PRECIPITATED SILICA AND PROCESS FOR ITS MANUFACTURE

Abstract

A precipitated silica having large particle size for use in tire applications. In particular, a precipitated silica characterised by a CTAB surface area S.sub.CTAB equal to or greater than 160 m.sup.2/g; a median particle size d50, measured by centrifugal sedimentation, such that

|d50|>25000/|S.sub.CTAB|  (I)

wherein |d50| represents the numerical value of the median particle size d50 measured by centrifugal sedimentation and expressed in nm and |S.sub.CTAB| represents the numerical value of the CTAB surface area S.sub.CTAB expressed in m.sup.2/g; and
an aluminium content not exceeding 4500 ppm.

Claims

1. A precipitated silica characterised by: a CTAB surface area S.sub.CTAB equal to or greater than 160 m.sup.2/g; a median particle size d50, measured by centrifugal sedimentation, such that:
|d50|>25000/|S.sub.CTAB|  (I) wherein: |d50| represents the numerical value of the median particle size d50 measured by centrifugal sedimentation and expressed in nm and |S.sub.CTAB| represents the numerical value of the CTAB surface area S.sub.CTAB expressed in m.sup.2/g; and an aluminium content not exceeding 4500 ppm.

2. The precipitated silica according to claim 1 wherein the CTAB surface area S.sub.CTAB is in the range from 180 to 400 m.sup.2/g.

3. The precipitated silica according to claim 1 wherein the width of the particle size distribution Ld, measured by centrifugal sedimentation, is from 1.2 to 3.5.

4. The precipitated silica according to claim 1 wherein the BET surface area S.sub.BET is equal to or greater than 170 m.sup.2/g.

5. The precipitated silica according to claim 1 wherein the difference between the BET surface area S.sub.BET and the CTAB surface area S.sub.CTAB is at least 30 m.sup.2/g.

6. The precipitated silica according to claim 1 characterised by: the CTAB surface area S.sub.CTAB from 200 to 350 m.sup.2/g; a width of the aggregate size distribution Ld, measured by centrifugal sedimentation, in the range from 1.5 to 2.8; the median particle size d50, measured by centrifugal sedimentation, such that:
|d50|>25000/|S.sub.CTAB|  (I), and the aluminium content not exceeding 4500 ppm.

7. The precipitated silica according to claim 1 wherein the aluminium content is less than 3500 ppm.

8. The precipitated silica according to claim 1 wherein the CTAB surface area S.sub.CTAB range from 200 to 400 m.sup.2/g and the median particle size d50 is greater than 60 nm.

9. A process for the preparation of a precipitated silica of claim 1, said process comprising: (i) providing a starting solution having a pH from 2.0 to 5.0, (ii) simultaneously adding a silicate and an acid to said starting solution such that the pH of the reaction medium is maintained in the range from 2.0 to 5.0, (iii) stopping the addition of the acid and of the silicate and adding a base to the reaction medium to raise the pH of said reaction medium to a value from 7.0 to 10.0, (iv) simultaneously adding to the reaction medium a silicate and an acid, such that the pH of the reaction medium is maintained in the range from 7.0 to 10.0, and (v) stopping the addition of the silicate while continuing the addition of the acid to the reaction medium to reach a pH of the reaction medium of less than 5.5 and obtaining a suspension of precipitated silica, wherein the amount of silicate added to the reaction medium during step (ii) is more than 55% of the total amount of silicate required for the reaction.

10. The process according to claim 9 wherein in step (iii) the addition of the acid is stopped while continuing the addition of the silicate to the reaction medium to raise the pH of said reaction medium to a value in the range from 7.00 to 10.00.

11. A composition comprising the precipitated silica of claim 1 and at least one polymer.

12. The composition of claim 11 wherein the polymer is an elastomer.

13. An article comprising the precipitated silica of claim 1.

14. The article of claim 13 being in the form of a tire or tire component.

15. The composition of claim 12 wherein the polymer is selected from the group consisting of the diene elastomers.

Description

EXAMPLES

Example 1

[0136] 928 liters of industrial water were introduced into a 2500 liter reactor and heated to 90° C. 14.7 kg of solid sodium sulfate were introduced into the reactor under stirring and sulfuric acid (concentration: 96 wt %) was then added until the pH reached the value of 4.2.

[0137] A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio: 3.43, density: 1.230 kg/L) was introduced into the reactor over a period of 50 minutes, at a flow rate of 352 L/h, simultaneously with sulfuric acid (concentration: 7.7 wt %). The flow rate of the acid was regulated so as to maintain the pH of the reaction medium at a value of 4.2. The amount of silicate added to the reaction medium was 79% of the total amount.

[0138] After 50 minutes the introduction of acid was stopped and the addition of silicate was continued to reach a pH value of 8.0. Meanwhile the temperature was increased to 94° C. A further simultaneous addition was then performed over the course of 7 minutes with a sodium silicate flow rate of 577 L/h (same sodium silicate as in the first simultaneous addition) and a flow rate of sulfuric acid (concentration: 7.7 wt %) regulated so as to maintain the pH of the reaction medium at a value of 8.0.

[0139] After this simultaneous addition, the pH of the reaction medium was brought to a value of 4.8 by introduction of sulfuric acid (concentration: 7.7 wt %) to obtain a suspension of precipitated silica. The suspension was filtered and washed on a filter press, to give a precipitated silica cake with a solids content of 20.7 wt %. The silica cake obtained was then subjected to a liquefaction step in a continuous vigorously stirred reactor with addition of 1360 grams of a sulfuric acid solution (conc.: 7.7 wt %). The resulting slurry was dried by means of a nozzle spray dryer to obtain precipitated silica S1. The properties precipitated silica S1 are reported in Table 1.

Example 2

[0140] 927 liters of industrial water were introduced into a 2500 liter reactor and heated to 90° C. 14.7 kg of solid sodium sulfate were introduced into the reactor under stirring followed by the addition of sulfuric acid (concentration: 96 wt %) until the pH reached the value of 4.1.

[0141] A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio: 3.43, density: 1.230 kg/L) was introduced into the reactor over a period of 50 minutes, at a flow rate of 352 L/h, simultaneously with sulfuric acid (concentration: 7.7 wt %). The flow rate of the acid was regulated so as to maintain the pH of the reaction medium at a value of 4.1. The amount of silicate added to the reaction medium was 78% of the total amount.

[0142] After 50 minutes the introduction of acid was stopped and the addition of silicate was continued until the pH reached a value of 8.0. Meanwhile the temperature was increased to 94° C. A further simultaneous addition was then performed over the course of 7 minutes with a sodium silicate flow rate of 577 L/h (same sodium silicate as in the first simultaneous addition) and a flow rate of sulfuric acid (concentration: 7.7 wt %) regulated so as to maintain the pH of the reaction medium at a value of 8.0.

[0143] After this second simultaneous addition, the reaction medium was brought to a pH of 4.7 by introduction of sulfuric acid (concentration: 7.7 wt %) and a suspension of precipitated silica was obtained. The suspension was filtered and washed on a filter press, to give a precipitated silica cake with a solids content of 19.5 wt %. Silica cake obtained was then subjected to a liquefaction step in a continuous vigorously stirred reactor with addition to the cake of 1751 grams of a sodium aluminate solution with an Al.sub.2O.sub.3 content of 22.5 wt %. The resulting slurry was dried by means of a nozzle spray dryer to obtain precipitated silica S2. The properties precipitated silica S2 are reported in Table 1.

Comparative Example 1

[0144] 960 liters of water were introduced into a 2500 liter reactor and heated to 90° C. 15 kg of solid sodium sulfate were introduced into the reactor under stirring. Sulfuric acid (concentration: 96 wt %) was then added until the pH reached the value of 3.7.

[0145] A sodium silicate solution (SiO.sub.2/Na.sub.2O weight ratio: 3.41, density: 1.231 kg/L) was introduced into the reactor over a period of 25 minutes, at a flow rate of 370 L/h, simultaneously with sulfuric acid (concentration: 7.7 wt %). The flow rate of the acid was regulated so as to maintain the pH of the reaction medium at a value of 3.7. The amount of silicate added to the reaction medium was less than 50% of the total amount of silicate required for the reaction.

[0146] After 25 minutes of simultaneous addition at 90° C., the introduction of acid was stopped and the pH of the reaction medium allowed to reach 8.0. Meanwhile the temperature was increased to 94° C. A further simultaneous addition was then performed over the course of 18 minutes with a sodium silicate flow rate of 600 L/h (same sodium silicate as for the first simultaneous addition) and a flow rate of sulfuric acid (concentration: 7.7 wt %) regulated so as to maintain the pH of the reaction medium at a value of 8.0.

[0147] After this simultaneous addition, the reaction medium was brought to a pH of 4.5 by introduction of sulfuric acid (concentration: 7.7 wt %) and a suspension of precipitated silica was obtained. The suspension was filtered and washed on a filter press, to give a precipitated silica cake with a solids content of 19.2 wt %. The silica cake was subjected to a liquefaction step in a continuous vigorously stirred reactor with simultaneous addition to the cake of sulfuric acid (concentration: 7.7 wt %) and of a sodium aluminate solution (Al/SiO.sub.2 ratio: 0.30 wt %).

[0148] The liquefied cake was subsequently spray dried using a nozzle atomizer to provide silica CS1. The properties of silica CS1 are reported in Table 1.

TABLE-US-00003 TABLE 1 S.sub.CTAB S.sub.BET d50 Al V.sub.(d5-d50)/ Silica (m.sup.2/g) (m.sup.2/g) (nm) Ld (ppm) V.sub.(d5-d100) S1 252 327 125 2.0 4000 0.75 S2 268 317 123 1.9 3400 0.75 CS1 258 280 97 1.4 3200 0.69

Examples 3-5—Comparative Example 2

[0149] The control composition CE2 comprises the silica CS1. The compositions E3 to E5 in accordance with the invention comprise silica S1 and S2.

[0150] Compositions were prepared according to the following recipes given in Table 2 (components are expressed in part by weight per 100 parts of elastomers (phr)).

TABLE-US-00004 TABLE 2 CE2 E3 E4 E5 SBR (1) 103.1 103.1 103.1 103.1 BR (2) 25.0 25.0 25.0 25.0 Silica CS1 70.0 Silica S1 70.0 80.0 Silica S2 70.0 Coupling agent (3) 8.8 8.8 8.8 10.0 Plasticizer (4) 7.5 7.5 7.5 7.5 Carbon black N234 3.0 3.0 3.0 3.0 ZnO 2.5 2.5 2.5 2.5 Stearic acid 2.0 2.0 2.0 2.0 Antioxydant (5) 2.5 2.5 2.5 2.5 Sulfur 1.4 1.4 1.4 1.4 CBS (6) 2.2 2.4 2.3 2.5 DPG (7) 2.4 2.8 2.7 3.1 (1) SSBR with 44.5% vinyl-1,2; 26% bound styrene; extended with 37.5 phr TDAE oil, Tg = −30° C. (« Buna VSL 4526-2 HM » from Arlanxeo); (2) BR (« Buna CB 25 » from Arlanxeo) (3) TESPD (« Luvomaxx TESPD » from Lehman & Voss) (4) TDAE (Treated distillate aromatic extract) (« Vivatec 500 » from Hansen & Rosenthal KG) (5) N-1,3-dimethylbutyl-N-phenyl-para-phenylenediamine (« Santoflex 6-PPD » from Flexsys) (6) N-Cyclohexyl-2-benzothiazyl-sulfenamide (« Rhenogran CBS-80 » from RheinChemie) (7) Diphenylguanidine (« Rheonogran DPG-80 » from RheinChemie)

[0151] Preparation of rubber compositions: The process for preparing the rubber compositions was conducted in three successive phases. First and second mixing stage (non-productive stages, NP1 and NP2) consists in a thermomechanical working at high temperature, followed by a third mechanical working stage (productive stage, P3) at temperatures below 110° C. The latter allows the introduction of the vulcanization system. The first and second phase were carried out by means of an internal mixer from Brabender (net chamber volume: 380 mL) with respectively a fill factor of 0.62 and 0.60. The initial temperature and the speed of the rotors were fixed each time so as to reach mixing drop temperatures of about 140-170° C. Duration of the first mixing stage was between 2 and 10 minutes. After cooling of the mixture (temperature below 100° C.), the second mixing phase allows the introduction of the vulcanization system (sulfur and accelerators). It was carried out on an open two roll mill, preheated to 50° C. The duration of this phase was between 2 and 6 minutes. The final rubber composition was then calendered in sheets at 2-3 mm thickness. An evaluation of rheological properties on the uncured compounds was first run to monitor processability indicators. Once the vulcanization characteristics were determined, uncured compounds were vulcanized at the vulcanization optimum (t98) and mechanical and dynamic properties were measured.

Viscosity of Uncured Compositions

[0152] Mooney viscosity was measured at 100° C. using a MV 2000 rheometer according to NF ISO289 standard. After one minute preheating, the value of the torque was read at 4 minutes (ML (1+4)−100° C.). Complementary, a strain sweep measurement from 0.9 to 50% was carried out using a D-MDR 3000 rheometer according to DIN 53529 standard, at a temperature of 100° C. and a frequency of 1 Hz. The results obtained with those two methods are shown in Table 3.

TABLE-US-00005 TABLE 3 CE2 E3 E4 E5 ΔG′ (0.9-50%) - 100° C. (kPa) - NP1 2756 1993 2729 3531 ΔG′ (0.9-50%) - 100° C. (kPa) - NP2 1181 746 1245 1632 ML (1 + 4) - 100° C. (M.U) - P3 106 97 112 116

[0153] As can be seen in Table 3, the uncured compositions in accordance with the invention show lower (improved) Payne effect ΔG′ (0.9-50%) and lower (improved) Mooney viscosity ML (1+4) at comparable CTAB values with respect to silica CS1 from control CE2. The uncured composition E4, because of the higher CTAB surface area of silica S2, has a slightly higher Mooney viscosity value but comparable Payne effect values ΔG′ (0.9-50%) reflecting comparable processability of the uncured compound. It can be concluded that the processability of the non-vulcanized rubber mixtures containing the silica of the present invention is comparable to that shown by compositions containing high surface area silica from the prior art, having equal CTAB surface area.

Mechanical Properties of Cured Compositions

[0154] Shore A hardness measurement of the cured compositions (Vulcanization time t98 at 170° C.) were performed according to ASTM D 2240 standard. The values were measured after 3 seconds.

[0155] The uniaxial tensile tests were performed in accordance with the NF ISO 37 standard with H2 specimens at a speed of 500 mm/min on an INSTRON 5564. Moduli M100 and M300 (respectively obtained at strains 100% and 300%) and tensile strength are expressed in MPa; elongation at break is expressed in %. A reinforcement index (RI) defined as the ratio between modulus obtained at 300% strain and the one obtained at 100% strain was calculated. The measured properties are summarized in Table 4.

TABLE-US-00006 TABLE 4 CE2 E3 E4 E5 Hardness Shore A-3s (pts) 64 59 61 63 Modulus M100 (Mpa) 2.1 2.3 2.4 2.6 Modulus M300 (Mpa) 8.3 9.5 9.4 10.7 RI = M300/M100 3.9 4.1 4.0 4.2

[0156] The results in Table 4 show that the use of silica 51 and S2 at same loading as silica CS1, provides compounds with lower Shore A hardness, higher moduli M300 and higher reinforcement index RI than composition CE2.

[0157] Increasing the loading of 51 (Composition E5) to reach same Shore A hardness as composition CE2, leads to the same conclusions regarding improved reinforcement potential of the inventive silica.

Dynamic Properties of Cured Compositions

[0158] Dynamic properties were measured on a viscoanalyzer (Metravib DMA+1000) according to ASTM D5992.

Dynamic Response of Cured Compounds Under Strain Sweep Conditions

[0159] Parallelepiped specimens (section 8 mm.sup.2 and height 7 mm) were subjected to a sinusoidal deformation in alternating double shear at a temperature of 40° C. and at a frequency of 10 Hz according to a cycle round trip, ranging from 0.1% to 50% for the forward cycle and from 50% to 0.1% for the return cycle. The values of the maximum loss factor (tan δ max), the shear storage modulus (G′0.1%, G*12%) and the Payne effect (G′0.1%-G′50%) were recorded during the return cycle. The results are shown in Table 5.

TABLE-US-00007 TABLE 5 CE2 E3 E4 E5 G′0.1% (MPa) 3.7 2.6 2.9 3.2 G′0.1%-G′50% (MPa) 2.6 1.5 1.8 2.0 G*12% (MPa) 1.5 1.5 1.6 1.6 tan δ max 0.245 0.190 0.203 0.218

[0160] The compositions in accordance with the invention E3 and E4 show drastically improved hysteresis properties at high temperature (40° C.) based on lower values of tan δ max and Payne effect (G′.sub.0.1%-G′.sub.50%) compared to composition CE2. In spite of the higher silica S1 loading in composition E5, the tan δ max value is kept substantially lower than state of the art composition CE2. Those indicators attest that compositions E3 to E5 to have excellent potential in tire tread compositions in particular in improving rolling resistance without deteriorating handling (steering) performance of the tire (G*12%).

Dynamic Response of Cured Compounds Under Temperature Sweep Conditions

[0161] The dynamic response of the vulcanized rubber compositions is measured by soliciting parallelepiped specimens (section 8 mm.sup.2 and height 7 mm) at a temperature sweep from −45° C. to +45° C. (temperature rise rate of +5° C./min), under an alternating double shear sinusoidal deformation of 1% and at a frequency of 10 Hz. The maximum loss factor (tan δ max) is then monitored. The results are summarized in Table 6:

TABLE-US-00008 TABLE 6 CE2 E3 E4 E5 tan δ max 0.720 0.862 0.832 0.832 T max (° C.) −26 −27 −27 −26

[0162] The compositions in accordance with the invention E3 and E4 show drastically improved hysteresis properties at low temperature, by increasing substantially the maximum loss factor (tan δ max) compared to composition CE2. In spite of the higher silica S1 loading in composition E5, the tan δ max value is improved over the state of the art composition CE2.

[0163] The examination of above described properties demonstrates that the compositions containing inventive silicas S1 and S2 have an excellent potential in tire tread compositions in particular in improving substantially rolling resistance and wet grip at comparable level of tire handling and without deteriorating wear performance and processability behavior performances.