NO901883L - SILICA, SPECIAL FOR USE IN DENTAL CARE, PROCEDURE FOR ITS MANUFACTURING, AND DENTAL CARE. - Google Patents

Abstract

Silica capable of being employed in particular in dentifrice compositions and compatible especially with organic amino compounds. The silica of the invention is characterised in that it results in an aqueous suspension whose pH varies according to defined equations, depending on its concentration and its electrical conductivity.

Description

It is known that silica is often used in dental care preparations and they can then play several roles.

It is primarily an abrasive in that it helps to remove dental plaque through its mechanical effect.

It can also act as a thickening agent to impart certain rheological properties to the dentifrice as well as an optical agent to give it the desired color.

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, further a binder such as an algae colloid such as carrageenan, guar gum or xanthan gum, a wetting agent which can be a polyalcohol such as glycerol , sorbitol, xyliton and propylene glycol. You will also find any ingredients such as surface-active agents, agents for reducing dental plaque or tartar deposits, taste-correcting agents, coloring agents and pigments, etc.

Among the various components in the composition of dental care products, you come across the organic amine compounds. By "organic amine compound" is meant any active molecule which is part of the dental care products and which contains at least one nitrogen atom. More specifically, the following compounds can be mentioned: fluorinated amines, used as anti-caries agents, in particular addition compounds between amines or long-chain amino acids, with hydrogen fluoride, such as cetylamine hydrogen fluoride, bis-(hydroxyethyl)-aminopropyl-N-hydroxy-ethyloctadecylamine dihydrogen fluoride, octadecylamine fluoride and N,N',N'-tri-(polyoxyethylene)-N -hexadecyl-propylenediamine-dihydrofluoride amino oxides used as non-ionic surfactants agents obtained by oxidation of tertiary aliphatic amines with hydrogen peroxide. Particular mention may be made of alkyl amino oxides with the formula: R(CH3)2N -> 0 in which R is a linear or branched alkyl radical with approximately 10 - 24 carbon atoms.

One can also amino oxides with the formula:

R(CH2CH2OH)2N - 0

alkylamines which can be primary, secondary, tertiary or quaternary aliphatic amines used as cationic surfactants. Examples include alkylamines with the formula R-CH2NH2 or dimethylalkylamines

of formula R-N(0^3)2; cetyltrimethylammonium bromide. alkyl betaines which are N-alkyl derivatives of N-dimethylglycine and alkylamidoalkyl betaines which are hereinafter referred to as "alkyl betaines".

As examples of this class of amphoteric surfactants can be given:

alkylbetaines of formula

alkylamidopropyl-dimethylbetaines with formula:

wherein R is a linear or branched alkyl radical with 10 - 24 carbon atoms.

The presence of these organic amine compounds presents the problem of their compatibility with silica. They may in fact, particularly due to their adsorbing ability, tend to react with the aforementioned compounds so that these cannot fulfill the function assigned to them. The purpose of the present invention is to provide a new silica which is compatible in particular with the above-mentioned organic amine compounds and in particular with those of the class of fluorinated amines and betaines and which are completely usable in the dentifrice mixtures. A further object of the invention is also to provide a silica which has a good compatibility with the various cations present in the toothpaste mixture, such as zinc, strontium, tin, etc.

A further object of the invention is also to provide a silica which is likewise more compatible with products of the guanidine type, particularly bis-guanides where the most representative element is chlorhexidine.

Finally, a further object of the invention is a method for producing such compatible silica.

For this, it is recognized in the context of the invention that they

the compatibility properties sought depend mainly on the surface chemistry of the silica used. A certain number of conditions have been determined on the surface of silica for this to be acceptable.

The properties of silica according to the invention are that it

leads to an aqueous suspension where the pH varies as a function of its concentration in the area defined by the two equations:

where pH varies as a function of its electrical conductivity in the range defined by the two equations:

where in equations (Ia) and (Ib):

(C) represents the concentration by weight of the aqueous suspension of silica expressed as % SIO 2

in equations (Ila) and (Ilb)

(D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm-<1>.

A further property of silica in accordance with the invention is that it has an acidity function Ho of at least 4.0.

A further property of silica in accordance with the invention is that it has a number of OH~ sites per nm<2> at most 12.

A further property of silica according to the invention is that it has an O charge point (PZC) of at least 4.

A property of silica in accordance with the invention is that it has a compatibility of at least 30% with organic amine compounds and in particular at least 50% and preferably at least 80% with organic amine compounds selected from the group consisting of fluorinated amines, amine oxides, alkylamines and alkylbetaines.

A further property of silica according to the invention is that it has a compatibility of at least 50% and more particularly at least 70% with metal cations.

A further property of silica according to the invention in that embodiment is that it has a compatibility with products of the guanidine type, especially chlorhexidine of at least 30% and more especially at least 60%.

The present invention also relates to a method for producing silica in accordance with the invention and is characterized in that a silicate is reacted with an acid which then 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, then a second ripening at pH 6.0 at most, and a third ripening at pH 5.0 at most, after which the silica is separated, subjected to a washing with water until an aqueous suspension is obtained where the pH measured in a suspension with 20% SiC >2 corresponds to the following equation:

where equation (III)

e is a constant above 0.6 and at most 1.0

d is a constant above 7.0 and at least 8.5 (D) stands for the electrical conductivity of the aqueous suspension of silica expressed in microsiemens cm-<1>.

After which the suspension is dried.

The invention finally relates to dental care agent mixtures characterized in that they contain silica as described above or produced in accordance with the method described above.

Other properties and advantages of the invention are better understood by the teachings of the following description and execution examples which illustrate the invention.

Silica, which is the subject of the present invention, is characterized in that its pH in aqueous suspension varies as a function of its concentration and its electrical conductivity in accordance with the previously stated equations.

The instructions for measuring pH as a function of concentration of the aqueous suspension of silica and its electrical conductivity are given below.

As indicated at the outset, the essential properties of silica according to the invention depend on the surface chemistry. In more detail, one of the aspects included in this surface chemistry is the acidity. For this, one of the properties of silica in accordance with the invention is the strength of the acidic surface sites.

Here the acidity is indicated as Lewis acidity, i.e. that which expresses the tendency of a site to receive an electron pair from a base according to the equation:

The term "acidity function" Ho, developed by

Hammett, to measure the tendency of the acid, in the present case silica, to accept an electron pair from a base.

The function Ho is thereby defined by the following classic equation:

To determine the strength of the acid sites in a silica according to the invention by means of Hammett's method, the method with indicators described originally by Walling (J. Am. Chem. Soc. 1950, 72, 1164) is used.

The strength of the acidic sites is determined by means of colored indicators for which, under the conditions of use, the pKa for the passage between the acidic and basic forms is known. Thus, the lower the pKa of the indicator undergoing the color change, the stronger the acidity of the seat. In the table below, a non-limiting list of Hammett indicators that can be used to judge the value for Ho in which form two subsequent dicators are adsorbed is listed as examples. The color of the indicators adsorbed on a silica is a measure of the strength of the acid sites. If the color corresponds to the acidic form of the indicator, then the value of the function Ho of the surface is equal to or lower than the pKa value of the indicator.

Low values for Ho correspond to acidic sites with high strength.

Thus, for example, a silica that gives a red coloration with p-dimethylaminoazobenzene and yellow coloration with 2-amino-5-azotoluene will give an acidity function Ho of between 3.3 and 2.

Experimentally, the determination is carried out with 0.2 g of silica placed in a test tube in the presence of an indicator solution with a 100 mg/l in cyclohexane.

Silica is previously dried at 190°C for two hours and preserved under the exclusion of moisture in a desiccator.

Upon stirring, adsorption, if this takes place, will take place within a few minutes and one notices the color change which is visible to the eye or possibly by examining the characteristic adsorption spectra of the adsorbed colored indicators, both in their acidic and in their basic form.

The first property of silica according to the invention is that they have an acidity function as determined above of at least 4.0.

The surface condition of silica in accordance with the invention is such that the conditions regarding the number of acidic sites on the surface are met. The number can be measured as the number of groups OH~ or silanols per nm<2>.

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

The samples of silica are previously dried at 105°C for two hours. A mass Pg of silica is placed in a thermobalance and brought to 190°C within two hours where P190 becomes the mass obtained.

The silica is then brought to 900°C for two hours so that P900 becomes the new mass obtained.

The number of seats OH~ is calculated by 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 measured by BET and expressed in m<2>/g.

In the present case, silica in accordance with the invention advantageously has a number of OH~/nm<2> of at most 12, more particularly at most 10 and especially between 6 and 10.

The nature of the OH~ sites in silica according to the invention also constitutes a characterization of their surface chemistry and can also be determined at the point of zero charge.

This point of zero charge (PZC) is defined by the pH of a suspension of silica for which the electric charge of the surface of the solid is zero regardless of the ionic strength of the mixture. This PZC measures the real pH of the surface to the extent that it is free of all ion-type contaminants.

The electrical charge is determined by potentiometry. The principle of the method is based on the total balance 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, in relation to a reference corresponding to a surface charge of 0, is given by the equation:

in which:

A stands for the specific surface of the solid i

m2/g,

M is the amount of solids in the suspension in g,

F is Faraday,

(H<+>) or (OH~) represents the variation per surface unit of the excess of ions H+ or

0H~ on the solid.

The test protocol for determining PZC is as follows: The method described by Berube and de Bruyn (J. Colloid Interface Sc. 1968, 27, 305) is used.

Silica is pre-washed in deionized water with high resistance (10 Mega.Ohm.cm) and dried and then degassed.

Practically, a number of solutions are prepared to pHo = 8.5 with the addition of KOH or HNO3 and containing an indifferent electrolyte (KNO3) to a concentration that varies between 10-<5> and 10_<1>mol/l.

A given mass of silica is added to these solutions and the pH in the resulting suspensions is allowed to stabilize under stirring, at 25°C and under nitrogen for 24 hours, and the pHo gets its value from this.

The calibration solutions are made up of the supernatant obtained by centrifugation for 30 minutes at 1,000 revolutions/minute of a part of the same suspensions and for these pHo will be the pH of the supernatants.

The pH of a known volume of these suspensions and corresponding calibration solutions is then brought to pH 0 by adding the required amount of KOH and leaving the suspensions and calibration solutions for four hours.

Accordingly, V0h~'NOH~number of base equivalents are added to bring pH'o to pHo in a known volume (V) of suspension or calibration solution.

The potentiometric determination of suspensions and calibration 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. Accordingly, VH+ • NH+ is the number of equivalents of acid to arrive at pHf.

From pHo, the expression (VH<+>• NH<+>- VQH- • N0h~) is recorded as a function of the pH increments for all suspensions (three ionic strengths or less) and all corresponding calibration solutions.

For each value of pH (not 0.2 units), the difference between the consumption of H+ or OH~ for the suspension and for corresponding calibration solutions is then determined. This operation is repeated to maintain the ionic strength.

This gives the expression (H<+>) - (0H~) corresponding to the consumption of protons in the surface. The surface charge is determined using the above-mentioned effect. One then records curves for surface charge as a function of pH for all ionic strengths considered. PZC is defined at the intersection of the curves. The concentration of silica is adjusted as a function of its specific surface area. For example, suspensions with 2% are used for silica with 50 m<2>/g for three ionic strengths (0.1, 0.01 and 0.001 mol/l), the determination is carried out with 100 ml suspension by using 0.1 M potassium hydroxide.

Practically, it is preferred that the value of this PZC should be at least 4, more particularly between 4 and 6. In the case of a better compatibility with metal cations, it is at most 6.5. For good compatibility with fluoride, it is preferred to have a PZC on

at most 7.

In order to improve compatibility, especially with respect to fluorine, it is always interesting that the content of divalent or multivalent cations contained in silica is at most 10 ppm. It is particularly desirable that aluminum in silica in accordance with the invention is at most 500 ppm.

Furthermore, the content of iron in silica in accordance with the invention can advantageously be no more than 200 ppm. The calcium content is preferably no more than 500 ppm and more particularly no more than 300 ppm.

Silica in accordance with the invention also preferably has a carbon content of no more than 50 ppm and more particularly no more than 10 ppm. Silica in accordance with the invention which is compatible with organic amine compounds is also compatible with the various metal cations included in dentifrice mixtures.

They can actually contain, among other things, metal cations with a valence above 1 added to more active molecules. As examples, divalent metal cations can be mentioned and particularly chosen in groups 2a, 3a, 4a and 8 in the periodic table. More specifically, cations of group 2a, calcium, strontium and barium can be mentioned, cations of group 3a, aluminum and indium, of group 4a, barium, tin, lead and group 8, manganese, iron, nickel, zinc, titanium, zirconium, palladium , etc.

The mentioned cations can also be in the form of mineral salts,

e.g. chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts such as acetate, citrate, etc.

As specific examples can be mentioned zinc citrate, zinc sulphate, strontium chloride, stannous fluoride in single gel (SnF2) or in the form of double salt (SnF, Kf), stannous chloride fluoride SnCIF, zinc fluoride (ZnF2).

Silica according to the invention is compatible with the various metal cations. The compatibility of said silicas with cations defined in accordance with the above described tests is at least about 50%, preferably about 70% and even more preferably at least 80%.

Silica in accordance with the invention can be advantageously used in dentifrice mixtures containing divalent and multivalent cations and particularly in mixtures containing at least one of the following components: zinc citrate, zinc sulphate, strontium chloride, stannous fluoride.

In a particular embodiment of the invention, silica in accordance with the invention can also be compatible with guanidines and especially with chlorhexidine. The compatibility measured by a test described above is at least 30%. It can be improved to be at least 60% and preferably at least 90%.

In this case, the silica has a content of anions of the type S04<2>~,Cl-, N03~, P04<3-,>C03<2>" of at most 5 • 10~<3> on 100 g of silica. This compatibility will be all the greater as this content becomes smaller In the case of preferred variants, it is at most 1 • 10-<3>mol and more particularly 0.2 • 10-<3>mol for 100 g of silica.

In the case of silica prepared from sulfuric acid, this content of anion is more conveniently expressed as a content expressed as SO 4 = and as weight. In this case, the content is at most 0.1%. In a preferred variant of the invention, this content is at most 0.05% and more particularly at most 0.01%.

The silica according to the invention is particularly well suited for use in dentifrice mixtures containing guanidines and bis-guanidines. Mention may be made of those described in US Patents 3,934,002 and 4,110,083, the teachings of which are incorporated herein by reference.

The pH in silica in accordance with the invention measured according to standard NFT 45-007 is generally at most 8 and is particularly between 6.0 and 7.5.

The above-mentioned properties make it possible to obtain a silica which is compatible with the aforementioned organic amine compounds, metal cations and in the individual case also with fluorides and guanidines, especially chlorhexidine.

In addition to the chemical surface properties which will be described below and which determine the compatibility, the silica in accordance with the invention also has physical properties which make them perfectly adapted to their use in dental care products. These properties of a structural 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 and more particularly between 40 and 350 m<2>/g. Their surface CTAB usually varies between 40 and 400 m<2>/g, more particularly between 40 and 200 m<2>/g.

The surface BET is determined according to the method of Brunauer-Emmet-Teller in the Journal of the Americam Chemical Society vol. 60, page 309, February 1938 and according to standard NF Xll-622 (3,3).

The CTAB surface is the outer surface determined according to standard ASTM D3765, but practically by adsorption of hexadecyltrimethylammonium bromide (CTAB) at pH 9 and assuming a projected molecular area of CTAB of 35 A<2>.

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

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

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

Silica in accordance with the invention can also have an oil absorption capacity of between 80 and 500 cm<*>/100 g determined according to standard NFT 30-022 (March 1953) using dibutyl phthalate. ;More precisely, this oil absorption is between 100 and ;140 cm<5>/100 g for abrasive silica, between 200 and 400 for thickener silica and between 100 and 300 for bifunctional silica. Furthermore, all the time with a view to its use in dentifrices, silica has preferred a particle size between 1 and 10 µm. ;This average particle size (&$ o) of the particles is measured using the Counter-Counter. ;The apparent density generally varies between 0.01 and 0.3. In the particular embodiment of the invention, silica is of the precipitation type. Finally, silica according to the invention has a refractive index generally between 1.440 and 1.465. The method for producing silica in accordance with the invention shall be described in more detail. As stated above, this method is of the type involving a reaction between a silicate and an acid which gives rise to the formation of a suspension or a gel of silica. ;It is noted that one can use any known method of working to arrive at this suspension or gel (addition of acid to an initial mixture of silicate, simultaneous addition of all or part of the acid and silicate to an initial mixture of water or solution of silicate, etc) as the choice is mainly made as a function of the physical properties of the silica that one wants to achieve. 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 starting mixture which can be water, a collidal dispersion of silica containing 0 - 150 g/l silica expressed as SiO2, a silicate or an inorganic or organic salt, preferably an alkali metal salt, such as sodium sulfate or sodium acetate. The addition of the two reaction components is then carried out simultaneously so that the pH is maintained constantly 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 in heating an aqueous solution of silicate, for example between 60 and 95°C and adding the acid to the said aqueous solution until a pH is achieved between 8.0 and 10.5 and preferably about 9.5. The concentration of aqueous solution of silicate expressed as SiC>2 is preferably between 20 and 150 g/l. A dilute or concentrated acid can be used and its normality can vary between 0.5 N and 36 N, preferably between 1 and 2 N. 1 the foregoing is understood by silicate advantageously to be a silicate of alkali metal and preferably a silicate of sodium or with a weight ratio of SiC >2/Na2O between 2 and 4, preferably 3.5. With regard to the acid, this can be gaseous, such as carbon dioxide or liquid, preferably sulfuric acid. In a further step of the method according to the invention, the suspension or gel is subjected to maturation operations. ;One carries out a first maturation at a pH of no more than 8.5 and ;f. e.g. between 6 and 8.5, preferably 8.0. The ripening is preferably carried out in a warm state, for example at a temperature between 60 and 100°C, preferably 95°C and for a duration which can be between ten minutes and two hours. Another variant for carrying out the method according to the invention consists in producing a suspension or a gel of silica by gradually adding the acid to a starting mixture containing the silicate until the desired pH for maturation is reached. This operation is carried out at a temperature preferably between 60 and 95°C. The maturation of the suspension of silica gel is then carried out under the conditions described above. A further ripening is then carried out at a pH below 6, preferably between 5 and 6, and even more preferably at a pH of 5.5. ;The temperature conditions and duration are the same as during the first ripening. For this, the pH is brought to the desired pH for ripening by adding acid and you can, for example, use a mineral acid such as nitric and hydrochloric acid, sulfuric acid, phosphoric acid or also carbonic acid formed by bubbling through carbon dioxide. A third maturation is carried out at a pH below 5.0, preferably between 3 and 5 and even more preferably at approximately 4.0. The conditions with temperature and duration are similar to the other two maturations. The desired pH is achieved during maturation by adding acid. You then proceed with the separation of silica from reaction mixtures using known means, such as vacuum filtration or using a filter press. A silica filter cake is then obtained. The method according to the invention can then be carried out using two basic variants. The first variant relates to the production of silica that is compatible with organic amine compounds and divalent or multivalent metal cations. ;In this case, the method involves a washing of the filter cake carried out under conditions such that the pH of the suspension or mixture before drying corresponds to the following equation: ; in which equation (III) ;e is a constant of at least 0.6 and at most 1.0 ;d is a constant of at least 7.0 and at most 8.5 ;(D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens .cm-<1>;For this, a washing with water, generally deionized and/or a washing using an acidic solution with a pH between 2 and 7 is foreseen. ;This acidic solution can, for example, be a solution of a mineral acid which e.g. nitric acid. In a particular embodiment of the invention, however, this acidic solution can also be a solution of an organic acid, in particular a complex-forming organic acid which can be selected from the group consisting of carboxylic acids, dicarboxylic acids, hydroxycarboxylic acids and aminocarboxylic acids. As an example of such acids, acetic acid can be mentioned and for complex-forming acid, tartaric acid, maleic acid, glyceric acid, gluconic acid, citric acid can be mentioned. It can be advantageous, particularly in the case of a solution of a mineral acid, to carry out a final washing with deionised water. Another variant relates to the production of silica which must also be compatible with guanidines and especially chlorhexidine. In such a case, a more vigorous washing is carried out. This must be continued until a filtrate from the washing is obtained where the conductivity is at most 200 microsiemens.cm-<1> and preferably below 100 microsiemens.cm-<1>. As mentioned above, this means that the concentration of anions must be no more than 5 IO-<3>mol per 100 g of silica. In the individual case, one or more washings can be carried out, generally two with water, preferably deionised water and/or with the help of an aqueous solution of an organic acid, especially those mentioned above. From a practical point of view, the washing operations can be carried out by passing the washing solution over the filter cake or by introducing this into the obtained suspension, after dividing the filter cake. Before the drying operation, the filter cake is practically subjected to a division which is carried out using known means, e.g. using a stirrer which rotates at high speed. The silica filter cake, before or after washing, is then split and dried using known measures. The drying is carried out, for example, in a tunnel oven or muffle oven or by atomization in a hot air stream where the temperature can vary between approximately 200 and 500°C and the exit temperature between approximately 80 and 100°C. The residence time is between ten seconds and five minutes. If necessary, the dried product can be ground to obtain the desired grain size. The operation is carried out in a conventional device either in a knife mill or 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 below. The amount of silica in accordance with the invention used in the dental care product sheets 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 an element selected from the group comprising fluorides, phosphates, guanidines, including chlorhexidine. They may actually have a compatibility using the tests described below of at least 90% for each of these elements. They are also well suited for dentifrice compositions containing at least one element selected from the group comprising organic amine compounds and compounds containing at least one divalent metal cation. They can then have a compatibility with the help of the tests described below which can reach 80% for each element. ;Silica in accordance with the invention is particularly suitable for dentifrice mixtures comprising at the same time at least one element selected from the group consisting of simple inorganic fluorides and organic fluorides, at least one element selected from the group of alkyl betaines and at least one element selected from the group of guanidines, especially chlorhexidine. One can mention in more detail mixtures containing a sodium fluoride and/or a stannous fluoride and/or a fluorinated amine, in particular cetylamine hydrofluoride, bis-(hydroxyethyl)-aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoride and an alkyl betaine and chlorhexidine. Silica in accordance with the invention is furthermore compatible with maleic acid-vinyl ethyl ether copolymers and can then be incorporated into dentifrice mixtures containing these copolymers. With regard to the fluorinated compounds, their amount preferably corresponds to a fluorine concentration in the mixture of between 0.01 and 1% by weight and in particular 0.1 - 0.5%. The fluorinated compounds are in particular salts of monofluorophosphoric acid and in particular sodium, potassium, lithium, calcium, aluminum and ammonium salts, mono- and difluorophosphate as well as various fluorides containing fluorine in ionic form, in particular alkali metal fluorides such as sodium , lithium and potassium fluoride, ammonium fluoride, stannous fluoride, manganese fluoride, zirconium fluoride, aluminum fluoride as well as addition products between these fluorides intermingle with or with other fluorides, such as the fluorides of potassium, sodium or manganese. Other fluorides can also be used for the invention such as zinc fluoride, germanium fluoride, palladium fluoride, titanium fluoride, alkali metal fluorozirconates such as sodium or potassium compounds, stannous fluorozirconate, sodium, potassium fluoroborate or fluorine sulfate. The organic fluorinated compounds mentioned at the beginning of the description can also be used, preferably cetylamine fluoride and bis-(hydroxyethyl)aminopropyl-N-hydroxyethyloctadecylamine dihydrofluoride. With regard to the compounds which supply at least divalent metal cations and especially those mentioned in the description, the most frequently used are zinc citrate, zinc sulphate, strontium chloride and stannous fluoride. For elements usable as anti-plaque agents of the type poly-phosphates or polyphosphonates, guanidines, bis-biguanides, mention can be made of those indicated in US patent document 3,934,002 or 4,110,083 whose teachings are considered incorporated herein. The dentifrice mixtures may also contain a binding agent. The main binders used are selected in particular from: cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, vegetable thickeners such as carrageenates, alginates, agar-agar and gels, vegetable resins such as gum arabic and tragacanth gum, xanthan gum, karaya gum, ;carboxyvinyl and acrylic polymers, ;polyoxyethylene resins. In addition to silica in accordance with the invention, the dental care agent mixtures may also contain one or more additional abrasives or polishing agents selected in particular from: ;precipitated potassium carbonate;di- and tricalcium phosphates;insoluble sodium metaphosphate;calcium pyrophosphate;titanium oxide (whitening agent);silicates;aluminum oxides and silico-aluminates ;oxides of zinc and tin ;talc ;kaolin. The toothpaste mixtures may also include surface-active agents, wetting agents, flavoring agents, molding agents and coloring agents as well as preservatives. ;The essential surfactants are selected in particular from: sodium lauryl sulfate; sodium lauryl ether sulfate and sodium lauryl sulfoacetate; sodium dioctyl sulfosuccinate; sodium lauryl sarcosinate; sodium ricinoleate; sulfated monoglycerides. ;The essential wetting agents used are selected in particular from among polyalcohols such as: ;glycerol ;sorbitol, generally a 70% solution in water propylene glycol ;The essential flavoring agents (perfumes) are selected in particular from anise essences, tonkin badian, menthol, juniper, cinnamon, cloves and rose. The essential sweeteners are chosen from among orthosulfobenzoimides and cyclamates. ;The essential dyes used are selected in particular from the desired colours: red and pink: amaranth, azorubin, cachou, cochineal, erythrosine, ;green colour: chlorophyll and chlorophyllin ;yellow colour: solar yellow (Orange S) and quinoline yellow. The essential concentrating agents are those most often used, namely parahydroxybenzoates, formalin and products that develop these, hexetidine, quaternary ammonium compounds, hexachlorophene, bromophene and hexamedine. ;Finally, the dentifrice mixture may contain therapeutic agents where the active principles are selected from among: ;antiseptic and antibiotic agents ;enzymes ;oligoelements .and fluorinated compounds as described in ;the foregoing. The following examples illustrate the invention. The protocol for measuring pH as a function of conductivity and concentration as well as the tests for measuring the compatibility of silica with the various elements are described. Protocol for measuring pH as a function of the concentration of silica and conductivity. ;Suspensions of silica with increasing concentration from 0-25% by weight are established by dispersing a mass M of silica that has previously been dried at 120°C for two hours in a mass 100 m of ionized and degassed water (Millipore quality). The suspensions are stirred for 24 hours at 25°C. The pH of the suspensions and the solution obtained after centrifugation of a part of the suspension at 8,000 revolutions per minute for 40 minutes and filtration through a filter Millipore 0.22<y>m, is measured at 25°C under a nitrogen atmosphere with a measuring system of the type Titroprosessor Metrohm 672. In the same way, the conductivity of suspensions and solutions obtained in the foregoing at 25°C is measured with a conductivimeter Radiometer (CDM83) equipped with a cell of the type CDC304 with a cell constant of 1 cm-1. The conductivity is measured as uSiemens/cm. ;The effect of the suspension (SE) is defined as the difference in pH between the pH of a suspension with 20% silica and the pH of its supernatant solution separated by centrifugation. ;Measurement of the compatibility with bis-(hydroxyethyl)-aminopropyl-N-hydroxyethyl- octadecylamine difluoride ;1) An aqueous solution (1) containing 1.65% amine fluoride is established by adding 5 g of a 33% amine fluoride solution ;in propanediol to 95 g of double-distilled water . 2) 4 g of silica is dispersed in 16 g of the solution obtained under 1). The resulting suspension is stirred for four weeks at 37°C. 3) The solution was then centrifuged at 8,000 revolutions/minute for 30 minutes and the obtained supernatant (3) was filtered through Millipore filter 0.22 um. 4) The concentration of free amine fluoride is determined by microanalysis of nitrogen in the solution obtained under ;1) and in the supernatant obtained under 3). ;5) The compatibility is determined by the following equation: ;;In the following, the % compatibility of amine fluoride with AF is denoted. ;Measurement of compatibility with cetylamine hydrogen fluoride ;1) An aqueous solution (1) containing 1.7 2% amine fluoride is established by dissolving 1.72 g of cetylamine hydrogen fluoride ;in 98.28 g of double-distilled water. 2) 4 g of silica is dispersed in 16 g of the solution obtained under 1). The resulting suspension is stirred for four weeks at 37°C. ;3) The suspension is then centrifuged at 8,000 revolutions/minute for 30 minutes and the supernatant obtained under (3) is filtered through Millipore filter 0.22 µm. ;4) The concentration of free amine fluoride is determined by microanalysis of nitrogen in the solution obtained under 1) and the supernatant obtained under 3). ;5) The compatibility is determined by the following equation: ;;In the following, % compatibility with amine fluoride is denoted by ;AFC. ;Measurement of compatibility with an alkyl betaine. Of the alkyl betaine used is a product sold by Akso under the name "ARMOTERIC" LB. 1) An aqueous solution (1) containing 2.0% alkyl betaine is established by dissolving 6.67 g of 30% alkyl betaine in ;9 3.33 g of double-distilled water. ; 2) 4 g of silica are dispersed in 16 g of solution obtained under 1) ; The thus obtained suspension is stirred for four weeks at 37°C. 3) The suspension is then centrifuged at 8,000 revolutions/minute for 30 minutes and the supernatant obtained under (3) is filtered through Millipore filter 0.22 µm. 4) The concentration of free alkyl betaine is determined by microanalysis of organic carbon in the solution obtained under 1) and in the supernatant obtained under 3). ;5) The compatibility is determined by the following equation: ;In the following, % compatibility with alkyl betaine is denoted by aBeta. ;Measurement of compatibility with an alkylamidoalkylbetaine ;1) An aqueous solution (1) containing 2.0% alkylamidoalkylbetaine is established by dissolving 6.67 g of alkylamidoalkylbetaine 30% in 93.33 g of double-distilled water. 2) 4 g of silica are dispersed in 16 g of the solution obtained under 1). The obtained suspension is stirred for four weeks at 37°C. ;3) The obtained suspension is centrifuged at 8,000 revolutions/minute for 30 minutes and the supernatant obtained during (3) is filtered through a Millipore filter; 0.22 pm. ;4) The concentration of free alkylamidoalkylbetaine is determined by microanalysis of organic carbon in the solution ;obtained under 1) and the supernatant obtained 3). ;5) The compatibility is determined by the following equation: ;In the following, % compatibility with alkylamidoalkylbetaine is denoted by Beta. ;Measurement of compatibility with chlorhexidine ;4 g of silica is dispersed in 16 g of an aqueous solution of chlorhexidine with a 1% concentration of chlorhexidine di-gluconate. The suspension is stirred for 24 hours at 37°C. The suspension is then centrifuged at 20,000 rpm for 30 minutes and the supernatant obtained is filtered through Millipore filter 0.2 µm. ;Then 0.5 ml of the filtered solution is taken out and diluted in 100 ml of water in a graduated measuring glass. This solution constitutes the experimental solution. ;A reference solution is mixed using the same protocol but in the absence of silica. An aqueous solution of 1% chlorhexidine digluconate is stirred for 24 hours at 37°C and then filtered at 20,000 rpm and the supernatant filtered through the Millipore filter 0.2 µm. 0.5 ml of the solution thus obtained is diluted in 100 ml of water in a graduated test glass. The absorption of the two solutions is then measured at 254 nm using a spectrophotometer (Uvikon 810/820). ;The amount of free chlorhexidine calculated as % compatibility is determined using the equation: ;Measurement of compatibility with fluorides ;4 g of silica is dispersed in 16 g of a 0.3% solution of sodium fluoride (NaF). The suspension is then stirred for 24 hours at 37°C. After centrifugation of the suspension at 20,000 revolutions/minute for 30 minutes, the supernatant is filtered through Millipore filter 0.2 µm. The solution thus obtained constitutes the experimental solution. ;A comparison solution is formed using the same protocol but in the absence of silica. ;The compatibility with. the fluoride is determined based on % free fluoride measured using a fluoride-selective electrode (Orion). It is determined using the following equation: Measurement of compatibility with zinc. ;4 g of silica is dispersed in 100 ml of a 0.06% solution of ZnSo4*7H20. A suspension is obtained where the pH is stabilized at 7 for 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 is filtered through Millipore filter 0.2 µm and constitutes the test solution.

A reference solution is mixed following the same protocol but in the absence of silica.

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

The compatibility is determined by the following equation:

Measurement of compatibility with pyrophosphates of sodium and potassium 4 g of silica are dispersed in 16 g of a 1.5% suspension of sodium or calcium pyrophosphate. The suspension is stirred for 24 hours at 37°C, after which it is centrifuged at 20,000 revolutions/minute for 30 minutes.

The supernatant is filtered through Millipore filter 0.2 µm. 0.2 g of diluted solution in 100 ml of water in a graduated measuring glass 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 by the ratio of the area of peaks in chromatograms and corresponds to the retention time of pyrophosphate for the test solution and the comparison solution.

EXAMPLE 1

In a reactor equipped with a system for regulating temperature and pH and a screw-shaped agitator system (Mixel), 8.32 1 of sodium silicate with a silica concentration of 130 g/l and molar ratio Si02/Na2<0 = 3.5 and 8.33 1 of ion-exchange treated water are introduced with conductivity 1 pS/cm.

After continuing the stirring (350 rpm), the resulting starting solution 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 bring the pH to 9.5.

You continue with the simultaneous addition of 45.23 1 sodium silicate with a silica concentration of 130 g/l with a molar ratio SiO2/Na20 = 3.5 and an amount of 0.754 l/minute and 29.64 1 80 g/l sulfuric acid. 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 a quantity 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 maintained at 8 by the addition of acid.

At the end of the maturation, the pH is changed to 5.5 by adding sulfuric acid in an amount of 0.494 l/minute and a maturation is then carried out for 30 minutes at this pH at 95°C.

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

After cessation of heating, the mixture is filtered and the resulting filter cake is washed with deionized water until a filtrate with a conductivity of 2,000 uS/cm is obtained. The filter cake obtained after washing is dispersed in the presence of deionized water to form a suspension of silica with a concentration of 10%. The pH in the suspension is adjusted to 6 by adding acetic acid.

A further 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 acetic acid so that the pH is set to pH 5.5.

In what follows, it is established that the following equation holds true:

pH < 8.20 - 0.91 log (D)

The filter cake is then divided and the silica is dried by atomization. The resulting silica is then finally ground on a knife mill to obtain a powder with an average diameter of the agglomerates measured by Counter-Coulter and 8 um.

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

Table I repeats the chemical surface properties of silica in accordance with the invention and table I gives the results for compatibility with organic amine compounds and in table II the results for compatibility with classic components in dentifrice mixtures such as chlorhexidine, fluoride, zinc, pyrophosphate.

EXAMPLE 2

In a reactor equipped with a system for regulating temperature and pH and a system with a screw agitator (Mixel), 5.30 1 sodium silicate with a silica concentration of 135 g/l and molar ratio SiO2/Na20 = 3.5 and 15.00 1 ion-exchange treated water is introduced with conductivity 1 uS/cm.

After starting stirring (350 revolutions/minute), the starting mixture thus established 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 change the pH to 9.5.

One continues with the simultaneous addition of 44.70 1 sodium silicate with silica concentration 135 g/l with molar ratio SiC>2/Na20 = 3.5 and supply quantity 0.745 l/minute and 25.30 1 sulfuric acid with 80 g/l. The addition of sulfuric acid is regulated 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 a quantity of 0.350 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 at 95°C. During ripening, the pH is maintained at 8 by the addition of acid.

After maturation, the pH is brought to 5.0 by adding sulfuric acid in a quantity of 0.400 l/minute and maturation is then carried out for 30 minutes at this pH at 95°C.

At the end of ripening, the pH is brought to 3.5 by adding sulfuric acid. This pH is maintained at 3.5 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 2,000 uS/cm is obtained.

The filter cake is then split in the presence of water to form a suspension with 20% silica and the pH adjusted to 5.1 to confirm the following equation:

Silica is dried by atomization and ground in a knife mill to obtain a powder with an average agglomerate diameter of 8 µm.

The physical properties of the silica thus obtained are listed in the following table:

In table I are listed the chemical surface properties of silica in accordance with the invention and in table I are also listed results for compatibility with organic amine compounds and in table II results for compatibility with classic components within dental care agents such as chlorhexidine, fluoride, zinc, pyrophosphate.

EXAMPLE 3

In a reactor equipped with a system for regulating temperature and pH and a system with a screw agitator (Mixel), 5.60 1 sodium silicate with silica concentration 135 g/l and molar ratio SiC>2/Na20 = 3.5 are introduced.

After starting stirring (350 revolutions/minute), the starting mixture thus established is heated to 85°C.

When this temperature is reached, continue with the addition of sulfuric acid with a concentration of 85 g/l heated to 70°C with a constant supply of 0.50 l/minute to bring the pH to 9.7.

You continue with the simultaneous addition of 52.64 1 sodium silicate with a silica concentration of 135 g/l, molar ratio SiO2/Na20 = 3.5 and addition 0.745 l/minute and 30.00 1

85 g/l sulfuric acid. The addition amount of sulfuric acid is set so that the pH in the reaction mixture is maintained at a constant value of 9.7. The simultaneous addition is carried out at 85°C using preheated reaction components at 70°C.

After 45 minutes of addition, the addition of sodium silicate is stopped and the addition of sulfuric acid is continued at a rate of 0.450 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 ten minutes at this pH and 95°C. During ripening, the pH is maintained at 8 by the addition of acid.

At the end of the ripening, the pH is brought to 5.0 by adding sulfuric acid in a quantity of 750 l/minute and then a ripening is carried out for 15 minutes at this pH and 95°C.

At the end of maturation, the pH is brought to 3.7 by adding sulfuric acid. This pH is maintained at 3.7 for 60 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 equal to 2,500 uS/cm is obtained.

The filter cake is then split in the presence of water to form a suspension with 20% silica and the pH is adjusted at 5.5 to confirm the following equation:

Silica is dried by atomization and ground on a knife mill to obtain a powder with an average agglomerate diameter of 8 µm.

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

In the following table I, the chemical surface properties of silica are listed in accordance with the invention described in examples 1 - 3.

Table I also lists the compatibility results for silica according to the invention with organic amine compound.

Table II mentions the results of the compatibility test with classic components included in toothpaste mixtures, namely chlorhexidine, fluoride, zinc, pyrophosphate.

For comparison, the properties and different compatibilities with commercial silica are listed in tables I and II, where the list defined above constitutes a representative range for classic silica.

TABLE I

The physicochemical properties and compatibility with organic amine compounds for silica according to the invention and ordinary silica.

The meaning of the symbols used in the preceding table is given in the following: pH/log(C) represents the equation pH = b - a log(C) where a and b are two constants and C is the weight percentage of

the silica suspension.

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 suspension of silica in uSiemens/cm.

SE stands for the effect of the suspension measured using the ratio SE = pH-suspension - pH-supernatant defined

previously.

She is Hammett's constant.

PZC represents the pH at which the surface charge of

silica is 0.

AF, AFc, Beta, aBeta and CHx represent percentage compatibility of fluorinated amines, alkylbetaine and chlorhexidine, respectively. The sizes have been defined previously.

Percent compatibility obtained with amine fluoride AFc and alkylbetaine aBeta are similar to those obtained for AF and Beta.

The results in the above-mentioned table I show that silica in accordance with the invention and which are compatible in particular with organic amine compounds are quite different from classical silica by means of the following equations:

Claims (26)

1. Silica, characterized in that it leads to an aqueous suspension where the pH varies as a function of its concentration in areas defined using the following equations: and where pH varies as a function of its electrical conductivity in the range defined by the two equations: in the two equations (Ia) and (Ib), represents (C) the weight concentration of that van dige silica suspension expressed as % SiO2, and in equations (Ila) and (Ilb), represents (D) the electrical conductivity of the aqueous suspension of silica expressed in microsiemens.cm-<1>. 2. Silica as specified in claim 1, characterized in that it has a surface chemistry such that its acidity function Ho is at least 4.0. 3. Silica as stated in claim 1 or 2, characterized in that it has a surface chemistry such that the number of OH~ expressed as OH-/nm<2> is at most 12, preferably between 6 and 10. 4. Silica as stated in one or more of claims 1-3, characterized in that it has a point of zero charge (PZC) of at least 4, preferably between 4 and 6. 5. Silica, characterized in that it has a compatibility with organic amine compounds of at least 30%. 6. Silica as specified in claim 5, characterized in that it has a compatibility with organic amine compounds of at least 50%, more particularly at least 80%. 7. Silica as stated in claim 5 or 6, characterized in that it has a compatibility with organic amino compounds selected from the group of amine fluorides, amine oxides, alkyl amines and alkyl betaines. 8. Silica as specified in claim 1, characterized in that it has a compatibility with divalent and multivalent metal cations selected from groups 2a, 3a, 4a and 8 in the periodic table of at least 50%, more particularly at least 70%. 9. Silica as specified in claim 1, characterized in that it has a compatibility with products of the guanidine type, in particular chlorhexidine of at least 30% and more particularly at least 60%. 10. Silica as specified in one or more of claims 1-9, characterized in that it has a content of anions of the type SC>4<2>~, Cl~, NC>2<_>, PC<3~, CO3< 2->at most 5•10~<3>mol for 100 g of silica. 11. Silica as stated in claim 10, characterized in that it has a content of the mentioned anions of no more than 1 • IO-<3>, more particularly no more than 0.2 • 10~<3> mol per 100 g of silica. 12. Silica as specified in one or more of the preceding claims, characterized in that it has a sulfate content expressed as SO4 of no more than 0.1%, preferably no more than 0.05% and most preferably no more than 0.01%. 13. Silica as stated in one or more of claims 1-12, characterized in that it has a content of divalent metal cations of no more than 1,000 ppm. 14. Silica as specified in claim 13, characterized in that the content of aluminum is at most 500 ppm, the content of iron is at most 200 ppm, the content of calcium is at most 500 ppm and most especially at most 300 ppm. 15. Silica as stated in one or more of claims 1-14, characterized in that it has a carbon content of no more than 50 ppm and more particularly no more than 10 ppm. 16. Silica as stated in one or more of claims 1-15, characterized in that it has a pH of at most 8 and more particularly between 6 and 7.5. 17. Method for producing a silica as stated in one or more of claims 1-16, characterized by reacting a silicate with an acid so that a suspension or gel of silica is obtained, after which a first maturation of pH at least 6 and not more than 8.5, and then a second ripening at pH not more than 6.0, and a third ripening at pH not more than 5.0, the said silica being separated and subjected to a washing with water until it leads to an aqueous suspension where the pH measured on a suspension with 20% SiC>2 corresponds to the following equation: in which the equation (III) e is a constant of at least 0.6 and at most 1.0, d is a constant of at least 7.0 and at most 8.5, (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsiemens, after which the suspension is dried. 18. Method as stated in claim 17, characterized in that the suspension of silica gel is produced by simultaneously adding the silicate and the acid to a starting mixture which can be water, a colloidal dispersion of silica containing 0 - 150 g/l silica expressed as SiC>2 , a silicate or an inorganic or organic salt, preferably of an alkali metal. 19. Method as stated in claim 18, characterized in that the addition of the reaction components is carried out simultaneously so that the pH is maintained constant between 4 and 10, preferably between 8.5 and 9.5. 20. Method as stated in claim 18 or 19, characterized in that the temperature is between 60 and 95°C. 21. Method as stated in claim 18, characterized in that the colloidal dispersion of silica containing 20 - 150 g/l silica is prepared by heating the aqueous solution of silicate between 60 and 95°C and adding the acid in the said aqueous solution until obtaining of a pH between 8.0 and 10.0, preferably 9.5. 22. Method as stated in claim 17, characterized by producing a suspension or a gel of silica by gradually adding the acid to a starting mixture containing the silicate until a desired pH is achieved during maturation at a temperature between 60 and 95°C. 23. Procedure as specified in one or more of claims 17 - 20, 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. 24. Procedure as specified in one or more of claims 17 - 23, characterized in that another maturation of the suspension or gel of silica is carried out at a pH between 5 and 6, preferably 5.5, at a temperature between 60 and 100°C, preferably 95°C. 25. Method as specified in one or more of claims 19 - 24, characterized in that a third maturation of the suspension or gel of silica is carried out at a pH between 3 and 5, preferably 4, at a temperature between 60 and 100°C, preferably 95°C. 26. Tooth care product, characterized in that it contains a silica as stated in one or more of claims 1-16 or a silica produced using the method as stated in one or more of claims 17-25.