Membrane for Capillary Microfiltration

Abstract

The present disclosure provides a hydrophilic, integrally asymmetric, semi-permeable hollow-fiber membrane made from a hydrophobic aromatic sulfone polymer and at least one hydrophilic polymer, the membrane comprising an inner surface facing towards its lumen, an outer surface facing outwards and an intermediate wall having a wall thickness and comprising an open-pore separating layer and an supporting layer having an asymmetric, sponge-like structure without finger pores, wherein adjoining to the wall of the inner surface the hollow-fiber membrane comprises an essentially isotropic zone; after which the pore size abruptly start increasing up to a maximum, after which the pore size decrease again, then adjoining an essentially isotropic supporting layer which then is adjoined by the outer surface, wherein the separating layer has a cut-off of greater than 300 000 Daltons. The present disclosure further provides a method for producing such membranes and a use of the membranes for microfiltration purposes.

Claims

1. Hydrophilic, integrally asymmetric, semi-permeable hollow-fiber membrane made from a hydrophobic aromatic sulfone polymer and at least one hydrophilic polymer, the membrane comprising an inner surface facing towards its lumen, an outer surface facing outwards and an intermediate wall having a wall thickness and comprising an open-pore separating layer and an supporting layer having an asymmetric, sponge-like structure without finger pores, wherein adjoining to the wall of the inner surface the hollow-fiber membrane comprises an essentially isotropic zone; after which the pore size abruptly start increasing up to a maximum, after which the pore size decrease again, then adjoining a an essentially isotropic supporting layer which then is adjoined by the outer surface, wherein the separating layer has a cut-off of greater than 300 000 Daltons.

2. The hollow-fiber membrane according to claim 1, wherein the membrane exhibits a nominal pore size in the separation layer in the range of from 45 to 150 nm, preferably from 50 to 140 nm, more preferably in the range of from 55 to 130 nm.

3. The hollow-fiber membrane according to claim 1, wherein the essentially isotropic zone adjoining to the wall of the inner surface the hollow-fiber membrane has a proportion in the range of from 1 to 8%, preferably from 2 to 7%, more preferably from 3 to 6% of the total thickness of the membrane wall.

4. The hollow-fiber membrane according to claim 1, wherein the essentially isotropic zone adjoining to the wall of the inner surface comprises the open-pore separation layer.

5. The hollow-fiber membrane according to claim 1, wherein the wall thickness is in the range of from 140 to 400 μm, preferably in the range of from 150 to 380 μm, more preferably in the range of from 160 to 360 μm.

6. The hollow-fiber membrane according to claim 1, wherein the inner diameter of the hollow-fiber membrane is in the range of from 700 to 2000 μm, preferably from 800 to 1800 μm, more preferably from 900 to 1600 μm.

7. The hollow-fiber membrane according to claim 1, wherein the pores of the outer surface exhibit maximum diameters of less than 1.5 μm, preferably less than 1.2 μm, more preferably of less than 1 μm, even more preferably less than 900 nm.

8. The hollow-fiber membrane according to claim 1, wherein the inner pores of the inner surface exhibit maximum diameters of less than 3 μm, preferably of less than 2.5 μm, more preferably of less than 2 μm.

9. The hollow-fiber membrane according to claim 1, wherein the zone with maximum pore size is located at a distance from the inner surface in the range between 15 and 40% of the wall thickness.

10. The hollow-fiber membrane according to claim 1, wherein the size of the maximum pores in the zone with maximum pore sizes is in the range of from 5 to 50 μm, preferably from 10 to 45 μm, more preferably from 15 to 50 μm.

11. The hollow-fiber membrane according to claim 1, wherein the membrane exhibits a trans membrane flow for water of at least 4 mL/(cm2.Math.min.Math.bar), preferably at least 5 mL/(cm2.Math.min.Math.bar), more preferably at least 6 mL/(cm2.Math.min.Math.bar), and even more preferably at least 7 mL/(cm2.Math.min.Math.bar).

12. The hollow-fiber membrane according to claim 1, wherein the membrane exhibits a tensile strength of at least 650 cN, preferably of at least 750 cN, and more preferably of at least 850 cN.

13. The hollow-fiber membrane according to claim 1, wherein the membrane exhibits an elongation in the range of from 20 to 60%, preferably from 22 to 52%, more preferably from 24 to 50%.

14. A process for producing a hollow-fiber membrane, comprising the following steps: (i) Providing a spinning solution comprising at least one hydrophobic aromatic sulfone polymer and at least one hydrophilic polymer; (ii) Providing a bore liquid comprising water and glycerol; (iii) Spinning a hollow fiber with a spinneret outer diameter for dope in the range of from 1100 to 3000 μm, a spinneret needle outer diameter in the range of from 600 to 2200 μm and a spinneret needle inner diameter in the range of from 400 to 1500 μm.

15. Use of the hollow-fiber membrane according to claim 1 for microfiltration of aqueous liquids.

Description

DESCRIPTION OF FIGURES

[0057] FIG. 1: SEM picture of 20000× magnification of the outer surface of a hollow-fiber membrane according to the present disclosure.

[0058] FIG. 2: SEM picture of 8000× magnification of the inner surface of a hollow-fiber membrane according to the present disclosure.

[0059] FIG. 3: SEM picture of 100× magnification of a cross-section of a hollow-fiber membrane according to the present disclosure.

[0060] FIG. 4: SEM picture of 1000× magnification of a cross-section of a hollow-fiber membrane according to the present disclosure.

EXAMPLES

[0061] The present disclosure is further described without however wanting to limit the disclosure thereto. The following examples are provided to illustrate certain embodiments but are not meant to be limited in any way. Prior to that some test methods used to characterize materials and their properties will be described. All parts and percentages are by weight unless otherwise indicated.

[0062] Test Methods

[0063] Volume Porosity:

[0064] A sample of at least 0.5 g of the membrane to be examined is dry weighed. The membrane sample is subsequently placed in a liquid that moistens the membrane material, however without causing swelling, for 24 hours such that the liquid penetrates into all pores. A silicone oil with a viscosity of 200 mPa s at 25° C. (Merck) is used. The permeation of liquid into the membrane pores is visually discernable in that the membrane sample changes from an opaque to a glassy, transparent state. The membrane sample is subsequently removed from the liquid, liquid adhering to the membrane sample is removed by centrifuging at approx. 1800 g, and the mass of the thus pretreated wet, i.e. liquid-filled, membrane sample is determined by weighing.

[0065] The volume porosity c is determined according to the following formula:

[00001]$\mathrm{Volume}\mathrm{porosity}\mathrm{.Math.}=\frac{\left(m_{\mathrm{wet}}-m_{\mathrm{dry}}\right)/\rho \mathrm{liquid}}{\left(m_{\mathrm{wet}}-m_{\mathrm{dry}}\right)/\rho \mathrm{liquid}+m_{\mathrm{dry}}/\rho \mathrm{polymer}}$

[0066] where: [0067] m.sub.dry=weight of the dry membrane sample after wetting and drying [g] [0068] m.sub.wet=weight of the wet, liquid-filled membrane sample [g] [0069] ρ.sub.liquid=density of the liquid used [g/cm.sup.3] [0070] ρ.sub.polymer=density of the membrane polymer [g/cm.sup.3]

[0071] Maximum Separating Pore:

[0072] The diameter of the maximum separating pore is determined by means of the bubble point method (ASTM No. 128-99 and F 316-03), for which the method described in DE-A-36 17 724 is suitable. Thereby, d.sub.max results from the vapor pressure P.sub.B associated with the bubble point according to the equation

d.sub.max=σ.sub.B/P.sub.B

[0073] where σ.sub.B is a constant that is primarily dependent on the wetting liquid used during the measurement. For IPA, σ.sub.B is 0.61 μm.Math.bar at 25° C.

[0074] Nominal Pore Size The nominal pore size in the separating layer is determined by perm porometry according to ASTM F 316-03 with the PMI Advanced Porometer CFP-1020-APLC-GFR (PMI, Ithaca, N.Y., US).

[0075] Transmembrane Flow (Water Permeability):

[0076] A test cell with a defined number of hollow fibers and length is produced from the hollow-fiber membranes to be tested. For this, both ends of the hollow fibers are embedded in a polyurethane resin. After setting of the resin, the embeddings are cut to a length of approx. 30 mm with the lumina of the hollow-fiber membranes being opened by the cut. The hollow-fiber lumina in the embeddings must be checked for free passage. The free length of the hollow-fiber membranes between the embeddings is normally 120+/−10 mm. The number of hollow-fiber membranes must be such that, allowing for the free length and inside diameter of the hollow-fiber membranes, a filtration surface area of approx. 30 cm.sup.2 is provided in the test cell.

[0077] The test cell is integrated into a test apparatus through which ultrafiltrated and deionised water conditioned to 25° C. flows with a defined test pressure (approx. 0.4 bar). The filtrated water volume obtained over a measuring time of 2 minutes, i.e. the permeate produced during the measurement, is determined gravimetrically or volumetrically. Before the start of the measurement, the system must be purged air-free. In order to determine the TMF, the inlet and outlet pressure at the test cell are measured in the test apparatus. The measurement is performed at 25° C.

[0078] The transmembrane flow TMF is calculated using formula (III)

[00002]$\begin{array}{cc}\mathrm{TMF}=\frac{\mathrm{Vw}}{\Delta t.Math.A_M.Math.\Delta p}\left[\frac{\mathrm{ml}}{{\mathrm{cm}}^2.Math.\min .Math.\mathrm{bar}}\right]& \left(\mathrm{III}\right)\end{array}$

[0079] where: [0080] V.sub.W=Water volume flowing through the membrane sample during the measuring time [ml] [0081] Δt=Measuring time [min] [0082] A.sub.M=Area of the membrane sample exposed to the flow (normally 30 cm.sup.2) [0083] Δ.sub.p=Pressure set during the measurement [bar]

[0084] Characterisation of the Cut-Off by Determination of the Retention Capacity for Dextran Molecules of Different Molar Mass

[0085] A polydisperse aqueous dextran solution (pool) flows in crossflow mode toward the membrane to be characterised. A defined wall shear rate and a defined filtrate flow density through the membrane is set. The content of dextran molecules of different molar mass MW in the filtrate flow or pool is determined by means of gel permeation chromatography (GPC).

[0086] The GPC spectrum of the pool or filtrate is thereby divided into 40 equidistant sections whose area is determined by numerical integration. A molar mass is assigned to each of these time intervals according to the calibration spectrum that is determined using monodisperse dextran molecules of known molar mass. The sieving coefficient of the membrane compared with dextran molecules of the molar mass MW is obtained by forming the ratio of the area segments of the GPC spectra of the filtrate and the pool assigned to this molar mass.

[00003]$\begin{array}{cc}{\mathrm{SK}}_{\mathrm{MW}}=\frac{\mathrm{Area}\left(\mathrm{MW},\mathrm{permeate}\right)}{\mathrm{Area}\left(\mathrm{MW},\mathrm{pool}\right)}& \left(\mathrm{IV}\right)\\ \mathrm{Retention}=\left(1-\mathrm{SK}\right).Math.100\left[\%\right]& \left(V\right)\end{array}$

[0087] The retention coefficient R.sub.MW for dextran molecules of the molar mass MW is calculated as follows:

R.sub.MW=1−SK.sub.MW  (VI)

[0088] Since the determined retention profile is highly dependent on the test conditions (concentration polarisation), the filtrate flow density and wall shear rate must be clearly defined when determining the retention profile. For a hollow-fiber membrane module of length l containing n hollow-fiber membranes, filtrate flow density Q.sub.F and axial volumetric flow Q.sub.L are calculated as follows:

[00004]$\begin{array}{cc}{Q}_L=\frac{n.Math.d^3.Math.y_w}{1.69.Math.{10}^{11}}& \left(\mathrm{VII}\right)\end{array}$ [0089] γ.sub.W: Wall shear rate=2000/s [0090] d: Inside diameter of the hollow-fiber membranes [μm] [0091] n: Number of hollow-fiber membranes in the membrane module [0092] Q.sub.L: Axial volumetric flow in the lumen of the hollow-fiber membranes [ml/min]

Q.sub.F=n.Math.π.Math.d.Math.I.Math.V.sub.L.Math.10.sup.−9  (VIII) [0093] Q.sub.F: Filtrate flow rate [ml/min] [0094] l: Free length of the hollow-fiber membrane in the membrane module [cm] [0095] V.sub.L: Velocity in the lumen [cm/min] (V.sub.L=4.Math.10.sup.8.Math.Q.sub.L/(n.Math.π.Math.d.sup.2)) [0096] n: Number of hollow fibers in the membrane module

[0097] Composition of the dextran solution employed (manufacturer: Pharmacia Biotech; article designations: T10, T40, T70, T500)

TABLE-US-00001 Dextran type: T10 T40 T70 T500 Weight: 0.50 g/l 0.60 g/l 0.7 g/l 0.7 g/l

[0098] The solutions are mixed with deionised water.

[0099] Breaking Force, Breaking Strength

[0100] The breaking force of the hollow-fiber membranes is measured using a standard universal testing machine from Zwick, Ulm.

[0101] The hollow-fiber membrane sample is drawn at constant speed in the longitudinal direction until it breaks. The force required is measured in relation to the change in length and recorded in a force/elongation diagram. The measurement is performed as a multiple determination on several hollow-fiber membrane samples with 100 mm clamped length and at a drawing speed of 500 mm/min. The pretension weight is 2.5 cN. The force BK required for the break is output as a mean numerical value in cN.

[0102] The breaking strength GB of the hollow-fiber membrane sample is obtained by standardisation of the breaking force BK to the cross-sectional area A.sub.Q of the membrane wall.

[0103] Bursting Pressure

[0104] An approx. 40 cm long hollow-fiber membrane sample is formed as a loop with its ends embedded e.g. in polyurethane resin.

[0105] The membrane is wetted on the lumen side with a test liquid of 1.5 g/l methyl cellulose in water in order to fill the pores of the membrane while maintaining the pore structure. This makes the membrane walls impermeable to gas. Nitrogen is then admitted to the lumen side of the hollow-fiber membrane sample, whereby a linear increase in pressure (2 bar/min) is generated at the sample using a pressure booster station, throttle valve and high-pressure reservoir.

[0106] The pressure at the inlet to the sample is measured and documented on a plotter. The pressure is increased until the membrane sample bursts. When the membrane sample bursts or explodes, the pressure at the test cell drops suddenly. The pressure at the reversing point of the pressure increase is read off as the bursting pressure.

[0107] Force and Elongation at Break:

[0108] Measuring the force at break of the membrane takes place using a standard, universal testing machine from Zwick (Ulm, Germany).

[0109] The hollow-fiber membrane sample is drawn at constant speed in the longitudinal direction until it breaks. The force required is measured in relation to the change in length and recorded in a force/elongation diagram. The measurement is performed as a multiple determination on several hollow-fiber membrane samples with 100 mm clamped length and at a drawing speed of 500 mm/min. The pretension weight is 2.5 cN. The elongation at break is output as a mean numerical value in % of the original length.

Example 1

[0110] A spinning solution was prepared by intensively mixing 21 wt.-% polyethersulfone (Ultrason E 6020, BASF), 12.6 wt.-% polyvinylpyrrolidone (PVP K30, ISP), 31.54 wt.-% ε-caprolactam, 31.54 wt.-% γ-butyrolactone, 3.32 wt.-% glycerol and 0.8% water at a temperature of about 100° C. The resulting spinning solution was cooled down to about 60° C., filtrated and degassed. A spinneret tempered to 35° C. and having and outer diameter for dope of 0.22 mm, a needle outer diameter of 0.12 mm and a spinneret needle inner diameter of 930 μm was used. By using the above-mentioned spinning solution and a mixture of ε-caprolactam, glycerol and water in a ratio of 47:37:16 as bore liquid in the spinneret needle of the spinneret, a hollow fiber was generated. This hollow fiber was transferred through a climate chamber conditioned to a temperature of 75° C. and 85% relative humidity such that a residual time of about 6 s was maintained. After that, the hollow fiber was transferred into a water-containing precipitation bath tempered to about 68° C., thereby fixing the membrane structure. Directly after this coagulation and fixation of the membrane, the wet hollow-fiber membrane was assembled to hollow-fiber membrane bundle having a length of about 1.3 m and comprising about 450 hollow-fiber membranes, extracted with water having a temperature of about 90° C. for about 1 h and subsequently dried with air at a temperature of about 90° C. for about 2 h. The hollow-fiber membranes obtained via this procedure had a physical inner diameter of about 1200 μm and a wall thickness of about 280 μm.

[0111] Further properties of the hollow-fiber membranes according to example 1 are summarized in table 1.

Comparative Example 1

[0112] In order to produce a homogeneous spinning solution, 21.00 wt. % polyether sulfone (Ultrason E 6020, BASF), 12.60 wt. % polyvinylpyrrolidone (PVP K30, ISP), 31.54 wt. % ε-caprolactam, 31.54 wt. % γ-butyrolactone and 3.32 wt. % glycerine were intensively mixed at a temperature of approx. 100° C. The resulting solution was cooled to approx. 60° C., degassed, filtered and conveyed to the annular gap of a hollow-fiber die maintained at 35° C. with a gap width of 0.24 mm and an inside diameter of the die needle of 0.6 mm. For the formation of the lumen and the lumen-side separating layer, an interior filler consisting of ε-caprolactam/glycerine/water in the ratio of 47:37:16 by weight was extruded through the needle of the hollow-fiber die. The hollow fiber formed was conducted through a conditioned climate-controlled channel (climate: 60° C.; 60% relative humidity, t=4 s), precipitated in a precipitation bath containing water conditioned to approx. 70° C., and the membrane structure fixed. Immediately after fixing, the wet membrane was made up to approx. 1 m long hollow-fiber membrane bundles with approx. 900 hollow fibers, extracted for 3 hours with approx. 90° C. hot water and subsequently dried for approx. 2 hours with 90° C. hot air. The hollow-fiber membranes contained in the bundles had a lumen diameter of approx. 0.75 mm and a wall thickness of approx. 0.22 mm.

[0113] The membrane exhibited a transmembrane flow in water TMF.sub.W of 1.28 ml/(cm.sup.2.Math.min.Math.bar). A cut-off of approx. 62 000 daltons was determined from the separation curve obtained with dextrans. In the tensile test, the membranes showed a breaking force of 510 cN, corresponding to a breaking strength of approx. 760 cN/mm.sup.2. The resulting product of the transmembrane flow and breaking force determined in this manner was 653 cN.Math.ml/(cm.sup.2.Math.min.Math.bar). The bursting pressure of the hollow-fiber membranes in this example was 15.75 bar.

[0114] The examination under the scanning electron microscope showed the membrane to have a separating layer with a thickness of approx. 6 μm on its lumen side, that was adjoined towards the outside by an approx. 160 to 170 μm thick supporting layer, within which the size of the pores increased sharply starting from the separating layer up to a zone with maximum pore size at approx. 20 to 25% of the wall thickness, and after passing through the maximum decreased towards the outside up to an outer layer. The supporting layer was adjoined by the outer layer with a thickness of approx. 50 μm, within which an essentially isotropic pore structure, i.e. an essentially constant pore size, prevailed.

Comp. Ex. 2

[0115] In order to produce a homogeneous spinning solution, 19.46 wt. % polyether sulfone (Ultrason E 6020, BASF), 13.65 wt. % polyvinylpyrrolidone (PVP K30, ISP), 31.91 wt. % ε-caprolactam, 31.61 wt. % γ-butyrolactone and 3.37 wt. % glycerine were intensively mixed at a temperature of approx. 100° C. The resulting solution was cooled to approx. 60° C., degassed, filtered and conveyed to the annular gap of a hollow-fiber die maintained at 35° C. with a gap width of 0.16 mm and an inside diameter of the die needle of 0.6 mm. For the formation of the lumen and the lumen-side separating layer, an interior filler consisting of ε-caprolactam/glycerine/water in the ratio of 45:37:18 by weight was extruded through the needle of the hollow-fiber die. The hollow fiber formed was conducted through a conditioned climate-controlled channel (climate: 60° C.; 60% relative humidity, t=4 s), precipitated in a precipitation bath containing water conditioned to approx. 75° C., and the membrane structure fixed. Immediately after fixing, the wet membrane was made up to approx. 1 m long hollow-fiber membrane bundles with approx. 900 hollow fibers, extracted for 3 hours with approx. 90° C. hot water and subsequently dried for approx. 2 hours with 90° C. hot air. The hollow-fiber membranes had a lumen diameter of approx. 0.70 mm and a wall thickness of approx. 0.15 mm.

[0116] The examination under the scanning electron microscope showed the membrane to have a separating layer with a thickness of approx. 6 μm on its lumen side, that was adjoined towards the outside by a supporting layer within which the size of the pores increased starting from the separating layer up to a zone with maximum pore size at approx. 25 to 30% of the wall thickness, and after passing through the maximum decreased towards the outside up to an outer layer. The pores in the zone of maximum pore size were smaller in the hollow-fiber membrane in Comp. Ex. 2 than in the hollow-fiber membrane produced according to Comp. Ex. 1. The supporting layer was adjoined by the outer layer with essentially isotropic pore structure, i.e. an essentially constant pore size, and a thickness of approx. 45 μm.

[0117] The properties of the membranes are summarized in table 1.

TABLE-US-00002 TABLE 1 Properties of the membranes according to the examples and comparative examples. Ex. 1 Comp. Ex. 1 Comp. Ex. 2 Inner diameter [μm] 1200 750 700 Wall thickness [μm] 280 220 150 TMF [mL/cm.sup.2 min bar] 7 1.28 1.36 Nominal pore diameter in 90 separation layer [nm] Tensile strength [cN] 930 510 253 Elongation [%] 27 Burst pressure [bar] 15.0 15.75 11.5 Implosion pressure [bar] 5.8 Bubble point in IPA [bar] 1.4 Dextrane cut-off [Dalton]  300,000 about 100,000 about 100,000