Abstract:
A porous membrane assembly includes, in an exemplary embodiment, a porous membrane substrate having hydrophobic properties. The membrane substrate includes a first side, an opposing second side and a plurality of pores extending therethrough. The membrane substrate also includes a discontinuous layer of an oleophobic coating applied to the first side so that the plurality of pores in the first side are substantially free of the oleophobic coating, and a continuous layer of an amine containing hydrophilic polymer coating applied to the second side of the membrane substrate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to asymmetric porous membranes, their laminates, and more particularly, to asymmetric porous membranes having oleophobic properties on one side and hydrophilic properties on an opposing side. 
         [0002]    It is generally known that a porous membrane may have at least one property that may be limited by the material from which the membrane is made. For example, a porous membrane made from polytetrafluoroethylene (PTFE) material may have excellent hydrophobic properties, have low surface energy, and may be extremely inert. Therefore, it may be difficult to chemically or mechanically attach a different material to the ePTFE material. For example, U.S. Pat. No. 5,976,380 describes bonding a hydrophilic coating to the surface of a porous PTFE membrane by wetting the membrane with the solution, or with a wetting agent such as methanol, ethanol, isopropanol, or acetone, and then applying the solution under pressure. 
         [0003]    Protective clothing designed for use against chemical and biological threats has been developed using many advanced materials available today. The joint service lightweight integrated suit technology (JSLIST) worn by military personnel, for example, consists of an air-permeable carbon-based chemical protective over-garment, with a water-repellent-treated cotton/nylon ripstop fabric shell and a carbon-bonded tricot liner. However, at least some known protective suits are considered heavy and uncomfortable because of the materials used in fabricating the suit, especially those that use carbon-based materials and/or have low perspiration permeability and a low moisture vapor transmission rate (MVTR). 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one aspect, a porous membrane assembly is provided that includes a porous membrane substrate having hydrophobic properties. The membrane substrate includes a first side, an opposing second side and a plurality of pores extending therethrough. The membrane substrate also includes a discontinuous layer of an oleophobic coating applied to the first side so that the plurality of pores in the first side are substantially free of the oleophobic coating, and a continuous layer of an amine containing hydrophilic polymer coating applied to the second side of the membrane substrate. 
         [0005]    In another aspect, a method of manufacturing a porous membrane assembly is provided. The method includes providing a porous membrane substrate having hydrophobic properties, where the membrane substrate includes a first side, an opposing second side and a plurality of pores extending therethrough. The method also includes forming a discontinuous layer of an oleophobic coating by applying the oleophobic coating to the first side of the membrane substrate so that the plurality of pores in the first side are substantially free of the oleophobic coating, and forming a continuous layer of an amine containing hydrophilic polymer coating by applying the amine containing hydrophilic polymer coating to the second side of the membrane substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional schematic illustration of an exemplary embodiment of an asymmetric porous membrane. 
           [0007]      FIG. 2  is an enlarged schematic illustration of a portion of the permeable membrane shown in  FIG. 1 . 
           [0008]      FIG. 3  is a cross-sectional schematic illustration of another exemplary embodiment of an asymmetric porous membrane. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    An asymmetric porous membrane in accordance with an exemplary embodiment and a method of making the porous membrane are described in more detail below. The asymmetric porous membrane, in an exemplary embodiment, is an expanded PTFE membrane that has an oliophobic coating on one side and a hydrophilic coating on the opposite side. The asymmetric porous membrane can be used in fabricating a chemical protective laminate. The asymmetric porous membrane is waterproof, when combined with appropriate textile fabrics, and provides a barrier against chemical and biological warfare agents, while possessing a good moisture vapor transmission rate (MVTR). 
         [0010]    Referring to the drawings,  FIG. 1  is a cross-sectional schematic illustration of an exemplary embodiment of an asymmetric porous membrane  10  that includes a base substrate  12 . Membrane substrate  12  can be formed from any suitable material, for example, PTFE. Substrate  12  formed from PTFE has hydrophobic properties due to the surface tension properties of PTFE. Substrate  12  has a first side  14  and an opposing second side  16 . First side  14  of substrate  12  is treated with a oleophobic material  18  to impart oleophobic properties to first side  14 . A hydrophilic coating  20  is deposited on second side  16  to impart hydrophilic properties to second side  16 . 
         [0011]    Oleophobic material  18  can be any suitable material, such as, a fluorinated polymer material that enhances the oleophobic properties without compromising the gas permeability of membrane  10 . Suitable examples of fluorinated polymers include, but are not limited to, fluorinated urethane polymers, perfluro alkyl acrylic(methacrylic) copolymers, random copolymers composed of fluorinated acrylate(methacylate), butyl acrylate or a comparable n-alkyl acrylate(methacrylate). Fluorinated polymers are available from DuPont under the trade name Zonyl®. Oleophobic material  18  is deposited on first side  14  of substrate  12  such that the membrane pores contain substantially none of oleophobic material  18  which leaves substrate  12  porous. 
         [0012]    Hydrophilic coating  20  can be any suitable coating, such as, an amine-containing hydrophilic polymer. Any suitable amine-containing hydrophilic polymer can be used in hydrophilic coating  20 , for example, polyvinyl alcohol-coamine polymers. Polyvinyl alcohol-coamine polymers are commercially available from, Celanese under the trade name Erkol®. Hydrophilic coating  20  is deposited on second side  16  of substrate  12  as a continuous film covering the surface of second side  16 , including the pores. In one embodiment, to impart wash durability to the end article, hyrdrophilic coating  20  may be cross-linked using a suitable cross-linking polymer, such as, but not limited to, Polycup® 172, commercially available from Hercules, Inc., Wilmington, Del. 
         [0013]    Referring also to  FIG. 2 , membrane substrate  12  includes a three-dimensional matrix or lattice type structure that is formed by a plurality of nodes  30  interconnected by a plurality of fibrils  32 . Surfaces of nodes  30  and fibrils  32  define a plurality of pores  34  in membrane substrate  12 . Membrane substrate  12  is made from any suitable material, and in the exemplary embodiment is made of expanded polytetrafluoroethylene (ePTFE) that has been at least partially sintered. Generally, the size of a fibril  32  that has been at least partially sintered is in the range of about 0.05 micrometer (μm) to about 0.5 μm in diameter taken in a direction normal to the longitudinal extent of the fibrin. In the exemplary embodiment, membrane substrate  12  has a weight of about 0.1 to about 1.0 ounces per square yard. 
         [0014]    Surfaces of nodes  30  and fibrils  32  define numerous interconnecting pores  34  that extend completely through membrane substrate  12  between opposite major side surfaces in a tortuous path. In the exemplary embodiment, the average effective pore size of pores  34  is sufficient to permit gases to pass through membrane substrate  12  and prevent liquid from passing through membrane substrate  12 . A suitable average effective pore size D for pores  34  is in the range of about 0.01 μm to about 10 μm, and in another embodiment, in the range of about 0.1 μm to about 5.0 μm. In the exemplary embodiment, membrane substrate  12  has an air permeability of at least about 0.10 cubic foot of air per minute per square foot of membrane substrate  12  as measured by ASTM D-737. Also, in the exemplary embodiment, membrane substrate  12  has a moisture vapor transmission rate greater than about 22,000 g/m 2 /day as measured by ISO 15496. 
         [0015]    In the exemplary embodiment, membrane substrate  12  is made by extruding a mixture of polytetrafluoroethylene (PTFE) fine powder particles and lubricant. The extrudate is calendered, and then the calendered extrudate is “expanded” or stretched in at least one and preferably two directions, MD and XD, to form fibrils  32  connecting nodes  30  to define a three-dimensional matrix or lattice type of structure. “Expanded” is intended to mean sufficiently stretched beyond the elastic limit of the material to introduce permanent set or elongation to fibrils  32 . Membrane substrate  12  is then heated or “sintered” to reduce and minimize residual stress in the membrane material by changing portions of the material from a substantially crystalline state to a substantially amorphous state. In an alternate embodiment, membrane substrate  12  is unsintered or partially sintered as is appropriate for the contemplated end use of the membrane. 
         [0016]    In the exemplary embodiment, membrane substrate  12  is made from polytetrafluoroethylene (PTFE) film that has been expanded to form an open pore structured membrane known as expanded polytetrafluoroethylene (ePTFE). In alternate embodiments, woven polytetrafluoroethylene and non-woven polytetrafluoroethylene are used to make membrane substrate  12 . Other materials and methods can be used to form a suitable base membrane substrate  12  that has an open pore structure. For example, other suitable materials include, but are not limited to, polyolefin, polyamide, polyester, polysulfone, polyether, acrylic and methacrylic polymers, polystyrene, polyurethane, polypropylene, polyethylene, and CaCO 3  filled polyethylene. Other suitable methods of making a porous membrane include foaming, skiving or casting any of the suitable materials. 
         [0017]      FIG. 3  is a cross-sectional schematic illustration of another exemplary embodiment of an asymmetric porous membrane  50  that includes a first base membrane substrate  52  and a second base membrane substrate  54  bonded together. The bonding may be done using any known appropriate adhesive or thermal lamination process that enables asymmetric porous membrane  50  to function as described herein. First membrane substrate  52  has a first side  56  and an opposing second side  58 . Second membrane substrate  54  has a first side  60  and a second side  62 . Second side  58  of first membrane substrate is bonded to first side  60  of second membrane substrate  54  with an adhesive  64 , or any other appropriate lamination process. First and second membrane substrates  52  and  54  is similar to membrane substrate  12  described above and includes nodes  30  and fibrils  32 . 
         [0018]    Membrane substrates  52  and  54  can be formed from any suitable material, for example, PTFE, similar to membrane substrate  12  described above. Substrates  52  and  54  formed from PTFE have hydrophobic properties due to the surface tension properties of PTFE. Membrane substrate  52  is treated with oleophobic material  18  to impart oleophobic properties. Other materials can be used to form a base membrane substrates  52  and  54  that have an open pore structure. For example, other suitable materials include, but are not limited to, polyolefin, polyamide, polyester, polysulfone, polyether, acrylic and methacrylic polymers, polystyrene, polyurethane, polypropylene, polyethylene, and CaCO 3  filled polyethylene. Other suitable methods of making a porous membrane include foaming, skiving or casting any of the suitable materials. 
         [0019]    Second membrane substrate  54  is filled with hydrophilic coating  20  before second membrane  54  is bonded to first membrane substrate  52 . Hydrophilic coating  20 , in an exemplary embodiment is dip coated onto second membrane substrate  52  and into pores  34  of membrane substrate  52  before adhesively laminating second substrate  54  to oleophobically treated membrane substrate  52 . 
         [0020]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.