Unsintered stretched polytetrafluoroethylene composite film with dimensional stability

Abstract

To provide a method of forming an un-sintered biaxially stretched PTFE / thermoplastic polymer composite film.SOLUTION: The method involves blending fibrillatable polytetrafluoroethylene (PTFE) particles and thermoplastic polymer particles, wherein a melting point of the thermoplastic polymer particles is lower than the melting point of the fibrillatable PTFE particles. The method further includes forming the blend into a tape, stretching the tape in a first direction at a first temperature and heating. The stretched tapes are then simultaneously or sequentially stretched in a second direction to form an ePTFE composite membrane. This method does not include a sintering temperature. Average particle sizes of the ePTFE particles and thermoplastic polymer particles are less than 1 μm. Additionally, the ePTFE composite membrane has a ratio of geometric mean matrix modulus to geometric mean matrix tensile strength of at least about 6 and an absolute dimensional change of less than about 1.5%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure generally relates to stretched polytetrafluoroethylene (ePTFE) composite films that are dimensionally stable without sintering, more specifically to ePTFE composite films comprising at least one thermoplastic polymer. Methods for producing such composite films are also provided. [Background technology]

[0002] ePTFE films and ePTFE composite films may be subjected to at least one sintering process before use to improve dimensional stability. While sintering above the melting point of ePTFE is known to improve the dimensional stability of ePTFE films, sintering adversely affects the film by reducing crystallinity and increasing amorphous content. Films containing amorphous PTFE often exhibit a rigid amorphous phase with a transition temperature of approximately 120°C. Above this transition temperature, the mechanical properties of sintered ePTFE products may deteriorate. For example, sintering an ePTFE film adversely affects the film's properties, including the mean matrix modulus, a decrease in dimensional stability over time, and a decrease in crystallinity. Those skilled in the art desire to produce dimensionally stable porous films derived from ePTFE without the need to sinter ePTFE and thereby lose crystallinity. Therefore, there is a need to provide ePTFE films and ePTFE composite films that are dimensionally stable over time at high temperatures, exhibit a high matrix modulus, and have a relatively high geometric mean matrix modulus / geometric mean matrix tensile strength ratio. [Overview of the project]

[0003] One embodiment relates to a method for forming an unsintered biaxially oriented ePTFE composite film. The method includes providing a blend comprising a first plurality of fibrillable polytetrafluoroethylene (PTFE) particles having a first melting point and a second plurality of thermoplastic polymer particles having a second melting point lower than the first melting point. The blend is then formed into a tape. The tape is then stretched in a first direction at a temperature lower than the first melting point, and then stretched in a second direction different from the first direction (e.g., orthogonal) to form an ePTFE composite film. Stretching in the first direction can be carried out at a temperature lower than the second melting point, such as about 170°C to about 300°C. Stretching in the second direction can be carried out at a temperature higher than the second melting point and lower than the first melting point, such as about 280°C to 327°C. The blend comprises 40% to 79.9% by mass of fibrillable PTFE particles and 20.1% to 60% by mass of thermoplastic polymer. The ePTFE composite film can have a geometric mean matrix modulus / geometric mean matrix tensile strength ratio of at least about 6. Furthermore, this method can omit heating steps above 327°C.

[0004] Another embodiment relates to a method for forming an unsintered biaxially oriented ePTFE composite film. This method includes providing a blend comprising a first plurality of fibrillable polytetrafluoroethylene (PTFE) particles having an average particle size of less than 1 μm and a second plurality of thermoplastic polymer particles having an average particle size of less than 1 μm. In an exemplary embodiment, the average particle size of the thermoplastic polymer particles is the same as or smaller than the average particle size of the fibrillable PTFE particles. Furthermore, the melting point of the thermoplastic polymer is lower than the melting point of the fibrillable PTFE particles. The blend comprises 40% to 79.9% by mass of fibrillable PTFE particles and 20.1% to 60% by mass of thermoplastic polymer particles. This method also includes paste-extruding a blend containing a lubricant to form a calendered tape, drying the calendered tape to remove the lubricant to produce a dry calendered tape, and stretching the dry calendered tape in a first direction at a temperature lower than the melting point of the thermoplastic polymer to form a uniaxially oriented ePTFE composite film. This method also includes heating a uniaxially oriented porous ePTFE composite film to a temperature higher than the melting point of the thermoplastic polymer and lower than the melting point of fibrillable PTFE particles, and simultaneously or sequentially stretching the uniaxially oriented porous ePTFE composite film in a second direction, where the second direction is different from the first direction, to form a biaxially oriented ePTFE composite film. The temperature of the stretching step in the first direction can be about 170°C to about 300°C, and the temperature of the heating step can be about 280°C to about 300°C. The step of stretching the film in the second direction may be simultaneous with the heating step. This method may omit the heating step at a temperature above the melting point of fibrillable PTFE particles, such as above 327°C. Examples of thermoplastic polymers include poly(ethene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and combinations thereof.

[0005] Another embodiment relates to an unsintered biaxial ePTFE composite film having multiple nodes and fibrils. The ePTFE composite film comprises 40% to 79.9% by mass of polytetrafluoroethylene (PTFE), 20.1% to 60% by mass of a thermoplastic polymer, and has a geometric mean matrix modulus / geometric mean matrix tensile strength ratio of at least about 6. The fibrils mainly consist of ePTFE, and the nodes contain a greater amount of thermoplastic polymer than the amount originally present in the ePTFE composite film. The ePTFE composite film can exhibit a dimensional change of less than 1.5% when measured by dynamic mechanical analysis (DMA) after heating from 25°C to 200°C at a rate of 5°C / min and holding at 200°C for 5 minutes. The fibrils may contain at least about 85% or about 90% by mass of the ePTFE composite film. The nodes may contain at least about 51% by mass of the thermoplastic polymer. Examples of thermoplastic polymers include poly(ethene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and combinations thereof. [Brief explanation of the drawing]

[0006] The accompanying drawings are included to provide a further understanding of this disclosure, are incorporated herein, constitute part thereof, illustrate embodiments, and, together with the description, help to illustrate the principles of this disclosure.

[0007] [Figure 1] Figure 1 is a flowchart showing a method for forming a biaxially oriented polytetrafluoroethylene (ePTFE) composite film according to at least one embodiment.

[0008] [Figure 2] Figure 2 is a differential scanning calorimetry (DSC) graph of Example 1 poly(ethene-co-tetrafluoroethene) (ETFE) resin having a melting point of approximately 260°C, according to at least one embodiment.

[0009] [Figure 3] Figure 3 is a DSC graph of Example 7's PTFE / ETFE composite film having a melting point of approximately 344°C, according to at least one embodiment, which indicates that the PTFE composite film was not pre-sintered.

[0010] [Figure 4] Figure 4 is a DSC graph of the FEP / PTFE composite film of Example 9 having a melting point of approximately 342°C, according to at least one embodiment.

[0011] [Figure 5] Figure 5 is a schematic diagram of the structure of an ePTFE composite film having fibrils mainly formed from ePTFE and nodes rich in thermoplastic polymer, according to at least one embodiment.

[0012] [Figure 6] Figure 6 is a scanning electron microscope (SEM) image showing the structure of an ETFE / ePTFE composite film according to at least one embodiment.

[0013] [Figure 7] Figure 7 is a scanning electron microscope (SEM) image showing the structure of an ETFE / ePTFE composite film according to another embodiment. [Modes for carrying out the invention]

[0014] Those skilled in the art will readily understand that various aspects of this 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 drawings referenced herein are not necessarily drawn to scale and may be exaggerated to illustrate various aspects of this disclosure, and in this regard, the drawings should not be construed as limiting. It should be understood that the terms “fibrillable PTFE particles” and “fibrillable PTFE” are interchangeable as used herein. Furthermore, the terms “thermoplastic polymer particles” and “thermoplastic polymer” are interchangeable as used herein. It should also be understood that stretched composite films are porous.

[0015] Referring first to Figure 1, a method 1000 for preparing an ePTFE composite film from PTFE particles and thermoplastic polymer particles, each having a different melting point, is shown. The resulting ePTFE composite film possesses structural characteristics of both PTFE and thermoplastic polymers. The resulting ePTFE composite film exhibits excellent dimensional stability and mechanical properties. It should be understood that the term "thermoplastic polymer" is intended to include a single thermoplastic polymer or multiple thermoplastic polymers.

[0016] Thermoplastic polymer particles are provided as shown in step 1010 of method 1000 in Figure 1. Fibrillable PTFE particles useful for forming ePTFE composite films according to this disclosure are disclosed, for example, in Gore's U.S. Patent No. 3,953,566 (e.g., homopolymers) and Bail's U.S. Patent No. 6,541,589 (e.g., modified polymers). The polymers may be modified with, for example, perfluorobutylethylene (PFBE), poly(methyl vinyl ether) (PMVE), perfluoro(propyl vinyl ether) (PPVE), etc. PTFE particles may have an average particle size of less than 1.0 μm. The particle size of PTFE can be about 0.1 μm, about 0.2 μm, about 0.3 μm, about 0.4 μm, about 0.5 μm, about 0.6 μm, about 0.7 μm, about 0.8 μm, about 0.9 μm, or about 1.0 μm. In some embodiments, the PTFE particle size is approximately 0.1 μm to approximately 1.0 μm, approximately 0.2 μm to approximately 0.9 μm, or approximately 0.3 μm to approximately 0.8 μm.

[0017] Thermoplastic polymer particles are provided as shown in step 1020 of method 1000 in Figure 1. Suitable thermoplastic polymer particles include, but are not limited to, poly(ethene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene polyethylene (FEP), perfluoroalkoxy (PFA), and combinations thereof. The particle size of the thermoplastic polymer may be approximately the same as that of fibrillable PTFE particles, or it may be smaller than that of PTFE particles. The particle size of the thermoplastic polymer may be about 0.1 μm, about 0.2 μm, about 0.3 μm, about 0.4 μm, about 0.5 μm, about 0.6 μm, about 0.7 μm, about 0.8 μm, about 0.9 μm, or about 1.0 μm. In some embodiments, the particle size of the thermoplastic polymer is approximately 0.1 μm to approximately 1.0 μm, approximately 0.2 μm to approximately 0.9 μm, or approximately 0.3 μm to approximately 0.8 μm.

[0018] As shown in step 1030 of method 1000 in FIG. 1, the composite resin is formed from PTFE particles from step 1010 and thermoplastic polymer particles from step 1020. The composite resin forming step 1030 includes blending fibrillatable PTFE particles from step 1010 with thermoplastic polymer particles from step 1020. In one embodiment, the PTFE particles can be polymerized in a suitable liquid (e.g., water) to form a first dispersion containing the PTFE particles. Invisible thermoplastic polymer particles can be polymerized in a suitable liquid (e.g., water) to form a second dispersion containing the thermoplastic polymer particles. Depending on the solids content of the dispersed particles in each dispersion, the two dispersions can be blended in a desired ratio (e.g., thermoplastic polymer particle dispersion / fibrillatable PTFE particle dispersion) to produce the desired mass percentage of each polymer in the blend. The blend can contain about 40 wt%, about 50 wt%, about 55 wt%, about 60 wt%, about 65 wt%, about 70 wt%, about 75 wt%, about 77 wt% or 79.9 wt% of PTFE. In some embodiments, the PTFE content of the blend can be about 40 wt% - 79.9 wt%, about 50 wt% - about 70 wt% or about 55 wt% - about 65 wt%. In certain embodiments, the PTFE content of the blend can be as high as 75 wt%, 76 wt%, 77 wt%, 78 wt%, 79 wt%, 79.5 wt% or 79.9 wt%. Further, the blend can contain 20.1 wt%, about 23 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 45 wt%, about 50 wt%, about 55 wt% or about 60 wt% of the thermoplastic polymer. In some embodiments, the thermoplastic polymer content of the blend can be about 23 wt% - about 60 wt%, about 30 wt% - about 50 wt% or about 35 wt% - about 45 wt%. In certain embodiments, the thermoplastic polymer content of the blend can be as low as 20.1 wt%, 20.5 wt%, 21 wt%, 22 wt%, 23 wt% or 24 wt%. In an exemplary embodiment, the blend contains 40 wt% - 79.9 wt% of ePTFE and 20.1 wt% - 60 wt% of the thermoplastic polymer.

[0019] The resin forming step 1030 can include blending other components and / or other steps for generating a blended dispersion having desired weight fractions and other desired properties. For example, the resin forming step 1030 can include coagulating the blended dispersion, for example, by adding HNO3, which is then stirred. The resin forming step 1030 can also or alternatively include drying the blended dispersion in a vacuum oven or by other suitable drying methods known in the art to produce a composite resin. Further, the resin forming step 1030 can include diluting the blended dispersion to a desired solids %. The solids % can be about 10 wt%, about 15 wt%, about 20 wt%, about 25 wt%, about 30 wt%, about 35 wt%, about 40 wt%, about 45 wt% or about 50 wt%. In some embodiments, the solids % is from about 10 wt% to about 50 wt%, from about 15 wt% to about 45 wt% or from about 20 wt% to about 40 wt%. In some embodiments, the solids % is about 15 wt%.

[0020] As shown in step 1040 of method 1000 of FIG. 1, the composite resin is then formed into a tape. The forming step 1040 begins with cooling the composite resin prepared in the composite resin forming step 1030 to about 10° C. before mixing the composite resin with a suitable lubricant such as light mineral oil to form a lubricated mixture. Other suitable lubricants include aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, etc. selected according to flammability, evaporation rate and economic considerations. As used herein, it should be understood that the term "lubricant" is intended to describe a processing aid consisting of a non-compressible fluid that is not a solvent for the polymer under the process conditions. One specific example of a suitable lubricant is an isoparaffinic hydrocarbon such as ISOPAR™ K SOLVENT (commercially available from Imperial Oil Chemicals, Calgary, Canada).

[0021] Next, the lubricated mixture can be compressed to create pellets, which can then be extruded through an extruder die to produce a wet tape. A non-limiting example of a method for applying pressure (compression) is ram extrusion (typically called paste extrusion or paste processing, e.g., when a lubricant is present). The resulting wet tape can then be calendered, and subsequently the calendered tape can be dried to remove the lubricant. Tapes can also be formed using other known methods, such as extruding a tube and then slitting the tube to a desired thickness. The thickness of the calendered tape can be about 0.10 mm, about 0.20 mm, about 0.30 mm, about 0.40 mm, about 0.50 mm, about 0.60 mm, about 0.70 mm, about 0.80 mm, about 0.90 mm, or about 1.0 mm. In some embodiments, the thickness of the calendered tape can be about 0.10 mm to about 1.0 mm, about 0.20 mm to about 0.60 mm, or about 0.30 mm to about 0.40 mm. The drying temperature is lower than the melting temperature of the thermoplastic polymer. The drying temperature can be about 145°C, about 150°C, about 155°C, about 160°C, about 165°C, about 170°C, or about 175°C. In some embodiments, the drying temperature is about 145°C to about 175°C, about 150°C to about 170°C, or about 155°C to about 165°C. In some embodiments, the drying temperature is about 160°C.

[0022] As shown in step 1050 of method 1000 in Figure 1, the tape is then stretched in a first direction. The stretching step 1050 may be carried out in a temperature-controlled environment, such as two ovens in series or a single temperature-adjustable oven. To stretch the tape in the first direction, the tape can be restrained, such as by a biaxial tenter frame. The restrained tape can then be heated to a first temperature, where the first temperature is lower than the melting point of the thermoplastic polymer. The appropriate temperature is specific to the thermoplastic polymer and is generally lower than the melting temperatures of both PTFE and the selected thermoplastic polymer. For PVDF with a melting temperature of 170°C, an appropriate temperature would be a temperature below approximately 170°C. In this example, the first temperature could be approximately 150°C, approximately 155°C, approximately 160°C, approximately 165°C, or approximately 170°C. In some embodiments, including PVDF, the first temperature could be approximately 150°C to approximately 170°C or approximately 155°C to approximately 165°C. For PCTFE with a melting temperature of 200°C, suitable temperatures include those below approximately 200°C. In such embodiments, the first temperature can be approximately 180°C, approximately 185°C, approximately 190°C, approximately 195°C, or approximately 200°C. In some embodiments, the first temperature can be approximately 180°C to approximately 200°C or approximately 185°C to approximately 195°C. For FEP with a melting temperature of 280°C, suitable temperatures include those below approximately 280°C. In this embodiment, the first temperature can be approximately 260°C, approximately 265°C, approximately 270°C, approximately 275°C, or approximately 280°C. In some embodiments, the first temperature can be approximately 260°C to approximately 280°C or approximately 265°C to approximately 275°C. For PFA with a melting temperature of 310°C, suitable temperatures include those below approximately 310°C. In this example, the first temperature can be about 290°C, about 295°C, about 300°C, about 305°C, or about 310°C. In some embodiments, the first temperature is about 290°C to about 310°C or about 295°C to about 305°C. For ETFE with a melting temperature of 260°C, suitable temperatures include those below about 260°C. In such embodiments, the first temperature can be about 240°C, about 245°C, about 250°C, about 255°C, or about 260°C.In some embodiments, the first temperature is about 240°C to about 260°C, about 245°C to about 255°C, or about 249°C to about 251°C. In other embodiments, a suitable first temperature may be less than about 290°C for ETFE, less than about 170°C for PVDF, less than about 200°C for PCTFE, less than about 280°C for FEP, or less than 310°C for PFA. In some embodiments, the restrained tape is heated to a first temperature of about 250°C.

[0023] As shown in steps 1060 and 1070 of method 1000 in Figure 1, the uniaxially stretched porous membrane from step 1050 is further stretched in a second direction (i.e., perpendicular to the first direction) at a temperature lower than the melting temperature of PTFE but higher than the melting temperature of the thermoplastic polymer after heating in step 1060 (or concurrently with the heating step 1060). In some embodiments, the second stretch in step 1070 is in a second direction different from the first direction. In further embodiments, the second direction is perpendicular to the first direction. In exemplary embodiments, the composite membrane is heated to a temperature lower than the melting temperature of PTFE and higher than the melting temperature of the thermoplastic polymer, and then stretched in the second direction. Heating step 1060 can be carried out in a second oven different from the first oven in step 1050, or heating can be carried out in the oven used in step 1050.

[0024] The second temperature is higher than the first temperature in step 1050 (i.e., the temperature is higher than the melting temperature of the thermoplastic resin) but lower than the melting point of PTFE (i.e., less than about 340°C). The second temperature can be about 280°C, about 285°C, about 290°C, about 295°C, about 300°C, about 305°C, about 310°C, or about 315°C. In some embodiments, the second temperature is about 280°C to about 300°C, about 290°C to about 310°C, or about 295°C to about 305°C. In an exemplary embodiment, the second temperature is about 300°C. The heating step 1060 may be carried out for about 30 seconds, about 60 seconds, about 90 seconds, or about 120 seconds. In certain embodiments, the heating step 1060 may be carried out for about 60 seconds to about 90 seconds.

[0025] The first and second stretching directions can correspond to (i) the machine direction (MD), also called the longitudinal direction or extrusion direction, or (ii) the transverse direction (TD), also called the cross direction, where TD is perpendicular to MD. In some embodiments, the first direction is oriented longitudinally and the second direction is transverse with respect to the first stretching direction. In other embodiments, the first direction is oriented transversely and the second direction is longitudinal with respect to the first stretching direction. To accommodate some standard equipment, it may be advantageous to stretch the tape in the machine direction before stretching it transversely. Conversely, it may also be possible to stretch the tape in the transverse direction before stretching it in the machine direction.

[0026] Method 1000 in Figure 1 also includes maintaining the tape at a temperature below 327°C in all steps. The fibrillable PTFE particles from step 1010 have an initial melting point of about 340°C and a second melting point after sintering of 327°C. Therefore, Method 1000 in Figure 1 does not involve heating the tape to a temperature above 340°C in any step. In some embodiments, Method 1000 lacks heating the tape to 327°C or above, or its sintering temperature. In some embodiments, Method 1000 can be formed from heating steps 1050 and 1070 without any additional heating. By maintaining the tape below the initial melting point of PTFE, the resulting ePTFE composite film 100 (Figure 5) is unsintered.

[0027] Method 1000 shown in Figure 1 is provided as an example of various features of Method 1000, and combinations of these exemplified features are clearly within the scope of the invention, but the steps shown in Method 1000 are not intended to suggest that the concepts of the invention provided herein are limited by fewer steps, additional steps, or alternative steps to one or more steps shown in Figure 1. For example, in various embodiments, the stretching steps 1050, 1070 and the heating steps 1050, 1060 can be performed simultaneously and / or in different orders.

[0028] A biaxially oriented ePTFE composite film 100, fabricated according to method 1000 in Figure 1, is schematically shown in Figure 5. In particular, the ePTFE composite film 100 includes a microstructure comprising fibrils 110, nodes 120, and pores 130. The nodes 120 are clumps of polymer material and ePTFE, and the fibrils 110 are formed from ePTFE, extending between the nodes 120 and interconnecting them. The microstructure of the ePTFE composite film can be modified, for example, by changing the viscosity of the thermoplastic polymer, in which case the microstructure includes substantially parallel nodes (as shown in Figure 7).

[0029] The fibril 110 is formed primarily from ePTFE, meaning that ePTFE constitutes more than (i.e., more than 50%) of the weight of the fibril 110. Therefore, the mass ratio of ePTFE to thermoplastic polymer in the fibril 110 is greater than 1:1. In some embodiments, the fibril 110 may be formed from 51% by mass of ePTFE, about 55% by mass of ePTFE, about 60% by mass of ePTFE, about 65% by mass of ePTFE, about 70% by mass of ePTFE, about 75% by mass of ePTFE, about 80% by mass of ePTFE, about 85% by mass of ePTFE, about 90% by mass of ePTFE, or about 95% by mass of ePTFE. In some embodiments, the fibril 110 may be formed from at least about 90% by mass of ePTFE or at least about 95% by mass of ePTFE or more. In some embodiments, the fibril 110 is substantially or entirely formed from ePTFE. As used herein, the term “substantially formed” is intended to mean that the fibril is entirely (e.g., 100%) or nearly entirely (e.g., 99%, 98%, 97%, 96%) formed from ePTFE. With respect to method 1000, a larger portion of the PTFE from providing step 1010 may be present within the fibril 110.

[0030] Node 120 can be rich in thermoplastic polymer, meaning that the thermoplastic polymer content in node 120 is higher than the thermoplastic polymer content of the ePTFE composite film 100 as a whole. In some embodiments, node 120 can have a thermoplastic polymer content greater than the thermoplastic polymer content provided in the composite resin (see step 1030 in Figure 1). In other words, when an ePTFE composite film with a thermoplastic polymer content of about 20% by mass is formed, the thermoplastic polymer content of node 120 is greater than about 20% by mass, and when a film with a thermoplastic polymer content of about 30% by mass is formed, the thermoplastic polymer content of the node is greater than about 30% by mass, and so on.

[0031] In certain embodiments, node 120 can be formed primarily from a thermoplastic polymer, meaning that the thermoplastic polymer constitutes a larger portion (i.e., more than 50%) of the mass of node 120. Thus, the mass ratio of the thermoplastic polymer to the ePTFE in node 120 is greater than 1:1. In some embodiments, node 120 may be formed from 51% by mass of thermoplastic polymer, about 55% by mass of thermoplastic polymer, about 60% by mass of thermoplastic polymer, about 65% by mass of thermoplastic polymer, about 70% by mass of thermoplastic polymer, about 75% by mass of thermoplastic polymer, or about 80% by mass of thermoplastic polymer.

[0032] The porosity of the ePTFE composite film 100 can vary as a function of the distance between nodes 120 (e.g., pore size). As the distance between nodes decreases, the porosity decreases, and vice versa. The porosity of film 100 can also vary as a function of the size of each node 120. As the size of each node 120 increases, the porosity decreases, and vice versa. The porosity of the ePTFE composite film can be about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 95%, about 96%, about 97%, about 98%, or about 99%. In some embodiments, the porosity is about 10% to about 99%, about 40% to about 90%, or about 50% to about 80%.

[0033] The thickness of the ePTFE composite film 100 can also vary. The thickness of the ePTFE composite film 100 can be approximately 10 μm, approximately 15 μm, approximately 20 μm, approximately 30 μm, approximately 40 μm, approximately 50 μm, approximately 60 μm, approximately 70 μm, approximately 80 μm, approximately 90 μm, approximately 100 μm, approximately 110 μm, approximately 120 μm, approximately 130 μm, approximately 140 μm, or approximately 150 μm. In some embodiments, the thickness of the ePTFE composite film 100 is approximately 10 μm to approximately 150 μm, approximately 15 μm to approximately 120 μm, or approximately 20 μm to approximately 100 μm.

[0034] A scanning electron microscope image of the ePTFE / thermoplastic polymer composite film 200, prepared according to method 1000 in Figure 1, is shown in Figure 6. Similar to film 100 in Figure 5, film 200 in Figure 6 has fibrils 210, nodes 220 and pores 230. As described above, the ePTFE / thermoplastic polymer composite film can be configured to be thinner and have substantially parallel nodes, as shown in Figure 7, which shows film 300, fibrils 310, nodes 320 and pores 330.

[0035] Films 100, 200, and 300 are dimensionally stable, exhibit high matrix modulus, and have a relatively high ratio of geometric mean matrix modulus to geometric mean matrix tensile strength. The absolute dimensional change rate was measured by dynamic mechanical analysis (DMA), which is described in detail in the "Dimensional Stability Test" below. In particular, the samples were heated from 25°C to 200°C at 5°C / min and then held at 200°C for 5 minutes. The absolute dimensional change rates of the ePTFE composite films can be approximately 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.25%, or 1.5%. In some embodiments, the absolute dimensional change rates are approximately 0.1% to 1.5%, 0.2% to 1.0%, or 0.3% to 0.9%. In some embodiments, the absolute dimensional change rate is less than 1.5%. The matrix modulus of the ePTFE composite film can be about 700 MPa, about 800 MPa, about 900 MPa, about 1000 MPa, about 1100 MPa, about 1200 MPa, about 1300 MPa, about 1400 MPa, about 1500 MPa, or about 1600 MPa. In some embodiments, the matrix modulus is about 700 MPa to about 1600 MPa, about 800 MPa to about 1500 MPa, or about 900 MPa to about 1400 MPa. The ratio of the geometric mean matrix modulus of the ePTFE composite film to the geometric mean matrix tensile strength can be about 6, about 8, about 10, about 12, about 14, or about 16. In some embodiments, the geometric mean matrix modulus is about 6 to about 16, about 8 to about 14, or about 10 to about 12. [Examples]

[0036] Test method Specific methods and apparatus are described below, but please understand that other methods or apparatus deemed appropriate by those skilled in the art may be used as alternatives.

[0037] Non-contact thickness measurement

[0038] The non-contact thickness of the film was measured using the KEYENCE LS-7600 laser system (commercially available from KEYENCE USA).

[0039] Calculation of membrane density

[0040] The sample was die-cut to form a rectangular section measuring 9.05 cm × 5.08 cm. Each sample was weighed using an A&D Model HF400 balance. Using the thickness calculated with a KEYENCE laser, the density of the sample was calculated using the following formula.

number

[0041] Matrix tensile strength (MTS)

[0042] To determine the MTS, sample ePTFE composite membranes were cut longitudinally and transversely using an ASTM D412-dogbone die type F (DD412F). Tensile breaking load was measured using an INSTRON® 5500R (Illinois Tool Works Inc., Norwood, MA) tensile testing machine equipped with a flat-face grip and a "200 lb" (approximately 90.72 kg) load cell. The grip gauge length was set to 8.26 cm, and a strain rate of 0.847 cm / s or 14.3% / s was used. After placing the sample in the grip, the sample was compressed by 1.27 cm to obtain a baseline, and then a tensile test was performed at the aforementioned strain rates. Two samples under each condition were tested individually, and the average of the maximum load (i.e., peak force) measurements was used to calculate the MTS. The longitudinal and transverse MTS were calculated using the following formulas. MTS = (Maximum load / Cross-sectional area) * (Density of resin / Density of film).

[0043] Next, the geometric mean of the MTS for each membrane was calculated using the following formula. Geometric mean MTS = square root [(longitudinal MTS) * (transverse MTS)]

[0044] Matrix tensile stress at specific strains

[0045] Matrix tensile stress = (Load at a specific strain / Cross-sectional area) * (Density of resin / Density of film)

[0046] Matrix elastic modulus

[0047] Raw data was imported into the data analysis program from the same tensile tests described for matrix tensile strength at specific strains. Because it is difficult to fully load the samples into the grips, the 0 strain point for each sample was calculated as the point where the standard deviation of the load was 2 deviations above zero. The modulus of elasticity was obtained from the 0 cm point up to an additional 0.18 cm (or 0-3% strain based on the gauge length of the D412-F dogbone). The modulus of elasticity is the stress / strain gradient. Matrix moduli in both the longitudinal and transverse directions were calculated using the following formulas. Matrix tensile stress at 3% strain = (Load at 3% strain / Cross-sectional area) * (Density of resin / Density of film) Matrix modulus = (Matrix tensile stress at 3%) / (0.03)

[0048] Next, the geometric mean of the matrix modulus of each membrane was calculated using the following formula. Geometric mean matrix modulus = square root [(longitudinal matrix modulus) * (transverse matrix modulus)]

[0049] Next, the ratio of the matrix modulus to the matrix tensile strength was calculated using the following formula. Ratio of matrix modulus to matrix tensile strength = Geometric mean matrix modulus / Geometric mean matrix tensile strength

[0050] Differential Scanning Calorimetry (DSC) Measurement

[0051] DSC measurements were performed using a Q2000 machine (TA Instruments, New Castle, Delaware). As shown in Figure 2, DSC was used to identify the melting point of the ETFE in Example 1, and the melting points of the ePTFE composite films manufactured according to Examples 7 and 9, as shown in Figures 3 and 4, respectively. In the ePTFE composite film samples, if the melting point measured by DSC of the resulting film was approximately 344°C, the sample was not pre-sintered. This provides an indicator that the PTFE experienced initial melting during DSC and therefore was not pre-sintered during stretching.

[0052] Dimensional stability test

[0053] Dynamic mechanical analysis (DMA) was performed using an RSA-G2 solid-state analyzer available from TA Instruments (New Castole, Delaware). ePTFE composite membrane samples were cut in the machine direction to a width of 13 mm and a gauge length of 15 mm. To maintain tautness, the samples were heated from 25°C to 200°C at 5°C / min with a minimum load of 1 g, and then held at 200°C for 5 minutes. Dimensional changes of each sample were measured over a period of time, and the final absolute change in dimensions was reported (see Table 2).

[0054] Example 1

[0055] Ethylene-tetrafluoroethylene (ETFE) polymerization was carried out as follows: 23 kg of deionized (DI) water, 5 g of oxalic acid, 20 mL of chloroform, and 0.6 kg of non-terogenic perfluorinated dispersant were added to a 50-liter horizontal polymerization reactor equipped with a three-blade stirrer. The reactor was repeatedly evacuated until the oxygen level dropped to below 20 ppm and pressurized to below approximately 1 atm (Atm) (approximately 101.325 kPa) with tetrafluoroethylene (TFEE). The contents were briefly stirred at approximately 60 rpm between evacuation and purging cycles to ensure that the water was deoxygenated. The reactor was cooled to 20°C and stirred at 60 rpm. Subsequently, 1550 kPa of TFE was added, followed by 550 kPa of ethylene. At this time, KMnO4 (0.6 g / L) in aqueous DI solution was continuously injected at 40 g / min. For every 200 kPa decrease in pressure, 110 kPa of TFE and 90 kPa of ethylene were returned to the reactor to maintain a constant pressure. After 322 minutes, the reaction was stopped and the pressure was released. 44.64 kg of dispersion was produced, with a solid content of 15.96 mass%, and the particle size of the untreated dispersion was 220 nm. As shown in Figure 2, the obtained ETFE resin had a melting point of 260°C as measured by DSC.

[0056] Example 2

[0057] Polytetrafluoroethylene (PTFE) polymerization was carried out as follows: 1.5 kg of paraffin wax, 28 kg of deionized (DI) water, 18 g of non-terogenic perfluorinated dispersant, and 5 g of succinic acid dissolved in approximately 50 g of DI water were added to a 50-liter horizontal polymerization reactor equipped with a three-blade stirrer. The reactor and its contents were heated above approximately 60°C, the melting point of the wax. The reactor was repeatedly evacuated until the oxygen level dropped to below 20 ppm and pressurized to below approximately 1 atmosphere (approximately 101.325 kPa) using TFE. The contents were briefly stirred at approximately 60 rpm between evacuation and purging cycles to ensure that the water was deoxygenated. 8 mL of perfluorobutylethylene (PFBE) was added to the vacuum reaction as a pre-loading comonomer. The reactor was then heated to 83°C.

[0058] Next, TFE was added to the reactor until the pressure reached 2.8 MPa (approximately 3.0 kg), and KMnO4 (0.063 g / L) in aqueous DI solution was injected at 80 mL / min until approximately 2.0 kg of TFE had been added. This was achieved in approximately 7 minutes. Approximately 100 g of non-terogenic perfluorinated dispersant was added in 12-gram increments, with the first increment added after approximately 1 kg of TFE had been added to the reactor, and subsequent increments added after each subsequent kg of TFE had been added, until the final increment was added after approximately 9 kg of TFE had been loaded. The rate of KMnO4 addition was reduced to 40 mL / min at the 2 kg TFE level and continued at this rate until approximately 3 kg of TFE had been added. Next, the rate of KMnO4 addition was further reduced to 20 mL / min until approximately 5 kg of TFE had been added. Next, the addition of KMnO4 was reduced to 10 mL / min and continued at this rate until approximately 7 kg of TFE had been added to the reactor, at which point the addition of KMnO4 was stopped. The resulting dispersion had an untreated dispersion particle size of 203 nm and a solid content of 35.0% by mass.

[0059] Example 3

[0060] The polymerization of polychlorotrifluoroethylene (mPCTFE) modified with vinylidene difluoride (VDF) was carried out as follows: 28 kg of DI water, 5 g of oxalic acid, 100 g of ammonium bisulfite monohydrate, and 0.5 kg of non-terogenic perfluorinated dispersant were added to a 50-liter horizontal polymerization reactor equipped with a three-blade stirrer. The reactor was repeatedly evacuated until the oxygen level dropped to below 20 ppm, and the reactor was pressurized to below approximately 1 atmosphere (approximately 101.325 kPa) with vinylidene difluoride (VDF). The contents were briefly stirred at approximately 60 rpm between the evacuating and purging cycles to ensure that the water was deoxygenated. Next, the reactor was cooled to 60°C and stirred at 60 rpm. Then, 100 kPa of VDF was added to the reactor, followed by 2.0 L of liquefied chlorotrifluoroethylene (CTFE) being added through a high-pressure liquid pump to a final reactor pressure of 1400 kPa. A solution of ammonium persulfate (5 g dissolved in 100 mL of DI water) was continuously added to the reactor at a rate of 5 g / min. CTFE was continuously added to the reactor to maintain a pressure of 1400 kPa. For every 100 mL of liquefied CTFE added, 10 kPa of VDF was added to the reactor. After 300 minutes, the reaction was stopped (a total of 3.6 L of CTFE monomer and 360 kPa of VDF were added), and the pressure in the reaction vessel was released. A dispersion with a solid content of 15.2 mass% and a total weight of 37.74 kg was produced. DSC analysis showed a melting point of 203 °C. A skeleton density of 2.177 g / mL was determined using helium pycnometry. Next, the dispersion of mPCTFE was co-coagulated with the dispersion of PTFE prepared in Example 2 using the general mixing method described in Example 4 to obtain a final composite resin consisting of 40 mass% PCTFE and 60 mass% PTFE.

[0061] Example 4

[0062] The ETFE and PTFE were mixed as follows. Based on the solid content of the two resins obtained from polymerization described in Examples 1 and 2, the two dispersions were mixed in a ratio of 1 kg of 15.96% solid ETFE dispersion to 0.684 kg of 35.0% solid PTFE. This produced a dispersion with 23.7% solid content, containing 40% ETFE and 60% PTFE. Next, this dispersion was diluted to 15% solid content. To solidify the two materials, 1.0 L of 65% HNO3 was added to 117 kg of dispersion being stirred at 100 rpm. The stirring speed was increased to 420 rpm and stirred for a further 12 minutes to produce an air-wet composite resin. Next, this composite resin was dried in a vacuum oven at 150°C for 28 hours to produce 17.7 kg of final resin containing 40% ETFE thermoplastic resin and 60% fibrillable PTFE.

[0063] Example 5

[0064] Fluorinated ethylene propylene (FEP) polymerization was carried out as follows: 23 kg of DI water and 0.2 kg of non-terogenic perfluorinated dispersant were added to a 50-liter horizontal polymerization reactor equipped with a three-blade stirrer. The reactor was repeatedly evacuated until the oxygen level dropped to below 20 ppm, and pressurized to below approximately 1 atm (approximately 101.325 kPa) with TFE. The contents were briefly stirred at approximately 60 rpm between evacuation and purging cycles to ensure that the water was deoxygenated. The reactor was heated to 85°C and stirred at 60 rpm. Subsequently, 3 kg of hexafluoropropylene (HFP) was added to the reactor. Next, TFE was added to the reactor until the total pressure reached 1100 kPa. At this time, 2.5 kg of a solution of ammonium persulfate in deionized water (1.13 g / L) was added to the reactor. The pressure was maintained at 1100 kPa by continuously adding TFE to the reactor as it was consumed. The reaction was stopped after 105 minutes to obtain a 35.65 kg dispersion with a solid content of 14.6% by mass. This resin is suitable for mixing with PTFE according to the general mixing process described in Example 4.

[0065] Example 6

[0066] ETFE / PTFE extrusion was performed as follows: 3.17 kg of ETFE / PTFE resin blend prepared from Example 4 was cooled to 10°C, then mixed with 0.80 kg of ISOPAR® K, an isoparaffinic hydrocarbon lubricant, and shaken in a drum tumbler for 10.5 minutes. Next, this mixture was compressed at 3.1 MPa in a 101 mm diameter tube to produce pellets. To prevent evaporation of ISOPAR® K, the pellets were heated overnight at 70°C in a concealed tube. Next, the pellets were extruded at a speed of 0.76 mm / second through an extruder heated to 49°C using a die with dimensions of 229 mm width × 0.46 mm thickness. The resulting wet tape was then calendered to 0.30 mm and subsequently dried in a dryer at 160°C to remove all ISOPAR® K from the tape. The mass per unit area of ​​the resulting dry tape was 420 g / m². 2 That was the case.

[0067] Example 7

[0068] ETFE / PTFE stretching was performed as follows. Next, sections of the dry extruded material from Example 6 were placed on a biaxial tenter frame equipped with two ovens to produce a biaxially stretched porous ETFE / PTFE film. After heating the constrained tape at 250°C for 120 seconds (i.e., below the melting point of ETFE), the tape was stretched longitudinally at a rate of 50% / second to a ratio of 10:1. Next, the stretched ETFE / PTFE film was heated in a second oven to 300°C (i.e., above the melting point of ETFE but below the melting point of PTFE) and stretched transversely at a rate of 50% / second to a ratio of 10:1. This biaxial stretching resulted in a mass per area of ​​6.2 g / m². 2A porous ETFE / PTFE film was produced with a thickness of 18.85 micrometers and an MTS of 185 MPa in the longitudinal direction and 92 MPa in the transverse direction. As shown in Figure 3, the resulting film had a melting point of approximately 344°C, as measured by DSC, indicating that the PTFE initially melted during DSC and was therefore not pre-sintered during stretching. It should be understood that if the PTFE were sintered, a melting point of 344°C would not be shown by DSC, and the melting point of PTFE would be 327°C.

[0069] Comparative Example 1

[0070] PTFE extrusion was performed as follows. The PTFE resin prepared in Example 2 was solidified without blending with a thermoplastic resin. 7.71 kg of this resin was mixed with 1.94 kg of ISOPAR® K for 10.25 minutes. Next, this mixture was compressed at 3.1 MPa in a 101 mm diameter tube to produce pellets. These pellets were extruded at a speed of 12.2 mm / second at approximately 22°C using a die with a width of 229 mm and a thickness of 0.46 mm. The wet tape was calendered to 0.30 mm and then dried in a dryer to remove all ISOPAR® K from the tape. The mass per unit area of ​​the resulting dry tape was 400 g / m². 2 That was the case.

[0071] Comparative Example 2

[0072] PTFE stretching was performed as follows. A section of tape from Comparative Example 1 was placed on a biaxial tenter frame containing two ovens to produce a biaxially oriented film. After heating the constrained tape at 300°C, the tape was simultaneously biaxially stretched in both the longitudinal and transverse directions at a rate of 700% / second in a 10:1 ratio, and then sintered and constrained at 365°C for 90 seconds. The resulting film had a mass per area of ​​6.2 g / m². 2 The thickness was 28.9 μm, and the MTS (Metal Structure Test) was 240 MPa in the longitudinal direction and 96 MPa in the transverse direction.

[0073] Comparative Example 3

[0074] Non-melting ETFE / PTFE blend stretching was performed as follows: Sections of the tape prepared in Example 6 were placed on a biaxial tenter frame containing two ovens to produce a biaxially oriented film. After heating, the constrained tape was stretched longitudinally at a rate of 250% / second at 250°C (below the melting point of ETFE) in a 10:1 ratio, and then stretched transversely at a rate of 250% / second at 250°C in a 10:1 ratio. This ETFE / PTFE composite film was never exposed to temperatures above the melting point of either ETFE or PTFE. The mass per unit area of ​​the film was 6.81 g / m². 2 The thickness was 120 μm.

[0075] Example 8

[0076] FEP / PTFE blend extrusion was performed as follows: 13.22 kg of mixed resin was prepared according to the general mixing method described in Example 4. The mixed FEP / PTFE resin contained 40% by mass of FEP (synthesized in Example 5) and 60% by mass of PTFE (synthesized in Example 2). This was cooled to 10°C, then mixed with 3.33 kg of ISOPAR® K lubricant and shaken in a drum tumbler for 10.5 minutes. Next, the mixture was compressed at 3.1 MPa in a 101 mm diameter tube to produce pellets. To prevent evaporation of ISOPAR® K, the pellets were heated overnight at 49°C in a concealed tube. The pellets were extruded at a speed of 2.54 mm / second through an extruder heated to 49°C using a die with a width of 229 mm and a thickness of 0.46 mm. Next, the wet tape was calendered to a thickness of 0.25 mm, and then dried in a dryer at a temperature of 180°C to remove all ISOPAR(trademark)K from the tape.

[0077] Example 9

[0078] The FEP / PTFE blend was stretched as follows: A dry tape from Example 8 was stretched longitudinally in an oven at a rate of 10 meters / min at a ratio of 4:1 at a temperature of 200°C (below the melting point of FEP). Next, this stretched tape was stretched again in the same longitudinal direction through the same oven at 200°C, a ratio of 2:1, and a rate of 2.5 meters / min. Finally, the tape was sent to an oven at 300°C (higher than the melting point of FEP but lower than the melting point of PTFE) to produce a uniformly melted uniaxially oriented tape. Next, a portion of this tape was placed on a tenter frame, heated to 300°C (higher than the melting point of FEP but lower than the melting point of PTFE), and stretched transversely at a rate of 100% / second at a ratio of 10:1. The resulting film contained a melting point of approximately 342°C for PTFE as measured by DSC, indicating that the PTFE initially melted during DSC and was therefore not pre-sintered during stretching (see Figure 4).

[0079] Example 10

[0080] The ETFE / PTFE blend was stretched as follows: A section of the dry extruded material from Example 6 was placed on a biaxially stretched porous membrane in a biaxial tenter frame with two ovens. After heating the constrained tape to 250°C (below the melting point of ETFE), the tape was stretched longitudinally at a rate of 50% / second in a 10:1 ratio. The stretched membrane was then heated in a second oven to 300°C (i.e., higher than the melting point of ETFE but lower than the melting point of PTFE) and stretched transversely at a rate of 50% / second in a 5:1 ratio. The resulting porous ETFE / PTFE composite membrane had a mass per area of ​​13.6 g / m². 2 The thickness was 67.8 micrometers, and the MTS pressure was 203 MPa in the longitudinal direction and 43 MPa in the transverse direction.

[0081] Example 11

[0082] The ETFE / PTFE blend was stretched as follows: A section of the dry extruded material from Example 6 was placed on a biaxially stretched porous membrane on a biaxial tenter frame with two ovens. After heating the constrained tape to 250°C (i.e., below the melting point of ETFE), the tape was stretched longitudinally at a rate of 50% / second in a 5:1 ratio. Next, this stretched membrane was heated in a second oven to 300°C (i.e., above the melting point of ETFE and below the melting point of PTFE) and stretched transversely at a rate of 50% / second in a 10:1 ratio. This resulted in a mass per area of ​​13.6 g / m². 2 A porous ETFE / PTFE composite film was produced with a thickness of 67.8 micrometers, an MTS of 108 MPa in the longitudinal direction, and 107 MPa in the transverse direction.

[0083] Example 12

[0084] The ETFE / PTFE blend was stretched as follows: A portion of the dry extruded piece from Example 5 was placed on a biaxial tenter frame and biaxially stretched longitudinally and then transversely at 250°C (i.e., below the melting point of ETFE), at a ratio of 10:1 and a rate of 250% / second. Stretching was not performed at temperatures above the melting point of ETFE (260°C). After the stretching operation, the film was heat-treated at 300°C (i.e., above the melting point of ETFE but below the melting point of PTFE) for 60 seconds. This resulted in a mass per area of ​​6.7 g / m². 2 We fabricated an ETFE / PTFE composite film with a thickness of 99 micrometers, an MTS of 164 MPa in the longitudinal direction, and 96 MPa in the transverse direction.

[0085] Comparative Example 4

[0086] The PTFE was stretched as follows: A dry extruded piece of Comparative Example 1 (PTFE - without thermoplastic resin) was placed on a biscrew tenter frame and heated to 300°C (i.e., below the melting point of PTFE), and then stretched longitudinally at a ratio of 10:1. Next, the sample was moved to a second oven and heated to 365°C (i.e., above the melting point of PTFE), and stretched transversely at a ratio of 6:1. This resulted in a mass per area of ​​10.9 g / m².2 and an ePTFE porous membrane with a thickness of 70.6 micrometers, an MTS of 251 MPa in the longitudinal direction and 192 MPa in the transverse direction was produced. The ePTFE membrane was observed to be dimensionally stable and did not shrink by itself when removed from the tenter frame pins.

[0087] Comparative Example 5

[0088] The resin was prepared by blending 15% by mass of ETFE prepared according to Example 1 and 85% by mass of PTFE prepared according to Example 2. The resin was paste extruded into a flat tape with a thickness of 0.76 mm and a width of 76 cm. Next, the tape was calendered between two metal nips to provide a tape with a thickness of 0.46 mm, an MTS of 30.6 MPa in the longitudinal direction and 15.0 MPa in the transverse direction. A dried extruded piece was placed on a biaxial tenter frame and biaxially stretched at 250 °C (i.e., below the melting point of ETFE) at a ratio of 10:1 in both the machine direction and the transverse direction. Next, the sample was transferred to a second oven and heated at 365 °C (i.e., above the melting point of PTFE) for 60 seconds. Thereby, 10.9 g / m 2 and an ETFE / PTFE composite membrane (ETFE content 15% by mass) with an MTS of 283 MPa in the longitudinal direction, 175 MPa in the transverse direction, and a geometric mean MTS of 222 MPa was produced. The matrix elastic modulus in the longitudinal direction was 1738 MPa, the matrix elastic modulus in the transverse direction was 1078 MPa, and the geometric mean was 1369 MPa.

[0089] Comparative Example 6

[0090] Sections of the dry extrudate according to Example 6 were placed on a biaxial tenter frame including two ovens to produce a biaxially stretched porous membrane. After heating the constrained tape at 250 °C (i.e., below the melting point of ETFE), the tape was stretched 10:1 in the longitudinal direction and then 10:1 in the transverse direction. Next, this ePTFE composite membrane was sintered at 365 °C (i.e., above the melting point of PTFE) for 60 seconds. Thereby, 6.3 g / m 2A porous membrane was produced with a matrix modulus (MTS) of 158 MPa in the longitudinal direction, 84 MPa in the transverse direction, and a geometric mean MTS of 108 MPa. The matrix modulus in the longitudinal direction was determined to be 1263 MPa, and in the transverse direction 337 MPa, resulting in a geometric mean matrix modulus of 652 MPa.

[0091] Example 13

[0092] The above samples were evaluated for tensile strength and dimensional stability, and the results are shown in Tables 1 and 2, respectively. [Table 1] [Table 2]

[0093] Examples 14-18

[0094] Various co-coagulated composite resins of thermoplastic resin and PTFE were produced using the thermoplastic resins prepared in Examples 1, 3, and 5. These resins were then paste-extruded in a manner similar to that of Example 6 or 7, but under various conditions as can be performed by those skilled in paste extrusion. Porous films were then stretched from these tapes by stretching in the MD direction at a specified stretch ratio below the melting point of the thermoplastic resin, and then stretching transversely at a temperature above the melting point of the thermoplastic resin but below 327°C, providing final properties with respect to geometric mean MTS, geometric mean matrix modulus, and the ratio of matrix modulus to MTS as shown in Table 3. The composite resins of Examples 14-18 were stretched longitudinally below the melting point of the thermoplastic resin, and then transversely at a temperature above the melting point of the thermoplastic resin but below the melting point of the PTFE. Comparative Examples 7-9, which did not contain thermoplastic resin, required heat treatment at 365°C for 60 seconds to produce dimensionally stable films. [Table 3]

[0095] The invention of this application has been described above, both in general and with respect to specific embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made in embodiments without departing from the scope of this disclosure. Accordingly, the embodiments are intended to cover modifications and variations of the invention, insofar as they fall within the scope of the appended claims and their equivalent forms. The following are aspects of the present invention. [Aspect 1] The present invention provides a blend comprising a plurality of fibrillable polytetrafluoroethylene (PTFE) particles having a first melting point and a plurality of thermoplastic polymer particles having a second melting point lower than the first melting point. The blend is formed into tape, The tape is stretched in the first direction at a temperature below the second melting point to form a stretched tape, and When forming a stretched PTFE composite film by stretching the stretched tape in a second direction at a melting point higher than the second but lower than the first, A method for forming a stretched composite stretched PTFE film, characterized in that the stretching occurs at a temperature below the first melting point so that the stretched PTFE composite film is not sintered. [Aspect 2] The method according to embodiment 1, wherein multiple stretching steps are carried out sequentially. [Aspect 3] The method according to embodiment 1, wherein multiple stretching steps are performed simultaneously. [Aspect 4] The method according to embodiment 1, wherein the thermoplastic polymer is a thermoplastic fluoropolymer. [Aspect 5] The method according to embodiment 4, wherein the thermoplastic fluoropolymer is selected from poly(ethene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), and combinations thereof. [Aspect 6] The method according to embodiment 1, wherein the ratio of the geometric mean matrix modulus to the geometric mean matrix tensile strength of the film is 6 or more. [Aspect 7] The method according to Embodiment 1, wherein the blend comprises 40% to 79.9% by mass of fibrillable polytetrafluoroethylene (PTFE) particles and 20.1% to 60% by mass of thermoplastic polymer particles. [Aspect 8] The method according to embodiment 1, wherein the fibrillable PTFE particles and thermoplastic polymer particles each have an average particle size of less than 1 μm. [Aspect 9] The method according to embodiment 1, wherein the stretched PTFE composite film has an absolute dimensional change of less than 1.5%. [Aspect 10] A first set of multiple particles comprising fibrillable polytetrafluoroethylene (PTFE) particles having an average particle size of less than 1 μm, A second plurality of particles comprising thermoplastic polymer particles having an average particle size of less than 1 μm, wherein the melting point of the thermoplastic polymer particles is lower than the melting point of the fibrillable PTFE particles. A blend comprising 40% to 79.9% by mass of fibrillable PTFE particles and 20.1% to 60% by mass of thermoplastic polymer particles is provided. The blend is extruded as a paste in a lubricant to form a calendered tape. By drying the calendered tape to remove the lubricant, a dried calendered tape is produced. By stretching the dried calendered tape in a first direction at a temperature lower than the melting point of the thermoplastic polymer, a uniaxially oriented PTFE composite film is formed. The uniaxially stretched porous membrane is heated to a temperature higher than the melting point of the thermoplastic polymer but lower than the melting point of the fibrillable PTFE, and A method for forming an unsintered biaxially oriented composite PTFE film, characterized by forming a biaxially oriented PTFE composite film by stretching the uniaxially oriented porous film in a second direction different from the first direction. [Aspect 11] The method according to embodiment 10, wherein multiple stretching steps are carried out sequentially. [Aspect 12] The method according to embodiment 10, wherein multiple stretching steps are performed simultaneously. [Aspect 13] The method according to embodiment 10, wherein the stretched PTFE composite film has an absolute dimensional change of less than 1.5%. [Aspect 14] The method according to embodiment 10, wherein the ratio of the geometric mean matrix modulus to the geometric mean matrix tensile strength of the film is 6 or more. [Aspect 15] 40% to 79.9% by mass of fibrillable polytetrafluoroethylene (PTFE) particles, Thermoplastic polymer particles, 20.1% to 60% by mass Multiple nodes interconnected by fibrils, and The ratio of geometric mean matrix modulus (6 or greater) to geometric mean matrix tensile strength. An unsintered, biaxially oriented PTFE composite film. [Aspect 16] The film according to embodiment 15, wherein the fibril contains stretched PTFE, and the node contains a thermoplastic polymer content higher than the total thermoplastic polymer content of the stretched PTFE composite film. [Aspect 17] The film according to embodiment 15, wherein the fibril contains 85% or more of the stretched PTFE. [Aspect 18] The film according to embodiment 15, wherein the node contains 51% by mass or more of the thermoplastic polymer. [Aspect 19] The film according to embodiment 15, wherein the stretched PTFE composite film has a dimensional change of less than 1.5% as measured by dynamic mechanical analysis (DMA) after heating from 25°C to 200°C at a rate of 5°C / min and holding at 200°C for 5 minutes. [Aspect 20] The film according to embodiment 15, wherein the thermoplastic polymer is selected from the group consisting of poly(ethylene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and combinations thereof.

Claims

1. 40% to 79.9% by mass of fibrillable polytetrafluoroethylene (PTFE), It comprises 20.1% to 60% by mass of a thermoplastic polymer, and An unsintered biaxially oriented PTFE composite film comprising multiple nodes interconnected by fibrils, The ratio of geometric mean matrix modulus (6 or greater) to geometric mean matrix tensile strength. An unsintered biaxially oriented PTFE composite film comprising the above, wherein the matrix elastic modulus is 700 MPa to 1600 MPa.

2. The film according to claim 1, wherein the fibril comprises stretched PTFE, and the node contains a thermoplastic polymer content higher than the total thermoplastic polymer content of the biaxially stretched PTFE composite film.

3. The film according to claim 1, wherein the fibril contains 85% by mass or more of the biaxially oriented PTFE.

4. The film according to claim 1, wherein the node contains 51% by mass or more of the thermoplastic polymer.

5. The film according to claim 1, wherein the thermoplastic polymer is selected from the group consisting of poly(ethylene-co-tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), and combinations thereof.

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