Heat treatment of ultrafiltration membranes

Abstract

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

Description

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

[0002] According to IUPAC, filtration membranes are classified based on their retentive properties and pore size. Based on this recommendation, average pore sizes of 0.1 μm to 10 μm are generally classified as microfiltration membranes, ultrafiltration membranes are those with an average pore size of 0.01 μm to 0.1 μm, nanofiltration membranes have an average pore size of 0.001 μm to 0.01 μm, and membranes with an average pore size below 0.001 μm are generally referred to as reverse osmosis membranes (see Shang-Tian Yang, Bioprocessing for Value-Added Products from Renewable Resources, 2007).

[0003] Similar classification can be carried out by the retention rate of membrane to specific molecules, because the accurate pore size determination for narrow membrane such as ultrafiltration membrane, nanofiltration membrane or reverse osmosis membrane is more difficult than the accurate pore size determination for more open membrane (such as microfilter), thereby more easily loses effectiveness. The typical retention rate measurement of ultrafiltration membrane is carried out using the molecule of protein or other applicable simulation typical process. Another method is to use widely distributed polymer to measure sieving curve or sieving coefficient. Such method can be applied to the aperture of wider range, and is not limited to a type of membrane, such as described in ASTM E1343-90.

[0004] Precipitation casting is a method for producing membranes ranging from reverse osmosis membranes to ultrafiltration membranes to microfiltration membranes. During precipitation, the solvent of the polymer solution is exchanged for a nonsolvent from the coagulation bath in a thermodynamic and kinetic process, and a porous structure is formed (see "Basic Principles of Membrane Technology" 2 nd Ed., Marcel Mulder, pp. 77ff). The applied solvent plays a key role in the formation of the porous structure of the membrane. Specific solvents allow the production of membranes with specific characteristics with regard to rejection and flux.

[0005] In recent years, some countries / regions have enacted regulations on the manufacture and import of chemical substances and their uses (e.g., the 2006 EU REACH Regulation). For example, REACH also addresses the continued use of chemical substances of very high concern (SVHC) due to their potential negative effects on human health and / or the environment. Some solvents commonly used in the production of ultrafiltration membranes may fall under this category. Some uses of SVHC may require prior authorization from the European Chemicals Agency, and applicants for authorization will have to include a plan to replace the use of SVHC with safer alternatives.

[0006] However, finding alternative solvents for producing ultrafiltration membranes based on thermoplastic polymers is quite difficult. It has been found that, with alternative solvents, it is often not possible to produce an ultrafiltration membrane with the same performance profile as before the solvent change. Therefore, there is a high demand for adjusting / changing the methods for producing filtration membranes, in particular via precipitation casting.

[0007] Retention rate and flux are determined by the different structural characteristics of membrane. Retention rate is determined by the size of the smallest pore in the membrane body. For ultrafiltration membranes, these pores are located in the top layer (epidermal layer). Flux is determined by the pore size (gradient) in the entire membrane body including the epidermal layer. Relatively high flux under a given retention rate characteristic can be achieved in particular by a membrane with a high degree of asymmetry, wherein the membrane has small holes in the membrane epidermal layer within the desired range, followed by a steep gradient of larger holes to the bottom layer.

[0008] To some extent, the pore size and pore size gradient at the skin layer can be tailored by the properties of the polymer solution and the membrane production process. Such processes are well known to those skilled in the art and are described in textbooks (e.g., "Basic Principles of Membrane Technology", Marcel Mulder, 2 nd Ed. p.77 ff. ). However, the ratio of achievable retention rate to flux is usually subject to the thermodynamic and kinetic limitations of film formation itself. The pore size at each position in the film is determined by the polymer content in the casting solution when phase separation begins and the coalescence time between phase separation and film solidification. Therefore, obtaining apertures at the epidermis of the film and obtaining larger pores in the film are known methods using two stratified polymer solutions for film formation, i.e., solutions comprising a large amount of polymers to form a thin layer of epidermis and solutions comprising a small amount of polymers to form a larger layer (for example, as described in EP 4 003579 A1) of larger pores in the film. However, such methods are laborious because mixing, pumping and stratification techniques of accurately controlling layer thickness and double amount are required.

[0009] The technical problem addressed by the present invention is therefore to provide an ultrafiltration membrane with an improved rejection to permeability ratio in an efficient and rapid manner, and preferably without using chemicals of very high concern, or preferably avoiding the inclusion of chemicals of very high concern in the final ultrafiltration membrane.

[0010] The solution to the above technical problem is achieved through the embodiments described in the claims.

[0011] In particular, the present invention relates to a method for treating an ultrafiltration membrane comprising the steps of:

[0012] providing an ultrafiltration membrane comprising a thermoplastic polymer, and

[0013] The ultrafiltration membrane is heat-treated at a temperature of at least 110° C. up to (glass transition temperature of the thermoplastic polymer + 10 K).

[0014] By means of the method of the present invention, the properties of an ultrafiltration membrane based on a thermoplastic polymer (completely rinsed and dried) can be modified by heat treating the ultrafiltration membrane at high temperature (heat treatment) to achieve a higher rejection rate. The process is schematically shown in Figure 1 The temperature used to heat the film is close to the glass transition temperature (T g ), but below its melting temperature. Without being bound by this theory, it is believed that the chain mobility of the polymer can be temporarily increased, thereby causing a relaxation of chain tension, resulting in a closer arrangement of the chains as polymers with similar properties are partially arranged together. The determination of the T of a polymer is described in ASTM E1356. g A common method, which "covers the specification for determining the glass transition temperature of a material using differential scanning calorimetry or differential thermal analysis". Due to the very dense polymer arrangement in the so-called skin layer of the ultrafiltration membrane, it is possible to achieve a narrowing of the pores even in the nanometer region, which leads to a higher retention rate (lower cut-off value) of the membrane. Thus, it is preferably possible to avoid significant changes in the microporous substructure, where the polymer chains or domains are further separated and cannot interact at the molecular level.

[0015] Preferably, no toxic solvents or other substances of very high concern are used during the production of the ultrafiltration membrane.A reduction in production costs due to the new process design is preferably possible, since the ultrafiltration membranes obtained by the single-layer casting process can be processed.

[0016] In the present invention, the term "ultrafiltration membrane" refers to a membrane having a MWCO of 1 kDa to 1000 kDa. The molecular weight cutoff can be determined according to the American standard ASTM E1343-90 ("Test method for molecular weight cutoff evaluation of flat ultrafiltration membranes"). A cutoff of 90 (retaining 90% of molecules of a given size) is typically used to determine the molecular weight cutoff (MWCO) of an ultrafiltration membrane. The terminology of membranes is, for example, summarized in "Terminology for membranes and membrane processes" (in: 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, capillary flow porometry is used to determine the pore size. This is a gas / liquid porosimetry method in which the gas pressure difference and flow rate through the membrane sample are measured first in a wet state and then in a dry state. Before the measurement, the membrane sample is contacted with a wet liquid so that all pores are filled with the liquid. After the pores have been filled and the sample has been introduced, the measuring cell must be closed and the measurement started. After the measurement is started, the gas pressure automatically and gradually increases, and the pore diameter corresponding to the applied pressure is emptied by the gas pressure. This continues until the relevant pore range has been covered, i.e., until even the smallest pores present in the measurement range are free of liquid. The pressure is then reduced again, and the measurement is automatically repeated on the now dried sample. The pore size distribution was calculated from the difference between the two pressure-flow rate 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] To determine pore sizes greater than 10 μm and up to 1 mm, the image analysis-based method described in Journal of Membrane Science 372 (2011), pages 66 to 74, can be used.

[0019] For pore sizes less than 0.1 μm, the cutoff value is determined by filtering a model substance and generating a sieve curve that indicates the cutoff performance of the membrane over a range of molecular weights (see ASTM E1343-90).

[0020] According to the present invention, the treatment method comprises the step of providing an ultrafiltration membrane comprising a thermoplastic polymer. The provided ultrafiltration membrane is preferably an ultrafiltration membrane that has been produced in the same process prior to the treatment method, which saves additional winding / unwinding and allows for further disposal. Thus, a drying step and / or a storage step has already been performed on the ultrafiltration membrane, if necessary. Suitable production methods for producing ultrafiltration membranes are known in the art. For example, methods described in "Basic Principles of Membrane Technology" 2 nd The corresponding ultrafiltration membranes are produced as described in , Ed., Marcel Mulder, pp. 89 ff. Preferably, the ultrafiltration membrane has been produced by precipitation casting, more preferably by precipitation casting of a single polymer layer.

[0021] The thermoplastic polymer contained in the ultrafiltration membrane is not subject to any particular restrictions. Therefore, any one or more thermoplastic polymers suitable for membrane formation can be used. The thermoplastic polymer can be, for example, selected from cellulose esters (e.g., cellulose acetates (monoacetate, diacetate, triacetate), cellulose propionate, cellulose butyrate, cellulose acetate butyrate, nitrocellulose), cellulose ethers (e.g., methyl cellulose and ethyl cellulose), nylon 6, nylon 6,6, polyethersulfone, and polysulfone. The ultrafiltration membrane can contain one or more of these polymers, preferably one. More preferably, the polymer layer contains a cellulose ester-based polymer, preferably cellulose acetate, and more preferably consists of a single diacetate cellulose.

[0022] Where necessary, the drying step is already carried out with an ultrafiltration membrane. Therefore, the ultrafiltration membrane is preferably a dry ultrafiltration membrane. A dry membrane is defined as a membrane having a residual moisture content of less than 20 wt% H2O, preferably less than 15 wt% H2O, and most preferably less than 10 wt% H2O. Drying steps are generally known in the art and are, for example, contacting the membrane with a hot surface (drum or plate), a hot gas stream, for example at a temperature of 60°C to 110°C to ensure removal of excess water or drying in a vacuum at even lower temperatures.

[0023] Where necessary, the ultrafiltration membrane has been subjected to a preservation step. 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 glycerol solutions, alcoholic solutions (e.g., 20% ethanol), caustic soda, and a solution containing sodium azide. The preservation step is generally known in the art and is, for example, dried from a glycerol solution and preserved with an alcoholic solution (e.g., 20% ethanol), caustic soda, or a solution containing sodium azide. The corresponding ultrafiltration membrane contains a preservative to prevent pore collapse. When the membrane is dried, the preservative needs to be retained in narrow pores. In addition, a preservative can be included to promote the hydrophilicity of the membrane (especially for hydrophobic polymers such as polyethersulfone) (see, for example, S. Arénillas et al., Journal of Membrane Science and Research 3 (2017) pp. 102-108).

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

[0025] The supporting layer is not particularly limited. Therefore, all supporting layers known to those skilled in the art can be used. For example, the supporting layer can be a nonwoven web, a woven fabric or an open microfiltration membrane. The example of the nonwoven web is a polyolefin nonwoven, for example, a PP / PE core-shell nonwoven and a polyester nonwoven. The supporting layer is preferably a polyolefin nonwoven or a polyolefin film.

[0026] The ultrafiltration membrane can be transported by means of a carrier or by means of a transport mechanism, and is not subject to any particular restrictions. Any carrier suitable for the membrane production method from the prior art can be used. The carrier preferably has a flat surface and is inert relative to the material used during film formation. Preferably, a conveyor belt (conveyor belt) is used as a carrier. As a transport mechanism, for example, an unwinding roller and a winding roller and an optional transport roller can be used.

[0027] According to the present invention, the treatment method further comprises the step of: gThe step of heat-treating the (provided) ultrafiltration membrane at a temperature of (100°C to 100°C) (or to (the glass transition temperature of the thermoplastic polymer mixture + 10K) if more than one thermoplastic polymer is present). The temperature applied during the heat treatment can be selected depending on the thermoplastic polymer of the membrane and the desired compaction level. The applied temperature is always below the melting temperature of the thermoplastic polymer (or below the melting temperature of the thermoplastic polymer mixture if more than one thermoplastic polymer is present). For example, a higher (lower) temperature can be selected for a higher (lower) compaction level. The temperature in the heat treatment step is preferably from at least 110°C to (glass transition temperature of the thermoplastic polymer + 5K) (or to (glass transition temperature of the thermoplastic polymer mixture + 5K) when more than one thermoplastic polymer is contained), more preferably from at least 110°C to below the glass transition temperature of the thermoplastic polymer (or to below the glass transition temperature of the thermoplastic polymer mixture when more than one thermoplastic polymer is contained), more preferably from 130°C to below the glass transition temperature of the thermoplastic polymer (or to below the glass transition temperature of the thermoplastic polymer mixture when more than one thermoplastic polymer is contained), more preferably from 130°C to 160°C, more preferably from 135°C to 150°C, most preferably from 140°C to 145°C.

[0028] The duration (residence time) of the heat treatment step is not particularly limited and can be selected depending on the temperature applied to the film and the desired compaction level. For example, a longer duration can be selected when a lower temperature is applied, and vice versa, and / or a longer (shorter) duration can be selected for a higher (lower) compaction level. For example, the heat treatment step is performed for a duration of 10 s to 10 min, preferably 30 s to 5.0 min, and most preferably 60 s to 3.0 min.

[0029] The mode of carrying out the heat treatment step is not particularly limited, and the mode of well-known heat treatment can be applied.For example, the heat treatment step can be carried out by applying one or more selected from the following: the hot surface of direct contact membrane, hot air flow, and infrared irradiation.The heat treatment step is preferably carried out in the epidermis (surface) of ultrafiltration membrane.Preferably, the ratio of the cut-off 90% at the epidermis after heat treatment to the cut-off 90% at the epidermis before heat treatment is at most 1.0:1.5 (0.67), more preferably at most 1.0:2.0 (0.50), more preferably at most 1.0:3.0 (0.33), most preferably at most 1.0:4.0 (0.25).

[0030] Preferably, only the heat treatment step is applied to the epidermis (tight) layer / top surface of membrane.More preferably, while heat treating, one side (surface) of the membrane relative to the epidermis (tight) layer / top is not heat treated or one side (surface) of the membrane relative to the epidermis (tight) layer / top is cooled.Suitable cooling mode is known in the art.When applying heat treatment from the epidermis side and the heated influence of the membrane body part was less, the better effect of the flux and the ratio of the rejection of ultrafiltration membrane was observed.

[0031] The heat treatment step can be performed at varying degrees along the length of the membrane, for example by varying the temperature and / or duration at different locations, for example gradually or stepwise, preferably gradually. Thus, a change in pore size can be achieved not only in the thickness direction of the membrane, but also in the longitudinal direction of the membrane. Preferably, the heat treatment step is performed uniformly along the length of the membrane.

[0032] Another aspect of the present invention relates to a method for producing an ultrafiltration membrane comprising the treatment method of the present invention. The above definitions and embodiments apply analogously to this aspect of the present invention. The following definitions and embodiments apply analogously to the treatment method of the present invention.

[0033] Preferably, a small amount of REACH material or no REACH material is used to produce the film. Thus, it is preferred to obtain a film with a REACH material content below the detectable limit or without REACH material.

[0034] REACH is a European regulation whose purpose is to improve the protection of human health and the environment from the risks that chemicals may bring. Based on this regulation of ECHA (European Chemical Agency), the production and / or use of certain substances are banned or strictly restricted in the European Union. Some substances tend to be or are most likely to be banned. These "candidates" are also not conducive to use. In this article, the term "REACH material" refers to two types of materials that have been banned and potential candidates. The term "non-REACH" means the beneficial status of substances (solvents) that have not been registered as materials of concern. Examples of current beneficial solvents are acetic acid, acetone, N-n-butyl-2-pyrrolidone / N-butyl-2-pyrrolidone, dihydro-levulinone, ethanol, isopropyl alcohol, ethyl formate, glycerol, glycerol-1,3-diacetate, triacetin, methyl acetate, 2-hydroxy-N, N-dimethylpropionamide, 2-pyrrolidone, 2,2-dimethyl-1,3-dioxolane-4-methanol, tetrahydrofuran and water. The term "detectable content" means, for example, that the amount of the extract is detected by 1 mL of an extractant (such as water or ethanol) at a temperature of 60°C to 80°C. 2 50 μg / mL or more after 24 hours of extraction on the membrane surface.

[0035] Another aspect of the present invention relates to an ultrafiltration membrane obtained by the treatment method of the present invention or by the production method of the present invention. The above definitions and embodiments apply analogously to this aspect of the invention. The following definitions and embodiments apply analogously to the method of the present invention. The ultrafiltration membrane of the present invention is preferably a flat sheet membrane.

[0036] The method of the present invention enables a higher polymer concentration and a more asymmetric membrane structure to be obtained in the membrane skin layer of the final membrane, which preferably produces a rejection ratio at a higher membrane permeability, enabling a faster separation process.

[0037] The ultrafiltration membrane of the present invention preferably comprises an epidermis layer and an asymmetric membrane portion adjacent to the epidermis layer. The epidermis layer corresponds to the portion of the ultrafiltration membrane that has been heat-treated. The epidermis layer and the membrane portion are continuous within the membrane (i.e., there is no (clear) interface). The first portion of the membrane portion adjacent to the epidermis layer has an average pore size smaller than the average pore size of the second (opposite) portion of the membrane portion, and the average pore size increases (continuously or discontinuously) from the first portion of the asymmetric membrane portion toward the second portion of the asymmetric membrane portion. The ratio of the average pore size of the second portion of the membrane portion to the first portion of the membrane portion (average pore size of the second portion of the membrane portion / average pore size of the first portion of the membrane 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 epidermis layer is smaller than the average pore size of the first portion of the membrane portion. The ratio of the average pore size of the second portion of the membrane portion to the epidermis layer (average pore size of the second portion of the membrane portion / average pore size of the epidermis 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 microscope (SEM).Compared with the tight epidermis (first major surface), holes can be distinguished in the membrane body part.For example, the average pore size of ten sections of the whole cross section can be determined by the binarization of image.The ratio of the 10th section (in the second part) to the 1st section (in the first part) provides the indication of overall asymmetry.The average pore size of a section can be determined by 50 holes.The asymmetric membrane body part is preferably uniform, i.e., (substantially) without macropores.

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

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

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

[0041] The film body portion may have a thickness of 70 μm to 390 μm, preferably 90 μm to 345 μm, 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, more preferably 120 μm to 280 μm.

[0043] Preferably, the film contains no detectable levels of REACH materials, and more preferably contains no REACH materials.

[0044] The possible uses of the membrane of the invention are not particularly limited. It can be used for filtration, more particularly for filtering viruses, proteins or macromolecules in cross-flow applications such as ultrafiltration and diafiltration or in static filtration.

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

[0046] Figure 1 : Schematic diagram of an exemplary stand-alone heat treatment method of the present invention. The ultrafiltration membrane is heat treated in a continuous process, whereby controlled heating is applied, for example, by contact with a heated surface, by direct heat radiation (infrared), or by dwelling in a hot chamber (air) for a defined time.

[0047] Figure 2 : Schematic representation of the heat treatment integrated in a typical membrane production process of ultrafiltration membranes by precipitation casting (NIPS - non-solvent induced phase separation). The heat treatment of the invention can be performed, for example, in a separate compartment within the machine or can be part of the drying process.

[0048] Figure 3Based on a single casting solution, the cutoff properties of the membrane are shown as a function of the casting conditions (D: precipitation at a coagulation bath temperature of 20°C, C: precipitation at a coagulation bath temperature of 5°C) and, in particular, by the thermal post-treatment (membrane completely rinsed and dried) (C: heat treatment at 120°C for 3.3 min, B: heat treatment at 140°C for 3.3 min, A: heat treatment at 145°C for 3.3 min). Cutoffs range from 9020 kDa (A) to 30 kDa (B) and 50 kDa (C) (D would represent 70 kDa but was achieved with different casting conditions (=different precipitation bath temperatures)).

[0049] Figure 4 Different heat treatments of cellulose membranes based on the same casting solution affect the retention of a polymer standard, PSS (polystyrene sulfonate with nominal molar masses ranging from 1 kilodalton to 67 kilodaltons (kDa)), relative to the flux 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-heat treatment duration: A and C were precipitated at 20°C and heat treated for 2.2 min, while B, D, and E were precipitated at 5°C and heat treated for 3.3 min. Figure 4 The results in

[15] show that heat treatment at lower temperatures has only a low impact on the membrane properties. g The influence on the film is particularly pronounced at the specific optimum temperature (of the polymer) (glass transition temperature; here in the case of cellulose diacetate, about 145-150° C.):

[0050] B → D → E / a → C.

[0051] Figure 5 : Scanning electron microscope (SEM) images showing cross sections through membrane A (a), membrane B (b), membrane D (c), and membrane E (d). The SEM images show that the overall structure remains essentially unchanged through post-heat treatment, and that changes in the membrane structure clearly occur within the retentive skin / top layer.

[0052] The present invention will be further described below, but the present invention is not limited thereto.

[0053] Example 1 : Effect of heat treatment on the cutoff properties of a given membrane

[0054] Based on the determination of dextran sieving curve, Figure 3 The figure shows how the properties of a given film can be shifted towards a tighter cutoff by thermal treatment of the initial film. Cellulose films produced by single casting and subsequent drying have been post-treated at different temperatures for a given period of time (here 3.3 min). For films A, B and C, the same precipitation conditions, in particular the same precipitation temperature (=coagulation bath temperature; 5°C), were used. Figure 3 The three sieving curves A, B, and C shown in Figure 3 show that with higher post-treatment temperatures, the performance of the membrane shifts toward a lower 90°C cutoff range. Membrane A post-heated at 145°C exhibited a 20 kDa 90°C cutoff range, membrane B post-heated at 140°C exhibited a 30 kDa 90°C cutoff range, and membrane C post-heated at 120°C exhibited only a 50 kDa cutoff range.

[0055] also, Figure 3 It has been shown that the membrane properties can also be modified by varying the precipitation temperature, whereby the lower the precipitation temperature, the tighter the membrane (i.e., the higher the retention rate). 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 In particular, the comparison of films C and D shows that, for given conditions (polymer concentration, solvent, non-solvent, etc.), the modification by lowering the precipitation temperature has physical and technical limitations, as the viscosity is high at very low temperatures and can negatively affect the casting process. The comparison of films C (precipitated at 5°C) and D (precipitated at 20°C) also shows that the compacting effect is less pronounced than that achieved by post-heat treatment.

[0056] Regarding the post-heat treatment, it should be noted that in addition to the temperature, the duration of the treatment can also be varied. Longer heat treatments result in higher rejection rates for the membrane.

[0057] Example 2 : Effect of heat treatment on the rejection and flux of a given membrane

[0058] The effects of different temperatures and the residence time of the membranes at these temperatures on membrane properties related to rejection and flux have been investigated. Membranes A and C differ from membranes B, D, and E in terms of the coagulation bath temperature and the duration and temperature of the post-heat treatment. Membranes A and C were precipitated at 20°C and heat-treated at 135°C and 140°C, respectively, for 2.2 minutes, while membranes B, D, and E were precipitated at 5°C and heat-treated at 120°C, 140°C, and 145°C, respectively, for 3.3 minutes.

[0059] Comparison of membranes B, D and E and membranes A and C shows that an increase in the temperature of the post-heat treatment (from 120°C to 145°C and from 135°C to 140°C, respectively) leads to a progressive compaction of the membranes as evidenced by an increase in the rejection and a parallel decrease in the flux across the membranes (see Figure 4 ).

[0060] Importantly, the overall structure remains similar to that indicated by the SEM images (see Figure 5 The changes must occur within the retaining skin / top layer, whose ultrafine structure cannot be resolved by SEM. Therefore, the asymmetry of the ultrafiltration membrane can be further manipulated and improved.

Claims

1. A method for treating an ultrafiltration membrane, comprising the following steps: An ultrafiltration membrane is provided, the ultrafiltration membrane comprising a thermoplastic polymer, and The ultrafiltration membrane is heat treated at a temperature of at least 110° C. to (glass transition temperature of the thermoplastic polymer + 10 K).

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

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

4. The treatment 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 an antiseptic and / or a pore filler.

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

6. The treatment method according to any one of claims 1 to 5, wherein the heat treatment step is performed for a duration of 10 s to 10 min.

7. The treatment method according to any one of claims 1 to 6, wherein the heat treatment step is performed by applying one or more selected from the group consisting of: a hot surface in direct contact with the film, a hot air flow, and infrared irradiation.

8. The treatment method according to any one of claims 1 to 7, wherein the heat treatment step is applied only to the skin surface of the film. 9 . The treatment method according to claim 8 , wherein the side of the film opposite to the skin layer is cooled while the heat treatment is being performed.

10. The treatment method according to any one of claims 1 to 9, wherein the ratio of the cut-off 90 at the skin layer of the membrane after heat treatment to the cut-off 90 at the skin layer of the membrane before heat treatment is at most 1.0:1.

5.

11. A method for producing an ultrafiltration membrane comprising the treatment method according to any one of claims 1 to 10.

12. Ultrafiltration membrane obtained by the treatment method according to any one of claims 1 to 10 or by the production method according to claim 11.

13. The ultrafiltration membrane of claim 12, wherein the membrane comprises a skin layer and an asymmetric membrane body portion adjacent to the skin layer, The skin layer and the membrane body portion are continuous within the membrane; a first portion of the body portion adjacent to the skin layer having an average pore size that is smaller than an average pore size of a second portion of the body portion, and the average pore size increases from the first portion of the asymmetric body portion toward the second portion of the asymmetric body portion; and 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 opposite to the first portion in the membrane body portion, and a ratio of an average pore size of the second portion of the membrane body portion to the first portion of the membrane body portion 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. Use of a heat treatment for reducing the pore size in the skin layer of an ultrafiltration membrane comprising a thermoplastic polymer, wherein the ultrafiltration membrane is heat treated at a temperature of at least 110°C to (glass transition temperature of the thermoplastic polymer + 10K).

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