MXPA97009949A - Redispersible powders in water of polymers film trainers with a denucleo / cor structure - Google Patents

Abstract

A water-redispersible powder based on water-insoluble, substantially insoluble film-forming polymers, prepared by the polymerization of aqueous emulsion, of ethylenically unsaturated monomer with a core / shell structure comprising a hydrophobic polymeric core with a temperature of glass transition (Tg) of -30-25 ° C, preferably -10-20 ° C, and a shell attached to the core through covalent bonds resulting from the reaction of the grafting agent to graft the shell over the remaining ethylenically unsaturated bonds in the nucleus. Such a shell is prepared by the emulsion polymerization on the particle core, on a monomer mixture comprising at least one monomer selected from styrene or a C1 C12 alkyl acrylate or methacrylate, at least one monomer having a group saccharide and at least one ethylenically unsaturated bond, and at least one binding agent

Description

REDISPERSIBLE POWDERS IN WATER OF POLYMER FILM FORMATORS WITH A "CORE / NUCLEUS" STRUCTURE DESCRI PTION OF THE I NVENTION The present invention relates to water-redispersible powders based on film-forming polymers, in a "core / shell" structure, the film-forming polymers are prepared through the polymerization of aqueous emulsion of monomers containing ethylenic unsaturation. The redispersible powders of the above type, which are generally obtained through spray-dried polymer network structures, are already known. These powders have a great commercial advantage, since they can be marked as they are to the various users who disperse them in water to prepare formulations which are suitable for the conventional uses of structural networks, in particular, especially in the preparation of coating compositions of paper, paint compositions, adhesive coatings (pressure sensitive adhesives, adhesives for floor coverings) and additives for hydraulic binders of the mortar or concrete type. It is understood that the sale of such powders, which can be redispersed by the user in pseudo- or neolátex, have, when compared with the sale of the latex initial, the immense advantage of not understanding water, which generally constitutes 50% by weight of latex, and this presents considerable savings in transportation and handling. However, such powders must exhibit the following properties to be salable: they must be stable to storage, must be easily redispersible in water at room temperature and must produce a neolátex of narrow particle size which is suitable for the intended application, the application as an additive to hydraulic binders, more particularly with the aim in the case of the present invention, the additive resulting from the redispersible powder must be compatible with the hydraulic binder, must allow the hydraulic binder to be fixed to solid, without delaying the process of fixing and impart to the coatings comprising the binder comprising in the additive a correct adhesion to the usual substrates, even in a wet state, as well as good mechanical properties, it must be redispersible at a broad pH, it must exhibit a good redispersion in a medium containing multivalent ions, in particular alkaline The object of the present invention is to propose a redispersible powder exhibiting all the desired properties indicated above, and a process for the preparation of this powder. Heretofore, the invention first relates to all water redispersible powder based on film-forming polymers, which are substantially insoluble in water and which are prepared through aqueous emulsion polymerization of monomers containing ethylenic unsaturation and with a "core / shell" structure comprising: a hydrophobic core based on a polymer exhibiting a Tg (vitreous transition temperature) of between -30 and 25 °, preferably between -10 and 20 ° C, and - a shell, the shell is attached to the core through covalent bonds resulting from the reaction of an agent to graft the shell over the ethylenic unsaturations remaining in the core, characterized in that the shell is prepared through the emulsion polymerization on the core of a mixture of monomers comprising: at least 90%, preferably 50 to 80% by weight of at least one monomer selected from styrene and one acr C.sub.1 -C.sub.2 alkyl alkyl methacrylate, at least 50%, preferably from 15 to 40% by weight of at least one monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation, and at least 10%, preferably at least 5% by weight of at least one grafting agent selected from (meth) acrylic (meth) acrylic esters, (meth) allyl esters of the mono- and diesters of maleic, fumaric and itaconic acids, and alkene derivatives of acrylic and methacrylic acid amides, such as N-metalylmaleimide. The invention also relates to a pseudolatex obtained by the redispersion of water of a redispersible powder according to the invention. The invention also relates to a process for the preparation of a powder according to the invention, which consists in carrying out the following steps: the core is prepared through the emulsion polymerization, then a mixture of monomers is polymerized in a aqueous emulsion on the core in the presence of a grafting agent and an organophilic polymerization initiator, whereby the shell is formed, then the water is removed from the reaction mixture. Finally, the invention relates to the use of structure pseudorredes and redispersible powders according to the invention as additives for hydraulic binders, adhesives, paper coating compositions and paints. The invention relates firstly to a water redispersible powder based on film-forming polymers, which are substantially insoluble in water and which are prepared by polymerization of the aqueous emulsion of monomers containing ethylenic unsaturation and with a structure of " core / shell "comprising: a hydrophobic core based on a polymer exhibiting a Tg (vitreous transition temperature) of between -30 and 25 ° C, preferably between -10 and 20 ° C, and a shell, the shell is attached to the nucleus through covalent bonds resulting from the reaction of an agent to graft the shell on the ethylenic unsaturations remaining in the core, characterized in that the shell is prepared by emulsion polymerization, on the core particles, of a mixture comprising: at least 90%, preferably 50 to 80% by weight of at least one monomer selected from styrene and an acrylate or methacrylate or C 12 alkyl at least 50%, preferably 15 to 40% by weight of at least one monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation, and - at least 10%, preferably at least 5% by weight of at least one grafting agent selected from (meth) acrylic (meth) acrylic esters, (meth) allyl esters of the mono- and diesters of maleic, fumaric and itaconic acids, and alkene derivatives of acrylic and methacrylic acid amides, such as N-methallyl maleimide. The powder according to the invention is composed of particles based on water insoluble film polymers, which exhibit a core / shell structure. These core / shell particles consist of a core formed by a hydrophobic and soft polymer, that is to say that it exhibits a Tg of between -30 and + 25 ° C, preferably between -1 0 and + 20 ° C, which comprises remaining in ethylenic unsaturations, and a shell based on a hard, hydrophilic polymer, which exhibits a Tg greater than 55 ° C, preferably greater than 90 ° C. The shell is attached to the nucleus through covalent bonds that result from the reaction of an agent to nip the shell over the etiological insatuities that remain in the nucleus. The vitreous transition temperature is determined by the differential thermal analysis method. Such core / shell particles, which are substantially insoluble in water, are prepared from a polymer latex through a two-stage polymerization process, such as, for example, that described in the US Pat. United States A 4 876 31 3, cited as a reference, according to which, during a first stage, the core formed by a hydrophobic and soft polymer is prepared and, during a second stage, the shell formed by a hard polymer , and hydrophilic is grafted onto the core. In general, the core is prepared by polymerizing a mixture of monomers comprising from 99.9 to 92% of at least one monomer selected from styrene, butadiene and C-C12 alkyl acrylates and methacrylates and from 0.1 to 8% of at least one monomer selected from a carboxylic acid containing ethylenic unsaturation, an unsaturated carboxylic acid ester containing a hydroxyalkyl functional group or a monomer which contains a functional amide group. The core is preferably prepared by polymerization of a mixture of styrene-butadiene-based monomers. The more or less hydrophobic nature of the core can be adjusted with the carboxylic acid monomers containing ethylenic unsaturation, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, or an unsaturated carboxylic acid monomer ester containing a functional hydroxyalkyl group, such as hydroxyethyl acrylate or hydroxypropyl acrylate or a monomer containing an amide functional group selected, for example, from methylolacrylamide, acrylamide, methacrylamide and methyl methacrylamide. A person skilled in the art knows how to choose the respective amounts of "hard" monomers (e.g., styrene and methyl methacrylate) and of "soft" monomers (e.g., butadiene and butyl acrylate) with the vision of obtaining a polymer that has the desired Tg between -30 and + 25 ° C. The monomers used during this first step can be introduced as a mixture or separately or simultaneously in the reaction mixture, either before the start of the polymerization, only once, or during the polymerization, using successive or continuously fractions. The polymerization of the monomers forming the core is carried out in the presence of an emulsifier and a polymerization initiator. The emulsifying agent used are generally the conventional anionic agents represented especially by the alkali metal salts of fatty acids, alkyl sulfates, alkylsulfonates, alkylarylsulfates, alkylaryl sulfonates, arylsulfates, arylsulfonates, sulfosuccinates and alkyl phosphates. These are used in a proportion of 0.01 to 5% by weight in relation to the total weight of the monomers. The initiator for the emulsion polymerization, which is soluble in water, is more particularly represented by hydroxy peroxides, such as aqueous hydrogen peroxide, tert-butyl hydroperoxide, and by persulfates such as sodium persulfate, potassium persulfate and persulfate of ammonium. Amounts of between 0.05 and 2% by weight are used in relation to the total of the monomers. These initiators are optionally used in combination with a reducing agent such as sodium bisulfite or formaldehyde sulfoxylate, polyethylene amines, sugars: dextrose, sucrose or metal salts. The amounts of the reducing agent that are employed will vary from 0 to 3% relative to the total weight of the monomers. The reaction temperature, which is a function of the initiator used, is generally between 0 and 100 ° C, and preferably between 50 and 80 ° C. A transfer agent can be employed in proportions ranging from 0 to 3% by weight relative to the monomers, and is generally chosen from mercaptans such as n-dodecyl mercaptan, tert-dodecyl mercaptan, cyclohexene and halogenated hydrocarbons such as chloroform, bromoform and carbon tetrachloride. At the end of the first stage of preparation of the core, the second stage of preparation of the shell begins by adding directly to the latex, the core particles comprising a mixture of monomers which is suitable for producing a hard and hydrophilic shell as defined above. . In addition, during the second stage, an effective amount of a grafting agent is added during the emulsion polymerization in order to bind the shell through covalent bonds to the ethylenic unsaturations remaining in the core in order to graft the shell firmly in the core. Up to 90% by weight of the base monomers of the shell are chosen from styrene and an alkyl acrylate or methacrylate of CpC1 2- The acrylate or alkyl methacrylate can be chosen, for example, of methyl or ethyl (meth) acrylate butyl (meth) acrylate and 2-ethylhexyl (meth) acrylate. The hydrophilic nature of the shell is adjusted by adding up to 50% by weight of a monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation. This monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation, preferably carries a saccharide group selected from mono and disaccharides. In the case of monosaccharide groups, these can be for example, glucose, galactose or fructose. In the case of disaccharide groups, these may be, for example, sucrose, lactose or maltose. Most advantageously, the saccharide group is sucrose. In the latter case, the monomer carrying the sucrose group and exhibiting at least one ethylenic unsaturation can be selected from O-methacrylosucrose, O-vinylbenzylsucrose or O-acetaltylsacarose and its alkylated and / or acetylated derivatives, wherein the alkyl groups they contain a carbon number between 1 and 4. The important point is that the shell must have a hydrophobic nature substantially higher than the core. According to one embodiment of the invention, the shell must be insoluble in an alkaline medium. A person skilled in the art will know how to choose the respective amounts of "hard" monomers and "soft" monomers with the vision of obtaining a polymer having a desired Tg of the shell greater than 55 ° C. The polymerization mixture further contains a grafting agent in a sufficient amount and which is sufficiently reactive to produce covalent bonds between the shell and the ethylenic bonds present in the core in order to graft the shell firmly onto the core. This grafting agent consists of an organic compound containing at least two ethylenic unsaturations which exhibit different reactivities during emulsion polymerization reactions, the more reactive unsaturation, reacts with the monomers with the result that the grafting agent forms an integral part of the polymer constituting the shell, the other unsaturation is insufficiently reactive to undergo the polyaddition with the ethylenic unsaturations of the other monomers, but being sufficiently reactive to react with the remaining remaining double bonds of the core to join the shell to the core through bonds covalent An effective grafting agent, which may be mentioned, is alkene and more particularly, (meth) allyl esters of (meth) acrylic acid, such as allyl methacrylate, allyl acrylate, mono (meth) allyl esters - and diesters of maleic, fumaric and itaconic acids, and alkene derivatives of acrylic and methacrylic acid amides, such as N-metalylmaleimide. The grafting agent can be added at an amount that is up to 10%, preferably at least 0.2%, most preferably between 0.5 and 5% or even 0.5 and 3% of the shell monomers. According to a preferred embodiment of the invention, the shell is prepared by emulsion polymerization of a mixture of methyl methacrylate, O-methacryloyl saccharose from a grafting agent. According to another preferred embodiment of the invention, the shell is prepared by emulsion polymerization of a styrene mixture of O-vinylbenzylsucrose and a grafting agent. Emulsion polymerization of the shell is carried out under similar conditions of the core. In general, in the case of particles of redispersible powder according to the invention, the shell constitutes 50 to 5%, preferably 30 to 10% by weight, of the total weight of the core + shell. The redispersible powder according to the invention may further include at least one inorganic filler. The latter may exhibit a variable particle size. It is recommended to use as an inorganic filler, a filler specially selected from calcium carbonate, kaolin, barium sulfate, titanium oxide, talcum, alumina hydrate, bentonite, silica, and calcium sulfoaluminate (white satin). The powder may include 0.5 to 40, preferably 2 to parts by weight of the filler per 100 parts by weight of the film-forming polymers. The particle size of the powder is generally less than 500 μm, preferably less than 100 μm. The powders according to the invention generally exhibit a pH between 4 and 9. These can be redispersed with a medium with a pH between 1 and 1 3, preferably between 4 and 10. In most cases, the Powders according to the invention are completely redispersible in water at room temperature, merely by agitation. Completely redispersible means a powder according to the invention, which after addition with the appropriate amount of water, makes it possible to obtain a pseudolatex, whose particle size of the particles, is substantially identical with the particle size of the particles of latex present in the starting emulsion. The powders according to the invention are storage stable; they can be easily redispersible in water in the form of pseudolatex and employed directly in the form of powder or in the form of pseudolatex in all known fields of application of network structures. The invention also relates to a pseudo-latex obtained by the redispersion in water of a redispersible powder as defined above. The invention also relates to the process for the preparation of the powders according to the invention, which consists of implementing the following steps: the core is prepared by the polymerization of aqueous emulsion, then a mixture of monomers is polymerized in an aqueous emulsion in the core in the presence of a grafting agent and an organophilic polymerization initiator, whereby the shell is formed, then - the water is removed from the reaction mixture. The steps described above are, therefore, implemented in order to form a latex of film-forming polymers with a core / shell structure and during the second stage of shell polymerization, as described above, is carried out the polymerization in the presence of a grafting agent and an organophilic polymerization initiator. The organophilic polymerization initiator exhibits a high affinity for the polymer particles that make up the core, in order to improve the graft. Generally, it is a peroxide or a perester such as especially eumeno hydroperoxide, dicumyl peroxide, tert-butyl perneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate, bis (3,5,5-trimethylhexanoyl) peroxide ), dioctanoyl peroxide, dilauroyl peroxide and dibenzoyl peroxide. In particular it is recommended to use the combination of tert-butyl perbenzoate / ascorbic acid. The content by weight of this combination can be between 0.5 to 5% by weight in relation to the total weight of the shell monomers. It should be noted that if a water-soluble initiator, such as ammonium persulfate, is used instead of the organophilic initiator, a powder of mediocre redispersibility is obtained. It is desirable to employ a transfer agent to adjust the masses of the shell polymer. The same transfer agents can be employed as those mentioned above for preparing the nucleus, as well as alkyl mercaptan alkanoate (C ^ C ^), such as methyl mercapto propionate. The latex obtained generally exhibits a particle size between 0.1 and 0.3 μm. Optionally, an inorganic filler, as defined above, can be added to the latex with the vision to make it easier to prepare the powder composition. The reactive contents of the various constituents are chosen in such a way that the dry pulverulent compositions have the previously defined composition. The water in this emulsion is then removed, and the product obtained pulverized to obtain a powder. The steps of removing the emulsion water from the latex and obtaining a powder can be separated and combined. Thus, it is possible to employ a method for freezing, followed by a sublimation or freeze drying step, or spray drying (spray drying). The water is preferably removed after the pH of the reaction mixture has been adjusted between 6 and 9. Drying by atomization is the preferred procedure, since it makes it possible to obtain the powder with the desired particle size without necessarily going to through the grinding stage. The drying by atomization can be carried out in a conventional manner in any known apparatus, such as, for example, a spray tower, combining a spray produced by a nozzle or a turbine with a hot gas stream. The inlet temperature of the hot gas (generally air) in the upper part of the column is preferably between 100 and 11 5 ° C and the outlet temperature is preferably between 55 and 65 ° C.

The inorganic fillers can be added to the aqueous emulsion of the starting polymer or anywhere directly in the final powdery composition. All or part of the inorganic filler is preferably introduced during the spraying step in the spray drying process (spray drying). The presence of these inorganic fillers in the emulsion promotes the preparation of the powder and its storage stability, avoiding the aggregation of the powder, ie its cake formation. The redispersible powders or structural pseudorredes according to the invention are particularly well suited in the construction industry, as additives for the mixtures of hydraulic mineral binders for the production of protective and decorative coatings and of adhesive mortars and of adhesive cements intended to form coatings and floor coverings, where they are used in a weight ratio of 1 to 10%, preferably 2 to 5%. They have been found particularly suitable for the preparation of powder products ready for use on the basis of cement and in plastes. They do not increase the residence time of these binders and do not exhibit or induce any cake formation upon storage. The powders of the invention or the pseudo-derived structural networks thereof are also usable in all other fields of application of structural networks, more particularly in the field of adhesives, paper coating and paints. The powders according to the invention can also comprise customary additives, in particular biocides, microbiostats, bacteriostats and silicones and organic foam suppressors. The powders of the invention can also be used as reactive powders with polymers that carry isocyanate, epoxide or acidic bonds for the entanglement for the paint and coating products. In all that follows and the foregoing, the parts and percentages are expressed by weight, unless otherwise indicated. Now examples will be presented, without limitation.

EXAMPLES Example 1: Overpolymerization of 4% by weight of O-methacryloylsacarose in a carboxylated styrene-butadiene copolymer seed in the presence of the grafting agent 1.4 kg of seed latex with a solids content of 49.5% was prepared by the emulsion polymerization in the presence of ammonium persulfate of a mixture of: 37% by weight of butadiene, 59% by weight of styrene, and 4% by weight of acrylic acid. Ter-butyl mercapto was used as a transfer agent in a proportion of 0.8% by weight with respect to the monomer mixture. 2. The following three solutions were prepared: Solution A: a mixture of 21 7.5 g of methyl methacrylate (MMA) and 1.9 g of allyl methacrylate (ALMA), which is a grafting agent. - Solution B: mixture of 79 g of O-methacryloylsacarose in 150 g of deionized water. Solution C: mixture of 2.9 g of ascorbic acid and 2 g of sodium dodecylbenzenesulfonate, which is an emulsifier. 3. The seed latex prepared in step 1 was charged to a polymerization reactor equipped with a stirrer, a condenser and reagent feed tubes. This was heated for 1 hour, with stirring until reaching a temperature of 80 ± 1 ° C. During this increase in temperature, they mixed 7. 4 g of tert-butyl perbenzoate (which forms the polymerization initiator with ascorbic acid) with 26.3 g of solution A and this mixture was introduced to the reactor during its increase in temperature, at that time when the latter reached 30 ± 1 C. Then, when the temperature reached 35 ± 1 ° C, 27.3 g of solution B were added to the reactor. Finally, when the temperature of the reactants reaches 80 ± 1 ° C and has stabilized, the rest of the solutions A and B are added to the reactor for a period of 3 hours, and the solution C for a period of 3 hours. minutes. At the end of the introduction of the reagents, solutions A, B and C of the reactor mixture were maintained at 80 ± 1 ° C for 1 hour. The latex is then cooled and discarded from the reactor. It was found that no grain or any scale formed on the reactor wall during polymerization. 4. The obtained latex exhibits the following characteristics: solids content: 51.1% by weight, - pH = 5.0, viscosity measured in Brookfield RTV-DV 1 1 at 50 rev / min. 1 80 m Pa s, - particle size measured by Brookhaven DCP 1000 photosedimenometer: 0.1 7 ± 0.01 μm.

Example 2: Overpolymerization of 6% by weight of O-methacryloylsacarose in a styrene-butadiene carboxylate copolymer shell in the presence of the grafting agent.

The operations performed in Example 1 were repeated, except that solution B includes a mass of 118.8 g of O-methacryloylsacarose. The obtained latex exhibits the following characteristics: - solids content: 51% by weight, - pH = 5.0, viscosity measured in Brookfield RTV-DV 11 at 50 rev / min: 320 mPa s, particle size measured by a Brookhaven DCP photosedimentometer 1000: 0.1 7 ± 0.01 μm.

Example 3: drying of the structural networks obtained in the preceding examples and redispersion of the obtained powders. The prepared structural networks of Examples 1 and 2 were converted to powder through the spray dried ones, after adjustment to their pH at 7 with a 10% sodium hydroxide solution. This drying was carried out in a spray tower, where the hot air inlet temperature is 105 ° C and the outlet temperature is 60 ° C. During the spraying operation, an anti-caking agent (kaolin) was added to the latex in a proportion at 12% by weight. The powders obtained exhibit the following characteristics: excellent flowability, high storage stability: without any cake formation appearing after storage for 2 weeks at 55 ° C, excellent water wetting, spontaneous, rapid and complete redispersion, both in deionized water as in a concentrated CaCl2 solution (1 M). The particle size distributions of the structural pseudorredes obtained by the redispersion of these powders in water, determined with a Brookhaven DCP 1 000 photosedimentometer, are identical to those of the network structures of Examples 1 and 2. The powders exhibit good properties when they are used in a proportion of 5% by weight in normal mortars. In particular, they improve their sedimentation time and their mechanical properties.

Comparative Example 4: overpolymerization of 4% by weight of O-methacryloylsacarose in a carboxylated styrene / butadiene seed without an injectant. The procedure of Example 1 was implemented, leaving the grafting agent, that is, the methacrylate of ali of solution A. The obtained latex exhibits the following characteristics: solids content: 51.1% by weight, - pH = 5.0, viscosity measured in Brookfield RTV-DV 11 at 50 rev / min: 520 mPa s, particle size measured through a Brookhaven DCP 1000 photosedimentometer: 0.16 ± 0.01 μm. After atomization by spray drying according to the procedure of Example 3, the powder obtained exhibits only partial redispersibility: the percentage of particles larger than 43 μm is 40%.

Claims (19)

REIVI NDICATIONS

1. A redispersible powder in water, based on polymers formed in water, which are substantially insoluble in water and which are prepared by polymerization of aqueous emulsion of monomers containing ethylenic unsaturation and with a "core / shell" structure, comprising : a hydrophobic core based on a polymer that exhibits a Tg (vitreous transition temperature) between -30 and 25 ° C, and a shell made of a hard, hydrophilic polymer, exhibiting a Tg greater than 55 ° C, The shell is joined to the core through covalent bonds resulting from the reaction of an agent to graft the shell in the ethylenic unsaturations remaining in the core, characterized in that the shell is prepared by emulsion polymerization on the core, of a mixture of monomers comprising: at least 90%, by weight of at least one monomer selected from styrene and an alkyl acrylate and methacrylate of (C ^, at least 50% by weight) or at least one monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation, and at least 10% by weight of at least one grafting agent selected from alkene esters, and alkene derivatives of acid amides acrylic and methacrylic.

2. The powder according to claim 1, characterized in that the core is prepared by polymerizing a mixture of monomers comprising from 99.9% to 92% by weight of at least one monomer chosen from styrene, butadiene and alkyl acrylates and methacrylates. of CpC., 2 and from 0.1 to 8% by weight of at least one monomer selected from a carboxylic acid containing an ethylenic unsaturation, an unsaturated carboxylic acid ester of a hydroxyalkyl functional group or a monomer containing an amide functional group .

3. The powder according to claims 1 or 2, characterized in that the core is prepared by polymerization of a mixture of monomers based on styrene and butadiene.

4. The powder according to one of claims 1 to 3, characterized in that the monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation carries a saccharide group selected from mono and disaccharides.

5. The powder according to claim 4, characterized in that the saccharide group is sucrose.

6. The powder according to claim 5, characterized in that the monomer carrying a saccharide group and exhibiting at least one ethylenic unsaturation is selected from O-methacryloylsacarose, O-vinylbenzylsucrose and O-acetaltylsacarose and its alkylated and / or acetylated derivatives in where the alkyl groups contain a carbon number between 1 and 4.

7. The powder according to one of claims 1 to 6, characterized in that the shell is prepared by emulsion polymerization of a mixture of methyl methacrylate, O-methacryloyl-saccharose and a grafting agent.

8. The powder according to one of claims 1 to 6, characterized in that the shell is prepared by emulsion polymerization of a mixture of styrene, O-vinylbenzyl sucrose and a grafting agent.

9. The powder according to one of claims 1 to 8, characterized in that the shell constitutes 50 to 5% by weight of the total core + shell.

10. The powder according to any of claims 1 to 9, characterized in that the grafting agent is chosen from esters of (meth) allyl of (meth) acrylic acid and esters of (meth) allyl of the mono- and diesters of maleic acid , fumaric and itaconic. eleven .

The powder according to any of claims 1 to 10, characterized in that it comprises at least one inorganic filler.

12. A pseudolatex obtained by the redispersion in water of a powder according to any of claims 1 to 11.

13. A process for preparing a powder according to one of claims 1 to 11, characterized in that the following steps are carried out: the core is prepared by aqueous emulsion polymerization, then a mixture of monomers is polymerized in an emulsion aqueous in the core in the presence of a grafting agent and an organophilic polymerization initiator, whereby the shell is formed, then the water is removed from the reaction mixture.

14. The process according to claim 1, characterized in that the organophilic polymerization initiator is chosen from a peroxide or a peryester.

15. The process according to claim 13 or 14, characterized in that the organophilic polymerization initiator is the combination of tert-butyl perbenzoate / ascorbic acid.

16. The process according to one of claims 13 to 15, characterized in that the water is imidated after the pH of the reaction mixture is adjusted between 6 and 9.

7. The process according to claim 11, characterized in that the water is removed by spray drying.

1 8. The use of networks of structures defined in claim 1 2, and of the powders defined in claims 1 to 1, as additives for hydraulic binders, adhesives, paper coating compositions and paints.

19. The use of powders according to claims 1 to 11, as reactive powders with polymers carrying isocyanate, epoxide and acidic bonds, for the interlacing of the products for painting and coating. SUMMARY OF THE INVENTION A water-redispersible powder based on water-insoluble, substantially insoluble film-forming polymers prepared by the polymerization of aqueous emulsion, of ethylenically unsaturated monomers with a core / shell structure comprising a hydrophobic polymeric core with a temperature of glass transition (Tg) of -3O-25pC, preferably of -1 O-20 ° C; and a shell attached to the core through covalent bonds resulting from the reaction of the grafting agent to graft the shell over the remaining ethylenically unsaturated bonds in the core. Such a shell is prepared by the emulsion polymerization on the particle core, on a monomer mixture comprising at least one monomer selected from styrene or an alkyl acrylate or methacrylate of C-, C 2; at least one monomer having a saccharide group and at least one ethylenically unsaturated bond; and at least one grafting agent.