Method for producing a charge-excluding filtration membrane and its use in dairy product applications

Abstract

The charged ultrafiltration membrane is synthesized by thermally initiated free radical polymerization of sodium styrene sulfonate within the pores of the ultrafiltration precursor membrane. The resulting graft chains of the charged UF membrane provide a significant negative charge to maintain almost complete rejection of proteins at a significant ultraflux.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application was filed as a PCT international patent application on July 6, 2023, and claims the benefit and priority of U.S. Provisional Patent Application No. 63 / 359,228, filed on July 8, 2022, the disclosure of which is hereby incorporated by reference in its entirety.

Background Art

[0002] The present invention generally relates to the use of ultrafiltration membranes for fractionating aqueous streams containing proteins, sugars, and minerals. The proteins are retained by the membrane, while the sugars, minerals, and water permeate the membrane.

Prior Art Documents

Non - Patent Documents

[0003]

Non - Patent Document 1

Summary of the Invention

Means for Solving the Problems

[0004] This summary is provided to introduce, in a simplified form, a selection of concepts that are further described herein. This summary is not intended to identify essential or key features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0005] In accordance with aspects of the present invention, a method for manufacturing an ultrafiltration membrane, comprising: (a) contacting a precursor membrane comprising polyethersulfone (PES) and / or polysulfone (PSF) with an aqueous solution comprising sodium styrenesulfonate and a free radical initiator; and (b) curing the precursor membrane to form an ultrafiltration membrane, wherein the ultrafiltration membrane has sulfonated polystyrene bound to the pores of the ultrafiltration membrane and / or bound to the outer surface of the ultrafiltration membrane, is disclosed herein.

[0006] Ultrafiltration membranes are also encompassed herein. Generally, an ultrafiltration membrane can include (I) a polyethersulfone (PES) and / or polysulfone (PSF) membrane, and (II) sulfonated polystyrene bound to the pores of the ultrafiltration membrane and / or bound to the outer surface of the ultrafiltration membrane. More often, the ultrafiltration membrane further includes a mechanical support layer disposed under (and attached to) the membrane. The mechanical support layer can include polypropylene (PP) or polyester (PET), such as non-woven PP or PET as an example.

[0007] According to other aspects of the present invention, an ultrafiltration module and a milk fractionation system are provided herein. A representative ultrafiltration module can include (1) an inlet for a feed stream (e.g., a dairy product such as whole milk or skim milk), (2) one or more of the ultrafiltration membranes disclosed herein in any suitable configuration such as a hollow fiber configuration, a tubular configuration, or a spiral wound configuration, (3) a first outlet for a UF retentate stream, and (4) a second outlet for a UF permeate stream. A representative milk fractionation system can include (A) one or more of the ultrafiltration modules disclosed herein, (B) a nanofiltration module, and (C) a reverse osmosis module.

[0008] Methods for manufacturing dairy compositions are also included herein. One such method may include: (i) ultrafiltering a dairy product using any of the ultrafiltration membranes disclosed herein (or any of the ultrafiltration modules disclosed herein) to produce a UF permeate fraction and a UF retentate fraction; (ii) nanofiltrating the UF permeate fraction to produce an NF permeate fraction and an NF retentate fraction; (iii) subjecting the NF permeate fraction to a reverse osmosis process to produce an RO permeate fraction and an RO retentate fraction; and (iv) combining at least two of the UF retentate fraction, the RO permeate fraction, the RO retentate fraction, and the fat-rich fraction to form a dairy composition.

[0009] Both the foregoing summary and the following detailed description are exemplary and for illustrative purposes only. Accordingly, neither the foregoing summary nor the following detailed description should be considered limiting. Further, features or variations may be provided in addition to those described herein. For example, a particular aspect may be directed to various combinations and sub-combinations of the features described in the detailed description.

[0010] The following figures form a part of this specification and are included to further illustrate certain aspects of the invention. The invention may be better understood by reference to these figures in combination with the detailed description and examples.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

BEST MODE FOR CARRYING OUT THE INVENTION

[0012] Definitions To more clearly define the terms used in this specification, the following definitions are provided. Unless otherwise indicated, the following definitions apply to the present disclosure. When a term used in the present disclosure is not specifically defined herein, its definition may be applied as long as it does not conflict with other disclosures or definitions applied herein and does not render the claims to which it applies unclear or invalid, IUPAC Compendium of Chemical Terminology, 2 nd Ed (IUPAC Compendium of Chemical Terminology, 2nd Edition) (1997) can be applied. As long as the definitions or usage provided by the documents incorporated herein by reference conflict with the definitions or usage provided herein, the definitions or usage provided herein shall prevail.

[0013] In this specification, the features of the subject matter are described such that within a particular aspect, combinations of different features can be envisioned. For all aspects and / or features disclosed herein, whether or not accompanied by an explicit description of a particular combination, all combinations that do not adversely affect the systems, compositions, processes, and / or methods described herein are contemplated. Further, unless explicitly listed separately, all aspects and / or features disclosed herein can be combined to describe an inventive system, composition, process, and / or method consistent with the present invention.

[0014] In the present disclosure, compositions, methods, and systems are often described in terms of "comprising" various materials, steps, or components, but unless otherwise specified, the compositions, methods, and systems can also "consist essentially of" or "consist of" various materials, steps, or components.

[0015] The terms "a", "an", and "the" are intended to include a plurality of alternatives, e.g., at least one, unless otherwise specified. For example, the disclosure of "an ultrafiltration membrane" means, unless otherwise specified, including one or more ultrafiltration membranes.

[0016] In the disclosed methods, the terms "combining" and "contacting" include combining or contacting materials in any order, in any manner, and for any length of time, unless otherwise specified. For example, the materials can be blended, mixed, treated, impregnated, etc.

[0017] In the present invention, several types of ranges are disclosed. When any type of range is disclosed or claimed, the intention is to individually disclose or claim each possible numerical value that such a range can reasonably encompass, including the endpoints of the range, as well as the sub-ranges and combinations of sub-ranges included therein. For example, in an embodiment of the present invention, the reaction aqueous solution in step (a) can contain 1 to 25 wt.% of sodium styrenesulfonate. By the disclosure that the aqueous solution contains 1 to 25 wt.% of sodium styrenesulfonate, the intention is to state that the mass percentage can be any amount within the range, e.g., any range or combination of ranges within 1 to 25 wt.%, such as 2 to 20 wt.% of sodium styrenesulfonate, or 5 to 15 wt.% of sodium styrenesulfonate, etc. Similarly, all other ranges disclosed herein should be interpreted in the same manner as this example.

[0018] Generally, quantities, sizes, formulations, parameters, ranges, or other quantities or characteristics are “about” or “approximately,” whether or not explicitly stated as such. Whether or not modified by the term “about” or “approximately,” the claims include equivalents of the quantity or characteristic.

[0019] Any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, but typical methods, devices, and materials are described herein.

[0020] All publications and patents mentioned herein are hereby incorporated by reference in their entirety for the purpose of, for example, describing and disclosing the configurations and methodologies described in the publications and patents, which may be used in connection with the invention described herein.

[0021] Disclosed herein are ultrafiltration membranes comprising a polyethersulfone (PES) and / or polysulfone (PSF) membrane and sulfonated polystyrene bound to the pores and / or on the outer surface of the ultrafiltration membrane; methods for producing ultrafiltration membranes; ultrafiltration modules containing a plurality of ultrafiltration membranes; milk fractionation systems containing a plurality of ultrafiltration modules; and methods for producing dairy compositions utilizing ultrafiltration membranes and ultrafiltration modules.

[0022] Ultrafiltration (UF) membranes are typically characterized by a molecular weight cut-off (MWCO, the molecular weight or size of the molecules retained by the membrane). For example, when separating and concentrating proteins in the dairy industry, the MWCO of the ultrafiltration membrane may be 10 kDa, and thus at least 90% of the material with a molecular weight above 10,000 daltons is retained (the retentate), and the lower molecular weight species pass through (the permeate).

[0023] The object of the present invention is to produce an ultrafiltration membrane that maintains a high overall flux while maintaining a high protein rejection rate. Thus, the membrane retains the same amount of protein at a higher throughput. For example, the charged ultrafiltration membranes disclosed herein can retain protein at the same level as a standard 10 kDa MWCO ultrafiltration membrane, but with a significantly higher flux (e.g., 20 - 70% higher). This is achieved by modifying a precursor PES or PSF membrane with attached sulfonated polystyrene functional groups to form a charged polymer membrane that rejects proteins based on both molecular size and charge.

[0024] This object is mainly directed towards dairy applications, such as milk-based beverages and related products, since the main component is protein. However, the methods, techniques, membranes, etc. described herein are not limited to milk or dairy applications only and can generally be applied to non-alcoholic and alcoholic beverages, as well as end-use applications and products based on these materials, including but not limited to beer, wine, water, wastewater treatment, juice, oatmeal, almonds / nuts, peas, rice, seeds, grains, and other plant material processing.

[0025] Another object of the present invention is to form a charged polymer membrane using a free radical polymerization process instead of a cationic polymerization process that creates an ionic bond between the polymer chain and the precursor membrane. Any curing method can be used, but in certain embodiments, a thermal initiator is used for the thermal curing of the free radical polymerization process to thereby form a charged UF membrane.

[0026] Yet another object of the present invention is to form a charged polymer membrane with a simple synthetic scheme using a limited number of process steps, in this case directly using sodium styrene sulfonate. Further, glycerin may be included in the reaction aqueous solution together with sodium styrene sulfonate to promote and maintain wetting of the substrate membrane and pores.

[0027] The present invention is described in detail in connection with ultrafiltration (UF) membranes and methods for their production, but the methods and techniques disclosed herein are also applicable to microfiltration (MF) membranes, as well as nanofiltration (NF) membranes, reverse osmosis (RO) membranes, and forward osmosis (FO) membranes. Thus, MF membranes - and NF membranes and RO membranes and FO membranes - having a combined sulfonated polystyrene moiety are included herein.

[0028] In addition to the food processing applications described above, other non-limiting uses of charged membranes (e.g., charged UF membranes, charged MF membranes) include, among other end-use applications, bacteria removal, virus removal, fermentation processes, ion exchange, isolation of rare elements, and isolation of charged paint particles.

[0029] Ultrafiltration membrane This specification provides various methods for manufacturing ultrafiltration membranes. For example, a method for manufacturing an ultrafiltration membrane can include (a) contacting a precursor membrane comprising polyethersulfone (PES) and / or polysulfone (PSF) with an aqueous solution comprising sodium styrenesulfonate and a free radical initiator, and (b) curing the precursor membrane to form an ultrafiltration membrane, which can include (or consist essentially of or consist of) an ultrafiltration membrane having sulfonated polystyrene bonded to the pores of the ultrafiltration membrane and / or bonded to the outer surface of the ultrafiltration membrane.

[0030] Generally, any feature of any of the methods disclosed herein (e.g., PES polymer, PSF polymer, composition of the aqueous solution, curing process, and temperature and time conditions at which any step is performed, etc.) is described independently herein, and these features can be combined in any combination to further describe the disclosed methods. Further, unless otherwise specified, other process steps can be performed before, during, and / or after any of the steps recited in the disclosed methods. Further, any ultrafiltration membrane produced according to any of the disclosed methods is within the scope of this disclosure and is included herein.

[0031] Referring to step (a), any suitable precursor membrane comprising a polyethersulfone (PES) polymer material and / or a polysulfone (PSF) polymer material can be utilized. Generally, the precursor PES and PSF membranes have a pure water permeability of at least 20% more than that of a standard 10 kDa MWCO PES membrane. The precursor membrane may be fully wet, partially dry, or fully dry before contacting with the reaction aqueous solution in step (a), and glycerin may or may not be used. The precursor membrane can be unsupported or supported. When supported, it is typically supported on a PP or PET non-woven fabric material.

[0032] The aqueous solution in step (a) contains a monomer, typically sodium styrenesulfonate, and a free radical initiator. Without being limited thereto, the aqueous solution often contains, in one embodiment, 1 to 25 wt.% of sodium styrenesulfonate, in another embodiment, 2 to 20 wt.% of sodium styrenesulfonate, and in yet another embodiment, 5 to 15 wt.% of sodium styrenesulfonate. The amount of sodium styrenesulfonate present in the aqueous solution of step (a) is often limited by the desired amount of diffusion into the pores or the desired degree of reaction. For example, if there is too much graft polymer, this may cause the flux loss to be too large.

[0033] Similarly, the amount of free radical initiator in the aqueous solution is not particularly limited. Typical ranges include, but are not limited to, 0.1 to 5 wt.%, 0.2 to 4 wt.%, or 0.5 to 2.5 wt.% of free radical initiator based on the mass of the aqueous solution. The amount of free radical initiator often varies depending on, among other considerations, the amount of sodium styrenesulfonate present in the solution, the type of curing used in step (b), and the general temperature.

[0034] For the aqueous solution, any suitable free radical initiator can be used, although the type of initiator may vary depending on the type of curing used in step (b). In the case of thermal curing, a representative and non-limiting example of a suitable curing agent is potassium persulfate, which is advantageously heat-activated and water-soluble. Other suitable alternatives include sodium persulfate and ammonium persulfate, both of which are water-soluble. Non-water-soluble initiators such as azobisisobutyronitrile (AIBN) can also be used, but are generally used with a suitable co-solvent, typically a water-soluble alcohol.

[0035] Optionally, the aqueous solution may further contain glycerin. Without being bound by theory, it may be advantageous for there to be a small amount of glycerin, generally in the range of 0.1 wt.% to a maximum of 15 - 20 wt.%, in the aqueous solution to improve pore wetting and prevent the pores from drying. If glycerin is utilized, more typically, the aqueous solution contains 0.2 - 8 wt.%, 0.5 - 5 wt.%, or 3 - 15 wt.% of glycerin.

[0036] Step (a) can be carried out at any suitable temperature, such as 10°C to 90°C, 20°C to 70°C, 15°C to 55°C, 20°C to 45°C, or 20°C to 30°C, etc., but is not limited thereto. In these and other embodiments, these temperature ranges mean that step (a) is carried out at a series of different temperatures rather than a single fixed temperature, and the situation where at least one temperature is within each range is also included. The pressure at which step (a) is carried out is not particularly limited and can be high pressure (e.g., 5 psig to 100 psig), atmospheric pressure, or any suitable pressure below atmospheric pressure. In some cases, step (a) is carried out at atmospheric pressure, thereby eliminating the need for a pressurized vessel and the associated costs and complexities. Step (a) can be carried out for any period sufficient for the aqueous solution to diffuse and / or be drawn into the pores of the precursor membrane. Exemplary and non-limiting periods include a wide range of periods such as 10 seconds to 6 hours, 10 seconds to 2 minutes, 15 seconds to 5 hours, 30 seconds to 2 hours, 1 minute to 24 hours, 1 minute to 1 hour, 5 minutes to 6 hours, 15 minutes to 5 hours, or 30 minutes to 2 hours, etc., but are not limited to only these periods. Other suitable temperature, pressure, and time ranges will be readily apparent from this disclosure.

[0037] For step (a), any suitable container or vessel can be used, and as long as the container or vessel can keep the precursor membrane in contact (e.g., immersed) with the aqueous solution for a period sufficient for the aqueous solution to diffuse and / or be drawn into the pores of the precursor membrane, any suitable container or vessel can be used. Step (a) can be carried out batchwise or continuously.

[0038] In one embodiment of the present invention, the precursor membrane may be dry before step (a), but in another embodiment, the precursor membrane may be wet before step (a), and in yet another embodiment, the precursor membrane is dry before step (a), but may be further wetted with water before step (a).

[0039] After the execution of step (a), the precursor membrane is in contact with an aqueous solution, but may be partially dried before step (b). In particular, before step (b), excess water can be removed from the precursor membrane. The precursor is only partially dried, and a suitable amount of water (and sodium styrenesulfonate monomer and initiator) remains in the pores.

[0040] Referring to step (b), the precursor membrane is cured to form an ultrafiltration membrane. Thus, the ultrafiltration membrane contains sulfonated polystyrene bound to the pores of the ultrafiltration membrane and bound on the outer surface of the ultrafiltration membrane. As used herein, the term sulfonated polystyrene encompasses substituted styrene moieties, such as methylated polystyrene groups.

[0041] In step (b), any suitable curing method can be utilized, such as subjecting the precursor membrane to UV irradiation (initiation of UV light reaction) or electron beam irradiation. However, it has been found that the precursor membrane can be conveniently cured by heat in this specification. Thus, in one aspect of the present invention, the curing in step (b) may include subjecting the precursor membrane to a high temperature, which is typically in the range of 70 to 200 °C, but is not limited thereto. Other suitable temperature ranges include 70 to 150 °C, 95 to 180 °C, or 80 to 130 °C. In these and other aspects, these temperature ranges mean that the thermosetting is carried out at a series of different temperatures rather than a single fixed temperature, and the situation where at least one temperature is within each range is also included. The maximum curing temperature often varies depending on the melting point or softening point of the polyethersulfone (PES) and / or polysulfone (PSF) used for the precursor membrane, as well as the melting point or softening point of any support layer (which may be, for example, PP-based or PET-based).

[0042] In one aspect, the ultrafiltration membrane after step (b) may be rinsed with water, which can be done at the same location as step (b) or at a different location. In another aspect, the ultrafiltration membrane after step (b) may be rinsed with water, contacted with an aqueous glycerin solution, and then dried.

[0043] Consistent with aspects of the present invention, the precursor membrane and the ultrafiltration membrane may further contain a mechanical support layer disposed under (and attached to) each membrane. This additional layer generally exists prior to step (a). The mechanical support layer can be composed of any suitable material, but is often PP-based or PET-based, for example, non-woven PP or non-woven PET. As described above, depending on the composition of the mechanical support layer and its melting point or softening temperature, the curing temperature of step (b) can vary. In order to obtain a wide pH range during membrane use, PP can often be used as the support layer.

[0044] Also included herein is an ultrafiltration membrane that can include (I) a polyethersulfone (PES) and / or polysulfone (PSF) membrane, and (II) sulfonated polystyrene bound to the pores of the ultrafiltration membrane and / or bound to the outer surface of the ultrafiltration membrane. These ultrafiltration membranes can be produced according to any of the methods and processes described herein. Further, the ultrafiltration membrane may contain, and often does contain, a mechanical support layer disposed under (and attached to) the membrane. As described above, suitable PP or PET non-woven materials are typically used as the mechanical support layer. A photograph of a representative precursor membrane is shown in Figure 1, with the upper layer being PES or PSF and the lower layer being a PP or PET mechanical support layer. Generally, the layers are interconnected and the two-layer configuration does not peel. Figure 2 shows polystyrene bound to the pores of the ultrafiltration membrane (sulfonic acid groups or sulfonate groups are not explicitly shown, only the polystyrene grafts are shown).

[0045] In one aspect, the disclosed ultrafiltration membrane can be characterized by a water permeability that is at least 20% higher, optionally at least 25% higher, at least 30% higher, at least 40% higher, at least 50% higher, at least 100% higher, or at least 200% higher than the water permeability of a 10 kDa molecular weight cut-off (MWCO) membrane. Additionally or alternatively, the ultrafiltration membranes disclosed herein have at least 120 L / m 2- Time-bar, for example at least 150, at least 200, at least 250, at least 300, at least 500, or at least 1000 L / m 2 - It can be characterized by the water permeability of the time-bar. The water permeability test will be further described in the following examples.

[0046] In one aspect, the disclosed ultrafiltration membrane is characterized by a rejection percentage greater than or equal to the rejection percentage of whey protein of a 10 kDa MWCO membrane, and in fact, typically greater than 90%. Additionally or alternatively, the ultrafiltration membranes disclosed herein can be characterized by a rejection percentage of at least 90%, and in some cases at least 92%, or at least 95% of whey protein for the second 5 mL (10 mL) and third 5 mL (15 mL) sample increments. The whey protein rejection test will be further described in the following examples.

[0047] To determine whether the sulfonated polystyrene graft is not covalently bonded within the pores of the membrane, the membrane can be subjected to high-concentration brine extraction, which disrupts the electrostatic interactions used in the cationic polymerization material. Next, when the extract is analyzed by UV-Vis, it has a peak in the range of 270 - 300 nm (the aromatic ring of sulfonated polystyrene can be identified in this nm range). This indicates that the graft is leaching from the membrane, and since the graft may enter the product stream, it is not acceptable for food applications. In contrast, the ultrafiltration membrane (including covalently bonded sulfonated polystyrene) does not have absorbance in this wavelength range due to covalent bonding.

[0048] Systems and processes using ultrafiltration membranes The disclosed UF membrane contains sulfonated polystyrene bound to the pores of the UF membrane and / or bound to the outer surface of the UF membrane and can be used in various devices, systems, and processes. One such device can be a microfiltration module including (1) an inlet for a feed stream, (2) one or more of any suitable configuration of the microfiltration membranes disclosed herein, such as a hollow fiber configuration, a tubular configuration, or a spiral wound configuration, (3) a first outlet for a UF retentate stream, and (4) a second outlet for a UF permeate stream. The feed stream entering the microfiltration module from the inlet can be any suitable dairy product, such as whole milk or skim milk, but is not limited thereto. Generally, the microfiltration module uses, for example, a plurality of microfiltration membranes in a spiral wound configuration.

[0049] A representative milk fractionation system can include (A) one or more of the microfiltration modules disclosed herein, (B) a nanofiltration module, and (C) a reverse osmosis module. Other milk fractionation systems encompassed herein can include one or more of any of the microfiltration modules disclosed herein, a nanofiltration module, and a forward osmosis module; or one or more of any of the microfiltration modules disclosed herein and a nanofiltration module; or one or more of any of the microfiltration modules disclosed herein and a reverse osmosis module; or one or more of any of the microfiltration modules disclosed herein and a forward osmosis module. Typically, the milk fractionation system uses a plurality of microfiltration modules.

[0050] Also provided herein is a method for manufacturing a dairy product composition using an ultrafiltration membrane. Exemplary methods can include the following steps: (i) ultrafiltering a milk product using any of the ultrafiltration membranes disclosed herein (or any of the ultrafiltration modules disclosed herein) to produce a UF permeate fraction and a UF retentate fraction; (ii) nanofiltrating the UF permeate fraction to produce an NF permeate fraction and an NF retentate fraction; (iii) subjecting the NF permeate fraction to a reverse osmosis step to produce an RO permeate fraction and an RO retentate fraction; and (iv) combining at least two of the UF retentate fraction, the RO permeate fraction, the RO retentate fraction, and the fat-rich fraction to form a dairy product composition. For example, in one aspect, the dairy product composition can contain at least the UF retentate fraction and the RO retentate fraction, and in another aspect, the dairy product composition can contain at least the UF retentate fraction, the RO retentate fraction, and the fat-rich fraction (cream).

Example

[0051] The present invention is further illustrated by the following examples, which should in no way be construed as limiting the scope of the present invention. After reading the description herein, those skilled in the art will be able to conceive of various other aspects, modifications, and equivalents without departing from the spirit of the present invention or the scope of the appended claims.

[0052] The general procedure for preparing the ultrafiltration membrane is as follows. First, a 4.70 cm diameter precursor membrane sample made of either polyethersulfone (PES) or polysulfone (PSF) with a nominal MWCO of 10 - 20 kDa was forcedly wetted with water in a dead-end flow cell at 90 psig for a specified time with the membrane side up. For a precursor membrane with a nominal MWCO of 10 - 20 kDa, the time was 2 minutes.

[0053] The precursor membrane was taken out of the dead-end cell and immersed in a reaction aqueous solution containing about 83.33 wt.% deionized (DI) water solution containing about 15.87 wt.% sodium styrenesulfonate, about 0.79 wt.% potassium persulfate, and 5 vol% glycerin at room temperature and normal pressure for 1 hour. A representative schematic diagram of the diffusion of the reaction aqueous solution into the pores of the precursor membrane before curing is shown in Figure 3. Diffusion is one mechanism for introducing monomers into the pores, and another mechanism is to suck up the reaction aqueous solution into the open and dry pores. The dry pores may be partially filled with glycerin from the drying process.

[0054] After 1 hour, the impregnated membrane was taken out of the solution, placed with the membrane surface up on a polypropylene mesh on a glass plate, and put into an oven at about 100 °C for 1 hour for thermal curing. After the cured ultrafiltration membrane was taken out of the oven and cooled, the membrane was put into DI water and rinsed forcibly at 90 psig for 2 minutes in a dead-end flow cell. Next, the ultrafiltration membrane was immersed in a DI aqueous solution containing 5 vol% glycerin for 30 minutes, taken out of the glycerin solution, and dried.

[0055] The precursor membranes were obtained from Solecta and are summarized in Table 1 (Table 1) together with the prepared ultrafiltration membranes having attached sulfonated polystyrene groups.

[0056]

Table 1

[0057] The pure water permeability of the membranes of Examples 1 to 5 was measured with a dedicated setup equipped with a stainless steel membrane holder. After rapidly increasing to 90 psig for 2 minutes, the water permeability was measured. The process fluxes and protein rejection tests of Examples 1 to 5 were carried out using an AMICON stirred cell. The protein rejection test was carried out at a feed concentration of 548 ppm using whey protein isolate in DI water. The solute concentration was measured by a UV-Vis spectrophotometer at 279 nm.

[0058] Figure 4 summarizes the results of the water permeability tests of the precursor membranes and the modified ultrafiltration membranes of Examples 1-5. The flux was measured using a stainless-steel dead-end cell supplied from a stainless-steel tank pressurized with compressed air. To ensure complete wetting of the membrane, the supply pressure was first rapidly increased to 90 psig for 2 minutes. Flux data were measured at five pressures in 5 psig increments, measured and collected over 60 seconds, and each pressure measurement was repeated three times. The permeabilities of the precursor membranes of Example 1 (PE10), Example 2 (PE20C), and Example 4 (PS35C) are shown as a single bar in Figure 4, while for Example 3 (PE20) and Example 5 (PS35), the permeabilities before impregnation / reaction (precursor membrane) and after impregnation / reaction (charged ultrafiltration membrane) are shown.

[0059] The PE20 and PS35 membranes of Example 3 and Example 5 had much higher ultra-pure water permeabilities prior to reaction / impregnation. However, the PE20 membrane of Example 3 had a much lower permeability after reaction / impregnation than the PE10 membrane of Example 1. The data in Figure 4 show the effect of sulfonated polystyrene bound to the pores on the pore size of the membrane and thus the decrease in water permeability.

[0060] Unexpectedly, the PS35 membrane of Example 5 maintained a significantly high water permeability even after reaction / impregnation, much higher than the PE10 membrane of Example 1, with a surprisingly high ultra-water permeability exceeding 300 L / m 2 -hour-bar.

[0061] Figures 5-6 summarize the protein rejection results of the precursor membranes and the modified ultrafiltration membranes of Examples 1-5. Protein isolates in deionized (DI) water were used to determine the protein rejection rate (%) of various membranes. The rejection study was performed using a dead-end AMICON stirred cell filled with approximately 50 mL of a protein solution, setting the stirrer to 3, and applying a pressure of 50 psig. The supply concentration of the whey protein was 548 ppm, and the permeate was collected in fractions, distinguishing the initial burst (first 5 mL increment) and the protein rejection rate after fouling of the membrane surface. The protein concentration was measured with a UV-visible spectrophotometer at 279 nm.

[0062] The protein rejection percentages of the precursor membranes of Example 1 (PE10), Example 2 (PE20C), and Example 4 (PS35C), and the modified / charged ultrafiltration membranes of Example 3 (PE20) and Example 5 (PS35) are shown as three bars in FIGS. 5-6. Each bar represents a 5 mL increment of the permeate collected from a 50 mL feed, so the second bar represents the second 5 mL increment of the permeate (10 mL total permeate), and the third bar represents the third 5 mL increment of the permeate (15 mL total permeate).

[0063] The rejection rates of the whey protein isolate were very high for all membranes tested with a feed containing 548 ppm whey protein. In the second and third increments, the rejection percentages of all whey proteins exceeded 90%, except for the unmodified precursor membrane (PS35C) of Example 4. In FIG. 5, the rejection rate of the first 5 mL of permeate after reaction (Example 3 - PE20) was lower than that of the control membrane (Example 2 - PE20C). The rejection percentages of the subsequent permeate fractions of Example 3 (PE20) (the next two 5 mL permeates) were not significantly different from those of Example 1 (10 kDa MWCO PE10).

[0064] Unexpectedly, for the PS35 membrane of Example 5 after reaction / impregnation, the rejection percentages of whey protein exceeded 98% in the second and third increments, representing a significant improvement in protein rejection over Example 4. Additionally, at all sample increment data points, the modified PS35 ultrafiltration membrane had a protein rejection rate equal to or slightly better than that of the unmodified PE10 membrane of Example 1.

[0065] Another way to confirm the success of the modification of the precursor membrane is by dye adsorption. Positively charged dyes are adsorbed by electrostatic interaction with the negatively charged membrane. Figure 7 shows photographs of the absorption of methylene blue dye at different curing times (heating times) at 100 °C by the modified PS35 ultrafiltration membrane of Example 5. The methylene blue dye was retained by the sulfonated polystyrene bound to the pores of the membrane. The sulfonated polystyrene moiety was not removed from the pores of the membrane even when the modified membrane was washed or rinsed with water.

Claims

1. A method for manufacturing an ultrafiltration membrane: (a) A step of contacting a precursor film containing polyethersulfone (PES) and / or polysulfone (PSF) with an aqueous solution containing 1 to 25 wt.% sodium styrenesulfonate and a free radical initiator; and (b) A step of forming the ultrafiltration membrane by thermal curing the precursor membrane, Includes, A method wherein the ultrafiltration membrane has sulfonated polystyrene bonded to the pores of the ultrafiltration membrane and / or bonded to the outer surface of the ultrafiltration membrane.

2. The method according to claim 1, wherein the precursor film is dried before step (a).

3. The method according to claim 1, wherein the precursor film is wet before step (a).

4. The method according to claim 1, wherein the precursor film is dried before step (a) and moistened in water before step (a).

5. The method according to any one of claims 1 to 4, wherein the aqueous solution further comprises glycerin.

6. The method according to any one of claims 1 to 4, wherein the aqueous solution further comprises 0.1 to 20 wt.%, 0.2 to 8 wt.%, 0.5 to 5 wt.%, or 3 to 15 wt.% of glycerin.

7. The method according to any one of claims 1 to 4, wherein the aqueous solution contains 2 to 20 wt.% or 5 to 15 wt.% sodium styrenesulfonate.

8. The method according to any one of claims 1 to 4, wherein the aqueous solution contains 0.1 to 5 wt.%, 0.2 to 4 wt.%, or 0.5 to 2.5 wt.% of a free radical initiator.

9. The method according to any one of claims 1 to 4, wherein the free radical initiator comprises potassium persulfate, sodium persulfate, ammonium persulfate, or any combination thereof.

10. The method according to any one of claims 1 to 4, wherein step (a) is performed for a time sufficient to allow the aqueous solution to diffuse into and / or be drawn up into the pores of the precursor membrane.

11. The method according to any one of claims 1 to 4, wherein the precursor film is partially dried before step (b).

12. The method according to any one of claims 1 to 4, wherein the thermosetting in step (b) includes subjecting the precursor film to a high temperature in the range of 70 to 200°C.

13. The method according to any one of claims 1 to 4, wherein the ultrafiltration membrane after step (b) is rinsed with water.

14. The method according to any one of claims 1 to 4, wherein the ultrafiltration membrane after step (b) is rinsed with water, contacted with an aqueous glycerin solution, and dried.

15. The method according to any one of claims 1 to 4, wherein the precursor membrane and the ultrafiltration membrane include a mechanical support layer positioned and mounted beneath each membrane.

16. The ultrafiltration membrane is characterized by a water permeability that is at least 20%, at least 25%, at least 30%, at least 40%, at least 50%, at least 100%, or at least 200% higher than that of a 10 kDa molecular weight cutoff (MWCO) membrane. The ultrafiltration membrane is characterized by a water permeability of at least 120, at least 150, at least 200, at least 250, at least 300, at least 500, or at least 1000 L / m²-hour-bar. The ultrafiltration membrane is characterized by a whey protein inhibition percentage greater than or equal to that of a 10 kDa MWCO membrane. The ultrafiltration membrane is characterized by a rejection percentage of at least 90%, at least 92%, or at least 95% of whey protein for the second 5 mL (10 mL) and third 5 mL (15 mL) sample increments. The ultrafiltration membrane is characterized by having an FTIR transmittance of less than 100% at a wavenumber (in cm⁻¹) of 950 cm⁻¹. The high brine concentration extract from the ultrafiltration membrane does not have absorbance when measured by UV-Vis at 270-300 nm, or Any combination of those, The method according to any one of claims 1 to 4.

Images