Method for preparing isoporous hollow fiber composite membranes

Abstract

The present invention provides a coated hollow fiber membrane which has an isoporous inner skin and a porous outer support membrane, i.e. an inside-out isoporous composite hollow fiber membrane, and to a method of preparing such membranes. The coated hollow fiber membrane is prepared by a method comprising providing a hollow fiber support membrane having a lumen surrounded by the support membrane, and coating and the inner surface thereof by first passing a polymer solution of at least one amphiphilic block copolymer in a suitable solvent through the lumen of the hollow fiber support membrane and along the inner surface thereof, thereafter pressing a core gas stream through the lumen of the coated hollow fiber mebrane, and thereafter passing a non-solvent (precipitant) through the lumen of the coated hollow fiber membrane. In order to remove the solvent or solvents completely, the membranes are kept in water for 1-2 days and washed prior to use. In order to maintain the porosity of support membrane, membrane pretreatment is advantageous prior to coating which reduces the infiltration of block copolymer solution. The membranes are useful infiltration modules, in particular microfiltration modules, ultrafiltration modules, nano-filtration modules.

Claims

1. A method for producing a coated hollow fiber membrane having an isoporous inner skin, comprising providing a hollow fiber support membrane having a lumen surrounded by the support membrane, and coating and the inner surface thereof by first passing a polymer solution of at least one amphiphilic block copolymer in a suitable solvent through the lumen of the hollow fiber support membrane and along the inner surface thereof, thereafter pressing a core gas stream through the lumen of the coated hollow fiber membrane to remove superfluous polymer solution and to provide sufficient evaporation time for self-assembly, and thereafter passing a non-solvent through the lumen of the coated hollow fiber membrane.

2. The method according to claim 1, wherein the hollow fiber support membrane having a lumen surrounded by the support membrane support membrane is polymeric material, selected from the group consisting of a cellulose acetate (CA) membrane, a polyethersulfone (PES) membrane, a polyetherimide (PEI) membrane, a polyvinylidene fluoride (PVDF) membrane, a polysulfone (PSf) membrane, a polyacrylonitrile (PAN) membrane, a polyamide-imide (PAI) membrane, a modified cellulose acetate (mCA) membrane, a modified polyethersulfone (mPES) membrane, a modified polyetherimide (mPEI) membrane, a modified polyvinylidene fluoride (mPVDF) membrane, a modified polysulfone (mPSf) membrane, a modified polyacrylonitrile (mPAN) membrane, a modified polyamide-imide (mPAI) membrane etc.; a ceramic membrane and a metallic membrane.

3. A self-supporting, coated hollow fiber composite membrane produced according to the method of claim 2.

4. A filtration module comprising at least one coated hollow fiber membranes having an isoporous inner skin prepared according to the method of claim 1.

5. A self-supporting, coated hollow fiber membrane produced according to the method of claim 2, which is electro-conductive.

6. A filtration module comprising at least one coated hollow fiber membranes having an isoporous inner skin prepared according to the method of claim 1.

7. The method according to claim 1, wherein the hollow fiber support membrane has an inner diameter ranging from 0.2 to 3.0 mm.

8. The method according to claim 1, wherein the hollow fiber support membrane has a length of from 5 cm to 80 cm.

9. The method according to claim 1, wherein the at least one amphiphilic block copolymer used for making the isoporous inner skin is a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer.

10. The method according to claim 1, wherein the polymer solution further comprises at least one metal salt or a carbohydrate, wherein the metal salt is an organic salt of Mg, Ca or Sr.

11. The method according to claim 1, wherein the core gas is selected from compressed air, nitrogen, a noble gas and/or carbon dioxide (CO2), whereby the core gas is pressed or sucked through the lumen of the support membrane with a flow rate between 0.1 mL/min and 5 ml/min.

12. The method according to claim 1, wherein the non-solvent comprises water, methanol, ethanol or a mixture of two or more thereof, and wherein the non-solvent is pressed or sucked through the lumen of the support membrane with a flow rate between 0.1 mL/min and 5 mL/min.

13. The method according to claim 1, further comprising the step of washing the fiber in a non-solvent such as water.

14. The method according to claim 1, wherein, prior to coating the hollow fiber support membrane is pre-treated by passing a non-solvent for support membrane through the lumen thereof, wherein the non-solvent comprises dioxane.

15. The method according to claim 1, wherein the lumen surrounded by the support membrane support membrane has a multi bore, triangular-polygon or star-shaped lumen architecture.

Description

DETAILED EXPLANATION OF PREFERRED EMBODIMENTS WITH REFERENCE TO THE FIGURES

(1) The invention is now described below in an exemplary manner, without restricting the general intent of the invention, based on exemplary embodiments with reference to the figures appended hereto, wherein:

(2) FIG. 1 is a schematic diagram of the method according to the present invention, wherein the hollow fiber support membrane is preferably coated from top to bottom.

(3) FIG. 2 is a schematic diagram of the method according to the present invention, wherein the hollow fiber support membrane is preferably coated from bottom to top.

(4) FIG. 3a shows an SEM of the cross-section of a PEI support membrane coated top-bottom with 1.5 wt % PS.sub.79-b-P4VP.sub.21.sup.70k in dioxane: Q.sub.dox=0.2 mL/min; Q.sub.p=0.2 mL/min; Q.sub.CO2=0.2 mL/min; Q.sub.w=0.5 mL/min; T.sub.dox=10 s; T.sub.p=15 s; T.sub.CO2=15 s.

(5) FIG. 3b shows the cross-section near the inner surface of the coated membrane of FIG. 3a. The coating thickness is about 13 μm.

(6) FIG. 3c shows the morphology of the inner surface of the coated membrane of FIG. 3a in top view.

(7) FIG. 4a shows an SEM of the cross-section of a PES support membrane coated top-bottom with 1.5 wt % PS.sub.79-b-P4VP.sub.21.sup.70k in dioxane: Q.sub.dox=1.0 mL/min; Q.sub.p=1.0 mL/min; Q.sub.CO2=1.0 mL/min; Q.sub.w=1.0 mL/min; T.sub.dox=15 s; T.sub.p=25 s; T.sub.CO2=15 s.

(8) FIG. 4b shows the cross-section near the inner surface of the coated membrane of FIG. 4a. The coating thickness is about 5 μm.

(9) FIG. 4c shows the morphology of the inner surface of the coated membrane of FIG. 4a in top view.

(10) FIG. 5a shows an SEM of the cross-section of a mPES support membrane (commercially available) coated top-bottom with 2 wt % PS.sub.79-b-P4VP.sub.21.sup.70k and 1 wt % MgAc in dioxane: Q.sub.p=1.0 mL/min; Q.sub.N2=0.5 mL/min; Q.sub.w=0.5 mL/min; T.sub.p=5 s; T.sub.N2=5 s.

(11) FIG. 5b shows the cross-section near the inner surface of the coated membrane of FIG. 5a. The coating thickness is about 3 μm.

(12) FIG. 5c shows the morphology of the inner surface of the coated membrane of FIG. 5a in cross-sectional view.

(13) FIG. 5d shows the morphology of the inner surface of the coated membrane of FIG. 5a in top view.

(14) FIG. 6a shows an SEM of the cross-section of a mPES (commercially available) support membrane coated bottom-top with 2 wt % PS.sub.82.7-b-P4VP.sub.17.3.sup.168k and 1 wt % MgAc in dioxane: Q.sub.p=1.0 mL/min; Q.sub.N2=0.5 mL/min; Q.sub.w=0.5 mL/min; T.sub.p=10 s; T.sub.N2=20 s.

(15) FIG. 6b shows the cross-section near the inner surface of the coated membrane of FIG. 6a. The coating thickness is about 3 μm.

(16) FIG. 6c shows the morphology of the inner surface of the coated membrane of FIG. 6a in cross-sectional view.

(17) FIG. 6d shows the morphology of the inner surface of the coated membrane of FIG. 6a in top view.

(18) With reference to FIGS. 1 and 2, there is shown a schematic diagram of the method according to the present invention, wherein in step A a hollow fiber support membrane having a lumen surrounded by the support membrane is provided.

(19) Prior to coating, a module which consists of transparent PVC U-tubes having an outer diameter of about 6 mm and thickness of about 1 mm were provided in the present case. The tubes were preferentially pierced at every 3 cm distance, using a bore of 2.4 mm in order to fasten the exchange of solvent/non-solvent, to lead away the water filtered through the coated membrane and to avoid floating of the modules in the precipitation bath. To hold and straighten the support fibers both ends of PVC modules were sealed using epoxy resin. The effective length was varied in the range of 10-20 cm and a typical preparation procedure started with modules having one support membrane. A larger module can contain bundle of longer fibers as well.

(20) The hollow fiber support membrane may thereafter be pretreated with a non-solvent for support membrane and which shows good miscibility with the block copolymer solution, such as dioxane or dioxane/acetone.

(21) In steps B, C and D, three fluids were pumped from top to bottom (FIG. 1) or bottom to top (FIG. 2), respectively, as indicated by the arrows, for a certain time period through the module to achieve isoporous surface on the inner side: in step B the polymer solution as coating material; in step C a core gas for removal of superfluous polymer solutions and for providing sufficient evaporation time for self-assembly; in step D a non-solvent, such as water, for precipitation of the coated layer. Fibers with newly developed thin selective layer were then washed and kept in DI water. In step C, nitrogen (N.sub.2) and carbon dioxide (CO.sub.2) were used as gaseous fluid. To control the flow rates high-precision syringe pumps were used. The type of polymer solutions, flow rates and purge times are indicated in the description of FIGS. 3a, 4a, 5a, and 6a, respectively. Further, the fluids can be either pumped through or sucked out for coating, depending on the requirements and availability of the system.

(22) The morphology of the membranes was investigated using scanning electron microscopy (SEM). Specimens for the SEM of cross-section measurements were prepared by freezing the membrane samples in liquid nitrogen. The membrane surfaces and cross-sectional pieces were coated with a 2 nm thin platinum layer.

(23) Following abbreviations were used to define the coating parameters:

(24) Dioxane pretreatment flow rate (Q.sub.dox); polymer solution flow rate (Q.sub.p); nitrogen (N.sub.2) flow rate (Q.sub.N2) or carbon dioxide (CO.sub.2) flow rate (Q.sub.CO2); water flow rate (Q.sub.w); time of dioxane pretreatment (T.sub.dox); time of flow for polymer solution (T.sub.p); time of flow for N.sub.2 (T.sub.N2); time of flow for water (T.sub.w.

(25) Additives: Magnesium acetate (MgAc) or α-cyclodextrin

(26) Polymer characteristics (where subscripts denote the amount of respective block in wt. % and the digits in brackets show the number average molar mass in kg/mol): 1. PS.sub.86.5-b-P4VP.sub.13.5 (82.8 kg/mol) 2. PS.sub.79-b-P4VP.sub.21 (70 kg/mol) 3. PS.sub.82.7-b-P4VP.sub.17.3 (168 kg/mol) 4. PS.sub.83-b-P4VP.sub.17 (139 kg/mol)

(27) Used Support Membranes: 1. Polyetherimide (PEI) membranes 2. Polyethersulfone (PES) membranes 3. Modified PES (mPES) membranes (commercial).

(28) FIGS. 3a to 6d show the results of four samples prepared using the method according to the present invention. FIGS. 3a to 5d display results of the coating experiments performed in top-bottom direction on three different support hollow fiber membranes: PEI, PES and mPES, while FIGS. 6a to 6d show the results of the coating performed in bottom-top direction on mPES support. The cross-section images highlight uniform coating of thicknesses ranging from 3 to 15 μm (see description of Figures). Inner surface morphology shows that the method provides inside-out isoporous composite hollow fiber membranes. The porosity on inner surface can be increased by e.g. increasing the amount of additives, and variation in coating parameters e.g., flow rate and time of flow for gas stream.