FR2876298A1 - PROCESS FOR THE MANUFACTURE OF RADIOGRAFT MEMBRANES AND APPLICATION OF MEMBRANES OBTAINED - Google Patents

Abstract

A process for the production of radiografted membranes having a low surface grafting rate, in particular ion exchange membranes, structured in thickness, wherein a membrane is irradiated and contacted with a grafting composition, the composition grafting agent comprising a polymerization inhibitor having a steric hindrance limiting its penetration into the membrane to a surface area having a thickness of at most 10% of the thickness of the membrane.

Description

Process for the manufacture of radiografted membranes and application of

  The invention relates to a process for the manufacture of radiografted membranes structured in thickness, particularly suitable for the production of ion exchange membranes.

  Membrane structured in the thickness means a membrane which has in its thickness a controlled gradient of properties such as for example its degree of grafting or in the case of ion exchange membranes its ion exchange capacity, the capacity of surface exchange being different from that at the heart of the membrane.

  Ion exchange membranes are well known in the art. Their ability to be selectively permeable to anions or cations gives them many applications. Membrane electrolysis of sodium chloride solutions for the joint production of chlorine and sodium hydroxide, fuel cells and finally electrodialysis, itself having many applications such as water treatment, are examples of technologies using ion exchange membranes.

  When ion exchange membranes are used in electrodialysis for the treatment of fluids containing substances to be removed, they are quickly fouled by these substances (fouling), which involves cleaning or replacement. For practical reasons, it is therefore important that they have sufficient resistance to fouling, for example by virtue of their anti-adhesive properties. On the other hand, they must have sufficient overall ionic conductivity in order to maintain economically attractive electrical consumption.

  It is known (US2003 / 0024816A1) to improve the fouling resistance of ion exchange membranes by fixing polyalkylene glycol chains therein. The fouling resistance of these known membranes is, however, insufficient when the fluid to be treated contains pollutants of low molecular weight.

  The object of the invention is to provide an improved method for obtaining ion exchange membranes resistant to fouling and having excellent ionic conductivity.

  Accordingly, the invention relates to a process for the production of radiografted membranes having a low surface grafting rate, in particular ion exchange membranes, structured in thickness, wherein a membrane is irradiated and contacted with a grafting composition, the grafting composition comprising a polymerization inhibitor having a steric hindrance limiting its penetration into the membrane to a surface area having a thickness of at most 10% of the thickness of the membrane.

  Radiografting is a technique for producing copolymers in which a base polymer (which may itself be a homopolymer or a copolymer) is subjected to high energy radiation to create reactive radicals therein. In combination with the irradiation, the polymer is brought into contact with a composition comprising the monomer that is to be grafted. This monomer will polymerize on the sites rendered active by the radiation. The grafted groups can then be converted into ion exchange groups by a complementary functionalization treatment (such as sulfonation, phosphonation, amination, carboxylation, etc.). The yield of the functionalization is usually very high, more than 80%, preferably 90% of the graft groups being functionalized in groups (also called sites) exchangers. The control of the experimental parameters of the radiografting step, such as those relating to the irradiation and to the nature of the composition comprising the monomer, therefore has a direct impact on the control of the structuring in the thickness of the capacity of the membrane exchange obtained since the exchange capacity is directly related to the number of sites / interchange groups. The radiation used for the irradiation may be electromagnetic, such as X-rays or gamma rays, or may be charged particles such as electrons. Beta radiation, consisting of electrons having sufficient energy, for example at least 0.5 MeV, preferably at least 1 MeV, may be suitable. Usually, it is preferable that the energy does not exceed 20 MeV, the energies between 1.5 and 10 MeV being recommended. This radiation must be applied to the base polymer for the time necessary to obtain a sufficient amount of reactive radicals. This results in a dose of irradiation, expressed in kGy (kilogray), a gray value of 104 erg / gram. The required irradiation dose depends on the sensitivity of the polymer to the radiation. When the base polymer is ETFE, it has been observed that times corresponding to doses of between 20 and 100 kGy are suitable. Irradiation can be performed when the polymer is in contact with the composition comprising the monomer. It can also be done beforehand. In this case, the irradiated polymer is advantageously kept at a low temperature, while waiting for it to come into contact with the composition comprising the monomer. For the production of membranes according to the invention, it is recommended to irradiate the base polymer, already used in membrane form. It is also recommended that the irradiation be carried out prior to bringing the polymer into contact with the composition comprising the monomer.

  According to the invention, the use of a polymerization inhibitor in the grafting composition makes it possible to reduce surface grafting. However in the general case the inhibitor also penetrates the membrane and exerts its effects. The overall grafting rate is then insufficient to obtain ion exchange membranes having good ionic conductivity. The inventors have observed that the selection of inhibitors having a sufficient steric hindrance prevents their penetration deep into the membrane while sufficiently inhibiting the grafting of the superficial zone. According to the invention, this surface area represents at most 10%, advantageously at most 5%, preferably at most 1% of the thickness of the membrane. The depth of penetration of the inhibitor can be measured by any analytical technique making it possible to detect the presence of the inhibitor, possibly after peeling of the superficial zone, also carried out by any appropriate known technique. According to the invention, the inhibitor is considered to be absent, at a certain depth level, if its concentration is at most equal to one tenth, advantageously one twentieth, preferably one fiftieth, particularly preferably one hundredth of its surface concentration. .

  The steric hindrance necessary to prevent the penetration of the inhibitor in depth depends on the nature of the base polymer constituting the membrane prior to radiografting and the nature of the grafting composition, including the presence of solvents which penetrate the membrane and inflate it. The determination of the inhibitor having the ideal steric hindrance must therefore in certain cases be done by trial and error.

  However, in an advantageous embodiment of the process according to the invention, the polymerization inhibitor has a steric hindrance of at least 250 Å / molecule. Inhibitors having a steric hindrance of between 300 and 350 Å3 / molecule have been found to be particularly useful in a large number of circumstances.

  On the other hand, in general, the use of methylene blue, consisting of tetramethylthionine hydrochloride (C16H18SN3Cl) as inhibitor has appeared extremely advantageous. It is recommended to use this inhibitor in concentrations ranging from 0.05 g / l, preferably 0.1 g / l to 0.75 g / l, preferably 0.5 g / l.

  The presence of polymerization inhibitor in the grafting composition also has the advantage of maintaining the effectiveness of the composition up to temperatures up to 80 C, advantageously 85 C, ideally 90 C, which ensures kinetics of grafting very interesting.

  According to an advantageous variant of the process according to the invention, the grafting composition further comprises a crosslinking agent. Under the action of the crosslinking agent, a three-dimensional network is formed within the membrane. This results in an improvement of the mechanical properties, a decrease in the permeability to certain substances but also a better chemical resistance. The nature of the crosslinking agent depends on the chemical composition of the irradiated membrane and the nature of the grafted monomer. In particular, when the grafted monomer is styrene and the irradiated membrane is based on fluoropolymer, it is recommended to use divinylbenzene (DVB) or triallylisocyanate (TAIC) as a crosslinking agent, or a mixture of both. DVB is preferred. In this variant, the membrane is at least partly crosslinked, the inhibitor penetrates more difficultly. All other things being equal, it is then possible to use an inhibitor having a smaller steric hindrance. In this variant, it is advisable to use the crosslinking agent in a proportion of at least 0.5%, preferably at least 1% by volume relative to the grafted monomer. Usually it is recommended not to exceed 15%, values between 1.5% and 10% being preferred.

  According to another advantageous variant of the process according to the invention, the grafting composition also comprises a chain transfer agent. This additive makes it possible to further improve graft control in the thickness of the membrane. The use of chain transfer agents of the thiol family gives excellent results. In this family, certain thiols such as hexanethiol diffuse easily and make it possible to obtain a homogeneous grafting profile inside the membrane (the surface excepted). Other thiols with larger steric hindrance such as dodecanethiol diffuse more slowly and with difficulty and give rise to greater grafting gradients, the grafting at the center of the membrane being greater than that in the more external zones. It is preferable to use the chain transfer agents in a very small quantity, of between 0.01%, advantageously 0.02% and 0.5%, advantageously 0.25% by volume relative to the total volume of the composition. grafting.

  The joint use of a polymerization inhibitor, a crosslinking agent and a chain transfer agent makes it possible to produce grafted membranes having an extremely low surface grafting rate, preferably less than 10%, and a controlled profile inside ensuring further an overall grafting rate greater than 20%, preferably 30%, preferably 40%, the values. between 50 and 60% being especially advantageous. The grafting rate is the weight gain following the grafting of an ungrafted membrane weight unit. It can be evaluated locally, in a given layer of the membrane, or globally, for the entire thickness. By surface is meant a zone having a thickness of at most 5u, preferably 1, more preferably 0.1u. When the grafted membranes according to the invention are functionalized to produce ion exchange membranes, the latter have few sites on the surface, to a depth of a few molecular layers. The extreme fineness of this superficial layer and the excellent control of the depth grafting profile, obtained according to the invention, maintain excellent electrical properties and in particular good overall ionic conductivity.

  In the process according to the invention, the diffusing properties of certain particular polymerization additives are exploited. The evolution of the grafting rate, from the surface to the core of the membrane is consequently continuous, which has numerous advantages, in particular when the grafted membranes are functionalized as ion exchange membranes. In order to best exploit this situation, in a preferred embodiment of the method according to the invention, the membrane is a monolayer membrane. Monolayer membrane means a membrane having no interface in its thickness. Such a membrane is therefore distinguished from multilayer membranes, which result from the superposition of two or more fine membranes or successive overlays of a membrane by additional outer layers. A monolayer membrane has a continuous evolution, in the direction of the thickness, of its main material macroscopic parameters, such as its density, its ionic conductivity or its mechanical properties. The absence of interfaces and discontinuities in a monolayer membrane treated by the process according to the invention has many advantages, such as for example a better ionic conductivity, the absence of risk of delamination, less internal tensions and therefore less deformation in use.

  When the membrane is functionalized as an ion exchange membrane, it may be of the cationic or anionic type, depending on whether it is selectively permeable to cations or anions, respectively. The ion exchange sites that it comprises may be of various known types, such as, for example and depending on the type of membrane, carboxyl, sulfonate or trimethylammonium groups.

  In the case of a cationic membrane obtained by the process according to the invention, comprising sulfonate ion exchange sites, the low density or preferably the absence of exchange sites in the superficial zone can be very quickly and easily put into operation. highlighted by contacting the surface of the membrane with methylene blue (tetramethylthionine hydrochloride - Cl 6111 8SN3C1). This dye has a high molecular weight which reduces its penetration inside the membrane. In the case of membranes obtained according to the invention, the methylene blue is not fixed in contact with the sulfonate groups on the surface and gives no blue coloration. In comparison, the membranes obtained without a polymerization inhibitor fix the methylene blue and take an intense blue color. An analogous test using Congo Red makes it possible to demonstrate the absence of quaternary ammonium exchange sites on the anion exchange membranes obtained according to the invention.

  In the membrane according to the invention, the ion exchange sites, for example carboxyl, sulfonate or trimethylammonium groups, are covalently bound to a base polymer. This polymer must ensure in particular the mechanical strength of the membrane, its dimensional stability and have the necessary chemical resistance adapted to the medium in which it is operating. Many basic polymers can be used successfully in the process according to the invention. Examples of this are fluorinated polymers such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), perfluorinated poly (ethylene-propylene) (FEP) and ethylene polytetrafluoroethylene (ETFE) copolymers. ; Polyolefins (polyethylene and polypropylene); É polyamides.

  According to a preferred variant of the process according to the invention, the base polymer comprises a fluoropolymer. Preferably it consists of a fluoropolymer. In this variant, hydrophobic and antiadhesive membranes are obtained, the surface properties of the base polymer being preserved by the process according to the invention.

  To obtain a membrane, the base polymer is implemented by any means adapted to the thickness and dimensions of the membrane to be obtained and to the necessary accuracy of the latter. For membranes of small dimensions and having a large thickness of high precision, the injection techniques can be interesting. Most often, the polymer is used by extrusion or calendering. The ion exchange sites can be bonded to the base polymer before or after its implementation as a membrane. It is preferred that they be bonded to the base polymer after it has been used as a membrane.

  When the irradiated membrane is brought into contact with the composition comprising the monomer, grafting proceeds by progressing a front from the surface towards the inside. Without wishing to be bound by a technical explanation, the inventors believe that the control of the forehead progression mechanism makes it possible to improve the structure of the membrane in its thickness and in particular to obtain excellent control of the grafting rate. In certain circumstances, the progression kinetics of the forehead is such that it is possible to optimize the structuring of the membrane by varying over time the composition comprising the monomer.

  Consequently, in a particularly advantageous variant of the process according to the invention, the irradiated membrane is contacted successively with at least two grafting compositions, at least one grafting composition contacted previously with the membrane having a content of at least a polymerization additive greater than that of at least one grafting composition subsequently contacted, the polymerization additive being a chain transfer agent and / or an inhibitor and / or a crosslinking agent. The polymerization additive being present in greater quantity at the beginning of the grafting, it preferentially affects the outer layers of the membrane. For example, when the crosslinking agent is present mainly at the beginning of the grafting, it is possible to obtain a membrane which is more reticulated in its surface areas relative to its inner layers.

  In this variant, it is also possible to produce membranes having a very low surface grafting rate of less than 10%, preferably less than 1%, while being highly grafted in depth, the maximum grafting rate exceeding 40%. preferably 50%. For this purpose, the membrane is first contacted with a graft composition containing sufficient polymerization inhibitor and chain transfer agent to limit surface grafting. Next, the membrane is brought into contact with a second grafting composition in which these additives are sufficiently absent to obtain a large grafting when the grafting front reaches the inner layers of the membrane. The precise contents of the grafting compositions in these additives will be determined case by case by the skilled person depending on the base polymer and the monomer to be grafted.

  The ion exchange membranes having a fluorinated base polymer, obtained from such very weakly surface-grafted membranes, consequently have a high concentration of fluorine at the surface while being strongly grafted at depth. They also have an advantageous crosslinking profile, the surface areas being more crosslinked than the internal zones. These membranes are particularly chemically resistant and especially dirty very little. They are especially useful in electrodialysis, for example for the treatment of wine.

  The monomers included in the grafting compositions may be different or identical. When they are different, the membrane will have in its thickness a structuring of its chemical composition. This spatial structuring in the thickness will be a function of the temporal variation of the grafting compositions, the monomer included in the grafting composition subsequently contacted with the membrane being grafted into the outermost layers of the membrane with respect to the monomer included in the grafting compositions. compositions contacted previously. When the grafting compositions comprise several monomers, and when the membrane is brought into contact with a greater number of different grafting compositions, it is possible to gradually vary the content of the various grafting compositions in the monomers and to obtain a membrane presenting in the thickness a chemical composition gradient which is all the more continuous as the content of the various graft compositions is progressively varied.

  In a preferred embodiment of the process according to the invention, at least one grafting composition subsequently contacted with the membrane comprises a barrier monomer that is absent from at least one composition contacted previously. By barrier monomer is meant a monomer which, when present in the grafting composition, gives rise to a grafted membrane having a lower permeability than it would have in the absence of the barrier monomer in the grafting composition. In the process according to the invention, the permeability considered is the fluid permeability with which the membrane is brought into contact when it is in operation.

  Although the grafting compositions may be in the gaseous state or even the plasma, in the variants of the process according to the invention in which the grafting composition is varied over time, it is recommended that the grafting compositions be liquid state and that the contacting is performed by immersing the membrane in at least two different baths. The composition of the baths may be constant over time, in which case the membrane is successively contacted with at least two separate baths. The contacting can in this case be discontinuous, the whole amount of membrane produced being at a given moment in a given bath (batch mode) or continuous, different parts of the membrane being in different baths, the membrane being mobile. The composition of the baths can also change over time, by addition or even removal of certain components. In this case the membrane can remain in contact with a single bath.

  In the process according to the invention, it is possible to use any grafting composition adapted to the membrane that it is desired to produce. In particular, in the case where it is desired to obtain ion exchange membranes, the grafting composition depends on the cationic or anionic nature of the desired membrane. The grafting compositions advantageously comprise chloromethylstyrene (anionic membranes) or styrene (cationic membranes). Good results can also be obtained with fluorostyrenes, optionally substituted. By way of example of fluorostyrenes, there may be mentioned fluorostyrene, α, β-difluorostyrene, α, β, β-trifluorostyrene and the corresponding fluoronaphthylenes. By substituted fluorostyrene is meant a fluorostyrene having a substituent in the aromatic ring.

  The process according to the invention is particularly suitable for the production of ion exchange membranes having a low density of surface exchange sites.

  These membranes have an excellent compromise between their resistance to fouling and sufficient ionic conductivity.

  Accordingly the invention also relates to the use of ion exchange membranes obtained by the method according to the invention in electrodialysis, in particular for the treatment of fluids such as water or preferably wine.

  On the other hand, it has been observed that, in general, the radiografted membranes obtained by the process according to the invention, probably due to their low surface grafting, have improved impermeability to certain fluids, in particular to alcohols, particularly methanol-when compared to analogous membranes grafted more strongly on the surface. This improved impermeability makes them advantageous in fuel cells, especially in batteries operating directly with methanol, without prior conversion of methanol into hydrogen. In fact, in these cells, called Direct Methanol Fuel Cells DMFC, the following reactions occur at the electrodes: At the cathode: 02 + 4H + +4 e '2H20 At the anode: CH3OH + H2O * CO2 + 6H + + 6e The membrane used as a separator in such a fuel cell must meet specific and strict requirements, because its physicochemical properties significantly influence the performance of the battery. In particular, important parameters are the proton conductivity and the impermeability to fuel, in this case methanol.

  The invention therefore also relates to the use of the ion exchange membranes obtained by the process according to the invention in fuel cells, in particular in fuel cells operating with methanol such as DMFCs.

  The following examples serve to illustrate the invention.

  In these examples, the procedure was as follows.

  Irradiation of the membranes was performed in the presence of air under an electron beam at a voltage of 1.5 MeV and at a dose rate of 10 kGy / s. The dose deposited in the film is 20 to 100 kGy. The irradiated membranes were stored at a temperature of less than or equal to 18 C until they were used. At -18 C irradiated membranes can be stored for 12 months with very little loss of reactivity.

  The monomers and more particularly those containing significant amounts of stabilizers such as divinylbenzene (DVB) and chloromethylstyrene (CMS) were destabilized by washing in a basic aqueous medium NaOH 0.1 M and then rinsed to neutral pH with water demineralized in a separating funnel. The destabilized monomers were stored at -18 C until used.

  The ionic conductivity, the water content and the exchange capacity of the membranes obtained were measured according to the French standard AFNOR NF X45-200 December 1995.

  The permeability to methanol was measured at ambient pressure by introducing the studied membrane into a measurement cell of section 8.55 cm 2. The membrane delimited two compartments of 10 ml. One of the faces of the membrane was exposed to a molar solution of methanol continuously renewed at a constant rate of 24 ml / h. The second compartment was flushed with helium at a flow rate of 400 ml / min. The measuring cell was maintained at 25 ° C. The entrained vapors were condensed in two consecutive traps containing acetone at 2 ° C. Condensate analysis was performed by gas chromatography. Under the same measurement conditions, the Nafion 117 membrane with a dry thickness of 1751.im used as a reference had a methanol permeability of 1215 g / m 2 and 11230 g / m 2 in water for an electrical resistance (NaCl 10 g / l) of 2 , 4 Ohm.cm2 and the Nafion 112 membrane of dry thickness 45 m a methanol permeability of 1770 g / m2j for an electrical resistance of 1.0 Ohm.cm2.

  The contact angle was measured in water and di-iodomethane (Kruss G2 measuring instrument).

  The profiles of carbon and fluorine concentrations and / or concentration of sulfur were measured by microanalysis X (SEM-EDX) on a section of the membrane obtained by cutting with a cryogenic microtome. The sample section is obtained by ultramicrotomic smoothing at room temperature.

  It is then sputtered with a thin conductive layer based on a platinum / palladium alloy. The examination is performed using a LEO 982 brand Field Effect Scanning Electron Microscope (FEG-SEM) equipped with an Oxford Instruments ISIS 300 X Microanalysis System. The electron energy used is 20 keV. To measure a concentration profile, the X signals of the elements to be monitored (sulfur, fluorine, possibly carbon and oxygen), emitted following the electron incidence along a line chosen in parallel with each other, are collected point by point. the thickness of the membrane, the displacement being effected by deflection of the electron beam using the coils used for imaging. The concentration of the elements whose profile is measured is proportional to the intensity of the signal X measured.

  The presence and accessibility of the sulphonate sites on the membrane surface was evaluated by immersing the samples for 1 minute in a 5 g / l aqueous solution of methylene blue and then measuring the L * a * b * coordinates in transmission. with illuminant D65 and an observation angle of 10. Example 1 A grafting solution containing 30% by volume of non-destabilized styrene and 70% of ethanol comprising 0.1 g / l of methylene blue was prepared. To this solution was added 3, 15% by volume of pure divinylbenzene based on the volume of styrene used and 0.055% by volume of 1-dodecanethiol relative to the total volume of the grafting solution. An ETFE membrane irradiated at a dose of 60 kGy was introduced into the grafting solution and the assembly was purged with nitrogen until an oxygen concentration of less than 100 ppm in the reactor air was obtained.

  The grafting solution was brought to a temperature of 80 C for 16 hours. The grafting rate obtained was 47%. The styrene grafted in the sample was then sulphonated for 12 hours at room temperature in a solution of 1,2-dichloroethane (DCE) containing 6% by weight of chlorosulfonic acid. The membrane was rinsed in DCE and then in ethanol for 1 hour. The sulfonate sites were finally obtained by hydrolysis of the chlorosulfonyl sites in a 0.1 M aqueous solution at 60 ° C. for 16 hours. The profile of the distribution of sulfur in the thickness of the grafted film showed that the graft penetrated to the heart of the film and had a less grafted surface area (Figure 1).

  Figure 1: Profile of the concentration of S, C, F on membrane thickness Measured in an aqueous solution at 10 g / l NaCl, the membrane has a resistance of 1.8 μCm2. The water content is between 34.4 and 36% and the exchange capacity between 2.17 and 2.19 meq / g. The methylene blue test results indicate a low concentration of surface sites (L: 74.5, a: 41.9, b: 26.5) and the contact angle (86 in water and 57 in diameter). iodomethane) indicate low wettability.

Example 2

  A grafting solution containing 20% by volume non-destabilized styrene and 80% ethanol at 0.3 g / l methylene blue was prepared. To this solution was added 4% by volume of pure divinylbenzene based on the volume of styrene used and 0.050% by volume of 1-dodecanethiol relative to the total volume of the grafting solution. An ETFE membrane irradiated at a dose of 60 kGy was introduced into the grafting solution and the whole was purged with nitrogen until an oxygen concentration <100 ppm in the reactor air was obtained. The grafting solution was brought to a temperature of 80 C for 6 hours. The grafting rate obtained is 35 to 40%. The membrane was then treated as in Example 1. The profile on the thickness of the membrane, the signal of carbon and fluorine and sulfur showed that the graft had penetrated to the heart of the film and had a gradient grafting depth of the film (Figure 3).

  Figure 2: S, C, F profile in the thickness of the membrane.

  Measured in an aqueous solution containing 10 g / l of NaCl, the membrane has a resistance of 3.5 S 2 cm 2. The water content is 30% and the exchange capacity 1.8 meq / g. The methylene blue test result indicated a low concentration of surface sites. The permeability to methanol was 566 g / m 2 and the water permeability was 14370 g / m 2.

Example 3

  A 100 m membrane irradiated at 100 kGy was immersed in a grafting solution containing, by volume, 20% of destabilized CMS, 80% of ethanol containing 0.3 g / l of methylene blue. To this solution was. added a volume. pure DVB corresponding to 2.4% of the CMS volume. After deoxygenation, the grafting reactor was heated to 75 ° C. for 16 hours. The grafting rate obtained was 48.8%. After amination in a solution of 45% trimethylamine in water and then equilibrated in NaCl solution 10 g / l for 24 hours, the resistance of the membrane was between 3.6 and 4.7 g / cm 2. The membrane immersed for one minute in an aqueous solution of Congo red did not stain confirming the absence of quaternary amine sites on the surface.

Example 4

  A grafting solution containing 30% by volume non-destabilized styrene and 70% ethanol comprising 0.1 g / I methylene blue was prepared. To this solution was added 3.15% by volume of pure divinylbenzene based on the volume of styrene used and 0.055% by volume of 1-dodecanethiol relative to the total volume of the grafting solution. An ETFE membrane irradiated at a dose of 60 kGy was introduced into the grafting solution and the assembly was purged with nitrogen until an oxygen concentration of less than 100 ppm in the reactor air was obtained.

  The grafting solution was brought to a temperature of 80 C for 15 hours. The grafting rate obtained was 39%. The styrene grafted in the sample was then sulphonated for 12 hours at room temperature in a solution of 1,2-dichloroethane (DCE) containing 6% by weight of chlorosulfonic acid. The membrane was rinsed in DCE and then in ethanol for 1 hour. The sulfonate sites were finally obtained by hydrolysis of the chlorosulfonyl sites in a 0.1 M aqueous solution at 60 ° C. for 16 hours.

  Measured in an aqueous solution containing 10 g / l of NaCl, the membrane has a resistance of 1.4 S 2 cm 2. The water content is between 35% and the exchange capacity between 1.19 meq / g. The methylene blue test results indicate a low concentration of surface sites. This membrane has a dry thickness of 139 m. The methanol permeability measured in the pervaporation cell is 682 g / m 2 at 25 ° C.

Claims (10)

  A process for the production of radiografted membranes having a low surface grafting rate, in particular ion exchange membranes, structured in thickness, wherein a membrane is irradiated and contacted with a grafting composition, the graft composition comprising a polymerization inhibitor having a steric hindrance limiting its penetration into the membrane to a surface area having a thickness of at most 10% of the thickness of the membrane.   2. Method according to the preceding claim wherein the grafting composition comprises a crosslinking agent.   3. A process according to any one of the preceding claims wherein the polymerization inhibitor has a steric hindrance of at least 300 Å.   The method of any of the preceding claims wherein the grafting composition comprises a chain transfer agent.   The process of any of the preceding claims wherein the base polymer of the membrane comprises a fluoropolymer.   The method of any one of the preceding claims wherein the membrane is a monolayer membrane.   7. Process according to any one of the preceding claims, in which the irradiated membrane is put in contact successively with at least two grafting compositions, at least one grafting composition contacted previously with the membrane having a content of at least one additive. greater than that of at least one grafting composition contacted subsequently, the polymerization additive being a chain transfer agent and / or an inhibitor and / or a crosslinking agent.   8. A method according to any one of the preceding claims wherein the irradiated membrane is contacted successively with at least two different grafting compositions, at least one grafting composition subsequently contacted with the membrane comprising a barrier monomer. absent from at least one composition contacted previously.   9. Use in electrodialysis of an ion exchange membrane obtained by a process according to any one of claims 1 to 8.   10. Use in fuel cells, particularly in methanol fuel cells such as the DMFCs of an ion exchange membrane obtained by a process according to any one of claims 1 to 8.