U.S. patent application number 09/737069 was filed with the patent office on 2002-08-15 for process for fabricating sol-gel article involving low-shrinkage formulation.
Invention is credited to Bhandarkar, Suhas, Fleming, Debra Anne, Johnson, David Wilfred JR..
Application Number | 20020108399 09/737069 |
Document ID | / |
Family ID | 24962456 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020108399 |
Kind Code |
A1 |
Bhandarkar, Suhas ; et
al. |
August 15, 2002 |
Process for fabricating sol-gel article involving low-shrinkage
formulation
Abstract
A silica sol-gel fabrication process is provided which allows
improved control of the shrinkage that takes place during the
drying of a gel body. In particular, the invention makes it
possible to attain extremely low shrinkage through the completion
of the drying stage, e.g., below 1% linear shrinkage, in relatively
large sol-gel bodies of (dry weight) 1 kg or more, typically 10 kg
or more, or even 40 kg or more, compared to the much higher
shrinkages typically encountered. Specifically, use of a particular
polymeric additive makes it possible for a gel body to experience
linear shrinkage at least 55% less than an identical process
without the polymeric additive.
Inventors: |
Bhandarkar, Suhas; (Glen
Gardner, NJ) ; Fleming, Debra Anne; (Berkeley
Heights, NJ) ; Johnson, David Wilfred JR.;
(Bedminster, NJ) |
Correspondence
Address: |
Docket Administrator (Rm. 3C-512)
Lucent Technologies Inc.
600 Mountain Avenue
P.O. Box 636
Murray Hill
NJ
07974-0636
US
|
Family ID: |
24962456 |
Appl. No.: |
09/737069 |
Filed: |
December 14, 2000 |
Current U.S.
Class: |
65/17.2 |
Current CPC
Class: |
C03C 1/006 20130101;
C03B 19/12 20130101 |
Class at
Publication: |
65/17.2 |
International
Class: |
C03B 008/02 |
Claims
What is claimed is:
1. A process for forming an article, comprising the steps of:
providing an aqueous silica dispersion comprising 40 to 75 wt. %
silica particles, 2 wherein the silica particles have a nominal
surface area of 5 to 100 m /g, and wherein the dispersion is
stabilized at a pH of at least 10; adding to the dispersion a
polymeric additive; adding to the dispersion a gelling agent to
induce gelation; and drying the resultant gel body, wherein the
polymer additive is added in an amount sufficient to affect the
drying mechanism such that at the completion of drying, the body
has undergone linear shrinkage at least 55% less than an identical
process without the polymeric additive.
2. The process of claim 1, wherein the polymer additive is added in
an amount sufficient to affect the drying mechanism such that at
the completion of drying, the body has undergone linear shrinkage
at least 90% less than an identical process without the polymeric
additive.
3. The process of claim 1, wherein the silica particles have a
nominal surface area of 10 to 50 m.sup.2/g.
4. The process of claim 1, wherein the dispersion comprises 50 to
65 wt. % silica particles.
5. The process of claim 1, wherein the dispersion is stabilized at
a pH of at least 11.
6. The process of claim 1, wherein the polymeric additive comprises
a hydrocarbon chain comprising an attached hydrogen-acceptor.
7. The process of claim 6, wherein the polymeric additive comprises
hydroxy propyl cellulose,
poly(2-ethyl-2-oxazoline)-co-(2-phenyl-2-oxazol- ine),
poly(2-ethyl-2-oxazoline), poly(ethylene oxide), poly(ethylene
glycol), poly(vinylpyrrolidone), or poly(vinyl alcohol).
8. The process of claim 7, wherein the polymeric additive is at
least one compound selected from the group consisting of hydroxy
propyl cellulose,
poly(2-ethyl-2-oxazoline)-co-(2-phenyl-2-oxazoline), and
poly(2-ethyl-2-oxazoline).
9. The process of claim 1, further comprising a step of adding a
compound that increases the isoelectric point of the
dispersion.
10. The process of claim 1, wherein the dispersion further
comprises a stabilizing agent comprising a tetraalkylammonium
hydroxide, where the alkyl is methyl, ethyl, propyl, or butyl.
11. The process of claim 1, further comprising the steps of: heat
treating the dried gel body; and sintering the heat treated
body.
12. The process of claim 1, wherein prior to gelation, the surfaces
of the silica particles are primarily hydrophilic, and during
gelation, the surfaces of the silica particles become primarily
hydrophobic.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to fabrication of silica bodies by
colloidal sol-gel techniques.
[0003] 2. Discussion of the Related Art
[0004] Optical transmission fiber typically contains a high-purity
silica glass core optionally doped with a refractive index-raising
element such as germanium, an inner cladding of high-purity silica
glass optionally doped with a refractive index-lowering element
such as fluorine, and an outer cladding of undoped silica glass. In
some manufacturing processes, the preforms for making such fiber
are fabricated by forming an overcladding tube for the outer
cladding, and separately forming a rod containing the core material
and inner cladding material. The core/inner cladding are fabricated
by any of a variety of vapor deposition methods known to those
skilled in the art, including vapor axial deposition (VAD), outside
vapor deposition (OVD), and modified chemical vapor deposition
(MCVD). MCVD is discussed in co-assigned U.S. Pat. Nos. 4,217,027;
4,262,035; and 4,909,816. MCVD involves passing a high-purity gas,
e.g., a mixture of gases containing silicon and germanium, through
the interior of a silica tube (known as the substrate tube) while
heating the outside of the tube with a traversing oxy-hydrogen
torch. In the heated area of the tube, a gas phase reaction occurs
that deposits particles on the tube wall. This deposit, which forms
ahead of the torch, is sintered as the torch passes over it. The
process is repeated in successive passes until the requisite
quantity of silica and/or germanium-doped silica is deposited. Once
deposition is complete, the body is heated to collapse the
substrate tube and obtain a consolidated core rod in which the
substrate tube constitutes the outer portion of the inner cladding
material. To obtain a finished preform, the overcladding tube is
typically placed over the core rod, and the components are heated
and collapsed into a solid, consolidated preform. It is possible to
sinter a porous overcladding tube while collapsing it onto a core
rod, as described in co-assigned U.S. Pat. No. 4,775,401.
[0005] Because the outer cladding of a fiber is distant from
transmitted light, the overcladding glass generally does not have
to meet the optical performance specifications to which the core
and the inner cladding must conform. For this reason, efforts to
both ease and speed manufacture of fiber preforms have focused on
methods of making overcladding tubes. One area of such efforts is
the use of a sol-gel casting process. Co-assigned U.S. Pat. No.
5,240,488 discloses a sol-gel process capable of producing
crack-free overcladding preform tubes of a kilogram or larger. In
this process, a colloidal silica dispersion, e.g., fumed silica, is
obtained having a pH of 2 to 4. To obtain adequate stability of the
dispersion and prevent agglomeration, the pH is raised to a value
of about 10 to about 14 by use of a base. Typically, a
commercially-obtained dispersion is pre- stabilized at such a pH
value by addition of a base such as tetramethylammonium hydroxide
(TMAH). Introduction of the TMAH raises the pH value. Other
quaternary ammonium hydroxides behave similarly. When the pH is so
raised, the silica takes on a negative surface charge due to
ionization of silanol groups present on the surface, in accordance
with the following reaction:
--Si--OH+OH.sup.-.fwdarw.--Si--O.sup.-+H.sub.2O.
[0006] The negative charge of the silica particles creates mutual
repulsion, preventing substantial agglomeration and maintaining the
stability of the dispersion. In this state, the surface charge, and
nominally the zeta potential, of the particles is at a negative
value. (Zeta potential is the potential across the diffuse layer of
ions surrounding a charged colloidal particle, and is typically
measured from electrophoretic mobilities--the rate at which
colloidal particles travel between charged electrodes placed in a
solution. See, e.g., C. J. Brinker and G. W. Scherer, Sol-Gel
Science, Academic Press, 242-243.)
[0007] At a later stage in the process, as discussed in Col. 15,
lines 39-65 of the '488 patent, a gelling agent such as methyl
formate is added to reduce the pH. It is possible to use other
esters, as well. The ester reacts to neutralize base, and the
negative character of the silica particles is neutralized according
to the following reaction:
--Si--O.sup.-+H.sup.+.fwdarw.--Si--OH.
[0008] A sufficient amount of the ester must be introduced to
neutralize the silica to a degree where gelation is induced.
(Gelation, as used herein, indicates that the colloidal silica
particles have formed a three-dimensional network with some
interstitial liquid, such that the dispersion becomes essentially
non-flowing, e.g., exhibiting solid-like behavior, at room
temperature.) Subsequent to gelation, the sol-gel body is typically
released from its mold, dried, heat treated, and sintered, as
reflected in the Table at Cols. 11-12 of the '488 patent.
[0009] As discussed in the '488 patent, a major problem that had
been encountered in sol-gel fabrication of relatively large bodies,
e.g., 1 kg or greater, was cracking of the bodies during drying,
heat treatment and/or sintering. In particular, the gel body
undergoes substantial shrinkage from its gel form to its sintered
form, e.g., typically greater than 10 linear percent shrinkage.
This shrinkage induces numerous stresses in the body, and these
stresses often lead to cracks. According to the '488 patent,
however, the inclusion in the sol of an extremely small amount of
polymer additive, referred to as binder, reduced such cracking,
particularly when a plasticizer was also used. (See, e.g., Col. 5,
lines 19-56.) For this reason, the formulation of the '488 patent
allowed fabrication of tubes of useful size in a commercially
feasible manner.
[0010] However, the gel bodies produced by such sol-gel processes
still undergo relatively substantial shrinkage from gel form to
sintered form. This shrinkage continues to exert stresses
throughout the body, and the gel bodies therefore require careful
and highly controlled drying processes. These careful, controlled
treatments are time- consuming and relatively costly. And the
shrinkage reduces the number of uses for such sol-gel bodies.
Processes which reduce the shrinkage and/or otherwise allow use of
less time-consuming and costly techniques would be highly
advantageous.
SUMMARY OF THE INVENTION
[0011] The invention provides a silica sol-gel fabrication process
of the type described in the '488 patent, but which allows improved
control of the shrinkage that takes place during the drying of a
gel body. Specifically, use of a particular polymeric additive
makes it possible for a gel body to experience linear shrinkage,
through the drying stage, at least 55% less than an identical
process without the polymeric additive (meaning 100.times.(percent
shrinkage without additive-percent shrinkage with additive)/percent
shrinkage without additive). For example, it is possible to attain
extremely low shrinkage--even below 1% linear shrinkage, in
relatively large sol-gel bodies of (dry weight) 1 kg or more,
typically 10 kg or more, and even 40 kg or more, by adding a
sufficient amount and type of additive. (Percent linear shrinkage
indicates 100.times.(initial length-final length)/initial length.
The drying stage is complete when the body contains about 3 wt.%
water or less.)
[0012] The shrinkage mechanism in silica sol-gel bodies has been
modeled using classical drying theory, and this modeled mechanism
is widely accepted. (See, e.g., C. J. Brinker and G. W. Scherer,
supra, 453-509.) Specifically, as drying occurs on the outer
surface of a gel body, the solid material at the drying front is
exposed to the ambient atmosphere. Because silica typically has
surface silanols and thereby is hydrophilic, solid-vapor
interfacial energy is greater than solid-liquid interfacial energy,
i.e., the solid prefers to be wetted by the liquid rather than the
vapor. To make this happen, liquid from the interior of the body
flows toward the exterior to replace liquid that is evaporating
from the outer surface. The flow of this liquid through a tortuous,
fine pore matrix induces a pressure gradient, thereby putting the
liquid in tension, and the resultant compressive force on the gel
body causes shrinkage, as well as the tendency for cracking--a
major obstacle in sol-gel manufacture. Thus, it is this hydrophilic
property of silica that leads to shrinkage and cracking.
[0013] The process of the invention controls this shrinkage
mechanism by providing in situ hydrophobicity to the silica during
gelation. The additive generally contains a hydrocarbon chain
having an attached hydrogen-acceptor moiety (e.g., an ether or
carboxylic acid group), and is believed to function as follows. At
the relatively high pH (>10) at which the colloidal silica sol
is initially stabilized, a relatively low number of silanol groups
are present on the surface of the silica particles due to
de-protonation. Thus, adsorption of the polymer additive onto the
silanols is also relatively low. However, as the gelling agent
lowers the pH, surface silanol groups are re-formed, and thus an
increasing amount of the additive adsorbs onto the silica
particles. See, e.g., H. D. Bijsterbosch et al., "Nonselective
Adsorption of Block Copolymers and the Effect of Block
Incompatibility," Macromolecules, Vol. 31, 7436-44 (1998). This
adsorption expectedly renders the silica surface increasingly
hydrophobic (i.e., in situ hydrophobicity), because the
oxygen-containing groups of the additive are oriented toward and
hydrogen-bonded to the silanols, such that the exposed hydrocarbon
chains form a hydrophobic coating. By making the silica
hydrophobic, the propensity of the liquid to wet the silica of the
gel body is substantially diminished, i.e., the liquid puts much
less tension on the drying gel body. And the result is less
shrinkage and substantially less cracking.
[0014] Surprisingly, one such polymeric additive is optionally the
so-called binder referred to in the '488 patent, but in higher
concentrations than called for in the patent. It was unexpectedly
discovered that providing an increased concentration of such
binder, relative to the amounts called for in the '488 patent,
provided a significant decrease in shrinkage of gel bodies. For
example, Col. 8, lines 37-55 of the '488 patent instruct one to
use, for a 40 to 50 wt. % silica dispersion, at most, 1 wt. %
polymer based on the weight of the silica, but suggest that 0.5 wt.
% or less is preferred. Higher amounts are indicated to lead to
void formation and associated degradation of optical quality. Yet,
it was discovered that higher amounts, for such silica loadings,
are actually advantageous--in contrast to the teachings of the '488
patent. The process of the present invention generally uses, for
such loadings, greater than 1 wt. % of additive, more typically at
least 3 wt. %, based on the weight of the silica, with the precise
amount depending on, among other things, the particular additive,
as discussed below.
[0015] By substantially lowering shrinkage, the invention makes it
possible to increase the speed of drying, heat treatment, and
sintering for sol-gel bodies. Moreover, the invention makes it
possible to form more intricate and delicate bodies that are not
feasible with current sol-gel techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows the effect on shrinkage of varying amounts of
the polymeric additive of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] According to one embodiment of the invention, an aqueous
stabilized silica dispersion, or sol, is provided or produced. The
dispersion typically contains about 40 to about 75 wt. % silica,
more typically about 50 to about 65 wt. % silica, based on the
weight of the sol. The surface area of the silica generally ranges
from (nominally) 5 to 100 m.sup.2/g, more typically 10 to 50
m.sup.2/g. Shrinkage that occurs during drying is controlled
largely by the starting concentration of silica particles of a
given size range. For example, following a technique such as
disclosed in the '488 patent, but with no binder or plasticizer
added, a gel body formed from fumed silica particles of nominal
surface area of 50 m.sup.2/g will shrink about 13 linear percent
using a sol with 43 wt. % silica, but only about 10 linear percent
with a sol containing 52 wt. % silica. (The feasible upper limit
for silica of this size is about 65 wt. % --at higher loadings the
rheology of the sol is generally too restrictive to all
processing.) The lowest attainable drying shrinkage for nominal 50
m.sup.2/g silica, without added polymer and with an acceptable
rheology for processing, is about 5%, using a 65 wt. % silica sol.
More concentrated sols are possible with larger silica particles,
and would be expected to further decrease shrinkage.
[0018] The silica dispersion is stabilized by conventional methods,
typically at a pH greater than 10, more typically greater than 11.
Stabilization is generally provided by adding TMAH, typically up to
about 3 wt. %, although other organic bases are also possible,
including other tetraalkylammonium hydroxides where the alkyl is
ethyl, propyl, or butyl. The dispersion is typically aged for at
least 18 hours, more typically at least 24 hours, to adequately
dissolve the silica, and then optionally centrifuged to remove
contaminants.
[0019] Optionally, the isoelectric point (EP) of the silica is
adjusted to about 9.0 (advantageously 10.0) or greater by addition
of an IEP-modifying compound, such that the gel point is about 10.5
or greater. (The isoelectric point is the point on the pH scale
where the zeta potential is zero, as discussed in C. J. Brinker and
G. W. Scherer, Sol-Gel Science, supra. The gel point is typically
about 1 to 2 pH units higher than the isoelectric point.
Specifically, as the pH approaches the IEP, the zeta potential, and
thus the mutual repulsion, of the particles diminishes to the point
where ordinary thermal energy, i.e., Brownian motion, is able to
break through the repulsive barrier such that gelation begins.) The
IEP-modifying compound is typically selected from ammonium
hydroxide, primary amines, secondary amines, tertiary amines, or
compounds containing a combination of primary, secondary, and/or
tertiary amines (examples of the latter combination including
N,N'-bis(2-aminoethyl) piperazine and N,N'-bis-(3-aminopropyl)
piperazine). Examples of useful compounds within this group include
diethylenetriamine, hexamethylenediamine, and
tris(2-aminoethyl)amine. The amount of IEP-modifying compound added
depends on the compound's particular effect. A small amount of some
compounds has an effect equivalent to several times more of another
compound. In addition, the properties of the gel depend largely on
the particular IEP-modifying compound used. Selection of the
IEP-modifying compound also depends on whether one desires to cast
or extrude the resultant gel. Use of such compounds to adjust IEP
is discussed in co-assigned U.S. Pat. No. 5,944,866, and
co-assigned U.S. Pat. application Ser. No. 09/280588, filed Mar.
29, 1999, the disclosures of which are hereby incorporated by
reference. The dispersion is stabilized, and IEP adjusted, such
that the pH of the dispersion is greater than the gel point.
[0020] The polymeric additive of the invention is then added
(before, with, or just after the gelling agent). As noted above,
the polymeric additive provides in situ hydrophobicity to the
silica. The additive is believed to function by the following
mechanism. At the relatively high pH (>10) at which the
colloidal silica sol is initially stabilized, a relatively low
number of silanol groups are present on the surface of the silica
particles. Thus, adsorption of the polymer additive onto the
silanols is also relatively low, and silica remains in its charged
and hydrophilic state. However, as the gelling agent lowers the pH,
more surface silanol groups are found, and thus an increasing
amount of the additive adsorbs onto the silica particles. This
adsorption renders the silica surface increasingly hydrophobic
(i.e., in situ hydrophobicity), because the oxygen-containing
groups of the additive are oriented toward and hydrogen-bonded to
the silanes, such that the exposed hydrocarbon chains form a
hydrophobic coating. By making the silica hydrophobic, the
propensity of the liquid to wet the silica of the gel body is
substantially diminished, i.e., the liquid puts much less tension
on the drying gel body. And the result is less shrinkage and less
cracking. (There is a continuum from hydrophilic to hydrophobic,
but silica is considered to be hydrophilic when there are at least
2.7 surface silanol groups per square nanometer, and hydrophobic
when at least 30% of these groups are removed or no longer
exposed)
[0021] The inclusion of the additive generally changes the rheology
of the sol, however. Specifically, when the polymer additive is
introduced in to the sol at a relatively high pH of about 11.5, the
silica particles have weak interactions driven by the well-known
depletion flocculation mechanism. This results in increased
viscosity of the sol. Thus, this increase must be factored in when
designing a specific process. The additive generally contains a
hydrocarbon chain having an attached hydrogen-acceptor moiety
(e.g., an ether or carboxylic acid group). Examples include hydroxy
propyl cellulose, poly(2-ethyl-2-oxazoline)-co-(-
2-phenyl-2-oxazoline), poly(2-ethyl-2-oxazoline), poly(ethylene
oxide), poly(ethylene glycol), poly(vinylpyrrolidone), and
poly(vinyl alcohol). The first three of these listed compounds are
particularly advantageous, as reflected in the Examples. In
general, the greater the hydrophobicity of the exposed chains, the
greater the effect of the polymer on lowering shrinkage. As
reflected in Example 8, larger molecular weight polymers also tend
to have an increased effect on reducing shrinkage, since they
adsorb more strongly to the silica surface. The surface bonding is
surmised to be dominated by the presence of isolated silanols
separated by siloxane regions, as is the case with fumed silica
particle surfaces. (See, e.g., J. Rubio and J. A. Kitchener, "The
Mechanism of Adsorption of Poly(Ethylene Oxide) Flocculant on
Silica, Journal of Colloid and Interface Science, Vol. 57, No. 1,
132 (1976).) However, such large polymers tend to be difficult to
dissolve in water, and thus molecular weights in the range of 20K
to 100K appear to be the most useful and feasible.
[0022] As noted above, the polymer additive affects the drying
mechanism, such that at the completion of drying, the gel body has
undergone linear shrinkage at least 55% less than an identical
process without the polymeric additive. Optionally, the linear
shrinkage is at least 90% less than such an identical process. The
amount of polymer will vary depending on, among other things, the
weight percent silica in the sol. For example, for a 55 wt. %
silica sol, a concentration of greater than 1 wt. % additive will
generally be useful. For higher loadings, e.g., greater than 60 wt.
% silica, less polymer is needed, e.g., 0.5 wt. % additive is
typically useful. Also, as reflected in the Examples, the
properties of the polymer additive determine its effect on reducing
shrinkage, and thus the precise amount of additive needed will also
depend on the particular additive and the amount of shrinkage that
is allowable.
[0023] A gelling agent is added to reduce the pH of the dispersion
to the gel point. Generally, up to about 5 wt. % (based on the
weight of the dispersion) of gelling agent is suitable, with the
requisite amount selected depending on the concentration of base
used for stabilizing the dispersion. The gelling agent is typically
a water-soluble liquid that undergoes hydrolysis to consume base,
e.g., an ester, amide, or an alkyl halide, and thereby lowers the
pH. After adding the gelling agent, the dispersion is typically
transferred into a mold or an extruder, where it is allowed to gel.
Gelling typically occurs over a time period of about 15 minutes to
about 20 hours. Where the gel body is molded, the gel is then
typically allowed to age in the mold for up to 30 hours. For
extrusion, the gel generally ages for a few hours or less. Aging
provides a desirable rearrangement of particles, leading to better
packing, expulsion of some liquid around the particles, and
associated shrinkage of the gel in the mold --a process known as
syneresis. Syneresis adds strength and, due to the shrinkage, eases
removal from a mold. Once aged, the gel is released from the mold,
or extruded into the desired shape. The gel is then dried,
typically starting under relatively moderate conditions, e.g.,
temperature less than 25.degree. C. and relative humidity greater
than 50%. A significant advantage of the invention is the ability
to dry the body more rapidly, due to the lower shrinkage. As noted
above, the drying stage is considered to be complete when about 3
wt. % water remains in the body.
[0024] Heat treatment of the body is then performed. (As used
herein, heat treatment includes any number or combination of steps
that provide removal of water, hydroxyl ions, organic materials,
metal contaminants, undesired refractory metal oxide particulates,
and/or other undesired elements.) Typically, the body is heated to
temperatures of 25 to 400.degree. C. to complete water removal and
remove organics in an inert atmosphere. Air is generally introduced
to oxidize remaining organics. Metal contaminants, hydroxyl ions,
and refractory metal oxide particulates are generally removed by
exposure to a chlorine-containing atmosphere at temperatures of 400
to 1000.degree. C. An additional air treatment is generally
performed to remove chlorine from the body, and the body is then
cooled in nitrogen, and kept in a dry atmosphere until sintering is
performed. Alternative treatments that provide sufficient removal
of undesired materials are also possible. See, e.g., co-assigned
U.S. Pat. No. 5,356,447 and co-assigned U.S. Pat. application No.
09/109,827.
[0025] The process is useful for a variety of applications,
including fabrication of overcladding tubes for optical fiber
preforms, as discussed, for example, in U.S. Pat. No. 5,240,488,
referenced previously, as well as substrate tubes. Because of the
extremely low shrinkage attained by the process of the invention,
however, the sol-gel technique has a broader application than
previous sol-gel techniques. For example, it is possible to form a
sol-gel film on a rigid substrate, since with low shrinkage, lower
stresses are induced during drying. Complex and precision parts are
also contemplated due to the lessened shrinkage. Also contemplated
are bodies comprising multiple compositions, e.g., preforms for
graded index fiber, by reducing differential shrinkage. In
particular, it is typically not possible to form such composite
fibers, since the different shrinkage rates of different
compositions in a preform tend to cause fracture during drying.
[0026] The invention will be further clarified by the following
examples, which are intended to be exemplary.
COMPARATIVE EXAMPLE 1
[0027] A sol containing about 43 wt. % commercial fumed silica
particles with a nominal specific surface area of 50 m.sup.2/gm was
made using 1.5 wt. % tetramethylammonium hydroxide (TMAH) as a
stabilizer. The sol was mixed with 0.6 wt. % tris(2-aminoethyl)
amine (STAR) as the IEP modifier and 2.4 wt % of methyl acetate as
the gelling agent. The sol was poured into a tubular mold about 30
cm long and 10 cm in diameter. The resultant gel was aged for about
15 hours and then extracted from the mold. It was dried at 65%
relative humidity (RH) and 22.degree. C. for about a week.
Isotropic shrinkage was observed until the critical point, where
shrinkage stopped. The net shrinkage was about 13 linear %. When
the starting sol was replaced by more concentrated sols containing
55% and 61% silica, the shrinkage dropped to about 10% and 6%
respectively.
[0028] Additional experiments, using the same technique, were
performed with different mold sizes, different drying rates, and/or
with STAR absent. The shrinkage was shown to be substantially
independent of these parameters.
COMPARATIVE EXAMPLE 2
[0029] A series of experiments were carried out to examine the
effect of surfactants, which might conventionally be expected lower
the surface tension of water in the sol and hence the capillary
forces. For gels made according to Comparative Example 1, using a
55 wt. % silica sol, 0.05-0.1 wt. % of several non-ionic
surfactants were added just prior to adding the gelling agent. The
surfactants were TritonX-100, a hydrocarbon surfactant, FC170C
(available from 3M Company) and Zonyl FSN (available from Dupont
Company), the latter two being fluorinated surfactants. Addition of
these surfactants did not change the shrinkage behavior, relative
to an identical process with no additive.
COMPARATIVE EXAMPLE 3
[0030] Following the procedure of Comparative Example 1, for a 55
wt.% silica sol, several experiments were performed in which up to
1 wt. % of trimethylbutoxy silane (dissolved in methyl acetate) was
added prior to adding the gelling agent. It was contemplated that
introduction of alkyl-alkoxy silanes would make the silica
hydrophobic due to the attachment of the silane to the silica
particles. Addition of the silane induced no change in shrinkage
relative to an identical process with no additive. It is believed
that the silane hydrolyzed and di-merized, rather than attaching to
the silica surface.
EXAMPLE 4
[0031] The procedure of Comparative Example 1 was followed, with a
55 wt.% silica sol. Varying amounts of poly(2-ethyl-2-oxazoline)
(MW 50,000) were added prior to gelation. This polymer had a
significant impact on shrinkage as shown in FIG. 1. This Figure
shows the shrinkage as a function of the degree of drying for
different concentrations of poly(2-ethyl-2-oxazoline), with the
weight % of the polymer based on the initial weight of the 55%
silica sol. The abscissa axis represents the mass of the gel as a
fraction of the initial mass of the gel as water evaporates. Water
recedes in to the pores at the critical point, which is also the
end point of shrinkage. As seen in the plot, with lowered
shrinkage, the critical point occurs earlier in the drying
stage.
[0032] At relatively small concentrations of the polymer, such as
below 0.3 wt. %, there was little change in the viscosity of the
sol. As the concentration increased, however, the viscosity
increased immediately after the addition of the polymer, due to
flocculation.
EXAMPLE 5
[0033] The procedure of Example 4 was followed, with the additive
being poly(2-ethyl-2-oxazoline)-co-(2-phenyl-2-oxazoline) (MW
50,000). Adding 0.5 wt. % of this additive decreased the shrinkage
to 1.8 linear %.
COMPARATIVE EXAMPLE 6
[0034] The procedure of Example 5 was followed, with the additive
being polyethylenimine (MW 18,000). There was no change in the
viscosity of the sol (no flocculation) or shrinkage of the gel
compared to an identical process with no additive, even with 2 wt.
% of the polymer added in. Given that the poly(ethyloxazoline) of
Example 4 is considered to be a N-propionyl substituted linear
polyethylimine, this difference in the behavior highlights the
importance of the side chain in providing the necessary
hydrophobicity.
EXAMPLE 7
[0035] The procedure of Example 4 was followed, with the additive
being hydroxy propyl cellulose (MW 80,000), which was highly
effective in lowering shrinkage. At an additive concentration of
1.0 wt. %, the shrinkage was only 0.5 linear %. As with the
poly(ethyloxazoline), addition of this molecule also increased the
viscosity of the sol.
EXAMPLE 8
[0036] Following the procedure of Example 4, 0.5 wt. %, based on
the weight of the entire sol, of various additives were added to
the sol. The effect on shrinkage is shown in the Table below.
1 TABLE Polymer Additive Linear Shrinkage None 9.7% Hydroxy propyl
cellulose (MW 80K) 1.2 Poly(2-ethyl-2-oxazoline)-co-(2-phenyl-2-
1.8 oxazoline) (MW 50K) Poly(ethylene oxide) (MW 100K) 2.5
Poly(2-ethyl-2-oxazoline) (MW 50K) 3.0 Poly(2-ethyl-2-oxazoline)
(MW 5K) 7.0 Poly(ethylene glycol) (MW 10K) 7.0
Poly(vinylpyrrolidone) (MW 55K) 7.3 Poly(vinyl alcohol) (MW 13-23K)
7.5 Polyethylenimine (MW 18K) 9.6 Polyethylenimine (ethoxylated)
(MW 80K) 9.6
[0037] Other embodiments of the invention will be apparent to those
skilled in the art from consideration of the specification and
practice of the invention disclosed herein.
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