COMBINATION OF PYRROLIDONE BASED SOLVENTS FOR THE PRODUCTION OF POROUS MEMBRANES

Abstract

The present invention relates to a method for producing a porous membrane, the method being characterized in that a solvent system comprising 2-pyrrolidone and N-alkyl-2-pyrrolidone is used, wherein the content ratio of 2-pyrrolidone to N-alkyl-2-pyrrolidone in the solvent system is from 90%:10% to 10%:90%, based on mass %, and wherein N-alkyl-2-pyrrolidone is N-propyl-2-pyrrolidone and/or N-butyl-2-pyrrolidone. Furthermore, the present invention relates to a porous membrane obtainable by said method. Moreover, the present invention relates to the use of a specific solvent system for the production of a porous membrane.

Claims

1. A method for producing a porous membrane, the method comprising the following steps (a) to (d): (a) providing a casting solution, wherein the casting solution comprises a membrane-forming polymer dissolved in a solvent system comprising 2-pyrrolidone and N-alkyl-2-pyrrolidone; (b) forming a polymer film from the casting solution provided in step (a); (c) contacting the polymer film formed in step (b) with a liquid precipitation bath, with a gaseous phase comprising a precipitation-inducing agent or with a combination thereof to induce membrane formation, thereby obtaining a porous membrane; and (d) drying the porous membrane obtained in step (c), wherein the content ratio of 2-pyrrolidone to N-alkyl-2-pyrrolidone in the solvent system is in the range of from 90%:10% to 10%:90%, based on mass %, and N-alkyl-2-pyrrolidone is N-propyl-2-pyrrolidone, N butyl-2-pyrrolidone, or a combination thereof.

2. The method for producing a porous membrane according to claim 1, wherein the membrane-forming polymer is selected from the group consisting of polyethersulfone, polysulfone, polyphenylsulfone, polyarylenesulfone, polybisphenylsulfone, cellulose acetate, polyamide, and polyvinylidene fluoride.

3. The method for producing a porous membrane according to claim 2, wherein the membrane-forming polymer is polyethersulfone.

4. The method for producing a porous membrane according to claim 1, wherein the casting solution further comprises at least one additive selected from the group consisting of polyvinylpyrrolidone, polyvinylpyrrolidone-co-poly-vinylacetate, and polyethylene glycol.

5. The method for producing a porous membrane according to claim 1, wherein the content ratio of 2-pyrrolidone to N-alkyl-2-pyrrolidone in the solvent system is in the range of from 80%:20% to 30%:70%, based on mass %.

6. The method for producing a porous membrane according to claim 5, wherein the content ratio of 2-pyrrolidone to N-alkyl-2-pyrrolidone in the solvent system is in the range of from 75%: 25% to 40%:60%, based on mass %.

7. The method for producing a porous membrane according to claim 1, wherein the solvent system does not comprise dimethyl sulfoxide.

8. The method for producing a porous membrane according to claim 1, wherein the solvent system consists of 2-pyrrolidone and N-alkyl-2-pyrrolidone.

9. The method for producing a porous membrane according to claim 1, wherein N-alkyl-2-pyrrolidone in the solvent system is N-butyl-2-pyrrolidone.

10. The method for producing a porous membrane according to claim 9, wherein N-butyl-2-pyrrolidone is N-n-butyl-2-pyrrolidone.

11. The method for producing a porous membrane according to claim 1, wherein step (c) is performed with a liquid precipitation bath.

12. The method for producing a porous membrane according to claim 11, wherein the liquid precipitation bath is an aqueous solution.

13. A porous membrane, obtainable by the method for producing a porous membrane according to claim 1, the porous membrane having a pore size in the range of from 1 nm to 5 μm and a thickness in the range of from 50 μm to 250 μm, and exhibiting a hybrid bulk structure of two distinct layers arranged on top of one another when viewed in the thickness direction thereof, wherein one of the two distinct layers has a sponge-like morphology without macrovoids.

14. The porous membrane according to claim 13, wherein the distinct layer having a sponge-like morphology without macrovoids has a specific surface area of more than 30 m.sup.2/g.

15. The porous membrane according to claim 13, wherein the distinct layer having a sponge-like morphology without macrovoids has a thickness in the range of from 3 μm to 20 μm.

16. (canceled)

17. The method for producing a porous membrane according to claim 1, wherein the solvent system has a combined content of solvents other than 2-pyrrolidone and N-alkyl-2-pyrrolidone of not more than 5 mass % with a total mass of the solvent system being 100 mass %.

Description

[0074] The Figures show:

[0075] FIG. 1 shows a SEM image of a membrane cross-section of a porous PESu ultrafiltration membrane obtained by the method for producing a porous membrane according to the present invention.

[0076] FIG. 2A shows a SEM image of a lacy sub-morphology with an open porous structure.

[0077] FIG. 2B shows a SEM image of a cellular sub-morphology with a (partially) closed porous structure.

[0078] FIG. 3 shows the specific surface area of the sponge-like layer as a function of the content of N-n-butyl-2-pyrrolidone in the solvent system used for the production of the porous membrane.

[0079] FIG. 4 shows the thickness of the sponge-like layer as a function of the content of N-n-butyl-2-pyrrolidone in the solvent system used for the production of the porous membrane.

[0080] FIG. 5 shows the thickness of the sponge-like layer as a function of the content of water in the solvent system used for the production of the porous membrane.

[0081] FIG. 6 shows a SEM image of a membrane cross-section, wherein the content of water in the solvent system used for the production of the porous membrane is 0.25 mass %.

[0082] FIG. 7 shows a SEM image of a membrane cross-section, wherein the content of water in the solvent system used for the production of the porous membrane is 1 mass %.

[0083] FIG. 8 shows a SEM image of a membrane cross-section, wherein the content of water in the solvent system used for the production of the porous membrane is 2 mass %.

[0084] FIG. 9 shows the membrane flux and the retention as a function of the content of water in the solvent system used for the production of the porous membrane.

[0085] FIG. 10 shows the performance factor of a porous PESu ultrafiltration membrane as a function of the content of N-n-butyl-2-pyrrolidone in the solvent system used for the production of the porous membrane.

[0086] FIG. 11 shows the performance factor of a porous CA ultrafiltration membrane as a function of the content of N-n-butyl-2-pyrrolidone in the solvent system used for the production of the porous membrane.

EXAMPLES

[0087] The present invention is further illustrated by the following Examples. However, the present invention is not to be construed as being limited thereto:

[0088] Hansen Solubility Parameters

[0089] A common approach to predict if one material will dissolve in another to form a solution is the use of Hansen Solubility Parameters (HSP). They are based on the idea that like dissolves like, where one molecule is defined as being ‘like’ another if it binds and interacts to itself in a similar way.

[0090] Specifically, a certain molecule can be described using three Hansen Solubility Parameters, each of which is given in units of MPa.sup.0.5, the three HSP being as follows: [0091] δd: The energy from dispersion forces between molecules. [0092] δp: The energy from dipolar intermolecular forces between molecules. [0093] δh: The energy from hydrogen bonds between molecules.

[0094] The Hansen Solubility Parameters of polyethersulfone (PESu), 2-pyrrolidone (2-P), N-n-butyl-2-pyrrolidone (NBP), N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMA) are given in Table 1:

TABLE-US-00001 TABLE 1 component δd/MPa.sup.0.5 δp/MPa.sup.0.5 δh/MPa.sup.0.5 PESu 19.6 10.8 9.2 2-P 18.0 16.6 7.4 NBP 17.8 5.9 8.2 NMP 18.0 12.3 7.2 DMA 16.8 11.5 10.2

[0095] In order to assess the similarity of two components, a distance function is commonly applied as given in Formula (1):

R=√{square root over (4.Math.(δd.sub.1−δd.sub.2).sup.2+(δp.sub.1−δp.sub.2).sup.2+(δh.sub.1−δh.sub.2).sup.2)} (1)

[0096] In Formula (1), R is the distance value. The lower the distance value, the higher the similarity between the two components.

[0097] Using an approximation of a linear contribution of solvent properties, the distance values R given in Table 2 can be calculated for pure solvents and mixtures of 2-P and NBP in relation to PESu:

TABLE-US-00002 TABLE 2 1.sup.st component 2.sup.nd component R 2-P — 6.86 NBP — 6.16 NMP — 4.06 DMA — 5.73 2-P (90%) NBP (10%) 5.99 2-P (80%) NBP (20%) 5.18 2-P (70%) NBP (30%) 4.49 2-P (60%) NBP (40%) 3.97 2-P (50%) NBP (50%) 3.70 2-P (40%) NBP (60%) 3.74 2-P (30%) NBP (70%) 4.06 2-P (20%) NBP (80%) 4.62 2-P (10%) NBP (90%) 5.34

[0098] This calculation shows that surprisingly, a mixture of 2-pyrrolidone (2-P) and N-n-butyl-2-pyrrolidone (NBP) with a specific content ratio can yield a lower distance value than conventional solvents like NMP and DMA.

[0099] Experimental Methods

[0100] Membrane Production on a Laboratory Scale

[0101] The production of membrane samples was performed on lab scale. The casting solution was provided using a stirred vessel with a temperature control unit. The casting solution components were added in the following order:

TABLE-US-00003 solvent system: 2-P, NBP or other solvents hydrophilic additive: PEG 400 (polyethylene glycol) hydrophilic additive: PVP K30 (polyvinylpyrrolidone) membrane-forming polymer: PESu E6020 P (polyethersulfone)

[0102] The resulting suspension was stirred under 250 rpm at 60° C. for 24 h to ensure complete dissolution of the non-solvent components. The homogenous solution obtained was then stirred under 5 rpm at 60° C. for another 3 h to perform a degassing procedure. After cooling to room temperature, a portion of the resulting solution was placed on a glass plate and formed to a homogenous film using a casting rake in an initial thickness reflecting the desired thickness of the final membrane, which was 150 μm.

[0103] With minimum contact time to ambient atmosphere, the polymer film was transferred to a gently agitated water bath to induce membrane formation by the exchange of solvent and non-solvent. The membrane was allowed to form for 5 minutes before being transferred to another bath containing glycerin in water. The membrane was allowed to impregnate for 10 minutes before being dried at 50° C. for 15 minutes.

[0104] After the formation, the membrane was stored at ambient conditions until being subject to further characterization.

[0105] Determination of Bulk Structure

[0106] Scanning electron microscopy (JEOL Benchtop 6000) was used to investigate the bulk structure of the porous membrane, and to evaluate the thickness of the distinct layer having a sponge-like morphology without macrovoids and the thickness of the distinct layer having a macrovoid dominated morphology. The respective membrane samples were coated with gold prior to investigation. SEM images were obtained using an acceleration voltage between 2 kV and 20 kV, and a spot-size of from 1.0 to 8.0 in a high vacuum. Secondary electrons were detected using an Everhart-Thornley detector,

[0107] Determination of Specific Surface Area

[0108] A normal BET procedure (Gemini apparatus, 11 point method) was applied for determining the specific surface area of the sponge-like layer in the respective membrane samples.

[0109] The volume of gas (usually nitrogen) adsorbed to the surface of the membrane was measured at the boiling point of nitrogen (−196° C.). At this temperature, the nitrogen gas was below its critical temperature and condensed on the surface of the membrane. It is assumed that the gas condenses on the surface in a monolayer so that, since the size of the gas atom/molecule is known, the amount of adsorbed (condensed) gas can be correlated with the total surface area including the pores at the surface (inaccessible pores are not detected).

[0110] When the gas (adsorptive) is pumped into the sample tube, the gas covers both the external and the accessible internal pore surface of the membrane. In BET theory, the sample is covered with a monolayer of adsorbate.

[0111] The BET equation can be used to calculate the surface area of the sample. Other equations are available to calculate surface areas from gas adsorption. However, BET is the most popular. The derivation of the BET equation is, for example, described in “Adsorption of Gases in Multimolecular Layers”, S. Brunauer et al., J. Am. Chem. Soc., 60(2), 309-319, 1938 (DOI: 10.1021/ja01269a023). Herein, it is sufficient to show that the measured inputs to this equation are: [0112] the equilibrium pressure p and the saturation pressure p.sub.0 of the adsorbate at the temperature of adsorption [0113] the adsorbed gas quantity V (in terms of volume)

[0114] The BET equation is represented by Formula (2):

[00001] $\begin{matrix} \frac{p}{V .Math. (p_{0} - p)} = \frac{1}{V_{m o n o} .Math. C} + \frac{C - 1}{V_{m o n o} .Math. C} .Math. \frac{p}{p_{0}} & (2) \end{matrix}$

[0115] The values to be calculated are: [0116] the monolayer capacity V.sub.mono (in terms of volume) [0117] the BET constant C

[0118] To calculate the above values V.sub.mono and C, the BET equation was plotted as an adsorption isotherm typically at a relative pressure p/p.sub.0 between 0.05 and 0.3. In this range, BET theory suggests that it should form a straight line. The values V.sub.mono and C can then be calculated from the slope and the intercept. Next, the total surface area S.sub.total can be calculated in accordance with Formula (3) using the molecular cross-sectional area:

[00002] $\begin{matrix} S_{total} = \frac{V_{m o n o} .Math. N_{A} .Math. s}{V_{molar}} & (3) \end{matrix}$

[0119] In Formula (3), NA is the Avogadro constant, s is the adsorption cross-section of the adsorbing species, and V.sub.molar is the molar volume of the adsorbate.

[0120] The specific surface area S.sub.specific can then be calculated in accordance with Formula (4) using the mass m of the sample:

[00003] $\begin{matrix} S_{specific} = \frac{S_{total}}{m} & (4) \end{matrix}$

[0121] Determination of Permeability (Membrane Flux)

[0122] The permeability of the porous membrane was determined in terms of the membrane flux by filtering a pure component or a mixture through a membrane sample at defined conditions. Specifically, the method applied for determining the membrane flux was as follows:

[0123] The standard operation procedure included the filtration under constant pressure using a round membrane sample with a diameter of 26 mm. The sample was checked for visible defects and was then integrated with a non-woven support into a stirring cell facing the sponge-like layer side up. The effective filtration area was 3.8 cm.sup.2.

[0124] The stirring cell was filled with 10.5 mL of an aqueous solution containing 0.9 mass % NaCl. The filtration was conducted under a pressure of 1 bar and under a stirring rate of 1100 rpm in order to simulate crossflow conditions. 10 mL of the filtrate were collected while the time was recorded in parallel.

[0125] The membrane flux of the sample under investigation can be calculated in accordance with Formula (5):

[00004] $\begin{matrix} J = \frac{V}{A .Math. Δ t .Math. Δ p} & (5) \end{matrix}$

[0126] In Formula (5), J is the membrane flux, V is the filtered volume (corresponding to the volume of the filtrate), A is the membrane filtration area, Δt is the measured time, and Δp is the applied pressure. The membrane flux is given in units of L/(m.sup.2×h×bar).

[0127] Determination of Rejection Rate (Retention)

[0128] The rejection rate of a membrane can be evaluated using various methods. Herein, the values determined rely on the determination of the retention for a protein marker molecule, either Cytochrome C or Bovine Serum Albumin (BSA), dissolved in an aqueous NaCl solution. The retention is a dimensionless quantity ranging from 0 to 1, wherein 0 indicates no retention and 1 indicates complete retention of the marker molecule. Specifically, the method applied for determining the retention was as follows:

[0129] A stirring cell was filled with 10 mL of the marker molecule solution. The filtration was conducted under a pressure of 1 bar and under a stirring rate of 1100 rpm in order to simulate crossflow conditions.

[0130] Table 3 lists the content of the marker molecule in the aqueous NaCl solution and further lists the salt concentration thereof.

TABLE-US-00004 TABLE 3 marker molecule content NaCl concentration Cytochrome C/ 0.1 mass % 0.15M Bovine Serum Albumin

[0131] In addition, Table 4 lists the absorption wavelengths of both Cytochrome C and Bovine Serum Albumin for UV/Vis spectroscopy.

TABLE-US-00005 TABLE 4 marker molecule wavelength Bovine Serum Albumin 280 nm Cytochrome C 550 nm

[0132] Herein, for determining the retention of polyethersulfone (PESu) ultrafiltration membranes, Cytochrome C was used as the marker molecule, and for determining the retention of cellulose acetate (CA) ultrafiltration membranes, Bovine Serum Albumin was used as the marker molecule.

[0133] In a first step, 9.5 mL of the protein solution was filtered through the membrane under constant pressure and the filtrate was collected. Afterwards, the stirring cell was flushed with an aqueous solution containing 0.9 mass % of NaCl. In a second step, the stirring cell was filled with 5 mL of the above aqueous solution containing 0.9 mass % of NaCl, and an additional volume of 2.5 mL was filtered through the membrane and collected in the filtrate. Subsequently, the extinction (absorbance) of the filtrate with a total volume of 12 mL was measured.

[0134] The retention R of the sample under investigation can be calculated using Formula (6):

[00005] $\begin{matrix} R = (1 - \frac{\ln (1 - \frac{E_{F} .Math. . V_{M}}{E_{s} .Math. V_{A}})}{\ln (1 - \frac{V_{F}}{V_{A}})}) & (6) \end{matrix}$

[0135] In Formula (6), R is the retention, E.sub.F is the extinction (absorbance) of the filtrate, E.sub.S is the extinction (absorbance) of the original marker molecule solution, V.sub.A is the starting volume of the original marker molecule solution (10 mL), V.sub.F is the volume of the filtrate after the first filtration step (9.5 mL), and VM is the volume of the filtrate after the second filtration step (12 mL).

[0136] Formula (6) can be derived as follows, wherein c.sub.R is the concentration of the marker molecule in the retentate and c.sub.F is the concentration of the marker molecule in the filtrate:

[00006] $R = \frac{c_{R} - c_{F}}{c_{R}} c_{F} = c_{R} .Math. (1 - R)$

[0137] The concentration of the marker molecule in the retentate C.sub.R and the concentration of the marker molecule in the filtrate c.sub.F can be expressed as follows, where P is the marker molecule mass in the retentate, dP is an infinitesimal change of the marker molecule mass in the retentate, V is the volume of the retentate, and dV is an infinitesimal change of the volume of the retentate:

[00007] $c_{F} = \frac{d P}{d V} c_{R} = \frac{P}{V}$

[0138] Rearrangement and integration leads to the following, where P.sub.0 is the marker molecule mass in the retentate at the start of the filtration (e.g. 10 mg in the original marker molecule solution, corresponding to 0.1 mass %), P.sub.E is the marker molecule mass in the retentate at the end of the filtration, V.sub.A is the volume of the retentate at the start of the filtration corresponding to the starting volume of the original marker molecule solution (10 mL), and V.sub.E is the volume of the retentate at the end of the filtration:

[00008] $\frac{d P}{d V} = \frac{P}{V} .Math. (1 - R)$ $\frac{d P}{P} = \frac{d V}{V} .Math. (1 - R)$ $\int_{P_{0}}^{P_{E}} \frac{1}{P} d P = (1 - R) .Math. \int_{V_{A}}^{y_{E}} \frac{1}{V} d V \ln (\frac{P_{E}}{P_{0}}) = (1 - R) .Math. \ln (\frac{V_{E}}{V_{A}})$

[0139] The marker molecule mass in the retentate at the end of the filtration P.sub.E and the volume of the retentate at the end of the filtration V.sub.E can be expressed as follows, where P.sub.F is the marker molecule mass in the filtrate, and V.sub.F is the volume of the filtrate after the first filtration step (9.5 mL), as mentioned above:

P.sub.E=P.sub.0−P.sub.F V.sub.E=V.sub.A−V.sub.F

[0140] Rearrangement leads to the following, taking into account that the extinction (absorbance) is proportional to the concentration:

[00009] $\begin{matrix} \ln (\frac{P_{0} - P_{F}}{P_{0}}) = (1 - R) .Math. \ln (\frac{V_{A} - V_{F}}{V_{A}}) \ln (1 - \frac{P_{F}}{P_{0}}) = (1 - R) .Math. \ln (1 - \frac{V_{F}}{V_{A}}) R = 1 - \frac{\ln (1 - \frac{P_{F}}{P_{0}})}{\ln (1 - \frac{V_{P}}{V_{A}})} R = (1 - \frac{\ln (1 - \frac{E_{F} .Math. . V_{M}}{E_{s} .Math. V_{A}})}{\ln (1 - \frac{V_{F}}{V_{A}})}) & (6) \end{matrix}$

[0141] Performance Factor

[0142] As an indicator of the filtration properties of a membrane sample, the performance factor P can be defined in accordance with Formula (7), which is the product of the membrane flux J and the retention R towards a specific marker molecule to be cut off.

P=J.Math.R (7)

[0143] The performance factor is given in units of L/(m.sup.2×h×bar), as it is the case for the membrane flux J, taking into account that the retention R is a dimensionless quantity ranging from 0 to 1. The membrane flux and the retention are measured as described above.

[0144] PESu Ultrafiltration Membranes

[0145] As an example, a reference casting solution for polyethersulfone (PESu) ultrafiltration membranes was prepared and the solvent system thereof was varied from pure 2-pyrrolidone to pure N-n-butyl-2-pyrrolidone. Table 5 lists the composition of the reference casting solution with its varying solvent contents.

TABLE-US-00006 TABLE 5 content thereof component of casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 20% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone x % N-n-butyl-2-pyrrolidone 71.75-x %

[0146] In the above reference casting solution, the content of the (hydrophilic) additives polyvinylpyrrolidone and polyethylene glycol amounted to 8.25 mass %, with the total mass of the membrane-forming polymer, the solvent system and the additives being 100 mass %. Further, the content of the membrane-forming polymer was 21.8 mass % and the content of the solvent system was 78.2 mass %, with the total mass of the membrane-forming polymer and the solvent system being 100 mass %.

[0147] The resulting performance factors of the PESu ultrafiltration membranes are shown in FIG. 10.

[0148] It can be taken from FIG. 10 that a maximum of the performance factor can be achieved when the content of N-n-butyl-2-pyrrolidone falls within the range of from 20 to 50 mass %, with the total mass of 2-pyrrolidone and N-n-butyl-2-pyrrolidone being 100 mass %. Surprisingly, this corresponds very well to the distance values R shown in Table 2. Furthermore, it can be taken from FIG. 10 that the performance factor can be significantly increased compared to the performance factors achieved by using the pure solvents 2-pyrrolidone and N-n-butyl-2-pyrrolidone. This becomes also evident from the following Working Examples and Comparative Examples:

[0149] Production of a porous membrane using a solvent system comprising 2-pyrrolidone and N-n-butyl-2-pyrrolidone (Working Examples)

[0150] (1) Content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system being 70% to 30%, based on mass %

[0151] A casting solution was prepared according to the method described above with the composition shown in Table 6:

TABLE-US-00007 TABLE 6 content thereof component of casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 20% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 50.25% N-n-butyl-2-pyrrolidone 21.5%

[0152] The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 233 L/(m.sup.2×h×bar) could be determined.

[0153] (2) Content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system being 50% to 50%, based on mass %

[0154] A casting solution was prepared according to the method described above with the composition shown in Table 7:

TABLE-US-00008 TABLE 7 component of content thereof casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 20% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 35.875% N-n-butyl-2-pyrrolidone 35.875%

[0155] The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 249 L/(m.sup.2×h×bar) could be determined.

[0156] Production of a porous membrane using pure 2-pyrrolidone or pure N-n-butyl-2-pyrrolidone (Comparative Examples)

[0157] (1) Content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system being 100% to 0%, based on mass %

[0158] A casting solution was prepared according to the method described above with the composition shown in Table 8:

TABLE-US-00009 TABLE 8 content thereof component of casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 20% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 71.75% N-n-butyl-2-pyrrolidone 0%

[0159] The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 51 L/(m.sup.2×h×bar) could be determined.

[0160] (2) Content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system being 0% to 100%, based on mass %

[0161] A casting solution was prepared according to the method described above with the composition shown in Table 9:

TABLE-US-00010 TABLE 9 of content thereof component casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 20% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 0% N-n-butyl-2-pyrrolidone 71.75%

[0162] The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 126 L/(m.sup.2×h×bar) could be determined.

[0163] Production of a porous membrane using a solvent system comprising 2-pyrrolidone, N-n-butyl-2-pyrrolidone and water (Working Example)

[0164] As discussed above, the addition of water to the solvent system of the casting solution has an influence on the thickness of the sponge-like layer, as it is shown in FIG. 5, which in turn has an influence on the permeability of the porous membrane (i.e. the membrane flux), but not on the rejection rate (i.e. the retention), as it is shown in FIG. 9.

[0165] For studying the effect of adding water to the casting solution, a reference casting solution was prepared, having the composition shown in Table 10.

TABLE-US-00011 TABLE 10 of content thereof component casting solution (in total 100 mass %) polyethersulfone (PESu E 6020 P) 18% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 55.3125% N-n-butyl-2-pyrrolidone 18.4375%

[0166] In the reference casting solution, the content of the (hydrophilic) additives polyvinylpyrrolidone and polyethylene glycol amounted to 8.25 mass %, with the total mass of the membrane-forming polymer, the solvent system and the additives being 100 mass %. Further, the content of the membrane-forming polymer was 19.6 mass % and the content of the solvent system was 80.4 mass %, with the total mass of the membrane-forming polymer and the solvent system being 100 mass %. The content ratio of 2-pyrrolidone to N-n-butyl-2-pyrrolidone in the solvent system was 75% to 25%, based on mass %.

[0167] Next, 2-pyrrolidone and N-n-butyl-2-pyrrolidone in the solvent system were equally replaced by water with a content ranging from 0.25 mass % to 2 mass %, with the total mass of the solvent system being 100 mass %. Membrane samples were prepared and characterized as described above.

[0168] As can be taken from FIG. 6 to FIG. 8, the thickness of the sponge-like layer increased with the amount of water in the solvent system and amounted to an average value of 9.87 μm for the membrane shown in FIG. 6, 13.38 μm for the membrane shown in FIG. 7, and 19.90 μm for the membrane shown in FIG. 8. Specifically, the thickness of the sponge-like layer was doubled by the addition of 2 mass % water. Due to the increased thickness of the sponge-like layer, the permeability of the porous membrane (i.e. the membrane flux) decreased. On the other hand, the rejection rate (i.e. the retention) remained almost constant within the measurement accuracy.

[0169] As found by the present inventors, a residual amount of water, which may be seen as always present in the solvent system, at least at a trace level, did not influence the morphology of the sponge-like layer. In particular, in all the membrane samples investigated, the addition of water to the casting solution did not alter the lacy sub-morphology of the sponge-like layer. That is, in each membrane sample under investigation, the specific surface area of the sponge-like layer was more than 30 m.sup.2/g, indicating an open porous structure.

[0170] CA Ultrafiltration Membranes

[0171] As another example, a reference casting solution for cellulose acetate (CA) ultrafiltration membranes was prepared in the same way as described above, and the solvent system thereof was varied from 25 mass % N-n-butyl-2-pyrrolidone to pure N-n-butyl-2-pyrrolidone, with the total mass of the solvent system being 100 mass %. Table 11 lists the composition of the reference casting solution with its varying solvent contents.

TABLE-US-00012 TABLE 11 content thereof component of casting solution (in total 100 mass %) cellulose acetate 10% (cellulose diacetate L50 Daicel) polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone x % N-n-butyl-2-pyrrolidone 81.75-x %

[0172] In the above reference casting solution, the content of the (hydrophilic) additives polyvinylpyrrolidone and polyethylene glycol amounted to 8.25 mass %, with the total mass of the membrane-forming polymer, the solvent system and the additives being 100 mass %. Further, the content of the membrane-forming polymer was 10.9 mass % and the content of the solvent system was 89.1 mass %, with the total mass of the membrane-forming polymer and the solvent system being 100 mass %.

[0173] The resulting performance factors of the CA ultrafiltration membranes are shown in FIG. 11.

[0174] It can be taken from FIG. 11 that a maximum of the performance factor can be achieved when the content of N-n-butyl-2-pyrrolidone is about 50 mass %, with the total mass of 2-pyrrolidone and N-n-butyl-2-pyrrolidone being 100 mass %. Furthermore, it can be taken from FIG. 11 that the performance factor can be significantly increased compared to the performance factor achieved by using the pure solvent N-n-butyl-2-pyrrolidone.

[0175] PSu Ultrafiltration Membranes

[0176] As yet another example, a reference casting solution for polysulfone (PSu) ultrafiltration membranes was prepared in the same way as described above, and the solvent system thereof contained 75 mass % 2-pyrrolidone and 25 mass % N-n-butyl-2-pyrrolidone, with the total mass of the solvent system being 100 mass %. Table 12 lists the composition of the reference casting solution with its varying solvent contents.

TABLE-US-00013 TABLE 12 content thereof component of casting solution (in total 100 mass %) polysulfone (PSf) 18% polyvinylpyrrolidone (PVP K30) 8.25% polyethylene glycol (PEG 400) (combined) 2-pyrrolidone 55.3125% N-n-butyl-2-pyrrolidone 18.4375%

[0177] A membrane performance factor of 255 L/(m.sup.2×h×bar) with respect to Cytochrome C could be determined.

COMBINATION OF PYRROLIDONE BASED SOLVENTS FOR THE PRODUCTION OF POROUS MEMBRANES

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B01D2325/026

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B01D2323/2185

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Abstract

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