AND METHOD FOR MANUFACTURING SAME USING SOLUBLE FIBERS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This PCT patent application claims priority to the pending provisional application serial number 61/697,656 filed on September 6, 2012 entitled "Phase Inversion Membrane and Method for Manufacturing Same Using Soluble Fibers," the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to a support fabric for thin film composite membranes and asymmetric membranes for use in water filtration processes.

BACKGROUND

[0003] Membranes formed by phase inversion of polymer solutions are widely used in water filtration. Essentially all reverse osmosis (RO) membranes are made using the process. In flat- sheet RO membrane production, a thin, dense, nonwoven fabric (100 - 150 microns thick) has a polymer solution cast onto it, and the coated fabric is introduced into a coagulation bath where the solvent is removed and the membrane is formed. The fabrics used in RO are relatively dense and highly calendered so that the coating solution does not penetrate the full thickness of the fabric before congealing. This provides a back layer of uncoated nonwoven fabric on the membrane which is required for adhesion of the membrane when it is formed into membrane elements.

[0004] The nonwoven fabrics are required in RO to provide mechanical strength and stability to the membrane. In comparison to the high pressure drops seen across the RO rejection layer, the density and impermeability of the nonwoven fabric backing layer contributes little to the system flow resistance. In other words, the use of a nonwoven fabric backing layer does not significantly impede flow in RO processes. [0005] In contrast, highly calendered nonwoven fabric backing layers do not work well in forward osmosis (FO) membrane processes. FO membranes are formed by phase inversion in a manner similar to RO membranes. However, the FO process differs from RO in that, instead of a being a pressure driven process, FO is a diffusion based process. A consequence of the diffusion driving force in FO is that the diffusion resistance of the nonwoven fabric is a major impediment to the FO process. Dense fabrics which provide negligible resistance in RO may actually drop FO productivity by as much as a factor of 10.

SUMMARY OF THE INVENTION

[0006] The disclosed invention is a nonwoven fabric backing for membranes, and method for making same. More specifically, the nonwoven backing fabric is comprised of a combination of two types of fibers, wherein the first type are fibers that are substantially insoluble in the coagulation and annealing baths, and the second type of fibers are partially or substantially completely soluble in the annealing bath. A membrane is produced by subjecting the backing fabric of the invention to phase inversion by casting a polymer solution onto the fabric to produce a coated fabric, introducing the coated fabric to a coagulation bath, and thereafter subjecting the coated fabric to annealing.

[0007] The resulting membrane possesses sufficient mechanical strength for the FO process while being thin, possesses an open structure to allow diffusion, and still provides an uncoated back surface for adhesion during the assembly of membrane elements.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The method involves producing a thin, highly-calendered dense, nonwoven backing fabric support material similar to those used in RO processes, but wherein a portion of the fibers in the fabric are composed of a material (e.g., plastic) that is at least partially soluble in the coagulation bath and/or annealing baths. The nonwoven backing fabric support material is used as a support for a polymeric layer.

[0009] The nonwoven backing fabric is comprised of a combination of water insoluble and water soluble fibers. The fibers of the first type are substantially insoluble in the coagulation and annealing baths. The fibers of the second type are at least partially soluble in the coagulation bath and are substantially completely soluble in the annealing bath.

[0010] The fibers of the first type may be a blend of fibers with different diameters, lengths, and polymer type, but at least one constituent must have a melting point that is lower than the melting point of the fibers of the second type.

[0011] The membrane is produced by casting a polymer solution onto the nonwoven backing fabric to produce a coated fabric, introducing the coated fabric to a coagulation bath, and thereafter subjecting the coated fabric to annealing. The substantially soluble fibers in the fabric dissolve at least partially in the coagulation and annealing baths, leaving a membrane comprising nonwoven backing fabric no longer containing the soluble fibers, wherein the fabric has a polymeric membrane coating.

[0012] The proportions of first (insoluble) and second (soluble) fibers in the nonwoven fabric backing according to the invention may vary. For example, the nonwoven fabric backing could be made of about 40% to about 50% fibers that are soluble, or other proportions.

[0013] The nonwoven fabric backing according to the invention may be comprised of only one type of substantially soluble fiber and one type of substantially insoluble fiber. The term "type" as used herein refers to the particular composition of the fiber. Alternatively, the fabric backing may be comprised of two or more types of substantially soluble fibers and/or two or more types of substantially insoluble fibers.

[0014] Nonlimiting examples of appropriate materials fibers for the nonwoven fabric backing support are as follows.

[0015] For the first type of fibers, i.e., the substantially insoluble (non-dissolving) fibers, polyester, co-polyester, polypropylene, PET or PVA, and mixtures of the foregoing may be used

[0016] For the second type of fibers, i.e., the substantially soluble fibers, water soluble PET or PVA may be used.

[0017] Other materials may also be used, provided that they have the solubility and melting point characteristics required to produce the backing described herein. [0018] In a preferred embodiment, the first type of fibers is a mixture of PET fibers and either EVA or polypropylene, and the second type of fibers is PVA.

[0019] When a polymer membrane is cast on this fabric, the polymer will not completely penetrate the fabric due to the high density of the fabric. As the cast polymer membrane travels through the coagulation and rinse baths, however, the soluble fibers in the fabric will dissolve and leave an open backing structure having an uncoated back. The uncoated backing is useful for adhering the membrane structure to other components in the membrane element.