Abstract:
A membrane module includes at least one hollow fibre membrane; a first resin layer formed around an end of the at least one hollow fibre membrane; and a second resin layer chemically bound to the first resin layer, wherein the second resin layer is of greater flexibility than the first resin layer.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to potting methods for porous hollow fibre membranes typically used in filtration systems. 
       BACKGROUND ART 
       [0002]    The potting materials used to support and hold arrays of porous hollow fibre membranes are usually a compromise between materials which have to sufficient rigidity to provide adequate support but sufficient softness and flexibility to avoid breakage of the fibres where they enter the pot. Too rigid a material produces rapid breakage of fibres adjacent the pot while too soft a material does not have sufficient mechanical strength to adequately support the fibres. The materials are also chosen to resist breakdown as a result of exposure to various types of feed as well as cleaning fluids used to maintain the fibres. 
         [0003]    Known systems employ single layers of epoxy, polyurethane or silicon materials, however, each suffer from the disadvantages outlined above. 
         [0004]    The present invention seeks to overcome or at least ameliorate one or more of the disadvantages of the prior art outlined above or at least provide a useful alternative. 
       DISCLOSURE OF THE INVENTION 
       [0005]    According to one aspect, the present invention provides a method of forming a pot for an array of hollow fibre membranes including the steps of: placing the ends of said fibre membranes in a mould; forming a first layer of curable resin material in a non-cured state around said fibre membrane ends, applying a second layer of polyurethane resin material to said first layer prior to full curing of said first layer, said second layer of polyurethane resin material being chemically reactive with said first layer material to form an adhesive bond therebetween; at least partially curing both layers and removing the pot formed from said mould, wherein said second layer material is of higher flexibility than said first layer material when each layer is fully cured. 
         [0006]    It will be appreciated that any known method of sealing the open fibres may be used prior to the potting process described above. 
         [0007]    Preferably, the curable resin material is an epoxy resin. The use of an epoxy resin has been found to be advantageous when potting fibre membranes to which typically contain hydroscopic liquids which may produce sealing problems between the fibres and the pot when polyurethane resin is used. 
         [0008]    In one preferred form the layer of higher flexibility is produced by adding one or more flexibilising agents to the components of the material forming the first layer of lower flexibility. 
         [0009]    Preferably, the method includes the step of the monitoring the curing process of the first layer to determine the optimal time in which to apply the second layer thereto. For preference, the step of monitoring includes monitoring the temperature changes within said first layer to determine the state of the curing process. 
         [0010]    It is important that the second layer be applied to the first layer prior to curing of the first layer being completed so that there are sufficient active sites available for the chemical reaction required to adhere one layer to the other can take place. Proper adhesion of the two layers is required to prevent ingress of feed between the layers and growth of bacteria etc. between the layers leading to damage and breakage of fibres and consequential contamination of the filtrate. The use of this process also eliminates the need for special adhesives and primers to produce adhesion between the layers. 
         [0011]    A necessary condition for the adhesion of the two layers is the chemical reaction between some of the components of the two different layers. For epoxy/polyurethane layers, it is desirable that one of the epoxy components is reactive with the isocyanate groups of the polyurethane (preferably amines or amides). 
         [0012]    According to a second aspect, the present invention provides an apparatus for potting hollow fibre membranes including: 
         [0013]    a mould for receiving the ends of said hollow fibre membranes; 
         [0014]    means for forming a first layer of curable resin material in a non-cured state around said fibre membrane ends in said mould, 
         [0015]    means for applying a second layer of polyurethane resin material to said first layer prior to full curing of said first layer, said second layer polyurethane resin material being chemically reactive with said first layer material to form an adhesive bond therebetween and said second layer polyurethane resin material being of higher flexibility than said first layer material when each layer is fully cured. 
         [0016]    Preferably, the mould includes separate means for flowing said first and second layer materials into the mould. For preference, said materials are fed into a centrifuge before being flowed along a conduit or tube into the mould. Preferably, a single centrifuge may be used having separate sections to receive the respective first and second layer materials. 
         [0017]    Although, a centrifuge is the preferred tool used in the method, it will be appreciated that other techniques such as static potting are equally applicable to the inventive method, 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0019]      FIG. 1  shows a simplified schematic diagram of the potting apparatus used to perform the potting method in accordance with one embodiment of the present invention; and 
           [0020]      FIG. 2  shows a cross-sectional view of one potted end of the fibres illustrating one embodiment of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    Referring to  FIG. 1 , the potting apparatus comprises a mould  5  for receiving the ends  6  of the fibres membranes  7 . The mould  5  includes a cylindrical cavity  8  for receiving the fibre membranes  7 . The cavity  8  has an to enlarged-diameter base portion  9  for holding the prepot  10  which serves to support the fibres during the potting process. A tube or hose  11  connects one bowl  12  of a centrifuge  13  to the base  14  of the mould cavity  8 . A second tube  15  connects a second bowl  16  of the centrifuge  13  to the upper portion  17  of the mould cavity  8 . 
         [0022]    In use, the material for forming the first relatively hard layer of the pot is dispensed Into bowl  12  of the centrifuge  13  and flowed along tube or hose  11  into the lower portion  14  of the mould cavity  8  to form a first layer around the fibre ends  6 . Prior to full curing of this material, a second material for forming the second softer layer Is dispensed Into bowl  16  and flowed along tube or hose  15  into the mould cavity  8  to apply the second layer of material to the first layer. The layers chemically react to form an adhesive bond between the first and second layers. At an appropriate curing stage the potted fibres are removed from the mould  5 . 
         [0023]    In some applications, it may be possible to use a single bowl and tube to flow the materials sequentially into the mould to form the two layers. 
         [0024]    In order to determine the optimal time during the curing of the first layer for applying the second layer, the temperature profile of the first layer material may be monitored, at least during the Initial set up of the process. The temperature profile indicates the various stages of the curing process and when the second layer may be applied to ensure that sufficient free sites are available for chemical bonding between the two layers. Once the optimal time has been determined for a particular material combination further monitoring is unnecessary. 
         [0025]    A steep rise in temperature indicates that the majority of the curing reaction is taking place. The optimal time for the addition of the second layer has been found to be within ±5 minutes of the time of this temperature rise and preferably, ±2 minutes. 
         [0026]    The embodiment shown in  FIG. 2  illustrates a method using a potting sleeve  23  to enhance the interfacial binding between the potting layers  20 ,  21 . The quantity of potting materials or the width of the potting sleeve  23  is selected to ensure that the interface  24  of the two layers is within the potting sleeve  23 . The potting sleeve  23  is formed from material which ensures good adhesion between its inner surface and the two potting materials. Besides the selection of the potting sleeve material, roughening the inner surface of the sleeve  23  can also enhance the binding of the potting materials with the sleeve  23 , Further enhancement can be achieved by forming grooves  25  in the potting sleeve  23 . Due to the adhesive force between the potting materials and the potting sleeve  23 , it serves to reduces any movement or detachment of the two potting layers  20 ,  21  at the interface  24 , and therefore enhances the binding of the two potting layers  20 ,  21 . 
         [0027]    The potting method outlined above provides a number of advantages over the prior art including improved support of the fibres serves to reduce breakages at the fibre/pot interface. In some applications it is also possible to dispense with the usual potting sleeve as the pot formed by this method has sufficient rigidity to be self-supporting. This produces a manufacturing saving as In some cases shrinkage of the potting materials relative to the potting sleeve produces gaps which are again undesirable in terms of leakage and bacterial growth. In such cases the pot must be discarded. 
         [0028]    It will be appreciated that further embodiments and exemplifications of the invention are possible without departing from the spirit or scope of the invention described.