Patent ID: 12226738

EXAMPLES

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

Hansen Solubility Parameters

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.

Specifically, a certain molecule can be described using three Hansen Solubility Parameters, each of which is given in units of MPa0.5, the three HSP being as follows:δd: The energy from dispersion forces between molecules.δp: The energy from dipolar intermolecular forces between molecules.δh: The energy from hydrogen bonds between molecules.

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 1componentδd/MPa0.5δp/MPa0.5δh/MPa0.5PESu19.610.89.22-P18.016.67.4NBP17.85.98.2NMP18.012.37.2DMA16.811.510.2

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·(δd1−δd2)2+(δp1−δp2)2+(δh1−δh2)2)}  (1)

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

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 21stcomponent2ndcomponentR2-P—6.86NBP—6.16NMP—4.06DMA—5.732-P (90%)NBP (10%)5.992-P (80%)NBP (20%)5.182-P (70%)NBP (30%)4.492-P (60%)NBP (40%)3.972-P (50%)NBP (50%)3.702-P (40%)NBP (60%)3.742-P (30%)NBP (70%)4.062-P (20%)NBP (80%)4.622-P (10%)NBP (90%)5.34

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.

Experimental Methods

Membrane Production on a Laboratory Scale

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:

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

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.

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.

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

Determination of Bulk Structure

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.

Determination of Specific Surface Area

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.

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).

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.

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:the equilibrium pressure p and the saturation pressure p0of the adsorbate at the temperature of adsorptionthe adsorbed gas quantity V (in terms of volume)

The BET equation is represented by Formula (2):

pV·(p0-p)=1Vm⁢o⁢n⁢o·C+C-1Vm⁢o⁢n⁢o·C·pp0(2)

The values to be calculated are:the monolayer capacity Vmono(in terms of volume)the BET constant C

To calculate the above values Vmonoand C, the BET equation was plotted as an adsorption isotherm typically at a relative pressure p/p0between 0.05 and 0.3. In this range, BET theory suggests that it should form a straight line. The values Vmonoand C can then be calculated from the slope and the intercept. Next, the total surface area Stotalcan be calculated in accordance with Formula (3) using the molecular cross-sectional area:

Stotal=Vm⁢o⁢n⁢o·NA·sVmolar(3)

In Formula (3), NAis the Avogadro constant, s is the adsorption cross-section of the adsorbing species, and Vmolaris the molar volume of the adsorbate.

The specific surface area Sspecificcan then be calculated in accordance with Formula (4) using the mass m of the sample:

Sspecific=Stotalm(4)
Determination of Permeability (Membrane Flux)

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:

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 cm2.

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.

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

J=VA·Δ⁢⁢t·Δ⁢⁢p(5)

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/(m2×h×bar).

Determination of Rejection Rate (Retention)

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:

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.

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

TABLE 3marker moleculecontentNaCl concentrationCytochrome C/0.1 mass %0.15MBovine Serum Albumin

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

TABLE 4marker moleculewavelengthBovine Serum Albumin280 nmCytochrome C550 nm

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.

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.

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

R=(1-ln⁡(1-EF·.VMEs·VA)ln⁡(1-VFVA))(6)

In Formula (6), R is the retention, EFis the extinction (absorbance) of the filtrate, ESis the extinction (absorbance) of the original marker molecule solution, VAis the starting volume of the original marker molecule solution (10 mL), VFis 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).

Formula (6) can be derived as follows, wherein cRis the concentration of the marker molecule in the retentate and cFis the concentration of the marker molecule in the filtrate:

R=cR-cFcR⁢⁢cF=cR·(1-R)

The concentration of the marker molecule in the retentate CRand the concentration of the marker molecule in the filtrate cFcan 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:

cF=d⁢Pd⁢V⁢⁢cR=PV

Rearrangement and integration leads to the following, where P0is 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 %), PEis the marker molecule mass in the retentate at the end of the filtration, VAis 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 VEis the volume of the retentate at the end of the filtration:

d⁢Pd⁢V=PV·(1-R)d⁢PP=d⁢VV·(1-R)∫P0PE⁢1P⁢d⁢P=(1-R)·∫VAyE⁢1V⁢d⁢V⁢⁢ln⁡(PEP0)=(1-R)·ln⁡(VEVA)

The marker molecule mass in the retentate at the end of the filtration PEand the volume of the retentate at the end of the filtration VEcan be expressed as follows, where PFis the marker molecule mass in the filtrate, and VFis the volume of the filtrate after the first filtration step (9.5 mL), as mentioned above:
PE=P0−PFVE=VA−VF

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

ln⁡(P0-PFP0)=(1-R)·ln⁡(VA-VFVA)⁢⁢ln⁡(1-PFP0)=(1-R)·ln⁡(1-VFVA)⁢⁢R=1-ln⁡(1-PFP0)ln⁡(1-VPVA)⁢⁢R=(1-ln⁡(1-EF·.VMEs·VA)ln⁡(1-VFVA))(6)
Performance Factor

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·R(7)

The performance factor is given in units of L/(m2×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.

PESu Ultrafiltration Membranes

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 5content thereofcomponent of casting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)20%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidonex %N-n-butyl-2-pyrrolidone71.75-x %

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 %.

The resulting performance factors of the PESu ultrafiltration membranes are shown inFIG.10.

It can be taken fromFIG.10that 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 fromFIG.10that 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:

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

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

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

TABLE 6content thereofcomponent of casting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)20%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone50.25%N-n-butyl-2-pyrrolidone21.5%

The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 233 L/(m2×h×bar) could be determined.

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

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

TABLE 7component ofcontent thereofcasting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)20%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone35.875%N-n-butyl-2-pyrrolidone35.875%

The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 249 L/(m2×h×bar) could be determined.

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

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

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

TABLE 8content thereofcomponent of casting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)20%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone71.75%N-n-butyl-2-pyrrolidone0%

The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 51 L/(m2×h×bar) could be determined.

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

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

TABLE 9of content thereofcomponent casting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)20%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone0%N-n-butyl-2-pyrrolidone71.75%

The membranes were prepared and characterized according to the methods described above. A membrane performance factor of 126 L/(m2×h×bar) could be determined.

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

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 inFIG.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 inFIG.9.

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 10of content thereofcomponent casting solution(in total 100 mass %)polyethersulfone (PESu E 6020 P)18%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone55.3125%N-n-butyl-2-pyrrolidone18.4375%

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 %.

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.

As can be taken fromFIG.6toFIG.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 inFIG.6, 13.38 μm for the membrane shown inFIG.7, and 19.90 μm for the membrane shown inFIG.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.

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 m2/g, indicating an open porous structure.

CA Ultrafiltration Membranes

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 11content thereofcomponent of casting solution(in total 100 mass %)cellulose acetate10%(cellulose diacetate L50 Daicel)polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidonex %N-n-butyl-2-pyrrolidone81.75-x %

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 %.

The resulting performance factors of the CA ultrafiltration membranes are shown inFIG.11.

It can be taken fromFIG.11that 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 fromFIG.11that the performance factor can be significantly increased compared to the performance factor achieved by using the pure solvent N-n-butyl-2-pyrrolidone.

PSu Ultrafiltration Membranes

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 12content thereofcomponent of casting solution(in total 100 mass %)polysulfone (PSf)18%polyvinylpyrrolidone (PVP K30)8.25%polyethylene glycol (PEG 400)(combined)2-pyrrolidone55.3125%N-n-butyl-2-pyrrolidone18.4375%

A membrane performance factor of 255 L/(m2×h×bar) with respect to Cytochrome C could be determined.