CURED EPOXYSILICONE LAYER MEMBRANE FOR NANOFILTRATION

Abstract

Processes for the preparation of composite membranes are disclosed, as well as the composite membranes obtainable by these processes. The processes employ a step of roller coating a porous support substrate with an essentially solventless coating mixture containing a cationically UV curable compound, which can then be cured in an oxygen-containing atmosphere. The process thereby dispenses withor greatly reduces the impact ofa number of the prominent processing constraints of prior art techniques, thereby affording a more streamlined and less energetically burdensome membrane manufacturing process.

Claims

1. A process for the preparation of a composite membrane, the process comprising the steps of: a) providing a porous support substrate, the porous support substrate having an upper major surface and a lower major surface; b) providing a coating mixture comprising: i. a photoinitiator, and ii. a UV-curable compound bearing one or more groups capable of undergoing cationic UV curing, wherein the coating mixture has a viscosity at 25 C. of 10-1000 cP and comprises less than 50% by weight of a solvent relative to the total weight of the coating mixture, and and wherein the photoinitiator and the UV-curable compound are such that the coating mixture is cationically curable upon exposure to UV radiation; c) applying a film of the coating mixture to the upper major surface of the porous support substrate to provide an uncured membrane assembly; d) subjecting the uncured membrane assembly to UV radiation in an oxygen-containing atmosphere to cause the film of coating mixture to cure; wherein in step c), the coating mixture is transferred from the surface of a rotating first roller to the upper major surface of the porous support substrate.

2. The process of claim 1, wherein the porous support substrate is polymeric.

3. The process of claim 1 or 2, wherein the porous support substrate is formed from one or more polymers selected from the group consisting of polyacrylonitrile, polyetherimide, polyimide, polyaniline, polyester, polyethylene, polypropylene, polyether ether ketone, polyphenylene sulphide, Ethylene-ChloroTriFluoroEthylene copolymer and crosslinked derivatives thereof.

4. The process of any one of claim 1, 2 or 3, wherein the porous support substrate is formed from one or more polymers selected from the group consisting of polyacrylonitrile, polyetherimide, polyimide, polyether ether ketone and crosslinked derivatives thereof.

5. The process of any preceding claim, wherein the porous support substrate is provided on a porous substructure, the porous substructure being in contact with the lower major surface of the porous support substrate.

6. The process of claim 5, wherein the porous substructure is a non-woven material.

7. The process of any preceding claim, wherein the photoinitiator is a cationic photoinitiator.

8. The process of any preceding claim, wherein the photoinitiator is an organic salt of a non-nucleophilic anion.

9. The process of claim 8, wherein the anion is selected from the group consisting of BF.sup.4, PF.sup.6, SbF.sup.6 and AsF.sup.6.

10. The process of claim 8 or 9, wherein the organic salt is a diaryliodonium salt.

11. The process of any preceding claim, wherein the one or more groups capable of undergoing cationic UV curing are selected from the group consisting of epoxy, oxetane, lactone and vinyl ether.

12. The process of any preceding claim, wherein the one or more groups that are capable of undergoing cationic UV curing is, or comprises, any one or more or the following moieties: ##STR00010## ##STR00011##

13. The process of any preceding claim, wherein the UV-curable compound is a siloxane bearing the one or more groups capable of undergoing cationic UV curing.

14. The process of claim 13, wherein the siloxane is a poly(siloxane) or a cyclic siloxane.

15. The process of any preceding claim, wherein the UV-curable compound has a structure according to formula (I) shown below: ##STR00012## wherein each R.sub.1 is independently (1-3C)alkyl, each R.sub.2 is independently (1-3C)alkyl or a moiety capable of undergoing cationic UV curing as defined in claim 12, each R.sub.3 is independently (1-3C)alkyl or a moiety capable of undergoing cationic UV curing as defined in claim 12, a ranges from 1 to 100, b ranges from 1 to 100, with the proviso that at least one R.sub.2 or R.sub.3 is a moiety capable of undergoing cationic UV curing as defined in claim 12.

16. The process of any preceding claim, wherein the UV-curable compound is one or more compounds selected from: ##STR00013##

17. The process of any preceding claim, wherein the weight ratio of UV-curable compound to photoinitiator in the coating mixture ranges from 95:5 to 99.99:0.01.

18. The process of any preceding claim, wherein coating mixture has a viscosity at 25 C. of 10-800 cP.

19. The process of any preceding claim, wherein coating mixture has a viscosity at 25 C. of 25-650 cP.

20. The process of any preceding claim, wherein coating mixture has a viscosity at 25 C. of 25-400 cP.

21. The process of any preceding claim, wherein the solvent that may be present in the coating mixture is an organic solvent.

22. The process of any preceding claim, wherein the coating mixture comprises less than 40% by weight of a solvent relative to the total weight of the coating mixture.

23. The process of any preceding claim, wherein the coating mixture comprises less than 25% by weight of a solvent relative to the total weight of the coating mixture.

24. The process of any preceding claim, wherein the coating mixture comprises less than 10% by weight of a solvent relative to the total weight of the coating mixture.

25. The process of any preceding claim, wherein the coating mixture comprises less than 5% by weight of a solvent relative to the total weight of the coating mixture.

26. The process of any preceding claim, wherein the coating mixture comprises substantially no solvent or no solvent.

27. The process of any preceding claim, wherein the surface of the rotating first roller comprises one or more depressions (e.g. grooves, dimples, notches or furrows).

28. The process of claim 27, wherein the one or more depressions have a total volume of 0.01-100 cm.sup.3 per m.sup.2 of the surface of the rotating first roller.

29. The process of any preceding claim, wherein during step c) at least a portion of the surface of the rotating first roller is in constant contact with a quantity of the coating mixture contained within a reservoir.

30. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate is metered using a doctor blade or a second roller.

31. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate during step c) is less than 50 g per square metre of the porous support substrate.

32. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate during step c) is less than 10 g per square metre of the porous support substrate.

33. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate during step c) is less than 1 g per square metre of the porous support substrate.

34. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate during step c) is less than 0.60 g per square metre of the porous support substrate.

35. The process of any preceding claim, wherein the quantity of coating mixture applied to the upper major surface of the porous support substrate during step c) is less than 0.55 g per square metre of the porous support substrate.

36. The process of any preceding claim, wherein step d) is conducted in an atmosphere containing greater than 1 vol % oxygen.

37. The process of any preceding claim, wherein step d) is conducted in an atmosphere containing greater than 10 vol % oxygen.

38. The process of any preceding claim, wherein step d) is conducted in air.

39. The process of any preceding claim, wherein the cured composite membrane resulting from step d) is subjected to electron beam treatment.

40. The process of any preceding claim, wherein the process is a continuous process.

41. A composite membrane obtainable, obtained or directly obtained by the process of any preceding claim.

42. A composite membrane comprising: a porous support substrate having an upper major surface and a lower major surface, and a polymeric separating layer disposed on the upper major surface of the porous support substrate and in contact therewith, wherein the polymeric separating layer comprises the polymerisation product of: i. a photoinitiator, and ii. a cationically UV-curable compound, and wherein the mass of polymeric separating layer is less than 10 g per square metre of the porous support substrate.

43. The composite membrane of claim 42, wherein the porous support is as defined in any one of claims 1-40.

44. The composite membrane of claim 42 or 43, wherein the photoinitiator is as defined in any one of claims 1-40.

45. The composite membrane of claim 42, 43 or 44, wherein the cationically UV-curable compound is as defined in any one of claims 1-40.

46. The composite membrane of any one of claims 42 to 45, wherein the mass of polymeric separating layer is less than 0.55 g per square metre of the porous support substrate.

47. The composite membrane of any one of claims 42 to 46, wherein the membrane has a molecular weight cut-off (MWCO) in the region of 200-5000 g mol.sup.1.

48. Use of a composite membrane as claimed in any one of claims 41 to 47 for performing a molecular separation process.

49. The use of claim 48, wherein the molecular separation process is a nanofiltration process.

Description

EXAMPLES

[0120] One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:

[0121] FIG. 1 shows a schematic of a gravure coating process.

[0122] FIG. 2 shows a silicone coated membrane of Example 5 with nominal thickness of 1.5 micron.

[0123] FIG. 3 shows a silicone coated membrane of Example 3 with nominal thickness below 0.5 micron.

[0124] FIG. 4 shows a silicone coated membrane of Example 11 with coat weight of 25 g m.sup.2.

[0125] FIG. 5 shows a silicone coated membrane of Example 6.

[0126] FIG. 6 shows a cross section image of the composite membrane prepared in Example 10 (top), with a corresponding light microscope image (bottom).

[0127] FIG. 7 shows the MWCO curve and heptane flux of the composite membrane described in Example 5.

[0128] FIG. 8 shows the MWCO curve and toluene flux of composite membranes prepared in Example 7 that were further subjected to electron beam radiation.

[0129] FIG. 9 shows the MWCO curve and toluene flux of composite membranes prepared in Example 9 that were further subjected to electron beam radiation.

[0130] FIG. 10 shows the MWCO curves in toluene and heptane of a composite membrane prepared in Example 10.

MATERIALS AND METHODS

Materials

[0131] The following materials were used in the examples:

Ultem 1000 is a polyetherimide (Sabic)
Polyacrylonitrile (230 k) was obtained from Goodfellow
ECMS-924 is an [8-10% (epoxycyclohexylethyl)methylsiloxane]-dimethylsiloxane copolymer having a viscosity of 300-450 cSt (Gelest)
ECMS-327 is an [3-4% (epoxycyclohexylethyl)methylsiloxane]-dimethylsiloxane copolymer having a viscosity of 650-850 cSt (Gelest)
Speedcure 937 is an iodonium hexafluoroantimonate salt (Lambson Limited)
Omnicat 445 is a iodonium hexafluorophosphate salt (IGM Resins)
1,3-bis(3,4-epoxycyclohexyl-1-ethyl)tetramethyldisiloxane (Gelest)

SEM Measurements

[0132] Membrane samples were freeze fractured and analysed by a high resolution scanning electron microscope (SEM), LEO 1525, Karl Zeiss.

Membrane MWCO and Flux

[0133] Flux and rejection measurements were used to characterise the performance of the fabricated membranes of the present invention. A laboratory scale cross-flow nanofiltration unit was used with 8 cross flow cells in series. Membrane discs of active area 14 cm.sup.2 were used. A 2 L feed tank was charged with a feed solution consisting of 1 g of styrene oligomers of nominal molecular weight 580 g mol.sup.1 and 1 g of styrene oligomers of nominal molecular weight 1000 g mol.sup.1 (Agilent) and 0.1 g of -methylstyrene dimer (Sigma Aldrich, UK). The styrene oligomers were all fully soluble in the tested solvents at this concentration and the feed solution was re-circulated at a flow rate of 120 L h.sup.1 using a diaphragm pump (Hydra-Cell, Wanner, USA). Pressure in the cells was generated using a backpressure regulator which was located downstream of a pressure gauge. The re-circulating liquid was kept at 30 C. by a heat exchanger. During operation, permeate samples were collected from individual sampling ports for each cross-flow cell and the retentate sample was taken from the feed tank. The solvent flux N.sub.v was calculated from the equation:

[00002]$\begin{array}{cc}{N}_v=\frac{V}{\mathrm{At}}& \left(1\right)\end{array}$

where V=volume of a liquid sample collected from the permeate stream from a specific cross-flow cell, t=time over which the liquid sample is collected, A=membrane area. Polystyrene rejection was measured using an Agilent HPLC machine. A reverse phase column (C18-300, 250 mm4.6 mm, ACE Hichrom) was used and the mobile phases were 10% THF and 90% MeOH. The HPLC pump flow rate was set at 1 ml min.sup.1 and the column temperature was set at 30 C. The rejection, R.sub.i, was calculated via the following equation:

[00003]$\begin{array}{cc}{R}_i=\left(1-\frac{c_{p,i}}{c_{f,i}}\right)100& \left(2\right)\end{array}$

where c.sub.p,i is the concentration of solute in permeate, and c.sub.f,i is the concentration of solute in the feed.

[0134] Testing was confined to either n-heptane or toluene with the polystyrenes as described above. Prior to HPLC analysis, a solvent swap was conducted of the polystyrenes to acetonitrile through evaporation of test solvent. In additional cases, diphenylanthracene (330 Da) was used as a solute at levels of up to 50 ppm in toluene or heptane within the same experimental set up, except that UV-Vis was used to analyze the concentration, with the rejection being calculated by Equation 2.

Example 1Preparation of PAN Support Membrane

[0135] A polyacrylonitrile (PAN) ultrafiltration membrane was prepared by creating a polymer solution of PAN:DMSO:1,3 dioxolane at a mass ratio of 22:89:89. This mixture was heated overnight at 75 C. Upon cooling the polymer solution was subject to two filtration steps (firstly 41 micron filter, and subsequently an 11 micron filter) through a nitrogen pressurised filtration cell (Merck Millipore, XX4004740) at pressures of up to 70 psi. The resultant polymer solution appeared free of particulates and had a viscosity of 20,000 cP. The membrane was cast on to a PET non woven backing material on a continuous casting machine so that the cast polymer film was subject to 30 seconds of atmospheric exposure prior to immersion into a water bath. The membrane was then dried. The membrane exhibited a mean flow pore size in the range of 18-25 nm with a pure heptane permeance of several hundred I m.sup.2 h.sup.1 bar.sup.1 as characterised by liquid liquid porometry (Porolux 1000).

Example 2Preparation of Crosslinked PEI Support Membrane

[0136] A solvent stable ultrafiltration membrane from Ultem 1000 polyetherimide was prepared by dissolving the Ultem 1000 in a 50:50 mixture of DMSO:1,4 Dioxane at 15 wt %. The powder dissolved readily and was cast on to a PET non-woven backing on a continuous casting machine at 8 metres/minute. The dope and nonwoven were immersed in water, and then transferred to IPA. The ultrafiltration membrane was then placed into a reactor vessel with 10 litre capacity, and propanediamine was added to the vessel at 0.5 wt %. The vessel was then heated to 60 C. by means of a heated jacket and left for 4 hours. The crosslinked membrane was then cooled and washed with IPA, and further dried. The membrane remained flexible in the dry state, and exhibited a mean flow pore size similar to that of the PAN membrane described in Example 1. The degree of crosslinking as measured by FTIR through the conversion of imide to amide groups was around 50%.

Example 3Gravure Coating on PAN Support Membrane

[0137] An epoxysilicone co polymer (ECMS-924, Gelest), characterised with 8-10 mol % epoxy was mixed with an antimonate based photoinitiator (Speedcure 937, Lamsbon chemicals) at a ratio of 99:1 polymer:initiator. After thorough mixing, this solution was filtered through a 0.65 m DVPP filter (Merck Millipore), subsequently sonicated and subjected to vacuum filtration ready for coating. The solution viscosity was 400 cP. A PAN ultrafiltration membrane described in Example 1 was wound into a pilot scale coating machine (RK Print, UK) that contained a gravure coating cylinder (1,900 Ipi with a nominal volume capacity of 1 cm.sup.3 m.sup.2) and a UV lamp (GEW, UK). The gravure coating head was operated in the forward configuration with the use of an impression roller (40 degree EPDM rubber) at 40 psi. The web was run through the machine such that the active side of the UF membrane was in contact with the gravure cylinder as it passed through the nip point at a speed of 4 m/min, and then immediately passed through a UV lamp employing a dosage to the substrate >500 mJ/cm.sup.2. The resultant film appeared tack free after leaving the UV lamp. A typical cross section SEM image of this membrane can be seen in FIG. 3.

Example 4Gravure Coating on Crosslinked PEI Support Membrane

[0138] The epoxysilicone coating mixture described in Example 3 was coated on to the crosslinked Ultem 1000 substrate via similar machine conditions. The intensity of the UV lamp was increased, such that the dose was this time >2,000 mJ/cm.sup.2 to render the coated film somewhat tack free.

Example 5Gravure Coating on PAN Support Membrane

[0139] The procedure described in Example 3 was repeated, except that a gravure head engraved at 400 Ipi was utilised, with a nominal volume capacity of 5 cm.sup.3 m.sup.2.

[0140] An SEM image of the membrane is shown in FIG. 2, and the corresponding membrane performance in heptane with polystyrenes is shown in FIG. 7.

Example 6Blend Coating on PAN Support Membrane

[0141] A blend of epoxysilicone co-polymers (ECMS-924:ECMS-327) was prepared at a mass ratio of 6:4, and mixed with the antimonate based photoinitiator (Speedcure 937) at the same ratio as described previously (99:1). The resultant solution appeared more cloudy than that described in Example 3 and exhibited a viscosity of 600 cP. This formulation was coated on to the PAN substrate following the methodology described in Example 3, except that a gravure head engraved at 400 Ipi was utilised, with a nominal volume capacity of 5 cm.sup.3 m.sup.2. With a single pass through the UV lamp, the resultant film appeared tack free. A cross section SEM image of this membrane can be seen in FIG. 5.

Example 7Blend Coating on PAN Support Membrane

[0142] An epoxysilicone co polymer (ECMS-924) was mixed with an epoxysilicone monomer (1,3-bis(3,4-epoxycyclohexyl-1-ethyl)tetramethyldisiloxane, Gelest) at a ratio of 8:2 polymer:monomer, and mixed with the antimonate based photoinitiator (Speedcure 937) at the same ratio as described previously (99:1). The resultant solution exhibited a viscosity of 250 cP. This formulation was coated on to the PAN substrate following the same methodology in Example 3. With a single pass through the UV lamp, the resultant film appeared tack free. The composite membrane was characterised by the fact that there was a higher level of intrusion as measured by SEM-EDS of the silicone coating into the support membrane than in previous examples.

[0143] Some membrane sheets from this coating run were additionally exposed to electron beam radiation (EB lab system, ebeam technologies, USA). Details of the applied dosage are given in the following table:

TABLE-US-00001 TABLE 1 Different electron beam treatments applied to Example 7 composite membranes Membrane # Dose (kGy) Accelerating Voltage (eV) 1 n/a n/a 2 50 80 3 75 80 4 100 80

[0144] The MWCO curve of these membranes in toluene is shown in FIG. 8.

Example 8Gravure Coating Under Heating

[0145] Example 3 was repeated, except this time prior to coating, the solution and coating head were heated to 80 C. in an oven. Upon removal from the oven, the coating process was quickly conducted to minimise heat losses. At the elevated temperature that this coating was conducted, the viscosity of the same formulation described in Example 3 is roughly half.

Example 9Gravure Coating on Crosslinked PEI Support Membrane

[0146] A commercially available epoxysilicone coating solution (Silicolease UV Poly 205, Bluestar silicones) was coated on to the crosslinked Ultem 1000 substrate via the same methodology given in Example 3, with the UV lamp intensity set such that the applied dosage on the substrate >2,000 mJ/cm.sup.2. Some membrane sheets from this coating run were additionally exposed to electron beam radiation (EB lab system, ebeam technologies, USA). Details of the applied dosage are given in the following table:

TABLE-US-00002 TABLE 2 Different electron beam treatments applied to Example 9 composite membranes Membrane # Dose (kGy) Accelerating Voltage (eV) 1 n/a n/a 2 100 80 3 200 80

[0147] The MWCO curve of these membranes in toluene is shown in FIG. 9

Example 10Gravure Coating on PAN Support Membrane

[0148] To compare, the commercially available epoxysilicone coating solution (Silicolease UV Poly 205, Bluestar silicones) utilised in Example 9 was also coated on to PAN substrate via the same methodology given in Example 3. The resultant membrane had a silicon active layer of around 500 nm as verified by SEM in FIG. 6. Light microscopy revealed that the coating had spread uniformly and contained a minimal amount of defects. The MWCO curves of this membrane in heptane and in toluene is given in FIG. 10.

Example 11Gravure Coating on PAN Support Membrane

[0149] A repeat coating was conducted via the same procedure as described in Example 3, except that a gravure head engraved at 55 Ipi was utilised, with a nominal volume capacity of 50 cm.sup.3 m.sup.2. An SEM image of the membrane is shown in FIG. 4, which had <0.1 I m.sup.2 h.sup.1 bar.sup.1 permeance for either toluene or heptane.

[0150] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.