Porous polytetrafluoroethylene membrane

Details Porous polytetrafluoroethylene membrane Porous polytetrafluoroethylene membrane

2000-04-07

2001-08-14

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Filter

Material

C210S500270, C428S316600

Reexamination Certificate

active

06274043

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to porous polytetrafluoroethylene (PTFE) membranes and methods for their preparation.
BACKGROUND OF THE INVENTION
Porous membranes are often employed as semi-permeable barriers between two or more miscible fluids. In these applications, the membranes control the transmission of components between the fluids, and in the absence of overriding intermolecular forces based on charge, magnetism, dipoles, etc., they can generally be thought of as acting like sieves. As such, fluid components smaller than pores of the membrane can travel from one membrane surface to the other, but substances larger than the pores are prevented from doing likewise. This function is exemplified by the use of membranes as filters to remove particles from liquids or gases.
If a membrane contains a range of pore sizes, the largest pore determines the largest and smallest fluid components that will pass through or be retained by the membrane, respectively. Membranes with maximum pore sizes smaller than about 0.02 &mgr;m are commonly referred to as ultrafine, while those with maximum pore sizes between about 0.02 &mgr;m and about 10 &mgr;m (more typically about 1 &mgr;m) are considered microporous. Such membranes are often used by the electronics and pharmaceutical industries to remove particulate impurities from fluids (i.e., liquids and gases), and for reasons of economics and convenience it is preferred that these filtrations be performed rapidly and reliably. Membrane permeability and strength are therefore properties that are almost as important as pore size.
It is difficult to produce strong, permeable, micro and ultrafine membranes, and it is particularly difficult to create such membranes from polytetrafluoroethylene (PTFE). This material has many attractive qualities for use in membrane applications, such as superior chemical inertness and high mechanical stability at a range of temperatures. Strong, highly porous PTFE membranes were first produced by about the 1970's, and their preparation is described in U.S. Pat. No. 3,953,566 (Gore) and U.S. Pat. No. 4,187,390 (Gore). The method described in these patents comprises two fundamental steps: (a) the rapid stretching of unsintered PTFE extrudates to create pores, and (b) heat treatment of the stretched material to increase the mechanical strength thereof.
Since these patents, two general strategies have dominated attempts to prepare strong, highly porous PTFE membranes with smaller pore sizes. One common strategy has been to adopt the basic method of the '566 patent, but to modify and optimize the individual steps thereof. For example, U.S. Pat. No. 5,476,589 (Bacino) discloses an improved protocol of transverse and longitudinal stretching of unsintered PTFE to provide thin PTFE membranes with maximum pore sizes reportedly as small as 0.125 &mgr;m.
The sintering level of the stretched PTFE also has been subject to variation. Maximum pore sizes allegedly as low as 0.3 &mgr;m are obtained when semisintered PTFE is stretched according to the method of U.S. Pat. No. 5,234,739 (Tanaru et al.), while maximum pore diameters allegedly as low as 0.052 &mgr;m are produced if two or more fused sheets of fully sintered PTFE are stretched as described in U.S. Pat. No. 5,510,176 (Nakamura et al.). However, the stretching of fully sintered PTFE is difficult to perform, as the resulting membrane can be fragile and easily damaged. Two advantages of the present inventive method are that porous PTFE membranes of any sintering level can be produced, and the inventive membranes possess excellent mechanical strength.
A second common strategy for preparing a porous PTFE membrane has been to use a pore-forming filler, such as NaCl, instead of using a stretching protocol such as described in the '566 patent. In these methods, the filler is mixed with particles of PTFE, the mixture is transformed into a thin film, and the filler is removed from the film (e.g., by washing with hot water, acids, etc. to dissolve the filler) to create pores. This procedure was used to prepare membranes with maximum pore sizes of at least 0.1 &mgr;m in U.S. Pat. No. 4,863,604 (Lo et al.).
The aforementioned procedures all require the execution of complex sequences, and the quality of the resulting membranes can vary considerably if the individual steps (e.g., heating, fusing, stretching, etc.) are not carefully performed. Moreover, the porous PTFE membranes prepared by these methods have maximum pore sizes larger than 0.05 &mgr;m.
There exists a need for microporous and ultrafine PTFE membranes of relatively high mechanical strength, permeability, and chemical inertness, and for methods of producing those membranes that is relatively simple and reproducible. The present invention provides such membranes and such methods. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to non-fused microporous and ultrafine polytetrafluoroethylene (PTFE) membranes of high permeability, excellent mechanical strength, and good chemical inertness, and to methods for their preparation. According to the method of the present invention, a porous PTFE substrate having a first pore rating is compressed to provide a porous membrane having a second pore rating, wherein the second pore rating is smaller than the first pore rating. The PTFE substrate can be contacted with one or more fibrous sheets during the compression step, which is preferably performed by passing the substrate, and fibrous sheets, if appropriate, between two calender rolls. The resulting porous PTFE membrane then can optionally be stretched and/or sintered, and preferably has a water permeability of at least about 0.5 l/hr/m
2
/kPa (about 0.005 l/min/ft
2
/psi).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The porous polytetrafluoroethylene (PTFE) membranes of the present invention and the methods for their preparation are described in detail hereinafter. According to the method of the present invention, a porous PTFE substrate having a first pore rating is compressed between two compression agents to provide a porous membrane having a second pore rating, wherein the second pore rating is smaller than the first pore rating.
The first and second pore ratings can be determined by measuring the titre reductions of the porous substrate and membrane, respectively, with respect to monodispersed latex beads. Monodispersed latex beads have diameters that are all substantially identical. At a minimum, the monodispersed latex beads have diameters that fall within a narrow range. For example, essentially all monodispersed 0.055 &mgr;m beads have a diameter of from about 0.050 &mgr;m to about 0.060 &mgr;m.
The titre reduction of a filtration medium with respect to a particular particle, e.g., 0.055 &mgr;m monodispersed beads, is determined by filtering an influent containing a known number of those particles, and counting the number of the particles that pass through the filtration medium into the effluent. A filtration medium that prevents 99.9% of 0.055 &mgr;m monodispersed beads from entering the effluent is said to have a titre reduction of 10
3
(99.9% removal) with respect to 0.055 &mgr;m monodispersed beads. For the purposes of the present invention, the first and second pore ratings are defined as the diameters of the smallest monodispersed beads for which each of the porous substrate and membrane, respectively, has a titre reduction of 10
3
(99.9% removal).
Another measure of pore size is the K
L
of the filter medium. The K
L
, which is inversely proportional to the effective pore diameter, is the applied air pressure at which a liquid used to wet the filter medium, e.g., isopropyl alcohol, begins to be forced from the pores of the medium. A porous PTFE membrane prepared according to the method of the present invention typically has an increased K
L
compared to the porous substrate from which it was prepa

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