Thermal treatment of ultrafiltration membranes

Abstract

The present invention relates to a method for treating an ultrafiltration membrane, a method for producing an ultrafiltration membrane including the treatment method, and an ultrafiltration membrane obtained by the treatment method or the production method.

Description

Technical Field

[0001] The present invention relates to a method for treating an ultrafiltration membrane, a method for manufacturing an ultrafiltration membrane including the treatment method, and an ultrafiltration membrane obtained by the treatment method or the manufacturing method.

Background Art

[0002] According to IUPAC, the classification of filtration membranes is performed based on their retention characteristics and pore sizes. Based on this recommendation, an average pore size of 0.1 μm to 10 μm is typically classified as a microfiltration membrane, those having an average pore size of 0.01 μm to 0.1 μm are ultrafiltration membranes, nanofiltration membranes have an average pore size of 0.001 μm to 0.01 μm, and membranes having an average pore size below 0.001 μm are typically called reverse osmosis membranes (see Non-Patent Document 1).

[0003] Similar classification can be performed based on the retention of the membrane for specific molecules. This is because accurately determining the pore size of a narrow membrane such as an ultrafiltration membrane, a nanofiltration membrane, or a reverse osmosis membrane is much more difficult and has a higher tendency to fail than in the case of a more open membrane such as a microfilter. The typical measurement of the retention of an ultrafiltration membrane is performed using proteins or other suitable molecules by simulating a typical process. Another method is to determine the sieving curve or sieving coefficient using a widely distributed polymer. Such a method is not limited to one type of membrane as described in ASTM E 1343-90 and can be applied to a wider range of pore sizes.

[0004] Precipitation casting is a method for manufacturing a wide range of filtration membranes, from reverse osmosis or ultrafiltration membranes to microfiltration membranes. During precipitation, the solvent in the polymer solution is replaced by a non-solvent from the coagulation bath through thermodynamic and kinetic processes, forming a porous structure (see Non-Patent Document 2, pages 77 onwards). In particular, the solvent used plays a crucial role in the formation of the membrane's porous structure. Specific solvents make it possible to create membranes with specific characteristics in terms of retention and permeation flux.

[0005] Over the past few years, certain countries / regions have issued regulations concerning the production and import of chemicals and their use (e.g., the EU regulation REACH dated 2006). For example, REACH addresses the continued use of substances of very high concern (SVHCs) because they may have adverse effects on human health and / or the environment. Some solvents frequently used in the manufacture of ultrafiltration membranes may fall into this category. The use of some SVHCs may be subject to prior authorization from the European Chemicals Agency, and applicants for authorization will need to include plans to replace the use of SVHCs with safer alternatives.

[0006] However, it has been found that finding alternative solvents for the manufacture of ultrafiltration membranes based on thermoplastic polymers is extremely difficult. Using alternative solvents often makes it impossible to manufacture ultrafiltration membranes with the same performance characteristics as those produced before the solvent change. Therefore, there is a high demand for adapting / modifying the manufacturing methods of filtration membranes, particularly those via precipitation casting.

[0007] Retention and permeation flux are determined by various structural properties of the membrane. Retention is determined by the size of the smallest pores in the membrane body. In the case of ultrafiltration membranes, the smallest pores are located in the top layer (skin layer). Permeation flux is determined by the pore size (gradient) throughout the entire membrane body, including the skin layer. Relatively high permeation flux with a given retention profile can be achieved, in particular, by a membrane with high asymmetry, having small pores in the skin layer within a desired range, followed by a steep gradient toward larger pores towards the bottom layer.

[0008] Not only the pore size in the skin layer, but also the pore size gradient can be adjusted to some extent by the properties of the polymer solution and the film manufacturing process. Such processes are well known to experts and are described in textbooks (e.g., Non-Patent Document 2, pp. 77 onwards). However, the achievable retention-to-permeation flux ratio is generally limited by the thermodynamic and kinetic aspects of film formation itself. The pore size at each location within the film is determined not only by the polymer content in the cast solution at the start of phase separation, but also by the aggregation time from the start of phase separation to the solidification of the film body. Therefore, a known approach to achieving larger pores in the film body along with small pores in the skin layer of the film is to use two layers of polymer solution for film formation, i.e., a thinner layer of solution containing a large amount of polymer to form the skin layer, along with a larger layer of solution containing a small amount of polymer to form larger pores in the film body (e.g., as disclosed in Patent Document 1). However, such an approach is laborious because it requires not only precise control of layer thickness but also doubles the amount of mixing, pumping, and layering techniques. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] European Patent Application Publication No. 4003579 [Non-patent literature]

[0010] [Non-Patent Document 1] Shang-Tian Yang, "Bioprocessing for Value-Added Products from Renewable Resources," 2007. [Non-Patent Document 2] "Basic Principles of Membrane Technology," 2nd edition, Marcel Mulder [Overview of the project] [Problems that the invention aims to solve]

[0011] Therefore, the fundamental technical problem of the present invention is to provide an ultrafiltration membrane having an improved inhibitory-to-permeability ratio efficiently and quickly, preferably without applying chemical substances of very high concern, or preferably without including chemical substances of very high concern in the final ultrafiltration membrane. [Means for solving the problem]

[0012] Solutions to the above technical problems are realized by embodiments characterized in the claims.

[0013] In particular, the present invention relates to a method for treating an ultrafiltration membrane, A step of preparing an ultrafiltration membrane containing a thermoplastic polymer, A step of thermally treating the ultrafiltration membrane at a temperature of at least 110°C (glass transition temperature of thermoplastic polymer + 10K), This includes methods.

[0014] According to the method of the present invention, it is possible to change the properties of an ultrafiltration membrane based on a thermoplastic polymer (thoroughly rinsed and dried) toward higher retention by heat treatment (thermal treatment) at a high temperature. This process is schematically illustrated in Figure 1. The temperature used for heat treatment of the membrane is the glass transition temperature (T) of the thermoplastic polymer.g It is close to ) but below its melting point. Although not bound by this theory, it is thought that because polymer parts with similar properties come together, the chain mobility of the polymer is temporarily increased, thereby inducing relaxation of chain tension and making it possible to obtain a tighter chain arrangement. g A general method for determining this is described in ASTM E1356, which "includes the assignment of glass transition temperatures of materials using differential scanning calorimetry or differential thermal analysis." Due to the extremely dense alignment of polymers in the so-called skin layer of the ultrafiltration membrane, even pore narrowing in the nanometer range can be achieved, resulting in higher retention (lower cutoff) of the membrane. This preferably avoids significant changes in the microporous substructure where polymer chains or polymer domains cannot interact at the molecular level due to further separation.

[0015] Preferably, no toxic solvents or other substances of high concern are used during the manufacture of the ultrafiltration membrane. Since the ultrafiltration membrane obtained by the single-layer casting process can be processed, it is preferable that manufacturing costs be reduced through a new process design.

[0016] In this invention, the term "ultrafiltration membrane" refers to a membrane having a MWCO of 1 kDa to 1000 kDa. The determination of the molecular weight cutoff can be carried out according to the US standard ASTM E1343-90 ("Standard test method for molecular weight cutoff evaluation of flat sheet ultrafiltration membranes"). Typically, a cutoff of 90 (where 90% of molecules of a given size are retained) is used to determine the molecular weight cutoff (MWCO) of an ultrafiltration membrane. Membrane terminology is summarized, for example, in "Terminology for membranes and membrane processes" (Journal of Membrane Science, 120, pp. 149-159, 1996).

[0017] For pore sizes of at least 0.1 μm, i.e., for microfiltration membranes with an average pore size of 0.1 μm to 10 μm, the pore size is determined using capillary flow porometry. This is a gas / liquid porometry method in which the differential pressure and flow rate of the gas passing through the membrane sample are first measured in a wet state and then in a dry state. Before measurement, the membrane sample is brought into contact with the wetting liquid so that all pores are filled with this liquid. After the pores are filled and the sample has been introduced, the measurement cell must be closed and the measurement started. After the measurement has started, the gas pressure is automatically and gradually increased, and the pore diameters corresponding to the applied pressure are emptied by the gas pressure. This is continued until the relevant pore range is covered, i.e., until the liquid is removed even from the smallest pores present in the measurement range. The pressure is then reduced again, and the measurement is automatically repeated for the currently dry sample. The pore size distribution is calculated from the difference between two pressure-flow curves using the Young-Laplace equation (see also A. Shrestha, "Characterization of porous membranes via porometry," 2012, Mechanical Engineering Graduate Theses & Dissertations, Paper 38, University of Colorado at Boulder).

[0018] For measuring pore sizes larger than 10 μm and up to 1 mm, the image analysis-based method described on pages 66-74 of the Journal of Membrane Science 372 (2011) may be used.

[0019] For pore sizes less than 0.1 μm, the cutoff is determined by filtering a model material and creating sieve curves that show the membrane's cutoff behavior across various molecular weights (see ASTM E1343-90).

[0020] According to the present invention, the treatment method includes the step of preparing an ultrafiltration membrane containing a thermoplastic polymer. The ultrafiltration membrane to be prepared is preferably an ultrafiltration membrane manufactured in the same process before the treatment method, whereby additional winding / unwinding and possibly further handling can be reduced. Thus, if necessary, the drying process and / or the storage process for the ultrafiltration membrane have already been carried out. Manufacturing methods suitable for manufacturing ultrafiltration membranes are known in the art. For example, each ultrafiltration membrane can be manufactured as described on pages 89 and subsequent of Non-Patent Document 2. Preferably, the ultrafiltration membrane is made by the precipitation casting method, more preferably by the precipitation casting method of a single polymer layer.

[0021] The thermoplastic polymer(s) contained in the ultrafiltration membrane is not subject to any special restrictions. Thus, any one or more thermoplastic polymers suitable for film formation can be used. The thermoplastic polymer can be selected from the group consisting of, for example, cellulose esters (such as cellulose acetate (cellulose monoacetate, cellulose diacetate, cellulose triacetate), cellulose propionate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, etc.), cellulose ethers (such as methyl cellulose and ethyl cellulose, etc.), nylon 6, nylon 6,6, polyethersulfone, and polysulfone. The ultrafiltration membrane may contain one or more of these polymers, preferably one kind. More preferably, the polymer layer contains a cellulose ester-based polymer, preferably cellulose acetate, and more preferably consists of a single cellulose diacetate.

[0022] If necessary, the drying process has already been carried out for the ultrafiltration membrane. Therefore, the ultrafiltration membrane is preferably a dry ultrafiltration membrane. A dry membrane is defined as a membrane containing residual moisture of less than 20% by weight of H2O, preferably less than 15% by weight of H2O, and most preferably less than 10% by weight of H2O. The drying process is generally known in the art and, for example, involves contacting the membrane with a hot surface (drum or plate), a hot air stream, for example, a hot air stream at a temperature of 60°C to 110°C, to ensure the removal of excess water, or drying it in a vacuum even at a lower temperature.

[0023] If necessary, the storage process has already been carried out for the ultrafiltration membrane. Therefore, the ultrafiltration membrane is preferably an ultrafiltration membrane treated with a preservative and / or a pore filler. Examples of preservatives and pore fillers are solutions containing glycerol solution, alcohol solution (e.g., 20% ethanol), caustic soda, and sodium azide. The storage process is generally known in the art and, for example, involves drying from a glycerol solution, storage using an alcohol solution (e.g., 20% ethanol), caustic soda, or a solution containing sodium azide. Each ultrafiltration membrane contains a preservative to prevent the collapse of pores. When the membrane is dried, the preservative needs to remain in the narrow pores. Furthermore, a preservative can be included to improve the hydrophilicity of the membrane (especially in the case of hydrophobic polymers such as polyethersulfone) (see, for example, pages 102 to 108 of S. Arenillas et al., Journal of Membrane Science and Research 3 (2017)).

[0024] The ultrafiltration membrane may comprise a support layer (supported ultrafiltration membrane). The thermoplastic polymer may be present partially or completely on and / or in the support layer. For example, the thermoplastic polymer layer may be present in the support layer at a level of at least 25% by volume, preferably at least 50% by volume, more preferably at least 75% by volume. Preferably, the polymer layer penetrates into the support layer at a level of less than 100% by volume.

[0025] The support layer is not particularly limited. Therefore, all support layers known to those skilled in the art from the prior art can be used. For example, the support layer may be a nonwoven web, a woven fabric, or an open-type microfiltration membrane. Examples of nonwoven webs are polyolefin nonwovens such as PP / PE core-shell nonwovens and polyester nonwovens. The support layer is preferably a polyolefin nonwoven or a polyolefin membrane.

[0026] The ultrafiltration membrane can be transported by a carrier or a transfer mechanism, and these are not subject to any special limitations. Any carrier suitable for membrane manufacturing methods from the prior art may be used. The carrier preferably has a flat surface and is inert to the substances used during membrane formation. Preferably, a moving belt (conveyor belt) functions as the carrier, which is preferably made of steel. As a transfer mechanism, for example, an unwinding roll and a hoisting roll, and optionally a transfer roll, may be used.

[0027] According to the present invention, the processing method involves heating the (prepared) ultrafiltration membrane from at least 110°C to (the glass transition temperature of the thermoplastic polymer + 10K (i.e., T gThe process further includes a step of thermal treatment at a temperature up to +10K (or, if two or more thermoplastic polymers are included, up to the glass transition temperature of the thermoplastic polymer mixture + 10K). The temperature applied during the thermal treatment may be selected depending on the thermoplastic polymer of the film and the desired level of densification. The temperature applied is always below the melting point of the thermoplastic polymer (or, if two or more thermoplastic polymers are included, always below the melting point of the thermoplastic polymer mixture). For example, a higher (lower) temperature may be selected for a higher (lower) level of densification. The temperature in the thermal treatment step is preferably at least 110°C to (glass transition temperature of the thermoplastic polymer + 5K) (or, if two or more thermoplastic polymers are included, to (glass transition temperature of the thermoplastic polymer mixture + 5K)), more preferably at least 110°C to below the glass transition temperature of the thermoplastic polymer (or, if two or more thermoplastic polymers are included, to below the glass transition temperature of the thermoplastic polymer mixture), more preferably 130°C to below the glass transition temperature of the thermoplastic polymer (or, if two or more thermoplastic polymers are included, to below the glass transition temperature of the thermoplastic polymer mixture), more preferably 130°C to 160°C, more preferably 135°C to 150°C, and most preferably 140°C to 145°C.

[0028] The duration (residence time) of the thermal treatment process is not particularly limited and can be selected depending on the temperature applied to the film and the desired level of densification. For example, a longer duration can be selected when applying a lower temperature, and vice versa, and / or a longer (shorter) duration can be selected for a higher (lower) level of densification. For example, the thermal treatment process may be carried out for a duration of 10 seconds to 10 minutes, preferably 30 seconds to 5 minutes, and most preferably 60 seconds to 3 minutes.

[0029] The means for carrying out the thermal treatment process are not particularly limited, and generally known thermal treatment methods can be applied. For example, the thermal treatment process can be carried out by applying one or more selected from the group consisting of a high-temperature surface in direct contact with the film, a high-temperature airflow, and infrared irradiation. The thermal treatment process is preferably carried out on the skin layer (surface) of the ultrafiltration film. Preferably, the ratio of the cutoff 90 on the skin layer after thermal treatment to the cutoff 90 on the skin layer before thermal treatment is a maximum of 1.0:1.5 (0.67), more preferably a maximum of 1.0:2.0 (0.50), more preferably a maximum of 1.0:3.0 (0.33), and most preferably a maximum of 1.0:4.0 (0.25).

[0030] The thermal treatment process is preferably applied only to the surface of the skin (tight) layer / top layer of the membrane. More preferably, the side (surface) of the membrane opposite to the skin (tight) layer / top layer is not thermally treated or is cooled while the thermal treatment is being performed. Suitable cooling means are known in the art. Applying the thermal treatment from the skin layer side, so that the main body of the membrane is not significantly affected by the heat, yields better results in terms of the permeate flux to retention ratio of the ultrafiltration membrane.

[0031] For example, the thermal treatment process can be carried out to varying degrees along the length of the film by changing the temperature and / or duration at different locations, for example, gradually or in stages, preferably gradually. This makes it possible to achieve changes in pore size not only in the thickness direction but also in the longitudinal direction of the film. Preferably, the thermal treatment process is carried out uniformly along the length of the film.

[0032] Further aspects of the present invention relate to a method for producing an ultrafiltration membrane, including the processing method of the present invention. The above definitions and embodiments apply similarly to this aspect of the present invention. The following definitions and embodiments also apply similarly to the processing method of the present invention.

[0033] Preferably, the film is manufactured by applying a small amount of REACH substance or by not applying any REACH substance at all. This preferably allows for the production of a film having a REACH substance content below the detectable limit or a film that does not contain any REACH substance.

[0034] REACH is a European regulation aimed at improving the protection of human health and the environment from the risks that can be posed by chemicals. Under this regulation by ECHA (European Chemicals Agency), the manufacture and / or use of certain substances is prohibited or severely restricted in the European Union. Some substances are likely to be banned, or will almost certainly be banned. These “candidates” are also undesirable for use. In this specification, the term “REACH substance” refers to both substances that are already banned and those that are potential candidates. The term “non-REACH” means a beneficial state in which a substance (solvent) is not registered as a substance of concern. Examples of currently useful solvents include acetic acid, acetone, Nn-butyl-2-pyrrolidone / N-butyl-2-pyrrolidone, dihydrolevoglucocenone, ethanol, isopropanol, ethyl formate, glycerol, glycerol-1,3-diacetate, glycerol triacetate, methyl acetate, 2-hydroxy-N,N-dimethylpropanamide, 2-pyrrolidone, 2,2-dimethyl-1,3-dioxolane-4-methanol, tetrahydrofuran, and water. The term "detectable content" refers to, for example, 6 cm. 2 This refers to a content of 50 μg / mL or more after extraction with 1 mL of extraction liquid (e.g., water or ethanol) on the membrane surface and 24 hours at a temperature of 60°C to 80°C.

[0035] Further aspects of the present invention relate to ultrafiltration membranes obtained by the processing method or manufacturing method of the present invention. The above definitions and embodiments apply similarly to this aspect of the present invention. The following definitions and embodiments apply similarly to the method of the present invention. The ultrafiltration membrane of the present invention is preferably a flat membrane.

[0036] The method of the present invention can achieve a higher polymer concentration in the membrane skin layer of the final membrane, and a more asymmetric membrane structure. This preferably results in a rejection rate that enables a faster separation process with higher membrane permeability.

[0037] The ultrafiltration membrane of the present invention preferably comprises a skin layer and an asymmetrical membrane body portion adjacent to the skin layer. The skin layer corresponds to the thermally treated portion of the ultrafiltration membrane. The skin layer and the membrane body portion are continuous within the membrane (i.e., they do not have a defined interface). The first portion of the membrane body portion adjacent to the skin layer has an average pore size smaller than the average pore size of the second (opposite) portion of the membrane body portion, and the average pore size increases (continuously or discontinuously) from the first portion to the second portion of the asymmetrical membrane body portion. The ratio of the average pore sizes between the second portion and the first portion of the membrane body portion (average pore size of the second portion of the membrane body portion / average pore size of the first portion of the membrane body portion) is preferably at least 5:1, more preferably at least 10:1, and most preferably at least 15:1. The average pore size of the skin layer is smaller than the average pore size of the first portion of the membrane body portion. The ratio of the average pore size between the second portion of the membrane body and the skin layer (average pore size of the second portion of the membrane body / average pore size of the skin layer) is preferably at least 20:1, more preferably at least 30:1, and most preferably at least 50:1. The membrane cross-section can be visualized by scanning electron microscopy (SEM). Pores can be resolved in the membrane body, in contrast to the dense skin (first main surface). The average pore size for, for example, 10 sections of the entire cross-section can be determined by binarizing the image. The ratio of the 10th section (in the second portion) to the 1st section (in the first portion) serves as an indicator of overall asymmetry. The average pore size for one section can be determined from 50 pores. The asymmetrical membrane body is preferably homogeneous, i.e., (substantially) free of macrovoids.

[0038] As used herein, an asymmetric membrane portion is a membrane portion in which a first portion of the asymmetric membrane portion has an average pore size smaller than the average pore size of a second portion (on the opposite side) of the asymmetric membrane portion, and the average pore size increases (continuously or discontinuously) from the first portion to the second portion of the asymmetric membrane portion.

[0039] The skin layer may be 5% or less, preferably 4% or less, and more preferably 3% or less (excluding 0.0%) of the total thickness of the membrane. The membrane body portion may be 95% or more, preferably 96% or more, and more preferably 97% or more (excluding 100%) of the total thickness of the membrane. Preferably, the ultrafiltration membrane consists of a skin layer and a membrane body portion. The membrane body portion includes an optional support layer.

[0040] The skin layer may have a thickness of 0.5 μm to 10 μm, preferably 0.5 μm to 5.0 μm, and more preferably 1.0 μm to 3.0 μm.

[0041] The main body of the film may have a thickness of 70 μm to 390 μm, preferably 90 μm to 345 μm, and more preferably 100 μm to 250 μm.

[0042] The film may have a total thickness of 80 μm to 400 μm, preferably 100 μm to 350 μm, and more preferably 120 μm to 280 μm.

[0043] Preferably, the membrane does not contain REACH substances in a detectable amount, and more preferably, it does not contain any REACH substances at all.

[0044] There are no particular limitations on the possible uses of the membrane of the present invention. The membrane of the present invention may be used for filtration, more specifically for cross-flow applications such as ultrafiltration and diafiltration, or for filtering viruses, proteins, or macromolecules in static filtration.

[0045] A further aspect of the present invention relates to the use of a thermal treatment means for reducing the pore size in the skin layer of an ultrafiltration membrane comprising a thermoplastic polymer, wherein the thermal treatment is performed on the ultrafiltration membrane at a temperature of at least 110°C to (glass transition temperature of the thermoplastic polymer + 10K). The above definitions and embodiments apply similarly to this aspect of the present invention. [Brief explanation of the drawing]

[0046] [Figure 1] This figure shows a schematic diagram of an exemplary standalone thermal treatment method of the present invention. An ultrafiltration membrane is thermally treated in a continuous process, in which controlled heat is applied, for example, by contact with a heated surface, direct thermal irradiation (infrared radiation), or residence in a high-temperature chamber (air) for a specified period of time. [Figure 2] This figure shows a schematic diagram of a thermal treatment incorporated into a typical film manufacturing process for ultrafiltration membranes produced by the precipitation casting method (NIPS-non-solvent-induced phase separation). The thermal treatment of the present invention can be carried out, for example, in a separate compartment within a machine, or it can be part of a drying process. [Figure 3] This figure shows how the cutoff behavior of a film based on a single cast solution is affected by the casting conditions (D: precipitation at a coagulation bath temperature of 20°C, C: precipitation at a coagulation bath temperature of 5°C), and especially by thermal post-treatment (complete rinsing and drying of the film) (C: thermal treatment at 120°C for 3.3 minutes, B: thermal treatment at 140°C for 3.3 minutes, A: thermal treatment at 145°C for 3.3 minutes). A cutoff of 90 was reached in the range from 20kDa (A) to 30kDa (B) and 50kDa (C) (D would represent 70kDa, but this was achieved under various casting conditions (= various precipitation bath temperatures)). [Figure 4]This figure shows how various thermal treatments of cellulose membranes based on the same cast solution affect the retention versus permeate flux of polymer standard PSS (polystyrene sulfonate with nominal molar masses ranging from 1 kilodalton (kDa) to 67 kDa) during concentration in cross-flow filtration. Membranes A and C differ from membranes B, D, and E in terms of coagulation bath temperature and post-thermal treatment duration: A and C were precipitated at 20°C and thermally treated for 2.2 minutes, while B, D, and E were precipitated at 5°C and thermally treated for 3.3 minutes. The results in Figure 4 show that lower thermal treatment temperatures have less effect on membrane behavior. The effect on the membranes is particularly pronounced at the (polymer) intrinsic optimal temperature close to Tg (glass transition temperature; here, approximately 145°C to 150°C for cellulose diacetate): B→D→E / A→C. [Figure 5] This figure shows scanning electron microscope (SEM) images of cross-sections of films A(a), B(b), D(c), and E(d). The SEM images show that the overall structure remains substantially unchanged after post-thermal treatment, but a clear change in the film structure occurs within the retaining skin layer / top layer. [Modes for carrying out the invention]

[0047] The present invention will be further illustrated by the following examples, but will not be limited thereto. [Examples]

[0048] Example 1: Effect of thermal treatment on the cutoff behavior of a given film Based on the determination of the dextran sieve curve, Figure 3 shows how the behavior of a given film shifts towards a tighter cutoff due to the initial thermal treatment of the film. Cellulose films prepared by a single-pass casting method followed by drying were post-treated at various temperatures for a given time (here, 3.3 minutes). Films A, B, and C were subjected to the same precipitation conditions, particularly the same precipitation temperature (=coagulation bath temperature, 5°C). The three sieve curves for A, B, and C shown in Figure 3 show that as the post-treatment temperature increases, the behavior of the films shifts to a lower cutoff range of 90. Film A, post-treated at 145°C, shows a cutoff range of 90 at 20 kDa; film B, post-treated at 140°C, shows a cutoff range of 90 at 30 kDa; and film C, post-treated at 120°C, shows only a cutoff range of 90 at 50 kDa.

[0049] Furthermore, Figure 3 shows that the behavior of the membrane can also be modified by changing the precipitation temperature, in which case the lower the precipitation temperature, the denser the membrane becomes (i.e., the higher its retention capacity). This phenomenon is already well known (see Nevstrueva, Daria et al., "Effect of precipitation temperature on the properties of cellulose ultrafiltration membranes prepared via immersion precipitation with ionic liquid as solvent," Membranes 8.4 (2018): 87). However, Figure 3, particularly the comparison between membrane C and membrane D, shows that there are physical and technical limitations to modifying the membrane by lowering the precipitation temperature under given conditions (polymer concentration, solvent, non-solvent, etc.). This is because at very low temperatures, the viscosity increases, which can negatively affect the casting process. A comparison of film C (deposition at 5°C) and film D (deposition at 20°C) also shows that the densification effect is not as significant as the effect achieved by post-thermal treatment.

[0050] Regarding post-thermal treatment, it should be noted that in addition to temperature, the duration of the treatment can also be varied. The longer the thermal treatment, the better the film retention.

[0051] Example 2: Effect of thermal treatment on the retention and permeation flux of a given film. The effects of various temperatures and the residence time of the films at these temperatures on film properties related to retention and permeation flux were investigated. Films A and C differ from films B, D, and E in terms of the deposition bath temperature and the duration and temperature of the post-thermal treatment. A and C were deposited at 20°C and thermally treated at 135°C and 140°C for 2.2 minutes, respectively, while B, D, and E were deposited at 5°C and thermally treated at 120°C, 140°C, and 145°C for 3.3 minutes, respectively.

[0052] Comparisons of films B, D, and E, as well as a comparison of films A and C, show that increasing the temperature of post-thermal treatment (from 120°C to 145°C and from 135°C to 140°C, respectively) gradually causes the films to become more compacted. This is demonstrated by increased retention and, in parallel, a decrease in permeation flux through the films (see Figure 4).

[0053] Importantly, the overall structure remains similar, as shown by the SEM images (see Figure 5). Changes must occur within the ultrastructure-retaining skin / top layer, which are not resolvable by SEM. This allows for further manipulation and improvement of the asymmetry of the ultrafiltration membrane.

Claims

1. A method for processing an ultrafiltration membrane, A step of preparing an ultrafiltration membrane containing a thermoplastic polymer, The ultrafiltration membrane is subjected to thermal treatment at a temperature of at least 110°C (the glass transition temperature of the thermoplastic polymer + 10K), Methods that include...

2. The processing method according to claim 1, wherein the thermoplastic polymer is selected from the group consisting of cellulose ester, cellulose ether, nylon 6, nylon 6,6, polyethersulfone, and polysulfone, and a mixture of at least two of the thermoplastic polymers.

3. The processing method according to claim 1 or 2, wherein the ultrafiltration membrane is an ultrafiltration membrane prepared by a deposition casting method.

4. The processing method according to any one of claims 1 to 3, wherein the ultrafiltration membrane is a dry ultrafiltration membrane and / or the ultrafiltration membrane is an ultrafiltration membrane treated with a preservative and / or a pore-filling agent.

5. The processing method according to any one of claims 1 to 4, wherein the temperature in the thermal processing step is 130°C to 160°C.

6. The processing method according to any one of claims 1 to 5, wherein the thermal processing step is carried out for a duration of 10 seconds to 10 minutes.

7. The processing method according to any one of claims 1 to 6, wherein the thermal processing step is carried out by applying one or more of the group selected from a high-temperature surface in direct contact with the film, a high-temperature airflow, and infrared irradiation.

8. The processing method according to any one of claims 1 to 7, wherein the thermal treatment step is applied only to the surface of the skin layer of the film.

9. The processing method according to claim 8, wherein the side surface of the film opposite to the skin layer is cooled while the thermal treatment is being performed.

10. The processing method according to any one of claims 1 to 9, wherein the ratio of the cutoff 90 in the skin layer of the film after thermal treatment to the cutoff 90 in the skin layer of the film before thermal treatment is a maximum of 1.0:1.

5.

11. A method for producing an ultrafiltration membrane, comprising the processing method described in any one of claims 1 to 10.

12. An ultrafiltration membrane obtained by the processing method described in any one of claims 1 to 10, or the manufacturing method described in claim 11.

13. The aforementioned film comprises a skin layer and an asymmetrical film body portion adjacent to the skin layer. The skin layer and the main body portion of the film are continuous within the film. The first portion of the membrane body adjacent to the skin layer has an average pore size smaller than the average pore size of the second portion of the membrane body, and the average pore size increases from the first portion to the second portion of the asymmetric membrane body, The ultrafiltration membrane according to claim 12, wherein the average pore size of the skin layer is smaller than the average pore size of the first portion of the membrane body.

14. The ultrafiltration membrane according to claim 13, wherein the second portion is located on the opposite side of the first portion of the membrane body, and the ratio of the average pore size between the second portion of the membrane body and the first portion of the membrane body is at least 5:

1.

15. The ultrafiltration membrane according to any one of claims 12 to 14, wherein the membrane does not contain REACH material.

16. The use of a thermal treatment means for reducing the pore size in the skin layer of an ultrafiltration membrane containing a thermoplastic polymer, wherein the thermal treatment is performed on the ultrafiltration membrane at a temperature of at least 110°C to (the glass transition temperature of the thermoplastic polymer + 10K).

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