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Many different precipitated silicas are known and have been used in a wide
variety of applications. Precipitated silicas are most commonly produced
by precipitation from an aqueous solution of sodium silicate using a
suitable acid such as sulfuric acid, hydrochloric acid, and/or carbon
dioxide. Processes for producing precipitated silicas are described in
detail in U.S. Pat. Nos. 2,657,149; 2,940,830; and 4,681,750, the entire
disclosures of which are incorporated herein by reference, including
especially the processes for making precipitated silicas and the
properties of the products.
Although both are silicas, it is important to distinguish precipitated
silica from silica gel inasmuch as these different materials have
different properties. Reference in this regard is made to R. K. Iler, The
Chemistry of Silica, John Wiley & Sons, New York (1979), Library of
Congress Catalog No. QD 181.S6144. Note especially pages 15-29, 172-176,
218-233, 364-365, 462-465, 554-564, and 578-579, the entire disclosures of
which are incorporated herein by reference. Silica gel is usually produced
commercially at low pH by acidifying an aqueous solution of a soluble
metal silicate, customarily sodium silicate, with acid. The acid employed
is generally a strong mineral acid such as sulfuric acid or hydrochloric
acid although carbon dioxide is sometimes used. Inasmuch as there is
essentially no difference in density between the gel phase and the
surrounding liquid phase while the viscosity is low, the gel phase does
not settle out, that is to say, it does not precipitate. Silica gel, then,
may be described as a non-precipitated, coherent, rigid, three-dimensional
network of contiguous particles of colloidal amorphous silica. The state
of subdivision ranges from large, solid masses to submicroscopic
particles, and the degree of hydration from almost anhydrous silica to
soft gelatinous masses containing on the order of 100 parts of water per
part of silica by weight, although the highly hydrated forms are only
rarely used.
Precipitated silica is usually produced commercially by combining an
aqueous solution of a soluble metal silicate, ordinarily alkali metal
silicate such as sodium silicate, and an acid so that colloidal particles
will grow in weakly alkaline solution and be coagulated by the alkali
metal ions of the resulting soluble alkali metal salt. Various acids may
be used, including the mineral acids and/or carbon dioxide. In the absence
of a coagulant, silica is not precipitated from solution at any pH. The
coagulant used to effect precipitation may be the soluble alkali metal
salt produced during formation of the colloidal silica particles, it may
be added electrolyte such as a soluble inorganic or organic salt, or it
may be a combination of both.
Precipitated silica, then, may be described as precipitated aggregates of
ultimate particles of colloidal amorphous silica that have not at any
point existed as macroscopic gel during the preparation. The sizes of the
aggregates and the degree of hydration may vary widely.
Precipitated silica powders differ from silica gels that have been
pulverized in ordinarily having a more open structure, that is, a higher
specific pore volume. However, the specific surface area of precipitated
silica as measured by the Brunauer, Emmett, Teller (BET) method using
nitrogen as the adsorbate, is often lower than that of silica gel.
Variations in the parameters and/or conditions during production result in
variations in the types of precipitated silicas produced. Although they
are all broadly precipitated silicas, the types of precipitated silicas
often differ significantly in physical properties and sometimes in
chemical properties. These differences in properties are important and
often result in one type being especially useful for a particular purpose
but of marginal utility for another purpose, whereas another type is quite
useful for that other purpose but only marginally useful for the first
purpose.
Reinforcement of precipitated silica, that is, the deposition of silica on
aggregates of previously precipitated silica, is itself known. It has now
been found, however, that by controlling the conditions of silica
precipitation and multiple reinforcement steps, new silicas may be
produced having properties that make them especially useful for clarifying
beer and for reinforcing silicone rubbers. They also may be used for many
of the purposes for which other types of precipitated silicas have been
used. For example they may be used as reinforcing fillers for
styrene-butadiene rubber and other organic rubbers. They may be used as
fillers and extenders in toothpaste; as carriers for vitamins; as paper
extenders and brighteners; and in a multitude of other uses.
Although it is not desired to be bound by any theory, it is believed that
as precipitated silica is dried, the material shrinks; consequently, pore
diameters are reduced, surface area is reduced, and the void volume is
reduced. It is further believed that by sufficiently reinforcing the
silica prior to drying, a more open structure is obtained after drying.
Irrespective of theory, the reinforced precipitated silica of the present
invention has, on balance, larger pore diameters and a larger total
intruded volume for the surface area obtained than is the case for
previous precipitated silicas, whether or not reinforced.
Accordingly, one embodiment of the invention is a process for producing
reinforced amorphous precipitated silica having, on a coating-free and
impregnant-free basis, a surface area of from about 220 to about 340
square meters per gram, a pore diameter at the maximum of the volume pore
size distribution function of from about 9 to about 20 nanometers, and a
total intruded volume of from about 2.6 to about 4.4 cubic centimeters per
gram, the process comprising: (a) establishing an initial aqueous alkali
metal silicate solution containing from about 0.5 to about 4 weight
percent SiO.sub.2 and having an SiO.sub.2 :M.sub.2 O molar ratio of from
about 1.6 to about 3.9; (b) over a period of at least about 20 minutes and
with agitation, adding acid to the initial aqueous alkali metal silicate
solution at a temperature below about 50.degree. C. to neutralize at least
about 60 percent of the M.sub.2 O present in the initial aqueous alkali
metal solution and thereby to form a first reaction mixture; (c) over a
period of from about 115 to about 240 minutes, with agitation, and at a
temperature of from about 80.degree. C. to about 95.degree. C.,
substantially simultaneously adding to the first reaction mixture: (1)
additive aqueous alkali metal silicate solution, and (2) acid, thereby to
form a second reaction mixture wherein the amount of the additive aqueous
alkali metal silicate solution added is such that the amount of SiO.sub.2
added is from about 0.5 to about 2 times the amount of SiO.sub.2 present
in the initial aqueous alkali metal silicate solution established in step
(a) and wherein the amount of the acid added is such that at least about
60 percent of the M.sub.2 O contained in the additive aqueous alkali metal
silicate solution added during the simultaneous addition is neutralized;
(d) adding acid to the second reaction mixture with agitation at a
temperature of from about 80.degree. C. to about 95.degree. C. to form a
third reaction mixture having a pH below 7; (e) aging the third reaction
mixture with agitation at a pH below 7 and at a temperature of from about
80.degree. C. to about 95.degree. C. for a period of from about 1 to about
120 minutes; (f) with agitation and at a temperature of from about
80.degree. C. to about 95.degree. C., adding to the aged third reaction
mixture additive aqueous alkali metal silicate solution to form a fourth
reaction mixture having a pH of from about 7.5 to about 9; (g) forming a
fifth reaction mixture by adding to the fourth reaction mixture with
agitation and at a temperature of from about 80.degree. C. to about
95.degree. C., a further quantity of additive aqueous alkali metal
silicate solution and adding acid as necessary to maintain the pH at from
about 7.5 to about 9 during the addition of the further quantity of the
additive aqueous alkali metal silicate solution, wherein: (1) the amount
of the additive aqueous alkali metal silicate solution added in steps (f)
and (g) is such that the amount of SiO.sub.2 added in steps (f) and (g) is
from about 0.05 to about 0.75 times the amount of SiO.sub.2 present in the
third reaction mixture, and (2) the additive aqueous alkali metal silicate
solution is added in steps (f) and (g) over a collective period of at
least about 40 minutes; (h) aging the fifth reaction mixture with
agitation at a temperature of from about 80.degree. C. to about 95.degree.
C. for a period of from about 5 to about 60 minutes; (i) adding acid to
the aged fifth reaction mixture with agitation at a temperature of from
about 80.degree. C. to about 95.degree. C. to form a sixth reaction
mixture having a pH below 7; (j) aging the sixth reaction mixture with
agitation at a pH below 7 and at a temperature of from about 80.degree. C.
to about 95.degree. C. for a period of at least about 1 minute; (k)
separating reinforced precipitated silica from most of the liquid of the
aged sixth reaction mixture; (1) washing the separated reinforced
precipitated silica with water; and (m) drying the washed reinforced
precipitated silica, wherein: (n) the alkali metal silicate is lithium
silicate, sodium silicate, potassium silicate, or a mixture thereof; and
(o) M is lithium, sodium, potassium, or a mixture thereof.
Optionally, prior to step (c) the first reaction mixture is aged with
agitation at a temperature of from about 30.degree. C. to about 95.degree.
C. for a period of from about 5 to about 180 minutes.
The composition of the initial aqueous alkali metal silicate solution
established in step (a) may vary widely. Generally the initial aqueous
alkali metal silicate solution comprises from about 0.5 to about 4 weight
percent SiO.sub.2. In many cases the initial aqueous alkali metal silicate
solution comprises from about 1 to about 3 weight percent SiO.sub.2. From
about 1.5 to about 2.5 weight percent SiO.sub.2 is preferred. Usually the
initial aqueous alkali metal silicate solution has an SiO.sub.2 :M.sub.2 O
molar ratio of from about 1.6 to about 3.9. Often the SiO.sub.2 :M.sub.2 O
molar ratio is from about 2.5 to about 3.6. Preferably the SiO.sub.2
:M.sub.2 O molar ratio is from about 2.9 to about 3.6. Often the SiO.sub.2
:M.sub.2 O molar ratio is from about 3.2 to about 3.3.
The composition of the additive aqueous alkali metal silicate solution may
also vary widely. Usually the additive aqueous alkali metal silicate
solution comprises from about 2 to about 30 percent by weight SiO.sub.2.
Often the additive aqueous alkali metal silicate solution comprises from
about 10 to about 15 percent by weight SiO.sub.2. From about 12 to about
13 weight percent SiO.sub.2 is preferred. Frequently the additive aqueous
alkali metal silicate solution has an SiO.sub.2 :M.sub.2 O molar ratio of
from about 1.6 to about 3.9. In many cases the SiO.sub.2 :M.sub.2 O molar
ratio is from about 2.5 to about 3.6. Preferably the SiO.sub.2 :M.sub.2 O
molar ratio is from about 2.9 to about 3.6. Often the SiO.sub.2 :M.sub.2 O
molar ratio is from about 3.2 to about 3.3. Additive aqueous alkali metal
silicate solution having the same composition may be used throughout the
various silicate additions, or additive aqueous alkali metal silicate
solutions having differing compositions may be used in different silicate
addition steps.
The acid used in the process may also vary widely. In general, the acid
added in steps (b), (c), and (g) should be strong enough to neutralize
alkali metal silicate and cause precipitation of silica. The acid added in
steps (d) and (i) should be strong enough to reduce the pH to desired
values within the specified ranges. The acid used in the various acid
addition steps may be the same or different, but preferably it is the
same. A weak acid such as carbonic acid produced by the introduction of
carbon dioxide to the reaction mixture may be used for precipitation of
silica, but a stronger acid must be used in steps (d) and (i) when it is
desired to reduce the pH to values below 7. It is preferred to use strong
acid throughout the process. Examples of the strong acids include sulfuric
acid, hydrochloric acid, nitric acid, phosphoric acid, and acetic acid.
The strong mineral acids such as sulfuric acid, hydrochloric acid, nitric
acid, and phosphoric acid are preferred; sulfuric acid is especially
preferred.
The acid addition of step (b) is made over a period of at least about 20
minutes. Frequently the acid addition of step (b) is made over a period of
from about 20 to about 60 minutes. From about 26 to about 32 minutes is
preferred.
The temperature of the reaction mixture during the acid addition of step
(b) is below about 50.degree. C. From about 30.degree. C. to about
40.degree. C. is preferred.
At least about 60 percent of the M.sub.2 O present in the initial aqueous
alkali metal silicate solution is neutralized during the acid addition of
step (b). As much as 100 percent of the M.sub.2 O may be neutralized if
desired. Preferably from about 75 to about 85 percent of the M.sub.2 O is
neutralized.
The additions made in step (c) are made over a period of from about 115 to
about 240 minutes. Preferably the additions are made over a period of from
about 115 to about 125 minutes.
The temperature of the reaction mixture during the additions of step (c) is
from about 80.degree. C. to about 95.degree. C. From about 90.degree. C.
to about 95.degree. C. is preferred.
In step (c), the amount of additive aqueous alkali metal silicate added is
such that the amount of SiO.sub.2 added is from about 0.5 to about 2 times
the amount of SiO.sub.2 present in the initial aqueous alkali metal
silicate solution established in step (a). From about 0.9 to about 1.1
times the SiO.sub.2 present in the initial aqueous alkali metal silicate
solution is preferred.
The amount of acid added in step (c) is such that at least about 60 percent
of the M.sub.2 O contained in the additive aqueous alkali metal silicate
solution added in step (c) is neutralized. As much as 100 percent of such
M.sub.2 O may be neutralized if desired. Preferably from about 75 to about
85 percent of the M.sub.2 O is neutralized.
The temperature of the reaction mixture during the acid addition of step
(d) is from about 80.degree. C. to about 95.degree. C. From about
90.degree. C. to about 95.degree. C. is preferred.
In step (d), the acid is added such that the pH of the third reaction
mixture is below 7. Often the pH is from about 2.5 to below 7. A pH of
from about a to about 5 is preferred.
Similarly, the third reaction mixture is aged in step (e) at a pH below 7.
Often the pH is from about 2.5 to below 7. A pH of from about 4 to about 5
is preferred.
The temperature of the third reaction mixture during the aging of step (e)
is from about 80.degree. C. to about 95.degree. C. From about 90.degree.
C. to about 95.degree. C. is preferred.
The aging in step (e) is for a period of from about 1 to about 120 minutes.
In many cases the third reaction mixture is aged for a period of from
about 15 to about 120 minutes. A period of from about 15 to about 30
minutes is preferred.
The temperature of the reaction mixture during the addition of additive
aqueous alkali metal silicate solution in step (f) is from about
80.degree. C. to about 95.degree. C. From about 90.degree. C. to about
95.degree. C. is preferred.
The pH of the fourth reaction mixture formed in step (f) is from about 7.5
to about 9. A pH of from about 8 to about 9 is preferred.
Acid is added in step (g) as necessary to maintain the pH of the reaction
mixture at from about 7.5 to about 9 during the addition of the further
quantity of additive aqueous alkali metal silicate solution. A pH of from
about 8 to about 9 is preferred.
The amount of additive aqueous alkali metal silicate solution added in
steps (f) and (g) is such that the amount of SiO.sub.2 added in steps (f)
and (g) is from about 0.05 to about 0.75 times the amount of SiO.sub.2
present in the third reaction mixture. Preferably the amount of additive
aqueous alkali metal silicate solution added in steps (f) and (g) is such
that the amount of SiO.sub.2 added in steps (f) and (g) is from about 0.25
to about 0.45 times the amount of SiO.sub.2 present in the third reaction
mixture.
The additive alkali metal silicate solution is added in steps (f) and (g)
over a collective period of at least about 40 minutes. A collective period
of from about 40 to about 240 minutes is often employed. A collective
period of from about 70 to about 100 minutes is preferred.
The temperature of the fifth reaction mixture during the aging of step (h)
is from about 80.degree. C. to about 95.degree. C. From about 90.degree.
C. to about 95.degree. C. is preferred.
In step (h), the fifth reaction mixture is aged for a period of from about
5 to about 60 minutes. A period of from about 30 to about 60 minutes is
preferred.
The temperature of the reaction mixture during the acid addition of step
(i) is from about 80.degree. C. to about 95.degree. C. From about
90.degree. C. to about 95.degree. C. is preferred.
In step (i), the acid is added such that the pH of the sixth reaction
mixture is below 7. Often the pH is from about 2.5 to below 7. A pH of
from about 4 to about 5 is preferred.
The sixth reaction mixture is aged in step (j) at a pH below 7. In many
cases the pH is from about 2.5 to below 7. A pH of from about 4 to about 5
is preferred.
The temperature of the sixth reaction mixture during the aging of step (j)
is from about 80.degree. C. to about 95.degree. C. From about 90.degree.
C. to about 95.degree. C. is preferred.
In step (j), the sixth reaction mixture is aged for a period of at least
about 1 minute. Often the aging period is at least about 30 minutes. An
aging period of at least about 50 minutes is preferred.
The separation of step (k) may be accomplished by one or more techniques
for separating solids from liquid such as, for example, filtration,
centrifugation, decantation, and the like.
The washing of step (1) may be accomplished by any of the procedures known
to the art for washing solids. Examples of such procedures include passing
water through a filter cake, and reslurring the reinforced precipitated
silica in water followed by separating the solids from the liquid. One
washing cycle or a succession of washing cycles may be employed as
desired. The primary purpose of washing is to remove salt formed by the
various neutralizations to desirably low levels. Usually the reinforced
precipitated silica is washed until the concentration of salt in the dried
reinforced precipitated silica is less than or equal to about 2 percent by
weight. Preferably the reinforced precipitated silica is washed until the
concentration of salt is less than or equal to about 0.2 percent by
weight.
The drying of step (m) may also be accomplished by one or more known
techniques. For example, the reinforced precipitated silica may be dried
in an air oven or in a vacuum oven. Preferably the reinforced precipitated
silica is dispersed in water and spray dried in a column of hot air. The
temperature at which drying is accomplished is not critical, but the usual
practice is to employ temperatures of at least 70.degree. C. Generally the
drying temperature is less than about 700.degree. C. In most cases drying
is continued until the reinforced precipitated silica has the
characteristics of a powder. Ordinarily the dried reinforced precipitated
silica is not absolutely anhydrous but contains bound water (from about 2
to about 5 weight percent) and adsorbed water (from about 1 to about 7
weight percent) in varying amounts, the latter depending partly upon the
prevailing relative humidity. Adsorbed water is that water which is
removed from the silica by heating at 105.degree. C. for 24 hours at
atmospheric pressure in a laboratory oven. Bound water is that water which
is removed by additionally heating the silica at calcination temperatures,
for example, from about 1000.degree. C. to about 1200.degree. C.
Another optional step which may be employed is size reduction. Size
reduction techniques are themselves well known and may be exemplified by
grinding and pulverising. Particularly preferred is fluid energy milling
using air or superheated steam as the working fluid. Fluid energy mills
are themselves well known. See, for example, Perry's Chemical Engineers'
Handbook, 4th Edition, McGraw-Hill Book Company, New York, (1963), Library
of Congress Catalog Card Number 6113168, pages 8-42 and 8-43; McCabe and
Smith, Unit Operations of Chemical Engineering, 3rd Edition, McGraw-Hill
Book Company, New York (1976), ISBN 0-07-044825-6, pages 844 and 845; F.
E. Albus, "The Modern Fluid Energy Mill", Chemical Engineering Progress,
Volume 60, No. 6 (June 1964), pages 102-106, the entire disclosures of
which are incorporated herein by reference. In fluid energy mills the
solid particles are suspended in a gas stream and conveyed at high
velocity in a circular or elliptical path. Some reduction occurs when the
particles strike or rub against the walls of the confining chamber, but
most of the reduction is believed to be caused by interparticle attrition.
The degrees of agitation used in the various steps of the invention may
vary considerably. The agitation employed during the addition of one or
more reactants should be at least sufficient to provide a thorough
dispersion of the reactants and reaction mixture so as to avoid more than
trivial locally high concentrations of reactants and to ensure that silica
deposition occurs substantially uniformly thereby avoiding gellation on
the macro scale. The agitation employed during aging should be at least
sufficient to avoid settling of solids to ensure that silica deposition
occurs substantially uniformly throughout the mass of silica particles
rather than preferentially on those particles at or near the top of a
settled layer of particles. The degrees of agitation may, and preferably
are, greater than these minimums. In general, vigorous agitation is
preferred.
Yet another optional step which may be employed is treating the reinforced
precipitated silica with one or more materials which coat, partially coat,
impregnate, and/or partially impregnate the silica. Many materials may be
used for this purpose. In general, the type of material used depends upon
the effect desired. Most often the materials are organic polymers.
Examples of suitable materials include hydrocarbon oils, polyesters,
polyamides, phenolic resins, aminoplast resins, polysiloxanes,
polysilanes, and the like. The treatment step may be accomplished at any
convenient time during or after formation of the reinforced precipitated
silica.
A preferred embodiment within the first embodiment of the invention is a
process for producing reinforced amorphous precipitated silica having, on
a coating-free and impregnant-free basis, a surface area of from about 220
to about 340 square meters per gram, a pore diameter at the maximum of the
volume pore size distribution function of from about 13 to about 18
nanometers, and a total intruded volume of from about 3 to about 4.4 cubic
centimeters per gram, the process comprising: (a) establishing an initial
aqueous alkali metal silicate solution containing from about 0.5 to about
4 weight percent SiO.sub.2 and having an SiO.sub.2 :M.sub.2 O molar ratio
of from about 1.6 to about 3.9; (b) over a period of at least about 20
minutes and with agitation, adding acid to the initial aqueous alkali
metal silicate solution at a temperature of from about 30.degree. C. to
about 40.degree. C. to neutralize from about 75 to about 85 percent of the
M.sub.2 O present in the initial aqueous alkali metal solution and to form
a first reaction mixture; (c) over a period of from about 115 to about 125
minutes, with agitation, and at a temperature of from about 90.degree. C.
to about 95.degree. C., substantially simultaneously adding to the first
reaction mixture: (1) additive aqueous alkali metal silicate solution, and
(2) acid, to form a second reaction mixture wherein the amount of the
additive aqueous alkali metal silicate solution added is such that the
amount of SiO.sub.2 added is from about 0.9 to about 1.1 times the amount
of SiO.sub.2 present in the initial aqueous alkali metal silicate solution
established in step (a) and wherein the amount of the acid added is such
that from about 75 to about 85 percent of the M.sub.2 O contained in the
additive aqueous alkali metal silicate solution added during the
simultaneous addition is neutralized; (d) adding acid to the second
reaction mixture with agitation at a temperature of from about 90.degree.
C. to about 95.degree. C. to form a third reaction mixture having a pH of
from about 4 to about 5; (e) aging the third reaction mixture with
agitation at a temperature of from about 90.degree. C. to about 95.degree.
C. for a period of from about 15 to about 30 minutes; (f) with agitation
and at a temperature of from about 90.degree. C. to about 95.degree. C.,
adding to the aged third reaction mixture additive aqueous alkali metal
silicate solution to form a fourth reaction mixture having a pH of from
about 8 to about 9; (g) forming a fifth reaction mixture by adding to the
fourth reaction mixture with agitation and at a temperature of from about
90.degree. C. to about 95.degree. C., a further quantity of additive
aqueous alkali metal silicate solution and adding acid as necessary to
maintain the pH at from about 8 to about 9 during the addition of the
further quantity of the additive aqueous alkali metal silicate solution,
wherein: (1) the amount of the additive aqueous alkali metal silicate
solution added in steps (f) and (g) is such that the amount of SiO.sub.2
added in steps (f) and (g) is from about 0.25 to about 0.45 times the
amount of SiO.sub.2 present in the third reaction mixture, and (2) the
additive aqueous alkali metal silicate solution is added in steps (f) and
(g) over a collective period of from about 70 to about 100 minutes; (h)
aging the fifth reaction mixture with agitation at a temperature of from
about 90.degree. C. to about 95.degree. C. for a period of from about 30
to about 60 minutes; (i) adding acid to the aged fifth reaction mixture
with agitation at a temperature of from about 90.degree. C. to about
95.degree. C. to form a sixth reaction mixture having a pH of from about 4
to about 5; (j) aging the sixth reaction mixture with agitation at a
temperature of from about 90.degree. C. to about 95.degree. C. for a
period of at least about 50 minutes; (k) separating reinforced
precipitated silica from most of the liquid of the aged sixth reaction
mixture; (1) washing the separated reinforced precipitated silica with
water; and (m) drying the washed reinforced precipitated silica, wherein:
(n) the alkali metal silicate is lithium silicate, sodium silicate,
potassium silicate, or a mixture thereof; and (o) M is lithium, sodium,
potassium, or a mixture thereof.
It is understood that one or more ranges in the preferred embodiment may be
used in lieu of the corresponding broader range or ranges in the broader
first embodiment of the invention.
A further embodiment of the invention is reinforced amorphous precipitated
silica having, on a coating-free and impregnant-free basis, a surface area
of from about 220 to about 340 square meters per gram, a pore diameter at
the maximum of the volume pore size distribution function of from about 9
to about 20 nanometers, and a total intruded volume of from about 2.6 to
about 4.4 cubic centimeters per gram. The concurrence of all three of
these properties is essential to the reinforced precipitated silica of the
present invention.
As used in the present specification and claims, the surface area of the
reinforced amorphous precipitated silica is the surface area determined by
the Brunauer, Emmett, Teller (BET) method according to ASTM C 819-77 using
nitrogen as the adsorbate but modified by outgassing the system and the
sample for one hour at 180.degree. C. The surface area is from about 220
to about 340 square meters per gram. In many cases the surface area is
from about 220 to about 300 square meters per gram. From about 220 to
about 270 square meters per gram is preferred. ASTM C 819-77 is, in its
entirety, incorporated herein by reference.
The volume average pore size distribution function of the reinforced
amorphous precipitated silica is determined by mercury porosimetry using
an Autoscan mercury porosimeter (Quantachrome Corp.) in accordance with
the accompanying operating manual. In operating the porosimeter, a scan is
made in the high pressure range (from about 103 kilopascals absolute to
about 227 megapascals absolute). The volume pore size distribution
function is given by the following equation:
##EQU1##
where: D.sub.v (d) is the volume pore size distribution function, usually
expressed in cm.sup.3 /(.mu.m.multidot.g);
d is the pore diameter, usually expressed in .mu.m;
P is the pressure, usually expressed in pounds per square inch, absolute;
and
V is the pore volume per unit mass, usually expressed in cm.sup.3 /g.
Dv(d) is determined by taking .DELTA.V/.DELTA.P for small values of
.DELTA.P from either a plot of V versus P or preferably from the raw data.
Each value of .DELTA.V/.DELTA.P is multiplied by the pressure at the upper
end of the interval and divided by the corresponding pore diameter. The
resulting value is plotted versus the pore diameter. The value of the pore
diameter at the maximum of the volume pore size distribution function is
then taken from the plotted graph. Numerical procedures may be used rather
than graphical when desired. For the reinforced amorphous precipitated
silica of the present invention the pore diameter at the maximum of the
volume pore size distribution function is from about 9 to about 20
nanometers. Preferably the pore diameter at the maximum of the function is
from about 13 to about 18 nanometers.
In the course of determining the volume average pore diameter by the above
procedure, the maximum pore radius detected is sometimes noted. The
maximum pore diameter is twice the maximum pore radius.
The total intruded volume is the total volume of mercury which is intruded
into the reinforced amorphous precipitated silica during the high pressure
scan described above divided by the mass of the reinforced amorphous
precipitated silica constituting the sample under test. The total intruded
volume of the reinforced amorphous precipitated silica is from about 2.6
to about 4.4 cubic centimeters per gram. Preferably the total intruded
volume is from about 3 to about 4.4 cubic centimeters per gram.
The reinforced amorphous precipitated silica may be in the form of
aggregates of ultimate particles, agglomerates of aggregates, or a
combination of both. Ordinarily, less than about 10 percent by weight of
the reinforced amorphous precipitated silica has gross particle sizes
greater than about 80 micrometers as determined by use of a Model TAII
Coulter counter (Coulter Electronics, Inc.) according to ASTM C 690-80 but
modified by stirring the precipitated silica for 10 minutes in Isoton II
electrolyte (Curtin Matheson Scientific, Inc.) using a four-blade, 4.5
centimeter diameter propeller stirrer. In many cases less than about 10
percent by weight of the reinforced amorphous precipitated silica has
gross particle sizes greater than about 40 micrometers. When size
reduction is employed usually less than about 10 percent by weight of the
reinforced amorphous precipitated silica has gross particle sizes greater
than about 20 micrometers. Preferably less than about 10 percent by weight
of the reinforced amorphous precipitated silica has gross particle sizes
greater than about 10 micrometers. When less than about 10 percent by
weight of the reinforced amorphous precipitated silica has gross particle
sizes greater than about 20 micrometers, it is preferred that the median
gross particle size be less than about 5 micrometers. When less than about
10 percent by weight of the reinforced amorphous precipitated silica has
gross particle sizes greater than about 10 micrometers, it is preferred
that the median gross particle size be less than about 2 micrometers. It
is expected that in some usages such as fillers for battery separators,
microporous materials, and rubbers, the sizes of reinforced amorphous
precipitated silica aggregates will be reduced during processing of the
ingredients to prepare the final articles. Accordingly, the distribution
of gross particle sizes in such articles may be smaller than in the raw
reinforced amorphous precipitated silica itself. ASTM C 690-80 is, in its
entirety, incorporated herein by reference.
The average ultimate particle size of the reinforced amorphous precipitated
silica (irrespective of whether or not the ultimate particles are
aggregated and/or agglomerated) is usually less than about 0.1 micrometer
as determined by transmission electron microscopy. Often the average
ultimate particle size is less than about 0.05 micrometer. Preferably the
average ultimate particle size is less than about 0.03 mi | | |
