Water wettable filtration membranes and their manufacture

Abstract

Filtration membranes prepared by adhering a poloxamer to a preformed sheet of microporous polyolefin are described. In an embodiment, the poloxamer is that supplied under the trade name PLURONIC™ P-123 and the sheet is microporous poly(ethylene). The membranes provide the advantage of being tolerant to the cleaning agents used in clean-in-place protocols.

Description

Filtration membranes are used in a range of industrial processes, including food processing, to recover or remove water from a feed stream. In one application the objective may be to separate the water from contaminating particulates. In another application the objective may be to concentrate high value solutes . In either application efficiency is increased by contacting the feed stream with a large surface area of the filter membrane. To this end the filtration membrane will often be assembled into a spiral wound filter element, which is then installed in the industrial plant. Such spiral wound membrane assemblies - or "filter elements" - are supplied by manufacturers such as Synder Filtration (Vacaville, California, USA). Further efficiencies are realised if cleaning can be performed in place without the need for removal and reinstallation of the filter element. Cleanin-place protocols use chemically aggressive solutions such as acid, alkali and hypochlorite. Alternatively, the feed streams to which the membrane is exposed may be chemically aggressive and durability under these conditions reduces the frequency with which the filter element needs to be replaced. Microporous sheets of polyolefin, such as poly (ethylene) are available commercially from suppliers such as Celgard (Charlotte, North Carolina, USA) and Targray (Kirkland, Quebec, Canada). One impediment to the use of these substrates as filtration membranes in the applications alluded to above is there inherent hydrophobicity. Where the objective is to provide a semipermeable membrane for use in concentrating high value solutes the required rejection properties may also be lacking. It has now been determined that the grafting of a microporous sheet of poly(ethylene) with the poloxamer supplied under the trade name PLURONIC-P123 provides a filtration membrane that is readily wetted with water and provides high flux rates at relatively low pressures (5 bar). The filtration membranes so produced have also been demonstrated to have the desired durability when exposed to chemically aggressive liquids. The retention of these desirable properties - attributable to the graft - is enhanced by the inclusion of a crosslinking agent in the working solution used in the method of preparation. Without wishing to be bound by theory low molecular weight crosslinking agents are favoured so as not to disrupt the favourable rejection properties also demonstrated for the membranes. The method of preparing the filtration membrane is readily adaptable to a continuous production process. In accordance with the methods described, working solutions of the following composition are used to impregnate the microporous substrate before it is irradiated with ultraviolet light at a wavelength in the range 250 nm to 360 nm, wavelengths at or toward the lower end of this range (250 nm) being preferred. Working solution: 3 to 5 % (w / v) poloxamer 0.5 to 1 % (w / v) photoinitiator 0 to 0.5 % (w / v) crosslinking agent 30 to 50 % (v / v) in alcohol or acetone in water The preferred poloxamer for use in the working solution is that supplied under the trade name PLURONIC P-123. The preferred photoinitiator for use in the working solution is benzophenone. The preferred crosslinking agent for use in the working solution is divinylbenzene. EXAMPLE A Preparation of filtration membrane (laboratory method) A volume of 5 mL of a solution in water of 10% (w / v) triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) was mixed with an equal volume of deionised water. A quantity of 0.1 g of the photoinitiator benzophenone (diphenylmethanone; Ph2O) was dissolved in a separate volume of 5 mL of ethanol before being added to the diluted solution of the triblock copolymer. The working solution was stored in the dark until use. Samples (13.5 x 18.5 cm) were cut from a sheet of microporous poly (ethylene) (TARGRAY™ wet process polyethylene separators, item no. SW320H (Targray, Kirkland QC, Canada)) and each sample coated with a volume of 5 mL of the working solution. The coated samples were then irradiated with ultraviolet (UV) light in the range 250 to 360 nm for a period of time of two minutes before rinsing with water and air-drying on top of a warm oven. The four replicate samples prepared according to this method were designated 040918Wiv, 040918Wv, 040918Wvi and 151018Wi. A small piece of the sample designated 040918Wiv was excised from the edge of the sample and submitted to scanning electronic microscopy (SEM). Each of the samples was readily wetted with water, being observed to become uniformly translucent when contacted with this solvent. Durability, flux and protein rejection A filtration membrane assembly (Sterlitech) as illustrated in Figure 1 was used to determine flux (LMH) for each of the samples designated 040918Wiv, 040918Wv and 040918Wvi. Samples were individually mounted in the filtration membrane assembly and the flux determined at 0 and 5 bar. The time to collect a predetermined volume of permeate at the specified pressure and temperature was recorded and the flux (J) was calculated according to the following equation: _ V txA where V was the volume of permeate (L), t was the time (h) for the collection of V and A was area of the sample (m2) exposed to the feed stream (water or skim milk). The results are summarised in Table 1. Sample Temperature 0 bar / 5 bar Flux 0 bar 5 bar 040918Wiv 9 / 9 10 367 040918WV 15 / 11 33 476 040918Wvi 10 / 9 32 428 Table 1. Fluxes (LMH) determined at 0 and 5 bar with water as the feed stream at the temperatures (°C) specified. To assess durability fluxes were also determined after repeated clean-in-place (CIP) protocols. The CIP protocol was based on that employed in a commercial processing operation for reverse osmosis (RO) membranes (Anon (2014)) and is summarised in Table 2. Step Feed stream Time (min) Temperature (°C) 1 water 5 Ambient 2 water 5 35 3 alkali 5 35 4 water 5 35 5 acid 10 35 6 water 5 Ambient 7 hypochlorite 5 35 8 water 5 Ambient Table 2. Clean-in-place (CIP) protocol adapted from Anon (2014). The 'alkali' was 2% (w / v) sodium hydroxide (NaOH). The 'acid' was 1.9% (w / v) nitric acid (H2NO3) and 0.6 (w / v) phosphoric acid (H3PO4) . For each sample a number of CIP protocols were repeated alternating with the use of water or skim milk as the feed stream. The fluxes and percentage protein rejection (with skim milk as the feed stream) determined for the samples designated 040918Wv and 040918Wvi are provided in Table 3. Total protein concentrations in permeate were calculated on the basis of HPLC analysis with UV absorbance monitoring. Sample Feed stream CIP protocols Temperature (°C) 0 bar / 5 bar Flux (LMH) Protein rejection (%) 0 bar 5 bar 040918Wiv water 0 9 / 9 10 367 040918WV water 0 15 / 11 33 476 040918WV water 1 10 / 10 6 139 040918WV water 2 - / 12 - 195 040918WV water 3 - / 11 - 171 040918WV water 6 - / 10 - 476 040918WV water 10 11 / 10 46 4 94 040918WV milk 3 - / 11 - 21 040918WV water 6 - / 10 - 476 040918WV water 10 11 / 10 46 4 94 040918WV milk 10 11 / 12 4 21 99.4 040918Wvi water 0 10 / 9 32 428 040918Wvi water 1 19 / 10 43 642 040918Wvi water 2 13 / 12 40 714 040918Wvi water 3 10 / 10 38 644 040918Wvi milk 3 9 / 11 3 24 040918Wvi water 4 - / 9 - 56(dry) 040918Wvi milk 4 - / 10 - 9 99.71 040918Wvi water 5 10 / 9 38 234 040918Wvi water 7 9 / 9 60 803 040918Wvi water 10 11 / 10 64 188 040918Wvi milk 10 - / 12 - 9 99.76 Table 3. Fluxes (LMH) and protein rejection determined at 0 and 5 bar with water or skim milk as the feed stream at the temperatures (°C) specified. Determinations were made for each of the samples following repeated clean-in-place (CIP) protocols. 10 The durability of the filtration membranes was further evaluated by contacting the sample designated 151018Wi with 2% (w / v) sodium hydroxide (NaOH) for 7 days. The fluxes and percentage protein rejection (with skim milk as the feed stream) determined for these samples are provided in Table 4 . Sample Feed stream Temperature (°C) 0 bar / 5 bar Flux (LMH) Protein rejection (%) 0 bar 5 bar 151018Wi water 9 / 9 54 257 151018Wi milk 10 / 10 - 8 99.65 Table 4. Fluxes (LMH) and protein rejection determined at 0 and 5 bar with water or skim milk as the feed stream at the temperatures (°C) specified. Determinations were made for the samples following exposure to 2% (w / v) sodium hydroxide (NaOH) for 7 days . Fourier transform infrared (FTIR) spectroscopy Spectra were recorded for each of the samples designated 040918Wiv, 040918Wv and 040918Wvi using a Thermo Electron Nicolet 8700 FTIR spectrometer equipped with a single bounce ATR and diamond crystal. Thirty-two scans at a resolution of 4 cm were averaged for each sample. A comparison of the spectra (3800 cm ' to 525 cm) recorded for: (i) the untreated microporous poly(ethylene) (TARGRAY™ wet process polyethylene separators, item no. SW320H (Targray, Kirkland QC, Canada))('PE virgin'); (ii) the triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) used in the preparation of the samples ('P123'); and (iii) the top (Etop) and back (Eback) sides of each of the samples designated 040918Wiv, 040918Wv and 040918Wvi is provided in Figure 2. Signals corresponding to the symmetrical stretch mode of C-O-C fragments (1108 cm ' ) and the C-H stretch mode of CH3 (2970 cm ' ) present in the spectrum of the triblock copolymer (PLURONIC™ P-123) were also present in the spectra recorded for each of the samples. Many signals characteristic of the triblock copolymer (PLURONIC™ P-123) were also observed at low intensity in the 'fingerprint' region of the spectra provided in Figure 3. Signals characteristic of the triblock copolymer (PLURONIC™ P-123) were retained in spectra recorded for regions of the sample designated 040918Wiv following exposure to the feed stream (water) as shown in Figure 4. • SEM Scanning electron micrographs of the small piece excised from the edge of the sample designated 040918Wiv are provided in Figure 5 and Figure 6. The fibres of poly(ethylene) of the microporous sheet appear to be coated. The observations from FTIR spectroscopy and SEM appeared to demonstrate the grafting of the poloxamer to the polyolefin matrix of the microporous sheet. The conversion of the inherently hydrophobic microporous sheet of polyolefin to a water-wettable permeable membrane is attributed to this grafting. EXAMPLE B Preparation of filtration membrane (laboratory method) A volume of 10 mL of a solution in water of 10% (w / v) triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) was mixed with an equal volume of deionised water. Quantities of 0.2 g of the photoinitiator benzophenone (diphenylmethanone; PhjO) and 0 or 0.1 g of the crosslinking agent divinylbenzene (DVB) were dissolved in separate volumes of 10 mL of ethanol (methylated spirits) before being added to a volume of 10 mL of the diluted solution of the triblock copolymer. These working solutions -excluding or including the crosslinking agent DVB - were stored in the dark until use. Samples (13.5 x 18.5 cm) were cut from a sheet of microporous poly (ethylene) (TARGRAY™ wet process polyethylene separators, item no. SW320H (Targray, Kirkland QC, Canada)) and each sample coated with a volume of one of the working solutions. The coated samples were then irradiated with ultraviolet (UV) light in the range 250 to 360 nm for a period of time of two minutes before rinsing with water and air-drying in open air. The three replicate samples prepared according to this method using the working solution excluding DVB were designated 110419Wi, 180419Wi and 180419Wii. The three replicate samples prepared according to this method with the working solution including DVB were designated 230419Wi, 230419Wii and 230419Wiii. Each of the samples was readily wetted with water, being observed to become uniformly translucent when contacted with this solvent. The water flux was determined for each of the samples with deionised water as the feed stream (Dll). The samples were then completely dried before again determining the water flux with deionised water as the feed stream (DI2). Each of the samples were then subjected to a clean-in-place (CIP) protocol before twice more determining the water flux with deionised water as the feed stream (DI3 and DI4) and an intervening drying of the samples. Each of the samples remained readily wettable with water. The results are summarised in Table 5 and Table 6 and compared in Figure 7. Sample DI# Flux 0 bar 5 bar 110419Wi 1 61 964 2 32 771 3 38 964 4 95 1446* 180419Wi 1 70 890 2 35 723 3 48 964 4 - 76 180419Wii 1 62 964 2 34 643 3 86 890 4 - 26 Table 5. Average fluxes (LMH) determined at room temperature (22 to 24 °C) at 0 and 5 bar with water as the feed stream (‘membrane failure). Sample DI# Flux 0 bar 5 bar 230419Wi 1 43 680 2 15 321 3 24 609 4 15 399 230419Wii 1 345 826 2 28 642 3 34 826 4 17 1285 230419Wiii 1 26 723 2 33 642 3 56 723 4 26 964 Table 6. Average fluxes (LMH) determined at room temperature (22 to 24 °C) at 0 and 5 bar with water as the feed stream. 5 EXAMPLE C Preparation of filtration membrane (prototype method) A volume of 300 mL of a solution of 10% (w / v) triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) in distilled water was dispensed into a reservoir protected from exposure to light. A further volume of 300 mL of 10 distilled water was then added to provide an initial solution of 5% (w / v) triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) in the reservoir. A solution of 1.5% (w / v) benzophenone in ethanol (methylated spirits) was prepared separately and a volume of the crosslinking agent divinylbenzene (DVB) added to provide a final concentration of 0.75% (v / v) 15 DVB. A volume of 400 mL of this separately prepared solution was then mixed with the solution of triblock copolymer (PLURONIC™ P-123; lot #MKCC2305, Sigma-Aldrich) in a reservoir to provide the working solution. Referring to Figure 7 of the accompanying drawings pages, peristaltic pumps (1,2) were used to deliver the working solution from the reservoirs (3,4) to the two hemicylindrical troughs (5,6) of a prototype production line. The reservoirs were periodically replenished with the working solution during the operation of the prototype production line. The width of a continuous microporous sheet (7) of microporous poly(ethylene) was fed from a dispensing roll of stock into a first impregnation station comprising an idler roller (8) co-axially mounted in the first of the two hemicylindrical troughs (5). The difference between the radii of the roller (8) and trough (5) was sufficient to permit free passage of the sheet (7) around the roller and through the trough, but not so great as to promote evaporation of the working solution in the trough. The surface of the roller (8) over which the sheet (7) passes may be spiral engraved to promote passage of the working solution across the length of the surface. The sheet (7) exiting the first impregnation station was then fed vertically into a first irradiating station comprising a slotted chamber (9) containing two opposed arrays (10,11) of ultraviolet light sources. The sheet (7) passed between the opposed arrays (10,11) so that both sides were irradiated. The rate at which the sheet (7) was fed was regulated to provide the required residence time within the slotted chamber (9). The irradiated sheet (7) was then passed through a second impregnation station (12) and second irradiating station (13) of the same configuration as the first impregnation station and first irradiating station. Following these repeated steps, the irradiated sheet (7) was fed around a plurality of idler rollers (14,15,16) immersed in water in a washing station (17). The water in the washing station (17) was circulated by an external pump (18) and the depth of the water controlled by a combination of level transmitter and solenoid valve (19). The combination of a plurality of idler rollers (14,15,16) and depth of water ensured sufficient residence time before the water washed sheet (7) was fed into the drying station. The drying station was a forced air dryer comprising two plenum chambers (20,21) having opposed perforated face plates between which the sheet of substrate passed. Hot air blowers (22,23) mounted in the wall of each chamber forced air through the perforated face plates. The dried sheet (7) of substrate was then rewound onto a receiving roll (not shown). Filter elements The membrane may be used in the manufacture of assemblies of various configurations. In one embodiment the membrane is used in the manufacture of a spiral wound filter element. The manufacture of such filter elements is well known in the art. Although the invention has been described with reference to embodiments or examples it should be appreciated that variations and modifications may be made to these embodiments or examples without departing from the scope of the invention. Where known equivalents exist to specific elements, features or integers, such equivalents are incorporated as if specifically referred to in this specification. Variations and modifications to the embodiments or examples that include elements, features or integers disclosed in and selected from the referenced publications are within the scope of the invention unless specifically disclaimed. The advantages provided by the invention and discussed in the description may be provided in the alternative or in combination in these different embodiments of the invention. INDUSTRIAL APPLICABILITY Methods of preparing filtration membranes and their use in recovering or removing water from feed streams are provided. The filtration membranes are advantageously used in recovering or removing water from feed streams where cleaning of the membranes in situ is desirable to increase efficiency of plant operations. INCORPORATION BY REFERENCE Where the claims, description or drawings of this specification are missing in their entirety or part, the corresponding portion of the specification accompanying the most recently filed application from which priority is claimed is to be incorporated by reference so as to complete this specification in accordance with Rules 4.18, 20.5 and 20.6 of the PCT Regulations (as in force from 1 July 2015 or subsequently amended). For the purposes of 37 C.F.R. 1.57 of the United States Code of Federal Regulations the disclosures of the following publications (as more specifically identified under the heading 'Referenced Publications') are incorporated by reference: Jones et al (2008) and Schmolka (1973). REFERENCED PUBLICATIONS Anon (2014) DOW FTLMTEC™ Membranes - Cleaning procedures for DOW FTLMTEC FT30 elements Tech Fact (Form No. 6 0 9-23 010-0211) . Carter et al (2018) Controlling external versus internal pore modification of ultrafiltration membranes using surface-initiated AGET-ATRP Journal of Membrane Science, 554, 109-116. Cheng et al (2017) Method for preparing mesoporous composite film Chinese patent application no. 201611226194 [Publ. no. CN 106731886 A]. Guo et al (2015) Coated microporous materials having filtration and adsorption properties and their use in fluid purification processes International application no. PCT / US2014 / 061326 [Publ. no. WO 2015 / 073161 Al] . 5 Jones et al (2008) Compendium of polymer terminology and nomenclature IUPAC Recommendations, RSC Publishing. Liu et al (2014) With multi-scale gradient microstructure surface preparation method of a microporous membrane Chinese patent application no. 201310479920 [Publ. no. CN 103611437 A]. 10 Schmolka (1973) Polyoxyethylene-polyoxypropylene aqueous gels United States patent no. 3,740,421. Wang et al (2006) Pluronic polymers and polyethersulfone blend membranes with improved fouling-resistant ability and ultrafiltration performance Journal of Membrane Science, 283, 440-447. 15 Yang et al (2014) Preparation and application of PVDF-HFP composite polymer electrolytes in LlNi0 5Co0 2Mn0,3O2 lithium-polymer batteries Electrochimica Acta 134, 258-265.

Claims

2020330484   10 Jun 20261) A water-wettable filtration membrane consisting of a microporous sheet of grafted polyolefin where the graft is a photoinitiated graft andcomprises a poloxamer of the structureHO(ethylene oxide)m-(propylene oxide)n-(ethylene oxide)mHwhere m is in the range 15 to 25 and n is in the range 50 to 90.2)   The membrane of claim 1 where the polyolefin is poly(ethylene).3)   The membrane of claim 1 or 2 where m is 20 and n is 70.4)   The membrane of any one of claims 1 to 3 where the graft comprises acrosslinking agent.5)   The membrane of claim 4 where the crosslinking agent has a molecularweight below 150 g mol-1.6)   The membrane of claim 5 where the crosslinking agent is divinylbenzene.7) A filter assembly comprising the membrane of any one of claims 1 to 7.8)   The filter element of claim 7 where the membrane is spiral wound.9)   A method of preparing a water-wettable filtration membrane comprising:(a) Contacting a microporous sheet of polyolefin with a solution of a poloxamer in a solvent to provide a contacted sheet;(b)  Irradiating the contacted sheet with ultraviolet light in thepresence of a photoinitiator to provide an irradiated sheet; and(c)  Drying and washing the irradiated sheet to provide the membrane,where the poloxamer is a polymer of the structureHO(ethylene oxide)m-(propylene oxide)n-(ethylene oxide)mHwhere m is in the range 15 to 25 and n is in the range 50 to 90.10) The method of claim 9 where the polyolefin is poly(ethylene).11)  The method of claim 9 or 10 where m is 20 and n is 70.12)  The method of any one of claims 9 to 11 where the solution comprises thephotoinitiator.13)  The method of any one of claims 9 to 12 where the photoinitiator is aType II photoinitiator.14) The method of any one of claims 9 to 13 where the photoinitiator is2020330484   10 Jun 2026benzophenone.15)  The method of any one of claims 9 to 14 where the solution comprises alow molecular weight crosslinking agent.16)  The method of claim 15 where the crosslinking agent is divinylbenzene.17)  The method of any one of claims 9 to 16 where the solvent is 30 to 50 %(v / v) alcohol or acetone in water.18)  The method of any one of claims 7 to 14 where the solvent is 30 to 50 %(v / v) ethanol in water.19)  A method of recovering or removing water from a feed stream comprisingthe step of contacting a first side of a membrane of any one of claims 1or 8 with the feed stream at a pressure sufficient to provide a permeate.20)  The method of claim 19 where the pressure is less than 10 bar and theflux is greater than 500 LMH.

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