Process for manufacturing a film by coating a block polymer ionomer

Abstract

The invention relates to a method for manufacturing a film by coating, which film has a thickness less than or equal to 200 µm and comprises an ionomer which is a block polymer of formula A-(BA)nBA in which A represents a block of a poly(vinylaromatic) bearing pendant ionic groups, B represents a block of a hydrogenated poly(1,3-diene) or of a hydrogenated copolymer of a 1,3-diene and a vinylaromatic, the vinylaromatic being a vinylaromatic monomer of formula ArCH=CH2 or ArC(Me)=CH2, the symbol Ar representing an aromatic hydrocarbon group, n is an integer greater than or equal to 0.

Description

Title of the invention: Method for manufacturing a film by coating a block polymer ionomer

[0001] The field of the invention is that of methods for manufacturing films comprising an ionomer intended to be used, for example, in a membrane of an electrochemical device such as a fuel cell or an electrolyzer.

[0002] To form ionomers which are polymers which carry ionic groups, it is known to modify with ionic groups block polymers consisting of rigid blocks such as polystyrenes and soft blocks such as hydrogenated homopolymers of 1,3-diene or hydrogenated copolymers of 1,3-diene and styrene. The ionic groups are for example groups comprising a sulfonate function or a quaternary amine. Reference may for example be made to patent applications WO9532236 and WO2019010290 which describe the synthesis of these ionomers based on block polymers and their use in the form of a film in ion-conducting membranes in electrochemical devices such as fuel cells and electrolysers. The ionomer film can be prepared by a coating process from an ionomer solution.One of the major challenges of the coating process is to prepare a film with a constant thickness throughout its extent and a homogeneous appearance.

[0003] The Applicant has developed a new process for coating these ionomers which allows the manufacture of ionomer films meeting these requirements of uniformity of thickness and appearance.

[0004] Thus the invention relates to a method for manufacturing a film having a thickness H less than or equal to 200 μm comprising an ionomer which is a block polymer of formula (I) A-(BA)nBA (I) in which: A represents a block of a poly(vinylaromatic) bearing pendant ionic groups, B represents a block of a hydrogenated poly(1,3-diene) or of a hydrogenated copolymer of a 1,3-diene and a vinyl aromatic, the vinylaromatic being a vinylaromatic monomer of formula ArCH=CH2 or ArC(Me)=CH2, the symbol Ar representing an aromatic hydrocarbon group, n is an integer greater than or equal to 0, which process comprises the following successive steps: a) preparing a solution of the ionomer in a solvent, b) pouring the solution onto a support to form a layer of solution with a thickness E less than or equal to 200 pm, c) drying the layer, d) pouring the solution again onto the surface of the previously dried layer to form a new layer of solution, knowing that the cumulative thickness of the new layer of solution and the previously dried layer is less than or equal to 200 pm, e) dry the new layer, f) repeating the sequence formed by steps d) and e) until a film of thickness H is obtained, knowing that the cumulative thickness of the new layer of solution and the previously dried layers is less than or equal to 200 pm, g) optionally peeling the film from the support. Detailed description of the invention

[0005] The polymers mentioned in the description may be of fossil or bio-sourced origin. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. In the same way, they may also come from the recycling of materials already used, that is to say they may be, partially or totally, derived from a recycling process, or obtained from raw materials themselves derived from a recycling process.

[0006] The ionomer useful for the purposes of the invention is a block polymer of formula (I) A-(BA)nBA (I) in which: A represents a block of a poly(vinylaromatic) bearing pendant ionic groups, B represents a block of a hydrogenated poly(1,3-diene) or of a hydrogenated copolymer of a 1,3-diene and a vinyl aromatic, vinylaromatic being a vinylaromatic monomer of formula ArCH=CH2 or ArC(Me)=CH2, the symbol Ar representing an aromatic hydrocarbon group, n is an integer greater than or equal to 0.

[0007] Poly(vinylaromatic) is understood to mean a homopolymer of vinylaromatic monomer. The term vinylaromatic monomer designates either a single vinylaromatic monomer or a mixture of vinylaromatic monomers which differ from one another by their chemical structure. Preferably, the term vinylaromatic monomer designates a single vinylaromatic monomer.

[0008] According to one embodiment of the invention, the poly(vinylaromatic) is a polystyrene or a copolymer of vinylaromatic monomers, one of which is styrene. preferably, the poly(vinylaromatic) is a polystyrene.

[0009] The essential characteristic of poly(vinylaromatic) is that it carries pendant ionic groups, which gives the block polymer useful for the purposes of the invention its name ionomer. The ionic group preferably comprises a sulfonate group or a quaternary amine group.

[0010] As is well known, the term poly(1,3-diene) denotes a homopolymer of a 1,3-diene. The 1,3-diene may be a single 1,3-diene monomer, in which case the monomer units constituting the poly(1,3-diene) are the monomer units of the 1,3-diene. Alternatively, the 1,3-diene may be a mixture of several 1,3-dienes which differ from one another in their chemical structure, in which case the monomer units constituting the poly(1,3-diene) are the units of each of the monomers constituting the mixture. Preferably, the poly(1,3-diene) is a homopolymer of a single 1,3-diene.

[0011] Any 1,3-diene containing 4 to 20 carbon atoms may be suitable as 1,3-diene. The 1,3-diene is preferably 1,3-butadiene or isoprene, more preferably 1,3-butadiene.

[0012] The hydrogenated copolymer of a 1,3-diene and a vinyl aromatic is preferably a hydrogenated copolymer of a 1,3-diene and a styrene, more preferably a hydrogenated copolymer of 1,3-butadiene and styrene.

[0013] Advantageously, in formula (I), the poly(vinylaromatic) is a polystyrene and B represents a block of a hydrogenated poly(1,3-butadiene) or of a hydrogenated copolymer of 1,3-butadiene and styrene, preferably a hydrogenated poly(1,3-butadiene).

[0014] According to a particular embodiment of the invention, n is equal to 0 or 1, preferably equal to 0.

[0015] The block polymer is preferably a triblock, more preferably a triblock in which the block of a poly(vinylaromatic) is a polystyrene and B represents a block of a hydrogenated poly(1,3-butadiene) or of a hydrogenated copolymer of 1,3-butadiene and styrene, even more preferably a triblock in which the block of a poly(vinylaromatic) is a polystyrene and B represents a block of a hydrogenated poly(1,3-butadiene).

[0016] The number-average molar mass of the block polymer before the introduction of the pendant ionic groups by modification of the poly(vinylaromatic) is not particularly limited, but generally it is preferably from 20,000 g / mol to 500,000 g / mol in polystyrene equivalent (size exclusion chromatography coupled with detection by a refractometer).

[0017] The mass proportion of poly(vinylaromatic) block in the block polymer before the introduction of the pendant ionic groups by modification of the poly(vinylaromatic) is not particularly limited, but generally it varies from 10% to 70% by mass of the mass of the block polymer before introduction of the pendant ionic groups, preferably from 10% to 45% by mass of the mass of the block polymer before introduction of the pendant ionic groups.

[0018] The amount of pendant ionic groups in the ionomer is adapted by a person skilled in the art depending on the use for which the film is intended. For example, for use in a membrane for an electrochemical device, the amount of pendant ionic groups in the ionomer preferably varies from 0.5 to 5 mmol per gram of ionomer.

[0019] The synthesis of the ionomer useful for the purposes of the invention is well known, for example from documents US 5239010, EP 1986257 and WO 2019010290. It consists of introducing the pendant ionic groups into a block polymer of formula (II) A'-(B'-A')n-B'-A' (II) in which: A' represents a block of a poly(vinylaromatic), B' represents a block of a hydrogenated poly(1,3-diene) or of a hydrogenated copolymer of a 1,3-diene and a vinylaromatic, the vinylaromatic being a vinylaromatic monomer of formula ArCH=CH2 or ArC(Me)=CH2, the symbol Ar representing an aromatic hydrocarbon group, n is an integer greater than or equal to 0.

[0020] The block polymer of formula (II) can be synthesized to order by well-known methods such as those described in patent application WO 03008467. Alternatively, the block polymer of formula (II) can be a commercially available product, for example from the company “Kraton” under the name “Kraton G” or from the company “Kuraray” under the trade name “SEPTON”.

[0021] Step a) of the process according to the invention makes it possible to prepare a solution of the ionomer capable of being spread on a flat surface of a support to form a homogeneous layer in step b). The concentration of the solution is adjusted by a person skilled in the art depending in particular on the viscosity of the ionomer. It generally ranges from 1 to 15% by mass of solid matter. The solvent is typically a good solvent for the ionomer. Suitable for this purpose are ethers, mixtures of ethers and alcohols and halogenated solvents. The solvent in step a) is preferably chosen from tetrahydrofuran, mixtures of tetrahydrofuran and an alcohol, chloroform, the alcohol preferably being ethanol.

[0022] At the end of step b), the cumulative thickness of the new layer of solution and the previously dried layer is less than or equal to 200 μm.

[0023] Step c) allows the formation on the support of a dry layer of ionomer which is of constant thickness and which, by transparency, has a uniform visual appearance over the entire surface of the film.

[0024] The sequence of steps d) and e), respectively the spreading of the solution to form a new layer of solution on the previously dried layer and the drying of the newly formed layer, leads to the formation of a new layer of ionomer which is superimposed on the previous one and which is also of constant thickness and which, by transparency, has a uniform visual appearance over the entire surface of the film.

[0025] Steps d) and e) are repeated successively one after the other until a film of the desired thickness is obtained. In the repetition of step d), the thickness of the new layer of solution spread on the previously dried layer is determined by the cumulative thicknesses of the new layer of solution and the previously dried layers, since the cumulative thicknesses of these must remain less than or equal to 200 μm.

[0026] The drying steps, in this case steps c) and e), are preferably carried out at a temperature ranging from room temperature (23°C) to the boiling point of the solvent reduced by 15°C under a stream of air or an inert gas. The thickness of each layer spread after drying is preferably less than or equal to 20 μm, more preferably less than or equal to 10 μm.

[0027] The film obtained which covers the support at the end of the repetition of the sequence formed by steps d) and e) has a constant thickness and a uniform visual appearance over the entire surface of the film. The film can be peeled off from the support, in particular for use in a membrane in an electrochemical device. The film preferably has a thickness H greater than or equal to 40 μm, more preferably a thickness H greater than or equal to 50 μm. The film has a thickness H preferably less than 200 μm, more preferably less than 100 μm, in particular for use in a membrane for a fuel cell or electrolyser.

[0028] The aforementioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of several exemplary embodiments of the invention, given for illustrative and non-limiting purposes. Examples

[0029] Preparation of the ionomer:

[0030] The ionomer is prepared from a block polymer, a polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) triblock “G1652” from Kraton. Table 1 gives the composition of the commercial SEBS used, Table 2 its macrostructure (number average molar mass Mn and dispersity Ip measured by size exclusion chromatographic analysis, PS calibration) and its glass transition temperature (Tg, measured by differential scanning calorimetry DSC according to the standard ASTM D3418 (1999)).

[0031] [Tables 1] Composition SEBS Styrene Butylene Ethylene % by mass 29.6 25.3 45.1 % by mole 12.1 19.5 68.4

[0032] [Tables2] Mn (g / mol) IP Tg (°C) 65,000 1.04 -53

[0033] 1st step: bromoalkylation of SEBS:

[0034] The reaction is carried out in a Steinie bottle; 22.2 g (62.87 mmol styrene) of SEBS are weighed and 460 mL of dichloromethane (DCM) are added. Stirring is carried out until the polymer is completely dissolved (4 h, room temperature RT), followed by bubbling the SEBS solution (3.6% by mass) with nitrogen in the cold for 10 min. The solution is cooled to 0°C in an ice bath for 30 min and 3.35 mL (37.95 mmol, 0.6 equivalent of styrene) of trifluoromethanesulfonic acid (TFSA) are added in a rapid dropwise manner at 0°C with stirring; the solution becomes orange-yellow. After the addition is complete, stirring is continued for 30 min at 0°C, then the solution is allowed to return to room temperature. 4.20 mL (31.42 mmol, 0.5 styrene equivalent) of 6-bromo-l-hexene is added dropwise using a syringe pump over approximately 50 min, the solution turns red. After the addition is complete, continue stirring for 1 h at room temperature, the solution turns dark red.5 mL of methanol is added as a reaction stopper, the medium becomes discolored. The acidity of the reaction medium is neutralized with a fractional addition of 40 mL of a solution of KOH in EtOH (IM) until the pH of the medium is 7 to 8. A little precipitate forms. Coagulate in MeOH by pouring the reaction medium into 3 times its volume of methanol, then wash the coagulate obtained which appears in the form of crumbs (in English . "crumbs") in demineralized water to remove the salt formed during neutralization at pH = 7-8. Dry in an oven for 24 hours at 50°C. Bromoalkylated SEBS is obtained in the form of white crumbs. 46 mol% of the styrene units were modified by the bromoalkylation reaction, i.e. 6 mol% of the total SEBS units.

[0035] 2nd step: quaternization reaction with methylpiperidine,

[0036] 15 g of bromoalkylated SEBS obtained in the first step are introduced into a steinie bottle and 460 mL of dichloromethane are added. After complete solubilization of the polymer, α-methylpiperidine (10 equivalents relative to the number of moles of Br atoms) is added. The reaction medium is stirred and heated at 40°C for 48 hours, the solution turns yellow. The reaction medium is poured into an aluminum tray covered with a polytetrafluoroethylene (PTFE) support, then the solvent is removed by evaporation. The crumbs obtained are washed in MeOH (until a colorless washing solution is obtained). They are dried in an oven (40°C, 24 hours): obtaining the ionomer in the form of slightly yellow crumbs.

[0037] Coating process: Five films F1 to F5 were prepared according to a process not in accordance with the invention using the ionomer synthesized according to the following procedure: a) a solution of the ionomer in a solvent is prepared, b) the solution is poured onto a flat polytetrafluoroethylene (PTFE) support framed by two shims 1500 pm high to form a layer of solution 1500 pm thick by passing a scraper over it, c) the solvent is evaporated at room temperature for 120 minutes under air sweeping to dry the layer.

[0038] For each of the films formed, Table 3 indicates the solvent used and the mass concentration of ionomer in the solution.

[0039] Table 3: Solvent Film Ionomer Concentration Fl THF / EtOH: 90 / 10 (by mass) 1% F2 THF / EtOH: 90 / 10 (by mass) 4% F3 THF / EtOH: 90 / 10 (by mass) 7% F4 THF / EtOH: 90 / 10 (by mass) 10% F5 CHC13 7%

[0040] Two films F6 and F7 were prepared according to a process in accordance with the invention using the ionomer synthesized by following the procedure described below: a) a solution of ionomer in a solvent is prepared, b) the solution is poured onto a flat PTFE support framed by two shims 200 pm high to form a layer of solution 200 pm thick by passing a scraper c) the solvent is evaporated at room temperature for 15 minutes under air flow to dry the layer, d) the solution is poured again onto the surface of the dried layer to form a new layer of solution so that its cumulative thickness with that of the previously dried layer is equal to 200 pm, e) the new layer is dried under the same conditions as in step c), f) the sequence formed by steps d) and e) is repeated until a film with a thickness of 50 pm is obtained.

[0041] For each of the films formed F6 and F7, table 4 indicates the solvent used, the mass concentration of ionomer in the solution, the number of times that the sequence formed from steps d) and e) was repeated (called the number of passes).

[0042] [Tables4] Solvent Film Ionomer Concentration Number of Passes F6 THF / EtOH: 90 / 10 (by mass) 7% 5 F7 CHC13 4.7% 7

[0043] The surface of the films F1 to F5 which is opposite the surface of the film in contact with the support has a visual appearance which is not homogeneous and the thickness of the films F1 to F5 is not constant over the entire surface of the film.

[0044] The surface of the films F6 and F7 which is opposite the surface of the film in contact with the support has a visual appearance which is homogeneous and the thickness of the films F6 to F7 is constant over the entire surface of the film.

[0045] Similarly, a polystyrene-poly(ethylene-butylene-styrene)-polystyrene block polymer modified with 12 mol% sulfonate function can be used as ionomer. Films prepared according to the compliant process are homogeneous and of constant thickness, unlike films prepared according to the non-compliant process. described previously.

[0046] A key point of the method of the invention is that it makes it possible to obtain a homogeneous film with a thickness greater than 50 μm. The method not in accordance with the invention makes it possible to obtain homogeneous films as long as their thickness remains less than 50 μm. However, for an application of the film in a fuel cell membrane, it can be very advantageous to be able to increase the thickness of the film to reduce the permeability of the membrane to gases without affecting the homogeneity of the film.

Claims

Claims

1. A method of manufacturing a film having a thickness H less than or equal to 200 pm comprising an ionomer which is a block polymer of formula (I) A-(BA)nBA (I) in which: A represents a block of a poly(vinylaromatic) bearing pendant ionic groups, B represents a block of a hydrogenated poly(1,3-diene) or of a hydrogenated copolymer of a 1,3-diene and a vinylaromatic, the vinylaromatic being a vinylaromatic monomer of formula ArCH=CH2 or ArC(Me)=CH2, the symbol Ar representing an aromatic hydrocarbon group, n is an integer greater than or equal to 0, which method comprises the following successive steps: a) preparing a solution of the ionomer in a solvent, b) casting the solution onto a support to form a layer of solution with a thickness E less than 200 pm, c) drying the layer, d) casting reapply the solution onto the surface of the previously dried layer to form a new layer of solution,knowing that the cumulative thickness of the new layer of solution and the previously dried layer is less than or equal to 200 pm, e) drying the new layer, f) repeating the sequence formed by steps d) and e) until a film of thickness H is obtained, knowing that the cumulative thickness of the new layer of solution and the previously dried layers is less than or equal to 200 pm, g) optionally peeling the film from the support.,

2. The method of claim 1 wherein the poly(vinylaromatic) is polystyrene or a copolymer of vinylaromatic monomers one of which is styrene.

3. A method according to any one of claims 1 to 2 wherein the hydrogenated copolymer of a 1,3-diene and a vinyl aromatic is a hydrogenated copolymer of a 1,3-diene and styrene.

4. A process according to any one of claims 1 to 3 wherein the 1,3-diene is 1,3-butadiene or isoprene.

5. A method according to any one of claims 1 to 4 wherein the ionic group comprises a sulfonate group or a quaternary amine group.

6. A method according to any one of claims 1 to 5 wherein n is 0 or 1.

7. A method according to any one of claims 1 to 6 wherein n is equal to 0.

8. A method according to any one of claims 1 to 7 wherein the amount of pendant ionic groups in the ionomer varies from 0.5 to 5 mmol per gram of ionomer.

9. Method according to any one of claims 1 to 8 in which the thickness H of the film is greater than or equal to 40 pm, preferably greater than or equal to 50 pm.