Asymmetric polymer porous filter membrane and related methods

Abstract

A porous polymer filter membrane having a highly asymmetric pore structure through the thickness of the membrane, and methods of making and using the porous polymer filter membrane are described.

Description

Technical Field

[0001] The following description relates to a porous polymeric filter membrane having a highly asymmetric pore structure over the thickness of the membrane, and methods of making and using the porous polymeric filter membrane.

Background Art

[0002] Many gaseous and liquid fluids are processed using filters to remove contaminants or impurities. Examples include air, drinking water, liquid industrial solvents and process fluids, industrial gases used for manufacturing or processing (e.g., in semiconductor fabrication), and liquids having medical or pharmaceutical applications. Undesirable substances removed from the fluid include impurities and contaminants such as particles, microorganisms, and dissolved or suspended chemical species. Specific examples of impurity removal applications for filter membranes include removing cell residue particles, bacteria, or other organics from therapeutic solutions in the pharmaceutical industry, or treating ultrapure aqueous and organic solvent solutions used in microelectronics and semiconductor processing, or their use in air and water purification processes.

[0003] To perform the filtration function, a filter product includes a filter membrane responsible for removing undesirable substances from the fluid passing through the filter. The filter membrane may, if desired, be in the form of a flat sheet, wound (e.g., spirally), or pleated, etc. Alternatively, the filter membrane may be in the form of hollow fibers. The filter membrane can be included within a housing having an inlet and an outlet, such that the fluid to be filtered enters through the inlet and passes through the filter membrane before passing through the outlet.

[0004] The filter membrane can be constructed of a porous polymer film having an average pore size that can be selected based on the expected use of the filter, i.e., the type of filtration to be performed using the filter. Typical pore sizes are in the micron or submicron range, e.g., from about 0.001 micron to about 10 microns. Membranes having an average pore size of about 0.001 to about 0.05 microns may be classified as ultrafiltration membranes. Membranes having a pore size of about 0.05 to 10 microns may be classified as microporous membranes.

[0005] For commercial use, the filter membrane must be of a type that can be efficiently manufactured and assembled into a filter product. The membrane must be capable of being efficiently manufactured and must have mechanical properties such as strength and flexibility that allow the membrane to withstand assembly into a filter cartridge form or other form of filter membrane structure. In addition to mechanical properties, the membrane should have appropriate chemical functionality, including stability and microstructure (pore size and morphology) for high-performance filtration.

[0006] Various techniques for forming porous filter membranes are known. Exemplary techniques include, among others, melt extrusion (e.g., melt casting) techniques, dip casting (phase inversion) techniques. Different techniques for forming porous polymer membranes can produce different membrane structures with respect to the size and distribution of pores formed within the membrane, i.e., different techniques produce different pore sizes and membrane structures, sometimes referred to as morphology, which means the uniformity, non-uniform shape, size, and distribution of pores within the membrane.

[0007] Examples of membrane morphologies include homogeneous (isotropic) and asymmetric (anisotropic). A membrane having pores of substantially uniform size distributed uniformly throughout the membrane (within a certain range) is often referred to as isotropic, or "homogeneous." An anisotropic (also known as "asymmetric") membrane can be considered to have a morphology in which a pore size gradient exists across the membrane. For example, the membrane can have a porous structure with relatively large pores on one membrane surface and relatively small pores on the opposite membrane surface, and the pore structure varies along the thickness of the membrane. The term "asymmetric" is often used interchangeably with the term "anisotropic." In many cases, the portion of the membrane having relatively small pores (compared to other regions of the membrane) is called the "dense" region. The portion of the membrane having larger pores is often called the "open" region.

[0008] For the continuous improvement of liquid material filtration, membranes having different morphologies are needed.

Summary of the Invention

[0009] Described below are "multi-asymmetric" porous polymer membranes that can be effective as porous polymer filter membranes, as well as methods for preparing and using the described multi-asymmetric porous polymer membranes.

[0010] The described membranes have a "multi-asymmetric" morphology, which means that the membrane includes pores having a pore size that varies across the thickness of the membrane such that the membrane produces at least three regions of different pore sizes. The membranes having a multi-asymmetric morphology described herein provide multiple alternating pore size regions for capturing particles of different sizes.

[0011] The membrane includes a plurality of "thickness regions" having different morphologies. A "thickness region" of the membrane is a portion of the membrane that extends in the length and width dimensions of the membrane over a defined portion of the membrane thickness. For the purposes of this specification and the claims, the membrane can be considered to include at least three thickness regions of the type that are identified as an "open pore region" ("open region") having relatively large-sized pores, or a "dense pore region" ("dense region") having relatively small-sized pores. The open regions and the dense regions are present within the membrane in an alternating order along the membrane thickness, for example, open-dense-open regions, dense-open-dense regions, and the like.

[0012] The multi-asymmetric membrane can be prepared from a sulfone polymer, sometimes referred to as polysulfone, which can be processed by the methods of this specification to form multi-asymmetric membranes, particularly including polysulfone and polyethersulfone.

[0013] The multi-asymmetric porous membrane can be prepared by the following method, according to which a liquid polymer composition is formed into a film, and subsequently the film is exposed to conditions that cause the polymer contained in the film to coagulate. The method includes first contacting the film with gaseous water vapor (e.g., air containing a certain amount of moisture) to cause initial phase separation within the film, and subsequently contacting the film with an aqueous liquid to coagulate the polymer contained in the film, thereby producing a multi-asymmetric polymeric porous membrane.

[0014] In one aspect, the present disclosure relates to a porous polymeric membrane having a membrane thickness and a multi-asymmetric morphology along the membrane thickness. The membrane includes a membrane average pore size across the membrane thickness, two open regions having an average pore size and a maximum pore size greater than the membrane average pore size, and a dense region having an average pore size and a minimum pore size less than the membrane average pore size, the dense region being located between the two open regions.

[0015] In another aspect, the present disclosure relates to a method for preparing a porous polymer membrane having a film thickness and a multi-asymmetric morphology along the film thickness, formed using polyethersulfone or polysulfone. The method includes forming a liquid polymer composition film, wherein the liquid polymer composition comprises a polymer selected from 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 percent to absorb moisture in the air with a liquid coating composition, concentrating the polymer in a polymer-rich phase and not concentrating it as much in a polymer-lean phase; and after exposing the film to air, immersing the film in an aqueous bath to precipitate the polymer as a porous polymer membrane.

Brief Description of the Drawings

[0016]

Figure 1A

Figure 1B

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 4

Mode for Carrying Out the Invention

[0017] The following description relates to the (described) "multi-asymmetric" porous polymer membrane, which may be effective as a porous polymer filter membrane, and methods for preparing and using the described multi-asymmetric porous polymer membrane.

[0018] The porous multi-symmetric membrane includes a porous polymer membrane body having a matrix wall and a continuous polymer matrix defining pores between the walls (including, consisting of, consisting essentially of), and the pores are highly asymmetric along the thickness of the membrane body. The matrix structure is "continuous", which means that the matrix is a structure made of a single type of polymer without a single (other than by pores) interruption throughout the matrix.

[0019] The porous polymer membrane has two opposing substantially 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 located across the thickness of the membrane, enabling the flow of fluid from one side of the membrane through the thickness of the membrane to the opposite side and through the opposite side of the membrane. When the fluid flows through the membrane, impurities or contaminants, such as particulate contaminants, are retained by the membrane and removed from the fluid.

[0020] This type of membrane may be referred to as an "open pore" membrane as compared to a "closed pore" membrane. An open pore membrane is in the form of a continuous open pore structure including a thin film or sheet of a porous polymer material having a relatively uniform thickness over an area (an area having a certain length and a certain width), and a polymer matrix defining a large number of open "pores" which are a three-dimensional void structure located between solid walls of a continuous matrix structure. The open pores constitute interconnected channels or passages between adjacent pores, enabling a liquid or gaseous fluid to flow from one side of the membrane through the thickness of the membrane to the other side.

[0021] The membrane has a "highly asymmetric" morphology, which means that the membrane includes pores having a pore diameter that varies across the thickness of the membrane such that at least three regions of different pore diameters are produced, and each region is distinguishable as an "open pore region" ("open region") having relatively large-sized pores, or a "dense pore region" ("dense region") having relatively small-sized pores, and the two types of regions exist in an alternating order along the membrane thickness, for example, open-dense-open regions, dense-open-dense regions, etc.

[0022] The membrane can be described as having an "average pore size" at different depth positions. The average pore size at a certain depth of the membrane is the average of the sizes of the pores that are all located similarly at a specific depth position of the membrane, that is, the average size of the pores that are all located at the same distance ("depth") from the membrane surface. The membrane can be described in terms of the average pore size at different individual depths along the thickness of the membrane.

[0023] The membrane also has a "membrane average pore size", which is the average of the sizes of the pores of the membrane across the thickness of the membrane, that is, the average of the sizes of the pores located at depth positions (distance from the membrane surface) across the entire thickness of the membrane.

[0024] The pore size, the average pore size (across the membrane or at individual depths of the membrane), the pore size variation (difference in average pore size) across the thickness of the membrane, etc. can be visually observed and measured using a microscope such as a scanning electron microscope. The pore size data of the membrane can be collected and electronically analyzed to evaluate the average pore size at different depths of the membrane, compare the average pore sizes at different depths within the membrane, and compare the average pore sizes at different depths within the membrane with the membrane average pore size. The analysis can be performed by a commercially available software product (for example, especially MatLab) that analyzes the matrix of pixels of the SEM image of the membrane using RGB (red, green, blue) coordinates, identifies the pores (black) of the membrane, and then determines the pore size and position of pores of different sizes as part of the membrane.

[0025] In a useful form, the data of the pore sizes at different depth positions within the membrane can be electronically analyzed and presented in the form of a graph that plots the average pore size against the depth position of the membrane measured at different depths of the membrane. For example, see Figures 1B and 2B. In the graph form showing the average pore size measured at different depths along the thickness of the membrane, the line on the graph representing the average pore size at each membrane depth can be called a "pore size function". Also, for simplicity, the pore size function of the membrane can be compared with the membrane average pore size on the graph. See Figures 1B and 2B.

[0026] The described membrane includes at least one region along the thickness of the membrane that is an "open pore region" or "open region". The membrane also includes at least one region along the thickness of the membrane that is a "dense pore region" or "dense region". Each dense region has a pore size that includes a minimum pore size smaller than the average pore size of the membrane. Each open region has a pore size that includes a maximum pore size larger than the average pore size of the membrane.

[0027] In an exemplary membrane, the minimum or maximum pore size is located in the central 1 / 3 of the thickness. Alternatively or additionally, the membrane may have at least two minimum pore sizes or at least two maximum pore sizes in the central 8 / 10 of the thickness.

[0028] The membrane herein more particularly includes at least one dense region, at least one open region, and at least one additional region that is either a dense region or an open region. The regions alternate between dense regions and open regions along the membrane thickness.

[0029] As an example, the membrane can include two dense regions and have an open region between the two dense regions. As another example, the membrane can include two open regions and have a dense region between the two open regions. See FIGS. 2A and 2B.

[0030] As another example, the membrane can include three dense regions and two open regions in an alternating order, i.e., in the order of dense-open-dense-open-dense. See FIGS. 1A and 1B. Also, as yet another example, the membrane can include three open regions and two dense regions in an alternating order, i.e., in the order of open-dense-open-dense-open.

[0031] The dense region is a region of the membrane along the thickness of the membrane that includes pores having a pore diameter smaller than the membrane average pore diameter (e.g., the average pore diameter at a specific depth). The pore diameter across the thickness of the dense region can be identified by a pore diameter function that plots the average pore diameter against the position of the membrane in the thickness direction of the membrane, and the pore diameter function is also compared to the membrane average pore diameter. Each dense region includes a distinguishable low point on the pore diameter function across the domain of the dense region between the ends of the dense region, which is referred to as the "dense region minimum". The ends of the dense region can be identified as the surface of the membrane or as the intersection of the pore diameter function and the membrane average pore diameter.

[0032] Similarly, the open region of the membrane is a region along the thickness of the membrane that includes pores having a pore diameter larger than the membrane average pore diameter (e.g., the average pore diameter). Each open region includes a distinguishable high point on the pore diameter function across the domain of the open region between the ends of the open region, which is referred to as the "open region maximum". The ends of the open region can be identified as the surface of the membrane or as the intersection of the pore diameter function and the membrane average pore diameter.

[0033] In an exemplary membrane, the average pore diameter of the membrane can be in the submicron range, e.g., from 0.1 micron to 1 micron, e.g., from 0.2 nanometers to 0.9 microns, or from 0.3 to 0.8 microns.

[0034] The multi-asymmetric porous membrane is considered to be "integral" or "continuous", which means that the membrane includes a polymer matrix made of a single type of polymer that forms a single matrix body not interrupted by anything other than pores. In a continuous or integral membrane, the entire thickness of the membrane and the opposing two surfaces are formed and constructed together as a structurally single and continuous membrane by a single forming process, e.g., forming a film (which may be by casting, coating, extrusion of ore), followed by a single process of solidifying the polymer of the film. When viewed using magnification, the size of the pores along the depth of the membrane gradually changes, such as between different thickness regions and between the open region and the closed region, and the boundary between the open region and the dense region is not visually distinguishable as in a membrane that is a "laminated membrane" or a multilayer membrane or a coextruded membrane.

[0035] In contrast to an integral or continuous porous membrane, other porous membranes may be non-integral or discontinuous. These include membranes, sometimes referred to as multilayer or "laminated" membranes, which are prepared by sequentially combining two separate (separately prepared) membrane layers, each of which may have a different morphology or chemical composition. These also include porous membranes formed using two different polymer compositions by co-extruding the two different polymer compositions to form a single "co-extruded" membrane from two or more different polymer materials. These types of laminated multilayer assemblies and co-extruded membrane structures are not considered "integral" or "continuous" membranes.

[0036] Examples of the useful multi-asymmetric membranes described can be used alone, in the absence of another membrane or layer and without applying a coating to the multi-asymmetric membrane. However, it is also possible to combine the continuous or integral multi-asymmetric membranes herein with another membrane layer or support structure, etc., to form a multilayer membrane structure that includes the multi-asymmetric membrane as one membrane layer of the multilayer structure. Alternatively or additionally, a coating of a separate material can be applied to the multi-asymmetric membranes herein to form a composite membrane that includes a continuous multi-asymmetric membrane having the coating applied to one or more surfaces of the multi-asymmetric membrane.

[0037] The multi-asymmetric membranes described can be prepared from a sulfone polymer, sometimes referred to as polysulfone, which can be processed by the methods herein to form the multi-asymmetric membranes.

[0038] The family of polysulfone polymers includes thermoplastic resin 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 herein can be prepared to contain (or for example, consist of, consist essentially of) any of a large amount of polysulfone polymer or polyethersulfone polymer, or a combination of these two types of polysulfone polymers. Exemplary asymmetric membranes herein can be made from polymers containing (e.g., consisting of, consisting essentially of) at least 80, 90, 95 or 99 percent polyethersulfone, polysulfone, or a mixture of these two polymers.

[0039] The polysulfone polymer contains (consists of, consists essentially of) a large amount (at least 80, 90, 95 or 99 percent) of polysulfone repeating units: TIFF2025521260000002.tif17170.

[0040] The polyethersulfone polymer contains (consists of, consists essentially of) a large amount (at least 80, 90, 95 or 99 percent) of polyethersulfone repeating units: TIFF2025521260000003.tif15170.

[0041] Examples of commercially available polyethersulfone polymers include, among others, those sold under the trade name VERADEL® by Solvay Specialty Polymers, ULTRASON® E by BASF, and RADEL®-A by AMOCO Polymers. Exemplary polysulfones include commercially available polymers from Solvay Specialty Polymers (Udel® PSU polysulfone), BASF (Ultrason® PSU), and PolyOne corporation (Edgetek® PSU).

[0042] The polyethersulfone polymer or polysulfone polymer for use in the methods and membranes described herein can have any effective molecular weight. For example, the polyethersulfone or polysulfone can have a number average molecular weight or weight average molecular weight in the range of about 1,000 grams / mole to about 1,000,000 grams / mole, such as 50,000 to 900,000 grams / mole, or 100,000 to 800,000 grams / mole.

[0043] The multi-asymmetric membranes herein can have any useful thickness, such as a thickness in the range of 50 to 300 microns, such as 25 microns or 40 microns, up to 250 microns or 200 microns.

[0044] Referring to FIG. 1A, a cross-section of an exemplary porous membrane 10 is shown having a surface 12, a second surface 14, and a thickness between these two surfaces.

[0045] Figure 1B is a plot showing the average pore diameter of pores at positions along the depth of the membrane 10 between two surfaces 12 and 14 (where surface 14 corresponds to a depth of 0 on the y-axis and surface 12 corresponds to a depth of 1). The average pore diameter at each position along the depth of the membrane 10 is shown by the pore diameter function 20 (the zigzag line). Figure 1B also shows the membrane average pore diameter 22 (the dashed line), which is the pore diameter calculated as the average size of all pores located on a line extending in the thickness direction of the membrane 10 between surface 12 and surface 14. The average size of the pores of the membrane 10 is about 0.5554 microns.

[0046] Figure 1B shows that the membrane 10 includes three dense regions and two open regions, that is, has the dense regions and the open regions alternately in the order of "dense-open-dense-open-dense". The dense regions are shown in Figure 1B as region 30, region 32 and region 34, and the pore diameter of the membrane 10 is measured to be smaller than the membrane average pore diameter 22. The open regions are shown in Figure 1B as region 40 and region 42, and the pore diameter of the membrane 10 is measured to be larger than the membrane average pore diameter 22. The membrane 10 also has three pore diameter minimum values shown as minimum values 52, 54 and 56 of the pore diameter function 20, as well as two pore diameter maximum values 62 and 64.

[0047] Referring to Figure 2A, a cross-section of an exemplary porous membrane 110 having a surface 112, a second surface 114, and a thickness between these two surfaces is shown.

[0048] Figure 2B is a plot showing the average pore diameter of pores at positions along the depth of the membrane 110 between two surfaces 112 and 114. The average pore diameter at each position along the depth of the membrane 110 is shown by the pore diameter function 120. Figure 2B also shows the membrane average pore diameter 122, which is the pore diameter calculated as the average size of all pores located on a line extending in the thickness direction of the membrane 110 between surface 112 and surface 114.

[0049] Figure 2B shows that the membrane 110 includes two open regions and one dense region, that is, has the dense region and the open regions alternating in the "open-dense-open" order. The dense region is shown in Figure 2B as region 130, and the pore diameter of the membrane 110 is measured to be smaller than less than the membrane average pore diameter 122. The open regions are shown in Figure 2B as regions 140 and 142, and the pore diameter of the membrane 110 is measured to be larger than the membrane average pore diameter 122. The membrane 110 also has one pore diameter minimum value shown as the minimum value 152 of the pore diameter function 120, and two pore diameter maximum values 162 and 164. The average size of the pores of the membrane 110 is about 0.487 microns.

[0050] To generate the data of Figures 1B and 2B, the images of Figures 1A and 2A were analyzed by the following procedure. First, using MatLab software, each SEM (scanning electron microscope) image was read into RGB (red, green, blue) coordinates. Each pixel contains RGB values indicating its color. Then, the program scans the SEM RGB matrix to identify the black pixels representing the membrane pores. The cropped SEM image can be pre-processed by the MatLab function imadjust to improve the contrast of the image.

[0051] The output contains the pixel coordinates (black pixels) representing the pores. The pixels can be classified into groups where each group represents an individual pore. This is achieved by checking whether the pixels are adjacent to each other. If so, they belong to the same pore. The program identifies the number of pixels within each pore and calculates the pore diameter. For example, if an irregularly shaped pore contains 500 black pixels, its equivalent pore diameter is d = ((S / pi)^0.5)*2 = ((500 / 3.14)^0.5)*2 = 25 pixels. From the known scale of the image, the unit length of each pixel and the pore diameter are calculated. Using the pore data, a pore diameter distribution along the y-axis is generated. This is achieved by examining each row of pixels. MatLab finds the position of the black pixels and determines which pore they belong to. Then, the pore diameter is recorded.

[0052] The multi-asymmetric porous membrane can be prepared by the novel method of the present invention. According to this method, 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 solidify. This method first contacts the film with gaseous water vapor (for example, air containing a certain amount of moisture) to cause initial phase separation in the film, and then contacts the film with an aqueous liquid to solidify the polymer contained in the film, including manufacturing a multi-asymmetric polymer porous membrane.

[0053] As in the background art, various porous membranes can be prepared by forming a polymer-containing liquid film and then solidifying the polymer contained in the membrane. Various different techniques for causing (inducing) the solidification of polymers are known. One technique called "non-solvent induced phase separation" (NIPS) exposes the film to a "non-solvent" to solidify the polymer in the film. A different technique called "thermal induced phase separation" (TIPS) uses a temperature change of the liquid film to solidify the polymer in the film.

[0054] The method described herein can include non-solvent induced phase separation, in contrast to thermal induced phase separation. A useful method does not require, and can particularly exclude, exposing the liquid polymer composition to a temperature change to induce phase separation or polymer solidification. According to the method herein, the polymer contained in the film of the liquid polymer composition can be solidified to form a multi-asymmetric polymer porous membrane by a process that does not cause or enable a significant change in the temperature of the liquid polymer composition from room temperature. In an exemplary method, the liquid polymer composition can be maintained at a temperature within the range of 20 to 32 degrees Celsius when the liquid polymer composition is formed into a film and when the film is processed to solidify the polymer in the liquid polymer composition to form a multi-asymmetric polymer porous membrane. The temperature of the liquid coating composition before and during the process of forming the film and solidifying the polymer in the film can be in the range of 20 to 32 degrees Celsius, for example, in the range of 20 to 30 degrees Celsius or 20 to 25 degrees Celsius.

[0055] In conventional non-solvent induced phase separation (NIPS), a liquid polymer composition containing a polymer dissolved or suspended in a liquid solvent is formed into a film. The film can be formed by applying the liquid polymer composition in a useful manner, such as by extrusion, coating, or other means onto a support surface such as the surface of a roller, a moving belt, etc. The film is then contacted with a "non-solvent", inducing phase separation of the components of the liquid polymer composition.

[0056] According to a specific technique called "immersion casting", the cast film is immersed in a coagulation bath containing a non-solvent to cause phase separation within the film. One common non-solvent is water, but an aqueous solution, or a pure organic solvent such as ethanol, isopropanol, or butanol can also be used as the non-solvent in the coagulation bath. The film separates into two phases during immersion in the non-solvent: one polymer-rich phase that forms a continuous film matrix and one solvent-rich (polymer-lean) phase that forms discontinuous pores in the membrane.

[0057] According to the present specification, a method for inducing coagulation within a film formed from a liquid polymer composition includes an immersion step, i.e., a step of immersing the film in an aqueous bath, but the immersion step is carried out after a 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 at a temperature in the normal temperature range that contains a certain amount of moisture, i.e., moist air. The amount of moisture in the air can preferably be in the range of 20 to 75 percent relative humidity (e.g., 20 to 50 percent relative humidity), and the air can be at a temperature in the range of 20 to 30 degrees Celsius, e.g., 20 to 25 degrees Celsius.

[0058] The air contains moisture, and the moisture in the air is absorbed by the film as water. The water absorbed by the liquid coating composition concentrates the polymer dissolved or suspended in the solvent of the liquid coating composition in the polymer-rich phase of the coating composition and does not concentrate it as much in the polymer-lean phase of the coating composition.

[0059] Before the dipping step of dipping the film of the liquid coating composition into an aqueous bath, the step of exposing the film to air containing moisture can be carried out for any useful amount of time. The desired amount of time can be an amount of time effective to produce a desired effect on the liquid coating composition. The desired effect can be to absorb water in the air by the liquid coating composition, for example, for an amount of time that results in a desired amount of polymer being concentrated in the polymer-rich phase. Examples of useful amounts of time for exposing the film to air containing moisture can be at least 30 seconds, such as 30 seconds to 5 minutes, or 30 seconds to 4, 3, or 2 minutes.

[0060] In certain methods, the liquid polymer composition contains a dissolved polymer (polysulfone, polyethersulfone, or a combination thereof) in a combination of solvents including a strong solvent and a co-solvent. A "strong solvent" is a solvent that can completely dissolve the polymer of an amount of the liquid polymer composition alone. A co-solvent is a solvent that by itself cannot completely dissolve the polymer of an amount of the liquid polymer composition but is used in combination with the strong solvent to affect (improve) the dissolution characteristics of the strong solvent in the liquid polymer composition.

[0061] Examples of strong solvents include n-methylpyrrolidone, dimethylformamide (DMF), dimethylacetate (DMAC), and dimethyl sulfoxide (DMSO). Examples of co-solvents that can be useful in combination with these or other strong solvents include polyols such as glycols like diethylene glycol (DEG), triethylene glycol (TEG), and the like.

[0062] The amount of polymer in the liquid polymer composition can be any amount useful for producing the described highly asymmetric membrane by the described method. Exemplary concentrations of the polymer in the liquid polymer composition can be in the range of 5 to 20 weight percent, such as in the range of 8 to 15 weight percent, based on the total weight of the liquid polymer composition.

[0063] The remainder 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 the total amount of two or more different types of solvents. Based on the total weight of the liquid polymer composition, it is in the range of 80 to 95 weight percent of the solvent (total), for example, in the range of 85 to 92 weight percent of the solvent (total). In some embodiments, the liquid polymer composition may be 5 to 20 weight percent polymer, 10 to 40 percent strong solvent, and 30 to 80 weight percent co-solvent.

[0064] In certain exemplary liquid polymer compositions, the composition can contain a strong solvent, such as n-methylpyrrolidone, in an amount in the range of 10 to 40 weight percent, for example 15 to 35 weight percent, based on the total weight of the liquid polymer composition.

[0065] In certain exemplary liquid polymer compositions, the composition can contain a co-solvent, such as a polyol (e.g., DEG, PEG), in an amount in the range of 30 to 80 weight percent, for example 40 to 70 weight percent, based on the total weight of the liquid polymer composition.

[0066] Referring to FIG. 3A, a general method 200 useful for forming a multi-asymmetric membrane can be carried out using a series of steps including forming or otherwise providing a polymer-containing liquid (204) comprising a polymer for preparing a porous polymer membrane dissolved or suspended in a solvent in step (202), forming a film from the liquid in step (210), exposing the film to air containing moisture in step (220), and immersing the film in an aqueous bath (230) to form a multi-asymmetric solidified porous polymer membrane. The useful method may also include drying the formed porous polymer membrane or otherwise further processing it.

[0067] To form a liquid polymer composition (204), a polymer is combined with a solvent (e.g., suspended or dissolved in the solvent) to form a polymer-containing liquid in a non-solidified or partially solidified form that can be formed into a film (referred to herein as a "liquid polymer composition" or a "polymer-containing liquid"). The step of forming the liquid polymer composition (204) can be carried out with the liquid composition (204) maintained at a temperature in the ambient range, e.g., 20 to 30 degrees Celsius, or 20 to 25 degrees Celsius, or otherwise having that temperature.

[0068] The liquid polymer composition can be formed into a film (210) by any useful method effective for forming a thin film of the liquid coating composition, such as an extrusion method (using a die), a casting method, a coating method, etc. In an exemplary film-forming process, the liquid coating composition can be applied onto a stationary or moving solid surface such as glass or metal to form a film. According to an example of a continuous process, the film may be formed using a die, a coater, or any other type of extrusion or film-forming apparatus, and the film may be continuously applied onto a moving belt, a roller surface, or another moving surface.

[0069] The step of forming a film from the liquid polymer composition (204) (210) can be carried out with the liquid composition (204) maintained at a temperature in the ambient range, e.g., 20 to 30 degrees Celsius, or 20 to 25 degrees Celsius, or otherwise having that temperature.

[0070] After forming the film, the film is exposed to air containing a certain amount of moisture (humidity) (220). Moisture in the air is absorbed by the liquid coating composition of the film. The moisture, i.e., water, acts as a poor solvent within the liquid coating composition, causing the polymer in the liquid coating composition to form a polymer-lean phase and a polymer-rich phase. The water absorbed from the air makes the concentration of the dissolved polymer in the film higher in the polymer-rich phase (the polymer is more concentrated) and lower in the polymer-lean phase (the polymer is less concentrated).

[0071] Subsequent to the step (220) of exposing the film to air containing moisture, the film is immersed in an aqueous bath (i.e., a "coagulation bath") (230). The aqueous bath can be a liquid bath mainly containing water, for example, containing at least 70, 80, 90, or 95 weight percent of water together with an optional organic solvent. The aqueous bath can be maintained at a temperature in the ambient range, for example, 20 to 30 degrees Celsius, or 20 to 25 degrees Celsius, or can have that temperature in other cases.

[0072] Referring to Figure 3B, a more specific example of a method 300 for manufacturing a multi-asymmetric membrane is shown, and the steps include the following. 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 onto a surface, such as 310. The device 306 can form the film 310 by any useful method, such as an extrusion method (using a die), a casting method, a coating method, etc. As shown, 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 (belt 320) onto which the film is applied are at a temperature in the ambient range, for example, 20 to 30 degrees Celsius, or 20 to 25 degrees Celsius.

[0073] After forming the film 320, the film is exposed to air containing a certain amount of moisture (humidity). The moisture in the air is absorbed by the liquid coating composition of the film. The moisture, i.e., water, acts as a poor solvent within the liquid coating composition, causing the polymer in the liquid coating composition to form a polymer-lean phase and a polymer-rich phase. Following the step of exposing the film 310 to air containing moisture, the film is immersed in an aqueous bath 330 (i.e., a "coagulation bath") having an ambient temperature, for example, 20 to 30 degrees Celsius, or 20 to 25 degrees Celsius.

[0074] According to one example, the multi-asymmetric porous polymer membrane 10 of FIG. 1A was prepared according to the described method by forming a film of the liquid polymer composition on a solid surface. The liquid polymer composition contained about 27 weight percent n-methylpyrrolidone, 63 weight percent triethylene glycol, and 10 weight percent polyethylene sulfone. The liquid coating composition and the film were brought to room temperature and the film was formed. Then, the film on the surface was exposed to ambient air with 50 percent relative humidity for 45 seconds. Then, the film was immersed in water at 20 degrees Celsius. The resulting porous polymer membrane 10 is shown in FIG. 1A.

[0075] The membrane 10 was tested and found to have a bubble point of 67.9 pounds per square inch (psi) and a flux of 35704 liters per square meter per hour per bar (LMHB). The flux was measured at 14.2 psi, 21 degrees Celsius, over a period of 1 minute, with a surface area of 13.8 cm 2It is measured by measuring the total volume of DI water passing through the membrane. The bubble point method is based on the premise that for a specific fluid and pore size, the pressure required to push air bubbles into the pores with a certain degree of wetting is inversely proportional to the size of the pores. Higher bubble point values are associated with smaller pore sizes. To determine the bubble point of a porous material, a sample of the porous material is immersed and wetted in DI water at a temperature of 20 - 25 degrees Celsius (e.g., 22 degrees Celsius). Compressed air is used to apply gas pressure to one side of the sample, and the gas pressure is gradually increased. The minimum pressure at which gas flows through the sample is called the bubble point.

[0076] According to another example, the multi-asymmetric porous polymer membrane 110 of FIG. 2A was prepared according to the described method by forming a film of a liquid polymer composition on a solid surface. The liquid polymer composition contained about 27 weight percent n-methylpyrrolidone, 63 weight percent triethylene glycol, and 10 weight percent polyethylene sulfone. The liquid coating composition and the film were brought to room temperature to form the film. The film on the surface was then exposed to ambient air at 50 percent relative humidity for 60 seconds. The film was then immersed in water at 20 degrees Celsius. The resulting porous polymer membrane 110 is shown in FIG. 2A. The membrane 110 was tested and found to have a bubble point of 48.9 psi and a flow rate of 90149 LMHB.

[0077] In some embodiments, the membranes described herein have a bubble point of at least 40 psi, at least 45 psi, at least 50 psi, at least 55 psi, at least 60 psi, at least 65 psi, at least 70 psi, and all ranges and sub-ranges therebetween, and / or a flow rate of at least 30,000 LMHB, at least 35,000 LMHB, at least 40,000 LMHB, at least 45,000 LMHB, at least 50,000 LMHB, at least 55,000 LMHB, at least 60,000 LMHB, at least 65,000 LMHB, at least 70,000 LMHB, at least 75,000 LMHB, at least 80,000 LMHB, at least 85,000 LMHB, at least 90,000 LMHB, at least 95,000 LMHB, at least 100,000 LMHB, and all ranges and sub-ranges therebetween.

[0078] The membranes described herein, or filters or filter components containing the filter membranes, can be useful in methods of filtering liquid materials to purify or otherwise remove unwanted particles from liquid chemical materials. Generally, the liquid chemical materials can be any of a variety of useful commercially available materials, can be liquid chemical materials useful in any of a variety of different industrial, commercial, or laboratory applications, or can be liquid chemical materials for use in medical, pharmaceutical, life science, and food industries.

[0079] The membrane can be included within a larger filter structure such as a filter or filter cartridge used in a filtration system. The filtration system arranges the filter membrane, for example, as part of a filter or filter cartridge in the liquid flow path so that the filter membrane can remove impurities and contaminants in the form of particles in the micron or sub-micron size range from the liquid, and flows the liquid through the filter membrane. The structure of the filter or filter cartridge includes one or more of various additional materials and structures that support the porous filter membrane within the filter so that the fluid flows from the filter inlet through the filter membrane and through the filter outlet, thereby passing through the filter membrane when passing through the filter. The filter membrane supported by the filter structure can be in any useful shape, such as a pleated cylinder, a cylindrical pad, one or more non-pleated (flat) cylindrical sheets, a pleated sheet, etc.

[0080] An example of a filter structure that includes the filter membrane in the form of a pleated cylinder can be prepared to include the following components, any or none of which may be included in the filter structure but may not be necessary, namely, a rigid or semi-rigid, typically cylindrical core that supports the pleated cylindrical membrane in the internal channel of the pleated cylindrical filter membrane; a rigid or semi-rigid cage that supports or surrounds the outside of the pleated cylindrical filter membrane outside the filter membrane; optional end pieces or "pucks" located at each of the two opposing ends of the pleated cylindrical filter membrane; and a filter housing that includes an inlet and an outlet. The membrane is supported at a position between the inlet and the outlet, and the liquid is flowed through the membrane and passed from the inlet to the outlet. The filter housing can have any useful desired size, shape, and material, and preferably can be made of a suitable polymeric material.

[0081] As an example, FIG. 4 shows a filter component 430, which is a product having a pleated cylindrical component 410 and end pieces 422, and having other optional components. The cylindrical component 410 includes the porous membrane 412 described herein and includes folds or pleats 420, i.e., is “pleated”. The end piece 422 is attached (e.g., “potted”) to one end of the cylindrical filter component 410. The end piece 422 can preferably be made of a melt processable polymer material. A core (not shown) can be disposed within the internal opening or “channel”) 424 of the pleated cylindrical component 410, and a cage (not shown) can be disposed around the outside of the pleated cylindrical component 410. A second end piece (not shown) can be attached (“potted”) to the second end of the pleated cylindrical component 420. The resulting pleated cylindrical component 420 having two opposed potted ends as well as optional core and cage can then be disposed within a filter housing, which includes an inlet and an outlet and is configured such that fluid entering the inlet must necessarily pass through the membrane 412 before exiting the filter at the outlet.

[0082] The filter housing can have any useful desired size, shape, and material, and preferably is a fluorinated or non-fluorinated polymer, such as nylon, polyethylene, or a fluorinated polymer, such as poly(tetrafluoroethylene-co-perfluoro(alkyl vinyl ether)), TEFLON® perfluoroalkoxy alkane (PFA), perfluoromethyl alkoxy (MFA), or another suitable fluoropolymer (e.g., a perfluoropolymer).

[0083] In a first aspect, a porous polymer membrane having a membrane thickness and having a multi-asymmetric morphology along the membrane thickness includes a membrane average pore size across the membrane thickness, two open regions having an average pore size and a maximum pore size value greater than the membrane average pore size, and a dense region having an average pore size and a minimum pore size value less than the membrane average pore size, the dense region being located between the two open regions.

[0084] In a second aspect according to the first aspect, the membrane has a pore size profile along the membrane thickness that includes an open region - a dense region - an open region.

[0085] In a third aspect according to the first aspect, the membrane has a pore size profile along the membrane thickness that includes an open region - a dense region - an open region - a dense region - an open region.

[0086] In a fourth aspect according to any of the previous aspects, the membrane has a bubble point exceeding 40 psi.

[0087] In a fifth aspect according to any of the previous aspects, the membrane has a flow rate of at least 30,000 LMHB.

[0088] In a sixth aspect according to any of the previous aspects, the membrane has a membrane average pore size in the range of 0.2 to 1 micron.

[0089] In a seventh aspect according to any of the previous aspects, the membrane includes polyethersulfone.

[0090] In an eighth aspect according to any of the previous aspects, the membrane includes polysulfone.

[0091] In a ninth aspect, the filter cartridge includes the membrane of any of the previous aspects.

[0092] In a tenth aspect, a method for preparing a porous polymer membrane having a film thickness and a multi-asymmetric morphology along the film thickness includes forming a liquid polymer composition film, wherein the liquid polymer composition contains a polymer selected from the group consisting of polyethersulfone and polysulfone dissolved in an organic solvent containing a strong solvent and a co-solvent, forming the liquid polymer composition film, exposing the film to air having a relative humidity of at least 20 percent to absorb moisture in the air by the liquid coating composition, concentrating the polymer by the polymer-rich phase and not concentrating it much in the polymer-lean phase, and after exposing the film to air, immersing the film in an aqueous bath to precipitate the polymer as a porous polymer membrane.

[0093] In an eleventh aspect according to the tenth aspect, the film is exposed to air having a relative humidity of at least 20 percent for at least 30 seconds.

[0094] 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 the range of 20 to 30 degrees Celsius.

[0095] In a thirteenth aspect according to any one of the tenth to twelfth aspects, the air has a temperature in the range of 20 to 30 degrees Celsius.

[0096] In a fourteenth aspect according to any one of the tenth to thirteenth aspects, the liquid polymer composition has a temperature in the range of 20 to 30 degrees Celsius.

[0097] In a fifteenth aspect according to any one of the tenth to fourteenth aspects, the aqueous bath has a temperature in the range of 20 to 30 degrees Celsius.

[0098] In a sixteenth aspect according to any one of the tenth to fifteenth aspects, the strong solvent is n-methylpyrrolidone.

[0099] In a 17th aspect according to any one of the 10th to 16th aspects, the co-solvent is a polyol.

[0100] In an 18th aspect according to any one of the 10th to 17th aspects, the co-solvent includes diethylene glycol, triethylene glycol, or a combination thereof.

[0101] In a 19th aspect according to any one of the 10th to 18th aspects, the coating composition includes 5 to 20 weight percent of a polymer, 10 to 40 weight percent of a strong solvent, and 30 to 80 weight percent of a co-solvent.

[0102] In a 20th aspect according to any one of the 10th to 19th aspects, the porous membrane has a membrane average pore diameter across the membrane thickness, two open regions having an average pore diameter and a maximum pore diameter larger than the membrane average pore diameter, and a dense region having an average pore diameter and a minimum pore diameter smaller than the membrane average pore diameter, the dense region being located between the two open regions.

Claims

1. A porous polymer membrane having a film thickness and a multi-asymmetric morphology along the film thickness, the average pore diameter across the film thickness, two open regions having an average pore diameter and a maximum pore diameter larger than the average pore diameter, a dense region having a minimum pore diameter smaller than the average pore diameter and the average pore diameter across the film thickness, the dense region being located between the two open regions and comprising a porous polymer membrane.

2. A membrane according to claim 1, having a pore diameter profile along the film thickness and comprising an open region - dense region - open region.

3. A membrane according to claim 1, having a pore diameter profile along the film thickness and comprising an open region - dense region - open region - dense region - open region.

4. The membrane according to claim 1, wherein the membrane has a bubble point exceeding 40 psi.

5. The membrane according to claim 1, wherein the membrane has a flow rate of at least 30,000 LMH B.

6. The membrane according to claim 1, wherein the membrane has an average pore diameter in the range of 0.2 to 1 micron.

7. The membrane according to claim 1, wherein the membrane contains polyethersulfone.

8. The membrane according to claim 1, wherein the membrane contains polysulfone.

9. A filter cartridge comprising the membrane according to any one of claims 1 to 8.

10. A method for preparing a porous polymer membrane having a film thickness and a multi-asymmetric morphology along the thickness, forming a liquid polymer composition film, wherein the liquid polymer composition contains a polymer selected from the group consisting of polyethersulfone and polysulfone dissolved in an organic solvent containing a strong solvent and a co-solvent, forming a liquid polymer composition film; exposing the film to air having a relative humidity of at least 20 percent in order to absorb moisture in the air with a liquid coating composition, concentrate the polymer with a polymer-rich phase and not concentrate it too much in the polymer-lean phase; after exposing the film to air, immersing the film in an aqueous bath to precipitate the polymer as a porous polymer membrane and comprising a method.

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

12. The method according to 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 20 to 30 degrees Celsius.

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

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

15. The method according to claim 10, wherein the aqueous bath has a temperature in the range of 20 to 30 degrees Celsius.

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

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

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

19. The coating composition comprises 5 to 20 weight percent of a polymer, 10 to 40 weight percent of a strong solvent, and 30 to 80 weight percent of a co-solvent The method according to claim 10.

20. The porous membrane comprises a membrane average pore size across the membrane thickness, two open regions having an average pore size and a maximum pore size value greater than the membrane average pore size, and a dense region having an average pore size and a minimum pore size value less than the membrane average pore size, the dense region being located between the two open regions The method according to claim 10.

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