Abstract:
A process for preparing an ion-permeable web-reinforced separator membrane includes the steps of: providing a web ( 2 A) and a suitable paste ( 5 ), guiding the web ( 2 A) in a vertical position, equally coating both sides of the web with the paste to produce a paste coated web ( 2 B), and applying a symmetrical surface pore formation step and a symmetrical coagulation step to the paste coated web to produce a web-reinforced separator membrane.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to a web-reinforced separator membrane for use in electrochemical applications.  
       STATE OF THE ART  
       [0002]     EP-A-0232 923 discloses a process for preparing an ion-permeable diaphragm comprising an organic fabric embedded in a film forming mixture of particulate inorganic hydrophilic material and an organic polymeric binder, comprising the steps of mixing the inorganic hydrophilic material with a solution of the polymeric binder in an appropriate solvent to form a slurry; uniformly spreading the slurry on an inert flat surface to form a sheet with a wet thickness of less than 2 mm; immersing any type of woven or non-woven stretched fabric into the wet sheet; removing the solvent by evaporation and or lixiviation while keeping the fabric stretched; removing the sheet from the said surface. The process is performed in the horizontal direction and the organic fabric is introduced after the sheet has been spread (cast) on an inert flat surface (glass plate). In practice the combination of (horizontal) spreading of the slurry on inert flat surface, followed by the immersing of a stretched organic fabric, leads to a product where the fabric is not embedded properly (nicely in the middle). Almost always the fabric appears at one side of the diaphragm, causing non-desired adhering gas bubbles when the diaphragm is used in e.g. a separator.  
         [0003]     Further, a prerequisite for being able to immerse any type of stretched fabric is to use a slurry with low viscosity. This inevitably leads to a less-desired asymmetric pore structure, which gives rise to diaphragms having different pore structure and pore diameter at both side. With this respect the topside during fabrication (the shiny side of the finished product) will always be the tightest side, whereas the dull side (the bottom side during fabrication) is always the more open side. Also, the process is a very laborious and non-economical way of fabrication, since it can not be implemented as a continuous process.  
         [0004]     EP-A-0692830 describes an extrusion process of thermoplastic sheet (300 μm thick) consisting of a blend of about 16.6 wt % of synthetic resin (e.g.: polyethylene), 66.6 wt % of an inorganic powder (e.g.: SiO2) and a 16.6 wt % of pore-forming agent (paraffin oil). Adhering to one side of the extruded thermoplastic sheet (in semi-molten state) an inorganic sheet (200 μm thick) containing 80 wt % of inorganic material (glass fiber) and 20 wt % of polyolefinic pulp as organic binder are added to give a laminate. The laminate is then pressure-moulded to unify the semi-molten sheet and cooled to remove the pore-forming agent, resulting in an inorganic material coated separator. This disclosure is thus a dry and continuous extrusion process wherein 2 materials are “joined” by a lamination process.  
         [0005]     U.S. Pat. No. 4,707,265 describes a web impregnating apparatus for wherein a web is fed vertically between calendar rollers through a casting solution. The membrane is then guided between a casting drum and a casting blade through a quench bath. This setup cannot lead to a symmetrical membrane, as one side of the web is facing the quench bath and the other side is facing the drum. Due to this setup, the resulting membrane has smaller pores at the quench side than at the drum side. Further, as indicated in the document, the described setup easily leads to so-called “terry clothing”, which can only be avoided by changing the thickness of the coating and the distance between casting drum and casting blade. Due to this limitation, it is very difficult to produce membranes of a broad thickness range.  
         [0006]     U.S. Pat. No. 2,940,871 describes a process for coating a web with a plastic composition. A resinous dispersion is coated on a backing strip and heated to fuse the thermoplastic compounds in the dispersion. The matrix is then removed in a solvent bath, and the resulting coated web is dried and stored. Due to the vertical and upward setup, only relatively thick dispersions can be used (otherwise they run down the backing strip before they reach the heating device), which leads to a small thickness range. Further, it is difficult to control the pore size of the resulting membrane, as it is mainly controlled by the composition and mixing of the dispersion. Pore characteristics will therefore not be uniform over the complete membrane. Furthermore, the heating step is relatively difficult to control and expensive compared to a coagulation step in a quench bath.  
         [0007]     There is a need for symmetrical web-reinforced separator diaphragms, where the web is nicely embedded in the diaphragm, without appearance of the web at a surface of the diaphragm. Also, a process for such fabrication should be continuous in order to be economically interesting. Further, it should be possible to produce products with various dimensions with equal properties over the complete surface of the diaphragm.  
       AIMS OF THE INVENTION  
       [0008]     The present invention aims to provide a continuous process for manufacturing web-reinforced separator diaphragms, where the web is nicely embedded in the diaphragm, without appearance of the web at the surfaces of the diaphragm.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention concerns a process for preparing an ion-permeable web-reinforced separator membrane, comprising the steps of: providing a web and a suitable paste, guiding said web, preferably downward, in a vertical position, preferably downward, equally coating both sides of said web with said paste to produce a paste coated web, and applying a symmetrical surface pore formation step and a symmetrical coagulation step to said paste coated web to produce a web-reinforced separator membrane. With symmetrical step is meant that the step is performed in the same way at both sides of the membrane. This leads to a membrane with symmetrical characteristics. Preferably, each step is also performed at the same time for both sides, and with the same exposure time (e.g. exposure to water vapour or coagulation bath).  
         [0010]     The process of the present invention preferably further comprises the steps of providing a liner to one side of the web-reinforced separator membrane and winding the web-reinforced separator membrane together with said liner on a roll. The rolled-up separator membrane with the liner makes it possible to wash the membrane easily, especially when a corrugated liner is used, that allows fluid access to the membrane surface when the roll is immersed.  
         [0011]     Further, the process preferably comprises a washing step. Said washing step can be performed in a washing bath ( 16 ) comprising e.g. water. Preferably, the washing bath ( 16 ) is at a temperature of between 10 and 80° C.  
         [0012]     The coating step in the present invention is preferably performed by guiding the web through a double-sided coating system with automatic paste feeding.  
         [0013]     The surface pore formation step preferably comprises symmetrically exposing both sides of the paste coated web to water vapour phase (e.g. water vapour or “cold steam” blanket).  
         [0014]     The coagulation step preferably comprises symmetrically exposing both sides of the paste coated web to a coagulation bath comprising water, mixtures of water and an aprotic solvent selected from the group consisting of NMP, DMF, DMSO, DMAc), water solutions of water-soluble polymers (selected from the group consisting of HPC, CMC, PVP, PVPP, PVA), or mixtures of water and alcohol (such as ethanol, propanol and isopropanol). The coagulation bath is preferably at a temperature of between 40° C. and 90° C.  
         [0015]     Another aspect of the present invention concerns a web-reinforced separator membrane, characterised in that the web is positioned in the middle of the membrane and both sides of the membrane have the same pore size characteristics.  
         [0016]     Another aspect of the present invention concerns an apparatus for providing a web-reinforced separator membrane, comprising a web-unwinding station for web-tension control, a spreader roller, a coater with a double-sided coating system with automatic paste feeding with vertical (guided) web transportation, and guiding rollers in a heated coagulation bath.  
         [0017]     The apparatus of the present invention can further comprise a liner feeder and a product winding station in a heated washing water bath, wherein the product winding station determines the rate of production. 
     
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  illustrates the process according to the present invention.  
         [0019]      FIGS. 2 and 3  represent a membrane fabricated according to the present invention with respectively a PES Spunlaid non-woven web and a PA Woven web.  
         [0020]      FIG. 4  illustrates a prior art membrane (such as in EP-A-0232 923) 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Fabrication Steps:  
         [0021]     Web preparation step: web ( 2 A, woven or non/woven) unwinding; web guiding into vertical position ( 18  and  9 ) and web spreading ( 3 ) to prevent fold formation (perpendicular to the fabrication direction)  
         [0022]     Web coating step: simultaneous double-side coating of paste  5  (e.g. organo-mineral) with double-sided coating system  4  and automatic paste feeding on both sides of the web (same level at both sides) to obtain a paste coated web ( 2 B)  
         [0023]     Surface pore formation step: contacting of the double-side coated web with water vapour phase  7 . It is also possible to obtain an asymmetric web-reinforced membrane with different pore size characteristics at both sides by applying different conditions on both sides of the paste coated web.  
         [0024]     Bulk formation step: coagulation of product into hot water bath  8   
         [0025]     Product winding step: liner  14  feeding and winding together with product  
         [0026]     Post-treatment step: washing out of chemicals in water reservoir  16   
         [0027]     Drying step: drying of the product  
         [0000]     Fabrication Parameters  
         [0028]     The following parameters can be envisaged when performing the method according to the present invention. It is evident that these parameters can be easily changed by the skilled in order to adapt the method to specific dimensions, products used or other considerations. 
        Production speed: between 0.2 and 20 m/min     Web tension: 15-35 N     Temperature of coagulation bath: 40-90° C.     Temperature of washing bath: 10-80° C.     Distance between the two coating rollers (of the coater): 450-1100 μm 
 
 Web Examples: 
       
 
         [0034]     Table 1 lists a few examples of web types that can be used for practising the present invention:  
                                                         TABLE 1                                       Web   Roller   Product               thickness   distance   thickness           Web type   (μm)   (μm)   (μm)                                        PES   450   1100   800           Spunlaid           non-woven           PA Woven   100   450   300           PEEK Woven   350   900   650                      
 
         [0035]      FIGS. 2 and 3  respectively show a membrane fabricated according to the present invention with respectively a PES Spunlaid non-woven web and a PA Woven web.  
         [0036]     As a liner, for example PES spunlaid non-woven web (thickness: 450 μm) can be used.  
         [0000]     Paste Examples:  
         [0037]     The paste can comprise 
    any hydrophilic inorganic filler material, such as TiO 2 , HfO 2 , Al 2 O 3 , ZrO 2 , Zr 3 (PO 4 ) 4 , Y 2 O 3 , SiO 2 , perovskite oxide materials, SiC, C(Pt/Rh/Ru) can be used;     any organic binder material such as PVC, C-PVC, PSf, PES, PPS, PU, PVDF, PI, PAN, PVA, PVAc and their grafted variants (sulphonated, acrylated, aminated, . . . ); and     a solvent such as NMP, DMF, DMSO or DMAc or a mixture thereof. 
 
 Equipment: 
   
 
         [0041]     A preferred set-up for practising the current invention is depicted in  FIG. 1 . The set-up comprises: 
        Motor driven web-unwinding station  1  for web-tension control     Spreader roller  3  in front of coater  4      Coater with double-sided coating system  4  with automatic paste feeding  5  with vertical (guided) web transportation     Guiding rollers (rollers  18 ,  9 ,  10 ,  12  and  13 ) in heated coagulation basin  8      Liner  15  feeding  14      Product winding station  17  in heated washing water basin  16  (the used roller is the driver roller of the whole system)        
 
         [0048]     The following exemplifies a possible practical embodiment of the present invention, using the equipment as described above. The following steps are executed: 
        Spreading of web  2  A in the perpendicular direction to the fabrication direction     Vertical simultaneous doubleside coating of paste  5      Use of two roll-coating system  4  with automatic paste feeding     Coated web  2  B is brought in contact with water vapour  7  for correct surface pore size     Liner  15  feeding  14  for enabling washing step in warm water bath  16         
 
         [0054]     In order to obtain good results, a paste with good flow behaviour/properties should be used.  
       EXAMPLES  
     Example 1  
     Separator with Reinforced Woven PA Web  
       [0055]     The reinforcing web used is a woven type of web based on PA-6.6 monofilaments, supplied by Sefar Inc. Filtration Division, CH-8803 Rueschlikon, Switzerland, of type Nitex 03-190/57. Its characteristics are as follows:  
                                                                                           Thickness   100   μm           Filament thickness   62   μm           Mesh size   190   μm                Open area   57%                Web width   50   cm                      
 
         [0056]     The composition of the used paste is: 
        46.99 wt. % of solvent (DMF)     13.25 wt. % of polymer (PSf Udel of type P-1800 NT)     39.75 wt. % of mineral filler (Ti-Pure® Dupont TiO 2 , type R-100 with particle size of 0.32 μm)        
 
         [0060]     The fabrication parameters are: 
        Production speed: 1 m/min     Web tension during fabrication: 15 N (in both directions (production direction and perpendicular direction thereon)     Distance between the two coating rollers  4 : 450 μm     Distance between two coater rollers  4  and the water level of the coagulation bath  8 : 40 cm     Temperature of coagulation bath  8 : 65° C.     Temperature of washing bath  16 : 35° C.        
 
         [0067]     The resulted product had the following characteristics:  
                                                       Thickness   250 μm           Web   nicely in the middle           Tensile strength   same as reinforcing web           Specific resistance   4 Ωcm (30 wt. KOH, 25° C.)                      
 
       Example 2  
     Separator with Reinforced Spunlaid Nonwoven PA Web  
       [0068]     The reinforcing web used is a spunlaid nonwoven type of web based on PA-6 bicomponent filaments (PET core; PA 6 skin), supplied by Colbond Inc. Nonwovens, Enka, N.C. 28728 USA, of type Colback® CDF 70. Its characteristics are as follows:  
                                                           Thickness   450   μm           Filament thickness   40   μm           Web width   50   cm                      
 
         [0069]     The composition of the used paste is: 
        54.55 wt. % of solvent (NMP)     13.64 wt. % of polymer (CPVC type H827, Mitsui, Japan)     31.82 wt. % of mineral filler (Al203 Alcoa type A-15SG with particle size of 1.7 μm)        
 
         [0073]     The fabrication parameters are: 
        Production speed: 1 m/min     Web tension during fabrication: 25 N (in both directions (production direction and perpendicular direction thereon)     Distance between the two coating rollers  4 : 1100 μm     Distance between two coater rollers  4  and the water level of the coagulation bath  8 : 40 cm     Temperature of coagulation bath  8 : 65° C.     Temperature of washing bath  16 : 35° C.        
 
         [0080]     The resulted product had the following characteristics:  
                                                       Thickness   850 μm           Web   nicely in the middle           Tensile strength   same as reinforcing web           Specific electrolyte resistance:   6 Ωcm (30 wt. KOH,               25° C.)                      
 
       INDUSTRIAL APPLICATIONS  
       [0081]     The present invention can be used in the following applications: 
    Alkaline water electrolysis     Batteries (acid and alkaline)     Fuel cells     And combinations thereof.