Process for producing chemically treated amorphous precipitated silica

1. In the process of producing a chemically treated siliceous tiller by contacting an acidic aqueous suspension of amorphous precipitated silica with a coupling agent, optionally in the presence of a surfactant and/or a water-miscible solvent, to form an acidic aqueous suspension of chemically treated precipitated silica, and recovering said chemically treated precipitated silica, the improvement comprising:

(a) providing an acidic aqueous suspension of amorphous precipitated silica having a pH of 2.5 or less,

(b) treating the precipitated silica with a coupling agent comprising a combination of:

(i) mercaptoorganometallic compound and

(ii) non-sulfur organometallic compound,

the weight ratio of (i) to (ii) being at least 0.05:1, under conditions which result in essentially complete hydrolysis of any alkoxy groups associated with the coupling agent, thereby to produce chemically treated precipitated silica,

(c) raising the pH of the acidic aqueous suspension of chemically treated precipitated silica to from 3 to 10 with acid neutralizing agent that does not adversely affect the chemically treated precipitated silica, and

(d) separating chemically treated precipitated silica from the aqueous suspension while retaining in the aqueous phase substantially all of any alcohol by-product produced by hydrolysis of alkoxy groups associated with the coupling agent, said chemically treated precipitated silica having a carbon content of greater than 1 weight percent, a mercapto content of greater than 0.15 weight percent, a Silane Conversion Index of at least 0.3 and a Standard Reinforcement Index of at least 4, as measured using the Standard Compounding Protocol.

2. The process of claim 1 wherein said treatment of precipitated silica with coupling agent produces by-product alcohol and said alcohol by-product is retained in said aqueous phase such that less than 4000 ppm of by-product alcohol is released from said separated chemically treated precipitated silica.

3. The process of claim 1 wherein the mercaptoorganometallic compound is represented by the following formula:

wherein M is silicon, L is halogen or —OR 7 , Q is hydrogen, C 1 -C 12 alkyl, or halosubstituted C 1 -C 12 alkyl, R 6 is C 1 -C 12 alkylene, R 7 is C 1 -C 12 alkyl or alkoxyalkyl containing from 2 to 12 carbon atoms, said halogen or (halo) group being chloro, bromo, iodo or fluoro, and n is 1, 2 or 3.

4. The process of claim 3 wherein the mercapto group of the mercaptoorganometallic compound is blocked.

5. The process of claim 1 wherein said non-sulfur organometallic compound is chosen from compound(s) represented by formulae II, III, IV and V:

R 1 a SiX (4−a)   II

R 2 2c+2 Si c O (c−1)   III

R 3 2d Si d O d   IV

(R 2 3 Si) k NR 4 (3−k)   V

and mixtures of said compounds; wherein each R 1 is independently a hydrocarbon group of from 1 to 18 carbon atoms or R 1 is an organofunctional hydrocarbon group of from 1 to 12 carbon atoms, said functional group being amino, carboxylic acid, carbinol ester or amido; each X is independently chosen from halogen, amino, alkoxy groups of from 1 to 12 carbon atoms and acyloxy groups of from 1 to 12 carbon atoms, a is the integer 1, 2 or 3; each R 2 is independently halo, hydroxy, or a hydrocarbon group containing from 1 to 18 carbon atoms, with the proviso that at least 50 mole percent of the R 2 substituents are hydrocarbon groups containing from 1 to 18 carbon atoms, c is an integer from 2 to 10,000; each R 3 is independently halo, hydroxy, or a hydrocarbon group containing from 1 to 18 carbon atoms and d is an integer from 3 to 20; each R 4 is independently hydrogen or a hydrocarbon group containing from 1 to 18 carbon atoms and k is 1 or 2; said halo or halogen being chosen from chloro, fluoro, bromo or iodo.

6. The process of claim 1 wherein the acidic aqueous suspension of amorphous precipitated silica has a pH of 2 or less, and the weight ratio of (i) mercaptoorganometallic compound to (ii) non-sulfur organometallic compound is from 0.05:1 to 10:1.

7. The process of claim 6 wherein the acid neutralizing agent is chosen from sodium hydroxide, potassium hydroxide, ammonium hydroxide and sodium bicarbonate.

8. The process of claim 6 wherein the chemically treated precipitated silica has a carbon content of at least 1.5 weight percent, a mercapto content of at least 0.3 weight percent, a Silane Conversion Index of at least 0.4, and a Standard Reinforcement Index of at least 4.5, as measured using the Standard Compounding Protocol.

9. The process of claim 6 wherein the mercaptoorganometallic compound is represented by the following formula:

wherein M is silicon, L is chloro or —OR 7 , Q is hydrogen, C 1 -C 12 alkyl, or chlorosubstituted C 1 -C 12 alkyl, R 6 is C 1 -C 3 alkylene, R 7 is C 1 -C 4 alkyl, and n is 1, 2 or 3.

10. The process of claim 6 wherein the non-sulfur organometallic compound is represented by the following formula:

R 1 a SiX (4−a)

wherein each R 1 is a hydrocarbon group of from 1 to 12 carbon atoms, X is chloro or C 1 -C 3 alkoxy, and a is the integer 1, 2, or 3.

11. The process of claim 1 wherein the treatment step (b) is performed at temperatures of from 30° C. to 150° C.