Patent Application
20220193620
The invention relates to modifying the chemical and physical properties of microporous polyolefin substrates and the use of these modified substrates in the preparation of durable asymmetric composite membranes. In particular, although not exclusively, the invention relates to asymmetric composite membranes for use in the recovery or removal of water from food processing feed streams, such as dairy.
Osmosis is generally seen as the movement of water from a solution of higher water chemical potential to one of lower water chemical potential. This movement, or flux, is moderated by a semi-permeable membrane, which allows the passage of water but not the passage of the species whose presence lowers the chemical potential of water in the receiving solution. This fundamental thermodynamic property of solutions is an essential component of many biological processes (McCutcheon and Wang (2012)).
The first viable semi-permeable membrane was made in the 1960s from cellulose acetate and used in reverse osmosis (Loeb (1981)). The further development of thin film composite membranes followed with the introduction of the concept of interfacial polymerisation (Mogan (1965)). In a thin film composite membrane, each individual layer can be optimised for its particular function. The thin “barrier layer” can be optimised for the desired combination of solvent flux and solute rejection, while the porous “support layer” can be optimised for maximum strength and compression resistance combined with minimum resistance to permeate flow. Numerous reviews concerning the preparation and properties of composite membranes developed for use in reverse osmosis, nano- and ultra-filtration are available (e.g., Petersen (1993)).
The desired properties of membranes used in water desalination or purification include high rejection of undesirable species, high filtration rate and good mechanical strength. Depending on the particular application in which the membrane is used other desired properties may also include resistance to fouling and chemical decomposition (McCutcheon and Wang (2013)). These latter properties are particularly desirable for membranes used in dairy processing operations where periodic cleaning and sterilisation is required.
Numerous attempts have been made to develop chlorine-tolerant, thin-film composite membranes. Thin-film composite membranes fabricated using sulfonated aromatic poly(ether sulfone) as the active layer have so far failed to satisfy flux and rejection performance requirements.
The publication of Colquhoun et al (2010) discloses a multilayer membrane with chlorine tolerance in reverse osmosis operation. The membrane consists of a film of high molecular weight poly(ether sulfone) ionomer deposited on a porous support membrane. A second layer of formaldehyde-cross-linked poly(vinyl alcohol) is deposited on the surface of the film to further increase salt rejection.
It is well-known to use photografting to modify the surface of films, sheets and molded objects formed from polyolefins. For example, the publication of Tazuke and Kimura (1978) discloses photografting onto poly(propylene), poly(ethylene) and several other polymer films using benzophenone as a sensitizer. In this publication the choice of solvent and sensitizer was noted to be very important. The publication of Ang et al (1980) discloses an irradiation procedure where the sensitizer is dissolved in the hydrophilicitizing agent solution and can be used for the photosensitized copolymerization in high yields of styrene, 4-vinyl pyridine and methyl methacrylate to poly(propylene). Again, this publication notes that the reaction was found to be very specific to certain types of sensitizers.
The publication of Ogiwara et al (1981) discloses the photografting on poly(propylene) and low-density poly(ethylene) (LDPE) films on which sensitizers were coated beforehand. The sensitizers coated on films enabled vinyl hydrophilicitizing agents, such as methyl methacrylate, acrylic acid and methacrylic acid to graft easily with high yields. The hydrophilic hydrophilicitizing agents acrylic acid and methacrylic acid were conveniently grafted using them in aqueous solution in a liquid phase system. The publication of Allmer et al (1988) discloses the modification of surfaces of LDPE, high-density poly(ethylene) (HDPE) and polystyrene by grafting with acrylic acid. The grafting is performed in the vapor-phase and increased the wettability of the polymer. It was observed that acetone was able to initiate grafting and was found to promote and direct grafting to the surface. The publication of Allmer et al (1989) discloses the grafting of the surface of LDPE with glycidyl acrylate and glycidyl methacrylate by photoinitiation. Acetone and ethanol were used as solvents, with acetone yielding slightly more grafting at the surface.
The publications of Yao and Ranby (1990a, 1990b and 1990c) disclose inter alia a process for the continuous photoinitiated graft copolymerization of acrylamide and acrylic acid onto the surface of HDPE tape film. The process is performed under a nitrogen atmosphere using benzophenone as the photoinitiator. It was noted that pre-soaking was very important for efficient photographing within short irradiation times. The application of this pre-soaking photografting method to poly(ethylene terephthalate) (PET) was also disclosed. In this context acetone was found to be a somewhat better solvent than methylethyl ketone and methylpropyl ketone. When applied to a continuous process for the photochemically induced graft polymerization of acrylamide and acrylic acid of poly(propylene) (PP) fiber surface under a nitrogen atmosphere, optimal concentrations of hydrophilicitizing agent and initiator in the pre-soaking solution were determined.
The publications of Kubota and Hata (1990a and 1990b) disclose an investigation of the location of methacrylic acid chains introduced into poly(ethylene) film by liquid and vapor-phase photograftings and a comparative examination of the photografting behaviours of benzil, benzophenone and benzoin ethyl ether as sensitizers. In these latter studies poly(methacrylic acid) was grafted onto initiator-coated LDPE film.
The publication of Edge et al (1993) discloses the photochemical grafting of 2-hydroxyethyl methacrylate (HEMA) onto LDPE film. A solution phase method is used to produce a material with increased wettability. The publication of Singleton et al (1993) discloses a method of making a polymeric sheet wettable by aqueous solvents and useful as an electrode separator in an electrochemical device. The polymeric sheet is formed from fibers which comprise poly(propylene) alone and is distinguished from a membrane formed from a microporous polymer sheet. The publication of Zhang and Ranby (1991) discloses the photochemically induced graft copolymerisation of acrylamide onto the surface of PP film. Acetone was shown to be the best solvent among the three aliphatic ketones tested.
The publications of Yang and Ranby (1996) disclose factors affecting the photografting process, including the role of far UV radiation (200 to 300 nm). In these studies benzophenone was used as the photoinitiator and LDPE film as the substrate. Added water was shown to favour the photografting polymerisation of acrylic acid on the surface of polyolefins, but acetone was shown to have a negative effect due to the different solvation of poly(acrylic acid) (PAA).
The publication of Hirooka and Kawazu (1997) discloses alkaline separators prepared from unsaturated carboxylic acid grafted poly(ethylene)-poly(propylene) fiber sheets. Again, the sheets used as a substrate in these studies are distinguished from a membrane formed from a microporous polymer sheet.
The publication of Xu and Yang (2000) discloses a study on the mechanism of vapor-phase photografting of acrylic acid in LDPE. The publication of Shentu et al (2002) discloses a study of the factors, including the concentration of hydrophilicitizing agent, affecting photo-grafting on low-density LDPE. The publication of El Kholdi et al (2004) discloses a continuous process for the graft polymerisation of acrylic acid from hydrophilicitizing agent solutions in water onto LDPE. The publication of Bai et al (2011) discloses the preparation of a hot melt adhesive of grafted low-density poly(ethylene) (LDPE). The adhesive is prepared by surface UV photografting of acrylic acid onto the LDPE with benzophenone as the photoinitiator.
The publication of Choi et al (2001) states that graft polymerisation is considered as a general method for modifying the chemical and physical properties of polymer materials.
The publication of Choi (2002) discloses a method for producing an acrylic graft polymer on the surface of a polyolefin article comprising the steps of immersing the article in a solution of an initiator in a volatile solvent, allowing the solvent to evaporate, and then immersing the article in a solution of an acrylic hydrophilicitizing agent before subjecting the article to ultraviolet irradiation in air or an inert atmosphere. Acrylic acid is used as the acrylic hydrophilicitizing agent in each one of the Examples disclosed in the publication, although the use of equivalent amounts of methacrylic acid, acrylamide and other acrylic hydrophilicitizing agents is anticipated.
The publication of Choi (2004) discloses the use of “ethylenically unsaturated hydrophilicitizing agents” in graft polymerisation. These other hydrophilicitizing agents are disclosed as hydrophilicitizing agents that are polymerisable by addition polymerisation to a thermoplastic polymer and are hydrophilic as a consequence of containing carboxyl (—COOH), hydroxyl (—OH), sulfonyl (SO3), sulfonic acid (—SO3H) or carbonyl (—CO) groups. No experimental results concerning the chemical and physical properties of graft polymers prepared by a method using these other hydrophilicitizing agents is disclosed.
The publication of Choi (2005) discloses a non-woven sheet of polyolefin fibres where opposed surfaces of the sheet are hydrophilic as a consequence of an acrylic graft polymerisation. The properties of the sheet are asymmetric, the ion exchange coefficient of the two surfaces being different. The method used to prepare these asymmetric acrylic graft polymerised non-woven polyolefin sheets comprises the steps of immersing the substrates in a solution of benzophenone (a photoinitiator), drying and then immersing the substrate in a solution of acrylic acid prior to subjecting to ultraviolet (UV) irradiation. The irradiation may be performed when the surfaces are in contact with either air or an inert atmosphere.
The publication of Gao et al (2013) discloses a method of preparing a radiation cross-linked lithium-ion battery separator. In an example, a porous polyethylene membrane is immersed in a solution of benzophenone and triallyl cyanurate in dichloromethane. The immersed membrane is dried at room temperature before being immersed in a water bath at 30° C. and irradiated on both sides using a high-pressure mercury lamp for three minutes.
The objective of the majority of these prior art methods is to improve the adhesion, biocompatibility, printability or wettability of the surface of a substrate. These improvements to surface characteristics are to be distinguished from the use of UV-initiated polymerisation to modify the permeability of preformed microporous polyolefin substrates, such as the substrates described in the publications of Fisher et al (1991) and Gillberg-LaForce (1994).
It is well-known to prepare thin film composite membranes to modify the permeability of a preformed microporous polyolefin substrate. For example, the publication of Jones (1990) discloses a composite permselective membrane comprising an ultrathin semipermeable layer comprising a polybenzimidazole polymer in occluding contact with at least one surface of a microporous polymer support layer. The membranes are asserted to provide better combinations of flux and rejection rates in reverse osmosis processes than do conventional semipermeable membranes of polybenzimidazole polymer alone.
The publication of Callahan and Johnson (1990) discloses a composite membrane having a microporous support which is coated with a UV curable polymer composition having a sufficiently high viscosity to prevent pore filling upon coating and curing.
The publication of Gillberg-LaForce and Gabriel (1991) discloses a pore modified microporous membrane which is made by a process of incorporating a polymerizable vinyl hydrophilicitizing agent within the pores of a microporous membrane followed by polymerization to secure the resulting polymer within the pores. The process is stated to be particularly suitable for modifying a hydrophobic microporous membrane with a hydrophilic polymer, as occurs for example when polyacrylic acid is secured into the pores of a polypropylene microporous membrane.
The publication of Callahan and Johnson (1992) discloses a composite membrane having a microporous support which is coated with a UV curable polymer composition having a sufficiently high viscosity to prevent pore filling upon coating and curing.
The publication of Cussler et al (1992) discloses a process for modifying the properties of a hydrophobic microporous membrane which includes the steps of treating a hydrophobic microporous membrane with a surfactant to render the membrane hydrophilic, permeating the membrane with a polyol, and crosslinking the polyol to yield a hydrophilic microporous membrane having pores filled with an aqueous gel. The modified membranes are asserted to be useful in carrying out chromatographic separations.
The publication of Donato and Phillips (1993) discloses a composite membrane having a microporous support which is coated with a polymer selected from the group consisting of polyethylene oxide, polyacrylic acid, poly(methyl methacrylate) and polyacrylamide wherein there is no pore filing of the microporous support. The publication of Donato (1994) discloses a composite membrane having a microporous support coated with an aqueous polyeurethane dispersion composition. The publication of Donato and Phillips (1994) discloses a composite membrane having a microporous support which is coated with a polymer composition and a contact adhesive layer applied to said polymer.
For the most part, the methods of preparing composite membranes disclosed in these publications use UV initiated polymerisation to form polymers in situ. Methods of adhering dissimilar preformed polymers to the surface of the microporous polyolefin substrates are less well known.
It is an object of the present invention to provide a method of decreasing the hydrophobicity of preformed microporous polyolefin sheets and thereby provide modified microporous polyethylene sheets suitable for use in the preparation of water permeable asymmetric composite membranes. It is an object of the present invention to provide an asymmetric composite membrane suitable for use in the recovery or removal of water from dairy and other feed streams. These objects are to be read in the alternative with the object at least to provide a useful choice.
In a first aspect a method of preparing an asymmetric composite membrane is provided, the method comprising:
In an embodiment of the first aspect, the method comprises:
where the wetted microporous sheet of polyolefin is wetted with a solution of a hydrophilicitizing agent in an aqueous solvent.
Preferably, the aqueous solvent is 40 to 60% (v/v) acetone in water.
Preferably, the hydrophilicitizing agent is 4-ethenyl-benzenesulfonic acid.
Preferably, the polyolefin of the hydrophilicitized microporous sheet is a graft polymer. More preferably, the side chains of the graft polymer are derived from one or more of 2-acrylamido-1-methyl-2-propanesulfonic acid, 2-propen-1-ol, 2-propenoic acid, 2-hydroxyethyl 2-methyl-2-propenoic acid ester and 4-ethenyl-benzenesulfonic acid. Yet more preferably, the side chains of the graft polymer are derived from either or both of 2-acrylamido-1-methyl-2-propanesulfonic acid and 4-ethenyl-benzenesulfonic acid. Most preferably, the side chains of the graft polymer are derived from 4-ethenyl-benzenesulfonic acid.
Preferably, the organic solvent is dimethylacetamide.
Preferably, the cross-linking agent is a di-, tri- or tetra-ethenyl compound with a molecular weight less than 260. More preferably, the cross-linking agent is a di- or tetra-ethenyl compound selected from the group consisting of: divinylbenzene, ethylene glycol dimethacrylate and glyoxal bis(diallyl acetal). Most preferably, the cross-linking agent is p-divinylbenzene.
Preferably, the ratio of cross-linking agent to sulfonated poly(ether ether ketone) is in the range 2:3 to 1:3. More preferably, the ratio of cross-linking agent to sulfonated poly(ether ether ketone) is 1:2.
Preferably, the dispersion additionally includes at least one hydrophilicitizing agent. More preferably, the at least one hydrophilicitizing agent is 2-acrylamido-1-methyl-2-propanesulfonic acid, 4-ethenyl-benzenesulfonic acid, or a salt thereof. Most preferably, the at least one hydrophilicitizing agent is 4-ethenyl-benzenesulfonic acid.
Preferably, the dispersion comprises a photoinitiator. Most preferably, the photoinitiator is benzophenone.
Preferably, the concentration of photoinitiator is greater than 2% (w/w). More preferably, the concentration of photoinitiator is greater than 4% (w/w).
Preferably, the irradiating is at wavelengths greater than 350 nm and at an intensity equivalent to 0.1 mW cm−2 at a distance of 50 mm.
Preferably, the irradiating is for a time of 60 to 120 seconds. More preferably, the irradiating is for a time of 80 to 100 seconds
In a second aspect the invention provides an asymmetric composite membrane consisting essentially of a film of cross-linked sulfonated poly(ether ether ketone) adhered to a sheet of hydrophilic microporous polyolefin.
Preferably, the film of cross-linked sulfonated poly(ether ether ketone) is an interpenetrating film of cross-linked sulfonated poly(ether ether ketone).
In a third aspect the invention provides a method of recovering water from a feed stream comprising the step of contacting the asymmetric composite membrane of the second aspect of the invention with the feed stream at a pressure sufficient to produce permeate.
Preferably, the feed stream is a dairy product. More preferably, the feed stream is milk. Most preferably, the feed stream is whole milk.
Preferably, the pressure is in the range 10 to 40 bar. More preferably, the pressure is in the range 15 to 35 bar. Most preferably, the pressure is 20±2.5 bar.
Preferably, the temperature is in the range 2 to 98° C. More preferably, the temperature is in the range 4 to 40° C. Most preferably, the temperature is in the range 4 to 20° C.
In a fourth aspect the invention provides a method of preparing a hydrophilic microporous polyolefin substrate comprising the steps of:
Preferably, the contacting is under an atmosphere of air.
Preferably, the microporous polyolefin substrate is a sheet of microporous polyolefin. More preferably, the polyolefin is selected from the group consisting of: polyethylene, polypropylene, polybutylene and polymethylpentene. Most preferably, the polyolefin is polyethylene.
Preferably, the microporous polyethylene substrate is prepared according to a method disclosed in the publications of Fisher et al (1991) and Gillberg-LaForce (1994).
Preferably, the solution is a solution in 40 to 60% (v/v) acetone in water. More preferably, the solution is a solution in 50% (v/v) acetone in water.
Preferably, the photoinitiator is selected from the group consisting of: aceto-phenone, anthraquinone, benzoin, benzoin ether, benzoin ethyl ether, benzil, benzil ketal, benzophenone, benzoyl peroxide, n-butyl phenyl ketone, iso-butyl phenyl ketone, fluorenone, propiophenone, n-propyl phenyl ketone and iso-propyl phenyl ketone. Most preferably, the photoinitiator is benzophenone.
Preferably, the UVA irradiating is at wavelengths greater than 350 nm.
Preferably, the UVA irradiating is for a time no greater than 5 minutes.
Preferably, the removing non-grafted polymer is by washing in water. More preferably, the removing non-grafted polymer is by washing in water at a temperature of 40 to 50° C.
In a fifth aspect the invention provides an asymmetric composite membrane comprising a cross-linked poly(vinyl alcohol) polymer or polymer blend coated on a film of cross-linked sulfonated poly(ether ether ketone) adhered to a sheet of hydrophilicitized microporous polyolefin.
Preferably, the cross-linked poly(vinyl alcohol) polymer or polymer blend is cross-linked poly(vinyl alcohol) polymer or a polymer blend with poly(vinyl pyrrolidone). More preferably, the cross-linked poly(vinyl alcohol) polymer or polymer blend is cross-linked poly(vinyl alcohol).
Preferably, the cross-linking agent of the cross-linked poly(vinyl alcohol) polymer or polymer blend is selected from the group consisting of formaldehyde, glutaraldehyde and glycidyl acrylate. More preferably, the cross-linking agent of the cross-linked poly(vinyl alcohol) is glutaraldehyde.
Preferably, the cross-linking agent of the cross-linked sulfonated poly(ether ether ketone) is selected from the group consisting of: o-divinylbenzene, m-divinylbenzene, p-divinylbenzene or ethylene glycol dimethacrylate. More preferably, the cross-linking agent of the cross-linked sulfonated poly(ether ether ketone) is divinylbenzene.
Preferably, the cross-linked sulfonated poly(ether ether ketone) is hydrophilicitized and the hydrophilicitizing agent is selected from the group consisting of: 2-acrylamido-1-methyl-2-propanesulfonic acid, 2-allyoxyethanol, 4-ethenyl-benzenesulfonic acid, 2-hydroxyethyl 2-methyl-2-propenoic acid ester, 2-propenoic acid (acrylic acid) and 2-propen-1-ol. More preferably, the hydrophilitizing agent of the hydrophilicitized cross-linked sulfonated poly(ether ether ketone) is 4-ethenyl-benzenesulfonic acid, 2-allyoxyethanol or 2-hydroxyethyl 2-methyl-2-propenoic acid ester. Most preferably, the hydrophilitizing agent of the hydrophilicitized cross-linked sulfonated poly(ether ether ketone) is 2-hydroxyethyl 2-methyl-2-propenoic acid ester.
Preferably, the degree of sulfonation (DS) of the cross-linked sulfonated poly(ether ether ketone) is 40 to 70%.
Preferably, the hydrophilicitizing agent of the hydrophilicitized microporous polyolefin is selected from the group consisting of: 2-acrylamido-1-methyl-2-propanesulfonic acid, 4-ethenyl-benzenesulfonic acid, 2-hydroxyethyl 2-methyl-2-propenoic acid ester, 2-propenoic acid (acrylic acid) and 2-propen-l-ol. More preferably, the hydrophilicitizing agent of the hydrophilicitized microporous polyolefin is 2-acrylamido-2-methylpropane sulfonic acid (AMPS) or 4-ethenyl-benzenesulfonic acid.
Preferably, the polyolefin of the hydrophilicitized microporous polyolefin is poly(ethylene).
In a sixth aspect the invention provides a method of preparing an asymmetric composite membrane of the fifth aspect of the invention comprising the steps:
Preferably, the method comprises the additional subsequent steps of:
Preferably, the wavelength is between 280 and 400 nm. More preferably, the wavelength is between 350 and 370 nm.
Preferably, the cross-linking agent is glutaraldehyde.
In a seventh aspect a method of recovering water from a feed stream is provided, the method comprising exposing a surface of the asymmetric composite membrane of the fifth aspect to the feed stream at a pressure sufficient to produce a permeate. The exposed surface of the membrane is the coated face of the membrane.
In the description and claims of this specification the following acronyms, terms and phrases have the meaning provided: “bursting strength” means the maximum uniformly distributed pressure applied at right angles to its surface, that a single sample location can withstand under test conditions; “close to its limit of solubility” means an increase in concentration of 5% (w/v) or more causes at least a portion of the solute to come out of solution, e.g. as a precipitate; “comprising” means “including”, “containing” or “characterized by” and does not exclude any additional element, ingredient or step; “consisting essentially of” means excluding any element, ingredient or step that is a material limitation; “consisting of” means excluding any element, ingredient or step not specified except for impurities and other incidentals; “crosslinking agents” means materials that are incorporated into the crosslinking bridge of a cross-linked polymer network; “crosslinking” means the formation of a three-dimensional polymer network by covalent bonding between the main chains of the polymer; “degree of sulfonation” means the ratio of moles of sulfonated structural repeating units to total moles of structural repeating units expressed as a percentage; “DMAc” means dimethylacetamide; “DS” means degree of sulfonation; “durable” means capable of maintaining performance during repeated clean-in-place (CIP) protocols; “ethenyl” means having a terminal ethylene function (vinyl); “flow” means the rate at which a feed stream is introduced; “flux” means the rate (volume per unit of time) of permeate transported per unit of membrane area; “gfd” means gallons per square foot per day; “graft polymer” means a polymer in which the linear main chain has attached to it at various points macromolecular side chains of a structure different from the main chain; “homopolymer” means a polymer formed by the polymerization of a single monomer; “hydrophilic” means having a tendency to mix with, dissolve in, or be wetted by water and “hydrophilicity”, “hydrophilicitized” and “hydrophilicitizing” have corresponding meanings; “hydrophilicitizing agents” means reagents that are incorporated as monomers, oligomers or polymers into a graft polymer network to impart hydrophilic properties; “hydrophobic” means having a tendency to repel or fail to mix with water and “hydrophobicity” has a corresponding meaning; “interpenetrating” means a comingling of two polymer networks; “LMH” means litres per square metre per hour; “ionomer” means a polymer that comprises both electrically neutral structurally repeating units and a fraction of ionized structurally repeating units; “microporous” means consisting of an essentially continuous matrix structure containing substantially uniform small pores or channels throughout the body of the substrate (such as may be manufactured using a cast (wet) process technology) and specifically excludes a discontinuous matrix of woven or non-woven fibres; “non-aqueous” means initially excluding water; “passage” means the percentage of dissolved species in the feed stream allowed to pass through the membrane; “PEEK” means poly(oxy-1,4-phenyleneoxy-1.4-phenylenecarbonyl-1,4-phenylene); “permeate” means the aqueous solution or purified water transported; “photoinitiator” means a photolabile compound which upon irradiation forms a radical; “polymer blend” means a homogenous or heterogeneous physical mixture of two or more polymers; “post-treated polymer” means a polymer that is modified, either partially or completely, after the basic polymer backbone has been formed; “preformed” means formed beforehand, i.e. prior to treatment; “recovery” means the percentage of the feed stream that emerges from the system as purified water or permeate; “rejection” means the percentage of solute concentration removed from the feed stream by the membrane; “sPEEK” means sulfonated PEEK; “structural repeating unit” means a smallest structural unit that repeats in the polymer backbone, e.g. oxy-1,4-phenyleneoxy-1.4-phenylenecarbonyl-1,4-phenylene is the structural repeating unit of PEEK; “tensile strength” means the maximum tensile stress sustained by a specimen at the yield point (tensile strength at yield) or at break (tensile strength at break) during a tension test; “UVA” means electromagnetic radiation having wavelengths between 320 and 400 nm; “UVB” means electromagnetic radiation having wavelengths between 290 and 320 nm; and “wettable” means becoming permeated with solvent, e.g. water, on being contacted with the solvent under standard laboratory conditions (i.e. 25° C. at 100 kPa) and “water-wettable” means wettable with water. Any reference to a “preformed microporous substrate” specifically excludes a preformed post-treated polymer.
In the absence of further limitation, the use of plain bonds in the representations of the structures of compounds encompasses the diastereomers, enantiomers and mixtures thereof of the compounds. The use of double bonds in the representations of aromatic ring structures does not exclude delocalisation of the π-electrons and encompasses alternative representations of the same aromatic ring structures.
The terms “first”, “second”, “third”, etc. used with reference to elements, features or integers of the subject matter defined in the Statement of Invention and Claims, or when used with reference to alternative embodiments of the invention are not intended to imply an order of preference.
The terms “film” and “sheet” are used to distinguish between planar materials of differing thickness, i.e., where a film is thinner than a sheet. Distinguishable material limitations are not to be inferred from the use of the terms “coating” and “film” or “adhered” and “coated”. Without additional qualification the terms “adhered” and “coated” are used synonymously. To facilitate understanding the terms “film” and “adhered” are generally used with reference to crosslinked, sulfonated poly(ether ether ketone) and the terms “coating” and “coated” are generally used with reference to crosslinked poly(vinyl alcohol).
Where a membrane is described as “water-wettable” a sample of the membrane is readily wetted with water, being observed to become uniformly translucent when contacted with this solvent.
Where concentrations or ratios of ingredients, reagents, solvents or substrates are specified, the concentration or ratio specified is the initial concentration or ratio of the ingredients, reagents, solvents or substrates. Where values are expressed to one or more decimal places standard rounding applies. For example, 1.7 encompasses the range 1.650 recurring to 1.749 recurring.
The invention will now be described with reference to embodiments or examples and the figures of the accompanying drawings pages.
) and after exposure to an acid (
) or alkali (▪) environment.
) and after exposure to an acid (
) or an alkali (▪) environment relative to the permeability of the unmodified polyolefin substrate (□).
The preparation of hydrophilicitized microporous sheets of polyolefin, such as polyethylene (μPE), is described. These hydrophilicitized microporous sheets are advantageously used as a backing layer in the preparation of durable asymmetric composite membranes. In turn, these membranes can be used as an asymmetric composite substrate and coated with at least partially crosslinked poly(ethenol) (poly(vinyl alcohol); PVA) to further improve the rejection properties of the asymmetric composite membranes.
The publication of Briggs et al (2015) describes the preparation of an asymmetric composite membrane consisting of a film of cross-linked sulfonated poly(ether ether ketone) (“rejection layer”) adhered to a sheet of sulfonated microporous poly(ethylene) (“backing layer”). The preparation of batches of the membrane is described using preformed sheets of microporous poly(ethylene) sulfonated by reaction with a phosphorus pentoxide-sulfuric acid “sulfonating agent”.
As described here, the backing layer is prepared by the photoinitiated graft polymerisation of a microporous sheet of polyolefin with selected hydrophilicitizing agents (Table 1). The hydrophilicitizing agent is selected to provide graft polymers with the chemical and physical properties dictated by the intended use of the asymmetric composite membrane. The use of 4-ethenyl-benzenesulfonic acid (SSS) has been found to be suitable for the preparation of a durable, i.e., chlorine tolerant, asymmetric composite membrane.
The method described here uses UV irradiation to reduce the risk of harm to operators and permit the rate and degree of modification of the microporous sheet of polyolefin to be readily controlled. The period of irradiation of the substrate is limited to less than 5 minutes. In addition, the use of a solvent system (e.g., 1:1 (v/v) acetone-water) in which the photoinitiator (e.g., benzophenone) is close to its limit of solubility is believed to promote the deposition of the photoinitiator on the walls of the pores of the substrate.
In the asymmetric composite membrane described here hydrophilicitization of the microporous sheet of poly(ethylene) is used to promote adherence between the film of sulfonated poly(ether ether ketone) that serves as the “rejection layer” and the sheet of poly(ethylene) that serves as the “backing layer”.
As noted above, the selectivity of the durable asymmetric composite membranes thus obtained can be further improved by adhering a coat of cross-linked poly(vinyl alcohol) to the surface of the film of cross-linked sulfonated poly(ether ether ketone). The coating may be applied in a single or in multiple steps. The improvement in the performance of the membrane with regard to its selectivity is achieved without significant loss of the favourable characteristic of durability during repeated clean-in-place (CIP) protocols observed for the asymmetric composite membranes. The phrase “composite substrate” is used to refer to the asymmetric composite membranes when they are being coated.
Poly(vinyl alcohol) (PVA) is a hydrophilic polymer that swells in water. In the method of preparing membranes described here, this swelling (and potential delamination from the underlying film of crosslinked sulfonated poly(ether ether ketone)) is controlled by cross-linking the PVA using the cross-linking agent glutaraldehyde. It is anticipated that other cross-linking agents may be employed, but glutaraldehyde has been found to be the suitable for cross-linking of PVA in the preparation of the membranes described here.
The Fourier transform infrared (FTIR) spectra obtained for PVA powder (
Preparation of Hydrophilicitized Microporous Sheets of Poly(Ethylene)
A microporous polyolefin substrate is contacted with a solution of 1% (w/v) photoinitiator and 6% (w/v) hydrophilicitizing agent in 1:1 (v/v) acetone-water. The contacted substrate is then UVA-irradiated at a peak wavelength of 368 nm for a maximum of 5 minutes. The irradiated substrate is finally washed using ultrasound in an excess of water followed by soaking in water. It was observed that a lower contact angle was achievable when irradiation of the contacted substrate occurred with the photoinitiator in solution (as opposed to being dried on the surface of the substrate).
For the preparation of samples A to D of modified polyolefin substrate according to the general method, sheets (20 μm thickness) of porous (45% porosity, 0.08 μm average pore diameter) poly(ethylene) (CELGARD™ K2045, Celgard LLC) were used as the polyolefin substrate. The solution was prepared by mixing benzophenone (photoinitiator) with acetone before adding water and then the selected hydrophilicitizing agent. The polyolefin substrate was contacted with the solution by placing a sheet of the substrate in a clear polyethylene bag and then using a threaded rod to apply the solution to the substrate. Any residual air was then removed from the bag before sealing and hanging from a frame. Irradiation was for three and a half minutes using UV fluorescent lamps (368 nm) having a bulb irradiance of 0.1 mW cm−2 at a distance of 50 mm. The ultrasound washing was for five minutes followed by soaking at 45° C. for three hours.
For the preparation of Sample E amounts of 0.6 g of the hydrophilicitizing agent sodium 4-vinylbenzene sulfonate and 0.1 g of the photoinitator benzophenone were dissolved in water (5 mL) and acetone (5 mL). The solution was then applied to a microporous sheet of polyethylene on a glass plate using a threaded rod. Three applications were made until the polyethylene was wetted out. The glass plate and sample were then placed in a polyethylene plastic bag then clamped to a frame and cured using fluorescent UV lamps at a distance of 5 cm on both sides of the sample. The peak wavelength of the lamps was 368nm and an irradiance power of 0.2 to 0.4 mW/m for each lamp. The lamps were placed in a line with 50 mm centres. The time the samples were irradiated was 210 seconds. The samples were then removed from the polyethylene bag and washed in 45° C. water for 10 seconds to removed excess polymer and unreacted hydrophilicitizing agent and put in an oven to dry for 30 minutes at 65° C. The samples were then removed from the glass plate by immersion in a water bath and extracted in a beaker of deionised water for three hours. Sample F was prepared by the same method as used for the preparation of Sample E, but with a volume of 0.6 mL of the hydrophilicitizing agent acrylic acid being substituted for the hydrophilicitizing agent sodium 4-vinylbenzene sulphonate and added after the benzophenone was dissolved in the solvent. Sample G was prepared by the same method as used for the preparation of Sample E, but with a volume of 0.6 mL of the hydrophilicitizing agent 2-hydroxyethyl methacrylate being substituted for the hydrophilicitizing agent sodium 4-vinylbenzene sulphonate and added after the benzophenone was dissolved in the solvent. The properties of samples of modified polyolefin substrate prepared using different hydrophilicitizing agents were assessed.
Evaluation of Hydrophilicitized Macroporous Sheets of Poly(Ethylene)
Fourier Transform Infrared (FTIR)
Spectra of the samples were recorded using a Thermo Electron Nicolet 8700 FTIR spectrometer equipped with a single bounce ATR and diamond crystal. An average of 32 scans with a 4 cm−1 resolution was taken for all samples.
Surface Analysis
The contact angles for the surfaces of the asymmetric composite membrane were determined in using the captive bubble method as described in the publication of Causserand and Aimar (2010). The samples were immersed in deionized water with the surface to be analysed facing downwards. An air bubble was trapped on the lower surface of the sample using a syringe. An image of the bubble was captured and the contact angle was calculated from its geometrical parameters.
Permeability and Flux Testing
Permeability was determined by measuring the flux in deionized water at various pressures starting at 20 bar and decreasing in 4 bar iterations. Flux JV was then graphed against effective pressure difference across the membrane, peff, and the slope of the permeability Lp.
Initial flux rates under pressure (20 bar) and no pressure were determined using the Sterlitech flux rig illustrated in
where V is the permeate volume (L), t is the time (h) for the collection of V and A is area of the sample (m2) which was determined to be 0.014 m2.
To assess durability in different environments tests were also performed on samples immersed for 60 to 70 hours in aqueous solutions of either 30% (w/v) potassium hydroxide (“alkali environment”) or 33% (w/v) hydrochloric acid (“acid environment”).
Dry weight increases were calculated by taking the dry weight of the sample after it had dried in an oven for half an hour and comparing the weight to the initial weight of the porous polyethylene before grafting. Dry weights were taken after loose polymer had been extracted from the membrane and at the end of testing after a clean in place (CIP) protocol.
Water absorption was measured after loose polymer from the membrane had been extracted. The wet membranes were dabbed dry with a paper towel to remove surface moisture and weighed.
Total solids rejection for whole milk samples was measured by pouring 20 mL of sample from the feed in a petri dish and measuring the dry weight after 2 hours in a 100° C. oven.
where mp,Ts is total milk solids in the permeate and mf,Ts is the mass of milk total solids in the feed.
Sodium chloride rejection was measured using a 2 g/L solution with a feed pressure of 16 bar. The conductivities from the feed and permeate were compared.
Where σp is the conductivity of permeate and σf is the conductivity of the feed.
The bubble point of the dry membranes was determined by gradually increasing the pressure of the feed until permeate started to flow through the membrane.
Results
The FTIR spectra for samples A, C and D generally showed faint peaks compared to the peaks observed in the FTIR spectrum of the unmodified polyolefin substrate (CELGARD™ K2045, Celgard LLC) (see
The contact angles for samples A to D showed an inverse relationship with the permeability determined for the same sample (see
All of samples A to D showed an increase in permeability compared to the unmodified membrane which measured 2.56 m s−1 Pa−1. When soaked for 66 hours in 30% (w/v) potassium hydroxide Sample A was stable based on a comparison of the permeability determined before and after exposure to the alkali environment. By comparison Sample D showed a large increase in permeability when exposed to the same alkali environment indicating the importance of the selection of the hydrophilicitizing agent when preparing modified polyolefin substrates for particular applications, e.g. alkaline battery separators. Furthermore, when immersed in 33% (w/v) hydrochloric acid Sample D turned the acid environment yellow and a strong odour of chlorine was detected, indicating oxidation of the modified polyolefin substrate. Notwithstanding this observation, the permeability of Sample D following exposure to the acid environment remained stable suggesting that the polyolefin substrate was not being degraded. When Sample B was exposed to the acid environment no colour change was observed, but the permeability decreased to less than the permeability of the polyolefin substrate, i.e. less than 2.56 m s−1 Pa−1).
As a general rule the higher the observed contact angle the lower the permeability determined for a sample. After Sample D was exposed to an alkali environment the sample developed a high initial flux even though the contact angle was determined to remain high. This observation indicates that the structure of the modified polyolefin is degraded. Water absorption was observed to be greatest for Sample B and Sample D, and of these two samples, Sample D had the largest water absorption. Sample A had a larger water absorption than Sample C (see
Based on the assessment the preparation of modified polyolefin substrates according to the general method using 2-hydroxyethyl 2-methyl-2-propenoic acid ester as the hydrophilicitizing agent is selected for use as a backing or support layer in osmosis membranes. Sample C has been determined to provide high initial flux and the ability to let permeate through at low pressure differentials. Use of this class of modified polyolefin is indicated for medical applications.
Based on the assessment the preparation of modified polyolefin substrates according to the general method using 2-propen-1-ol as the hydrophilicitizing agent is selected for use in applications having an alkali environment. Sample A maintained a relatively high permeability under these conditions.
Based on the assessment the preparation of modified polyolefin substrates according to the general method using 4-ethenyl-benzenesulfonic acid as the hydrophilicitizing agent is selected for use in applications having an acid environment. Under these conditions Sample D maintained a more stable flux than Sample B exposed to the same conditions.
The assessments of replicates (i, ii, iii, . . . ) of samples E, F and G are presented in Table 3 and
Sample F was observed to provide a water permeable membrane with the highest rejection of salt (sodium chloride) (
The combination of a cross-linked sulfonated poly(ether ether ketone) rejection layer and a hydrophilic polyethylene backing layer provides a durable asymmetric composite membrane suitable for use in commercial processing operations.
Preparation of an Asymmetric Composite Membrane
The membrane is prepared by adhering a hydrophilicitized microporous sheet of poly(ethylene) (μPE) to a film of putatively cross-linked sulfonated poly(ether ether ketone) (sPEEK). The adherence is augmented by the interpenetration of the two polymers. In the laboratory the membrane may be prepared according to the following method in which the sheet of hydrophilic μPE is nominally referred to as the ‘backing layer’ and the film of putatively cross-linked sPEEK is nominally referred to as the ‘rejection layer’. (The backing layer may alternatively be referred to as the ‘support layer’ and the rejection layer alternatively referred to as the ‘barrier layer’.) The method provides the advantage of being adaptable to the continuous production of the asymmetric composite membrane. The method is described in detail in respect of the preparation of a single sample.
Rejection Layer
Poly(ether ether ketone) (PEEK) (VICTREX™ 450P, Victrex, England) was sulfonated by heating to 70° C. in concentrated sulfuric acid (95%) for 8 hours. The sulfonated PEEK (sPEEK) was precipitated and washed in ice water several times before being dried in a vacuum oven. Without wishing to be bound by theory it is believed the small amount of water present in the concentrated sulfuric acid prevents cross-linking attributable to the formation of sulfone bridges. The degree of sulfonation of the sPEEK was determined by titration according to a modified form of the method disclosed in the publication of Drioli et al (2004). The sPEEK was leached for three days in a 3M solution of sodium chloride (NaCl) and the resulting solution titrated against a 0.2 M solution of sodium hydroxide (NaOH) using phenolphthalein as indicator. An amount of sPEEK (0.2 g) with a 69% DS was then added to a volume of dimethylacetamide (DMAc) (2.7 mL) and sonicated until a clear to slightly cloudy dispersion was obtained.
A volume (0.1 mL) of divinylbenzene (DVB) as crosslinking agent and an amount (0.14 g) of sodium styrene sulfonate (SSS) as hydrophilicitizing agent were added to a dispersion of sPEEK in DMAc. The dispersion contained 8% (w/w) sPEEK (0.216 mol/L) to provide a mixture containing a molar ratio of DVB to sPEEK of 1:2 and a molar ratio of SSS to sPEEK of 1:2. To increase the rate of the photoinitiated reaction an amount of benzophenone (BP) (8 μg) was added to the mixture before pouring onto aluminium foil on a glass plate, directly onto a glass plate or directly onto a stainless steel surface. The poured mixture was then exposed to 0.1 mW cm−2 UVA fluorescent lamps (368 nm) at a distance of 50 mm for a limited time of 60 to 90 seconds to provide a semi-cured film. The photoinitiated reaction is conveniently performed under an atmosphere of air (without the need to provide an inert, e.g. nitrogen (N2), atmosphere). The structures of DVB and alternative di- and tetra-ethenyl cross-linking agents are provided in Table 4.
Backing Layer
The sheet of sμPE to which a film of xsPEEK is adhered was prepared from a preformed sheet of microporous poly(ethylene) (μPE). The formation of sheets μPE is described, for example, in the publications of Fisher et al (1991) and Gillberg-LaForce (1994). In the present studies a preformed sheet of μPE (20 μm thickness, 45% porosity, 0.08 μm average pore diameter) (CELGARD™ K2045, Celgard LLC) was contacted with a solution of 1% (w/v) benzophenone and 6% (w/v) 4-ethenyl-benzenesulfonic acid (as the sodium salt) (SSS) as hydrophilicitizing agent in 1:1 (v/v) acetone-water. The solution was prepared by mixing benzophenone with acetone before adding water and then the hydrophilicitizing agent. The use of SSS is preferred due to the greater chlorine tolerance of membranes prepared using this hydrophilicitizing agent. This advantage applies to both the preparation of the hydrophilicitized backing layer and the asymmetric composite membrane.
Asymmetric Composite Membrane
The sheet of μPE contacted with the solution was laid on top of the semi-cured film (the nascent ‘rejection layer’). The composite of μPE contacted with the solution and semi-cured film of putative xsPEEK was then exposed as before to 0.1 mW cm−2 UVA fluorescent lamps (368 nm) at a distance of 50 mm, but for a limited time of 210 seconds. The UVA-irradiated composite was then dried in an oven at 60° C. for 30 minutes to promote adherence of the film and sheet before releasing the composite membrane from the aluminium foil by immersion in a solution of 2% w/w sodium hydroxide or, if cured on a glass plate, by immersing the membrane in a water bath at room temperature until the membrane releases and floats to the surface (typically for 10 to 15 minutes). Where the nascent
rejection layer is cured on a stainless-steel surface it may be necessary to soak in water overnight. The structures of AMPS, SSS and alternative mono-ethenyl hydrophilicitizing agents are provided in Table 1. Before evaluation the laboratory prepared composite membrane was rinsed at 50° C. with a large excess of deionised (DI) water.
Samples of the asymmetric composite membrane were prepared according to the foregoing method consisting of a rejection layer and a backing layer prepared using the compositions and conditions provided in Table 3 and Table 4.
Evaluation of the Asymmetric Composite Membrane
The performance of the asymmetric composite membrane was evaluated using a reverse osmosis (RO) filter assembly of the type illustrated in
Permeability was determined by measuring the flux in deionized water at various pressures starting at 20 bar and decreasing in 4 bar iterations. Flux JV was then graphed against effective pressure difference across the membrane, peff, and the slope of the permeability Lp.
Initial flux rates under pressure (20 bar) and no pressure were determined. The asymmetric composite membrane was mounted in the flux cell and bolted. Deionized water was fed into the rig at 2.5 L min−1 and 4 to 8° C. The time to collect a predetermined volume of permeate was noted. The flux rate (J) was calculated according to the following equation:
where V is the permeate volume (L), t is the time (h) for the collection of V and A is area of the sample (m2) which was determined to be 0.014 m2.
To mimic commercial processing operations the asymmetric composite membrane was subjected to ‘clean-in-place’ (CIP) protocols between each use of milk as the feed stream. The CIP protocols were based on those employed in a commercial processing operation for reverse osmosis (RO) membranes (Anon (2014)) and summarised in Table 7. The CIP protocols were repeated alternating with the use of milk as a feed stream. Samples were taken from the feed and permeate for each intervening use of milk as a feed stream to determine any deterioration in the performance of the membrane attributable to repeated CIP protocols. The asymmetric composite membrane was also evaluated for its tolerance to a CIP protocol including sodium hypochlorite (Table 8).
The following measurements relating to the performance of the asymmetric composite membrane before and after repeated application of the CIP protocols were made:
The total solids content was determined gravimetrically for both the feed and permeate. Samples were weighed in Petri dishes and dried in an oven at 60° C. for two hours and then 102° C. for a further two hours. The results are summarised in Table 9.
Sample 1
The sample was subjected to repeated CIP protocols according to the schedule provided in Table 8 with the exception that Step 1 and Step 6 were also performed at 35° C. The maximum total solids rejection (standard milk) was observed after three CIP protocols with flux and total solids rejection stabilising after four to five CIP protocols (
1alkali (2% (w/v) NaOH) and acid (1.9% (w/v) H2NO3 and 0.6 (w/v) H3PO4).
1alkali (2% (w/v) NaOH), acid (1.9% (w/v) H2NO3 and 0.6 (w/v) H3PO4) and chlorine (0.05% (w/v) sodium hydroxide and 0.09% (w/v) sodium hypochlorite).
Sample 2
The sample was subjected to repeated sequential CIP protocols according to the schedules provided in Table 7 (10×) and Table 8 (12×). The sample was then dried for several days before being subjected to further CIP protocols. The lactose rejection remained high throughout the sequential CIP protocols, the moderate decline in performance being recoverable following drying of the sample (
Sample 3
The sample was subjected to repeated CIP protocols (25×) according to the schedule provided in Table 8. A total solids rejection (standard milk) comparable with that obtained for sample 1 was observed. A greater variability in flux was observed (
Sample 4
The sample was subjected to repeated CIP protocols (17×) and exhibited an unacceptable decline in the rejection of total solids (
Sample 5
The performance of the sample was evaluated when used to recover permeate from fresh raw milk over a prolonged period of time (18 hours) at a constant pressure of 16 bar. A performance comparable with that of existing commercial operations was observed.
Sample 6
The sample was prepared to demonstrate the advantage provided by the inclusion of both cross-linking and hydrophilicitizing agents in the preparation of the rejection layer. The performance of the sample before and after a single CIP protocol according to the schedule provided in Table 8 was compared with that of Sample 1. Whereas the performance of the latter in terms of total solids rejection improved, the performance of Sample 6 deteriorated. The poor durability of the sample is attributed to the absence of cross-linking and interpenetration of the polymers of the backing layer and rejection layer of the composite membrane.
Samples 7 to 10
These samples were prepared to evaluate the influence the proportion of SPEEK used in the preparation of the rejection layer had on performance (in the absence of the hydrophilicitizing agent SSS). The non-linear relationship between the proportion of SPEEK used and sodium chloride rejection is consistent with an expected increase in the electric field gradient of the membrane and corresponding rejection of charged species (
Sample 11
The sample was prepared using a high (greater than 80%) solids content when preparing the rejection layer. In addition, HEMA was substituted for SSS as the hydrophilicitizing agent due to the poor solubility of the latter in methanol. An extended curing period of 10 minutes was employed. At a pressure of 20 bar the sample provided a comparable sodium chloride rejection (
Sample 12
The sample was prepared using an unmodified μPE as the backing layer. This necessitated the use of acetone/water as the solvent for the rejection layer formulation. Pursuant to the use of this solvent the proportion of sPEEK was reduced and the proportion of SSS increased with a total solid content of 6% (w/w). The curing was performed in a sealed polyethylene bag to prevent flush evaporation of acetone during the curing period of five minutes. The performance of the sample at 20 bar in terms of flux and sodium chloride and sucrose rejection was poor when compared with the performance of an analogous sample prepared using a grafted, hydrophilicitized backing layer.
Preparation of a Coated Composite Substrate
The asymmetric composite membrane can be advantageously used as a composite substrate in the preparation of an asymmetric composite membranes. A coating of at least partially crosslinked poly(ethenol) (poly(vinyl alcohol; PVA) is applied to the surface of the film of crosslinked sulfonated poly(ether ether ketone) (xsPEEK) of the composite substrate.
Method I
Preparation of Sulfonated Poly(Ether Ether Ketone)
An amount of poly(ether ether ketone) (PEEK) (VICTREX™ 450 P, Victrex Manufacturing Limited, England) was sulfonated by heating to 70° C. in concentrated sulfuric acid (95%) for 8 h. The sulfonated PEEK (sPEEK) was then precipitated and washed in ice water several times before being dried in a vacuum oven. The degree of sulfonation of the sPEEK was determined by titration according to a modified form of the method disclosed in the publication of Drioli et al (2004). The sPEEK was leached for three days in a 3M solution of sodium chloride (NaCl) and the resulting solution titrated against a 0.2 M solution of sodium hydroxide (NaOH) using phenolphthalein as indicator.
Preparation of a Film of Semi-Cured Cross-Linked sPEEK
An amount of sPEEK (0.2 g) with a 69% degree of sulfonation (DS) was added to a volume of dimethylacetamide (DMAc) (2.7 mL) and sonicated until a clear to slightly cloudy dispersion was obtained. A volume (0.1 mL) of the crosslinking agent divinylbenzene (DVB) and an amount (0.14 g) of the hydrophilicitizing agent 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA) were added to the dispersion of sPEEK in DMAc to provide a mixture containing 8% (w/w) sPEEK (0.216 mol/L) and molar ratios of DVB to sPEEK of and HEMA to sPEEK of 1:2. An amount of the photoinitiator benzophenone (8 μg) was added to the mixture before pouring onto a glass plate and exposing to 0.1 mW cm−2 UVA fluorescent lamps (368 nm) at a distance of 50 mm for a limited time of 90 s to provide the semi-cured film of cross-linked sPEEK.
Preparation of a Hydrophilicitized Microporous Sheet of Poly(Ethylene)
A preformed sheet (20 μm thickness) of microporous (45% porosity, 0.08 μm average pore diameter) poly(ethylene) (PE) (CELGARD™ K2045, Celgard LLC) was wetted with a solution in 1:1 (v/v) acetone-water of 1% (w/v) benzophenone and 6% (w/v) 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA). The solution was prepared by mixing benzophenone with acetone before adding water and then HEMA. The wetted sheet was then UVA-irradiated at a peak wavelength of 368 nm for a maximum of 5 min before washing in an excess of water using ultrasound and soaking to provide the hydrophilicitized sheet of microporous PE.
Preparation of the Composite Substrate
The hydrophilicitized microporous sheet of PE was laid on top of the semi-cured film of semi-cured cross-linked sPEEK and exposed to 0.1 mW cm−2 UVA fluorescent lamps (368 nm) at a distance of 50 mm for a limited time of 210 s. The UVA-irradiated composite substrate was then dried in an oven at 60° C. for 30 min to promote adherence of the film and sheet before releasing the composite substrate from the glass plate by immersing in a water bath at room temperature for 10 to 15 min and rinsing with a large excess of deionised (DI) water at 50° C. to provide the composite substrate.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a solution in water of 5% (w/w) poly(vinyl alcohol) (PVA). The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with an ice cooled mixture in acidified (H2SO4) water of 5% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA) and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a solution in 7:3 (v/v) water-isopropanol of 0.5% (w/w) PVA. The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with an ice cooled mixture in acidified (H2SO4) water of 2% (w/w) poly(vinyl pyrrolidone) (PVP), 2% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA) and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a cooled mixture in acidified (H2SO4) water of 1% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA). The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with the cooled mixture and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
Method II
Preparation of Sulfonated Poly(Ether Ether Ketone)
An amount of poly(ether ether ketone) (PEEK) (VICTREX™ 150 P, Victrex Manufacturing Limited, England) was sulfonated by heating to 70° C. in concentrated sulfuric acid (98%) for 4 h. The sulfonated PEEK (sPEEK) was then precipitated and washed in ice water several times before being dried in a vacuum oven overnight. The degree of sulfonation of the sPEEK was determined by titration according to a modified form of the method disclosed in the publication of Drioli et al (2004). The sPEEK was leached for three days in a 3M solution of sodium chloride (NaCl) and the resulting solution titrated against a 0.2 M solution of sodium hydroxide (NaOH) using phenolphthalein as indicator.
Preparation of a Composite Substrate
The polyethylene (PE) is first cut, using a craft knife, into rectangles measuring 185 mm×135 mm. The corners are removed to allow for it to fit within the testing rig. The initial weights of the cut PE are taken.
Backing layer solution is prepared using an amount of 0.6 g of 4-ethenyl-benzenesulfonic acid (SSS) and an amount of 0.1 g of benzophenone (BP). The measured amounts of SSS and BP are transferred to a vial and a volume of 5 mL of DI and a volume of 5 mL of acetone are added. The vial is sealed and shaken/stirred until the materials have completely dissolved.
Rejection layer solution is prepared using an amount of sPEEK (0.24 g) with a degree of sulfonation (DS) in the range of 50-70%. An amount of sPEEK is added to a volume of 5 mL methanol (MeOH) and sonicated until a clear to slightly cloudy dispersion was obtained. An amount of the crosslinking agent divinylbenzene (DVB) and a volume of 2 mL the hydrophilicitizing agent 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA) are added to the dispersion of sPEEK in MeoH.
Rejection layer solution is applied to aluminium foil and left to flash off for 10 min. The rejection layer is then cured under fluorescent lamps for 12 min. A sheet of microporous PE film is wet-out with the backing layer solution. The wet-out PE film is then laid on top of the cured rejection layer. The composite substrate is then cured together under fluorescent lamps for 3.5 min. The cured composite is washed under tap water for 10 s.
The composite substrate is dried in an oven at 65° C. for 30 min and then lifted from the aluminium foil by immersion in a solution of 2% w/w sodium hydroxide (NaOH).
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a solution in water of 5% (w/w) poly(vinyl alcohol) (PVA). The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with an ice cooled mixture in acidified (H2SO4) water of 5% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA) and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a solution in 7:3 (v/v) water-isopropanol of 0.5% (w/w) PVA. The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with an ice cooled mixture in acidified (H2SO4) water of 2% (w/w) poly(vinyl pyrrolidone) (PVP), 2% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA) and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a cooled mixture in acidified (H2SO4) water of 1% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA). The coated composite substrate was then dried at 65° C. for 10 to 15 min.
The coated composite substrate was then recoated with the cooled mixture and further cured at 65° C. for 15 min to provide the asymmetric composite membrane.
The membrane was washed with tap water and lifted from the glass plate before assessment.
Method III
Preparation of Sulfonated Poly(Ether Ether Ketone) [Sample 171214-17.5]
Sample 171214-17.5 was made using 450P PEEK at a concentration of 5% w/v PEEK to sulphuric acid (H2SO4). The PEEK is stirred at room temperature for 17.5 h before dropping out in an ice bath and washing until pH˜6.5. The material is dried at 65° C. in vacuum oven overnight. The dried product is soluble in DMAc but not in MeOH or MeOH/water.
Preparation of a Composite Substrate
This procedure requires a pre-cured, modified PE backing layer to be placed on a semi-cured rejection layer. The modification of the backing layer will be described first.
Backing layer preparation: The PE is first cut, using a craft knife, into rectangles measuring 185 mm×135 mm. The corners are removed to allow for it to fit within the testing rig. The initial weights of the cut PE are taken.
A volume of 1.2 mL of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and an amount of 0.1 g benzophenone (BP) are weighed out. Then, AMPS and BP are transferred to a vial and a volume of 5 mL of DI and a volume of 5 mL of acetone are added. The vial is sealed and shaken/stirred until the materials have completely dissolved. Once the solution is made exposure to light should be minimised due to the photoreactivity of the BP.
A PE sheet is placed onto a glass plate within a fume hood and a small amount (˜3-5 mL) is placed on the top of the sheet. It will be observed that the sheet will appear semi-transparent upon contact with the solution. The solution is quickly spread over the sheet to form a uniform coating using either a finger or a threaded rod. Due to evaporation of the solvent dry patches may appear after application. If this occurs more solution should be applied so that the entire sheet is semi-transparent. Once the solution has been evenly applied the glass plate, with the wetted PE sheet, is placed inside a sealable PE bag. It should be noted that the plate should be placed in the bag and sealed rapidly after even application of the solution is achieved due to risk of solvent evaporation in an open environment. The PE bag is then placed into a UVA light source with a wavelength >350 nm at a distance of ˜50 mm for 90 s to cure.
After curing the plate and membrane are left in the bag while the rejection layer is semi cured, this prevents the membrane from drying, allowing for a flat, even application of the backing layer to the rejection layer.
Rejection layer solution is prepared using an amount of sPEEK (0.23 g), an amount of 0.1125 g of DVB, an amount of 0.158 g of SSS and an amount of 0.0204 g of BP. These materials are transferred to a vial and a volume of 2 mL of DMAc was added. The vial is sealed and shaken/stirred until the materials have completely dissolved. Once the solution is made contact with light should be minimised due to the photoreactivity of the BP.
A small amount of solution, ˜0.6 mL, is placed on the smooth aluminium foil surface and spread evenly over an area slightly larger than that of the membrane. The plate with the solution is then cured with a UVA light source with a wavelength >350 nm at a distance of ˜50 mm for 45 s to cure the rejection layer.
While curing the modified backing layer is removed from the bag/plate. The backing layer is then laid flat on to the cured rejection layer, ensuring no wrinkles or bubbles in the backing layer. The plate with the backing layer and rejection layer is then placed in to a drying oven for 30 min at 65° C.
Once dried the membrane may be stuck to the aluminium foil. The membranes can be separated from the foil by soaking in 1-2% NaOH. After the membrane is removed from the foil the sheets are rinsed in DI and extracted in DI at 50° C. for 3 h. The extracted membranes are dried and stored for testing.
The composite substrate was placed on a glass plate with the film of cross-linked sPEEK uppermost and coated with a solution in water of 5% (w/w) poly(vinyl alcohol) (PVA). The coated composite substrate was then dried at 65° C. for 10 to 15 min. The coated composite substrate was then recoated with an ice cooled mixture in acidified (H2SO4) water of 1% (w/w) PVA and 2.5% (w/w) of the crosslinking agent glutaraldehyde (GA) and further cured at 65° C. for 15 min to provide the asymmetric composite membrane. The membrane was washed with tap water and lifted from the glass plate before assessment.
A volume of 0.25 mL of 98% H2SO4 was added to 10 mL of 1% (w/w) PVA in water. The vial was cooled in an ice bath before the addition of 0.25 mL of GA. The composite substrate was placed on a glass plate with the film of crosslinked sPEEK uppermost and coated with the PVA solution. The coated composite substrate was then dried at 65° C. for 10-15 min. The coated composite substrate was then recoated with the cooled PVA solution and further cured at 65° C. for 10-15 min to provide the PVA asymmetric composite membrane. The membrane was washed with tap water, lifted from the glass plate and dried at room temperature before assessment.
Method IV
This procedure uses a pre-cured, modified PE backing layer placed on a semi-cured rejection layer.
Preparation of Sulfonated Poly(Ether Ether Ketone) [Sample 24/11]
An amount of 15 g (PEEK) (VICTREX™ 150 P, Victrex Manufacturing Limited, England) was sulfonated by heating to 70° C. in concentrated sulfuric acid (98%).
The PEEK is stirred at room temperature for 17.5 h before dropping out in an ice bath and washing until pH ˜6.5. The material is dried at 65° C. in vacuum oven overnight.
Preparation of a Composite Substrate
The polyethylene (PE) is first cut, using a craft knife, into rectangles measuring 185 mm×135 mm. The corners are removed to allow for it to fit within the testing rig. The initial weights of the cut PE are taken.
Backing layer solution is prepared using an amount of 0.6 g of 4-ethenyl-benzenesulfonic acid (SSS) and an amount of 0.1 g of benzophenone (BP). The measured amounts of SSS and BP are transferred to a vial and a volume of 5 mL of DI and a volume of 5 mL of acetone are added. The vial is sealed and shaken/stirred until the materials have completely dissolved. Once the solution is made contact with light should be minimised due to the photoreactivity of the BP.
A PE sheet is placed onto a glass plate within a fume hood and a small amount (˜3-5 mL) is placed on the top of the sheet. It will be observed that the sheet will appear semi-transparent upon contact with the solution. The solution is quickly spread over the sheet to form a uniform coating using either a finger or a threaded rod. Due to evaporation of the solvent dry patches may appear after application. If this occurs more solution should be applied so that the entire sheet is semi-transparent. Once the solution has been evenly applied the glass plate, with the wetted PE sheet, is placed inside a sealable PE bag. It should be noted that the plate should be placed in the bag and sealed rapidly after even application of the solution is achieved due to risk of solvent evaporation in an open environment. The PE bag is then placed into a UVA light source with a wavelength >350 nm at a distance of ˜50 mm for 210 s to cure. After curing the plate is removed from the bag and the modified PE sheet is then rinsed, while on the plate, in warm water for 10 s. The plate is then placed into a drying oven for 30 min at 65° C. Once dry the plates are removed from the oven and allowed to cool to room temperature.
After drying the PE sheet is then placed into 50° C. DI for 3 h to allow extraction of non-crosslinked/grafted materials within the sheet. The sheets are left in the DI but allowed to cool to room temperature.
The glass plates are wrapped in tinfoil, ensuring one side has a smooth flat surface. Rejection layer solution is prepared using an amount of sPEEK (0.3 g), an amount of 0.111 g of DVB, an amount of 0.2127 g of allyl oxy-ethanol (AOE) and an amount of 0.017 g of BP. These materials are transferred to a vial and a volume of 4 mL of DMAc was added. The vial is sealed and shaken/stirred until the materials have completely dissolved. Once the solution is made exposure to light should be minimised due to the photoreactivity of the BP.
A small amount of solution, ˜0.5 mL, is placed on the smooth aluminium foil surface and spread evenly over an area slightly larger than that of the membrane. The plate with the solution is then cured with a UVA light source with a wavelength >350 nm at a distance of ˜20 mm for 90 s to cure the rejection layer. While curing the modified backing layer is removed from the DI and the excess water is removed, while leaving the membrane wetted.
The backing layer is then laid flat on to the cured rejection layer, ensuring no wrinkles or bubbles in the backing layer. The plate with the backing layer and rejection layer is then placed into a drying oven for 30 min at 65° C.
Once dried the membrane may be stuck to the aluminium foil. The membranes can be separated from the foil by soaking in 1-2% NaOH. Once removed from the foil the sheets are rinsed in fresh DI and stored either dry or in a PE bag with DI.
The procedure for the preparation of Example 6 is used. A volume of 0.25 mL of 98% H2SO4 was added to 10 mL of 1% (w/w) PVA in water. The vial was cooled in an ice bath before the addition of 0.25 mL of GA. The composite substrate was placed on a glass plate with the film of crosslinked sPEEK uppermost and coated with the PVA solution. The coated composite substrate was then dried at 65° C. for 10-15 min. The coated composite substrate was then recoated with the cooled PVA solution and further cured at 65° C. for 10-15 min to provide the PVA asymmetric composite membrane. The membrane was washed with tap water, lifted from the glass plate and dried at room temperature before assessment.
Evaluation of the Coated Composite Substrate
The performance of samples of coated composite substrate selected from Examples 1 to 9 was evaluated.
Salt and Sucrose Rejection
The performance of the coated composite substrate was evaluated using a flux test unit of the type illustrated in
The permeate flux (J), i.e. the volume (V) of permeate passing through a sample of asymmetric composite membrane of area (A) during a period of time (t) was calculated according to the following equation:
Conductivities of the feed stream (Fσ) and permeate (Pσ) were measured using a multi parameter meter (Oakton PCS Tester 35, Cole-Parmer, New Zealand) at ambient temperature. Salt rejection (R) was calculated according to the following equation:
Sucrose rejection was calculated based on the dry weights of residues obtained after evaporating the solvent from known volumes of feed and permeate samples.
In Situ Cleaning of Coated Asymmetric Substrate
To mimic commercial processing operations the coated composite substrate was subjected to the ‘clean-in-place’ (CIP) protocols summarised in Tables 10 and 11. The salt and sucrose rejections were determined following repeated CIP protocols.
This sample of the asymmetric composite membrane exhibited initial rejections of 79.3% NaCl (at a flux of 0.64 LMH), 98% MgSO4 (at a flux of 0.85 LMH) and 94.2% sucrose. A series of CIP protocols were conducted to test the stability of the rejection layer of the sample. After the fourth CIP protocol stabilized rejections of 66.5% NaCl and 94.7% sucrose were obtained. A rejection of 84.5% MgSO4 was obtained, but then jumped to 91.7% after the fifth CIP. The stabilized performances indicate the membrane had survived the harsh CIP conditions. The performance of the membrane after each CIP cycle is provided in
This sample of the asymmetric composite membrane exhibited initial rejections of 73% NaCl, 93% MgSO4 and 92% sucrose. The flux was in the range 3 to 4 LMH, an improvement compared to Example 1A, Method II. The performance of the membrane after repeated CIP protocols is provided in
This sample of the asymmetric composite membrane exhibited initial rejections of 88% NaCl, 100% MgSO4 and 99% sucrose rejections. The membrane performed well during repeated CIP protocols until the fourth CIP protocol. At this stage the sample showed a sudden marked decrease in the rejection of MgSO4 and sucrose (cf. Example 4).
The results (Table 12) are from a sample [#060716-2] that survived 10 CIP cycles.
Membranes were subjected to 10 CIP cycles. The flux rate and rejection on 2 g/L (±-5%) salt solutions was monitored. The feed flowrate and temperature was 2±0.2 LPM and 16±3° C. respectively. Table 13 gives the results for three samples of the membrane: 140616-1, 140616-2 and 140616-3.
Samples of the membrane (140616-1 and 140616-2) were retested after repeated drying to determine if the performance of the asymmetric composite membrane was adversely affected with either salt solution (MgSO4) or milk as the feed stream (Table 14).
The performance of a sample [#210815] of the asymmetric composite membrane prepared according to Example 9 was assessed. The results are presented in Table 15.
The performance of samples of membranes prepared omitting one or more of the steps employed in the preparation of the asymmetric composite membranes of Examples 1 to 9 was assessed (Table 16). Comparative Example 1 (C1) was prepared excluding the crosslinked (glutaraldehyde) from the solution of poly(vinyl alcohol). Comparative Example 2 (C2) was prepared using microporous poly(ethylene) hydrophilicitised by UV initiated grafting of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) as the substrate, i.e. excluding the film of crosslinked, sulfonated poly(ether ether ketone) of the composite substrate used in the preparation of the asymmetric composite membranes of Examples 1 to 9. Comparative Example 3 (C3) was prepared with a single coating of the poly(vinyl alcohol) solution. Comparative Example 4 (C4) was prepared using an increased
concentration (0.5 mL) of cross-linking agent (glutaraldehyde). Comparative Example 5 (C5) was a sample of composite substrate, i.e. excluding the poly(vinyl alcohol) coating.
Although the invention has been described with reference to embodiments or samples it should be appreciated that variations and modifications may be made to these embodiments or samples 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. In particular, variations and modifications to the embodiments or samples 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.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2014905278 | Dec 2014 | AU | national |
This application is a Continuation-in-Part of U.S. application Ser. No. 16/298,264 filed Mar. 11, 2019, which is a continuation-in-part of U.S. application Ser. No. 15/539,903 filed Jun. 26, 2017 and issued as U.S. Pat. No. 10,335,742 on Jul. 2, 2019 and is a National Stage Application, filed under 35 U.S.C. § 371, of International Patent Application No. PCT/IB2015/060001 filed Dec. 28, 2018, which claims priority to Australian Application No. 2014905278 filed Dec. 24, 2014. This application is also a continuation-in-part of U.S. application Ser. No. 16/338,786 filed Apr. 2, 2019, which is a National Stage Application, filed under 35 U.S.C. § 371, of International Patent Application No. PCT/IB2016/05589 filed Oct. 3, 2016. Each of the previously noted applications is hereby incorporated by reference herein in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16298264 | Mar 2019 | US |
| Child | 17653900 | US | |
| Parent | 15539903 | Jun 2017 | US |
| Child | 16298264 | US | |
| Parent | 16338786 | Apr 2019 | US |
| Child | 15539903 | US |
