CN117258567A - Asymmetric polymeric porous filter membranes and related methods - Google Patents

Abstract

The present application relates to asymmetric polymeric porous filter membranes and related methods. Porous polymeric filter membranes having multiple asymmetric pore structures through the membrane thickness are described, as well as methods of making and using the same.

Description

Asymmetric polymeric porous filter membranes and related methods

Technical Field

The following description relates to porous polymeric filter membranes having multiple asymmetric pore structures across the membrane thickness, and methods of making and using the same.

Background

Filters are used to treat many gaseous and liquid fluids to remove contaminants or impurities. Examples include air, potable water, liquid industrial solvents and process fluids, industrial gases for manufacturing or processing (e.g., in semiconductor manufacturing), and liquids having medical or pharmaceutical uses. Unwanted materials removed from the fluid include impurities and contaminants such as particulates, microorganisms, and dissolved or suspended molecular chemical species. Specific examples of impurity removal applications for filtration membranes include the use of the filtration membranes to remove cell residual particles, bacteria, or other organic matter from therapeutic solutions in the pharmaceutical industry, or to treat ultrapure water solutions and organic solvent solutions for use in microelectronics and semiconductor processing, or for air and water purification processes.

To perform the filtering function, the filter product contains a filter membrane responsible for removing unwanted material from the fluid as it passes through the filter. The filter membrane may be in the form of a flat sheet, which may be wound (e.g., spiral) or pleated, etc., as desired. The filter membrane may alternatively be in the form of hollow fibers. The filter membrane may be contained within a housing comprising an inlet and an outlet such that filtered fluid enters through the inlet and passes through the filter membrane before passing through the outlet.

The filter membrane may be composed of a porous polymer film having an average pore size that may be selected based on the intended use of the filter, i.e., the type of filtration performed using the filter. Typical pore sizes are in the micrometer or sub-micrometer range, for example, from about 0.001 micrometers to about 10 micrometers. Membranes having an average pore size of from about 0.001 to about 0.05 microns are sometimes classified as ultrafiltration membranes. Membranes having pore sizes between about 0.05 and 10 microns are sometimes classified as microporous membranes.

For commercial use, the filter membranes should be of a type that can be efficiently manufactured and assembled into a filter product. The membrane must be capable of being produced efficiently and must have mechanical properties, such as strength and flexibility, that allow the membrane to withstand the assembly into a cartridge form or other form of filter membrane structure. In addition to mechanical properties, the membrane should have suitable chemical functionality, including stability and microstructure (pore size and morphology) for high performance filtration.

Various techniques for forming porous filter membranes are known. Example techniques include melt extrusion (e.g., melt casting) techniques, immersion casting (phase inversion) techniques, and the like. Different techniques for forming porous polymer membranes can produce different membrane structures depending on the size and distribution of the pores formed within the membrane, i.e., different techniques produce different pore sizes and membrane structures, sometimes referred to as morphologies, which represent the uniformity, non-uniformity, shape, size, and distribution of pores within the membrane.

Examples of film morphologies include homogeneous (isotropic) and asymmetric (anisotropic). Films having substantially uniform size (within a range) of pores uniformly distributed throughout the film are commonly referred to as isotropic or "homogeneous". Anisotropic (also referred to as "asymmetric") films can be considered to have a morphology in which there is a pore size gradient across the film. For example, the membrane may have a porous structure with relatively large pores at one membrane surface and relatively small pores at the opposite membrane surface, wherein the pore structure varies along the thickness of the membrane. The term "asymmetric" is generally used interchangeably with the term "anisotropic". In general, a portion of the membrane having relatively small pores (as compared to other regions of the membrane) is referred to as a "dense" region. The portion of the membrane having larger pores is commonly referred to as the "open" area.

Membranes having different morphologies are needed to continuously improve filtration of liquid materials.

Disclosure of Invention

The following describes "multi-asymmetric" porous polymer membranes that can be effectively used as porous polymer filtration membranes, and methods of making and using the described multi-asymmetric porous polymer membranes.

The described membrane has a "multi-asymmetric" morphology, which means that the membrane comprises pores having pore sizes that vary across the thickness of the membrane in a manner that creates at least three regions of different pore sizes. As described herein, a membrane having multiple asymmetric morphologies provides multiple alternating pore size regions to capture particles of different sizes.

The film includes a plurality of "thickness regions" having different morphologies. The "thickness zone" of a film is a portion of the film that extends over a constant portion of the film's thickness, in both the length and width dimensions of the film. With respect to the present specification and claims, a film may be considered to include at least three thickness regions of the type identified as "open regions" ("open regions") having relatively large-sized pores or "dense regions" ("dense regions") having relatively smaller-sized pores. Open and dense regions are present in the membrane in alternating order along the membrane thickness, e.g., as open-dense-open regions, as dense-open-dense regions, etc.

The multi-asymmetric membranes can be prepared from sulfone polymers (sometimes referred to as polysulfones) that can be processed by the methods of the present description to form multi-asymmetric membranes, particularly those comprising polysulfones and polyethersulfones.

A multi-asymmetric porous membrane may be prepared by a method according to which a liquid polymer composition is formed into a film, and then exposing the film to conditions that cause the polymer contained in the film to condense, comprising first contacting the film with gaseous water vapor (e.g., air containing an amount of moisture) to cause initial phase separation within the film, and then contacting the film with an aqueous liquid to cause the polymer contained in the film to condense and produce a multi-asymmetric polymer porous membrane.

In one aspect, the present disclosure is directed to a porous polymer film having a film thickness and multiple asymmetric morphologies along the thickness. The film comprises: a membrane average pore size over the membrane thickness; two open regions having an average pore size and a pore size maximum greater than the average pore size of the membrane; and a dense region having an average pore size and a pore size minimum that is less than the average pore size of the membrane, wherein the dense region is located between the two open regions.

In another aspect, the present disclosure is directed to a method of making a porous polymeric membrane having a membrane thickness and multiple asymmetric morphologies along the thickness, the membrane being formed using polyethersulfone or polysulfone. The method comprises the following steps: forming a film of a liquid polymer composition comprising a polymer selected from polyethersulfone and polysulfone dissolved in an organic solvent including a strong solvent and a co-solvent; exposing the film to air having a relative humidity of at least 20% to allow moisture in the air to be absorbed by the liquid coating composition and cause the polymer to become more concentrated in a polymer-rich phase and less concentrated in a polymer-lean phase; and immersing the film in a water bath to cause the polymer to precipitate as a porous polymer film after exposing the film to the air.

Drawings

Fig. 1A and 1B show examples of membranes as described, and data relating to the pore size of the membranes.

Fig. 2A and 2B show examples of membranes as described, and data relating to the pore size of the membranes.

Fig. 3A and 3B show example steps of a method for producing a multi-asymmetric membrane as described.

Fig. 4 shows an example of a filter product as described.

Detailed Description

The following description relates to "multi-asymmetric" (as described) porous polymer membranes that can be effectively used as porous polymer filtration membranes, and also relates to methods of making and using the described multi-asymmetric porous polymer membranes.

Porous, multi-symmetric membranes comprise (consist of, consist essentially of) a porous polymeric membrane body having a continuous polymeric matrix defining matrix walls and open pores between the walls, wherein the pores are multi-asymmetric along the thickness of the membrane body. The matrix structure is "continuous", meaning that the matrix is a single, uninterrupted (except for the pores) structure made of a single type of polymer throughout the matrix.

The porous polymer film has two opposing, effectively parallel surfaces (or opposing "sides") extending in both the length and width directions, and a thickness extending in a third direction and located between the two opposing surfaces. The pores of the porous membrane are positioned across the thickness of the membrane and allow fluid to flow through the thickness of the membrane from one side of the membrane to and through the opposite side of the membrane. As the fluid flows through the membrane, impurities or contaminants (e.g., particulate contaminants) are trapped by the membrane and removed from the fluid.

This type of membrane is sometimes referred to as an "open cell" membrane, as opposed to a "closed cell" membrane. The apertured film may be in the form of a film or sheet of porous polymeric material having a relatively uniform thickness over a region (region having a length and width) and a continuous open-celled structure comprising a polymeric matrix defining a plurality of open "cells", the open-cells being three-dimensional void structures located between the solid walls of the continuous matrix structure. The openings constitute interconnecting channels or passages between adjacent holes to allow liquid or gaseous fluid to flow through the thickness of the membrane from one side of the membrane to the other.

The membrane has a "multi-asymmetric" morphology, meaning that the membrane comprises pores of pore size that vary across the thickness of the membrane in a manner that produces at least three regions of different pore sizes, wherein each region can be identified as either an "open region" having pores of relatively large size ("open region") or a "dense region" having pores of relatively smaller size ("dense region"), and wherein the two types of regions exist in alternating order along the membrane thickness, e.g., as open-dense-open regions, such as dense-open-dense regions, and the like.

The membrane may be described as having "average pore sizes" at different depth locations. The average pore size at the depth of the membrane is the average of the pore sizes all similarly located at a particular depth position of the membrane, i.e., the average size of pores all located at the same distance ("depth") from the membrane surface. The film may be described in terms of average pore size at different individual depths along the thickness of the film.

The membrane also has a "membrane average pore size", which is the average of the pore size of the membrane across the thickness of the membrane, i.e., the average of the pore size at a depth location (distance from the membrane surface) across the entire thickness of the membrane.

A microscope may be used, such as with a scanning electron microscope, to visually observe and measure pore size, average pore size (across the membrane or at individual depths of the membrane), pore size variation in the thickness of the membrane (difference in average pore size), and the like. Pore size data for the membrane can be electronically collected and analyzed to evaluate average pore sizes at different depths of the membrane, to compare average pore sizes at different depths within the membrane, and to compare average pore sizes at different depths within the membrane to the membrane average pore size. Analysis may be performed by a commercially available software product (e.g., from MatLab, etc.) that uses RGB (red, green, blue) coordinates to analyze a matrix of pixels of an SEM image of a film to identify the pores of the film (which are black), and then determine the pore sizes and locations of the different sized pores as part of the film.

In a useful format, the pore size data at different depth locations within the membrane can be electronically analyzed and presented in the form of a graph plotting the average pore size measured at different depths of the membrane relative to the depth locations of the membrane. See, for example, fig. 1B and 2B. In a graph format depicting average pore sizes measured at different depths along the thickness of a membrane, lines on the graph representing average pore sizes at the respective membrane depths may be referred to as "pore size functions. Also conveniently, the pore size function of the membrane can be compared graphically with the average pore size of the membrane. See fig. 1B and 2B.

The film as described includes at least one region along the thickness of the film that is an "open area" or "open area". The film also includes at least one region along the thickness of the film that is a "dense hole region" or "dense region". Each densified region will have a pore size, including a minimum pore size, that is less than the average pore size of the membrane. Each open region will have a pore size greater than the average pore size of the membrane, including a maximum pore size.

In the example film, the pore size minimum or pore size maximum is located at the middle 1/3 of the thickness. Alternatively or additionally, the membrane may have at least two pore size minima or at least two pore size maxima at the middle 8/10 of the thickness.

More particularly, the films of the present description include at least one dense region, at least one open region, and at least one additional region that is a dense region or an open region. The regions alternate along the film thickness between dense and open regions.

As an example, the membrane may comprise two dense regions with an open region between the two dense regions. As another example, the membrane may comprise two open regions with a dense region between the two open regions. See fig. 2A and 2B.

As another example, a film may include three dense regions and two open regions in alternating order: compact-open-compact. See fig. 1A and 1B. And as yet another example, the membrane may comprise three open regions and two dense regions in alternating order: open-dense-open.

A dense zone is a region of a film along the thickness of the film that contains pores having a pore size (e.g., average pore size at a particular depth) that is less than the average pore size of the film. The pore size across the thickness of the dense region can be identified by a pore size function that plots the average pore size relative to the position of the membrane in the thickness direction of the membrane, wherein the pore size function is also compared to the membrane average pore size. Each densified region includes identifiable low points on the pore size function over the area of the densified region, between the ends of the densified region, which is referred to as the "densified region minimum". The end of the dense region may be identified as the surface of the membrane or as the intersection of a pore size function with the average pore size of the membrane.

Similarly, an open region of a membrane is a region along the thickness of the membrane that contains pores having a pore size (e.g., average pore size) that is greater than the average pore size of the membrane. Each open region includes identifiable high points on the pore size function over the area of the open region, between the ends of the open region, which is referred to as an "open region maximum". The ends of the open regions may be identified as the surface of the membrane or as the intersection of a pore size function with the average pore size of the membrane.

In example membranes, the average pore size of the membrane may be in the submicron range, such as from 0.1 microns to 1 micron, such as in the range from 0.2 nanometers to 0.9 microns or from 0.3 to 0.8 microns.

Multiple asymmetric porous membranes are considered "monolithic" or "continuous," meaning that the membrane comprises a polymer matrix made of a single type of polymer that forms a single matrix body that is uninterrupted except for the pores. In continuous or integral films, the entire thickness of the film and both opposing surfaces are formed and built together as a structurally unitary and continuous film by a single forming step, such as by a single step of forming the film (which may be by casting, coating or extrusion), followed by coagulation of the polymer of the film. The size of the pores along the depth of the film (including between the regions of different thickness and between the open and dense regions) gradually changes when viewed in magnification; there is no visible boundary between the open and dense regions, as would be visually identifiable in a film that is a "stacked" or multi-layer or coextruded film.

Other porous membranes may be non-integral or discontinuous as compared to integral or continuous porous membranes. These non-integral or discontinuous porous membranes include membranes prepared by combining together in series two separate (separately prepared) membrane layers, sometimes referred to as multilayer membranes or "stacked" membranes, each of which may have a different morphology or chemical composition. These non-integral or discontinuous porous films also include porous films formed by coextruding two different polymer compositions using two different polymer compositions to form a single "coextruded" film from two or more different polymer materials. These types of stacked multilayer assemblies and coextruded film structures are not considered "integral" or "continuous" films.

Examples of multi-asymmetric membranes as described can be used alone in the absence of another membrane or layer, and without any coating applied to the multi-asymmetric membrane. However, the continuous or integral multiple asymmetric membranes of the present description may also be combined with layers of another membrane or with a support structure or the like to form a multilayer membrane structure containing multiple asymmetric membranes as one membrane layer of the multilayer structure. Alternatively or additionally, a coating of a separate material may be applied to the multiple asymmetric membranes of the present description to form a composite membrane comprising a continuous multiple asymmetric membrane and having a coating applied to one or more surfaces of the multiple asymmetric membrane.

The multi-asymmetric membranes as described can be prepared from sulfone polymers (sometimes referred to as polysulfones) that can be processed by the methods of the present specification to form multi-asymmetric membranes.

The polysulfone polymer family comprises thermoplastic polymers containing the common structural unit "diphenyl sulfone". Examples of polysulfones include polysulfone ("PS" or "PSU") polymers, polyarylsulfone polymers, polyethersulfone ("PES") polymers, and polyphenylsulfone polymers. The membranes of the present description can be particularly prepared to comprise (e.g., consist of, or consist essentially of) a plurality of polysulfone polymers or polyethersulfone polymers, or a combination of both types of polysulfone polymers. Examples of the present disclosure multiple asymmetric membranes may be made from polymers comprising (e.g., including, consisting of, or consisting essentially of) at least 80%, 90%, 95%, or 99% polyethersulfone, polysulfone, or a mixture of both polymers.

The polysulfone polymer contains (consists of, or consists essentially of) a plurality (at least 80%, 90%, 95%, or 99%) of polysulfone repeating units:

the polyethersulfone polymer comprises (consists of, or consists essentially of) a plurality (at least 80%, 90%, 95%, or 99%) of polyethersulfone repeat units:

commercially available polyethersulfone polymers comprise a polymer blend of the following components under the trade nameFrom Suwhist Polymer Co., ltd (Solvay Specialty Polymers) under the trade name +.>From BASF and under the trade name +.>Polyethersulfone Polymers sold from AMOCO Polymers (AMOCO Polymers), and the like. Exemplary polysulfones include those available from Suvigter Polymer Inc. (-)>PSU polysulfone), basf company (++>PSU) and privet (Polyone) Inc. (A->PSU) a commercially available polymer.

The polyethersulfone or polysulfone polymers used in the methods and membranes described herein may have any effective molecular weight. For example, the polyethersulfone or polysulfone may have an average molecular weight or weight average molecular weight in the range of about 1,000 g/mole to about 1,000,000 g/mole, such as from 50,000 to 900,000 or from 100,000 to 800,000 g/mole.

The multi-asymmetric membranes of the present description can have any useful thickness, such as a thickness in the range from 50 to 300 microns, such as in the range from 25 or 40 microns up to 250 or 200 microns.

Referring to fig. 1A, a cross section of an example porous membrane 10 is shown having a surface 12, a second surface 14, and a thickness therebetween.

Fig. 1B is a graph showing the average pore size of the pores between two surfaces 12 and 14 (surface 14 corresponds to depth 0 on the y-axis and surface 12 corresponds to depth 1), at a location along the depth of film 10. The average pore size at each location along the depth of the membrane 10 is shown as a function of pore size 20 (jagged line). Also shown at fig. 1B is a membrane average pore size 22 (straight dashed line), which is a pore size calculated as the average size of all pores between surfaces 12 and 14 that lie on a line extending in the thickness direction of membrane 10. The average pore size of the membrane 10 was about 0.5554 microns.

Fig. 1B shows that the membrane 10 includes three dense regions and two open regions, i.e., with alternating dense and open regions in the following order: "dense-open-dense". The dense regions are shown at fig. 1B as regions 30, 32 and 34, where the pore size of the membrane 10 is measured to be smaller than the membrane average pore size 22. The open regions are shown at fig. 1B as regions 40 and 42, where the pore size of the membrane 10 is measured to be greater than the membrane average pore size 22. The membrane 10 also has three pore size minima shown as minima 52, 54 and 56 of the pore size function 20, and two pore size maxima 62 and 64.

Referring to fig. 2A, a cross section of an example porous membrane 110 is shown having a surface 112, a second surface 114, and a thickness therebetween.

Fig. 2B is a graph showing the average pore size of pores between the two surfaces 112 and 114 at locations along the depth of the membrane 110. The average pore size at each location along the depth of the membrane 110 is shown as a function of pore size 120. Also shown at fig. 2B is a membrane average pore size 122, which is a pore size calculated as the average size of all pores between surfaces 112 and 114 that lie on a line extending in the thickness direction of membrane 110.

Fig. 2B shows that the membrane 110 includes two open regions and one dense region, i.e., has alternating dense and open regions in the following order: "open-dense-open". The dense region is shown at fig. 2B as region 130, where the pore size of membrane 110 is measured to be smaller than membrane average pore size 122. The open regions are shown at fig. 2B as regions 140 and 142, where the pore size of the membrane 110 is measured to be greater than the membrane average pore size 122. The membrane 110 also has one pore size minimum, shown as minimum 152 of the pore size function 120, and two pore size maxima 162 and 164. The average pore size of the membrane 110 is about 0.487 microns.

To generate the data of fig. 1B and 2B, the images of fig. 1A and 2A were analyzed using the following procedure. First, matLab software was used to read each SEM (scanning electron microscope) image into RGB (red, green, blue) coordinates. Each pixel contains RGB values showing its color. Next, the program scans the SEM RGB matrix to identify black pixels representing film holes. The cropped SEM image may then be pre-processed by MatLab function imadjust to enhance the contrast of the image.

The product contains pixel coordinates representing the aperture (black pixel). Pixels may be categorized into groups, with each group representing an individual aperture. This is achieved by checking whether the pixels are adjacent to each other. If so, the pixels belong to the same aperture. The program identifies the number of pixels in each well to calculate the well size. For example, if an irregularly shaped hole contains 500 black pixels, then its equivalent hole diameter is d= ((S/pi) ≡0.5) = ((500/3.14) ≡0.5) ×2=25 pixels. The unit length of each pixel, and the pore size, is calculated from the known image dimensions. The pore data is used to generate a pore size distribution along the y-axis. This is achieved by examining each row of pixels. MatLab finds the location of the black pixel and looks at which hole it belongs to. The hole size was then recorded by MatLab.

The multiple asymmetric porous membrane may be prepared by a novel and inventive method according to which a liquid polymer composition is formed into a film, and then the film is exposed to conditions that cause the polymer contained in the film to condense, including first contacting the film with gaseous water vapor (e.g., air containing an amount of moisture) to cause initial phase separation within the film, and then contacting the film with an aqueous liquid to cause the polymer contained in the film to condense and produce the multiple asymmetric polymer porous membrane.

By way of background, different kinds of porous films can be prepared by forming a polymer-containing liquid film and then causing the polymer contained in the film to coagulate. Various different techniques for causing (inducing) coagulation of polymers are known. One technique, known as "non-solvent induced phase separation" (NIPS), exposes the film to a "non-solvent," which causes the polymer in the film to coagulate. A different technique known as "thermally induced phase separation" (TIPS) uses temperature changes of a liquid film to cause coagulation of polymers in the film.

In contrast to thermally induced phase separation, the methods described herein may involve non-solvent induced phase separation, and useful methods do not require and may explicitly exclude exposure of the liquid polymer composition to temperature changes to induce phase separation or polymer coagulation. According to the methods of the present disclosure, the polymer contained in the film of the liquid polymer composition may be coagulated to form a multi-asymmetric polymeric porous membrane by a step that does not cause or allow the temperature of the liquid polymer composition to vary significantly from ambient temperature. In example methods, as the liquid polymer composition is formed into a film, and as the film is treated to cause the polymer in the liquid polymer composition to coagulate to form a multi-asymmetric polymeric porous membrane, the liquid polymer composition may be maintained at a temperature in the range of from 20 to 32 degrees celsius. The temperature of the liquid coating composition may be in the range of from 20 to 32 degrees celsius, such as in the range of from 20 to 30 or 20 to 25 degrees celsius, before and during the step of forming the film and causing the polymer in the film to coagulate.

Liquid polymer compositions containing polymers dissolved or suspended in liquid solvents are formed into films by conventional non-solvent induced phase separation (NIPS). The film may be formed by useful methods, such as by extruding, coating, or otherwise applying the liquid polymer composition onto a support surface (e.g., a roll surface, a moving belt, etc.). The film is then allowed to contact a "non-solvent," which induces phase separation of the components of the liquid polymer composition.

According to some specific techniques known as "immersion casting", cast films are immersed in a coagulation bath containing a non-solvent to cause phase separation within the film. One common non-solvent is water, but aqueous solutions or pure organic solvents such as ethanol, isopropanol or butanol may also be used as non-solvents in the coagulation bath. The film separates into two phases during immersion in the non-solvent: a polymer-rich phase forming a continuous membrane matrix, and a solvent-rich (polymer-poor) phase forming discontinuous pores of the membrane.

According to the present specification, the method of inducing coagulation in a film formed from a liquid polymer composition comprises an immersion step, i.e. a step of immersing the film in a water bath, but the immersion step is performed after the step of exposing the film to air containing a certain amount of moisture. After the film is formed (by any useful method), the film is exposed to air, i.e., humid air, containing an amount of moisture and at a temperature within the ambient temperature range. The amount of moisture in the air may preferably be in the range from 20% to 75% relative humidity (e.g., from 20% to 50% relative humidity), and the air may be at a temperature in the range from 20 to 30 degrees celsius, such as from 20 to 25 degrees celsius.

The air contains moisture, and the moisture in the air is absorbed as water by the film. The water absorbed by the liquid coating composition causes the polymer dissolved or suspended in the solvent of the liquid coating composition to become more concentrated in the polymer-rich phase of the coating composition and less concentrated in the polymer-poor phase of the coating composition.

The step of exposing the film of the liquid coating composition to air containing moisture may be performed for any useful amount of time prior to the immersing step of immersing the film in a water bath. The desired amount of time may be an amount of time effective to produce the desired effect on the liquid coating composition. The desired effect may be to allow water in the air to be absorbed by the liquid coating composition, for example, for an amount of time that will produce a desired amount of polymer that thickens in the polymer-rich phase. Examples of useful amounts of time for exposing the film to moisture-containing air may be at least 30 seconds, such as from 30 seconds to 5 minutes, or from 30 seconds to 4, 3, or 2 minutes.

In a particular method, the liquid polymer composition contains a dissolved polymer (polysulfone, polyethersulfone, or a combination of these) in a solvent combination comprising a strong solvent and a co-solvent. A "strong solvent" is a solvent that is capable of completely dissolving, alone, a quantity of the polymer of the liquid polymer composition. Cosolvents are polymers that do not themselves completely dissolve a certain amount of the liquid polymer composition, but are used in combination with strong solvents to achieve (improve) the solubility properties of the strong solvents in the liquid polymer composition.

Examples of strong solvents include n-methylpyrrolidone, dimethylformamide (DMF), dimethyl acetate (DMAC), and Dimethylsulfoxide (DMSO). Examples of co-solvents that may be used with these or other strong solvents include polyols, such as glycols, e.g., diethylene glycol (DEG), triethylene glycol (TEG), and the like.

The amount of polymer in the liquid polymer composition can be any amount that can be used to produce a multi-asymmetric membrane as described by the method as described. Example concentrations of the polymer in the liquid polymer composition may range from 5 to 20 wt%, such as a concentration ranging from 8 to 15 wt%, based on the total weight of the liquid polymer composition.

The balance of the liquid polymer composition, i.e., the liquid polymer composition that is not a polymer, may be a solvent; for example, the liquid coating composition may contain a solvent, which means a total amount of two or more different types of solvents, in an amount ranging from 80 to 95 wt% solvent (total), based on the total weight of the liquid polymer composition, such as an amount ranging from 85 to 92 wt% solvent (total). In some embodiments, the liquid polymer composition may be 5 to 20 weight percent polymer, 10 to 40 weight percent strong solvent, and 30 to 80 weight percent co-solvent.

In certain example liquid polymer compositions, the composition may contain a strong solvent (e.g., n-methylpyrrolidone) in an amount ranging from 10 to 40 wt%, such as from 15 to 35 wt%, based on the total weight of the liquid polymer composition.

In certain example liquid polymer compositions, the compositions can contain a co-solvent (e.g., a polyol (e.g., DEG, PEG)) in an amount ranging from 30 to 80 wt%, such as from 40 to 70 wt%, based on the total weight of the liquid polymer composition.

Referring to fig. 3A, a general method 200 that may be used to form a multi-asymmetric membrane may be performed using a series of steps including: forming or otherwise providing (202) a polymer-containing liquid (204) comprising a polymer for preparing a porous polymer film dissolved or suspended in a solvent; forming (210) a film from the liquid; exposing (220) the film to moisture-containing air; and immersing (230) the film in a water bath to form a multi-asymmetrically coagulated porous polymer film. Useful methods may also include drying or otherwise further treating the formed porous polymer film.

To form (202) the liquid polymer composition (204), the polymer may be combined with a solvent (e.g., suspended or dissolved in a solvent) to form a polymer-containing liquid containing the polymer in a non-coagulated or partially coagulated form (referred to herein as a "liquid polymer composition" or "polymer-containing liquid") that may be formed into a film. The step of forming the liquid polymer composition (204) may be performed with the liquid composition (204) maintained at or otherwise having a temperature within an ambient range (e.g., from 20 to 30 degrees celsius or from 20 to 25 degrees celsius).

The liquid polymer composition can be formed into a film (210) by any useful method effective to form a film of the liquid coating composition, such as by extrusion (using a die), casting, coating, and the like. In an example film-forming step, the liquid coating composition may be applied to a stationary or moving solid surface, such as glass or metal, to form a film. According to examples of continuous processes, a die, coater, or any other type of extrusion or film forming device may be used to form the film, and the film may be applied to a moving belt, a roller surface, or another moving surface in a continuous manner.

The step of forming (210) a film from the liquid polymer composition (204) may be performed with the liquid composition (204) maintained at or otherwise having a temperature within an ambient range (e.g., from 20 to 30 degrees celsius or from 20 to 25 degrees celsius).

After forming the film, the film is exposed (220) to air containing an amount of moisture (humidity). Moisture in the air is absorbed by the liquid coating composition of the film. Moisture, i.e., water, acts as an anti-solvent within the liquid coating composition and causes the polymers in the liquid coating composition to form a polymer-poor phase and a polymer-rich phase. The water absorbed from the air causes the concentration of dissolved polymer in the film to become higher in the polymer-rich phase (more polymer rich) and lower in the polymer-lean phase (less polymer rich).

After the step of exposing (220) the film to air containing moisture, the film is immersed (230) in a water bath (i.e., a "coagulation bath"). The water bath may be a liquid bath containing predominantly water, for example at least 70, 80, 90 or 95% by weight water, with optionally an organic solvent. The water bath may be maintained at a temperature within an ambient range, such as from 20 to 30 degrees celsius or from 20 to 25 degrees celsius, or may otherwise have such a temperature.

Referring to fig. 3B, a more specific example of a method 300 is shown that includes steps for producing a multi-asymmetric membrane. As shown, the liquid polymer composition 304 is delivered to a coating or extrusion device 306. The device 306 is used to deliver the liquid polymer composition 304 as a film 310 onto a surface. The apparatus 306 may form the film 310 by any useful method, such as by extrusion (using a die), casting, coating, and the like. As illustrated, the liquid coating composition 304 is applied to a moving surface, such as a moving belt 320. During the step of applying the film 310 to the moving belt 320, the liquid coating composition 304 and the surface to which the film is applied (belt 320) are at a temperature in the ambient range, such as from 20 to 30 degrees celsius or from 20 to 25 degrees celsius.

After the film 310 is formed, the film is exposed to air containing a certain amount of moisture (humidity). Moisture in the air is absorbed by the liquid coating composition of the film. Moisture, i.e., water, acts as an anti-solvent within the liquid coating composition and causes the polymers in the liquid coating composition to form a polymer-poor phase and a polymer-rich phase. After the step of exposing the film 310 to moisture-containing air, the film is immersed in a water bath 330 (i.e., a "coagulation bath") having a temperature in the ambient range, such as from 20 to 30 degrees celsius or from 20 to 25 degrees celsius.

According to one example, the multiple asymmetric porous polymer membrane 10 of fig. 1A is prepared according to the method as described by forming a thin film of a liquid polymer composition on a solid surface. The liquid polymer composition contained about 27 wt% of n-methylpyrrolidone, 63 wt% of triethylene glycol and 10 wt% of polyvinyl sulfone. The liquid coating composition and the film form a film at ambient temperature. The film was then exposed to ambient air having a relative humidity of 50% on the surface for 45 seconds. The film was then immersed in water at 20 degrees celsius. The resulting porous polymer film 10 is shown in fig. 1A.

Membrane 10 was tested to have a bubble point of 67.9 pounds per square inch (psi) and a flow rate of 35704 liters per square meter per hour per bar (LMHB). By measuring at 14.2psi and at a temperature of 21 degrees celsius over a period of one minute having 13.8cm ² The total volume of DI water for the surface area of the membrane was measured for flow rate. The bubble point method is based on the following preconditions: for a particular fluid with constant wetting and pore size, the pressure required to force a bubble through a pore is inversely proportional to the size of the pore. Higher bubble point values are related to the pore size. To determine the bubble point of the porous material, a sample of the porous material is immersed in DI water at a temperature of 20 to 25 degrees Celsius (e.g., 22 degrees Celsius)And wetting is performed. The gas pressure is applied to one side of the sample by using compressed air and gradually increases. The minimum pressure at which the gas flows through the sample is known as the bubble point.

According to another example, the multiple asymmetric porous polymer membrane 110 of fig. 2A is prepared by forming a thin film of a liquid polymer composition on a solid surface according to the method as described. The liquid polymer composition contained about 27 wt% of n-methylpyrrolidone, 63 wt% of triethylene glycol and 10 wt% of polyvinyl sulfone. The liquid coating composition and the film form a film at ambient temperature. The film was then exposed to ambient air having a relative humidity of 50% on the surface for 60 seconds. The film was then immersed in water at 20 degrees celsius. The resulting porous polymer film 110 is shown in fig. 2A. Membrane 110 was tested to have a bubble point of 48.9psi and a flow rate of 90149 LMHB.

In some embodiments, the membranes described herein have a bubble point of at least 40psi, at least 45psi, at least 50psi, at least 55psi, at least 60psi, at least 65psi, at least 70psi, and all ranges and subranges therebetween, and/or a flow rate of at least 30,000LMHB, at least 35,000LMHB, at least 40,000LMHB, at least 45,000LMHB, at least 50,000LMHB, at least 55,000LMHB, at least 60,000LMHB, at least 65,000LMHB, at least 70,000LMHB, at least 75,000LMHB, at least 80,000LMHB, at least 85,000LMHB, at least 90,000LMHB, at least 95,000LMHB, at least 100,000LMHB, and all ranges and subranges therebetween.

A membrane as described herein, or a filter or filter assembly containing a filter membrane, may be used in a method of filtering a liquid material to purify or otherwise remove unwanted particles from the liquid chemical material. In general, the liquid chemical may be any of a variety of useful commercial materials, and may be a liquid chemical that may be used in any of a variety of different industrial, commercial, or laboratory applications; or in the medical, pharmaceutical, life sciences and food industries.

The membrane may be housed within a larger filter structure, such as a filter or cartridge used in a filtration system. The filtration system places a filter membrane (e.g., as part of a filter or cartridge) in the flow path of the liquid to cause the liquid to flow through the filter membrane such that the filter membrane is capable of removing impurities and contaminants from the liquid, for example, in the form of particles in the micrometer or sub-micrometer size range. The structure of the filter or cartridge may include one or more of a variety of additional materials and structures that support the porous filter membrane within the filter to cause fluid to flow from the filter inlet through the filter membrane and through the filter outlet, thereby passing through the filter membrane as it passes through the filter. The filter membrane supported by the filter structure may be of any useful shape, for example, pleated cylinders, cylindrical pads, one or more non-pleated (flat) cylindrical sheets, pleated sheets, and the like.

One example of a filter structure comprising a filter membrane in the form of a pleated cylinder may be prepared to comprise the following component parts, any of which may be included in the filter construction but may not be necessary: a rigid or semi-rigid typically cylindrical core supporting the pleated cylindrical membrane at the interior channel of the pleated cylindrical membrane; a rigid or semi-rigid cage supporting or surrounding the exterior of the pleated cylindrical filter membrane at the exterior of the filter membrane; an optional end piece or "disc" located at each of the two opposite ends of the pleated cylindrical filter membrane; and a filter housing including an inlet and an outlet, wherein the membrane is supported at a position between the inlet and the outlet that causes liquid to flow through the membrane to flow from the inlet to the outlet. The filter housing may be of any useful and desired size, shape and material, and may preferably be made of a suitable polymeric material.

As one example, fig. 4 shows a filter assembly 430, which is a product of the pleated cylindrical assembly 410 and the end piece 422 with other optional components. As described herein, the cylindrical component 410 includes a porous membrane 412 and contains folds or pleats 420, i.e., is "pleated". An end piece 422 is attached (e.g., "packaged") to one end of the cylindrical filter assembly 410. The end piece 422 may preferably be made of a melt processable polymeric material. A core (not shown) may be placed at the interior opening or "channel" 424 of the pleated cylindrical assembly 410, and a cage (not shown) may be placed around the exterior of the pleated cylindrical assembly 410. A second end piece (not shown) may be attached ("packaged") to the second end of the pleated cylindrical assembly 410. The resulting pleated cylindrical assembly 410 with two opposing potted ends and optional cores and cages may then be placed into a filter housing that includes an inlet and an outlet and is configured such that fluid entering the inlet must pass through the membrane 412 before exiting the filter at the outlet.

The filter housing can be of any useful and desired size, shape and material, and can preferably be a fluorinated or non-fluorinated polymer, such as nylon, polyethylene or a fluorinated polymer, such as poly (tetrafluoroethylene-co-perfluoro (alkyl vinyl ether)),Perfluoroalkoxyalkane (PFA), perfluoromethylalkoxy (MFA), or another suitable fluoropolymer (e.g., perfluoropolymer).

In a first aspect, a porous polymer film having a film thickness and a multi-asymmetric morphology along the thickness comprises: a membrane average pore size over the membrane thickness; two open regions having an average pore size and a pore size maximum greater than the average pore size of the membrane; and a dense region having an average pore size and a pore size minimum that is less than the average pore size of the membrane, wherein the dense region is located between the two open regions.

In a second aspect according to the first aspect, wherein the membrane has a pore size profile along the thickness, the pore size profile comprising: open region-dense region-open region.

In a third aspect according to the first aspect, wherein the membrane has a pore size profile along the thickness, the pore size profile comprising: open region-dense region-open region.

In a fourth aspect according to any one of the preceding aspects, the membrane has a bubble point of greater than 40 psi.

In a fifth aspect according to any one of the preceding aspects, the membrane has a flow rate of at least 30,000LMHB.

In a sixth aspect according to any one of the preceding aspects, the membrane has a membrane average pore size in the range from 0.2 to 1 micron.

In a seventh aspect according to any one of the preceding aspects, the membrane comprises polyethersulfone.

In an eighth aspect according to any one of the preceding aspects, the membrane comprises polysulfone.

In a ninth aspect, a filter cartridge comprises a membrane according to any one of the preceding aspects.

In a tenth aspect, a method of preparing a porous polymer film having a film thickness and multiple asymmetric morphologies along the thickness comprises: forming a film of a liquid polymer composition comprising a polymer selected from the group consisting of polyethersulfone and polysulfone dissolved in an organic solvent comprising a strong solvent and a co-solvent; exposing the film to air having a relative humidity of at least 20% to allow moisture in the air to be absorbed by the liquid coating composition and cause the polymer to become more concentrated in the polymer-rich phase and less concentrated in the polymer-lean phase; and immersing the film in a water bath to cause the polymer to precipitate as a porous polymer film after exposing the film to the air.

In an eleventh aspect according to the tenth aspect, the film is exposed to air having a relative humidity of at least 20% for at least 30 seconds.

In a twelfth aspect according to the tenth or eleventh aspect, the liquid polymer composition film is formed by casting the liquid polymer composition as a film onto a surface having a temperature in a range from 20 to 30 degrees celsius.

In a thirteenth aspect according to any one of the tenth to twelfth aspects, the air has a temperature in a range from 20 to 30 degrees celsius.

In a fourteenth aspect according to any one of the tenth to thirteenth aspects, the liquid polymer composition has a temperature in the range from 20 to 30 degrees celsius.

In a fifteenth aspect according to any one of the tenth to fourteenth aspects, the water bath has a temperature in the range from 20 to 30 degrees celsius.

In a sixteenth aspect according to any one of the tenth to fifteenth aspects, the strong solvent is n-methylpyrrolidone.

In a seventeenth aspect according to any one of the tenth to sixteenth aspects, the co-solvent is a polyol.

In an eighteenth aspect according to any one of the tenth to seventeenth aspects, the co-solvent comprises diethylene glycol, triethylene glycol, or a combination thereof.

In a nineteenth aspect according to any one of the tenth to eighteenth aspects, the coating composition comprises: from 5 to 20 wt% of a polymer, from 10 to 40 wt% of a strong solvent, and from 30 to 80 wt% of a co-solvent.

In a twentieth aspect according to any one of the tenth to nineteenth aspects, the porous film includes: a membrane average pore size over the membrane thickness; two open regions having an average pore size and a pore size maximum greater than the average pore size of the membrane; and a dense region having an average pore size and a pore size minimum that is less than the average pore size of the membrane, wherein the dense region is located between the two open regions.

Claims (20)

1. A porous polymer film having a film thickness and a multi-asymmetric morphology along the thickness, the film comprising:

a membrane average pore size over the membrane thickness;

two open regions having an average pore size and a pore size maximum greater than the average pore size of the membrane; a kind of electronic device with high-pressure air-conditioning system

A dense region having an average pore size and a pore size minimum that is less than the average pore size of the membrane, wherein the dense region is located between the two open regions.

2. The film of claim 1 having a pore size profile along the thickness, the pore size profile comprising: open region-dense region-open region.

3. The film of claim 1 having a pore size profile along the thickness, the pore size profile comprising: open region-dense region-open region.

4. The membrane of claim 1, wherein the membrane has a bubble point greater than 40 psi.

5. The membrane of claim 1, wherein the membrane has a flow rate of at least 30,000LMHB.

6. The film of claim 1, wherein the film has a film average pore size in a range from 0.2 to 1 micron.

7. The membrane of claim 1, wherein the membrane comprises polyethersulfone.

8. The membrane of claim 1, wherein the membrane comprises polysulfone.

9. A filter cartridge comprising the membrane of any preceding claim.

10. A method of preparing a porous polymer film having a film thickness and multiple asymmetric morphologies along the thickness, the method comprising:

forming a film of a liquid polymer composition comprising a polymer selected from the group consisting of polyethersulfone and polysulfone dissolved in an organic solvent comprising a strong solvent and a co-solvent;

Exposing the film to air having a relative humidity of at least 20% to allow moisture in the air to be absorbed by the liquid coating composition and cause the polymer to become more concentrated in the polymer-rich phase and less concentrated in the polymer-lean phase; a kind of electronic device with high-pressure air-conditioning system

After exposing the film to the air, the film is immersed in a water bath to cause the polymer to precipitate as a porous polymer film.

11. The method of claim 10, wherein the film is exposed to air having a relative humidity of at least 20% for at least 30 seconds.

12. The method of claim 10, wherein the liquid polymer composition film is formed by casting the liquid polymer composition as a film onto a surface having a temperature in the range of from 20 to 30 degrees celsius.

13. The method of claim 10, wherein the air has a temperature in a range from 20 to 30 degrees celsius.

14. The method of claim 10, wherein the liquid polymer composition has a temperature in a range from 20 to 30 degrees celsius.

15. The method of claim 10, wherein the water bath has a temperature in a range from 20 to 30 degrees celsius.

16. The method of claim 10, wherein the strong solvent is n-methylpyrrolidone.

17. The method of claim 10, wherein the co-solvent is a polyol.

18. The method of claim 10, wherein the co-solvent comprises diethylene glycol, triethylene glycol, or a combination thereof.

19. The method of claim 10, wherein the coating composition comprises:

from 5 to 20% by weight of a polymer,

from 10 to 40% by weight of a strong solvent, and

from 30 to 80% by weight of a co-solvent.

20. The method of claim 10, wherein the porous membrane comprises:

a membrane average pore size over the membrane thickness;

two open regions having an average pore size and a pore size maximum greater than the average pore size of the membrane; a kind of electronic device with high-pressure air-conditioning system

A dense region having an average pore size and a pore size minimum that is less than the average pore size of the membrane, wherein the dense region is located between the two open regions.