Abstract:
A method of manufacturing a fiber-reinforced article. The method includes the steps of providing a mat fiber structure defining voids therein. A membrane structure is applied over at least a portion of the fiber structure. The membrane structure includes a microporous membrane. At least the microporous membrane of the membrane structure has an oil resistance rating of at least a number 8 determined by AATCC 118 testing. A resin and hardener mix is provided. The resin is infused into voids of the fiber structure by applying a vacuum to the fiber structure and the membrane structure wherein the membrane structure inhibits the flow of resin therethrough.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to fabricating a fiber-reinforced article and particularly to fabricating the article by a vacuum assisted molding process using membrane. 
     Composite articles made from a fiber-reinforced resin matrix that are to be used at relatively elevated temperatures are known. By way of example, such composite articles can include jet engine blades, jet engine nacelles, boat hulls, car bodies and components, wind turbine blades, aircraft structures such as wings, wing parts, radar domes, fuselage components, nose cones, flap tracks, landing gear and rear bulkhead. The reinforcing fibers used in the composite article may be any suitable technical fiber such as fiberglass, carbon, aramid, ceramic, hybrid and the like. Depending on the service that the composite article is going to be put into, the article may be manufactured with a resin applied at an elevated temperature so the surface tension of the resin is relatively low. 
     The known composite articles may be fabricated by infusing resin into a fiber-reinforced layer with vacuum. Laminated sheet material is placed adjacent the fiber-reinforced layer. The laminated sheet material includes a membrane. It is known that the resin is introduced to the fiber-reinforced layer at relatively high temperatures so that the resin has a relatively low surface tension. The resin can, at times, wet or leak through the membrane. When wetting or leaking occurs the molding process is rendered less effective. 
     Therefore, a need exists for an improved membrane structure that can better resist wetting or leaking through the membrane for use in vacuum assisted molding operations with resins at a relatively high temperature and/or that have relatively low surface tensions. 
     BRIEF DESCRIPTION OF THE INVENTION 
     One aspect of the invention is a method of manufacturing a fiber-reinforced article. The method includes the steps of providing a mat fiber structure defining voids therein. A membrane structure is applied over at least a portion of the mat fiber structure. The membrane structure includes a microporous membrane. At least the microporous membrane of the membrane structure has an oil resistance rating of at least a number 8 determined by AATCC 118 testing. A resin and hardener mix is provided. The resin is infused into voids of the mat fiber structure by applying a vacuum to the mat fiber structure and the membrane structure wherein the membrane structure inhibits the flow of resin therethrough. 
     Another aspect of the invention is a method of manufacturing a fiber reinforced article. The method includes the steps of providing mat fiber structure defining voids therein. A membrane structure is applied over the mat fiber structure. The membrane structure has an oil resistance rating of at least a number 8 determined by AATCC 118 testing and an air permeability of at least 0.005 CFM per square foot at 125 Pascals as determined by ASTM D737 testing. A resin and hardener mix is provided. The resin is infused into voids of the mat fiber structure by applying a vacuum to the mat fiber structure and the membrane structure wherein the membrane structure inhibits the flow of resin therethrough. 
     Another aspect of the invention is a membrane structure for use in a transfer molding operation in which a resin and hardener mix is used. The membrane structure includes a microporous membrane. A porous fabric is laminated to the microporous membrane. A treatment material is applied to at least the macroporous membrane. The microporous membrane has an oil resistance of at least a number 8 determined by AATCC 118 testing and having an air permeability of at least 0.005 CFM per square foot determined by ASTM D737 testing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the invention will be better understood when the following detailed description is read with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating a composite article made with fiber-reinforcement according to one aspect of the invention; 
         FIG. 2  is an exploded perspective view illustrating the manufacture of a portion of the composite article shown in  FIG. 1 , according to the one aspect; 
         FIG. 3  is an enlarged cross-sectional view of a membrane structure illustrated in  FIG. 2  used in the manufacture of at least a the portion of the composite article, according to the one aspect; 
         FIG. 4  is a view similar to  FIG. 3  illustrating a membrane structure according to another aspect of the invention; 
         FIG. 5  is a view similar to  FIG. 3  illustrating a membrane structure according to another aspect of the invention; 
         FIG. 6  is a view similar to  FIG. 3  illustrating a membrane structure according to yet another aspect of the invention; and 
         FIG. 7  is a graph illustrating the surface tension of a resin/hardener mix as a function of temperature. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method of fabricating a composite article made with a fiber-reinforced resin matrix utilizing a new and improved oleophobic microporous membrane structure is described below in detail. The new and improved oleophobic microporous membrane structure resists, to an extent that is heretofore unknown, the leaking, “wetting” or the passage of resin with relatively low surface tensions while permitting gas to pass through it. The membrane structure permits a vacuum to be applied relatively evenly to the entire composite article or desired select portions of the composite article during a molding process. The membrane structure also enables the use of resin at operating conditions so the resin has relatively low surface tensions. The membrane structure further facilitates a controlled flow of resin and reduces defects in the article that could result from uneven resin flow. Production cycle time along with labor time is reduced along with a reduction in the cost of process consumable materials. The use of the new and improved oleophobic microporous membrane structure provides, for example, improved quality of the finished part, lower void content, reduced manual rework and optimized fiber to resin ratios. 
     Composite articles made from a fiber-reinforced resin matrix may include, by way of a non-limiting example, an article  20 , such as a carbon reinforced nacelle or housing for an aircraft jet engine  22 , as illustrated in  FIG. 1 . The article  20  or nacelle is positioned about the jet engine  22 . Because of its proximity to the jet engine  22 , at least portions of the article  20  or nacelle may be exposed to elevated temperatures (for example 180° C.) for extended durations as during operation of the jet engine. While a jet engine nacelle is disclosed and described, the article  20  can include, without limitation, jet engine blades, jet engine nacelles, boat hulls, car bodies and components, wind turbine blades, aircraft structures such as wings, wing parts, radar domes, fuselage components, nose cones, flap tracks, landing gear and rear bulkhead. 
     The application for the article  20  may require relatively high strength over the time exposed to elevated temperatures or relatively low weight for which a technical fiber is used, such as fiberglass, carbon, aramid, ceramic, hybrid and the like or mixtures thereof. The article  20  may be manufactured with a vacuum assisted molding process when relatively high dimensional tolerance is required, low void content or high reinforcing fiber content is desired. Vacuum assisted molding processes often apply a resin, or resin and hardener mix, at a relatively elevated temperature so its surface tension is relatively low. 
     The article  20 , according to one aspect of the invention, is made from a carbon fiber reinforced resin matrix structure. The carbon fiber reinforced structure of the article  20  can be made from one or more layers of carbon fiber reinforced material. The article  20  is fabricated by infusing resin, with or without hardener, into the carbon fiber reinforced structure with vacuum. The resin is introduced to the carbon fiber reinforcing structure at a relatively high temperature so that the resin has a relatively low surface tension. 
     The article  20  may be made from a pair of parts. The parts of the article  20  are made separately. The parts of the article  20  are then attached together by suitable means to form the finished article  20 , as illustrated in  FIG. 1 . 
     Referring to  FIG. 2 , each part of the article  20  may include a core  40 . The core  40  typically includes a plurality of grooves to facilitate the flow of resin through core and the remainder of the article  20  during manufacture. The core  40  may be made from any suitable material, such as polymeric foam, wood, and/or a metal honeycomb. Examples of suitable polymeric foams include, but are not limited to, PVC foams, polyolefin foams, epoxy foams, polyurethane foams, polyisocyanurate foams, and mixtures thereof. 
     The article  20  also includes at least one layer of structural reinforcing material  42  located adjacent the core  40 . Each layer of the structural reinforcing material  42  is formed from a mat of reinforcing fibers. Typically, the mat is a woven mat of reinforcing fibers or a non-woven mat of directionally oriented reinforcing fibers. The mat of reinforcing fibers has voids throughout the structural reinforcing material  42  that are to be completely filled with resin. Examples of suitable reinforcing fibers include, but are not limited to, glass fibers, graphite fibers, carbon fibers, polymeric fibers, ceramic fibers, aramid fibers, kenaf fibers, jute fibers, flax fibers, hemp fibers, cellulosic fibers, sisal fibers, coir fibers and, hybrid fibers and the like or mixtures thereof. 
     During manufacture, a resin, which may also be a resin and hardener mix, is infused into the structural reinforcing material  42  of the article  20  to fill the voids in the mat of reinforcing fibers and then cured. The infused resin may be cured with heat and/or time in order to provide the article  20 . The infused resin may be cured with heat and/or time in order to provide the article  20 . The cured resin provides integrity and strength to each finished article  20 . Examples of suitable resins include, but are not limited to, vinyl ester resins, epoxy resins, polyester resins, diglycidyl ether of bishpenol-A based resins, and mixtures thereof. The choice of a resin may depend upon the intended service of the article  20 , the reinforcing fiber used and the manufacturing process. 
     One particularly suitable resin and hardener mix is commercially available under the tradename RTM-6 from Hexcel Corporation. The resin is a monocomponent material having a crosslinker or hardener mixed with the resin. An example of a suitable crosslinker is a blend of cycloaliphatic amines. When cured, the resin displays a three-dimensional structural network with stable properties at the relatively high temperature it may be exposed to. 
     The resin and hardener mix has a relatively low surface tension at the elevated temperature it is typically infused into the reinforcing fiber mat, as illustrated in  FIG. 7 . For example, if applied at 100° C., the surface tension of the resin and hardener mix is in the range of about 21 to 22 dynes/cm. At 80° C. the surface tension of the resin and hardener mix is in the range of about 23 to 24 dynes/cm. Approaching 180° C. the estimated surface tension of the resin and hardener mix is about 13 dynes/cm. The resin and hardener mix with this relatively low surface tension can leak through, or “wet”, heretofore known membrane structures that have been used in vacuum assisted resin transfer molding processes. 
     During manufacture of the article  20 , the structural reinforcing material  42  is arranged relative to the core  40 , if any is used, and then positioned in a mold  60 . A release material  80  is applied to the exposed or outer surface of the structural reinforcing material  42  of the article  20 . The release material  80  is in the form of a release film and peel ply. An oleophobic and gas-permeable membrane structure or membrane assembly  82  is then applied over the release material  80  and the article  20  to facilitate the resin infusion process. 
     An air transport material  84  may be positioned over membrane structure or assembly  82  to further assist in degassing the work-piece by permitting gas displaced during the infusion of resin to escape the voids in the structural reinforcing material  42 . Air transport material  84  can be formed from any suitable mesh or fabric material, for example, a polyethylene mesh. 
     A gas-impermeable vacuum bagging film or vacuum film  86  formed from a suitable material, for example, a polyamid, is positioned over air transport material  84 . A vacuum connection  100  extends through a lateral edge of the vacuum bagging film  86 . A seal  102  extends around the periphery of the mold  60  between the mold and vacuum bagging film  86  to prevent leakage of gas and resin. The seal  102  is in fluid connection with the vacuum connection  100 . 
     A resin infusion input connection  104  extends through a central portion of the vacuum bagging film  86 . The resin infusion connection  104  is in fluid connection with a resin supply tube  106  running essentially for the extent of the mold  60 . The resin supply tube  106  is positioned adjacent the article  20 . 
     The resin is introduced into the resin infusion connection  104 , the resin supply tube  106  and structural reinforcing materials  42  while a vacuum is established through vacuum connection  100 . The vacuum facilitates resin flow and infuses the resin into the voids in the structural reinforcing material  42 . Membrane assembly  82  prevents the resin from flowing away from structural reinforcing materials  42  while permitting gas displaced by the infused resin to escape to the vacuum connection  100 . The supply of resin and vacuum to the article  20  is stopped. The resin is then cured. Resin input connection  104  and supply tube  106 , air transport material  84 , vacuum bagging film  86 , membrane assembly  82  and release material  80  are removed from the article  20 . The article  20  can then be removed from the mold  60  and permitted to further cure, be used, finished or assembled with other components. 
     In one aspect of the invention, the membrane assembly  82  ( FIG. 3 ) includes a microporous membrane  120  thermally or adhesively laminated to a porous fabric backing material  122 . The membrane assembly  82  has a membrane side  124  and a fabric backing side  126 . 
     The membrane  120  is preferably a microporous polymeric membrane that allows the flow of gases, such as air or water vapor, into or through the membrane and is hydrophobic. The membrane  120  is formed from any suitable material, for example, polytetrafluoroethylene, polyolefin, polyamide, polyester, polysulfone, polyether, acrylic and methacrylic polymers, polystyrene, polyurethane, polypropylene, polyethylene, polyphenelene sulfone, and mixtures thereof. A preferred microporous polymeric membrane for use as the membrane  120  is an expanded polytetrafluoroethylene (ePTFE) that has preferably been at least partially sintered. 
     An ePTFE membrane typically comprises a plurality of nodes interconnected by fibrils to form a microporous lattice type of structure, as is known. The membrane  120  has an average pore size of about 0.01 micrometer (μ) to about 10μ. Surfaces of the nodes and fibrils define numerous interconnecting pores that extend completely through the membrane  120  between the opposite major side surfaces of the membrane in a tortuous path. Typically, the porosity (i.e., the percentage of open space in the volume of the membrane  120 ) of the membrane  120  is between about 50% and about 98%. The material, average pore size and surface energy of the membrane  120  help establish the oleophobicity of the membrane. 
     The membrane  120  is preferably made by extruding a mixture of polytetrafluoroethylene (PTFE) fine powder particles (available from DuPont under the name TEFLON® fine powder resin) and lubricant. The extrudate is then calendered. The calendered extrudate is then “expanded” or stretched in at least one direction and preferably two orthogonal directions, to form the fibrils connecting the nodes in 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 the fibrils. The membrane  120  is preferably then heated or “sintered” to reduce and minimize residual stress in the membrane material. However, the membrane  120  may be unsintered or partially sintered as is appropriate for the contemplated use of the membrane. An example of suitable membrane  120  properties includes a unit weight of about 0.42 ounce per square yard, an air permeability of about 1.5 CFM, a Mullen Water Entry pressure of about 15 PSI and a moisture vapor transmission rate (MVTR) of about 60,000 grams per square meter per day (gr/m 2 /day). 
     It is known that porous ePTFE membrane  120 , while having excellent hydrophobic properties, is oleophilic. That is, the material making up the membrane  120  is susceptible to not holding out challenge agents, such as resin and hardener mix at relatively elevated temperature so it has a relatively low surface tension, such as 24 dynes/cm or lower. 
     Other materials and methods can be used to form a suitable membrane  120  that has an open pore structure. For example, other suitable materials that may be used to form a porous membrane include, but are not limited to, polyolefin, polyamide, polyester, polysulfone, polyether, acrylic and methacrylic polymers, polystyrene, polyurethane, polypropylene, polyethylene, cellulosic polymer and combinations thereof. Other suitable methods of making a microporous membrane  120  include foaming, skiving, casting or laying up fibers or nano-fibers of any of the suitable materials. 
     According to one aspect of the invention, at least the membrane  120  of the membrane assembly  82  is treated with an oleophobic fluoropolymer material in such a way that enhanced oleophobic properties result without compromising its gas permeability. According to one non-limiting aspect, the entire membrane assembly  82  is preferably treated with the oleophobic fluoropolymer material. The oleophobic fluoropolymer coating adheres to the nodes and fibrils that define the pores in the membrane  120  and the surfaces of the pores in the fabric backing material  122 . 
     Substantially improved oleophobic properties of at least the microporous membrane  120  can be realized if the surfaces defining the pores in the membrane  120  are treated with the oleophobic fluoropolymer material. The treatment may be applied by to the membrane  120 , the backing material  122  or the membrane assembly  82  any suitable means, such as those disclosed and described in U.S. Pat. No. 6,228,477 or U.S. Patent Application Publication 2004/0059717. The increased oleophobic property of the membrane  120 , backing material  122  or the membrane assembly  82  is important as resins and hardener mixes that are being used that have relatively low surface tensions. 
     The term “oleophobic” is used to describe a material property that is resistant to wetting by liquid challenge materials, such as resin. An “oleophobic property” or “oleophobicity” of the membrane assembly  82  is typically rated on a scale of 1 to 8 according to AATCC test 118. This test objectively evaluates a test specimen&#39;s resistance to wetting by various standardized challenge liquids having different surface tensions. Eight standard challenge liquids, labeled #1 to #8, are used in the test. The #1 challenge liquid is mineral oil (surface tension: 31.5 dynes/cm at 25° C.) and the #8 challenge liquid is heptane (surface tension: 19.61 dynes/cm at 25° C.). According to the test method, five drops of each challenge liquid are placed on one side of the membrane assembly  82  to be tested. Failure occurs when leaking or wetting of the membrane assembly  82  by a selected challenge liquid occurs within 30 seconds. 
     The oleophobic rating number of the membrane assembly  82  corresponds to the last challenge liquid successfully tested. The higher the oleophobic number rating, the greater the oleophobic property, or oleophobicity, as evidenced by resistance to penetration by challenge liquids of relatively lower surface tension. It was found that both the membrane side  124  and the fabric side  126  of the membrane assembly  82  were able to pass a challenge by hexane that has a relatively lower surface tension than heptane. See the table below for the relation of surface tension and temperature of challenge agents. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Surface tension in dynes/cm at ° C. 
               
             
          
           
               
                 Oil repellency 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 grade 
                 Challenge 
               
               
                 number 
                 liquid 
                 20 
                 25 
                 40 
                 50 
                 60 
                 70 
                 80 
                 90 
                 100 
               
               
                   
               
             
          
           
               
                 0 
                 none 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 1 
                 Kaydol 
                   
                 31.5 
               
               
                 2 
                 65 v % Kaydol: 
                   
                 28 
               
               
                   
                 35 v % n- 
               
               
                   
                 hexadecane 
               
               
                 3 
                 n-hexadecane 
                 27.50 
                 27.07 
                 25.79 
                 24.94 
                 24.08 
                 23.23 
                 22.38 
                 21.52 
                 20.67 
               
               
                 4 
                 n-tetradecane 
                 26.60 
                 26.17 
                 24.86 
                 23.99 
                 23.12 
                 22.26 
                 21.39 
                 20.52 
                 19.65 
               
               
                 5 
                 n-dodecane 
                 25.40 
                 24.96 
                 23.63 
                 22.75 
                 21.86 
                 20.98 
                 20.10 
                 19.21 
                 18.33 
               
               
                 6 
                 n-decane 
                 23.80 
                 23.34 
                 21.96 
                 21.04 
                 20.12 
                 19.20 
                 18.28 
                 17.36 
                 16.44 
               
               
                 7 
                 n-octane 
                 21.60 
                 21.12 
                 19.70 
                 18.75 
                 17.80 
                 16.85 
                 15.89 
                 14.94 
                 13.99 
               
               
                 8 
                 n-heptane 
                 20.10 
                 19.61 
                 18.14 
                 17.16 
                 16.18 
                 15.20 
                 14.22 
                 13.24 
                 12.26 
               
               
                   8+ 
                 n-hexane 
                 18.40 
                 18.35 
               
               
                   
               
             
          
         
       
     
     Therefore, a new non-standard rating number of “8+” was adopted to indicate that a tested specimen resisted penetration of hexane under standard test conditions. Thus, the term “preferably at least a number 8 rating” means that a standard number 8 rating or more, such as 8+, is achieved by the tested specimen. This is a significant improvement over known membrane assemblies used in vacuum assisted molding processes. 
     Sample laminates in the form of membrane assemblies  82  were treated according to one aspect of the invention. The properties that resulted from the treatment are reported in the table below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Oil Hold Out 
                   
               
             
          
           
               
                 Membrane 
                 Fabric 
                 Air Perm 
               
               
                 Side 
                 Side 
                 (CFM) 
               
               
                   
               
               
                 8+ 
                 8+ 
                 0.1 
               
               
                   
               
             
          
         
       
     
     Some membranes  120  are relatively thin and fragile. The fabric backing material  122  is included in the membrane assembly  82  to provide support to the membrane  120 . The backing material  122  may have other or alternative functions including, for example, restricting or preventing the flow of the same and/or different particles and fluids as the membrane  120  and/or protecting the membrane  120  or other layers in the membrane assembly  82  from damage from handling. 
     The fabric backing material  122  may be formed from non-woven or woven polymeric fibers, for example, polyester fibers, nylon fibers, polyethylene fibers and mixtures thereof. The backing material  122  is typically made from a porous woven, non-woven or scrim of polymeric material. Often the backing material  122  is made using a fibrous material, however, other porous materials may also be used. The average pore size of the backing material  122  is usually larger than the average pore size of the membrane  120 , although this is not necessary in some applications. Thus, in some applications, the backing material  122  acts to at least partially filter the fluid flowing into or through the laminated article. Typically, the average pore size of the support fabric is about 500 μm (micron) or less and often at least about 0.5 μm. The porosity of the support fabric is often in the range of about 20% to almost 90%. 
     Suitable polymeric materials for the porous backing material  122  include, for example, stretched or sintered plastics, such as polyesters, polypropylene, polyethylene, and polyamides (e.g., nylon). These materials are often available in various weights including, for example, 0.5 oz/yd 2  (about 17 gr/m 2 ), 1 oz/yd 2  (about 34 gr/m 2 ), and 2 oz/yd 2  (about 68 gr/m 2 ). Woven fabric such as 70 denier nylon woven taffeta pure finish may also be used. Another suitable fabric is a non-woven textile such as a 1.8 oz/yd 2  co-polyester flat-bonded bi-component non-woven media. 
     The membrane assembly  82  is gas permeable and oleophobic. That is, the membrane assembly  82  permits the passage of gases through it. The addition of the oleophobic treatment increases the resistance of the membrane assembly  82  to being wet by resin, oil or oily substances. The membrane assembly  82  has an oil hold out or resistance rating of at least a number 8 as determined by AATCC 118 testing. The membrane assembly  82  also has an air permeability of at least 0.01 CFM/ft 2  at 125 Pascals per square foot of membrane measured by ASTM D737 testing. 
     The resulting membrane assembly  82 , according to the aspect illustrated in  FIG. 3 , has a membrane side  124  and a fabric side  126 . The membrane assembly  82  is hydrophobic on both the membrane side  124  and the fabric side  126 . That is, the membrane assembly  82  prevents or resists the passage of liquids, such as water, through the laminated article. The membrane assembly  82  is gas permeable and moisture vapor transmissive. That is, the membrane assembly  82  permits the passage of gases, such as air, carbon dioxide and water vapor, through it. 
     An oleophobic treatment is applied to the entire membrane assembly  82  from an inorganic solvent, according to one aspect of the invention, to provide improved oleophobicity to the entire membrane assembly. The addition of the oleophobic treatment increases the resistance of the membrane assembly  82  to the passage of the resin from either the membrane side  124  or the fabric side  126 . It will be apparent, however, that just the membrane  120  may be treated to increase its oleophobicity. 
     The backing material  122  and the membrane  120  are laminated together. The lamination of the hacking material  122  and the membrane  120  can be accomplished by a variety of methods, such as thermal lamination or adhesive lamination.  FIG. 3  illustrates one aspect of a membrane assembly  82  in which the backing material  122  and membrane  120  are adhered by thermal lamination. The membrane  120  is preferably a microporous expanded polytetrafluoroethylene (ePTFE) membrane available from BHA Group, Inc. as part number QM902. The fabric backing material  122  is preferably a porous layer of spunbond material available from Freudenberg as part number Novatexx 2425. The membrane  120  and fabric backing material  122  are laminated together. The laminated membrane assembly  82  is then treated to increase oleophobic properties, according to one aspect. 
     It has been found that an inorganic fluid under supercritical conditions can dissolve the preferred fluorinated polymer treatment material. The resulting solution is capable of wetting the membrane assembly  82  and entering pores in the microporous membrane  120  with the dissolved fluorinated polymer treatment material. The solution with dissolved fluorinated polymer treatment material has a surface tension, viscosity and relative contact angle that permit the dissolved treatment material to be easily carried into the smallest pores of the membrane  120  and the backing material  122  with the inorganic solvent. 
     The inorganic solvent is preferably carbon dioxide in a supercritical phase. The surface tension of the supercritical carbon dioxide (SCCO 2 ) solution is less than 1 dyne/cm and most preferably less than 0.1 dyne/cm so it can enter very small areas of the membrane assembly  82  to be treated, such as the pores of the membrane  120 . Supercritical carbon dioxide also has a viscosity of less than about 0.1 centipoise. The viscosity and surface tension of the solution are extremely low so very little resistance to flow is encountered, thus, lending itself to the possibility of entering even the smallest pores of the membrane  120 . Effective treatment is possible even if the membrane assembly  82  is in a confined state, such as in a tightly wound roll of sheet material. 
     The fluorinated polymer treatment material, or fluoropolymer, is deposited on and around surfaces of the nodes and fibrils that define the interconnecting pores extending through the membrane  120  and pores of the backing material  122 . This results in a relatively thin and even coating being applied to virtually all the surfaces of the membrane assembly  82 . Once a predetermined proper amount of fluorinated polymer treatment material is deposited on the membrane assembly  82  the pores are not dramatically reduced in flow area from that of an untreated laminated article. Improved oleophobic properties are realized on both the membrane side  124  and the fabric side  126  of the membrane assembly  82 . 
     Examples of suitable fluorinated polymer treatment materials include those having a fluoroalkyl portion or, preferably, a perfluoroalkyl portion. One such fluorinated polymer treatment material is a perfluorakyl acrylic copolymer refereed to as Fabati 100 and was designed and synthesized by Micell Technologies, Inc. Fabati 100 was synthesized in MIBK (methyl isobutyl ketone) utilizing TAN (1,1,2,2,-tetrahydroperfluorooctyl acrylate); butyl acrylate; a cross-linking agent TMI (isopropenyl-a,a-dimethylbenzyl isocyanate); Vazo 52 initiator (2,4-dimethyl-2,2′-azobispentanenitrile). The Fabati 100 treatment material is cross-linked by a post-treatment cure with heat. Another suitable perfluorakyl acrylic copolymer is Fabati 200. Fabati 200 is similar to Fabati 100 but does not have the cross-linking agent (TMI) and HBA (4-hydroxybbutyl acrylate) is used instead of butyl acrylate. Thus, the Fabati 200 treatment material does not require post-treatment heating. 
     A variety of inorganic solvents can be used in the solution containing the oleophobic fluorinated polymer treatment material. The term “inorganic solvent” refers to non-aqueous solvents and combinations of non-aqueous solvents, and, in particular, to solvents comprising inorganic compounds. Suitable inorganic solvents include, for example, carbon dioxide (CO2), ammonia (NH 3 ), urea [(NH 2 ) 2 CO], inorganic acids, such as hydrochloric acid, sulfuric acid, carbon tetrachloride and carbon tetrafluoride and oxides of carbon such as carbon dioxide (CO 2 ), carbon monoxide (CO), potassium carbonate and sodium bicarbonate. A choice of solvent or solvents may be affected by a variety of factors including solubility of the treatment material in the solvent, molecular weight of the solvent and polarity of the solvent. In preferred aspects of the invention, the treatment material is completely dissolved in the inorganic solvent. In other aspects of the invention, the treatment material is not fully dissolved in the inorganic solvent. 
     The amount of fluorinated polymer treatment material in the solution may vary over a wide range. Typically, the amount of fluorinated polymer treatment material in the solution affects the resultant oleophobicity of the membrane assembly  82 . Typically, the amount of fluorinated polymer treatment material, or fluoropolymer, in the solution is about 25 wt % or less and preferably, about 10 wt % or less. For many applications, that the membrane assembly  82  is used in, the amount of fluoropolymer treatment material in the inorganic solvent ranges from about 0.8 wt % to about 10.0 wt % and preferably, from about 2.0 wt % to about 5.0 wt %. 
     According to one aspect of the invention, the backing material  122  and membrane  120  of the membrane assembly  82  are treated together subsequent to lamination of the backing material  122  and membrane  120 . Typically, during treatment, the fluorinated polymer solution wets and, preferably, saturates, the backing material  122  and membrane  120  of the membrane assembly  82 . The use of an inorganic solvent facilitates the relatively uniform distribution of the fluorinated polymer treatment material throughout the backing material  122  and membrane  120  of the laminated article. The inorganic solvent is then removed. The fluorinated polymer treatment material attaches to the backing material  122  and membrane  120  and enhances the oleophobicity at both sides  124 ,  126  of the membrane assembly  82 . 
     Optionally, the treated membrane assembly  82  may then be “cured” by heating. The “curing” process increases the oleophobicity by allowing rearrangement of the fluoropolymer into an oleophobic orientation. The curing temperature varies among fluoropolymers. 
     The membrane assembly  82  has a relatively high moisture vapor transmission rate (MVTR) and air permeability while its oleophobic properties are improved by the treatment material. Both sides  124 ,  126  of the membrane assembly  82  have an oil hold out rating of at least a number 8 rating as determined by AATCC 118 testing and preferably at least a number 8+ rating. The membrane assembly  82  preferably has a moisture vapor transmission rate (MVTR) of at least 1500 gr/m 2 /day and more preferably at least 15,000 g/m 2 /day measured by JISL-1099B2 testing. The membrane assembly  82  preferably has an air-permeability of at least 0.005 CFM per square foot of membrane, preferably at least 0.01 CFM per square foot of membrane and more preferably at least 0.05 CFM per square foot of membrane measured by ASTM D737 testing. 
     Improved oleophobic properties of the membrane assembly  82  are realized according to one aspect of the invention by treating surfaces defining the pores in the membrane  120  and backing material  122  as well as the surfaces of the membrane side  124  and the fabric side  126  of the membrane assembly  82  with a fluorinated polymer treatment material, or fluoropolymer. The membrane assembly  82 , according to one aspect of the invention, has the treatment material coating even the smallest pores of the membrane  120  of the laminated article. The applied treatment material modifies properties of the entire membrane assembly  82 , such as oleophobicity. 
     An alternate aspect of the invention is to use two or more membrane assemblies  82  physically overlaid together instead of being laminated together. In testing, a sample membrane structure included three membrane assemblies  82  overlaid together. The resultant membrane structure resisted leaking and wetting with the RTM-6 resin and hardener mix until the temperature of the resin and hardener mix reached 180° C. The surface tension at 180° C. for the RTM-6 resin and hardener mix is about 13 dynes/cm. In terms of AATCC 118 testing, the membrane assembly  82  has a hold out rating of at least a number 8 and even a number 8+. Air permeability of this membrane structure was 0.1 CFM at 125 Pascal per square foot of membrane measured by ASTM D737 testing. 
     A membrane assembly  82   a , according to another aspect of the invention illustrated in  FIG. 4 , includes two membrane assemblies  82  as illustrated in  FIG. 3  laminated together and then subjected to the oleophobic treatment. The membrane assembly  82   a  has a membrane side  124  and a fabric side  126 . The membrane assembly  82   a  is hydrophobic on both the membrane side  124  and the fabric side  126 . That is, the membrane assembly  82   a  prevents or resists the passage of liquids, such as water, through the laminated article. The membrane assembly  82   a  is gas permeable and moisture vapor transmissive. That is, the membrane assembly  82   a  permits the passage of gases, such as air, carbon dioxide and water vapor, through it. 
     The membrane assembly  82   a  is oleophobic. The addition of the oleophobic treatment increases the resistance of the membrane assembly  82   a  to leakage or being wet by liquid resin, oil or oily substances. The membrane assembly  82   a  has an oil hold out or resistance rating of at least a number 8 as determined by AATCC 118 testing and preferably at least a number 8+ rating. The membrane assembly  82   a  also has an air permeability of at least 0.01 CFM/ft 2  at 125 Pascal as determined by ASTM D737 testing. 
     The membrane assembly  82   b , according to another aspect illustrated in  FIG. 5 , includes two membrane assemblies laminated together. The membrane assembly  82   b  is similar to membrane assembly  82   a . However, the membranes  140  used have an oleophobic treatment applied differently than the membranes  120 . The membranes  140  are treated by the method disclosed in U.S. Pat. No. 6,288,477. The treated membranes  140  are then laminated to untreated fabric backing material to form the membrane assembly  82   b . The membrane assembly  82   b  has a membrane side  144  and a fabric side  126 . The membrane assembly  82   b  is hydrophobic on both the membrane side  144  and the fabric side  126 . That is, the membrane assembly  82   b  prevents or resists the passage of liquids, such as water, through the laminated article. The membrane assembly  82   b  is gas permeable and moisture vapor transmissive. That is, the membrane assembly  82   b  permits the passage of gases, such as air, carbon dioxide and water vapor, through it. 
     The membrane assembly  82   b  is oleophobic. The addition of the oleophobic treatment increases the resistance of the membrane assembly  82   b  to being leakage or by resin, oil or oily substances. The membrane assembly  82   b  has an oil hold out or resistance rating of at least a number 8 as determined by AATCC 118 testing and preferably at least a number 8+ rating. The membrane assembly  82   b  also has an air permeability of at least 0.01 CFM/ft 2  at 125 Pascal per square foot of membrane measured by ASTM D737 testing. 
     The membrane assembly  82   c , according to yet another aspect illustrated in  FIG. 6 , includes two membranes  120  and  160  laminated to the backing material  122 . The membranes  120  and  160  may be laminated together or integrally formed as one piece during manufacturing. The membranes  120 ,  160  may be identical or have different thicknesses, pore sizes, void spaces, or other properties. Either one, none or both of the membranes  120 ,  160  may be treated to enhance their oleophobic properties. The membranes  120 ,  160  are laminated to the fabric backing material  122 . The membrane assembly  82   c  could be treated to increase oleophobicity after lamination. The membranes  120 ,  160  could be located on opposite sides of the fabric backing material  122 . 
     The membrane assembly  82   c  has a membrane side  164  and a fabric side  126 . The membrane assembly  82   c  is hydrophobic on both the membrane side  164  and the fabric side  126 . That is, the membrane assembly  82   c  prevents or resists the passage of liquids, such as water, through the laminated article. The membrane assembly  82   c  is gas permeable and moisture vapor transmissive. That is, the membrane assembly  82   c  permits the passage of gases, such as air, carbon dioxide and water vapor, through it. 
     The membrane assembly  82   c  has enhanced oleophobic properties. The addition of the oleophobic treatment increases the resistance of the membrane assembly  82   c  to leakage or being wet by resin, oil or oily substances. The membrane assembly  82   c  has an oil hold out or resistance rating of at least a number 8 as determined by AATCC 118 testing and preferably at least a number 8+ rating so it can inhibit resin flow through the membrane assembly when the surface tension of the resin at 25 C is lower than 19.61 dynes/cm. The membrane assembly  82   c  also has an air permeability of at least 0.01 CFM/ft 2  at 125 Pascal per square foot of membrane measured by ASTM D737 testing. 
     An alternate aspect of the invention is to use two or more membrane assemblies  82   c  physically overlaid together instead of being laminated together. In testing, a sample membrane structure included three membrane assemblies  82   c  overlaid together. The resultant membrane structure resisted leakage and wetting with the RTM-6 resin and hardener mix until the temperature of the resin and hardener mix reached 180° C. The surface tension at 180° C. for the RTM-6 resin and hardener mix is about 13 dynes/cm. In terms of AATCC 118 testing, the membrane assembly  82  has a hold out rating of at least a number 8 and even a number 8+. Air permeability of this membrane structure was 0.1 CFM at 125 Pascal per square foot of membrane measured by ASTM D737 testing. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the systems, techniques and obvious modifications and equivalents of those disclosed. It is intended that the scope of the invention disclosed should not be limited by the particular disclosed embodiments described above.