Abstract:
A hollow fiber membrane is made by covering a tubular supporting structure with a membrane dope and converting the membrane dope into a solid porous membrane wall. Optionally, a textile reinforcing structure in the form of a circular knit may be added around the supporting structure before it is covered in dope. The reinforcing structure thereby becomes embedded in the membrane wall. The supporting structure may be soluble in a non-solvent of the membrane wall, for example water, and may be removed from the membrane. Alternatively, the supporting structure may be porous. A porous supporting structure may be made by a non-woven textile process, a sintering process within an extrusion machine, or by extruding a polymer mixed with a second component. The second component may be a soluble solid or liquid, a super-critical gas, or a second polymer that does not react with the first polymer.

Description:
FIELD 
     This specification relates to hollow fiber membranes, to methods of making hollow fiber membranes, and to supporting structures for hollow fiber membranes. 
     BACKGROUND 
     The following discussion is not an admission that anything described below is common general knowledge or citable as prior art. 
     U.S. Pat. No. 5,472,607 describes a hollow fiber polymeric membrane supported by a braided textile tube. The braided tube is sufficiently dense and tightly braided that it is round stable before being coated with a membrane dope. The membrane polymer is located primarily on the outside of the braid. This type of structure has been used very successfully in the ZeeWeed™ 500 series membrane products currently sold by GE Water and Process Technologies. In particular, this type of supported membrane has proven to be extremely durable in use. 
     INTRODUCTION 
     The following paragraphs are intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention. 
     The inventors have observed that the number of filaments required to make a round stable braided support tends to result in braids that have an over-abundance of tensile strength. Braiding a round stable support is also a slow process, typically done at a speed of about 20-30 meters per hour. In comparison, the dope coating process may be done at speeds up to 20-40 meters per minute and so the braid must be wound onto a spool as it is made, and then taken off again during the coating process. Reducing the number of filaments in a braided support could reduce the cost of the membrane while still providing adequate tensile strength. However, with fewer filaments the braided support might not be round stable and might not provide enough support for the membrane dope during coating. Further, even if fewer filaments were used in the braid, the braiding speed would still be much less than the coating speed and the braiding and coating operations still could not be combined in the same production line. 
     Various methods of making hollow fiber membranes are described herein in which a membrane dope is extruded onto a tubular supporting structure, alternatively called a support or tubular support. In a first set of membranes, the tubular support is porous and remains as a part of the finished membrane. The porous supporting structure may be made, for example, by a non-woven textile process, a sintering process using an extruder, or by extruding a polymer mixed with a second component. The second component may be a soluble solid or liquid, a super-critical gas, or a second polymer that does not react with the first polymer. The supporting structure supports the membrane dope until the dope is solidified into a membrane wall. Depending on the type and material of the tubular support, the resulting composite structure, comprising the supporting structure and the membrane wall, may have more tensile strength than a membrane wall of the same thickness made with the dope alone. 
     In a second set of membranes, the tubular support is surrounded by a textile reinforcement formed over a tubular support. For example, the textile reinforcement may be knitted around the tubular support. In the second set of membranes, the tubular support may be dissolved out of the finished membrane after the dope has formed a membrane wall, leaving the textile reinforcement embedded in the membrane wall. Alternatively, a porous support as in the first set of membranes may be surrounded by a textile reinforcement and remain in the finished membrane. The tubular support supports the membrane dope until the dope is solidified into a membrane wall. The textile reinforcement increases the tensile strength of the membrane, relative to a membrane wall of the same thickness made with the dope alone. The tubular support, if retained, may also contribute to an increase in textile strength. Optionally, the textile reinforcement may be knitted around the tubular support at the same speed as, and in line with, a dope coating process. 
    
    
     
       FIGURES 
         FIG. 1  is a schematic cross section of a first hollow fiber membrane. 
         FIG. 2  is a schematic crass section of a second hollow fiber membrane. 
         FIG. 3  is a cross section of a coating head. 
         FIG. 4  is a photograph of a soluble tubular supporting structure below a soluble tubular supporting structure with a knitted textile reinforcement around it. 
         FIG. 5  is a cross section of a hollow fiber membrane made using a soluble supporting structure with a knitted textile reinforcement layer after the soluble tubular supporting structure has been washed away. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , a hollow fiber membrane  10  is made by forming a membrane wall  14  having an outer separation layer  15  over a tubular support  12 . The tubular support  12  may be pre-made and stored on a spool until it will be used to manufacture the hollow fiber membrane  10 . To make the hollow fiber membrane  10 , the tubular support  12  is pulled through the centre of a coating head  16  while a membrane dope  18  is pumped through a die  19  onto the tubular support  12 . The tubular support  12  passes through a central bore  17  of the coating head  16 . The central bore  17  allows the tubular support to pass through the die  19  and also centers the tubular support  12  relative to the die  19  so that the dope  18  is applied evenly around the outside of the tubular support  12 . The cylindrical surface of the support  12  allows the dope coating thickness, dope viscosity and other parameters to be chosen without being restricted to values that could be used in creating a membrane with the dope alone. After exiting from the coating head  16 , the tubular support  12  and dope  18  pass into a quenching bath wherein the dope  18  is converted into a porous solid membrane wall  14 . The formation of the membrane wall  14  may be by way of a thermally induced phase separation (TIPS) or non-solvent induced phase separation (NIPS) process. The separation layer  15  may have a nominal pore size in the microfiltration range or smaller. 
     The tubular support  12  may be porous and may remain inside of the finished hollow fiber membrane  10  in use. The tubular support  12  may have a higher tensile strength than a membrane wall  14  of the same thickness. Accordingly, the composite membrane  10  may also have a higher tensile strength than a membrane of the same thickness made with the dope  18  alone. A suitable permanent porous tubular support  12  can be made by extruding a mixture comprising two primary components though an annular die. A first primary component is a thermoplastic extrusion grade polymer that will remain as the porous support  12 . A second primary component is a compatible substance that is soluble in a non-solvent of the first primary component. For example, the first primary component may be a water insoluble polymer and the second primary component may be a water-soluble substance such as sugar, salt, PVA or glycerol. The first and second primary components are chosen such that they do not react with each other. Instead, the first and second primary components are mixed with smooth agitation prior to extrusion so as to produce a homogenous mixture, for example an emulsion, of small particles of the second primary component in the first primary component. The mixture is extruded through an annular die and, optionally, a further outside diameter calibration die. The first primary component of the extruded mixture solidifies to form the support  12 . The tubular support  12  is placed in a bath of a solvent of the second primary component, for example water, so that the second primary component can be leached out in the bath. This leaching step may occur before or after the membrane wall  14  is formed over the support  12 . The first primary component could be an inexpensive (relative to the membrane dope  18 ) polymer, such as polyethylene (PE), if the tubular support  12  is required primarily to support the membrane dope  18  during coating. However, if a membrane  10  of greater strength is desired, then the first primary component may be a stronger polymer such as polyester (polyethylene terephthalate, PET). 
     Alternatively, a tubular support  12  may be made by extruding a mixture of two different thermoplastic materials. The two materials are chosen such that they are not reactive with each other. The two materials are also incompatible in the sense that the second material does not mix or dissolve well in the first material and instead the second material forms clusters with low adherence to a matrix provided by the first material. For example, one known second material is polymethylpentene (PMP). The first material may be, for example, PE or PET depending on whether cost or strength is more important for the finished membrane  10 . After the extruded tube has solidified, it is stretched to set the final outside diameter of the tubular support  12 , and to create small cracks between the two materials. The cracks provide the tubular support  12  with the desired porosity. A similar tubular support  12  can be made by mixing the first material with a non solute that is not a polymer such as CaCO 3 . 
     As further extrusion based alternatives, a commercially available filtration tube can be used for the support  12 . A tubular support  12  with an open cell structure can also be extruded out of a polymer mixed with a super critical gas such as carbon dioxide. As the mixture comes out of the extrusion die, a reduction in pressure allows the super critical gas to escape leaving an open cell structure. An extruded solid tube can also be punched out mechanically, or with one or more pulsating laser beams, to form the support  12 . 
     A tubular support  12  can also be made without extrusion. For example, a non-woven support can be made by directing electro-spun, melt blown or spray blown fiber segments onto a rotating mandrel. The mandrel is porous with its inner bore connected to a source of suction such that the semi-solidified fiber segments collect on the mandrel. The fiber segments fully solidify and bond to each other on the mandrel to form a non-woven tube. One end of the tube is continually pulled off of the mandrel to create the tubular support  12 . Optionally, a non-woven structure may have sufficient density to increase the strength of the membrane  10 . A support  12  may also be made by a continuous sintering process with an extrusion machine. In this process, semi molten polymer granules are pressed together in an extruder and sintered under pressure pushing them through the extrusion die in a manner similar to metallic sintering, but into a tubular shape. This tube is than used as a support  12 . The sintering process does not produce a support  12  with as much strength as a non-woven support, but sintering and non-woven techniques may be performed in line with the coating process. 
     Referring to  FIG. 2 , a second hollow fiber membrane  20  has a membrane wall  14 , a permanent or temporary tubular support  12 , and a textile reinforcement  22 . The textile reinforcement  22  is preferably embedded in the membrane wall  14 . In the second membrane  20  shown, the textile reinforcement  22  is knitted from monofilament or multifilament yarns or threads. In general, knitting may be done faster than braiding and requires a less complicated machine. In the textile reinforcement  22  shown, the knit is also designed to produce a looser structure, relative to current braided membrane support structures, and so requires less material than a braided membrane support. The textile reinforcement  22  is not dense enough to support the dope  18  by itself and so the textile reinforcement  22  is used in combination with a tubular support  12 . However, the textile reinforcement  22  increases the strength of the resulting membrane  10 . 
     The knitted reinforcement  22  is applied to the tubular support  12  by passing the support  12  through a knitting machine while knitting one or more mono or multi-filaments  24  in a tube around the support  12 . The knitting may be done at the same speed as the dope coating process described in relation to  FIG. 3  thus allowing the knitting to be done in line with the coating. In that case, the tubular support  12  may be drawn from a spool and pass directly, that is without being re-spooled, through the knitting machine and the coating head  16 . 
     Alternatively, the textile reinforcement  22  may be provided in the form of one or more spiral wrappings made by passing a support  12  through a wrapping machine. 
     Further alternatively, a textile reinforcement  22  may be provided in the form of many short segments of filaments  24 . For example, the textile reinforcement may be made by mixing micro fibers into the membrane dope  18 . Micro fibers can also be applied to the outside surface of a semi-solidified membrane dope  18  by spraying chopped, electro-spun or melt blown fibers on the surface of the dope  18 . The fibers are sprayed onto the membrane dope  18  as it leaves the coating head  16  or soon after such that the dope  18  may have started to solidify, but it is not yet solid. 
     Optionally, the one or more filaments  24  in the reinforcement  22  may be coated or bi-component filaments  24 , or a yarn or other multi-filament form of filament  24  may have two or more types of component mono-filaments. Crossing or adjacent filaments  24  may be fixed to each other at points of contact. For example, filaments  24  may be fixed to each other by heat setting, ultraviolet light, welding, plasma, resins or other adhesives. The material of the filaments  24 , the coating of coated filaments  24 , one or more exposed material in bi-component filaments  24 , or one or more mono-filaments in multi-filaments may be chosen to be suitable for a chosen fixation method if required. The flexibility of the textile reinforcement  22  may be adjusted by altering the density of the filaments  24 , by deciding whether or not to fix filaments  24  together at points of contact or intersection, and by adjusting the density of points of contact or intersection between filaments  24 . 
     The support  12  may remain inside the second membrane  20 . In that case, the support  12  may be made by any of the methods described for the first membrane  10 . Regardless of the method of making the permanent support  12 , the reinforcement  22  is formed around the support  12 , for example by being knit or cable wrapped around the support  12 . Since the support  12  keeps the reinforcement  22  in a round shape and at a desired diameter, the density of the reinforcement  22  can be chosen based on the intended use of the membrane  20 . For example, a membrane  20  intended for use in drinking water filtration may have a reinforcement  22  that is less dense than in a membrane  20  intended for use in waste water. After the reinforcement  22  is placed around the support  12 , the combined structure is passed through the coating head  16  as described above in relation to  FIG. 3 . 
     If the tubular support  12  will not remain inside the finished membrane  20 , then the support  12  is made from a material that is soluble in a non-solvent of the separating layer  14 . For example, the support  12  may be water-soluble. The reinforcement  22  is applied around the support  12  as described above and the combined structure is passed through the coating head  16  as described in relation to  FIG. 3 . The support  12  provides a base keeping the inner surface shape of the reinforcement  22  in a tubular shape and keeping the combined structure centered in the coating head  16 . This assists in creating a uniform coating of dope  18  and so a membrane wall  14  of uniform thickness. 
     After a solid membrane wall  14  has formed, the support  12  is washed out in the coagulation bath or a separate solvent. The reinforcement  22  is preferably loose enough that the dope  18  will have penetrated through the reinforcement  22  to the outer surface of the support  12 . At least outer filaments  24 , or segment of filaments  24 , are encapsulated in the membrane wall  14 . The reinforcement  22  can also be very thin such that the outside diameter of the membrane  20  can be 1 mm or less. The total amount of material used in the support  12  and the reinforcement  22  may be similar to or less than the amount of bore fluid required to make an unsupported membrane. Accordingly, a light duty supported membrane  22  can be made for a similar price as an unsupported membrane but with increased strength. The dissolved material from the support  12  can be recovered from the coagulation bath or other solvent. 
     With a knit reinforcement  22  over a tubular support  12 , for example in the form of a water soluble tube, the applied tension during knitting and during the coating process helps ensure that the circular knit reinforcement  22  lies smoothly on the surface of the support  12 . The wall thickness of the support  12  is chosen such that the support  12  is not fully dissolved until the membrane material in the dope  18  coagulates. Accordingly, although the filaments  24  may be fixed to each other before the dope  18  is applied to the support  12 , this is not mandatory since the coagulated dope  18  is sufficient for maintaining the position of the filaments in the knit in the finished second membrane  20 . 
     A knit reinforcement  22  is preferably made by circular warp knitting. Warp knits consist of multiple yarn systems (using mono or multi-filament yarns), which run longitudinally along the surface of the cylindrical knit. Each yarn system has a designated needle and the yarn guiding elements alternate between the loop forming needles. In comparison, a weft knit is formed from one or multiple yarn systems, which are interconnected by loops, formed by needles. In weft knitting the adjacent loops (stitches) run in a spiral shaped line around the length axis of the circular knit. A plurality of the loop (stitch) forming needles and the yarn guiding elements rotate with reference to each other. A weft knit provides a more nearly closed and cylindrical surface than a warp knit, but uses more filament  24  material and is less stable in length. While the weft material might be better if its primary purpose was to support the dope  18 , in the present membrane  10  the dope is cast on to a tubular support  12  and the knit is intended to increase the strength of the membrane  10 . A warp knit is more rigid or stable in length than a weft knit, and so the warp knit is preferred for use as the reinforcement  12 . 
       FIG. 4  shows, in the lower part of the figure, an example of a tubular support made by extruding PVAL. In the upper part of  FIG. 4 , fine polyester multifilament yarns (133 dtex f 32) where knit on a 4 needle warp knitting machine, by forming the loops on the adjacent needles, over a soluble tubular support  12 . The stitch length was 2.5-3.5 mm (2.8-4 loops/cm). The combined structure was then coated with a PVDF based membrane dope  18 . After the dope was formed into a solid separation layer  14 , the support  12  was dissolved out. A cross section of the resulting membrane  20  is shown in  FIG. 5 .