SILICA FOR ORAL CARE COMPOSITIONS

Abstract

A precipitated silica is provided having high compatibility with stannous and fluoride ions and good balance of abrasion properties. A precipitated silica characterised by a CTAB surface area S.sub.CTAB comprised between 5 and 50 m.sup.2/g; an abrasion value Hm between 3.5 and 14.0 μm; and a stannous ion compatibility of at least 40%.

Claims

1. A precipitated silica which is characterised by: a CTAB surface area S.sub.CTAB comprised between 5 and 50 m.sup.2/g; an abrasion value Hm between 3.5 and 14.0 μm; and a stannous ion compatibility of at least 40%.

2. The precipitated silica according to claim 1 characterised by a number of OH per surface area equal to or greater than 15 OH/nm.sup.2.

3. The precipitated silica according to claim 1 characterised by: a CTAB surface area S.sub.CTAB comprised between 15 and 50 m.sup.2/g; an abrasion value Hm between 4.0 and 14.0 μm; a stannous ion compatibility of at least 40%, and a number of OH per surface area equal to or greater than 30 OH/nm.sup.2.

4. The precipitated silica according to claim 1 wherein the stannous ion compatibility is between 40 and 60%.

5. The precipitated silica according to claim 1 wherein the CTAB surface area S.sub.CTAB is comprised between 15 and 45 m.sup.2/g.

6. The precipitated silica according to claim 1 wherein the number of OH per surface area is between 30 and 60 OH/nm.sup.2.

7. The precipitated silica according to claim 1 which has a fluoride ion compatibility of at least 80%.

8. A process for preparing the precipitated silica of claim 1, which comprises the steps of: (i) providing an aqueous silicate solution in a vessel, the concentration of silicate, expressed as SiO.sub.2, in said vessel being less than 150 g/L and the pH of the aqueous suspension being comprised between 8.0 and 10.0; (ii) adding an acid to said aqueous silicate suspension to obtain a pH value for the reaction medium between 5.0 and 9.0; (iii) perform an ageing step for a time period from 10 to 120 minutes; (iv) adding an acid to the reaction medium to lower the pH to a value of less than 5.0 to obtain a silica suspension; (v) submitting said silica suspension to filtration to provide a filter cake; and optionally drying.

9. The process of claim 8 wherein the silicate concentration in the aqueous silicate solution in step (i) is between 10 and 50 g/L.

10. The process of claim 8 wherein the time period of step (iii) is between 30 and 100 minutes.

11. The process of claim 8 wherein step (iii) is performed at a temperature higher than the temperature of steps (i) and (ii).

12. The process of claim 8 wherein step (iii) is performed at a pH between 5.0 and 9.0.

13. An oral care composition comprising the precipitated silica of claim 1.

14. A toothpaste composition comprising the precipitated silica of claim 1 and a therapeutically active amount of stannous fluoride.

Description

EXAMPLE 1

[0100] In a 2500 L stainless steel reactor were introduced: 156.6 L of water and 32.8 kg of a sodium silicate solution (SiO.sub.2/Na.sub.2O ratio=3.43; SiO.sub.2 concentration=19.6 wt %, used in all the steps of the process). Initial pH of the solution was 9.4.

[0101] The solution obtained was stirred and heated to reach 95° C. After this first step, a 7.7 wt % sulfuric acid solution was added at a flow rate of 4.2 kg/min to reach a pH of 8.9 and a neutralization ratio of 88%. The same sulfuric acid solution was used in all steps of the process.

[0102] The neutralization ratio is defined by the following relation:

Neutralization ratio (%)=(number of H.sup.+ (mol))/(number of OH.sup.− (mol)).

[0103] Over a period of 38.3 min, were simultaneously introduced sodium silicate, (flow rate: 10.1 kg/min), water (flow rate: 8.6 kg/min) and sulfuric acid solution to obtain a total neutralization ratio of 90% and a pH value of 8.9.

[0104] At the end of the first simultaneous addition, over a period of 21 min, were introduced: sodium silicate (flow rate: 10.1 kg/min) and sulfuric acid to obtain a neutralization ratio of 90% and a pH value 8.9.

[0105] The pH of the reaction medium was then brought to a value of 7.0 with sulfuric acid (flow rate: 5.0 kg/min). An ageing step was carried out over a period of 82 min at pH 7.0. After 82 min, the pH of the reaction medium was brought to a value of 4.0 with sulfuric acid (flow rate: 11.9 kg/min). A suspension of precipitated silica was obtained.

[0106] The suspension was filtered and washed on a drum filter. The moisture of the cake was more than 45 wt %. The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO.sub.2 suspension at 30 wt % of silica. The pH was adjusted with sulfuric acid to reach value less than 5.0.

[0107] The product was dried by atomization. The product was obtained in powder form with a moisture less than 5 wt %. The physicochemical properties of the product are reported in Table I.

EXAMPLE 2

[0108] In a 2500 L stainless steel reactor were introduced: 158.3 L of water and 32.7 kg of a sodium silicate solution (SiO.sub.2/Na.sub.2O ratio=3.45; SiO.sub.2 concentration=19.5 wt %, used in all the steps of the process). Initial pH of the solution was 9.4.

[0109] The solution obtained was stirred and heated to reach 95° C. After this first step, sulfuric acid solution (at a concentration of 7.7 wt %; the same solution was used in all steps of the process) was added at a flow rate of 4.0 kg/min to reach a pH of 8.1 and a neutralization ratio of 85%.

[0110] Over a period of 38.3 min, were simultaneously introduced: sodium silicate, (flow rate: 11.7 kg/min), water (flow rate: 8.6 kg/min) and sulfuric acid solution to obtain a neutralization ratio of 90% and a pH of 8.1.

[0111] At the end of the first simultaneous addition, over a period of 21.1 min, were introduced sodium silicate (flow rate: 10.1 kg/min), and sulfuric acid to obtain a neutralization ratio of 90% and a pH of 8.1

[0112] The pH of the reaction medium was brought to a value of 7.0 with sulfuric acid (flow rate: 4.9 kg/min). An ageing step was carried out over a period of 77 min at a pH of 7.0. After 77 min, the pH of the reaction medium was brought to a value of 4.0 with sulfuric acid (flow rate: 11.8 kg/min).

[0113] A suspension of precipitated silica was obtained. The suspension was filtered and washed on a filter plate. The moisture of the cake was more than 45 wt %. The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO.sub.2 suspension at 30 wt % of silica. The pH was adjusted with sulfuric acid to reach a value of less than 5.0.

[0114] The product was dried by atomization. The product obtained was in powder form with a moisture of less than 5 wt %. The physicochemical properties of the product are reported in Table I.

EXAMPLE 3

[0115] In a 2500 L stainless steel reactor were introduced: 156.6 L of water and 32.76 kg of a sodium silicate solution (SiO.sub.2/Na.sub.2O ratio=3.45; SiO.sub.2 concentration=19.5 wt %, used in all the steps of the process). Initial pH of the solution was 9.4.

[0116] The solution obtained was stirred and heated to reach 95° C. After this first step, a 7.8 wt % sulfuric acid solution (the same solution was used in all steps of the process) was added at a flow rate of 4.0 kg/min to reach a pH of 7.3 and a neutralization ratio of 85.8%.

[0117] Over a period of 38.9 min, were simultaneously introduced: sodium silicate (flow rate: 9.9 kg/min), water (flow rate: 8.6 kg/min) and sulfuric acid to obtain a total neutralization ratio equal to 89.0% and a pH of 7.3.

[0118] At the end of the first simultaneous addition, over a period of 16.4 min, were introduced: sodium silicate (flow rate: 10.2 kg/min) and sulfuric acid to obtain a neutralization ratio of 88.9% and a pH of 7.3.

[0119] The pH of the reaction medium was brought to a value of 7.0 with sulfuric acid at a flow rate of 4.9 kg/min. At pH 7.0, an ageing step was carried out over a period of 80 min. After 80 min, the pH of the reaction medium was brought to a value of 4.0 with sulfuric acid at a flow rate of 11.8 kg/min.

[0120] A suspension of precipitated silica was obtained. The suspension was filtered and washed on a filter plate. The moisture of the cake was more than 45 wt %.

[0121] The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO.sub.2 suspension at 30 wt % of silica. Then pH was adjusted with sulfuric acid to reach value less than 5.0.

[0122] The product was dried by atomization. The product obtained was in powder form with a moisture of less than 5 wt %. The physicochemical properties of the product are reported in Table I.

COMPARATIVE EXAMPLE 1

[0123] In a 170 L stainless steel reactor were introduced: 17.8 L of water and 7 kg of a sodium silicate solution (SiO.sub.2/Na.sub.2O ratio=3.44; SiO.sub.2 concentration=12 wt %). The same sodium silicate solution was used in all the steps of the process.

[0124] The obtained solution was stirred and heated to reach 90° C. Once the set temperature was reached sulfuric acid (7.7 wt % solution) was added (flow rate: 491 g/min) until the reaction medium reached the pH value of 9.0. The same sulfuric acid solution was used in all the steps of the process.

[0125] Simultaneously, over a period of 60 min, were introduced: sodium silicate, at a flowrate of 1429 g/min, and sulfuric acid. The flow rate of the sulfuric acid was regulated so that the pH of the reaction medium was maintained at a value of 9.0.

[0126] At the end of the simultaneous addition, the pH of the reaction medium was brought to a value of 7.0 sulfuric acid. Simultaneously, the reaction medium was heated to 95° C. The rest of the process was carried out at this temperature. A first ageing step was carried out at pH 7.0 over a period of 75 min. After 75 min, the pH of the reaction medium was brought to a value of 4.0 with sulfuric acid (flow rate: 680 g/min). At pH 4.0, a second ageing step was carried out over a period of 10 min to obtain a suspension of precipitated silica.

[0127] The suspension of precipitated silica was filtered and washed on a filter plate. The moisture of the cake was more than 30 wt %.

[0128] The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO.sub.2 suspension having 30 wt % of silica content.

[0129] The product was dried by spray drying. The product obtained, in powder form, had a moisture content of less than 7 wt %. The physicochemical properties of the product are reported in Table I.

COMPARATIVE EXAMPLE 2

[0130] In a 10 L stainless steel reactor were introduced: 0.957 L of water and 385 g of a sodium silicate solution (SiO.sub.2/Na.sub.2O ratio=3.43; SiO.sub.2 concentration=12 wt %). The same sodium silicate solution was used in all the steps of the process.

[0131] The obtained solution was stirred and heated to reach 90° C. At this temperature, a sulfuric acid (7.7 wt % solution) was added at a flow rate of 22 g/min until the reaction medium reached the pH value of 9.5. The same sulfuric acid solution was used in all the steps of the process.

[0132] Simultaneously, over a period of 55 min, were introduced: sodium silicate (flow rate: 85.2 g/min) and sulfuric acid. The flow rate of the sulfuric acid was regulated so that the pH of the reaction medium was maintained at a value of 9.5.

[0133] At the end of the simultaneous addition, the pH of the reaction medium was brought to a value of 7.0 with sulfuric acid. Simultaneously, the reaction medium was heated to 95° C. The rest of the process was carried out at this temperature. A first ageing step was performed at pH 7.0 over a period of 50 min. After 50 min, the pH of the reaction medium was brought to a value of 4.0 with sulfuric acid at a flow rate of 45.4 g/min. At pH 4.0, the reaction medium was submitted to a second ageing step over a period of 30 min and a suspension of precipitated silica was obtained.

[0134] The suspension was filtered and washed on a filter plate. The moisture of the cake was more than 30 wt %.

[0135] The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO.sub.2 suspension containing 30 wt % of silica.

[0136] The product was dried by spray drying. The product obtained, in powder form, had a moisture content of less than 7 wt %. The physicochemical properties of the product are reported in Table I.

TABLE-US-00001 TABLE I Exam- Exam- Exam- Comp. Comp. ple 1 ple 2 ple 3 Example 1 Example 2 BET (m.sup.2/g) 35 38 48 64 72 CTAB (m.sup.2/g) 31 32 37 60 52 Abrasion depth 8.8 13.1 11.1 4.8 6.8 Hm (μm) DOA (g/100 g) 113 105 110 129 122 pH 5.9 6.1 6.3 7.0 7.1 Compatibility 45 48 45 37 29 Sn(II) (%) Compatibility F.sup.− 92 94 91 92 92 (from NaF) (%) OH/nm.sup.2 51 51 35 25 27

[0137] The data in Table 1 show that the inventive silicas have a higher stannous ion compatibility with respect to silicas obtained with a process as disclosed in EP396460A1.

[0138] The compatibility of a known precipitated silica for use in toothpaste compositions, Zeodent® 113 (commercially available from Huber) having a CTAB surface area S.sub.CTAB of 53 m.sup.2/g, to Sn(II) ions was found to be 24%.

[0139] The inventive silica thus provides a high availability of both Sn(II) and fluoride ions in compositions comprising stannous fluoride as well as good abrasion levels (good balance between plaque removal and abrasion).