WO2006063426A1

WO2006063426A1 - Reinforced hollow fibre membrane

Info

Publication number: WO2006063426A1
Application number: PCT/CA2004/002133
Authority: WO
Inventors: Steven Kristian Pedersen; John David Ireland; Hamid R. Rabie
Original assignee: Zenon Environmental Inc.
Priority date: 2004-12-15
Filing date: 2004-12-15
Publication date: 2006-06-22

Abstract

A hollow fiber membrane has a hollow, cylindrical microporous membrane wall having a first density and one or more generally parallel reinforcement structures embedded in spaced apart relationship within the wall. The reinforcements structure span the length of the membrane and have a second density greater than the first density to function as reinforcements for the membrane. The membrane may be made by extruding strands of a melted polymer into an annular stream of a dope and then solidifying the melted polymer and dope to form a porous membrane wall embedded with solid strands of polymer.

Description

Reinforced Hollow Fibre Membrane Field of the invention

[0001] This invention relates to hollow fibre filtering membranes and methods of manufacturing membranes.

Background of the invention [0002] By its very nature of being porous, a membrane is a weak structure. To address this problem, US Patent No. 5,472,607 describes a hollow fiber membrane having a thin porous membrane film coated onto a tubular support. The tubular support is a woven or knitted braid of ends or yarns of non-porous fibres having spaces between the ends larger than the pores in the film. The film does not fully penetrated into the braid, but is reinforced by the braid to create a strong composite structure. However, the braid adds to the cost of the membrane and there are occasionally problems with the film peeling away from the braid.

Summary of the invention [0003] It is an object of the invention to improve on, or at least provide a useful alternative to, the prior art. It is another object of the invention to provide a reinforced membrane and a process for making such a membrane. The following summary is intended to introduce the reader to the invention which may reside in a combination or sub-combination of elements or steps described below or in other parts of this specification, for example the claims.

[0004] In one aspect, the invention provides a membrane fiber having one or more reinforcing structures wholly or partially embedded within its otherwise porous walls. The reinforcing structures may also traverse the lumen. A single reinforcement support or a plurality of supports may be used. The hollow fiber membrane has a hollow, cylindrical polymer membrane having a first density, and a plurality of reinforcement structures, for example strands of a solid polymer, longitudinally spanning the membrane and having a second density greater than the first density. The membrane material is sufficiently porous to allow filtration of a liquid through the walls of the membrane and the reinforcing structures are stronger than the porous membrane material, even if the reinforcing strands and membrane material are of the same polymer. Accordingly, the tensile strength of the reinforced fiber may be two or more, for example four to eight, times the tensile strength of the porous material alone. This results in numerous benefits, such as reduced rates of breakage, when the membranes are in use.

[0005] In another aspect, the invention relates to a process for making a reinforced membrane fiber. A single reinforcing support strand or a plurality of supports, for example comprising a melted polymer is co-extruded or spun through a draw plate, nozzle or spin pack with a dope used to form a porous membrane wholly or partially surrounding the reinforcing strands and, optionally, a centering fluid or centering polymer may be used, temporarily occupying the lumen of the fiber. The resulting fiber may be quenched after leaving the draw plate or otherwise treated to solidify the reinforcing strands and coagulate, phase separate, or otherwise transform the membrane dope into a material that is, or can be made to be, microporos. The reinforcing strands may be melt spun or extruded. A Thermal Induced Phase Separation (TIPS) process, a Non-solvent Induced Phase Separation (NIPS) or hybrid process may be used to create the porous membrane. Brief description of the drawings

[0006] For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which:

[0007] Figures 1 and 1A-F show cross sections of reinforced hollow fiber membranes.

[0008] Figure 2 shows a cross-sectional elevation view of a nozzle and bottom plan views of various nozzles for producing the hollow fiber membranes of Figure 1.

[0009] Figure 3 shows a system including a nozzle of Figure 2 for producing a hollow fiber membrane of Figure 1. Detailed description of the invention

[0010] Figures 1 and 1A-F show various cross sections of reinforced hollow fiber membranes 10. Each membrane 10, which is a reinforced composite system, includes a porous wall 12 in which is embedded a plurality of solid reinforcement structures 14, which can be strands or fibers of one or more solid fibers. The reinforcing structure 14 can also be a single structure which traverses the lumen. The fiber has an outside diameter in the range of about 0.5 - 2.5 mm and an inside diameter range of about 0.25 to 1.5 mm. The diameter of each of the structures 14 is about 0.05-0.10 mm, or about 20- 45% of the wall thickness. There may be about 1 to 30 or more structures 14, depending on reinforcing structure 14 chosen, which comprise about 10-30%, about 10-25% or about 12-25% of the cross sectional area, the remaining cross sectional area being comprised of the porous wall 12.

[0011] The porous wall 12 may have pores in the Microfiltration,

Ultrafiltration, or Nanofiltration ranges and can use NIPS based or TIPS based chemistries and manufacturing systems. Examples include but are not limited to: PP, PE, PVDF, ECTFE, PES, THV, Polyesters, LCP; essentially all polymers, copolymers and terpolymers for which a true solvent or latent solvent exists. The reinforcing structures 14 supporting the porous wall can be made out of any of the above polymers, or other polymers that can be co- extruded with the membrane forming dope. Membrane-reinforcement combinations may be of the same polymer, or different polymers, such as PET reinforcement in an ECTFE or PVDF membrane. Reinforcing structures 14 may be of polymer blends as well, for example PET/PC or PET/PEN (miscible and will trans-esterify at high processing temperatures). In selecting polymers, the melt temperature ranges, drawing rates and melt viscosities of the polymers is considered along with the viscosity of the dope for the porous wall 12. In particular, the melt viscosity of the structures 14 should be higher than the viscosity of the dope used to create the membrane walls 12. This tends to occur naturally when the same polymer is used for the walls 12 and structures 14 and the walls 12 are created by a TIPS process since the polymer for the walls 12 is diluted to create a dope having lowered viscosity. Other process for forming the walls 12 that result in a dope of suitable viscosity may also be used.

[0012] The inclusion of the structures 14 substantially increases the strength of the membrane 10. For example, in a typical TIPS process, the base polymer is mixed with a diluent. The mixture is heated, extruded and quickly cooled to quench the membrane forcing the diluent and polymer to phase separate. The diluted polymer is significantly lower in melt viscosity than the pure polymer. Because of the low fraction of polymer typically used, the final membrane has a large void fraction. As a result, the membrane has significantly reduced strength. Taking the specific example of a TIPS manufactured PVDF microfiltration hollow membrane of size 1.4mm (outer diameter) and 0.9mm (inner diameter), the tensile break strength is about 1 Newton. A pure PVDF fiber of the same size would typically have a break strength of about 31 Newtons. Thus, making the fiber porous results in a 30- fold reduction in the mechanical strength. However, when reinforcement structures 14 of the same undiluted polymer present in the membrane 12 are introduced, only a five-fold decrease in strength occurs when the structures occupy only about 20% of the cross sectional area. In other words, the addition of the reinforcement structures 14 results in a six-fold increase in strength as compared to the non-reinforced porous fiber. Other example structures may have differing tensile strength contributions due to adhesion area and structural mechanics.

[0013] Figure 2 shows a nozzle 20, alternately called a draw plate, spinneret or spin pack, used to form the reinforced membrane system 10, specifically the membrane system 10 of Figure 1. The nozzle 20 has a top plate 22 and a bottom plate 24. The top plate 22 has a centering fluid inlet 26, a polymer melt inlet 28 and a dope inlet 30. The centering fluid inlet 26 communicates with a central bore 32. The polymer melt inlet 28 communicates with an annular polymer melt distribution ring 34. The dope inlet 30 communicates with an annular dope distribution ring 36. The bottom plate 24 comprises an outer ring 38, needle retaining ring 40, needles 42 and central nozzle 44. These structures interact to form an annular dope channel 46 concentric with and outside of the central nozzle 44 and pierced by the needles 42. The central nozzle 44 communicates with the central bore 32. The needles 42 communicate with the polymer melt distribution ring 34. The dope channel 46 communicates with the dope distribution ring 36. In this way, centering fluid injected into the centering fluid inlet 26 flows out of the central nozzle 44 to occupy the space that will be the lumen of the membrane 10. Dope for creating the porous wall 12 flows into the dope inlet 30 and out through the bottom of the dope channel 46 around the centering fluid. Melted polymer for the structures 14 flows into the polymer melt inlet 28 and out through the needles 42 at a location with the walls 12. The alternate bottom views of the nozzles in Figure 2, labeled 20A-F, depict the various configurations needed to produce the structures 14 in Figures 1A-F. Bottom view of nozzle 2OA corresponds with the sectional side view in the top of Figure 2 and shows needles 42 that produce a round structure 14 as shown in Figures 1 and 1A. Alternate needles 47, 48, 49 in nozzles 2OB, 2OC, and 2OD respectively, depict the other extrusion profiles that are needed to produce the supporting structures depicted in Figure 1 B, 1 E and 1 D. The die profiles depicted for nozzles 2OE and 2OF are needed to produce the support structures shown in figures 1F and 1C. In this case the support material flows from channel 51 , the centering fluid from passage 50, and the membrane forming dope from 46.

[0014] Figure 3 shows a system 100 for producing the hollow fiber membrane 10. The system 100 includes the nozzle 20 described above. The system 100 also has a dope supply system 100A in communication with the dope inlet 30, a melt extrusion system 100B in communication with the polymer melt inlet 28 and a centering fluid system 100C in communication with the centering fluid inlet 26. According to a TIPS process, a polymer solution (diluent) blend 109 in the shape of the wall of the membrane 10 is produced at the bottom of the nozzle 20, the polymer solution blend 109 having strands of a denser liquid composition comprising a polymer wholly or partially embedded therein. The blend 109 is then cooled in a bath 111 , to a temperature between 40 and 150⁰C, to induce pore formation in the walls 12, for example by phase separation, and complete solidification of the walls 12 and structures 14 if necessary. The resulting membrane 10 passes over rollers 110 and is taken up on spool 112, for example at a rate between 20 and 200 ft per minute. The resulting membrane 10 may later be soaked in a second solvent to remove the diluent from the membrane 10 so as to open up the pores of the membrane 10.

[0015] Returning to the melt extrusion system 100B, polymer for the structures 14 is fed into a hopper 116 leading to an extruder 108 where the polymer is melted at a temperature range of 180 to 28O⁰C and pressurized to between 50 and 400psi to flow through a zenith gear pump 102 and Mott filter pack 103 in the range of 10 to 100 micron. The pipes 104 conveying the melt are heated to a temperature in the range of 180 to 28O⁰C and reach the nozzle 20 at an appropriate temperature and viscosity. The melted polymer forms the solid reinforcement structures 14 upon cooling to the bath temperature between 40 and 150°C after leaving the nozzle 20.

[0016] In the centering fluid system 100C, a centering fluid, such as water, EG, or PEG is pushed from a heated (50 to 28O⁰C) and pressurized (50 to 400 psi) centering fluid tank 107 to a centering fluid metering pump 105 such as a zenith spin finish pump. The centering fluid passes through a 200 mesh stainless or plastic filter 103 and heated pipes 104 to arrive at the centering fluid inlet 26 of the nozzle 20 at a temperature in the range of 50 to 280⁰C.

[0017] In the dope supply system 100A, a solvent (diluent) and an inorganic filler are blended together in a blender 113. Other elements, such as a nucleating agent, may also be included in the solvent and inorganic filler blend. The resultant blend is homogenized with a polymer such as those mentioned earlier for making membranes, in a heated (50 to 280⁰C) and pressurized (50 to 400 psi) dope mixer 101 or alternatively through a secondary extruder (not shown) and fed through a zenith gear pump 102 and Mott filter 103. The dope flows through heated (50 to 280⁰C) piping 104 to arrive at the dope inlet 30 at an appropriate temperature and viscosity. In particular, the dope should be 5 to 30% less viscous than the polymer used for the structures 14. The blend temperature is in the range of about 50 to 28O⁰C depending on the dope formulation. The concentrations of the diluent in the dope or blend may also be adjusted to alter the viscosity of the dope provided that the concentration remains suitable for the target pore size and transport rate of the wall 12. For example, a blend may be made between ECTFE, a trimellitate plasticizer and an inorganic filler in the ratio of 20 to 50% polymer, 80 to 50% trimellitate plasticizer and 0 to 15% inorganic filler for the dope. This dope blend can be co-extruded with a lower viscosity 100% ECTFE, or a blend of ECTFE and trimellitate plasticizer in a ratio of 70 to 95% polymer for the supporting structure 14. As mentioned earlier, the centering fluid may be any of a family of glycols such PEG or PEG and water in a ratio of 70 to 80% PEG. Other suitable membrane chemistries and methods of manufacture are described in International Application No. PCT/CA2004/001846 which is incorporated herein in its entirety by this reference to it.

[0018] The description above is by way of example and does not limit the invention which may be practiced with alternate apparatus elements or process steps. The scope of the invention as protected by an issued patent is defined by the following claims, as they have been amended, if applicable, prior to issue.

Claims

Claims:What is claimed is:

1. A membrane comprising a) a hollow tubular porous membrane wall having a first density; and, b) one or more reinforcement structures generally parallel to the membrane wall and wholly or partially embedded in the membrane wall; the reinforcement structures having a second density greater than the first density.

2. The membrane of claim 1 having a plurality of the reinforcement structures.

3. The membrane of claim 1 or 2 wherein the one or more reinforcement structures are wholly embedded in the membrane wall.

4. The membrane of any preceding claim having a plurality of reinforcing structures wholly embedded in the membrane wall and distributed radially throughout the membrane wall.

5. The membrane of any preceding claim wherein the reinforcing structures occupy 10-30% of the cross-sectional area of the membrane.

6. The membrane of any preceding claim having an outside diameter in the range of about 0.5 to 2.5 mm and an inside diameter in the range of about

0.25 to 0.5 mm.

7. The membrane of any preceding claim wherein the reinforcing structures have a diameter of about 20-45% of the thickness of the membrane wall.

8. The membrane of any preceding claim wherein the membrane wall has pores in the microfiltration, ultrafiltration or nanofiltration ranges.

9. The membrane of any preceding claim wherein the one or more reinforcing structures spans a lumen of the membrane wall.

10. A method of forming a membrane comprising the steps of co-extruding one or more strands of a composition comprising a molten polymer wholly or partially within a stream of annular cross-section of a dope; solidifying the molten polymer of the composition to form a solid polymer of a first density; and, solidifying and forming pores in the dope so as to form a porous membrane wall of a second density less than the first density wholly or partially embedded with one or more strands of the solid polymer of the first density.

11. The method of claim 10 wherein a plurality of strands of the composition are co-extruded, each wholly within the dope.

12. The method of claims 10 or 11 wherein the porous membrane wall has pores in the microfiltration, ultrafiltration or nanofiltration ranges.

13. The method of any of claims 10-12 wherein the dope comprises a polymer and a diluent and the porous membrane wall is formed by thermally induced phase separation.

14. The method of any of claims 10 to 13 further comprising a step of forcing a centering fluid into any area within the annular cross-section of the dope.

15. The method of any of claims 10 to 14 further comprising quenching the composition and the dope.

16. The method of claim 15 wherein quenching the dope causes a phase separation of a polymer and a diluent within the dope and further comprising extracting the diluent from the quenched dope with a solvent.

17. The method of any of claims 10 to 16 wherein, during the step of co- extruding, the viscosity of the dope is 5 to 30% less than the viscosity of the composition.

18. The method of any of claims 10 to 17 wherein the dope comprises ECTFE and a trimellitate.

19. The method of claim 18 wherein the composition comprises ECTFE.

20. The method of claim 19 wherein the composition is composed of a polymer in an amount between 70 and 95% by weight and further comprises a solvent of the polymer.