Title of Invention

A BLEND MEMBRANE

Abstract

A process for the manufacture of a blend membrane comprises the steps of preparing a solution of at least one ion exchange material selected from perf1uorinated sulphonic acid resin, perf1uorinated carboxylic acid resin, polyvinyl alcohol, sulphonated diviny1benzene-styrene based polymers, sulphonated polysulphones. A separate solution of hydrophobic fluorinated polymers comprising poly(viny1idene fluoride), poly(tetra f1uoroethy1ene), poly(viny1idene f1uoride-co-hexaf1uoro propylene)is prepared.The former solution and the latter solution are homogeneously mixed in a weight ratio range of 1:1 to 20:1 to form a blend solution. The blend solution is cast on a clean surface to a thickness between 10 micrometers and 200 micrometers, while removing the excess solvent and surfactant by means such as herein described to obtain the blend membrane.

Full Text

This invention relates to a blend membrane Solid polymer electrolytes comprising a perfluorosulfonic acid polymer blended with poly(vinylidene fluoride) in varied ratios is found to have improved membrane characteristics. These membranes have similar conductivities as perfluorosulphonic acid membrane, are slightly inexpensive and offer less methanol permeability. They are suitable for polymer electrolyte membrane fuel cell and direct methanol fuel cell. This invention refers to homogeneously formed blends of perfluorosulphonic acid polymer with hydrophobic thermoplastic polymer like poly(vinylidene fluoride) for use as ion exchange membrane in electrochemical cells such as solid polymer electrolyte fuel cells and directs methanol fuel cells. Solid polymer electrolyte (SPE) cells are electrochemical cells, which employ an ion exchange polymer in the form of membrane to separate the anode and the cathode while serving as an electrolyte. SPE cells may be used as electrolytic cells for production of electrochemical products or may be operated as fuel cells for the production of electrical energy. In fuel cells these membrane separate the anode fuel, hydrogen, from the cathode oxidant, pure oxygen or air. Membrane based fuel cells are advantageous as they operate at lower temperatures than the other fuel cells. In membrane based fuel cells a cation exchange membrane is employed and protons are transported across the membrane as the cell is operated. Hence these cells are known as proton exchange membrane (PEM) fuel cells. In a cell employing hydrogen / oxygen couple, hydrogen molecules at the anode get oxidized donating electrons to the anode, while at the cathode oxygen is reduced accepting electrons from the cathode. The H"^ ions (protons ) formed at the anode migrate through the membrane to the cathode and combine with oxygen to form water. SPE membranes must have the requisite strength for their use in various applications. Normally, thicker membranes are used in order to increase the strength of the membranes. This results in decrease in ionic conductivity and hence performance. Membranes can also be reinforced to increase their strength . In fuel cells where a direct feed of liquid or gaseous organic fuel such as methanol is used, fuel crossover is known to occur. Fuel crossover refers to the transport of methanol, the fuel, from the anode side to the cathode side (oxygen) of the fuel cell. This fuel in the cathode lowers the performance of the cathode (lowers its operating potential) and also results in fuel wastage (loss due to fuel efficiency). This reduces the specific cell power output and also overall cell efficiency. Therefore it is desirable to have cation exchange membrane with low fuel crossover rate. Simpler method of tailor-making a polymer with desired properties to give a distinct material is polymer blending compared to methods of synthesizing different polymer segments via copolymerisation, formation of block copolymers or formation of interpenetrating materials. Compatible polymer blends have two polymers that are missoible at the molecular level and combine the properties of the two polymers to give better properties to the blend. However polymers may be incompatible even when they are prepared from the same solution, i.e., blending does not always result in formation of homogenous polymer blends or polymer alloys. Entropy of mixing of polymers with molecular weight higher than 10,000 approaches zero there by making Gibbs free energy (AG=AH-TAS) of mixing for most of the polymers positive . The blend formed is miscible, only when the enthalpy of mixing is negative or at least zero. In immiscible blends both the homopolymers retain their distinct phases and phase separation is inevitable. These immiscible blends have poor mechanical properties. Miscibility of polymers occurs only in the amorphous regions. If one of the polymers, say poly(vinylidiene fluoride), is semi crystalline, the crystal structure of the polymer is inherently retained. In miscible blends, melting point Tm decreases on blending. The different amorphous phases of the two homopolymers do not separate. The fraction of crystalline portion extends itself into the amorphous part and interacts with the non-crystalline regions. Thus, the film with homogenous blend of semicrystalline polymer as one of its components will have higher tensile strength . Also, the miscible polymers have excellent improvement in properties . The objective of this invention is to provide a low cost, easy to prepare membranes with suitable chemical and mechanical properties for use in electrochemical cells. Another objective of the invention is to provide a less methanol permeable membrane for use in direct methanol fuel cells. It is also an object of this invention to provide a process for preparing the novel blends based on a hydrophobic material and an ion exchange resin. The present invention is an advancement over the ion exchange membranes known. In one aspect the blend membrane has lower cost than the present commercially available membranes for fuel cells . In other aspect these membranes have lower methanol absorption and lower permeability to methanol, hence a good prospect for use in direct methanol fuel cells. Membranes with varying thicknesses can be prepared by adjusting the amount of various materials judiciously to suit the requirement. The ion exchange material may be selected from a group consisting of perfluorinated sulphonic acid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol, sulphonated divinylbenzene and styrene based polymers, and sulphonated polysulphones. The hydrophobic material used for preparation of the blend can be chosen from poly(vinylidene fluoride), poIy(tetra fluoroethylene) and poly (vinylidene fluoride -co-hexafluoropropylene). Briefly, the process of making an ion exchange membrane of the present invention comprises > Separately dissolving the ion exchange material and poly(vinylidene fluoride) in a suitable solvent or a mixture of solvents. > Mixing the polymer solutions together in a weight ratio of ion exchange material to PVDF between I to 1 and about 20 to 1 to form a blend solution > Casting the blend solution onto a clean surface and > Drying the cast blend solution for a time sufficient to evaporate the solvents and form blend membrane having a thickness between 10 micrometers and about 200 micrometers. Blend membrane constitutes thermoplastic hydrophobic microreinforcing polymeric material and cation exchangeable polymeric material or an ion exchange resin. The blend membrane of the present invention has various applications including electrolysis, fuel cells and batteries and use as super acid catalyst . The cast membrane is essentially pinhole free, air impermeable, homogeneous and of desirable mechanical strength with properties as of the ion exchange resin it is blended with. Broad spectrum of ion exchange polymeric material is available for use as resins in the blend membrane. These include perfluorinated sulphonic acid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol, sulphonated divinylbenzene and styrene based polymers, and sulphonated polysulphones. A mixture of ion exchange polymers may also be used for blending with thermoplastic hydrophobic polymers . Solvents for dissolution of ion exchange polymeric material includes alcohols, dimethyl formamide, dimethyl sulphoxides, water, tetrahydrofuran and combinations there of Surfactant molecules with both hydrophilic and hydrophobic regions may be used. A surfactant with a molecular weight greater than 1 GO is employed with the ion exchange resin while blending. Commercially available anionic, nonionic, or amphoteric surfactants which may be like Merpol® a hydrocarbon based surfactant or Zonyl® a fluorocarbon based surfactant may be used. The final blend membrane has uniform thickness. The membrane is impermeable to non polar gases and bulk flow of liquids. The blended membrane may be processed to remove the sufactant which may have been employed while formation of the membrane. This may be accomplished by soaking and submerging the membrane in water, isopropyl alcohol, hydrogen peroxide, methanol etc. This causes the membrane to absorb water and swell. Since the ion exchange material is blended homogenously with the hydrophobic material it does not get leached out. A blend membrane with desired mechanical strength and good conductivity is obtained when the membrane is swollen in suitable solvent like water. Although the membrane has long term chemical stability it is poisoned by organics and metal ions. Hence the membrane can be boited in acids or hyrogen peroxides to remove these. To prepare the membranes solvent casting technique has been adopted. The blends are made by dissolving PVDF in suitable solvent and the ion exchange resin in a suitable solvent and mixing two solutions together and casting on a clean glass plate. The solvents may be removed by heating them in an oven in the temperature range 60-100°C in vacuum. The preferred temperature is 80°C. The membranes are then pressed at SOOOpsi at a temperature from 30°C-140°C in a hydraulic press. This introduces crystallites in the membrane and a stable membrane is obtained. The membrane is then processed to remove surfactants , organics and metal impurities as described above and soaked in solvent to give blend membranes that are ready for use. PVDF is a hydrophobic thermoplastic polymer. PVDF is a semi-crystalline polymer that displays up to 50% crystallinity. It is soluble in solvents like dimethylsulphoxide(DMSO), N-methyl pyrrolidone(NMP), dimethylacetamide(DMAc) and dimethylformamide (DMF). Its crystalline melting point is between 155°C and 163°C and its glass transtition temperature is between -30°C and -10°C. Therefore PVDF is a crosslinked, rubbery polymer where the hard crystalline domains serve as crosslinking junctures in the blend. These blend membranes have reduced swelling in solvents . It is believed that PVDF in the blend membrane reduces the absorption of water when submerged in water.The percentage water absorbed is about 9 % compared to Nafion®! 15 where the percentage of water absorption is 36% . The weight ratio of the ion exchange material to PVDF is about 1 to 1 and above, preferably about 2 to 1 or 3 to 1 .The ion exchange material comprises from 50-90% by weight of the blend and PVDF comprises 50-10% of the blend. Most preferred blends have the ion exchange material between 50-70% the balance being PVDF. The mechanical strength of these blends is increased by thermal treatment. The homogenous blend membrane of the said ion exchange material with thermoplastic PVDF of the present invention has the following advantages 1. Blend membranes have lower swelling ratios than pure ion exchange material. 2. Blend membranes have higher flexibility and strength than pure ion exchange material. 3. Blend membranes are cheaper than the ion exchange material by 30-50% depending on the percentage of the hydrophobic material added. 4. Blend membranes have lower permeability to methanol. Example 1 An ion exchange material/surfactant solution was prepared comprising 48% by volume of a perfluorosulphonic acid resin solution (in H+ form which itself is comprised of less than 10% resin and the rest being water and mixture of lower molecular weight alcohols commercially available from E.LDupont de Nemours.Inc. under the trade name of Nafion® solution) and 48% of dimethyl sulphoxide(DMSO) and 4% of a nonionic surfactant. 20 wt% of solution of polyvinylidene fluoride in DMSO was prepared separately. 0.5g of the resin in solution was mixed with 0.5 g of PVDF in DMSO solution at room temperature. This blend has a 1.1 weight ratio of ionomer to PVDF. This blend solution was poured into clean glass plate and cast This was then placed in a chamber to evaporate off the solvents. The final membrane was a dry, translucent membrane. The dry membrane was pressed in a press at 5000 psi at increasing temperatures of 30°C-140°C for 15 mins. This dry membrane was put in water for 1 hr and tested the conductivity and methanol permeability. The conductivity of the wet membrane was 0.06 S/cm and the permeability factor was 12.5 cm^/s (Nafion 115 membrane has a conductivity of 0.077S/cm and a permeability factor of 48.8 cmVs) Example 2 The same procedure in example 1 was repeated to give a resin - hydrophobic polymer a ratio of 9:1, 4.i,7:3and 3:2 by weight. The properties of these polymers are listed in Table 1 Table 1 The process, according to this invention, for the manufacture of a blend membrane comprises the steps of preparing a solution of at least one ion exchange material selected from perfluorinated sulphonic acid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol, sulphonated divinylbenzene-styrene based polymers, sulphonated polysulphones and a separate solution of hydrophobic fluorinated polymers comprising poly (vinylidene fluoride), poly (tetrafluoroethylene) poly(vinylidene fluoride-co-hexafluoro propylene); homogeneously mixing the former sohition with the latter in a weight ratio range of 1:1 to 20:1 to form a blend sohition; casting on a clean surface to a thickness between 10 micrometers and 200 micrometers while removing the excess solvent and surfactant by means such as herein described to obtain the blend membrane. We Claim: 1. A process for the manufacture of a blend membrane comprising the steps of preparing a solution of at least one ion exchange material selected from perfluorinated sulphonic acid resin, perfluorinated carboxylic acid resin, polyvinyl alcohol, sulphonated divinylbenzene-styrene based polymers, sulphonated polysulphones and a separate solution of hydrophobic fluorinated polymers comprising poly (vinylidene fluoride), poly (tetrafluoroethylene) poly(vmylidene fluoride-co-hexa fhloro propylene); homogeneously mixing the former solution with the latter in a weight ratio range of 1:1 to 20:1 to form a blend solution; casting on a clean surface to a thickness between 10 micrometers and 200 micrometers while removmg the excess solvent and surfactant by means such as herein described to obtain the blend membrane. 2.A process as claimed in Claim 1 wherein the blend material comprises a surfactant with both hydrophobic and hydrophilic regions. 3.A process as claimed in Claim 2 wherein the surfactant has a molecular weight greater than 100. 4.A process as claimed in Claim 2 or Claim 3 wherein the surfactant is commercially available surfactant such as herein described. 5.A process as claimed in Claim 1 or Claim 2 wherein the solvents for the preparation of the solution of the ion exchange material comprises at least one of alcohols, dimethyl form amide, dimethyl sulphoxides, water, tetrahydrofuran. 6A process for the manufacture of a blend membrane substantially as herein described and illustrated with reference to the Examples. 7.A blend membrane whenever manufactured in accordance with a process as claimed in any one of the preceding Claims. Dated this 9th April 2001

Documents:

0303-mas-2001 abstract duplicate.pdf

0303-mas-2001 abstract.pdf

0303-mas-2001 claims duplicate.pdf

0303-mas-2001 claims.pdf

0303-mas-2001 correspondence others.pdf

0303-mas-2001 correspondence po.pdf

0303-mas-2001 description (complete) duplicate.pdf

0303-mas-2001 description (complete).pdf

0303-mas-2001 form-1.pdf

0303-mas-2001 form-19.pdf

0303-mas-2001 form-26.pdf

0303-mas-2001 form-3.pdf