NOVEL METHOD FOR PREPARING PRECIPITATED SILICAS, NOVEL PRECIPITATED SILICAS, AND USES THEREOF, PARTICULARLY FOR POLYMER REINFORCEMENT

Abstract

The invention relates to a novel process for preparing a precipitated silica, in which: a silicate is reacted with an acidifying agent, so as to obtain a suspension of precipitated silica, said suspension of precipitated silica is filtered, so as to obtain a filter cake, said filter cake is subjected to a liquefaction operation, after the liquefaction operation, a drying step is performed,
characterized in that at least one polycarboxylic acid is added to the filter cake, during or after the liquefaction operation.

The invention also relates to novel precipitated silicas and to uses thereof.

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 part of a total amount of alkali metal M silicate engaged in the reaction, the concentration of silicate, expressed as SiO.sub.2, in said initial feedstock being less than 20 g/l, (ii) adding acidifying agent to said initial feedstock, until at least 50% of the amount of M.sub.2O present in said initial feedstock is neutralized, (iii) simultaneously adding alkali metal M silicate and acidifying agent to the reaction medium, such that the ratio of the amount of silicate added expressed as SiO.sub.2/amount of silicate present in the initial feedstock expressed as SiO.sub.2 is greater than 4 and not more than 100, (iv) discontinuing the addition of the silicate while continuing the addition of the acidifying agent to the reaction medium, until a value of the pH of the reaction medium of between 2.5 and 5.3, is obtained, thus providing 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, drying the second filter cake, wherein at least one polycarboxylic acid is added to the filter cake, either during the liquefaction operation, or after the liquefaction operation and before the drying step.

2. The process as claimed in claim 1, wherein, during the liquefaction operation, at least one polycarboxylic acid is added to the filter cake.

3. The process as claimed in claim 1, wherein at least one polycarboxylic acid is added to the filter cake after the liquefaction operation.

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

5. The process as claimed in claim 4, wherein, during the liquefaction operation, at least one polycarboxylic acid and at least one aluminum compound are simultaneously added to the filter cake.

6. The process as claimed in claim 4, wherein, during the liquefaction operation, at least one aluminum compound is added to the filter cake prior to the addition of at least one polycarboxylic acid.

7. The process as claimed in claim 4, wherein at least one polycarboxylic acid is added to the filter cake after the liquefaction operation.

8. The process as claimed in claim 1, wherein said polycarboxylic acid is selected from linear or branched, saturated or unsaturated aliphatic polycarboxylic acids containing from 2 to 20 carbon atoms and aromatic polycarboxylic acids.

9. The process as claimed in claim 1, wherein a mixture of polycarboxylic acids is added to the filter cake.

10. The process as claimed in claim 9, wherein the mixture of polycarboxylic acids comprises: adipic acid, glutaric acid and succinic acid.

11. The process as claimed in claim 9, wherein the mixture of polycarboxylic acids comprises: methylglutaric acid, ethylsuccinic acid and adipic acid.

12. A precipitated silica, characterized in that it has: a BET specific surface area of between 100 and 240 m.sup.2/g, a CTAB specific surface area of between 100 and 240 m.sup.2/g, a content (C) of polycarboxylic acid+corresponding carboxylate, expressed as total carbon, of at least 0.15% by weight, an object size distribution width Ld ((d84d16)/d50), measured by XDC particle size analysis after ultrasound deagglomeration, of at least 0.70, and a pore distribution width ldp of less than 0.65, and optionally an aluminum (Al) content of at least 0.20% by weight, especially of at least 0.25% by weight.

13. (canceled)

14. A precipitated silica, characterized in that it has: a BET specific surface area of between 100 and 240 m.sup.2/g, a CTAB specific surface area of between 100 and 240 m.sup.2/g, a content (C) of polycarboxylic acid+corresponding carboxylate, expressed as total carbon, of at least 0.15% by weight, a pore distribution width ldp of less than 0.49, and optionally an aluminum (Al) content of at least 0.20% by weight.

15-17. (canceled)

18. The precipitated silica as claimed in claim 12, wherein the precipitated silica has a pore distribution such that the pore volume generated by the pores whose diameter is between 175 and 275 represents at least 55% of the pore volume generated by the pores with diameters of less than or equal to 400 .

19. The precipitated silica as claimed in claim 12, wherein the precipitated silica has a dispersive component of the surface energy .sub.s.sup.d of less than 52 mJ/m.sup.2.

20. (canceled)

21. A method for reinforcing a polymers, the method comprising adding a precipitated silica as claimed in claim 12 to the polymer as a reinforcing filler.

22. A method for reducing the viscosity of a polymer composition, the method comprising adding the precipitated silica as claimed in claim 15 to the polymer composition.

23. A polymer composition comprising a precipitated silica as claimed in claim 12.

24. An article comprising at least one composition as claimed in claim 23, wherein the article is selected from 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 and a tire.

25. The article as claimed in claim 24, wherein the article is a tire.

Description

EXAMPLES

Example 1

[0305] 15 226 liters of industrial water and 465 kg of sodium silicate (SiO.sub.2/NaO.sub.2 weight ratio equal to 3.4) with a density at 20 C. equal to 1.2300.006 are placed in a reactor.

[0306] The concentration of silicate, expressed as SiO.sub.2, in the initial feedstock is then 5.5 g/l.

[0307] The mixture is then brought to 70 C. with continued stirring. Sulfuric acid (mass concentration equal to 7.7%) is then introduced into the mixture, over 7 minutes, until the pH of the reaction medium reaches a value of 8.7. Once the acidification is complete, sodium silicate of the type described above is introduced into the reaction medium at a flow rate of 8.5 m.sup.3/h, simultaneously with sulfuric acid (mass concentration equal to 7.7%) at a flow rate regulated so as to maintain the pH of the reaction medium at a value of 8.7. After 50 minutes of simultaneous addition, the introduction of the sodium silicate is stopped and the addition of sulfuric acid is continued so as to bring the pH of the reaction medium to a value equal to 5.2.

[0308] 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 22% by weight.

Example 2

[0309] Part of the filter cake obtained in Example 1 (7500 g) is then subjected to a liquefaction operation.

[0310] 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).

[0311] The cake obtained in the filtration step is subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 44.8 grams of a sodium aluminate solution (Al/SiO.sub.2 weight ratio of 0.3%) and 51 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1.0%).

[0312] This disintegrated cake (with a solids content of 23% 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:

[0313] Mean inlet temperature: 250 C.

[0314] Mean outlet temperature: 135 C.

[0315] Mean flow rate: 16.5 l/h.

[0316] The silica obtained, which is in powder form, is then formed into granules using a compactor (Alexanderwerk WP 120*40). A precipitated silica in the form of granules is then obtained.

[0317] The characteristics of the silica S1 obtained (in the form of granules) are then as follows:

TABLE-US-00002 BET (m.sup.2/g) 148 Content of polycarboxylic acid + carboxylate (C) (%) 0.45 Aluminum (Al) content (%) 0.47 Ratio (R) 0.7 CTAB (m.sup.2/g) 156 .sub.s.sup.d (mJ/m.sup.2) 48.5 V2/V1 (%) 66.0 Pore distribution width ldp 0.43 Water uptake (%) 7.3 .sub.50M (m) after ultrasound deagglomeration 2.7 F.sub.DM after ultrasound deagglomeration 19.9 pH 5.65

Example 3

[0318] Part of the filter cake obtained in Example 1 (7500 g) is then subjected to a liquefaction operation.

[0319] 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.

[0320] 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 57.5 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1%).

[0321] This disintegrated cake (with a solids content of 23% 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:

[0322] Mean inlet temperature: 250 C.

[0323] Mean outlet temperature: 135 C.

[0324] Mean flow rate: 16.5 l/h.

[0325] The silica obtained, which is in powder form, is then formed into granules using a compactor (Alexanderwerk WP 120*40). A precipitated silica in the form of granules is then obtained.

[0326] The characteristics of the silica S2 obtained (in the form of granules) are then as follows:

TABLE-US-00003 BET (m.sup.2/g) 151 Content of polycarboxylic acid + carboxylate (C) (%) 0.36 CTAB (m.sup.2/g) 162 .sub.s.sup.d (mJ/m.sup.2) 40.3 V2/V1 (%) 66.0 Pore distribution width ldp 0.44 Water uptake (%) 7.4 .sub.50M (m) after ultrasound deagglomeration 4.4 F.sub.DM after ultrasound deagglomeration 19.1 pH 3.86

Example 4 (Comparative)

[0327] Part of the filter cake obtained in Example 1 (7500 g) is then subjected to a liquefaction operation.

[0328] The cake obtained in the filtration step is subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 43.8 grams of a sodium aluminate solution (Al/SiO.sub.2 weight ratio of 0.3%) and 48.5 grams of a sulfuric acid solution at 7.7% by mass.

[0329] This disintegrated cake (with a solids content of 23% 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:

[0330] Mean inlet temperature: 250 C.

[0331] Mean outlet temperature: 135 C.

[0332] Mean flow rate: 16.5 l/h.

[0333] The silica obtained, which is in powder form, is then formed into granules using a compactor (Alexanderwerk WP 120*40). A precipitated silica in the form of granules is then obtained.

[0334] The characteristics of the silica C1 obtained (in the form of granules) are then as follows:

TABLE-US-00004 BET (m.sup.2/g) 157 Content of polycarboxylic acid + carboxylate (C) (%) Aluminum (Al) content (%) 0.45 Ratio (R) 0.0 CTAB (m.sup.2/g) 152 .sub.s.sup.d (mJ/m.sup.2) 62.9 V2/V1 (%) 62.0 Pore distribution width ldp 0.47 Water uptake (%) 7.8 .sub.50M (m) after ultrasound deagglomeration 1.9 F.sub.DM after ultrasound deagglomeration 19.5 pH 6.92

Example 5

[0335] A second filter cake with a solids content of 23% by weight is prepared according to the procedure of Example 1.

Examples 6 and 7

[0336] A first part of the silica cake obtained in Example 5 (8000 g) is then subjected to a liquefaction step.

[0337] 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.

[0338] The cake obtained in the filtration step of Example 5 is thus subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 51.3 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1.0%) and 144 g of water.

[0339] This disintegrated cake (with a solids content of 23% 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:

[0340] Mean inlet temperature: 250 C.

[0341] Mean outlet temperature: 140 C.

[0342] Mean flow rate: 9.7 l/h.

[0343] The powder thus generated is separated by screening at 100 microns on an S079 screen (Chauvinscreening area of 0.3 m.sup.2).

[0344] The characteristics of the silica S3 obtained (in the form of substantially spherical beads) are indicated in the table below.

[0345] A second part of the silica cake obtained in Example 5 (8000 g) is then subjected to a liquefaction step using 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).

[0346] The cake obtained in the filtration step of Example 5 is thus subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 51.3 grams of the MGA solution (MGA mixture/SiO.sub.2 weight ratio of 1.0%), 39.7 g of a sodium aluminate solution (Al/SiO.sub.2 weight ratio of 0.3%) and 144 g of water.

[0347] This disintegrated cake (having a solids content of 23% by weight) is subsequently dried using a two-fluid nozzle atomizer as described above for the first part of the cake, under the following mean conditions of flow rate and of temperatures:

[0348] Mean inlet temperature: 250 C.

[0349] Mean outlet temperature: 140 C.

[0350] Mean flow rate: 10.8 l/h.

[0351] The powder thus generated is separated by screening at 100 microns on an S079 screen (Chauvinscreening area of 0.3 m.sup.2).

[0352] The characteristics of the silica S4 obtained (in the form of substantially spherical beads) are indicated in the table below.

TABLE-US-00005 Characteristics S3 S4 BET (m.sup.2/g) 164 160 Content of polycarboxylic acid + carboxylate (C) (%) 0.35 0.34 Aluminum (Al) content (%) 0.41 CTAB (m.sup.2/g) 162 157 .sub.s.sup.d (mJ/m.sup.2) 40.6 47.6 Width Ld (XDC) 1.15 0.98 V2/V1 (%) 59.0 62.0 Pore distribution width ldp 0.60 0.57 Width L'd (XDC) 1.06 0.92 Water uptake (%) 7.2 7.3 .sub.50M (m) after ultrasound deagglomeration 2.6 1.6 F.sub.DM after ultrasound deagglomeration 19.7 20.0 pH 4.5 5.6

Example 8 (Comparative)

[0353] Part of the silica cake (8000 g) obtained in Example 5 is then subjected to a liquefaction step.

[0354] The cake obtained in the filtration step is subjected to a liquefaction operation in a continuous vigorously stirred reactor with simultaneous addition to the cake of 39.8 grams of a sodium aluminate solution (Al/SiO.sub.2 weight ratio of 0.3%), 51.8 grams of a sulfuric acid solution at 7.7% by mass and 144 g of water.

[0355] This disintegrated cake (with a solids content of 23% 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:

[0356] Mean inlet temperature: 250 C.

[0357] Mean outlet temperature: 140 C.

[0358] Mean flow rate: 9.6 l/h.

[0359] The powder thus generated is separated by screening at 100 microns on an S079 screen (Chauvinscreening area of 0.3 m.sup.2).

[0360] The characteristics of the silica C2 obtained (in the form of substantially spherical beads) are then the following:

TABLE-US-00006 BET (m.sup.2/g) 169 Content of polycarboxylic acid + carboxylate (C) (%) Aluminum (Al) content (%) 0.40 CTAB (m.sup.2/g) 159 .sub.s.sup.d (mJ/m.sup.2) 66.4 Width Ld (XDC) 1.02 V2/V1 (%) 62.0 Pore distribution width ldp 0.60 Width L'd (XDC) 0.97 Water uptake (%) 7.6 .sub.50M (m) after ultrasound deagglomeration 1.4 F.sub.DM after ultrasound deagglomeration 20.1 pH 7.3

Example 9

[0361] 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 Brabender type (380 ml):

TABLE-US-00007 TABLE I Composition Control 1 Composition 1 Composition 2 SBR (1) 103 103 103 BR (1) 25 25 25 Silica C2 (2) 80 Silica S3 (3) 80 Silica S4 (4) 80 Coupling agent (5) 6.4 6.4 6.4 Plasticizer (6) 5 5 5 Carbon black (N330) 3 3 3 ZnO 2.5 2.5 2.5 Stearic acid 2 2 2 Antioxidant (7) 1.9 1.9 1.9 DPG (8) 1.5 1.5 1.5 CBS (9) 2 2 2 Sulfur 1.1 1.1 1.1 (1) Solution SBR (Buna V5L4526-2 from the company Lanxess) with 44.5 4% of vinyl units; 26 2% of styrene units; Tg in the region of 30 C.; 100 phr of SBR extended with 37.5 2.8% by weight of oil/BR (Buna CB 25 from the company Lanxess) (2) Silica C2 (liquefaction with simultaneous addition of sodium aluminate and sulfuric acid (Example 8-comparative)) (3) Silica S3 according to the present invention (liquefaction with addition of a mixture of MGA acids (Example 6 above)) (4) Silica S4 according to the present invention (liquefaction with simultaneous addition of sodium aluminate and of a mixture of MGA acids (Example 7 above)) (5) TESPT (Luvomaxx TESPT from the company Lehvoss France sarl) (6) Plasticizing oil of TDAE type (Vivatec 500 from the company Hansen & Rosenthal KG) (7) N-(1,3-dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex 6-PPD from the company Flexsys) (8) Diphenylguanidine (Rhenogran DPG-80 from RheinChemie) (9) N-cyclohexyl-2-benzothiazolylsulfenamide (Rhenogran CBS-80 from the company RheinChemie)

[0362] Process for preparing the elastomeric compositions:

[0363] 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.

[0364] The first phase is carried out using a mixing device, of internal mixer type, of Brabender brand (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 140-160 C.

[0365] 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.

[0366] 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.

[0367] 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.

[0368] Each final mixture is subsequently calendered in the form of plates with a thickness of 2-3 mm.

[0369] An evaluation of the rheological properties of these crude mixtures obtained makes it possible to optimize the vulcanization time and the vulcanization temperature.

[0370] The mechanical and dynamic properties of the mixtures vulcanized at the curing optimum (T98) are then measured.

[0371] Rheological Properties [0372] Viscosity of the crude mixtures:

[0373] 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.

[0374] 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 233 C.

TABLE-US-00008 TABLE II Compo- Compo- References Control 1 sition 1 sition 2 ML (1 + 4) - 100 C. Initial 75 68 68 Mooney relaxation Initial 0.327 0.338 0.353 ML (1 + 4) - 100 C. After 21 days 96 77 78 (23 3 C.) Mooney relaxation After 21 days 0.264 0.297 0.304 (23 3 C.)

[0375] It is found that the silicas S3 and S4 of the present invention (Compositions 1 and 2) allow a substantial reduction in the initial crude viscosity, relative to the value of the mixture with the reference (Control 1).

[0376] It is also found that the silicas S3 and S4 of the present invention (Compositions 1 and 2) make it possible to retain the advantage in crude viscosity, relative to the value of the mixture with the reference (Control 1), after 21 days of storage.

[0377] 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. [0378] Rheometry of the compositions:

[0379] The measurements are performed on the compositions in crude form. The results relating to the rheology test, which is performed at 160 C. using a Monsanto ODR rheometer according to the standard NF ISO 3417, are given in Table III.

[0380] 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.

[0381] The following are determined from the curve of variation in the torque as a function of time: [0382] the minimum torque (Tmin), which reflects the viscosity of the composition at the temperature under consideration; [0383] the maximum torque (Tmax); [0384] the delta torque (T=TmaxTmin), which reflects the degree of crosslinking brought about by the action of the crosslinking system and, if the need arises, of the coupling agents; [0385] the time T98 necessary to obtain a vulcanization state corresponding to 98% of complete vulcanization (this time is taken as the vulcanization optimum); [0386] 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 implement the raw mixtures at this temperature without having initiation of vulcanization (the mixture cures at and above TS2).

[0387] The results obtained are shown in Table III.

TABLE-US-00009 TABLE III Compositions Control 1 Composition 1 Composition 2 Tmin (dN .Math. m) 18.7 16.5 16.5 Tmax (dN .Math. m) 58.9 59.6 63.8 Delta torque (dN .Math. m) 40.2 43.1 47.3 TS2 (min) 4.2 5.4 5.1 T98 (min) 25.3 26.6 26.3

[0388] The use of silicas S3 and S4 of the present invention (Compositions 1 and 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.

[0389] It is also found that the use of silicas S3 and S4 of the present invention (Compositions 1 and 2) makes it possible to improve the scorch time TS2 relative to the control mixture (Control 1) without impairing the time T98. The stability of the mixtures is thus improved.

[0390] Mechanical properties of the vulcanizates:

[0391] The measurements are performed on the optimally vulcanized compositions (T98) for a temperature of 160 C.

[0392] 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.

[0393] 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.

[0394] The properties measured are collated in Table IV.

TABLE-US-00010 TABLE IV Compositions Control 1 Composition 1 Composition 2 10% Modulus (MPa) 0.5 0.5 0.5 100% Modulus (MPa) 1.8 1.6 1.7 300% Modulus (MPa) 8.3 6.8 7.9 Ultimate strength (MPa) 20.7 19.9 20.9 Elongation at break (%) 538 588 550 RI 4.6 4.3 4.6 Shore A hardness - 15 s 59 55 57 (pts)

[0395] The use of silicas S3 and S4 of the present invention (Compositions 1 and 2) makes it possible to obtain a satisfactory level of reinforcement, relative to the control mixture (Control 1) and in particular to conserve a high level of the 300% strain modulus without penalizing the reinforcement index and the ultimate properties (breaking strength and elongation at break).

[0396] Dynamic Properties of the Vulcanizates:

[0397] The dynamic properties are measured on a viscosity analyzer (Metravib VA3000) according to standard ASTM D5992.

[0398] The values for the loss factor (tan ) and the dynamic shear elastic modulus (G*.sub.12%) 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 performed according to an outward-return cycle, proceeding outward from 0.1% to 50% and then returning from 50% to 0.1%.

[0399] 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 also the elastic modulus G*.sub.12%.

TABLE-US-00011 TABLE V Compositions Compo- Control 1 Composition 1 sition 2 G*.sub.12%, 40 C., 10 Hz (MPa) 1.4 1.4 1.4 Tan max return, 40 C., 10 Hz 0.224 0.201 0.212

[0400] The use of the silicas S3 and S4 of the present invention (Compositions 1 and 2) makes it possible to achieve improved dynamic properties at 40 C. when compared with those of the control mixture (Control 1), the stiffness/dissipation compromise thus being improved.

[0401] Examination of the various Tables II to V shows that the compositions in accordance with the invention (Compositions 1 and 2) make it possible to improve the processing/reinforcement/hysteresis properties at 40 C. compromise, relative to the control composition (Control 1), and in particular to achieve a substantial gain in crude viscosity, which remains stable on storage over time.