IE883324L - Silica for use in dentifrice compositions - Google Patents

Abstract

The invention relates to a silica which is usable, in particular, in dentifrice compositions and compatible, more particularly, with chlorhexidine. The silica of the invention possesses an at least 65%, and more especially at least 90%, compatibility with products of the guanidine type, and especially chlorhexidine, as well as an acid value Ho of at least 3.3. [EP0315503A1]

Description

f < r r -i O u v 1 -1- The present indention relates to a silica nore particularly usable 1b dentifrice compositions e its preparation process and to dentifrice compositions eesat&iaiag said silica.

It is known that silica is widely used in the preparation of dentifrice 5 compositions, where it can fulfil several functions.

It firstly acts es an abrasive agent by aiding the elimination of dental plaque through its mechanical action.

It can also have a thickening agent function for giving predeteraiaed rhaological properties to the dentifrice and can serve as an optical 10 agent to give it the desired colouriag.

Moreover, it is known that dentifrices contain various agents particularly for the prevention of caries, for reducing the formation of dental plaque or tartar deposits on teeth. Among these agents particular reference is aade to fluorides. Other elements are also used, such as phos-15 phates, pyrophosphates, polyphosphates, polyphosphonates, guanidines, especially bis-biguanides, whereof one of the most widely used eleaents is chlorhexidine. The dentifrice femulations can also contain zinc, flavours, perfumes,, etc.

The presence ©f these agents in the1 dentifrice leads to the problem 20 of their coopatability with silica. Particularly as a result of its absorbing characteristics, silica tends to react with these agents in such a way that they are no longer available for exerting the aforementioned therapeutic effects.

The object of the invention is therefore to find silicas which ere comp-25 atible with the aforementioned agents and in particular guanidine and which can be readily used in the fornulation of dentifrices. Another - 2 - object of the invention is a process permitting the preparation of such compatible silicas.

The Applicant has found that the sought coapatability properties were essentially dependent on the surface chemistry of the silica used. 5 Thus, the Applicant has established a certain number of conditions regarding the surface of the silicas in such a way that they are compatible.

The silica according to the invention is characterized in that it has a compatability with guanidine type products and in particular chlorhexidine of at least 65% and more particularly at least 90%. 10 Moreover, the silica according to the invention, which is compatible with products of the guanidine type and especially chlorhexidine, is also characterised in that it has a surface chemistry such that its acidity function Ho is at least 3.3.

According to a first variant, the inventive process for the preparation 15 of silica of the above type, comprises a reaction of a silicate with en acid, with a view to obtaining a silica gel or suspension, followed by the separation and drying of the silica and is characterized in that following the separation of the silica, the cake resulting from this separation is washed with water until the conductivity of the filtrate 20 is at the raost 200 nicrosiemens es According to a second variant, the inventive silica preparation process comprises a reaction of a silicate with an acid, with a view to obtaining a silica gel or a suspension, followed by the separation and drying of the silica end is characterized in that the cake resulting from this 25 separation undergoes a first washing with water and then a second washing or treatment with an acid solution» Finally, the invention relates to dentifrice compositions containing a silica of the type described hereinbefore or prepared by the aforementioned process. - 3 ~ Other features and advantages of the invention will become more apparent from reading the following description relative to specific examples.

As has been stated in the introduction, the essential characteristics 5 of the silicas according to the invention are based on their surface chemistry. More specifically* oae of the aspects to be taken into account in said surface chemistry is the acidity. In this connection one of the characteristics of the silicas according to the invention is the strength of their surface acid sites. 10 Acidity here is understood in the Lewis sense, i.e. it translates the tendency of a site to accept a pair of electrons from a base in accordance with the equilibrium: 3 : + A ■ BA In order to characterize the silicas according to the invention, use 15 is made of the "acidity function" Ho developed by Harnett for measuring the tendency of the acid, silica ia the present case, to accept a pair of electrons from a base. Thus, the Ho function is defined by the standard relation: pKa 4- log (B :) - Ho (B :) (A) 20 For determining the strength of the acid sites of a silica according to the invention using the Hamaett method, use is made of the indicators Initially described by Walling (J.Am. chea. Soc. 1950, 72, 1164). The strength of the acid sites is determined by the colour indicators whereof, under the conditions of use, the passage pKa between the acid and basic forms is known.

Thusthe lower the pKa of the indicator undergoing the colour change, the stronger the acidity of the site. The following table gives ia - 4 - an exemplified Banner • a list of Hamaett iodicacors which can be used for surrounding the Ho value by determining In which form two successive indicators are adsorbed.

Golcmr basic forn scid form pEa Indicator 5 _____ Neutral red Methyl red Pheay lazonaphthy lemlne !>-Diaethylamiaoa2ob Experimentally, dosing takes place with 0.2g of silica placed in a test tube in the presence of a 100 uag/l indicator solution in cyclohexene. 30? The silica is previously dried at 190°C for 2 hours and kept protected - 5 - from noisture in a desiccator. As a result of stirring, the adsorption, if it takes place,, occurs within a few ednutes and note is takea of the colour change which is visible with the salted eye or possibly by studying the absorption spectra characterizing adsorbed colour indic~ 5 ators, both ia their acid form aad in their basic forn.

Ihe first feature of the silicas according to the Invention is that they have en acidity functions as determined hereinbefore, of at least 3.3.

Hie strength and nature of the acid surface sites can also be Measured 10 by the infrared spectrometry of the pyridine adsorbed on the silica.

It is known that the pyridine quantity adsorbed on a solid Makes it possible to determine the nature of the acid surface sites. Pyridine is a relatively strong base (pKb - 9) and unlike HH^ (pXb - 5) does not react with weak sites. The formation of a pyridinium ion (ryH+) 15 will also make it possible to differentiate Levis and Breasted sites.

Information on the acidity of the surface of a solid caB also be obtained by studying the absorption bands of pyridine in the range 1700 to 1400 cm""*. Moreover, quantification of the strength of the acid sites is made possible through the value of the displacement of the bends chara- 2o cteristic of pyridine sad its iodized forn, before ®ad after adsorption. -1 Hie pyrldlniuiD ion gives a band at 1540 cm „ whilst the pyridine bonded by H bonds or coordination gives bands in the range 1440 to 1465 cm~*. Moreover, it would appear that the pyridine band at 1583 caf* is displaced when the pyridine is adsorbed. This band indicates the presence 25 of Levis acid sites. The acid strength of the latter is proportional to the displacement of the band., Ia summarizing, it is possible to use bands at 1540, 1640 a^d 1485 cm"1 for defining the acidity of the Bronsted type and bands in the —1! range 1440 to 1465 cm * for the Lewis acidity. - 6 - Experimentally Measurements were carried out on a silica suspension ia carbon tetrachloride in the presence of pyridine.

The silice is dried beforehand at 190°C for 2 hours and kept protected from moisture. After cooling, Ig of silica is dispersed in 50 &1 of CCl, by magnetic stirring, followed by ultrasonic dispersion (10 nin). ■4 ' j 0.8 mg of pyridine is added per a" of silica introduced. Reflating accompanied by stirring is carried out for 1 hour.

The same protocol is used for preparing a control solution with the sasae pyridine concentration „ but without silica and a control suspension of the same silica concentration, but without pyridine.

The adsorption spectrum of pyridine is obtained by infrared spectroscopy on the suspension, the silica-free pyridine solution and the pyrl-dine-free silica suspension. Rroa the spectrum obtained on the basis of the suspension is removed the spectrum corresponding to the control 15 solution and the spectrum corresponding to the control suspension.

The silica is characterized by the position of the remaining bands and the displacement of the absorption bands of the pyridine and the pyri-dinium ion compared with the position of the bands of their non-adsorbed forms. In general terms, the spectrum obtained must not have a pyri-20 dinium peak (band at 1540 cm*""), the absence of said peak indicating that the silica present has an acid function Ho of at least 3.3.

The importance of the displacement of the pyridine and adsorbed pyridine bands makes it possible to evaluate the varying acidity of the acid —1 surface sites. In general terms, for the 1440 cm band, said displace- 25 ment ( ) must be at the aost 10 oaf" and aora particularly at the most 5 em***According to the preferred embodiment of the invention „ it is zero.

The silicae as defined hereinbefore„ has a good compatebility with chlorhexidine . When measured by the test described hereinafter, said 10 A - 7 - compatability car. be at least 65%. particularly at least 80% and preferably at least 901.

However, according to a particular embodiment of the invention, the silicas csjpi also be compatible with fluorioe. Is, this case, they have 5 a content of anions of the type S0^" , CI , HO^**, PO^*""* OOj"" of at the most 5-10~^ soles per lOOg of silica.

The lower the said content, the greater said compatability. According -3 to oreferred variants, it will be at the sost 1.10 ^oles and more -3 especially 0.2.10 moles per lOOg of silica. 10 In the case of silicas prepared fro® sulphuric acid,, said anion contest is more conveniently expressed by a content in S04 and by weight. In ■this case, the content is at the saost 0.3%.

According to a preferred variant of the invention, said content is at the most 0.1% and more especially at the sost 0.021. 15 This compatability can be further improved, particularly relative to certain elements like sine, on observing the conditions on the nuaber of acid surface sites. This number eaa be measured as a isusber of CM groups or silanols per nm~. This number is determined in the following way. the number of surface OH sites is likened to the quantity of water 20 freed by the silica between 190 and 900°C. The silica samples are previously dried at 105°C for 2 hours- A silica mass Po i® placed in a theraobalance and heated to 190°C for 2 hours, i.e. the mass obtained is P190. The silica is then heated to 900°C for 2 hours., i.e. P900 is the sew mass obtained,. 25 The number of OH sites is calculated by the following equation: - 8 « HOH - 66922.2 r PI90 - P900 A ? 190 10 15 20 In the present case, the silicas according to the invention advantag- at the aost 12 and especially between 3 asd 12.

The nature of the OH sites of the silicas according to the invention, which also constitutes a characterization of their surface cheaistry, can also be evaluated by the point zero charge (PZC). The latter is defined by the pH of a silica suspension for which the electric charge of the surface of the solid is zero and this applies so natter what, the ionic strength of the nediuia. This PZC aeasures the real pH of the surface , to the extent that the latter is free froa all ionic impurities.

The electric charge is deternined by potentlonetry. The principle of the oethod is based on the overall balaacs of the protons adsorbed or desorbed on the surface of the silica at a given pH.

On the basis of equations describing the overall balance of the operation, it is easy to demonstrate that the electric charge C of the surface relative to a reference corresponding to a aero surface charge is given by the equation: F C » (I - 01) A.M in which: o A represents the specific surface of the solid in m"fg, H is the solid quantity in the suspension in g, - 9 - F is the Faraday constant, 1 ©r OH represents the variation per surface unit ©£ the excess of fif or OH" ions respectively on the solid.

The experimental protocol for determining the FZC is as follows. Use 5 is nade of the oethod described by Berube and Bruyn (J- Colloid Interface Sc» 1968, 27,, 305). The silica is washed beforehand ia, high resistivity deionized water (10 segeobja^cs)„ dried and then degassed..

In practice,, a series of solutions is prepared at pHo:8.5 by adding KOH or HHO, end containing an indifferent electrolyte (KN0~) at a concen- . 3 ^ 10 tration varying betveen 10" and 10"* moleA - To these solutions is added a given silica mass and the pH of the suspensions obtained is allowed to stabilise under stirring at 25°C and under nitrogen for 24 hours8 i.e. its value is pH?o» Standard solutions are formed by the supernatant obtained by centri-15 fuging for 30 min at 1000 r.p.m. of a part of said same suspensions, i.e. plTo is the pH of these superastasts - The pH of a volume of these suspensions and the corresponding standard solutionis is then brought to pHo by adding the necessary KOE quantity and the suspensions and standard solutions are allowed to stabilize for 4 hours. Thus 20 Voh-Hoh is the number of base equivalents added to pass from pH'o to pHo a k&OMn wolvme (¥) of suspension or standard solution.

Th® potectioaetric dosing of the suspeasic-as aad standard solutions is carried out on. the basis of pHo by adding nitric acid up to pHf«2.0. The preferred procedure involves the addition of an acid increment corr-25 esponding to a pH variation of 0.2 unit and after each addition the pH is stabilised for 1 minute. Thus, Vh>Nh is the aumbar of acid equivalents required to obtain pHf. 30 On the basis of pHo, the term (Vh-Nh ~ Voh.Noh) is plotted as a function of the incremented pH values for all the suspensions (at least 3 ionic strengths) and for all the corresponding standard solutions™ For each - 10 - pH value (0.2 mtz step),, th® difference is then forned between th® flf or 0H~ consumption for the suspension end the corresponding staadard solution. This operation is repeated for all the ionic strengths.

This gives the term (H - OH) corresponding to the consumption of protons 5 by the surface. The surface charge is calculated by the above equation.

This is followed by the plotting of the surface charge curves as a function of the pH for all the considered ionic strengths. The FZC is defined by the intersection of the curves.

The silica concentration is adjusted es a function of its specific sur-10 face. For example, 2% suspensions ere issed for 50 af/g silicas with 3 ionic strengths (0.1; 0.01 and 0.001 mole/i-). Itosiag is carried out on 100 ml of suspension using 0.1 H potassium hydroxide.

Ia practice, it is preferable for th® value of said PZC to be at least 3 and more particularly between 4 and 6. In the case of a better compat-1 5 ability with zinc, it e aost 6„5„ For fluorine compatability, it is preferable to have a FZC of at the nost 7.

For Improving the compatability* particularly with respect to fluorine* it is of interest for the al^miniKus contest of the silicas according to the invention to be at the post 500 ppsa. Is. addition* the ires c©n~ 20 tent of the silicas according to the Invention is advantageously at the most 200 ppa. In a preferred manner, the calcium content is at the most 500 ppm and aore particularly at the sost 300 ppa.

The silicas according to the invention also preferably have a carbon content of at the most 50 ppm and more especially at the most 10 ppm. 25 The pH of the silicas according to the invention measured according to NFT Standard 45-007 is generally at the most 8 sad ©ore particularly between 6,0 sad 7.5. - 11 - The above characteristics sake it possible to obtain a silica which is compatible with at least guanidimes and ia particular chlorhexidine aad,„ as a function of the particular case,, also with fluorides, phosphates and their derivatives aad is particular zinc* 5 Apart fro® the surface chemistry characteristics described hereinbefore and which condition the compatabilities, the silicas according to the invention also have physical characteristics making thea perfectly suitable for their use in dentifrices. These structural characteristics will be described hereinafter „ 10 Generally the BET surface of the silicas according to the invention t j is between 40 and 600 m"/g end wore especially between 40 and 350 af/g- t Their CTAB surface normally varies betveen 40 and 400 m""/8» aore part- 2 icularly between 40 end 200 m /g.

The BET surface is determined according to the BRUNAUER-EMMET-TELLER 15 method described ia the Journal of the American Chendeal Society,, rol„ 60s p.309, February 1938 aad accordlag to NF Staadard HI-622 (3.3).

The CTAB surface is the outer surface determined according to ASiH Standard D3765, but whilst carrying out the adsorption of cetriaonium bromide (CTAB) at pH 9 and taking as the projected area of the CTAB molecule 35 A°2. 2o The silicas according to the invention can obviously correspond to the three standard types in the dentifrice field.

Thus 5 the silicas according to the invention can be of the abresive f type. They then have a BET surface between 40 and 300 m~/g. In this t case, the CTAB surface is betveen 40 and 100 m~/g. 25 The silicas according to the invention can also be of the thickening 2 type. They then have a BET surface between 120 and 450 a /g, more espec-ially 120 and 200 m~/g. They could then have a CTAB surface betveen - 12 - 2 ■? 120 and 400 m /g, more especially between 120 and 200 sT/g.

Finally, according to a third type, the silicas accordlag to the invention can be bifunctional. 'They thea fea^e a BUT surface between 80 and 7 •> 200 oT/g. The CTAB surface is then between 80 and 200 a"/g. 5 The silicas according to the invention can also have an oil sample between 80 and 500 cra^/iOOg determined according to NFT Standard 30-022 (March 1953) using dibutylphthalate. 3 More specifically, said oil sample is between 100 and 140 em /100g for abrasive silicas, 200 and 400 for thickening silicas aa,d 100 and 300 •JO for bifunctional silicas.

Moreover and still with a view to use ia deatifrices „ the silicas preferably have a particle size between 1 and 10 pa. This mean particle size is measured with the Coulter counter.

The apparent density generally varies between 0.01 and 0.3. According 15 to a particular embodiment of the invention, the silicas are precipitation silicas.

Finally, the silicas according to the invention have a refractive index which is generally betveen 1.440 and 1.465.

The process for the preparation of the silicas according to the invention 20 will nov be described in greater detail.

As stated hereinbefore, this process is of the type in^olwiag the reaction of a silicate with ea acid, which gives rise to the formation of a silica gel or a suspeasioa „ It should be noted that it is possible to use any known operating procedure for arriving at said suspension 25 or gel (addition of acid to a silicate sediment, siavltanecms total or partial addition of acid and silicate to © water sediment or silicate solution, etc.), the choice essentially being made as a function of 13 - the desired physical characteristics of the silica. It can be advantageous to bring the pH of the solutioa or gel obtained to a value of at the saost 6 gmd ia particular betveen 4 sad 6.

This is followed by the separation of the silica fro® the reaction aediva 5 by any knows aeans, such as e.g. a vacuus filter or filter press. Thus, a silica cake is collected.

The process according to the invention can then be realised according to two sain variants.

The first variant relates to the preparation of silicas essentially 10 compatible with guenidines and particular chlorhexidine. In this case, the process involves the washing of the cake. This washing takes place with water, generally deionized water, until a washing filtrate is obt-ained, whose conductivity is at the roost 200 aicrosieaens ca If it is desired to further iapro^e the compatability ©f the silicas o'bc-15 ained by the process, said washing can be continued to a greater extent. In particular, according to a preferred eabodiaent of the invention, said washing is cositiaued to a conductivity of at the most 100 eicro-siemens an *.

I Once the silica cake has been washed according to the aforeaentioned 20 operating procedure, the cake or, if it is disintegrated, the disintegration suspension is dried by any known aecuts. Drying can in particular take place by stondLs&tion. The dried product is grsaarsd if necessary in order to obtain the desired grain size.

The second variant of the process relates to the preparation of silicas 25 which are compatible with elements such as fluorine, sirac and phosphates,, as well as guenidines.

In this case, the process also involves a washing with water, generally deionized water, as in the first variant- However, this washing can be less Barked. For ex&mpleit caa be carried out until a filtrate _ l^- is obtained with a conductivity of at the most 200 -1 microsiemens cm Following onto said first washing with water, according to the second variant a second washing or treatment of the cake is carried out with 5 an acid solution or acidulated water. This second washing or treatment aims at obtaining, at the end of the preparation, a silica with a PH of at the most 8 and more particularly between 6.0 and 7-5 and also a PZC of at least 3 and more particularly between 4 and 6.

The washing or treatment can talcs place by passing the acid solution 10 onto the cake, or introducing the latter into the suspension obtained following the disintegration of the cake.

This acid solution can e.g. be a solution of a mineral acid, such as nitric acid However, according to a special embodiment of the invention, said acid 15 solution can also be a solution of an organic acid, particularly a completing organic acid. This acid can be chosen from carboxylic* dicarbosylic „ hydrocarbon lie and erai rtoc arboxy lie acids.

An example of such acids is acetic acid and in the case of tha completing acids tartaric, maleie9 glyceric, gluconic end citric acids. 20 Particularly when using a solution of a mineral acid, it can be advant ageous to carry out a final washing with deoinised water.

Following the washing or treatment operations according to said second variant, drying is carried out in th® same way as described hereinbefore for the first variant. - 15 - According to another variant of the invention, following the acid-silicate reaction and just prior to the separation of the silica, the suspension ©r gel is aged. This ageing generally takes place at a pH of at the sost 6 sad e.g. between & and 6. 5 It is also possible to carry oet an ageing during the reaction, e.g. at a pH betveen 6 and 8. Said ageing operations are preferably carried out hot, e.g. at a teaperature betveen 80 and 100°C and over a period of betveen 15 minutes aad 2 hours.

Finally, it has been found that it was possible to further improve the 10 compatability of the inventive products by another complinentary treatment. This treatment consists of ?jsiag an alkaline-earth product, which can he introduced either ioto the suspension or the silica gel, or, preferably, onto the cake, especially following the crumbling of the letter, e.g. in the form of a salt or hydroxide. 15 In particular» use is Bade of an organic salt and in particular an alkaline-earth completing agent, generally e barium salt, e.g. barium acetate.

The invention also relates to dentifrice compositions containing the silicas of the type described hereinbefore or obtained by the process which has been discussed. 20 The silica quantity according to the invention used in the dentifrice compositions can vary within wide limits and it is generally between 5 end 35%.

The silicas according to the invention can be ©ore particularly used 1b dentifrice compositions eostaiaiag at least one element chosen from 25 the group including fluorides, phosphates, guanidines and ia particular chlorhexidine. According to the following tests, they can have s compatability of at least 90% for each of these The silicas according to the invention are also compatible with maleic - 16 - acld-vinylethyl ether copolymers end can also be incorporated into dentifrice expositions containing these copolymers. Finally, they can have a compatability with zinc of at least 50% and preferably at least set. 5 ifeth regards to the fluoriB.e~eoEtsiai.ag compounds, their quantity preferably corresponds to a fluorine coaceatratioa in the conpositios between, 0.01 end 1 sad sore particularly 0.1 and 0.5% by weight. Us® fluorine-containing compounds are in particular the salts of monofluorophosphoric acid and especially those of sodium, potassium,, lithium, calcium, alumin-10 ium and ammonium, the monofluorophosphate and difluorophosphate, as veil as the various fluorides containing ftusorima is the form of the ioa more particularly linked with the alkaline fluorides, such as those of sodium* lithium, potassium, ammonium fluoride, stennous fluoride, manganese fluoride* zirconium fluoride„ aluminium fluoride,, as well 15 as the addition products of said fluorides with one another and with other fluorides, such as potassium, sodium or manganese fluorides.

Other fluorides are also usable for the present invention, such as e.g. ssiae fluoride, germanium fluoride, palladium fluoride, titanium fluoride, alkaline fluozlrconates, e.g. of sodium or potassium, stasaous fluozir-20 coaate, fluoborate or fluosulphates of ©odium aad potassium- Organic fluorine-containing compounds can also be used and preferably chose known as long chain amine or amino acid addition products with hydrogen fluoride, cetylaalne fluoride, bis-(hydroxyethyl)~emlnopropyl~ H-hydroxyethyl-octadecylaaine dihydrofluoride, octadecylamlne fluoride 25 and M.H'.N'-tri-CpolyoxyethyleneJ-H-hexadecylpropylenedlamine dihydrofluoride.

The zinc is more particularly present in the citrate or sulphate form.

With regards to the elements usable es plaque-preventing agents of the polyphosphate, polyphosphonate, guanidine and bis-biguanlde type, refer-30 ©ace can be ®ad® to those described in US Patent 3 934 002. - - 17 - The dentifrice compositions can also contain a binder. Th© nain binders used are in particular those chosen fro® aaong: cellulose derivatives: aethyl cellulose, hydroxyethyl cellulose, ©odiKB carboxyaethyl cellulose. 5 mucilages: carraghenates, algiaates., agar-agar sod geloses, guns : gun Arabic* gun eragacaatb* gun xanthane and gua Xaraya, carboxyvlnyl and acrylic polyners* polyoxyethylene resins.

Apart from the silicas according to the Invention* the dentifrice coop-10 ositions can also contain one or nore other abrasive polishing agents chosen from among : precipitated calcium carbonate, magnesium carbonate* calcium, dicalcium and tricalcium phosphates, 15 insoluble sodiua aetaphosphate, calcium pyrophosphate, titaniua oxide (vhiteniag age^e)., silicates, aluainers aad ®ilico aluainates* 20 sisc sad tin oxides,, talc, kaolin.

The dentifrice compositions can also contain detergents, huaectants* aromatizing agaats* sweeteners, dyes sad preservatives. 25 The aaln detergents used are in particular chosen from aaong: sodlira laurylsulphate, sodium laury1sulfoacetcte and laurylether sulphate, sodJUua dioctylsulfosuccinate* sodium laurylsarcosinate, 30 sodium ricinoleate, - 18 - sulphated oonoglycerides.

The ss&Ib huaectents used ere is particular chosen froa aaong polyalcohols such as; glycerol, 5 .sorbitol® generally la 70S solutioa la water, propylene glycol,.

The WB.±n aroeig.tl.giag egents (parfuses) used are ia particular chosen from among aniseed, mint,, bay, juniper, star anise, cinnaoon, clove and rose oils. 10 The aain sweeteners are ia particular chosen froa aaong orthosulfobenzoic imides and cycleaates.

The mala dyes are chosen as a function of the desired colour froa aaong: red sad piak colours amaranth, asoruby,, catechu* new c&ccla (PONCEAU 4R), cochineal and erythrosin, 15 green colour: chlorophyll a^d chlorophyllin, yellow colour-* ssis yellow (Crasg-s S) acd quinoline fellow.

The aost widely used preservatives are parahydroxybenzoates, foraol and products giving off the sasae, hexetidine, quaternary aaaaoniuss, hexachlorophene, broaophene and hexaoedine. 20 Finally, the dentifrice compositions contain therapeutic agents, the Eaost important of these being selected from among antiseptics and antibiotics, enzymes, oligo-eleaents and fluorine-containing compounds described hereinbefore.

Specific, but non-liaitative ezaaples will now be given and before this 25 tests for maasuriag the compatability of the silica with the various elements will be described. - 19 ~ Measurement of the comaetab!lit? vdth chlorhexidine 4g of silica are dispersed in 16g of as aqueous chlorhexidine solution with a IS chlorhexidine digluconate concentration. The suspensions is stirred for 24 hours at 37°C. The suspension is then ceiotrifuged at 5 20000 r.p.si. for 30 siia asid the supernatant obtained is filtered on a 0»2 pm Millipore filter, 0.5 Ml of the thus filtered solution is then removed and diluted is 100 ml of water ia a graduated flask. This ©ol^tioo constitutes the test solution. 10 A reference solutioa is formed in accordance with the same protocol* hut in the absence of silica. A IX aqueous chlorhexidine dlgluconate solution is stirred for 24 hours at 37°C. then centrifuged at 20000 r.p.n. and the supernatant is filtered on the 0-2 pm Mllliposre filter. 0.5 ml of the thus obtained solution is diluted in 100 al of water in -j 5 « graduated flask- This is followed by the measurement of the absorbance of the two solutions at 254 ;om with the aid of a spectrophotometer (Uvikon 810/820), The free chlorhexidine quantity designated % compatibility is iSetenaised by the relation: Absortsaee ©f the test , 2q S compataoliity * . x 100 Absorbance of the reference Measuring the compatability with fluorides 4g of silica are dispersed ©£ 0.3% sodium fluoride (HaF) solu tion. The suspension is stirred for 24 hours at 37°C. After centri-fugiag the suspension at 200-00 r.p.n. for 30 min, the sapersatant is 25 filtered on a 0.2 jo® Millipore filter. The thus obtained solution constitutes the test solution.

A reference solution is formed by using the same protocol, bet in the absence of silica. - 20 - The compatability with the fluorides is determined by the free fluoride percentage measured by a fluoride-selective electrode (Orion). It is determined by the follcwlsg relation: F concentration of test (ops) % compatability - — — x ICO 5 F concentration of reference (ppm) Measurement of the competability vith zinc 4g of silica are dispersed in 100 ml of Q.06% Zn S0^s 7Ho0 solution, which gives a suspension., whose pH is stabilised to 7 for 15 minutes by the addition of NaOH or H^SO^. The suspension is then stirred for 10 24 hours at 37°C, followed by centrifuging at 20000 r.p.m. for 30 min.

The supernatant filtered on the 0.2 jim Millipore filter constitutes the test solution. A reference solution is then formed by using same protocol, but in the absence of silica. tne The free sine concentration of the two solutions is determined toy atomic 15 absorption (214 no).

The compatability is determined by the follovl&g relation: Zn concentration of test % compatability - > x 100 Zn concentration of reference Measuring the compatability with potassium aasd sodium pyrophosphates 20 4g of silica are dispersed ia 16g of 1.5% potassium or sodium pyrophosphate suspension. The suspension is stirred for 24 hours at 37®C and then ceatrifuged at 20000 r.p.m. for 30 tain. Hie supernatant is filtered oa a 0.2 jjm Millipore filter. 0.2 g of solution diluted in 100 si of water in a, graduated flask forms the test solution. A refer-25 ence solution is formed by following the sane protocol, but in the absence ©£ silica. - 21 - The free pyrophosphate ion (P^O-, ) concentration of the two solutions is deterained by ion chromatography (DIOHEI 2000i systen) equipped with an integrator,, Th® compatability is deterediaed by the relationship of the areas of 5 the peaks obtained on the chronatograns and corresponding to the pyrophosphate retention tiae with respect to the test sad reference. area of peak of test % compatability » 100 x —~—: area of peak of reference EXAMPLE 1 1 o This example relates to the preparation of a compatible silica of the abrasive type. 6 litres of deionized water are introduced into a reactor equipped with a temperature and pH regulating system aad a tyrbisae scirriBg system. After starting up the stirring operation (300 r.p.a.), the thus foraed 1 5 sediment is heated at 85°C.

When the temperature is reached, there is a simultaneous addition ©f 8„5l of sodiup silicate with a 120 g/L silica concentration with a SiO^/ Ha09 ratio equal to 3-5 and a flow rate of 0.34 IL/adn* and 13.51 of sulphuric acid of concentration 80 g/l» The acid flow rate is adjusted 20 in such a way as to sain tain the pH of the sediua at a constant value ©f 8»0» Following an addition over a period of 40 oinutes, the silicate addition is stopped and the acid addition is continued until the pH of the reaction oixture is stabilized at 4. This is followed by a 15 ninute ageing 25 at said pE and at 85°C„ The aixture is then filtered and the moist cake washed with deionized water, until the conductivity of the filtrate is below 100 aicrosienens. - 22 - This is followed by a washing of the cake with water brought to pH 4 by the addition of acetic acid. ,1 final washing operation is carried out with deionised water.

The product is thea dried by stosiisatioo and ground ©s a Forplex grinder to obtain a grain size of 10 laicrojas.

The physicochemical characteristics of th® thus obtained silica are as follows? BET surface CTAB surface 1o Oil sample pH 100 s2/g 55 a2/g 120 CM3/100 g 6.5 The chemical analyses of the silica are given in the following table. ions SO ^ Al Fe Ca C ppM 100 250 130 300 10 15 The surface cheaistry is quantified by the following parameters: Ho above 3.3 PZC « 4 d'O M 5 cm"*" number of 0H/na~: 9 20 The following table gives the conpetabilities of the silica with the different ingredients of a dentifrice foraulation.

Ingredients Fluoride Pyrophosphate Chlorhexidine Zinc MeP Ha/I diglucoaat© ZbSO % Compatability 95 98 75 70 - 23 - EXAMPLE 2 Example 1 is repeated to obtain a ©ilica cake washed with water at pH 4 through the addition of acetic acid. Th,® thsis obtained ceke is dis-integrated ia a disintegrator to obtain a fluid suspension. 0„2 g of bariua acetate is added, accompanied by stirring. The silica is thea dried by atosization and ground to obtain an average ©article siae of 10 pm.

The physieocheaical characteristics of the thus obtained silica are as follows: 2 10 BET surface 100 m /g CTAB surface 60 n"/j Oil sample 110 ca^/ pH 6.5 The ch'asical analyses of the silica are givea in the following table. ioBS 80^ AL Fe Ca C ppM 100 250 12-0 400 40 The surface chemistry is quantified by tile following parameters; Ho above 3.3 PZC - 4.5 « 2 ca"" ty aeaber of GH/mT - 8 - 24 Compatibilities Ingredients fluoride HaF Pyrophosphate He/K Chlorhexidine Zicc digluconate Ea 80^ I Compatability §5 98 90 70 5 EXAMPLE 3 This example relates to the preparation of a thickening silica gel. 14 litres of sodium silicate with a 86 g/t silica concentration and a SiO^/NejO ratio of 3„5 are introduced, into a reactor equipped with a temperature and pH regulating system and a turbine stirring system. 10 After starting up stirring (100 r.p.m.), 1.451 of 28% ammonia is added in 2 min. This is followed by the addition of 0.81 of sulphuric acid with a concentration of 200 g/L a&d a flow rat© ©f 0.2 l/mln. The fixture is then reacted for 5 min at a temperature controlled to 20°C. this is followed by the addition of 5.21 of sulphuric acid with a conc-15 entration of 200 g/L and a flow rate of 0.2 L/min.

After the appearance ©f the gel (visual ©r by turbidity measurement), the mixture Is allowed to age for 10 mi&utes. the gel is then dispersed by stirring the eistwre at 400 r.p.m. for 30 mln.

The pH of the medium is then lowered to 3.5 by adding sulphuric acid 20 (200 g/t) with a flow rate of 0.2 l/min. The reaction mixture is allowed to stabilize for 1 hour at 60°C.

The preparation of the gel is completed by filtering said mixture and by washing the cake obtained using twice 201 of deionlsed water at 60°C The silica is dried by atcaissatien on, an Anhydro etosiizer. The silica is thea nicronized on s JET Pulveriser grinder t© hare a grain size 5 of 1.5 jm.

The physicocheaical characteristics of the thos obtained silica are as follows: 9 BET surface 450 a~/g CTi^B surface 350 a^/g Oil sample 300 esfVlGOg pH 6.8 Apparent density 0.270 Refractive index 1.445 The chemical analyses of the silica are given in the following table. ions SO^ Al Fe Ca C ppH 500 200 120 300 10 Tne surface chemistry is quantified by the following parameters: Ho above 3.3 PZC - 3.6 Ho pyridine adsorption bands.

Compatabilities Ingredients Fluoride NaF % Compatability 90 Pyrophosphate Chlorhexidine Zinc HaK digluconate & Si 95 65 50 « 26 ~ EXAMPLE 4 Ibis example relates to a thickening silica. 6 litres of water are introduced into a reactor equipped wish a pH sad teaperature regulating system, followed by the additioa of 150 g of 5 sodium sulphate accompanied by stirring.

The mixture is heated to 60°C aad there is a simultaneous addition of lOt of sodium silicate (Km * 3™5 and Si07 « 220 gA ) &nd sulphuric acid with e concentration of 30 g/L ia 40 min.

The sulphuric acid flow rate is adjusted ia such 0 %?&¥ S2.S 1£0 keep the 10 pH of the reaction medium at a consteat value ©f 7.8. The pH of the medium is stabilized at the value 4.0 by the very rapid addition of sulphuric acid and ageing takes place for 15 ain at 60°C.

The reaction mixture is filtered at 60°C and the silica cake is washed with deionized water so as to have a conductivity of the filtrate ©£ _T 15 900 microsiemens.ca *. The cake is then washed with water at pR-4, adjusted by the addition of acetic acid and a final washing with deion- ized water takes place., The thus obtained silica, cake is cruiabled asd then 2g of calcium acetate are added thereto, accompanied by stirring.

The silica is dried by atcmization and micronization takes place on 20 a Jet Pulverizer grinder to adjust the silica particle si2e to 1.5 jm.

Th® physicochemicsl characteristics of the thus obtained silica areas follows: 25 BET surface CTAB surface Oil sample pH 320 a"/g 120 m2/g 250 ca3/200g 6.5 _ 7'7 - IS* * The chemical analyses of the silica appear la the following table.

Ioes SO^ At Be Ca C ppM 100 200 120 mo 20 The surface chemistry is quantified by the following parameters: 5 Bo above 3.3 PZC « 4 •? number of OFi/nsT « 11 1© pyridine adsorption bands.

Couipata'oilities 10 Ingredients Fluoride Pyrophosphate HaF Ma/K % compatible 90 95 Chlorhexidine Zinc digluconate EnS04 90 80 EXAMPLE 5 This example relates to the preparation of an abrasive silica. 6 litres of deionized water are introduced into a reactor equipped with a temperature and pH reg?ilatiag system and a turbine stirring sy®t When the teaperature is reached there is a simultaneous addition of 8-5L <©£ sodium silicate with a 120 g/L silica concentration and ratio of Si00 to Ha70 of 3.5 and at a flow rate of 0.34 l/ain, and 13.51 of sulphuric acid with a concentration of 80 gA.. The acid flour rate is adjusted so as to oaintain the pH ©f the aediua at a constant value of 8-0- - 28 - Following 40 sain addition, silicate addition is stopped aad the .tdxtuxe is allowed to age for 13 sain at 85°C aad pH 8. The acid addition is continued until the pH of the reaction sistere is stabilized at 4. Ageing thea takes place for 15 Bin at said pH aad 85°C. 5 The aixture is thea filtered and the aoist cake washed with deionized water until the conductivity of the filtrate is 100 aicrosieaens.cEf * -The cake is thea washed twice with water brought to pH 4 by the addition of acetic acid. A final washing takes place with deiosiged water.

Tn® product is then dried by eto-misatio^ and grouad on a Forplex grinder 10 to obtain a grain size of 9.0 aicrons.

The physicocheaical characteristics of the thus obtained silica are as follows: 2 BET surfece 60 a /, CTAB surface 50 nT/ 15 Oil staple 120 ea3/100g pH 6.0 The cheaical analyses of the silica are given below. less SO/, AL Fe €a C 100 250 100 200 10 20 The surface chemistry is quantified by the following parameters; Ho above 3.3 PZC - 4.5 Mo pyridine adsorption bands. „ 70 _ OB Compatability Ingredients Fluoride Pyrophosphate Chlorhexidine Ziac NaF Ha/I diglucosate ,2s,S0^ %■Compatible 95 98 70 80 5 KMMPLB 6 This example relates to the preparation of a thickening silica is a reactor equipped with a turbine stirring system, ioto which are introduced 5,07L of sodium silicate of silica concentration 120 g/i, and SiOj/ Na90 ratio of 3.5 and 3.8L of deionized vater. After starting the stir-10 ring operation (300 r.p.m.), the thus formed sediment is heated at 68 C,„ When the temperature is reached, 2.61 of sulphuric acid of concentration 80.5 g/t are added, the acid flow rate is 0.120 l/min.

After 22 min addition, the acid addition is stopped and the mixture is allowed to age for 10 nin (a sudden turbidity increase is observed). 15 This is followed by the addition of 4.21 of sulphuric acid £n 35 min.

The temperature is then raised to 87°C sad there is a simultaneous addition of sodium silicate at a rate of 30 ml/ain and sulphuric acid at a rate of 52 al/min for 3-0 sin.

'Use temperature is raised to 95°C and 0.523L of sulphuric acid is added 20 in 10 nin. The mixture is then allowed to age for 10 min. Finally, the pH of the oedica is brought to 4 by adding acid.

The mixture is thea filtered aad the aoist cake washed with deionized water until the conductivity of the filtrate is 100 nicrosiemens cb~"~.

The cake is then washed with water brought to pH 5 by the addition of 25 acetic acid. A final washing takes place with deionized water.

The product is then dried by atomlzation and sdcronized ©a a Jet Pulverizer grinder to obtain a graia size of 1.2 microns. - 30 - The physico-chemical characteristics of the thus obtained silica are as follows; BET surface 180 if/® 2-k 2 CTAB surface 170 a /g 5 Oil sample 350 csa^/iOOg pH 6.3 The chemical analyses of the silica appear ia the following table. ions S0A AL Fe Ca C ppH 500 200 120 300 10 10 The surface chemistry is quantified by the following parameters: Ho above 3„3 PZC * 3.6 No pyridine adsorption bends.

ZnS°4 60 EXAMPLE 7 This example describes the preparation of a silica cake,, which will 20 serve as the starting prod act for the silicas of examples 8 to 12.

Ccmpatability 15 Ingredients Fluoride Pyrophosphate Chlorhexidine HaF Na/K digluconate % compatible 90 95 70 Imto a 30t reactor equipped with a temperature and pH regulatiag system end a Mixel type stirring system are introduced 1.41 of sodium silicate of SiQ7/Fe,,0 rati© of 3.5 sad as SiOj concentration ©f 135 g/(^, preheated to 7S°C* After starting up th® stirring (500 r.p.m.)* the thus formed 25 sediment i© heated to 85°C. When the temperature is reached, there - 31 - is a simultaneous addition of said sasae sodium silicate at a flew rate ©f 0.28 Umln aad e sulphuric acid of concentration 8-0 g/L preheated to 75°C assd at a flow rate of 0.16 l/min.

The average pH of the medium during the sinultaneous addition is 9.8.

- Follo^d^g 47 sia of sinultaneous addition, silicate addition is ©topped and acid addition continued at the same rate until a pH of 8 is reached. The temperature of the reaction mixture is then raised to 95°C in 10 bbLb« whilst continuing the acid addition in order to stabilise the pH of the reaction mixture at 4„2 in this seme time. -j 0 This is followed by ageing for 15 sad si at this pH and at 95°C.

The mixture filtered and the ssoist cake washed with deioaised water until the conductivity of the filtrate is 2000 mlcroslemens cm The cake obtained will be used as a base for producing the silicas of controlled surface chemistry of examples 8 to 12. 15 BSAHPUE 8 The cak© of example 7 is used aM dispersed in deioisized water to form a 100 g/L silica suspension, which is then filtered. The operation is repeated until a filtrate conductivity of 100 saicrosiecaens ea~" is obtained. 2o Ihe cake is then redispersed ia the fona of a ISO g/L. suspension lis deionized water and the pH of the latter is brought to 6 by adding acetic acid™ After filtering, a fiaal washing take® place with deionized water. The product is then dried by atooization and grossed on a Forplex grinder to obtain a grain sise of 9.0 pa. 25 EXAMPLE 9 A cake obtained according to the operating procedure of example 7 is washed with deionized water until a filtrate conductivity of 500 micro-siemens.cm * is obtained. The cake is then washed with 10L of water - 32 - brought to pH 4 by gluconic acid addition. A fiaal washing then takes place with deionized water.

The cake is disintegrated to obtain a homogeneous suspension and, accoo-panied by stirring. 8.3 g of barium acetate ar© added (SBa C2H2°2S H20)* 5 "Hie aixture is aged for 30 sain.

The product is thea dried toy atomlzation and ground on a Forplex grinder to obtain a grain size of 9.0 pm.

EXAMPLE 10 A cake obtained according to the operating procedure of example 7 is washed with deionized water until a filtrate conductivity of 500 micro-1 o siemens. cm""" is obtained.

Ihe cake is then redispersed in the form of a 150 g/L suspension in deionized water and the pH of the latter is brought to 6 by the eddition of acetic acid.

Accompanied by stirring, 25g of barl«n hydroxide Ba(0H)2» 03,0 are added -I5 and the oixture is allowed to age for 30 sain. The suspension is thea filtered and washed with 51 of water- The product is then dried by atoalzatlon end ground on a Forplex grinder to obtain a grain size of 9.0 saicrons.

SIMPLE 11 2o A cake obtained according to the operating procedure of example 7 is washed with deionized water until a conductivity of the filtrate of 100 taicrosiemezts,is obtained. 25 The cake is then redispersed ia the form of a 150 g/l, suspension ia deionized water and the pH of the latter is brought to 6.3 by acetic acid eddition. A final washing then takes place 'with deionised water. - 33 - Accompanied by stirring, Chi g of bariuia acetate BaCC^H^O^)^, H^O is then added .aad the fixture is allowed to age for 30 sain.

Use product is then dried by atonlsatlon aad ground on a Forplex grinder to obtain a grain sise of 9.0 aicross. 5 BIAMPIB 12 Toe cake of exampl / is used, which is dispersed ia deionized water to fora a 100 g/L silica suspension, vhich is then filtered. The opera-tion is repeated viotil the filtrate conductivity is 200 microsieaens* ca The pro-duct is than dried by etomizetion and ground on a Forplex grinder 10 to obtain a grain sis® of 9.0 micros®., Tne characteristics of the silicas of examples 8 to 12 are given in the following tables: Example BET CTAB pH Oil PZC Ho refractive Surfaces sample index 15 m2/g OE/nm2 8 250 50 6.9 12 102 4.5 >3.3 1.460 9 250 45 6.8 10 110 4 >3-3 1.457 10 250 60 7.2 12 105 3.8 >3.3 1.458 11 260 55 7.0 10 105 6 >3.3 1.437 12 250 55 7.5 12 100 3.6 >3.3 1-446 Chemical Analysis Iocs (pp'M.) Examples % At -65 1 95 0 95 0.50 1 96 0 95 1~56 0 90 40 95 0.7 4 90 2 90 1.0 0 90 20 90 0 60 0 90 CHx«chlorhexidine, F - fluorine, Zn » sine, Pyr «» pyrophosphate.

It should be ooted that the PZC is below 3 for all the products given in the table.

EXAMPLE 14 20 This esEEiple relates to the formulation of a translucent dentifrice paste of the gel type using silicas according to the invention. The formula is as follows: Sorbitol (70% aqueous) 65.00 Glycerin 0-00 7mFd CMC 0„8Q Sodium saccharinete 0.20 Sodiu© fluoride 0.24 Sodium benzoate 0.08 Aroma 2.00 Abrasive silica, example 2 15.00 - 36 Thickening silica, example 6 Chlorhexidine digluconate Distilled water 8.00 1.00 7*m Th® dentifrice has satisfactory rheologlcal properties .aad is extruded 5 la an appropriate manner Initially aad after storage (2 aoaths).

Visual examination of the dentifrice coafiras that the behaviour of the extrudete is correct and that it is ia the £ox» of a translucent gel. This dentifrice also has the following properties: There is an anti-bacterial activity.

EXAMPLE 15 This example relates to the femulation of as opaque dentifrice ax the 15 paste type. The fonaula is as follows: 10 pH, 10% dilution Abrasive power on copper LNE Standard (ag) Plastic viscosity (Pe.s) 0.5 6.8 5.6 Glycerin 7nFD CMC Sodiua s&cchariaate Sodium Boaoflcorophosphate Sodism fluoride Sodium laurylsulphate (30% aqueous) Sodium benzoate Aroma Titanium dioxide Abrasive silica, example 5 22.00 1„0 0,20 0.76 20 25 0.10 4.67 0.10 0.90 1.00 31.50 2aS04t, 7 H,0 0.48 Distilled water 37.29 - 3/ - The rheological and visual examination of the dentifrice paste obtained reveals that the usual properties of the dentifrice are good. - 38 -

Claims (6)

1. CLAIMS 1. Silica, characterised ia that it has eoapatability with products of the guanidine type aad ia particular chlorhexidine of at least 651 and sore particularly at least 90%.

2. Silica, according to claim 1, characterized ia that It also has a compatability with zinc of at least 50% .said preferably 80%« 3. Silica more particularly compatible with products of the guanidine type and especially chlorhexidine, characterised ia that it has a surface chemistry such that its acidity function Ho exceeds 3.3. 10 A. Silica according to claim 3, characterised ia that it has en infrared pyridine adsorption spectrum having compared with the pyridine spectrum alone a displacement AV of at the most 10 cm and in particular at the aost 5 cm~*. 5. Silica according to claim A, characterized ia that the aforement-15 ioned displacestent is zero. 6. Silica according to any one of the preceding claims* characterised in that it has an anion content ©f the type S0A~ * CL * H0« , PQ, ead OO^""" of at the most 5.10~J soles for lOQg of silica. ?'» Silica according to claim 6, characterized in that it has a content -3 20 of the aforementioned anions of at the most 1,10 * aore Darticul- -3 arly et the most 0.2.10 moles per 100 g of silica. 8, Silica according to claims 6 or 7, characterised ia that it has e sulphate content* expressed in S0A, of at the most 0.5%, preferably at the most 0.1% aad aore especially at the mcst 0„02%„ 25 9. Silica according to any one of the preceding claims, characterized in that it has a surface chemistry such that the number of OH, - 39 - 9 expressed ia GH/nn"", is at the sost 13 sad nore particularly at the sost 12. 10. Silica according to one of 'die preceding claims, characterised is that it ha.® a poiot aero charge (PZC) of at least 3 aad core 5 particularly between 4 and 6. 11. Silica according to one of the preceding claims, characterized ia that it has an aluminium content of at the most SCO ppa. 12. Silica according to oae of the preceding claims,, characterized ia that it has aa i ron cos tent of at the most 200 ppou TO 13» Silica according to one of the preceding claims, characterized in that it has a calcium content of at the most 500 ppa, in particular at the nost 300 ppE8„ 14. Silica according to one of the preceding claims,, characterized ia that it has a carbon content of at the most 50 ppm and sore 15 particularly at the ®ost 10 ppn. 15. Silica according to one of the preceding claims, characterized ia that it has a pH of at the sost 8 aad note particularly between 8-0 aad 7.5. 16. Silica according to one ©£ the preceding claims, characterized t 20 in that it has a BUT surface between 40 and 600 a~/g- 17- Silica according to one of the preceding claims, characterized 2 in that it has a CTAB surface between 40 end 400 a /g. 18. Silica according to one of the preceding claims, of the abrasive type, characterized in that it has a BET surface between 40 and 25 300 m2/g. 19, 20 21 22 23 25 26 27 28 - 40 - Silica according to claim 18, characterized in that it has a CTAB surface between AO and 100 *T/g. Silica according to one of the claias 1 to 17, of the thickening type, characterised ia that it has a BET surface between 120 and 450 a~/g, sore particularly betveen 120 aad 200 s"7g. Silica accordlag to claim 20, characterised in that it has a CTAB surface between 120 end 40-0 af/g. Silica according to any one of the claims 1 to 17, of th® bifunctional type, characterized in that it has a BET surface between 30 and 200 n^/g. Silica according to claim 22, characterized ia that it has a CTAB surface between 80 end 200 ra"Yg. Silica according to on® of the preeediag claias, characterized *? in that it has ara oil ©aaple between 80 sad 500 tea"/. 100 g. Silica according to one of the preceding claias, characterized in that it has as?, average particle size between 1 mi 10 ps- Silica according to one of the preceding claias, characterized in that it has an apparent density betveen 0.01 and 0.

3. Silica according to one of the preceding claias, characterized Jin that it is a, precipitation silica. Process for the preparation of a silica compatible with guaaidiaes, aore particularly chlorhexidine, according to one of the preceding cleiais of the type involving reacting a silicate with an acid to obtain a silica gel or suspension, separating .and drying the silica, characterized in that following the separation of the silica, the cake resulting froa this separation is washed with water until - 41 - 10 the conductivity of the filtrate is at the aost 200 aicrosiemena.ca~*. 29. Process according to claim 28,, characterised in that the afor®-centioned cake is wasfesd with water until the conductivity ©f the filtrate is at the aost 100 aicrosieaens*ca~". 30. Process for the preparation of a silica compatible with guanidine aad in particular chlorhexidine, according to one of the preceding claias, of the type involving reacting a silicate with an acid, so as to obtain a suspension or a silica gel, characterised in that the cake resulting from said separation undergoes a first washing with water aad then a second vashing or treataent with an acid Bolution. 31. Process according to claim 30, characterized in that the first vashing is carried out until the conductivity of the filtrate'is at the sost 2000 microeieaens-cffl 15 32. Process according to claias 30 or 31, characterised in that the aforementioned acid solutioa is a solutioa of en organic aeids particularly © cooplexing ©cid. 33. Process according to claia 32, characterised in that the aforementioned organic acid is chosen froa aaong carboxylic, dicarboxylic, asaino carboxylic aad faydroxycarboxyllc acids. 20 3

4. Process according to claias 32 or 33, characterised ia that the organic acid is chosen froa aaong acetic, gluconic, tartaric, citric, meleic and glyceric acids. 3

5. Process according to any o^e of the claims 28 to 34, characterized in that the precipitated silica gel or suspension undergoes ageing 25 prior to separation. - 42 - 3

6. Process (according to any one of the claias 28 to 35, characterized Ijs that following the reaction of the silicate with an. add, the pB of the suspension or gel obtaiaed is adjusted to a value of at the most 6 end in particular between 4 sad 6_ ^ 37 „ Process accordlag to one of the clains 28 to 36, characterized ia that addition tekes place to either the suspension or gel, or to the cake, possibly following the disintegration of the latter,, of an alkaline-earth salt. 3-8. Process according to claim 37, characterised in that use is issade -jO of an organic salt,, particularly as alkaline-earth completing agent. 39. Dentifrice composition, characterized 1b that it contains a silica according to one of the claims 1 to 27, or a silica prepared by the process according to on& of the claias 28 to 38. 40. Dentifrice composition according to claira 39, characterized in 20 that it coo,tela,s at least os.e elejaeat chosen from the group inclu- ding fluorine, phosphates, guenidiae3, sine aad osaleic acid-vinyl-ethyl ether copolymers. 43 41. Silica according to claim 1, substantially as hereinbefore described. 42. A process for the preparation of a silica according to claim 1, substantially as hereinbefore described and exemplified. 43. A silica according to claim 1e whenever prepared by a process claimed in a preceding claim. 44. A dentifrice composition according to claim 39, substantially as hereinbefore described and exemplified. P. E. KELLY a CO-, AGSNTS FOR THE APPLICANTS.