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EP-3860746-B1 - UNSINTERED EXPANDED POLYTETRAFLUOROETHYLENE COMPOSITE MEMBRANES HAVING DIMENSIONAL STABILITY

EP3860746B1EP 3860746 B1EP3860746 B1EP 3860746B1EP-3860746-B1

Inventors

  • EBATA, Yuri
  • SAYLER, Todd, S.

Dates

Publication Date
20260506
Application Date
20181004

Claims (14)

  1. A method of forming an expanded composite polytetrafluoroethylene (PTFE) membrane comprising: providing a blend including a plurality of fibrillatable polytetrafluoroethylene (PTFE) particles having a first melting point and a plurality of thermoplastic polymer particles having a second melting point that is less than the first melting point; forming the blend into a tape; expanding the tape in a first direction below the second melting point to form an expanded tape; and expanding the expanded tape in a second direction above the second melting point but below the first melting point to form an expanded composite PTFE membrane, wherein the expanding occurs at temperatures below the first melting point such that the expanded composite PTFE membrane is not sintered.
  2. The method of claim 1, wherein the thermoplastic polymer is a thermoplastic fluoropolymer; and optionally wherein the thermoplastic fluoropolymer is selected from poly(ethene- co- tetrafluoroethene) (ETFE), polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), and combinations thereof.
  3. The method of claim 1, wherein the blend includes from 40 wt% to 79.9 wt% fibrillatable polytetrafluoroethylene (PTFE) particles and from 20.1 wt% to 60 wt% thermoplastic polymer particles.
  4. The method of claim 1, wherein the fibrillatable PTFE particles and the thermoplastic polymer particles each have an average particle size of less than 1 µm.
  5. The method of claim 1, wherein the blend comprises a first plurality of particles, the first plurality of particles comprising the plurality of fibrillatable polytetrafluoroethylene (PTFE) particles having an average particle size of less than 1 µm; and a second plurality of particles, the second plurality of particles comprising the plurality of thermoplastic polymer particles having an average particle size of less than 1 µm, wherein the melting point of the thermoplastic polymer particles is less than the melting point of the fibrillatable PTFE particles, wherein the blend includes from 40 wt% to 79.9 wt% fibrillatable PTFE particles and from 20.1 wt% to 60 wt% thermoplastic polymer particles; wherein the step of forming the blend into a tape comprises paste extruding the blend in a lubricant to form a calendared tape; the method further comprising the step of drying the calendared tape to remove the lubricant to produce a dried calendared tape; and heating the expanded porous membrane formed by expanding the expanded tape to a temperature greater than the melting point of the thermoplastic polymer and less than the melting point of the fibrillatable PTFE; and, expanding the uniaxially expanded porous membrane in a second direction, wherein the second direction is different from the first direction, to form a biaxially expanded ePTFE composite membrane.
  6. The method of claims 1 or 5, wherein the expansion steps are performed sequentially.
  7. The method of claims 1 or 5, wherein the expanded composite PTFE membrane has an absolute dimensional change of less than 1.5% as measured using the method described herein.
  8. The method of claims 1 or 5, wherein the expanded composite PTFE membrane has a geometric mean matrix modulus to geometric mean matrix tensile strength ratio of at least 6 as measured using the method described herein.
  9. An unsintered biaxially expanded composite PTFE membrane comprising: from 40 wt% to 79.9 wt% fibrillatable polytetrafluoroethylene (PTFE) particles; from 20.1 wt% to 60 wt% thermoplastic polymer particles; a plurality of nodes interconnected by fibrils; and a geometric mean matrix modulus to geometric mean matrix tensile strength ratio of at least 6 as measured using the method described herein.
  10. The membrane of claim 9, wherein the fibrils include ePTFE and the nodes include a higher thermoplastic polymer content than the total thermoplastic polymer content of the expanded composite PTFE membrane.
  11. The membrane of claim 9, wherein the fibrils comprise at least 85% of the ePTFE.
  12. The membrane of claim 9, wherein the nodes comprise at least 51 wt% of the thermoplastic polymer.
  13. The membrane of claim 9, wherein the expanded composite PTFE membrane has a dimensional change of less than 1.5% as measured by dynamic mechanical analysis (DMA) upon heating from 25 °C to 200 °C at a rate of 5 °C/min and upon holding at 200 °C for 5 minutes as measured using the method described herein.
  14. The membrane of claim 9, 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.

Description

FIELD The present disclosure relates generally to expanded polytetrafluoroethylene (ePTFE) composite membranes that are dimensionally stable without sintering, and more specifically, to ePTFE composite membranes that include at least one thermoplastic polymer. Methods of producing such composite membranes are also provided. BACKGROUND ePTFE membranes and ePTFE composite membranes may be subjected to at least one sintering step to improve dimensional stability prior to use. While a sintering step above the melting point of ePTFE has been known to provide ePTFE membranes with increased dimensional stability, sintering deleteriously affects the membrane by reducing crystallinity and increasing amorphous content. Membranes having amorphous PTFE often exhibit a rigid amorphous phase that has a transition temperature of about 120 °C. Above this transition temperature, a sintered ePTFE article may have reduced mechanical properties. For example, sintering an ePTFE membrane adversely impacts properties of the membrane such as mean matrix modulus, a loss of dimensional stability over time, and a loss in crystallinity. It is desired by those skilled in the art to produce a dimensionally stable porous membrane derived from ePTFE without having to sinter the ePTFE, thereby losing crystallinity. As such, there is a need to provide ePTFE membranes and ePTFE composite membranes that are dimensionally stable over time at elevated temperatures, exhibit a high matrix modulus, and have a relatively high ratio of geometric mean matrix modulus to geometric mean matrix tensile strength. EP 2 067 814 A1 relates to an expanded membrane incorporating PTFE and PVDF. SUMMARY One embodiment relates to a method of forming an unsintered biaxially expanded ePTFE composite membrane. The method includes providing a blend that includes a first plurality of fibrillatable polytetrafluoroethylene (PTFE) particles having a first melting point and a second plurality of thermoplastic polymer particles having a second melting point that is less than the first melting point. The blend is then formed into a tape. Next, the tape is expanded in a first direction and then in a second direction that is different from the first direction (e.g., orthogonal) at a temperature less than the first melting point to form an ePTFE composite membrane. Expanding in the first direction may be performed at a temperature below than the second melting point, such as from about 170 °C to about 300 °C. Expanding in the second direction may be performed at a temperature above the second melting point and below the first melting point, such as from about 280 °C to 327 °C. The blend includes from 40 wt% to 79.9 wt% fibrillatable PTFE particles and from 20.1 wt% to 60 wt% thermoplastic polymer. The ePTFE composite membrane may have a geometric mean matrix modulus to geometric mean matrix tensile strength ratio of at least about 6. Additionally, the method may be devoid of a heating step above 327 °C. Another embodiment relates to a method of forming an unsintered biaxially expanded ePTFE composite membrane. The method includes providing a blend that includes a first plurality of fibrillatable polytetrafluoroethylene (PTFE) particles having an average particle size less than 1 µm and a second plurality of thermoplastic polymer particles having an average particle size of less than 1 µm. In exemplary embodiments, the average particle size of the thermoplastic polymer particles is the same as or smaller than the average particle size of the fibrillatable PTFE particles. In addition, the melting point of the thermoplastic polymer is less than the melting point of the fibrillatable PTFE particles. The blend includes from 40 wt% to 79.9 wt% of fibrillatable PTFE particles and from 20.1 wt% to 60 wt% of thermoplastic polymer particles. The method also includes paste extruding the blend with a lubricant to form a calendared tape, drying the calendared tape to remove the lubricant and produce a dried calendared tape, expanding the dried calendared tape in a first direction at a temperature below the melting point of the thermoplastic polymer to form a uniaxially expanded ePTFE composite membrane. The method also includes heating the uniaxially expanded porous ePTFE composite membrane to a temperature greater than the melting point of the thermoplastic polymer and less than the melting point of the fibrillatable PTFE particles and, either concurrently or sequentially, expanding the uniaxially expanded porous ePTFE composite membrane in a second direction, where the second direction is different from the first direction, to form a biaxially expanded ePTFE composite membrane. The temperature of the step of expanding in the first direction may be from about 170 °C to about 300 °C and the temperature of the heating step may be from about 280 °C to about 300 °C. The step of expanding the membrane in the second direction may be concurrent with the step of heating. The method may be devoid