PREPARATION OF PRECIPITATED SILICAS USEFUL AS FILLERS IN SILICON MATRICES

Abstract

The invention relates to the use of carboxylic acid during the preparation of precipitated silica or a suspension of precipitated silica and to the precipitated silicas thus obtained, particularly with low water uptake, which can be used, for example, as a reinforcing filler in silicon matrices.

Claims

1.-2. (canceled)

33. A process for the preparation of a precipitated silica or a suspension thereof, the process comprising mixing a carboxylic acid with or adding same to the filtration cake resulting from the reaction of precipitation of the silica, during the disintegration of the filtration cake, the resulting mixture subsequently being optionally dried, wherein the precipitated silica comprises: a water absorption lower than 6%, a level of residual anion, expressed as alkaline metal sulfate, of less than 1% by weight based on the mass of the precipitated silica, and a content of carboxylic acid+corresponding carboxylate, expressed as carbon, of at least 0.02% by weight based on the mass of the precipitated silica.

34. The process as defined by claim 33, in which said carboxylic acid is selected from among maleic acid, formic acid, octanoic acid, acetic acid, oxalic acid, propionic acid or succinic acid.

35. The process as defined by claim 33, in which said carboxylic acid is selected from among maleic acid, formic acid, octanoic acid, propionic acid or succinic acid.

36. A process for the preparation of the precipitated silica or a suspension thereof, the process comprising: reacting at least one silicate with at least one acidifying agent, to produce a precipitated silica suspension, filtering said precipitated silica suspension, to provide a filtration cake, and disintegrating said filtration cake, and mixing at least one carboxylic acid with said filtration cake or adding said at least one carboxylic acid thereto, during the disintegration operation, the mixture obtained subsequently being optionally dried; wherein the precipitated silica comprises: a water absorption lower than 6%, a level of residual anion, expressed as alkaline metal sulfate, of less than 1% by weight based on the mass of the precipitated silica, and a content of carboxylic acid+corresponding carboxylate, expressed as carbon, of at least 0.02% by weight based on the mass of the precipitated silica.

37. The process as defined by claim 36, in which said carboxylic acid is selected from among maleic acid, formic acid, the octanoic acid, acetic acid, oxalic acid, propionic acid or succinic acid.

38. The process as defined by claim 36, in which said carboxylic acid is selected from among maleic acid, formic acid, octanoic acid, propionic acid or succinic acid.

39. The process as defined by claim 36, wherein the precipitated silica has a water uptake of greater than 3%.

40. The process as defined by claim 36, wherein the precipitated silica has a level of residual anion, expressed as alkaline metal sulfate, of less than 0.5% based on the mass of the precipitated silica.

41. The process as defined by claim 36, wherein the precipitated silica has a content of carboxylic acid+corresponding carboxylate, expressed as carbon, of at least 0.03% by weight based on the mass of the precipitated silica.

42. The process as defined by claim 36, wherein the precipitated silica has a CTAB specific surface of from 50 to 260 m.sup.2/g.

43. The process as defined by claim 36, wherein the precipitated silica has a DOP oil uptake of greater than 300 ml/100 g.

44. The process as defined by claim 36, wherein the precipitated silica has a mean particle size or a median particle diameter of less than 30 μm.

45. The process as defined by claim 36, wherein the precipitated silica has a pH from 3.5 to 7.5, where the pH is measured in a 5% by weight suspension of the silica in deionized water according to ISO standard 787/9.

46. The process as defined by claim 36, wherein the precipitated silica has a BET specific surface and a CTAB specific surface such that the BET specific surface-CTAB specific surface difference is at most 60 m.sup.2/g.

47. The process as defined by claim 36, wherein the precipitated silica is in the form of a powder.

48. A process for the preparation of a precipitated silica, the process comprising: (a) providing a starting aqueous vessel heel with a temperature of from 80° to 100° C. and which comprises a silicate, the concentration of silicates in said starting vessel heel, expressed as SiO.sub.2 equivalent, being less than or equal to 15 g/l, (b) adding an acidifying agent to said starting vessel heel, at a temperature of from 80° to 100° C., to adjust the pH of the reaction medium to a value of from 7 to 8, (c) adding a silicate and an acidifying agent, optionally simultaneously, to the medium thus produced, at a temperature of from 80° to 100° C., the respective amounts of silicate and of acidifying agent, added over time being such that, throughout the duration of the addition: the pH of the reaction medium remains from 7 to 8, and the concentration of silicate in the medium, expressed as SiO.sub.2 equivalent, is less than or equal to 35 g/l, (d) adding an acidifying agent to the medium obtained, at a temperature of from 80° to 100° C., to adjust the pH of the medium to a value of from 3 to 6.5, (e) filtering the suspension of silica, (f) drying the filtration cake obtained on conclusion of the filtration (e), said process further comprising, prior to the drying thereof in stage (f), mixing the filtration cake, or adding thereto, at least one carboxylic acid, and the filtration cake thus obtained exhibiting prior to drying thereof in stage (f), a loss on ignition at 1000° C. of greater than 80%; wherein the filtration cake obtained on conclusion of the filtration (e) is subjected, prior to the drying thereof in stage (f), to a disintegration operation, the carboxylic acid being added to said cake during said disintegration operation.

49. The process as defined by claim 48, further comprising: (g) milling or micronizing the silica obtained on conclusion of stage (f).

50. The process as defined by claim 48, wherein the drying stage (f) is carried out by atomization.

51. A reinforcing filler for an organosilicon matrix comprising the precipitated silica prepared by the process as defined by claim 48.

52. A shoe sole, polyamide matrix, elastomeric matrix, cement or concrete composition, liquid carrier, dentifrice composition or battery separator comprising the reinforcing filler as defined by claim 48.

Description

EXAMPLE

[0158] 14 000 g of water and 630 g of a 236 g/l (as SiO.sub.2 equivalent) aqueous sodium silicate solution were introduced into a reactor equipped with a system for regulating the temperature and pH and with a system for stirring with a 3-bladed propeller, the SiO.sub.2/Na.sub.2O ratio by weight (Rw) of the sodium silicate used being 3.46.

[0159] After starting to stir (250 revolutions per minute), the vessel heel thus formed was heated to 95° C. and the pH was brought to 7.5, over 11 minutes, by addition of an 80 g/l aqueous sulfuric acid solution (mean flow rate of 61 g per minute).

[0160] Once the pH of 7.5 was reached, a simultaneous addition of 4320 g of a 236 g/l (as SiO.sub.2 equivalent) aqueous sodium silicate solution (Rw=3.46), at a flow rate of 48 g/min (duration of the addition: 90 minutes), and of 4770 g of an 80 g/l aqueous sulfuric acid solution, at a flow rate of 53 g/min, was carried out, so as to maintain the pH of the medium at a value equal to 7.5 (to within about 0.1 pH unit).

[0161] After the period of addition of 90 minutes, the addition of silicate was halted and addition of acid was continued until the pH of the reaction mixture had stabilized at 3.4. Maturing was carried out by leaving the solution to stir for 5 minutes.

[0162] The slurry obtained was subsequently filtered on a flat filter and then the filtration cake obtained was disintegrated with water and with added maleic acid.

[0163] The disintegrated cake obtained exhibits a loss on ignition at 1000° C. of greater than 83%. It was subsequently dried by rotary atomization.

[0164] The dried silica was then milled using a classifier mill.

[0165] The physicochemical characteristics of the precipitated silica obtained, in the powder form, are as follows: [0166] water uptake: 5.5% [0167] Na.sub.2SO.sub.4 content: 0.20% (with respect to the total weight of the material in the dry state) [0168] content of maleic acid+corresponding maleate, expressed as carbon: 0.04% [0169] CTAB specific surface: 160 m.sup.2/g [0170] BET specific surface: 160 m.sup.2/g [0171] DOP oil uptake: 320 ml/100 g [0172] mean particle size: 12 μm [0173] pH: 5.5 [0174] loss on ignition at 1000° C.: 4.9% [0175] residual water content after 2 hours at 105° C.: 2.6%