Fluorinated membranes

0001The present invention relates to membranes comprising semicrystalline or amorphous ionomeric (per)fluorinated polymers, usable in electrochemical applications, for example fuel cells, electrochemical cells or electrolyzers, for example for hydrochloric acid.

0002Specifically, the invention relates to ionomer membranes having also very thin thickness, up to 5 <i>µ</i>m and having an improved solvation degree, good mechanical properties under the use conditions, combined with an improved conductivity.

0003It is known in the prior art to use ionomeric polymers, for example perfluorosulphonic and/or perfluorocarboxylic in electrochemical applications, as in fuel cells, electrochemical cells for example chloro-soda cells, electrolyzers for HCl, energy accumulation systems using fuel cells (regenerative fuel cell technology), lithium batteries, electrodialysis. In these applications the ionomer is in contact with a liquid, having affinity with the ionic functional groups of the ionomer, selected from aqueous or organic polar solvents. The membranes used according to the prior art are generally obtained by melt extrusion.

0004It is known in the prior art that in electrochemical applications it is desirable to have membranes having the following combination of properties: good ionic conductivity and good mechanical properties. Furthermore it is known to use ionomeric membranes having very thin thickness, since by decreasing the thickness, the resistance to the membrane ionic transport decreases. These membranes with a reduced thickness better resist to the critical dehydration conditions, taking place for example in fuel cells when the cell works at high temperature, for example at a temperature higher than 100°C. Another factor affecting the ionic transport is the water retention by the membrane. In fact it is known that, by increasing the hydration degree, the conductivity increases being equal the amount of ionic groups of the membrane.

0005To improve the membrane mechanical properties it is known to use high equivalent weight (EW) ionomers. However these polymers have the drawback to have an unsatisfactory hydration, and therefore their ionic conductivity is not high.

0006Summing up, the ionomeric membranes described in the prior art do not show the desired combination of good mechanical properties and good ionic conductivity.

0007It is therefore desirable to have available membranes having, also when used at high temperatures, for example at temperatures higher than 100°C, e.g. 120°-150°C, the following combination of properties: <ul id="ul0001" list-style="dash" compact="compact"><li>good mechanical properties;</li><li>improved ionic conductivity.</li></ul>

0008The Applicant has surprisingly and unexpectedly found ionomeric membranes solving the above technical problem.

0009It is an object of the invention ionomeric membranes comprising (per)fluorinated, semicrystalline or amorphous, ionomeric polymers, having equivalent weight (EW) from 380 g/eq to 1,800 g/eq, preferably from 450 to 1,650 g/eq, which when used in fuel cell, under the following conditions: <ul id="ul0002" list-style="dash" compact="compact"><li>membrane thickness 50 <i>µ</i>m assembled between two electrodes catalyzed with 0.6 mg/cm² Pt supported on Carbon (Pt/C) and treated with 0.7 mg/cm² of Nafion® , having 10 cm² area; hydrogen and air feeding, both at the pressure of 0.25 MPa, both saturated with water at 80°C; cell temperature 75°C;</li></ul> for membranes formed of copolymers TFE/F₂C=CF-O-(CF₂)₂-SO₂F, give the following maximum specific power values P_MAX (maximum power for electrode surface unit), at the indicated EW values: <ul id="ul0003" list-style="none" compact="compact"><li>at EW = 670   P_MAX higher than 0.55 Watt/cm²;</li><li>at EW = 830   P_MAX higher than 0.66 Watt/cm²;</li><li>at EW = 1,160   P_MAX higher than 0.50 Watt/cm²;</li><li>at EW = 1,600   P_MAX higher than 0.32 Watt/cm².</li></ul>

0010The electrodes are formed of a carbon cloth having a thickness of 350 <i>µ</i>m and weight for surface unit of 116 g/cm², said carbon cloth being treated on one side as described hereinafter: <ul id="ul0004" list-style="dash" compact="compact"><li>a first treatment is carried out with a PTFE/carbon mixture so as to make the sheet surface hydrophobic;</li><li>subsequently, one side of the surface is catalyzed with Pt supported on carbon powder having surface area of 250 m²/g, the Pt concentration being 30% by weight with respect to the carbon powder and the Pt surface concentration being 0.6 mg/cm²;</li><li>the catalyzed side is treated with the ionomeric polymer having the following structure: <chemistry id="chem0001" num="0001"><img file="EP1589062A2_D0001.tif" /></chemistry> having equivalent weight 1,100 eq/g and a" and b" being such to give the required equivalent weight; so as to have a surface concentration of said polymer of 0.7 g/cm². The ionomeric polymer used in this step is commercially known with the trademark Nafion®.</li></ul>

0011Electrodes with these characteristics are marketed with the trademark ELAT® (E-TEK, Inc.).

0012The membranes according to the present invention contain ionomers, in particular (per)fluorinated ionomers. More preferably sulphonic ionomers are used having an equivalent weight from 380 to 1,800 g/eq, comprising: <ul id="ul0005" list-style="none" compact="compact"><li>(A) monomeric units deriving from one or more fluorinated monomers containing at least one ethylene unsaturation;</li><li>(B) fluorinated monomeric units containing sulphonyl groups -SO₂F in amount to give an equivalent weight in the above range.</li></ul>

0013The fluorinated monomers of type (A) are selected from: <ul id="ul0006" list-style="dash" compact="compact"><li>vinylidene fluoride (VDF);</li><li>C₂-C₈ perfluoroolefins, preferably tetrafluoroethylene (TFE);</li><li>C₂-C₈ chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;</li><li>CF₂=CFOR_f1 (per) fluoroalkylvinylethers (PAVE), wherein R_f1 is a C₁-C₆ (per)fluoroalkyl, for example trifluoromethyl, bromodifluoromethyl, pentafluoropropyl;</li><li>CF₂=CFOX perfluoro-oxyalkylvinylethers, wherein X is a C₁-C₁₂ perfluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.</li></ul> The fluorinated monomers of type (B) are selected from one or more of the following: <ul id="ul0007" list-style="dash" compact="compact"><li>         F₂C=CF-O-CF₂-CF₂-SO₂F; </li><li>         F₂C=CF-O- [CF₂-CX_AF-O] _nA- (CF₂) _nB-SO₂F wherein X_A= Cl, F or CF₃; nA = 1-10, nB = 2, 3;</li><li>         F₂C=CF-O-CF₂-CF₂-CF₂-SO₂F; </li><li>         F₂C=CF-Ar-SO₂F wherein Ar is an aryl ring.</li></ul>

0014Optionally the invention sulphonic fluorinated ionomers can contain from 0.01% to 2% by moles of monomeric units deriving from a bis-olefin of formula:         R₁R₂ C = CH - (CF₂)_m- CH = CR₅R₆     (I) wherein: <ul id="ul0008" list-style="none" compact="compact"><li>m = 2-10, preferably 4-8;</li><li>R₁, R₂, R₅, R₆, equal to or different from each other, are H or C₁-C₅ alkyl groups.</li></ul>

0015The introduction as comonomer of the bis-olefin of formula (I), having a number of unsaturations higher than the unit, is advantageous since said comonomer has the function to pre-crosslink the ionomer during the polymerization. The bis-olefin introduction has also the advantage to increase the length of the primary chains forming the final reticule.

0016Other ionomers which can be used are those containing groups which in the activation phase (hydrolysis) are transformed into -COOH groups. Ionomers comprising -COOH and -SO₃H groups, after hydrolysis, can also be used.

0017The fluorinated monomers (B) used to prepare the ionomers containing -COOH groups have the following structures: <ul id="ul0009" list-style="dash" compact="compact"><li>         F₂C=CF-O-CF₂-CF₂-Y; </li><li>         F₂C=CF-O- [CF₂-CX_AF-O] _nA- (CF₂) _nB-Y wherein X_A = Cl, F o CF₃; nA = 1-10; nB = 2, 3;</li><li>         F₂C=CF-O-CF₂-CF₂-CF₂-Y </li><li>         F₂C=CF-Ar-Y wherein Ar is an aryl ring;</li></ul> wherein Y is a precursor group of the carboxylic group, selected from the following: CN, COF, COOH, COOR_B, COOM, CONR_2BR_3B, wherein R_B is C₁-C₁₀, preferably C₁-C₃ alkyl and R_2B and R_3B, equal or different, are H or have the R_B meaning.

0018As said, the fluorinated monomers (B) with terminal Y having the above formulas can be in admixture with the fluorinated monomers containing sulphonyl groups -SO₂F, the total amount of monomers (B) being such to give the equivalent weight as above indicated.

0019Preferably the membranes of the present invention contain sulphonic perfluorinated ionomers comprising: <ul id="ul0010" list-style="dash" compact="compact"><li>monomeric units deriving from TFE;</li><li>monomeric units deriving from CF₂=CF-O-CF₂CF₂SO₂F.</li></ul>

0020The ionomers, when they are amorphous (per)fluorinated polymers, can be crosslinked. To carry out the crosslinking, the ionomer is mixed with crosslinking agents.

0021The sulphonic fluorinated ionomers are crosslinked for example by peroxidic route. In this case they must contain radical attack sites in the backbone and/or in end position in the macromolecules, for example iodine and/or bromine atoms. Preferably the crosslinkable fluorinated sulphonic ionomers comprise: <ul id="ul0011" list-style="dash" compact="compact"><li>monomeric units deriving from TFE;</li><li>monomeric units deriving from CF₂=CF-O-CF₂CF₂SO₂F;</li><li>monomeric units deriving from the bis-olefin of formula (I) ;</li><li>iodine atoms in terminal position.</li></ul>

0022The introduction in the chain of said iodine and/or bromine atoms, can be carried out by addition, in the reaction mixture, of brominated and/or iodinated "cure-site" comonomers, as bromo and/or iodo olefins having from 2 to 10 carbon atoms, as described for example in USP 4,035,565 and USP 4,694,045, or iodo and/or bromo fluoro-alkylvinylethers, as described in the patents US 4,745,165, US 4,564,662 and EP 199,138, in such amounts that the "cure-site" comonomer content in the final product is generally between 0.05 and 2 moles per 100 moles of the other basic monomeric units.

0023Alternatively or also in combination with "cure-site" comonomers, the introduction of iodine and/or bromine terminal atoms can be carried out by addition to the reaction mixture of iodinated and/or brominated chain transfer agents as, for example, the compounds of formula R_f1(I)_x(Br)_y, wherein R_f1 is a (per)fluoroalkyl or a (per)fluorochloroalkyl having from 1 to 8 carbon atoms, while x and y are integers between 0 and 2, with 1 s x+y s 2 (see for example the patents US 4,243,770 and US 4,943,622). It is also possible to use as chain transfer agents alkaline or alkaline-earth metal iodides and/or bromides, according to what described in the patent US 5,173,553.

0024Preferably the crosslinking of radical type uses ionomers containing units of the bis-olefin of formula (I) and iodine in terminal position.

0025The sulphonic ionomer is crosslinked by radical way at a temperature between 100°C and 200°C, in function of the type of peroxide used, adding a peroxide capable to generate radicals by heating. Generally, the peroxide amount is comprised between 0.1% and 5% by weight with respect to the polymer. Among them, the following can be mentioned: dialkylperoxides, as for example di-terbutyl-peroxide and 2,5-dimethyl-2,5-di-(terbutylperoxy)hexane; dicumyl peroxide; dibenzoyl peroxide; diterbutyl perbenzoate; di-1,3-dimethyl-3-(terbutyl-peroxy)butylcarbonate. Other peroxidic systems are described, for example, in patent EP 136,596 and EP 410,351.

0026Furthermore to the ionomer mixture with the crosslinking agents the following components can optionally be added: <ul id="ul0012" list-style="dash" compact="compact"><li>a crosslinking co-agent, in an amount between 0.5 and 10%, preferably between 1 and 7% by weight with respect to the polymer; among crosslinking co-agents it can be mentioned: triallyl-cyanurate; triallyl-isocyanurate (TAIC); tris(diallylamine)-s-triazine; triallylphosphite; N,N-diallyl-acrylamide;N,N,N',N'-tetraallyl-malonamide; trivinyl-isocyanurate;2,4,6-trivinyl-methyltrisiloxane; N,N'bisallylbicyclo-oct-7-ene-disuccinimide (BOSA); bis olefin of formula (I), triazine;</li><li>a metal compound, in amounts between 1% and 15%, preferably between 2% and 10% by weight with respect to the polymer, said metal compound selected from divalent metal oxides or hydroxides as, for example, Mg, Zn, Ca or Pb, optionally combined with a weak acid salt as, for example, stearates, benzoates, carbonates, oxalates or Ba, Na, K, Pb, Ca phosphites;</li><li>conventional additives as thickeners, pigments, antioxidants, stabilizers and the like;</li><li>inorganic or polymeric reinforcing fillers, preferably optionally fibrillable PTFE. Preferably fillers have a particle size from 10 to 100 nm, preferably from 10 to 60 nm.</li></ul>

0027The ionomer can optionally be mixed with another fluoropolymer. For example crystalline fluoropolymers as PTFE, optionally modified with a comonomer as HFP (hexafluoropropene), VE (vinylethers), for example MFA, PFA, FEP, opzionally modified with VE; PVDF, ECTFE, ETFE, PCTFE, can be mentioned.

0028Fluoroelastomers, preferably perfluoroelastomers, co-curable with the ionomer, can also be used. Preferably for the co-curing the fluoroelastomer contains iodine and/or bromine atoms. The TFE/perfluoromethylvinylether copolymer having a ratio by moles between the two monomers between 80/20 and 60/40 can for example be mentioned. Said copolymer is for example described in EP 661,304 and is used in an amount between 0 and 50% by weight with respect to the ionomer.

0029The ionomer and fluoroelastomer mixture can be, for example, a physical blend of solid polymers or of polymerization latexes. In this case the peroxide percentages to be used are to be referred to the mixture formed by the ionomer and fluoroelastomer. Also for the optional agents added, their respective percentages by weight are referred to said mixture.

0030The crosslinking blend is prepared, for example, by using mechanical mixers.

0031When in the crosslinking phase iodine has been used, its residual traces can be removed from the membrane by thermal post-treatment, at temperatures preferably between 200°C and 250°C.

0032The ionomer preparation can be carried out by a radical polymerization process in bulk, suspension, emulsion.

0033The aqueous emulsion or microemulsion polymerization can, for example, be mentioned. The surfactants usable in these polymerizations are (per)fluorinated surfactants, for example salts (as defined below) of the perfluorooctanoic, p-erfluorononanoic, perfluorodecanoic acid, or their mixtures, etc., (per)fluoropolyethers with an acid end group (example COOH, SO₃H), salified with NH₄⁺ or with alkaline metal cations, the other end group being (per)fluorinated, optionally containing one H or Cl atom. The number average molecular weights of the perfluoropolyether surfactants are generally in the range 300-1,800, preferably 350-750.

0034The microemulsion polymerization is well known in the prior art.

0035In particular the ionomer preparation is carried out by using an aqueous emulsion wherein in the reaction medium as surfactants those of formula:         R_f-X₁^-M⁺ are used, wherein: <ul id="ul0013" list-style="none" compact="compact"><li>X₁ is equal to -COO, -SO₃;</li><li>M is selected from H, NH₄ or an alkaline metal; <ul id="ul0014" list-style="dash" compact="compact"><li>R_f represents a (per)fluoropolyether chain, preferably having number average molecular weight between about 230 and about 1,800, preferably between 300 and 750, said (per)fluoropolyether chain comprising repeating units selected from one or more of the following: <ul id="ul0015" list-style="none" compact="compact"><li>a) - (C)F₆O)-;</li><li>b) - -(CF₂CF₂O)-;</li><li>c) - (CFL₀O) -, wherein L₀ = -F, -CF₃;</li><li>d) -CF₂(CF₂)_z,CF₂O-, wherein z' is an integer 1 or 2;</li><li>e) -CH₂CF₂CF₂O-.</li></ul></li></ul></li></ul>

0036R_f is monofunctional and has a (per)fluorooxyalkyl terminal T, for example CF₃O-, C₂F₅O-, C₃F₇O-; optionally in the perfluoroalkyl terminals one fluorine atom can be substituted with one chlorine or hydrogen atom. Examples of said terminals are Cl(C₃F₆O)-, H(C₃F₆O)-. The unit a) C₃F₆O is -CF₂-CF (CF₃) O- or - CF (CF₃) CF₂O^-.

0037In the above indicated formula R_f preferably has one of the following structures: <ul id="ul0016" list-style="none" compact="compact"><li>1)         T- (CF₂O)_a- (CF₂CF₂O) _b-CF₂- with b/a between 0.3 and 10, extremes included, a being an integer different from 0;</li><li>2)         T- (CF₂ - (CF₂)_z' -CF₂O)_b'-CF₂- wherein z' is an integer equal to 1 or 2;</li><li>3)         T- (C₃F₆O)_r- (C₂F₄O)_b - (CFL₀O)_t -CF₂- with r/b = 0.5-2.0 (r+b)/t = 10-30, b and t being integers different from 0 when all the units with r, b, and t indexes are present; or b = t = 0, or b = 0;</li></ul> a, b, b', r, t are integers, the sum of which is such that R_f has the above values of number average molecular weight.

0038The compounds wherein R_f has the following formula:         T- (CF₂CF (CF₃) O)_m(CF₂O)_n-CF₂- are still more preferred, wherein m/n = 1-30; wherein T = -OCF₃ or -OCF₂Cl.

0039The (per)fluoropolyethers R_f are obtainable with the well known processes of the prior art, see for example the following patents herein incorporated by reference: US 3,665,041, US 2,242,218, US 3,715,378 and the European patent EP 239,123. The functionalized fluoropolyethers with hydroxyl termination are obtained for example according to patents EP 148,482, US 3.810.874, from which the functional end groups are obtained with the processes mentioned in said patents.

0040It is also possible to use in the polymerization chain transfer agents. For example alkaline or alkaline-earth metal iodides and/or bromides, according to patent US 5,173,553. Preferably chain transfer agents containing hydrogen, such as hydrocarbons, alcohols, in particular ethyl acetate and ethane, I are used.

0041The polymerization initiators used in the process of the present invention are preferably radical inorganic initiators, as for exmple ammonium and/or potassium and/or sodium persulphate, optionally in combination with ferrous, cupreous or silver salts. The modalities of the initiator feeding to the polymerization reactor can be continuous or by a single addition at the polymerization starting.

0042The polymerization reaction is generally carried out at temperatures between 25°C and 70°C, preferably between 50° and 60°C, under pressure up to 30 bar (3 MPa), preferably higher than 8 bar (0.8 MPa).

0043The monomer (B) is fed to the polymerization reactor in a continuous way or by steps.

0044When the polymerization is completed, the ionomer is isolated by conventional methods, as the coagulation by addition of electrolytes or by freezing.

0045The membranes of the invwention are obtainable through the process described hereinbelow.

0046A further object of the present invention is a process to obtain the (per) fluorinated ionomeric membranes, comprising the following steps: <ul id="ul0017" list-style="none" compact="compact"><li>a) loading of the semicrystalline or amorphous (per)fluorinated polymer, solid or liquid, in a mould comprising (Fig. 1 and Fig. 1a): <ul id="ul0018" list-style="dash" compact="compact"><li>a hollow cylinder (1), equipped with thermocouple, not reported in the Figure, to measure the temperature on the internal surface of the cylinder (1) ;</li><li>a cylinder (2) (chuck) having a diameter lower than that of the cylinder (1), equiaxial with cylinder (1), equipped with thermocouple, not reported in the Figure, to measure the temperature on the external surface of the cylinder (2);</li><li>two rings (3) and (4), (Fig. 1) perpendicular to the axis of the cylinders closing the mould; at least one of said rings (3) and (4) being sliding along the chuck (2) to convey inside the cylinder the pressure applied on their surfaces;</li></ul> said mould being equipped with heating means to heat the mould external surfaces;</li><li>b) degassing the mould containing the polymer, with the following steps: <ul id="ul0019" list-style="none" compact="compact"><li>b') when the loaded polymer is solid, heating up to the polymer is in the liquid form, exerting inside the mould a pressure not lower than 0.5 MPa, preferably not lower than 1 MPa; generally by operating at pressures not higher than 50 MPa, preferably not higher than 30 MPa;</li><li>b") when the loaded polymer is in the liquid state in the mould, the heating step is omitted and the pressures indicated in b') are used;</li></ul> the step b) being ended when the difference of temperature between the internal surface of the cylinder (1) and the external surface of the cylinder (2) is lower than 10°C;</li><li>c) cooling of the mould until a temperature lower than 30°C, preferably to a temperature between 15°C and 25°C, by operating at a pressure in the range 1 MPa-10 MPa; after the solidification of the polymer, extraction from the mould of the polymer manufactured article (sleeve);</li><li>d) obtainment of the membrane by sleeve exfoliation, carried out by rotating the sleeve on its longitudinal axis at a speed from 1 rpm to 500 rpm, putting the sleeve surface into contact with a blade, parallel to the cylinder axis, to obtain a film with a constant thickness.</li></ul>

0047The pressure requested in steps b) and c) is applied to the rings (3) and/or (4) of the mould.

0048As said, in step b) the degassing of the mould containing the polymer takes place through the interstices comprised beetween the cylinders (1) and (2) and the rings (3) and (4).

0049Preferably in b') a heating gradient from 0.5°C/min to 10°C/min is used.

0050Preferably b') is carried out with the following steps: <ul id="ul0020" list-style="none" compact="compact"><li>S₁) polymer heating from room temperature to a temperature T₁ ranging from 50°C to 130°C, with a heating rate v₁ ranging from 0.5°C/min to 10°C/min, by operating at a pressure P₁ preferably ranging from 10 MPa to 30 MPa;</li><li>S₂) residence time at temperature T₁ and pressure P₁ until the difference of temperature between the internal wall of the cylinder (1) and the external wall of the cylinder (2) (ΔT) is lower than 20°C;</li><li>S₃) polymer heating up to a temperature T₂ higher than T₁, T₂ ranging from 100°C to 220°C, with a heating rate v₂ ranging from 0.5°C/min to 10°C/min, by operating at a pressure P₂ between 7 MPa and 12 MPa;</li><li>S₄) residence time at temperature T₂ and pressure P₂ until the difference of temperature ΔT is lower than 20°C.</li><li>S₅) polymer heating at a temperature T₃ higher than T₂, wherein T₃ ranges from 150°C to 300°C, with a heating rate v₃ ranging from 0.5°C/min to 10°C/min, by operating at a pressure P₃ ranging from 1 MPa to 7 MPa;</li><li>S₆) residence time at temperature T₃ and pressure P₃ until ΔT in the mould containing the liquid polymer is lower than 10°C.</li></ul>

0051Preferably in c) one operates with a cooling gradient ranging from 0.1°C/min to 10°C/min.

0052Optionally step c) can be carried out with the following steps: <ul id="ul0021" list-style="none" compact="compact"><li>S₇) polymer cooling in the mould at temperature T₄ lower than the temperature T₃, wherein T₄ ranges from 70°C to 200°C, the cooling rate v₄ ranging from 0.1°C/min to 1°C/min; by operating at a pressure P₄ from 1 MPa to 7 MPa;</li><li>S₈) residence time at temperature T₄ and pressure P₄ until ΔT is lower than 20°C;</li><li>S₉) cooling from T₄ to a temperature lower than 30°C, preferably to a temperature ranging from 15°C to 25°C, with a cooling rate v₅ from 0.5°C/min to 10°C/min; by operating at a pressure P₅ from 1 MPa to 7 MPa.</li></ul>

0053When a semicrystalline solid ionomer is used, in powder or in pellets, preferably in powder, it is obtained by coagulation and drying of the polymerization latex.

0054With semicrystalline ionomer it is meant a polymer having at least one melting endothermy at DSC (differential scanning calorimetry).

0055When an amorphous ionomer is used (i.e. a polymer not showing a melting endothermy), from the latex coagulation process a compact polymer is obtained; in this case the ionomer can be loaded into the mould in pieces, or the polymer can be heated to become liquid and then to fill the mould.

0056Optionally the coagulated polymer can be milled. When the amorphous ionomers are crosslinkable, step b) can optionally be carried out in the presence of crosslinking agents, to obtain a cured polymer. In this case the crosslinking agents are premixed with the polymer before step a).

0057When the ionomer is a powder, the premixing can, for example, be carried out in an open mixer or in a jar-turner. When the ionomer is a compact mass, an open mixer as those used in the rubber processing, is used.

0058The sleeve obtained after step c) can have, for example, the following sizes: <ul id="ul0022" list-style="dash" compact="compact"><li>OD, as defined in Fig. 1, from 60 mm to 1000 mm;</li><li>ID, as defined in Fig. 1, from 20 mm to 900 mm;</li><li>sleeve height from 100 mm to 1,500 mm.</li></ul>

0059The Applicant has found that for a sleeve having the following sizes: ID = 80 mm, OD = 120 mm, heigth = 50 mm, the preferred cycles which can be used in steps b') and c) are the following: <ul id="ul0023" list-style="none" compact="compact"><li>b') <ul id="ul0024" list-style="none" compact="compact"><li>S₁ heating from T = 20°C to T₁ = 120°C with rate v₁ = 3.3°C/min and pressure P₁ = 20 MPa;</li><li>S₂ residence time at T₁ for 120 min; pressure = 20 MPa;</li><li>S₃ heating from T₁ to T₂ = 190°C with rate v₂ = 2°C/min and pressure P₂ = 10 MPa;</li><li>S₄ residence time at T₂ for 90 min; pressure = 10 MPa;</li><li>S₅ heating from T₂ to T₃ ranging from 210°C to 260°C with a rate V₃ = 2.3°C/min and pressure P₃ = 5 MPa;</li><li>S₆ residence time at T₃ for 180 min; pressure P₃ = 5 MPa;</li></ul></li><li>c) cooling at T₄ = 20°C at a rate v₄ = 0.5°C/min and pressure P₃ = 5 MPa.</li></ul>

0060The membranes according to the above process are obtained by exfoliation of a polymeric ionomer sleeve, obtained in the mould wherein the ionomer is initially loaded in melted form and then solidified.

0061With the process of the present invention continuous exfoliated membranes can be obtained, with productivity up to 30 m/min, having a constant thickness.

0062The exfoliated membrane thickness is in the range from 5 <i>µ</i>m to 1,500 <i>µ</i>m, preferably from 20 <i>µ</i>m to 500 <i>µ</i>m.

0063The exfoliated membranes are subjected to a treatment to obtain the acid groups from the corresponding precursor functional groups. In this step the sulphonyl group -SO₂F is converted into sulphonic group -SO₃H and the precursor groups Y into carboxylic groups.

0064For example for the sulphonic ionomers said treatment can be carried out in 2 steps: <ul id="ul0025" list-style="dash" compact="compact"><li>hydrolysis with aqueous alkales and obtainment of the salified acid group; for example the -SO₂F group is converted into -SO₃K;</li><li>acidification to convert the salt into the corresponding acid; for example the -SO₃K group into -SO₃H.</li></ul>

0065For example the hydrolysis is carried out by immersing the membrane (film) into an aqueous solution containing 10% by weight of KOH, at a temperature between 60°C and 80°C, for a time not lower than 2 hours. The membrane is then transferred into a distilled water bath at room temperature to remove the residual alkalinity. The subsequent acidification step is carried out, for example, by immersing the membrane into an aqueous solution containing 20% by weight of HCl, by operating at room temperature for a time not lower than 30 minutes. At the end a washing with demineralized water is carried out.

0066The membranes of the present invention can optionally be reinforced. In this case the membranes, before the conversion of the functional precursor groups of the acid groups, are adhered by hot lamination to a reinforcement net, preferably a PTFE net. One generally operates in a range of temperatures comprised between the starting melt and the end melt of the ionomer.

0067The ionomeric membranes of the present invention can be used in fuel cell applications or in electrolyzers for HCl, or also in electrolyzers for chloro-soda process.

0068As said, the membranes obtainable with the process of the present invention have an improved hydration, improved ionic conductivity and, used in cell, show improved mechanical properties. Tests carried out by the Applicant have shown that membranes obtained by extrusion, even with a reduced thickness, have a lower hydration, and the mechanical and dimensional properties have different values in the planar X and in planar transversal direction Y. In particular the elongation at break value in X direction is very low. Consequently, when used in fuel cells, during the start and stop cycles of the cell, breaks can happen due to dimensional variations of the membrane giving asimmetric tensions in the polymeric structure. It has been found that the membranes obtained by extrusion are prone to break in X direction, in particular in correspondence of the gasket border the place wherein the greatest mechanical tensions develop.

0069The Applicant has found that the membranes of the invention are substantially isotropic in the planar direction X and Y as regards both the dimensional variations due to the membrane hydration and to the mechanical properties. The Applicant has furthermore found that the membranes of the invention have improved elongation at break, higher than 150% (ASTM D 1708), even with very thin thicknesses, in X and Y directions, compared with membranes having the same composition but obtained by extrusion.

0070As said, the membranes of the invention can also be obtained with very thin thickness. In this way it is possible to obtain lower ohmic drops in the electrochemical cell.

0071Unexpectedly and surprisingly the Applicant has found that the membranes obtained with the process of the present invention have an improved performance in cell in comparison with the membranes obtained by extrusion, since they show an improved maximum specific power.

0072Surprisingly and unexpectedly, the Applicant has found that the membranes of the present invention have an improved hydration in comparison with the membranes obtained by extrusion. See the comparative Examples.

0073An improved hydration allows to use ionomers having higher equivalent weights (EW), in comparison with those used in the membranes obtained by extrusion, obtaining the same conductivity. This is advantageous since it allows to obtain still improved mechanical properties, therefore a longer life of the membrane.

0074The membranes of the present invention can in particular be used in the car industry, wherein cells having a high power density obtained with very thin membranes are required.

0075The membranes of the invention, as said, in comparison with the membranes obtained by extrusion, show an improved reliability to thermal cycles during the use of the membranes.

0076In electrochemical applications, for example in the hydrochloric acid electrolysis, it is known to use ionomeric membranes having high sizes. According to the prior art, for this application membranes having a high width, even over the metre, and length higher than about 2 metres, and furthermore having high thickness, generally of the order of 100-500 <i>µ</i>m, preferably 100-300 <i>µ</i>m, can be used to guarantee suitable mechanical properties. It is also known to use supports for said membranes, as for example PTFE nets. In this application membranes of the present invention having reduced sizes compared with those of the prior art, can be used. In addition, the use of supports is optional with the membranes of the present invention.

0077This represents an advantage from the industrial point of view.

0078Further the membranes of the present invention can also be obtained with ionomers having even very low MFI values (ASTM D 1238-52T), for example equal to 0.6 g/10 min (280°C, 10 Kg) or lower, therefore having very high molecular weight. As known very low MFI ionomers are hardly processable by extrusion. Therefore from an industrial point of view it is not practically possible to obtain extruded membranes with said ionomers. Therefore the present invention makes available membranes of ionomers having a very high molecular weight and therefore with still improved mechanical properties.

0079The present invention will be better illustrated by the following embodiment Examples, having a merely indicative but not limitative purpose for the scope of the invention itself.

EXAMPLES

Characterization

Hydration percentage and determination of the membrane

dimensional variation

0080A membrane rectangular piece, cut so that one side is parallel to MD (machine direction) and the other to TD (transversal direction), is initially dried at 105°C under vacuum for 1 hour. The piece is weighed and its sizes are determined. Successively the membrane is hydrated in distilled water at 100°C for 30 minutes. After water removal, the water excess on the surface is removed and the membrane is weighed again and the sizes measured.

0081The hydration percentage (Hydr.%) of the membrane is evaluated according to the following formula:<maths id="math0001" num=""><math display="block"><mrow><mtext>Hydr.% = (final weight-initial weight)/initial weight x 100.</mtext></mrow></math><img file="EP1589062A2_D0002.tif" /></maths>

0082The dimensional variations in the two orthogonal directions are determined as per cent referred to the corresponding initial sizes obtained after drying at 105°C under vacuum for 1 hour.

Stress and elongation at break

0083These properties are determined at the temperature of 23°C and 50% of relative humidity, according to ASTM D 1708.

Melt Flow Index (MFI) determination

0084The detrmination is carried out according to the ASTM D 1238-52T standard.

Equivalent weight determination

0085A polymer film in acid form, obtained with the processes indicated in the Examples, is dried at 105°C up to constant weight and weighed; then the film is suspended in a hydroalcohlic, or aqueous, solution, an excess of a titrated NaOH solution is added and it is titrated back with a titrated HCl solution. The equivalent weight is determined from the ratio between the film weight, expressed in grams, and the number of titrated acid group equivalents.

Power determination in fuel cell

0086The membrane is assembled between two electrodes ELAT® (E-TEK, Inc.) catalyzed with 0.6 mg/cm² Pt/C and treated with 0.7 mg/cm² of Nafion®, having a 10 cm² area; the cell is fed from the anode side with hydrogen and from the cathode side with air. The pressure of the gas fed to the cell is equal and is 0.25 MPa. The cell temperature is maintained at 75°C and the feeding gases are previously saturated with water at 80°C.

0087By a load applied to the external circuit to the cell the current intensity is regulated (current for electrode surface unit) and the voltage is measured at the two cell poles. The operation is repeated by using various external loads. The voltage obtained at different current intensities in the cell is determined. The specific power (Watt/cm²) or power for electrode surface unit is determined by the product between the current intensity and the voltage.

EXAMPLE 1

Ionomer preparation

0088In a 22 litre autoclave the following reactants are introduced: <ul id="ul0026" list-style="dash" compact="compact"><li>11.5 litres of demineralized water;</li><li>980 g of the monomer of formula CF₂=CF-O-CF₂CF₂-SO₂F;</li><li>3,100 g of an aqueous solution at 5% by weight of a fluoropolyoxyalkylene with acid terminal having number average molecular weight 521 potassium salified, of formula:         CF₂ClO(CF₂CF(CF₃)O)_n(CF₂O)_mCF₂COOK wherein n/m = 10 ;</li></ul>

0089The autoclave, kept under stirring at 540 rpm, is heated to 60°C. Then 225 ml of an aqueous solution at concentration 6 g/l of potassium persulphate (KPS) are fed into the autoclave. The pressure is brought to absolute 1.3 MPa by introducing TFE. The reaction starts after 4 min. The pressure is maintained at absolute 1.3 MPa by feeding TFE. When 1,000 g of TFE have been fed to the reactor, 175 g of the sulphonyl monomer of formula CF₂=CF-O-CF₂-CF₂-SO₂F are introduced. Subsequently 175 g of the same sulphonic monomer are introduced in the reactor every 200 g of TFE fed. The total TFE mass fed to the reactor is 4,000 g.

0090The reaction is stopped after 233 min by interrupting the TFE feeding, cooling and venting the reactor under vacuum. The produced latex has a solid content of 28.5% by weight. The latex is coagulated by freezing and defreezing, the polymer separated from the mother liquors, washed with water up to a constant pH of the washing waters, and dried at 150°C for 40 h at room pressure.

0091The polymer has MFI = 2.5 g/10 min (280°C, 5 kg).

0092By DSC it is determined that the polymer is semicrystalline. The second melting enthalpy value is 5.2 J/g.

EXAMPLE 1A

Preform preparation and exfoliation

00932 kg of the polymer powder obtained in the Example 1 are fed into a stainless steel cylindrical mould having the following sizes: OD = 120 mm; ID = 80 mm; height = 340 mm. The two opposite cylinder faces are closed with two bronze rings having the following sizes: OD = 119.8 mm; ID = 80.2 mm; height = 20 mm.

0094The mould is placed in a vertical press and on the upper bronze ring a pressure of 20 MPa is applied to remove the most part of air and of the gases contained in the powder. After 20 minutes the applied pressure is decreased and the mould is wound with an electrical heater and put in vertical press with heated plates.

0095The mould is then subjected to the following thermal cycle: <ul id="ul0027" list-style="none" compact="compact"><li>S₁) heating from room temperature to 120°C with a heating rate of 3.3°C/min, constant pressure of 20 MPa;</li><li>S₂) residence time at 120°C and constant pressure of 20 MPa for 2 hours, so that the temperature difference between the internal wall of the cylinder (1) and the external wall of the cylinder (2) (ΔT) is lower than 20°C;</li><li>S₃) heating from 120°C to 190°C with heating rate of 2°C/min and pressure of 10 MPa;</li><li>S₄) residence time at 190°C and constant pressure of 10 MPa for 1.5 hours, so that the difference of temperature ΔT is lower than 20°C.</li><li>S₅) heating from 190°C to 250°C with rate of 2°C/min, pressure of 5 MPa;</li><li>S₆) residence time at 250°C, pressure of 5 MPa for 3 hours, so that ΔT is lower than 10°C;</li><li>c) cooling from 250°C to room temperature at a pressure of 5 MPa with rate of 0.3°C/min.</li></ul>

0096When the mould is cooled at room temperature, a sleeve is extracted having the following sizes: OD = 120 mm; ID = 80 mm; height = 150 mm.

0097The sleeve is let rotate along its central longitudinal axis at a rotation speed of 30 rpm and exfoliated on the surface, along the whole length, with a blade having a length at least equal to or higher than the sleeve heigth. The exfoliation is carried out at room temperature (20°C) so as to obtain thin films having the thickness as indicated hereunder. During the exfoliation the sleeve is let advance towards the blade so as to maintain the thickness of the exfoliated film unchanged. Films of various thickness, depending on the advancement rate of the sleeve towards the blade, were obtained. The thicknesses of the obtained films were, respectively, of 25 <i>µ</i>m, 50 <i>µ</i>m, 80 <i>µ</i>m, 100 <i>µ</i>m and 150 <i>µ</i>m.

EXAMPLE 1B

Preparation of the membrane in acid form and determination of the equivalent weight, per cent hydration and tensile properties of the membrane

0098From the film having a 50 <i>µ</i>m thickness prepared in the Example 1A, rectangular-shaped specimens were cut out. The specimens are treated at 80°C for 24 h with KOH at 10% by weight, followed by washing with demineralized water, subsequent treatment at room temperature for 24 h with HCl at 20% by weight and final washing in demineralized water. In this way the film sulphonyl groups are converted into acid sulphonic groups.

0099The copolymer equivalent weight is 830 g/eq, corresponding to a 84.6% molar composition of TFE and 15.4% molar of sulphonic monomer.

0100The hydration percentage is 65% and the dimensional increase, which is equal in both the directions (MD and TD), is 28%.

0101The film mechanical properties in MD and TD direction are the following (thickness 50 <i>µ</i>m): <ul id="ul0028" list-style="dash" compact="compact"><li>stress at break (MPa)   30</li><li>elongation at break (%)   178</li></ul>

EXAMPLE 1C

Measurements in fuel cell

0102The membrane obtained in the Example 1B having a 50 µm thickness is assembled between two electrodes ELAT® (E-TEK, Inc.) catalyzed with 0.6 mg/cm² Pt/C and treated with 0.7 mg/cm² of Nafion®, having 10 cm² area; the cell is fed with hydrogen and air both at the pressure of 0.25 MPa. The cell temperature is maintained at 75°C and the feeding gases are saturated with water at 80°C. It is found that under said conditions the cell supplies a current of 1.65 A/cm² at 0.4 V.

0103The maximum specific power supplied is of 0.70 W/cm² at 0.47 V.

EXAMPLE 1D (comparative)

0104The polymer of the Example 1 is subjected to extrusion at 245°C by Braebender extruder to obtain granules. Successively the granules are extruded at 250°C, obtaining a film having a 50 <i>µ</i>m thickness.

0105The film sulphonyl groups are hydrolyzed to acid groups as described in the Example 1B.

0106The hydration percentage is 49% and the dimensional increase is 12% in MD and 25% in TD.

0107The film mechanical properties in MD and TD direction are the following (50 <i>µ</i>m thickness): <ul id="ul0029" list-style="none" compact="compact"><li>MD direction: <ul id="ul0030" list-style="dash" compact="compact"><li>Stress at break (MPa)   28</li><li>Elongation at break (%)   125</li></ul></li><li>TD direction: <ul id="ul0031" list-style="dash" compact="compact"><li>Stress at break (MPa)   26</li><li>Elongation at break (%)   132</li></ul></li></ul>

Comments to the Example 1D Comp.

0108The extruded membrane shows anisotropy as regards the size variations and the mechanical properties. Indeed in both cases the obtained values depend on the considered direction. In particular it is noticed that the elongation at break in MD and also the hydration percentage give values clearly lower than the corresponding values obtained in the Example 1B according to the invention. Therefore the membrane obtained by extrusion has not very good mechanical and hydration propeties.

EXAMPLE 1E (comparative)

Measurements in fuel cell

0109The Example 1C is repeated by using the membrane of the Example 1D (comparative) having 50 <i>µ</i>m thickness. Under these conditions the cell supplies a current of 1.47 A/cm² at 0.4 V.

0110The maximum specific power supplied is 0.66 W/cm² at 0.54 V.

Comments to the Example 1E Comp.

0111The data obtained in the Example show that the fuel cell performance working with extruded membrane are lower than those of the fuel cell of the Example 1C which uses the membrane according to the present invention.

EXAMPLE 2

Polymerization

0112In a 22 litre autoclave the following reactants are introduced: <ul id="ul0032" list-style="dash" compact="compact"><li>11.5 litres of demineralized water;</li><li>980 g of the monomer of formula CF₂=CF-O-CF₂CF₂-SO₂F;</li><li>3, 100 g of an aqueous solution at 5% by weight of a fluoropolyether having formula:         CF₂ClO (CF₂CF(CF₃)O)_n(CF₂O)_mCF₂COOK and average molecular weight 559.</li></ul>

0113The autoclave, kept under stirring at 540 rpm, is heated to 60°C. The pressure in autoclave is brought to 0,02 MPa with ethane. 150 ml of an aqueous solution at concentration 6 g/l of potassium persulphate (KPS) are then fed into the autoclave. The pressure is brought to 1.7 MPa by introducing TFE. The reaction starts after 4 min. The pressure is maintained at 1.7 MPa by feeding TFE. When 1,000 g of TFE have been fed, 126 g of the sulphonyl monomer of formula CF₂=CF-O-CF₂-CF₂-SO₂F are introduced in the reactor. Subsequently 126 g of the same sulphonyl monomer are introduced in the reactor every 200 g of TFE fed. The total TFE mass fed to the reactor is 3,400 g.

0114The reaction is stopped after 320 min from the start, by lessening the stirring, cooling and venting the reactor under vacuum. The produced latex has a solid content of 24.2% by weight. The latex is coagulated by freezing and defreezing, the polymer separated from the mother liquors, washed with water until a constant pH of the washing waters, and dried at 150°C for 40 h at room pressure. The polymer has MFI = 1.7 g/10 min (280°C, 5 kg).

0115By DSC it is determined that the polymer is semicrystalline. The second melting enthalpy value is of 10.1 J/g.

EXAMPLE 2A

Preform preparation and exfoliation

0116The Example 1A is repeated with the ionomer prepared in the Example 2 but by using the following cycle: <ul id="ul0033" list-style="none" compact="compact"><li>S₁) heating from room temperature to 120°C with a heating rate of 3.3°C/min and constant pressure of 20 MPa;</li><li>S₂) residence time at 120°C for 2 hours at the constant pressure of 20 MPa;</li><li>S₃) heating from 120°C to 190°C with heating rate of 2°C/min, pressure of 10 MPa;</li><li>S₄) residence time at 190°C for 1.5 hours at the pressure of 10 MPa;</li><li>S₅) heating from 190°C to 260°C with heating rate of 2°C/min at the pressure of 5 MPa;</li><li>S₆) residence time at 260°C for 3 hours at the pressure of 5 MPa;</li><li>S₇) cooling from 260°C to room temperature with cooling rate of 0.3°C/min at the pressure of 5 MPa.</li></ul>

0117When the mould is cooled, a sleeve is extracted having the same sizes as that obtained in the Example 1A.

0118The exfoliation is carried out likewise as described in the Example 1A. Films having the same thicknesses as those indicated in the Example 1A are obtained.

EXAMPLE 2B

Preparation of the membrane in acid form and determination of the equivalent weight, per cent hydration and tensile properties of the membrane

0119From the films prepared in the Example 2A and having a thickness of 50 <i>µ</i>m and 80 <i>µ</i>m, respectively, rectangular-shaped specimens were cut out. The specimens were treated at 80°C for 8 h with a mixture of H₂O/KOH/dimethylsulphoxide in the weight ratios 55/15/30, then washed with demineralized water and subsequently treated at room temperature for 24 h with HCl at 20% by weight. Lastly a washing with demineralized water was carried out. In this way the film sulphonyl groups were converted into acid sulphonic groups.

0120The copolymer equivalent weight is 1,160 g/eq, corresponding to a 89.8% molar composition of TFE and 10.2% molar of sulphonic monomer.

0121The hydration percentage is 36% and the size increase, which is equal in both the directions (MD and TD), is 15%.

EXAMPLE 2C

Measurements in fuel cell

0122The Example 1C is repeated but by using the membrane of the Example 2B having a thickness of 50 <i>µ</i>m. Under these conditions the cell supplies a current of 1.33 A/cm² at 0.4 V and 0.85 A/cm² at 0.6 V.

0123The maximum specific power supplied is 0.56 W/cm² at 0.47 V.

EXAMPLE 2D (comparative)

0124The polymer of the Example 2 is subjected to extrusion at 280°C by Braebender extruder to obtain granules. Subsequently the granules are extruded at 280°C, obtaining a film having a thickness of 50 <i>µ</i>m.

0125The film sulphonyl groups are hydrolyzed to acid groups as described in the Example 2B.

0126The hydration percentage is 22% and the size increase in MD is 2% and in TD 12%.

EXAMPLE 2E (comparative)

0127The Example 1C is repeated but by using the membrane of the Example 2D (comparative) having a thickness of 50 <i>µ</i>m. Under these conditions the cell supplies a current of 1.20 A/cm² at 0.4 V and 0.75 A/cm² at 0.6 V.

0128The maximum specific power supplied is 0.50 W/cm² at 0.48 V.

0129The data herein obtained, compared with those of the Example 2C, confirm the previous comments to the Example 1E (comparative).

EXAMPLE 3

Polymerization

0130In a 22 litre autoclave the following reactants are introduced: <ul id="ul0034" list-style="dash" compact="compact"><li>11.5 litres of demineralized water;</li><li>980 g of the monomer of formula CF₂=CF-O-CF₂CF₂-SO₂F;</li><li>3,100 g of an aqueous solution at 5% by weight of a fluoropolyether having formula:         CF₂ClO(CF₂CF(CF₃)O)_n(CF₂O)_mCF₂COOK and average molecular weight 559.</li></ul>

0131The autoclave is kept under stirring at 540 rpm and heated to 60°C. The pressure in the autoclave is brought to 0,04 MPa with ethane. Then 150 ml of an aqueous solution at concentr-ation 6 g/l of potassium persulphate (KPS) are fed into the autoclave. The pressure is brought to 2 MPa by introducing TFE. The reaction starts after 2 min. The pressure is then maintained constant at 2 MPa by feeding TFE. 1,000 g of TFE are then fed, 126 g of the sulphonyl monomer of formula CF₂=CF-O-CF₂-CF₂-SO₂F are introduced in the reactor. Subsequently 126 g of the same sulphonyl monomer are introduced in the reactor every 200 g of TFE fed. The total TFE mass fed to the reactor is equal to 4,000 g. The reaction is stopped after 284 min from the start, by lessening the stirring, cooling and venting the reactor under vacuum. The produced latex has a solid content of 25.1% by weight. The latex is coagulated by freezing and defreezing, the polymer separated from the mother liquors, washed with water until a constant pH of the washing waters, and dried at 150°C for 40 h at room pressure. The polymer has a MFI value equal to 0.6 g/10 min (280°C, 10 kg), value much lower than those obtained in the previous polymerization Examples (Examples 1 and 2).

0132By DSC it is determined that the polymer is semicrystalline. The second melting enthalpy value is of 16.1 J/g.

EXAMPLE 3A

Preform preparation and exfoliation

0133The Example 2A is repeated but by using the ionomer prepared in the Example 3.

0134When the mould has been cooled, a sleeve is extracted having the same sizes as that obtained in the Example 1A.

0135The exfoliation is carried out likewise as described in the Example 1A. Films having the same thicknesses as those indicated in the Example 1A are obtained.

Comments to the Example 3A

0136The technology for obtaining the polymer film by exfoliation allows to prepare thin films also from polymers having very low MFI values. As a matter of fact, from a polymer having a MFI value as that indicated in the Example 3, it would be very difficult to obtain a thin film by extrusion.

EXAMPLE 3B

Preparation of the membrane in acid form and determination of the equivalent weight per cent hydration and tensile properties of the membrane

0137The Example 2B was repeated but by using the films obtained in the Example 3A, having a thickness of 50 <i>µ</i>m, 100 <i>µ</i>m and 150 <i>µ</i>m, respectively.

0138The copolymer equivalent weight is 1, 600 g/eq, corresponding to a 93.0% molar composition of TFE and 7.0% molar of sulphonic monomer.

0139The hydration percentage is 18% and the size increase, equal in both the directions (MD and TD), is 10%.

0140The film mechanical properties in MD and TD direction are the following (thickness 150 <i>µ</i>m): <ul id="ul0035" list-style="dash" compact="compact"><li>stress at break (MPa)   34</li><li>elongation at break (%)   180</li></ul>

EXAMPLE 3C

Measurements in fuel cell

0141The Example 1C is repeated by using the membrane of the Example 3B having a thickness of 50 <i>µ</i>m. Under said conditions the cell supplies a current of 0.85 A/cm² at 0.4 V and 0.57 A/cm² at 0.6 V.

0142The maximum specific power supplied is 0.37 W/cm² at 0.48 V.

EXAMPLE 4

Polymerization

0143In a 22 litre autoclave the following reactants are introduced: <ul id="ul0036" list-style="dash" compact="compact"><li>11.5 litres of demineralized water;</li><li>980 g of the monomer of formula CF₂=CF-O-CF₂CF₂-SO₂F;</li><li>3,100 g of an aqueous solution at 5% by weight of a fluoropolyether having formula:         CF₂ClO(CF₂CF (CF₃)O)_n(CF₂O)_mCF₂COOK and average molecular weight 559.</li></ul>

0144The autoclave is kept under stirring at 540 rpm and heated to 50°C. Then 300 ml of an aqueous solution at concentration 28 g/l of potassium persulphate (KPS) are fed into the autoclave. The pressure is brought to 1.2 MPa by introducing TFE. The reaction starts after 1 min. The pressure is maintained constant at 1.2 MPa by feeding TFE. When 1,000 g of TFE have been fed, 175 g of the sulphonyl monomer of formula CF₂=CF-O-CF₂-CF₂-SO₂F are introduced in the reactor. Subsequently 175 g of the same sulphonyl monomer are introduced in the reactor every 200 g of TFE fed. The total TFE mass fed to the reactor is 4,000 g. The reaction is stopped after 224 min from the start by lessening the stirring, cooling and venting the reactor under vacuum. The produced latex has a solid content of 28.8% by weight. The latex is coagulated by freezing and defreezing, the polymer separated from the mother liquors, washed with water until a constant pH of the washing waters, and dried at 150°C for 40 h at room pressure. The polymer has a MFI = 1 g/10 min (280°C, 10 kg).

0145By DSC it is determined that the polymer is semicrystalline. The second melting enthalpy value is of 7.9 J/g.

EXAMPLE 4A (comparative)

0146The polymer of the Example 4 is subjected to extrusion at 320°C by Braebender extruder to obtain granules. The extrusion temperature used is high in connection with the rather low MFI value of the polymer. Successively the granules are extruded in films at 320°C. It has been found that the minimum film thickness obtainable under the used extrusion conditions is 60 <i>µ</i>m.

0147The film sulphonyl groups are hydrolyzed to acid groups as described in the Example 1B.

0148The copolymer equivalent weight is 970 g/eq, corresponding to a molar composition 87.3% of TFE and 12.7% of sulphonic monomer.

0149The hydration percentage is 33% and the size increase is 8% in MD and 19% in TD.

0150The film mechanical properties in MD and TD direction are the following (140 <i>µ</i>m thickness): <ul id="ul0037" list-style="none" compact="compact"><li>MD direction: <ul id="ul0038" list-style="dash" compact="compact"><li>Stress at break (MPa)   34</li><li>Elongation at break (%)   99</li></ul></li><li>TD direction: <ul id="ul0039" list-style="dash" compact="compact"><li>Stress at break (MPa)   25</li><li>Elongation at break (%)   142</li></ul></li></ul>

Comments to the Example 4a Comp.

0151The results of the determinations of the mechanical properties show that the membrane has an anisotropic behaviour. Furthermore the elongation at break value in MD direction is the lowest among those reported in the above Examples. Therefore the mechanical properties are not suitable to the film use as a membrane.

0152Furthermore the hydration percentage (33%) of the membrane of the Example 4A Comp., prepared with a copolymer having equivalent weight 970 g/eq, is comparable with that obtained in the Example 2B according to the invention (36%), wherein a membrane having a higher equivalent weight, of 1,160 g/eq, was used.

0153The Example confirms therefore that the membranes of the present invention show the same hydration percentage of the membranes obtained by extrusion but with a higher equivalent weight.

EXAMPLE 5

Polymerization

0154In a 22 litre autoclave the following reactants are introduced: <ul id="ul0040" list-style="dash" compact="compact"><li>11.5 litres of demineralized water;</li><li>980 g of the monomer of formula CF₂=CF-O-CF₂CF₂-SO₂F;</li><li>3,100 g of an aqueous solution at 5% by weight of a fluoropolyether having formula:         CF₂ClO(CF₂CF(CF₃)O)_n(CF₂O)_mCF₂COOK and average molecular weight 559.</li></ul>

0155The autoclave, kept under stirring at 540 rpm, is heated to 50°C. Then 300 ml of an aqueous solution at concentration 28 g/l of potassium persulphate (KPS) are fed into the autoclave. The pressure is brought to 0.9 MPa by introducing TFE. The reaction starts after 10 min. The pressure is maintained at 0.9 MPa by feeding TFE. When 600 g of TFE have been fed, 210 g of the sulphonyl monomer of formula CF₂=CF-O-CF₂-CF₂-SO₂F are introduced in the reactor. Subsequently 200 g of the same sulphonyl monomer are introduced in the reactor every 150 g of TFE fed. The total TFE mass fed to the reactor is 3,000 g. The reaction is stopped after 230 min from the start by lessening the stirring, cooling and venting the reactor under vacuum. The produced latex has a solid content of 25.0% by weight. The latex is coagulated by freezing and defreezing, the polymer separated from the mother liquors, washed with water until a constant pH of the washing waters, and dried at 120°C for 40 h at room pressure. The polymer has a MFI = 87 g/10 min (280°C, 5 kg).

0156By DSC it is determined that the polymer is semicrystalline. The second melting enthalpy value is very low and equal to 0.1 J/g.

EXAMPLE 5A

Preform preparation and exfoliation

0157The Example 1A is repeated with the ionomer prepared in the Example 5 but using the following cycle: <ul id="ul0041" list-style="none" compact="compact"><li>S₁ heating from room temperature to 120°C with a heating rate of 3.3°C/min and constant pressure of 20 MPa;</li><li>S₂ residence time at 120°C for 2 hours at the constant pressure of 20 MPa;</li><li>S₃ heating from 120°C to 190°C with heating rate of 2°C/min, pressure 10 MPa;</li><li>S₄ residence time at 190°C for 1.5 hours at the pressure of 10 MPa;</li><li>S₅ heating from 190°C to 210°C with heating rate of 2°C/min at the pressure of 5 MPa;</li><li>S₆ residence time at 210°C for 3 hours at the pressure of 5 MPa;</li><li>S₇ cooling from 210°C to room temperature with rate of 0.3°C/min at a pressure of 5 MPa.</li></ul>

0158When the mould has been cooled, a sleeve is extracted having the same sizes as that obtained in the Examnple 1A.

0159The exfoliation is carried out likewise as described in the Example 1A. Films having the same thicknesses as those indicated in the Example 1A are obtained.

EXAMPLE 5B

Preparation of the membrane in acid form and determination of the equivalent weight, per cent hydration and tensile properties of the membrane

0160The Example 1B was repeated but by using the films obtained in the Example 5A, having a thickness of 50 <i>µ</i>m, 100 <i>µ</i>m, respectively.

0161The copolymer equivalent weight is 670 g/eq, corresponding to a molar composition 79.6% of TFE and 20.4% of sulphonic monomer.

0162The hydration percentage is 123% and the size increase, equal in both the directions (MD and TD), is 42%.

EXAMPLE 5C

Measurements in fuel cell

0163The Example 1C is repeated but by using the membrane of the Example 5B having a thickness of 50 <i>µ</i>m. Under said conditions the cell supplies a current of 1.46 A/cm² at 0.4 V and 0.74 A/cm² at 0.6 V.

0164The maximum specific power supplied is 0.60 W/cm² at 0.46 V.

EXAMPLE 6

Preparation of a reinforced membrane

0165The ionomer film in sulphonyl fluoride form having a thickness of 150 micrometers obtained in the Example 3A is placed on a PTFE net constuituted by interlaced PTFE threads and having a mesh opening of 42.5 mm²/cm². The obtained composite is placed between two aluminum plates and positioned in press for the welding treatment. A pressure of 1 MPa is applied for 10 minutes at a temperature of 260°C.

0166At the end of the treatment the composite film is extracted and it is cooled under pressure of 0.1 MPa until reaching the room temperature. A film is thus obtained with a PTFE support which is homogeneously adhered and englobed in the polymeric film. The sulphonyl fluoride groups are transformed into acid form with the process described in the Example 2B. The obtained reinforced membrane Keeps planar. The PTFE net remains perfectly adhered to the membrane ionomeric layer.

0167The reinforced membrane of the present Example results particularly useful for the electrolysis process of the hydrochloric acid.