Readily dispersible precipitated silica

Details Readily dispersible precipitated silica Readily dispersible precipitated silica

1998-09-15

2001-01-30

Cain, Edward J. (Department: 1714)

Chemistry of inorganic compounds

Silicon or compound thereof

Oxygen containing

C423S338000, C524S493000

Reexamination Certificate

active

06180076

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a readily dispersible precipitated silica, the process for its preparation and its use in rubber mixtures.
BACKGROUND OF THE INVENTION
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 tire mixtures, especially with high filler contents.
The document EP-A 0 520 862 discloses precipitated silicas which are employed as a filler in rubber mixtures for tires.
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 above mentioned precipitated silicas. Due to increased requirements by the tire industry, even the improved dispersion of this precipitated silica is no longer adequate for use in tire mixtures.
SUMMARY OF THE INVENTION
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:
BET surface area
120-300 m
2
/g
CTAB surface area
100-300 m
2
/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
<1.0 &mgr;m
particles
Particle size of the non-
1.0-100 &mgr;m
degradable particles
The physico-chemical data are determined by the following methods:
BET surface area
Areameter, Ströhlein, in
accordance with ISO
5794/Annex D
CTAB surface area
at pH 9, in accordance with
Jay, Janzen and Kraus in
“Rubber Chemistry and
Technology” 44 (1971) 1287
Sears index
in accordance with G.W.
Sears, Analyt. Chemistry 12
(1956) 1982
DBP index
ASTM D 2414-88
wk coefficient
Cilas Granulometer 1064 L
In a particularly preferred embodiment, the precipitated silica according to the invention can have the following physico-chemical data:
BET surface area
120-200 m
2
/g
CTAB surface area
100-200 m
2
/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
<1.0 &mgr;m
particles
Particle size of the non-
1.0-30 #m
degradable particles
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:
BET surface area
120-300 m
2
/g
CTAB surface area
100-300 m
2
/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
<1.0 &mgr;m
particles
Particle size of the non-
1.0-100 &mgr;m
degradable particles
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/l-110 g/l, 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 30 to 90 minutes and then continued.
In one embodiment of the invention, the solids concentration can be less than 80 g/l 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 m
2
/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 can be 130 to 140 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 140 to 160 m
2
/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 to 8
Addition of water-glass and sulfuric acid over a period of 38 to 50 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 160 to 180 m
2
/g and a WK coefficient of less than 3.4:
Solids concentration: 68 to 84 g/l
Temperature: 59 to 65° C.
pH: 8 to 9, preferably 8.5
Addition of water-glass and sulfuric acid over a period of 150 to 170 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 180 to 200 m
2
/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.
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 m
2
/g, preferably 200 to 240 m
2
/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.
The precipitated silica according to the invention can be modified with organosilanes of the formulae I to III:
in which the symbols denote:
Z: —SCN, —SH, —Cl, —NH
2
(if q=1) or —Sx—(if q=2),
R and R
1
: an alkyl group having 1 to 4 carbon atoms, the phenyl radical, wherein all the radicals R and R
1
can each have the same or a different meaning,
R: a C
1
-C
4
-alkyl, C
1
C
4
-alkoxy group,
n: 0; 1 or 2,
Alk: a divalent straight-chain or branched hydrocarbon radical having 1 to 6 carbon atoms,
m: 0 or 1,
Ar: an arylene radical having 6 to 12 C atoms, preferably 6 C atoms,
p: 0 or 1, with the proviso that p and n do not simultaneously denote 0,
x: a number from 2 to 8,
alkyl: a monovalent straight-chain or branched unsaturated hydrocarbon radical having 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms,
alkenyl: a monovalent straight-chain or branched unsaturated hydrocarbon radical having 2 to 20 c

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