Abstract:
A nozzle for making a reinforced hollow fibre membrane discharges one or more reinforcing yarns at or near a plane where the dope exits the nozzle, or in the same plane as a discharge opening of dope passage, or both. Multiple discrete yarn discharge openings may be spaced around a longitudinal axis of the nozzle. A reinforcing yarn passage remains generally free of dope during use. The dope may be discharged in an annular ring inside of the one or more reinforcing yarns, in an annular ring outside of the reinforcing yarns, or both. Minimal tension is required to pull the yarns through the nozzle, which helps to reduce distortion of the membrane cross section in a coagulation bath.

Description:
FIELD 
       [0001]    This specification relates to devices and processes for producing reinforced hollow fiber membranes, for example hollow fiber membranes for use in microfiltration or ultrafiltration. 
       BACKGROUND 
       [0002]    Hollow fiber membranes can be made from a variety of polymers by different methods. One method involves non-solvent induced phase separation (NIPS) as taught for example in U.S. Pat. Nos. 3,615,024; 5,066,401; and, 6,024,872. Another method involves thermally induced phase separation (TIPS) as taught for example in U.S. Pat. Nos. 4,702,836 and 7,247,238. The membranes may have separation layers on their inner surfaces or their outer surfaces and may be used, for example, for microfiltration (MF) or ultrafiltration (UF). 
         [0003]    The strength of a hollow fiber membrane can be increased by coating a membrane dope on a pre-formed tubular braid. U.S. Pat. Nos. 5,472,607 and 6,354,444 to Mahendran et al. teach coating a membrane on the outside of a braid with limited penetration. U.S. Pat. No. 4,061,861 to Hayano et al., U.S. Pat. No. 7,267,872 to Lee et al., and U.S. Pat. No. 7,306,105 to Shinada et al. also teach braid supported membranes. Hollow fiber membranes made according to the teachings of Mahendran et al. have been successfully commercialized. 
         [0004]    Another approach that has been proposed for making a reinforced hollow fiber membrane involves embedding fibers within the wall of a hollow fiber while the hollow fiber is being cast. US Publication 2002/0046970 to Murase et al., International Publication WO 03/097221 to Yoon et al. and U.S. Pat. No. 6,454,943 to Koenhen describe methods of embedding a monofilament or a multi-filament yarn longitudinally within the wall of a hollow fiber. 
       INTRODUCTION 
       [0005]    The following introduction is intended to introduce the reader to the detailed description to follow and not to limit or define the claims. 
         [0006]    This specification will describe an alternative device and process for making a reinforced membrane in which one or more reinforcing yarns are embedded in the walls of the hollow fibre membrane. The resulting membrane may be used, for example, to provide water treatment by microfiltration or ultrafiltration. 
         [0007]    A hollow fibre spinning nozzle described in the specification discharges one or more reinforcing yarns from one or more yarn passages with openings at or near a plane where the dope exits the nozzle. Alternatively or additionally, a discharge opening of a yarn passage may be in or near the same plane as a discharge opening of dope passage. A dope passage may be located inside of the one or more yarn passages, outside of the one or more yarn passages, or both. There is no dope inlet to the one or more yarn passages and, preferably, there is essentially no dope in the one or more yarn passages when the nozzle is in use. 
         [0008]    A reinforcing yarn is typically a multifilament yarn, but may also be a monofilament. If there are multiple reinforcing yarns, there may be multiple discrete yarn discharge openings spaced in a ring around a longitudinal axis of the nozzle. Optionally, a reinforcing yarn comprises filaments having a polymer, at least on the outer surface of the filaments, that is wetted by the dope; a surface treatment that increases wetting by the dope; or, both. Optionally, a reinforcing yarn may be wetted with a solvent before it contacts the dope. 
         [0009]    In some of the prior art methods of making reinforced hollow fibre membranes, the reinforcement is pulled into a passage through a spinneret, passes through dope that is injected into the same passage, and then exits the passage with some of the dope. In these prior art methods, dope tends to leak out of the spinneret from an inlet orifice where the reinforcement enters the spinneret. This problem is inherent because the dope is under pressure and the reinforcement enters the spinneret from ambient atmospheric pressure. Attempting to solve this problem with sealing devices is complicated and can cause fraying and damage to the reinforcement. In contrast, discharging a reinforcing yarn from a yarn passage at or near a plane where the dope exits a nozzle, or in or near the plane of a discharge opening of a dope passage, as described in this specification, places the reinforcing yarn in communication with the dope where the dope is at or near atmospheric pressure. This at least reduces the tendency for dope to leak out of the nozzle through the yarn passage. 
         [0010]    Some of the prior art methods pass reinforcements through an annular dope passage. Although the reinforcements may enter the dope passage with a preferred spacing or placement, the reinforcements may move relative to each other in the annular dope passage. Accordingly, multiple reinforcements do not necessarily exit the spinneret equally spaced from each other. Optionally discharging multiple reinforcing yarns through discrete spaced openings at or near a plane where the dope exits the nozzle, as described in this specification, tends to result in more evenly spaced reinforcing yarns. 
         [0011]    The inventors have also observed that a material amount of force is required to pull a reinforcement through a passage filled with dope in a spinneret. The cross section of a membrane made from such a spinneret also tends to be distorted relative to a desired annular cross section. Without intending to be limited to any particular theory, the inventors believe that tension applied to a forming membrane as it passes around a roller in a coagulation bath tends to distort the cross section of the resulting hollow fibre membrane. Since the dope is viscous, pulling the reinforcement through a reservoir of dope that is not flowing at the membrane making line speed puts the reinforcing yarns under a material amount of tension. A nozzle tested in this specification resulted in a significant reduction in the force required to pull reinforcing yarns through the nozzle at a given line speed. These results suggest that a nozzle that discharges a reinforcing yarn from a yarn passage at or near a plane where the dope exits the nozzle, or in or near the plane of a discharge opening of a dope passage, without injecting dope into the yarn passage, will help produce a hollow fibre membrane with an embedded reinforcing yarn and a generally annular cross section. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0012]      FIG. 1  is an isometric view of a nozzle for making a reinforced hollow fibre membrane, with the nozzle cut along its longitudinal axis. 
           [0013]      FIG. 2  is a back view of another nozzle for making a reinforced hollow fibre membrane. 
           [0014]      FIG. 3  is a side view of the nozzle of  FIG. 2 . 
           [0015]      FIG. 4  is a front view of a nozzle of  FIG. 3 . 
           [0016]      FIG. 5  is a cross section of the nozzle of  FIG. 2  along the line A-A in  FIG. 2 . 
           [0017]      FIG. 6  is a cross section of the nozzle of  FIG. 2  along the line B-B in  FIG. 2 . 
           [0018]      FIG. 7  is a cross section of the nozzle of  FIG. 2  along the line C-C in  FIG. 2 . 
           [0019]      FIG. 8  is an enlarged view of the area G shown in  FIG. 4 . 
           [0020]      FIG. 9  is an enlarged view of the area H shown in  FIG. 5 . 
           [0021]      FIG. 10  is a cross section of a hollow fibre membrane that may be produced from the nozzle of  FIG. 1  or the nozzle of  FIGS. 2 to 9 . 
           [0022]      FIG. 11  is a schematic view of a coagulation bath with a tension gauge used in an experimental example. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a nozzle  100  cut open along a plane parallel to its longitudinal axis  102 . Internal passages in the nozzle  100  provide a number of zones for moving different materials through the nozzle  100 . These passages all discharge through discharge openings located in a common plane defined by the front face  104  of the nozzle  100 . The nozzle  100  is typically located in use with its front face  104  oriented horizontally. The different materials are discharged vertically downwards from the nozzle  100  and fall through an air gap into a coagulation bath to form a membrane. 
         [0024]    Starting at the longitudinal axis  102 , the first zone A carries a bore fluid along the longitudinal axis  102 . The bore fluid may be a liquid or a gas, such as air, and is used to form a lumen within the resulting membrane. 
         [0025]    The second zone B carries a membrane dope. In general, the dope is a mixture of one or more polymers which will form the membrane wall in a solvent. There may also be other minor ingredients such as a non-solvent or weak non-solvent and a hydrophilic additive. 
         [0026]    The third zone C carries one or more reinforcing yarns. The reinforcing yarns are described further below in relation to  FIG. 10 . Zone C is preferably subdivided, at least where it opens to the front face  104  of the nozzle  100 , into a plurality of discrete passages. Typically, one or more reinforcing yarns pass through each discrete passage although one or more of the passages may optionally be left empty. The passages in zone C also communicate with a solvent passage  106 . The solvent passage  106  is used to inject a solvent, typically the same solvent that is used in the membrane dope, into the reinforcing yarn passages. This solvent pre-wets the reinforcing yarns, reduces a flow of air through zone C, and also helps prevent dope from entering the reinforcing yarn passages. 
         [0027]    The fourth zone D carries a second flow of membrane dope. Optionally, zones B and D may be in communication with each other inside of the nozzle  100  so that one dope inlet can feed both of zones B and D. Dope can be injected into the nozzle  100  from a pot pressurized with nitrogen, or using a positive displacement pump. The dope may be provided at a temperature in the range of about 15 to 200 degrees C. and at a pressure in the range of about 20 to 400 kPa. 
         [0028]    In use, annular streams of membrane dope are discharged from zones B and D through the front face  104  of the nozzle  100 . At the same time, one or more reinforcing yarns are pulled through the nozzle by a force applied by a take up winder on the resulting membrane. The one or more reinforcing yarns are discharged from the front face  104  of the nozzle  100  between the two dope flows. The two dope flows merge with each other immediately outside of the nozzle  100  to form a single annular flow of dope. The one or more reinforcing yarns are embedded in the dope. 
         [0029]    The dope and reinforcing yarn drop through an air gap into a coagulation bath. The coagulation bath is typically a tank equipped with rollers at the bottom and at the top as is known for membrane coagulation. A powered take-up winder receives the membrane emerging from the coagulation bath, optionally after the membrane passes through other unit processes such as a rinsing bath. The take up winder typically has a traverse guide to evenly populate a bobbin. The take up winder operates at an adjustable speed, typically between 1 and 30 m/min, that is matched to the downward velocity of dope being discharged through the nozzle  100 . The take up winder also pulls the one or more reinforcing yarns through the nozzle  100 . This results in the one or more reinforcing yarns being under tension between the nozzle  100  and the take up winder in an amount equal to the force required to pull the one or more reinforcing yarns through the nozzle  100 . 
         [0030]      FIGS. 2 to 9  show a second nozzle  110 . The second nozzle  110  is similar to nozzle  100  but it has an additional plate  112  at the front of the second nozzle  110 . The front face  104  of the second nozzle  110  is defined by the front of the plate  112 . A bore fluid needle  114 , providing a zone A, is also extended to the front of the plate  112 . Zones B, C and D as described in relation to the nozzle  100  of  FIG. 1  are also provided in the second nozzle  110 . However, the discharge openings for zones B, C and D are set back from the front face  104  by the thickness of the plate  112 . Other features of the detailed construction of the second nozzle  110  shown in  FIGS. 2 to 9  are also used with the nozzle  100  of  FIG. 1 . 
         [0031]    Referring for example to  FIGS. 2 and 3 , the primary components of the second nozzle  110  are a main body  116 , the bore fluid needle  114 , a first insert  120 , a second insert  122  and the plate  112 . The bore fluid needle  114  threads into the main body  116  from the back. The first insert  120 , second insert  122  and plate  112  are inserted into a recess at the front of the main body  116  and held in with screws (not shown) to be threaded into screw holes  124 . A bypass connector  126  is drilled into the side of the main body  116  to connect zones D and B inside of the main body  116  and then plugged. 
         [0032]    Referring to  FIG. 2 , a dope inlet  130  provides dope to both of zones B and D. A bore fluid inlet  132  allows bore fluid to be provided to the needle  114 . Reinforcing yarns enter the main body through ceramic guides  134 , one for each distinct reinforcing yarn passage. Solvent enters the main body  116  through a plurality of solvent inlets  136 . Referring primarily to  FIGS. 8 and 9 , bore fluid exits the needle  114  from a bore fluid outlet  140 . Dope exits zones B and D through first and second dope passage discharge outlets  142  and  144  respectively. Reinforcing yarns exit zone C through yarn discharge outlets  146 . Dope from the two zones B and D merges together around the reinforcing yarns in an annular space between the needle  114  and an outlet bore  148  in the plate  112 . 
         [0033]    Referring to  FIGS. 1 to 9 , both nozzles introduce reinforcement yarns into the membrane dope from yarn passages that are not connected to a supply of dope. The yarn discharge openings  146  are at or near the plane where the dope exits the nozzle  100 ,  110  at the front face  104 , for example within 5 mm of the front face  104 . Alternatively or additionally, the yarn discharge openings  146  are at or near to the dope discharge openings  142 ,  144 , for example within 3 mm of the dope discharge openings  142 ,  144 . When multiple reinforcement yarns are deployed they can be discharged from discrete yarn discharge openings  146  spaced evenly around the needle  114  at or near the front face  104  of the nozzle  100 ,  110 . 
         [0034]    The two nozzles  100 ,  110  differ in that in the second nozzle  110  the yarn discharge openings  146  are setback from the front face  104 , which defines the discharge plane of the second nozzle  110  as a whole. Without setback, as in nozzle  100 , the reinforcing filaments are introduced to the dope at the discharge plane of the nozzle. In this case, the reinforcing yarns exit from their passageways where the dope pressure is essentially atmospheric. As the yarn discharge openings  146  and dope discharge openings  142 ,  144  are moved back from the discharge plane, as in the second nozzle  110 , the reinforcing yarn is discharged into an area of higher dope pressure. Some dope pressure may be desirable to minimize air entrained into the product membrane with the reinforcing yarn. However, the dope pressure at the yarn discharge openings  146  is preferably kept below a threshold at which the dope would flow back through the reinforcing yarn passage when the nozzle is in operation. 
         [0035]    Referring to  FIG. 10 , a hollow fiber membrane  10  produce from either of the nozzles  100 ,  110  has a membrane wall  16  made from the dope of zones B and D. The membrane wall  16  has one or more reinforcing yarns  12  embedded in it. The reinforcing yarns  12  may be made up of individual filaments  14 . Individual filaments  14  are preferably long continuous filaments rather than, for example, staple fibres. 
         [0036]    The specific membrane  10  shown in  FIG. 1  has one reinforcing yarn  12 , but there may be a plurality of reinforcing yarns  12 , for example between two and eight. Each reinforcing yarn  12  is preferably a multi-filament yarn made of continuous thermoplastic filaments  14 . The filaments are preferably grouped together but without sufficient twisting to be classified as a twisted yarn. Other types of yarns or threads, or a monofilament, might also be used but they are not preferred. 
         [0037]    Filaments  14  can be made from polymeric fibers such as polyethylene, polypropylene, polyester, nylon or PVDF. Filaments  14  can be bi-component filaments with a first part, preferably a complete outer layer or sheath, made of a first polymer that is wetted by a membrane forming dope. For example, a reinforcing filament  14  may have an outer layer or other part made of a polymer that is soluble in a solvent used in the membrane dope. In particular, the outer layer or other part may comprise a polymer that is also present in the membrane dope. A second part, for example a core, of a bi-component filament  14  may be made of second polymer that is provides an improvement over using the first polymer alone. For example, the second polymer may be stronger, or less expensive, or both, relative to the first polymer. 
         [0038]    The filaments  14  shown in  FIG. 1  are bi-component fibers spun with a core of polyethylene terephthalate (PET) and a sheath of polyvinylidene fluoride (PVDF). The core is about 70-90% of the cross-sectional area. The PET is a strong material that has mechanical characteristics suitable for reinforcing or supporting membranes. PVDF by comparison is a relatively weak material. However, the PVDF sheath has an affinity for a PVDF and NMP based membrane dope. Such a dope may be used to form the membrane wall  16  using a NIPS process. 
         [0039]    The affinity between the outer surface polymer of the filaments  14  and the dope discourages air bubbles and encourages contact between the filaments  14  and the membrane wall. Alternatively, the surface of filaments  14  may also be modified or treated to promote bonding to the membrane dope. Such treatments can include, for example, plasma or chemical etching. The treatment is chosen to be appropriate for the materials of the filament  14  and the dope. Alternatively or additionally, as described above, filling the reinforcing filament passages with a solvent compatible with the dope also discourages air bubbles and encourages contact between the filaments  14  and the polymer wall. 
       Example 
       [0040]      FIG. 11  shows a modified coagulation bath  200  that was used in experimental tests. A tank  202  was filled with a quenching solution  204 , primarily water, to form membranes from a PVDF in NMP based dope by a NIPS process. Experimental coating nozzles  208  was placed over the tank  202  and oriented such that a precursor fibre  206  would fall vertically through an air gap  210  and then into the quenching solution  204 . In the tank  202 , the precursor fibre  206  passed over a tension gauge  214  and a lower roller  216 . After leaving the tank, the product fibre  212  passed over an upper roller before being taken up on a winder  218 . The winder  218  applied the force necessary to pull reinforcing yarns through the experimental nozzles  208  at a constant line speed. The tension gauge  214  measured the applied force, and therefore the tension on the precursor fibre  206 . 
         [0041]    Three nozzles  208  were tested. The first two nozzles were generally as shown in U.S. patent application Ser. No. 13/328,761 filed on Dec. 16, 2011, which is incorporated by reference. In these nozzles, reinforcing yarns pass through a middle passage containing dope. The middle passage ends in a first annular region located around a bore fluid needle and upstream of the exit plane of the nozzle. Dope wetted filaments pass from the first annular region into a downstream second annular region surrounding the bore fluid needle. A second flow of dope is injected into this second annular region. The dope with embedded reinforcing filaments leaves the second annular region and the exit plane of the nozzle. A third nozzle was a modified version of a second of the first two nozzles. In this modified nozzle, (a) the first annular region carrying reinforcing filaments from the middle passage was extended to the exit plane of the nozzle, (b) the supply of dope to the middle passage and first annular region was stopped, and (c) the bore fluid needle was replaced with an inner dope needle. Accordingly, the third nozzle resembled the nozzle  100  of  FIG. 1  except that it had no bore fluid needle and so produced a solid fibre rather than a hollow fibre. Although it produces a solid fibre, the third nozzle confirms that two flows of dope can envelope a reinforcing yarn when both flows of dope and the reinforcing yarn are discharged from a common plane. 
         [0042]    In preliminary tests, it was determined that there was no material tension (less than 10 g) when passing reinforcing yarns alone through the first or third nozzle, and when passing dope alone through the first nozzle. However, when two reinforcing yarns and dope were passed through the nozzles at a line speed of 90 feet per minute (fpm), the first and second nozzles required a tension of about 118 and 130 g respectively. The third nozzle, however, required a tension of only about 22 g. At a line speed of 50 fpm with two reinforcing yarns, the first nozzle required a tension of about 93 fpm and the third nozzle required a tension of about 17 g. The second nozzle was not tested under these conditions. These tests indicated that the third nozzle resulted in a significant reduction in tension on the precursor fibre  206 . 
         [0043]    In one other test, the second nozzle was used with one reinforcing yarn at a line speed of 90 fpm and required about 62 g of tension. The first nozzle was tested with one reinforcing yarn at a line speed of 50 fpm and required about 58 g of tension. Considered in combination with the tests described above, these tests indicate that for a given nozzle the required tension is influenced mostly by, and roughly proportional to, the number of reinforcing yarns. Tension is also influenced by line speed, although to a lesser extent. Overall, these tests suggest that the primary cause of tension is the movement of a reinforcing yarn at line speed through a relatively slow moving volume of dope in a chamber or passage of the nozzle. 
         [0044]    The experiments also demonstrated that changing to the third nozzle design resulted in a greater reduction in tension than either a reduction in line speed or a reduction in the number of reinforcing yarns. We expect that this will result in less distortion of a hollow fibre membrane particularly as it is pulled around a lower roller  216 . Alternatively, a faster line speed or reduced coagulation tank depth could be used with the third nozzle while producing a similar quality membrane compared to the first or second nozzle. It was also observed that no dope leaked out of the third nozzle through the inlets to the reinforcing yarn passages even though no seals were provided at the reinforcing yarn inlets. 
         [0045]    This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention. The scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.