WO2021069256A1

WO2021069256A1 - Silica for oral care compositions

Info

Publication number: WO2021069256A1
Application number: PCT/EP2020/077218
Authority: WO
Inventors: Jules VALENTE; Cédric FERAL-MARTIN; Caroline FAYOLLE
Original assignee: Rhodia Operations
Priority date: 2019-10-07
Filing date: 2020-09-29
Publication date: 2021-04-15

Abstract

A precipitated silica is provided having good balance between thickening ability, low abrasion properties and high compatibility with stannous and zinc ions.

Description

SILICA FOR ORAL CARE COMPOSITIONS

This application claims priority from European patent application No. 19306296.5 filed on 7 October 2019, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

[0001] The present invention relates to a precipitated silica for use in oral care compositions. In particular the invention relates to a precipitated silica having a good compromise between thickening and abrasive properties as well as good compatibility with therapeutic agents such as stannous fluoride and zinc.

Background of invention

[0002] 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.

[0003] 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 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.

[0004] Toothpaste compositions commonly contain so-called thickening silica for controlling the rheology of the toothpaste composition. Silica is known to interact with other co-ingredients of the compositions such as fluorides and zinc compounds. Silica is 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.

[0005] EP396460A1 discloses precipitated silica, suitable either as abrasive or as thickening agent, having good compatibility with metallic cations, in particular Zn, as well as with the fluoride ion. However thickening silicas prepared according to EP396460A1 do not possess an optimal balance of properties like thickening ability and low abrasion on one end and good compatibility to ions such as zinc and tin.

[0006] To improve the compatibility to stannous ions it has also been suggested to reduce the small pore porosity in the precipitated silica, see for instance WO201 8/114280. Reducing the porosity has however the consequence that the abrasivity of the silica is increased and its ability to control the rheology of the toothpaste is limited.

[0007] Objective of the present invention is to provide a silica that has improved compatibility with the active ingredients of the toothpaste composition and that is, at the same time, an effective rheology control agent for the composition.

Summary of invention

[0008] Object of the invention is a precipitated silica which is characterised by: a CTAB surface area SCTAB of at least 80 m²/g; a volume of the pores having a diameter of 1 pm or less of at least 1.90 ml_/g; and a stannous ion compatibility equal to or greater than 40%. [0009] 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.

[0010] The CTAB surface area SCTAB is at least 85 m²/g, typically at least 90 m²/g.

[0011] The CTAB surface area does not exceed 190 m²/g. The CTAB surface area SCTAB may be lower than 185 m²/g, preferably lower than 180 m²/g.

[0012] For applications as thickener in toothpaste formulations, advantageous ranges of CTAB surface area SCTAB are from 85 to 180 m²/g, preferably from 90 to 170 m²/g.

[0013] The BET surface area SBET of the inventive silica is not particularly limited, but it is at least 85 m²/g, typically at least 90 m²/g. BET surface area SBET is generally at most 200 m²/g. The BET surface area may advantageously be from 85 to 190 m²/g, even from 90 to 185 m²/g, preferably from 95 to 180 m²/g.

[0014] The volume generated by the pores of the inventive silica is measured by mercury porosimetry as described in detail hereafter. The volume of the pores having a diameter of 1 pm or less, hereinafter referred to as volume “Vi “, is at least 1.95 ml_/g, even at least 2.00 mL/g. Volume Vi is at most

4.50 mL/g, preferably at most 4.00 mL/g, and even at most 3.50 mL/g. Volume V 1 may conveniently be from 1.90 to 4.00 mL/g, even from 1.95 to

3.50 mL/g.

[0015] The inventive silica is advantageously characterised by a compatibility with stannous ions, as determined using the stannous ion compatibility method described hereafter, equal to or greater than 40%. Stannous ions are generally Sn(ll) ions deriving from SnF2_.

[0016] The stannous ion compatibility may advantageously be 100%, even up to 80%.

[0017] In an advantageous embodiment, the inventive silica has a stannous ion compatibility greater than 40% and up to 80%, even greater than 40% and up to 75%.

[0018] Advantageously, the inventive silica has a compatibility with zinc, as determined using the Zn compatibility method described hereafter, greater than 70%, even at least 75%. [0019] The inventive silica exhibits in general a high compatibility with respect to other cations which are customarily present in toothpaste compositions. Notable non limiting examples of said cations are for instance, calcium, strontium, barium, manganese, indium, nickel, 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.

[0020] 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 (SnF2), potassium fluoride, (KF), potassium stannous fluoride (SnF2-KF), indium fluoride (lnF3), zinc fluoride (ZnF2), ammonium fluoride (NFUF), and stannous chlorofluoride (SnCIF).

[0021] The fluoride ion compatibility of the inventive silica, measured on NaF solutions, is typically greater than 80%, preferably equal to or greater than 90%.

[0022] The inventive silica is characterised by a number of OFI groups per surface area, expressed as mmol of OFI/g of silica, which is less than 3.2, even less than 3.0 mmol of OFI/g of silica, preferably less than 2.8 mmol of OFI/g of silica. The number of OFI groups per surface area typically is not less than 1.0 mmol of OFI/g of silica.

[0023] The inventive silica is further characterised by an oil absorption, measured as bis(2-ethylhexyl)adipate (DOA) absorption, which is between 200 and 400 mL/100 g, typically between 220 and 380 mL/100 g, preferably between 240 and 350 ml_/100g, even between 250 and 350 ml_/100g.

[0024] The inventive silica is characterised by a high thickening capacity when tested in standard toothpaste applications. Toothpaste Brookfield viscosity measured after 50 days at 20°C is typically above lOOPa.s.

[0025] The precipitated silica of the invention does not possess abrasive properties. The inventive silica is characterised by an abrasion depth value, Flm, as determined using the PMMA abrasion test described hereafter, of less than 3.5 miti, preferably less than 3.0 pm. The abrasion depth value, Hm, is typically from 0.1 to less than 3.5 pm.

[0026] A second object of the present invention is a process for the preparation of a precipitated silica having high stannous ion compatibility and good rheology control abilities. 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 of a wet precipitated silica and the drying of the wet precipitated silica.

[0027] The process comprises the steps of:

(i) providing an aqueous silicate solution in a vessel, the concentration of silicate, expressed as S1O2, in said vessel being less than 15 g/L and the pH of the aqueous suspension being comprised between 8.5 and 10.0;

(ii) adding an acid to said aqueous silicate suspension to obtain a pH value for the reaction medium between 6.0 and 9.0;

(iii) simultaneously adding a silicate and an acid such that the pH of the reaction medium is maintained in the range from 6.0 to 9.0,

(iv) continuing the simultaneous addition of the acid and of the silicate to the reaction medium in such a way that the pH of the reaction medium is lowered to a value of less than 7.0, preferably to a value between 5.0 and 7.0;

(v) once the pH value is achieved, simultaneously adding a silicate and an acid to the reaction medium such that the pH is maintained in the range from 5.0 to 7.0, and

(vi) stopping the addition of silicate and adding an acid to the reaction medium to lower the pH to a value of less than 5.0, preferably between 3.0 and 5.0 to obtain a silica suspension.

[0028] The silica suspension obtained at the end of step (vi) is generally submitted to a liquid/solid separation step to provide a filter cake. The wet precipitated silica thus obtained is subsequently dried.

[0029] 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.

[0030] In the case where sodium silicate is used, the latter generally has an

Si0₂/Na₂0 weight ratio of from 2.0 to 4.0, in particular from 2.4 to 3.9, for example from 3.1 to 3.8.

[0031] The silicate may have a concentration of from 3.9 wt% to 25.0 wt%, for example from 5.6 wt% to 23.0 wt%, in particular from 5.6 wt% to 20.7 wt%. The silicate concentration is expressed in terms of % by weight of S1O2.

[0032] 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.

[0033] 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.

[0034] 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 S1O2, is used in all of the stages of the process.

[0035] In step (i) of the process an aqueous silicate solution having a pH from 8.0 to 10.0 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 8.5 to 10.0. The pH is measured at the temperature of the reaction.

[0036] The concentration of the aqueous silicate solution provided in the reaction vessel in step (i) is less than 15 g/L. The silicate concentration is typically less than 12 g/L, preferably less than 10 g/L. The silicate concentration is at least 1 g/L. [0037] 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 8.0 to 10.0.

[0038] 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.

[0039] 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 6.0 and 9.0, for example between 7.0 and 8.5, even between 7.0 and 8.0, is reached.

[0040] The temperature of the reaction medium during step (i) and (ii) of the process is typically between 70 and 97°C, typically between 80 and 95°C.

[0041] Once the desired pH value has been reached a simultaneous addition of acid and silicate is made to the reaction medium.

[0042] The rates of addition of the acid and of the silicate during step (iii) are controlled in such a way that the pH of the reaction medium is maintained in the range from 6.0 to 9.0. The pH of the reaction medium is preferably maintained in the range from 7.0 to 8.5, more preferably from 7.0 to 8.0.

[0043] The simultaneous addition in step (iii) is advantageously performed in such a manner that the pH value of the reaction medium is always equal (to within ± 0.2 pH units) to the pH reached at the end of step (ii).

[0044] At the end of step (iii) the simultaneous addition of acid and silicate is continued, however the flow rates of either the silicate or the acid are modified in such a way that the pH of the reaction medium is lowered to a value of less than 7.0, preferably between 5.5 and 7.0 and, even more preferably, between 6.0 and 7.0 (step (iv)).

[0045] In a first, preferred, embodiment of the process, the flow rate of the acid is increased with respect to the flow rate in step (iii) while the flow rate of the silicate is maintained as during step (iii). Alternatively, the flow rate of the silicate can be decreased while maintaining the flow rate of the acid constant.

[0046] The pH at the end of step (iv) is lower than the pH of the reaction medium in step (iii).

[0047] Once the pH has reached the desired value, at the end of step (iv), a further simultaneous addition of acid and silicate is made to the reaction medium in step (v). The rates of addition of the acid and of the silicate during step (v) are controlled in such a way that the pH of the reaction medium is maintained in the range from 5.0 to 7.0. The pH of the reaction medium is preferably maintained in the range from 5.5 and 7.0 and, more preferably, between 6.0 and 7.0. The value of the pH in step (v) is the same as the value at the end of step (iv).

[0048] At the end of step (v) an acid is added to the reaction medium to lower the pH to a value of less than 5.0, preferably between 3.0 and 5.0 (step (vi)) to obtain a suspension of precipitated silica.

[0049] A maturing step may optionally be performed between step (v) and step (vi). The maturing step typically lasts 1 to 120 minutes, preferably 1 to 60 minutes.

[0050] A liquid/solid separation step is subsequently carried out on the suspension of precipitated silica.

[0051] 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 2.0 and 7.0. Depending on the case, one or more washing steps may be carried out.

[0052] 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.

[0053] 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 0.1 wt%, even at least 0.5 wt%. The cation concentration generally does not exceed 2.0 wt%.

[0054] 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.

[0055] When a nozzle spray dryer is used, the precipitated silica is usually in the form of approximately spherical beads.

[0056] 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.

[0057] The invention also relates to oral care compositions, preferably toothpaste compositions, containing the inventive precipitated silica.

[0058] Examples of suitable oral care compositions are for instance those described in US5578293, US5004597, US5225177.

[0059] Typically toothpaste compositions will contain the inventive silica in an amount from 0.1 to 40 wt%, preferably from 1 to 20 wt%, more preferably from 5 to 15 wt%. [0060] Toothpaste compositions comprising the inventive silica have a good balance between good compatibility to cations, notably Sn(ll) and Zn(ll), to fluoride ions as well as high thickening capability and low abrasion.

[0061] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0062] 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.

[0063] ANALYTICAL METHODS

[0064] The physicochemical properties of the precipitated silica of the invention were determined using the methods described hereafter.

[0065] Determination of CTAB surface area

[0066] CTAB surface area (SCTAB) values were determined according to an internal method derived from standard NF ISO 5794-1 , Appendix G.

[0067] Determination of BET surface area

[0068] BET surface area SBET was determined according to the Brunauer -

Emmett - Teller method as detailed in standard NF ISO 5794-1 , Appendix E (June 2010) with the following adjustments: the sample was pre-dried at 200°C±10°C; the partial pressure used for the measurement P/P° was between 0.05 and 0.3.

[0069] Determination of abrasion depth Hm

[0070] 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 60-70) 89 x 20 x 7.5 mm were used as substrate. On each plate a 3 mm wide zone for brushing (Testing area) was defined using adhesive tape and then submitted to brushing for 10000 cycles using toothbrushes Brasserie Frangaise, held at 15° angle and under a 240 g load, in the presence of slurries of abrasive silica prepared according to IS011609:2010 protocol.

[0071] The abrasion depth (Hm, expressed in pm) at the end of the brushing cycles was measured across a 20 x 10 mm area including the Testing area by optical profilometry (Altimet Altisurf 500) on rinsed plates. The area around the Testing area was used to define the baseline for the optical profilometry determination.

[0072] Thickening capability determination

[0073] Thickening capability of silicas was determined by measuring Brookfield viscosity of toothpaste formulations at 20°C after 50 days +/-1day ageing at 40°C in a climatic chamber, using Brookfield DVII+PRO rheometer and a spindle at 5 rpm.

[0074] Toothpaste formulations described in Table 1 were prepared in a GUEDU reactor (4.5 L) according to the protocol detailed Table 1 below. Silicas from the inventions were used in the formulation in an amount of 8 wt%. Pre-gel was prepared in a RAYNERI mixer, adding ingredients at 1000- 1500 rpm, the day before mixing on the GUEDU reactor.

[0075] Inorganic fillers and other additives were incorporated when mixing was stopped and before mixing under vacuum (Pressure p=100mbar). A final mixing under vacuum was performed after surfactant addition (p<50mbar) during at least 4 minutes.

Table 1

[0076] Determination of Sn(ll) ion compatibility

[0077] The compatibility of precipitated silica with Sn (II) ions was determined using a modification of the method disclosed in EP2349488B1.

[0078] The method was performed as follows:

[0079] (1) a solution in sodium gluconate was prepared containing 0.45 wt % of SnF2 and from 0.6 to 1.0 wt %of sodium gluconate in a solvent composed of a mix of water and sorbitol (the amount of sorbitol in at most 70%). The amount of sorbitol and sodium gluconate were adjusted to prevent precipitation of stannous hydroxide.

[0080] (2) 2 g of silica were dispersed in 200 g of the solution obtained in (1). The pH was adjusted to 6.2-6.3 by addition of NaOH 0.1 N. The suspension thus obtained was stirred for 5 hours at 50°C under nitrogen bubbling to avoid any oxidation.

[0081] (3) The suspension was filtered through a 0,45 pm PVDF filter.

[0082] (4) The concentration of Sn(ll) was measured in the solution prepared in (1) and in the supernatant obtained in (3) using inductively coupled plasma (ICP-OES) using a PlasmaQuant PQ9000 Elite instrument.

[0083] The Sn(ll) compatibility was calculated as the ratio of Sn(ll) ions available in the solution obtained at the end of step (3) with respect to the theoretical value according to the following formula:

Sn(II)in supernatant (3)

Sn(ll) compatibility (%)

Sn(II)in solution (1)

[0084] Determination of Zn(ll) ion compatibility

[0085] The compatibility of precipitated silica with Zn(ll) ions was determined using a modification of the method disclosed in EP2349488B1.

[0086] The method was performed as follows:

[0087] (1) a solution of ZnSC>₄.7 H2O at 0,06%wt was prepared by dissolution of the solid into water [0088] (2) 4 g of silica were dispersed in 100 mL of the solution obtained in (1). The suspension thus obtained was stirred for 24 hours at 40°C.

[0089] (3) The suspension was filtered through a 0.45 pm PVDF filter.

[0090] (4) The concentration of Zn(ll) was measured in the solution prepared in

(1) and in the supernatant obtained in (3) using inductively coupled plasma (ICP-OES) using a PlasmaQuant PQ9000 Elite instrument.

[0091] The Zn(ll) compatibility was calculated as the ratio of Zn(ll) ions available in the solution obtained at the end of step (3) with respect to the theoretical value according to the following formula:

Zn(II) in supernatant (3)

Zn(ll) compatibility (%) = x 100 Zn(II) in solution (1)

[0092] Determination of fluoride ion compatibility

[0093] 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 (190 ppm and 1900 ppm).

[0094] The determination of fluoride ion compatibility of the precipitated silica was determined as follows:

[0095] (1) an initial solution containing NaF was prepared in a polypropylene vial of 1000 mL by adding 19.3 g (± 0.1) of NaFhPC^, 51.1 g (± 0.1) of Na2HP04 and 90 mL of a solution of NaF at 40 g/L and then bringing the volume of the solution to 1000 mL with distilled water. The initial solution has a fluoride ion concentration of 1628 ppm and 0.5 M phosphate buffer.

[0096] (2) Preparation of the sample solution: 26 g (± 0.05) TISAB-F- were added to 10 mL of initial solution prepared in (1).

[0097] (3) Testing of the sample of silica: 7 g (± 0.05) of silica were added to 30 g (± 0.05) of initial solution prepared in (1) and stirred for 1 hour at 60°C.

The silica was separated from the solution by centrifugation at 10,000 rpm for 15 minutes. The fluoride ion concentration in the supernatant solution was then determined. [0098] (4) 26 g (± 0.05) TISAB-f - were added to 10 mL of supernatant solution obtained in step 3 and fluoride ion measured at room temperature.

[0099] The fluoride ion compatibility was calculated as the ratio of fluoride ions available in the solution after contact with silica (supernatant of step (3)) with respect to the theoretical value according to the following formula:

F — in supernatant (3)

F — compatibility (%) = x 100 F — in solution (1)

[00100] Oil absorption determination

[00101] Oil absorption (DOA number) was determined using a method based on ASTM D 2414 for carbon black modified for precipitated silica. 12.5 g (+/- 0,1 g) of precipitated silica are added to the kneading chamber (Brabender Absorptometer “C”) with help to the spatula. Bis(2-ethylhexyl) adipate (DOA, CAS [103-23-]) 12.5 g (+/- 0.1 g) is added dropwise with a dosing rate of 4 mL/min at room temperature into the mixture with continuous mixing (rotation rate of kneader blades 125 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. The measurement is stopped 100 seconds after the force reaches a maximum (this maximum has to be superior to 100 mNm), and the kneader and the DOA metering are both switched off via an electrical contact. The experimental recording of the force was fitted with a polynomial curve and the DOA value was taken as the first experimental point corresponding to 70% of the maximum value reached by the polynomial curve. This value is determined and calculated by the software ABSORPTOMETER.

[00102] The DOA absorption capacity (in ml_/100g) of the silica is defined as : 100

[00103] Where Vzo_% is the added volume of DOA when the torque value reaches 70% of the maximum value of the fitted polynomial curve and m_Siiica is the introduced mass of silica (typically, 12.50 g).

[00104] Silanol density determination [00105] The samples were analyzed using ATD-ATG technique on Meiiler's

LF1100 thermobalance and a Tensor 27 Bruker spectrometer equipped with a gas cell, with the following program: Temperature rise from 25°C to 1100°C at 10°C/min, under air (60 mL/min), in AI2O3 crucible of 150 pL The silanol density is directly related to the loss of mass between 200°C and 800°C. The loss of mass (%) between 200°C and 800°C is identified as AW% this value.

[00106] The silanol ratio (mmol/g) is defined by:

TsiO_H = AW^*2^*1000 / (18.015^*100) = 1.11 ^*AW

[00107] Example 1

[00108] In a 25 L stainless steel reactor were introduced: 14.5 L of purified water and 0.660 kg of a sodium silicate solution (Si02/Na20 ratio = 3.46; S1O2 concentration = 19.5 wt%, used in all the steps of the process). The obtained solution was stirred at 390 rpm and heated to reach 95°C. A 7.7 wt% sulfuric acid solution at a flow rate of 57.7 g/min was then introduced in the reactor to reach a pH of 7.5.

[00109] Simultaneously, over a period of 40 min, were introduced in the reactor: sodium silicate, at a flow rate of 50.2 g/min, and a 7.7 wt% sulfuric acid solution. The flow rate of the 7.7 wt% sulfuric acid solution was regulated so that the pH of the reaction medium was maintained at a value of 7.5.

[00110] The flow rate of the 7.7 wt% sulfuric acid was then increased to reduce the pH to 6.5. The sodium silicate flow rate was kept constant at 50.2 g/min during the transition.

[00111] Over a period of 57 minutes, sodium silicate flow rate was kept constant at

50.2 g/min while the 7.7 wt% sulfuric acid solution flow rate was regulated to maintain the pH of the reaction medium at a value of 6.5.

[00112] At the end of the simultaneous addition, the pH of the reaction medium was brought to a value of 3.4 with 7.7 wt% sulfuric acid at a flow rate of

57.2 g/min. The reaction mixture was allowed to stand for 5 minutes. A suspension of precipitated silica was obtained

[00113] The suspension was filtered and washed on a filter press. The filter cake thus obtained was disintegrated mechanically while adding deionized water in order to reach a silica concentration of 15 wt% in the mixture. The pH of the resulting silica suspension was brought to 6.2 by the addition of 7.7 wt% sulfuric acid solution.

[00114] The resulting suspension was dried by means of a nozzle spray dryer to obtain precipitated silica S1. The properties of precipitated silica S1 are reported in Table 2.

[00115] Example 2

[00116] In a 25 L stainless steel reactor were introduced: 14.5 L of purified water and 0.660 kg of a sodium silicate solution (Si02/Na20 ratio = 3.46; S1O2 concentration = 19.5 wt%, used in all the steps of the process). The obtained solution was stirred at 390 rpm and heated to reach 95°C. A 7.7 wt% sulfuric acid solution at a flow rate of 57.3 g/min was then introduced in the reactor to reach a pH of 7.5.

[00117] Simultaneously, over a period of 40 min, were introduced in the reactor: sodium silicate, at a flow rate of 50.2 g/min, and a 7.7 wt% sulfuric acid solution. The flow rate of the 7.7 wt% sulfuric acid solution was regulated so that the pH of the reaction medium was maintained at a value of 7.5.

[00118] The flow rate of the 7.7 wt% sulfuric acid was then increased to reduce the pH to 6.5. The sodium silicate flow rate was kept constant at 50.2 g/min during the transition.

[00119] Over a period of 57 minutes, sodium silicate flow rate was kept constant at 50.2 g/min while the 7.7 wt% sulfuric acid solution flow rate was regulated to maintain the pH of the reaction medium at a value of 6.5.

[00120] At the end of the simultaneous addition, the suspension was aged for 60 minutes at the constant pH of 6.5.

[00121] At the end of the maturing step, the pH of the reaction medium was brought to a value of 3.4 with 7.7 wt% sulfuric acid at a flowrate of 54.7 g/min. The reaction mixture was allowed to stand for 5 minutes. A suspension of precipitated silica was obtained.

[00122] The suspension was filtered and washed on a filter press. The filter cake thus obtained was disintegrated mechanically while adding deionized water in order to reach a silica concentration of 15 wt% in the mixture. The pH of the resulting silica suspension was brought to 6.2 by the addition of 7.7 wt% sulfuric acid solution. [00123] The resulting suspension was dried by means of a nozzle spray dryer to obtain precipitated silica S2. The properties of precipitated silica S2 are reported in Table 2.

[00124] Comparative examples

[00125] Silica CS1 is Tixosil® 43, commercially available from Solvay.

[00126] Silica CS2 was prepared by following the procedure of Example 2 in EP396460A1 adapted to fit a 170L stainless steel reactor.

Table 2

[00127] The results in Table 2 show that silica S1 and S2 of the invention have an overall better compatibility to Sn(ll) and/or Zn(ll) cations than silica CS1 and CS2 of the prior art.

[00128] Silica S1 , CS1 and CS2 were milled to achieve a median particle size between 6 and 12 pm. Their thickening abilities were evaluated in toothpaste formulation following the protocol described above. The results are presented in Table 3 below.

Table 3

[00129] Silicas S2 and CS2 were milled to achieve a median particle size between 6 and 12 pm. Their abrasive properties were evaluated and are presented in Table 4 below. Table 4

[00130] Silica S2, in addition to a high compatibility with both Sn(ll) and Zn(ll) cations has higher thickening ability combined with low abrasive characteristics than silica CS2, which is an advantage for silicas used for thickening purposes.

[00131] The inventive silicas present a higher thickening ability and an enhanced compatibility to actives (Zn and Sn) with respect to both silica CS1 and CS2, as well as very low abrasion ability.

Claims

1. A precipitated silica which is characterised by:

- a CTAB surface area SCTAB of at least 80 m²/g;

- a volume of the pores having a diameter of 1 pm or less (Vi) of at least 1.90 ml_/g; and

- a stannous ion compatibility equal to or greater than 40%.

2. Precipitated silica according to claim 1 which has a Zn ion compatibility greater than 70%.

3. Precipitated silica according to claim 1 or 2 which has an abrasion value Hm lower than 3.5 pm.

4. Precipitated silica according to anyone of the preceding claims which has less than 3.2 mmol of OH /g of silica.

5. Precipitated silica according to anyone of the preceding claims characterised by:

- a CTAB surface area SCTAB comprised between 80 and 190 m²/g;

- a volume of the pores having a diameter of 1 pm or less (Vi) froml .90 to 4.00 ml_/g;

- a Zn ion compatibility of at least 75%

- from 1.0 to 3.2 mmol of OH /g of silica.

6. Precipitated silica according to anyone of the preceding claims wherein the stannous ion compatibility is from 40 to 80%.

7. Precipitated silica according to anyone of the preceding claims wherein the CTAB surface area SCTAB is from 90 to 170 m²/g.

8. Precipitated silica according to anyone of the preceding claims wherein the (DOA) oil absorption is from 220 to 400 mL/100 g.

9. A process for the preparation of a precipitated silica of anyone of claims 1 to 8 which comprises the steps of:

(ii) adding an acid to said aqueous silicate suspension to obtain a pH value for the reaction medium between 6.0 and 9.0; (iii) simultaneously adding a silicate and an acid such that the pH of the reaction medium is maintained in the range from 6.0 to 9.0,

10. The process of claim 9 wherein the silicate concentration in the aqueous silicate solution in step (i) is between 1 and 15 g/L.

11. The process of anyone of claims 9 and 10 wherein the pH in steps (ii) and (iii) is between 7.0 and 8.0.

12. The process of anyone of claims 9 to 11 wherein step (v) is performed at a pH between 5.0 and 7.0.

13. An oral care composition comprising a precipitated silica of anyone of claims 1 to 8.

14. A toothpaste composition comprising the precipitated silica of anyone of claims 1 to 8 and a therapeutically active amount of stannous fluoride.