
765 et seq.).
For wane yeaus, organic aerogels have also bees known which are derived from
tt~e most
widely diverse staring materials, e.~. melaatine foz>nsldelryde (tt W.
Pelcala, J. Mate,
Sci. I9$9, 24, 3221 ).
Tnorganic aa-ogels tea thereby he produced i~a di$aent ways.
Oa the one hand, Si02 aerogels cao for exs~aplc be produced by acid hydrolysis
ind
condensation of teas-ethyl orthosiiicate is c~b~uwl. During this process, a gd
is produced
which can be dtiod by super-caitical drying whilo its suuc~ture is mamtaiaed.
Production
methods based on this drying tochaique are k~wwn for example from EP-A-0 396
O~d,
wo 92103378 or w~ 95/06517.
The high pressure techaiQue irrvolvad io the super-critical dryi»g of serogeis
is however
as expensive process and irivolyes a high security risk. In addition, however,
supar-
critica! drying of acrogels is a vrty cost-inte~ive production method.
Ia principle, an ahelaative to super-critical drying is afforded by a method
for the suh-
eritical dying of SiOZ ~ls. The costs invin Sub-critical drying are
substantially less
by ressorr of the simpler iecbnology, the lower energ~r costs and the lesser
secmiiy risk_
The SQL gels can for example be obtaitKd by acid hydrolysis of teQa-alkoacy
silaaes in a
suitable organic solvent by mesins of waxcr. Once the solvent has been
exc~hsngod for a
~~ . the gel obtained is in a further step reamed with a sdyla>ti~ agent.
The SiO~ gel rasulting from this can then, from an organic solwe~nt, be dried
in air. Thus,
aerogeis with densities of les$ than 0.4 g/cu.cau and porosities about 60~/e
can be achieved.
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The production method based oa this dryia,g technique is descn'b~d is detail
in WO
94/25149.
Furthermore, the shouts-described gala can, prior to drying sad in the ~cohol-
aqueous
sohuio~ be mixed with te~a~.aikoxy silaae3 and aged, ~n order to irrc~ese the
gal. lattice
strength, as disclosed in WO 92/20b23 ,
1"he recta-stllcoxy silwres used ac craning t»~erials in the about-deaaibad
processes do,
however, likewise represent an extretaely high cast factor
A considerable cost reduadon eau be achievmd by using water-glass as s
starting malaria!
for the produaiop of Si4i gds. To this end, it is possible for example to
produce a silicic
acid from an aqueous water-glass soluoon with the help of an ion atchauge
resin, the
siIicia acid that be3ag polycondensod by the addition of a base to produce an
SiOz eel.
After exch8pge of the aqueous modiuta for a suitable orgatuc solvem, it is
rhea possible
in a step to react the resulting gel with $ silylati>ng agent containing
chlorine. The
SiOZ gel which is surface modified for example with methyl silyl groups cap
they and
likewise from an organic solvent, be dried in air. The pradueticm method beard
~t this
teclu~ique is known from DE-A-43 42 548.
Ahet~ve methods with regard to the prodtu;aon of an Si0= aervgcl on a basis of
water-
glass with ~bsequent sub-critical drying are desrn'bcd in German Patent
Application
195 ~i 1 715.1 arid 195 41 992. 8.
Disclosed in Crerman Pate~rJt Application No. 196 48 798.6 is a method of
producing
ae~e~ in which hydrogals are su~soe mn~d without prior solvsrt exeha~ i. e.
with
essentially water in the pores, after which they are dried.
From DE-PS 896189, it is lcnuvrm that globular siliac acid hydrogels can be
produced by
produc~,ng iioru a rsw ttiaterial, e.g. water glass, which contains >iilicic
acid, and by
reaction with axe add, for exarr~ls sulphuric acid, a gel-forming silicic acid
bydrosol which
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is then, in the form of individual drop, passed through a gaseous or liquid
mediwu, erg.
a ttiinerai oil, which is not mi~,'ble with, water and the hpdrosol. The
bydrosol drops
thereby acquire a zswre or lees globular form and tamale in the oal layer
cieotly long
for the conversion from the sol to the solid hydmgel to take place. The
hydrogd bast
produced by the indicated method do however comma mineral oil coutarninatioas
which
c$:uiot be completely eliminated ever by very axpe~re washing processes.
to the case of this method, if the mixture is iajectod irno a gaseous tedium,
then the
praxd~,u~ adopted is one whereby srstly hydrosol droplets are produced by a
mixing jet
from water-glass, sulphuric acid and aluminium sulphate, these droplets then
being
irgecxed irao sa sir-filled vessel. Under the conditions applied, conversion
of the hydrosol
into a hydrogel occurs within about 1 second so that the tiny hydrogel
droplets tea be
trapped in s layer of water in the bottom of the vessel and further processed.
DE-C-21 03 243 describes a method ~ and au apparatus for producing
substantially
globular hydrogels which contain silicic acid, the s>7icic acid hydrosol being
forazed irt a
special mixes jet from a raw material comaira»g silicie acid and an sad
sohrtioa. The
hydrosol thus formed is, for the purpose of drop fonnauorr, sprayed into a
gaseous
medium which does not aotioeably dissolve in the hydrosol, far exaxaple air.
By reason of the necessary fall dare as a reaction time for gel formation,
however,
depcndrnt upon the particle size so the overall height for the apps into which
the
Izydrosol is injected is Vafottw~tely considerable As a resuh, this prior art
method is very
~ ~~ once co~espp~Bly substantial n~at~al is required, the spay requir~rt
is considerable and it takes longs to produce the apparatus. Furthermore, it1
the case of
this known method, lyogel Particles of a aoa-homog~us size distnbutioa arise
which
pos$ibly then have to be sorted according to their size so that all in all
this method
becomes more time arid cost iutcnsjvc,
~om~ou to all the aforesaid methods is that in order to initiate g~ formation,
two or
~ ts~ e.g. wares-glass solution and mistral acid, have to be brought
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together. is this rrspcxx, iv is apparently favourable for the properties of
the gel particles,
particularly for their subsequcnt atabi>ityty, if ve form and size of the
particles can be
adjusted already poor to the gel formation process. it is in particular
advanageous for
the 9ubseguem stages of the process following gel iorlcnatioa and shaping,
tech as for
exaaap~ for washing, for possibly subsc-dueat reactions and for the later
drying proocss,
if the particles are p~~er~ in an easily-handled form, in other words for
e~taa~ple as balls.
Globular particles are in tams of sabiliry, supeior to all outer shapea_ By
virtue of tat
regular geotrreuy and the tack of edges and corAers, it is poaslble
substsniieily to avoid
undesirable abrasiop during the foIlow~ng stages of the process_ Substantially
globular
lyog<ds have the advantage that the particla site diatribuiion of the end
product produced
from the Iyobel can be adjustcd pardculariy easily by the shaping pmceas.
~~~'S >~ preseui ittvcntioa is bascd ors the problem of providing a method of
producing 3ubscautiaiiy globular lyogels in which the disadvantages of the
prior art
~aethods are avoiaed.
This problem is resolved is that the gel forming components are mixcd together
to form
a lyosol and rhea tbc lrosol is introduced into a moving medium for forming
the lyoget,
the said nbring not noticxable is the iyosol.
In the preset Appiicatiori, the term lyosol or lyogel must be undett;tood to
moan a sol or
a gel in which the sol or ~l interstices ere filled with uuid. 1f the quid
consists essentially
of wafer, then ors speaks of a hydrosol or hydrogel, as the case gray bc.
Incorporation into s moving atmosphere greatly increascs the dwell tittle of
the lyoaol
particles iu the medium so that the overall height. of the appliance can be
n~k~yy
reduced. s ha-efare, the appliances or apparatus r~,quire considerably lets
azaterial and
space so that the co3rs of the method ecaording to the invention ere
considera~uly reduced.
ideally, the modiu~u is art air atmosphere> whweby fiuther substances can be
added to the
lyosot before it is int~~uduced into tha sir atmosphere. The air ~n thereby
also coruain
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other g,~eout media. Aany apper~uSes known to s man Skilled in the art for
this pwpose
can be used for mixictg the gel forming component and for incorporating the
lyosol.
Expediently, the lyosol is dripped or sprayed into the sir, pr~erably in the
direction of
gravity.
Acaordi~g to a preferred embodiment, the lyosol is added to au air scream
which flows
substantially against the direction of gravity. The air Bow can also contain
otherwise
directed velocity cotripoaerus. Thus, the dwell time of the particles in ttte
air csa be
incrcased under control, which resuhs in a furtl~r savir~ on the overall
height of the
apparaws into which the lyosol is introduced. The air flow which is opposite
to the
direction of gravity can more Ix used for any grading or classifying of the
drops or
particles duri4g gel formatioct. Iaarticle$ with a diatr~etet below the limit
grain diameter
which corresponds to tho flow vclocity are delivered upwardly whereas larger
particles
ara delivered downwardly. ConSequeatly, no additional step is needed for
sorting the gel
particles according to tbeir size so that the coats of the iuethod according
to the iavemioa
arc stiii further reduced.
A futtha development of this embodiment providcs for the Iyosol to be
introduced imo
an air flow, the velocity of which diminishes in the direction of flow_
A further preferred embodituettt of the method resides in that the lyosol
droplets, after
being converted to lyogel, are trapped in a layer of waver_
A 8ullar effect ofthe reduced falling rate of the balls is due to the air
flowing against tla:
direction of fall; the elect is cue of so,>tiening the introduction of the
lyogel balls in a I~yet
of water for example.
Suitablo starting substances for the method according to the invernioa are
basically say
which are usable for the prior art ways of synthesising lyogela, for example
as a preata$e
for art aerogel (see for exaaipie 1. Brinkar, G. VIt. Schero, Sol-Gel Science;
Tbc Physic
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sad ~hemisuy of SoUGai Proc~sir~, Academic Press Ltd., Lorcdoa 1990;
pFrA-43 4Z 548; US-A-5 081 i 63; US-A-4 873 2I 8).
Pre~tred thereby are the pra-stages of SiOz bydrosols, e.g. silicic acid and
rniaeral aoid.
Sodium waterglass solutions and hydrochloric acid are partiwlariy pr~acred.
A pcoblern oa which the present i>nvesuion is based resides in providi~ a
method
of producing substantially globular aerogels This problem is resolved by a
method in
uvhich a subs~tially globular lyogd, as can be produced in aecordaace with the
present
iavsutiorr, is convened to an aerogel.
The method of oonvertiug the lyogel to an aerogel is irc oa way limited. Any
alternative
mrt6ods .l~nown to a rnan skilled in the art can be applied;
In a prefetrod embodis;rent, the subsitaacially globular lyogel is reacted
with a silylatiug
agent. Any silylatiqg agenss such as for example trimethyl chlorosilane, which
are known
to a man sicihod is zhe art, may be uscd. Prior to silylatioa, the lyogels can
be washed
and/or the soivrant of the lyogel may be exchanged for another organic solvem.
Washing
of the ly~el or hydro~l and the solvent e~d~nge can also be carried out by say
methods
desctibed in thr state of the art.
Dtyi:ag can likewise be carried out by any methods Irnowa vo a man skilled in
the arc. In
tlris respect, super-cxitical as well as sub-critical drying processes known
for aetogels are
preferred, sub-critically drying being pstticularly pr~erred.
The mothod according to the invcniiort is described is greater detail
hereinafter with
rrFcreaoe to as example of emboditaeat.
A sodirttn watea:glasos solution is ptoduc~ by dilWioa of 53. 5 kg
commercially available
sodavm water-glaxs solution with 25_5°~o SiOZ arid 7.6°~ro NazO
with 31.7 kg of ddonised
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water. A dilute hydroa6loric said is produced by diwiion of X9.3 kg
cammer,;iaily
available 25% hydrochloric acid with 65.8 kg deionssed watar_ 1n each cast 3o
kg/hr of
the dilute hydcoehloric acid and oC the dilute water-glass solution are fed to
a tniacing and
sprayi>Ag apparatus in precisely rneagured atnouats. The outlet of the mixing
nozzle is
situated xt the top end of a pipe through which heatai sir bows vertically
upwards. The
bocr~orn third of the pipe is find with water. Above the surf eve of the
water, the pipe has
sir inla aperaua. The flow of air is adjusted to an empty Pipe velocity of 4
ml3ec. Thr
temperature inside the pipo is 100°c. 'ire lrydrogel spheres are
capnucd in the water
lays, settle t>uotigh the layer of water acrd are delivered from the spray
tower is a. stream
of water.
The small hydrogel bays are conisru~ously v~ashed with 0.1 atol hydrochloric
acid and rhea
with deiorvised water. Subsequently, the lyogei balls are washed with acetone
in several
stages uat~ the watr con»t in the gel is less than 1 %_ The a~ceione-moist gd
is exposod
to a rtruQUre w"acetotx: and 5% ttttnethyl chiarneilat~e for i0 bouts. 'fhea,
again is scvet8l
stages, ~e gel is washed with acetone. T'he acetono-uroist gel balls are dried
iu a fluidised
bed with nitrogen at 180°C for 5 mina. The aerogel bsDs obtained have a
density
of 7 30 kg/cu, m and their beat cor~ductiviry is 0. ~ 1 WimK.
