ASYMMETRIC COMPOSITE MEMBRANES AND MODIFIED SUBSTRATES USED IN THEIR PREPARATION

Abstract

Durable asymmetric composite membranes comprising of a film of cross-linked poly(ether ether ketone) adhered to a sheet of hydrophilicitized microporous polyolefin are disclosed. The membranes are suitable for use in the recovery or removal of water from feed streams where repeated clean-in-place protocols are required such as in the processing of dairy products. The membranes are also suitable for use in the preparation of durable asymmetric composite membranes with improved rejection characteristics.

Claims

1. A method of recovering water from a feed stream comprising exposing a surface of a membrane to the feed stream at a pressure sufficient to produce a permeate, where the membrane comprises: (a) a water-wettable microporous sheet of grafted polyolefin; (b) a film of cross-linked sulfonated poly(ether ether ketone) adhered to the sheet; and (c) a coating of cross-linked poly(vinyl alcohol) adhered to the film, and the exposed surface of the membrane is the coating.

2. The method of claim 1 where the sulfonated poly(ether ether ketone) is crosslinked with a cross-linking agent comprising divinylbenzene.

3. The method of claim 2 where the poly(vinyl alcohol) is crosslinked with a cross-linking agent comprising glutaraldehyde.

4. The method of claim 3 where the polyolefin is grafted with a hydrophilicitizing agent selected from the group consisting of: 2-acrylamido-1-methyl-2-propanesulfonic acid (AMPS), 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA) and 4-ethenyl-benzenesulfonic acid (SSS).

5. The method of claim 4 where the polyolefin is grafted with the hydrophilicitizing agent 4-ethenyl-benzenesulfonic acid (SSS).

6. The method of claim 5 where the sulfonated poly(ether ether ketone) is crosslinked with a cross-linking agent comprising a hydrophilicitizing agent selected from the group consisting of: 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA); 4-ethenyl-benzenesulfonic acid (SSS), and allyl oxyethanol (AOE).

7. The method of claim 6 where the sulfonated poly(ether ether ketone) is crosslinked with a cross-linking agent comprising the hydrophilicitizing agent 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA).

8. The method of claim 7 where the polyolefin is poly(ethylene).

Description

BRIEF DESCRIPTION OF FIGURES

[0093] FIG. 1. Comparison of the FTIR spectra obtained for Sample A (lower trace), Sample C (middle trace) and Sample D (upper trace). An FTIR spectrum was not obtained for Sample B.

[0094] FIG. 2. Comparison of the contact angles determined for Sample A (Ally [sic] alcohol), Sample C (HEMA), Sample D (SSS) and Sample B (Acrylic acid) before () and after exposure to an acid () or alkali (.square-solid.) environment.

[0095] FIG. 3. Comparison of the permeability determined for Sample A (Allyl alcohol), Sample C (HEMA), Sample D (SSS) and Sample B (Acylic [sic] Acid) before () and after exposure to an acid () or an alkali (.square-solid.) environment relative to the permeability of the unmodified polyolefin substrate (□).

[0096] FIG. 4. Correspondence between contact angle and permeability determined for samples before (.diamond-solid.) and after exposure to an acid (.box-tangle-solidup.) or an alkali (.square-solid.) environment. The outlier is Sample D (SSS) after exposure to an alkali (.square-solid.) environment.

[0097] FIG. 5. Water absorption determined for Sample A (Allylic alcohol), Sample B (Arylic [sic] Acid), Sample C (HEMA) and Sample D (SSS).

[0098] FIG. 6. The dry weight increase determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0099] FIG. 7. The water absorption determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0100] FIG. 8. The contact angles determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0101] FIG. 9. The bubble points determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0102] FIG. 10. The sodium rejection determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0103] FIG. 11. The milk flux determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0104] FIG. 12. The total milk solids rejection determined for Sample E (SSS), Sample F (AA), Sample G (HEMA) and the untreated microporous polyethylene substrate (CELGARD™ K2045).

[0105] FIG. 13. Exploded view of the filter assembly (Sterlitech Corp.) used in the flux testing of samples of sheets of hydrophilic microporous polyethylene and asymmetric composite membrane.

[0106] FIG. 14. Flux (LMH) (.diamond-solid.) and total solids rejection (%) (.square-solid.) for Sample 1 during repeated CIP protocols (10× according to the schedule provided in Table 8). The feed stream was whole milk.

[0107] FIG. 15. Lactose rejection (%) detected by FTIR for Sample 2 during sequential CIP protocols (10 times according to the schedule provided in Table 7 followed by 12 times according to the schedule provided in Table 8), drying of the sample and further CIP protocols (8 times according to the schedule provided in Table 8). The feed stream was whole milk.

[0108] FIG. 16. Flux (LMH) (.diamond-solid.) and total solids rejection (%) (.square-solid.) for Sample 3 during repeated CIP protocols (25× according to the Schedule provided in Table 8. The feed stream was whole milk.

[0109] FIG. 17. Flux (LMH) (.diamond-solid.) and total solids rejection (%) (.square-solid.) for Sample 4 during repeated CIP protocols (17 times according to the schedule provided in Table 8). The feed stream was whole milk.

[0110] FIG. 18. Flux (LMH) (X) for Sample 5 measured over a period of eight hours using raw milk as the feed stream.

[0111] FIG. 19. Comparison of the total solids rejection (%) for Sample 6 and Sample 1 before (left hand bar) and after (right hand bar) a single CIP protocol according to the schedule provided in Table 8. The feed stream was whole milk.

[0112] FIG. 20. Sodium chloride (NaCl) rejection (%) by Samples 7 to 10 of an asymmetric composite membrane prepared using different ratios of sPEEK and DVB in the preparation of the rejection layer. The feed stream was whole milk.

[0113] FIG. 21. Lactose rejection (%) by Samples 7 to 10 of an asymmetric composite membrane prepared using different ratios of sPEEK and DVB in the preparation of the rejection layer. The feed stream was whole milk.

[0114] FIG. 22. Total solids rejection (%) by Samples 7 to 10 of an asymmetric composite membrane prepared using different ratios of sPEEK and DVB in the preparation of the rejection layer. The feed stream was whole milk.

[0115] FIG. 23. Flux (LMH) for Samples 7 to 10 of an asymmetric composite membrane prepared using different ratios of sPEEK and DVB in the preparation of the rejection layer. The feed stream was either deionised water (.diamond-solid.) or whole milk (⋄).

[0116] FIG. 24. Comparison of the sodium chloride (NaCl) rejection (%) for Sample 11 of an asymmetric composite membrane prepared using a different combination of solvent and hydrophilicitizing agent.

[0117] FIG. 25. Flux (LMH) for Sample 11 of an asymmetric composite membrane prepared using a different combination of solvent and hydrophilicitizing agent.

[0118] FIG. 26. Comparison of the sodium chloride (NaCl) rejection (%) (coarse diagonal hatching), flux (LMH) (fine diagonal hatching) and sucrose rejection (%) (medium diagonal hatching) for Sample 13 of an asymmetric composite membrane prepared using unmodified μPE as the backing layer and a sample of a symmetric composite membrane prepared using hydrophilic μPE as the backing layer.

[0119] FIG. 27A. FTIR spectra of poly(vinyl alcohol) (PVA) before (a; lower trace) and after (b; upper trace) crossliking with glutaraldehyde (GA). Spectra were recorded using a Thermo Electron Nicolet 8700 Fourier transform infrared spectrometer equipped with a single bounce ATR and diamond crystal. An average of 32 scans with a 4 cm.sup.−1 resolution were recorded for each sample.

[0120] FIG. 27B. FTIR spectra of a sample of asymmetric composite membrane after four clean-in-place (CIP) protocols. Spectra were recorded using a Thermo Electron Nicolet 8700 Fourier transform infrared spectrometer equipped with a single bounce ATR and diamond crystal. An average of 32 scans with a 4 cm.sup.−1 resolution were recorded for each sample.

[0121] FIG. 28. Flux rates (.square-solid.) and rejections (.circle-solid.) of (A) sodium chloride (NaCl) and (B) magnesium sulfate (MgSO.sub.4) for a sample of the asymmetric composite membrane (Example 4) following repeated clean-in-place (CIP) protocols.

[0122] FIG. 29. Flux rate (.square-solid.) and rejection of sucrose (.circle-solid.) for a sample of the asymmetric composite membrane (Example 4) following repeated clean-in-place (CIP) protocols.

[0123] FIG. 30. Flux rates (.circle-solid.) and rejections (.box-tangle-solidup.) of (A) sodium chloride (NaCl) and (B) magnesium sulfate (MgSO.sub.4) for a sample of the asymmetric composite membrane (Example 5) following repeated clean-in-place (CIP) protocols.

[0124] FIG. 31. Flux rate (.circle-solid.) and rejection of sucrose (.box-tangle-solidup.) for a sample of the asymmetric composite membrane (Example 5) following repeated clean-in-place (CIP) protocols.

[0125] FIG. 32. Flux rates (.circle-solid.) and rejections (.box-tangle-solidup.) of (A) magnesium sulfate (MgSO.sub.4) and (B) sucrose for a sample of the asymmetric composite membrane (Example 6) following repeated clean-in-place (CIP) protocols.

[0126] FIG. 33. Flux rate (.diamond-solid.) and rejection (.square-solid.) of monovalent salt, sodium chloride (NaCl) for a sample of the asymmetric composite membrane (Example 9) following repeated clean-in-place (CIP) protocols.

[0127] FIG. 34. Flux rate (.diamond-solid.) and rejection (.square-solid.) of divalent salt, magnesium sulfate (MgSO.sub.4) for a sample of the asymmetric composite membrane (Example 9) following repeated clean-in-place (CIP) protocols.

DETAILED DESCRIPTION

[0128] 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.

[0129] 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”.

[0130] 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.

[0131] 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.

[0132] 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”.

[0133] 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.

[0134] 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.

[0135] The Fourier transform infrared (FTIR) spectra obtained for PVA powder (FIG. 27A) and samples of the asymmetric composite membrane were consistent with cross-linking via acetalization. Notably not all available hydroxyl (-OH) groups of the PVA are consumed in the cross-linking process. This ensures the coating of cross-linked PVA retains a hydrophilic character. The FTIR spectra obtained for samples of membrane following repeated CIP protocols were consistent with stable cross-links being formed (FIG. 27B). The combination of a cross-linked poly(vinyl alcohol) coating on a film of poly(ether ether ketone) adhered to a sheet of hydrophilic microporous polyethylene provides a durable asymmetric composite membrane suitable for use in commercial processing operations with improved selectivity. Several methods of preparing these membranes will now be described.

[0136] Preparation of Hydrophilicitized Microporous Sheets of Poly(Ethylene)

[0137] 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).

[0138] 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.sup.−2 at a distance of 50 mm. The ultrasound washing was for five minutes followed by soaking at 45° C. for three hours.

[0139] 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.

[0140] Evaluation of Hydrophilicitized Macroporous Sheets of Poly(Ethylene)

[0141] Fourier Transform Infrared (FTIR)

[0142] 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.sup.−1 resolution was taken for all samples.

TABLE-US-00001 TABLE 1 Structure of alternative hydrophilicitizing agents. Hydrophilicitizing agents Structure 2-acrylamido-1-methyl-2-propanesulfonic acid (AMPS) [00001] 2-propen-1-ol (allyl alcohol) [00002] 2-propenoic acid (acrylic acid) [00003] 2-hydroxyethyl 2-methyl-2-propenoic acid ester (HEMA) [00004] 4-ethenyl-benzenesulfonic acid (as the sodium salt) (SSS) [00005]

[0143] Surface Analysis

[0144] 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.

[0145] Permeability and Flux Testing

[0146] Permeability was determined by measuring the flux in deionized water at various pressures starting at 20 bar and decreasing in 4 bar iterations. Flux J.sub.V was then graphed against effective pressure difference across the membrane, p.sub.eff, and the slope of the permeability L.sub.p.

[00001]$L_p=\frac{J_V}{\Delta p_{\mathrm{eff}}}$

[0147] Initial flux rates under pressure (20 bar) and no pressure were determined using the Sterlitech flux rig illustrated in FIG. 13 equipped with a PolyScience cooling unit. The samples were mounted in the flux cell and bolted. Deionized water was fed into the rig at 2.5 L min.sup.−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:

[00002]$J=\frac{V}{t\times A}$

[0148] where V is the permeate volume (L), t is the time (h) for the collection of V and A is area of the sample (m.sup.2) which was determined to be 0.014 m.sup.2.

[0149] 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”).

[0150] 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.

[00003]$\Delta m_{\mathrm{dry}}=\frac{m_{\mathrm{dry}}-m_{\mathrm{initial}}}{m_{\mathrm{initial}}}\times 100$

[0151] 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.

[00004]$\Delta m_{\mathrm{Wet}}=\frac{m_{\mathrm{wet}}-m_{\mathrm{initial}}}{m_{\mathrm{initial}}}\times 100$

[0152] 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.

[00005]$\%R_{\mathrm{TS}}=\left(\begin{array}{c}1-m_{p,\mathrm{TS}}\\ m_{f,\mathrm{TS}}\end{array}\right)\times 100$

[0153] where m.sub.p,Ts is total milk solids in the permeate and m.sub.f,Ts is the mass of milk total solids in the feed.

[0154] 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.

[00006]$\%R_{\mathrm{NaCl}}=\left(1-\frac{{\sigma }_p}{{\sigma }_f}\right)\times 100$

[0155] Where σ.sub.p is the conductivity of permeate and σ.sub.f is the conductivity of the feed.

[0156] 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.

[0157] Results

[0158] 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 FIG. 1). However, the ester and carbonyl groups of Sample C were clearly discernible. The hydroxyl group peaks of Sample A and Sample D were barely evident. The FTIR spectrum for Sample B was not determined.

[0159] The contact angles for samples A to D showed an inverse relationship with the permeability determined for the same sample (see FIGS. 2 to 4). Sample C was observed to have the lowest contact angle and the highest permeability prior to exposure to an acid or alkali environment. Following exposure to an acid environment the contact angle for Sample D increased. The contact angle of the unmodified polyolefin substrate (CELGARD™ K2045, Celgard LLC) was determined to be 89°, so modification of the surface tension is shown for all the samples despite the absence of definitive FTIR spectra. The observed initial flux rates were also consistent with modification of the polyolefin substrate (see Table 2).

TABLE-US-00002 TABLE 2 Initial flux rates of samples of modified polyolefin substrate (CELGARD ™ K2045, Celgard LLC). Initial flux (Lm.sup.2min.sup.−1) Sample No pressure Pressure (20 bar) A (Allyl alcohol) 50 484 B (Acrylic acid) 43 555 C (HEMA) 61 772 D (SSS) 44 577

[0160] All of samples A to D showed an increase in permeability compared to the unmodified membrane which measured 2.56 m s.sup.−1 Pa.sup.−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.sup.−1 Pa.sup.−1).

[0161] 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 FIG. 5).

[0162] 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.

[0163] 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.

[0164] 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.

[0165] The assessments of replicates (i, ii, iii, . . . ) of samples E, F and G are presented in Table 3 and FIGS. 6 to 12.

TABLE-US-00003 TABLE 3 Assessments of replicates of Samples E, F and G. Hydrophilicitizing Sample B. Pt Flux.sub.Milk agent (replicate) Δm.sub.dry Δm.sub.wet bar B. Pt.sub.CIP 1 Θ Flux.sub.DI % R.sub.NaCl (Lm.sup.-2hr.sup.-1) % R.sub.TS 4-ethyenyl- E(i)  9% 10% 4 0 32 429 2% 16 66% benzenesulfonic E(ii)  7% 13% 4 4 60 114 3% 15 71% acid, Na salt E(iii)  7% 155%  4 0 55 213 5% 15 65% (SSS) Acrylic acid F(i) 10% 158%  0 0 33 208 9% 13 72% (AA) F(ii) 13% 165%  0 0 32 167 13%  147  8% F(iii) 16% 158%  0 0 30 208 41%  12 71% 2-hydroxyethyl 2- G(i) 13% 64% 0 0 32 303 4% 20 50% methyl- G(ii) 14% 57% 0 0 35 405 3% 44 46% 2-propenoic acid ester G(iii) 10% 68% 4 4 27 147 4% 51 46% (HEMA) G(iv) 10% 68% 0 0 31 385 2% 97 16%

[0166] Sample F was observed to provide a water permeable membrane with the highest rejection of salt (sodium chloride) (FIG. 10) combined with a relatively high flux (FIG. 11) and rejection of total milk solids (FIG. 12). Based on this assessment the preparation of modified polyolefin substrates according to the general method using acrylic acid as the hydrophilicitizing agent is indicated for use as a membrane in the ultrafiltration of feed streams such as milk.

[0167] 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.

[0168] Preparation of an Asymmetric Composite Membrane

[0169] 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.

[0170] Rejection Layer

[0171] 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.

[0172] 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.sup.−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 (N.sub.2), atmosphere). The structures of DVB and alternative di- and tetra-ethenyl cross-linking agents are provided in Table 4.

[0173] Backing Layer

[0174] 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.

TABLE-US-00004 TABLE 4 Structures of cross-linking agents. Cross-linking agents Structure o-Divinylbenzene (o-DVB) [00006] m-Divinylbenzene (m-DVB) [00007] p-Divinylbenzene (p-DVB) [00008] Ethylene glycol dimethacrylate (EGDMA) [00009] glyoxal bis (diallyl acetal) (GBDA) [00010]

[0175] Asymmetric Composite Membrane

[0176] 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.sup.−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

TABLE-US-00005 TABLE 5 Rejection layer formulations and cure conditions used in the preparation of each of the samples. The rejection layer of sample 12 was prepared using 1:1 (v/v) acetone-water as solvent. sPEEK DVB SSS BP % solids Cure time Number of Sample DS % of solids Solvent (w/w) (s) applications 1 69 45 22 31 2 DMAc 12 90 1 2 69 45 22 31 2 DMAc 12 60 1 3 69 45 15 33 6 DMAc 15 90 2 4 >80 41 17 30 11 DMAc 15 90 1 5 69 45 15 33 6 DMAc 15 90 2 6 69 98 0 0 2 DMAc 15 90 1 7 69 70 21 0 9 DMAc 9 90 1 8 69 57 35 0 8 DMAc 9 90 1 9 69 47 46 0 6 DMAc 9 90 1 10 69 42 52 0 6 DMAc 9 90 1 11 >80 63 32 0 5 MeOH 29 90 1 12 >80 15 10 70 5 acetone/water 6 300 1 13 69 45 19 34 2 DMAc 15 90 2

TABLE-US-00006 TABLE 6 Backing layer formulations used in the preparations of each of the samples. All backing layers (except for sample 11 and sample 12) were prepared using 1:1 (v/v) acetone-water as solvent. Hydrophilicitizing H.A. BP % Cure agent % of solids time Number of Sample (H.A.) solids (w/w) (s) applications 1 AMPS 86 14 7 90 1 2 AMPS 86 14 7 600 2 3 SSS 86 14 7 90 1 4 SSS 86 14 7 90 1 5 SSS 86 14 7 90 1 6 SSS 86 14 7 90 1 7 SSS 86 14 7 90 1 8 SSS 86 14 7 90 1 9 SSS 86 14 7 90 1 10 SSS 86 14 7 90 1 11 n.a. n.a. n.a. n.a. n.a. n.a. 12 n.a. n.a. n.a. n.a. n.a. n.a. 13 SSS 86 14 7 90 1
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.

[0177] 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.

[0178] Evaluation of the Asymmetric Composite Membrane

[0179] The performance of the asymmetric composite membrane was evaluated using a reverse osmosis (RO) filter assembly of the type illustrated in FIG. 13. A section of the asymmetric composite membrane (1) was pre-wetted by dipping in distilled water and then placed on a coarse support mesh (2) located in the lower half (3) of the filter assembly housing, with a shim (4) optionally interposed. The section was placed with the rejection layer of the asymmetric composite membrane facing downwards. A fine mesh (5) located in the upper half of the filter assembly (6 housing was placed over the upper surface of the section of the asymmetric composite membrane (1). The filter assembly was sealed by sealing rings (7, 8) and held in a hydraulic press pressurised to 60 Bar. The inlet port (9) of the lower half of the filter assembly housing (3) was in fluid connection with a feed reservoir (not shown) from which a feed stream was pumped (425 rpm) at a rate to maintain the feed stream pressure measured on the pressure gauge (10). Permeate was collected from the outlet port (11) of the upper half of the filter assembly housing (6) in a graduated cylinder (not shown). Collection was started at least 5 minutes after the commencement of permeate being discharged from the outlet port (11) in order to exclude water from the pre-wetting of the membrane or permeate from previously used feed streams. Flow rates of approximately 2 L/min were obtained.

[0180] Permeability was determined by measuring the flux in deionized water at various pressures starting at 20 bar and decreasing in 4 bar iterations. Flux J.sub.V was then graphed against effective pressure difference across the membrane, p.sub.eff, and the slope of the permeability L.sub.p.

[00007]$L_p=\frac{J_V}{\Delta p_{\mathrm{eff}}}$

[0181] 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.sup.−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:

[00008]$J=\frac{V}{t\times A}$

where V is the permeate volume (L), t is the time (h) for the collection of V and A is area of the sample (m.sup.2) which was determined to be 0.014 m.sup.2.

[0182] 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).

[0183] The following measurements relating to the performance of the asymmetric composite membrane before and after repeated application of the CIP protocols were made: [0184] 1. initial flux rates with water or whole milk as the feed stream after equilibration for 30 minutes; [0185] 2. rejection levels for fat, lactose and protein; [0186] 3. total solids content; [0187] 4. salt (NaCl or Na2SO4) retention; and [0188] 5. Sucrose retention.

[0189] 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.

[0190] Sample 1

[0191] 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 (FIG. 14). Microscopic examination of the surface of the sample exposed to repeated CIP protocols indicated an increase in crystallinity of the membrane. It was found that increasing the concentration of the photoinitiator benzophenone (BP) used in the subsequent preparation of samples improved the reproducibility of these observations.

TABLE-US-00007 TABLE 7 Clean-in-place (CIP) protocol adapted from Anon (2014). Time Temperature Step Wash.sup.1 (min) (° C.) 1 Water 5 Ambient 2 Water 5 35 3 Alkali 10 35 4 Water 5 35 5 Acid 10 35 6 Water 5 Ambient 7 Alkali 10 35 8 Water 5 Ambient .sup.1alkali (2% (w/v) NaOH) and acid (1.9% (w/v) H.sub.2NO.sub.3 and 0.6 (w/v) H.sub.3PO.sub.4).

TABLE-US-00008 TABLE 8 Clean-in-place (CIP) protocol including 200 ppm free chlorine (as sodium hypochlorite). Time Temperature Step Wash.sup.1 (min) (° C.) 1 Water 5 Ambient 2 Water 5 35 3 Alkali 10  35 4 Water 5 35 5 Acid 10  35 6 Water 5 Ambient 7 Chlorine 10  35 8 Water 5 35 9 Water 1-2 35 10 Water 1-2 Ambient .sup.1alkali (2% (w/v) NaOH), acid (1.9% (w/v) H.sub.2NO.sub.3 and 0.6 (w/v) H.sub.3PO.sub.4) and chlorine (0.05% (w/v) sodium hydroxide and 0.09% (w/v) sodium hypochlorite).

[0192] Sample 2

[0193] 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 (FIG. 15).

TABLE-US-00009 TABLE 9 Performance of the samples of the asymmetric composite membrane measured at 20 bar. Deionised Standard milk water Rejection Flux Flux Rejection Rejection (total L/m.sup.2/h Sample L/m.sup.2/h (gfd) (NaCl) (lactose) solids) (gfd) 1 40 (11.7) 52 99 99 12.1 (3.5) 2 18.1 (5.3) 47 98 99 10.1 (3.0) 3 9.5 (2.8) 46 90 97 9.4 (2.8) 4 50 (14.7) 64 75 97 14.7 (4.3) 5 9.5 (2.8) 46 91 6 (1.8) 6 1051 (308) 82 13.5 (4.0) 7 3.3 (1.0) 19 42 73 8.7 (2.6) 8 56 (16) 17 91 83 12.4 (3.6) 9 65 (19) 13 59 79 14 (4.1) 10 107 (31) 5 32 71 12.7 (3.7) 11 1.6 (0.5) 50 n.a. n.a. n.a. 12 83 (24) 25 13 100 (29) 38

[0194] Sample 3

[0195] 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 (FIG. 16).

[0196] Sample 4

[0197] The sample was subjected to repeated CIP protocols (17×) and exhibited an unacceptable decline in the rejection of total solids (FIG. 17). The unacceptable performance of this sample was attributed to the high DS (greater than 80%) of the sPEEK used in the preparation of the rejection layer.

[0198] Sample 5

[0199] 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.

[0200] Sample 6

[0201] 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.

[0202] Samples 7 to 10

[0203] 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 (FIG. 20). The optimal lactose and total solids rejection was obtained for the sample with a molar ratio of sPEEK:DVB of 0.6 (FIGS. 21 and 22). The molar ratio of sPEEK:DVB that provided optimal flux was dependent on the feed stream (FIG. 23). For water the flux was highest for the sample with the lowest molar ratio of 0.3. For milk the flux was highest for the samples with the lower molar ratios. For both feed streams a high molar ratio of sPEEK:DVB was incompatible with a high flux.

[0204] Sample 11

[0205] 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 (FIG. 24) but at a negligible flux (FIG. 25).

[0206] Sample 12

[0207] 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.

[0208] Preparation of a Coated Composite Substrate

[0209] 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.

[0210] Method I

[0211] Preparation of Sulfonated Poly(Ether Ether Ketone)

[0212] 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.

[0213] Preparation of a Film of Semi-Cured Cross-Linked sPEEK

[0214] 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.sup.−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.

[0215] Preparation of a Hydrophilicitized Microporous Sheet of Poly(Ethylene)

[0216] 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.

[0217] Preparation of the Composite Substrate

[0218] 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.sup.−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.

Preparation of the Coated Composite Substrate—Example 1

[0219] 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 (H.sub.2SO.sub.4) 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.

Preparation of the Coated Composite Substrate—Example 2

[0220] 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 (H.sub.2SO.sub.4) 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.

Preparation of the Coated Composite Substrate—Example 3

[0221] 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 (H.sub.2SO.sub.4) 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.

[0222] Method II

[0223] Preparation of Sulfonated Poly(Ether Ether Ketone)

[0224] 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.

[0225] Preparation of a Composite Substrate

[0226] 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.

[0227] 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.

[0228] 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.

[0229] 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.

[0230] 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).

Preparation of the Coated Composite Substrate—Example 4

[0231] 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 (H.sub.2SO.sub.4) 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.

Preparation of the Coated Composite Substrate—Example 5

[0232] 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 (H.sub.2SO.sub.4) 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.

Preparation of the Coated Composite Substrate—Example 6

[0233] 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 (H.sub.2SO.sub.4) 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.

[0234] 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.

[0235] The membrane was washed with tap water and lifted from the glass plate before assessment.

[0236] Method III

[0237] Preparation of Sulfonated Poly(Ether Ether Ketone) [Sample 171214-17.5]

[0238] Sample 171214-17.5 was made using 450P PEEK at a concentration of 5% w/v PEEK to sulphuric acid (H.sub.2SO.sub.4). 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.

[0239] Preparation of a Composite Substrate

[0240] 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.

[0241] 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.

[0242] 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.

[0243] 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.

[0244] 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.

[0245] 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.

[0246] 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.

[0247] 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.

[0248] 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.

Preparation of the Coated Composite Substrate—Example 7

[0249] 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 (H.sub.2SO.sub.4) 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.

Preparation of the Coated Composite Substrate—Example 8

[0250] A volume of 0.25 mL of 98% H.sub.2SO.sub.4 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.

[0251] Method IV

[0252] This procedure uses a pre-cured, modified PE backing layer placed on a semi-cured rejection layer.

[0253] Preparation of Sulfonated Poly(Ether Ether Ketone) [Sample 24/11]

[0254] 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%).

[0255] 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.

[0256] Preparation of a Composite Substrate

[0257] 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.

[0258] 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.

[0259] 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.

[0260] 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.

[0261] 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.

[0262] 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.

[0263] 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.

[0264] 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.

Preparation of the Coated Composite Substrate—Example 9

[0265] The procedure for the preparation of Example 6 is used. A volume of 0.25 mL of 98% H.sub.2SO.sub.4 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.

[0266] Evaluation of the Coated Composite Substrate

[0267] The performance of samples of coated composite substrate selected from Examples 1 to 9 was evaluated.

[0268] Salt and Sucrose Rejection

[0269] The performance of the coated composite substrate was evaluated using a flux test unit of the type illustrated in FIG. 13 (Sterlitech Corporation, 22027 70th Avenue S, Kent, Wash. 98032-1911 USA). The sample of the asymmetric composite membrane (1) was pre-wetted by dipping in distilled water and then placed on a coarse support mesh (2) located in the lower half (3) of the flux test unit housing with a shim (4) optionally interposed. The sample was placed with the PVA coated side of the asymmetric composite membrane facing downwards. A fine mesh (5) located in the upper half of the filter assembly (6) housing was placed over the upper surface of the sample of the asymmetric composite membrane (1). The filter assembly was sealed by sealing rings (7, 8) and held in a hydraulic press pressurised to 60 Bar. The inlet port (9) of the lower half of the filter assembly housing (3) was in fluid connection with a feed reservoir (not shown) from which a feed stream was pumped at a rate to maintain the feed stream pressure measured on the pressure gauge (10). Permeate was collected from the outlet port (11) of the upper half of the filter assembly housing (6) in a graduated cylinder (not shown). Feed streams consisting of solutions in water of (i) 2000 ppm sodium chloride (NaCl) and 2000 ppm sucrose or (ii) 2000 ppm magnesium sulfate (MgSO.sub.4) were passed through the asymmetric composite membrane under pressure at a temperature of about 18° C. The permeate flux and rejection of salts and sucrose at a predetermined highest pressure (usually 20 Bar) were measured once the system had reached steady (typically after 1 h).

[0270] 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:

[00009]$J=\frac{V}{A.Math.t}$

[0271] 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:

[00010]$R=\left(1-\frac{P\sigma }{F\sigma }\right)\times 100$

[0272] Sucrose rejection was calculated based on the dry weights of residues obtained after evaporating the solvent from known volumes of feed and permeate samples.

[0273] In Situ Cleaning of Coated Asymmetric Substrate

[0274] 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.

Example 4

[0275] This sample of the asymmetric composite membrane exhibited initial rejections of 79.3% NaCl (at a flux of 0.64 LMH), 98% MgSO.sub.4 (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% MgSO.sub.4 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 FIGS. 29 and 30.

Example 5

[0276] This sample of the asymmetric composite membrane exhibited initial rejections of 73% NaCl, 93% MgSO.sub.4 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 FIGS. 32 and 33. The observed decline in the rejections may be attributed to the leaching of PVP from the polymer blend coating during repeated CIP protocols.

TABLE-US-00010 TABLE 10 Clean-in-place (CIP) protocol employed for the assessment of samples of coated composite substrate (Examples 1 to 6). Time Temperature Step Wash pH (min) (° C.) 1 Water 6-7 5 35 2 Water 6-7 5 35 3 Alkali 12 10  35 (2% (w/w) NaOH) 4 Water 6-7 5 35 5 Acid   1.5 10  35 (2% (w/w) H.sub.2NO.sub.3) 6 Water 6-7 5 35 7 1,000 ppm pH > 10 10  35 sodium hypochlorite 8 Water 6-7 5 35 9 Water 6-7 1-2 35 10 Water 6-7 1-2 Ambient

Example 6

[0277] This sample of the asymmetric composite membrane exhibited initial rejections of 88% NaCl, 100% MgSO.sub.4 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 MgSO.sub.4 and sucrose (cf. Example 4).

TABLE-US-00011 TABLE 11 Clean-in-place (CIP) protocol employed for the assessment of samples of coated composite substrate (Examples 7 to 9). Time Temperature Step Wash pH (min) (° C.) 1 Water 6-7 5 24 ± 3 2 Alkali   12 ± 0.5 5 31 ± 3 (2% (w/w) NaOH) 3 Water 12.1 ± 0.3 5 33 ± 2 4 Acid   2 ± 0.1 10 33 ± 3 (2% (w/w) HNO.sub.3) 5 Water  2.7 ± 0.3 5 26 ± 2 6 Sodium 12 ± 1 5 22 ± 2 Hypochlorite (200 ppm) 7 Water 10 ± 1 5 21 ± 2 8 Water 6-7 5 20 ± 2

Example 7

[0278] The results (Table 12) are from a sample [#060716-2] that survived 10 CIP cycles.

TABLE-US-00012 TABLE 12 Flux (LMH) and salt rejection (%) determined following repeated CIP protocols for samples of asymmetric composite membrane prepared according to Example 7. All values determined at 20 Bar. Flux [rejection] Flux Sodium chloride Magnesium sulfate DI (NaCl) (MgSO.sub.4) CIP 1 3.0 ± 0.1 6.4 ± 0.0 [53 ± 2] 7.1 ± 0.0 [82.2 ± 0.4] CIP 3 7.1 ± 0.0 [53.6 ± 0.1] 7.0 ± 0.1 [83.1 ± 0.1] CIP 5 7.6 ± 0.1 [50 ± 1] 7.6 ± 0.1 [83.1 ± 0.1] CIP 7 7.9 ± 0.1 [50.0 ± 0.2] 8.0 ± 0.0 [81 ± 1] CIP 9 8.5 ± 0.2 [52 ± 1] 8.6 ± 0.0 [83.5 ± 0.2] CIP 10 9.3 ± 0.2 [50 ± 1] 10.0 ± 0.2 [81 ± 2]

Example 8

[0279] 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.

TABLE-US-00013 TABLE 13 Flux (LMH) and salt rejection (%) determined following repeated CIP protocols for samples of asymmetric composite membrane prepared according to Example 8. All values determined at 20 Bar. Flux [rejection] Flux Sodium chloride Magnesium sulfate DI (NaCl) (MgSO.sub.4) CIP 1 9 ± 3  7 ± 4 [56 ± 11]  9 ± 3 [89 ± 5] CIP 3 11 ± 3 [57 ± 14]  12 ± 3 [87 ± 11] CIP 5 13 ± 4 [57 ± 13] 12 ± 4 [89 ± 5] CIP 7 13 ± 4 [56 ± 12] 14 ± 3 [90 ± 5] CIP 9 14 ± 5 [57 ± 10] 15 ± 5 [90 ± 3] CIP 10 13 ± 3 [55 ± 11] 14 ± 4 [89 ± 4]

[0280] 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 (MgSO.sub.4) or milk as the feed stream (Table 14).

TABLE-US-00014 TABLE 14 Flux (LMH) and salt rejection (%) determined for samples of the membrane (140616-1 and 140616-2) following drying with either a salt solution (MgSO.sub.4) or milk as the feed stream. All values determined at 20 Bar. Flux Flux [rejection] DI Magnesium sulfate (MgSO.sub.4) Milk 1.sup.st Dry  6 ± 1 6 ± 1 [93 ± 1] 5 ± 1 [95 ± 1 (total solids)] [95 ± 6 (lactose)] 2.sup.nd Dry 10 ± 7 7 ± 2 [93 ± 2] Not determined

Example 9

[0281] 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.

TABLE-US-00015 TABLE 15 Flux (LMH) and salt rejection (%) determined for various feed streams using a sample [#210815] of asymmetric composite membrane prepared according to Example 9 before and after a single CIP protocol. All values determined at 16 Bar. Flux Flux [rejection] DI NaCl MgSO.sub.4 Sucrose Milk — 1.7 ± 0.1 1.73 ± 0.03 2.50 ± 0.01 3 ± 0 2.0 ± 0.1 [42] [78] [83 ± 2] [93 ± 2] CIP 1 3.15 ± 0.02 1.91 ± 0.03 4.00 ± 0.04 [79] [86 ± 1] [94.5 ± 0.1]

Comparative Examples

[0282] 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

TABLE-US-00016 TABLE 16 Flux (LMH) and salt rejection (%) determined for samples of the Comparative Examples. All values determined at 20 Bar. Flux [rejection] Comparative Flux Sodium chloride Magnesium sulfate Example DI (NaCl) (MgSO.sub.4) C1 56 ± 39 48 ± 36 46 ± 28 [19 ± 11] [19 ± 10] C2 108 ± 81  123 ± 57  120 ± 57  [7 ± 5] [10 ± 7]  C3 39 ± 13 31 ± 10 35 ± 10 [45 ± 5]  [42 ± 3]  C4 8 ± 1 8 ± 1 9 ± 2 [61 ± 1]  [83 ± 5]  C5 65 ± 6  45 ± 4  65 ± 15 [35 ± 3]  [17 ± 1] 
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.

[0283] 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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