WO2005095505A1

WO2005095505A1 - Colloidal dispersion of silica coated cerium composition particles, solid compound obtainable from said dispersion and methods for the preparation and the use thereof

Info

Publication number: WO2005095505A1
Application number: PCT/FR2005/000437
Authority: WO
Inventors: Franck Fajardie; David Fauchadour
Original assignee: Rhodia Chimie
Priority date: 2004-03-02
Filing date: 2005-02-24
Publication date: 2005-10-13
Also published as: FR2867181B1; FR2867181A1

Abstract

The invention relates to a colloidal dispersion of cerium composition particles characterised in that said cerium composition particles are coated at least partially with a silica layer which is obtainable by a hot reaction of a colloidal dispersion of a cerium composition with an alkali silicate at a pH equal to or greater than 7 or by the reaction of said dispersion with a silicon alkoxide or by the reaction of a colloidal dispersion of a cerium composition whose pH is equal or less than 4 with the alkali silicate followed, if necessary, by the pH increase to a basic value or by ageing. The inventive dispersion can be used for cosmetics in the form of an anti-UV agent, in polymers, paints, coating materials or in fibres.

Description

COLLOIDAL DISPERSION OF PARTICLES OF A CERIUM COMPOUND

SILICA COATINGS, SOLID COMPOUND OBTAINED FROM THIS DISPERSION, METHODS OF PREPARATION AND USE

The present invention relates to a colloidal dispersion of particles of a cerium compound coated with silica, methods of preparing this dispersion and its uses. It also relates to a solid compound obtained from this dispersion. Colloidal dispersions or cerium soils are well known. They are of great interest in applications, for example for applications, such as anti-UV, in cosmetics or in polymers. More particularly, these floors can be incorporated into paints, and in particular stains and varnishes, which are commonly used for the protection of metals or the protection of wood, in particular in the furniture, carpentry, parquet and of construction, to protect the wood from damage due to light and bad weather in particular. However, there may be interactions between the cerium oxide particles and the medium in which the dispersions are incorporated. For example, in the case of uses in wood stains, cerium oxide can react with extractable elements (tannins, etc.) in the wood, which can cause the color of the stain to vary. In cosmetics, the possible reduction of cerium IV to cerium III can induce oxidation of the components of the formulations which can also result in color variations or destabilization of these formulations. Problems of degradation of polymers, for example plastics or fibers, can still be encountered when these cerium soils are used as additives in these materials. These problems arise from oxidative catalysis reactions due to the cerium alone or to the cerium in association with impurities contained in these formulations or materials. The object of the invention is to provide a colloidal dispersion of cerium which makes it possible to attenuate or eliminate this interaction problem while retaining the properties of the dispersion, in particular as regards the particle size. For this purpose, the colloidal dispersion of the invention is a dispersion of particles of a cerium compound in a liquid phase which is characterized in that the particles of the cerium compound are coated at least in part with a layer of silica. The invention also relates to the preparation of such a dispersion. According to a first embodiment, the preparation process is characterized in that a colloidal dispersion of the cerium compound is reacted with an alkaline silicate at hot and at a pH of at least 7. According to a second embodiment, the method of the invention is characterized in that a colloidal dispersion of a cerium compound is reacted with a silicon alkoxide. According to a third embodiment, the method of the invention is characterized in that it comprises the following steps:

- A colloidal dispersion of a cerium compound whose pH is at most 4 is reacted with an alkaline silicate;

- Then, if necessary, the pH of the medium obtained is raised to a basic value;

- the medium obtained is subjected to ripening. The dispersion of the invention comprises particles which are encapsulated by silica which therefore reduces or prevents any interaction of the cerium with the medium in which the dispersion is incorporated. Furthermore, the particles of the dispersion retain a size of colloidal dimension and, preferably, remain perfectly individualized and not aggregated, which thus keeps a well dispersed and stable soil. Finally, the optical properties of cerium oxide (property of UV filter) are preserved. Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, the various concrete but nonlimiting examples intended to illustrate it as well as the appended drawing in which: - Figure 1 is the photo of a stain obtained from a known formulation; - Figure 2 is the photo of a stain obtained from a formulation incorporating a dispersion according to the invention; - Figure 3 is the photo of a stain obtained from a formulation incorporating a dispersion according to the prior art. The expressions “colloidal dispersion of a cerium compound” or “sol of a cerium compound” designate any system made up of fine solid particles of a cerium compound, of colloidal dimensions, in stable suspension in a liquid phase. These particles can also optionally contain residual amounts of bound or adsorbed ions such as for example nitrates, acetates or ammonium ions. By colloidal dimensions is meant dimensions between about 1 nm and about 100 nm, more particularly between 1 nm and 85 nm. It will be noted that in such a dispersion, the cerium can be found either, preferably, completely in the form of colloids, or simultaneously in the form of ions and in the form of colloids. The cerium is present in the dispersion generally in the form of oxide and / or hydrated oxide (hydroxide) of cerium, preferably in the form of ceric oxide CeO ₂ . The main characteristic of the dispersion of the invention is the fact that the particles of the cerium compound are coated at least in part with a layer of silica. The particles of the cerium compound therefore form a core or a heart which is surrounded by a bark or a silica envelope. The layer or envelope of silica may not be perfectly continuous or homogeneous, this layer however being continuous according to a preferred embodiment. The quantity of silica present in the dispersion, on the particles, must be such that it is possible to obtain, preferably, a complete coating of the particles. This quantity will therefore depend on the size of the particles and it will be all the greater the finer the particles. By way of example, this quantity, expressed by the SiO ₂ / CeO ₂ mass ratio, is generally at least 0.01, more particularly at least 0.1 and even more particularly at least 0.2 or at least 0.3. In particular, this amount can be between 0.01 and 2, in particular between 0.1 and 1.5, and even more particularly between 0.2 and 0.5. The encapsulation of the particles of the cerium compound by silica can be demonstrated by measuring the zeta potential of the dispersions. When the variation of this potential is measured by varying the pH of a dispersion from an acid value to a basic value, the latter goes from a positive value to a negative value for a dispersion of particles of a composed of non-encapsulated cerium, the change to zero of the potential constituting the isoelectric point. In the case of a dispersion according to the invention, this potential is in fact either always negative or, when there is an isoelectric point, this is offset with respect to the isoelectric point of a dispersion with non-encapsulated particles. In addition to measuring the zeta potential, the encapsulation can also be demonstrated by transmission electron microscopy (TEM). The particles of the dispersion, that is to say the particles of the cerium compound coated with silica, preferably have an average size of at most 85 nm. This average size can in particular be at most 50 nm and more particularly at most 25 nm. This size can be even more particularly at most 20 nm. More specifically, the particles can have a size between 5 nm and 25 nm or between 5 nm and 20 nm. The sizes given here must be understood as designating the mean hydrodynamic diameter of the particles, as determined on the dispersion by quasi-elastic light scattering (DQEL) according to the method described by Michael L. Me CONNELL in the journal Analytical Chemistry 53, No.

8, 1007 A, (1981). These sizes can also be determined by TEM in direct measurement, this technique, in the case of the products of the invention, giving values similar or slightly lower than those obtained by the DQEL technique. The average size values given above apply to particles in suspension in the liquid phase of the dispersion. According to a preferred embodiment, these particles (so-called primary particles) can be composed of a certain number of coherent domains measured by the RX analysis (elementary particles) of dimension less than 5 nm, from 2 to 3 nm for example , and they are presented in an individualized form, that is to say not aggregated. According to another embodiment, these particles (so-called secondary particles) correspond to an aggregate of a few primary particles of the above type. The dispersions according to the invention can have a pH in a wide range from 3 to 11 for example, this pH being chosen according to the subsequent use of the dispersions. The dispersions of the invention can have a concentration which also varies over a wide range. This concentration can be at least 1 g / L, in particular at least 10 g / L, more particularly at least 50 g / L and even more particularly at least 100 g / L. This concentration is expressed in mass of particles. It is determined after drying and calcination in air of a given volume of dispersion. According to a particular embodiment, the dispersion comprises an organic acid having at least three acid functions and whose third pK is at most 10 or a salt of this acid. In the dispersion according to this embodiment, the acid interacts with the cerium cation by any type of chemical bond. Therefore, the acid is therefore present on, or within, cerium compound particles but it can also be present in the liquid phase. As suitable acid, there may be mentioned very particularly citric acid. The acid salt may more particularly be an alkaline salt. The acid level in the dispersion expressed by the mole of acid / moles of cerium ratio can be for example between approximately 0.01 and approximately 1.5, more particularly between 0.2 and 0.5. It should be noted here that this rate depends on the size of the particles of the dispersion, this rate being all the lower as the size of the particles increases. It will also be noted that the lower limit of this rate is that below which the dispersion is no longer stable. The upper limit in fact depends on the solubility of the acid used and also on the maximum amount of acid beyond which there is a risk of seeing the dispersion transform into a gel. The dispersions according to the invention are generally aqueous dispersions, the liquid phase being water. However, according to certain variants, the dispersions of the invention can have a mixed aqueous / organic liquid phase, for example hydroalcoholic based on a mixture of water and alcohol, or else a liquid phase constituted by an organic solvent such as a alcoholic liquid phase. Mention may be made, as possible alcohol, of methanol, ethanol or propanol. As organic solvent, there may also be mentioned aliphatic hydrocarbons such as hexane, heptane, octane, nonane, inert cycloaliphatic hydrocarbons such as cyclohexane, cyclopentane, cycloheptane, aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes, liquid naphthenes. Also suitable are petroleum cuts of the Isopar or Solvesso type (trademarks registered by the company EXXON), in particular Solvesso 100 which essentially contains a mixture of methylethyl- and trimethyl-benzene, Solvesso 150 which contains a mixture of alkylbenzenes, in particular dimethylbenzene and of tetramethylbenzene and Isopar which essentially contains iso- and cyclo-paraffinic C-11 and C-12 hydrocarbons. It is also possible to use chlorinated hydrocarbons such as chloro- or dichlorobenzene, chlorotoluene. The ethers as well as the aliphatic and cycloaliphatic ketones such as for example diisopropyl ether, dibutyl ether, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, mesityl oxide, can be envisaged. The organic solvent can in particular also be a cosmetically acceptable compound which is immiscible in water and / or miscible in a cosmetically acceptable organic phase. The organic solvent or the organic phase may, for example, be chosen from oils of mineral or vegetable origin and their derivatives, essential oils, silicone oils, waxes, and any other cosmetically acceptable water-immiscible carrier. The methods for preparing the dispersion of the invention will now be described. According to a first mode, the process is implemented by hot reaction and at a pH of at least 7, of a colloidal dispersion of the cerium compound with an alkaline silicate. The starting colloidal dispersion may have been obtained for example by the methods described in European patent applications EP 206906, EP 208581, EP 316205, EP 700870 and EP 700871. It is possible to use very particularly the colloidal dispersions obtained by thermohydrolysis of a aqueous solution of a cerium IV salt such as a nitrate, in an acid medium in particular. Such a process is described in European patent applications EP 239477 or EP 208580. In this latter application, for example, a colloidal dispersion of a cerium IV compound is first prepared by reacting an aqueous solution of cerium IV salt with a base, then this dispersion is heat treated, whereby a precipitate is obtained. This precipitate can be resuspended in water and thus give a colloidal dispersion. In the case of the particular embodiment described above in which the dispersion comprises an organic acid, or a salt thereof, having at least three acid functions and whose third pK is at most 10, it is possible to start from colloidal dispersions of the type described above and at basic pH. By basic pH is meant here a pH greater than 7 and which is preferably between 7.5 and 9.5, more particularly between 7.5 and 9. Such pH values can be obtained by the presence of a base in the dispersion. This base can be ammonia but also an amino. Any type of amine may be suitable, primary, secondary or tertiary. Note that it is possible to use amino alcohols such as for example 2-amino-2-methyl-1-propanol. An acid (for example citric acid) as defined above is added to the dispersion so as to obtain the necessary pH value. The acid can be added to the starting dispersion first and then the base in a second step. It is however possible to add the acid and the base simultaneously. The dispersions which are suitable for the implementation of this particular embodiment can also be those described in patent application WO 01/38225. These dispersions have the advantage of being stable over a wide pH range, which facilitates their implementation in the process of the invention. According to a variant of the preparation process according to this first mode, a starting dispersion having a high purity is used. By this is meant a dispersion having a low ionic strength, in particular a dispersion with a low nitrate content, for example a dispersion in which the mass ratio NO ₃ 7CeO ₂ is at most 5%. Dispersions of this type are for example those described in EP-A-700870. This variant is particularly advantageous in the case of the preparation of dispersions in which the particles are fine (of the order of 10 nm) and / or at high concentration (at least 100 g / L) and / or with a high silica content. (at least 0.4). The alkali silicate can more particularly be a sodium silicate. The amount of alkali silicate used depends on the SiO ₂ / CeO ₂ ratio desired in the final dispersion. The reaction between the starting dispersion and the silicate is carried out hot. By this is meant a reaction at a temperature of at least 60 ° C, preferably at least 80 ° C and which, generally, can be between 80 ° C and 100 ° C. The reaction must also take place under pH conditions of at least

7, generally between 8 and 11. More particularly, this reaction can be carried out at constant pH, that is to say that the pH is maintained at a value varying by ± 0.5 pH unit around the fixed value, this control pH can be achieved by adding a base or an acid to the reaction medium. The reaction is preferably carried out by introducing the silicate into the dispersion. The dispersion may have been brought to a pH of at least 7 prior to this introduction. A base can be added to the dispersion simultaneously with the introduction of the silicate to obtain a suitable pH value. In the context of the process of the invention, the base used to adjust the pH can be chosen in particular from alkali or alkaline-earth hydroxides and ammonia. It is also possible to use secondary, tertiary or quaternary amines. Ammonia can be used in particular. At the end of the reaction of the dispersion with the silicate, it is possible to subject the medium obtained to maturing. By ripening is meant here the maintenance of the medium at a temperature close to, or identical to, that of the medium during the reaction. The medium can be stirred for this ripening. Curing is more particularly useful in the case of the preparation of dispersions with a high concentration of cerium. The duration and the temperature of the ripening depend on the silica content. Generally, the duration is between 10 minutes and 10 hours, this upper limit not being critical and being given as an indication. The temperature is similar to that at which the reaction was carried out, it is preferably at least 60 ° C and it can be between 80 ° C and 100 ° C more particularly. At the end of the reaction and of possible ripening, the medium obtained is allowed to cool and a dispersion according to the invention is thus obtained. This dispersion can be washed by adding a volume of water to the dispersion and then be subjected to ultrafiltration to return to the initial volume. The washing step can be repeated if necessary. In particular, in the case of a preparation using as starting dispersion the specific dispersion described above comprising an organic acid, or a salt thereof, having at least three acid functions and whose third pK is at most 10 , several washes can be carried out to reduce the amount of organic acid in the final dispersion or possibly eliminate it. In a final step, a concentration of the dispersion can be carried out, if necessary, by any known means, for example by ultrafiltration. A second type of process can be used. The method includes at least one step of reacting a colloidal dispersion of the cerium compound with a silicon alkoxide. The starting colloidal dispersion may be of the type mentioned above in the case of the method according to the first mode. However, acid dispersions are preferably used, for example those of the type mentioned above and which are obtained by thermohydrolysis of an aqueous solution of a cerium IV salt in an acid medium and described in EP-A-239477 or EP-A- 208580. The acid dispersions preferably have a pH of at most 4, more particularly at most 3 and which can be between 1.5 and 2.5 for example. As silicon alkoxide which can be used, mention may in particular be made of tetraethylorthosilicate. The amount of silicon alkoxide used depends on the ratio

SiO ₂ / CeO ₂ desired in the final dispersion. The reaction is usually carried out at room temperature, for example at 25 ° C. This reaction takes place in an aqueous or essentially aqueous route. he it should in fact be noted that the reaction concerned is a hydrolysis of the silicate which, in addition to the silica, produces an alcohol which remains in the reaction medium. This can be done by introducing the silicon alkoxide into the starting dispersion. At the end of the step which has just been described, called step (a) in the following description, a dispersion according to the invention is obtained. However, it may be advantageous to complete the method according to the second embodiment with an additional step. This makes it easier to obtain a dispersion in which the silica layer enveloping the particles of the cerium compound is continuous. This second optional step of the process, called step (b) in the following description, consists in raising the pH of the medium obtained at the end of the reaction between the dispersion and the silicon alkoxide. This pH, which is acidic, is brought back to a basic value, ie greater than 7 by addition of a base, for example ammonia. The pH can more particularly be at least 8 and it is generally at most 11. It is possible, at the end of step (a) and before step (b), to undergo a ripening in the medium obtained as in the case of the method according to the first mode. This ripening is usually done at room temperature and with stirring for a period which can also be between 30 minutes and 10 hours. As in the method according to the first mode, the dispersion can be washed, one or more times, using the same technique as that described above. However, in the case of the method of this second mode, the washing can be carried out before or after step (b). Likewise, the dispersion can be concentrated as indicated in the case of the first embodiment. The invention also relates to a third embodiment of the method which will be described below. The first step of the method according to this third mode is of the same type as that which has been described as the first step either in the first embodiment of the method, or in the second embodiment, step (a), since it is essentially acts of a reaction between a dispersion of a cerium compound and a silicate. However, here, the starting dispersion must have a pH of at most 4, preferably at most 2. What has been said previously for the first and second embodiments also applies here. The second step of the process consists, if necessary, in raising the pH of the medium obtained at the end of the previous step to a basic value, that is to say to a value greater than 7, preferably at least 9 This step can be carried out in the same way as that which has been described for analogous step (b) of the second embodiment. It will be noted that this second step may not be necessary because in certain cases, for a starting dispersion of high purity for example, the reaction between this dispersion and the silicate can lead to a medium having at the end of this reaction a pH already basic. The dispersion can be washed in the same manner as that which has already been described above. Finally, the third step of the process consists of a ripening of the same type as that which has been described previously for the other embodiments. At the end of this last step, a dispersion according to the invention is obtained. A solid / liquid separation can be carried out by any known technique to remove any solid residue which may sediment. Here again, the dispersion can be concentrated as indicated in the case of the first embodiment. This latter embodiment has the advantage of easily leading to dispersions of high concentration, in particular greater than 50 g / L. In the case of a dispersion in which the liquid phase is not only water but a water / organic solvent mixture or in which the liquid phase consists of an organic solvent, this dispersion can be prepared from an aqueous dispersion as obtained by the methods described above and by adding the organic solvent to the aqueous dispersion then distillation to remove the water. In the case of an organic solvent which is not soluble in water, a transfer agent can be introduced into the organic solvent and the transfer agent / organic solvent mixture is then brought into contact with the aqueous sol. Reference may be made here to the teaching of WO 01/10545 which describes a process for the preparation of an organic soil and of acids (in particular organic acids, amphiphiles, of 10 to 50 carbon atoms) usable as transfer agents. As indicated above, the invention also covers a solid compound, which is characterized in that it is obtained from a dispersion such as described above and by separation of the solid particles of the cerium compound from the liquid phase. This separation can be done by any suitable technique. It is thus possible to carry out the separation by flocculation with a third solvent or also by distillation. Drying can also be done by atomization or lyophilization or in an oven. Following this separation, a solid phase is collected which can be dried and which occurs, depending on the drying level reached, either in the form of a powder or in the form of a paste, which constitutes a solid compound according to the invention. This compound comprises aggregates whose size can vary within wide limits depending on the drying method used. This size, determined by MET, can be between 0.1 μm and 100 μm. These aggregates are formed by the particles of the dispersion and these particles retain well, in this compound, their structure with a core of cerium compound and with a silica shell. The dispersions or the solid compound according to the invention can be used in various fields such as cosmetics, polymers, paints, coating materials or fibers. As a result, the invention also relates to compositions of matter of the polymer material, cosmetic composition, paint, coating, fiber type, which are characterized in that they comprise or in that there is incorporated, a dispersion or a solid compound according to the invention. Thus, the dispersions or the solid compound according to the invention can be used in cosmetic compositions, for example in sun protection formulations, intended to be applied to the skin. These compositions can be formulated in particular in the form of creams, milks, oils, sprays. Such compositions and formulations, as well as the ingredients which can be used (oily and / or aqueous and / or alcoholic vectors, surfactants, viscosity agents, active agents beneficial for the skin, cosmetic sensation agents, stabilizers, perfumes, etc.) for implement them are known to those skilled in the art. The dispersions or the solid compound can be used in these compositions as a sole UV protection agent, or in combination with other sun protection agents, for example other inorganic agents such as dioxide agents. titanium, or organic agents. The other sun protection agents are known to those skilled in the art. The combination of different agents can provide optimal protection over different wavelength ranges (UV-A, UV-B etc). The cosmetic compositions can in particular be formulated in the form of emulsions, water in oil or oil in water. The dispersion or the solid compound may be present in the aqueous phase and / or in the oily phase. If the dispersion is a dispersion in water, the emulsion is preferably a water-in-oil emulsion, the dispersion being present in the aqueous phase. If the dispersion is a dispersion in an organic solvent immiscible with water, the emulsion is preferably an oil-in-water emulsion, the dispersion being present in the oil phase. It is not excluded to combine these last two embodiments of cosmetic compositions. If the compositions are formulated in the form of a sun oil, a dispersion is preferably used in an organic solvent, miscible or immiscible in water, and miscible with the oil used. Likewise, the dispersions or the solid compound according to the invention can be used as anti-UV protection agent in plastics which can be of the thermoplastic or thermosetting type. Examples of thermoplastic polymers that may be mentioned: polycarbonates such as poly [methane bis (4-phenyl) carbonate], poly [1, 1-ether bis (4-phenyl) carbonate], poly [diphenylmethane bis (4-phenyl) carbonate], poly [1, 1-cyclohexane bis (4-phenyl) carbonate] and polymers of the same family; polyamides such as poly (4-amino butyric acid), poly (hexamethylene adipamide), poly (6-aminohexanoic acid), poly (m-xylylene adipamide), poly (p-xylylene sebacamide), poly ( 2,2,2-trimethylhexamethylene terephthalamide), poly (metaphenylene isophthalamide), poly (p-phenylene terephthalamide), and polymers of the same family; polyesters such as poly (ethylene azelate), poly (ethylene-1,5-naphthalate, poly (1,4-cyclohexane dimethylene terephthalate), poly (ethylene oxybenzoate), poly (para-hydroxy benzoate), poly (1,4-cyclohexylidene dimethylene terephthalate), poly (1,4-cyclohexylidene dimethylene terephthalate), polyethylene terephthalate, polybutylene terephthalate and polymers of the same family; vinyl polymers and their copolymers such as polyvinyl acetate , polyvinyl alcohol, polyvinyl chloride; polyvinyl butyral, polyvinylidene chloride, ethylene-vinyl acetate copolymers, and polymers of the same family; acrylic polymers, polyacrylates and their copolymers such as acrylate polyethyl, poly (n-butyl acrylate), polymethylmethacrylate, polyethyl methacrylate, poly (n-butyl methacrylate), poly (n-propyl methacrylate), polyacrylamide, polyacrylonitrile, poly (acrylic acid), ethylene-acrylic acid copolymers, ethylene-vinyl alcohol copolymers , acrylonitrile copolymers, methyl methacrylate-styrene copolymers, ethylene-ethyl acrylate copolymers, methacrylate-butadiene-styrene copolymers, ABS, and polymers of the same family; polyolefins such as low density poly (ethylene), poly (propylene) and in general the alpha olefins of ethylene and propylene copolymerized with other alpha olefins such as 1-butene, and 1-hexenes which can be used up to less than 1%. Other comonomers used can be cyclic olefins such as 1,4-hexadiene, cyclopentadiene and ethylidenenorbornene. The copolymers can also be a carboxylic acid such as acrylic acid or methacrylic acid. Finally, mention may be made of low density chlorinated poly (ethylene), poly (4-methyl-1-pentene), poly (ethylene), poly (styrene). Among these thermoplastic polymers, polyethylenes are particularly preferred, including LDPEs (low density polyethylenes), LLDPEs (linear low density polyethylenes), polyethylenes obtained by metallocene synthesis, PVC (polyvinyl chloride), PET (polyethylene terephthalate) , polymethylmethacrylate, copolyolefins such as EVA (polyethylene of vinyl alcohol or ethylene vinyl acetate), mixtures and copolymers based on these (co) polymers, polycarbonate As regards thermosetting materials, mention may be made, for example, phenoplasts, aminoplasts, epoxy resins and thermosetting polyesters. The products of the invention can be introduced for example in the form of a solid powder or in the form of a dispersion in water or in an organic dispersant in the medium for synthesis of the polymer compound, or in a molten thermoplastic polymer in any form. The quantity of product of the invention, expressed by mass of CeO ₂ , in the polymer material can be in particular between 0.01% and 10% by mass relative to the total mass of the material, more particularly between 0.1% and 2%. These values are given by way of example only and they may vary depending on the nature of the polymer. The low value is fixed according to the intensity of the desired effect which one wishes to obtain. The high value is not critical, generally one does not exceed the value beyond which an additional quantity does not bring any advantage or additional effect compared to other constraints, for example cost constraints. The dispersions or the solid compound according to the invention can be used in paints, in particular varnishes or stains. By paints, stains or varnishes is meant the formulations or compositions usually designated by this term in the technical field of paints and which are for example based on the following emulsion resins: alkyd resins, the most common of which is called glycerophthalic; long or short oil modified resins; acrylic resins derived from acrylic (methyl or ethyl) and methacrylic acid esters optionally copolymerized with ethyl acrylate, 2-ethylhexyl or butyl acrylate as well as acrylic-isocyanate resins; vinyl resins such as, for example, polyvinyl acetate, polyvinyl chloride, butyralpolyvinyl, formalpolyvinyl, and the copolymers of vinyl chloride and vinyl acetate or vinylidene chloride; the aminoplast or phenolic resins most often modified; polyester resins; polyurethane resins; epoxy resins; silicone resins; cellulosic or nitrocellulosic resins. The products of the invention are used by simple mixing of the colloidal dispersion or of the solid compound with the paint, in particular the stain or the varnish. The amount of colloidal dispersion or solid compound used depends on the desired final content of cerium oxide in the paint. This content can be arbitrary. Generally, the products of the invention are used in an amount such that the content of cerium oxide is at most 25% by weight, preferably at most 10% by weight and even more preferably at most 3% by weight compared to the whole colloidal dispersion or solid compound and paint. The products of the invention are particularly suitable for paints for wood as well as for paints for metals, in particular for cars, trains or boats. Examples will now be given.

EXAMPLE 1 This example relates to a dispersion according to the invention prepared by a process according to the first embodiment. We start from a basic soil of pH 9.5 of cerium oxide particles, 9 nm in size, at a concentration of 10 g / L and with a low nitrate content (NO ₃ ^" / CeO ₂ of 5% by weight), at ambient temperature (25 ° C.) This sol is introduced at the bottom of the vessel in the reactor and brought to 90 ° C. The pH changes to a value of 8. The temperature is maintained at 90 ° C. for throughout the test. Sodium silicate (SiO ₂ = 236 g / L) is gradually added with mechanical stirring at 600 rpm The amount of sodium silicate determined by the mass ratio SiO ₂ / CeO ₂ is 0.4 The pH is kept constant during the addition of the silicate to a value of 8 if necessary with ammonia or sulfuric acid. The mixture then undergoes a maturing step at 90 ° C. minutes with mechanical stirring (600 rpm). The mixture is then allowed to cool freely in the reactor. A colloidal dispersion is obtained, the characteristics of which are have the following: pH: 9 Particle size: 11 nm (DQEL) Concentration: 10 g / L in CeO ₂ Measuring the Zeta potential by varying the pH of the dispersion in a range between 9 and 1 gives a potential always negative, there is no isoelectric point. No flocculation is observed up to a pH of 1.

This indicates good encapsulation of the particles of the cerium compound by silica.

EXAMPLE 2 This example relates to a dispersion according to the invention prepared by a process according to the second embodiment. Tetraethylorthosilicate (TEOS) is gradually incorporated at room temperature (25 ° C), with mechanical stirring (300 rpm), into a sol of cerium oxide particles of 10 nm, of acid pH (1.8) and at 10 g / L CeO ₂ concentration. The SiO ₂ / CeO ₂ mass ratio is fixed at 1.05. The mixture then undergoes a one hour maturing step at room temperature (25 ° C) with mechanical stirring (430 rpm). The dispersion is then washed by mixing 100 ml of the sol obtained with 200 ml of water, then ultrafiltration until it returns to a volume of 100 ml. Two successive washing cycles are carried out. At the end of the washing, the pH of the medium obtained is raised to 9 by addition of ammonia with mechanical stirring (300 rpm). A colloidal dispersion is obtained, the characteristics of which are as follows: pH: 9 Particle size: 8nm (DQEL) Concentration: 10 g / L of CeO ₂ Measurement of the Zeta potential by varying the pH of the dispersion in a range between 9 and 2.5 gives an always negative potential, there is no isoelectric point. No flocculation is observed up to a pH of

2.5. This indicates good encapsulation of the particles of the cerium compound by silica.

EXAMPLE 3 This example relates to the use of a dispersion of the invention in an aqueous stain. We start with a stain of conventional industrial formulation for exterior finish of the dispersed acrylic-polyurethane type having the commercial reference AZ930 from the company ARCH COATI NGS

France. The colloidal dispersion of Example 1 is incorporated by simple mixing at a content of 1% of active material (cerium oxide) relative to the total of the formulation. A cerium sol corresponding to that used as the starting product in Example 1, therefore based on particles not encapsulated by silica, is incorporated in the same manner as above at a content of 1% relative to the total to give a comparative formulation. The formulations thus produced are applied by brush to oak with a grammage of 150gr / m ² and are subjected to air conditioning for one week before their evaluation. We see in the photo of Figure 1 the stain obtained by applying the formulation as is. The photo of Figure 2 is that of the stain obtained from the formulation incorporating a dispersion of the invention and the photo of Figure 3 is that of the stain obtained from a formulation incorporating the cerium-based sol of particles not encapsulated by silica. It can be seen that the stain added to the comparative soil undergoes a significant variation in color compared to the stain alone. On the other hand, the stain added with the dispersion according to the invention undergoes practically no variation in coloring, its color is similar to that of the stain alone.

EXAMPLE 4 This example relates to the use of a solid compound according to the invention in a PVC resin. The PVC formulation used is as follows:

Processing conditions: All the ingredients of the PVC formulation (except the organic co-stabilizer and the epoxidized soybean oil which are liquid) are mixed in a Papenmeier type mixer of 8L so as to have a base compound of 2.5kg. The mixture is brought to 112 ° C by stirring at 2500 rpm, then cooled to 40 ° C at minimum speed. Each ingredient is incorporated into one hundred grams of the compound, including organic cost-effective and epoxidized soybean oil. The whole is mixed for 5 minutes using an IKA paddle mixer. Sheeting is done in 3 minutes on a Troester calender at 180 ° C, the distance between the cylinders being fixed at the 0.8 scale. The PVC sheet obtained is thermocompressed in a mold of

1mm thick, allowing to form rectangles of 90X50 mm. This thermocompression is carried out with the Schwabenthan plate press, 180 ° C. The cycle followed is: 2 minutes without pressure, 1 minute at 100 bars, 1 minute at 150 bars. Cooling takes place in 3 minutes at 150 bars. Four plates are produced, a first as described above, a second with an organic UV absorber which is Tinuvin P326 from Ciba as an additional additive, a third with a solid compound resulting from spray drying. of the dispersion according to the invention of Example 1 and a fourth with the solid compound resulting from the spray drying of the starting dispersion of Example 1 (cerium oxide sol not encapsulated with silica). These solid compounds and Tinuvin are used in an amount of 0.2 part in active material (Tinuvin or cerium oxide). The yellow index is measured on the plates in the Yxy system with a Spectra-Magic spectrocolorimeter (B2).

It can be seen from the table above that the PVC plates added with the encapsulated cerium oxide according to the invention have a yellow index very much lower than that measured in the case of a non-encapsulated cerium oxide. In addition, the value of the yellow index obtained in the case of a encapsulated cerium oxide according to the invention is similar to that obtained with an organic UV absorber conventionally used in PVC application and also close to that of the PVC matrix. reference.

EXAMPLE 5 This example relates to the preparation of a dispersion according to the invention by a process according to the third embodiment described above. The following reagents are used: a sol of cerium oxide particles of 10 nm at 82.2 g / L of CeO ₂ ; d

= 1.067, pH = 1.5 - a sodium silicate Na ₂ O = 5.64%; SiO ₂ = 18.57% - a 28% NH ₄ OH solution, i.e. 14.8 mol / L 22.13 g of the sodium silicate solution diluted in 39 mL of water are added, with mechanical stirring of 300 rpm, at 200 mL of the cerium sol placed in a 1 liter glass reactor, at a flow rate of 5 mL / min. The target mass ratio SiO ₂ / CeO ₂ is 0.25. The pH at the end of the addition of the sodium silicate is 4.9. 1.0 ml of ammonia is then added to obtain a pH of 9. The reaction mixture is homogenized for 15 minutes. The dispersion obtained is then washed in the following manner. Centrifuged at 2000 rev / min for 10 minutes then the solid product obtained is redispersed in a volume of water at pH 9 and equal to the volume of water in the dispersion before washing. We repeat this operation once. The washed dispersion is matured for 2 hours at 100 ° C. At the end of ripening, it is observed that a small fraction of solid is deposited at the bottom of the bottle. This fraction is separated by centrifugation at 2000 rpm. The supernatant resulting from centrifugation constitutes the dispersion of the invention, the characteristics of which are as follows: pH = 9.0 Concentration: 51 g / L of CeO ₂ particle size: 82 nm (DQEL)

Claims

1- Colloidal dispersion of particles of a cerium compound in a liquid phase, characterized in that the particles of the cerium compound are coated at least in part with a layer of silica.

2- Dispersion according to claim 1, characterized in that the coated particles have an average size of at most 85 nm, more particularly at most 50 nm.

3- Dispersion according to claim 2, characterized in that the coated particles have an average size of at most 25 nm.

4- Dispersion according to one of the preceding claims, characterized in that it contains silica in an amount, expressed by the mass ratio SiO ₂ / CeO ₂ , of at least 0.01, more particularly between 0, 01 and 2.

5- Dispersion according to one of the preceding claims, characterized in that it comprises an organic acid having at least three acid functions and whose third pK is at most 10 or a salt of this acid.

6- Dispersion according to one of the preceding claims, characterized in that the liquid phase is water.

7- Dispersion according to one of claims 1 to 5, characterized in that the liquid phase is an organic phase.

8- A method of preparing a dispersion according to one of claims 1 to 6, characterized in that reacts hot and at pH of at least 7, a colloidal dispersion of a cerium compound with a silicate alkaline.

9- Process according to claim 8, characterized in that after the reaction of the colloidal dispersion of the cerium compound with the alkali silicate, the medium obtained is subjected to ripening. 10- Process according to claim 8 or 9, characterized in that the colloidal dispersion of the cerium compound is reacted with the alkali silicate while keeping the pH of the reaction medium constant.

11- Method according to one of claims 8 to 10, characterized in that the colloidal dispersion of the cerium compound is reacted with the alkali silicate by introducing the silicate into the dispersion.

12- A method of preparing a dispersion according to one of claims 1 to 6, characterized in that a colloidal dispersion of a cerium compound is reacted with a silicon alkoxide.

13- The method of claim 12, characterized in that after the reaction between the colloidal dispersion and the silicon alkoxide is then raised the pH of the medium obtained to a basic value.

14- A method according to claim 13, characterized in that the pH of the medium is raised to a value of at least 9.

15- Method according to one of claims 12 to 14, characterized in that at the end of the reaction between the colloidal dispersion and the silicon alkoxide and before possibly raising the pH to a basic value, it is subjected ripening in the middle obtained.

16- Method according to one of claims 12 to 15, characterized in that at the end of the reaction between the colloidal dispersion and the silicon alkoxide or after having raised the pH to a basic value, it is subjected to a dispersion wash obtained.

17- Method for preparing a dispersion according to one of claims 1 to 6, characterized in that it comprises the following steps:

- Then, if necessary, the pH of the medium obtained is raised to a basic value;

- the medium obtained is subjected to ripening. 18- Solid compound, characterized in that it is obtained from a dispersion according to one of claims 1 to 7 by separation of the particles from the liquid phase.

19- Composition of matter of the polymer material type, cosmetic composition, paint, coating, fiber, characterized in that it comprises or in that one has incorporated therein, a dispersion or a solid compound according to one of claims 1 at 7 or 18.