Method for preparing precipitated silicas, novel precipitated silicas and uses thereof, particularly for the reinforcement of polymers

Abstract

The invention relates to a process for preparing a precipitated silica, to precipitated silicas, and to uses thereof. The process of the invention generally includes reacting a silicate with an acidifying agent, so as to obtain a suspension of precipitated silica; filtering the precipitated silica to obtain a filter cake; subjecting the filter cake to a liquefaction operation to form a second filter cake; and drying the second filter cake. In the process of the invention, at least one polycarboxylic acid is added to the filter cake, during or after the liquefaction operation.

Claims

1. A process for preparing a precipitated silica, the process comprising: precipitating a silicate and an acidifying agent by: (i) forming an initial feedstock comprising a silicate and an electrolyte, the concentration of silicate expressed as SiO.sub.2 in said initial feedstock being less than 100 g/l and the concentration of electrolyte in said initial feedstock being less than 17 g/l, (ii) adding the acidifying agent to said feedstock to form a reaction medium, wherein the acidifying agent is added until a value for the pH of the reaction medium of at least 7 is obtained, (iii) simultaneously adding acidifying agent and a silicate to the reaction medium to form a silica suspension, filtering the silica suspension to form a filter cake, subjecting the filter cake to a liquefaction operation to form a second filter cake, and drying the second filter cake, said process comprising one of the three following operations (a), (b) or (c): (a) adding at least one aluminum compound A and, subsequently or simultaneously, a basic agent to the reaction medium, after step (iii), (b) simultaneously adding a silicate and at least one aluminum compound A to the reaction medium, after step (iii) or in place of step (iii), (c) performing step (iii) by simultaneously adding to the reaction medium, acidifying agent, a silicate and at least one aluminum compound B, and wherein a mixture of polycarboxylic acids is added to the filter cake, either during the liquefaction operation, or after the liquefaction operation and before the drying step, the mixture of polycarboxylic acids comprising adipic acid, ethylsuccinic acid, and methylglutaric acid.

2. The process as claimed in claim 1, wherein the process comprises: precipitating a silicate and an acidifying agent by: (i) forming an initial feedstock comprising a silicate and an electrolyte, the concentration of silicate expressed as SiO.sub.2 in said initial feedstock being less than 100 g/l and the concentration of electrolyte in said initial feedstock being less than 17 g/l, (ii) adding the acidifying agent to said feedstock to form a reaction medium, wherein the acidifying agent is added until a value for the pH of the reaction medium of at least 7 is obtained, (iii) simultaneously adding acidifying agent and a silicate to the reaction medium, (iv) simultaneously adding at least one aluminum compound A and a basic agent to the reaction medium, and (v) adding acidifying agent to the reaction medium, to form a silica suspension, filtering the silica suspension to form a filter cake, subjecting the filter cake to a liquefaction operation to form a second filter cake, and drying the second filter cake, wherein the mixture of polycarboxylic acids is added to the filter cake, either during the liquefaction operation, or after the liquefaction operation and before the drying step.

3. The process as claimed in claim 1, wherein the process comprises: precipitating a silicate and an acidifying agent by: (i) forming an initial feedstock comprising a silicate and an electrolyte, the concentration of silicate expressed as SiO.sub.2 in said initial feedstock being less than 100 g/l and the concentration of electrolyte in said initial feedstock being less than 17 g/l, (ii) adding the acidifying agent to said feedstock to form a reaction medium, wherein the acidifying agent is added until a value for the pH of the reaction medium of at least 7 is obtained, (iii) simultaneously adding a silicate and at least one aluminum compound A to the reaction medium to form a silica suspension, filtering the silica suspension to form a filter cake, subjecting the filter cake to a liquefaction operation to form a second filter cake, and drying the second filter cake, wherein the mixture of polycarboxylic acids is added to the filter cake, either during the liquefaction operation, or after the liquefaction operation and before the drying step.

4. The process as claimed in claim 1, wherein the process comprises: precipitating a silicate and an acidifying agent by: (i) forming an initial feedstock comprising a silicate and an electrolyte, the concentration of silicate expressed as SiO.sub.2 in said initial feedstock being less than 100 g/l and the concentration of electrolyte in said initial feedstock being less than 17 g/l, (ii) adding the acidifying agent to said feedstock to form a reaction medium, wherein the acidifying agent is added until a value for the pH of the reaction medium of at least 7 is obtained, (iii) simultaneously adding acidifying agent, a silicate and at least one aluminum compound B to the reaction medium to form a silica suspension, filtering the silica suspension to form a filter cake, subjecting the filter cake to a liquefaction operation to form a second filter cake, and drying the second filter cake, wherein the mixture of polycarboxylic acids is added to the filter cake, either during the liquefaction operation, or after the liquefaction operation and before the drying step.

5. The process as claimed in claim 1, wherein, during the liquefaction operation, the mixture of polycarboxylic acids is added to the filter cake.

6. The process as claimed in claim 1, wherein the mixture of polycarboxylic acids is added to the filter cake after the liquefaction operation.

7. The process as claimed in claim 1, wherein the liquefaction operation comprises the addition of at least one aluminum compound C.

8. The process as claimed in claim 7, wherein, during the liquefaction operation, the mixture of polycarboxylic acids and at least one aluminum compound C are simultaneously added to the filter cake.

9. The process as claimed in claim 7, wherein, during the liquefaction operation, at least one aluminum compound C is added to the filter cake prior to the addition of the mixture of polycarboxylic acids.

10. The process as claimed in claim 7, wherein the mixture of polycarboxylic acids is added to the filter cake after the liquefaction operation.

Description

EXAMPLES

Example 1

(1) The following are introduced into a stainless-steel reactor equipped with an impeller stirring system and a heating jacket: 65.7 liters of water, 1.27 kg of Na.sub.2SO.sub.4 (electrolyte), 40.2 kg of silicate (SiO.sub.2/Na.sub.2O weight ratio equal to 3.42) with a density at 20° C. equal to 1.230±0.006.

(2) The solution is brought to 82° C. Sulfuric acid (mass concentration equal to 7.7%) is then introduced with stirring into the mixture until the pH of the reaction medium reaches a value of 8.0 at a flow rate of 835 g/min for 20 minutes, and then at a flow rate of 1502 g/min for 15 minutes. At the same time, the temperature of the mixture is increased to 92° C.

(3) Once the acidification is complete, sodium silicate of the type described above is introduced into the reaction medium, over 20 minutes, at a flow rate of 485 g/min, simultaneously with sodium aluminate (content of Al.sub.2O.sub.3=20.5±0.5%; content of Na.sub.2O.sub.3=19.4±0.4%) at a flow rate of 54.8 g/min and sulfuric acid (mass concentration equal to 7.7%) at a flow rate regulated so as to bring the pH of the reaction medium to and then maintain it at a value of 8.0.

(4) After 20 minutes of simultaneous addition, the introduction of the sodium silicate and of the sodium aluminate is stopped and the addition of sulfuric acid is continued, increasing the flow rate to 692 g/min so as to bring the pH of the reaction medium to a value equal to 6.0.

(5) The total duration of the reaction is 61 minutes.

(6) A reaction slurry of precipitated silica is thus obtained after the reaction, which is filtered and washed using a filter press so as to recover a silica cake with a solids content of 23% by weight.

Example 2

(7) Part of the filter cake obtained in Example 1 (6070 g) is then subjected to a liquefaction operation.

(8) During the liquefaction operation, a solution of an MGA mixture at 34% by mass (mixture of polycarboxylic acids: 94.8% by weight of methylglutaric acid, 4.9% by weight of ethylsuccinic anhydride, 0.2% by weight of adipic acid, 0.1% others) is used.

(9) The cake obtained in the filtration step is thus subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 35.6 grams of a sodium aluminate solution (Al/SiO.sub.2 weight ratio of 0.3%) and of 46.9 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1.0%).

(10) This disintegrated cake (with a solids content of 22% by weight) is subsequently dried using a two-fluid nozzle atomizer by spraying the disintegrated cake through a 2.54 mm SU5 nozzle (Spraying System) with a pressure of 1 bar under the following mean conditions of flow rate and of temperatures:

(11) Mean inlet temperature: 250° C.

(12) Mean outlet temperature: 135° C.

(13) Mean flow rate: 15 l/h.

(14) The characteristics of silica S1 obtained (in the form of substantially spherical beads) are then the following:

(15) TABLE-US-00002 BET (m.sup.2/g) 151 Content of polycarboxylic acid + carboxylate (C) (%) 0.37 Aluminum (Al) content (%) 1.4 Ratio (R) 0.19 CTAB (m.sup.2/g) 161 γ.sub.s.sup.d (mJ/m.sup.2) 63.4 V2/V1 (%) 42.0 Water uptake (%) 10.2 Ø.sub.50M (μm) after ultrasound deagglomeration 2.0 F.sub.DM after ultrasound deagglomeration 18.6 pH 7.54

Example 3

(16) Part of the filter cake obtained in Example 1 (6030 g) is then subjected to a liquefaction operation.

(17) During the liquefaction operation, a solution of an MGA mixture at 34% by mass (mixture of polycarboxylic acids: 94.8% by weight of methylglutaric acid, 4.9% by weight of ethylsuccinic anhydride, 0.2% by weight of adipic acid, 0.1% others) is used.

(18) The cake obtained in the filtration step is subjected to a liquefaction operation in a continuous vigorously stirred reactor with addition to the cake of 6.4 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1%).

(19) This disintegrated cake (with a solids content of 22% by weight) is subsequently dried using a two-fluid nozzle atomizer by spraying the disintegrated cake through a 2.54 mm SU5 nozzle (Spraying System) with a pressure of 1 bar under the following mean conditions of flow rate and of temperatures:

(20) Mean inlet temperature: 300° C.

(21) Mean outlet temperature: 130° C.

(22) Mean flow rate: 15 l/h.

(23) The characteristics of silica S2 obtained (in the form of substantially spherical beads) are then the following:

(24) TABLE-US-00003 BET (m.sup.2/g) 148 Content of polycarboxylic acid + carboxylate (C) (%) 0.41 Aluminum (Al) content (%) 1.0 Ratio (R) 0.3 CTAB (m.sup.2/g) 152 γ.sub.s.sup.d (mJ/m.sup.2) 44.6 V2/V1 (%) 50.8 Water uptake (%) 10.0 Ø.sub.50M (μm) after ultrasound deagglomeration 4.1 F.sub.DM after ultrasound deagglomeration 15.8 pH 5.88

Example 4

Comparative

(25) Part of the filter cake obtained in Example 1 (6020 g) is then subjected to a liquefaction operation.

(26) The cake obtained in the filtration step is subjected to a liquefaction operation in a continuous vigorously stirred reactor with addition to the cake of 77.7 grams of a sulfuric acid solution at 7.7% by mass.

(27) This disintegrated cake (with a solids content of 22% by weight) is subsequently dried using a two-fluid nozzle atomizer by spraying the disintegrated cake through a 2.54 mm SU5 nozzle (Spraying System) with a pressure of 1 bar under the following mean conditions of flow rate and of temperatures:

(28) Mean inlet temperature: 250° C.

(29) Mean outlet temperature: 135° C.

(30) Mean flow rate: 15 l/h.

(31) The characteristics of silica C1 obtained (in the form of substantially spherical beads) are then the following:

(32) TABLE-US-00004 BET (m.sup.2/g) 177 Content of polycarboxylic acid + carboxylate (C) (%) — Aluminum (Al) content (%) 1.1 Ratio (R) 0.0 CTAB (m.sup.2/g) 157 γ.sub.s.sup.d (mJ/m.sup.2) 97.5 V2/V1 (%) 46.5 Water uptake (%) 10.4 Ø.sub.50M (μm) after ultrasound deagglomeration 1.9 F.sub.DM after ultrasound deagglomeration 18.3 pH 6.91

Example 5

(33) The elastomer compositions, the make up of which, expressed as parts by weight per 100 parts of elastomers (phr), is shown in Table I below, are prepared in an internal mixer of Haake type (380 ml):

(34) TABLE-US-00005 TABLE I Composition Control 1 Composition 1 Composition 2 NR (1) 100 100 103 Silica C1 (2) 55 Silica S1 (3) 55 Silica S2 (4) 55 Coupling agent (5) 4.4 4.4 4.4 ZnO 3 3 3 Stearic acid 2.5 2.5 2.5 Antioxidant 1 (6) 1.5 1.5 1.5 Antioxidant 2 (7) 1 1 1 Carbon black (N330) 3 3 3 CBS (8) 1.9 1.9 1.9 TBzTD (9) 0.2 0.2 0.2 Sulfur 1.5 1.5 1.5 (1) Natural rubber SMR - CV60 (supplied by the company Safic-Alcan) (2) Silica C1 (liquefaction with addition of sulfuric acid (Example 4 - comparative)) (3) Silica S1 according to the present invention (liquefaction with simultaneous addition of sodium aluminate and of a mixture of MGA acids (Example 2 above)) (4) Silica S2 according to the present invention (liquefaction with addition of a mixture of MGA acids (Example 3 above)) (5) TESPT (Luvomaxx TESPT from the company Lehvoss France sarl) (6) N-(1,3-dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex 6-PPD from the company Flexsys) (7) 2,2,4-trimethyl-1H-quinoline (Permanax TQ from the company Flexsys) (8) N-cyclohexyl-2-benzothiazylsulfenamide (Rhenogran CBS-80 from the company RheinChemie) (9) Tetrabenzylthiuram disulfide (Rhenogran TBzTD-70 from the company RheinChemie)

(35) Process for Preparing the Elastomeric Compositions:

(36) The process for preparing the rubber compositions is performed in two successive preparation phases. A first phase consists of a phase of high-temperature thermomechanical working. It is followed by a second phase of mechanical working at temperatures below 110° C. This phase allows the introduction of the vulcanization system.

(37) The first phase is performed using a Haake brand mixing device, of internal mixer type (capacity of 380 ml). The filling coefficient is 0.6. The initial temperature and speed of the rotors are set on each occasion so as to achieve mixture dropping temperatures of approximately 130-160° C.

(38) Broken down here into two passes, the first phase makes it possible to incorporate, in a first pass, the elastomers and then the reinforcing filler (portionwise introduction) with the coupling agent and the stearic acid. For this pass, the duration is between 4 and 10 minutes.

(39) After cooling the mixture (temperature of less than 100° C.), a second pass makes it possible to incorporate the zinc oxide and the protecting agents/antioxidants (in particular 6-PPD). The duration of this pass is between 2 and 5 minutes.

(40) After cooling the mixture (temperature of less than 100° C.), the second phase allows the introduction of the vulcanization system (sulfur and accelerators, such as CBS). It is performed on an open mill, preheated to 50° C. The duration of this phase is between 2 and 6 minutes.

(41) Each final mixture is subsequently calendered in the form of plates with a thickness of 2-3 mm.

(42) An evaluation of the rheological properties of these “crude” mixtures obtained makes it possible to optimize the vulcanization time and the vulcanization temperature.

(43) The mechanical and dynamic properties of the mixtures vulcanized at the curing optimum (T98) are then measured.

(44) Rheological Properties Viscosity of the crude mixtures:

(45) The Mooney consistency is measured on the compositions in the crude state at 100° C. using an MV 2000 rheometer and also the determination of the Mooney stress-relaxation rate according to standard NF ISO 289.

(46) The value of the torque, read at the end of 4 minutes after preheating for one minute (Mooney Large (1+4)—at 100° C.), is shown in Table II. The test is performed after preparing the crude mixtures and then after aging for 3 weeks at a temperature of 23±3° C.

(47) TABLE-US-00006 TABLE II References Control 1 Composition 1 Composition 2 ML (1 + 4) - Initial 58 57 56 100° C. Mooney Initial 0.472 0.468 0.501 relaxation ML (1 + 4) - After 21 66 59 60 100° C. days (23 ± 3° C.) Mooney After 21 0.401 0.468 0.459 relaxation days (23 ± 3° C.)

(48) It is found that silica S1 and silica S2 of the present invention (Composition 1 and Composition 2) make it possible to conserve the initial crude viscosity values, relative to the control mixture (Control 1).

(49) It is also found that silica S1 and silica S2 of the present invention (Composition 1 and Composition 2) allow a reduction in the reduced crude viscosity, relative to the control mixture (Control 1), after 3 weeks of storage.

(50) This type of behavior over time is of great use to a person skilled in the art in the case of using rubber mixtures containing silica. Rheometry of the Compositions:

(51) The measurements are performed on the compositions in crude form. The results relating to the rheology test, which is performed at 150° C. using a Monsanto ODR rheometer according to the standard NF ISO 3417, are given in Table III.

(52) According to this test, the test composition is placed in the test chamber regulated at a temperature of 150° C. for 30 minutes, and the resistive torque opposed by the composition to a low-amplitude (3°) oscillation of a biconical rotor included in the test chamber is measured, the composition completely filling the chamber under consideration.

(53) The following are determined from the curve of variation in the torque as a function of time: the minimum torque (Tmin), which reflects the viscosity of the composition at the temperature under consideration; the maximum torque (Tmax); the delta torque (ΔT=Tmax−Tmin), which reflects the degree of crosslinking brought about by the action of the crosslinking system and, if the need arises, of the coupling agents; the time T98 necessary to obtain a vulcanization state corresponding to 98% of complete vulcanization (this time is taken as the vulcanization optimum); and the scorch time TS2, corresponding to the time necessary in order to have a rise of 2 points above the minimum torque at the temperature under consideration (150° C.) and which reflects the time during which it is possible to implement the raw mixtures at this temperature without having initiation of vulcanization (the mixture cures at and above TS2).

(54) The results obtained are shown in Table III.

(55) TABLE-US-00007 TABLE III Compositions Control 1 Composition 1 Composition 2 Tmin (dN .Math. m) 13.2 11.8 12.0 Tmax (dN .Math. m) 84.9 86.9 86.5 Delta torque (dN .Math. m) 71.7 75.1 74.5 TS2 (min) 6.9 7.8 7.7 T98 (min) 12.5 15.1 14.3

(56) It is found that the use of silica S1 and silica S2 of the present invention (Composition 1 and Composition 2) makes it possible to reduce the minimum viscosity (sign of an improvement in the crude viscosity) relative to the control mixture (Control 1) without impairing the vulcanization behavior.

(57) It is also found that the use of silica S1 and silica S2 of the present invention (Composition 1 and Composition 2) makes it possible to improve the scorch time TS2 relative to the control mixture (Control 1).

(58) Mechanical Properties of the Vulcanizate

(59) The measurements are performed on the optimally vulcanized compositions (T98) for a temperature of 150° C.

(60) Uniaxial tensile tests are performed in accordance with the instructions of standard NF ISO 37 with test specimens of H2 type at a rate of 500 mm/min on an Instron 5564 machine. The x % moduli, corresponding to the stress measured at x % of tensile strain, and the ultimate strength are expressed in MPa; the elongation at break is expressed in %. It is possible to determine a reinforcing index (RI) which is equal to the ratio of the modulus at 300% strain to the modulus at 100% strain.

(61) The Shore A hardness measurement of the vulcanizates is performed according to the instructions of standard ASTM D 2240. The given value is measured at 15 seconds.

(62) The properties measured are collated in Table IV.

(63) TABLE-US-00008 TABLE IV Compositions Control 1 Composition 1 Composition 2 10% Modulus (MPa) 0.8 0.9 0.8 100% Modulus (MPa) 3.5 4.0 3.7 300% Modulus (MPa) 14.8 15.9 15.5 Ultimate strength (MPa) 29.0 29.4 29.3 Elongation at break (%) 523 516 525 RI 4.2 4.0 4.2 Shore A hardness - 70 70 69 15 s (pts)

(64) It is found that the compositions resulting from the invention (Composition 1 and Composition 2) have a good compromise of mechanical properties, with respect to what is obtained with the control mixture.

(65) The use of silica S1 and of silica S2 of the present invention (Composition 1 and Composition 2) makes it possible to improve the 300% modulus values while maintaining a level of reinforcement equivalent to that of the control mixture (Control 1) without impairing the strength or elongation at break properties.

(66) Dynamic Properties of the Vulcanizates:

(67) The dynamic properties are measured on a viscosity analyzer (Metravib VA3000) according to standard ASTM D5992.

(68) The values for loss factor (tan δ) and compressive dynamic complex modulus (E*) are recorded on vulcanized samples (cylindrical test specimen with a cross section of 95 mm.sup.2 and a height of 14 mm). The sample is subjected at the start to a 10% prestrain and then to a sinusoidal strain in alternating compression of ±2%. The measurements are performed at 60° C. and at a frequency of 10 Hz.

(69) The values for the loss factor (tan δ max return) are recorded on vulcanized samples (parallelepipedal test specimen with a cross section of 8 mm.sup.2 and a height of 7 mm). The sample is subjected to a double alternating sinusoidal shear strain at a temperature of 60° C. and at a frequency of 10 Hz. The strain amplitude sweeping processes are performed according to an outward-return cycle, proceeding outward from 0.1% to 50% and then returning from 50% to 0.1%.

(70) The results, presented in Table V, are thus the compressive complex modulus (E*, 60° C., 10 Hz) and the loss factor (tan δ, 60° C., 10 Hz).

(71) TABLE-US-00009 TABLE V Compositions Control 1 Composition 1 Composition 2 E*, 60° C., 10 Hz (MPa) 9.1 8.9 8.9 Tan δ, 60° C., 10 Hz 0.080 0.080 0.082 Tan δ max return, 60° C., 0.133 0.121 0.119 10 Hz

(72) The use of a silica S1 and of a silica S2 of the present invention (Composition 1 and Composition 2) makes it possible to improve the maximum value of the loss factor, with respect to the control mixture (Control 1), without impairing the other dynamic properties.

(73) Examination of the various Tables II to V shows that the compositions in accordance with the invention (Composition 1 and Composition 2) make it possible to obtain a good processing/reinforcement/hysteresis properties compromise, with respect to the control composition (Control 1).