Readily dispersible precipitated silica
The invention relates to a readily dispersible precipitated
silica, the process for its preparation and its use in
rubber mixtures.
It is known to incorporate precipitated silicas into rubber
mixtures (S. Wolff, Kautschuk und Gummikunstst. 7 (1988) p.
674). For use in rubber mixtures, precipitated silicas must
be readily dispersible. A poor dispersibility is often the
reason why precipitated silicas are not employed in tyre
mixtures, especially with high filler contents.
The document EP-A 0 52 0 862 discloses precipitated silicas
which are employed as a filler in rubber mixtures for
tyres.
The document EP-A 0 157 703 discloses a precipitated silica
which can be prepared in accordance with the document EP-A
0 501 227.
The known precipitated silicas have the disadvantage that
they have a poor dispersion.
EP-A 0 647 591 and EP-A 0 157 703 describe a precipitated
silica which has an improved dispersion compared with the
abovementioned precipitated silicas. Due to increased
requirements by the tyre industry, even the improved
dispersion of this precipitated silica is no longer
adequate for use in tyre mixtures.
There was thus the object of developing a precipitated
silica which can be dispersed significantly better in
rubber mixtures.
The invention provides a precipitated silica, characterized
by the following physico-chemical data:

The precipitated silica according to the invention has a
particle size distribution which ensures a very good
dispersion after incorporation into rubber mixtures. A very-
low wk coefficient is characteristic of a very good
dispersion.
The invention also provides a process for the preparation
of the precipitated silica having the following physico-
chemical parameters:

which is characterized in that an alkali metal silicate is
reacted with mineral acids at temperatures of 60 - 95°C at
a pH of 7.0 - 11.0 with continuous stirring, the reaction
is continued up to a solids concentration of 40 g/1 - 110
g/1, the pH is adjusted to a value between 3 and 5, and the
precipitated silica is filtered off, washed and then dried,
and if appropriate ground or granulated.
In one embodiment of the invention, the addition of the
acid and of the water-glass can be interrupted for 3 0 to 90
minutes and then continued.
In one embodiment of the invention, the solids
concentration can be less than 80 g/1 and/or the
temperature can be less than 80 °C and/or the precipitation
time can be less than 76 minutes and/or the precipitation
can be interrupted and/or a dilute or concentrated mineral
acid can be employed.
In a further preferred embodiment, commercially available
soda water-glass (modulus 3.2 - 3.5) can be reacted with
sulfuric acid at a pH of between 7.5 and 10.5, some of the
soda water-glass already being added to the initial mixture
to adjust the pH. Simultaneous addition of water-glass and
sulfuric acid is maintained over a period of up to 120
minutes, it being possible for the addition to be
interrupted for 30 to 90 minutes in a particularly
preferred form. The mixture can then be acidified to pH 3 -
5, filtered, washed and dried. To achieve a particularly
good dispersibility, the simultaneous addition of soda
water-glass and sulfuric acid preferably takes place for
between 40 - 90 minutes. The surface area of the silica can
be established here via the duration of the precipitation.
In a particularly preferred form, the precipitated silicas
can be subjected to flash drying, it being possible for the
precipitated silica to be filtered off, washed and
redispersed.
In one embodiment of the invention, the following
conditions can be maintained for the preparation of a
precipitated silica having a BET surface area of 120 to 140
m2/g and a WK coefficient of less than 3.4:

1. Addition of water-glass and sulfuric acid over a period
of 15 to 25 minutes,
2. Interruption of the addition for 30 to 90 minutes,
3. Addition of water-glass and sulfuric acid over a period
I of 50 to 70 minutes,
where the total precipitation time can be 130 to 140
minutes.
In another preferred embodiment of the invention, the
following conditions can be maintained for the preparation
Addition of water-glass and sulfuric acid over a period of
60 to 70 minutes.
In another preferred embodiment of the invention, the
following conditions can be maintained for the preparation
of a precipitated silica having a BET surface area of 200
to 300 m2/g, preferably 200 to 240 m2/g, and a WK
coefficient of less than 3.4:
The modification with organosilanes can be carried out in
mixtures of 0.5 to 50 parts, based on 100 parts of silica,
in particular 2 to 15 parts, based on 100 parts of
precipitated silica, it being possible for the reaction
between the precipitated silica and silane to be carried
out during the preparation of the mixture (in situ) or
externally (premodification).
In a preferred embodiment of the invention,
bis(triethoxysilyl-propyl)-tetrasulfane is employed as the
silane.
The precipitated silica according to the invention can be
mixed into vulcanizable rubber mixtures as a reinforcing
filler in amounts of 5 to 200 parts, based on 100 parts of
rubber, as a powder, microbeads or granules, both with
silane modification and without silane modification.
The addition of one or more of the abovementioned silanes
to the rubber mixture can be carried out together with the
silicas according to the invention, the reaction between
the filler and silane proceeding during the mixing process
at elevated temperatures (in situ modification), or in an
already premodified form (for example in accordance with
DE-PS 40 04 781), that is to say the two reaction partners
are reacted outside the actual preparation of the mixture.
Another mixture comprises modifying the precipitated
silicas with organosilanes in mixtures of 0.5 to 50 parts,
based on 100 parts of precipitated silica, in particular 2
to 15 parts, based on 100 parts of precipitated silica, the
reaction between the precipitated silica and organosilane
being carried out during the preparation of the mixture (in
situ) or externally by spraying on and subsequent heat
treatment of the mixture, or by mixing the silane and the
silica suspension with subsequent drying and heat
treatment.
In addition to mixtures which comprise exclusively the
silicas according to the invention, with and without
organosilanes according to formula I to III, as fillers,
the rubber mixtures can additionally have a filler content
of one or several more or less reinforcing fillers. A blend
between carbon blacks (for example furnace, gas, flame and
acetylene blacks) and the silicas according to the
invention, with and without silane, and also between
naturally occurring fillers, such as, for example, clays,
silica chalks or other commercial silicas, and the silicas
according to the invention would primarily be customary
here.
The ratio in the blend also depends here, as with the
metered amount of the organosilanes, on the profile of
properties to be achieved in the finished rubber mixture. A
ratio of 5 - 95 % can be maintained between the silicas
according to the invention and the other abovementioned
fillers. In addition to the silicas according to the
invention, the organosilanes and other fillers, the
elastomers form a further important constituent of the
rubber mixture. The silicas according to the invention can
be employed in all types of rubbers which can be
crosslinked with accelerators/sulfur, and also
peroxidically. There would be mentioned here elastomers,
naturally occurring and synthetic, possibly extended with
oil, as an individual polymer or blend with other rubbers,
such as, for example, natural rubbers, butadiene rubbers,
isoprene rubbers, butadiene/styrene rubbers, in particular
SBR, prepared by means of the solution polymerization
process, butadiene/acrylonitrile rubbers, butyl rubbers,
terpolymers of ethylene, propylene and non-conjugated
dienes. The following additional rubbers are furthermore
possible for rubber mixtures with the rubbers mentioned:
carboxyl rubbers, epoxide rubbers, trans-polypentenamer,
halogenated butyl rubbers, rubbers of 2-chloro-butadiene,
ethylene/vinyl acetate copolymers, ethylene/propylene
copolymers, optionally also chemical derivatives of natural
rubber and modified natural rubbers.
The conventional further constituents, such as
plasticizers, stabilizers, activators, pigments, anti-
ageing agents and processing auxiliaries, in the
conventional metered amounts are also known.
The silicas according to the invention, with and without
silane, are employed in all rubber applications, such as,
for example, tyres, conveyor belts, seals, V-belts, hoses,
shoe soles etc. The precipitated silica according to the
invention can moreover be employed in battery separators,
in silicone rubber and as a silica support.
Examples
To achieve a good profile of values in a polymer mixture,
the dispersion of precipitated silica in the matrix, the
polymer, is of decisive importance. It has been found that
the wk coefficient is a measure of the dispersibility of a
precipitated silica.
The wK coefficient is determined as follows:
The measurement is based on the principle of laser
diffraction. The measurement is performed using a CILAS
Granulometer 1064 L. For the determination, 1.3 g of the
precipitated silica is transferred into 25 ml water and
treated with ultrasound at 100 W (90 % pulsed) for 4.5
minutes. Thereafter, the solution is transferred to the
measuring cell and treated with ultrasound for a further
minute. The detection with the aid of two laser diodes at
different angles to the sample is carried out during the
ultrasonic treatment. The laser beams are diffracted in
accordance with the principle of diffraction of light. The
diffraction pattern formed is evaluated with the aid of a
computer. The method enables the particle size distribution
to be determined over a wide measurement range (approx. 40
nm - 5 00 urn).
An essential point here is that the energy introduced by
ultrasound represents a simulation of the energy introduced
by mechanical forces in the kneaders of the tyre industry.
Figures 1-4 show the results and measurements of the
particle size distribution of precipitated silicas
according to the invention and of comparison silicas.
The curves show a first maximum in the particle size
distribution in the range of 1.0 - 100 [im, and a further
maximum in the range of < 1.0 urn. The peak in the range of
1.0 - 100 p.m indicates the proportion of non-comminuted
silica particles after the ultrasonic treatment. These
quite coarse particles are poorly dispersed in the rubber
mixtures. The second peak of significantly smaller particle
size (< 1.0 |im) indicates that portion of particles of the
silica which has been comminuted during the ultrasonic
treatment. These very small particles are excellently
dispersed in rubber mixtures.
The wK coefficient is thus the ratio of the peak height of
the non-degradable particles (B), the maximum of which lies
in the range of 1.0 - 100 urn, to the peak height of the
degraded particles (A), the maximum of which lies in the
range of < 1.0 mm.

The wk coefficient is therefore a measure of the
"degradability" (=dispersibility) of the precipitated
silica. Therefore: A precipitated silica is more readily
dispersible the smaller the wk coefficient and the more
particles are degraded during incorporation into rubber.
The silicas according to the invention have wk coefficients
of < 3.4. The maximum in the particle size distribution of
the non-degradable particles of the precipitated silica
according to the invention lies in the range of 1.0 - 100
Jim. The maximum in the particle size distribution of the
degraded particles of the precipitated silica according to
the invention lies in the range of < 1.0 \xm.
Known precipitated silicas have significantly higher wk
coefficients and other maxima in the particle size
distributions measured with the CILAS Granulometer 1064 L
and are therefore more poorly dispersible.
The following substances are employed in the examples:
First Latex Crepe - Natural rubber
CBS - Benzothiazyl-2-cyclohexylsulfenamide
TMTM - Tetramethylthiuram monosulfide
SI 69 - Bis(3-triethoxysilylpropyl)tetrasul-
fane (Degussa AG)
DEG - Diethylene glycol
VSL 1955 S 25 - Styrene/butadiene rubber based on
solution polymerization with a
styrene content of 25 % and a
vinyl content of 55 % (Bayer AG)
DPG - Diphenylguanidine
Vulkanox 4020 - N-(1,3-Dimethylbutyl)-N'-phenyl-p-
phenylenediamine (Bayer AG)
Protector G 35 - Ozone protection wax
ZBED - Zinc dibenzyldithiocarbamate
Buna CB 24 - Butadiene rubber from Bunawerke Huls
Naftolen ZD - Aromatic mineral oil plasticizer

Example 1
Preparation of a precipitated silica in the N2 surface area
range of 120 - 140 m3/g
17.6 1 water are mixed with soda water-glass (modulus 3.42,
density 1.346) up to pH 8.5, while stirring, in a vat and
the mixture is heated to 78 °C. 1.18 1 water-glass and 0.28
1 50 % sulfuric acid are added in the course of 20 min with
constant stirring, while maintaining the temperature of
78 °C and the pH of 8.5. The addition of water-glass and
acid is then stopped for 60 min. Thereafter, further water-
glass solution and sulfuric acid are added until, after 138
min, a solids content of 75 g/1 is reached.
Sulfuric acid is then added until a pH of between 3 and 5
is reached. The solid is separated off on a filter press,
washed and then subjected to brief or long-term drying, and
if appropriate ground.
The resulting precipitated silica has an N2 surface area of
127 m2/g, a CTAB surface area of 12 0 m2/g, a DBP index of
252 ml/100 g and a Sears index of 10.5.
Example 2
Preparation of a precipitated silica in the N2 surface area
range of 140 - 160 m2/g
45.5 m3 water are heated to 95 °C in a vat, while stirring.
Soda water-glass (modulus 1.342, density 1.348) and 96 %
sulfuric acid are added, with constant stirring and while
maintaining the temperature of 95 °C, at a pH of 7.5 in the
course of 48 min in an amount such that a solids content of
56 g/1 is reached after 48 min. Sulfuric acid is then added
until a pH of between 3 and 5 is reached. The solid is
separated off on a filter press, washed and then subjected
to brief or long-term drying, and if appropriate ground.
The resulting precipitated silica has an N2 surface area of
141 m2/g, a CTAB surface area of 121 m2/g, a DBP index of
288 ml/100 g and a Sears index of 7.5.
Example 3
Preparation of a precipitated silica in the N2 surface area
range of 160 - 180 m2/g
20.6 1 water are mixed with soda water-glass (modulus 3.42,
density 1.350) until pH 8.5 is reached, while stirring, in
a vat and the mixture is heated to 62 °C. 5.6 1 water-glass
and 1.3 1 50 % sulfuric acid are added, with constant
stirring and while maintaining the temperature of 62 °C and
the pH of 8.5, in the course of 158 min in an amount such
that a solids content of 76 g/1 is reached after 158 min.
Sulfuric acid is then added until a pH of between 3 and 5
is reached. The solid is separated off on a filter press,
washed and then subjected to brief or long-term drying, and
if appropriate ground.
The resulting precipitated silica has an N2 surface area of
171 m2/g, a CTAB surface area of 139 m2/g, a DBP index of
275 ml/100 g and a Sears index of 17.6.
Example 4
Preparation of a precipitated silica in the N2 surface area
range of 180 - 200 m2/g
46 m3 water are mixed with soda water-glass (modulus 1.342,
density 1.348) until pH 9 is reached, while stirring, in a
vat and the mixture is heated to 80 °C. Soda water-glass
and 96 % sulfuric acid are added, with constant stirring
and while maintaining the temperature of 80 °C, at a pH of
9.0 in the course of 67 min in an amount such that a solids
content of 8 9 g/1 is reached after 67 min. Sulfuric acid is
then added until a pH of between 3 and 5 is reached. The
solid is separated off on a filter press, washed and then
subjected to brief or long-term drying, and if appropriate
ground.
The resulting precipitated silica has an N2 surface area of
185 m2/g, a CTAB surface area of 163 m2/g, a DBP index of
269 ml/100 g and a Sears index of 17.0.
Example 5
Preparation of a precipitated silica in the N2 surface area
range of 200 - 300 m2/g
46 m3 water are mixed with soda water-glass (modulus 1.342,
density 1.348) until pH 9 is reached, while stirring, in a
vat and the mixture is heated to 69 °C. Soda water-glass
and 96 % sulfuric acid are added, with constant stirring
and while maintaining the temperature of 69 °C, at a pH of
9.0 in the course of 76 min in an amount such that a solids
content of 96.5 g/1 is reached after 76 min. Further
sulfuric acid is then added until a pH of between 3 and 5
is reached. The solid is separated off on a filter press,
washed and then subjected to brief or long-term drying, and
if appropriate ground.
The resulting precipitated silica has an N2 surface area of
218 m2/g, a CTAB surface area of 186 m2/g, a DBP index of
299 ml/100 g and a Sears index of 21.6.
Example 6
Determination of the wk coefficient with the Cilas
Granulometer 1064 L on a silica according to the invention
with a CTAB surface area of 12 0 m /g and comparison with
standard silicas in the same surface area range. The values
B, A, B' and A1 according to graph 1 are additionally
given.

Example 7
Determination of the wk coefficient with the Cilas
Granulometer 1064 L on a silica according to the invention
with a CTAB surface area in the range of 130 - 150 m2/g and
comparison with standard silicas in the same surface area
range. The values B, A, B' and A1 according to graph 1 are
additionally given.

Example 8
Determination of the wk coefficient with the Cilas
Granulometer 1064 L on a silica according to the invention
with a CTAB surface area in the range of 150 - 180 m2/g and
comparison with standard silicas in the same surface area
range. The values B, A, B' and A' according to graph 1 are
additionally given.

The precipitated silica 3370 shown in the table corresponds
to the precipitated silica according to EP-A 0 647 591,
example 3. It has a substantially poorer WK coefficient
than the precipitated silica according to the invention.
Furthermore, the Phillips value - as can be seen from
example 10 - is significantly poorer. This means: The
precipitated silica according to the invention has a
significantly better dispersibility and therefore a lower
abrasion in the tyre mixture.
Example 9
Measurement results of the precipitated silica according to
the invention from example x and x in comparison with
standard silicas (see figures 1 - 4 in the appendix).
Example 10
Silica according to the invention from example 4 in
comparison with standard silicas in L-SBR/BR running tread
mixtures:
The silica according to the invention from example 4 has a
lower viscosity, higher elongation at break, higher wet
antiskid properties at a low rolling resistance and,
particularly importantly, a higher dispersion coefficient
compared with both Ultrasil VN3 and Ultrasil 3370. Ultrasil
3370 is described in the document EP-A 0 647 591, example
3. A higher dispersion coefficient means a lower abrasion,
and a lower abrasion means a longer life of the tyres.
Example 11
Silica according to the invention from example 1 in
comparison with standard silica in an NR/SBR mixture for
the tyre carcass:

The silica according to the invention from example 1 leads
to lower viscosities, higher modulus values, a higher
elongation at break, a lower heat build up and a higher
dispersion coefficient compared with Ultrasil VN2, which
has a similar surface area.
The figures describe:
Figure 1 - Result of the measurement of Ultrasil 3380 by
the laser diffraction method
Figure 2 - Result of the measurement of silica according to
the invention from example 3 by the laser
diffraction method
Figure 3 - Result of the measurement of Ultrasil VN 3 by
the laser diffraction method
Figure 4 - Result of the measurement of Zeosil 1165 MP by
the laser diffraction method
Figure 5 - Result of the measurement of Perkasil KS 4 08 by
the laser diffraction method
Figure 6 - Graph of the wk coefficient
WE CLAIM:
1. Process for the preparation of the precipitated silica having the following
physico-chemical parameters:
BET surface area 120 - 300 m2/g
CTAB surface area 100 - 300 m2/g
BET/CTAB ratio 0.8 - 1.3
Sears index (consumption of 6-25 ml
0.1 NNaOH)
DBP index 150 - 300g/100g
wk coefficient < 3.4
Particle size of the degraded particles < 1.0 urn
Particle size of the non-degradable
particles 1.0 - 100 urn
characterized in that an alkali metal silicate such as herein described,
(preferably soda water-glass) is reacted with mineral acids such as herein
described (preferably sulfuric acid) at temperatures of 60 - 95 °C at a pH
of 7.0 - 11.0 with continuous stirring, the reaction is continued up to a
solids concentration of 40 g -110 g.the pH is adjusted to a value between
3 and 5, and the precipitated silica is filtered off, washed and then dried,
and if appropriate ground or granulated.
2. Process as claimed in claim 2, wherein the solids concentration is less
than 80 g/1 and/or the temperature is less than 80 °C and/or the
precipitation time is less than 76 minutes and/or the precipitation is
interrupted and/or a dilute or concentrated mineral acid is employed.
3. Process as claimed in claims 2 or 3, wherein the following conditions are
maintained for the preparation of a precipitated silica having a BET
surface area of 120 to 140 m2/g and a WK coefficient of less than 3.4:
Solids concentration 68 to 85 g/l
Temperature 74 to 82 °C
PH 8 to 9, preferably 8.5
1. Addition of water-glass and sulfuric acid over a period of 15 to 25
minutes.
2. Interruption of the addition for 30 to 90 minutes,
3. Addition of water-glass and sulfuric acid over a period of 50 to 70
minutes,
where the total precipitation time is 130 to 140 minutes.
4. Process as claimed in claims 2 or 3, wherein the following conditions are
maintained for the preparation of a precipitated silica having a BET
surface area of 140 to 160 m2/g and a WK coefficient of less than 3.4:
Solids concentration 40 to 60 g/l
Temperature 88 to 96 °C
PH 7 to 9, preferably 7.5
Addition of water-glas and sulfuric acid over a period of 38 to 50 minutes.
5. Process as claimed in claims 2 or 3, wherein the following conditions are
maintained for the preparation of a precipitated silica having a BET
surface area of 160 to 180 m2/g and a WK coefficient of less than 3.4:
Solids concentration 68 to 85 g/l
Temperature 74 to 82 C
pH 8 to 9, preferably 8.5
Addition of water-glass and sulfuric acid over a period of 150 to 170
minutes.
6. Process as claimed in claims 2 or 3, wherein the following conditions are
maintained for the preparation of a precipitated silica having a BET
surface area of 180 to 200 m2/g and a WK coefficient of less than 3.4:
Solids concentration 74 to 94 g/l
Temperature 75 to 83 °C
pH 8 to 10
Addition of water-glass and sulfuric acid over a period of 60 to 70 minutes.
7. Process as claimed in claims 2 or 3, wherein the following conditions are
maintained for the preparation of a precipitated silica having a BET
surface area of 200 to 300 m2/g, preferably 200 to 240 m2/g and a WK
coefficient of less than 3.4:
Solids concentration 70 to 110 g/l
Temperature 60 to 76 °C
pH 8 to 10, preferably 9
Addition of water-glass and sulfuric acid over a period of 60 to 86 minutes.
8 Process as claimed in claims 2 to 7, wherein chamber filler presses or
membrane filter presses or belt filters or rotary filters or automatic
membrane filter presses or two of the filters In combination are employed
For the nitration.
9. Process as claimed In claims 2 to 8, wherein a flow dryer, rack dryer, flash
dryer, spin-flash dryer or similar equipment is employed for the drying.
10. Process as claimed in claims 2 to 9, wherein liquefied niter cakes are dried
In a spray dryer with an atomizer or two-component nozzle or one-
component nozzle and/or integrated fluidized bed.
11. Process as claimed In claims 2 to 10, wherein a roller compactor or similar
equipment is employed for the granulation.
12. Process for the preparation of the silicas as claimed in claim 11, wherein
the precipitated silicas are modtfted with organosllanes In mixtures of 0.5
to 50 parts, based on 100 parts of precipitated silica, in particular 2 to 15
parts, based on 100 parts of precipitated silica, the reaction between the
precipitated silica and organosilane being carried out during the
preparation of the mixture (in situ) or externally by spraying on and
subsequent heat treatment of the mixture, or by mixing the silane and the
silica suspension with subsequent drying and heat treatment.
13. Vulcanizable rubber mixtures and vulcanisates which comprise, as a filler,
the precipitated filler as claimed In claim 1 with the following physico-
chemical parameters
BET surface area 120 - 300 m2/g
CTAB surface area 100 - 300 m2/g
BET/CTAB ratio 0.8 - 1.3
Sears index (consumption of 6-25 ml
O.INNaOH)
DBP index 160 - 300g/100g
wk coefficient < 3.4
Particle size of the degraded particles < 1.0 urn
Particle size of the non-degradable
particles 1.0 • 100 urn

Precipitated silica, characterized by the following
physico-chemical parameters:
BET surface area 120 - 300 m2/g
CTAB surface area 100 - 3 00 m2/g
BET/CTAB ratio 0.8 - 1.3
Sears index (consumption of 0.1 N NaOH) 6 - 25 ml
DBP index 150 - 300 g/100 g
wk coefficient < 3.4
Particle size of the degraded
particles < 1.0 μm
Particle size of the non-
degradable particles 1.0 - 100 μm
It is prepared by a process in which an alkali metal
silicate (preferably soda water-glass) is reacted with
mineral acids (preferably sulfuric acid) at temperatures of
60 - 95°C at a pH of 7.0 - 11.0 with continuous stirring,
the reaction is continued up to a solids concentration of
40 g - 110 g, the pH is adjusted to a value between 3 and
5, and the precipitated silica is filtered off, washed and
then dried, and if appropriate ground or granulated.
It is employed as a filler in vulcanizable mixtures for the
production of tyres.