JP-7856722-B2 - Porous PTFE membrane

Inventors

• スザー， アニル
• ブランディマルト， リチャード
• パンネパッカー， ジュニア， ロバート ジェイ．
• ラジガリア， スツティ エス．

Assignees

• ドナルドソン カンパニー，インコーポレイティド

Dates

Publication Date

20260511

Application Date

20241004

Priority Date

20181217

Claims (4)

1. A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membrane is a nonwoven web having a microstructure of microfibrils having less than 100 nodes/100 μm² , the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ±10% of the tensile peak stress in the transverse direction, and the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction are 130 MPa or less at room temperature.
2. A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membrane is joined at structural points to form a nonwoven web having a microstructure of microfibrils having less than 100 nodes/100 μm² , the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, and the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ± 5% of the tensile peak stress in the transverse direction.
3. A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membrane is a nonwoven web having a microstructure of microfibrils having less than 100 nodes/100 μm² , the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ± 10% of the tensile peak stress in the transverse direction, and the membrane has a thickness of 1 to 3.8 micrometers.
4. A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membrane is a nonwoven web having a microstructure of microfibrils having less than 100 nodes/100 μm² , the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ± 10% of the tensile peak stress in the transverse direction, and the membrane has a thickness of 7.8 to 10 micrometers.

Description

Cross-reference of related applications This application claims the interests relating to U.S. Provisional Patent Application No. 62/780,776, filed on 17 December 2018, the disclosure of which is incorporated in its entirety by reference herein. Porous polytetrafluoroethylene (PTFE) membranes are known to have asymmetrical strength (i.e., in the X-Y plane or longitudinal and transverse directions) and related properties. For certain applications, such as in fuel cells, membranes with symmetrical strength are often desirable. Figure 1 is a SEM image of the membrane from Example 1.Figure 2 is an SEM image of the membrane from Example 2. This disclosure provides a unique porous polytetrafluoroethylene (PTFE) membrane having a microstructure substantially composed solely of microfibrils (i.e., substantially nodeless) and substantially symmetrical tensile peak stress (i.e., tensile peak stress in the longitudinal direction being substantially the same as in the transverse direction). In this context, “substantially composed solely of microfibrils” (i.e., substantially nodeless) means less than 100 nodes/100 microns² , and “substantially symmetrical” tensile peak stress means that the membrane has a longitudinal tensile peak stress that is no more than 10% of the transverse tensile peak stress. Where the longitudinal and transverse directions cannot be distinguished, substantially symmetrical tensile peak stress means that the membrane has a balanced ratio of orthogonal dimensions of no more than 10%. More specifically, this disclosure provides a porous PTFE membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less. This means that, for a membrane having tensile peak stress in both the longitudinal and transverse directions, the tensile peak stress in the longitudinal direction is within 10% of the tensile peak stress in the transverse direction. In certain embodiments, the PTFE membrane has a balance ratio of orthogonal dimensions of 5% or less, or 1% or less. That is, the membrane has a tensile peak stress in the longitudinal direction that is 5% or less of the tensile peak stress in the transverse direction, or 1% or less. Peak stress is defined as the peak force relative to the cross-sectional area of the test specimen. The ratio for determining the balance between orthogonal dimensions (longitudinal ("MD") versus transverse ("CD" or "CMD")) is calculated as the ratio of the difference in peak stress between the orthogonal dimensions relative to the longitudinal direction. Alternatively, the balance between orthogonal dimensions (longitudinal vs. transverse) can be calculated as the ratio of the difference in peak stress between the orthogonal dimensions relative to the dimension with higher strength (either longitudinal or transverse). The balance ratio is shown below in the form of a mathematical formula. If the longitudinal strength is higher than the transverse strength, the strength balance ratio can instead be calculated as follows: The properties of substantially symmetrically strong membranes with virtually no nodes are often desirable for improving membrane performance in applications such as fuel cells, filters (e.g., for aeration and microfiltration), and semiconductors. For example, symmetrically strong membranes can improve membrane permeation flux by reducing resistance to fluid ion flow. Similarly, for applications where the membrane can be cut to smaller sizes, balanced strength provides dimensional stability under thermal and mechanical stress. In certain embodiments, the membrane has a tensile peak stress of at least 5 MPa in both the longitudinal and transverse directions at room temperature. In certain embodiments, the membrane has a tensile peak stress of 130 MPa or less in both the longitudinal and transverse directions at room temperature. The tensile peak stress can be measured using an EJA series tensile testing machine (available from Thwing-Albert, West Berlin, NJ) according to ASTM D882-18 (grip spacing of 60 mm at the start of the test; sample cut to a width of 15 mm; and fracture strength tested at 0.24 inches/minute (6.09 mm/minute)). In certain embodiments, the PTFE membrane has a thickness of at least 1 micron. In certain embodiments, the membrane has a thickness of 30 microns or less, or 20 microns or less, or 10 microns or less. Typically, when a membrane is thinned (e.g., by stretching), its strength is lost. In certain embodiments of the membranes described herein, when the membrane is thinned (e.g., by stretching), its strength increases up to a maximum value and then decreases. In certain embodiments, the PTFE membrane has a pore size of at least 0.05 microns (i.e., 1 micrometer). In certain embodiments, the membrane has a pore size of 2 microns or less. The pore size is the average of the average pore sizes of multi