Abstract:
A process comprising: depositing a liquid bonding layer comprising an ionomer and a solvent over a carrier film; placing a decal substrate over the liquid bonding layer and drying the liquid bonding layer to provide a solid bonding layer comprising the ionomer, and the solid bonding layer bonding the decal substrate and carrier film together.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/777,197, filed Feb. 27, 2006. 
     
    
     FIELD  
       [0002]     The disclosed embodiments generally relate to methods of laminating a decal substrate to a carrier film.  
       BACKGROUND  
       [0003]     In the fuel cell art, catalysts have been coated onto polymer electrolyte membranes. The catalyst layer may be deposited directly on the membrane, or indirectly applied to the membrane by first coating the catalyst on a decal substrate. The catalysts have been coated on the decal substrate as a slurry using a rolling process. The membrane with the catalyst coated thereon is known as a catalyst coated membrane (CCM).  
         [0004]     After the catalyst is coated on the decal substrate, an ionomer may be sprayed over the catalyst before the catalyst is transferred to membrane. Even though both the catalyst and the membrane contain the ionomer, the ionomer spray layer provides a better contact between the catalyst and the membrane. This increases the proton exchange between the membrane and the catalyst, thus increasing the performance of the fuel cell.  
         [0005]     The decal substrate may be of porous material, such as a porous expanded polytetrafluoroethylene (ePTFE) decal substrate. However, the ePTFE substrate is expensive and not reusable. Particularly, when the catalyst is transferred to the membrane on the ePTFE substrate, a certain portion of the ionomer remains in the ePTFE substrate. Further, the ePTFE substrate may stretch, deform and absorb solvents during the process, and thus the ePTFE substrates are discarded after one use.  
         [0006]     The decal substrate can also be a non-porous material, such as an ethylene tetrafluoroethylene (ETFE) decal substrate. The ETFE decal substrate provides minimal loss of catalyst and ionomer to the substrate because virtually all of the coating and ionomer are transferred in the decal process. The ETFE decal substrates do not deform and may be reused.  
         [0007]     In another fabrication technique, the membrane electrode assembly (MEA) is prepared as a catalyst-coated diffusion media (CCDM) instead of a CCM. The diffusion media is porous material so that gas and water may be transported through the MEA. The diffusion media is typically a carbon paper substrate that is coated with a microporous layer, wherein the microporous layer is a mixture of carbon and a fluoropolymer (FEP, PVDF, HFP, PTFE etc.). A catalyst ink is typically coated on top of the microporous layer, and may be overcoated, for example, by spraying with an ionomer solution. A piece of bare perfluorinated membrane is sandwiched between two pieces of CCDM with the catalyst sides facing the membrane and then hot-pressed to bond the CCDM to the membrane.  
         [0008]     One approach to manufacturing robust MEAs can be found in commonly assigned U.S. Pat. No. 6,524,736 to Sompalli, et al. This approach includes a process to manufacture MEAs by coating catalyst inks on porous expanded—PTFE supports or webs to generate electrodes with a uniform distribution of the ionomeric binder as shown in  FIGS. 1-2   a.  The concept of over-spraying to aid good transfer of catalyst to the membrane is also described. Sompalli, et al., U.S. Pat. No. 6,524,736, discloses the following. The very thin microporous substrates, onto which the catalyst is deposited, may be difficult to handle, and a porous carrier film may be bonded to the thin microporous substrates to provide a layer of strength and support. The porous carrier film may be sufficiently bonded to the microporous substrate during the process of removing the substrate from the membrane electrode assembly. The bond must be maintained between the microporous substrate and the porous carrier film throughout the entire process, and must be sustained through temperatures of the hot-pressing step.  
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION  
       [0009]     One embodiment of the invention includes a process comprising: depositing a liquid bonding layer comprising an ionomer and a solvent over a carrier film; placing a decal substrate over the liquid bonding layer and drying the liquid bonding layer to provide a solid bonding layer comprising the ionomer, and solid bonding layer bonding the decal substrate and carrier film together.  
         [0010]     Other embodiments of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]      FIG. 1  illustrates one embodiment of the invention including a method of depositing a liquid bonding layer including an ionomer over a carrier film.  
         [0013]      FIG. 2  illustrates one embodiment of the invention including depositing a decal substrate on the liquid bonding layer.  
         [0014]      FIG. 3  illustrates one embodiment of the invention including a method of allowing the liquid bonding layer to solidify and depositing a catalyst slurry layer on the decal substrate. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0015]     The following description of embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0016]     Referring now to  FIG. 1 , one embodiment of the invention includes a method of depositing a liquid bonding layer  12  over a carrier film  10 . In one embodiment of the invention the carrier film  10  has a thickness ranging from 25 to 100 micrometers. In one embodiment of the invention, the carrier film may be PET, Polyimide (Kapton®), PEN, PVDF or any polymeric films that can withstand temperatures up to 150° C. The liquid bonding layer  12  includes an ionomer and a solvent. In alternative embodiments of the invention, the liquid bonding layer  12  may be deposited to a thickness ranging from 5-200 microns, 5-100 microns, or thickness therebetween.  
         [0017]     Referring now to  FIG. 2 , in one embodiment of the invention, before the solvent in the liquid bonding layer  12  evaporates, a decal substrate  14  is placed on the liquid bonding layer  12  under minimal or elevated compression. Thereafter, the solvent in the liquid bonding layer  12  is allowed to evaporate or is driven off using heat, for example from an oven or drying lamps. In one embodiment the liquid bonding layer  12  is dried at a temperature ranging from room temperature to 120° C., and preferable at 60-120° C.  
         [0018]     The decal substrate  14  may be any of a variety of materials having a pore size below 2 microns, preferable below 0.5 microns and most preferably below 0.2 microns. The void volume of the porous material may range from 50-80 percent. Preferably the porous material is temperature compatible for approximately 5 minutes at a temperature ranging from 130-150° C. Additional surface coatings may be applied on the decal substrate  14  to aid in the release of a catalyst formed over the decal substrate  14 . The decal substrate  14  may be of variety of porous materials including, for example, porous polyethylene or porous polypropylene. The carrier film  10  has a greater mechanical strength and resistance to tearing than the decal substrate  14 . In one embodiment of the invention, the decal substrate  14  may be ePTFE and the carrier film  10  may be PET.  
         [0019]     Referring now to  FIG. 3 , in one embodiment of the invention, after the solvent in the liquid bonding layer  12  has evaporated, a solid bonding layer  12 ′, which includes an ionomer, holds the carrier film  10  and the decal substrate  14  together. In alternative embodiments of the invention, the solid bonding layer  12 ′ may have a thickness ranging from 0.05-10 microns, 0.05-2 microns, 0.1-1 microns, or a thickness therebetween. A catalyst slurry layer  16  is deposited on the decal substrate  14  and is dried to produce a decal assembly  18  which includes the dried catalyst layer  16  attached to the decal substrate  14  bonded to the carrier film  10 .  
         [0020]     The catalyst layer preferably includes a group of finely divided carbon particles supporting finely divided catalytic particles such as platinum and a proton conductive material intermingled with the particles. The proton conductive material may be an ionomer such as a perfluorinated sulfonic acid polymer. Preferred catalyst materials include metals such as platinum, platinum alloys and other catalyst known to those in the fuel cell art.  
         [0021]     The liquid bonding layer  12  includes an ionomer and a solvent. The solvent may be water, a low boiling alcohol or mixtures thereof. For example, the alcohol has a boiling point below 120° C. and has a finite drying time. Examples of low boiling alcohols are methanol, ethanol, iso and n-propanol. Bonding the decal substrate  14  and the carrier film  10  together using a liquid bonding layer  12  that includes an ionomer eliminates the need to laminating the decal substrate  14  and carrier film  10  under high temperature which can damage porous decal substrates such as ePTFE.  
         [0022]     When the terms “over”, “overlying”, “overlies”, or “under”, “underlying”, “underlies” are used with respect to the relative position of a first component or layer with respect to a second component or layer, such shall mean that the first component or layer is in direct contact with the second component or layer, or that additional layers or components may be interposed between the first component or layer and the second component or layer.  
         [0023]     The description of the invention is merely exemplary in nature and, thus, variations thereof not to be regarded as a departure from the spirit and scope of the invention.