Patent Publication Number: US-2022234008-A1

Title: Highly oriented expanded polytetrafluoroethylene with superior stiffness

Description:
FIELD 
     The present invention is directed to microporous fluoropolymer membranes, and more specifically, to self-supporting, highly expanded polytetrafluoroethylene (ePTFE) membranes that have high intrinsic strength, high matrix modulus, a high crystallinity index, and aligned fibrils. 
     BACKGROUND 
     Polytetrafluoroethylene is an attractive material due to one or more properties such as inertness to many chemicals, biocompatibility, thermal stability, low surface energy, low coefficient of friction, and the ability to be processed into a variety of form factors, such as membranes, fibers, tubes, and the like. Expanded polytetrafluoroethylene (ePTFE) may be used alone or used in composites and/or laminates to produce articles for use in a variety of applications. Many of these applications may benefit from using materials that are lighter, thinner, stronger, and/or have improved optical properties. As such, there is an ever present need to provide ePTFE articles with improved properties. 
     SUMMARY 
     According to one aspect, (“Aspect 1”), an expanded polytetrafluoroethylene (ePTFE) membrane includes a matrix tensile strength at least about 1000 MPa in the machine direction, a matrix modulus of at least about 100 GPa at a temperature of 20° C., and a crystallinity index of at least about 94%. 
     According to another aspect, (“Aspect 2”) further to Aspect 1, the ePTFE membrane has an areal density less than about 30 g/m 2 . 
     According to another aspect, (“Aspect 3”) further to Aspect 1 and Aspect 2, the ePTFE membrane has a &lt;P2&gt; orientation greater than or equal to 0.98. 
     According to another aspect, (“Aspect 4”) further to any one of the preceding Aspects, the ePTFE membrane has a bulk denier less than about 750 g/9000 m. 
     According to another aspect, (“Aspect 5”) further to any one of the preceding Aspects, the ePTFE membrane has a tenacity greater then about 5 gf/d. 
     According to another aspect, (“Aspect 6”) further to any one of the preceding Aspects, the ePTFE membrane is self-supporting. 
     According to another aspect, (“Aspect 7”) further to any one of the preceding Aspects, the ePTFE membrane is uniaxially oriented. 
     According to another aspect, (“Aspect 8”) further to any one of the preceding Aspects, the ePTFE membrane is at least partially coated with a polymer, at least partially imbibed with a polymer, or a combination thereof. 
     According to another aspect, (“Aspect 9”) further to any one of the preceding Aspects, the ePTFE membrane is in the form of a fiber, a sheet, a tube, a three dimensional self-supporting structure, a diced fiber, a diced sheet, a diced tube, or a diced three dimensional self-supporting structure. 
     According to another aspect, (“Aspect 10”) further to any one of the preceding Aspects, the ePTFE membrane includes a spacing layer. 
     According to another aspect, (“Aspect 11”) further Aspect 10, the spacing layer is selected from a porous polymer, a non-porous polymer, a fluoropolymer, a porous polyolefin, and a non-porous polyolefin. 
     According to another aspect, (“Aspect 12”) a composite includes the expanded polytetrafluoroethylene membrane of any one preceding Aspect. 
     According to another aspect, (“Aspect 13”) a laminate includes the expanded polytetrafluoroethylene membrane of any one preceding Aspect. 
     According to another aspect, (“Aspect 14”) an article includes the expanded polytetrafluoroethylene membrane of Aspects 1-11, the composite of Aspect 12 or the laminate of Aspect 13. 
     According to another aspect, (“Aspect 15”), a method of forming a uniaxially oriented ePTFE membrane includes (1) sectioning at least a first piece from a first expanded polytetrafluoroethylene (ePTFE) membrane, (2) biaxially stretching said at least a first piece to obtain a second expanded polytetrafluoroethylene membrane, (3) sectioning at least a second piece from said second expanded membrane, (4) positioning said at least one first piece and said at least one second piece in a stacked orientation to form stacked sample, (5) repeating steps (1) through (4) until a desired biaxially oriented ePTFE membrane is obtained, and (6) uniaxially stretching the biaxially oriented ePTFE membrane. 
     According to another aspect, (“Aspect 16”) further to Aspect 15, the method further includes adding a spacing layer. 
     According to another aspect, (“Aspect 17”) further to Aspect 16, the spacing layer is selected from a porous polymer, a non-porous polymer, a fluoropolymer, a porous polyolefin, and a non-porous polyolefin. 
     According to another aspect, (“Aspect 18”) further to Aspect 15 to Aspect 17, the ePTFE membrane is uniaxially stretched in the machine direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  is a scanning transmission electron microscopy (STEM) image of Sample E1G from Example 1 taken at 2,000× magnification with a full horizontal field width of approximately 63 microns in accordance with embodiments described herein; 
         FIG. 2  is a STEM image of Sample E1G from Example 1 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 3  is a STEM image of Sample E1G from Example 1 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 4  is a STEM image of Sample E1H from Example 1 taken at 20,000× magnification with a full horizontal field width of approximately 6 microns in accordance with embodiments described herein; 
         FIG. 5  is a STEM image of Sample E1I from Example 1 taken at 2,000× magnification with a full horizontal field width of approximately 63 microns in accordance with embodiments described herein; 
         FIG. 6  is a STEM image of Sample E2E from Example 2 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 7  is a STEM image of Sample E2F from Example 2 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 8  is a STEM image of Sample E2G from Example 2 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 9  is a STEM image of Sample E3A from Example 3 taken at 100,000× magnification with a full horizontal field width of approximately 1.27 microns in accordance with embodiments described herein; 
         FIG. 10  is a STEM image of Sample E3B from Example 3 taken at 20,000× magnification with a full horizontal field width of approximately 6 microns in accordance with embodiments described herein; 
         FIG. 11  is a STEM image of Sample E1H (taken at 20,000× magnification with a full horizontal field width of approximately 6 microns) from Example 5 where manual image analysis was used to measure fibril width in accordance with embodiments described herein; 
         FIG. 12  is a histogram of the fibril width data in nanometers and a fit of a lognormal distribution from the manual image analysis of  FIG. 11  of Example 5 in accordance with embodiments described herein; 
         FIG. 13  is a STEM image of Sample E5A from Example 5 taken at 6,000× magnification with a full horizontal field width of approximately 21 microns in accordance with embodiments described herein; 
         FIG. 14  is a STEM image of Sample E5B from Example 5 taken at 6,000× magnification with a full horizontal field width of approximately 21 microns in accordance with embodiments described herein; 
         FIG. 15  is a STEM image of Sample E5C from Example 5 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 16  is a STEM image of Sample E5D from Example 5 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 17  is a STEM image of Sample E5E from Example 5 taken at 6,000× magnification with a full horizontal field width of approximately 21 microns in accordance with embodiments described herein; 
         FIG. 18  is a STEM image of Sample E5F from Example 5 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 19  is a STEM image of Sample E5G from Example 5 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 20  is a scanning electron microscope (SEM) image of Sample E5H from Example 5 taken at 10,000× magnification with a full horizontal field width of approximately 21 microns in accordance with embodiments described herein; 
         FIG. 21  is an SEM image of Sample E5I from Example 5 taken at 20,000× magnification with a full horizontal field width of approximately 6 microns in accordance with embodiments described herein; 
         FIG. 22  is an SEM image of Sample E5H from Example 5 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 23  is an SEM image of Sample E5I from Example 5 taken at 5,000× magnification with a full horizontal field width of approximately 25 microns in accordance with embodiments described herein; 
         FIG. 24  is a graphical illustration of the quality factor vs. the particle diameter for Samples E6A, E6B, E6D, and E6E from Example 6 in accordance with embodiments described herein; 
         FIG. 25  is a STEM image of Sample E6A from Example 6 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein; 
         FIG. 26  is a STEM image of Sample E6B from Example 6 taken at 10,000× magnification with a full horizontal field width of approximately 12 microns in accordance with embodiments described herein 
         FIG. 27  is a STEM image of Sample E6A from Example 6 taken at 1500× magnification with a full horizontal field width of approximately 84 microns in accordance with embodiments described herein; 
         FIG. 28  is a STEM image of Sample E6B from Example 6 taken at 1500× magnification with a full horizontal field width of approximately 84 microns in accordance with embodiments described herein; 
         FIG. 29  is a graphical illustration of % transmission vs. wavelength for Sample E7A (grey line; 48 layers) and Sample E7B (black line; 3 layers) in accordance with embodiments described herein; 
         FIG. 30  is an x-ray diffraction (XRD) pattern for Sample E8C from Example 8 in accordance with embodiments described herein; 
         FIG. 31  is an x-ray diffraction pattern (XRD) for Sample E8D (heat treated) from Example 8 in accordance with embodiments described herein; 
         FIG. 32  is a graphical illustration of q (nm −1 ) versus intensity (10-45 nm −1 ) for Samples E8C (bottom trace—not heat treated) and E8D (top trace—heat treated) from Example 8 in accordance with embodiments described herein; 
         FIG. 33  is a graphical illustration of q(nm −1 ) versus intensity (focusing on 10-20 nm −1  range) for Samples E8C (bottom trace—not heat treated) and E8D (top trace—heat treated) from Example 8 in accordance with embodiments described herein; 
         FIG. 34  is a graphical illustration of matrix storage modulus vs. temperature for Sample E9A from Example 9 in accordance with embodiments described herein; 
         FIG. 35  is an x-ray diffraction pattern (XRD) for Sample E9A from Example 9 in accordance with embodiments described herein; 
         FIG. 36  is a graphical illustration of intensity vs 2-theta for Sample E9A from Example 9 in accordance with embodiments described herein; and 
         FIG. 37  is an SEM image of Sample E9A from Example 9 taken at 5000× with a full horizontal field width of approximately 23 microns in accordance with embodiments described herein; 
         FIG. 38  is a graphical illustration of nanoparticle vs. filtrate permeability retention for Example 10 samples in accordance with embodiments described herein; and 
         FIG. 39  is a diagram generally depicting a method of making a biaxially expanded membrane in accordance with embodiments described herein. 
     
    
    
     GLOSSARY 
     Average Fibril Width:  w  (nm). 
     Median Fibril Width: w m  (nm). 
     Area Weighted Fibril Width (AWFW):  w   AWFW  (nm). 
     Area Weighted Fibril Width was calculated utilizing the following Equation: 
     
       
         
           
             
               
                 w 
                 ¯ 
               
               
                 A 
                 ⁢ 
                 W 
                 ⁢ 
                 F 
                 ⁢ 
                 W 
               
             
             = 
             
               
                 ∑ 
                 
                   
                     n 
                     i 
                   
                   ⁢ 
                   
                     w 
                     i 
                     2 
                   
                 
               
               
                 ∑ 
                 
                   
                     n 
                     i 
                   
                   ⁢ 
                   
                     w 
                     i 
                   
                 
               
             
           
         
       
     
     Specific Surface Area (SSA) 
     
       
         
           
             
               A 
               
                 V 
                 ⁢ 
                 
                   ρ 
                   x 
                 
               
             
             ⁢ 
             
               ( 
               
                 
                   m 
                   2 
                 
                 / 
                 g 
               
               ) 
             
           
         
       
     
     was calculated with the following Equation: 
     
       
         
           
             
               S 
               ⁢ 
               S 
               ⁢ 
               A 
             
             = 
             
               
                 A 
                 
                   V 
                   
                     ρ 
                     x 
                   
                 
               
               . 
             
           
         
       
     
     where: 
     Surface area: A (m 2 ); 
     Volume: V (m 3 ); and 
     Density of Crystalline PTFE ρ x  (g/m 3 ). 
     Specific Surface Area (based on w m ) (m 2 /g) was calculated with the following Equation: 
     
       
         
           
             
               S 
               ⁢ 
               S 
               ⁢ 
               
                 A 
                 
                   w 
                   m 
                 
               
             
             = 
             
               
                 4 
                 
                   
                     ρ 
                     x 
                   
                   ⁢ 
                   
                     w 
                     m 
                   
                 
               
               . 
             
           
         
       
     
     Specific Surface Area (based on  w   AWFW ) (m 2 /g) was calculated with the following Equation: 
     
       
         
           
             
               S 
               ⁢ 
               S 
               ⁢ 
               
                 A 
                 
                   
                     w 
                     ¯ 
                   
                   
                     A 
                     ⁢ 
                     W 
                     ⁢ 
                     F 
                     ⁢ 
                     W 
                   
                 
               
             
             = 
             
               
                 4 
                 
                   
                     ρ 
                     x 
                   
                   ⁢ 
                   
                     
                       w 
                       ¯ 
                     
                     
                       A 
                       ⁢ 
                       W 
                       ⁢ 
                       F 
                       ⁢ 
                       W 
                     
                   
                 
               
               . 
             
           
         
       
     
     Areal Density (Mass per area) (g/m 2 ):
         Areal Density (initial): MPA o , and   Areal Density (final): MPA f .       

     Number of layers (n). 
     Area Ratio (AR) was calculated by the following Equation: 
     
       
         
           
             
               A 
               ⁢ 
               R 
             
             = 
             
               
                 
                   n 
                   ⁢ 
                   M 
                   ⁢ 
                   P 
                   ⁢ 
                   
                     A 
                     o 
                   
                 
                 
                   M 
                   ⁢ 
                   P 
                   ⁢ 
                   
                     A 
                     f 
                   
                 
               
               . 
             
           
         
       
     
     DETAILED DESCRIPTION 
     Persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatus configured to perform the intended functions. It should also be noted that the accompanying figures referred to herein are not necessarily drawn to scale, but may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawing figures should not be construed as limiting. 
     In addition, the terms “adjacent” and “adjacent to” as used herein are meant to denote that when an element is “adjacent” to another element, the element may be directly adjacent to the other element or intervening elements may be present. As used herein, the singular forms “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. The term “on” as used herein is meant to denote that when an element is “on” another element, it can be directly on the other element or intervening elements may also be present. It is to be appreciated that the terms “fine powder” and “powder” may be used interchangeably herein. Also, the terms “ePTFE membrane(s)” and “membrane(s)” may be used interchangeably herein. Further, in this application, the term “ePTFE membrane” is meant to include a single layer or multiple layers of ePTFE membrane(s). It is to be understood that the machine direction and the longitudinal direction are the same and may be interchangeably used herein. In addition, the terms “microporous ePTFE membrane” and “ePTFE membrane” may be used interchangeably herein. 
     In one aspect, the present invention is directed to thin, self-supporting biaxially oriented polytetrafluoroethylene (ePTFE) membranes that have a high crystallinity index, high intrinsic strength, low areal density (i.e., lightweight), and high optical transparency. In particular, the ePTFE membrane may have a crystallinity index of at least about 94% and a matrix tensile strength of at least about 600 MPa in both the longitudinal and transverse directions. The ePTFE membrane may also have an areal density less than about 100 mg/m 2  and a total luminous transmittance of at least 98%. In addition, the ePTFE membrane is transparent or invisible to the naked eye. Further, the ePTFE membrane is stackable, which may be used to control permeability, pore size, and/or bulk mechanical properties. The ePTFE membrane may be used to form composites, laminates, fibers, sheets, tubes, or other three-dimensional objects, which may or may not be subsequently diced or otherwise cut or sectioned into smaller portions. Additionally, the biaxially oriented ePTFE membranes may be used in filtration applications. In another aspect, the biaxially oriented ePTFE membrane may be further uniaxially expanded, which aligns the fibrils in one direction (hereafter a uniaxially oriented ePTFE membrane). Such an ePTFE membrane may have a tenacity greater then about 5 grams force per denier (gf/d) and a bulk denier less than about 750 grams per 9000 meters (g/9000 m). 
     With polytetrafluoroethylene (PTFE) polymers, the particle size, shape, and distribution thereof are important to obtain desired porous structures. These particle characteristics affect the packing density as well as connection density, thereby affecting the porous structures that can be produced from the particles. The PTFE resin is provided in a particulate form, for example, in the form of a fine powder. PTFE fine powders are formed of primary particles. 
     In forming the ePTFE membrane, the PTFE fine powder is first mixed with a lubricant, such as a light mineral oil. One particular example of a suitable lubricant is an isoparaffinic hydrocarbon, such as ISOPAR™ K (ExxonMobil Chemical, Spring, Tex.). Other suitable lubricants include aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, and the like, and are selected according to flammability, evaporation rate, and economic considerations. It is to be appreciated that the term “lubricant”, as used herein, is meant to describe a processing aid that includes (or consists of) an incompressible fluid that is not a solvent for the polymer at the process conditions. The fluid-polymer surface interactions are such that it is possible to create a homogenous mixture. It is also to be noted that that choice of lubricant is not particularly limiting and the selection of lubricant is largely a matter of safety and convenience. The lubricant may be added to the PTFE powder in an amount from about 242 mL/kg to about 340 mL/kg. 
     In at least one embodiment, the PTFE fine powder and lubricant are mixed so as to uniformly or substantially uniformly distribute the lubricant with the PTFE powder. It is to be appreciated that various times and mixing methods may be used to distribute the PTFE powder in the lubricant. Once the lubricant and PTFE powder are sufficiently distributed, the lubricated powder is compressed into a cylindrical form (i.e., a pellet). The pellet may then be ram extruded (e.g., typically called paste extrusion or paste processing when lubricant is present) through an extruder die to produce a cohesive, flexible PTFE tape. As used herein, the term “cohesive” is meant to describe a tape that is sufficiently strong for further processing. The ram extrusion occurs below the melting temperature PTFE polymer (e.g., below 327° C.). The tape formed has an indeterminate length and a thickness less than about 1.0 mm, less than about 0.8 mm, less than about 0.5 mm, or less than about 0.4 mm. The cohesive, flexible tapes are referred hereafter simply as “tape”. 
     In a subsequent step, the lubricant is removed from the tape. In instances where ISOPAR™ K is the lubricant, the tape may be heated to about 200° C. In other embodiments, the lubricant may be removed by washing the tape in hexane or other suitable solvent. If the lubricant is of sufficient volatility, the lubricant may be removed without a washing step, or it may be removed by heat and/or vacuum. It is to be appreciated, however, that any conventional drying method may be used. 
     The tape is then expanded in the longitudinal and transverse directions simultaneously (i.e., biaxially expanded). As used herein, the terms “biaxially expanded”, “biaxial expansion”, and “biaxially oriented” are meant to describe a polymer, membrane, preform, or article that is expanded in at least two orthogonal directions such that the fibrils are substantially oriented in-plane. In one embodiment, the tape is subsequently expanded only in the machine direction (i.e., uniaxially expanded). As used herein, the term “uniaxial”, “uniaxially oriented” or “uniaxial expansion” is meant to describe a polymer, membrane, preform, or article that is expanded in only one direction (e.g., either the machine direction (MD) or the transverse direction (TD)). The expansion may be conducted with or without heat at strain rates up to about up to about 10,000%/second, up to about 5,000%/second, up to about 2,500%/second, up to about 1,000%/second, up to about 750%/second, up to about 500%/second, up to about 250%/second, up to about 150%/second, up to about 100%/second, up to about 75%/second, up to about 50%/second, up to about 40%/second, up to about 35%/second, up to about 30%/second, up to about 20%/second, up to about 10%/second, or up to about 5%/second. Additionally, the tape may be expanded (with or without heat) from about 1%/second to about 10,000%/second, from about 1%/second to about 5,000%/second from about 1%/second to 2,500%/second, from about 1%/second to about 1,000%/second, from about 1%/second to about 750%/second, from about 1%/second to about 500%/second, from about 1%/second to about 250%/second, from about 1%/second to about 150%/second, from about 1%/second to about 100%/second, from about 1%/second to about 75%/second, from about 1%/second to about 50%/second, from about 1%/second to about 40%/second, from about 1%/second to about 35%/second, from about 1%/second to about 30%/second, from about 1%/second to about 20%/second, from about 1%/second to about 10%/second, or from about 1%/second to about 5%/second. It is to be appreciated that an increase in intrinsic strength concurrently occurs upon expansion. The increase in intrinsic strength of the PTFE polymer is dependent upon the strength of the tape prior to expansion, the quality of the PTFE resin (e.g., particle size, molecular weight, distribution of particle size and/or molecular weight, degree of crystallinity, composition of polymer, and the like), the temperature at which expansion is performed, the rate of expansion, and/or the total amount of expansion. 
     The tape is biaxially and in some embodiments, additionally uniaxially expanded, to form an ePTFE membrane. The tape may be expanded at the same or different strain rates and at the same or different temperatures to achieve the microporous ePTFE membrane. As used herein, the term “microporous” is meant to define an article, such as a membrane, that has pores that are not visible to the naked eye. It has been discovered that the material properties of an ePTFE membrane produced in this manner exceed comparative properties of conventional membranes through an efficient and complete conversion of the PTFE primary particles (i.e., PTFE fine powder) into fibrils. Advantageously, the ePTFE membranes discussed herein retain the properties of conventional ePTFE membranes, such as, but not limited to, chemical inertness, thermal stability, low surface energy, low coefficient of friction, biocompatibility, and a wide range of use temperatures. The ePTFE membrane may optionally be heat treated at a temperature up to about 390° C. Uniaxially stretching the ePTFE membrane creates an ePTFE membrane with uniaxially-oriented fibrils, a high crystallinity index, and a high matrix tensile strength in the direction in which it was stretched (i.e., the machine direction (MD) or the transverse direction (TD)). Hereafter, the ePTFE membrane is described with respect to expansion in the machine direction, but it is to be appreciated that expanding in the transverse direction is considered to be within the purview of the invention. 
     The biaxially oriented ePTFE membrane is very thin, and may have a total membrane thickness less than about 2 mm, less than about 1.5 mm, less than about 1.0 mm, less than about 0.5 mm, less than about 0.3 mm, less than about 0.1 mm, less than 0.05 mm, less than 0.005 mm, less than 0.001 mm, less than about 500 nm, less than about 400 nm, less than about 300 nm, less than about 200 nm, less than about 100 nm, less than about 50 nm, less than about 25 nm, less than about 10 nm, less than about 5 nm, or less than about 1 nm. As used herein, the term “about” is meant to denote a range+/−10% of the number or amount being described. The biaxially oriented ePTFE membrane may be formed to have a total membrane thickness from about 1 nm to about 100 nm, from about 1 nm to about 90 nm, from about 1 nm to about 80 nm, from about 1 nm to about 70 nm, from about 1 nm to about 60 nm, from about from about 1 nm to about 50 nm, from about 1 nm to about 40 nm, from about 1 nm to about 30 nm, from about 1 nm to about 20 nm, or from about 1 nm to about 10 nm. 
     In at least one embodiment, the biaxially oriented ePTFE membrane has a thickness per layer less than about 100 nm, less than about 90 nm, less than about 80 nm, less than about 70 nm, less than about 60 nm, less than about 50 nm, or less than about 40 nm, less; than about 30 nm, less than about 20 nm, less than about 10 nm, less than about 5 nm, less than about 4 nm, less than about 3 nm, less than about 2 nm, less than about 1 nm. In some embodiments, the ePTFE membrane has a thickness per layer from about 1 nm to about 100 nm, from about 1 nm to about 90 nm, from about 1 nm to about 80 nm, from about 1 nm to about 70 nm, from about 1 nm to about 60 nm, from about 1 nm to about 50 nm, from about 1 nm to about 40 nm, from about 1 nm to about 30, from about 1 nm to about 20 nm, from about 1 nm to about 10 nm, from about 1 nm to about 5 nm, from about 1 nm to about 4 nm, from about 1 nm to about 3 nm, or from about 1 nm to about 2 nm. The biaxially oriented ePTFE membrane, unlike conventional ePTFE membranes, is so thin as to be invisible to the naked eye. 
     The “invisibility” of the biaxially oriented ePTFE membranes is also at least partially due to the fibril microstructure of the ePTFE membranes. Generally, the fibrils are substantially cylindrical in shape. The term “substantially cylindrical” as used herein is meant to denote that the fibrils in the biaxially oriented ePTFE membranes have an aspect ratio in cross section of about 1:1 to about 10:1. In addition, the fibrils in the biaxially oriented ePTFE membranes are thin, and have a median fibril width that is not greater than about 80 nm. In some embodiments, the median fibril width is less than about 70 nm, less than about 60 nm, less than about 50 nm, less than about 40 nm, less than about 30 nm, less than about 20 nm, or less than about 10 nm. In some embodiments, the median fibril width is from about 10 nm to about 80 nm, from about 10 nm to about 70 nm, from about 10 nm to about 60 nm, from about 10 nm to about 50 nm, from about 10 nm to about 40 nm, from about 10 nm to about 30 nm, or from about 10 nm to about 20 nm. In some embodiments, the median fibril width is from about 20 nm to about 70 nm, from about 30 nm to about 60 nm, from about 40 nm to about 50 nm. In other embodiments, the median fibril width is from about 30 nm to about 80 nm, from about 40 nm to about 80 nm, from about 50 nm to about 80 nm, from about 60 nm to about 80 nm, or from about 70 nm to about 80 nm. The intersection or overlap of two or more fibrils are called “crossover points” herein. In some embodiments, the thickness of a biaxially oriented ePTFE membrane may be the thickness of the crossover point of two fibrils. 
     In addition, the biaxially oriented ePTFE membranes are extremely light, having an areal density per layer less than about 100 mg/m 2  (0.1 g/m 2 ), less than about 90 mg/m 2  (0.09 g/m 2 ), less than about 80 mg/m 2  (0.08 g/m 2 ), less than about 70 mg/m 2  (0.07 g/m 2 ), less than about 60 mg/m 2  (0.06 g/m 2 ), less than about 50 mg/m 2  (0.05 g/m 2 ), less than about 40 mg/m 2  (0.04 g/m 2 ), less than about 30 mg/m 2  (0.03 g/m 2 ), less than about 20 mg/m 2  (0.02 g/m 2 ), less than about 15 mg/m 2  (0.015 g/m 2 ), less than about 10 mg/m 2  (0.01 g/m 2 ), less than about 5 mg/m 2  (0.005 g/m 2 ), less than about 4 mg/m 2  (0.004 g/m 2 ), less than about 3 mg/m 2  (0.003 g/m 2 ), less than about 2 mg/m 2  (0.002 g/m 2 ), less than 1.0 mg/m 2  (0.001 g/m 2 ), less than about 0.50 mg/m 2  (0.0005 g/m 2 ), less than about 0.40 mg/m 2  (0.0004 g/m 2 ), less than about 0.30 mg/m 2  (0.0003 g/m 2 ), less than about 0.20 mg/m 2  (0.0002 g/m 2 ), less than about 0.10 mg/m 2  (0.0001 g/m 2 ), less than about 0.05 mg/m 2  (0.00005 g/m 2 ) or less than about 0.003 mg/m 2  (0.000003 g/m 2 ). In some embodiments, the areal density per layer is about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 100 mg/m 2  (0.1 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 90 mg/m 2  (0.09 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 80 mg/m 2  (0.08 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 70 mg/m 2  (0.07 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 60 mg/m 2  (0.06 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 50 mg/m 2  (0.05 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 40 mg/m 2  (0.04 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 30 mg/m 2  (0.03 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 20 mg/m 2  (0.02 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 10 mg/m 2  (0.01 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 1.0 mg/m 2  (0.001 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.5 mg/m 2  (0.0005 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.4 mg/m 2  (0.0004 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.3 mg/m 2  (0.0003 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.2 mg/m 2  (0.0002 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.1 mg/m 2  (0.0001 g/m 2 ) or from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.05 mg/m 2  (0.00005 g/m 2 ). In some embodiments, the areal density per layer is from about 5 mg/m 2  (0.005 g/m 2 ) to about 100 mg/m 2  (0.1 g/m 2 ), from about 20 mg/m 2  (0.02 g/m 2 ) to about 90 mg/m 2  (0.09 g/m 2 ), or from about 30 mg/m 2  (0.03 g/m 2 ) to about 80 mg/m 2  (0.08 g/m 2 ). 
     Further, the biaxially oriented ePTFE membrane has an area ratio from about 2:000:1 to about 300,000,000:1. In some embodiments, the biaxially oriented ePTFE membrane has an area ratio from about 20,000:1 to about 300,000,000:1, from about 40,000:1 to about 300,000,000:1, from about 60,000:1 to about 300,000,000:1, from about 80,000:1 to about 300,000,000:1, from about 100,000:1 to about 300,000,000:1, from about 250,000:1 to about 300,000,000:1, from about 500,000:1 to about 300,000,000:1, from about 1,000,000:1 to about 300,000,000:1, or from about 2,500,000:1 to about 300,000,000:1. 
     Additionally, the biaxially oriented ePTFE membranes may have a total areal density less than about 100 g/m 2 , less than about 10 g/m 2 , less than about 5 g/m 2 , less than about 1 g/m 2 , less than about 0.5 g/m 2 , less than about 0.1 g/m 2 , less than about 50 mg/m 2  (0.05 g/m 2 ), less than about 10 mg/m 2  (0.01 g/m 2 ), less than about 5.0 mg/m 2  (0.005 g/m 2 ), less than about 4.0 mg/m 2  (0.004 g/m 2 ), less than about 3.0 mg/m 2  (0.003 g/m 2 ), less than about 2.0 mg/m 2  (0.002 g/m 2 ), less than about 1.0 mg/m 2  (0.001 g/m 2 ), less than about 0.50 mg/m 2  (0.0005 g/m 2 ), less than about 0.40 mg/m 2  (0.0004 g/m 2 ), less than about 0.30 mg/m 2  (0.0003 g/m 2 ), less than about 0.20 mg/m 2  (0.0002 g/m 2 ) less than about 0.10 mg/m 2  (0.0001 g/m 2 ), less than about 0.07 mg/m 2  (0.00007 g/m 2 ), less than about 0.05 mg/m 2  (0.00005 g/m 2 ), less than about 0.03 mg/m 2  (0.00003 g/m 2 ), less than about 0.007 mg/m 2  (0.000007 g/m 2 ), or less than about 0.003 mg/m 2  (0.000003 g/m 2 ). In some embodiments, the biaxially oriented ePTFE membranes have a total areal density from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 100 g/m 2 , from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 10 g/m 2 , from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 1.0 g/m 2 , from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.5 g/m 2 , from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.1 g/m 2 , from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 50 mg/m 2  (0.05 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 10 mg/m 2  (0.01 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 5 mg/m 2  (0.005 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 4.0 mg/m 2  (0.004 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 3.0 mg/m 2  (0.003 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 2.0 mg/m 2  (0.002 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 1.0 mg/m 2  (0.001 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.50 mg/m 2  (0.0005 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.40 mg/m 2  (0.0004 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.30 mg/m 2  (0.0003 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.20 mg/m 2  (0.0002 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.10 mg/m 2  (0.0001 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.07 mg/m 2  (0.00007 g/m 2 ), from about 0.003 mg/m 2  (0.000003 g/m 2 ) to about 0.05 mg/m 2  (0.00005 g/m 2 ) or from about 0.10 mg/m 2  (0.010 g/m 2 ) to about 10 g/m 2 . 
     The uniaxially oriented ePTFE membranes are also extremely light, having an areal density per layer less than about 500 mg/m 2  (0.5 g/m 2 ), less than about 400 mg/m 2  (0.4 g/m 2 ), less than about 300 mg/m 2  (0.3 g/m 2 ), less than about 200 mg/m 2  (0.2 g/m 2 ), less than about 100 mg/m 2  (0.1 g/m 2 ), less than about 70 mg/m 2  (0.07 g/m 2 ), less than about 50 mg/m 2  (0.05 g/m 2 ), less than about 30 mg/m 2  (0.03 g/m 2 ), less than about 25 mg/m 2  (0.025 g/m 2 ), less than about 20 mg/m 2  (0.02 g/m 2 ), less than about 15 mg/m 2  (0.015 g/m 2 ), less than about 10 mg/m 2  (0.01 g/m 2 ), less than about 5 mg/m 2  (0.005 g/m 2 ), less than about 4 mg/m 2  (0.004 g/m 2 ), less than about 3 mg/m 2  (0.003 g/m 2 ), less than about 2 mg/m 2  (0.002 g/m 2 ), less than about 1.0 mg/m 2  (0.001 g/m 2 ), less than about 0.50 mg/m 2  (0.0005 g/m 2 ), less than about 0.40 mg/m 2  (0.0004 g/m 2 ), less than about 0.30 mg/m 2  (0.0003 g/m 2 ), less than about 0.20 mg/m 2  (0.0002 g/m 2 ), or less than about 0.10 mg/m 2  (0.0001 g/m 2 ). In some embodiments, the areal density is from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 500 mg/m 2  (0.5 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 400 mg/m 2  (0.4 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 300 mg/m 2  (0.3 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 200 mg/m 2  (0.2 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 100 mg/m 2  (0.1 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 70 mg/m 2  (0.07 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 50 mg/m 2  (0.05 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 30 mg/m 2  (0.03 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 20 mg/m 2  (0.02 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 15 mg/m 2  (0.015 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 10 mg/m 2  (0.01 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 5 mg/m 2  (0.005 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 0.40 mg/m 2  (0.0004 g/m 2 ), from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 0.30 mg/m 2  (0.0003 g/m 2 ), or from about 0.10 mg/m 2  (0.0001 g/m 2 ) to about 0.20 mg/m 2  (0.0002 g/m 2 ). 
     Despite being thin and lightweight, the ePTFE membranes that are biaxially expanded possess high intrinsic strength properties. The ePTFE membranes have a matrix tensile strength (MTS) of at least about 600 MPa in both the longitudinal and transverse directions, at least about 650 MPa, at least about 700 MPa, at least about 750 MPa, at least about 800 MPa, at least about 850 MPa, at least about 900 MPa, or at least about 1000 MPa in both the longitudinal and transverse directions. In at least one embodiment, the biaxially oriented ePTFE membranes have a matrix tensile strength (MTS) from about 600 MPa to about 1000 MPa, from about 650 MPa to about 1000 MPa, from about 700 MPa to about 1000 MPa, from about 750 MPa to about 1000 MPa, from about 800 MPa to about 1000 MPa, from about 850 MPa to about 1000 MPa, or from about 900 MPa to about 1000 MPa in both the longitudinal and transverse directions. 
     Additionally, ePTFE membranes having been additionally uniaxially expanded have even higher intrinsic strength properties. In some embodiments the ePTFE membranes have a matrix tensile strength (MTS) greater than about 1000 MPa in the machine direction, greater than about 1100 MPa in the machine direction, greater than about 1200 MPa in the machine direction, greater than about 1200 MPa in the machine direction, greater than about 1300 MPa in the machine direction, or greater than about 1400 MPa in the machine direction. In some embodiments, the uniaxially oriented ePTFE membrane has a matrix tensile strength from about 1000 MPa to about 1400 MPa in the machine direction, from about 1100 MPa to about 1400 MPa in the machine direction, from about 1200 MPa to about 1400 MPa in the machine direction, from about 1200 MPa to about 1300 MPa in the machine direction, or from about 1300 MPa to about 1400 MPa in the transverse direction. It is to be appreciated that although the matrix tensile strength is given herein for the machine direction, it is equally applicable for ePTFE membranes expanded in the transverse direction. 
     In addition, uniaxially oriented ePTFE membrane has a matrix storage modulus of at least 100 GPa at ambient temperature (i.e., about 20° C.). In some embodiments, the uniaxially oriented ePTFE membrane has a matrix storage modulus at ambient temperature (i.e., about 20° C.) from about 100 GPa to about 111 GPa, from about 101 GPa to about 111 GPa, from about 102 GPa to about 111 GPa, from about 103 GPa to about GPa, from about 104 GPa to about 111 GPa, from about 105 GPa to about 111 GPa, from about 106 GPa to about 111 GPa, from about 107 GPa to about 111 GPa, from about 108 GPa to about 111 GPa, from about 109 GPa to about 111 GPa, or from about 110 GPa to about 111 GPa. The uniaxially oriented ePTFE membrane also has a bulk denier less than about 750 g/9000 m. In some embodiments, the uniaxially oriented ePTFE membrane has a bulk denier from about 0.5 g/9000 m to about 750 g/9000 m, from about 0.5 g/9000 m to about 650 g/9000 m, from about 0.5 g/9000 m to about 500 g/9000 m, from about 100 g/9000 m to about 450 g/9000 m, from about 0.5 g/9000 m to about 400 g/9000 m, from about 0.5 g/9000 m to about 350 g/9000 m, from about 0.5 g/9000 m to about 250 g/9000 m, from about 0.5 g/9000 m to about 200 g/9000 m, from about 0.5 g/9000 m to about 150 g/9000 m, from about 0.5 g/9000 m to about 100 g/9000 m, from about 0.5 g/9000 m to about 50 g/9000 m, from 0.5 g/9000 m to about 25 g/9000 m, from about 0.5 g/9000 m to about 15 g/9000 m, from about 0.5 g/9000 m to about to about 10 g/9000 m, from about 0.5 g/9000 m to about 5 g/9000 m, from about 0.5 g/9000 m to about 3 g/9000 m, or from about 0.5 g/9000 m to about 1 g/9000 m. 
     In addition, the uniaxially oriented ePTFE membranes have a tenacity of at least about 5 gf/d. In some embodiments, the uniaxially oriented ePTFE membranes have a tenacity from about 5 gf/d to about 8 gf/d from about 6 gf/d to about 8 gf/d, or from about 6 gf/d to about 7 gf/d. Further, the uniaxially oriented ePTFE membranes have a &lt;P2&gt; orientation greater than or equal to 0.985. 
     In addition, biaxially oriented ePTFE membranes have little air resistance. In some embodiments, the air resistance of the ePTFE membrane may be less than about 30,000 Pa·s/m, less than about 25,000 Pa·s/m, less than about 20,000 Pa·s/m, less than about 15,000 Pa·s/m, less than about 10,000 Pa·s/m, less than about 7,500 Pa·s/m, less than about 5000 Pa·s/m, less than about 2000 Pa·s/m, less than about 1500 Pa·s/m, less than about 1000 Pa·s/m, less than about 750 Pa·s/m, less than about 500 Pa·s/m, less than about 250 Pa·s/m, or less than about 150 Pa·s/m. In some embodiments, the air resistance is from about 100 Pa·s/m to about 2000 Pa·s/m, from about 100 Pa·s/m to about 1500 Pa·s/m, from about 100 Pa·s/m to about 1000 Pa·s/m, from about 100 Pa·s/m to about 750 Pa·s/m, from about 100 Pa·s/m to about 500 Pa·s/m, from about 100 Pa·s/m to about 250 Pa·s/m, or from about 250 Pa·s/m to about 500 Pa·s/m. The low air resistance in combination with the high surface area of the ePTFE membrane provides for a high performance filtration device. 
     The biaxially oriented ePTFE membranes are also highly optically transmissive, with a total luminous transmittance (measured from 380 nm to 780 nm) that is greater than or equal to about 90%, greater than or equal to about 95%, greater than or equal to about 98%, greater than or equal to about 99%. In exemplary embodiments, the biaxially oriented ePTFE membrane may have a total luminous transmittance from about 90% to about 99%, from about 95% to about 99%, or from about 98% to about 99%. In some embodiments, the ePTFE membrane has a total luminous transmittance of nearly 100%. 
     The fibrils of the ePTFE membrane (biaxially oriented and uniaxially oriented) may optionally be coated such that the ePTFE is porous or such that the ePTFE is non-porous with at least one coating composition, such as, but not limited to, a polymer or a biologic coating. A coating composition can be applied to the ePTFE membrane by any conventional coating method such as solvent coating, spray coating, spin coating, vapor deposition, atomic layer deposition (ALD), or dip coating. Additionally, a coating may be applied to the ePTFE membrane by applying compression with heat between sheets of a component, such as, but not limited to, fluorinated ethylene propylene (FEP), polyfluoroacrylate (PFA) and silicone. 
     In some embodiments, the coating composition occupies or fills at least a portion of the spaces through the thickness of the biaxially or uniaxially oriented ePTFE membrane. Suitable polymers and/or biologic coatings that may be coated and/or imbibed on or into the ePTFE membrane include, but are not limited to, polyesters; polystyrene; polyamides; polyphthalamides; polyamide-imides; polycarbonates; polyethersulphones; polysulfones; polyphenylenesulfides; liquid crystalline polymers; polyetherketones; polyetheretherketones; polysiloxanes; epoxies; polyurethanes; polyimides; polyetherimides; polyacrylates; polyparaxylylene; terpolymers of tetrafluoroethylene (TFE), VDF (vinylidenefluoride), and HFP (hexafluoropropylene); copolymers of tetrafluoroethylene (TFE) and perfluoroalkylvinylethers (PAVEs); a copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole; perfluoroalkylvinylethers; perfluoroalkylethers; polyvinylidenefluoride (PVDF); ethylene tetrafluoroethylene (ETFE); polychlorotrifluoroethylene (PCTFE); fluorinated ethylene propylene (FEP); perfluoroalkoxyalkanes (PFA), polyvinyl alcohol (PVA), CBAS®/Heparin coating (commercially available from W.L. Gore &amp; Associates, Inc.), antimicrobial agents, antibodies, pharmaceuticals, biologic entities, vascularization stimulators, and any combination thereof. The amount of coating applied will be dependent upon the desired application. 
     The biaxially or uniaxially oriented ePTFE membranes are self-supporting, and in some embodiments the ePTFE membrane is used to reinforce a polymer film such as porous polymers, non-porous polymers, fluoropolymers, polyolefins, films, tapes, and other membranes. By “self supporting” is it meant that the ePTFE membrane does not require a backing or carrier layer. However, because the ePTFE membrane is so thin, the edges of the ePTFE membrane are often restrained at macroscale lengths. In other words, the ePTFE membrane is restrained around the perimeter of the membrane (e.g., “picture framed”) to maintain the integrity of the ePTFE membrane. The intrinsic strength of the membrane is connected across distances and holds itself together without a backing or supporting layer behind or under the membrane. 
     The biaxially oriented and uniaxially oriented ePTFE membranes may be formed as a single layer of ePTFE membrane. In other embodiments, the biaxially and uniaxially oriented ePTFE membranes may have tens, hundreds, or thousands of layers of ePTFE membrane in the ePTFE membrane. In some embodiments, 2 layers to 4 layers may be present. In other embodiments, 2 layers to 16 layers may be present in the ePTFE membrane. In further embodiments, 2 layers to 500 layers, 2 layers to 1,000 layers, 2 layers to 5,000 layers, 2 layers to 10,000 layers, 2 layers to 25,000 layers, 2 layers to 50,000 layers, 2 to 100,000 layers, 2 to 500,000 layers, 2 to 1,000,000 layers (or more) may be present in the ePTFE membrane. Although not wishing to be bound by theory, it is believed that the only limiting factor for the number of ePTFE layers present in the ePTFE membrane is the time spent to stack the layers and expand. Typically, the ePTFE membrane stack “grows” four-fold each time the ePTFE membrane is biaxially expanded. It is to be appreciated that no adhesive or other bonding agent is typically used to connect the individual ePTFE membranes in the stacked ePTFE membranes, although the inclusion of an adhesive or other bonding material is not precluded from use herein and is considered to be within the purview of the invention. 
     In another embodiment, the ePTFE membrane (both biaxially oriented and uniaxially oriented) may include ePTFE membranes having the same mechanical properties, ePTFE membranes having differing mechanical properties, and/or a spacing layer (e.g. a different polymeric layer such as, a porous polymer, a non-porous polymer, a fluoropolymer, a porous polyolefin, or a non-porous polyolefin. In other words, the ePTFE membrane may be engineered to include different polymeric and/or non-polymeric layer(s) in the ePTFE membrane. Additionally, one ePTFE membrane layer may differ from another ePTFE layer by the amount of expansion and/or strain rate, and/or total work it has been through. By varying the membrane type, expansion, mechanical properties of any additional layers within a ePTFE membrane, the ePTFE membrane may be formed to meet specific bulk properties while maintaining transport, filtration, or separation requirements. 
     Through the formation of the ePTFE membranes (biaxially and uniaxially oriented) and/or the optional additional spacing layers within the ePTFE membrane, the permeability, the pore size, and bulk mechanical properties may be controlled. As used herein, the term “permeability” means the ability to transmit fluids (i.e., liquid or gas) through the pores of a membrane or filter material when the material is subjected to a differential pressure across it. In one instance, the ePTFE membranes allow for a range of pore sizes, such as, for example, pore sizes less than about 6 microns in diameter. As used herein, the term “pore size” means the size of the pores in ePTFE membrane. The pore size may range from about 2 nm to about 6 microns. Additionally, the ePTFE may have a specific surface area (SSA) as measured by area weighted fibril width (AWFW) from about 35 m 2 /g to about 120 m 2 /g, from about 45 m 2 /g to about 120 m 2 /g, from about 55 m 2 /g to about 120 m 2 /g, from about 65 m 2 /g to about 120 m 2 /g, from about 75 m 2 /g to about 120 m 2 /g, from about 80 m 2 /g to about 120 m 2 /g, from about 90 m 2 /g to about 120 m 2 /g, from about 100 m 2 /g to about 120 m 2 /g, or from about 110 m 2 /g to about 120 m 2 /g. 
     In some embodiments, the biaxially oriented ePTFE membranes may be used in air filtration applications. In such applications, the ePTFE membrane demonstrates a quality factor at a face velocity of 5.33 cm with a 0.1 micron diameter challenge particle of at least 65 (kPa −1 ). It is to be appreciated that the strength to weight ratio (intrinsic strength) of the ePTFE membrane is higher than that of conventional ePTFE membranes. Higher quality factor values are associated with better filtration performance. In certain embodiments, the biaxially oriented ePTFE membrane may have a quality factor from about 65 (kPa −1 ) to about 180 (kPa −1 ), from about 70 (kPa −1 ) to about 180 (kPa −1 ), from about 80 (kPa −1 ) to about 180 (kPa −1 ), from about 90 (kPa −1 ) to about 180 (kPa −1 ), from about 100 (kPa −1 ) to about 180 (kPa −1 ), from about 110 (kPa −1 ) to about 180 (kPa −1 ), from about 120 (kPa −1 ) to about 180 (kPa −1 ), from about 130 (kPa −1 ) to about 180 (kPa −1 ), from about 140 (kPa −1 ) to about 180 (kPa −1 ), from about 150 (kPa −1 ) to about 180 (kPa −1 ), from about 160 (kPa −1 ) to about 180 (kPa −1 ), or from about 170 (kPa −1 ) to about 180 (kPa −1 ). 
     The biaxially oriented ePTFE membranes may be used in applications where it is desirable to filter nanoparticles (e.g., from about 1 nm to about 200 nm) from a liquid media, even when the liquid media is traveling at high flow rate. Thus, the ePTFE membrane may be used as a filtration material, and by the nature of polytetrafluoroethylene, is resistant to chemical attack, is biocompatible, and demonstrates a high matrix tensile strength (MTS). The filterable matrix may be selected from a solution, a suspension, a colloid, a biological fluid, a component of a biological fluid, an aqueous material, or a non-aqueous material. To filter a filterable matrix, the matrix is passed through the ePTFE membrane and the resulting filtrate is collected. In one embodiment, the biaxially oriented ePTFE membrane includes a nanoparticle retention percent (%) equal to or greater than the line defined by Equation (1). 
         y&gt;= 9.70-10.76*ln( x )  Equation (1)
 
     where 
     y=Nanoparticle Retention %; and 
     x=Filtrate Permeability [g/cm 2 /s/MPa]. 
     TEST METHODS 
     Non-Contact Thickness Measurements 
     The non-contact thickness of the membranes was measured using a KEYENCE LS-7600 laser system (commercially available from KEYENCE America). 
     Membrane Density Calculation 
     Samples were cut to form square sections 15.2 cm by 15.2 cm. Each sample was weighed using a MettlerToledo AT20 balance. Using the thickness calculated by the KEYENCE laser, the densities of the samples were calculated using Equation (2). 
       ρ= m /( w*l*t )  Equation (2).
 
     where: 
     ρ=density (g/cm 3 ); 
     m=mass (g); 
     w=width (cm); 
     l=length (cm); and 
     t=thickness (cm). 
     Matrix Tensile Strength (MTS) (Method 1) 
     To determine the MTS of biaxial ePTFE membranes, a sample ePTFE membrane was cut in the longitudinal and transverse directions using an ASTM D412-Dogbone Die Type F (D412F). To determine MTS of uniaxial membranes, a sample ePTFE membrane was loaded in the longitudinal direction. Tensile break load was measured using an INSTRON® 5567 (Illinois Tool Works Inc., Norwood, Mass.) tensile test machine equipped with flat-faced grips and a “22 lb” (˜100 N) load cell. The gauge length for the grips was set to 8.26 cm and the strain rate was 0.847 cm/s. After placing the sample in the grips, the sample was retracted 1.27 cm to obtain a baseline followed by a tensile test at the aforementioned rate. The peak force measurements were used for the MTS calculation. The longitudinal and transverse MTS were calculated using Equation (3): 
       MTS=(maximum load/cross sectional area)×(density of resin/density of the membrane).  Equation (3).
 
     Matrix Tensile Strength (MTS) (Method 2) 
     To determine the MTS of uniaxial ePTFE membranes, a sample ePTFE was loaded in the longitudinal direction using cord and yarn grips. Tensile break load was measured using an INSTRON® 5567 (Illinois Tool Works Inc., Norwood, Mass.) tensile test machine equipped with cord and yard grips and a “22 lb” (˜100 N) load cell. The gauge length for the grips was set to 15.24 cm and the strain rate was 0.254 cm/s. After placing the sample in the grips, the sample was retracted 1.27 cm to obtain a baseline followed by a tensile test at the aforementioned rate. The peak force measurement was used for the MTS calculation. 
     Scanning Transmission Electron Microscope (STEM) 
     Low voltage STEM (scanning transmission electron microscopy) is a technique used to visualize thin samples by accelerating a focused beam of electrons through the sample and collecting the transmitted electrons with a suitable detector. Low voltage refers to the use of a beam accelerating voltage of less than 100 kV (&lt;30 kV as exemplified herein). The image contrast is due to differences in the electron absorption by the membrane due to either composition or thickness. 
     A scanning electron microscope (Hitachi, SU8000; Hitachi Ltd, Tokyo, Japan) with a transmission adapter (STEM) was used and operated at an accelerating voltage no higher than 30 kV. No prior or additional treatment (staining) was applied to the samples. The samples for the analysis of the thin porous films were prepared on a copper grid (PELCO® Center-Marked Grids, 400 mesh, Copper, Product #1GC400, Ted Pella Inc., Redding, Calif.) with a carbon support layer (Carbon Type-B, 300 mesh, Copper, Product #01813, Ted Pella, Inc.). 
     X-Ray Diffractograms for Biaxial Samples 
     Two dimensional (2-d) x-ray diffractograms were obtained using the X27C beamline of the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, N.Y.). The beamline provided a well collimated, monochromatic x-ray beam with a wavelength of 0.1371 nm, a nominal flux of 10 12  photons/s, and a diameter of 0.39 mm. The detector was a Rayonix MAR-CCD 2-d Image System (Rayonix LLC, Evanston, Ill.). The system was set with a sample-detector distance of 67.97 mm and calibrated using a Al 2 O 3  powder standard. The samples were mounted between the beam and detector and the transmission geometry scattered/diffracted x-ray image collected for between 480 and 540 seconds. In addition, a background image without the sample present was recorded for the same time period immediately after each sample was imaged. The background image was then subtracted from the sample image to remove the effect of air scattering and create the desired diffractogram. 
     X-Ray Scattering Methodology for Uniaxial Samples 
     The wide-angle x-ray scattering experiments were carried out on a Xenocs brand Xeuss 2.0 SAXS/WAXS Laboratory Beamline system (Xenocs SAS, Sassenage, France). The instrument uses a GeniX3D Cu k a  source (0.154 nm wavelength) operating at 50 kV and 0.6 mA, and a Dectris brand Pilatus 300 K detector (Dectris Ltd., Baden-Daettwil, Switzerland). The beam was collimated with 2 in-line slits, each open to an area of 0.5 mm×0.5 mm. The sample-detector distance was 71.0 mm (calibrated by lanthanum hexaboride standard). The “Virtual Detector” feature of the Xeuss 2.0 system was used to erase blind spots in the detector as well as to extend its angular range. This is achieved by translating the detector in the horizontal direction, then averaging multiple scans. Here, 4 scans were taken at varying horizontal detector offsets, each with exposure times of 15 minutes. Averaging these four scans provided a scattering profile. The orientation was quantified from the I vs. φ Azimuthal scans utilizing Equation (4). 
     
       
         
           
             
               
                 
                   
                     〈 
                     
                       
                         
                           cos 
                             
                         
                         2 
                       
                       ⁢ 
                       
                         ( 
                         φ 
                         ) 
                       
                     
                     〉 
                   
                   = 
                   
                     
                       
                         
                           ∫ 
                           0 
                           
                             π 
                             / 
                             2 
                           
                         
                         
                           
                             I 
                             ⁡ 
                             ( 
                             φ 
                             ) 
                           
                           ⁢ 
                           
                             
                               cos 
                               2 
                             
                             ( 
                             φ 
                             ) 
                           
                           ⁢ 
                           
                             sin 
                             ⁡ 
                             ( 
                             φ 
                             ) 
                           
                           ⁢ 
                           d 
                           ⁢ 
                           φ 
                         
                       
                       
                         
                           ∫ 
                           0 
                           
                             π 
                             / 
                             2 
                           
                         
                         
                           
                             I 
                             ⁡ 
                             ( 
                             φ 
                             ) 
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                             ( 
                             φ 
                             ) 
                           
                           ⁢ 
                           d 
                           ⁢ 
                           φ 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     As &lt;P2&gt; approaches 1, nearly perfect orientation in the machine direction is achieved as determined by Equation (5). 
     
       
         
           
             
               
                 
                   
                     〈 
                     
                       P 
                       ⁢ 
                       2 
                     
                     〉 
                   
                   = 
                   
                     
                       
                         
                           3 
                           ⁢ 
                           
                             〈 
                             
                               
                                 
                                   cos 
                                     
                                 
                                 2 
                               
                               ⁢ 
                               
                                 ( 
                                 φ 
                                 ) 
                               
                             
                             〉 
                           
                         
                         - 
                         1 
                       
                       2 
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
     Crystallinity Index 
     The crystallinity index was obtained by peak fitting of Intensity vs. q scans using JMP® 14.1.0 statistical analysis software (SAS institute). The range of integration was limited to q=8.74 to 15.4 (nm −1 ), and a linear background was defined such that it coincided with the measured intensity from about q=8.74 to 10 (nm −1 ) and q=14.1 to 15.4 (nm −1 ). The Pearson VII function was used to fit both peaks after the linear background was subtracted. 
     As defined in U.S. Patent Publication No. 2004/0173978 to Bowen, et al., the crystallinity index was calculated from the area under the fitted 100 crystalline peak (A 100 ) and the area under the fitted amorphous peak (A amorphous ) according to the following Equation (6): 
       Crystallinity Index (%)={ A   100 /( A   100   +A   amorphous )}×100  Equation (6).
 
     Bubble Point 
     The bubble point was measured according to the general teachings of ASTM F31 6-03 using a Capillary Flow Porometer (Model CFP 1500 AE from Porous Materials, Inc., Ithaca, N.Y.). The sample membrane was placed into a sample chamber and wet with SilWick Silicone Fluid (commercially available from Porous Materials, Inc.) having a surface tension of 19.1 dynes/cm. The bottom clamp of the sample chamber consists of a 40 micron porous metal disc insert (Mott Metallurgical, Fannington, Conn.) with the following dimensions (2.54 cm diameter, 3.175 mm thickness). The top clamp of the sample chamber consists of an opening, 12.7 mm in diameter. Using the Capwin software version 6.74.70, the following parameters and set points were used: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Set Point 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Maxflow 
                 200000 
                 (cc/m) 
               
               
                   
                 Bubflow 
                 10-127 
                 (cc/m) 
               
            
           
           
               
               
               
            
               
                   
                 F/PT 
                 50 
               
            
           
           
               
               
               
               
            
               
                   
                 Minbppres 
                 0.1 
                 (psi) 
               
               
                   
                 Zerotime 
                 1 
                 (sec) 
               
               
                   
                 V2incr 
                 10 
                 (cts) 
               
               
                   
                 Preginc 
                 1 
                 (cts) 
               
               
                   
                 Pulse Delay 
                 2 
                 (sec) 
               
               
                   
                 Maxpress 
                 500 
                 (psi) 
               
               
                   
                 Pulse Width 
                 0.2 
                 (sec) 
               
               
                   
                 Mineqtime 
                 30 
                 (sec) 
               
               
                   
                 Presslew 
                 10 
                 (cts) 
               
               
                   
                 Flowslew 
                 50 
                 (cts) 
               
            
           
           
               
               
               
            
               
                   
                 Eqiter (0.1 sec) 
                  3 
               
               
                   
                 Aveiter (0.1 sec) 
                 20 
               
            
           
           
               
               
               
               
            
               
                   
                 Maxpdif 
                 0.1 
                 (psi) 
               
               
                   
                 Maxfdif 
                 50 
                 (cc/m) 
               
               
                   
                 Startp 
                 1 
                 (psi). 
               
               
                   
                   
               
            
           
         
       
     
     The values presented for bubble point were the average of two measurements. 
     ATEQ Airflow 
     The ATEQ airflow test measures laminar volumetric flow rates of air through membrane samples. Each membrane sample was clamped between two plates in a manner that seals an area of 2.99 cm 2  across the flow pathway. An ATEQ® (ATEQ Corp., Livonia, Mich.) Premier D Compact Flow Tester was used to measure airflow rate (L/hr) through each membrane sample by challenging it with a differential air pressure of 1.2 kPa (12 mbar) through the membrane. 
     Airflow Resistance 
     A Textest FX 3300 Air Permeability Tester device manufactured by Textest AG (Zurich, Switzerland) was used to test airflow resistance. The Frazier permeability reading is the rate of flow of air in cubic feet per square foot of sample area per minute at a differential pressure drop across the test sample of 12.7 mm water column. Where noted the pressure drop was reduced for the characterization of lightweight unsupported membranes. Air permeability was measured by clamping a test sample into a circular, flanged fixture which provided a circular opening of 7 cm diameter (38.5 cm 2  area). The upstream side of the sample fixture was connected to a flow meter in line with a source of dry compressed air. 
     Optical Transmittance Measurements 
     Optical transmittance measurements were carried out using a spectrophotometer (Jasco V-670; JASCO Deutschland GmbH, Pfungstadt, Germany) with double-beam integrating sphere attachment (150 mm diameter, ILN-725). The spectrophotometer is comprised of a deuterium &amp; tungsten-halogen lamps, a single Czerny-Turner type monochromator (1200 line/mm diffraction grating) and a photomultiplier tube (PMT) detector. The light from the monochromator is split into a sample and reference beam before entering the integrating sphere. The integrating sphere is configured for unidirectional illumination and diffuse detection. The sample beam illuminates, at normal incidence, a 20 mm×20 mm sample mounted on the integrated sphere entrance port; while the reference beam passes through an open port on the integrating sphere. The sample and reference beams are alternately incident upon the PMT detector and converted to a digital signal after being subjected to synchronous rectification. 
     The monochromator bandwidth was set to 10 nm and the grating wavelength was scanned from 250 nm to 800 nm at a scan rate of 2000 nm/min. The source was changed from the deuterium to the tungsten-halogen lamp at 340 nm. The signal was recorded at intervals of 2 nm. A ‘dark correction’ spectrum (blocking the sample beam) and a ‘baseline correction’ spectrum (sample beam passes through an open port) was collected: these spectra were used to report the transmittance spectrum expressed as a percentage of the incident light. 
     The total luminous transmittance was calculated by weighting the transmittance spectrum by a CIE Standard Illuminant and CIE Standard Colorimetric Observer (see ASTM D1003-13: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.) The D65 illuminant and the 1931 2-deg standard observer were used in the calculation presented here. The % Transmittance in the UVA and UVB ranges were calculated by computing the average % Transmittance for the 315-400 nm and 280-315 nm wavelength ranges, respectively. 
     Average Fibril Width Determination 
     Selected samples were imaged by STEM and characterized manually by 50 measurements of the projected width of the fibrils (e.g.,  FIG. 11 ). Uniform sampling was facilitated by using a random number generator to highlight 50 regions, then the operator would trace the outline of the nearest fibril—preferably a fragment of a fibril which has not already been characterized. In general, the marked fibril was rectangular in shape, with an aspect ratio greater than 1. The nominal projected width of the fibril was calculated from the ratio of the area divided by the length of the object. This is thought to be more representative and informative than a single width measurement as it naturally forces the projected width measurement to be perpendicular to the major axis of the rectangular shape. To confirm the method, lines were drawn through the centroid of the manually identified region, orthogonal to the major axis at the calculated width as calculated according to Equation (7). 
     
       
         
           
             
               
                 
                   
                     
                       W 
                       ¯ 
                     
                     
                       A 
                       ⁢ 
                       W 
                       ⁢ 
                       F 
                       ⁢ 
                       W 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           
                             n 
                             i 
                           
                           ⁢ 
                           
                             w 
                             i 
                             2 
                           
                         
                       
                       
                         ∑ 
                         
                           
                             n 
                             i 
                           
                           ⁢ 
                           
                             w 
                             i 
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     
                       ( 
                       7 
                       ) 
                     
                     . 
                   
                 
               
             
           
         
       
     
     Dynamic Mechanical Analyzer (DMA) Matrix Storage Moduli and Loss Moduli 
     Measurement of the matrix storage and loss moduli was carried out using a TA Instruments Q800 system (TA Instruments, New Castle, Del.) outfitted with tension sample clamps. The DMA was calibrated according to standard TA Instruments procedures. Samples dimensions were obtained using a 10× microscope with a 0.1 mm graduated reticule for width and a KEYENCE LS7010 high accuracy non-contact micrometer (Keyence Corp., Itasca, Ill.) for thickness. The sample mass was measured using a Mettler-Toledo A120 micro-balance (Mettler-Toledo, LLC, Columbus, Ohio). The sample was then mounted in the instrument and a 5 mN pre-load applied. The sample length was obtained from the calibrated DMA clamp position at 25° C. A sinusoidal strain with a true strain amplitude of 0.001 and frequency of 1 Hz was applied with an additional constant load just sufficient to maintain the sample in tension throughout the imposed sinusoidal strain. The sample was equilibrated at −50° C. for 10 minutes and then the temperature was ramped at 2° C./min to 150° C. The magnitude and phase angle of the resulting sinusoidal force acting on the sample was measured once a second throughout the heating ramp and used to calculate the storage and loss moduli. The desired matrix moduli were obtained by multiplying the storage and loss moduli by the ratio ρ true /ρ sample . ρ true  was assumed to be that of crystalline poly(tetrafluoroethylene), 2.3 g/cm 3 , while ρ sample  was calculated from the measured sample dimensions and mass. 
     Air Filtration Performance Measurement 
     Particle Filtration Efficiency Membrane filtration efficiency testing was performed using dioctyl phthalate (DOP) aerosol on a TSI CERTITEST® Model 8160 Automated Filter Tester (TSI Incorporated, St. Paul, Minn.), according to the procedure specified in the CERTITEST® Model 8160 Automated Filter Tester Operation and Service Manual. The sample test area was 77.8 cm and face velocity was 5.32 cm/sec. 
     The Quality Factor Q f  was determined using Equation (8): 
         Q   f =Ln(1/ P )/Δ p   Equation (8).
 
     The penetration, P, is the fraction of particles penetrating or passing through the sample and Δp is the pressure drop in kPa at an air velocity of 5.33 cm/s. Higher quality factor values are associated with better filtration performance (see William C. Hinds,  Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles , Second Ed., John Wiley &amp; Sons; Hoboken, N.J. (1994)). Comparisons of the quality factor are made using the same face velocity and test aerosol particle size. Quality factor is defined in units of reciprocal pressure (kPa −1 ). The efficiency, E (%)=100*(1-P). 
     Determination of Liquid Permeability and Retention with Bead Tests 
     The bead test measures permeability and bead retention of the membrane sample. The membrane sample was restrained in a 25 mm filter holder. The membrane was first wetted with an isopropyl alcohol (IPA)-DI water solution (70:30 v/v IPA:water). Air pressure was used to force this solution through the membrane. 7 grams of solution was flowed through the sample, followed by 10 grams of aqueous solution made of 1% by volume of the non-ionic surfactant TRITON™ X-100 (CAS 9002-93-1; Sigma Aldrich, St. Louis, Mo.) in DI water. The membrane was then challenged with a solution of 0.025 μm diameter polystyrene latex beads (Fluoro-Max R25 red fluorescent polymer microspheres; Thermo Fisher Scientific, Waltham, Mass.) dispersed in an aqueous solution made of 1% by volume of TRITON™ X-100 in DI water, such that the membrane was challenged with a quantity of beads sufficient to cover the membrane surface area with a single monolayer of beads. The concentration of the beads in the challenge solution and filtrate was determined using an Agilent Technologies Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies, Santa Clara, Calif.). 
     The permeability of the membrane was calculated using Equation (9): 
     
       
         
           
             
               
                 
                   
                     k 
                     = 
                     
                       g 
                       
                         A 
                         ⁢ 
                         t 
                         ⁢ 
                         P 
                       
                     
                   
                   . 
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     In Equation (9), k is the permeability of the membrane, g is the mass of an aliquot of filtrate, A is the physical area of the membrane sample in the filter holder, t is the time require to collect the aliquot of filtrate, and P is the pressure differential across the membrane. In Equation (9), g/t is the mass flow rate through the membrane and g/At is the mass flux through the membrane. 
     The percent of beads in the solution that were retained by the membrane was calculated using Equation (10): 
     
       
         
           
             
               
                 
                   
                     % 
                     ⁢ 
                         
                     Retention 
                   
                   = 
                   
                     100 
                     ⁢ 
                     
                       
                         
                           
                             C 
                             challe𝔫ge 
                           
                           - 
                           
                             C 
                             filtrate 
                           
                         
                         
                           C 
                           challenge 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                       
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     In Equation (10), C challenge  is the concentration of beads in the challenge solution, and C filtrate  is the concentration of beads in the filtrate. 
     EXAMPLES 
     Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various uses and conditions. 
     Example 1 
     The following example discloses the production of a single layer PTFE membrane having very low areal density (e.g., areal densities of less than 10 mg/m 2 ). 
     Polytetrafluorethylene (PTFE) fine powder (E.I. DuPont de Nemours; Wilmington, Del.) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant (ExxonMobil Chemical; Spring, Tex.) at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. to form dried tape (the “initial tape”). A 98 mm square was cut from the initial tape. The initial tape areal density (prior to pantograph expansion) was determined to be 1130 grams per square meter (g/m 2 ). As used herein, all initial tape areal densities are meant to denote 1150+/−100 g/m 2 . A summary of the process parameters used in Example 1 is provided in Table 1. 
     First Pass 
     Using a pantograph machine, the 98 mm square of dried tape was heated in an oven set to 300° C. (set point) for 120 seconds and then expanded in the longitudinal direction (machine direction (MD)) and transverse direction (TD) simultaneously (biaxial expansion) at a target ratio of about 4:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 8 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     A piece of the cooled ePTFE membrane from the first pass was harvested for further expansion, i.e. a “second pass.” Using the same pantograph machine, the selected membrane was heated in an oven set to 300° C. for a target of 120 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 10:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 9%/second. The pantograph opened at a constant velocity target for approximately 100 seconds. The second pass ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     A piece of the cooled ePTFE membrane from the second pass was selected for further expansion, i.e. a “third pass.” Using the same pantograph machine, the selected membrane was again heated in an oven set to 300° C. for 120 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 10:1 in each direction while maintaining a temperature of about 300° C. The constant acceleration rate set point was 1%/second. The pantograph opened at a constant acceleration rate set point for approximately 230 seconds. 
     The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. A summary of Example 1 process parameters is provided in Table 1. 
     The cooled expanded ePTFE membrane from the third pass was harvested from the pantograph and placed onto (152.4 m×152.4 mm) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed and the average areal density was calculated to be 4.3 mg/m 2  with the lightest sample weighing 2.4 mg/m 2  (Table 2). The area ratio is defined as the ratio of the areal density before and after the series of expansion operations. The ePTFE membrane from the third pass exhibited area ratios ranging from 122,690:1 to 459,273:1; depending upon process conditions (Table 2).  FIGS. 1-3  are from the same sample (2.40 mg/m 2 ; Sample E1G) at 3 different magnifications. No remaining primarily particles can be observed.  FIG. 4  is from a second piece where the same strain path was used, while the oven was set to 322° C. (Sample E1H). A STEM image of Sample E1I is provided as  FIG. 5 . Table 1 consolidates the process parameters. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Process Parameters Example 1 
               
            
           
           
               
               
               
               
            
               
                   
                 First Pass 
                 Second Pass 
                 Third Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                 Sample 
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 (FIG.) 
                 ° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 E1A 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1B 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1C 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 10 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1D 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 10 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1E 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1F 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 E1G 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 (FIGS. 
               
               
                 1-3) 
               
               
                 E1H 
                 322 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 (FIG.) 4 
               
               
                 E1I 
                 320 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
               
               
                 (FIG. 5) 
               
               
                   
               
               
                   1 “s” = constant speed; “r” = constant acceleration rate set point. 
               
               
                   2 Set point 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Single Layer Sample Data after 3 Passes 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Areal Density 
                   
                   
               
               
                   
                 Sample 
                 (mg/m 2 ) 
                 Area Ratio 
               
               
                   
                   
               
               
                   
                 E1A 
                 2.48 
                 459273:1 
                   
               
               
                   
                 E1B 
                 4.13 
                 275564:1 
               
               
                   
                 E1C 
                 3.57 
                 319494:1 
               
               
                   
                 E1D 
                 5.63 
                 202249:1 
               
               
                   
                 E1E 
                 9.15 
                 122690:1 
               
               
                   
                 E1F 
                 4.37 
                 256995:1 
               
               
                   
                 E1G 
                 2.40 
                 467013:1 
               
               
                   
                   
                 4.53 
                 300468:1 
                 Average 
               
               
                   
                   
                 2.40 
                 122690:1 
                 Minimum 
               
               
                   
                   
                 9.15 
                 467013:1 
                 Maximum 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     The following examples disclose the production of an ePTFE membrane having very low areal density per layer with layering up to 256 layers and area ratios up to about 34 million to 1. 
     PTFE fine powder (E.I. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating the tape to approximately 200° C. A 98 mm square was cut from the dry tape. A summary of the process parameters used in Example 2 is provided in Table 3. 
     First Pass 
     Using a pantograph machine, four squares of tape were heated in an oven set to 300° C. for 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about 7:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 16.7 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. Four pieces (4 layers each) were harvested from the cooled ePTFE membrane and set aside for further expansion, i.e. a second pass. The first pass process was repeated 1 more time to create another 16 layers. Both 16 layer samples were combined to make a 32-layer sample. 
     Second Pass 
     Using the same pantograph machine, after both stacks of 16 layers (32 layers total) were heated in an oven set to 300° C. for a target of 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 7:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 5%/second. The pantograph opened at a constant velocity target for approximately 120 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Four samples (32 layers each) were harvested from the cooled ePTFE membrane and layered (128 total layers) for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 8:1 in each direction while maintaining a temperature of about 300° C. The simultaneous expansion was conducted at a constant acceleration rate set point of 1%/s for a target ratio of 8:1 in each direction. The pantograph opened for approximately 208 seconds for the third pass. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Fourth Pass 
     A sample (128 layers) was harvested from the cooled ePTFE membrane and layered (128 total layers) for further expansion, i.e. a fourth pass. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 120 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 3:1 in each direction while maintaining a temperature of about 300° C. The simultaneous expansion was conducted at a constant acceleration rate set point set point of 1%/s for a target ratio of 3:1 in each direction. The pantograph opened for approximately 110 seconds for the fourth pass. The expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The ePTFE membrane was harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed to calculate an areal density of 0.00047 g/m 2 /layer, while the ePTFE membrane weighed 0.0605 g/m 2  (Sample E2A; Table 4). Area ratios and areal density (for both ePTFE membrane and stacks of the ePTFE membrane) for other ePTFE membranes were set and are set forth in Table 4. 
     Additionally three more ePTFE membranes were generated (E2B-D) for Example 2, each consisting of 128 layers using the first 3 passes described above. Each ePTFE membrane was individually loaded for a fourth and final expansion. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 120 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 4:1 (E2B), 5:1 (E2C) or 6:1 (E2D), in each direction while maintaining a temperature of about 300° C. The simultaneous expansion was conducted at a constant acceleration rate set point of 1%/s for Examples E2B-D. The pantograph opened for approximately 139 (E2B), 161 (E2C) or 179 (E2D) seconds for the fourth pass. At the end of each expansion (E2B-E2D), the expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed. Table 4 includes the fourth pass ratio settings, area ratio, areal density of the stack of ePTFE membranes, the areal density of each layer, and translation time during the final pass. 
     Additionally three more ePTFE membranes were generated (E2E-G) for Example 2, primarily for observations of images from STEM ( FIGS. 6-8 ). 
     Example E2E was processed using the same steps as Example E2D with the following 2 exceptions. The dwell time prior to expansion was reduced from 240 (E2D) to 120 (E2E) seconds prior to the third pass and the fourth pass target ratio was increased from a set point of 6:1 in both directions (E2D) to 8:1 in both directions (E2E) for an area ratio set point for the last pass. The pantograph opened for approximately 208 (E2E) seconds for the fourth pass. The expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. 
     Example E2F was processed using the same steps as Example E2E with the following 2 exceptions. The number of layers loaded for the fourth expansion was increased from 128 (E2E) to 256 (E2F) and the fourth pass target ratio was increased from a set point of 8:1 in each direction (E2E) to 9:1 in each direction (E2F). The expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. 
     Example E2G was processed using the same steps as Example E2E with the following 2 exceptions. The number of layers loaded for the second expansion was decreased from 32 (E2E) to 16 (E2G) and no fourth pass was used. The expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed to calculate an areal density of 0.009 g/m 2 /layer, while the ePTFE membrane weighed 1.175 g/m 2  (Sample E2G; Table 4). Area ratios and areal density (for both ePTFE membrane and stacks of the ePTFE membrane) for these and other ePTFE membranes were calculated and are set forth in Table 4. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Process Conditions for Example 2 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                    ample 
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                    FIG.) 
                 (° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
               
                 E2A 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E2B 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E2C 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E2D 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E2E 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 
                   
                 
               
               
                 E2F 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                    FIG. 7) 
               
               
                 E2G 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 S 
                 16 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                    FIG. 8) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Third Pass 
                 Fourth Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                   
                    ample 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                   
                    FIG.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
                   
               
               
                   
                 E2A 
                 128 
                 240 
                 8 
                 8 
                 1 
                 r 
                 128 
                 120 
                 3 
                 3 
                 1 
                 r 
               
               
                   
                 E2B 
                 128 
                 240 
                 8 
                 8 
                 1 
                 r 
                 128 
                 120 
                 4 
                 4 
                 1 
                 r 
               
               
                   
                 E2C 
                 128 
                 240 
                 8 
                 8 
                 1 
                 r 
                 128 
                 120 
                 5 
                 5 
                 1 
                 r 
               
               
                   
                 E2D 
                 128 
                 240 
                 8 
                 8 
                 1 
                 r 
                 128 
                 120 
                 6 
                 6 
                 1 
                 r 
               
               
                   
                 E2E 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
               
               
                   
                 
                   
                 
               
               
                   
                 E2F 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
                 256 
                 120 
                 9 
                 9 
                 1 
                 r 
               
               
                   
                    FIG. 7) 
               
               
                   
                 E2G 
                     128 3   
                 120 
                 8 
                 8 
                 1 
                 r 
                 na 
               
               
                   
                    FIG. 8) 
               
               
                   
                   
               
               
                   
                   1 “s” = constant speed; “r” = constant acceleration rate set point; 
               
               
                   
                   2 Set point; 
               
               
                   
                   3 final number of layers in final pass. 
               
               
                   
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Fourth pass ratio set-point, area ratio and 
               
               
                 areal density data for 128 layer samples 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                 Areal 
                 Areal 
                 Fourth 
               
               
                   
                 Fourth 
                   
                 Density 
                 Density 
                 Pass 
               
               
                   
                 Pass MD 
                   
                 for Stack 
                 per 
                 Translation 
               
               
                 Sam- 
                 and TD 
                 Area Ratio 
                 of layers 
                 layer 
                 Times 
               
               
                 ple 
                 ratio SP 
                 (AR) 
                 (g/m 2 ) 
                 (g/m 2 ) 
                 (s) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 E2A 
                 3 × 3 
                 2,452,746:1 
                 0.060 
                 0.0004729 
                 110 
               
               
                 E2B 
                 4 × 4 
                 8,289,810:1 
                 0.018 
                 0.0001399 
                 139 
               
               
                 E2C 
                 5 × 5 
                 8,452,355:1 
                 0.018 
                 0.0001372 
                 161 
               
               
                 E2D 
                 6 × 6 
                 34,485,608:1  
                 0.004 
                 0.0000336 
                 179 
               
               
                 E2G 
                 NA 
                     126,383:1 
                 1.175 
                 0.009178 
               
               
                   
               
            
           
         
       
     
     Example 3 
     The following example discloses the production of an ePTFE membrane having very low areal density per layer having up to 1024 ePTFE layers and area ratios of up to nearly 300 million to 1. 
     PTFE fine powder (E.I. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating the tape to approximately 200° C. A 98 mm square was cut from the dry tape. A summary of the process parameters used in Example 3 is provided in Table 5. 
     First Pass 
     Using a pantograph machine, the four squares of tape were stacked upon each other and these four squares (layers) of tape were heated in an oven set to 322° C. (set point) for 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about 10:1 in each direction while maintaining a temperature of about 322° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 25 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The first pass process was repeated 1 more time to create another 32 layers. 
     Second Pass 
     Using the same pantograph machine, both stacks of 32 layers (64 layers total) were heated in an oven set to 322° C. for a target of 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 10:1 in each direction while maintaining a temperature of about 322° C. The average engineering strain rate target was set to 3.6%/second. The pantograph opened at a constant velocity target for approximately 250 seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Four samples (64 layers each) were harvested from the cooled ePTFE membrane and stacked (256 total layers) for further expansion, i.e. a third pass. Using the same pantograph machine, the stacked ePTFE membrane layers were again heated in an oven set to 322° C. for a target of 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 8:1 (E3B) or 10:1 (E3A) in each direction while maintaining a temperature of about 322° C. The simultaneous expansion was conducted at a constant acceleration rate target of 1%/s until 400% strain (λ sp =5:1 in both directions) as the pantograph accelerated to a velocity target of 3.5 mm/s, and the expansions were completed at a constant velocity set point of 5%/s (3.5 mm/s in this specific case based on the original length input of 70 mm (“r/s” rate mode)). The pantograph opened for approximately 221 (E3B) or 261 (E3A) seconds for the third pass. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Fourth Pass 
     A sample (256 layers) was harvested from the cooled ePTFE membrane and stacked (1024 total layers) for further expansion, i.e. a fourth pass. Using the same pantograph machine, the membrane was again heated in an oven set to 322° C. for a target of 120 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 7:1 in each direction while maintaining a temperature of about 322° C. The simultaneous expansion was conducted at a constant acceleration rate set point of 1%/s until 400% strain (λ sp =5:1 in both directions) as the pantograph accelerated to a velocity set point of 3.5 mm/s, and the expansions were completed at a constant velocity set point of 5%/s (3.5 mm/s in this specific case based on the original length input of 70 mm) (“r/s” rate mode). The pantograph opened for approximately 201 seconds for the fourth pass to a (λ sp =7:1 in both directions). The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The ePTFE membranes were harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membranes were weighed to calculate an areal density ranging from 0.005 to 0.016 mg/m 2 /layer, while the ePTFE membrane had a mass per area (MPA; areal density) ranging from 3.88 mg/m 2  to 16.6 mg/m 2  (Table 6). Two different locations were measured for sample E3A (i.e., E3A-1 and E3A-2) while three different locations were measured for sample E3B (i.e., E3B-1, E3B-2, and E3b-3). STEM imaging was conducted for samples E3A ( FIG. 9 ) and E3B ( FIG. 10 ). 
     Area ratios of up to 298,611,016:1 were possible (Table 6). The lowest areal density for a complete stack of ePTFE membranes was ˜3.9 mg/m 2 . 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Process Conditions Table for Example 3 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                 Sample 
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 1   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 (FIG.) 
                 (° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
               
                 E3A 
                 322 
                 4 
                 240 
                 10 
                 10 
                 36 
                 s 
                 64 
                 240 
                 10 
                 10 
                 3.6 
                 s 
               
               
                 (FIG. 9) 
               
               
                 E3B 
                 322 
                 4 
                 240 
                 10 
                 10 
                 36 
                 s 
                 64 
                 240 
                 10 
                 10 
                 3.6 
                 s 
               
               
                 (FIG. 10) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Third Pass 
                 Fourth Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                 Sample 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 (FIG.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 N 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
               
                 E3A 
                 256 
                 120 
                 10 
                 10 
                 1-5 
                 r/s 
                 1024 
                 120 
                 7 
                 7 
                 1-5 
                 r/s 
               
               
                 (FIG. 9) 
               
               
                 E3B 
                 256 
                 120 
                 8 
                 8 
                 1-5 
                 r/s 
                 1024 
                 120 
                 7 
                 7 
                 1-5 
                 r/s 
               
               
                 (FIG. 10) 
               
               
                   
               
               
                   1 “s” = constant speed; “r” = constant acceleration rate set point; “r/s”—constant acceleration rate step followed by constant speed step; 
               
               
                   2 Set point; 
               
               
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Area ratio, areal density, and areal 
               
               
                 density per layer data for Example 3 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Areal Density 
                 Areal Density 
               
               
                   
                   
                 Area Ratio 
                 Stack of layers 
                 per layer 
               
               
                   
                 Sample 
                 (AR) 
                 (g/m 2 ) 
                 (g/m 2 ) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 E3A-1 
                 233,800,735:1 
                 0.00508 
                 0.00000496 
               
               
                   
                 E3A-2 
                 162,285,216:1 
                 0.00732 
                 0.00000714 
               
               
                   
                 E3B-1 
                 298,611,016:1 
                 0.00388 
                 0.00000379 
               
               
                   
                 E3B-2 
                  83,984,348:1 
                 0.01377 
                 0.00001345 
               
               
                   
                 E3B-3 
                  69,624,330:1 
                 0.01662 
                 0.00001623 
               
               
                   
                   
               
            
           
         
       
     
     Example 4 
     The following example discloses production of ePTFE membranes with areal densities on the order of 0.6-2.0 grams/m 2  to facilitate membrane thickness measurements (per layer). 
     PTFE fine powder (E.I. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. A 98 mm square was cut from the dry tape. A summary of the process parameters used in Example 4 is provided in Table 7. 
     First Pass 
     Using a pantograph machine, four squares of tape were heated in an oven set to 300° C. for a target of 120 (set point) (Sample E4B) or 240 (set point) (E4A, E4C and E4D) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about 7:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 16.6 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Using the same pantograph machine, stacks (16, 32 or 48 layers total—details in Table 7) were heated in an oven set to 300° C. for a target of 120 (E4B) or 240 (E4A, E4C and E4D) seconds then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 7:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 5%/second. The pantograph opened at a constant velocity target for approximately 120 seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Four samples were harvested from the cooled ePTFE membrane and stacked 128 (E4A-C) or 192 (E4D) total layers for further expansion, i.e. a third pass. Using the same pantograph machine, stacks of 128 or 192 layers were loaded. The ePTFE membrane was again heated in an oven set to about 300° C. for a target of 120 (E4A-C) or 180 (E4D) seconds then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 8:1 in each direction while maintaining a temperature of about 300° C. The simultaneous expansion was conducted at a constant acceleration rate set point of 1%/s until 400% strain (λ sp =5:1 in both directions) as the pantograph accelerated to a velocity set point of 3.5 mm/s, and the expansions (E4A, E4C-D) were completed at a constant velocity set point of 5%/s (3.5 mm/s in this specific case based on the original length input of 70 mm). The pantograph opened for approximately 221 seconds during the expansions of E4A and E4C-D. The simultaneous expansion of example (E4B) was conducted at a constant acceleration rate target of 1%/s to a target ratio of 8:1 in each direction. The entire third pass translation took approximately 210 seconds for Example E4B. The ePTFE membranes were allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The ePTFE membranes were harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames. 
     Using the frame as a cutting guide, ePTFE membranes E4A-E4D were weighed and measured. ePTFE membrane E4A was weighed to calculate an areal density of 0.0076 g/m 2 /layer, while the membrane weighed 0.974 g/m 2 . The average total thickness measurement of the 128 layer ePTFE membrane was 7.76 microns which corresponded to approximately 60 nm per layer. Table 7 includes the process details for this and a similar sample which was exposed to approximately 350° C. for a target of 5 minutes (i.e., the “thermal treatment”) to promote dimensional stability. ePTFE membrane E4B was weighed to calculate an areal density of 0.0049 g/m 2 /layer, while the membrane weighed 0.632 g/m 2 . The average total thickness measurement of the 128 layer ePTFE membrane was 4.95 microns which corresponded to approximately 39 nm per layer. Table 7 includes the process details for two additional, similar samples which were compressed to reduce thickness using the methods described herein. ePTFE membrane E4C was a compressed region of ePTFE membrane E4A. ePTFE membrane E4C was placed in a laboratory press under 2.07 MPa (300 psi) for approximately 30 minutes at approximately 22° C. ePTFE membrane E4D was placed in an autoclave at 1.73 MPa (250 psi) for 40 minutes under pressure at approximately 200° C. ePTFE membrane E4C was weighed to calculate an areal density of 0.0076 g/m 2 /layer, while the membrane weighed 0.974 g/m 2 . The average total thickness measurement of the 128 layer ePTFE membrane was 1.50 microns which corresponded to approximately 11.7 nm per layer. ePTFE membrane E4D was weighed to calculate an areal density of 0.016 g/m 2 /layer, while the membrane weighed 2.038 g/m 2 . The average total thickness measurement of the 192 layer ePTFE membrane was 3.50 microns which corresponded to approximately 18.2 nm per layer. 
     Table 8 indicates that the 128 and 192 layer ePTFE membranes were heavy enough and thick enough for wall thickness measurements. The calculated thickness per layer of each uncompressed ePTFE membrane is consistent with approximately twice the width of a typical fibril as measured from microscopic images from the STEM to be approximately 20-30 nm. The solid volume fraction and porosity are calculated using 2.2 g/cc as the density of PTFE. The compressed ePTFE membrane indicated a reduced porosity and thickness per layer. 
     Densification Methods 
     Method 1: Lab Press 
     ePTFE membrane E4C was placed in a Caver Laboratory Press Model M (Fred S. Carver Inc., Menomonee Falls, Wis.). The laboratory press was operated at room temperature (˜22° C.) with a 3″ diameter (˜0.0762 m) anvil on top of to generate approximately 300 psi (˜2.07 MPa) for approximately 30 minutes. 
     Method 2: Lab Autoclave 
     ePTFE membrane E4D was placed in an autoclave bag assembled from KAPTON® polyimide film (E.I. DuPont de Nemours Inc., Wilmington, Del.). The assembly was placed in an Econoclave™ 3 feet×5 feet laboratory autoclave (ASC Process Systems; Valencia, Calif.) using a temperature set point of 200° C. with an applied pressure of 250 psi (˜1.72 MPa) for approximately 70 minutes. 
     
       
         
           
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Process parameters for Example 4 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                   
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 Sample 
                 (° C.) 
                 N 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
               
                 E4A 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E4B 
                 300 
                 4 
                 120 
                 7 
                 7 
                 36 
                 s 
                 16 
                 120 
                 7 
                 7 
                 5 
                 s 
               
               
                 E4C 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                 E4D 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 48 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Third Pass 
                   
                 Heat 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 Rate 
                   
                 Heat 
                 Treatment 
                   
               
               
                   
                   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                 Treatment 
                 Time 
                 Densification 
               
               
                   
                 Sample 
                 N 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 (° C.) 2   
                 (s) 
                 Method 
               
               
                   
                   
               
               
                   
                 E4A 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r/s 
                 na 
                 na 
                 na 
               
               
                   
                 E4B 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
                 350 
                 300 
                 na 
               
               
                   
                 E4C 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r/s 
                 na 
                 na 
                 Method 1 
               
               
                   
                 E4D 
                 192 
                 180 
                 8 
                 8 
                 1 
                 r/s 
                 350 
                 300 
                 Method 2 
               
               
                   
                   
               
               
                   
                   1 “s” = constant speed; “r” = constant acceleration rate set point; “r/s”—constant acceleration rate step followed by constant speed step; 
               
               
                   
                   2 Set point; 
               
               
                   
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Example 4 sample data 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Stack 
                   
                 Areal 
                   
                   
                 Layer 
                 Bulk 
                 Solid Volume 
                   
               
               
                   
                 thickness 
                   
                 Density 
                 Area 
                 Heat 
                 thickness 3   
                 Density 
                 Fraction 
                 Porosity 
               
               
                 Sample 
                 (μm) 
                 n 1   
                 (g/m 2 ) 2   
                 Ratio 
                 Treatment 
                 (nm) 
                 (g/cm 3 ) 
                 (SVF) 
                 (1-SVF) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 E4A 
                 7.76 
                 128 
                 0.974 
                 148,501:1 
                 N 
                 60.6 
                 0.1256 
                 0.06 
                 0.94 
               
               
                 E4B 
                 4.95 
                 128 
                 0.632 
                 228,861:1 
                 Y 
                 38.7 
                 0.1276 
                 0.06 
                 0.94 
               
               
                 E4C 
                 1.50 
                 128 
                 0.974 
                 148,501:1 
                 N 
                 11.7 
                 0.6489 
                 0.29 
                 0.71 
               
               
                 E4D 
                 3.50 
                 192 
                 2.038 
                 106,447:1 
                 Y 
                 18.2 
                 0.5823 
                 0.26 
                 0.74 
               
               
                   
               
               
                   1 number of layers in stack; 
               
               
                   2 Areal density of sample; 
               
               
                   3 per individual layer of final expanded sample. 
               
            
           
         
       
     
     Example 5 
     The following example discloses the production of stacked ePTFE membranes (stacks of up to 192 layers by layering and co-expansion) and the measurement of various membrane parameters including: mean fibril width, area weighted fibril width (AWFW), median fibril width, specific surface area, bubble point, airflow resistance, and areal density. 
     High permeability is shown by relatively high air flow at a particular pressure, or stated another way, less pressure is required for higher flow. Airflow resistance is a function of structure and most simple models use the solid volume fraction and a representative fibril radius as the primary factors. More sophisticated models address slip as fibril radii decrease such that they are a small fraction of the mean free path of air at standard conditions, taken here to be 65 nm. Other contributing factors to generate a membrane with high air flow are the uniformity of the distribution of the fibrils, fibril shape and orientation. Uniform distribution of fibrils would be maximized if each fibril was separated by an identical distance, where a less uniform distribution would be represented by clumped or aggregated collections of fibrils, the later exhibiting higher permeability. Fibril shape can alter air flow resistance as well. 
     One way to determine the average fibril width is to the manually measure the width of fibrils within a representative sample.  FIG. 4  (ePTFE membrane E1H) was used to manually measure the fibril widths (50 fibrils measured) to calculate average width and median width ( FIG. 11 ). The fibril measurements were in nanometers (nm). From  FIG. 11  (ePTFE membrane E1H), it is obvious due to grey scale intensity changes across the width of the fibril, that indeed, the projected width is an over simplification as smaller fibrils are observed aggregating or clumping on larger fibrils. A histogram of the fibril measurements from  FIG. 11  is provided as  FIG. 12  where the data was fitted to a lognormal distribution. 
     PTFE fine powder (E.I. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. The dry tape was cut in to 98 mm squares. A summary of the process parameters used in Example 5 is provided in Table 9. 
     First Pass 
     Using a pantograph machine, up to four squares of tape were heated in an oven set to 300° C. (set point) for between 120 (E5A-G) or 240 (E5H-J) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio (λ sp ) of 4:1, 7:1, or 9:1 (Table 9) while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 8.3, 16.6 or 22 seconds based on the target ratio (Table 9). The expanded ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. The specific number of layers stacked for the second pass of each sample are provided in Table 9. Using the same pantograph machine, stacks of layers of ePTFE were heated in an oven set to 300° C. for a target of 120 (E5A-G) or 240 (E5H-J) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of 6.35:1 (E5J), 7:1 (E5H-I) or 10:1 (E5A-G) in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target ranged from 4%/s to 9%/s (Table 9). The pantograph opened at a constant velocity target for approximately 100 (E5A-BG), 150 (E5H), 120 (E5I) and 134 (E5J) seconds (Table 9). The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Samples were harvested from the cooled ePTFE membrane and stacked if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 120 (E5A-G), 180 (E5I) or 240 (E5H and E5J) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of 7:1 (E5H and E5J), 8:1 (E5I) or 10:1 (E5A-G) (Table 9) in each direction while maintaining a temperature of about 300° C. The biaxial expansions were conducted at a constant acceleration rate set point of 1%/s (E5A-E5H and E5J). For Example E5I, the biaxial expansion was conducted at a constant acceleration rate set point of 1%/s until 400% strain target (λ sp =5:1 in both directions) as the pantograph accelerated to a velocity set point of 3.5 mm/s, and the expansion was completed at a constant velocity set point of 5%/s (3.5 mm/s in this specific case based on the original length input of 70 mm) (“r/s” rate mode) to the target ratio, λ sp =8:1 in both directions. The pantograph opened for approximately 221 (E5I), 230 (E5A-G) and 195 (E5H and E5J) seconds). Selected samples (E5E-G and E5I) were thermally conditioned (heat treated) in an oven at a set point of 350° C. for a target of 300 seconds while restrained on the pantograph. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The cooled expanded ePTFE membrane from the third pass was harvested from the pantograph and placed onto (152.4 mm×152.4 mm) adhesive backed frames. The mean fibril width, area weighted fibril width (AWFW), median fibril width, specific surface area, bubble point, airflow resistance, and areal density are set forth in Table 10. 
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Process parameters for Example 5 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Rate 
                   
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
               
               
                 Sample 
                 (° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
               
               
                   
               
               
                 E5A 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
               
               
                 (FIG. 13) 
               
               
                 E5B 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 4 
                 120 
                 10 
                 10 
               
               
                 (FIG. 14) 
               
               
                 E5C 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 8 
                 120 
                 10 
                 10 
               
               
                 (FIG. 15) 
               
               
                 E5D 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 16 
                 120 
                 10 
                 10 
               
               
                 (FIG. 16) 
               
               
                 E5E 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 4 
                 120 
                 10 
                 10 
               
               
                 (FIG. 17) 
               
               
                 E5F 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 8 
                 120 
                 10 
                 10 
               
               
                 (FIG. 18) 
               
               
                 E5G 
                 300 
                 2 
                 120 
                 4 
                 4 
                 36 
                 s 
                 16 
                 120 
                 10 
                 10 
               
               
                 (FIG. 19) 
               
               
                 E5H 
                 300 
                 4 
                 240 
                 9 
                 9 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
               
               
                 (FIGS. 
               
               
                 20 &amp; 22) 
               
               
                 E5I 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 48 
                 240 
                 7 
                 7 
               
               
                 (FIGS. 
               
               
                 21 &amp; 23) 
               
               
                 E5J 
                 300 
                 4 
                 240 
                 9 
                 9 
                 36 
                 s 
                 32 
                 240 
                 6.35 
                 6.35 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Second Pass 
                 Third Pass 
                 Heat 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
                 Heat 
                 Treat. 
               
               
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                 Treat. 2   
                 Time 
               
               
                 Sample 
                 (%/s) 
                 Mode 1   
                 n 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 (° C.) 
                 (s) 
               
               
                   
               
               
                 E5A 
                 9 
                 s 
                 1 
                 120 
                 10 
                 10 
                 1 
                 r 
                 na 
                 na 
               
               
                 (FIG. 13) 
               
               
                 E5B 
                 9 
                 s 
                 4 
                 120 
                 10 
                 10 
                 1 
                 r 
                 na 
                 na 
               
               
                 (FIG. 14) 
               
               
                 E5C 
                 9 
                 s 
                 8 
                 120 
                 10 
                 10 
                 1 
                 r 
                 na 
                 na 
               
               
                 (FIG. 15) 
               
               
                 E5D 
                 9 
                 s 
                 16 
                 120 
                 10 
                 10 
                 1 
                 r 
                 na 
                 na 
               
               
                 (FIG. 16) 
               
               
                 E5E 
                 9 
                 s 
                 4 
                 120 
                 10 
                 10 
                 1 
                 r 
                 350 
                 300 
               
               
                 (FIG. 17) 
               
               
                 E5F 
                 9 
                 s 
                 8 
                 120 
                 10 
                 10 
                 1 
                 r 
                 350 
                 300 
               
               
                 (FIG. 18) 
               
               
                 E5G 
                 9 
                 s 
                 16 
                 120 
                 10 
                 10 
                 1 
                 r 
                 350 
                 300 
               
               
                 (FIG. 19) 
               
               
                 E5H 
                 4 
                 s 
                 128 
                 240 
                 7 
                 7 
                 1 
                 r 
                 na 
                 na 
               
               
                 (FIGS. 
               
               
                 20 &amp; 22) 
               
               
                 E5I 
                 5 
                 s 
                 192 
                 180 
                 8 
                 8 
                 1 
                 r/s 
                 350 
                 300 
               
               
                 (FIGS. 
               
               
                 21 &amp; 23) 
               
               
                 E5J 
                 4 
                 s 
                 128 
                 240 
                 7 
                 7 
                 1 
                 r 
                 na 
                 na 
               
               
                   
               
               
                   1 “s” = constant speed; “r” = constant acceleration rate set point; “r/s”—constant acceleration rate step followed by constant speed step 
               
               
                   2 Set point; 
               
               
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Fibril Width, Area Weighted Fibril Width, Specific Surface Area, and Bubble Point 
               
            
           
           
               
               
            
               
                   
                 Samples 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 E1H (FIG. 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 11 &amp; 12) 
                 E5A 
                 E5B 
                 E5C 
                 E5D 
                 E5E 
                 E5F 
                 E5G 
                 E5H 
                 E5I 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Fibril Width Mean (n) 
                 1.7 
                 1. 
                 19. 
                 19.9 
                 19.2 
                 25. 
                 22.7 
                 27.1 
                 7.4 
                 1.9 
               
               
                 Standard Deviation 
                 10.4 
                 7.7 
                 10.2 
                 9.9 
                 7.2 
                 14. 
                 1.9 
                 14.9 
                 14.1 
                 10.5 
               
               
                 Min(n) 
                 4.9 
                 2.9 
                 7.2 
                 5.2 
                 .4 
                 .1 
                 2.5 
                 7.2 
                 1.1 
                 10. 
               
               
                 Max(n) 
                 47.0 
                 4.1 
                 7.0 
                 59.5 
                 44.7 
                 79.2 
                 7. 
                 7.7 
                 7.5 
                 4.2 
               
               
                 Area Wei hted Fibril 
                 24.4 
                 19.9 
                 24.7 
                 24. 
                 21.9 
                 .7 
                 1.2 
                 5. 
                 42.7 
                 5.4 
               
               
                 Width (AWFW) (n) 
               
               
                 Ln ean (u) 
                 2.7 
                 2. 
                 2.7 
                 2.9 
                 2.9 
                 .12 
                 2.91 
                 .17 
                 .55 
                 .41 
               
               
                 In si a 
                 0.5 
                 0.55 
                 0.42 
                 0.4 
                 0. 
                 0.49 
                 0.71 
                 0.51 
                 0.7 
                 0. 
               
               
                 u (n) 
                 1.19 
                 14.29 
                 17. 
                 17.99 
                 17.9 
                 22.7 
                 1.27 
                 2.2 
                 4.9 
                 0.2 
               
               
                 Median (n) 
                 1.17 
                 14.0 
                 17.7 
                 1.2 
                 17. 
                 22. 
                 1.70 
                 24. 
                 4.9 
                 0. 
               
               
                 Specific Surface Area 
                 71 
                 7 
                 70 
                 70 
                 79 
                 52 
                 5 
                 49 
                 41 
                 49 
               
               
                 ( 2 /) (AWFW) 
               
               
                 Specific Surface Area 
                 10 
                 119 
                 9 
                 107 
                 9 
                 7 
                 9 
                 71 
                 50 
                 57 
               
               
                 ( 2 /) (Median) 
               
               
                 Mass per Area (/ 2 ) 
                   
                 0.00 
                 0.015 
                 0.0 
                 0.04 0 
                 0.010 
                 0.02 1 
                 0.0 2 
                 0.771 
                 2.1 2 
               
               
                 Air Resistance (Pa s/) 
                 na 
                 125 
                 17 
                 1225 
                 19 
                 202 
                 27 
                 4 2 
                 2202 
                 2 95 
               
               
                 dP (Pa) 
                 na 
                 5 
                 20 
                 125 
                 125 
                 22 
                 45 
                 75 
                 125 
                 125 
               
               
                 PMI Para eter 
                 na 
                 na 
                 10 
                 10 
                 10 
                 10 
                 10 
                 10 
                 na 
                 127 
               
               
                 Bubflow (L/in) 
               
               
                 Bubble Point [BPP] psi 
                 na 
                 na 
                 4.2 (0.029) 
                 .2 (0.057) 
                 40. (0.2 0) 
                 na 
                 2.2 (0.015) 
                 14.9 (0.10) 
                 na 
                 0.1 (0.552) 
               
               
                 (MPa) 
               
               
                 / 2 /layer 
                   
                 . 
                 4.0 
                 4.5 
                 2.9 
                 2. 
                 2.9 
                 2.1 
                 .0 
                 11.1 
               
               
                 of layers (n) 
                 1 
                 1 
                 4 
                   
                 1 
                 4 
                   
                 1 
                 12 
                 192 
               
               
                 Heat Treat ent Used 
                 n 
                 n 
                 n 
                 n 
                 n 
                 y 
                 y 
                 y 
                 n 
                 y 
               
               
                 (yes/no) 
               
               
                   
               
               
                 Bubble Point [BPP] psi (MPa) E5J = 129 psi (0.9) 
               
            
           
         
       
     
     Example 6 
     The following example discloses the production of ePTFE membranes and the measurement of various membrane parameters including: Quality Factor, airflow resistance, areal density, particle capture efficiency and penetration. Air filtration performance was measured as described in Test Methods section. 
     PTFE fine powder (E.I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. The dry tape was cut in to 98 mm squares. A summary of the process parameters used in Example 6 is provided in Table 11. 
     First Pass 
     Using a pantograph machine, a single (E6A-C) layer or up to four (E6D) squares of tape were layered and heated in an oven set to 300° C. (set point) for 120 (E6A-C) or 240 (E6D) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a selected target ratio (λ sp ) of 4:1, 7:1, or 9:1 (Table 11) in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 8.3, 16.6, or 22 seconds based on the target ratio (Table 11). The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Samples were harvested from the cooled membrane for further expansion, i.e. a second pass. The specific number of ePTFE membranes loaded for the second pass of each sample is provided in Table 11. Using the same pantograph machine, stacks of ePTFE membranes were heated in an oven set to 300° C. for a target of 120 (E6A-C) or 240 (E6D) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at selected target ratio of 7:1 or 10:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was 5%/s, 9%/s or 4%/s (Table 11). The pantograph opened at a constant velocity target for approximately 120 (E6A-B), 100 (E6C) and 150 (E6D) seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven to 300° C. for a target of 120 (E6A-C) or 240 (E6D) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of 8:1 (E6A-B), 10:1 (E6C) or 7:1 (E6D) in both longitudinal and transverse directions while maintaining a temperature of about 300° C. The average strain rate target was 1%/s. The pantograph opened at a constant acceleration rate target for approximately 208 (E6A-B), 230 (E6C) or 195 (E6D) seconds. Two samples, E6B and E6D were exposed to heat (approximately 350° C.) for 5 minutes. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Samples of the ePTFE membrane were harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed for further testing. Samples were tested for air flow resistance and filtration efficiency as described in the Test Method section. The air filtration results are provided in Table 12. A plot of particle diameter versus quality factor (Q f ) for samples E6A, E6B, E6C, E6D, and E6E (Comparative Example 1) is provided as  FIG. 24 .  FIG. 24  shows the improvement in quality factor for Samples E6A through E6D relative to the comparative example, E6E. 
     The ePTFE samples were cut free from the tape and weighed on a Mettler Toledo AT 20. The fibril width was measured for Samples E6A and E6B and shown in  FIGS. 25 and 26 .  FIGS. 27 and 28  show Samples E6A and E6B at lower magnification, respectively. Fibril width measurement results are provided in Table 13. 
     Comparative Example 1 
     An ePTFE membrane was manufactured according to the general teachings described in U.S. Pat. No. 3,953,566 to Gore. The ePTFE membrane (Sample E6E) had a mass per area of 5.6 g/m 2 , an air flow resistance of 6.68 mm H 2 O, and a particle capture efficiency of 98.344% for 0.1 micron DOP challenge particles tested with a face velocity of 5.33 cm/s (Table 12). 
     
       
         
           
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 Process parameters for Example 6 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Rate 
                   
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 Temp 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
               
               
                 (FIG.) 
                 (° C.) 2   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
               
               
                   
               
               
                 E6A 
                 300 
                 1 
                 120 
                 7 
                 7 
                 36 
                 s 
                 4 
                 120 
                 7 
                 7 
               
               
                 (FIGS. 
               
               
                 24&amp; 26) 
               
               
                 E6B 
                 300 
                 1 
                 120 
                 7 
                 7 
                 36 
                 s 
                 4 
                 120 
                 7 
                 7 
               
               
                 (FIGS. 
               
               
                 25 &amp; 27) 
               
               
                 E6C 
                 300 
                 1 
                 120 
                 4 
                 4 
                 36 
                 s 
                 1 
                 120 
                 10 
                 10 
               
               
                 E6D 
                 300 
                 4 
                 240 
                 9 
                 9 
                 36 
                 s 
                 16 
                 240 
                 7 
                 7 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Second Pass 
                 Third Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
                   
               
               
                   
                 Sample 
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                 Heat 
               
               
                   
                 (FIG.) 
                 (%/s) 
                 Mode 1   
                 n 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 Treat. 
               
               
                   
                   
               
               
                   
                 E6A 
                 5 
                 s 
                 4 
                 120 
                 8 
                 8 
                 1 
                 r 
                 n 
               
               
                   
                 (FIGS. 
               
               
                   
                 24&amp; 26) 
               
               
                   
                 E6B 
                 5 
                 s 
                 4 
                 120 
                 8 
                 8 
                 1 
                 r 
                 y 
               
               
                   
                 (FIGS. 
               
               
                   
                 25 &amp; 27) 
               
               
                   
                 E6C 
                 9 
                 s 
                 3 
                 120 
                 10 
                 10 
                 1 
                 r 
                 n 
               
               
                   
                 E6D 
                 4 
                 s 
                 16 
                 240 
                 7 
                 7 
                 1 
                 r 
                 y 
               
               
                   
                   
               
               
                   
                   1 “s” = constant speed; “r” = constant acceleration rate set point; 
               
               
                   
                   2 Set point; 
               
               
                   
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Air Filtration Data 
               
            
           
           
               
               
            
               
                   
                 Sample 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 E6E 
               
               
                   
                   
                   
                   
                   
                 (Comparative 
               
               
                   
                 E6A 
                 E6B 
                 E6C 
                 E6D 
                 Example 1) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Mass per area (g/m 2 ) 
                 0.0320 
                 0.0290 
                 0.0330 
                 0.0925 
                 5.6000 
               
               
                 Air Resistance (Pa s/m) 
                 955 
                 280 
                 804 
                 1273 
                 1248 
               
               
                 MPA/Layer (mg/m 2 /n) 
                 8.00 
                 7.25 
                 11.00 
                 5.78 
                 5600 
               
               
                 Number of layers (n) 
                 4 
                 4 
                 3 
                 16 
                 1 
               
               
                 Heat treatment 
                 n 
                 y 
                 n 
                 y 
                 na 
               
               
                 Pressure Drop (mm H2O) 
                 5.198 
                 1.522 
                 4.363 
                 7.204 
                 6.784 
               
               
                 Penetration % (0.1 μm) 
                 0.056 
                 24.877 
                 0.150 
                 0.552 
                 1.656 
               
               
                 E (%) 
                 99.944 
                 75.123 
                 99.850 
                 99.448 
                 98.344 
               
               
                 QF @ 5.33 cm/s with 
                 147 
                 93 
                 152 
                 74 
                 62 
               
               
                 0.1 μm (kPa −1 ) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 13 
               
             
            
               
                   
               
               
                 Fibril Properties 
               
            
           
           
               
               
               
            
               
                   
                 Sample 
                   
               
            
           
           
               
               
               
            
               
                   
                 E6A 
                 E6B 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Mean (nm) 
                 18.3 
                 35.2 
               
               
                   
                 Standard Deviation 
                 8.2 
                 21.3 
               
               
                   
                 Min (nm) 
                 3.5 
                 7.7 
               
               
                   
                 Max (nm) 
                 40.8 
                 101.0 
               
               
                   
                 Area Weighted Fibril Width (nm) 
                 22.0 
                 48.1 
               
               
                   
                 Ln mean (mu) 
                 2.80 
                 3.39 
               
               
                   
                 ln sigma 
                 0.49 
                 0.59 
               
               
                   
                 mu nm 
                 16.46 
                 29.68 
               
               
                   
                 Median (nm) 
                 17.47 
                 29.67 
               
               
                   
                 Specific Surface Area (m 2 /g) (AWFW) 
                 79 
                 36 
               
               
                   
                 Specific Surface Area (m 2 /g) (Median) 
                 100 
                 59 
               
               
                   
                   
               
            
           
         
       
     
     Example 7 
     The following example discloses the production of ePTFE membranes subsequently used for optical transmittance measurements. 
     PTFE fine powder (E.I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. The dry tape was cut into 98 mm squares. A summary of the process parameters used in Example 7 is provided in Table 14. 
     First Pass 
     Using a pantograph machine, one or four squares of tape were heated in an oven set to 300° C. (set point) for 240 (E7A) or 120 (E7B) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at various target ratios (Table 13). The average engineering strain rate target was determined for samples E7A and E7B (Table 13). The pantograph opened at a constant velocity target for approximately 16.6 (E7A) or 8.4 (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. Using the same pantograph machine, a single layer (E7B) or a stack of 16 layers (E7A) were heated in an oven set to 300° C. for a target of 120 (E7B) or 240 (E7A) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of 7:1 (E7A) or 10:1 (E7B) in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 5%/s (E7A) or 9%/s (E7B). The pantograph opened at a constant velocity target for approximately 120 (E7A) or 100 (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Samples were harvested from the cooled ePTFE membrane and stacked as needed for further expansion, i.e. a third pass. Using the same pantograph machine, a 3 layer (E7B) and a 48 (E7A) layer sample were heated in an oven set to 300° C. for a target of 120 seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of 7:1 (E7B) or 8:1 (E7A) (Table 14) in each direction, while maintaining a temperature of about 300° C. The average strain rate target was set to 1%/s. The pantograph opened at a constant acceleration rate target for approximately 208 (E7A) or 195 (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The ePTFE membranes were harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames for further testing. Samples were tested for air flow resistance as described in the Test Method section. The ePTFE membrane was cut free from the tape and weighed on a Mettler Toledo AT 20. Selected samples were also tested for optical transmission as described in the Test Method section. The results of optical transmission testing are provided in Table 15 and  FIG. 29 .  FIG. 29  is a plot of wavelength vs. % transmission for both a 3-layer sample (E7B; black line) and a 48-layer sample (E7A; grey line). 
     
       
         
           
               
             
               
                 TABLE 14 
               
               
                   
               
               
                 Process parameters for Example 7 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Rate 
                 Rate 
                   
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 Temp  2   
                   
                 Time 
                   
                   
                 Input 1 
                 Input 2 
                 Rate 
                   
                 Time 
               
               
                 (FIG.) 
                 (° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
               
               
                   
               
               
                 E7A 
                 300 
                 4 
                 240 
                 7 
                 7 
                 36 
                 36 
                 s 
                 16 
                 240 
               
               
                 (FIG. 29 - 
               
               
                 grey line) 
               
               
                 E7B 
                 300 
                 1 
                 120 
                 3.38 
                 4.79 
                 28.5 
                 44.8 
                 s 
                 1 
                 120 
               
               
                 (FIG. 29 - 
               
               
                 black line) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Second Pass 
                 Third Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                   
                 Sample 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                   
                 (FIG.) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
                   
               
               
                   
                 E7A 
                 7 
                 7 
                 5 
                 s 
                 48 
                 120 
                 8 
                 8 
                 1 
                 r 
               
               
                   
                 (FIG. 29 - 
               
               
                   
                 grey line) 
               
               
                   
                 E7B 
                 10 
                 10 
                 9 
                 s 
                 3 
                 120 
                 7 
                 7 
                 1 
                 r 
               
               
                   
                 (FIG. 29 - 
               
               
                   
                 black line) 
               
               
                   
                   
               
               
                   
                   1 “s” = constant speed; “r” = constant acceleration rate set point; 
               
               
                   
                   2  Set point; 
               
               
                   
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 15 
               
             
            
               
                   
               
               
                 Optical Measurements for Example 7 
               
            
           
           
               
               
            
               
                   
                 Sample 
               
            
           
           
               
               
               
            
               
                   
                 E7A 
                 E7B 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 MPA (g/m 2 ) 
                 0.4388 
                 0.0237 
               
               
                 Air Resistance (Pa s/m) 
                 10606 
                 726 
               
               
                 mg/m 2 /layer 
                 9.1 
                 7.9 
               
               
                 Layers (n) 
                 48 
                 3 
               
               
                 Heat Treatment 
                 n 
                 n 
               
               
                 Total Luminous % Transmittance (380-780 nm) 
                 98.0 
                 99.9 
               
               
                 UVA % Transmittance (315-400 nm) 
                 92.7 
                 99.8 
               
               
                 UVB % Transmittance (280-315 nm) 
                 88.5 
                 99.6 
               
               
                   
               
            
           
         
       
     
     Example 8 
     This example highlights the enhanced strength to weight ratio for relatively balanced ePTFE membranes consisting of extremely slender and similar fibrils exhibiting exceptionally high crystallinity index that is at least 94%. Stacking and co-expansion was employed to generate ample mass for bulk mechanical characterization and to reduce the time in the synchrotron for structural characterization. The amorphous content and relative strength balance was determined using x-ray diffraction (XRD). 
     PTFE fine powder (DuPont) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. The dry tape was cut in to 98 mm squares. A summary of the process parameters used in this example is provided in Table 16. 
     First Pass 
     Using a pantograph machine, up to four squares of tape were heated in an oven set to 300° C. (Samples E8A and E8B) or 322° C. (Samples E8C and E8D) for 240 seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of either 7:1 (E8C-D) or 9:1 (E8A-B) (Table 16) in each direction. The average engineering strain rate target was set to 36%/s. The pantograph opened at a constant velocity target for approximately 16.6 (E8C-D) and 22.2 (E8A-B) seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. Table 16 lists the specific number of layers loaded for the second pass of each condition. Using the same pantograph machine, samples having 16 (E8B) or 32 (E8A and E8C-D) layers were heated in an oven set to 300° C. (E8A-B) or 322° C. (E8C-D) for 240 seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a ratio 7:1 each in direction while maintaining the set point temperature. The average engineering strain rate target was 4%/s (E8A-B) or 5%/s (E8C-D) (Table 16). The pantograph opened at a constant velocity target for approximately 150 (E8A-B) or 120 (E8C-D) seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Samples were harvested from the ePTFE membrane and stacked if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the ePTFE membrane (using 64 (E8B) or 128 (E8A and E8C-D) layers) was again heated in an oven set to 300° C. (E8A-B) or 322° C. (E8C-D) for 120 (E8C-D) or 240 (E8A-B) seconds, respectively, and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of 7:1 (E8A-B) or 8:1 (E8C-D) in each direction while maintaining the set point temperature (Table 16). The average strain rate target was 1%/s. The pantograph opened at a constant acceleration for approximately 195 (E8A-B) or 208 (E8C-D) seconds. Samples E8B and E8D were thermally conditioned in an oven at a set point of 350° C. for a target of 300 seconds while restrained on the pantograph. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The ePTFE membranes were harvested from the machine onto (152.4 mm×152.4 mm) adhesive backed frames for further testing. Tensile test results included in Table 17 show the intrinsic strength to weight metrics exceed values previously reported in the art (see Comparative Examples—Table 18). Samples E8C and E8D were further characterized by X-ray diffraction (XRD) ( FIG. 30 , Sample E8C (not heat treated) and  FIG. 31 , Sample E8D (heat treated) and the results are consistent with isotropic orientation in the MD-TD plane. These results are consistent with the balanced strength results.  FIG. 32  is a plot of q(nm −1 ) vs. intensity over a range of 10-45 nm −1  for both a heat treated sample (Sample E8D; top trace) and a sample that was not heat treated (Sample E8C; bottom trace).  FIG. 32  is a plot of q (nm −1 ) vs. intensity (10-20 nm −1 ) over a range of 10-20 nm −1  for samples E8D (heat treated, top trace) and E8C (not heat treated, bottom trace).  FIGS. 32 and 33  demonstrate that the ePTFE membranes have a very high crystallinity index. In addition, the narrowness of the peaks centered at q=12.8 nm −1  ( FIG. 33 ) suggest that the crystalline packing of these ePTFE membranes have few defects. Example E8C had a crystallinity index of 99%. Example E8D had a crystallinity index of 99.2%. 
     
       
         
           
               
             
               
                 TABLE 16 
               
               
                   
               
               
                 Process parameters for Example 8 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 First Pass 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Rate 
                   
                 Second Pass 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Temp 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
               
               
                 Sample 
                 (° C.) 2   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
               
               
                   
               
               
                 E8A 
                 300 
                 4 
                 240 
                 9 
                 9 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
               
               
                 E8B 
                 300 
                 4 
                 240 
                 9 
                 9 
                 36 
                 s 
                 16 
                 240 
                 7 
                 7 
               
               
                 E8C 
                 322 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
               
               
                 (FIG. 30) 
               
               
                 E8D 
                 322 
                 4 
                 240 
                 7 
                 7 
                 36 
                 s 
                 32 
                 240 
                 7 
                 7 
               
               
                 (FIG. 31) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Second Pass 
                 Third Pass 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Rate 
                   
                   
                   
                   
                   
                 Rate 
                   
                 Heat 
               
               
                   
                   
                 Input 
                 Rate 
                   
                 Time 
                   
                   
                 Input 
                 Rate 
                 Treat. 
               
               
                   
                 Sample 
                 (%/s) 
                 Mode 1   
                 n 3   
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
                 (y/n) 
               
               
                   
                   
               
               
                   
                 E8A 
                 4 
                 s 
                 128 
                 240 
                 7 
                 7 
                 1 
                 r 
                 n 
               
               
                   
                 E8B 
                 4 
                 s 
                 64 
                 240 
                 7 
                 7 
                 1 
                 r 
                 y 
               
               
                   
                 E8C 
                 5 
                 s 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
                 n 
               
               
                   
                 (FIG. 30) 
               
               
                   
                 E8D 
                 5 
                 s 
                 128 
                 120 
                 8 
                 8 
                 1 
                 r 
                 y 
               
               
                   
                 (FIG. 31) 
               
               
                   
                   
               
               
                   
                   1 “s” = constant speed; “r” = constant acceleration rate set point; 
               
               
                   
                   2 Set point; 
               
               
                   
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 17 
               
             
            
               
                   
               
               
                 Properties of Example 8 ePTFE Membranes 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Areal 
                   
                 Bulk 
                 Solid Volume 
                   
                   
                   
               
               
                   
                   
                 MTS Psi 
                 MD × TD 
                 Area 
                 Density 
                 Thickness 
                 Density 
                 Fraction 
                 Porosity 
                 Layers 
                 Tenacity 
               
               
                 Sample 
                 Direction 
                 (MPa) 
                 (MPa 2 ) 
                 Ratio 
                 (g/m 2 ) 
                 (μm) 
                 (g/cm 3 ) 
                 (SVF) 
                 (1-SVF) 
                 (n) 
                 (gf/d) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 E8A 
                 MD 
                  92761 (640) 
                 449238 
                 153893:1 
                 0.929 
                 9.67 
                 0.096 
                 0.044 
                 0.96 
                 128 
                 3.23 
               
               
                   
                 TD 
                 101876 (702) 
                   
                   
                   
                   
                   
                   
                   
                   
                 3.55 
               
               
                 E8B 
                 MD 
                  91382 (630) 
                 469092 
                 140647:1 
                 0.518 
                 3.76 
                 0.138 
                 0.063 
                 0.94 
                 64 
                 3.19 
               
               
                   
                 TD 
                 107985 (745) 
                   
                   
                   
                   
                   
                   
                   
                   
                 3.76 
               
               
                   
               
            
           
         
       
     
     Comparative Examples 2-4 
     The matrix tensile strength of Comparative ePTFE Examples 2-4 in the art are set forth in Table 18. 
     
       
         
           
               
             
               
                 TABLE 18 
               
             
            
               
                   
               
               
                 Strength to Weight Metrics - Comparative Examples 2-4 
               
            
           
           
               
               
               
               
               
            
               
                   
                 MD 
                 TD 
                   
                   
               
               
                 Comparative 
                 MTS 
                 MTS 
                 MD × TD 
               
               
                 Example 
                 (MPa) 
                 (MPa) 
                 (MPa 2 ) 
                 Source 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 2 
                 414 
                 460 
                 190,440 
                 Example 4 of U.S. 
               
               
                   
                   
                   
                   
                 Pat. No. 7,306,729 to 
               
               
                   
                   
                   
                   
                 Bacino, et al. 
               
               
                 3 
                 586 
                 552 
                 323,472 
                 Example 2 of U.S. 
               
               
                   
                   
                   
                   
                 Pat. Publication No. 
               
               
                   
                   
                   
                   
                 2013/0231733 to 
               
               
                   
                   
                   
                   
                 Knisley, et al. 
               
               
                 4 
                 705 
                 385 
                 271,425 
                 Example 1 of U.S. 
               
               
                   
                   
                   
                   
                 Pat. Publication No. 
               
               
                   
                   
                   
                   
                 2014/0172083 to 
               
               
                   
                   
                   
                   
                 Bruchman, et al. 
               
               
                   
               
            
           
         
       
     
     Example 9 
     The following example describes the preparation and analysis of uniaxially orientated ePTFE membranes having low mass with high intrinsic strength in the fibril direction. 
     PTFE fine powder (E.I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (˜0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. A set of 98 mm square was cut from the dry tape. A summary of the process parameters used in Example 9 is provided in Table 19. 
     First Pass 
     Using a pantograph machine, two different samples, each having four layers of tape were heated in an oven set to 300° C. (set point) for 240 seconds and then expanded in the longitudinal direction (machine direction) and transverse direction simultaneously at a target ratio of about 10:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 25 seconds. The ePTFE membranes were allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Second Pass 
     Four pieces (4 layers each) were harvested from the cooled membrane for further expansion, i.e. a second pass. Using the same pantograph machine, samples containing a stack of 16 layers were heated in an oven set to 300° C. for a target of 240 seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of 10:1 in each direction while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 3.6%/second. The pantograph opened at a constant velocity target for approximately 250 seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Third Pass 
     Four samples (16 layers each) were harvested from the cooled membrane and 2 stacks of 16 layers (32 layers total) were loaded for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was heated in an oven set to 300° C. for a target of 120 seconds and then stretched in the longitudinal direction and transverse directions simultaneously at a target ratio of 3:1 (Example E9A) or 5:1 (Example E9B) in each direction (specific details for each example are included in Table 19) while maintaining a temperature of about 300° C. The average strain rate target was set to 1%/second. The pantograph opened at a constant acceleration for approximately 110 (E9A) or 161 (E9B) seconds. The ePTFE membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Optional Fourth Pass 
     Using the same pantograph machine, for Sample E9B, the 32 layer sample was again heated in an oven set to 300° C. for a target of 120 seconds and then stretched in the longitudinal direction and transverse direction simultaneously at a target ratio of 3:1 in each direction while maintaining a temperature of about 300° C. The simultaneous expansion was conducted at a constant acceleration rate set point target of 1%/s. The pantograph opened at a constant acceleration for approximately 110 (E9B) seconds. 
     Second to Last Pass 
     Using the same pantograph machine, the ePTFE membrane is freed from constraint in the transverse direction, while remaining fixed in the machine direction. The ePTFE membrane was heated in an oven set to 300° C. for a target of 120 seconds and then stretched only in the longitudinal direction at a target ratio of 6:1 (in the machine direction) while the ePTFE membrane in the transverse direction was allowed to neck down (i.e., narrow) freely. The pantograph opened with a constant acceleration rate set point for approximately 170 seconds (E9A-B). The expansion was conducted with a constant acceleration rate set point of 1%/s. 
     Last Pass 
     Using the same pantograph machine the 32 layer sample was heated in an oven set to 350° C. for a target of 300 seconds and then stretched at a target ratio of 1.5:1 (Sample E9A) or 1.67:1 (Sample E9B) only in the machine (longitudinal) direction while maintaining a temperature of about 350° C. The expansion was conducted with a constant acceleration rate set point of 1%/s. The pantograph opened with a constant acceleration rate set point for approximately 40 (E9A) or 51 (E9B) seconds. 
     The membranes were harvested from the machine onto adhesive backed frames of known dimensions (152.4 mm×152.4 mm). Using the frame as a cutting guide, the ePTFE membranes were weighed to calculate the linear density (bulk denier) and mechanical data was collected using the Matrix Tensile Test from the Test Methods section set forth above. Sample E9A was also characterized using dynamic mechanical analysis (DMA) indicating a matrix modulus of 100 GPa ambient temperature (i.e., about 20° C.) ( FIG. 33 ). Sample E9A was further characterized by XRD ( FIG. 34 ) which is consistent with an extremely high degree of crystalline orientation, where the &lt;P2&gt; orientation function is 0.989 where 1.0 would be consistent with perfect parallel alignment ( FIG. 35 ). The crystallinity index was determined to be 94.6%. An SEM of Sample E9A is provided as  FIG. 36 . 
     
       
         
           
               
             
               
                 TABLE 19 
               
             
            
               
                   
               
               
                 Process Parameters for Example 9 Membranes 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                 Sample 
                   
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 (FIGS.) 
                 Pass 
                 (° C.) 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 E9A 
                 1 
                 300 
                  4 
                 240 
                 10 
                 10 
                 36 
                 s 
               
               
                 (FIGS. 
                 2 
                 300 
                 16 
                 240 
                 10 
                 10 
                 3.6 
                 s 
               
               
                 34-37) 
                 3 
                 300 
                 32 
                 120 
                 3 
                 3 
                 1 
                 r 
               
               
                   
                 2 nd   
                 300 
                 32 
                 120 
                 6 
                 na 
                 1 
                 r 
               
               
                   
                 from 
               
               
                   
                 last 
               
               
                   
                 pass 
               
               
                   
                 Last 
                 350 
                     32 3   
                 300 
                 1.5 
                 na 
                 1 
                 r 
               
               
                 E9B 
                 1 
                 300 
                  4 
                 240 
                 10 
                 10 
                 36 
                 s 
               
               
                   
                 2 
                 300 
                 16 
                 240 
                 10 
                 10 
                 3.6 
                 s 
               
               
                   
                 3 
                 300 
                 32 
                 120 
                 5 
                 5 
                 1 
                 r 
               
               
                   
                 4 
                 300 
                 32 
                 120 
                 3 
                 3 
                 1 
                 r 
               
               
                   
                 2 nd   
                 300 
                 32 
                 120 
                 6 
                 na 
                 1 
                 r 
               
               
                   
                 from 
               
               
                   
                 last 
               
               
                   
                 pass 
               
               
                   
                 Last 
                 350 
                     32 3   
                 300 
                 1.67 
                 na 
                 1 
                 r 
               
               
                   
               
               
                   1 “s” = constant speed; “r” = constant acceleration rate set point; 
               
               
                   2 Set point; 
               
               
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 20 
               
             
            
               
                   
               
               
                 Properties of the Uniaxially Oriented Membranes 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Matrix Tensile 
                   
                 Bulk 
                 Areal 
                   
                 Bulk 
                 Solid Volume 
                   
                   
               
               
                   
                   
                 Strength Psi 
                 Tenacity 
                 Denier 
                 Density 
                 Thickness 
                 density 
                 Fraction 
                 Porosity 
                 Layers 
               
               
                 Sample 
                 Direction 
                 (MPa) 
                 (gf/d) 
                 (g/9000 m) 
                 (g/m 2 ) 
                 (μm) 
                 (g/cm 3 ) 
                 (SVF) 
                 (1-SVF) 
                 (n) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 E9A 
                 Uniaxial 
                 167,474 (1155) 
                 5.9 
                 160 
                 6 
                 3.81 
                 1.58 
                 0.716 
                 0.284 
                 32 
               
               
                 E9B 
                 Uniaxial 
                 202,649 (1397) 
                 7.2 
                 431 
                 16 
                 8.61 
                 1.86 
                 0.845 
                 0.155 
                 32 
               
               
                   
               
            
           
         
       
     
     Example 10 
     The following example discloses the production of multilayered ePTFE membranes having very low mass with high intrinsic strength and the measurement of nanoparticle retention. Nanoparticle retention is tested using the bead test disclosed in the methods section which measures permeability and bead retention of the membrane sample. 
     PTFE fine powder (E.I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of 110 mL per pound (0.454 kg) of fine powder (0.156 g lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at 49° C. to provide a tape. The tape was 16.2 cm wide and 0.762 mm thick. The ISOPAR™ K was removed by heating to approximately 200° C. The dry tape was cut in to 98 mm squares. A summary of the process parameters used in Example 10 is provided in Table 21. 
     First Pass 
     Using a pantograph machine, four squares of tape were layered and heated in an oven set to 300° C. for a target of 120 (E10A-C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a selected target ratio (λ sp ) of 7:1 (E10A and E10C) or 2:1 in each direction for E10B, while maintaining a temperature of about 300° C. The average engineering strain rate target was set to 36%/second. The pantograph opened at a constant velocity target for approximately 16.6 (E10A and E10C) or about 2.8 (E10B) seconds based on the target ratio (Table 21). The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. The first pass is repeated until 64 (E10A), or 16 (E10B) or 32 (E10C) layers are available for the second pass. 
     Second Pass 
     The specific number of layers, 64 (E10A), 16 (E10B) or 32 (E10C) were loaded for the second pass of each condition as provided in Table 21. Using the same pantograph machine, stacks of layers were heated in an oven set to 300° C. for a target of 240 (E10A and B) or 120 (E10C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a selected target of 7:1 (E10A) or 10:1 (E10B) or 6:1 (E10C) while maintaining a temperature of about 300° C. The average engineering strain rate target was 5%/s (E10A and E10C) or 18%/s (E10B) (Table 21). The pantograph opened at a constant target velocity for approximately 120 (E10A), 50 (E10B) and 100 (E10C) seconds. The expanded membrane was allowed to cool to room temperature (−22° C.) under restraint of the pantograph. 
     Third Pass 
     Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a third pass. The specific number of layers, 256 (E10A), 120 (E10B) or 128 (E10C) were loaded for the second pass of each condition as provided in Table 21. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 120 (E10A-C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of 8 (E10A), 10 (E10B) or 6:1 (E10C) in both longitudinal and transverse directions while maintaining a temperature of about 300° C. The average constant acceleration strain rate set point was 1%/s. The pantograph opened at a constant acceleration rate target for approximately 221 (E10A), 261 (E10B) or 179 (E10C) seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     The samples were harvested from the machine onto (152.4×152.4 mm) adhesive backed frames for further testing (E10A and E10B) or further expansion (E10C). 
     Fourth Pass 
     Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a fourth pass. The specific number of layers, 2056 (E10C) were loaded for the fourth pass of each condition as provided in Table 21. Using the same pantograph machine, the membrane was again heated in an oven set to 300° C. for a target of 120 (E10C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of 4.75:1 (E10C) in both longitudinal and transverse directions while maintaining a temperature of about 300° C. The constant acceleration strain rate set point was 1%/s. The pantograph opened at a constant acceleration rate target for approximately 156 (E10C) seconds. The expanded membrane was allowed to cool to room temperature (˜22° C.) under restraint of the pantograph. 
     Densification 
     The samples (E10A-C) were densified while restrained in the MD and TD plane, by gently pouring Isopropyl Alcohol (IPA) onto the restrained membrane and allowing the IPA to evaporate. 
     Each membrane sample (samples E10A, E10B, and E10C) was evaluated for mean filtrate permeability (See Equation (9) above), and bead retention (See Equation(10) above) according to the Determination of Permeability and Retention with Bead Test set forth above in the Test Methods section. The results are provided in Table 22 and in  FIG. 38 . 
     Comparative Examples 5-7 
     Three comparative liquid filtration samples were prepared as follows. 
     Comparative Example 5 
     A fine powder of polytetrafluoroethylene polymer made in accordance with the teachings of U.S. Pat. No. 6,541,589 to Baillie was combined with 0.184 lb/lb of an isoparaffinic hydrocarbon lubricant (ISOPAR™ K, Exxon, Houston, Tex.). The resultant mixture was then blended, compressed into a cylindrical pellet, and thermally conditioned for at least 8 hours at a temperature of 49° C. The cylindrical pellet was then extruded through a rectangular orifice die at a reduction ratio of 72:1 to form a tape. The tape was then calendered between rolls at a calendering ratio of 3:1. The calendered tape was then transversely stretched at a ratio of 3.6:1 and dried at a temperature of 200° C. 
     The dried tape was then expanded at 330° C. in the machine direction to an expansion ratio of 7:1. The resulting material was subsequently expanded in the transverse direction to an expansion ratio of 12:1 at temperature of about 310° C. 
     This biaxially expanded membrane was compressed between rollers (at 25° C.) at a speed of 1 m/minute and with a compression force of 10 N/mm. 
     Comparative Example 6—Sample E10D2 
     A fine powder of polytetrafluoroethylene polymer made in accordance with the teachings of U.S. Pat. No. 6,541,589 to Baillie was combined with 0.151 lb/lb of lubricant (ISOPAR™ K, Exxon, Houston, Tex.). The resultant mixture was then blended, compressed into a cylindrical pellet, and thermally conditioned for at least 8 hours at a temperature of 49° C. The cylindrical pellet was then extruded through a rectangular orifice die at a reduction ratio of 72:1 to form a tape. The tape was then calendered between rolls at a calendering ratio of 3:1. The calendered tape was then transversely stretched at a ratio of 3.6:1 and dried at a temperature of 200° C. The dried tape was then expanded at 330° C. in the machine direction to an expansion ratio of 5:1. The resulting material was subsequently expanded in the transverse direction to an expansion ratio of 10.8:1 at temperature of about 310° C. The membrane was then heat treated at a temperature of approximately 380° C. for a target of 25 seconds. 
     This biaxially expanded membrane was compressed between rollers (at 25° C.) at a speed of 1 m/minute and with a compression force of 20 N/mm. 
     Comparative Example 7—Sample E10D3 
     A fine powder of polytetrafluoroethylene polymer made in accordance with the teachings of U.S. Pat. No. 6,541,589 to Baillie was combined with 0.145 lb/lb of lubricant (ISOPAR™ K, Exxon, Houston, Tex.). The resultant mixture was then blended, compressed into a cylindrical pellet, and thermally conditioned for at least 8 hours at a temperature of 49° C. The cylindrical pellet was then extruded through a rectangular orifice die at a reduction ratio of 72:1 to form a tape. The tape was then calendered between rolls at a calendering ratio of 3:1. The calendered tape was then transversely stretched at a ratio of 3.6:1 and dried at a temperature of 230° C. The dried tape was then expanded at 325° C. in the machine direction to an expansion ratio of 5:1. The resulting material was subsequently expanded in the transverse direction to an expansion ratio of 12.3:1 at temperature of about 300° C. This biaxially expanded membrane was compressed between rollers (at 90° C.) at a speed of 5 m/minute and with a compression force of 80 N/mm. 
     Each Comparative Example membrane sample (samples E10D1, E10D2, and E10D3) was evaluated for mean filtrate permeability (See Equation (9)), and bead retention (See Equation (10)) using the testing procedures described above. The results are provided in Table 22 and in  FIG. 38 . 
     
       
         
           
               
             
               
                 TABLE 21 
               
             
            
               
                   
               
               
                 Process parameters for Example 10 - Samples E10A-E10C 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Rate 
                   
               
               
                   
                   
                 Temp 2   
                   
                 Time 
                   
                   
                 Input 
                 Rate 
               
               
                 Sample 
                 Pass 
                 ° C. 
                 n 
                 (s) 
                 λ 1sp   
                 λ 2sp   
                 (%/s) 
                 Mode 1   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 E10A 
                 1 
                 300 
                  4 
                 120 
                 7 
                 7 
                 36 
                 s 
               
               
                   
                 2 
                 300 
                 64 
                 240 
                 7 
                 7 
                 5 
                 s 
               
               
                   
                 3 
                 300 
                 256 3       
                 120 
                 8 
                 8 
                 1 
                 r/s 
               
               
                 E10B 
                 1 
                 300 
                  4 
                 120 
                 2 
                 2 
                 36 
                 s 
               
               
                   
                 2 
                 300 
                 16 
                 240 
                 10 
                 10 
                 18 
                 s 
               
               
                   
                 3 
                 300 
                 120 3       
                 120 
                 10 
                 10 
                 1 
                 r/s 
               
               
                 E10C 
                 1 
                 300 
                  4 
                 120 
                 7 
                 7 
                 36 
                 s 
               
               
                   
                 2 
                 300 
                 32 
                 120 
                 6 
                 6 
                 5 
                 s 
               
               
                   
                 3 
                 300 
                 128  
                 120 
                 6 
                 6 
                 1 
                 r 
               
               
                   
                 4 
                 300 
                 2048 3    
                 120 
                 4.75 
                 4.75 
                 1 
                 r 
               
               
                   
               
               
                   1 “s” = constant speed; “r” = constant acceleration rate set point; “r/s”—combination of constant acceleration rate step followed by constant speed step; 
               
               
                   2 Set point; 
               
               
                   3 final number of layers in final pass. 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 22 
               
             
            
               
                   
               
               
                 Nanoparticle retention and Filtrate Permeability for Example 10 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Nanoparticle 
                 Filtrate 
               
               
                   
                   
                 Retention 
                 Permeability 
               
               
                   
                 Sample 
                 (%) 
                 (g/cm 2 /m/MPa) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 E10A 
                 15.2 
                 1.38 
               
               
                   
                 E10A 
                 13.3 
                 1.45 
               
               
                   
                 E10B 
                 19.1 
                 0.92 
               
               
                   
                 E10B 
                 22.7 
                 0.54 
               
               
                   
                 E10C 
                 11.3 
                 1.45 
               
               
                   
                 E10C 
                 14.6 
                 1.49 
               
               
                   
                 E10D1 
                 2.9 
                 1.04 
               
               
                   
                 (Comparative Example) 
               
               
                   
                 E10D2 
                 13.0 
                 0.45 
               
               
                   
                 (Comparative Example) 
               
               
                   
                 E10D3 
                 34.1 
                 0.04 
               
               
                   
                 (Comparative Example) 
               
               
                   
                   
               
            
           
         
       
     
     The invention of this application has been described above both generically and with regard to specific embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made in the embodiments without departing from the scope of the disclosure. Thus, it is intended that the embodiments cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.