HK8194A

HK8194A - Silica for dentifrice compositions which is compatible with organic amino-compounds

Info

Publication number: HK8194A
Application number: HK81/94A
Authority: HK
Inventors: Jacques Persello
Original assignee: Rhone-Poulenc Chimie
Priority date: 1989-05-03
Filing date: 1994-01-21
Publication date: 1994-02-04
Also published as: CA2015922C; SG126693G; EP0396459B2; PT93949B; FI91386C; EP0396459A1; FR2646664A1; ES2045840T5; FR2646664B1; AU633083B2; EP0396459B1; TR24265A; IL94246A0; TNSN90058A1; CN1024527C; KR920008564B1; CN1047066A; NO901883D0; DE69000328D1; CN1081100A

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

The present invention relates to a silica which can be used in toothpaste, its preparation process and in toothpaste compositions containing this silica.

Silica is known to be a common ingredient in toothpaste and can play several roles.

It acts primarily as an abrasive by helping to remove plaque by its mechanical action.

It can also act as a thickening agent to give toothpaste certain rheological properties and as an optical agent to give it the desired colour.

It is also known that toothpaste generally contains a source of fluoride, most often sodium fluoride or monofluorophosphate used as an anti-causal agent; a binding agent, for example, an algal colloid such as carrageenan, guar gum or xanthan gum; a moisturizing agent which may be a polyalcohol, for example, glycerin, sorbitol, xylitol and propylene glycol.

Err1:Expecting ',' delimiter: line 1 column 157 (char 156)

Err1:Expecting ',' delimiter: line 1 column 558 (char 557)

Examples of this class of amphoteric surfactants are: . alkylbetaines with the formula: Alkylamidopropyldimethylbetanes with the formula: - What?

R being a linear or branched alkyl radical with 10 to 24 carbon atoms.

The presence of these organic amino compounds raises the problem of their compatibility with silica, which, in particular because of its adsorbent capacity, may tend to react with these compounds in such a way that they can no longer fulfil their function.

The purpose of the present invention is therefore to provide a new silica which is compatible with the above mentioned organic amino compounds, particularly those of the fluorine and betaine classes, and which can therefore be used in the formulation of toothpaste.

Another object of the invention is also to provide a silica which is well compatible with the various cations present in the toothpaste formulation, such as zinc, strontium, tin, etc.

Another object of the invention is to provide a silica which is also most compatible with products of the guanidine type, in particular bis-biguanides, the most representative element of which is chlorhexidine.

Finally, another subject matter of the invention is the process of preparing such compatible silica.

The applicant therefore found that the compatibility properties sought depended essentially on the surface chemistry of the silica used and established a number of conditions on the surface of the silica to ensure that it was compatible.

The silica of the present invention is characterized by the fact that it leads to an aqueous suspension whose pH varies according to its concentration in the area defined by the two equations: and and whose pH varies according to its electrical conductivity in the area defined by the two inequalities: and - in the inequalities (a) and (b), . (C) is the weighted concentration of the aqueous suspension of silica, expressed as % Si02,- in the equations (Ila) and (Ilb), . (D) is the electrical conductivity of the aqueous suspension of silica expressed in microseconds.cm-1.

Another characteristic of the silica of the invention is that it has an acidity function Ho of at least 4.0.

Another feature of the silica of the invention is that it has a number of OH- sites per nm2 less than or equal to 12.

Another feature of the silica of the invention is that it has a zero charge point (ZCP) of at least 4.

A characteristic of the silica of the invention is that it has a compatibility of at least 30% with the amino organic compounds and in particular of at least 50% and preferably at least 80% with the amino organic compounds selected from the group formed by the fluorine amines, the amine oxides, the alkylamines and the alkylbetaines.

Another characteristic of the silica of the invention is that it has a compatibility of at least 50%, and more particularly of at least 70% with metal cations.

Another feature of the silica of the invention in one embodiment is that it has a compatibility with products of the guanidine type, in particular chlorhexidine, of at least 30% and more particularly of at least 60%.

The present invention also relates to one of the processes for preparing the silica of the invention which is characterized by the reaction of a silicate with an acid leading to a silica suspension or gel; first ripening at a pH of 6 or less or 8.5; second ripening at a pH of 6.0 or less; third ripening at a pH of 5.0 or less; separation of the silica to a water wash until it is subjected to an aqueous suspension with a pH measured on a 20 Si02 suspension of 20 % if the following equation is satisfied: in equation (III) . e is a constant greater than or equal to 0.6 and less than or equal to 1.0. d is a constant greater than or equal to 7.0 and less than or equal to 8.5. (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microsie-mens.cm-1; finally, to dry.

Finally, the invention relates to toothpaste compositions characterized by the presence of silica as described above or prepared by the process just mentioned above.

Other features and advantages of the invention will be better understood by reading the description and the practical examples which follow.

The silica of the present invention is characterized by the fact that the pH of its aqueous suspension varies according to its concentration and electrical conductivity according to the equations given above.

The protocol for measuring pH in relation to the concentration of the aqueous suspension of silica and its electrical conductivity is given below.

As stated in the introduction, the essential characteristics of the silices of the invention lie in their surface chemistry. More specifically, one of the aspects to be taken into account in this surface chemistry is acidity.

Here the acidity is taken in the Lewis sense, that is, it reflects the tendency of a site to accept a pair of electrons from a base according to the equilibrium:

Err1:Expecting ',' delimiter: line 1 column 112 (char 111)

The function Ho is defined by the classical relation:

To determine the strength of the acid sites of a silica of the invention by the Hammett method, the indicator method originally described by Walling (J. Am. Chem. Soc. 1950, 72, 1164) is used.

The strength of the acid sites is determined by coloured indicators, which under the conditions of use give the pKa of the transition between acid and basic forms.

The lower the pKa of the indicator undergoing the change of colour, the higher the acidity of the site. - What?

The color of the adsorbed indicators on a silica is a measure of the strength of the acid sites. If the color is that of the acid form of the indicator, then the value of the Ho function of the surface is equal to or less than the pKa of the indicator.

Low Ho values correspond to high strength acid sites.

Thus, for example, silica giving a red colour with p-dimethylaminoazobenzene and yellow with 2-amino-5-azotoluene will have an acidity function of between 3.3 and 2.

Experimentally, the dosage is given with 0,2 g of silica in a test tube in the presence of a 100 mg/I indicator solution in cyclohexane.

The silica is first dried at 190°C for 2 hours and stored in a desiccator away from moisture.

By agitation, adsorption, if it occurs, occurs within a few minutes and the change in colour visible to the naked eye or possibly by studying the adsorption spectra characteristic of the adsorbed coloured indicators, both in their acidic and basic forms, is noted.

The first characteristic of the silices of the invention is that they have an acidity function as determined above of at least 4.0.

The surface state of the silica of the invention is such that conditions on the number of surface sites are met.

The number of surface OH- sites is determined as follows: the number of surface OH- sites is equal to the amount of water released by silica between 190°C and 900°C.

The silica samples are dried at 105 °C for 2 hours.

A mass Po of silica is placed in a thermobalance and raised to 190°C for 2 hours: or P190, the mass obtained.

The silica is then heated to 900°C for 2 hours, or P900 the new mass obtained.

The number of OH- sites is calculated by the following equation: in which: - NαH- is the number of OH- sites per nm2 of surface- A is the specific surface of the solid as measured by BET and expressed in m2/g.

In the present case, the silices of the invention have an advantageous number of OH-/nm2 less than or equal to 12, especially not more than 10 and in particular between 6 and 10.

The nature of the OH- sites of the silicates of the invention which is also a characterization of their surface chemistry can also be appreciated from the zero charge point.

This zero charge point (Zpc) is defined as the pH of a silica suspension for which the electrical charge on the surface of the solid is zero regardless of the ionic strength of the medium.

The principle of the method is based on the overall balance of adsorbed or desorbed protons on the surface of the silica at a given pH.

From the 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 zero surface charge, is given by the equation: in which: - A is the specific surface area of the solid in m2/g,- M is the amount of solid in the suspension in g,- F is the Faraday-, (H+) or (OH-) represents the change per unit area of the excess H+ or OH- ions on the solid respectively.

The experimental protocol for determining the CCP is as follows:

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

The silica is first washed in highly resistive deionized water (10 Mega. Ohm. cm), dried and then degassed.

In practice, a series of solutions at pHo = 8,5 is prepared by adding KOH or HNO3 and containing an indifferent electrolyte (KN03) at a concentration varying between 10-5 and 10-1 Mole/1.

A given mass of silica is added to these solutions and the pH of the resulting suspensions is allowed to stabilise under agitation at 25°C and nitrogen for 24 hours; or pH o its value.

Standard solutions are the supernatant obtained by centrifuging for 30 min at 1000 t/min of a part of these same suspensions; or pH'o, the pH of these supernatants.

The pH of a known volume of its suspensions and corresponding standard solutions is then brought to pHo by adding the required amount of KOH and the suspensions and standard solutions are allowed to stabilise for 4 hours.

So VoH- NoH- the number of base equivalents added to change from pH'o to pHo of a known volume (V) of suspension or standard solution.

Potentiometric dosing of suspensions and standard solutions shall be carried out from pHo by addition of nitric acid to pHf = 2,0.

Preferably, an acid increment is added corresponding to a pH change of 0,2 pH units. After each addition, the pH is stabilised for 1 min.

So VH+. NH+ is the number of acid equivalents to get to pHf.

From pHo, the term (VH+ . NH+ - VOH- . NOH-) is plotted as a function of the incremented pH for all suspensions (at least 3 ionic forces) and for all corresponding standard solutions.

For each pH value (not 0.2 units), the difference between the consumption of H+ or OH- for the suspension and the corresponding standard solution is then made and this operation is repeated for all ionic forces.

This gives the term (H+) - (OH-) corresponding to the surface's proton consumption.

The surface charge curves are then plotted as a function of pH for all the ionic forces considered.

The silica concentration is adjusted according to the specific silica surface.

For example, 2% suspensions are used for silices of 50 m2/g at 3 ionic forces (0.1 ; 0.01 and 0.001 mole/1).

The dosage is performed on 100 ml of suspension using 0.1 M potassium hydroxide.

In practice, it is preferable to have a CZP of at least 4, especially between 4 and 6.

Also for the improvement of compatibility, particularly with fluorine, it is of interest that the content of bivalent and higher cations contained in the silica be at most 1000 ppm.

On the other hand, the iron content of the silicates of the invention can be advantageously up to 200 ppm.

In addition, the calcium content may be preferably not more than 500 ppm and more particularly not more than 300 ppm.

The silicates of the invention also preferably have a carbon content of not more than 50 ppm and more particularly not more than 10 ppm.

The silices of the invention which are compatible with the amino organic compounds are also compatible with the various metal cations involved in toothpaste compositions.

The most important examples are the bivalent metal cations, which are more commonly found in groups 2a, 3a, 4a and 8 of the periodic table, and in particular the cations of group 2a, calcium, strontium, barium, cations of group 3a, aluminium, indium, group 4a, germanium, tin, lead and group 8 manganese, iron, nickel, zinc, titanium, zirconium, palladium, etc.

These cations may be in the form of mineral salts, e.g. chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts acetate, citrate, etc.

More specific examples include zinc citrate, zinc sulphate, strontium chloride, tin fluoride in the form of single salt (SnF2) or double salt (SnF2, KF), tin fluoride (SnCIF), zinc fluoride (ZnF2).

The silices of the invention are compatible with the various metal cations, the compatibility of which with the cations as determined by the tests described below is at least about 50%, preferably at least 70%, and even more preferably at least 80%.

The silicates of the invention can therefore be used advantageously in toothpaste compositions containing bivalent cations and more and, more particularly, in compositions containing at least one of the following components: zinc citrate, zinc sulphate, strontium chloride, tin fluoride.

Depending on a particular mode of the invention, the silicates of the invention may be further compatible with guanidins and, in particular, with chlorhexidine. The compatibility measured by the test described below is at least about 30%. It may be improved to at least 60%, and preferably at least 90%.

In this case, the silica has a content of anions of the type S042-, CI-, N03-, P043-, C032- of not more than 5.10-3 moles per 100 g of silica.

In the case of silices prepared from sulphuric acid, this anion content is more conveniently expressed by a content expressed in S04= and by weight. In this case this content is not more than 0.1%; according to a preferred variant of the invention, this content is not more than 0.05% and more particularly not more than 0.01%.

The silica of the invention is therefore particularly well suited for use in toothpaste formulations containing guanidines, bisguanides, and the silica described in US Patent 3 934 002 or 4 110 083.

The pH of the silicates of the invention measured in accordance with NFT 45-007 is generally greater than 8.

The above characteristics allow a silica compatible with the above-mentioned amino organic compounds, metal cations and as appropriate, in addition to fluorides and guanidines, in particular chlorhexidine.

In addition to the surface chemistry characteristics described above which are the conditions for compatibility, the silices of the invention also have physical characteristics which make them particularly suitable for use in toothpaste.

Generally the BET surface area of the silica of the invention is between 40 and 600 m2/g, more particularly between 40 and 350 m2/g. Their CTAB surface area usually varies between 40 and 400 m2/g, more particularly between 40 and 200 m2/g.

The BET surface is determined by the BRU NAUER-EMM ET-TELLER method described in the Journal of the American Chemical Society, vol. 60, page 309, February 1938, and by the standard NF X11-622 (3.3).

The CTAB surface is the outer surface determined according to ASTM D3765 but by adsorption of hexadecyltrimethylammonium bromide (CTAB) at pH 9 and by taking as the projected area of the CTAB 35 Â2 molecule.

The silicates of the invention may of course correspond to the three types usually distinguished in the field of toothpaste.

Thus, the silica of the invention can be of the abrasive type, and have a BET surface area of 40 to 300 m2/g. In this case, the CTAB surface area is 40 to 100 m2/g.

The silica of the invention may also be of the thickening type, and have a BET surface area of 120 to 450 m2/g, and in particular 120 to 200 m2/g. They may then have a CTAB surface area of 120 to 400 m2/g, and in particular 120 to 200 m2/g.

Finally, according to a third type, the silica of the invention may be bifunctional, where the BET surface is between 80 and 200 m2/g. The CTAB surface is then between 80 and 200 m2/g.

The silica of the invention may also have an oil intake of 80 to 500 cm3/100 g as determined by NFT 30-022 (March 53) using dibutyl phthalate.

Specifically, this oil intake will be between 100 and 140 cm3/100 g for abrasive silica, 200 and 400 for thickening silica and 100 and 300 for bifunctional silica.

In addition, silicates are preferably of a particle size of 1 to 10 μm, also for use in toothpaste.

This mean particle size (d50) is measured by Counter-Coulter.

The apparent density will generally vary between 0.01 and 0.3.

Finally, the silicates of the invention have a refractive index generally between 1.440 and 1.465.

The method of preparation of the silices of the invention will now be described in more detail.

As mentioned above, this process is of the type involving the reaction of a silicate with an acid resulting in the formation of a silica suspension or gel.

It should be noted that any known method of operation can be used to obtain this suspension or gel (addition of acid to a silicate tank base, simultaneous total or partial addition of acid and silicate to a water tank base or silicate solution, etc.), the choice being essentially based on the physical characteristics of the silica to be obtained.

A preferred embodiment of the invention is to prepare the silica suspension or gel by simultaneously adding silicate and acid to a tank base which may be water, a colloidal silica dispersion containing 0-150 g/I silica expressed as Si02, a silicate or mineral or organic salt, preferably of an alkaline metal such as, for example, sodium sulphate, sodium acetate.

The two reagents are added simultaneously so that the pH is kept constant between 4 and 10, preferably between 8.5 and 9.5.

A method of preparing colloidal silica dispersion preferably in a concentration of 20 to 150 g/1 is to heat an aqueous silicate solution, e.g. between 60 °C and 95 °C, and to add acid to the aqueous solution until a pH of 8,0 to 10,0 is obtained, preferably around 9,5.

The concentration of the aqueous silicate solution expressed as Si02 is preferably between 20 and 150 g/I. A dilute or concentrated acid may be used: its normal range may be between 0.5 N and 36 N, preferably between 1 and 2 N.

In the above, silicate is to be understood as an alkaline metal silicate and preferably a sodium silicate with a Si02/Na20 ratio of between 2 and 4, preferably 3.5.

In another step of the invention process, the suspension or freeze is subjected to ripening operations.

The first ripening is carried out at a pH of not more than 8.5 and between, for example, 6 and 8.5, preferably 8.0.

Another embodiment of the invention is to prepare a silica suspension or gel by gradually adding the acid to a silicate-containing tank foot until the desired ripening pH is reached. This operation is carried out at a temperature of preferably 60°C to 95°C. The silica gel suspension is then ripened under the conditions described above.

A second ripening is then carried out at a pH below 6, preferably between 5 and 6, and even more preferably at 5.5.

The temperature and duration of the conditions are those of the first ripening.

To this end, the pH is brought to the desired ripening pH by the addition of acid.

For example, a mineral acid such as nitric, hydrochloric, sulphuric, phosphoric or carbonic acid can be used.

A third ripening is carried out at a pH below 5.0, preferably between 3 and 5, and even more preferably around 4.0.

The temperature and duration of the other two maturing processes are the same, and the desired pH is reached by adding acid.

The silica is then separated from the reaction medium by any known means, such as a vacuum filter or a press filter.

This is how you collect a silica cake.

The process according to the invention can then be implemented in two main ways.

The first variant concerns the preparation of silices compatible with organic amino compounds and bivalent metal cations and more.

In this case, the process involves washing the cake under such conditions that the pH of the suspension or medium before drying must meet the following equation: in equation (III) . e is a constant greater than or equal to 0.6 and less than or equal to 1.0. d is a constant greater than or equal to 7.0 and less than or equal to 8.5. (D) represents the electrical conductivity of the aqueous suspension of silica expressed in microseconds.cm-1.

For this purpose, it may be considered to wash with water, usually deionized, and/or wash with an acid solution with a pH between 2 and 7.

This acid solution may be, for example, a solution of a mineral acid such as nitric acid, but depending on the particular embodiment of the invention, this acid solution may also be a solution of an organic acid, in particular a complexing organic acid, which may be chosen from the group of carboxylic, dicarboxylic, hydroxycarboxylic and amino-carboxylic acids.

Examples of such acids are acetic acid and, for complexing acids, tartaric acid, maleic acid, glyceric acid, gluconic acid and citric acid.

It may be advantageous, especially in the case of a solution of mineral acid, to wash the final product with deionized water.

The second variant relates to the preparation of silicates which are also compatible with guanidins and, in particular, chlorhexidine.

In this case, further washing is carried out and must be continued until a washing filtrate with a conductivity of not more than 200 microsiemens.cm-1 is obtained, preferably less than 100 microsiemens.cm-1.

One or more washes, usually two with water, preferably deionized and/or an aqueous solution of organic acid, including the above, may be carried out as appropriate.

From a practical point of view, washing operations can be carried out by passing the washing solution over the cake or by introducing it into the resulting suspension after the cake has been removed.

The filter cake is subjected to a de-drying process before drying, which can be carried out by any known means, for example by means of a high-speed stirrer.

The silica cake is therefore delimbed before or after washing and then dried by any known means, for example in a tunnel oven or a muffle oven or by atomization in a hot air stream, the temperature of which may vary between 200°C and 500°C at the inlet and between 80°C and 100°C at the outlet, with a resting time of between 10 seconds and 5 minutes.

The dried product can be ground if necessary to obtain the desired particle size.

The invention also relates to toothpaste compositions containing silica of the type described above or obtained by the process just studied.

The amount of silica according to the invention used in toothpaste compositions can vary widely: it is usually between 5 and 35%.

The silicates of the invention are particularly suitable for toothpaste compositions containing at least one element selected from the group comprising fluorides, phosphates, guanidins, including chlorhexidine, as they can show compatibility according to the tests described below of at least 90% for each of their elements.

They are also well suited to toothpaste compositions containing at least one element selected from the group comprising the organic amino components and the components providing a bivalent metal cation and above.

The silicates of the invention are particularly well suited to toothpaste formulations containing at least one element from the group of simple mineral fluorides and organic fluorides, at least one element from the group of alkylbetaines and at least one element from the group of guanidines, by particularization of chlorhexidine.

Specifically, formulations containing sodium fluoride and/or tin fluoride and/or a fluoridated amine include cetylamine hydrofluoride, bis ((hydroxyethyl) -aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoride and an alkylbetaine and chlorhexidine.

The silicates of the invention are also compatible with maleic acid-vinylethyl ether copolymers and can therefore also be incorporated into toothpaste compositions containing these copolymers.

As regards fluorine compounds, their quantity should preferably be between 0.01 and 1% by weight in the composition and in particular between 0.1% and 0.5% by weight. Fluorinated compounds are in particular the salts of monofluorophosphoric acid and in particular those of sodium, potassium, lithium, calcium, aluminium and ammonium, mono and difluorophosphate, as well as various fluorides containing fluorine in the form of a bound ion, in particular alkaline fluorides such as sodium, lithium, potassium, ammonium fluoride, tin fluoride, manganese fluoride, zirconium fluoride, aluminium fluoride and products of addition of these fluorides with each other or with other fluorides, such as sodium fluoride or potassium fluoride.

Other fluorides are also usable for the present invention, such as zinc fluoride, germanium fluoride, palladium fluoride, titanium fluoride, alkaline fluozirconates such as sodium or potassium, stannous fluozirconate, fluoborate or sodium, potassium fluosulfates.

The organic fluorine compounds mentioned at the beginning of the description may also be used, preferably cetylamine fluoride and bis- ((hydroxyethyl) aminopropyl N-hydroxyethyl octadecylamine dihydrofluoride.

As regards the components carrying the bivalent metal cations and more, among those mentioned in the description, the most commonly encountered are zinc citrate, zinc sulphate, strontium chloride, tin fluoride.

For the elements usable as antiplaque agents of the type polyphosphates or polyphosphonates, guanidins, bis-biguanides, the ones mentioned in US Patents 3.934.002 or 4.110.083 may be mentioned.

Toothpaste compositions may also contain a binder.

The main binders used are selected from among:- cellulosic derivatives: methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose sodium,- mucilages: carrageenates, alginates, agar-agar and gels,- gums: arabic and adragante gums, xanthan gum, karaya gum,- carboxyvinyl and acrylic polymers,- polyoxyethylene resins.

In addition to the silicates of the invention, toothpaste compositions may also contain one or more other polishing abrasives selected from amongst others: - precipitated calcium carbonate, - magnesium carbonate, - calcium, di- and tricalcium phosphates, - insoluble sodium metaphosphates, - calcium pyrophosphate, - titanium oxide (bleaching agent), - silicates, - alumines and silicon-aluminates, - zinc and tin oxides, - talc, - kaolin.

Toothpaste compositions may also include surfactants, moisturizers, flavoring agents, sweeteners and colours and preservatives.

The main surfactants used are selected from: - sodium lauryl sulphate,- sodium lauryl ether sulphate and sodium lauryl sulphoacetate,- sodium dioctyl sulfosuccinate,- sodium lauryl sarcosinate,- sodium ricinoleate,- sulphate monoglycerides.

The main moisturizers used are selected from among polyalcohols such as: - glycerol,- sorbitol, usually in 70% water solution,- propylene glycol.

The main flavouring agents (perfumes) are selected from: anise, badian, mint, juniper berry, cinnamon, cloves and rose essential oils.

The main sweetening agents are selected from orthosulfobenzoic imides and cyclamates.

The main dyes used are chosen according to the desired colour from: - red and pink colouring: amaranth, azorubine, cashew, coccine nouvelle (PONCEAU 4 R), cochineal, erythrosine,- green colouring: chlorophyll and chlorophylline,- yellow colouring: sun yellow (Orange S) and quinoline yellow.

The main preservatives most commonly used are: parahydroxybenzoates, formaldehyde and its derivatives, hexetin, quaternary ammonium, hexachlorophen, bromophen and hexamedin.

Finally, toothpaste compositions contain therapeutic agents, the main of which are selected from: - antiseptics and antibiotics,- enzymes,- trace elements and fluorine compounds which have been described above.

Concrete examples of the invention will now be given.

The protocol for measuring pH by conductivity and concentration and the tests for measuring the compatibility of silica with different elements will be described.

The pH measurement protocol is based on the concentration of silica and conductivity.

Suspensions of silica of increasing concentration varying from 0 to 25% by weight are obtained by dispersing a mass m of silica previously dried at 120°C for 2 hours in a mass 100 m of deionized and degassed water (Millipore quality).

The pH of the suspensions and solutions obtained after centrifugation of part of the suspension at 8000 rpm for 40 min and filtration on a Millipore filter of 0,22 μm, shall be measured at 25°C under nitrogen atmosphere with a Titroprocessor Metrohm 672 measuring system.

In the same way, the conductivity of the suspensions and solutions obtained is measured as before at 25°C with a Radiometer conductivity meter (CDM83) equipped with a CDC304 cell of cell constant 1 cm-1.

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

Measure of compatibility with bis (hydroxyethyl) -aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoride

1) An aqueous solution (1) containing 1.65 per cent amine fluoride is obtained by adding 5 g of 33 per cent amine fluoride solution in propanediol to 95 g of bi-distilled water.2) 4 g of silica is dispersed in 16 g of solution obtained from 1).

The resulting suspension is agitated for 4 weeks at 37°C. 3) The suspension is then centrifuged at 8000 rpm for 30 min and the resulting surfactant (3) is filtered on a Millipore filter 0.22 μm.4) The concentration of free fluorine amine is determined by microanalysis of nitrogen in the solution obtained at 1) and in the surfactant obtained at 3).5) Compatibility is determined by the following relation:

The percentage of compatibility with fluorine amine is then denoted by AF.

Measure of compatibility with cetylamine hydrofluoride

1) An aqueous solution (1) containing 1.72 per cent amine fluoride is obtained by dissolving 1.72 g of cetylamine hydrofluoride in 98.28 g of bi-distilled water.2) 4 g of silica is dispersed in 16 g of solution obtained from 1).

The percentage of compatibility with fluorine amine is then denoted by AFc.

Measurement of compatibility with an alkylbetaine

The alkylbetaine used is a product marketed by AKSO under the brand name ARMOTERIC LB. 1) An aqueous solution (1) containing 2.0% alkylbetaine is obtained by dissolving 6.67 g of 30% alkylbetaine in 93.33 g of bi-distilled water.2) 4 g of silica is dispersed in 16 g of solution obtained from 1).

The resulting suspension is agitated for 4 weeks at 37°C. 3) The suspension is then centrifuged at 8000 rpm for 30 min and the resulting supernatant (3) is filtered on a Millipore 0.22 μm filter.4) The free alkylbetaine concentration is determined by micro-analysis of the organic carbon in the solution obtained at 1) and in the supernatant obtained at 3).5) Compatibility is determined by the following relation:

The % alkylbetaine compatibility is then denoted by aBeta.

Measurement of compatibility with an alkylamidoalkylbetaine

1) An aqueous solution (1) containing 2.0% alkylamidoalkylbetaine is obtained by dissolving 6.67 g of 30% alkylamidoalkylbetaine in 93.33 g of bi-distilled water.2) 4 g of silica is dispersed in 16 g of solution obtained from 1).

The resulting suspension is agitated for 4 weeks at 37°C. 3) The suspension is then centrifuged at 8000 rpm for 30 min and the resulting surfactant (3) is filtered on a Millipore filter 0.22 μm.4) The concentration of free alkylamidoalkylbetaine is determined by micro-analysis of the organic carbon in the solution obtained at 1) and in the surfactant obtained at 3).5) Compatibility is determined by the following relation:

The % Alkylamidoalkylbetaine Compatibility is then denoted by Beta.

Measurement of compatibility with chlorhexidine

4 g of silica is dispersed in 16 g of an aqueous chlorhexidine solution of 1% chlorhexidine digluconate concentration.

The suspension is agitated for 24 hours at 37°C.

The suspension is then centrifuged at 20000 t/min for 30 min and the resulting supernatant is filtered on a 0,2 μm Millipore filter.

Then 0.5 ml of the solution thus filtered is taken and diluted in 100 ml of water in a measured vial.

A reference solution is prepared according to the same protocol but without silica. A 1% aqueous solution of chlorhexidine digluconate is agitated for 24 hours at 37°C, then centrifuged at 20000 t/min and the supernatant is filtered on a 0.2 μm Millipore filter.

The absorbance of the two solutions is then measured at 254 nm by a spectrophotometer (Uvikon 810/820).

The amount of free chlorhexidine rated % Compatibility is determined by the report:

Measurement of compatibility with fluorides

4 g of silica is dispersed in 16 g of 0.3% sodium fluoride (NaF) solution. The suspension is agitated for 24 hours at 37°C. After centrifugation of the suspension at 20000 t/min for 30 min, the supernatant is filtered on a 0.2 μm Millipore filter. The resulting solution is the test solution.

A reference solution is prepared using the same protocol but without silica.

Compatibility with fluorides is determined by the percentage of free fluoride measured by a fluoride selective electrode (Orion) and is determined by the following relationship.

Measurement of compatibility with zinc

4 g of silica is dispersed in 100 ml of 0.06% ZnS04, 7H20 solution to obtain a suspension with pH stabilized at 7 for 15 minutes by addition of NaOH or H2SO4.

The supernatant filtered on a Millipore 0,2 μm filter is the test solution.

A reference solution is prepared following the same protocol but without silica.

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

Compatibility is determined by the following relationship:

Measurement of compatibility with sodium and potassium pyrophosphate

4 g of silica is dispersed in 16 g of suspension of 1.5% sodium or potassium pyrophosphate, agitated for 24 hours at 37°C and centrifuged at 20000 t/min for 30 min.

The supernatant is filtered on a 0,2 μm Millipore filter. 0,2 g of solution diluted in 100 ml of water in a measured vial is the test solution.

The concentration of free pyrophosphate ion (P207--) in both solutions is determined by ion chromatography (DIONEX 2000i system) with an integrator.

Compatibility is determined by the ratio of the peak areas on the chromatograms to the pyrophosphate retention time, test and reference.

The following paragraphs are added:

In a reactor equipped with a temperature and pH control system and a propeller stirring system (Mixel), 8,32 sodium silicate of silica concentration of 130 g/I and molar ratio Si02/Na20 = 3,5 and 8,33 l of water of 1 μS/cm permeable are introduced.

After starting the agitation (350 t/min), the resulting tank foot is heated to 90°C.

When the temperature is reached, add sulphuric acid at a concentration of 80 g/I with a constant flow rate of 0,40 I/min to bring the pH to 9,5.

Then, 45,25 g/l sodium silicate with a silica concentration of 130 g/l, a molar ratio of Si02/Na20 = 3,5 and a flow rate of 0,754 I/min, and 29,64 1 g/l sulphuric acid at 80 g/l are added simultaneously. The flow rate of sulphuric acid is adjusted to maintain the pH of the reaction medium at a constant value of 9,5.

After 60 minutes of addition, the addition of sodium silicate is stopped and sulphuric acid is continued at a rate of 0.494 I/min until the pH of the reaction mixture is stabilized at 8.0. During this phase the temperature of the medium is raised to 95°C. A 30 min ripening is then carried out at this pH and at 95°C. During ripening the pH is maintained at 8 by adding acid.

At the end of the ripening, the pH is brought to 5,5 by the addition of sulphuric acid with a flow rate of 0.494 I/min and a ripening of 30 min is then carried out at this pH and 95 °C.

At the end of ripening, the pH is brought to 3.5 by the addition of sulphuric acid and maintained at 3.5 for 30 min.

After heating, the mixture is filtered and the resulting cake is washed in deionised water to a filtrate with a conductivity of 2000 μS/cm. The resulting cake is washed and dispersed in the presence of deionised water to form a 10% silica suspension. The pH of the suspension is adjusted to 6 by the addition of acetic acid.

A second filtration is carried out followed by a wash with water to adjust the conductivity to 500 μS/cm and a wash with water with pH adjusted to 5 by acetic acid to adjust the pH to 5,5.

The following relationship is then checked:

The cake is then de-sliced and the silica is dried by atomization, and the resulting silica is finally ground on a knife grinder to a powder with an average agglomerate diameter of 8 μm measured at the COUNTER-COULTER.

The physicochemical characteristics of the silica thus obtained are summarized in the table below. - What?

The surface chemistry characteristics of the silica of the invention are summarized in Table I and the results of compatibility with the organic amino compounds are given in Table I and the results of compatibility with the classical components of toothpaste formulations: chlorhexidine, fluoride, zinc, pyrophosphate are given in Table II.

The following paragraphs shall apply:

In a reactor equipped with a temperature and pH control system and a propeller stirring system (Mixel), 5,30 I of sodium silicate of silica concentration 135 g/I and molar ratio Si02/Na20 = 3,5 and 15,00 1 of water of 1 μS/cm permeable are introduced.

When the temperature is reached, add sulphuric acid at a concentration of 80 g/I with a constant flow rate of 0,38 I/min to bring the pH to 9,5.

Then 44,70 I of sodium silicate with a silica concentration of 135 g/l, a molar ratio of Si02/Na20 = 3,5 and a flow rate of 0,745 I/mn and 25,30 1 of sulphuric acid at 80 g/l are added simultaneously. The flow rate of sulphuric acid is adjusted to maintain the pH of the reaction medium at a constant value of 9,5.

After 60 minutes of addition, the addition of sodium silicate is stopped and sulphuric acid is continued at a rate of 0.350 I/min until the pH of the reaction mixture is stabilized at 8.0. During this phase the temperature of the medium is raised to 95°C. A 30 min ripening is then carried out at this pH and at 95°C. During ripening the pH is maintained at 8 by adding acid.

At the end of the ripening, the pH is brought to 5,0 by the addition of sulphuric acid with a flow rate of 0,400 I/min and a ripening of 30 min is then carried out at this pH and 95 °C.

After heating, the mixture is filtered and the resulting cake is washed with deionised water until a filtrate with a conductivity of 2000 μS/cm is obtained.

The cake is then de-liquidated in the presence of water to form a 20% silica suspension and the pH is adjusted to 5.1 so that the following ratio is checked:

The silica is dried by atomization and crushed on a knife grinder to a powder with an average agglomerate diameter of 8 μm.

The physicochemical characteristics of the silica thus obtained are summarised in the table below: - What?

The Commission has

In a reactor equipped with a temperature and pH control system and a propeller stirring system (Mixel), 5,60 sodium silicate is introduced at a silica concentration of 135 g/I and a molar ratio of Si02/Na20 = 3,5.

After starting the agitation (350 t/min), the resulting tank foot is heated to 85°C.

When the temperature is reached, 85 g/I sulphuric acid is added, preheated at 70°C at a constant flow rate of 0,50 I/min, to bring the pH to 9,7.

Then, 52.64 I of sodium silicate with a silica concentration of 135 g/l, a molar ratio of Si02/Na20 = 3.5 and a flow rate of 0.745 I/min, and 30.00 1 of sulphuric acid at 85 g/l are added simultaneously.

After 45 minutes of addition, the addition of sodium silicate is stopped and sulphuric acid is continued at a rate of 0.450 I/min until the pH of the reaction mixture is stabilized at 8.0. During this phase the temperature of the medium is raised to 95°C. A maturation of 10 minutes at this pH and at 95°C is then carried out. During the maturation the pH is maintained at 8 by addition of acid.

At the end of the ripening, the pH is brought to 5,0 by the addition of sulphuric acid with a flow rate of 0,750 I/min and a ripening of 15 min is then carried out at this pH and 95°C.

At the end of ripening, the pH is brought to 3.7 by the addition of sulphuric acid and maintained at 3.7 for 60 min.

After heating, the mixture is filtered and the resulting cake is washed with deionised water until a filtrate with a conductivity of 2500 μS/cm is obtained.

The cake is then de-coated in the presence of water to form a 20% silica suspension and the pH is adjusted to 5.5 so that the following ratio is checked: The test chemical is used to determine the pH of the test medium.

The physicochemical characteristics of the silica thus obtained are summarized in the table below.

Table 1 below summarizes the surface chemistry characteristics of the silicates of the invention described in examples 1 to 3.

The results of the compatibility of the silices of the invention with the amino organic compounds are also given in Table I.

The results of compatibility with the conventional components used in toothpaste formulations are given in Table II below: chlorhexidine, fluoride, zinc, pyrophosphate.

For comparison purposes, the characteristics and various compatibilities of commercial silica, the list below of which is a representative range of classical silica, are recorded in Tables I and II. - What?

The meanings of the symbols used in the above table are given below: - pH/logC is the equation pH = b-a.logC where a and b are two constants and C is the percentage by weight of silica in the suspension- pH/logD is the equation pH = b'-a'IogD where b' and a' are two constants and D is the conductivity of the silica suspension in μSiemens/cm- SE is the suspension effect measured by the relation SE = pH suspension- pH surfacing defined elsewhere- Ho is the Hammett constant- PZC is the surface pH for which the silica load is zero- AF, AFc, B, aB and CHx are the percentages of Compatibility in fluoride amines, alkylbeta and chlorine. These are the deoxidizers defined elsewhere respectively.

The compatibility percentages obtained with the fluorine amine AFc and the alkylbetaine aBeta are similar to those obtained for AF and Beta respectively.

The results of Table 1 above show that the silices of the invention which are compatible with organic amino compounds in particular differ markedly from conventional silices by the following relations:

Claims

1. Process for the preparation of a silica, said silica leading to an aqueous suspension, whose pH varies as a function of its concentration in the area defined by the two inequations:andand whose pH varies as a function of its electrical conductivity in the area defined by the two inequations:andin the inequations (la) and (Ib),

(C) represents the weight concentration of the aqueous silica suspension as a percent of Si0₂, in the inequations (Ila) and (llb),

(D) represents the electrical conductivity of the aqueous silica suspension in microsiemens-cm-¹, characterized in that it consists of reacting a silicate with an acid thus leading to a suspension or silica gel, carrying out a first aging at a pH equal to or above 6 and equal to or below 8.5, then a second aging at a pH equal to or below 6.0, a third aging at a pH equal to or below 5.0, separating the silica, washing it with water until an aqueous suspension is obtained, whose pH, measured on a 20% Si0₂ suspension, complies with the following equation:in the equation (III)

e is a constant equal to or above 0.6 and equal to or below 1.0,

d is a constant equal to or above 7.0 and equal to or below 8.5, (D) represents the electrical conductivity of the aqueous silica suspension in microsiemens.cm-¹, followed by the washing thereof.

2. Process according to claim I, characterized in that the silica has a surface chemistry such that its acidity function Ho is at least 4.0.

3. Process according to either of the claims 1 and 2, characterized in that the silica has a surface chemistry such that the OH- number expressed in OH-/nm² is equal to or below 12 and preferably between 6 and 10.

4. Process according to one of the claims 1 to 3, characterized in that the silica has a zero charge point (ZCP) of at least 4 and preferably between 4 and 6.

5. Process according to one of the claims 1 to 4, characterized in that the silica has a compatibility with the organic amino compounds of at least 30%.

6. Process according to claim 5, characterized in that the silica has a compatibility with the organic amino compounds of at least 50% and more particularly at least 80%.

7. Process according to either of the claims 5 and 6, characterized in that the silica has a compatibility with the organic amino compounds chosen from within the group formed by fluorine-containing amines, amine oxides, alkyl amines and alkyl betaines.

8. Process according to claim 7, characterized in that the organic amino compound is cetylamine hydrofluoride, bis-(hydroxyethyl)-amino propyl-N-hydroxyethyl octadecylamine dihydrofluoride, an amine oxide of formula R(CH₃)₂N→ O, an alkyl betaine of formula R-N⁺(CH₃)₂-CH₂-COO-, an alkyl amido alkyl betaine of formula R-CO-NH₂-(CH₂)₃-N⁺ (CH₃)₂-CH₂-COO-, in said formula R representing a straight or branched-chain alkyl radical with 10 to 24 carbon atoms.

9. Process according to claim I, characterized in that the silica has a compatibility with the divalent and higher metal cations chosen from within groups 2a,3a,4a and 8 of the periodic classification of at least 50% and more particularly at least 70%.

10. Process according to claim 9, characterized in that the metal cation is calcium, strontium, barium, aluminium, indium, germanium, tin, lead, manganese, iron, nickel, zinc, titanium, zirconium and palladium.

11. Process according to either of the claims 9 and 10, characterized in that the metal cation is in the form of mineral salts, chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts acetate and citrate.

12. Process according to claim 11, characterized in that the metal cation is in the form zinc citrate, zinc sulphate, strontium chloride or tin fluoride.

13. Process according to claim I, characterized in that the silica also has a compatibility with guanadine-type products and in particular chlorhexidine of at least 30% and more particularly at least 60%.

14. Process according to one of the claims 1 to 13, characterized in that the silica has a content of anions of type SO₄ ^2-, CI-, NO₃ ^- PO₃ ^-, CO₃ ^2- of at the most 5.10-³ moles per 100g of silica.

15. Process according to claim 14, characterized in that the silica has a content of the above anions of at the most 1-10-³, more particularly at the most 0.2.10-³ moles per 100g of silica.

16. Process according to one of the claims 1 to 13, characterized in that the silica has a sulphate content, expressed as SO₄ ^2- of at the most 0.1 %, preferably at the most 0.05% and more particularly at the most 0.01%.

17. Process according to one of the claims 1 to 16, characterized in that the silica has a content of divalent and higher metal cations of at the most 1000 ppm.

18. Process according to claim 17, characterized in that the aluminium content is at the most 500 ppm, the iron content at the most 200 ppm, the calcium content at the most 500 ppm and more particularly at the most 300 ppm.

19. Process according to one of the claims 1 to 18, characterized in that the silica has a carbon content of at the most 50 ppm and more particularly at the most 10 ppm.

20. Process according to one of the claims 1 to 19, characterized in that the silica has a pH of at the most 8 and more particularly between 6.0 and 7.5.

21. Process according to one of the claims 1 to 20, characterized in that the silica has a BET surface between 40 and 600 m2/g.

22. Process according to one of the claims 1 to 21, characterized in that the silica has a CTAB surface between 40 and 400 m2/g.

23. Process according to one of the claims 1 to 22 of the abrasive type, characterized in that the silica has a BET surface between 40 and 300 m²/g.

24. Process according to claim 23, characterized in that the silica has a CTAB surface between 40 and 100 _m2/g.

25. Process according to one of the claims 1 to 22 of the thickening type, characterized in that the silica has a BET surface between 120 and 450 m²/g and more particularly between 120 and 200 m²/g.

26. Process according to claim -25, characterized in that the silica has a CTAB surface between 120 and 400 _m2/g.

27. Process according to one of the claims 1 to 22 of the bifunctional type, characterized in that the silica has a BET surface between 80 and 200 m²/g.

28. Process according to claim 27, characterized in that the silica has a CTAB surface between 80 and 200 _m2/g.

29. Process according to one of the claims 1 to 28, characterized in that the silica has an oil absorption between 80 and 500 cm³/100g.

30. Process according to one of the claims 1 to 29, characterized in that the silica has an average particle size between 1 and 10 f,.lm.

31. Process according to one of the claims 1 to 30, characterized in that the silica has an apparent density between 0.01 and 0.3.

32. Process according to one of the claims 1 to 31, characterized in that the silica is a precipitation silica.

33. Process according to one of the claims 1 to 32, characterized in that it consists of preparing the silica gel or suspension by simultaneously adding the silicate and acid to a sediment, which can be water, a colloidal silica dispersion containing 0 to 150 g/I of silica, expressed as Si0₂, a silicate or a mineral or organic salt, preferably alkali metal salt.

34. Process according to claim 33, characterized in that the addition of the two reagents takes place simultaneously, so that the pH is kept constant between 4 and 10 and preferably between 8.5 and 9.5.

35. Process according to one of the claims 33 and 34, characterized in that the temperature is between 60 and 95°C.

36. Process according to claim 33, characterized in that the colloidal silica dispersion containing 20 to 150 g/I of silica is prepared by heating an aqueous silicate solution between 60 and 95°C and adding the acid to said aqueous solution until a pH is obtained which is between 8.0 and 10.0 and preferably is 9.5.

37. Process according to one of the claims 1 to 32, characterized in that it consists of preparing a silica gel or suspension by progressively adding the acid to a sediment containing the silicate until the desired aging pH is obtained at a temperature between 60 and 95°C.

38. Process according to one of the claims 1 to 35, characterized in that a first aging of the silica gel or suspension takes place at a pH between 6 and 8.5 and preferably of 8 at a temperature between 60 and 100°C and preferably at 95C.

39. Process according to one of the claims 1 to 38, characterized in that the silica gel or suspension undergoes a second aging at a pH between 5 and 6 and preferably at 5.5 and at a temperature between 60 and 100°C and preferably at 95°C.

40. Process according to one of the claims 1 to 39, characterized in that a third aging of the silica gel or suspension takes place at a pH between 3 and 5 and preferably at 4 and at a temperature between 60 and 100°C and preferably at 95°C.

41. Process according to one of the claims 1 to 32, characterized in that washing takes place with water or using an acid solution.

42. Process according to one of the claims 1 to 32, characterized in that washing takes place until the conductivity of the washing filtrate is at the most 200 microsiemens.cm-¹,

43. Process according to claim 42, characterized in that washing takes place with water or using an acid solution.

44. Process according to one of the claims 41 and 43, characterized in that the aforementioned acid solution is a solution of an organic acid, particularly a complexing acid.

45. Process according to claim 44, characterized in that the aforementioned organic acid is chosen from among the group of carboxylic acids, dicarboxylic acids, aminocarboxylic acids and hydrocarboxylic acids.

46. Process according to one of the claims 44 and 45, characterized in that the organic acid is chosen from the group including acetic, gluconic, tartaric, citric, maleic and glyceric acids.

47. Dentifrice composition, characterized in that it contains a silica prepared by the process according to one of the claims 1 to 46.

48. Dentifrice composition according to claim 47, characterized in that it comprises at least one element chosen from within the group including fluorine, phosphates and guanadines.

49. Dentifrice composition according to claim 47, characterized in that it comprises at least one element chosen from within the group incorporating organic amino compounds and divalent and higher cations.

50. Dentifrice composition according to claim 47, characterized in that it comprises at least one organic amino compound chosen from within the group formed by fluorine-containing amines, amine oxides, alkyl amines and alkyl betaines.

51. Dentifrice composition according to claim 50, characterized in that the organic amino compound is cetyl amine hydrofluoride, bis-(hydroxgethyl)-aminopropyl-N-hydroxyethyl-octadecyl amine dihydrofluoride, , an amine oxide of formula R(CH₃)₂-N → O an alkyl betaine of formula R-N⁺(CH₃)₂-CH₂-COO-, an alkylami- doalkylbetaine of formula R-CO-NH₂-(CH2)₃-N⁺(CH₃)₂-CH₂-COO-, in which R represents a straight or branched-chain alkyl radical with 10 to 24 carbon atoms.

52. Dentifrice composition according to claim 47, characterized in that it comprises at least one divalent and higher metal cation chosen from the group 2a,3a,4a and 8 of the periodic classification.

53. Dentifrice composition according to claim 52, characterized in that the metal cation is calcium, strontium, barium, aluminium, indium, germanium, tin, lead, manganese, iron, nickel, zinc, titanium, zirconium or palladium.

54. Dentifrice composition according to one of the claims 52 and 53 characterized in that the metal cation is in the form of mineral salts, chloride, fluoride, nitrate, phosphate, sulphate or in the form of organic salts acetate and citrate.

55. Dentifrice composition according to one of the claims 52 to 54, characterized in that the metal cation is in the form zinc citrate, zinc sulphate, strontium chloride or tin fluoride.

56. Dentifrice composition according to claim 47, characterized in that it comprises chlorhexidine.