WO2008087212A1

WO2008087212A1 - Emulsion stabilized with block polymers

Info

Publication number: WO2008087212A1
Application number: PCT/EP2008/050585
Authority: WO
Inventors: Florence L'alloret; Jean-Thierry Simonnet
Original assignee: L'oreal
Priority date: 2007-01-19
Filing date: 2008-01-18
Publication date: 2008-07-24

Abstract

The present patent application relates to a composition for topical application to keratin materials, in the form of an oil-in-water emulsion comprising, in a physiologically acceptable medium, an oily phase dispersed in an aqueousphase, characterized in thatit contains at least one (block A)-(block B) diblock copolymer in which: -block A comprises at least 90% by weight of units derived from styrene relative to the weight of the block A; -block B comprises at least (a)units derived from acrylic acid in acid or salified form, in a proportion of not more than 90% by weight relative to the total weight of the block B, and (b)units derived from a C1-C4alkyl acrylate, in a proportion of at least 10% by weight relative to the totalweight of the block B, -the weight proportion of block B relative to the copolymer being greater than 60%. The invention also relates to the use of the said composition in cosmetics or dermatology.

Description

Emulsion stabilized with block polymers

The present patent application relates to a composition for topical application to keratin materials, in emulsion form, containing at least one amphiphilic block polymer, and to the use of the said composition in cosmetics or dermatology.

For various reasons associated in particular with better comfort of use (softness, emollience and the like) , the current cosmetic or dermatological compositions are usually in the form of an emulsion of the oil-in-water (O/W) type (i.e. a support constituted of an aqueous dispersing continuous phase and of an oily dispersed discontinuous phase) or an emulsion of the water-in-oil (W/O) type (i.e. a support constituted of an oily dispersing continuous phase and of an aqueous dispersed discontinuous phase) . Oil-in-water emulsions are the ones more in demand in cosmetics due to the fact that they afford a softer, less greasy and lighter feel than water-in-oil emulsion systems when applied to the skin.

In the case of O/W emulsions in which an oily phase is dispersed in an aqueous phase, this dispersion requires the use of an amphiphilic compound in order to stabilize the interface between the droplets of the oily phase and the aqueous phase, and thus in order to stabilize the dispersion. Conventionally, emulsifiers are used, which are amphiphilic compounds of low molar mass, which may pose problems of harmfulness, problems of stability, the latter possibly being limited as a function, for example, of the nature of the oils and of the content of dispersed phase, problems of cosmetic cleanliness (for example soaping problem during application or tack problem) or even preparation process problems if the phases need to be heated, for example . Moreover, the use of emulsifying compounds of low molar mass does not by itself make it possible to obtain emulsions that are stable over time, and the introduction of gelling agents is usually necessary in order to avoid "creaming", i.e. decantation of the oily dispersed phase. Although the use of amphiphilic polymers or of particles is also known, it remains limited to particular textures.

There is thus a real need for O/W emulsions that are stable, irrespective of the nature and amount of oils, that have good cosmetic properties, including good harmlessness, and that can have diverse textures.

The Applicant has found, surprisingly and unexpectedly, that certain diblock copolymers of polystyrene- polyalkyl acrylate type, partially hydrolysed, make it possible to disperse oily phases of varied nature and over a very wide concentration range, the emulsions obtained being stable without it being necessary to add a co-surfactant and/or an aqueous-phase gelling agent.

Furthermore, the emulsions stabilized with the copolymers used in the present invention have very novel textures, with an aqueous, slippery and non-tacky feel after application to the skin.

Admittedly, document EP-A-I 279 398 discloses cosmetic compositions comprising an aqueous phase containing a water-soluble or water-dispersible polymer, of A-B diblock structure in which A is an ionic water-soluble polymer block and B is a hydrophobic polymer block, the amount of polymer block A being greater than or equal to 60% of the total weight of the diblock polymer. However, the said document describes as diblock copolymer, a polystyrene-sodium polyacrylate copolymer, which means that the block A does not contain any alkyl acrylate units and that the degree of hydrolysis of the block A is equal to 100%, whereas the copolymer of the present invention includes a hydrophilic block that is only partially hydrolysed since it contains alkyl acrylate units. The polymers used in the present invention are thus a selection from the polymers described generally in the said document, and they have higher emulsion-stabilizing properties, as shown by the comparison presented below, between Example 10 according to the invention and the comparative example 1.

Thus, the present invention relates to a composition for topical application to keratin materials, in the form of an oil-in-water emulsion comprising, in a physiologically acceptable medium, an oily phase dispersed in an aqueous phase, characterized in that it contains at least one (block A) - (block B) diblock copolymer in which:

- block A comprises at least 90% by weight of units derived from styrene relative to the total weight of the block A;

- block B comprises at least (a) units derived from acrylic acid in acid or salified form, in a proportion of not more than 90% by weight relative to the total weight of the block B, and (b) units derived from a Ci-C₄ alkyl acrylate, in a proportion of at least 10% by weight relative to the total weight of the block B,

- the weight proportion of block B relative to the copolymer being greater than 60%.

In the present patent application, the expression "physiologically acceptable medium" means a medium that is compatible with any keratin material such as the skin, nails, mucous membranes and the hair or any other area of bodily skin (acceptable tolerance, toxicology and feel) . The composition of the invention may especially be a cosmetic or dermatological composition.

The composition of the invention has a pH that may preferably range from 2 to 9, more preferentially from 3 to 8 and better still from 5 to 8.

The composition of the invention may also be a W/O/W emulsion, prepared by dispersing a W/O emulsion in an aqueous phase containing the copolymer according to the invention .

One of the great advantages of the copolymers used according to the invention is that they make it possible to disperse oils of any nature, either based on triglycerides or on alkanes, esters, silicones, perfluoro compounds, or the like, either alone or as a mixture .

In addition, as indicated above, these copolymers make it possible to obtain stable emulsions, even if they are free of emulsifiers. The term "stable emulsions" means emulsions which, after storage for 2 months at any temperature between 4⁰C and 50⁰C, do not show any macroscopic change in colour, odour or viscosity or any variation in pH.

According to one preferred embodiment of the invention, the composition of the invention is free of emulsifier. The term "emulsifier" means in this case an amphiphilic compound capable of emulsifying an oily phase in an aqueous phase, such an emulsifier being a compound with a molar mass of less than 500, comprising a polar part and a chain containing from 6 to 30 carbon atoms. The emulsifiers of O/W emulsions generally have an HLB (hydrophilic-lipophilic balance) of at least 7 and preferably ranging from 9 to 18.

A subject of the present invention is also the use of at least one diblock copolymer as defined above, in an oil-in-water emulsion for topical application to keratin materials, to stabilize the said emulsion. This stabilization is achieved even when the emulsion is free of emulsifier. The term "stabilize" refers to the fact that the composition obtained remains in emulsion form without coalescing or releasing oil after a storage time of at least two months at any temperature, especially at 45°C.

Definitions

In the present patent application, the term "diblock copolymer" relates to a block copolymer architecture consisting of two blocks, and being substantially free of any other sequence of blocks.

In the present patent application, the expression "unit derived from a monomer" denotes a unit that may be obtained directly from the said monomer by polymerization. Thus, for example, a unit derived from an acrylic or methacrylic acid ester does not cover a unit of formula -CH₂-CH(COOH)-, or -CH₂-C(CH₃) (COOH)-, obtained, for example, by polymerizing an acrylic or methacrylic acid ester, and then by hydrolysing. Thus, the terminology "unit derived from a monomer" relates only to the final constitution of the polymer and is independent of the polymerization process used to synthesize the polymer.

In the present patent application, the weight ratio between the blocks corresponds to the ratio between the masses of the monomers (or monomer mixtures) used for the preparation of the blocks (taking into account the variations in masses associated with a subsequent hydrolysis) . The weight proportions of the blocks are the proportions relative to the total diblock copolymer, and correspond to the weight proportions of the monomers (or monomer mixtures) used for the preparation of the blocks, relative to the total amount of monomers used to prepare the diblock copolymer (taking into account the variations in masses associated with a subsequent hydrolysis) .

In the present patent application, the masses and ratios associated with the blocks are indicated as acid equivalents (units derived from acrylic acid in acid form, as opposed to a salified form of sodium acrylate type) .

In the present patent application, the term "hydro- philic monomer" means a monomer that has affinity for water, and that is typically incapable of forming a two-phase macroscopic solution in distilled water at 25°C, at a concentration of 1% by weight.

In the present patent application, the molar mass MA of a mixture of monomers Al and A2 with respective molar masses MAl and MA2, present in respective numbers of nAl and nA2, denotes the number-average molar mass

MA = MAl nAl/ (nAl + nA2 ) + MA2 nA2/(nAl + nA2 ) . The molar mass of a mixture of units in a macromolecular chain or a part of a macromolecular chain (for example a block) is defined similarly, with the molar masses of each of the units and the number of each of the units.

In the present patent application, the measured average molecular mass of a first block or of a copolymer denotes the number-average molecular mass in polystyrene equivalents of a block or of a copolymer, measured by steric exclusion chromatography (SEC) in THF, with calibration using polystyrene standards. The measured average molecular mass of a same block in a copolymer containing n blocks is defined as being the difference between the measured average molecular mass of the copolymer and the measured average molecular mass of the copolymer containing (n-1) blocks from which it is prepared.

For the sake of simplicity, it is common to express the average molecular masses of the blocks as "theoretical" or "target" average molecular masses, assuming a complete and fully controlled polymerization. In this case, a macromolecular chain is formed via the transfer function of a transfer agent; to obtain the molecular mass, it suffices to multiply the average molar mass of the units of a block by the number of units per block

(number amount of monomer by number amount of transfer agent) . In these calculations, the differences induced by small amounts of comonomers such as methacrylic acid may be ignored. The theoretical or target average molecular masses of the block B are expressed assuming a total hydrolysis (the masses are expressed assuming a degree of hydrolysis of 1) .

The theoretical average molecular mass Mblock of a block is typically calculated according to the following formula:

M block =ΣM: precursor

in which Mi is the molar mass of a monomer i, n_± is the number of moles of the monomer i, n_precursor is the number of moles of functions to which the macromolecular chain of the block will be attached. The functions may originate from a transfer agent (or a transfer group) or an initiator, a preceding block, etc. If it is a matter of a preceding block, the number of moles may be considered as being the number of moles of a compound to which the macromolecular chain of the said preceding block has been attached, for example a transfer agent

(or a transfer group) or an initiator. In practice, the theoretical average molecular masses are calculated from the number of moles of monomers introduced and from the number of moles of precursor introduced.

The "theoretical" or "target" average molecular mass of a block copolymer is considered as being the addition of the average molecular masses of each of the blocks, assuming a total hydrolysis (the masses are expressed assuming a degree of hydrolysis of 1), if such a hydrolysis has been performed. In the present patent application, the degree of hydrolysis T is defined as the ratio between the number of units derived from acrylic acid or an acrylic acid salt, and the number of units derived from the C1-C4 alkyl acrylate, present in a copolymer before hydrolysis. The number of units derived from the C1-C4 alkyl acrylate is considered as being equal to the number amount of alkyl acrylate monomer used for the preparation of the copolymer before hydrolysis. The number of units derived from acrylic acid or from an acrylic acid salt may be determined by any known method, especially by acid-base potentiometric titration of the number of -COONa groups using a strong acid, for example using hydrochloric acid.

In the present patent application, the term "transfer agent" means an agent capable of inducing a controlled free-radical polymerization in the presence of unsaturated monomers and optionally of a source of free radicals .

Block copolymers

The diblock copolymers in accordance with the invention are advantageously linear. The block B comprises two different units. They will generally be statistically distributed in block B. Block B is thus a statistical block.

The weight proportion of block B relative to the copolymer is greater than 60%.

The average molecular mass of the diblock copolymers in accordance with the invention is greater than or equal to 20 000 g/mol. It may especially be greater than or equal to 25 000 g/mol or even 28 000 g/mol. It may be less than 50 000 g/mol or even 40 000 g/mol. It is preferably between 20 000 and 50 000 g/mol and more preferentially between 25 000 and 50 000 g/mol.

The diblock copolymers in accordance with the invention are more particularly characterized in that they are linear diblock copolymers (block A) - (block B) in which:

- block A comprises at least 90% by weight of units derived from styrene relative to the total weight of block A; - block B is a statistical block comprising, relative to the total weight of block B:

(i) from 34% to 90% by weight of units derived from acrylic acid in acid form or in salified form;

(ii) from 10% to 66% by weight of units derived from a Ci-C₄ alkyl acrylate.

Block A may comprise as little as less than 10% of units other than the units derived from styrene relative to the total weight of block A. Block B may comprise units other than the units derived from acrylic acid and the alkyl acrylate units. Such units are included in the composition of block B (proportion of the various units) , the total of the units being 100%.

The weight ratio between the units derived from acrylic acid and the units derived from the Ci-C₄ alkyl acrylate is preferably between 34/66 and 90/10 and preferably between 64/36 and 75/25.

The Ci-C₄ alkyl acrylate is preferably an alkyl acrylate that may be hydrolysed to acrylic acid. The units derived from the Ci-C₄ alkyl acrylate are preferably derived from an alkyl acrylate that may be hydrolysed to acrylic acid. Thus, via a preferred process, the units derived from acrylic acid may be generated from the units derived from the alkyl acrylate, during a partial hydrolysis. As Ci-C₄ alkyl acrylates, mention may be made especially of ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate and tert-butyl acrylate.

Ethyl acrylate and tert-butyl acrylate are especially known as being readily hydrolysable .

The Ci-C₄ alkyl acrylate of the copolymer of the invention is advantageously ethyl acrylate (often noted EA or EtA) .

Block A and/or block B may comprise up to 10% by weight (especially from 0.1% to 10% by weight) and preferably comprise up to 5% by weight (especially from 0.1% to 5% by weight, for example about 2%, or from 0.1% to 1.8%) of an additional ionic or nonionic hydrophilic comonomer relative to the total weight of block A or of block B containing the said hydrophilic comonomer.

The term "hydrophilic comonomer" means a monomer that has affinity for water and that typically is incapable of forming a two-phase macroscopic solution in distilled water at 25°C, at a concentration of 1% by weight .

Among the additional ionic or nonionic hydrophilic comonomers, examples that may be mentioned include acrylamide, hydroxyethyl (meth) acrylate and methacrylic acid (MAA) in acid or salified form. It is more particularly preferred to use methacrylic acid in acid or salified form. Block A may also comprise as additional hydrophilic monomer acrylic acid in acid or salified form.

Preferably, the copolymer does not comprise any boronic functions, in acid or salified form.

The diblock copolymers in accordance with the invention that are particularly preferred are (block A) - (block B) diblock copolymers in which the weight proportion of block B relative to the copolymer is greater than or equal to 85%.

Among these diblock copolymers, two types of copolymer are advantageously distinguished:

- Type (a) : in which the weight proportion of block B relative to the copolymer is greater than or equal to 87%, especially greater than or equal to 87% and less than 94%; - Type (b) : in which the weight proportion of block B relative to the copolymer is greater than or equal to 94%, especially ranging from 94% to 97%.

Among the diblock copolymers of type (a) , two types of copolymer are advantageously distinguished:

- Type (al) : in which, in block B:

- the weight proportion of units derived from acrylic acid in free or salified form is between 64% (obtained, for example, via hydrolysis to a degree T = 0.7) and 75% (obtained, for example, via hydrolysis to a degree T = 0.8) , and

- the weight proportion of units derived from the Ci-C₄ alkyl acrylate is between 25% (obtained, for example, via hydrolysis to a degree T = 0.8) and 36% (obtained, for example, via hydrolysis to a degree T = 0.7);

- Type (a2) : in which, in block B:

- the weight proportion of units derived from acrylic acid in free or salified form is between 75% (obtained, for example, via hydrolysis to a degree T = 0.8) and 95% (obtained, for example, via hydrolysis to a degree T = 0.96) , and

- the weight proportion of units derived from the Ci-C₄ alkyl acrylate is between 5% (obtained, for example, via hydrolysis to a degree T = 0.96) and 25% (obtained, for example, via hydrolysis to a degree T = 0.8) .

The emulsion according to the invention may contain one or more block copolymers of types (a) and/or (b) . The copolymer of the invention may especially be in one of the following forms:

- in solid or dry form, or

- in the form of a fluid concentrated ingredient comprising a vector, at a concentration preferably of greater than 8% by weight.

If it is in the form of a fluid concentrated ingredient comprising a vector, at a concentration preferably greater than 8% by weight, the vector may especially comprise water and/or an alcohol solvent, the alcohol preferably being chosen from ethanol and isopropanol. The alcohol may especially contribute towards fluidizing the copolymer and making it easier to use industrially. The vector may be water or a mixture of more than 50% by weight of water and of less than 50% by weight of alcohol. The copolymer concentration may be at least 25% by weight and preferably not more than 75% by weight.

Process

The copolymer of the invention may be prepared via any suitable process, comprising a polymerization phase. The copolymer according to the invention may be obtained via any known method, whether via controlled or uncontrolled free-radical polymerization, via ring- opening (especially anionic or cationic) polymerization, via anionic or cationic polymerization or via chemical modification of a polymer.

One advantageous process includes the following steps: 1) step I) : the following are prepared:

- a (block A) - (block B' ) diblock copolymer, or - a triblock or star copolymer of architecture (core) - [(block A) - (block B')]x or (core) -[ (block B' )- (block A) ] x in which x is an average number greater than or equal to 2, in which - block A comprises the units derived from styrene, and

- block B' comprises the units derived from a Ci-C₄ alkyl acrylate, step I') optionally, for a triblock or star copolymer, the (core) - (block B') or (core) - (block A) bonds are broken so as to obtain a (block A) - (block B' ) diblock copolymer,

2) step II) block B' is hydrolysed to a block B to obtain the diblock copolymer (block A) - (block B) , the hydrolysis inducing, where appropriate for a triblock or star copolymer, cleavage of the (core) - (block B' ) or (core) - (block A) bonds, so as to obtain a diblock copolymer (block A) - (block B) .

Optionally, the process may include a step III) during and/or after step II), of deactivation of transfer groups borne by macromolecular chains and/or of purification of the diblock copolymer (block A) - (block B) and/or of destruction of hydrolysis by-products and/or of deactivation.

The polymerization process as described above, applied to the preparation of copolymers resembling those of the invention, is especially described in document WO-A-01/16187.

The terminologies of the type (block A) - (block B' ) do not, however, exclude the presence of chemical groups that are useful (transfer groups or residues) for the polymerization, especially at the ends of chains or at the centre of the chains. Thus, the diblock copolymer may in reality have a formula of the type R- (block A) -

(block B' ) -X in which X is a transfer group (for example X is a transfer group of formula -S-CS-Z or a residue of such a group) .

Step I) is a polymerization step. Step I') is optional and may optionally be applied if the copolymer prepared in step I) is a triblock or star copolymer. However, if the copolymer prepared in step I) is a triblock or star copolymer, it is possible to cleave the bonds during a hydrolysis step. If the cleavage may be performed during the hydrolysis step, then step I') will not be of great utility and will preferably be omitted.

According to one advantageous mode, step I) is performed via emulsion polymerization in water.

The diblock copolymers (block A) - (block B) used in the context of the invention may especially be obtained as follows :

- during step I), a diblock copolymer (block A) - (block B' ) is prepared via a process including the following intermediate steps Ia) and Ib) :

Ia) a first block A is prepared by placing the following in contact:

- n_τ mol of a transfer agent comprising only one transfer group; - n_A mol of styrene or of a mixture of monomers comprising at least 90% by weight of styrene and in which n_A/n_T > 5 and preferably < 5000; and

- optionally a free-radical initiator;

Ib) a second block B' is prepared to obtain a diblock copolymer (block A) - (block B' ) , by placing the following in contact:

- block A obtained in the preceding step,

- n_B mol of a Ci-C₄ alkyl acrylate or of a mixture of monomers comprising at least 90% by weight of a Ci-C₄ alkyl acrylate such that n_B/n_T > 5 and preferably < 5000; and

- optionally a free-radical initiator,

- during step II) following step I), a hydrolysis of block B' is then performed to a degree T in moles of between 0.4 and 0.96 to obtain the said diblock copolymer (block A) - (block B) and

- 20 000 = n_A/n_T M_A + M_AA n_B/n_T

- [T M_M n_B + (1-T) M_B n_B]/[M_An_A + T M_M n_B + (1-T) M_B n_B] = 50% in which MA is the molar mass of styrene or of the mixture of monomers comprising styrene used in step Ia) and MB is the molar mass of the Ci-C₄ alkyl acrylate or of the mixture of monomers comprising the C1-C4 alkyl acrylate used in step Ib) .

The process may also optionally comprise a step III) during and/or after step II), of deactivation of transfer groups borne by the macromolecular chains and/or of purification of the diblock copolymer (block A) - (block B) and/or of destruction of hydrolysis by-products and/or of deactivation.

According to one advantageous mode, steps Ia) and Ib) of step I) are performed via emulsion polymerization in water.

The degree of hydrolysis T may advantageously be between 0.7 and 0.8; preferably, T is equal to 0.75. It may, however, be outside this range.

Transfer agents that are useful for performing the process (during step I)) are known to those skilled in the art and especially include compounds comprising a transfer group -S-CS-, for the implementation of polymerization processes known under the terms RAFT and/or MADIX. Such processes and agents are detailed later .

During step I) described above, the preparation of a first block may be performed starting with monomers or a mixture of monomers, initiators and/or agents for promoting control of the polymerization (transfer agents containing -S-CS- groups, nitroxides, etc.), and growth of a second block on the first block may then be performed to obtain a diblock copolymer with monomers different from those used for the preparation of the preceding block, and optionally with addition of initiators and/or agents for promoting control of the polymerization. These processes for preparing block copolymers are known to those skilled in the art. It is mentioned that the copolymer may have at the end of the chain a transfer group or a residue of a transfer group, for example a group comprising an -S-CS- group (for example derived from a xanthate, a dithioester, a dithiocarbamate or a trithiocarbonate) or a residue of such a group.

During step II), the units derived from the hydrolysable monomers of the block B' are partially hydrolysed, to form a block B comprising units derived from acrylic acid or from a salt (hydrolysed units) , and units derived from the alkyl acrylate monomer (non- hydrolysed units) . These two types of units are statistically distributed in the block B; it may thus be considered that block B is a block in the form of a statistical copolymer comprising units derived from the alkyl acrylate and units derived from acrylic acid or from an acrylic acid salt. Naturally, block B may comprise other units, in small amounts, if a mixture of monomers is used during the implementation of step Ib) .

Block A comprises units derived from styrene. Block A may be obtained from a mixture of monomers comprising at least 90% by weight and preferably at least 95% by weight of styrene ("St") and from one or more hydrophilic comonomer(s) . Block A may thus be a statistical copolymer comprising at least 90%

(especially from 90% to 99.9% by weight) and preferably at least 95% by weight (especially from 95% to 99.9% by weight) of units derived from styrene, and up to 10% by weight (especially from 0.1% to 10% by weight) and preferably up to 5% by weight (especially from 0.1% to 5% by weight) of other units derived from hydrophilic comonomer (s) .

Block B' comprises units derived from a hydrolysable Ci-C₄ alkyl acrylate. Block B' may be obtained from a mixture of monomers comprising at least 90% (especially from 90% to 99.9%) and preferably at least 95%

(especially from 95% to 99.9%) by weight of a Ci-C₄ alkyl acrylate and of one or more hydrophilic comonomer(s) . Block B' may thus be a statistical copolymer comprising at least 90% (especially from 90% to 99.9%) and preferably at least 95% (especially from

95% to 99.9%) by weight of units derived from the Ci-C₄ alkyl acrylate and up to 10% (especially from 0.1% to

10%) and preferably up to 5% (especially from 0.1% to 5%), for example about 2% or from 0.1% to 1.8% by weight of other units derived from hydrophilic comonomer (s) .

Block B obtained from block B' after hydrolysis comprises units derived from the hydrolysable Ci-C₄ alkyl acrylate, units derived from acrylic acid in acid or salified form, and optionally units derived from a hydrophilic comonomer used during the step Ib) of growth of the block B' , for example units derived from methacrylic acid. The acrylic acid is generally present in block B in the form of a salt. This form generally results from the hydrolysis conditions used and from the reagents used. It is generally a salt of an alkali metal such as sodium or potassium. Consequently, block B generally comprises units derived from acrylic acid in the form of sodium or potassium acrylate.

Among the hydrolysable Ci-C₄ alkyl acrylates, mention is made in particular of ethyl acrylate (EA or EtA) .

Among the hydrophilic comonomers that may be useful for the preparation of block A and/or of block B' , mention is made of hydrophilic comonomers capable of stabilizing an emulsion of monomers and/or of stabilizing the polymer obtained via emulsion polymerization. Mention may be made especially of ionic or nonionic hydrophilic comonomers, for instance acrylamide, hydroxyethyl (meth) acrylate and methacrylic acid (MAA), and salts thereof. Methacrylic acid or salts thereof is (are) preferably used. Methacrylic acid is not sensitive to hydrolysis. However, it may be salified during the hydrolysis. For the preparation of block A, acrylic acid in acid or salified form may also be used as hydrophilic comonomer.

According to one particular embodiment, block A and/or block B' or B comprises from 0.1% to 10% and preferably from 0.1% to 5% by weight of hydrophilic comonomer, in particular methacrylic acid in acid or salified form, relative to the total weight of block A, of block B' or of block B containing the said hydrophilic comonomer.

Thus, during step Ia) , a mixture of monomers comprising at least 90% and preferably at least 95% by weight of styrene, and up to 10% and preferably up to 5% by weight of methacrylic acid in acid or salified form may be used.

During step Ib) , a mixture of monomers comprising at least 90% and preferably at least 95% by weight of Ci-C₄ alkyl acrylate such as ethyl acrylate, and up to 10% and preferably up to 5% by weight of methacrylic acid in acid or salified form may be used.

A few characteristics of the process for preparing the copolymers of the invention are detailed below.

Step I)

Preferably, for the polymerization step I), living or controlled free-radical polymerization methods are performed, and particularly preferably controlled or living free-radical polymerization methods using a transfer agent comprising a transfer group of formula -S-CS-, especially known under the names RAFT and MADIX.

As examples of living or controlled polymerization processes, reference may be made especially:

- to the processes of patent applications WO 98/58974, WO 00/75207 and WO 01/42312, which use a free-radical polymerization controlled with control agents of xanthate type,

- to the free-radical polymerization process controlled with control agents of dithioester or trithiocarbonate type of patent application WO 98/01478,

- to the free-radical polymerization process controlled with control agents of dithiocarbamate type of patent application WO 99/31144,

- the free-radical polymerization process controlled with control agents of dithiocarbazate type of patent application WO 02/26836, - to the free-radical polymerization process controlled with control agents of dithiophosphoroester type of patent application WO 02/10223,

- to the process of patent application WO 99/03894, which uses a polymerization in the presence of nitroxide precursors, or the processes using other nitroxides or nitroxide/alkoxyamine complexes,

- to the process of patent application WO 96/30421, which uses an atom-transfer radical polymerization

(ATRP) , - to the free-radical polymerization process controlled with control agents of iniferter type according to the teaching of Otu et al., Makromol. Chem. Rapid. Commun., 3, 127 (1982),

- to the free-radical polymerization process controlled by degenerative iodine transfer according to the teaching of Tatemoto et al . , Jap. 50, 127, 991 (1975), Daikin Kogyo Co. Ltd. Japan and Matyjaszewski et al . , Macromolecules, 28, 2093 (1995),

- to the free-radical polymerization process controlled with tetraphenylethane derivatives, disclosed by

D. Braun et al . , in Macromol. Symp. Ill, 63 (1996), or alternatively

- to the free-radical polymerization process controlled with organocobalt complexes, described by Wayland et al . , in J . Am . Chem . S oc . 1 1 6 , 7 973 ( 1 994 ) ,

- to the free-radical polymerization process controlled with diphenylethylene (see WO 00/39169 or WO 00/37507) .

The polymerizations may be performed in emulsion in water ("latex" process) . These processes may use emulsifiers, usually surfactants. Without wishing to be bound by any theory, it is thought that the emulsion preparation processes lead to the formation of nodules of blocks A, which may have an influence on the physicochemical properties of the copolymer.

The polymerizations may be formed in the presence of free-radical initiators, which are known to those skilled in the art. Sodium persulfate may be used, for example. Amounts of initiators of from 5% to 50% by number relative to the amount of transfer agent may typically be used.

It would not constitute a departure from the context of the invention to use and to adapt preparation processes leading to triblock or star copolymers, modified thereafter (during a step I') or during step II) so as to obtain diblock copolymers. It may especially be envisaged to use transfer agents comprising several transfer groups (for example trithiocarbonates Z-S-CS-S-Z) leading to telechelic copolymers of the type R- [ (block B' ) - (block A) ] w as triblocks or stars of the type (core) -[ (block A) - (block B')]x (for example (block A) - (block B' ) -R- (block B' )- (block A) as triblocks (block A) - (block B' )- (core) - (block B' ) - (block A) ) , and then to break (section or "cleave") the telechelic copolymers, to obtain diblock copolymers (block A) - (block B' ) . The sectioning may take place during the hydrolysis, in which case diblock copolymers

(block A) - (block B) are obtained directly. In such cases, a person skilled in the art will adapt the working conditions to target average molecular masses equivalent to those indicated, for example by multiplying the amounts of monomers introduced by the number of transfer groups included in the transfer agent. It is pointed out that during step I), a triblock copolymer is not typically prepared by a succession of 3 polymerization phases in which at least one of the blocks might not be separated from the others by cleavage during a hydrolysis. Thus, the copolymer prepared during step I) is typically not obtained via a polymerization process comprising a step of polymerization of styrene or of a mixture of monomers based on styrene, followed by a step of polymerization of ethyl acrylate or of a mixture of monomers based on ethyl acrylate, and then a step of polymerization of styrene or of a mixture of monomers based on styrene, the polymerizations being performed using a monofunctional transfer agent bearing a group of formula -S-CS-.

Step II)

During step II), the respective amounts of the various units in block B are controlled by means of the degree of hydrolysis. The composition of block A may remain unchanged during the hydrolysis, if block A does not comprise any hydrolysable units. However, it is not excluded for block A to be slightly modified during the hydrolysis step.

Preferentially, the hydrolysis step II) is performed by adding a strong base such as sodium hydroxide or potassium hydroxide. Typically, a numerical proportion of base relative to the amount of hydrolysable monomer used during step Ib) , corresponding approximately to the target degree of hydrolysis, is added, optionally with an excess of a few percents. For example, an amount of sodium hydroxide of 75% by number of the amount of hydrolysable ethyl acrylate used during step Ib) is introduced in order to obtain 75% hydrolysis. The process is preferably performed via homogeneous hydrolysis by gradually adding the sodium hydroxide to the copolymer.

The hydrolysis step may especially lead to the deactivation and/or the sectioning of certain transfer groups or of other groups attached to the macro- molecular chains. Step II) may thus generate by-products that it is desirable to remove, or generate groups on the macromolecular chains that it is desirable to chemically modify. Such operations may be performed during a step III) .

Step III)

Step III) is a step of deactivation of transfer groups borne by macromolecular chains, and/or of purification of the diblock copolymer (block A) - (block B) and/or of destruction of hydrolysis by-products and/or of deactivation .

During the optional step III), the block copolymers obtained or the hydrolysis by-products may undergo a purification reaction or a reaction for the destruction of certain species, for example via processes of the type such as hydrolysis, oxidation, reduction, pyrolysis, ozonolysis or substitution. An oxidation step with aqueous hydrogen peroxide solution is particularly suitable for treating sulfur-containing species. It is mentioned that some of these reactions or operations may take place totally or partially during step II) . In this case, for these reactions or operations, the two steps are simultaneous.

As indicated above, the average molecular masses of the diblock copolymer (block A) - (block B' ) before hydrolysis, typically depend on the relative amounts of the monomers and on the transfer agent used during step I) . Needless to say, the average molecular masses of the diblock copolymer (block A) - (block B) after hydrolysis, depends on these same relative amounts and also on the degree of hydrolysis, for example as a function of the amount of reagent introduced, generally a base, for this hydrolysis.

For the process whose steps Ia) and Ib) have been detailed above, the theoretical total mass of block A may be expressed by:

M_An_A.

For the process whose steps Ia) and Ib) have been detailed above, the theoretical or target average molecular mass of block A may be expressed by:

M_An_A/n_T.

For the process whose steps Ia) and Ib) have been detailed above, the theoretical total mass of block B' may be expressed by:

M_Bn_B .

For the process whose steps Ia) and Ib) have been detailed above, the theoretical or target average molecular mass of block B' may be expressed by:

M_Bn_B/n_T.

For the process whose steps Ia) and Ib) have been detailed above, the theoretical total mass of block B may be expressed by:

T M_M n_B + (1-T) M_B n_B

For the process whose steps Ia) and Ib) have been detailed above, the theoretical or target average molecular mass of block B may be expressed by:

M_AA n_B/n_T. (since T = I according to the definition of the theoretical or target average molecular mass) .

For the process whose steps Ia) and Ib) have been detailed above, the total theoretical mass of the copolymer may be expres sed by :

M_An_A + T M_AA n_B + ( 1 -T ) M_B n_B

For the process whose steps Ia) and Ib) have been detailed above, the theoretical average molecular mass of the copolymer may be expressed by: n_A/n_T M_A + M_M n_B/n_T

In the above expressions: - M_A is the molar mass of styrene or of the mixture of monomers comprising styrene used in step Ia) ,

- NU_A is the molar mass of acrylic acid,

- M_B is the molar mass of the Ci-C₄ alkyl acrylate or of the mixture of monomers comprising the Ci-C₄ alkyl acrylate used in step Ib) .

As guides, the following correspondences are given:

- n_A/n_T = 5 corresponds to a theoretical average molecular mass of block A of about 500 g/mol - n_A/n_T = 5000 corresponds to a theoretical average molecular mass of block A of about 500 000 g/mol,

- n_B/n_T = 5 corresponds to a theoretical average molecular mass of block B' of about 500 g/mol

- n_B/n_T = 5000 corresponds to a theoretical average molecular mass of block B' of about 500 000 g/mol,

- n_A/n_T M_A + M_M n_B/n_T = 13 000 g/mol (resp. 2000, resp. 8000, resp. 20 000, resp. 50 000) corresponds to a theoretical average molecular mass of the diblock (block A) - (block B) of about 13 000 g/mol (resp. 2000, resp. 8000, resp. 20 000, resp. 50 000), assuming a total hydrolysis and for the case where the alkyl acrylate is ethyl acrylate.

The weight ratios between the blocks are defined as the ratios between the theoretical or target total masses (the assumption of a degree of hydrolysis of 1 is not used for this descriptor). Thus:

- M_An_A < T M_AA n_B + (1-T) M_B n_B, indicates that the weight ratio (block B) / (block A) = 1. This is a characteristic of the copolymer used according to the invention,

- M_An_A/ [M_An_A + T MAA n_B + (1-T) M_B n_B] indicates the weight amount of block A in the diblock copolymer (block A) - (block B), i.e. the proportion of block A,

- [T M_ΛA n_B + (1-T) M_B n_B] / [M_An_A + T M_M n_B + (1-T) M_B n_B] indicates the weight amount of block B in the diblock copolymer (block A) - (block B), i.e. the proportion of block B.

Preferentially, the diblock linear copolymers (block A) - (block B) used according to the invention have a weight proportion of block B relative to the copolymer: [T M_M n_B + (1-T) M_B n_B]/[M_An_A + T M_M n_B + (1-T) M_B n_B] greater than or equal to 85%, and they generally have a theoretical average molecular mass:

(n_A/n_T M_A + M_M n_B/n_T) of greater than or equal to 13 000 g/mol.

Among these copolymers, mention may be made in particular of:

- the particular copolymers of the type (a) , in which the weight proportion of block B relative to the copolymer is greater than or equal to 87%, especially greater than or equal to 87% and less than 94%; they generally have a theoretical average molecular mass of between 13 000 and 20 000 g/mol. This is the case for the polymer denoted under the name Synthetic Example 2 ;

- the particular copolymers of the type (b) in which the weight proportion of block B relative to the copolymer is greater than or equal to 94%, especially ranging from 94% to 97%; they generally have a theoretical average molecular mass of greater than or equal to 20 000 g/mol and preferably greater than or equal to 25 000 g/mol, more preferentially between 20 000 and 50 000 g/mol, even more preferentially between 25 000 and 50 000 g/mol and more particularly between 28 000 and 40 000 g/mol. This is the case for the polymer denoted under the name Synthetic Example 1. The diblock copolymer (s) in accordance with the invention is (are) preferably present in a concentration (of active material) ranging from 0.1% to 10% of the total weight of the composition. More preferentially, this amount ranges from about 0.2% to 5% by weight and better still from 0.3% to 3% by weight relative to the total weight of the composition.

Oily phase

The amount of oily phase in the emulsion according to the invention may range, for example, from 2% to 70% by weight, preferably from 2% to 50% by weight and better still from 2% to 40% by weight relative to the total weight of the composition. The weight ratio of the amount of oily phase to the amount of block copolymer may range, for example, from 2 to 80 and preferably from 5 to 50. This ratio shows that a small amount of block copolymer suffices to obtain stable dispersions.

The oily phase of the composition of the invention contains at least one oil. The term "oil" means a fatty substance that is liquid at room temperature (25°C).

As oils that may be used in the composition of the invention, examples that may be mentioned include: - hydrocarbon-based oils of animal origin, such as perhydrosqualene (or squalane) ; - hydrocarbon-based oils of plant origin, such as liquid triglycerides of fatty acids containing from 4 to 10 carbon atoms, for instance heptanoic or octanoic acid triglycerides or alternatively, for example, sunflower oil, corn oil, soybean oil, marrow oil, grape seed oil, sesame seed oil, hazelnut oil, apricot oil, macadamia oil, arara oil, coriander oil, castor oil, avocado oil, caprylic/capric acid triglycerides such as those sold by the company Stearineries Dubois or those sold under the names Miglyol 810, 812 and 818 by the company Dynamit Nobel, jojoba oil, shea butter oil and the liquid fractions of shea butter;

- synthetic esters and ethers, especially of fatty acids or of fatty alcohols, for instance the oils of formulae R¹COOR² and R¹OR² in which R¹ represents a fatty acid residue containing from 8 to 29 carbon atoms and R² represents a branched or unbranched hydrocarbon- based chain containing from 3 to 30 carbon atoms, for instance purcellin oil, isononyl isononanoate, isopropyl myristate, 2-ethylhexyl palmitate (or octyl palmitate) , 2-octyldodecyl stearate, 2-octyldodecyl erucate or isostearyl isostearate; hydroxylated esters, for instance isostearyl lactate, octyl hydroxystearate, octyldodecyl hydroxystearate, diisostearyl malate, triisocetyl citrate, and fatty alkyl heptanoates, octanoates and decanoates; polyol esters, for instance propylene glycol dioctanoate, neopentyl glycol diheptanoate and diethylene glycol diisononanoate; pentaerythritol esters, for instance pentaerythrityl tetraisostearate; lipophilic derivatives of amino acids, such as isopropyl lauroyl sarcosinate (INCI name Isopropyl Lauroyl sarcosinate) sold under the name Eldew SL 205 by the company Ajinomoto;

- linear or branched hydrocarbons, of mineral or synthetic origin, such as mineral oils (mixture of hydrocarbon-based oils derived from petroleum; INCI name: mineral oil), volatile or non-volatile liquid paraffins, and derivatives thereof, petroleum jelly, polydecenes, isohexadecane, isododecane, hydrogenated isoparaffin, such as hydrogenated polyisobutene, for instance Parleam® oil sold by the company NOF Corporation (INCI name: hydrogenated polyisobutene);

- fatty alcohols containing from 8 to 26 carbon atoms, for instance cetyl alcohol, stearyl alcohol and mixtures thereof (cetearyl alcohol) , octyldodecanol, 2-butyloctanol, 2-hexyldecanol, 2-undecylpentadecanol or oleyl alcohol;

- partially hydrocarbon-based and/or silicone-based fluoro oils, such as those described in document JP-A- 2 2 95 912 ;

- silicone oils, such as volatile or non-volatile polydimethylsiloxanes (PDMS) with a linear or cyclic silicone chain, which are liquid or pasty at room temperature, especially volatile silicone oils which are either linear or cyclic, for instance cyclopoly- dimethylsiloxanes (cyclomethicones) such as cyclopenta- siloxane and cyclohexadimethylsiloxane; polydimethylsiloxanes comprising alkyl, alkoxy or phenyl groups, which are pendent or at the end of a silicone chain, these groups containing from 2 to 24 carbon atoms; phenyl silicones, for instance phenyl trimethicones, phenyl dimethicones, phenyltrimethylsiloxydiphenyl- siloxanes, diphenyl dimethicones, diphenylmethyl- diphenyltrisiloxanes, 2-phenylethyltrimethyl siloxy silicates and polymethylphenylsiloxanes;

- mixtures thereof.

According to one particular embodiment of the invention, the oily phase contains at least one oil chosen from fatty acid esters and silicone oils.

The other fatty substances that may be present in the oily phase are, for example, fatty acids containing from 8 to 30 carbon atoms, for instance stearic acid, lauric acid or palmitic acid; gums such as silicone gums (dimethiconol) ; silicone resins such as trifluoro- methyl-Cl-4-alkyldimethicone and trifluoropropyl dimethicone, and silicone elastomers, for instance the products sold under the name KSG by the company Shin- Etsu, under the name Trefil by the company Dow Corning or under the name Gransil by the company Grant Industries; pastes such as Petrolatum; waxes such as microcrystalline waxes, paraffin waxes, lignite waxes, ceresin, ozokerite, montan wax, beeswax, lanolin and derivatives thereof, candelilla wax, ouricury wax, carnauba wax, Japan wax, cocoa butter, palm oil in paste form at 20⁰C, cork fibre wax or sugar cane wax, hydrogenated oils that are solid at 25°C, fatty esters and glycerides that are solid at 25°C, polyethylene waxes, the waxes obtained by Fischer-Tropsch synthesis, silicone waxes; and mixtures of these fatty substances.

Aqueous phase

The emulsion comprises an aqueous phase, the amount of which may range, for example, from 30% to 98% by weight, preferably from 40% to 98% by weight, better still from 50% to 98% by weight and even better still from 55% to 98% by weight relative to the total weight of the emulsion.

The aqueous phase of the composition of the invention contains at least water. According to one preferred embodiment of the invention, the amount of water is at least 20% and preferably at least 30% of the total weight of the composition.

Conventionally, the aqueous phase may contain, besides water, one or more water-soluble solvents chosen from polyols (or polyhydric alcohols) and water-soluble lower alcohol (s) , and mixtures thereof. The term "lower alcohol" means an alcohol containing from 1 to 8 and preferably from 1 to 6 carbon atoms. Examples of lower alcohols that may be mentioned include ethanol, isopropanol and butanol, and mixtures thereof.

Examples of polyols that may be mentioned include glycerol; glycols such as propylene glycol or butylene glycol; sorbitol; sugars such as glucose, fructose, maltose, lactose and sucrose; and mixtures thereof.

The amount of water-soluble solvents (lower alcohols and polyols) may range, for example, from 0.5% to 30% by weight, preferably from 0.5% to 20% by weight and better still from 1% to 15% by weight relative to the total weight of the composition. According to one advantageous embodiment, the composition according to the invention comprises at least one wetting agent.

The term "wetting agent" means herein any compound which, when introduced into an aqueous solution at 0.05% (by weight), makes it possible to reduce the surface tension of water to a value of less than 35 mN/m and preferably less than 30 mN/m.

Such an agent combined with the diblock copolymer of the composition according to the invention makes it possible to obtain uniform spreading of the composition on the skin. The wetting agents in accordance with the invention are chosen from water-soluble silicones comprising at least one terminal or pendent monovalent polyoxyalkylene group, and which, when introduced at 0.05% by weight into an aqueous solution, are able to reduce the surface tension of water to a value of less than 35 mN/m and preferably less than 30 mN/m.

Accordingly, the present invention also relates to a composition for topical application to keratin materials, in the form of an oil-in-water emulsion comprising, in a physiologically acceptable medium, an oily phase dispersed in an aqueous phase, characterized in that it contains: (1) at least one diblock copolymer (block A) - (block B) in which:

- block A comprises at least 90% by weight of units derived from styrene relative to the total weight of block A;

- block B comprises at least (a) units derived from acrylic acid in acid or salified form, in a proportion of not more than 90% by weight relative to the total weight of block B, and (b) units derived from a C1-C4 alkyl acrylate, in a proportion of at least 10% by weight relative to the total weight of block B, the weight proportion of block B relative to the copolymer being greater than 60%;

(2) at least one wetting agent chosen from water- soluble silicones comprising at least one terminal or pendent monovalent polyoxyalkylene group, and which, when introduced at 0.05% by weight into an aqueous solution, are capable of reducing the surface tension of water to a value of less than 35 mN/m and preferably less than 30 mN/m.

The wetting agents in accordance with the invention are more preferentially chosen from water-soluble silicones comprising at least one polyoxyalkylene group of general formula (a) below: R² ₃SiO(R² ₂SiO)p(R²PESiO)_qSiR²3 (a) in which

- the radicals R², which may be identical or different, denote a monovalent hydrocarbon-based radical chosen from alkyl, aryl and aralkyl radicals containing not more than 10 carbon atoms; some of the radicals R² may also additionally contain an ethylcyclohexylene monoxide group of formula

and are in low proportion in the polysiloxane chain; - p ranges from 0 to 150, preferably from 0 to 100 and more preferentially from 0 to 30;

- q ranges from 1 to 12, preferably from 1 to 10 and more preferentially from 1 to 8,

- the polyether group PE has the formula (b) below -C_xH_2x (OC₂H₄) _y (OC₃H₆) _ZOR³ (b) in which: x ranges from 1 to 8 and preferably from 2 to 4 and is more preferentially equal to 3; y is greater than 0; z is greater than or equal to 0; the values of y and z are such that the total molecular weight of the polyoxyalkylene portion of the polyether group PE ranges from 200 to 10 000 and more preferentially from 350 to 4000;

R³ denotes hydrogen, a Ci-Cs alkyl group or a C2-C8 acyl group .

It should be noted that when z is other than 0, the polyoxyethylene and polyoxypropylene units may be randomly distributed along the polyether chain PE or distributed in blocks, or alternatively distributed both in blocks and randomly.

Preferably, the radicals R² are chosen from Ci-Cε lower alkyls, for instance methyl, ethyl, butyl, hexyl; phenyl and benzyl. More particularly, the radicals R² are chosen from C1-C4 lower alkyls and even more particularly denote the methyl group.

Preferably, the radicals R³ are chosen from Ci-C₄ lower alkyls and even more particularly denote the methyl group.

The number of oxyethylene units in the group PE should be sufficient to produce a cloudpoint in water of between 25 and 90⁰C and more preferentially from 40 to 70⁰C.

The water-soluble silicones of formula (a) may be obtained according to the process described in document US-A-4 847 398.

Among the water-soluble silicones of formula (a) that are preferably used are those of formula (a') below:

MeSiO (MeSiO) p (MePESiO) _qSiMe₃ (a' ) in which Me denotes the methyl group; PE denotes: - (CH₂) 3 (OC₂H₄) _y (OC₃Hg)₂OR³ (b' ) in which x, z and z have the same values indicated above and R³ denotes hydrogen or a Ci-C₄ alkyl group, and more particularly methyl. As another family of water-soluble silicones that may be used according to the invention, mention may be made of the branched silicones of formula (c) below:

Me₃SiO (Me₂SiO) _q_₃ [ (SiOMe₂) _P/qOPE] _q (c) in which p and q have the same values indicated above in formula (a) ; Me means the methyl group; PE denotes the group of formula (d) below:

- (OC₂H₄) _y (OC₃Hg)₂R³ (d) in which y and z have the same values indicated above in formula (b) and R³ denotes a Ci-C₄ alkyl group and more particularly the methyl group.

The water-soluble silicones in accordance with the invention are known and described especially in patent US-A-5 338 352 and their mode of preparation is described especially in patent US-A-4 847 398.

Such silicones are sold, for example, by the company OSI under the trade names Silwet L-720®, Silwet L-7002®, Silwet L-7600®, Silwet L-7604®, Silwet L-7605®, Silwet L-7607®, Silwet 1614, Silwet L-7657®, Silwet L-7200®, Silwet L-7230, Silsoft 305, Silsoft 820 and Silsoft 880, or by the company Goldschmidt under the trade names Tegowet 260, Tegowet 500, Tegowet 505 and Tegowet 510®.

The table below collates the surface tension values at 25°C of aqueous solutions comprising 0.05% (by weight) of various wetting agents.

According to the invention, the wetting agent (s) is (are) present in concentrations ranging, for example, from 0.01% to 10% by weight, preferably from 0.05% to 5% by weight and more particularly from 0.1% to 3% by weight relative to the total weight of the composition.

Adjuvants

In a known manner, the composition of the invention may also contain adjuvants that are common in cosmetics and/or dermatology, such as active agents, preserving agents, antioxidants, complexing agents, pH regulators

(acidic or basic) , fragrances, fillers, bactericides, odour absorbers, dyestuffs (pigments and dyes) , hydro- philic polymers, and also lipid vesicles. The amounts of these various adjuvants are those conventionally used in the field under consideration, for example from 0.01% to 20% of the total weight of the composition. Depending on their nature, these adjuvants may be introduced into the fatty phase, into the aqueous phase and/or into the lipid vesicles.

Needless to say, a person skilled in the art will take care to select the optional compound (s) to be added to the composition according to the invention such that the advantageous properties intrinsically associated with the composition in accordance with the invention are not, or are not substantially, adversely affected by the envisaged addition.

Hydrophilic polymers that may be mentioned include modified or unmodified carboxyvinyl polymers, such as the products sold under the names Carbopol (INCI name: carbomer) and Pemulen (INCI name: acrylates/ClO-30 alkyl acrylate crosspolymer) by the company Noveon; polymers derived from 2-acrylamido-2-methylpropane- sulfonic acid such as poly (2-acrylamido-2-methyl- propanesulfonic acid) sold by the company Hoechst under the name Hostacerin AMPS (INCI name: ammonium polyacryldimethyltauramide) , crosslinked anionic copolymers of acrylamide and of AMPS, which are in the form of a W/0 emulsion, such as those sold under the name Sepigel 305 (CTFA name: polyacrylamide/C13-14 isoparaffin/Laureth-7) and under the name Simulgel 600 (CTFA name: acrylamide/sodium acryloyldimethyltaurate copolymer/isohexadecane/Polysorbate 80) by the company SEPPIC, AMPS copolymers sold under the name Aristoflex by the company Clariant; synthetic neutral polymers such as poly-N-vinylpyrrolidone; polysaccharides such as guar gum, xanthan gum and cellulose derivatives; water-soluble or water-dispersible silicone derivatives such as acrylic silicones, polyether silicones and cationic silicones.

The compositions of the invention may contain all the active agents usually used in cosmetics in the field of care, makeup and bodily hygiene.

Active agents that may especially be used include vitamins (A or retinol, C or ascorbic acid, E or tocopherol, K, PP or B3 or niacinamide), alone or as mixtures, and also derivatives thereof and especially esters thereof; keratolytic agents and/or desquamating agents such as β-hydroxy acids, for instance salicylic acid and derivatives thereof, α-hydroxy acids, for instance glycolic acid, lactic acid and citric acid, ascorbic acid and derivatives thereof; sunscreens; anti-inflammatories; calmatives; depigmenting agents; plant proteins; polysaccharides of plant origin optionally in the form of microgels; starches; wax dispersions; mixed silicates and colloidal particles of mineral fillers; matting agents; hair-loss counter- actants and/or hair restorers, or alternatively anti- wrinkle agents, and mixtures thereof.

The compositions according to the invention may be free of lipophilic organic UV-screening agent and/or of mineral UV-screening agent. The mean size of the oil droplets of the emulsions obtained according to the invention may range from

100 nm to 10 μm (0.1 to 10 μm) . The viscosity of the emulsions obtained may range from very fluid (spray = sprayable product) to very viscous (cream) , and this viscosity is adjusted as a function of the content of introduced block copolymer, of the amount of emulsified oily phase and of the emulsification process used

(micron-sized and submicron-sized emulsions) .

The compositions of the invention may be prepared via any known emulsification process. The emulsification processes that may be used may be performed, for example, with paddles, with impellers, with a rotor- stator or under high pressure (HPH = high-pressure homogenization) .

To obtain stable emulsions with a low content of block copolymer used according to the invention, i.e. with a ratio of amount of oily phase/amount of block copolymer of greater than 25, it is possible to make the dispersion in concentrated phase and then to dilute the dispersion with the rest of the aqueous phase.

It is also possible, via HPH (between 50 and 800 bar) , to obtain stable dispersions with droplet sizes that may be as low as 100 nm.

To make the emulsion and to obtain a fine, sparingly polydispersed and stable dispersion, it is necessary to neutralize the block copolymer to a pH ranging from 6.5 to 7.5. However, it is possible after emulsification to modify the pH in order to have a pH ranging from 5 to 8, without the dispersion being affected.

It is also known that polyacrylic acid (PAA) is sensitive to ionic species such as salts and certain sunscreens. It will then be necessary to add them after the emulsification step. Although the compositions according to the invention are preferably free of emulsifiers, it is possible, in order to facilitate the emulsification of the oily phase when the block copolymer is in low amount, for example in an amount of less than 2% by weight, to add at least one co-emulsifier on condition that the content of co-emulsifier (s) is less than 50% by weight relative to the weight of diblock copolymer used according to the invention. Without this being limiting, the HLB of the emulsifiers is generally greater than 5 and, as a guide, the emulsifier (s) may belong to the following families: alkylpolyglucosides

(APG), oxyethylenated C8-C18 fatty acid esters, oxyethylenated C8-C18 fatty alkyl ethers, esters of C8- C18 fatty acids and of glycerol, ethers of C8-C18 fatty alcohols and of glycerol, optionally oxyethylenated esters of C8-C18 fatty acids and of sorbitan, optionally oxyethylenated ethers of C8-C18 fatty alcohols and of sorbitan, dimethicone copolyols, gemini surfactants and monosodium or disodium acylglutamates . When a co-emulsifier is added, this co-emulsifier is preferably chosen from esters of a branched-chain fatty acid and of polyol, especially of glycerol and of sorbitan, such as glyceryl isostearate.

The compositions of the invention may be used in any cosmetic or dermatological application, for example in cosmetics for caring for the skin, the hair, the scalp, the eyelashes, the eyebrows, the nails or mucous membranes (the lips) , for example as products for protecting, treating or caring for the face, the hands or the body, as skin cleansing products (for the face or the body) , as makeup products (for example foundations) and as haircare products.

Another subject of the invention is a cosmetic process for treating a keratin material such as the skin, the scalp, the hair, the eyelashes, the eyebrows, the nails or mucous membranes, characterized in that a composition as defined above is applied to the keratin material, according to the usual technique for the use of this composition.

The examples that follow illustrate the invention without being limiting in nature.

I . Synthetic example

Preparation of a polystyrene-block-poly (ethyl acrylate- stat-sodium acrylate) diblock copolymer via synthesis of a polystyrene-block-poly (ethyl acrylate) diblock copolymer with target Mn values of 2000-block-42 000 (g/mol) followed by 75% hydrolysis of the ethyl acrylate groups

Step Ia) : Preparation of a polystyrene first block with a theoretical molecular mass of about 2000 g/mol

3000 g of water, 17.6 g of sodium dodecyl sulfate and 0.290 g of sodium carbonate Na2CO3 are introduced into the reactor, at room temperature. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then raised to 75°C and a mixture 1 is then added, comprising:

- 10.00 g of styrene (St),

- 0.200 g of methacrylic acid (MAA), and

- 10.42 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃.

The mixture is brought to 85°C, and a solution of 1.19 g of sodium persulfate Na₂S₂Os dissolved in 20.0 g of water is then introduced.

After 5 minutes, addition is commenced of a mixture 2 comprising:

- 90.0 g of styrene (St) and

- 1.80 g of methacrylic acid (MAA). The addition is continued over 60 minutes. After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

A sample (5 g) is then taken and analysed by steric exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 2000 g/mol in polystyrene equivalents (calibration with linear polystyrene standards) . Its polydispersity index Mw/Mn is equal to 2.0.

An analysis of the sample by gas chromatography reveals that the conversion of the monomers is greater than 99%.

Step Ib: Preparation of a poly (ethyl acrylate) second block with a theoretical molecular mass of about 42 000 g/mol to obtain a polystyrene-block-poly (ethyl acrylate) diblock copolymer of type (b)

The copolymer in emulsion obtained previously in step Ia, after having removed 5 g for analysis and without stopping the heating, is used as starting material. 1.19 g of sodium persulfate Na2S2θs diluted in 50.0 g of water are introduced continuously over three hours .

Simultaneously over three hours, a mixture 3 is added at 85°C, comprising:

- 200.0 g of water,

- 2.20 g of sodium carbonate Na2CO3, and

- 4.40 g of sodium dodecyl sulfate.

Simultaneously, a mixture 4 is added, comprising:

- 2100 g of ethyl acrylate (EA), and

- 42.0 g of methacrylic acid (MAA).

After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour. Next, 4.40 g of tert-butylbenzyl peroxide are introduced in a single portion and the addition is commenced of a mixture 5 comprising: - 2.20 g of erythorbic acid, - 50.0 g of water.

The addition is continued over 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to ~25°C for one hour. A sample (5 g) is then taken and analysed by steric exclusion chromatography (SEC) in THF. Its measured number- average molecular mass Mn is equal to 41 000 g/mol in polystyrene equivalents (calibration with linear polystyrene standards) . Its polydispersity index Mw/Mn is equal to 6.

An analysis of the sample by gas chromatography reveals that the conversion of the monomers is greater than 99.8%. The product obtained is a dispersion in water of the copolymer (latex) , with a solids content of about 41%.

Step II: Partial hydrolysis (target 75%) of the poly (ethyl acrylate) block of the copolymer obtained previously in step Ib to obtain the polystyrene-block- poly (ethyl acrylate-stat-sodium acrylate) diblock

750 g of water, 250 g of 2-propanol and 1347 g of copolymer in emulsion (i.e. 550 g of dry copolymer) obtained previously in step 2 are introduced into the reactor, at room temperature. The mixture obtained is stirred for 15 minutes. The temperature is then raised to 75°C, followed by addition of 678 g of sodium hydroxide (solution in water at 23.2% by mass) continuously over one hour. Thirty minutes after the start of addition of the sodium hydroxide, the continuous addition over one hour of 12 g of aqueous hydrogen peroxide solution (30% solution) is commenced. After complete addition of the various ingredients, the copolymer solution obtained is maintained at 75°C for four hours. The reaction mixture is then cooled to 25°C for one hour.

The product recovered at the end of the reaction is a translucent gel in water with a solids content of about 20%.

The copolymer thus obtained has the following characteristics :

- theoretical average molecular mass of block A: 2000 g/mol

- theoretical average molecular mass of block B: 30 000 g/mol

- weight proportion of block B: 96%

- weight proportion of block A: 4%

- weight amount of units derived from ethyl acrylate in block B: 31%

2. Preparation of a polystyrene-block-poly (ethyl acrylate-stat-sodium acrylate) diblock copolymer via synthesis of a polystyrene-block-poly (ethyl acrylate) diblock copolymer with target Mn values of 2000-block- 20 000 (g/mol) , followed by 75% hydrolysis of the ester groups of the ethyl acrylate.

Step Ia) : Preparation of a first block of polystyrene with a theoretical molecular mass of about 2000 g/mol

2150 g of water, 6.O g of sodium dodecyl sulfate and 0.650 g of sodium carbonate Na2CO3 are introduced, at room temperature, into the reactor. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then raised to 75°C and a mixture 1 is then added, comprising:

- 18.2 g of styrene (St),

- 0.360 g of methacrylic acid (MAA), and

- 18.9 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃. The mixture is brought to 85°C and a solution of 2.16 g of sodium persulfate Na2S2θs dissolved in 20.O g of water is then introduced.

After 5 minutes, the addition of a mixture 2 is started, comprising:

- 163.6 g of styrene (St) and

- 3.30 g of methacrylic acid (MAA).

The addition is continued for 60 minutes. After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

A sample (~5 g) is then taken and analysed by steric exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 2000 g/mol as polystyrene equivalents (calibration with linear polystyrene standards) . Its polydispersity index Mw/Mn is equal to 2.1.

Step Ha) : growth of a second block of poly (ethyl acrylate) with a theoretical molecular mass of about 20 000 g/mol to obtain a polystyrene-block-poly (ethyl acrylate) diblock copolymer

The emulsion copolymer obtained above in step Ia is used as starting material, after having removed 5 g for analysis, and without stopping the heating.

2.16 g of sodium persulfate Na2S2θs diluted in 50.0 g of water are introduced continuously over three hours. A mixture 3 comprising the following is then simultaneously added over three hours at 85°C: - 200.0 g of water,

- 4.00 g of sodium carbonate Na2CO3, and

- 8.00 g of sodium dodecyl sulfate.

Simultaneously, a mixture 4 is added, comprising:

- 1818 g of ethyl acrylate (EA), and

- 33.4 g of methacrylic acid (MAA).

After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

4.00 g of tert-butylbenzyl peroxide are then introduced in a single portion and addition of a mixture 5 is commenced, this mixture comprising:

- 2.00 g of erythorbic acid,

- 50.0 g of water.

The addition is continued over 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to ~25°C for one hour.

A sample (5 g) is then taken and analysed by steric exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 17 500 g/mol as polystyrene equivalents (calibration with linear polystyrene standards) . Its polydispersity index Mw/Mn is equal to 2.9.

An analysis of the sample by gas chromatography reveals that the conversion of the monomers is greater than 99.8%.

The product obtained is a dispersion in water of the copolymer (latex) with a solids content of about 44%.

Step II: Partial hydrolysis (target 75%) of the poly (ethyl acrylate) block of the copolymer obtained above in step 2 to obtain the polystyrene-block- poly (ethyl acrylate-stat-sodium acrylate) diblock

900 g of water, 300 g of 2-propanol and 1563 g of copolymer emulsion (c.f. 700 g dry weight of copolymer) obtained above in step Ib are introduced into the reactor, at room temperature. The mixture obtained is stirred for 15 minutes. The temperature is then raised to 75°C and 822 g of sodium hydroxide (solution in water at 23.2% by mass) are then added continuously over one hour.

30 minutes after the start of addition of the sodium hydroxide, continuous addition over one hour of 25 g of aqueous hydrogen peroxide solution (30% solution) is commenced.

After complete addition of the various ingredients, the copolymer solution obtained is maintained at 75°C for four hours. The reaction mixture is then cooled to ~25°C for one hour.

The product recovered at the end of the reaction is a translucent gel in water, with a solids content of about 17%.

The copolymer thus obtained has the following characteristics : theoretical average molecular mass of block A:

2000 g/mol - theoretical average molecular mass of block B:

14 000 g/mol

- weight proportion of block B: 90%

- weight proportion of block A: 10%

- weight amount of ethyl acrylate-based units in block B: 31%.

II. Composition examples Procedure for preparing the emulsions:

The diblock copolymer of the invention, supplied as an aqueous-alcoholic solution, was diluted with water for 30 minutes with stirring at 25°C; the solution obtained was macroscopically homogeneous. The copolymer of Comparative Example 1, supplied in powder form, was dissolved in water over 2 hours with stirring at 25°C; the solution obtained was macroscopically homogeneous. The emulsion was prepared by slow introduction of the oily phase into the aqueous phase with stirring using a homogenizer of Moritz type at a stirring speed of 2000 rpm (revolutions per minute) over: - 15 minutes for the emulsions comprising drops greater than 1 μm in size,

5 minutes for the emulsions comprising drops less than 1 μm in size, and the emulsion is then refined using a homogenizer of Rannie type at a pressure equal to 500 bar (3 passes) .

The size of the drops of the micron-sized emulsions was measured using a Hydro 2000 S/G laser granulometer

(Malvern) , assuming the mean surface diameter D (3.2), and the size of the drops of the submicron-sized emulsions was measured using a BI-90 size analyser

(Brookhaven Instrument) .

The polymers used in the examples of the invention are chosen from: a polystyrene (PS) /polyacrylic acid (PAA) diblock copolymer (PS/PAA copolymer) in which the PS block has a molar mass of 2000 g/mol and the hydrophilic block has a molar mass of 30 000 g/mol, the degree of hydrolysis being 75 mol%, i.e. a proportion of ethyl acrylate of 25%. (The percentage of the copolymer as active material differs according to the batches) . This polymer corresponds to Synthetic Example 1 ; - a PS/PAA diblock copolymer in which the PS block has a molar mass of 2000 g/mol and the hydrophilic block has a molar mass of 30 000 g/mol, the degree of hydrolysis being 40 mol%, i.e. a proportion of ethyl acrylate of 60%; - a PS/PAA diblock copolymer in which the PS block has a molar mass of 2000 g/mol and the hydrophilic block has a molar mass of 14 000 g/mol, the degree of hydrolysis being 75 mol%, i.e. a proportion of ethyl acrylate of 25%. This polymer corresponds to Synthetic Example 2.

Example 1: Fluid body milk

Phase A; Distilled water 80.264%

Glycerol 3.375%

Preserving agent 0.113%

Citric acid 0.028%

PS/PAA copolymer (2000/30 000), 75% hydrolysed (25% ethyl acrylate) containing 18% active material (i.e. 1.125% active material) 6.22%

Phase B;

Parleam oil 6.5% Cyclohexadimethylsiloxane 3.5%

Phase B was added with very vigorous stirring to phase A, at room temperature.

A fluid milk that is stable for at least 2 months was obtained, the mean size of the oil drops of which was 3 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spin speed of the spindle 3 of 200 rpm, was equal to 0.295 Pa. s.

Example 2: Sprayable body mist

Phase A:

Distilled water 83.692% Glycerol 3.375%

Preserving agent 0.113%

Citric acid 0.028% PS/PAA copolymer (2000/30 000), 75% hydrolysed

(25% ethyl acrylate) , containing 18% active 2.76% material (i.e. 0.5% active material)

Phase B

Parleam oil 6.5%

Cyclohexadimethylsiloxane 3.5%

Phase B was added with vigorous stirring to phase A reduced in water by 40%. The missing 40% water was then added at the end of emulsification, as a simple dilution.

A very fluid, sprayable dispersion, which is stable for at least 2 months and the size of the oil droplets of which was about 2.5 μm, was obtained. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spindle 2 spin speed of 200 rpm, was 0.058 Pa. s.

Example 3 : Care fluid

Phase A:

Distilled water 70.550%

Glycerol 3%

Preserving agent 0.1%

Citric acid 0.1% PS/PAA copolymer (2000/30 000), 75% hydrolysed (25% ethyl acrylate) containing 16% active material (i.e. 1% active material) 6.25%

Phase B: Parleam oil 13%

Cyclohexadimethylsiloxane 7%

Phase B was added with very vigorous stirring to phase A, at room temperature. A dispersion that is stable for at least 2 months was obtained, the pH of which was 6.7; the size of the oil droplets was about 4 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spin speed of the spindle 2 of 200 rpm, was equal to 0.205 Pa. s.

Example 4 : Care fluid

Phase A:

Distilled water 70.219%

Glycerol 3%

Preserving agent 0.1%

Citric acid 0.061%

PS/PAA copolymer (2000/30 000), 40% hydrolysed (60% ethyl acrylate) , containing 15.1% active 6.62% material (i.e. 1% active material)

Phase B

Parleam oil 13%

Cyclohexadimethylsiloxane 7%

Phase B was added with very vigorous stirring to phase A at room temperature.

A dispersion which is stable for at least 2 months, the pH of which was 6.8, was obtained; the size of the oil droplets was about 2 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spindle 2 spin speed of 200 rpm, was 0.076 Pa. s.

Example 5: Dispersion rich in fatty phase (40%)

Phase A:

Distilled water 55.516%

Glycerol 2.25%

Preserving agent 0.075%

Citric acid 0.019%

PS/PAA copolymer (2000/30 000), 75% hydrolysed (25% ethyl acrylate) , containing 18.1% active 4.14% material (i.e. 0.75% active material)

Phase B

Parleam oil 26%

Cyclohexadimethylsiloxane 14%

Phase B was added with very vigorous stirring to phase A at room temperature.

A viscous dispersion which is stable for at least 2 months, the pH of which was 7.6, was obtained; the size of the oil droplets was about 3 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spindle 3 spin speed of 200 rpm, was 0.570 Pa. s.

This dispersion may constitute, for example, a care cream for dry skin.

Example 6: Dispersion rich in fatty phase (40%)

Phase A:

Distilled water 43.575

Glycerol 2.25%

Preserving agent 0.075%

Citric acid 0.05%

PS/PAA copolymer (2000/30 000), 75% hydrolysed

(25% ethyl acrylate), containing 18.1% active 11.05% material (i.e. 2% active material)

Phase B

Parleam oil 26%

Cyclohexadimethylsiloxane 14%

Phase B was added with very vigorous stirring to phase A at room temperature.

A viscous dispersion which is stable for at least 2 months, the pH of which was 7.2, was obtained; the size of the oil droplets was about 3 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spindle 4 spin speed of 200 rpm, was 3.5 Pa. s.

This fluid may be used especially for treating dry skin, for example as a night cream.

Example 7: Dispersion very rich in fatty phase (50%)

Phase A:

Distilled water 39.693%

Glycerol 1.875%

Preserving agent 0.063%

Citric acid 0.079%

PS/PAA copolymer (2000/30 000), 75% hydrolysed (25% ethyl acrylate) , containing 18.1% active 8.29% material (i.e. 1.5% active material)

Phase B

Parleam oil 32.5%

Cyclohexadimethylsiloxane 17.5%

Phase B was added with very vigorous stirring to phase A at room temperature.

A very viscous dispersion which is stable for at least 2 months, the pH of which was 6.9, was obtained; the size of the oil droplets was about 2 μm. Its viscosity, measured using a Rheomat 180 viscometer at 25°C at a spindle 4 spin speed of 200 rpm, was 4.125 Pa. s.

This fluid may be used especially for very dry skin, for example as a face cream for very dry skin.

Example 8 : Sprayable fluid obtained via a pressurization process

Phase A:

Distilled water 74.155% Glycerol 3%

Preserving agent 0.1%

Citric acid 0.015% PS/PAA copolymer (2000/30 000), 75% hydrolysed

(25% ethyl acrylate) , containing 18.1% active 2.73% material (i.e. 0.5% active material)

Phase B

Parleam oil 13%

Cyclohexadimethylsiloxane 7%

A sprayable emulsion of very low viscosity (< 35 mPa.s), which is stable for at least 2 months, and the pH of which was 7.5, was obtained. The mean size of the oil droplets was equal to 570 nm.

This fluid may constitute a moisturizing balm for the body.

Example 9: Body spray obtained via a pressurization process

Phase A:

Distilled water 74.155%

Glycerol 3%

Preserving agent 0.1%

Citric acid 0.015%

PS/PAA copolymer (2000/14 000), 75% hydrolysed (25% ethyl acrylate), containing 18.3% active 2.73% material (i.e. 0.5% active material)

Phase B

Parleam oil 13%

Cyclohexadimethylsiloxane 7%

A sprayable emulsion (viscosity < 35 mPa.s), which is stable for at least 2 months, and the pH of which was 7.5, was obtained. The mean size of the oil droplets was equal to 320 nm. Example 10 and Comparative Example 1:

The comparison between Example 10 and Comparative Example 1 shows the emulsion-stabilizing superiority of the polymers of the present invention over polymers whose hydrophilic block is 100% hydrolysed and which therefore contain no ethyl acrylate.

The polymer of the present invention (PS/PAA copolymer) is a polystyrene (PS) /polyacrylic acid (PAA) diblock copolymer (PS/PAA copolymer) in which the PS block has a molar mass of 2000 g/mol and the hydrophilic block has a molar mass of 30 000 g/mol, the degree of hydrolysis being 75 mol%, i.e. a proportion of ethyl acrylate of 25%, with a mass proportion of 96% of block

The polymer of Comparative Example 1 (PS/sodium polyacrylate copolymer) is a diblock copolymer comprising a polystyrene (PS) lipophilic block with a molar mass of 1800 g/mol and a sodium polyacrylate hydrophilic block with a molar mass of 42 430 g/mol. The hydrophilic block does not comprise any ethyl acrylate (it contains 100% sodium acrylate) since it is completely hydrolysed. The mass proportion of the hydrophilic block in the copolymer is equal to 95.93%. This polymer is sold by the company Polymer Source.

Procedure: Phase B was added with vigorous stirring to phase A reduced in water by 40%. This missing water was added at the end of emulsification, as a simple dilution .

Fluid emulsions in which the size of the oil droplets depended on the diblock polymer used were obtained: - for the 75% hydrolysed PS/PAA copolymer (2000/30 000) used according to the invention, the mean size of the oil drops was equal to 5 μm; for the PS/sodium polyacrylate (100%) copolymer outside the present invention, the mean size of the oil droplets was equal to 12 μm.

Stability of the emulsions:

The two emulsions (Example 10 and Comparative Example 1) were subjected to an accelerated stability test according to the following protocol: 2 g of emulsion were centrifuged using an ultracentrifuge of Optima TLX type (Beckmann) for 30 minutes at a spin speed of 100 000 rpm. The stability was assessed via the absence of coalescence.

The emulsion of Example 10 had no surface oil film, and the drop size was not modified, which shows the stability of the emulsion with respect to coalescence.

The emulsion of Comparative Example 1 had an oily surface film, which shows the presence of coalescence and thus instability. The stability S of the emulsions was evaluated by means of the following parameter:

S = 10Ox (1 - volume of surface oil/ (volume of surface oil + volume of the emulsion) ) .

The following results were obtained:

Example 10: S = 100%

Comparative Example 1: S = 62.5%.

The polymers of the present invention thus make it possible to obtain emulsions that are more stable than those obtained with the polymers described in the prior art .

Examples 11 to 14 :

Procedure for preparing the emulsions:

The diblock copolymer of the invention, supplied as an aqueous-alcoholic solution, was diluted with water for 30 minutes with stirring at 25°C; the solution obtained was macroscopically homogeneous. The emulsion was prepared by introducing at room temperature of the oily phase B into the aqueous phase A with vigorous stirring using a homogenizer of Moritz type at a stirring speed of 2000 rpm for 15 minutes. Phase C was then introduced into the emulsion with gentle stirring. The size of the drops of the emulsions was measured using a Hydro 2000 S/G laser granulometer (Malvern) , on the basis of the mean surface diameter D (3.2) .

The polymer used in Examples 11 to 14 of the invention is a polystyrene (PS) /polyacrylic acid (PAA) diblock copolymer (PS/PAA copolymer) in which the PS block has a molar mass of 2000 g/mol and the hydrophilic block has a molar mass of 30 000 g/mol, the degree of hydrolysis being 75 mol%, i.e. a proportion of ethyl acrylate of 25%. This polymer, sold by the company Rhodia, is obtained by hydrolysis in basic medium of a polystyrene/polyethyl acrylate diblock polymer according to Synthetic Example 1.

Examples 11 to 14: Fluid body milks

Examples 11 to 13 are oil-in-water emulsions stabilized with a partially hydrolysed PS/polyethyl acrylate diblock copolymer and comprising a wetting agent of water-soluble silicone type. Example 14 according to the invention is an oil-in-water emulsions stabilized with a PS/partially hydrolysed polyethyl acrylate diblock copolymer but not comprising any wetting agent of water-soluble silicone type. They are stable for at least 2 months and are in the form of fluid milks.

Spreading properties

An in vitro evaluation was performed, using Bioskin as skin model, which is a polyurethane elastomer whose surface properties are similar to those of skin. The critical surface tension of the skin was determined in vivo on the forearm. The measurements were taken in a glove box at 30⁰C and 75% humidity, according to the Zisman method using, as liquids with known surface tensions: water (γ = 72 mN/m) and diiodomethane (γ = 50.8 mN/m) . The critical surface tension of water on the forearm is equal to 40 mN/m and that of Bioskin is equal to 41±2 mN/m.

0.2 g of emulsion was spread by finger onto a portion of Bioskin and left to dry out partially for 5 minutes. The uniformity of the spreading was evaluated by the naked eye.

Examples 11 to 13 lead to films that are more continuous on Bioskin than Example 14 (emulsion not containing wetting agent) , which is in agreement with their behaviour on the skin.

Claims

1. Composition for topical application to keratin materials, in the form of an oil-in-water emulsion comprising, in a physiologically acceptable medium, an oily phase dispersed in an aqueous phase, characterized in that it contains at least one (block A) - (block B) diblock copolymer in which:

- block A comprises at least 90% by weight of units derived from styrene relative to the weight of block A;

- block B comprises at least (a) units derived from acrylic acid in acid or salified form, in a proportion of not more than 90% by weight relative to the total weight of block B, and (b) units derived from a Ci-C₄ alkyl acrylate, in a proportion of at least 10% by weight relative to the total weight of block B,

2. Composition according to Claim 1, characterized in that, in the copolymer:

(i) from 34% to 90% by weight of units derived from acrylic acid in acid form or in salified form, (ii) from 10% to 66% by weight of units derived from a Ci-C₄ alkyl acrylate, the weight proportion of block B relative to the copolymer being 60%.

3. Composition according to Claim 1 or 2, characterized in that the said diblock copolymer is linear, and block B is a statistical block.

4. Composition according to any one of the preceding claims, characterized in that the Ci-C₄ alkyl acrylate is ethyl acrylate.

5. Composition according to any one of the preceding claims, characterized in that block A and/or block B comprises up to 10% by weight and preferably up to 5% by weight of an ionic or nonionic hydrophilic comonomer relative to the total weight of block A or of block B containing the said hydrophilic comonomer.

6. Composition according to the preceding claim, characterized in that the hydrophilic comonomer is methacrylic acid in acid or salified form.

7. Composition according to any one of the preceding claims, characterized in that the copolymer is of the type (a) in which the weight proportion of block B relative to the copolymer is greater than or equal to 87% and less than 94%.

8. Composition according to any one of Claims 1 to 7, characterized in that the copolymer is of the type (b) in which the weight proportion of block B relative to the copolymer is greater than or equal to 94%, especially ranging from 94% to 97%.

9. Composition according to any one of the preceding claims, characterized in that the diblock copolymer

(block A) - (block B) may be obtained via a polymerization process comprising at least the following steps: step I) : the following are prepared:

- a (block A) - (block B' ) diblock copolymer, or

- a triblock or star copolymer of architecture (core) - [(block A) - (block B')]x in which (core) -[ (block B' ) - (block A) ] x or x is an average number greater than or equal to 2, in which

- block A comprises the units derived from styrene, and

- block B' comprises the units derived from a C1-C4 alkyl acrylate, step I' ) optionally, for a triblock or star copolymer, the (core) - (block B') or (core) - (block A) bonds are broken so as to obtain a (block A) - (block B' ) diblock copolymer, step II) block B' is hydrolysed to a block B to obtain the diblock copolymer (block A) - (block B) , the hydrolysis inducing, where appropriate for a triblock or star copolymer, cleavage of the (core) - (block B' ) or (core) - (block A) bonds, so as to obtain a diblock copolymer (block A) - (block B) .

10. Composition according to the preceding claim, characterized in that: during step I) , a diblock copolymer (block A) - (block

B' ) is prepared via a process including the following intermediate steps Ia) and Ib) :

Ia) a first block A is prepared by placing the following in contact: - n_τ mol of a transfer agent comprising only one transfer group;

- n_A mol of styrene or of a mixture of monomers comprising at least 90% by weight of styrene and in which n_A/n_T > 5 and preferably < 5000; and - optionally a free-radical initiator;

- block A obtained in the preceding step, - n_B mol of a hydrolysable Ci-C₄ alkyl acrylate or of a mixture of monomers comprising at least 90% by weight of a Ci-C₄ alkyl acrylate such that n_B/n_T > 5 and preferably < 5000; and

- optionally a free-radical initiator, during step II), a hydrolysis of block B' is then performed to a degree T in moles of between 0.4 and 0.96 to obtain the said diblock copolymer (block A)- (block B) .

11. Composition according to Claim 9 or 10, characterized in that an additional step III) is performed, during and/or after step II) of deactivation of transfer groups borne by the macromolecular chains and/or of purification of the diblock copolymer (block A) - (block B) and/or of destruction of hydrolysis by-products and/or of deactivation.

12. Composition according to any one of Claims 9 to 11, characterized in that step I) is performed via emulsion polymerization in water.

13. Composition according to any one of Claims 9 to

12, characterized in that the degree of hydrolysis T is between 0.7 and 0.8.

14. Composition according to any one of Claims 9 to

13, characterized in that block A and/or block B' or block B comprises from 0.1% to 10% by weight and preferably from 0.1% to 5% by weight of an ionic or nonionic hydrophilic comonomer relative to the total weight of block A, of block B or of block B' containing the said hydrophilic comonomer.

15. Composition according to the preceding claim, characterized in that the hydrophilic comonomer is methacrylic acid in acid or salified form.

16. Composition according to one of Claims 9 to 15, characterized in that the Ci-C₄ alkyl acrylate is ethyl acrylate .

17. Composition according to one of Claims 9 to 16, characterized in that the diblock copolymer (block A) - (block B) is such that:

- the weight proportion of block B relative to the copolymer is greater than or equal to 85%, and

- its theoretical average molecular mass is greater than or equal to 13 000 g/mol.

18. Composition according to the preceding claim, characterized in that the diblock copolymer (block A) - (block B) is of the type (a) in which: - the weight proportion of block B relative to the copolymer is greater than or equal to 87% and less than 94%, its theoretical average molecular mass is between 13 000 g/mol and 20 000 g/mol.

19. Composition according to Claim 17, characterized in that the diblock copolymer (block A) - (block B) is of the type (b) in which: the weight proportion of block B relative to the copolymer is greater than or equal to 94%, especially ranging from 94% to 97%, and its theoretical average molecular mass is greater than or equal to 20 000 g/mol and preferably between 20 000 and 50 000 g/mol.

20. Composition according to any one of the preceding claims, characterized in that the diblock copolymer (s) is (are) present in a concentration ranging from 0.1% to 10% by weight and preferably from 0.2% to 5% by weight relative to the total weight of the composition.

21. Composition according to any one of the preceding claims, characterized in that the amount of oily phase ranges from 2% to 70% by weight relative to the total weight of the composition.

22. Composition according to any one of the preceding claims, characterized in that it comprises at least one wetting agent chosen from water-soluble silicones comprising at least one polyoxyalkylene group of general formula (a) below:

R² ₃SiO(R² ₂SiO) p (R²PESiO) _qSiR²3 (a) in which - the radicals R², which may be identical or different, denote a monovalent hydrocarbon-based radical chosen from alkyl, aryl and aralkyl radicals containing not more than 10 carbon atoms; some of the radicals R² may also additionally contain an ethylcyclohexylene monoxide group of formula

and are in low proportion in the polysiloxane chain;

- p ranges from 0 to 150, preferably from 0 to 100 and more preferentially from 0 to 30;

- the polyether group PE has the formula (b) below

-C_xH_2x (OC₂H₄) _y (OC₃Hg)₂OR³ (b) in which: x ranges from 1 to 8 and preferably from 2 to 4 and is more preferentially equal to 3; y is greater than 0; z is greater than or equal to 0; the values of y and z are such that the total molecular weight of the polyoxyalkylene portion of the polyether group PE ranges from 200 to 10 000 and more preferentially from

350 to 4000;

R³ denotes hydrogen, a Ci-Cs alkyl group or a C₂-Cs acyl group.

23. Composition according to the preceding claim, cchhaarraacctteerriizzeedd iinn tthhaatt tthhee rraaddiiccaallss FR² are chosen from Ci-Cε lower alkyls; phenyl and benzyl.

24. Composition according to the preceding claim, characterized in that the radicals R² are chosen from Ci-C₄ lower alkyls and even more particularly denote the methyl group.

25. Composition according to either of Claims 24 and 25, characterized in that the radicals R³ are chosen from C1-C4 lower alkyls and even more particularly denote the methyl group.

26. Composition according to any one of Claims 22 to 25, characterized in that the water-soluble silicones of formula (a) are chosen from those of formula (a' ) below:

MeSiO (MeSiO) p (MePESiO) _qSiMe₃ (a' ) in which Me denotes the methyl group; PE denotes the group of formula (b' ) :

- (CH₂) ₃0 (OC₂H₄) _y (OC₃Hg)₂OR³ (b' ) in which x, z and z have the same values indicated in

Claim 30 and R³ denotes hydrogen or a Ci-C₄ alkyl group and more particularly methyl.

27. Composition according to any one of Claims 22 to

26, characterized in that the wetting agent is chosen from the branched silicones of formula (c) below:

(MeSiO) _q_₂[ (SiOMe₂) _p/qOPE]_q (c) in which

- q ranges from 1 to 12, preferably from 1 to 10 and more preferentially from 1 to 8; - Me means the methyl group;

- PE denotes the group of formula (d) below:

- (OC₂H₄) _y (OC₃Hg)₂R³ (d) in which

- y is greater than 0; - z is greater than 0; the values of y and z are such that the total molecular weight of the polyoxyalkylene portion of the polyether group PE ranges from 200 to 10 000 and more preferentially from 350 to 4000;

- R³ denotes a Ci-C₄ alkyl group.

28. Composition according to any one of Claims 22 to

27, characterized in that the wetting agent (s) is (are) present in a concentration ranging from 0.01% to 10% by weight and preferably from 0.05% to 5% by weight relative to the total weight of the composition.

29. Composition according to any one of the preceding claims, characterized in that it is a protecting, treating or care product for the face, a product for the hands or for the body, a skin cleansing product, a makeup product or a haircare product.

30. Cosmetic process for treating a keratin material, characterized in that a composition according to any one of Claims 1 to 29 is applied to the keratin material .

31. Use of at least one diblock copolymer in an oil- in-water emulsion for topical application to keratin materials, to stabilize the said emulsion, the said diblock copolymer (block A) - (block B) comprising:

- block A comprises at least 90% by weight of units derived from styrene relative to the weight of block A; - block B comprises at least (a) units derived from acrylic acid in acid or salified form, in a proportion of not more than 90% by weight relative to the total weight of block B, and (b) units derived from a C1-C4 alkyl acrylate, in a proportion of at least 10% by weight relative to the total weight of the block B,

32. Use according to Claim 31, characterized in that the emulsion is free of emulsifier.