Patent Description:
Oral care products such as toothpastes can provide both therapeutic and cosmetic hygiene benefits. Therapeutic benefits include caries prevention which is typically provided by the use of various fluoride salts; gingivitis prevention by the use of antimicrobial agents; or hypersensitivity control. Cosmetic benefits provided by oral products include the control of plaque and calculus formation, removal and prevention of tooth stain, tooth whitening, breath freshening, and overall improvements in mouth feel impression.

One differentiating factor among oral care products, such as toothpaste, is the active ingredient, fluoride. The most commonly used fluoride sources are typically sodium fluoride, sodium monofluorophosphate, and stannous fluoride. Sodium fluoride and sodium monofluorophosphate are effective sources of fluoride ions that remineralize and strengthen weakened enamel thus allowing fighting cavities. By comparison, stannous fluoride not only delivers cavity fighting fluoride, but it also has antibacterial properties, and it provides an anti-sensitivity mechanism of action. Stannous fluoride has both bactericidal and bacteriostatic properties, which fight plaque and treat/prevent gingivitis. Stannous fluoride also deposits a protective mineral barrier over exposed dentinal tubules to help prevent sensitivity pain from triggers such as hot or cold liquids and foods.

Toothpaste compositions commonly contain a silica abrasive for controlled mechanical cleaning, plaque removal and polishing of teeth. Such silica abrasives are known to interact with other co-ingredients of the compositions such as notably fluorides and zinc compounds. Silicas are also not adequately compatible with tin, strontium and other metallic cations. Such incompatibilities have the consequence that these ingredients are no longer available to elicit their beneficial effects.

<CIT> discloses precipitated silica having improved compatibility with metallic cations, in particular Zn, as well as with the fluoride ion. The process disclosed in <CIT> for the preparation of said precipitated silica comprises two ageing steps performed at different values of pH: a first ageing step performed at a pH between <NUM> and <NUM>, and a second ageing step performed at a pH between <NUM> and <NUM>. It has surprisingly been found that by eliminating from the process the second ageing step, the one performed at pH between <NUM> and <NUM>, it is possible to increase the compatibility of the precipitated silica to stannous ions at levels which are desirable in today's oral care applications.

Objective of the present invention is to provide a silica that has improved compatibility with the active ingredients of the toothpaste composition, in particular with stannous fluoride, and that is effective at removing undesirable plaque deposits with optimum abrasion level.

The above-mentioned objective has been met by a precipitated silica which is characterised by:.

The CTAB surface area SCTAB is a measure of the external specific surface area as determined by measuring the quantity of N hexadecyl-N,N,N-trimethylammonium bromide adsorbed on the silica surface at a given pH.

The CTAB surface area SCTAB is at least <NUM><NUM>/g, typically at least <NUM><NUM>/g. The CTAB surface area SCTAB may be greater than <NUM><NUM>/g.

The CTAB surface area does not exceed <NUM><NUM>/g. The CTAB surface area SCTAB may be lower than <NUM><NUM>/g, preferably lower than <NUM><NUM>/g.

For applications as an abrasive in toothpaste formulations, advantageous ranges of CTAB surface area SCTAB are from <NUM> to <NUM><NUM>/g, preferably from <NUM> to <NUM><NUM>/g.

The BET surface area SBET of the inventive silica is not particularly limited but it is at least <NUM><NUM>/g, typically at least <NUM><NUM>/g. BET surface area SBET may in certain instances be greater than <NUM><NUM>/g, even greater than <NUM><NUM>/g. BET surface area SBET is generally at most <NUM><NUM>/g. The BET surface area may advantageously be from <NUM> to <NUM><NUM>/g, even from <NUM> to <NUM><NUM>/g, even from <NUM> to <NUM><NUM>/g, preferably from <NUM> to <NUM><NUM>/g.

The precipitated silica of the invention is characterised by a good balance of abrasive properties, that is ability to remove the pellicle deposit of the teeth without damaging the enamel.

The inventive silica is characterised by an abrasion depth value Hm, as determined using the PMMA abrasion test described hereafter, between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The inventive silica is advantageously characterised by a compatibility with stannous ions, as determined using the stannous ion compatibility method described hereafter, of at least <NUM>%. Stannous ions are generally Sn(II) ions deriving from SnF<NUM>.

The stannous ion compatibility is preferably at least <NUM>%, even at least <NUM>%. The stannous ion compatibility may advantageously be as high as <NUM>%, although values of up to <NUM> are suitable for most applications.

In an advantageous embodiment the inventive silica has a stannous ion compatibility of <NUM> to <NUM>%.

The inventive silica exhibits in general a high compatibility with respect to cations which are customarily present in toothpaste compositions. Notable non limiting examples of said cations are for instance, calcium, strontium, barium, manganese, indium, nickel, zinc, titanium, zirconium, silver, palladium, ammonium or amino cations. These cations may be in the form of mineral salts, for example chloride, fluoride, nitrate, phosphate, sulfate or in the form of organic salts such as acetates, citrates.

Advantageously, the inventive silica has a compatibility with zinc, as determined using the Zn compatibility method described hereafter, of at least <NUM>%.

The inventive silica is also provided with good compatibility towards the fluoride ion. A wide variety of fluoride ion-yielding materials can be employed as sources of soluble fluoride in toothpaste compositions. Examples of suitable fluoride ion-yielding materials include, for instance, sodium fluoride, (NaF), stannous fluoride (SnF<NUM>), potassium fluoride, (KF), potassium stannous fluoride (SnF<NUM>-KF), indium fluoride (InF<NUM>), zinc fluoride (ZnF<NUM>), ammonium fluoride (NH<NUM>F), and stannous chlorofluoride (SnCIF).

The fluoride ion compatibility of the inventive silica, measured on NaF solutions, is typically greater than <NUM>%, preferably equal to or greater than <NUM>%.

The inventive silica is further characterised by an oil absorption, measured as bis(<NUM>-ethylhexyl)adipate (DOA) absorption, which is between <NUM> and <NUM>/<NUM>, typically between <NUM> and <NUM>/<NUM>.

Typically the inventive silica is characterised by a pH between <NUM> and <NUM> and preferably between <NUM> and <NUM>.

The inventive silica is characterised by a number of OH groups per surface area, expressed as number of OH/nm<NUM>, which is equal to or greater than <NUM>, even greater than <NUM>, preferably greater than <NUM>. The number of OH groups per surface area typically does not exceed <NUM> OH/nm<NUM>.

A second object of the present invention is a process for the preparation of a precipitated silica of the first object. Said process comprises the general steps of a precipitation reaction between a silicate and an acid whereby a silica suspension is obtained, followed by the separation and the drying of the suspension.

The choice of the acid and of the silicate used in the various steps of the process is made in a way well known in the art. The term "silicate" is used herein to refer to one or more than one silicate which can be added during the course of the inventive process. The silicate is typically selected from the group consisting of the alkali metal silicates. The silicate is advantageously selected from the group consisting of sodium and potassium silicate. The silicate may be in any known form, such as metasilicate or disilicate.

In the case where sodium silicate is used, the latter generally has an SiO<NUM>/Na<NUM>O weight ratio of from <NUM> to <NUM>, in particular from <NUM> to <NUM>, for example from <NUM> to <NUM>.

The silicate may have a concentration of from <NUM> wt% to <NUM> wt%, for example from <NUM> wt% to <NUM> wt%, in particular from <NUM> wt% to <NUM> wt%. The silicate concentration is expressed in terms of % by weight of SiO<NUM>.

The term "acid" is used herein to refer to one or more than one acid which can be added during the course of the inventive process. Any acid may be used in the process. Use is generally made of a mineral acid, such as sulfuric acid, nitric acid, phosphoric acid or hydrochloric acid, or of an organic acid, such as carboxylic acids, e.g. acetic acid, formic acid or carbonic acid.

The acid may be metered into the reaction medium in diluted or concentrated form. The same acid at different concentrations may be used in different stages of the process. Preferably the acid is sulfuric acid.

In a preferred embodiment of the process sulfuric acid and sodium silicate are used in all of the stages of the process. Preferably, the same sodium silicate, that is sodium silicate having the same concentration expressed as SiO<NUM>, is used in all of the stages of the process.

In step (i) of the process an aqueous silicate solution having a pH from <NUM> to <NUM> is provided in the reaction vessel. The starting solution is an aqueous solution, the term "aqueous" indicating that the solvent is water. Preferably, the starting solution has a pH from <NUM> to <NUM>.

The silicate concentration of the aqueous silicate solution provided in the reaction vessel in step (i) is less than <NUM>/L. The silicate concentration is typically less than <NUM>/L, preferably less than <NUM>/L, even less than <NUM>/L. The silicate concentration is at least <NUM>/L, preferably at least <NUM>/L. The silicate concentration may conveniently be between <NUM> and <NUM>/L, typically between <NUM> and <NUM>/L, for example between <NUM> and <NUM>/L.

The starting aqueous silicate solution may be obtained in different manners.

In a first embodiment the starting aqueous silicate solution is obtained by adding an acid to a sodium silicate solution so as to obtain a pH value from <NUM> to <NUM>.

Alternatively, the starting aqueous silicate solution may be obtained by simultaneously adding an acid and a silicate to water or an initial silicate solution in such a way that the desired pH and initial silicate concentration are achieved.

The addition of an acid in stage (ii) of the process leads to a drop in the pH of the reaction medium. The pH at the end of step (ii) is lower than the pH of the initial silicate solution. Addition of the acid is carried out until a value of the pH of the reaction medium between <NUM> and <NUM>, for example between <NUM> and <NUM>, is reached.

The temperature of the reaction medium during step (i) and (ii) of the process is typically between <NUM> and <NUM>, typically between <NUM> and <NUM>.

Once the desired pH value has been reached an ageing of the reaction medium is performed at the pH obtained at the end of stage (ii). This ageing step is generally carried out with stirring of the reaction medium. The duration of ageing step (iii) is from <NUM> to <NUM> minutes, in particular from <NUM> to <NUM> minutes, more preferably from <NUM> to <NUM> minutes.

Ageing step (iii) may be performed at the same temperature as steps (i) and (ii) or at a different temperature. Advantageously, step (iii) is performed at a temperature higher than the temperature of steps (i) and (ii), generally at a temperature between <NUM> and <NUM>.

At the end of step (iii) an acid is added to the reaction medium to lower the pH to a value of less than <NUM>, preferably between <NUM> and <NUM> and, even more preferably, between <NUM> and <NUM> (step (iv)). A suspension of precipitated silica is obtained at the end of step (iv).

A liquid/solid separation step is subsequently carried out on the suspension of precipitated silica. No intermediate ageing step is performed between steps (iv) and (v).

The separation step normally comprises a filtration, followed, if necessary, by a washing operation. The filtration is performed according to any suitable method, for example by means of a belt filter, a rotary filter, for example a vacuum filter, or, preferably a filter press. The washing is typically carried out with water and/or with an aqueous acidic solution having a pH of between <NUM> and <NUM>. Depending on the case, one or more washing steps may be carried out.

The filter cake may optionally be subjected to a liquefaction operation. The term "liquefaction" is intended herein to indicate a process wherein a solid, namely the filter cake, is converted into a fluid-like mass. The expressions "liquefaction step", "liquefaction operation" or "disintegration" are interchangeably intended to denote a process wherein the filter cake is transformed into a flowable suspension, which can then be easily dried. After the liquefaction step the filter cake is in a flowable, fluid-like form and the precipitated silica is again in suspension.

Preferably, in this preparation process, the suspension of precipitated silica obtained after the liquefaction step exhibits, immediately before it is dried, a solids content of at least <NUM> wt%, in particular of at least <NUM> wt% The solid content may be up to <NUM> wt%. Typically the solid content is between <NUM> and <NUM> wt%.

In an advantageous embodiment of the invention during the washing and/or during the liquefaction step of the process an aqueous solution containing salts of metallic cations is added to the precipitated silica. Notable non-limiting examples of suitable metallic cations are for instance selected from the group consisitng of the divalent and/or tetravalent cations of Ti, Sn, Zr, Mg, Ca, Sr, Zn, Ba. The cations are preferably selected from the group consisting of the divalent cations of Zn and Sn. Suitable salts for the preparation of the solutions are for instance sulfates. Typically the concentration of the metallic cation in the solution is at least <NUM> wt%, even at least <NUM> wt%. The cation concentration generally does not exceed <NUM> wt%.

The disintegrated filter cake is subsequently dried. The precipitated silica suspension obtained at the end of the liquefaction step is typically dried. Drying may be carried out using any means known in the art. Preferably, drying is carried out by spray drying. For this purpose, any suitable type of spray dryer may be used, especially a turbine spray dryer or a nozzle spray dryer (liquid-pressure or two-fluid nozzle). In general, when the filtration is carried out by means of a filter press, a nozzle spray dryer is used, and when the filtration is carried out by means of a vacuum filter, a turbine spray dryer is used.

When a nozzle spray dryer is used, the precipitated silica is usually in the form of approximately spherical beads.

After drying, a milling step may then be carried out on the recovered product. The precipitated silica that can then be obtained is generally in the form of a powder.

The invention also relates to oral care compositions, preferably toothpaste compositions, containing the inventive precipitated silica.

Examples of suitable oral care compositions are for instance those described in <CIT>, <CIT>, <CIT>.

Typically toothpaste compositions will contain the inventive silica in an amount between <NUM> and <NUM>% by weight, preferably between <NUM> and <NUM>% by weight. Toothpaste compositions comprising the inventive silica have a high compatibility to both Sn(II) and fluoride ions as well as a balanced abrasion level.

The invention will be now described in more detail with reference to the following examples whose purpose is merely illustrative and not limitative of the scope of the invention.

The physicochemical properties of the precipitated silica of the invention were determined using the methods described hereafter.

CTAB surface area (SCTAB) values were determined according to an internal method derived from standard NF ISO <NUM>-<NUM>, Appendix G.

BET surface area SBET was determined according to the Brunauer - Emmett - Teller method as detailed in standard NF ISO <NUM>-<NUM>, Appendix E (June <NUM>) with the following adjustments: the sample was pre-dried at <NUM>±<NUM>; the partial pressure used for the measurement P/P<NUM> was between <NUM> and <NUM>.

The pH is measured according to the following method deriving from the standard ISO <NUM>/<NUM> (pH of a <NUM>% suspension in water):.

Equipment: calibrated pH meter (accuracy of reading to <NUM>/100th), combined glass electrode, <NUM> beaker, <NUM> measuring cylinder, balance accurate to within about <NUM>.

Procedure: <NUM> grams of silica are weighed to within about <NUM> gram into the <NUM> beaker. <NUM> of water, measured from the graduated measuring cylinder, are subsequently added to the silica powder. The suspension thus obtained is vigorously stirred (magnetic stirring) for <NUM> minutes. The pH measurement is then carried out.

Abrasivity of silica was determined according to an internal method using poly(methyl methacrylate) (PMMA) plates as a substrate. Cast PMMA plates (Altuglas CN, Atoglas, Shore D hardness <NUM>-<NUM>) <NUM> x <NUM> x <NUM> were used as substrate. On each plate a <NUM> wide zone for brushing (Testing area) was defined using adhesive tape and then sumitted to brushing for <NUM> cycles using toothbrushes Brosserie Frangaise, held at <NUM>° angle and under a <NUM> load, in the presence of slurries of abrasive silica prepared according to ISO11609:<NUM> protocol. The abrasion depth (Hm, expressed in µm) at the end of the brushing cycles was measured across a <NUM> x <NUM> area including the Testing area by optical profilometry (Altimet Altisurf <NUM>) on rinsed plates. The area around the Testing area was used to define the baseline for the optical profilometry determination.

The compatibility of precipitated silica with Sn (II) ions was determined using a modification of the method disclosed in <CIT>.

The Sn(II) compatibility was calculated as the ratio of Sn(II) ions available in the solution obtained at the end of step (<NUM>) with respect to the theoretical value according to the following formula: <MAT>.

The amount of fluoride ions in solution was determined using a fluoride ion selective electrode (Perfection or equivalent). The amount of fluoride ions was determined by means of the software LabX. A calibration curve was done by measuring the potential of two standard solutions (<NUM> ppm and <NUM> ppm).

The determination of fluoride ion compatibility of the precipitated silica was determined as follows:.

The fluoride ion compatibility was calculated as the ratio of fluoride ions available in the solution after contact with silica (supernatant of step (<NUM>) with respect to the theoretical value according to the following formula: <MAT>.

Oil absorption (DOA number) was determined using a method based on ASTM D <NUM> for carbon black modified for precipitated silica. <NUM> (+/-<NUM>,<NUM>) of precipitated silica are added to the kneading chamber (Brabender Absorptometer "C") with help to the spatula. Bis(<NUM>-ethylhexyl) adipate (DOA, CAS [<NUM>-<NUM>-]) <NUM> (+/- <NUM>,<NUM>) is added dropwise with a dosing rate of <NUM>/min at room temperature into the mixture with continuous mixing (rotation rate of kneader blades <NUM> rpm). Only very little force is needed for the mixing incorporation process which is followed by using the digital display. Toward the end of the determination, the mixture becomes pasty, and this is indicated by means of a steep rise in the mixing force required. When the display reaches <NUM>, the kneader and the DOA metering are both switched off via an electrical contact. The synchronous motor for DOA input has coupling to a digital counter, and DOA consumption in mL can therefore be read off. The DOA absorption of a silica is determined as the amount of DOA where the torque is at <NUM>% of its maximum value. This value is determined and calculated by the software ABSORPTOMETER. DOA absorption is reported in g/(<NUM>).

The samples were analyzed using ATD-ATG technique on Mettler's LF1100 thermobalance and a Tensor <NUM> Bruker spectrometer equipped with a gas cell, with the following program: Temperature rise from <NUM> to <NUM> at <NUM>/min, under air (<NUM>/min), in Al<NUM>O<NUM> crucible of <NUM>µL. The silanol density is directly related to the loss of mass between <NUM> and <NUM>. The loss of mass (%) between <NUM> and <NUM> is identified as ΔW% this value.

The silanol ratio (mmol/g) is defined by: <MAT>.

Silanol density (OH/nm<NUM>) is calculated by : <MAT> wherein Na : Avogadro's number.

In a <NUM> stainless steel reactor were introduced: <NUM> of water and <NUM> of a sodium silicate solution (SiO<NUM>/Na<NUM>O ratio = <NUM>; SiO<NUM> concentration = <NUM> wt%, used in all the steps of the process). Initial pH of the solution was <NUM>.

The solution obtained was stirred and heated to reach <NUM>. After this first step, a <NUM> wt% sulfuric acid solution was added at a flow rate of <NUM>/min to reach a pH of <NUM> and a neutralization ratio of <NUM>%. The same sulfuric acid solution was used in all steps of the process.

The neutralization ratio is defined by the following relation:<MAT>.

Over a period of <NUM>, were simultaneously introduced sodium silicate, (flow rate: <NUM>/min), water (flow rate: <NUM>/min) and sulfuric acid solution to obtain a total neutralization ratio of <NUM>% and a pH value of <NUM>.

At the end of the first simultaneous addition, over a period of <NUM>, were introduced: sodium silicate (flow rate: <NUM>/min) and sulfuric acid to obtain a neutralization ratio of <NUM>% and a pH value <NUM>.

The pH of the reaction medium was then brought to a value of <NUM> with sulfuric acid (flow rate: <NUM>/min). An ageing step was carried out over a period of <NUM> at pH <NUM>. After <NUM>, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid (flow rate: <NUM>/min). A suspension of precipitated silica was obtained.

The suspension was filtered and washed on a drum filter. The moisture of the cake was more than <NUM> wt%. The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO<NUM> suspension at <NUM> wt% of silica. The pH was adjusted with sulfuric acid to reach value less than <NUM>.

The product was dried by atomization. The product was obtained in powder form with a moisture less than <NUM> wt%. The physicochemical properties of the product are reported in Table I.

The solution obtained was stirred and heated to reach <NUM>. After this first step, sulfuric acid solution (at a concentration of <NUM> wt%; the same solution was used in all steps of the process) was added at a flow rate of <NUM>/min to reach a pH of <NUM> and a neutralization ratio of <NUM>%.

Over a period of <NUM>, were simultaneously introduced: sodium silicate, (flow rate: <NUM>/min), water (flow rate: <NUM>/min) and sulfuric acid solution to obtain a neutralization ratio of <NUM>% and a pH of <NUM>.

At the end of the first simultaneous addition, over a period of <NUM>, were introduced sodium silicate (flow rate: <NUM>/min), and sulfuric acid to obtain a neutralization ratio of <NUM>% and a pH of <NUM>.

The pH of the reaction medium was brought to a value of <NUM> with sulfuric acid (flow rate: <NUM>/min). An ageing step was carried out over a period of <NUM> at a pH of <NUM>. After <NUM>, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid (flow rate: <NUM>/min).

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 <NUM> wt%. The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO<NUM> suspension at <NUM> wt% of silica. The pH was adjusted with sulfuric acid to reach a value of less than <NUM>.

The product was dried by atomization. The product obtained was in powder form with a moisture of less than <NUM> wt%. The physicochemical properties of the product are reported in Table I.

The solution obtained was stirred and heated to reach <NUM>. After this first step, a <NUM> wt% sulfuric acid solution (the same solution was used in all steps of the process) was added at a flow rate of <NUM>/min to reach a pH of <NUM> and a neutralization ratio of <NUM>%.

Over a period of <NUM>, were simultaneously introduced: sodium silicate (flow rate: <NUM>/min), water (flow rate: <NUM>/min) and sulfuric acid to obtain a total neutralization ratio equal to <NUM>% and a pH of <NUM>.

At the end of the first simultaneous addition, over a period of <NUM>, were introduced: sodium silicate (flow rate: <NUM>/min) and sulfuric acid to obtain a neutralization ratio of <NUM>% and a pH of <NUM>.

The pH of the reaction medium was brought to a value of <NUM> with sulfuric acid at a flow rate of <NUM>/min. At pH <NUM>, an ageing step was carried out over a period of <NUM>. After <NUM>, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid at a flow rate of <NUM>/min.

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 <NUM> wt%.

The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO<NUM> suspension at <NUM> wt% of silica. Then pH was adjusted with sulfuric acid to reach value less than <NUM>.

In a <NUM> stainless steel reactor were introduced: <NUM> of water and <NUM> of a sodium silicate solution (SiO<NUM>/Na<NUM>O ratio = <NUM>; SiO<NUM> concentration = <NUM> wt%). The same sodium silicate solution was used in all the steps of the process.

The obtained solution was stirred and heated to reach <NUM>. Once the set temperature was reached sulfuric acid (<NUM> wt% solution) was added (flow rate: <NUM>/min) until the reaction medium reached the pH value of <NUM>. The same sulfuric acid solution was used in all the steps of the process.

Simultaneously, over a period of <NUM>, were introduced: sodium silicate, at a flowrate of <NUM>/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 <NUM>.

At the end of the simultaneous addition, the pH of the reaction medium was brought to a value of <NUM> sulfuric acid. Simultaneously, the reaction medium was heated to <NUM>. The rest of the process was carried out at this temperature. A first ageing step was carried out at pH <NUM> over a period of <NUM>. After <NUM>, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid (flow rate: <NUM>/min). At pH <NUM>, a second ageing step was carried out over a period of <NUM> to obtain a suspension of precipitated silica.

The suspension of precipitated silica was filtered and washed on a filter plate. The moisture of the cake was more than <NUM> wt%.

The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO<NUM> suspension having <NUM> wt% of silica content.

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

The obtained solution was stirred and heated to reach <NUM>. At this temperature, a sulfuric acid (<NUM> wt% solution) was added at a flow rate of <NUM>/min until the reaction medium reached the pH value of <NUM>. The same sulfuric acid solution was used in all the steps of the process.

Simultaneously, over a period of <NUM>, were introduced: sodium silicate (flow rate: <NUM>/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 <NUM>.

At the end of the simultaneous addition, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid. Simultaneously, the reaction medium was heated to <NUM>. The rest of the process was carried out at this temperature. A first ageing step was performed at pH <NUM> over a period of <NUM>. After <NUM>, the pH of the reaction medium was brought to a value of <NUM> with sulfuric acid at a flow rate of <NUM>/min. At pH <NUM>, the reaction medium was submitted to a second ageing step over a period of <NUM> and 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 <NUM> wt%.

The filter cake obtained was disintegrated mechanically and water was added to obtain a SiO<NUM> suspension containing <NUM> wt% of silica.

The data in Table <NUM> show that the inventive silicas have a higher stannous ion compatibility with respect to silicas obtained with a process as disclosed in <CIT>.

The compatibility of a known precipitated silica for use in toothpaste compositions, Zeodent® <NUM> (commercially available from Huber) having a CTAB surface area SCTAB of <NUM><NUM>/g, to Sn(II) ions was found to be <NUM>%.

Claim 1:
A precipitated silica which is characterised by:
- a CTAB surface area SCTAB comprised between <NUM> and <NUM><NUM>/g,
- an abrasion value Hm between <NUM> and <NUM>; and
- a stannous ion compatibility of at least <NUM>%, wherein SCTAB , Hm and stannous ion compatibility are determined according to the methods disclosed in the description.