Higly dispersible silica for using in rubber

Abstract

The present invention relates to highly disperse precipitated silicas which exhibit an extremely high level of reinforcement of rubber vulcanizates, to a process for their preparation, and to their use as filler for rubber mixtures.

Claims

1. A process for preparing a precipitated silica comprising (1) simultaneously adding at a metered rate and with stirring water glass and an acid into an aqueous solution comprising at least one of an alkali metal silicate, an alkaline earth metal silicate, and a base, and having an alkali value from 7 to 30; wherein an alkali value of the aqueous solution is maintained from 7 to 30, a temperature of the aqueous solution is from 55 to 95 C., and a time of the metered addition is from 10 to 120 minutes, (2) further adding the acid to acidify the composition obtained in (1) to a pH of from approximately 2.5 to 6, thereby obtaining a suspension comprising precipitated silica, and (3) filtering the precipitated silica, washing the precipitated silica to reduce a sodium sulfate content of the precipitated silica to less than 4% by weight, (4) liquidizing the precipitated silica in aqueous sulfuric acid, and (5) drying the liquidized precipitated silica with addition of ammonia employing a spray tower drying method, wherein the base in (1), if present, is at least one selected from the group consisting of an alkali metal hydroxide, an alkaline earth metal hydroxide, an alkali metal carbonate, and an alkali metal hydrogen carbonate, a conductivity of the dried precipitated silica is from 580 to 930 S/cm, and a ratio of BET surface area to CTAB surface area of the precipitated silica is from 0.9 to 1.15.

2. The process as claimed in claim 1, wherein the alkali value in (1) is from 15 to 30.

3. The process as claimed in claim 1, wherein the alkali value in (1) is from 18 to 30.

4. The process as claimed in claim 1, wherein (1) is carried out in a manner which comprises stopping the feed for from 30 to 90 minutes while maintaining the temperature, and then, at the same temperature, for from 10 to 120 minutes, simultaneously adding water glass and an acid in such a way that the alkali value remains constant during the precipitation.

5. The process as claimed in claim 4, wherein the simultaneous addition of water glass and acid is carried out for from 10 to 60 minutes.

6. The process as claimed in claim 1, wherein during (1), an organic or inorganic salt is added.

7. The process as claimed in claim 4, wherein during (1) an organic or inorganic salt is added.

8. The process as claimed in claim 1, wherein after drying, pelletizing is carried out with a roller compactor.

9. The process as claimed in claim 1, wherein the precipitated silica has the following physical and chemical properties: TABLE-US-00017 CTAB surface area 100-160 m.sup.2/g BET surface area 100-190 m.sup.2/g DBP value 180-300 g/(100 g) Sears value V.sub.2 15-28 ml/(5 g) Moisture level 4-8% Ratio of Sears value V.sub.2 to 0.150 to 0.280 ml/(5 m.sup.2). BET surface area

10. The process as claimed in claim 9, wherein the surface of the precipitated silica has been modified with an organosilane of one of the formulae I to III
[SiR.sup.1.sub.n(OR).sub.r(Alk).sub.m(Ar).sub.p].sub.q[B](I),
SiR.sup.1n(OR).sub.3-n(Alkyl)(II),
SiR.sup.1.sub.n(OR).sub.3-n(Alkenyl)(III), where B is SCN, SH, Cl, NH.sub.2, OC(O)CHCH.sub.2, OC(O)C(CH.sub.3)CH.sub.2 (if q=1), or S.sub.w (if q=2), B being chemically bonded to Alk, R and R.sup.1 are an aliphatic, olefinic, aromatic, or arylaromatic radical having 2-30 carbon atoms, optionally with substitution by the following groups: the hydroxyl, amino, alcoholate, cyanide, thiocyanide, halo, sulfonic acid, sulfonic ester, thiol, benzoic acid, benzoic ester, carboxylic acid, carboxylic ester, acrylate, methacrylate, or organosilane radical, where the meaning or substitution of R and R.sup.1 may be identical or different, n is 0, 1, or 2, Alk is a bivalent unbranched or branched hydrocarbon radical having from 1 to 6 carbon atoms, m is 0 or 1, Ar is an aryl radical having from 6 to 12 carbon atoms, which may have substitution by the following groups: the hydroxyl, amino, alcoholate, cyanide, thiocyanide, halo, sulfonic acid, sulfonic ester, thiol, benzoic acid, benzoic ester, carboxylic acid, carboxylic ester, acrylate, methacrylate or organosilane radical, p is 0 or 1, with the proviso that p and n are not simultaneously 0, q is 1 or 2, w is a number from 2 to 8, r is 1, 2, or 3, with the proviso that r+n+m+p=4, Alkyl is a monovalent unbranched or branched saturated hydrocarbon radical having from 1 to 20 carbon atoms, Alkenyl is a monovalent unbranched or branched unsaturated hydrocarbon radical having from 2 to 20 carbon atoms, the process comprising modifying the precipitated silica with said organosilane in a mixture of from 0.5 to 50 parts, based on 100 parts of precipitated silica, where the reaction between the precipitated silica and organosilane is carried out during the preparation of the mixture (in situ) or externally via spray application and subsequent heat-conditioning of the mixture, via mixing of the organosilane and the silica suspension with subsequent drying and heat-conditioning.

11. The process as claimed in claim 10, wherein the mixture is from 1 to 15 parts of the organosilane, based on 100 parts of precipitated silica.

12. The process as claimed in claim 10, wherein Ar is an aryl radical having 6 carbon atoms, Alkyl is a monovalent unbranched or branched saturated hydrocarbon radical having from 2 to 8 carbon atoms, and Alkenyl is a monovalent unbranched or branched unsaturated hydrocarbon radical having from 2 to 8 carbon atoms.

13. The process as claimed in claim 1, additionally comprising incorporating the precipitated silica into a vulcanized rubber.

14. The process as claimed in claim 1, wherein the alkali value in (1) is from 15 to 25.

15. The process as claimed in claim 1, wherein the alkali value in (1) is from 18 to 22.

Description

EXAMPLE 1

Preparation of Silicas

Example 1.1

(1) 1550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) form an initial charge in a reactor made from stainless steel with a propeller-stirrer system and jacket heating. 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then metered in with vigorous stirring at 92 C. over a period of 80 minutes. This metering of sulfuric acid is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The water glass addition is then stopped, and the addition of sulfuric acid is continued until a pH of 5.0 (measured at room temperature) has been achieved. The resultant suspension is filtered, using a membrane filter press, and the product is washed with water. The filter cake, with 21% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed with 18% solids content and with a pH of 4.0 is then spray-tower dried with addition of ammonia.

(2) The resultant microbead product has a BET surface area of 123 m.sup.2/g and a CTAB surface area of 119 m.sup.2/g.

Example 1.2

(3) 1550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) form an initial charge in a reactor made from stainless steel with a propeller-stirrer system and jacket heating. 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then metered in with vigorous stirring at 88.5 C. over a period of 80 minutes. This metering of sulfuric acid is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The water glass addition is then stopped, and the addition of sulfuric acid is continued until a pH of 4.5 (measured at room temperature) has been achieved. The resultant suspension is filtered, using a membrane filter press, and the product is washed with water. The filter cake, with 19% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed with 17% solids content and with a pH of 3.0 is then spray-tower dried with addition of ammonia.

(4) The resultant microbead product has a BET surface area of 168 m.sup.2/g and a CTAB surface area of 148 m.sup.2/g.

Example 1.3

(5) 1550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) form an initial charge in a reactor made from stainless steel with a propeller-stirrer system and jacket heating. 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then metered in with vigorous stirring at 93 C. over a period of 80 minutes. This metering of sulfuric acid is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The water glass addition is then stopped, and the addition of sulfuric acid is continued until a pH of 5.0 (measured at room temperature) has been achieved. The resultant suspension is filtered, using a membrane filter press, and the product is washed with water. The filter cake, with 21% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed with 18% solids content and with a pH of 4.0 is then spray-dried with addition of ammonia and roller-granulated.

(6) The resultant granulated product has a BET surface area of 126 m.sup.2/g and a CTAB surface area of 118 m.sup.2/g.

Example 1.4

(7) 1 550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) formed an initial charge in a stainless steel reactor with propeller-stirrer system and jacket heating.

(8) 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then added at 92 C. over a period of 100 minutes, with vigorous stirring. This sulfuric acid addition is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The addition of water glass is then stopped, and the supply of sulfuric acid is continued until a pH of 5.0 (measured at room temperature) has been reached.

(9) The resultant suspension is filtered, using a membrane filter press, and washed with water. The filter cake, with 22% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed, with 19% solids content and with a pH of 3.8, is then spray-dried with ammonia feed and roller-granulated.

(10) The resultant granular product has a BET surface area of 130 m.sup.2/g and a CTAB surface area of 113 m.sup.2/g.

Example 1.5

(11) 1 550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) formed an initial charge in a stainless steel reactor with propeller-stirrer system and jacket heating.

(12) 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then added at 92.0 C. over a period of 100 minutes, with vigorous stirring. This sulfuric acid addition is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The addition of water glass is then stopped, and the supply of sulfuric acid is continued until a pH of 5.0 (measured at room temperature) has been reached.

(13) The resultant suspension is filtered, using a membrane filter press, and washed with water. The filter cake, with 21% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed, with 19% solids content and with a pH of 4.0 is then spray-tower dried with ammonia feed.

(14) The resultant microbead product has a BET surface area of 110 m.sup.2/g and a CTAB surface area of 108 m.sup.2/g.

Example 1.6

(15) 1 550 l of water and 141.4 kg of water glass (density 1.348 kg/l, 27.0% SiO.sub.2, 8.05% Na.sub.2O) formed an initial charge in a stainless steel reactor with propeller-stirrer system and jacket heating.

(16) 5.505 kg/min of the abovementioned water glass and about 0.65 kg/min of sulfuric acid (density 1.83 kg/l, 96% H.sub.2SO.sub.4) are then added at 88.0 C. over a period of 100 minutes, with vigorous stirring. This sulfuric acid addition is regulated in such a way that the alcaline number prevailing in the reaction mixture is 20. The addition of water glass is then stopped, and the supply of sulfuric acid is continued until a pH of 5.0 (measured at room temperature) has been reached.

(17) The resultant suspension is filtered, using a membrane filter press, and washed with water. The filter cake, with 22% solids content, is liquidized, using aqueous sulfuric acid and a shearing assembly. The silica feed, with 20% solids content and with a pH of 3.0 is then spray-tower dried with ammonia feed.

(18) The resultant microbead product has a BET surface area of 143 m.sup.2/g and a CTAB surface area of 131 m.sup.2/g.

(19) Further physico-chemical data for the abovementioned silicas are given in the following table.

(20) TABLE-US-00008 Silica from Sears value Sears value Example BET CTAB DBP Moisture pH Conductivity V.sub.2 V.sub.2/BET No. [m.sup.2/g] [m.sup.2/g] [g/(100 g)] [%] [] [S/cm] [ml/(5 g)] [ml/(5 m.sup.2)] 1.1 123 119 272 4.8 5.6 610 24 0.195 1.2 168 148 265 5.5 6.0 700 26 0.155 1.3 126 118 207 5.1 5.1 810 22 0.175 1.4 130 113 204 5.2 6.2 720 22 0.169 1.5 110 108 271 5.1 5.5 930 25 0.227 1.6 143 131 258 4.8 5.7 580 26 0.182

EXAMPLE 2

Example 2.1

(21) The precipitated silicas 1.1 and 1.3 of the invention from Example 1 were studied in an SBR emulsion rubber mixture. The silica Ultrasil VN2 GR from Degussa AG with a CTAB surface area of 125 m.sup.2/g was selected as prior art and reference.

(22) The mixing specification used for the rubber mixtures is given in Table 2.1 below. The unit phr here means parts by weight, based on 100 parts of the untreated rubber used.

(23) TABLE-US-00009 TABLE 2.1 Reference A B 1st stage Buna SBR 1712 137.5 137.5 137.5 Ultrasil VN2 GR 50 Silica as per Ex. 1.1 50 Silica as per Ex. 1.3 50 X50-S 3 3 3 ZnO 3 2 3 Stearic acid 1 1 1 Vulkanox 4020 2 2 2 Protector G 3108 1.5 1.5 1.5 2nd stage Stage 1 batch 3rd stage Stage 2 batch Vulkacit D/C 1.5 1.5 1.5 Vulkacit CZ/EG 1.5 1.5 1.5 Sulfur 2.2 2.2 2.2

(24) Polymer Buna 1712 is an emulsion-polymerized SBR copolymer from Buna DOW Leuna with a styrene content of 23.5% by weight and with an oil content of 37.5 phr. X50-S is a 50/50 blend of Si 69 [bis(3-triethoxysilylpropyl)tetrasulfane] and carbon black obtainable from Degussa AG. Vulkanox 4020 is 6PPD from Bayer AG, and Protektor G 3108 is an ozone-protection wax from HB-Fuller GmbH. Vulkacit D/C (DPG) and Vulkacit CZ/EG (CBS) are commercially available products from Bayer AG.

(25) The rubber mixtures are prepared in an internal mixer using the mixing instructions in Table 2.2. Table 2.3 gives the methods used for rubber testing. The mixtures are vulcanized at 160 C. for 18 minutes. Table 2.4 shows the results of testing on the vulcanized rubber.

(26) TABLE-US-00010 TABLE 2.2 Stage 1 Settings Mixing assembly Werner & Pfleiderer 1.5N Rotation rate 45 rpm Friction 1:1.11 Ram pressure 5.5 bar Capacity 1.6 1 Fill level 0.73 Chamber temp. 70 C. Mixing procedure 0-1 min Polymer 1-2 min 1st stage constituents 2 min Purging 2-3 min Mixing, aerating 3-4 min Mixing at 70 rpm, aerating 4-5 min Mixing at 75 rpm, discharging Aging 24 h at room temperature Stage 2 Settings Mixing assembly as in stage 1 except: Rotation rate 70 rpm Fill level 0.71 Mixing procedure 0-1 min stage 1 batch, plasticize 1-3 min maintain batch temperature 150 C. by varying rotation rate 3 min Discharge Aging 4 h at room temperature Stage 3 Settings Mixing assembly as in stage 1 except: Rotation rate 40 rpm Chamber temp. 50 C. Fill level 0.69 Mixing procedure 0-2 min stage 2 batch, stage 3 constituents 2 min discharge and form milled sheet on laboratory roll mill, (diameter 200 mm, length 450 mm, roll temperature 50 C.) homogenization: cut the material 3 times toward the left, 3 times toward the right; fold the material over 3 times with wide roll gap (3.5 mm) and 3 times with narrow roll gap (1 mm): peel milled sheet away

(27) TABLE-US-00011 TABLE 2.3 Physical testing Standard/conditions Vulcameter testing, 160 C. DIN 53529/3, ISO 6502 Torque difference Dmax Dmin [dNm] t10% and t90% [min] Ring tensile test, 23 C. DIN 53504, ISO 37 Modulus values 100% and 500% [MPa] Reinforcement factor: modulus value 500%/100% [] Elongation at break [%] Shore A hardness, 23 C. [] DIN 53 505 Ball rebound [%], 0 C. and 60 C. DIN EN ISO 8307 steel ball, 19 mm, 28 g Dispersion coefficient [%] see text

(28) The dispersion coefficient was determined using the surface topography inc. Medalia correction (A. Wehmeier, Filler Dispersion Analysis by Topography Measurements Technical Report TR 820, Degussa AG, Advanced Fillers and Pigments Division). The dispersion coefficient thus determined correlates directly at reliability level>0.95 with the optically determined dispersion coefficient, for example as determined by the Deutschen Institut far Kautschuktechnologie e.V. [Germany Institute for Rubber Technology], Hanover, Germany (H. Geisler, Bestimmung der Mischgte, presented at DIK Workshop, 27-28 Nov. 1997, Hanover, Germany).

(29) TABLE-US-00012 TABLE 2.4 Data for untreated mixture Reference A B Dmax Dmin 11.5 11.8 11.6 t10% 4.9 4.6 4.6 t90% 9.8 9.6 9.5 Vulcanizate data 100% modulus 1.0 1.0 1.0 500% modulus 9.1 9.9 10.3 500%/100% modulus 9.1 9.9 10.3 Elongation at break 530 500 520 Shore A hardness 51 51 51 Ball rebound 0 C. 22.1 21.2 21.3 Ball rebound 60 C. 71.0 70.4 70.3 Dispersion coefficient 98 99 97

(30) As can be seen from the data in Table 2.4, with the mixtures A and B the silicas of the invention have a lower vulcanization time t90% than the reference mixture. In addition to the lower vulcanization time, advantages are to be found in particular in a higher 500% modulus value and the increased reinforcement factor. The ball rebound values at 0 and 60 C. are comparable, and no shortcomings in the hysteresis behavior of the mixtures are therefore to be expected. The dispersion of the silicas of the invention is very good.

Example 2.2

(31) The precipitated silica 1.2 of the invention from example 1 was studied in an SSBR/BR rubber mixture. The prior art and reference selected was the silica Ultrasil 3370 GR from Degussa AG with a CTAB surface area of 160 m.sup.2/g. The mixture used represents a model mixing specification for a car tire tread mixture.

(32) The mixing specification used for the rubber mixtures is given in Table 2.5 below. The unit phr here means parts by weight, based on 100 parts of the unprocessed rubber used.

(33) TABLE-US-00013 TABLE 2.5 Reference C 1st Stage Buna VSL 5025-1 96 96 Buna CB 24 30 30 Ultrasil 3370 80 GR Silica of Ex. 80 1.2 X50-S 12.8 12.8 ZnO 2 2 Stearic acid 2 2 Naftolen ZD 10 10 Vulkanox 4020 1.5 1.5 Protektor G 1 1 3108 2nd Stage Stage 1 batch 3rd Stage Stage 2 batch Vulkacit D/C 2.0 2.0 Vulkacit 1.5 1.5 CZ/EG Perkazit 0.2 0.2 TBZTD Sulfur 1.5 1.5

(34) The polymer VSL 5025-1 is a solution-polymerized SBR copolymer from Bayer AG with a styrene content of 25% by weight and a butadiene content of 75% by weight. The copolymer comprises 37.5 phr of oil and has a Mooney viscosity (ML 1+4/100 C.) of 504. The polymer Buna CB 24 is a cis-1,4-polybutadiene (neodymium type) from Bayer AG with cis-1,4 content of at least 97% and a Mooney viscosity of 445. X50 S is a 50/50 blend of Si 69 [bis(3-triethoxysilylpropyl)tetrasulfane] and carbon black obtainable from Degussa AG. The aromatic oil used comprises Naftolen ZD from Chemetall. Vulkanox 4020 is a 6PPD from Bayer AG, and Protektor G 3108 is an ozone-protection wax from HB-Fuller GmbH. Vulkacit D/C (DPG) and Vulkacit CZ/EG (CBS) are commercially available products from Bayer AG. Perkazit TBZTD is obtainable from Akzo Chemie GmbH. The rubber mixtures are prepared in an internal mixture, using the mixing specification in Table 2.6. In addition to the methods indicated in Table 2.3 for rubber testing, the methods given in Table 2.7 were used. The mixtures were vulcanized at 165 C. for 15 minutes. Table 2.8 shows the results of testing on the vulcanized rubber.

(35) TABLE-US-00014 TABLE 2.6 Stage 1 Settings Mixing assembly Werner & Pfleiderer 1.5N Rotation rate 70 rpm Friction 1:1.11 Ram pressure 5.5 bar Capacity 1.6 1 Fill level 0.73 Chamber temp. 70 C. Mixing procedure 0-1 min Buna VSL 5025-1 + Buna CB 24 1-3 min 1/2 of filler, X50-S 3-4 min 1/2 of filler, remaining stage 1 constituents 4 min Purging 4-5 min Mixing and discharge Aging 24 h at room temperature Stage 2 Settings Mixing assembly as in stage 1 except: Rotation rate 80 rpm Chamber temp. 80 C. Fill level 0.70 Mixing procedure 0-2 min Plasticize stage 1 batch 2-5 min Maintain batch temperature at 150 C. by varying rotation rate 5 min Discharge Aging 4 h at room temperature Stage 3 Settings Mixing assembly as in stage 1 except Rotation rate 40 rpm Chamber temp. 50 C. Fill level 0.69 Mixing procedure 0-2 min Stage 2 batch, stage 3 constituents 2 min Discharge and form milled sheet on laboratory mixing rolls, (diameter 200 mm, length 450 mm, roll temperature 50 C.) Homogenize: Cut the material 3 times toward the left, 3 times toward the right Fold the material over 5 times with narrow roll gap (1 mm) and 5 times with wide roll gap (3.5 mm) and peel milled sheet away

(36) TABLE-US-00015 TABLE 2.7 Physical testing Standard/conditions Vulcameter testing, 165 C. DIN 53529/3, ISO 6502 Torque difference Dmax Dmin [dNm] t10% and t90% [min] Viscoelastic properties, DIN 53 513, ISO 2856 0 and 60 C., 16 Hz, initial force 50 N and amplitude force 25 N Test value recorded after 2 min of test time, i.e. 2 min of conditioning Complex modulus E* [MPa] Loss factor tan []

(37) TABLE-US-00016 TABLE 2.8 Unprocessed mixture data Reference C Dmax Dmin 18.6 18.5 t10% 1.5 1.5 t90% 6.3 6.1 Vulcanizate data Modulus value 100% 2.8 2.8 Modulus value 300% 13.4 14.7 Modulus value 300%/100% 4.8 5.3 Elongation at break 370 330 Shore A hardness 66 66 Ball rebound 0 C. 15.3 15.2 Ball rebound 60 C. 61.4 61.6 E* (0 C.) 23.4 31.8 E* (60 C.) 8.8 9.0 tan (0 C.) 0.360 0.441 tan (60 C.) 0.129 0.110 Dispersion coefficient 95 99

(38) As is seen from the data in Table 2.8, the advantages also found in example 2.1 are confirmed in the vulcanization kinetics and a higher level of reinforcement for the mixture C, using the silica of the invention. In addition, advantages are found in the hysteresis behavior of the mixture C. There is an increase in the loss factor tan (0 C.), indicating improved wet skid performance, and there is a decrease in tan (60 C.), indicating reduced rolling resistance. The dispersion quality of the silicas of the invention is moreover exceptionally high, with resultant advantages in road abrasion.