NO901884L - SILICA, SPECIAL FOR USE IN DENTAL CARE, PROCEDURE FOR ITS MANUFACTURING, AND DENTAL CARE. - Google Patents

Abstract

Silica compatible with metal ions such as zinc, tin or strontium, characterised by a surface OH<-> number expressed as OH<->/nm<2> lower than or equal to 10, a zero charge point (ZCP) of between 3 and 6.5 and resulting in an aqueous suspension whose pH varies as a function of its electrical conductivity according to the equation - pH = b - a log (D), in which a is a constant lower than or equal to 0.6, b is a constant lower than or equal to 8.5 and (D) represents the electrical conductivity of the aqueous suspension of silica, expressed in microsiemens cm<-><1>. Process for preparing the silica, consisting in reacting a silicate with an acid, thus resulting in a silica suspension or gel, in performing a first aging at a pH higher than or equal to 6 and lower than or equal to 8.5, then a second aging at a pH lower than or equal to 5.0, in separating off the silica, in subjecting it to washing with hot water until it produces an aqueous suspension whose pH measured on a suspension containing 20 % of SiO2 obeys the equation - pH = d - c log (D> in which c is a constant lower than or equal to 1.0, d is a constant lower than or equal to 8.5 and (D) denotes the electrical conductivity of the aqueous suspension of silica, expressed in microsiemens cm<-><1>; and finally in drying it. Dentifrice composition containing the silica described.

Description

The present invention relates to a silica which is particularly useful in dentifrice mixtures and also relates to the method for its production as well as dentifrice mixtures containing this silica.

It is known that silica is often used in toothpaste mixtures and then plays several roles.

It primarily acts as an abrasive and promotes the removal of dental plaque through its mechanical action.

It can also play a role as a thickening agent to impart certain rheological properties to the dental care agent as well as an optical agent to give it the desired colour.

It is also known that dental care products usually contain a fluoride source, most often sodium fluoride or monofluorophosphate used as an anti-caries agent, and a binding agent such as e.g. colloids such as carrageenan, guar gum or xanthan gum, a wetting agent which can be a polyalcohol such as e.g. glycerol, sorbitol, xylitol or propylene glycol. One also finds arbitrary components such as e.g. surface-active agents, agents to reduce dental plaque or tartar deposits, taste correcting agents, coloring agents and pigments, etc.

A certain number of metal cations can be included in dental care product mixtures. Examples include alkaline earth metal cations, especially calcium, strontium, barium, cations of group 3a, aluminium, indium, from group 4a, germanium, tin, lead and group 8, manganese, iron, nickel, zinc, titanium, zirconium, palladium , etc.

The mentioned cations can be in the form of inorganic salts such as e.g. chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts such as acetate, citrate, etc.

As more specific examples, zinc citrate, zinc sulphate, strontium chloride, stannous fluoride in the form of a simple salt can be mentioned

(S11F4) or the double salt (SnF4, KF), stannous chloride SnCIF, zinc fluoride, (SnF2).

The presence of agents containing these metal cations poses the problem of their compatibility with silica. In particular, due to its absorbent capacity, it may actually tend to react with these agents so that they are no longer available to perform the function assigned to them.

In French patent application 87/15276, silica is described which has the property that it is compatible with zinc. However, these described silica types do not have sufficient compatibility with other metal cations such as tin, strontium, etc.

The purpose of the present invention is to provide new silica which is compatible with zinc and other of the above-mentioned metal cations.

A further object of the invention is to provide a silica which is also compatible with the fluoride anion. The improvement of the compatibility with these cations actually decreases the compatibility with the fluoride anion. Accordingly, it is important that the proposed silica remains compatible with the anionic fluoride present in all dentifrice compositions.

Finally, a further object of the invention is a method for the production of such colloidal silica types.

For this reason, it is recognized that the compatibility properties sought depend mainly on the surface chemistry of the silica used. In this connection, a number of conditions have been established for the silica surfaces in order for them to be compatible.

For this purpose, silica in accordance with the invention is particularly useful in dentifrice mixtures, characterized in that it has a surface chemistry such that the number of 0H~ expressed as OH_/nm<2> is at most 10, its zero point charge (PZC) is between 3 and 6.5 and that it leads to an aqueous suspension whose pH varies as a function of its electrical conductivity according to the following equation (I):

- pH = b - a log (D) (I)

in which equation (I) :

. a is a constant at most 0.6

. b is a constant at most 8.5

. (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm

A further property of silica in accordance with the invention is that it has a compatibility with at least one bivalent or higher valent metal cation selected from group 2a, 3a, 4a and 8 in the periodic table of at least 30%, more particularly at least 50% and preferably at least 80%.

The present invention also relates to a method for producing silica in accordance with the invention, characterized in that a silicate is reacted with an acid which leads to a suspension or a gel of silica, an initial maturation is carried out at a pH of at least 6 and at most 8.5, and then another ripening at a pH of no more than 5.0, the silica separates and is subjected to a washing with hot water until an aqueous suspension is obtained where the pH measured in a suspension with 20% SiC>2 corresponds to the following equation:

- pH = d - c log (D) (II)

where equation (II) :

. c is a constant at most 1.0

. d is a constant at most 8.5

. (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm<->''',

after which the suspension is dried.

Finally, the invention relates to dental care agent mixtures characterized by the fact that they contain silica as described above or produced using the method mentioned above.

Other properties and advantages of the invention will become better apparent from the teaching in the subsequent description and execution examples.

As indicated at the outset, the essential properties of silicas according to the invention are due to their surface chemistry. More precisely, one of the aspects taken into account in this surface chemistry is acidity. For this, one of the properties of silica in accordance with the invention is the number of acid sites on the surface.

This number is measured as the number of groups 0H~ or silanol groups per nm<2>. Practically, the measurement is carried out as follows.

The determination of this number is carried out in the following way: The number of 0H~ sites on the surface corresponds approximately to the amount of water released from silica between 190 and 900°C.

Sample pieces of silica are previously dried at 105°C for 2 hours.

A mass Pq of silica is placed in a thermobalance and kept at 190°C for 2 hours. P^gg is & a ^en achieved mass.

Silica is then brought to 900°C for 2 hours so that Pggg is the new mass obtained.

The number of OH~ seats is calculated using the following equation:

in which:

- NQH<_>is the number of OH~ sites per nm<2>surface

- A is the specific surface area of the solid sofa measured by BET and expressed in m<2>/g.

In the present case, silica in accordance with the invention advantageously has a number of 0H~/nm<2> of at most 10 and in particular between 4 and 10.

The nature of the OH~ sites in silica in accordance with the invention and which also constitutes a characterization of their surface chemistry can also be judged from the O charge point.

This point of 0 charge (PZC) is defined as the pH in a suspension of silica for which the electric charge on the surface of the solid is 0 regardless of the ionic strength in the environment. This PZC measures the real pH on the surface, to the extent that it is free of all ionic contaminants.

The electric charge is determined by potentiometry. The principle of this method is based on the total equilibrium between protons adsorbed or desorbed on the surface of silica at a given pH. From equations describing the overall balance of the operation, it is easy to show that the electric charge C of the surface, taken in relation to a reference corresponding to a surface charge 0, is given by the equation:

in which:

- A represents the specific surface of the solid in m2 /g

- Add the amount of solids in the suspension in g,

- F is Faraday's constant,

- (H<+>) or (OH~) represents the variation per surface unit of the excess of ions H+ or OH~ respectively on the solid.

The test protocol for determining PZC is as follows:

The method described by Berube and Bruyn (J. Colloid Interface Sc. 1968, 27, 305) is used.

Silica is washed in advance in deionized water with a high resistance (10 mega.ohm.cm) and which has been dried and degassed.

Practically, a number of solutions to pHo = 8.5 are prepared by adding KOH or HNO3 and containing an indifferent electrolyte (KNO3) with a variable concentration between 10~5 and 10_1 mol/l.

A given mass of silica is added to these solutions and the pH in the obtained suspensions is allowed to stabilize with stirring at 25°C and under nitrogen for 24 hours and this gives the pH'o its value.

The sample solutions are made up of the supernatant obtained by centrifugation for 30 minutes at 1000 revolutions/min. of a part of the same suspensions and pH'o is then the pH in these supernatants.

The pH is then changed in a known volume in these suspensions and corresponding sample solutions to pHo by adding the required amount of KOH and the suspensions and sample solutions are allowed to stabilize for 4 hours.

Thereby, VQH<_>•<N>QH<->number of base equivalents added to determine pH'o from the pHo of a known volume (V) of the suspension or sample solution.

The potentiometric determination of suspensions or sample solutions is carried out from pHo by adding nitric acid to pHf = 2.0.

It is preferred to proceed by stepwise addition of acid corresponding to a variation of pH of 0.2 pH units. After each addition, the pH is stabilized for one minute.

Thus VH+• NH+ becomes the number of acid equivalents to arrive at pHf.

From pHo the condition (V„, • N„, - • N_„ ) is derived as

ri+ ri+ Url— Uri—

function of pH steps for all suspensions (at least 3 ionic strengths) and for all corresponding sample solutions.

For each value of pH (tirn of 0.2 units) the difference between the consumption of H+ or 0H~ for the suspension and for the corresponding sample solution is then determined. This operation is repeated for all ionic strengths.

This gives the condition (H<+>) - (0H~) corresponding to the consumption of protons at the surface. The surface charge is calculated using the previous equation.

Curves are then drawn for surface charge as a function of pH for all relevant ionic strengths. PZC is defined at the intersection of the curves.

The concentration of silica is set as a function of the specific surface of this. E.g. suspensions with 2% for silica with 50 m<2>/g with 3 ionic strengths (0.1, 0.01 and 0.001 mol/l) are used.

The determination is carried out with 100 ml of suspension using potassium hydroxide 0.1 M.

For silica according to the invention, this PZC is between 3 and 6.5.

Furthermore, in order to improve the compatibility of silica according to the invention with other elements, in particular with fluorine, it is interesting that the content of bivalent or multivalent cations contained in silica is at most 1000 ppm. In particular, it is desirable that the content of aluminum in silica in accordance with the invention is no more than 500 ppm.

The content of iron in silica in accordance with the invention should advantageously be no more than 200 ppm.

The calcium content is preferably no more than 500 ppm and especially no more than 300 ppm.

Silica in accordance with the invention also has a preferred carbon content of no more than 50 ppm, and in particular no more than 10 ppm.

Finally, the pH in silica in accordance with the invention measured using standard NFT 45-007 is generally no more than 7, and is particularly between 6 and 7.

The above-mentioned properties enable a silica that is compatible with bivalent and higher valent metal cations, especially zinc, strontium, tin. This compatibility measured according to the test given below is at least 30%, more particularly at least 50% and preferably at least 80%.

Furthermore, silica in accordance with the invention has a good compatibility with the fluoride anion of at least approximately 80% and preferably at least 90%.

In addition to the properties of the surface chemistry which will be described in the following and which determine the compatibility, silica in accordance with the invention also has physical properties which make it particularly suitable for applications in dental care products. These properties of the structure type are described in the following.

In general, the surface BET of silica in accordance with the invention is between 40 and 600 m<2>/g. Their surface CTAB usually varies between 40 and 400 m<2>/g.

The surface BET is determined according to the method of Brunhauer-Emmet-Teller described in the Journal of the American Chemical Society vol. 60, page 309, February 1938 and according to standard NF xll-622(3.3).

The surface CTAB is the outer surface determined according to standard ASTM D37 65 by carrying out adsorption of hexadecyltrimethyl-ammonium bromide (CTAB) at pH9, and assuming a projected area for the CTAB molecule of 3 5 A<2>.

Silica in accordance with the invention can of course correspond to the three types that are usually distinguished between in the area of dental care agents.

Thus, silica in accordance with the invention can be of the abrasive type and then have a surface BET between 40 and 300 m<2>/g. In this case, the surface CTAB is between 40 and 100 m<2>/g.

Silica in accordance with the invention can also be of the thickener type and then have a surface BET between 120 and 450 m<2>/g, more particularly between 120 and 200 m<2>/g. They may have a surface CTAB between 120 and 400 m<2>/g, more particularly between 120 and 200 m2/g.

Finally, according to a third type, silica in accordance with the invention can be bifunctional, and then have a surface BET between 80 and 200 m<2>/g and the CTAB surface is then between 80 and 200 m2/g.

Silica in accordance with the invention can also have an oil absorption capacity between 80 and 500 cm<5>/100 g determined according to standard NFT 30-022 (March 53) by using dibutyl phthalate.

In more detail, this oil absorption is between 100 and 140 cm<5>/100 g for abrasive silica, between 200 and 400 cm<5>/100 g for thickening silica and between 100 and 300 cm<5>/100 g for bifunctional silica.

Furthermore, all the time with a view to use as a dentifrice, silica has preferred a particle size between 1 and 10 µm.

Apparent density generally varies between 0.01 and 0.3.

In a particular embodiment of the invention, the silica is of the precipitation type.

Silica according to the invention may finally have a refractive index generally between 1.440 and 1.465.

The method for producing silica in accordance with the invention will now be described in more detail.

As indicated above, the method is of the type which comprises reaction of a silicate with an acid which gives rise to the formation of a suspension or a gel of silica.

It should be noted that one can use any known working method to obtain this suspension or gel (addition of acid to a container of silicate, total or partial simultaneous addition of acid and silicate to a container of water or a solution of silicate, etc .) as the choice is made mainly as a function of the physical properties of the silica that is desired to be obtained.

A preferred embodiment of the invention consists in producing the suspension or gel of silica by the simultaneous addition of silicate and acid to a container that can contain water, a colloidal dispersion of silica containing 0 to 150 g/l silica expressed as SiC>2, a silicate or an inorganic or inorganic salt, preferably an alkali metal salt such as e.g. sodium sulfate or sodium acetate.

The addition of the two reaction components is carried out simultaneously so that the pH is maintained between 4 and 10, preferably between 8.5 and 9.5. The temperature is advantageously between 60 and 95°C.

A production method for the colloidal dispersion of silica with a preferred concentration between 20 and 150 g/l consists of heating an aqueous solution of silicate, e.g. between 60 and 95°C and adding the acid to this aqueous solution until a pH of between 8.0 and 10.0 is obtained, preferably about 9.5. The concentration of the aqueous solution of silicate expressed as SiC>2 is preferably between 20 and 150 g/l. A diluted or concentrated acid can be used and its normality can vary between 0.5N and 36N, preferably between 1 and 2N.

In the foregoing, silicate advantageously means an alkali metal silicate and preferably sodium silicate with a weight ratio SiC>2/Na2O of between 2 and 4, preferably 3.5. For this, the acid, which can be gaseous, such as carbon dioxide gas, or liquid such as e.g. sulfuric acid.

In a further step of the method according to the invention, the suspension or gel is subjected to a double maturation operation.

A first maturation is carried out at a pH of no more than 8.5 and advantageously between 6 and 8.5, e.g. 8.0. The maturation is preferably carried out during heating, e.g. at a temperature between 60 and 100°C, and preferably 95°C and for a time which can vary between 10 minutes and 2 hours.

A further variant of the invention consists in producing a suspension or a gel of silica by gradual addition of the acid in a container containing the silicate until the desired pH is reached during maturation. This operation is carried out at a temperature preferably between 60 and 95°C. The maturation of the suspension of the silica gel is then carried out under the previously described conditions."

Another maturation is then carried out at a pH below 5, preferably between 3 and 5, and most preferably between 3.5 and 4.0.

Temperature conditions and duration are the same as for the first ripening.

For this, the pH is brought to the desired pH during maturation by adding acid.

One can e.g. use a mineral acid such as nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid or also carbonic acid formed by bubbling through carbon dioxide.

One then proceeds with the separation of silica from the reaction mixture using known measures, e.g. using a vacuum filter or filter press.

A silica filter cake is then obtained.

In the subsequent step of the method according to the invention, washing of the obtained silica filter cake is carried out. The washing is carried out under conditions such that the pH and the suspension or mixture before drying correspond to the following equation:

in which equation (II):

. e is a constant at most 1.0,

. d is a constant at most 8.5,

. (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm~\

The washing is carried out with water which preferably has a temperature between 40 and 80°C. In the individual case, one or more washings can be carried out, usually two, with water, preferably deionised water and/or with the help of an acidic solution which has a pH of 2 and 7.

This acidic solution can e.g. be a solution of a mineral acid such as nitric acid.

In a particular embodiment of the invention, however, this solution can be a solution of an organic acid, especially a complex-forming organic acid. This acid can belong to the group consisting of carboxylic acid, dicarboxylic acid, hydroxycarboxylic acid and aminocarboxylic acids.

Examples of acids include acetic acid and tartaric acid, maleic acid, glycerine acid, gluconic acid, citric acid as complex-forming acids.

It can also, particularly in the case where a solution of mineral acid is used, be advantageous to carry out a final washing with deionised water.

From a practical point of view, the washing operations can be carried out by passing the washing solution through the filter cake or by introducing this into the obtained suspension, after dividing the filter cake.

Practically, the filter cake is subjected to a division before the drying operation, which can be carried out using known means, e.g. using an agitator that rotates at high speed.

The silica filter cake, before or after washing, is then divided and then dried using known measures. The drying can be carried out e.g. in a tunnel furnace or muffle furnace or by atomization in a stream of hot air with an inlet temperature that can vary between about 200 and 500°C and an outlet temperature that can vary between about 80 and 100°C. The residence time is between 10 seconds and 5 minutes.

If necessary, the dried product can be ground to obtain the desired grain size. The operation is carried out in a classic device, e.g. a knife mill or an air jet mill.

The invention also relates to dental care agent mixtures containing silica of the type described above or obtained by means of the method to be explained in more detail.

The amount of silica in accordance with the invention used in dentifrice mixtures can vary within wide limits, but is usually between 5 and 35%.

Silica in accordance with the invention is particularly suitable for dentifrice mixtures containing at least one element selected from the group comprising fluorides, phosphates, metal cations.

With regard to fluorinated compounds, their amount preferably corresponds to a concentration of fluorine in the mixture of between 0.01 and 1% by weight and in particular 0.01 to 0.5%. The fluorine compounds are in particular salts of monofluorophosphoric acid and in particular salts of sodium, potassium, lithium, calcium, aluminum and ammonium, mono- and difluoro-phosphate as well as the various fluorides containing fluorine in the form of an associated ion, in particular alkali metal fluorides such as sodium, lithium, ammonium fluoride, stannous fluoride, manganese fluoride, zirconium fluoride, aluminum fluoride as well as addition products between these fluorides with each other or with other fluorides, such as potassium fluoride or sodium fluoride or manganese fluoride.

Other fluorides can also be used in the invention, such as e.g. zinc fluoride, germanium fluoride, palladium fluoride, titanium fluoride, alkali metal fluorozirconates such as sodium or potassium, stannous fluorozirconate, sodium or potassium fluoroborate or fluorosulphate.

Fluorinated organic compounds can also be used, preferably those known as addition products between amines or long-chain amino acids with hydrogen fluoride, such as cetylamine hydrofluoride, bis-(hydroxyethyl)aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoride, octadecyl fluoride and N,N',N '-tri-(polyoxyethylene)-N-hexadecylpropylenediamine dihydrofluoride.

With regard to the components which supply bivalent or multivalent metal cations, of those mentioned earlier in the description, the most frequently used are zinc citrate, zinc sulphate, strontium chloride and stannous fluoride.

For elements usable as antiplaque agents of the type polyphosphates or polyphosphonates, guanidines, biguanides, those specified in US patent document 3,934,002 can be mentioned.

The dentifrice mixtures may further comprise a binder.

The essential binders used are selected in particular from: cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose,

vegetable resins such as carrageenides, alginates, agar-agar and gels,

gum types such as gum arabic and tragacanth, xanthan gum,

karaya gum,

carboxyvinyl and acrylic polymers,

polyoxyethylene resins.

In addition to silica in accordance with the invention, the toothpaste mixture in accordance with the invention may also contain one or more abrasive or polishing agents selected in particular from: precipitated calcium carbonate,

magne s ium carbonate,

mono-, di- and tri-calcium phosphates.

insoluble sodium metaphosphate,

calcium pyrophosphate,

titanium oxide (whitening agent),

silicates,

aluminum oxides and silico-aluminates,

zinc oxide and tin oxide,

talc,

kaolin.

The dentifrice mixtures may also include surfactants, wetting agents, flavoring agents, sweeteners, and coloring agents and preservatives.

The essential surfactants used are selected in particular from:

sodium lauryl sulfate,

sodium lauryl ether sulfate and

lauryl sulfoacetate,

sodium dioctyl sulfosuccinate,

sodium lauryl sarcosinate,

sodium ricinolate,

sulfated monoglycerides.

The essential wetting agents used are selected in particular from among the following polyalcohols:

glycerol,

zorbitol, generally 70% solution in water, propylene glycol.

Anise essences, badian anise, menthol, juniper berries, cinnamon, cloves or rose are used as flavoring agents (perfumes).

The essential sweeteners are selected in particular from orthosulfo-benzoic acid imides and the cyclamates.

The essential dyes are selected in particular from: red color and pink color: amaranth, azorubin, catechu, new cochine (PONCEAU 4R),

cochineal, erythrosine,

green coloring: chlorophyll and chlorophyllin,

yellow color, solar yellow (orange S) and kinoline yellow.

The most commonly used preservatives are parahydroxybenzoates, formaldehyde and products that emit formaldehyde, hexetidine, quaternary ammonium compounds, hexachlorophene, bromophene and hexamedine.

Finally, the dentifrice mixture may contain therapeutic agents selected in particular from among:

antiseptic and antibiotic agents,

enzymes,

oligoelements and fluorinated compounds as described in

the preceding.

The invention will now be described with the help of design examples, but first the protocol for measuring pH as a function of conductivity and concentration as well as the tests for measuring the compatibility of silica with different elements will be described.

Protocol for measuring pH as a function of the concentration of silica and the conductivity

Suspensions of silica with increasing concentration varying from 0 to 25% by weight are established by dispersing a mass m of silica previously dried at 120°C for 2 hours in a mass of 100 m<J>of water that has been deionized and degassed (millipore quality) . The suspension is stirred for 24 hours at 25°C.

The pH of the suspensions and solutions obtained after centrifugation of a part of the suspension at 8,000 revolutions/minute for 40 minutes and filtration through a millipore filter 0.22 µm is measured at 25°C under a nitrogen atmosphere with a measurement system of the type Titroprocessor Metrohm 672".

In the same way, the conductivity of suspensions and solutions obtained as above is measured at 25°C with a conductivity meter "Radiometer" (CDM83) equipped with a cell of type CDC304 with a cell constant of 1 cm-1. The conductivity is expressed in uSiemens/cm.

The suspension effect (SE) is defined by the difference in pH between the pH of a suspension with 20% silica and the pH of the supernatant solution separated by centrifugation.

Measuring the compatibility with zinc

4 g of silica are dispersed in 100 ml of solution 0.06% of ZnSO4, 7H2O. A suspension is obtained where the pH is stabilized at 7 within 15 minutes by adding NaOH or H2SO4.

The suspension is then stirred for 24 hours at 37°C, after which it is centrifuged at 20,000 revolutions/minute for 30 minutes.

The supernatant after filtration through a filter "Millipore" 0.2 µm, constitutes the sample solution.

A reference solution is made up by following the same protocol, but in the absence of silica.

The concentration of free zinc in the two solutions is determined by atomic absorption (214 nm).

Compatibility is determined by the following conditions:

In the following, percentage compatibility with zinc is denoted with Zn.

Measurement of compatibility with stannous fluoride SnF

1) An aqueous solution (1) containing 0.40% SnF2 and 20% glycerol is formed by dissolving 0.40 g SnF and 20 g glycerol in

79.60 g double distilled water.

2) 4 g of silica is dispersed in 16 g of the solution obtained under 1). The pH in the suspension is adjusted to 9 by adding 0.1 N.

The obtained suspension is stirred for 4 weeks at 37°C.

3) The suspension is then centrifuged at 8000 revolutions/-

minute for 30 minutes and the obtained supernatant 3) is filtered through filter "Millipore" 0.22 µm.

4) The concentration of free tin is determined by atomic absorption in the solution obtained under 1) and in the supernatant obtained under 3). 5) Compatibility is determined by the following conditions:

In the following, % compatibility with tin is denoted by Sn.

Measurement of compatibility with strontium chloride SrCl2.6H2O

1) An aqueous solution (1) containing 1% SrCl2-6H20 is formed by dissolving 1 g of SrCl2-6H20 in 99 g of double-distilled water. The pH in the suspension is adjusted to 7.0 by adding 0.1 NaOH. 2) 4 g of silica are dispersed in 16 g of the solution obtained under 1).

The obtained suspension is stirred for 4 weeks at 37°C.

3) The suspension is then centrifuged at 8000 rpm for 30 minutes and the supernatant obtained during 3) is filtered through a filter "Millipore" 0.22 µm. 4) the concentration of free strontium is determined by atomic absorption in the solution obtained under 1) and in the supernatant obtained under 3). 5) Compatibility is determined by the following conditions:

In what follows, % compatibility with strontium is denoted with Sr.

Measurement of compatibility with fluoride

4 g of silica are dispersed in 16 g of a 0.3% solution of sodium fluoride (NaF). The suspension is stirred for 24 hours at 37°C. After centrifugation of the suspension at 20,000 rpm, the supernatant is filtered through a "Millipore" 0.2 µm filter. The solution thus obtained constitutes the experimental solution.

A reference solution is made up using the same protocol, but in the absence of silica.

The compatibility with fluorides is determined as % free fluoride measured by an electrode that is fluoride selective (Orion). It is determined using the following conditions.

Measurement of compatibility with sodium and potassium pyrophosphates

4 g of gilica is dispersed in 16 g of a 1.5% suspension of sodium or potassium pyrophosphate. The suspension is stirred for 24 hours at 37°C and centrifuged at 20,000 revolutions/minute for 30 minutes.

The supernatant is filtered through "Millipore" 0.2 µm. =.2 g of solution diluted in 100 ml of water in a graduated flask constitutes the test solution.

A reference solution is prepared by following the same protocol, but in the absence of silica.

Concentration of free pyrophosphation (P2O7) in the two solutions is determined by ion chromatography (system DIONEX 2000i) equipped with an integrator.

The compatibility is determined as the ratio between the areas of peaks obtained in the chromatograms and the corresponding retention time for pyrophosphate in the experiment and the reference.

EXAMPLE 1

In a reactor equipped with a system for regulating temperature and pH and an agitator system with a screw (Mixel), 8.32 1 of sodium silicate with a silica concentration of 130 g/l and molar ratio Si02/Na20 = 3.5 and 8.33 1 of deionized water are introduced with conductivity 1 us/cm.

After initiation of stirring (350 revolutions/minute), the starting mixture formed in this way is heated to 90°C. When this temperature is reached, sulfuric acid with a concentration of 80 g/l is added at a constant rate of 0.40 l/minute to change the pH to 9.5.

One continues with the simultaneous addition of 45.25 1 sodium silicate with a silica concentration of 130 g/l with a molar ratio SiC>2/Na20 = 3.5 and a feed rate of 0.754 l/minute and 29.64 1 80 g/l sulfuric acid. The supply quantity of sulfuric acid is adjusted so that the pH in the reaction mixture is maintained at a constant value of 9.5.

After 60 minutes of addition, the addition of sodium silicate is stopped and the addition of sulfuric acid is continued at a rate of 0.494 l/minute until stabilization of the pH in the reaction mixture at 8.0. During this phase, the temperature of the mixture is brought to 95°C. A maturation is then carried out for 30 minutes at this pH and 95°C. During ripening, the pH is kept at 8 by adding acid.

At the end of maturation, the pH is brought to 3.5 by adding sulfuric acid. This pH is maintained at 3.5 for 30 minutes.

After the heating has stopped, the mixture is filtered and the resulting filter cake is washed with 20 1 of deionized water and heated to 80°C. The filter cake obtained after washing is dispersed in the presence of deionized water to form a suspension with a silica concentration of 10%.

Another filtration is carried out, followed by a wash with water so that the conductivity is set to 500 uS/cm and a wash with water with a pH set to 5 using citric acid so that the pH is set to a value below 6.

final washing with deionized water.

It is determined from the pH in the aqueous suspension of the dispersed filter cake that it has a content of SiC>2 of 20% and satisfies the following conditions:

pH = < 8.20 - 0.91 log (D).

Silica is dried by atomization. The resulting silica is finally ground on a knife mill to obtain a powder with an average diameter of the agglomerates measured on the COUNTER-COULTER is 8 µm.

The physicochemical properties of silica obtained in this way are listed in the following table:

Table I shows the chemical surface properties in accordance with the invention and table II shows the results of the compatibility test with metal cations such as zinc, tin, strontium and with classic ingredients in toothpaste mixtures such as fluoride and pyrophosphate.

EXAMPLE 2

In a reactor equipped with a system for temperature control and pH and a system with a screw agitator (Mixel), 5.3 0 1 sodium silicate with a silica concentration of 135 g/l and molar ratio SiC>2/Na20 = 3.5 and 15.00 1 deionized is introduced water with conductivity 1 uS/cm.

After starting stirring (350 revolutions/minute), the starting mixture established in this way is heated to 90°C.

When this temperature is reached, sulfuric acid with a concentration of 80 g/l is added at a constant rate of 0.38 l/minute to bring the pH to 9.5.

One continues with the simultaneous addition of 44.7 0 1 sodium silicate with a silica concentration of 135 g/l, with molar ratio SiC>2/Na20 = 3.5 and an amount equal to 0.745 l/minute and 25.30 1 sulfuric acid with 80 g/l. The amount of sulfuric acid is adjusted so that the pH in the reaction mixture is maintained at a constant value of 9.5.

After 60 minutes of addition, the addition of sodium silicate is stopped and the addition of sulfuric acid is continued in an amount of 0.350 l/minute until stabilization of the pH in the reaction mixture at 7.0. During this phase, the temperature of the mixture is brought to 95°C. In the following, a maturation is carried out for 30 minutes at this pH and 95°C. During ripening, the pH is maintained at 7 by the addition of acid.

At the end of maturation, the pH is brought to 4.0 by adding sulfuric acid. This pH is maintained at 4.0 for 30 minutes.

After stopping the heating, the mixture is filtered and the resulting filter cake is washed with deionized water until a filtrate with a conductivity of 2000 uS/cm is obtained. The filter cake is then dispersed in the presence of water to form a suspension with 20% silica.

A final washing is carried out with deionized water so that the pH in the aqueous suspension of the dispersed filter cake has a content of SiO2 of 20%, confirmed using the following conditions:

pH < 8.20 - 0.91 log (D)

Silica is dried at 120°C for 24 hours, after which it is ground on a knife mill to obtain a powder with an average aggregate diameter of 8 µm.

The physicochemical properties of silica obtained in this way are listed in the following table:

The following table I shows the chemical surface properties of silica in accordance with the invention described in examples 1 and 2.

Table I also gives results from compatibility tests with silica in accordance with the invention with the metal cations, zinc, tin, strontium and with common ingredients in toothpaste mixtures in the form of fluoride and pyrophosphate.

For comparison, Tables I and II indicate the properties and the different compatibilities with commercial silicas, in which the list defined below constitutes a representative range for classic silicas.

S81 : Syloblanc 81 (GRAGE)

Z113 : Zeodent 113 (HUBER)

Sidl2 : Page 12 (DEGUSSA)

T7 3 : Tixosil 7 3 (RHONE-POULENC)

T83 : Tixosil 83 (RHONE-POULENC)

The meaning of the symbols used in the table above is given as follows: - pH/log(D) represents the equation pH = b-a log(D) where b and a are two constants and D is the conductivity of the silica suspension in uSiemens/cm. - SE represents the suspension effect measured by the equation SE = pH suspension - pH supernatant defined earlier.

- Ho is the Hammett constant

- PZC represents the pH at which the surface charge of silica is zero.

Claims (24)

1. Silica, characterized in that it has a surface chemistry such that the number of 0H~ expressed as OH""/nm <2> is at most 10, its point zero charge (PZC) is between 3 and 6.5 and that it leads to an aqueous suspension where the pH varies as function of its electrical conductivity according to the following equation (I): - pH = b - a log (D) (I) where equation (I): . a is a constant at most 0.6 . b is a constant at most 8.5 . (D) represents the electrical conductivity of the aqueous _in suspension of silica expressed in microsiemens cm 2. Silica as specified in claim 1, characterized in that it has a surface chemistry such that the number of OH~ expressed as OH~/nm <2> is between 4 and 10. 3. Silica as stated in claim 1 or 2, characterized in that its PZC is between 3 and 6.5. 4. Silica, characterized in that it has a compatibility with divalent and tetravalent metal cations selected from groups 2a, 3a, 4a and 8 in the periodic table of at least 30%, preferably at least 50% and more particularly at least 80%. 5. Silica as specified in claim 4, characterized in that the metal cation is calcium, strontium, barium, aluminium, indium, germanium, tin, lead, manganese, iron, nickel, zinc, titanium, zirconium, pallasdsium. 6. Silica as stated in claim 4 or 5, characterized in that the metal cation is in the form of inorganic salts, such as chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts such as acetate or citrate. 7. Silica as specified in claim 5, characterized in that the metal cation is in the form of zinc citrate, zinc sulphate, strontium chloride or stannous fluoride. 8. Silica as specified in claim 1, characterized in that it has a compatibility with fluoride anion of at least 80% and preferably at least 90%. 9. Silica as stated in one or more of claims 1-8, characterized in that it has a content of divalent and multivalent metal cations of no more than 1000 ppm. 10. Silica as specified in claim 9, characterized in that the aluminum content is at most 500 ppm, the iron content is at most 200 ppm, the calcium content is at most 500 ppm and more particularly at most 300 ppm. 11. Silica as stated in one or more of claims 1-10, characterized in that it has a carbon content of no more than 50 ppm and more particularly no more than 10 ppm. 12. Silica as specified in one or more of claims 1-11, characterized in that it has a pH of at most 7.0 and more particularly between 6.0 and 7.0. 13. Silica as specified in one or more of claims 1-12, characterized in that it has a surface BET of between 40 and 600 m <2> /g. 14. Process for producing silica as specified in one or more of claims 1-13, characterized in that a silica is reacted with an acid and leads to a suspension or a gel of silica, a first maturation is carried out at a pH of at least 6 and at most 8.5, followed by a second maturation at a pH of at most 5.0, the silica is separated and is subjected to a washing with hot water until it leads to the achievement of an aqueous suspension where the pH measured on a suspension with 20% SiC>2 corresponds to the following equation: - pH = d - c log (D) (II) where equation (II): . c is a constant at most 1.0 . d is a constant at most 8.5 . (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm and finally the silica is dried. 15. Method as stated in claim 14, characterized in that a suspension or gel of silica is produced by the simultaneous addition of silicate and acid to a starting mixture which can be water, a colloidal silica dispersion containing 0 to 150 g/l silica expressed as SiC> 2> a silicate or an inorganic or organic salt preferably of an alkali metal. 16. Method as stated in claim 14, characterized in that the addition of the two reaction components is carried out simultaneously so that the pH is maintained constant between 4 and 10, preferably between 8.5 and 9.5. 17. Method as stated in claim 14 or 15, characterized in that the temperature is kept between 60 and 95°C. 18. Method as stated in claim 14, characterized in that a colloidal dispersion of silica containing 20 to 150 g/l silica is prepared by heating an aqueous solution of silicate between 60 and 95°C, and that the acid is added to this aqueous solution until a pH of between 8.0 and 10.0 is achieved, preferably 9.5. 19. Procedure as specified in one or more of claims 14 - 18, characterized in that a first maturation of the suspension or gel of silica is carried out at a pH between 6 and 8.5, preferably 8.0, at a temperature between 60 and 100°C, preferably 95°C. 20. Method as stated in claim 14, characterized in that a suspension or a gel of silica is produced by gradually adding the acid to a starting mixture containing the silicate until the desired pH is achieved during maturation at a temperature between 60 and 95°C. 21. Method as stated in claim 14, characterized in that washing is carried out with water or with an acidic solution at a temperature between 40 and 80°C, preferably between 60 and 80°C. 22. Method as stated in claim 21, characterized in that the said acidic solution is a solution of an organic acid, in particular of a complex-forming organic acid. 23. Method as stated in claim 22, characterized in that the mentioned organic acid is selected from the group of carboxylic acids, dicarboxylic acids, aminocarboxylic acids and hydroxycarboxylic acids. 24. Method as stated in claim 22 or 23, characterized in that the organic acid is selected from the group comprising acetic acid, gluconic acid, tartaric acid, citric acid, maleic acid and glycerine acid. 2 5. Tooth care product, characterized in that it contains a silica as stated in one or more of claims 1 to 13 or silica produced using the method as stated in one or more of claims 14 to 24.