WO2011142726A1 - Nanofiltration membrane - Google Patents

Nanofiltration membrane Download PDF

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Publication number
WO2011142726A1
WO2011142726A1 PCT/SG2011/000186 SG2011000186W WO2011142726A1 WO 2011142726 A1 WO2011142726 A1 WO 2011142726A1 SG 2011000186 W SG2011000186 W SG 2011000186W WO 2011142726 A1 WO2011142726 A1 WO 2011142726A1
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WIPO (PCT)
Prior art keywords
nanofibrous
thickness
μπι
diamine
nanofiltration membrane
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PCT/SG2011/000186
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French (fr)
Inventor
Rajendrakumar Suresh Barhate
Seeram Ramakrishna
Balamurugan Ramalingam
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National University Of Singapore
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Publication of WO2011142726A1 publication Critical patent/WO2011142726A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B5/00—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts
    • B32B5/22—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed
    • B32B5/24—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed one layer being a fibrous or filamentary layer
    • B32B5/26—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by the presence of two or more layers which are next to each other and are fibrous, filamentary, formed of particles or foamed one layer being a fibrous or filamentary layer another layer next to it also being fibrous or filamentary
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/125—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction
    • B01D69/1251—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction by interfacial polymerisation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00412—Inorganic membrane manufacture by agglomeration of particles in the dry state by deposition of fibres, nanofibres or nanofibrils
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/1218—Layers having the same chemical composition, but different properties, e.g. pore size, molecular weight or porosity
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/12—Composite membranes; Ultra-thin membranes
    • B01D69/125—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/56—Polyamides, e.g. polyester-amides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/02—Layered products comprising a layer of synthetic resin in the form of fibres or filaments
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/06—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • B32B27/08—Layered products comprising a layer of synthetic resin as the main or only constituent of a layer, which is next to another layer of the same or of a different material of synthetic resin
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/12—Layered products comprising a layer of synthetic resin next to a fibrous or filamentary layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/30—Layered products comprising a layer of synthetic resin comprising vinyl (co)polymers; comprising acrylic (co)polymers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/34—Layered products comprising a layer of synthetic resin comprising polyamides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B27/00—Layered products comprising a layer of synthetic resin
    • B32B27/36—Layered products comprising a layer of synthetic resin comprising polyesters
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B5/00—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts
    • B32B5/02—Layered products characterised by the non- homogeneity or physical structure, i.e. comprising a fibrous, filamentary, particulate or foam layer; Layered products characterised by having a layer differing constitutionally or physically in different parts characterised by structural features of a fibrous or filamentary layer
    • B32B5/022—Non-woven fabric
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/442—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by nanofiltration
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M15/00—Treating fibres, threads, yarns, fabrics, or fibrous goods made from such materials, with macromolecular compounds; Such treatment combined with mechanical treatment
    • D06M15/19—Treating fibres, threads, yarns, fabrics, or fibrous goods made from such materials, with macromolecular compounds; Such treatment combined with mechanical treatment with synthetic macromolecular compounds
    • D06M15/37—Macromolecular compounds obtained otherwise than by reactions only involving carbon-to-carbon unsaturated bonds
    • D06M15/59—Polyamides; Polyimides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/39—Electrospinning
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/02—Reverse osmosis; Hyperfiltration ; Nanofiltration
    • B01D61/027—Nanofiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2262/00—Composition or structural features of fibres which form a fibrous or filamentary layer or are present as additives
    • B32B2262/02—Synthetic macromolecular fibres
    • B32B2262/0246—Acrylic resin fibres
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2262/00—Composition or structural features of fibres which form a fibrous or filamentary layer or are present as additives
    • B32B2262/02—Synthetic macromolecular fibres
    • B32B2262/0253—Polyolefin fibres
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2262/00—Composition or structural features of fibres which form a fibrous or filamentary layer or are present as additives
    • B32B2262/02—Synthetic macromolecular fibres
    • B32B2262/0276—Polyester fibres
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2262/00—Composition or structural features of fibres which form a fibrous or filamentary layer or are present as additives
    • B32B2262/14—Mixture of at least two fibres made of different materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/50—Properties of the layers or laminate having particular mechanical properties
    • B32B2307/582—Tearability
    • B32B2307/5825—Tear resistant
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/70—Other properties
    • B32B2307/726—Permeability to liquids, absorption
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2307/00—Properties of the layers or laminate
    • B32B2307/70—Other properties
    • B32B2307/732—Dimensional properties

Definitions

  • Nanofiltration is an emerging technology in water purification, wastewater treatment, and seawater desalination.
  • Hoek et al. Nanofiltration separations, Dekker Encyclopedia of Nanoscience and Nanotechnology (2nd edition), Taylor and Francis Group, 2009, pp. 2749-2762. Comparing with traditional technologies, nanofiltration offers many benefits including low operation cost, reduced energy consumption, and avenues for integrated and environmentally friendly processing. Progress has been made to perfect this technology in industrial applications. See, e.g., Freger et al., J. Membr. Sci. 209(1 ) (2002) 283. However, many problems still exist, for example, membrane fouling and low membrane stabilities and durability.
  • a nanofiltration membrane which includes (i) a supporting fabric layer, (ii) an engineered nanofibrous structural layer deposited on the top of the supporting fabric layer, and (iii) a polyamide thin film deposited on the top of the engineered nanofibrous structural layer.
  • the supporting fabric layer is 100- 200 ⁇ thick
  • the engineered nanofibrous structural layer is 30-75 ⁇ thick and contains pores of the size of 0.4-1.5 ⁇
  • the polyamide thin film is 100-300 ran thick.
  • the pore size used herein is referred to the diameter at the maximum of the pore size distribution. See Nimmo J.R., Encyclopedia of Soils in the Environment, Ed. by Hillel, D., London, Elsevier, 2004, v.
  • the nanofiltration membrane has one or more of the following features: the supporting layer has a thickness of 100-150 ⁇ and/or an air permeability of 0.0015-0.01 1 cubic foot per minute, the engineered nanofibrous structural layer has a thickness of 30-60 ⁇ and/or contains pores of the size of 0.7-1 ⁇ , and the polyamide thin film has a thickness of 100-200 nm and/or contains pores of the size of 0.001-0.1 ⁇ (or 0.001-0.01 ⁇ ). In one embodiment, the engineered nanofibrous structural layer has a thickness of 50-75 ⁇ and contains pores of the size of 0.7-1 ⁇ . In another embodiment, the supporting fabric layer has a thickness of about 100 ⁇ , the engineered nanofibrous structural layer has a.thickness of about
  • the polyamide thin film has a thickness of about 100 nm.
  • the supporting fabric layer can be made of polyester or polypropylene.
  • the engineered nanofibrous structural layer can be made of polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,
  • the polyamide thin film can be made by interfacial polymerization between aromatic or cyclic aliphatic diamine (e.g., ortho-, meta-, or para-phenylenediamine, piperazine, or bipiperidine) and acid halide (e.g., trimesoyl chloride, isophthaloyl chloride, or terephthoyl chloride).
  • aromatic or cyclic aliphatic diamine e.g., ortho-, meta-, or para-phenylenediamine, piperazine, or bipiperidine
  • acid halide e.g., trimesoyl chloride, isophthaloyl chloride, or terephthoyl chloride.
  • Another aspect of this invention relates to a process of preparing the above-described nanofiltration membrane.
  • the method includes (i) applying nanofibers onto a supporting fabric layer to form a nanofibrous structural layer, (ii) pressing the nanofibrous structural layer and the supporting fabric layer at a predetermined temperature and under a pre-determined pressure, and (iii) coating the pressed nanofibrous structural layer with a polyamide thin layer via interfacial polymerization between diamine and acid halide.
  • the applying step (i) can be performed by electrostatic spinning; melt electrospinning; modified melt spinning;
  • the pressing step (ii) can be performed at a temperature in a range between 10°C below and 10°C above the glass transition temperature of the nanofibers of the nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in.
  • nanofiltration membrane of this invention is a process of treating water by passing it through the nanofiltration membrane of this invention to remove salts or impurities.
  • This process can be performed under atmospheric or elevated pressure or under vacuum.
  • the nanofiltration membrane can be of any shape, e.g., tubular or sheet.
  • Fig. 1 is a cross-section illustration of a nanofiltration membrane of this invention.
  • Fig. 2 shows schematic filtration testing set-up.
  • Figs. 3 A and 3B show membrane stability of a NF membrane prepared without hot-pressing and a NF membrane prepared with hot-pressing, respectively.
  • Figs. 4A and 4B show filtration results for a NF membrane prepared without hot- pressing and a NF membrane prepared with hot-pressing, respectively.
  • Fig. 5 shows mechanical testing results of NF membranes DETAILED DESCRIPTION OF THE INVENTION
  • This invention relates to a nanofiltration membrane as shown in Fig. 1 , which has a supporting fabric layer (1), an engineered nanofibrous structural layer (2), and a polyamide thin film (3).
  • This membrane exhibits superior features, including high energy efficiency, structural durability and pressure tolerance, and operational stability for water treatment applications.
  • Nonwoven fabric is a fabric-like material which has a random fibrous web structure bonded together by entangling fibers or filaments mechanically, thermally, or chemically.
  • the nonwoven fabric has interconnecting open area throughout cross-section to allow liquid or gaseous fluids, but not particulate, to follow through it.
  • a commonly-used example is
  • spunbonded calendered nonwoven fabric made of fibers of an average diameter of 30- 40 microns. It has a 20-30 microns mean flow pore ratings, a basis weight ranging from 60 to 70 g/m 2 , an air flow permeability of 0.0015-0.01 1 cubic foot per minute, a thickness of 100-200 microns, and a high tear strength and tensile strength across machine direction (e.g., 4028 N/m MD) and cross direction (e.g., 2451.8 N/m CD).
  • the Frazier The Frazier
  • permeability of the nonwoven spunbonded calendered fabric can be around 640 L/m 2 /sec at 196 Pa differential pressures.
  • nanofibers via a known technique such as electrostatic spinning (electrospinning), melt electrospinning, modified melt spinning, electroblowing, splitting of jet of melt/solution and multicomponent fiber spinning or an 'islands-in-the-sea' method to form a nanofibrous structural layer:
  • nanofibers of a diameter of 50-500 nm having very high homogeneity are used. They can be made from polymer such as polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,
  • the thus-obtained nanofibrous structural layer is then pressed at a predetermined temperature and under a pre-determined pressure via a conventional method to form the engineered nanofibrous structural layer (2).
  • This step is preferably conducted within ⁇ 20°C (or ⁇ 10°C) of the glass transition temperature of the nanofibers of the
  • nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in.
  • This treatment improves not only the adhesion between the nanofibrous structural layer and the supporting fabric layer, but also the pore interconnectivity in nanofibrous structural layer.
  • the pore size is reduced.
  • the engineered nanofibrous structural layer has a thickness of 30-75 ⁇ (or 30- 60 ⁇ ) and contains pores of the size of 0.4-1.5 ⁇ (or 0.7-1.0 ⁇ ). It has a water permeability ranging from 30,000 l/(m 2 h) to 50,000 l/(m 2 h) at transmembrane pressure of 70 psi, mostly near to 36,0001/(m 2 h).
  • the engineered nanofibrous structural layer is coated with a polyamide thin layer (3) via the technique of interfacial polymerization.
  • the polyamide thin film is 100-300 nm thick.
  • Interfacial polymerization has been described in, e.g., in Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492. In this reaction, polymerization occurs at or near the interfacial boundary of two solutions, one containing a diamine monomer and the other containing an acid halide monomer.
  • the diamine monomer can be aromatic diamines (such as ortho, meta and para- phenylenediamine) or cyclic aliphatic diamines (such as piperazine and bipiperidine).
  • the acid halide monomer can be acid chloride (such as trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride).
  • the diamine monomer and acid halide monomer have different dissociation constants with trace additives so as to create different sizes of directed channels in the polyamide layer, which lead to varying rejection of monovalent, bivalent, and multivalent ions. Examples are
  • Solvents used in the interfacial polymerization can be selected from water and organic solvents.
  • the solvent used to dissolve the diamine and the solvent used to dissolve the acid halide are immiscible.
  • the diamine is dissolved in water and the acid chloride is dissolved in methylene chloride, chloroform, toluene, ester, ether, alkane (e.g., pentane or hexane), a mixture thereof.
  • the resulting membrane is dried with convective air circulation flow at a temperature ranging from 65 P C to 90°C and then thoroughly washed with de-ionized water.
  • the membrane of this invention unexpectedly offers several benefits such as structural durability, high pressure tolerance and operational stability, considerable energy saving efficiency due to high permeabilities/fluxes even at relatively lower driving pressure differentials.
  • One skilled in the art can translate the process parameters described herein to those suitable for commercial manufacture of the nanofiltration membrane of this invention.
  • the nanofiltration membrane of this invention can be further processed to make various types of articles useful for performing water treatments such as surface, ground, brackish, or sea water treatments, wastewater treatments, partial and hybrid desalination operation, industrial desalting operations, and industrial scale low molecular weight organics separation and purification.
  • a tubular membrane can be used to purify water by separating salts or impurities from water.
  • Nonwoven poly( ethylene terephthalate (PET) Hollytex 3242 supplied by Ahlstrom Mount Holly Springs, USA), polyacrylonitrile (PAN) average Mw 150,000 (Aldrich Product Number 181315), N,N-Dimethylformamide GR ACS (DMF) (Merck Ltd., Product code 1.03053), piperazine (Sigma-Aldrich Product Number P- 45907), bipiperidine dihydrochloride (Sigma-Aldrich Product Number 180742), triethylamine (Sigma-Aldrich Product Number T0886), NaOH (Sigma-Aldrich Product Number S8045), Water (Milli Q), 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride or TMC) (Sigma-Aldrich Product Number 147532), hexane anhydrous (Sigma aldrich Product Number 296090), and washing water for membrane (Milli Q), MgS
  • NF 90 and NF270 were supplied by the FilmTec Corporation (Edina, M , USA).
  • NF 90 is an aromatic polyamide, which contains carboxylic acid and primary amines (-NH2)
  • NF 270 is a mixed aromatic, aliphatic polyamide (polypiperazine amide) with secondary amine (-NH) and carboxylic acid.
  • -NH2 carboxylic acid and primary amines
  • NF 270 is a mixed aromatic, aliphatic polyamide (polypiperazine amide) with secondary amine (-NH) and carboxylic acid.
  • ENSA 1 -LF and ENSA 1 -LF2 membranes were supplied by Hydranautics a Nitto Denko Company (Oceanside, CA, USA).
  • ENSAl-LF and ENSA1- LF2 membranes are composite polyamide type similar to existing reverse osmosis membranes but chemically treated to adjust the hardness rejection and impart fouling resistance. See Bartels et al., Desalination 22 (1-3) (2008) 158.
  • Spunbonded polyester sheet structured from the continuous polyester fibers is commonly used for supporting a microporous layer. Calendering imparts high tear strength and tensile strength to the spunbonded polyester fabric across machine direction (MD) and cross direction (CD). Polyester materials of spunbonded sheet show a good thermal and chemical resistance particularly to acids, oxidizing agents and solvents. Balanced structural properties across the machine direction and cross direction and controlled thickness are an important perquisite feature of calendered nonwoven fabric. The properties of such a layer used to support the nanofibrous layer are given in Table 1 below.
  • the nanofibrous structural layer and the supporting fabric layer were exposed to mild convective air flow in fumehood for at least 3 hours to remove the residual solvent.
  • the nanofibrous structural layer and the supporting fabric layer were then hot-pressed in thermal transfer press (Hotronix, Carmichaels, PA 15320, USA) at light pressure (setting value of 3) at 85°C (near to glass transition temperature of PAN) for 16.65 mins.
  • the pore size of the nanofibrous structural layer was measured by a capillary flow porometer (CFP-1200-A, PMI- Porous Materials Inc., Ithaca, NY, USA) using GalwickTM of surface tension 0.0159 N/m as wetting liquid.
  • the nanofibrous structural layer was coated with a polyamide film by interfacial polymerization reaction using bipiperidine dihydrochloride and piperazine (as diamine monomers) and TMC (as aromatic acid monomer) according to Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492.
  • Pre-cut nanofibrous structural layer (15 cm x 15 cm) was placed on the flat metallic surface in a tray.
  • Aqueous diamine solution containing bipiperidine dihydrochloride (0.3 %w/v), piperazine (0.7 % w/v), NaOH (1.5 equivalent to bipiperidine dihydrochloride concentration), and triethylamine (1% w/v) was prepared.
  • the nanofibrous structural layer in the tray was wetted by aqueous diamine solution for 1 min. The excess amine solution was drained by keeping the tray in vertical position for 3 minutes. The wetted membrane was taken in separate dried tray and edges of the support membrane were sealed by an adhesive tape (3M, USA). The wetted nanofibrous structural membrane was covered completely by an organic solution containing TMC (0.1 % w/v) for 1 min. After draining TMC solution, the membrane was air dried in fumehood for 15 minutes followed by drying in a hot air oven with convective airflow at temperature of 65 °C for 20 minutes. Finally, the membrane was exposed to convective airflow at 25°C in the fume hood for 3.5 hours.
  • membrane was just air dried in fumehood for 1 hr. Dried TFC NF membrane was washed with water three times for 1/2 to 1 day. The dried membrane was used for filtration testing. Each membrane was fabricated three times for filtration testing to account the variations. Preparation of membranes without hot-pressing
  • the membranes were prepared according to the methods described above except that hot-pressing step was omitted. See also Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492.
  • Morphological observations of the membranes were performed by a versatile high performance and low vacuum scanning electron microscope (SEM) (quanta 200F, FEI).
  • SEM high performance and low vacuum scanning electron microscope
  • the samples were prepared by cutting the membranes into small pieces (approximately 5 mm x 5 mm in size) with a sharp surgical blade.
  • the samples for thickness measurement were prepared by cutting very thin pieces (rectangular approximately 4 mm x 15 mm in size).
  • the samples were fixed on a flat sample holder (stubs).
  • the samples for thickness measurement were hold vertically in a metallic clip.
  • a conductive adhesive carbon tap was applied on the sample holder (except in the thickness measurement) before mounting the membrane samples over it.
  • the samples for SEM were coated by platinum in the auto fine coater (JFC-1600, JEOL, Japan) for 60-120 sec at the current of 10 mA and vacuum of 8 to 10 Pa. If the samples were not coated enough, they were recoated again.
  • the distance between electron gun and sample, accelerating voltage, spot size and vacuum used for capturing good SEM images were 10 mm, 10 kV, 3 and 0.9 mbar, respectively.
  • the magnification, sharpness and contrast of images were adjusted before capturing the images.
  • the planer images were analyzed for the fiber diameter, orientations of fibers and surface analysis, while the vertical or cross sectional images were analyzed for the thickness of samples.
  • the Java image processing software (Image J 1.29 x (222 commands) was used for measuring the diameter of fibers and thickness of mat from stored SEM images. At least four pictures were used for calculating the mean values of diameter of fibers and thickness of web. Analysis using Atomic Force Microscope
  • Membrane surface imaging and roughness measurement was performed by the Atomic Force microscope (AFM) Dimensional 0.0 AFM,.Veeco.Instruments, Edina, M ). Imaging was performed in the tapping mode with silicon probe (NanosensorsTM PPP-NCH, non-contact/tapping mode type probe, Nanosensors, Switzerland).
  • the cantilever had nominal thickness of 4 ⁇ , nominal mean width of 30 ⁇ , a nominal spring constant of 42 N/m, nominal resonant frequency of 330 Hz, nominal tip radius of ⁇ 10 nm and nominal cantilever length of 125 ⁇ .
  • the air-dried membrane of 10 ⁇ x 10 ⁇ area was scanned at scan rate of 1 Hz, tip velocity of 10 ⁇ /sec, sample/lines 128 and lines 64.
  • the best images were captured by adjusting the amplitude set point, contrast, data scale, and resolution to highest value 512 samples/lines and 512 lines.
  • the captured images were processed by nanoscope ® III, version 6.13 Rl software. The roughness measurement was performed over the entire image. The following
  • Rq The Root Mean Square (RMS) Roughness is the standard deviation of the Z values within a given area.
  • Z ave is the average of Z value within the given area
  • Zi is the current Z value
  • N is the number of points within a given area.
  • Ra The mean Roughness (Ra) represents the arithmetic average of the deviations from the centre plan.
  • Z cp is the Z value of the center plan and Zi is the current Z value and N is the number of points within a given area.
  • Rmax The difference in height between the highest and lowest points on the surface relative to the mean plane.
  • SA Surface area
  • the mechanical testing of membranes was done using the Instron (UK) make 3345 Single Column Testing Systems.
  • the specimen of size 8 cm x 2 cm was used for tensile strength measurement.
  • the thickness of membrane was measured prior to analysis.
  • the appropriate load cell of 100 N or 1000 N capacity was used for the measurement, depending upon the membrane strength.
  • the test method was established using particular load cell at 10 mm/min stretching rate.
  • the area of 2 cm x 2 cm at both ends of the membrane was used to hold the specimen in the Instron machine.
  • the measurement was done using established test method.
  • the data acquisition/processing was done by using the Bluehill® 2 is a fully-integrated modular software package.
  • the plot of tensile stress (MPa) vs. tensile strain (%) was obtained to compare mechanical strengths of membranes.
  • NF membranes of this invention and commercial membranes were cut into circular pieces (0.065 m in diameter) with a membrane cutter. They were fixed in the cross flow cell. The exposed area of the membranes was 0.001963 m .
  • the schematic filtration testing set-up is shown in the Fig. 2.
  • TMP Trans Membrane Pressure
  • TMP [(Pi-P 2 )/2]-P 3
  • MgS0 4 .7H 2 0 2000 ppm
  • membrane was compacted at TMP pressure of 70 psi using the 2000 ppm MgS0 4 feed solution.
  • TMP pressure 70 psi
  • Conductivity and temperature of feed before starting the filtration experiment and after the filtration run was measured by the digital conductivity meter (Thermo Scientific Orion 3-star Plus). Conductivity and temperature of permeate was also measured.
  • R (%) [l- (Cp/C f )]*100
  • C p is the conductivity of permeate ( ⁇ 8/ ⁇ )
  • C f is the conductivity of feed ( ⁇ 8/ ⁇ ) after the filtration.
  • the capillary flow porosimetry results and pure water flux are summarized in the Table 3 below.
  • the hot pressing treatment to nanofibrous structural layer narrowed the pore size distribution of membrane.
  • the membranes prepared without hot pressing could not withstand the impact of high pressure, particularly when the TMP was more than 130 psi. See Fig. 3 A and Fig. 4A.
  • MgS0 4 rejection was dropped from 88.78 % to 53.60 % and permeate flux was increased from 218.2 to 451.7 1/m h, when the TMP was increased from 130 psi to 160 psi.
  • the nanofibrous structural layer (of 1.759 ⁇ mean flow pore diameter) was capable of supporting the polyamide thin film, as shown from the high values of MgS0 4 rejection. See Fig. 4A.
  • the thickness of thin film When the thickness of thin film
  • the polyamide layer became smaller than the diameter of fibers of supporting film, the polyamide film showed imprints of the nanofiber morphology.
  • the membrane prepared with hot pressing had improved pressure tolerance and mechanical and structural integrity. See Figs. 3B and 4B. It was robust to withstand the higher pressure more than the 130 psi. See Fig. 4B.
  • the membrane prepared with hot pressing had higher flux than commercial membranes (i.e. 10-12 % higher than the best commercial membrane).
  • Fig. 5 shows tensile stress vs. tensile strain relationships of various commercial membranes and the membrane of this invention.
  • Nanofibrous layer is too weak to impart suitable mechanical strength to the polyamide NF membrane (black curve). However, when it is supported by commercially available engineered nonwoven backing material, it imparts adequate mechanical strength to polyamide NF membrane (violet curve).
  • the membrane of this invention has comparable mechanical property with commercially available NF membranes.

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Abstract

A nanofiltration membrane, which includes (i) a supporting fabric layer, (ii) an engineered nanofibrous structural layer deposited on the top of the supporting fabric layer, and (iii) a polyamide thin film deposited on the top of the engineered nanofibrous structural layer. The supporting fabric layer is 100-200 µm thick, the engineered nanofibrous structural layer is 30-75 µm thick and contains pores of the size of 0.4-1.5 µm, and the polyamide thin film is 100-300 nm thick. Also disclosed is a method of preparing this nanofiltration membrane.

Description

NANOFILTRATION MEMBRANE
BACKGROUND OF THE INVENTION
Nanofiltration (NF) is an emerging technology in water purification, wastewater treatment, and seawater desalination. See Hoek et al., Nanofiltration separations, Dekker Encyclopedia of Nanoscience and Nanotechnology (2nd edition), Taylor and Francis Group, 2009, pp. 2749-2762. Comparing with traditional technologies, nanofiltration offers many benefits including low operation cost, reduced energy consumption, and avenues for integrated and environmentally friendly processing. Progress has been made to perfect this technology in industrial applications. See, e.g., Freger et al., J. Membr. Sci. 209(1 ) (2002) 283. However, many problems still exist, for example, membrane fouling and low membrane stabilities and durability.
There is still a need to develop nanofiltration membranes having improved features.
SUMMARY OF THE INVENTION
One aspect of this invention relates to a nanofiltration membrane, which includes (i) a supporting fabric layer, (ii) an engineered nanofibrous structural layer deposited on the top of the supporting fabric layer, and (iii) a polyamide thin film deposited on the top of the engineered nanofibrous structural layer. The supporting fabric layer is 100- 200 μιη thick, the engineered nanofibrous structural layer is 30-75 μπι thick and contains pores of the size of 0.4-1.5 μπι, and the polyamide thin film is 100-300 ran thick. The pore size used herein is referred to the diameter at the maximum of the pore size distribution. See Nimmo J.R., Encyclopedia of Soils in the Environment, Ed. by Hillel, D., London, Elsevier, 2004, v. 3, p. 295-303. Preferably, the nanofiltration membrane has one or more of the following features: the supporting layer has a thickness of 100-150 μπι and/or an air permeability of 0.0015-0.01 1 cubic foot per minute, the engineered nanofibrous structural layer has a thickness of 30-60 μπι and/or contains pores of the size of 0.7-1 μπι, and the polyamide thin film has a thickness of 100-200 nm and/or contains pores of the size of 0.001-0.1 μπι (or 0.001-0.01 μηι). In one embodiment, the engineered nanofibrous structural layer has a thickness of 50-75 μηι and contains pores of the size of 0.7-1 μηι. In another embodiment, the supporting fabric layer has a thickness of about 100 μπι, the engineered nanofibrous structural layer has a.thickness of about
50 μηι, and the polyamide thin film has a thickness of about 100 nm.
The supporting fabric layer can be made of polyester or polypropylene. The engineered nanofibrous structural layer can be made of polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,
polyetheretherketone, or a mixture thereof. The polyamide thin film can be made by interfacial polymerization between aromatic or cyclic aliphatic diamine (e.g., ortho-, meta-, or para-phenylenediamine, piperazine, or bipiperidine) and acid halide (e.g., trimesoyl chloride, isophthaloyl chloride, or terephthoyl chloride).
Another aspect of this invention relates to a process of preparing the above-described nanofiltration membrane. The method includes (i) applying nanofibers onto a supporting fabric layer to form a nanofibrous structural layer, (ii) pressing the nanofibrous structural layer and the supporting fabric layer at a predetermined temperature and under a pre-determined pressure, and (iii) coating the pressed nanofibrous structural layer with a polyamide thin layer via interfacial polymerization between diamine and acid halide. The applying step (i) can be performed by electrostatic spinning; melt electrospinning; modified melt spinning;
electroblowing, and splitting of jet of melt/solution. The pressing step (ii) can be performed at a temperature in a range between 10°C below and 10°C above the glass transition temperature of the nanofibers of the nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in.
Also within the scope of this invention is a process of treating water by passing it through the nanofiltration membrane of this invention to remove salts or impurities. This process can be performed under atmospheric or elevated pressure or under vacuum. The nanofiltration membrane can be of any shape, e.g., tubular or sheet.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the detailed description of several embodiments and also from the appending claims. DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 is a cross-section illustration of a nanofiltration membrane of this invention. Fig. 2 shows schematic filtration testing set-up.
Figs. 3 A and 3B show membrane stability of a NF membrane prepared without hot-pressing and a NF membrane prepared with hot-pressing, respectively.
Figs. 4A and 4B show filtration results for a NF membrane prepared without hot- pressing and a NF membrane prepared with hot-pressing, respectively.
Fig. 5 shows mechanical testing results of NF membranes DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a nanofiltration membrane as shown in Fig. 1 , which has a supporting fabric layer (1), an engineered nanofibrous structural layer (2), and a polyamide thin film (3). This membrane exhibits superior features, including high energy efficiency, structural durability and pressure tolerance, and operational stability for water treatment applications.
To fabricate the nanofiltration membrane of this invention, one can use
commercially available nonwoven fabric as the supporting fabric layer (1 ). Nonwoven fabric is a fabric-like material which has a random fibrous web structure bonded together by entangling fibers or filaments mechanically, thermally, or chemically. The nonwoven fabric has interconnecting open area throughout cross-section to allow liquid or gaseous fluids, but not particulate, to follow through it. A commonly-used example is
spunbonded calendered nonwoven fabric made of fibers of an average diameter of 30- 40 microns. It has a 20-30 microns mean flow pore ratings, a basis weight ranging from 60 to 70 g/m2, an air flow permeability of 0.0015-0.01 1 cubic foot per minute, a thickness of 100-200 microns, and a high tear strength and tensile strength across machine direction (e.g., 4028 N/m MD) and cross direction (e.g., 2451.8 N/m CD). The Frazier
permeability of the nonwoven spunbonded calendered fabric can be around 640 L/m2/sec at 196 Pa differential pressures.
Onto the supporting fabric layer (1), one deposits nanofibers via a known technique such as electrostatic spinning (electrospinning), melt electrospinning, modified melt spinning, electroblowing, splitting of jet of melt/solution and multicomponent fiber spinning or an 'islands-in-the-sea' method to form a nanofibrous structural layer:
Preferably, nanofibers of a diameter of 50-500 nm having very high homogeneity are used. They can be made from polymer such as polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides,
polyetheretherketone, or a mixture thereof. Production and deposition of nanofibers can carried out in a synchronized fashion.
The thus-obtained nanofibrous structural layer is then pressed at a predetermined temperature and under a pre-determined pressure via a conventional method to form the engineered nanofibrous structural layer (2). This step is preferably conducted within ± 20°C (or ± 10°C) of the glass transition temperature of the nanofibers of the
nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in. This treatment improves not only the adhesion between the nanofibrous structural layer and the supporting fabric layer, but also the pore interconnectivity in nanofibrous structural layer. After the hot-pressing step, the pore size is reduced.
The engineered nanofibrous structural layer has a thickness of 30-75 μπι (or 30- 60 μπι) and contains pores of the size of 0.4-1.5 μπι (or 0.7-1.0 μπι). It has a water permeability ranging from 30,000 l/(m2 h) to 50,000 l/(m2 h) at transmembrane pressure of 70 psi, mostly near to 36,0001/(m2 h).
Next, the engineered nanofibrous structural layer is coated with a polyamide thin layer (3) via the technique of interfacial polymerization. Preferably, the polyamide thin film is 100-300 nm thick. Interfacial polymerization has been described in, e.g., in Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492. In this reaction, polymerization occurs at or near the interfacial boundary of two solutions, one containing a diamine monomer and the other containing an acid halide monomer.
The diamine monomer can be aromatic diamines (such as ortho, meta and para- phenylenediamine) or cyclic aliphatic diamines (such as piperazine and bipiperidine). The acid halide monomer can be acid chloride (such as trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride). Preferably, the diamine monomer and acid halide monomer have different dissociation constants with trace additives so as to create different sizes of directed channels in the polyamide layer, which lead to varying rejection of monovalent, bivalent, and multivalent ions. Examples are
piperazine/bipiperidine dihydrochloride and trimesoyl chloride.
Solvents used in the interfacial polymerization can be selected from water and organic solvents. Preferably, the solvent used to dissolve the diamine and the solvent used to dissolve the acid halide are immiscible. For example, the diamine is dissolved in water and the acid chloride is dissolved in methylene chloride, chloroform, toluene, ester, ether, alkane (e.g., pentane or hexane), a mixture thereof.
After interfacial polymerization, the resulting membrane is dried with convective air circulation flow at a temperature ranging from 65PC to 90°C and then thoroughly washed with de-ionized water.
The membrane of this invention unexpectedly offers several benefits such as structural durability, high pressure tolerance and operational stability, considerable energy saving efficiency due to high permeabilities/fluxes even at relatively lower driving pressure differentials.
One skilled in the art can translate the process parameters described herein to those suitable for commercial manufacture of the nanofiltration membrane of this invention. For example, to continuously produce the membrane, one can synchronize the nanofiber production machineries with thermally and mechanically driven flat presses or rollers, thin film coating machines, washing/rinsing lines, and convective air drying chambers.
The nanofiltration membrane of this invention can be further processed to make various types of articles useful for performing water treatments such as surface, ground, brackish, or sea water treatments, wastewater treatments, partial and hybrid desalination operation, industrial desalting operations, and industrial scale low molecular weight organics separation and purification. For example, a tubular membrane can be used to purify water by separating salts or impurities from water.
Without further elaboration, it is believed that one skilled in the art can, based on the disclosure herein, utilize the present invention to its fullest extent. The following specific embodiments are, therefore, to be construed as merely descriptive, and not limitative of the remainder of the disclosure in any way whatsoever. All publications cited herein are incorporated by reference. Examples
Chemicals/solvents: Nonwoven poly( ethylene terephthalate (PET) (Hollytex 3242 supplied by Ahlstrom Mount Holly Springs, USA), polyacrylonitrile (PAN) average Mw 150,000 (Aldrich Product Number 181315), N,N-Dimethylformamide GR ACS (DMF) (Merck Ltd., Product code 1.03053), piperazine (Sigma-Aldrich Product Number P- 45907), bipiperidine dihydrochloride (Sigma-Aldrich Product Number 180742), triethylamine (Sigma-Aldrich Product Number T0886), NaOH (Sigma-Aldrich Product Number S8045), Water (Milli Q), 1,3,5-benzenetricarbonyl trichloride (trimesoyl chloride or TMC) (Sigma-Aldrich Product Number 147532), hexane anhydrous (Sigma aldrich Product Number 296090), and washing water for membrane (Milli Q), MgS047 H20 (Sino Chemicals Co. Pte Ltd ).
NF 90 and NF270 were supplied by the FilmTec Corporation (Edina, M , USA). NF 90 is an aromatic polyamide, which contains carboxylic acid and primary amines (-NH2), whereas NF 270 is a mixed aromatic, aliphatic polyamide (polypiperazine amide) with secondary amine (-NH) and carboxylic acid. See ArtuAY, et al., Sep. Sci. Technol. 42(13) (2007) 2947. ENSA 1 -LF and ENSA 1 -LF2 membranes were supplied by Hydranautics a Nitto Denko Company (Oceanside, CA, USA). ENSAl-LF and ENSA1- LF2 membranes are composite polyamide type similar to existing reverse osmosis membranes but chemically treated to adjust the hardness rejection and impart fouling resistance. See Bartels et al., Desalination 22 (1-3) (2008) 158.
Preparation of the nanofibrous structural layer over the supporting fabric
Spunbonded polyester sheet structured from the continuous polyester fibers is commonly used for supporting a microporous layer. Calendering imparts high tear strength and tensile strength to the spunbonded polyester fabric across machine direction (MD) and cross direction (CD). Polyester materials of spunbonded sheet show a good thermal and chemical resistance particularly to acids, oxidizing agents and solvents. Balanced structural properties across the machine direction and cross direction and controlled thickness are an important perquisite feature of calendered nonwoven fabric. The properties of such a layer used to support the nanofibrous layer are given in Table 1 below.
Table 1. Properties of the spunbonded polyester sheet
Figure imgf000008_0001
DMF was treated with molecular sieves to remove moisture. PAN was dissolved in dried DMF to form a PAN solution (8 % w/w). This solvent was subsequently subjected to electrospinning to deposit a nanofibrous structural layer onto the spunbonded polyester sheet. The electrospinning conditions were similar to those described in Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492, which are shown in Table 2 below. A Nanon-01 A electrospinning machine (MECC CO, Ltd, Japan) was used for electrospinning. Table 2: Electrospinning conditions
Figure imgf000009_0001
* selected close to 135 rpm as machine resolution was 50 rpm.
After electrospinning, the nanofibrous structural layer and the supporting fabric layer were exposed to mild convective air flow in fumehood for at least 3 hours to remove the residual solvent. The nanofibrous structural layer and the supporting fabric layer were then hot-pressed in thermal transfer press (Hotronix, Carmichaels, PA 15320, USA) at light pressure (setting value of 3) at 85°C (near to glass transition temperature of PAN) for 16.65 mins. The pore size of the nanofibrous structural layer was measured by a capillary flow porometer (CFP-1200-A, PMI- Porous Materials Inc., Ithaca, NY, USA) using Galwick™ of surface tension 0.0159 N/m as wetting liquid.
Deposition of polvamide thin layer
The nanofibrous structural layer was coated with a polyamide film by interfacial polymerization reaction using bipiperidine dihydrochloride and piperazine (as diamine monomers) and TMC (as aromatic acid monomer) according to Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492. Pre-cut nanofibrous structural layer (15 cm x 15 cm) was placed on the flat metallic surface in a tray. Aqueous diamine solution containing bipiperidine dihydrochloride (0.3 %w/v), piperazine (0.7 % w/v), NaOH (1.5 equivalent to bipiperidine dihydrochloride concentration), and triethylamine (1% w/v) was prepared. The nanofibrous structural layer in the tray was wetted by aqueous diamine solution for 1 min. The excess amine solution was drained by keeping the tray in vertical position for 3 minutes. The wetted membrane was taken in separate dried tray and edges of the support membrane were sealed by an adhesive tape (3M, USA). The wetted nanofibrous structural membrane was covered completely by an organic solution containing TMC (0.1 % w/v) for 1 min. After draining TMC solution, the membrane was air dried in fumehood for 15 minutes followed by drying in a hot air oven with convective airflow at temperature of 65 °C for 20 minutes. Finally, the membrane was exposed to convective airflow at 25°C in the fume hood for 3.5 hours. In some cases, membrane was just air dried in fumehood for 1 hr. Dried TFC NF membrane was washed with water three times for 1/2 to 1 day. The dried membrane was used for filtration testing. Each membrane was fabricated three times for filtration testing to account the variations. Preparation of membranes without hot-pressing
The membranes were prepared according to the methods described above except that hot-pressing step was omitted. See also Yoon et al., Journal of Membrane Science, 2009. 326(2): p. 484-492.
Microscopy of membranes
Morphological observations of the membranes were performed by a versatile high performance and low vacuum scanning electron microscope (SEM) (quanta 200F, FEI). The samples were prepared by cutting the membranes into small pieces (approximately 5 mm x 5 mm in size) with a sharp surgical blade. The samples for thickness measurement were prepared by cutting very thin pieces (rectangular approximately 4 mm x 15 mm in size). The samples were fixed on a flat sample holder (stubs). The samples for thickness measurement were hold vertically in a metallic clip. To prevent excessive charging of the samples during analysis, a conductive adhesive carbon tap was applied on the sample holder (except in the thickness measurement) before mounting the membrane samples over it.
The samples for SEM were coated by platinum in the auto fine coater (JFC-1600, JEOL, Japan) for 60-120 sec at the current of 10 mA and vacuum of 8 to 10 Pa. If the samples were not coated enough, they were recoated again. The distance between electron gun and sample, accelerating voltage, spot size and vacuum used for capturing good SEM images were 10 mm, 10 kV, 3 and 0.9 mbar, respectively. The magnification, sharpness and contrast of images were adjusted before capturing the images. The planer images were analyzed for the fiber diameter, orientations of fibers and surface analysis, while the vertical or cross sectional images were analyzed for the thickness of samples. The Java image processing software (Image J 1.29 x (222 commands)) was used for measuring the diameter of fibers and thickness of mat from stored SEM images. At least four pictures were used for calculating the mean values of diameter of fibers and thickness of web. Analysis using Atomic Force Microscope
Membrane surface imaging and roughness measurement was performed by the Atomic Force microscope (AFM) Dimensional 0.0 AFM,.Veeco.Instruments, Edina, M ). Imaging was performed in the tapping mode with silicon probe (Nanosensors™ PPP-NCH, non-contact/tapping mode type probe, Nanosensors, Switzerland). The cantilever had nominal thickness of 4 μπι, nominal mean width of 30 μπι, a nominal spring constant of 42 N/m, nominal resonant frequency of 330 Hz, nominal tip radius of <10 nm and nominal cantilever length of 125 μπι. The air-dried membrane of 10 μπι x 10 μπι area was scanned at scan rate of 1 Hz, tip velocity of 10 μηι/sec, sample/lines 128 and lines 64. The best images were captured by adjusting the amplitude set point, contrast, data scale, and resolution to highest value 512 samples/lines and 512 lines. The captured images were processed by nanoscope ® III, version 6.13 Rl software. The roughness measurement was performed over the entire image. The following
measurements were done to estimate surface topology of the membranes.
(i) Rq: The Root Mean Square (RMS) Roughness is the standard deviation of the Z values within a given area.
N /
RSM = i=l
N
Zave is the average of Z value within the given area, Zi is the current Z value and N is the number of points within a given area.
(ii) Ra: The mean Roughness (Ra) represents the arithmetic average of the deviations from the centre plan.
Figure imgf000012_0001
Zcp is the Z value of the center plan and Zi is the current Z value and N is the number of points within a given area.
(iii) Rmax: The difference in height between the highest and lowest points on the surface relative to the mean plane. (iv) Surface area (SA): This value is the sum of the area of all triangles formed by three adjacent data points.
(v) Surface Area Difference (SAD): The percentage increase of the three dimensional surface over the two dimensional surface area.
Mechanical strength determination
The mechanical testing of membranes was done using the Instron (UK) make 3345 Single Column Testing Systems. The specimen of size 8 cm x 2 cm was used for tensile strength measurement. The thickness of membrane was measured prior to analysis. The appropriate load cell of 100 N or 1000 N capacity was used for the measurement, depending upon the membrane strength. The test method was established using particular load cell at 10 mm/min stretching rate. The area of 2 cm x 2 cm at both ends of the membrane was used to hold the specimen in the Instron machine. The measurement was done using established test method. The data acquisition/processing was done by using the Bluehill® 2 is a fully-integrated modular software package. The plot of tensile stress (MPa) vs. tensile strain (%) was obtained to compare mechanical strengths of membranes.
Filtration testing
NF membranes of this invention and commercial membranes were cut into circular pieces (0.065 m in diameter) with a membrane cutter. They were fixed in the cross flow cell. The exposed area of the membranes was 0.001963 m . The schematic filtration testing set-up is shown in the Fig. 2.
Permeate controlling valve (V3) was kept fully open and Trans Membrane Pressure (TMP- the driving force for filtration) was varied by adjusting the feed controlling valve (V i) and retentate controlling valve (V2). TMP was calculated by using the following equation.
TMP = [(Pi-P2)/2]-P3 The aqueous solution of MgS04.7H20 (2000 ppm) at ambient condition (25 °C) - was used as feed solution. Before starting the filtration experiment, membrane was compacted at TMP pressure of 70 psi using the 2000 ppm MgS04 feed solution. Each filtration experiment was run for 1 hr at various TMP pressures (70, 100, 130, 160 and 190 psi). Conductivity and temperature of feed before starting the filtration experiment and after the filtration run was measured by the digital conductivity meter (Thermo Scientific Orion 3-star Plus). Conductivity and temperature of permeate was also measured. Volume of retentate (1) and permeate collected (1) was measured. Permeate flux at various TMP was calculated using the values of permeate collected (1), exposed area of membrane (0.001963 m2) and time of filtration (1 hr). Rejection of MgS04 (R %) was calculated using the following expression.
R (%) = [l- (Cp/Cf)]*100 Where, Cp is the conductivity of permeate (μ8/αη) and Cf is the conductivity of feed (μ8/α ) after the filtration.
Results
The capillary flow porosimetry results and pure water flux are summarized in the Table 3 below. The hot pressing treatment to nanofibrous structural layer narrowed the pore size distribution of membrane.
Table 3. Capillary Flow Porosimetry results of nanofibrous structural layer
Measurement Nanofibrous structural Nanofibrous
layer before hot structural layer
pressing treatment after hot pressing
treatment
Mean flow pore 1.7587 μπι 0.7998 μπι
diameter
Standard 1.3861 0.346
deviation of
average pore
diameter
Figure imgf000015_0001
The membranes prepared without hot pressing could not withstand the impact of high pressure, particularly when the TMP was more than 130 psi. See Fig. 3 A and Fig. 4A. MgS04 rejection was dropped from 88.78 % to 53.60 % and permeate flux was increased from 218.2 to 451.7 1/m h, when the TMP was increased from 130 psi to 160 psi. See Fig. 4A. However, the nanofibrous structural layer (of 1.759 μπι mean flow pore diameter) was capable of supporting the polyamide thin film, as shown from the high values of MgS04 rejection. See Fig. 4A. When the thickness of thin film
polyamide layer became smaller than the diameter of fibers of supporting film, the polyamide film showed imprints of the nanofiber morphology.
By contrast, the membrane prepared with hot pressing had improved pressure tolerance and mechanical and structural integrity. See Figs. 3B and 4B. It was robust to withstand the higher pressure more than the 130 psi. See Fig. 4B. The membrane prepared with hot pressing had higher flux than commercial membranes (i.e. 10-12 % higher than the best commercial membrane).
Fig. 5 shows tensile stress vs. tensile strain relationships of various commercial membranes and the membrane of this invention. Nanofibrous layer is too weak to impart suitable mechanical strength to the polyamide NF membrane (black curve). However, when it is supported by commercially available engineered nonwoven backing material, it imparts adequate mechanical strength to polyamide NF membrane (violet curve). The membrane of this invention has comparable mechanical property with commercially available NF membranes. OTHER EMBODIMENTS
All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.
From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.

Claims

WHAT IS CLAIMED IS:
1. A nanofiltration membrane comprising:
(1) a supporting fabric layer, which has a thickness of 100-200 μm;
(2) an engineered nanofibrous structural layer, which has a thickness of 30- 75 μm and contains pores of the pore size of 0.4-1.5 μπι; and
(3) a polyamide thin film, which has a thickness of 100-300 ran;
wherein the engineered nanofibrous layer is deposited on the top of the supporting fabric layer, and the polyamide thin film is deposited on the top of the engineered nanofibrous structural layer.
2. The nanoflitration membrane of claim 1 , wherein the supporting layer has a thickness of 100-150 μηι and the engineered nanofibrous structural layer has a thickness of 30-60 μπι and contains pores of the size of 0.7-1 μπι.
3. The nanofiltration membrane of claim 1 , wherein the engineered nanofibrous structural layer has a thickness of 50-75 μπι and contains pores of the size of 0.7-1 μηι.
4. The nanofiltration membrane of claim 1, wherein the supporting fabric layer has a thickness of 100-150 μπι and an air permeability of 0.0015-0.01 1 cubic foot per minute.
5. The nanofiltration membrane of claim 1 , wherein the polyamide thin film has a thickness of 100-200 nm and contains pores of the size of 0.001-0.01 μπι.
6. The nanofiltration membrane of claim 1 , wherein the supporting fabric layer is made of polyester or polypropylene.
7. The nanofiltration membrane of claim 1, wherein the engineered nanofibrous structural layer is made of polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides, polyetheretherketone, or a mixture thereof.
8. The nanofilration membrane of claim 1 , wherein the polyamide thin film is made by interfacial polymerization between diamine and acid chloride.
9. The nanofiltration membrane of claim 8, wherein the diamine is aromatic diamine or cyclic aliphatic diamine.
10. The nanofiltration membrane of claim 9, wherein the diamine is ortho, meta or para-phenylenediamine, piperazine, or bipiperidine and the acid chloride is trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride.
1 1. The nanofiltration membrane of claim 1 , wherein the supporting fabric layer has a thickness of 100-150 μπι and has an air flow permeability of 0.0015-0.01 1 cubic foot per minute, the engineered nanofibrous structural layer has a thickness of 50-75 μπι and contains pores of the size of 0.7-1 μπι, and the polyamide thin film has a thickness of 100-200 nm and contains pores of the size of 0.001-0.01 μιη.
12. The nanofiltration membrane of claim 1 1 , wherein the supporting fabric layer has a thickness of about 100 μπι, the engineered nanofibrous structural layer has a thickness of about 50 μπι, and the polyamide thin film has a thickness of 100 nm.
13. The nanofiltration membrane of claim 12, wherein the supporting fabric layer is made of polyester or polypropylene, and the engineered nanofibrous structural layer is made of polyacrylonitrile, polysulfone, polyethersulfone, Nylon 4,6, Nylon 6, Nylon 6,6, Nylon 12, polyamides, polyetheretherketone, or a mixture thereof.
14. The nanofilration membrane of claim 13, wherein the polyamide thin film is made¾y interfacial polymerization between diamine and acid chloride.
15. The nanofiltration membrane of claim 14, wherein the diamine is aromatic diamine or cyclic aliphatic diamine.
16. The nanofiltration membrane of claim 15, wherein the diamine is ortho, meta or para-phenylenediamine, piperazine, or bipiperidine and the acid chloride is trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride.
17. A process of preparing a nanofiltration membrane, comprising
applying nanofibers onto a supporting fabric layer to form a nanofibrous structural layer;
pressing the nanofibrous structural layer and the supporting fabric layer at a predetermined temperature and under a pre-determined pressure; and
coating the pressed nanofibrous structural layer with a polyamide thin layer via interfacial polymerization between diamine and acid chloride;
wherein the supporting fabric layer has a thickness of 100-200 μπι, the engineered nanofibrous structural layer has a thickness of 30-60 μπι and contains pores of the size of 0.4-1.5 μηι, and the polyamide thin film has a thickness of 100-300 nm.
18. The process of claim 17, wherein the applying step is performed by electrostatic spinning; melt electrospinning; modified melt spinning; electroblowing, and splitting of jet of melt/solution.
19. The process of claim 18, wherein the pressing step is performed at a temperature in a range between 10°C below and 10°C above the glass transition temperature of the nanofibers of the nanofibrous structural layer and under a pressure of 0.55-1.1 1 lb/in.
20. The process of claim 19, wherein the diamine is aromatic diamine or cyclic aliphatic diamine.
21. The process of claim 20, wherein the diamine is ortho, meta or para- phenylenediamine, piperazine, or bipiperidine and the acid chloride is trimesoyl chloride, isophthaloyl chloride, or terephthaloyl chloride.
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