Method for preparing precipitated silicas, novel precipitated silicas and their uses, in particular for strengthening polymers

Abstract

The present disclosure relates to a novel process for the preparation of a precipitated silica, in which: a silicate is reacted with at least one acidifying agent, so as to obtain a suspension of precipitated silica, said suspension of precipitated silica is filtered, so as to obtain a filtration cake, said filtration cake is subjected to a liquefaction operation comprising the addition of an aluminum compound, after the liquefaction operation, a drying stage is carried out, characterized in that methylglutaric acid is added to the filtration cake, during or after the liquefaction operation. It also relates to novel precipitated silicas and to their uses.

Claims

1. A precipitated silica having: a BET specific surface area of between 45 and 550 m.sup.2/g, a content (C) of polycarboxylic acid and corresponding carboxylate, expressed as total carbon, of at least 0.15% by weight, and an aluminum (Al) content of at least 0.20% by weight, wherein the polycarboxylic acid comprises methylglutaric acid.

2. The precipitated silica as claimed in claim 1, wherein the BET specific surface area is between 100 and 240 m.sup.2/g.

3. The precipitated silica as claimed in claim 1, wherein the content (C) of polycarboxylic acid and corresponding carboxylate, expressed as total carbon, is at least 0.25% by weight.

4. The precipitated silica as claimed in claim 1, wherein the aluminum (Al) content is at least 0.30% by weight.

5. The precipitated silica as claimed in claim 1, wherein the precipitated silica exhibits a dispersive component of the surface energy ?.sub.s.sup.d of less than 43 mJ/m.sup.2.

6. The precipitated silica as claimed in claim 1, wherein the precipitated silica exhibits a dispersive component of the surface energy ?.sub.s.sup.d of at least 40 mJ/m.sup.2 and of less than 43 mJ/m.sup.2.

7. The precipitated silica as claimed in claim 1, wherein the precipitated silica exhibits a dispersive component of the surface energy ?.sub.s.sup.d of less than 40 mJ/m.sup.2.

8. The precipitated silica as claimed in claim 1, wherein the precipitated silica exhibits a water uptake of greater than 6%.

9. A process for the preparation of the precipitated silica of claim 1, the process comprising: reacting at least one silicate with at least one acidifying agent, so as to obtain a suspension of precipitated silica, filtering said suspension of precipitated silica, so as to obtain a filtration cake, subjecting said filtration cake to a liquefaction operation comprising the addition of an aluminum compound adding methylglutaric acid to the filtration cake, during or after the liquefaction operation, and optionally drying the filtration cake after the liquefaction operation.

10. The process as claimed in claim 9, wherein the aluminum compound is added to the filtration cake during the liquefaction operation prior to the addition of the methylglutaric acid.

11. The process as claimed in claim 9, wherein the methylglutaric acid and the aluminum compound are simultaneously added to the filtration cake during the liquefaction operation.

12. The process as claimed in claim 9, wherein the methylglutaric acid is added to the filtration cake after the liquefaction operation.

13. The process as claimed in claim 9, wherein a portion or all of the methylglutaric acid is in the anhydride, ester, alkali metal salt (carboxylate) or ammonium salt (carboxylate) form.

14. The process as claimed in claim 9, wherein the aluminum compound is an alkali metal aluminate.

15. The process as claimed in claim 9, wherein reacting at least one silicate with at least one acidifying agent comprises: (i) forming an initial vessel heel comprising at least a portion of the total amount of the silicate involved in the reaction and an electrolyte, the concentration of silicate in said initial vessel heel being less than 100 g/l, (ii) adding the acidifying agent to said vessel heel until a value for the pH of the reaction medium of at least 7.0 is obtained, and (iii) adding additional acidifying agent and, if appropriate, simultaneously adding the remaining amount of silicate to the reaction medium; and adding methylglutaric acid to the filtration cake, either during the liquefaction operation, or after the liquefaction operation and before the drying stage.

16. A method for reducing the viscosity of a polymer composition, the method comprising adding a precipitated silica as claimed in claim 1 to said composition.

17. A polymer composition comprising a precipitated silica as claimed in claim 1.

18. An article comprising at least one polymer composition as claimed in claim 17, wherein the article is selected from the group consisting of a footwear sole, a floor covering, a gas barrier, a flame-retardant material, a roller for cableways, a seal for domestic electrical appliances, a seal for liquid or gas pipes, a braking system seal, a pipe, a sheathing, a cable, an engine support, a battery separator, a conveyor belt, a transmission belt or a tire.

Description

EXAMPLES

Example 1

(1) The suspension of precipitated silica used is a silica cake (having a solids content of 23% by weight), obtained on conclusion of the filtration stage during the process for the preparation of the Z1165MP silica.

(2) Prior to the liquefaction operation, a 50% by weight solution of methylglutaric acid is prepared by dissolving methylglutaric acid in water (at 35? C.) with stirring.

(3) The cake obtained in the filtration stage (303 kilograms) is subjected to a liquefaction operation in a continuous vigorously stirred reactor (for approximately 3 hours) with 1740 grams of the sodium aluminate solution (Al/SiO.sub.2 ratio by weight of 0.32%).

(4) Once the liquefaction has been carried out, 730 grams of the previously prepared methylglutaric acid solution (methylglutaric acid/SiO.sub.2 ratio by weight of 1.20%) are added to a portion (125 liters) of the disintegrated cake.

(5) This treated portion (having a solids content of 22% by weight) of the disintegrated cake is subsequently dried using a nozzle atomizer by spraying the disintegrated cake through a 1.5 mm nozzle with a pressure of 25 bar for 35 minutes under the following mean conditions of flow rate and of temperatures:

(6) Mean inlet temperature: 569? C.

(7) Mean outlet temperature: 159? C.

(8) Mean flow rate: 211 l/h.

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

(10) TABLE-US-00002 BET (m.sup.2/g) 157 Content of polycarboxylic acid + carboxylate (C) (%) 0.5 Aluminum (Al) content (%) 0.34 Ratio (R) 1.2 CTAB (m.sup.2/g) 151 ?.sub.s.sup.d (mJ/m.sup.2) 34 Water uptake (%) 8.7 ?.sub.50 (?m) after deagglomeration with ultrasound 3.1 Fd after deagglomeration with ultrasound 13.5 V2/V1 (%) 57 pH 5.2

Example 2 (Comparative)

(11) The suspension of precipitated silica used is a silica cake (having a solid content of 23% by weight) obtained on conclusion of the filtration stage during the process for the preparation of the Z1165MP silica.

(12) Prior to the liquefaction operation, a 100 g/l maleic acid solution is prepared by dissolving maleic acid in water (at 35? C.) with stirring.

(13) The cake obtained in the filtration stage is subjected to a liquefaction operation in a continuous vigorously stirred reactor (for approximately 90 minutes) with addition to the cake of 4400 grams of the 100 g/l maleic acid solution (maleic acid/SiO.sub.2 ratio by weight of 1.0%).

(14) This disintegrated cake (having a solids content of 22% by weight) is subsequently dried using a nozzle atomizer by spraying the disintegrated cake through a 1.5 mm nozzle with a pressure of 25 bar under the following mean conditions of flow rate and of temperatures:

(15) Mean inlet temperature: 577? C.

(16) Mean outlet temperature: 157? C.

(17) Mean flow rate: 220 l/h.

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

(19) TABLE-US-00003 BET (m.sup.2/g) 169 Content of polycarboxylic acid + carboxylate (C) (%) 0.19 Aluminum (Al) content (%) <0.05 Ratio (R) >4.3 CTAB (m.sup.2/g) 178 ?.sub.s.sup.d (mJ/m.sup.2) 51 ?.sub.50 (?m) after deagglomeration with ultrasound 3.6 Fd after deagglomeration with ultrasound 19.3 V2/V1 (%) 58 pH 3.8

Example 3

(20) The elastomeric 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):

(21) TABLE-US-00004 TABLE I Composition Control 1 Composition 1 SBR (1) 103 103 BR (1) 25 25 Silica C1 (2) 80 Silica S1 (3) 80 Coupling agent (4) 6.4 6.4 Carbon black (N330) 3.0 3.0 ZnO 2.5 2.5 Stearic acid 2 2 Antioxidant (5) 1.9 1.9 DPG (6) 1.5 1.5 CBS (7) 2 2 Sulfur 1.1 1.1 (1) Solution SBR (Buna VSL5025-2 from Lanxess) with 50 +/? 4% of vinyl units; 25 +/? 2% of styrene units; Tg in the vicinity of ?20? C.; 100 phr of SBR extended with 37.5 +/? 2.8% by weight of oil/BR (Buna CB 25 from Lanxess) (2) Silica C1 (liquefaction with addition of maleic acid (example 2- comparative)) (3) Silica S1 according to the present invention (liquefaction with addition of sodium aluminate, then addition of methylglutaric acid after liquefaction (example 1 above)) (4) TESPT (Luvomaxx TESPT from Lehvoss France sarl) (5) N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex 6-PPD from Flexsys) (6) Diphenylguanidine (Rhenogran DPG-80 from RheinChemie) (7) N-Cyclohexyl-2-benzothiazolesulfenamide (Rhenogran CBS-80 from RheinChemie)

(22) Process for the Preparation of the Elastomeric Compositions:

(23) The process for the preparation of the rubber compositions is carried out 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 of less than 110? C. This phase makes possible the introduction of the vulcanization system.

(24) The first phase is carried out using a mixing device, of internal mixer type, of Thermofischer Haake brand (capacity of 380 ml). The filling coefficient is 0.6. The initial temperature and the speed of the rotors are set on each occasion so as to achieve mixture dropping temperatures of approximately 140-160? C.

(25) 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 (introduction in installments) with the coupling agent and the stearic acid. For this pass, the duration is between 4 and 10 minutes.

(26) 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.

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

(28) Each final mixture is subsequently calandered in the form of plaques with a thickness of 2-3 mm.

(29) With regard to these raw mixtures obtained, an evaluation of their rheological properties makes it possible to optimize the vulcanization time and the vulcanization temperature.

(30) Subsequently, the mechanical and dynamic properties of the mixtures vulcanized at the curing optimum (T98) are measured.

(31) Rheological Properties

(32) Viscosity of the Raw Mixtures:

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

(34) 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 carried out after preparing the raw mixtures and then after aging for 3 weeks at a temperature of 23+/?3? C.

(35) TABLE-US-00005 TABLE II Compositions Control 1 Composition 1 ML (1 + 4) - 100? C. Initial 96 87 Mooney relaxation Initial 0.303 0.322 ML (1 + 4) - 100? C. After 3 weeks 106 94 (23 +/? 3? C.) Mooney relaxation After 3 weeks 0.260 0.290 (23 +/? 3? C.)

(36) It is found that the silica S1 of the present invention (Composition 1) makes possible a sizeable reduction in the initial raw viscosity, with respect to the value of the control mixture.

(37) It is also found that the silica S1 of the present invention (Composition 1) makes it possible to retain the advantage in reduced raw viscosity, with respect to the value of the control mixture, after 3 weeks of storage.

(38) This type of behavior over time is of great use to a person skilled in the art in the case of the processing of silica-comprising rubber mixtures.

(39) Rheometry of the Compositions:

(40) The measurements are carried out on the compositions in the raw state. The results relating to the rheology test, which is carried out at 160? C. using a Monsanto ODR rheometer according to the standard NF ISO 3417, have been given in table III.

(41) According to this test, the test composition is placed in the test chamber regulated at the temperature of 160? 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.

(42) 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 (?T=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 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 (160? C.) and which reflects the time during which it is possible to process the raw mixtures at this temperature without having initiation of vulcanization (the mixture cures from TS2).

(43) The results obtained are shown in table III.

(44) TABLE-US-00006 TABLE III Compositions Control 1 Composition 1 Tmin (dN .Math. m) 21.9 19.7 Tmax (dN .Math. m) 64.9 69.7 Delta torque (dN .Math. m) 43.0 50.0 TS2 (min) 4.7 6.1 T98 (min) 26.9 25.1

(45) It is found that the composition resulting from the invention (Composition 1) exhibits a satisfactory combination of rheological properties.

(46) In particular, while having a reduced raw viscosity, it exhibits a lower minimum torque value and a higher maximum torque value than those of the control mixture, which reflects a greater processability of the prepared mixture.

(47) The use of the silica S1 of the present invention (Composition 1) makes it possible to reduce the minimum viscosity (sign of an improvement in the raw viscosity) with respect to the control mixture without damaging the vulcanization behavior.

(48) Mechanical Properties of the Vulcanisates:

(49) The measurements are carried out on the optimally vulcanized compositions (T98) for a temperature of 160? C.

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

(51) The Shore A hardness measurement on the vulcanisates is carried out according to the instructions of the standard ASTM D 2240. The given value is measured at 15 seconds.

(52) The properties measured are collated in table IV.

(53) TABLE-US-00007 TABLE IV Compositions Control 1 Composition 1 10% Modulus (MPa) 0.7 0.6 100% Modulus (MPa) 2.4 2.5 300% Modulus (MPa) 12.5 15.4 RI 5.3 6.1 Shore A hardness - 15 s (pts) 63 59

(54) It is found that the composition resulting from the invention (Composition 1) exhibits a satisfactory compromise in mechanical properties, with respect to what is obtained with the control mixture.

(55) Composition 1 thus exhibits relatively low 10% and 100% moduli and a relatively high 300% modulus, hence a good reinforcing index.

(56) The use of a silica S1 of the present invention (Composition 1) makes it possible to obtain a satisfactory level of reinforcement, with respect to the control mixture, by significantly improving the 300% modulus.

(57) Dynamic Properties of the Vulcanisates:

(58) The dynamic properties are measured on a viscosity analyser (Metravib VA3000) according to the standard ASTM D5992.

(59) The values for loss factor (tan ?) 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 plus or minus 2%. The measurements are carried out at 60? C. and at a frequency of 10 Hz.

(60) The results, presented in table V, are the loss factor (tan ?, 60? C., 10 Hz).

(61) The values for the loss factor (tan ?) and for amplitude of dynamic shear elastic modulus (?G) 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 40? C. and at a frequency of 10 Hz. The strain amplitude sweeping processes are carried out according to an outward-return cycle, proceeding outward from 0.1% to 50% and then returning from 50% to 0.1%.

(62) The results, presented in table V, result from the return strain amplitude sweep and relate to the maximum value of the loss factor (tan ? max return, 40? C., 10 Hz) and to the amplitude of the elastic modulus (?G, 40? C., 10 Hz) between the values at 0.1% and 50% strain (Payne effect).

(63) TABLE-US-00008 TABLE V References Control 1 Composition 1 Tan ?, 60? C., 10 Hz 0.143 0.124 ?G, 40? C., 10 Hz (MPa) 1.9 1.0 Tan ? max return, 40? C., 10 Hz 0.228 0.183

(64) The use of a silica S1 of the present invention (Composition 1) makes it possible to improve the maximum value of the loss factor in dynamic compression, the amplitude of the elastic modulus or Payne effect and the Tan ? max return loss factor, with respect to the control mixture.

(65) The examination of the various tables II to V shows that the composition in accordance with the invention (Composition 1) makes it possible to obtain a good processing/reinforcement/hysteresis properties compromise, with respect to the control mixture, and in particular a sizeable gain in raw viscosity, which remains stable on storage over time.