Note: Descriptions are shown in the official language in which they were submitted.
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HYDROPHOBIC SILICA GELS WITH REDUCED SURFACE AREA
BACKGROUND OF INVENTION
The present invention is hydrophobic silica gels having reduced surface area
and a
method for their preparation. The method comprises three steps, where in the
first step a
mixture comprising a silica hjrdrosol and a colloidal silica is formed. In the
second step, the
mixture is heat treated in the presence of a strong mineral acid at a pH less
than about 1 to
form a silica hydrogel having incorporated therein colloidal silica. In the
third step the
silica hydrogel is contacted with an organosilicon compound in the presence of
a catalytic
amount of a strong acid to effect hydrophobing of the silica hydrogel thereby
forming a
hydrophobic silica gel having a surface area within a range of about 100 m''/g
to 450 m2/g
as measured in the dry state. In a preferred process the hydrophobic silica
gel is contacted
with a sufficient quantity of a water-immiscible organic solvent to convert
the hydrophobic
silica hydrogel into a hydrophobic silica organogel. The organic solvent can
then be
1 S removed from the organogel to form a hydrophobic silica gel having a
surface area within a
range of about 100 m2/g to 450 m2/g as measured in the dry state. A water
soluble
compound of cerium or iron may be added in the second step to improve the heat
stability
of the hydrophobic silica gel.
Although hydrophobic silica gels prepared by the present method are useful in
many
2 0 applications such as thermal insulating, reinforcing and extending filler
in natural rubbers,
and as filler in floatation devices, they are particularly useful as
reinforcing fillers in
silicone rubber compositions. It is well known that silicone rubber formed
from the
vulcanization of polydiorganosiloxane fluids or gums alone generally have low
elongation
and tensile strength values. One means for improving the physical properties
of such
2 5 silicone rubber involves the incorporation of a reinforcing silica filler
into the fluid or gum
prior to curing. However, silica reinforcing fillers have a tendency to
interact with the
polydiorganosiloxane fluid or gum causing a phenomenon typically referred to
as "crepe
hardening." A great deal of effort has been made in the past to treat the
surface of
reinforcing silica fillers with organosilanes or organosiloxanes to make the
surface of the
3 0 silica hydrophobic. This surface treatment reduces or diminishes the
tendency of the
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compositions to crepe harden and improves the physical properties of the cured
silicone
rubber.
Brown, U. S. Pat. No. 3,024,126, teaches a method for making a pre-formed
reinforcing silica filler hydrophobic by treating it in an organic solvent
with an
organosilicon compound, such as an organosilane or low-molecular weight
organosiloxane
containing 0.1 to 2 total hydroxyl and/or alkoxy radicals per silicon atom,
and a small
amount of amine, quaternary ammonium, or organometallic compound.
Lewis, U. S. Pat. No. 3,979,546, teaches a method for making the surface of
reinforcing silica fillers hydrophobic through the use of alpha-alkoxy-omega-
siloxanols
with alcohols under mild conditions. The fillers taught are pre-formed solids.
Tyler, U. S. Pat. No. 3,015,645, teaches the making of hydrophobic silica
powders ,
by reacting an organosilicon compound such as dimethyldichlorosilane or
trimethylmethoxysilane with a silica organogel in the presence of an acidic
catalyst and then
removing the volatile materials. The method requires the preparation of a
silica hydrogel
which is converted to a silica organogel by replacing the water in the
hydrogel with an
organic solvent.
Lentz, U. S. Pat. No. 3,122,520, teaches a procedure where an acidic silica
hydrosol
is first heated to develop a reinforcing silica structure and then mixed with
an organosilicon
compound, an acid catalyst, and a water-immiscible organic solvent to produce
a
2 0 hydrophobic silica filler. The organosilicon compounds taught by Lentz are
limited to those
compounds in which the organic radicals bonded to silicon atoms have less than
6 carbon
atoms, organosilicon compounds having no organofunctional substituents bonded
to silicon
atoms, and to organosilicon compounds having no hydrogen bonded to silicon
atoms.
Alexander et al., U. S. Pat. No. 2,892,797, describe silica sols modified by
treatment
2 5 with a solution of a metalate so that the silica particles are coated with
no more than a
molecular layer of a combined metal which forms an insoluble silicate at a pH
between S
and 12. Aluminum, tin, zinc, and lead are taught as the preferred metals.
Alexander et al.
teach that silica sols which carry a metal upon the surface of the particles
according to their
invention have increased stability at pH extremes.
3 0 Termin et al., U. S. Pat. No. 3,850,971, and Termin et al. U. S. Pat. No.
4,006,175
teach that porous silicic acid having a specific surface area of about 50 m2/g
to 1000 m'/g
2
............._ . ...~_.e... ... _ ~ .........r.,_.. _. r.. _....... ..
.........._. . .......
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can be made by hydrolyzing methyl or ethyl silicate or polymethyl or polyethyl
silicate with
about 70 to 120% of the stoichiometric amount of water with moderate stirring.
Termin et
al. teach that transition metals such as iron oxides and chronuum oxides may
be used as
hydrolysis activators and that such metals may appear in the end product.
Nauroth et al., U. S. Pat. No. 4,360,388, teach cerium containing precipitated
silica.
Nauroth et al. teach that silicone rubber compositions reinforced with the
cerium
containing precipitated silica exhibit excellent heat stability and that the
cerium containing
precipitated silica acts as a fire retardant agent.
Nauroth et aL, U. S. Pat. No. 4,208,316, teach the use of hydrophobic
precipitated
silica as a reinforcing filler in plastic masses which are hardenable to form
elastomers.
Such elastomers include silicone elastomers.
The hydrophobic silica gels prepared by the present method have improved
compatibility with polydiorganosiloxane polymers, when compared to silica gels
prepared
in the absence of the colloidal silica. Therefore, the present hydrophobic
silica gels
25 incorporating the colloidal silica are especially suited for use as
reinforcing fillers in
compositions curable to form silicone rubber. Such cured compositions can have
improved
physical properties such as tear and tensile strength, when compared to
compositions using
hydrophobic silica gels as reinforcing filler without the incorporation of
colloidal silica.
2 0 SUMMARY OF INVENTION
The present invention is hydrophobic silica gels having reduced surface area
and a
method for their preparation. The method comprises three steps, where in the
first step a
mixture comprising a silica hydrosol and colloidal silica is formed. In the
second step, the
mixture is heat treated in the presence of a strong mineral acid at a pH less
than about 1 to
2 5 form a silica hydrogel having incorporated therein colloidal silica. In
the third step the
silica hydrogel is contacted with an organosilicon compound in the presence of
a catalytic
amount of a strong acid to effect hydrophobing of the silica hydrogel thereby
forming a
hydrophobic silica gel having a surface area within a range of about 100 m2/g
to 450 m2/g
as measured in the dry state. In a preferred method the hydrophobic silica gel
is contacted
3 0 with a sufficient quantity of an organic solvent immiscible with water to
convert the
hydrophobic silica hydrogel into a hydrophobic silica organogel. The organic
solvent can
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be removed from the hydrophobic silica organogel to form a hydrophobic silica
gel having
a surface area within a range of about 100 mz/g to 450 m2/g as measured in the
dry state.
DESCRIPTION OF INVENTION
The present invention is hydrophobic silica gels having reduced surface area
and a
method for their preparation. ~'he method for preparing the hydrophobic silica
gels
comprises:
(A) forming a mixture comprising (i) a silica hydrosol comprising about 0.02 g
to 0.5 g of
Si02 per milliliter and having an average particle size less than 4 nm and
(ii) about 0.1 to
50 weight percent of colloidal silica having an average particle size of at
least 4 nm,
(B) heating the mixture in the presence of a strong mineral acid at a pH less
than about 1
and a temperature within a range of about 20°C to 250°C to form
a silica hydrogel having
the colloidal silica incorporated therein, and
(B) contacting the silica hydrogel with ( 1 ) a catalytic amount of a strong
acid in an amount
sufficient to effect reaction of (2) an organosiIicon compound selected from
the group
consisting of organosilanes described by formula
R~aHbSlX4_a_b (1)
and organosiloxanes described by formula
R~~SiO~d_"y2 (2)
2 0 with the silica hydrogel, where each R1 is independently selected from a
group consisting of
hydrocarbon radicals comprising about 1 to 12 carbon atoms and
organofunctional
hydrocarbon radicals comprising about 1 to 12 carbon atoms, each X is
independently
selected from a group consisting of halogen and alkoxy radicals comprising
about 1 to 12
carbon atoms, a=0, 1, 2, or 3, b=0 or 1, a+b=1, 2, or 3 with the proviso that
when b=1 then
2 5 a+b=2 or 3, and n is an integer of from 2 to 3 inclusive to form a
hydrophobic silica
hydrogel having a surface area within a range of about 100 mz/g to 450 m2/g as
measured in
the dry state.
The method of the present invention is a three-step procedure, comprising
steps (A),
(B), and (C) for making hydrophobic silica gels having colloidal silica
incorporated therein.
3 0 Step (A) of the method comprises forming a mixture comprising a preformed
silica
hydrosol having an average particle size less than 4 nanometers (nm) and a
preformed
4
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silica hydrosol having an average particle size of at least 4 nm, referred to
herein as
"colloidal silica".
Step (B) of the method comprises heating the mixture comprising the silica
hydrosol
and colloidal silica under strong acid conditions to form a silica hydrogel
having the
colloidal silica incorporated therein. Step (C) comprises contacting the
silica hydrogel
prepared in step (B) with an organosiiicon compound which reacts with the
silica hydrogel
to give a hydrophobic silica hydrogel. In a preferred method, sufficient water-
immiscible
organic solvent is added in step (C) to convert the silica hydrogel or
hydrophobic silica
hydrogel to the corresponding organogel. The solvent can then be removed from
the
hydrophobic silica organogel to form a hydrophobic silica gel. Hydrophobic
silica gels
prepared by the present method have reduced surface area which improves their
ease of
incorporation into silicone rubber compositions and make them suitable as
reinforcing
fillers in such compositions.
The method used to prepare the silica hydrosol is not critical and can be any
of those
known in the art. As used herein the term "silica hydrosols" means those
hydrosols of silica
having an average particle size less than 4 nanometers (nm). Silica hydrosols
useful in the
present method can be prepared by, for example, deionizing sodium silicate by
a method
such as the use of an ion exchange resin. The silica hydrosol may be prepared
by
hydrolyzing a silane at a low temperature. The silica hydrosol may be prepared
by
2 0 acidifying a sodium silicate mixture.
In the present method, the silica hydrosol provides about 0.02 g to 0.5 g of
Si02 per
ml of the mixture. Preferably, the silica hydrosol provides about 0.05 g to
0.2 g of Si02 per
ml of the mixture.
The mixture of the present method requires the presence of about 0.1 to 50
weight
2 5 percent of colloidal silica, based on the total weight of the mixture. As
used herein the term
"colloidal silica" refers to hydrosols of silica having an average particle
size of at least 4
nm. Preferred is when the mixture comprises about 10 to 30 weight percent of
colloidal
silica, based on the total weight of the mixture. Generally, the colloidal
silica useful in the
present method and compositions can be described as a colloidal amorphous
silica that has
3 0 not at any point existed as a gel during its preparation. The method of
preparation of the
colloidal silica is not critical to the present method and compositions and
can be any of
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those known in the art. The colloidal silica can be prepared by, for example,
combining an
aqueous solution of a soluble metal silicate, such as sodium silicate, and an
acid so that the
colloidal particles grow in a weakly alkaline solution until the desired
particle size is
achieved. Preferred is a colloidal silica having an average particle size
within a range of 4
to about 300 nm. Even more preferred is a colloidal silica having an average
particle size
within a range of about 6 to 100 nm.
In step (B), the mixture comprising the silica hydrosol and the colloidal
silica must
comprise a sufficient concentration of a strong mineral acid such that the pH
of the mixture
is less than about pH 1. Preferably, there should be a sufficient amount of
the strong
mineral acid present so that the pH is essentially 0, that is so that the pH
cannot be
measured. For the purpose of this invention any strong mineral acid can be
used. As used
herein, the term "strong mineral acid" refers to those acids which ionize to
the extent of at
least 25 percent in 0.1 N aqueous solution at 18°C. The strong mineral
acid may be, for
example, hydrochloric, hydroiodic, sulfuric, nitric, and phosphoric acid.
In step (B), the mixture comprising the silica hydrosol and the colloidal
silica is
heated at a temperature within a range of about 20°C to 250°C.
Preferred is when the
mixture is heated at a temperature within a range of about 75°C to
150°C. Even more
preferred is when, in step (A), the mixture is heated at a temperature within
a range of about
90°C to 110°C.
In step (B), the heating time required varies with the temperature and acid
concentration. Generally, the higher the temperature and the greater the acid
concentration
the shorter the heating time needed. The heating of step (B) must be continued
until the
silica hydrogel having the colloidal silica incorporated therein acquires a
structure such that
the final product after. hydrophobing has a surface area as measured in the
dry state within a
2 5 range of about 100 m2lg to 450 m2/g as determined by the Brunauer Emmett
and Teller
(BET) method described in the Jour. Am. Chem. Soc. 60:309 (1938) and as
further
described in Lentz, U. S. Pat. No. 3,122,520, which is hereby incorporated by
reference for
such a teaching.
The surface area of the silica hydrogel at the conclusion of step (B) is
immaterial
3 0 provided it is such that the surface area of the dried product after the
hydrophobing of step
(C) is within the above described range. Generally the surface area of the
silica hydrogel is
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reduced by the hydrophobing reaction, since the organosilyl groups which
become attached
to the surface of the silica hydrogel increase the average particle size. The
surface of the
silica hydrogel can be above 450 m2/g provided that the hydrophobing treatment
brings it
within a range of about I00 m2/g to 450 m2/g. To determine the proper heating
conditions
during conduct of step (B) it is necessary to proceed with the hydrophobing of
step (C) and
then measure the surface area of the resulting product in the dry state. If
the surface area of
the resulting product in the dry state is above 450 mZ/g, then the acid
heating conditions of
step (B) were too mild. If the surface area of the resulting product in the
dry state is below
100 m2/g, then the acid heating conditions of step (B) were too severe.
Examples of
suitable acid concentrations, temperatures, and times for conduct of step (B)
are provided in
the Examples herein. If the surface area of the hydrophobic silica gel in the
dry state is
above or below the described range, the hydrophobic silica gels have
diminished reinforcing
properties in silicone elastomers.
If desired, the silica organogel of step (B) may be subjected to a shearing
force to
reduce aggregate particle size and to improve the uniformity of the particle
size distribution
prior to the conduct of the hydrophobing reaction of step (C). The shearing
force may be
applied to the silica organogel by any of those methods known in the art. The
shearing
force may be applied, for example, by a mechanical means such as a high-speed
mixer or by
ultrasound. This reduction in aggregate particle size and improved uniformity
of the
2 0 particle size can provide for hydrophobic silica gels which when
compounded into silicone
elastomer compositions provide for lower viscosity compositions, more stable
compositions, and for cured silicone elastomers having improved clarity and
physical
properties.
In step (C) of the present method the silica hydrogel of step (B) is contacted
with
2 5 one or more of the defined organosilicon compounds described by formulas (
1 ) and (2) in
the presence of a catalytic amount of a strong acid to effect hydrophobing of
the silica gel.
In step (C), the strong acid can be the same acid which was used in step (B).
However, if
desired the silica hydrogel can be washed free of acid and a catalytic amount
of strong acid
added either prior to, simultaneously with, or subsequent to the addition of
the
3 0 organosilicon compound. In the case where the organosilicon compound is,
for example, a
chlorosilane, the catalytic amount of the strong acid can be generated in situ
by hydrolysis
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of the chlorosilane or the reaction of the chlorosilane directly with
hydroxyls of the silica
hydrogel. In step (C) the limitations on pH as described for step (B) do not
apply. It is only
necessary that a catalytic amount of a strong acid be present in an amount
sufficient to
effect reaction of the organosilicon compound with the silica hydrogel.
Examples of useful
acids include hydrochloric, sulfuric, and benzene sulfonic acids. It is
preferred that in step
(C) the strong acid catalyst provide a pH less than about 2.5.
The temperature at which the hydrophobing of step (C) is conducted is not
critical
and can be from about 20°C to 250°C. Generally it is preferred
that the hydrophobing of
step (C) be conducted at a temperature within a range of about 30°C to
150°C. The
hydrophobing of Step (C) can be conducted at the reflux temperature of the
water-
immiscible organic solvent when it is present.
In step (C), the silica hydrogel of step (B) is reacted with an organosilicon
compound described by formula ( 1 ) or (2). In formulas ( 1 ) and (2), each R'
can be
independently selected from a group consisting of hydrocarbon radicals
comprising about 1
to 12 carbon atoms and organofunctional hydrocarbon radicals comprising about
1 to 12
carbon atoms. R1 can be a saturated or unsaturated hydrocarbon radical. R' can
be a
substituted or non-substituted hydrocarbon radical. R' can be, for example,
alkyl radicals
such as methyl, ethyl, propyl, t-butyl, hexyl, heptyl, octyl, decyl, and
dodecyl; alkenyl
radicals such as vinyl, allyl, and hexenyl; substituted alkyl radicals such as
chloromethyl,
2 0 3,3,3-trifluoropropyl, and 6-chlorohexyl; and aryl radicals such as
phenyl, naphthyl, and
tolyl. R1 can be an organofunctional hydrocarbon radical comprising 1 to about
12 carbon
atoms where the the organic portion of the radical is substituted with
reactive atoms or
groups such as mercapto, disulfide, polysulfide, amino, carboxylic acid,
carbinol, ester, or
amido. A preferred organofunctional hydrocarbon radical is one having
disulfide or
2 5 polysulfide functionality.
In formula (1) each X is independently selected from a group consisting of
halogen
and alkoxy radicals comprising about 1 to 12 carbon atoms. When X is a
halogen, it is
preferred that the halogen be chlorine. When X is an alkoxy radical, X may be,
for
example, methoxy, ethoxy, and propoxy. Preferred is where each X is selected
from a
3 0 group consisting of chlorine atoms and methoxy.
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The viscosity of the organosiloxanes described by formula (2) is not limiting
and
can range from that of a fluid to a gum. Generally, higher molecular weight
organosiloxanes will be cleaved by the acidic conditions of the present method
allowing
them to react with the silica hydrogel.
The organosilicon compound may be provided to the present method as a single
compound as described by foimulas ( 1 ) or (2) or as a mixture of two or more
organosilicon
compounds described by formulas ( 1 ) and {2).
Examples of useful organosilicon compounds include diethyldichlorosilane,
allylmethyldichlorosilane, methylphenyldichlorosilane,
phenylethyldiethoxysilane, 3,3,3-
trifluoropropylmethyldichlorosilane, trimethylbutoxysilane, sym-
diphenyltetramethyldisiloxane, trivinyltrimethylcyclotrisiloxane,
hexaethyldisiloxane,
pentylmethyldichlorosilane, divinyldipropoxysilane, vinyldimethylchlorosilane,
vinylmethyldichlorosilane, vinyldimethylmethoxysilane, trimethylchlorosilane,
hexamethyldisiloxane, hexenylrnethyldichlorosilane,
hexenyldimethylchlorosilane,
dimethylchlorosilane, dimethyldichorosilane,
mercaptopropylmethyldimethoxysilane, and
bis { 3-(triethoxysilyl)propyl ) tetrasulflde. When the hydrophobic silica gel
is to be used as a
filler in silicone rubber, it is preferred that the organosilicon compound be
hexamethyldisiloxane or dimethyldichlorosilane.
The amount of organosilicon compound added to the method is that sufficient to
2 0 adequately hydrophobe the silica hydrogel to provide a hydrophobic silica
gel suitable for
its intended use. Generally the organosilicon compound should be added to the
method in
an amount such that there is at least 0.04 organosilyl unit per Si02 unit in
the silica
hydrogel, the Si02 units including both those provided by the silica hydrosol
and the
colloidal silica. The upper limit of the amount of organosilicon compound
added to the
2 5 process is not critical since any amount in excess of the amount required
to saturate the
silica gel will act as a solvent for the method.
The hydrophobic silica hydrogel of step (C) may be used as is or may be
recovered
for use by such methods as centrifugation or filtration. The hydrophobic
silica hydrogel
may be dried by the use of such methods as heating or reducing pressure or a
combination
3 0 of both heating and reducing pressure.
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In a preferred method a water-immiscible organic solvent in sufficient amount
to
convert the silica hydrogel or hydrophobic silica hydrogel to the
corresponding organogel is
added. The organic solvent can be added prior to, simultaneously with, or
subsequent to the
addition of the organosilicon compound. That is the silica hydrogel can be
first converted
into an organogel by replacement of the water with the organic solvent and
then
hydrophobed. Alternatively, the organosilicon compound and the organic solvent
can be
added simultaneously to the silica hydrogel. Under these conditions the
reaction of the
silica hydrogel with the organosilicon compound and the replacement of the
water in the
hydrophobic silica hydrogel with the organic solvent may occur simultaneously.
Finally the
organosilicon compound can be added prior to the organic solvent, in which
case the silica
hydrogel reacts with the organosilicon compound and the resulting product is
then
converted into an organogel by an addition of an organic solvent. In the
latter two cases the
conversion to a silica organogel is accomplished by a phase separation, in
which the
hydrophobic silica organogel passes into the organic solvent phase. A
preferred method is
where a water-immiscible organic solvent is added after formation of the
hydrophobic silica
hydrogel thereby effecting formation of a hydrophobic silica organogel.
For purpose of this invention any organic solvent immiscible with water can be
employed. Suitable water-immiscible solvents include low molecular weight
siloxanes
such as hexamethyldisiloxane, octamethylcyclotetrasiloxane,
diphenyltetramethyldisiloxane
2 0 and trimethylsilyl endblocked polydimethylsiloxane fluids. When a siloxane
is employed
as a solvent it may serve both as a solvent and as a reactant with the silica
hydrogel. In
addition, suitable solvents include aromatic hydrocarbons such as toluene and
xylene;
heptane and other aliphatic hydrocarbon solvents; cycloalkanes such as
cyclohexane; ethers
such as diethylether and dibutylether; halohydrocarbon solvents such as
methylene chloride,
2 5 chloroform, ethylene chloride, and chlorobenzene; and ketones such as
methylisobutyiketone.
The amount of water-immiscible organic solvent is not critical so long as
there is
sufficient solvent to convert the hydrophobic silica hydrogel into a silica
organogel.
Preferably the solvent should have a boiling point below about 250°C to
facilitate its
3 0 removal from the hydrophobic silica organogel, however the boiling point
is not critical
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since the solvent may be removed from the hydrophobic silica organogel by
centrifuging or
other suitable means.
After the silica hydrogel has been converted to the hydrophobic silica
organogel the
resulting product may be employed per se. That is the hydrophobic silica
organogel may be
used directly as a reinforcing agent in silicone rubber or in any other uses
for which this
type of product can be used. Alternatively, the solvent may be removed from
the
hydrophobic silica organogel and the resulting dry hydrophobic silica gel
used.
During the conduct of step (C) it may be desirable to add a surfactant or a
water-
miscible solvent to facilitate the reaction of the organosilicon compound with
the silica
hydrogel. The surfactant or water-miscible solvent may be added in the
presence or
absence of any water-immiscible organic solvent added to the method. Suitable
surfactants
can include, for example, anionic surfactants such as dodecylbenzene sulfonic
acid,
nonionic surfactants such as poiyoxyethylene(23)lauryl ether and
(Me3Si0)2MeSi(CHZ)3(OCHZCH2)~OMe where Me is methyl, and cationic surfactants
such
as N-alkyltrimethyl ammonium chloride. Suitable water-miscible solvents can
include, for
example, alcohols such as ethanol, propanol, isopropanol, n-butanol, and
tetrahydrofuran.
In step (C) of the present method an effective amount of a heat stabilizing
agent
selected from a group consisting of water soluble compounds of cerium and iron
may be
added. By the term "effective amount" it is meant that the water soluble
compound of
2 0 cerium or iron is present in the hydrophobic silica gel at a concentration
sufficient to
provide improved heat stability to those compositions in which the hydrophobic
silica gel is
incorporated. Such compositions can include, for example, silicone rubber,
natural rubber,
and synthetic organic rubber.
Generally, about 0.01 percent weight/volume (% Wt./Vol.) to 10 %Wt./Vol. of
the
2 5 water soluble compound of cerium or iron in relation to the volume of
components in step
(C), excluding solvents, is considered useful in the present process.
Preferred is where the
water soluble compound of cerium or iron comprises about 0.1 %Wt./Vol. to 1
%Wt./Vol.
on the same basis.
Examples of water soluble compounds which may be useful in the present method
3 0 include FeCl3, FeBr2, FeBr3.6H20, FeC12.4H20, FeI2.4H20, Fe(N03)3.6H20,
FeP04.2H20,
CeC13.9H~0, CeBr3.H20, CeI3.9H20, Ce(N03)3.6H20, and Ce(S04)2.2H20. A
preferred
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water soluble compound of cerium or iron for use in the present method is
selected from the
group consisting of FeCl3 and CeC13.9H20.
The following examples are provided to illustrate the present invention. These
examples are not intended to limit the scope of the present claims.
Example 1. A silica geI having incorporated therein colloidal silica was
hydrophobed with hexamethyldisiloxane. 750 ml of PQ N Clear Sodium Silicate
(PQ
Corporation, Valley Forge, PA) was diluted with 1350 ml of deionized water.
This solution
was added at a rate of 420 ml per minute to a rapidly stirred solution
comprising 280 ml of
concentrated hydrochloric acid (HCl) (Fisher Certified, Fisher Scientific,
Fair Lawn, NJ)
diluted with 620 ml of deionized water. The resulting mixture was stirred for
2 minutes and
then the pH adjusted to 2.5 using a sodium silicate solution. The resulting
3100 ml of silica
hydrosol contained 0.1 g of Si02 per milliliter.
The silica hydrosol prepared as described above was deionized by pumping
through
a 1.5 m x 5 cm column packed with 1500 ml of Dowex SOWXB-100 ion exchange
resin in
the acid form (The Dow Chemical Company, Midland, MI) at a rate of 60 ml per
minute.
The pH of the column effluent was monitored until the pH dropped below 0.5, at
which
point the next 2000-2400 ml of deionized silica hydrosol effluent was
collected.
The deionized silica hydrosol was agglomerated by placing 1 L of the deionized
silica hydrosol in a 5 L flask and, while stirring, adding 273 ml of colloidal
silica (Ludox~
2 0 SM, DuPont Chemicals, Wilmington, DE, average particle size of 10 nm) and
392 ml of
concentrated HCl (Fisher Certified). The silica hydrogel which formed within a
few
minutes of addition of the HCI was broken-up by additional stirring to form a
suspension
comprising an agglomerated silica hydrogel having incorporated therein the
colloidal silica.
The silica hydrogel suspension was heat treated at 100°C for 3 hours
and then cooled to
2 5 room temperature.
The heat-treated silica hydrogel suspension was hydrophobed as follows. To the
heat-treated silica hydrogel suspension, with stirring, was added 555 ml of
isopropanoI
followed by 288 ml of hexamethyldisiloxane. The resulting mixture was stirred
for 1 hour
at room temperature. Then, 1 L of toluene was added to the mixture. This
mixture was
3 0 mildly stirred for an additional 5 minutes, stirring stopped, and the
aqueous phase drained
from the bottom of the flask. The toluene phase was washed with 500 ml of
deionized
12
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CA 02280795 1999-08-12
WO 98/37020 PCT/US98/03273
water. The flask was then fitted with a Dean-Stark trap and the toluene phase
refluxed to
remove residual water. The toluene phase was removed by distillation under
reduced
pressure leaving as product a hydrophobic silica gel. The hydrophobic silica
gel was dried
overnight at 150°C to remove residual toluene. The yield of hydrophobic
silica gel was 202
g. Example 2. A silica gel having incorporated therein colloidal silica was
hydrophobed with hexamethyldisiloxane. A deionized silica hydrosol was
prepared as
described in Example 1. The deionized silica hydrosol was agglomerated by
placing 1 L of
the deionized silica hydrosol in a 5 L flask and, while stirring, adding 216
ml of colloidal
silica (Nalco~ 1050, Nalco Chemical Co., Chicago, IL) and 375 ml of
concentrated HCl
(Fisher Certified). The silica hydrogel which formed within a few minutes of
addition of
the HCl was broken-up by additional stirring to form a suspension comprising
an
agglomerated silica hydrogel having incorporated therein the precipitated
silica. The silica
hydrogel suspension was heat treated by refluxing for 3 hours and then cooled
to room
temperature.
The heat-treated silica hydrogel suspension was hydrophobed as follows. To the
heat-treated silica hydrogel suspension, with stirring, was added 530 ml of
isopropanol
followed by 100 ml of hexamethyldisiloxane. The resulting mixture was stirred
for 1 hour
at room temperature. Then, 1.3 L of toluene were added to the mixture. This
mixture was
mildly stirred for an additional 5 minutes, stirring stopped, and the aqueous
phase drained
2 0 from the bottom of the flask. The toluene phase was washed with 500 ml of
deionized
water. The flask was then fitted with a Dean-Stark trap and the toluene phase
refluxed to
remove residual water. The toluene phase was transferred to an open container
in an
exhaust hood and the toluene allowed to evaporate leaving as product a
hydrophobic silica
gel. The hydrophobic silica gei was dried for 4 hours at 150°C to
remove residual toluene.
2 5 The yield of dried hydrophobic silica gel was 2b7 g. The BET surface area
of the dried
hydrophobic silica gel was determined by the method described supra, and the
result is
reported in Table 1.
Example 3. A silica gel having incorporated therein colloidal silica was
hydrophobed with hexamethyldisiloxane. A deionized silica hydrosol was
prepared as
3 0 described in Example 1. The deionized silica hydrosol was agglomerated by
placing 1 L of
the deionized silica hydrosol in a 5 L flask and, while stirring, adding about
130 ml of
13
CA 02280795 1999-08-12
- WO 98/37020 PCT/US98/03273
colloidal silica (Nalco~ I 140, Nalco Chemical Co.) and 375 ml of concentrated
HCl
(Fisher Certified). The silica hydrogel which formed within a few minutes of
addition of
the HCl was broken-up by additional stirring to form a suspension comprising
an
agglomerated silica hydrogel having incorporated therein the colloidal silica.
The silica
hydrogel suspension was heat treated by refluxing for 3 hours and then cooled
to room
temperature.
The heat-treated silica hydrogel suspension was hydrophobed as follows. To the
heat-treated silica hydrogel suspension, with stirring was added 530 ml of
isopropanol
followed by 100 ml of hexamethyldisiloxane. The resulting mixture was stirred
for 1 hour
at room temperature. Then, 1750 ml of toluene were added to the mixture. This
mixture
was stirred for an additional 5 minutes, stirring stopped, and the aqueous
phase drained
from the bottom of the flask. The toluene phase was washed with 500 ml of
deionized
water. The flask was fitted with a Dean-Stark trap and the toluene phase
refluxed to
remove residual water. The toluene phase was transferred to an open container
in an
exhaust hood and the toluene allowed to evaporated leaving as product a
hydrophobic silica
gel. The hydrophobic silica gel was dried for 4 hours at 150°C to
remove residual toluene.
The yield of dried hydrophobic silica gel was 186 g. The BET surface area of
the dried
hydrophobic silica gel was determined by the method described in Example 2 and
the result
is reported in Table 1.
2 0 Example 4. A silica gel having incorporated therein colloidal silica,
hydrophobed
with hexamethyldisiloxane, and heat stabilized by the addition of FeCI~ was
prepared. 400
ml of PQ N Clear Sodium Silicate (PQ Corporation) was diluted with 600 ml of
deionized
water. This solution was added at a rate of 375 ml per minute to a stirred
solution
comprising 440 ml of concentrated HCl (Fisher Certified) diluted with 560 ml
of deionized
2 5 water to form a silica hydrosol comprising 0.08 g Si02/ml. Immediately
after completion of
the addition of the sodium silicate to the HCl solution, 309 ml of Ludox~ HS
(DuPont
Chemicals) was added with continuous stirring. The resulting 2 L of silica
hydrosol was
filtered through a fritted glass filter funnel and the silica hydrosol poured
into pans. The
silica hydrosol gelled in approximately 35 minutes and was let set for an hour
after gelation.
3 0 The resulting silica hydrogel was cut into approximately I cm squares and
washed with
deionized water until the pH of the effluent was about pH 2.1. The washed
silica hydrogel
14
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CA 02280795 1999-08-12
WO 98/37020 PCT/US98I03273
was placed in a 5 L flask, 839 ml of concentrated HCl (Fisher Certified)
added, and the
resulting mixture heated to reflux for 5 hours. The refluxed silica hydrogel
was cooled to
room temperature.
The heat-treated silica hydrogel suspension was hydrophobed as follows. To the
silica hydrogel, with stirring, was added 1049 ml of isopropanol, 543 ml of
hexamethyldisiloxane and 8.3 g of FeCI~. After stirring the flask content 1
hour at room
temperature, 2 L of toluene were added. After mild stirring the flask content
for an
additional 5 minutes, stirring was stopped and the aqueous bottom phase
drained from the
flask. The toluene phase was washed with 500 ml of deionized water. The flask
was fitted
with a Dean-Stark trap and the toluene phase refluxed to remove residual
water. After
refluxing the toluene was removed by distillation under reduced pressure
leaving as product
a hydrophobic silica gel. The hydrophobic silica gel was dried overnight at
150°C. The
yield of dried hydrophobic silica gel was 299 g. The BET surface area was
determined by
the method described in Example 2. The carbon and hydrogen content of the
dried
hydrophobic silica gel was determined by CHN analysis using a Perkin Elmer
Model 2400
CHN Elemental Analyzer (Perkin Elmer Corporation, Norwalk, CT). The iron
content of
the dried hydrophobic silica gel was determined by atomic adsorption. The
results of these
analysis are reported in Table 1.
Example 5. A silica gel having incorporated therein colloidal silica,
hydrophobed
2 0 with hexamethyldisiloxane, and heat stabilized by the addition of FeCl3
was prepared. A
silica hydrosol comprising about 0.1 g of SiOz/ml was prepared and deionized
as described
in Example 1. One liter of the deionized silica hydrosol was placed in a 5 L
flask and while
stirring 273 ml of Ludox~ SM (DuPont Chemicals) were added, followed by 392 ml
of
concentrated HCI. The silica hydrogel which formed within a few minutes of
addition of
2 5 the HCl was broken-up by additional stirring to form a silica hydrogel
suspension. The
silica hydrogel suspension was heat-treated at 100°C for 3 hours and
then cooled to room
temperature.
The heat-treated silica hydrogel was hydrophobed as follows. To the heat-
treated
silica hydrogel, with stirring, was added 555 ml of isopropanol, 288 ml of
3 0 hexamethyldisiloxane, and 2.7 g of FeCl3. The resulting mixture was
stirred for 1 hour at
room temperature and then 1 L of toluene was added to the mixture. This
mixture was
CA 02280795 1999-08-12
WO 98/37020 PCT/US98/03273
mildly stirred for an additional 5 minutes, then stirring stopped and the
aqueous phase
drained from the bottom of the flask. The toluene phase was washed with 500 ml
of
deionized water. The flask was fitted with a Dean-Stark trap and the toluene
phase refluxed
to remove residual water. The toluene phase was distilled under reduced
pressure leaving
as product a hydrophobic silica gel. The hydrophobic silica gel was dried
overnight at
150°C to remove residual toluene. The yield of dried hydrophobic silica
gel was 210 g.
The dried hydrophobic silica gel was analyzed to determine surface area,
carbon and
hydrogen content, and iron content by the methods described in Example 4. The
results of
this analysis are reported in Table 1.
Example 6. A silica gel having incorporated therein colloidal silica,
hydrophobed
with hexamethyldisiloxane and vinyldimethylchlorosilane, and heat stabilized
by the
addition of FeCl3 was prepared. A deionized silica hydrosol was prepared as
described in
Example 1. The deionized silica hydrosol was agglomerated by placing 1 L of
the
deionized silica hydrosol in a 5 L flask and while stirring adding 273 ml of
colloidal silica
(Ludox~ SM, DuPont Chemicals) and 392 ml of concentrated HCl (Fisher
Certified). The
mixture was heat-treated by refluxing for 3 hours with stirring to form a
suspension
comprising a silica hydrogel having incorporated therein the colloidal silica.
The resulting
heat-treated silica hydrogel was cooled to room temperature.
The heat-treated silica hydrogel was hydrophobed as follows. To the heat-
treated
2 0 silica hydrogel, with stirring, was added 555 ml of isopropanol, 78 ml of
hexamethyldisiloxane, and 2.7 g of FeCl3. The resulting mixture was stirred
for 1 hour at
room temperature. Then, 2 L of toluene were added to the mixture. This mixture
was
stirred for several minutes, stirring stopped, and the aqueous phase drained
from the bottom
of the flask. The toluene phase was washed with 1 L of deionized water. The
treatment
2 5 flask was then fitted with a Dean-Stark trap and the toluene phase
refluxed to remove
residual water. The toluene phase was heated at 110°C to remove
residual
hexamethyldisiloxane and then 5 ml of vinyldimethylchlorosilane were added.
This
mixture was refluxed for 1 hour and cooled to room temperature. About 50 ml of
deionized
water were added to the flask to washout residual HCl and the toluene phase
refluxed to
3 0 remove residual water. The toluene phase was transferred to an open
container in an
exhaust hood and the toluene allowed to evaporate leaving as product a
hydrophobic silica
16
~ , .
CA 02280795 1999-08-12
WO 98/37020 PCTIUS98/03273
gel. The hydrophobic silica gel was dried overnight at 85°C. The yield
of dried
hydrophobic silica gel was 214 g. Selected physical parameters of the dried
hydrophobic silica gel were characterized by standard methods and the results
are reported
in Table 2.
Example 7. A silica gel having incorporated therein colloidal silica,
hydrophobed
with hexamethyldisiloxane arid vinyldimethylchlorosilane, and heat stabilized
by the
addition of FeCl3 was prepared. The silica hydrogel was sheared prior to
hydrophobing to
reduce aggregate particle size and to improve the uniformity of the particle
size distribution.
A deionized silica hydrosol was prepared by a method similar to that described
in Example
1. The deionized silica hydrosol was agglomerated by placing 1 L of the
deionized silica
hydrosol in a 5 L flask and while stirring adding 273 ml of colloidal silica
(Ludox~ SM,
DuPont Chemicals) and 392 m1 of concentrated HCl (Fisher Certified). The
mixture was
heat treated by refluxing for 3 hours with stirring to form a suspension
comprising an silica
hydrogel having incorporated therein the colloidal silica.
After cooling to room temperature the silica hydrogel was sheared in a Waring
Blender
(Model 7011, Waring Products Division of Dynamics Corporation of America, New
Hartford, CT} for 2 minutes and then returned to the 5 L flask.
The heat-treated and sheared silica hydrogel was hydrophobed as follows. To
the
silica hydrogel, with stirring, was added 555 ml of isopropanol, 117 ml of
2 0 hexamethyldisiloxane, and 2.7 g of FeCl3. The resulting mixture was
stirred for 1 hour at
room temperature. Then, 2 L of toluene were added to the mixture. This mixture
was
mildly stirred for several minutes, stirring stopped, and the aqueous phase
drained from the
bottom of the flask. The toluene phase was washed with 1 L of deionized water.
The flask
was then fitted with a Dean-Stark trap and the toluene phase refluxed to
remove residual
2 5 water. The toluene phase was heated at 110°C to remove residual
hexamethyldisiloxane
and then 5 ml of vinyldimethylchlorosilane were added to the flask. This
mixture was
refluxed for 1 hour and cooled to room temperature. About 50 ml of deionized
water were
added to the flask to washout residual HCI and the toluene phase refluxed to
remove
residual water. The toluene phase was transferred to an open container in an
exhaust hood
3 0 and the toluene allowed to evaporate leaving as product a hydrophobic
silica gel. The
hydrophobic silica gel was dried overnight at 85°C. The yield of dried
hydrophobic silica
17
CA 02280795 1999-08-12
WO 98/37020 PCT/US98103273
gel was 209 g. Selected physical parameters of the dried hydrophobic silica
gel were
characterized by standard methods and the results are reported in Table 2.
Example 8. A silica gel having incorporated therein colloidal silica,
hydrophobed
with hexamethyldisiloxane and vinyldimethylchlorosilane, and heat stabilized
by the
addition of FeCl3 was prepared. The silica hydrogel was sheared prior to
hydrophobing to
reduce aggregate particle size and to improve the uniformity of the particle
size distribution.
A deionized silica hydrosol was prepared by a method similar to that described
in Example
1. The deionized silica hydrosol was agglomerated by placing 1.5 L of the
deionized silica
hydrosol in a 5 L flask and while stirring adding 409.5 ml of colloidal silica
(Ludox~ SM,
DuPont Chemicals) and 588 ml of concentrated HCl (Fisher Certified). The
mixture was
heat treated by refluxing for 3 hours with stirring to form a suspension
comprising a silica
hydrogel having incorporated therein the colloidal silica.
After cooling to room temperature, the silica hydrogel was sheared in a
blaring Blender
(Model 7011) for 2 minutes and then returned to the 5 L flask.
The heat-treated and sheared silica hydrogel suspension was hydrophobed as
follows. To the silica hydrogel suspension, with stirring, was added 832.5 m1
of
isopropanol, 175 ml of hexamethyldisiloxane, and 4 g of FeCl3. The resulting
mixture was
stirred for 1 hour at room temperature. Then, 3.2 L of toluene were added to
the mixture.
This mixture was mildly stirred for several minutes, stirring stopped, and the
aqueous phase
2 0 drained from the bottom of the flask. The toluene phase was washed with 1
L of deionized
water. The flask was then fitted with a Dean-Stark trap and the toluene phase
refluxed to
remove residual water. The toluene phase was heated at 110°C to remove
residual
hexamethyldisiloxane and then 3.75 ml of vinyldimethyIchlorosilane were added
to the
flask. This mixture was refluxed for 1 hour and then cooled to room
temperature. About
2 5 50 ml of deionized water were added to the flask to washout residual HCl
and the toluene
phase refluxed to remove residual water. The toluene phase was transferred to
an open
container in an exhaust hood and the toluene allowed to evaporate leaving as
product a
hydrophobic silica gel. The hydrophobic silica gel was dried overnight at
85°C. The yield
of dried hydrophobic silica gel was 292 g. Selected physical parameters of the
dried
3 0 hydrophobic silica gel were characterized by standard methods and the
results are reported
in Table 2.
18
CA 02280795 1999-08-12
WO 98/37020 PCTIUS98/03273
Example 9. Each of the dried hydrophobic silica gels prepared in Examples 1
through 5 were compounded into a liquid silicone rubber composition, the
composition
cured, and the physical properties determined. Each of the dried hydrophobic
silica gels
was compounded at 38 parts per hundred (pph) by weight into a
polydimethylsiloxane gum
containing about 0.15 mole percent vinyl radicals substituted on silicon atoms
and having a
plasticity of about 55 to 65. Into this base composition was blended 0.7 pph
by weight of
2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, based on the weight of the
polydimethylsiloxane gum. The catalyzed base composition was cured in
appropriate
configurations for physical property testing by hot pressing at 34.5 MPa for
15 minutes at
175°C. The following test methods were used to test the cured silicone
rubber: Tensile,
ASTM D412; Elongation, ASTM D412; 100% Modulus, ASTM D412; 50% Modulus,
ASTM D412; Durometer (Shore A), ASTM 2240; Tear (Die B), ASTM D624; Tear (Die
C}, ASTM D624; Compression set (22 h at 177°C}, ASTM D395. Plasticities
of the
uncured compositions were measured on samples weighing two times the specific
gravity of
the composition that were formed into balls and rested one hour before
measurement by
ASTM 926. The results of this testing are provided in Table 1.
Example 10. Each of the dried hydrophobic silica gels prepared in Examples 6
through 8 were compounded into a silicone rubber composition, the composition
cured, and
the physical properties determined. Each of the dried hydrophobic silica gels
was
2 0 compounded at the parts per hundred (pph) by weight described in Table 2
into a siloxane
mixture. The temperature at which this compounding was effected is also
provided in
Table 2. The siloxane mixture comprised 83.8 weight percent
vinyldimethylsiloxy end-
blocked polydimethylsiloxane having a viscosity of 55 Paxs at 25°C and
16.2 weight
percent of a vinyldimethylsiloxy end-blocked poly(vinylmethyl)dimethylsiloxane
2 5 copolymer having 2 mole percent vinyl substitution on silicon and a
viscosity of 0.35 Paxs
at 25°C. Into this base composition was blended a cure system
comprising a low-molecular
weight polydimethyl(methylhydrogen)siloxane fluid, neutralized complex of
platinum
dichloride with sym-divinyltetramethyldisiloxane, and 1-ethynyl-cyclohexanol.
The
19
CA 02280795 1999-08-12
WO 98137020 PCT/US98/03273
catalyzed base composition was cured in appropriate configurations for
physical property
testing by hot pressing at 34.5 MPa for 10 minutes at 150°C. The cured
compositions
where post-cured for 1 hour at 177°C. Physical properties of the cured
compositions were
determined by test methods described in Example 9 and the results are reported
in Table 2.
20
_ ,.
CA 02280795 1999-08-12
WO 98/37020 PCT/US98/03273
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CA 02280795 1999-08-12
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