Process for preparing membranes

Abstract

A process for preparing a composite membrane comprising the steps of: a) applying a radiation-curable composition to a porous support; b) irradiating the composition and thereby forming a layer of cured polymer of thickness 20 to 400 nm on the support; c) forming a discriminating layer on the layer of cured polymer; and d) optionally forming a protective layer on the discriminating layer; wherein the radiation-curable composition comprises a partially crosslinked, radiation-curable polymer comprises dialkylsiloxane groups. Composite membranes are also claimed.

Claims

1. A process for preparing a composite membrane comprising the steps of: a) applying a radiation-curable composition to a porous support; b) irradiating the composition and thereby forming a layer of cured polymer of thickness 20 to 400 nm on the support; c) forming a discriminating layer on the layer of cured polymer; and d) optionally forming a protective layer on the discriminating layer; wherein the radiation-curable composition comprises a partially crosslinked, radiation-curable polymer comprising dialkylsiloxane groups.

2. The process according to claim 1 wherein the partially crosslinked, radiation-curable polymer is free from phenyl siloxane groups.

3. The process according to claim 1, which further comprises the step of preparing the partially crosslinked, radiation-curable polymer by thermally curing a composition comprising one or more curable components, at least one of which comprises a dialkylsiloxane group.

4. The process according to claim 1, wherein the discriminating layer comprises a polyimide, cellulose acetate, polyethyleneoxide or polyetherimide.

5. The process according to claim 1, wherein the discriminating layer comprises a polyimide comprising trifluoromethyl groups.

6. The process according to claim 1, wherein the radiation-curable composition comprises a cationic photoinitiator.

7. The process according to claim 1, which further comprises the step of preparing the partially crosslinked, radiation-curable polymer by a process comprising the reaction of epoxy groups with a crosslinking agent thereby forming the partially crosslinked, radiation-curable polymer.

8. The process according to claim 1, wherein the radiation-curable composition comprises an epoxy-modified polydimethyl siloxane.

9. The process according to claim 1, which further comprises the step of treating the cured polymer with corona discharge or a plasma treatment before forming the discriminating layer thereon.

10. The process according to claim 1, wherein: the radiation-curable composition is applied continuously to the porous support in step a) by means of a manufacturing unit comprising a radiation-curable composition application station, step b) is performed using an irradiation source located downstream from the radiation-curable composition application station, the discriminating layer is formed on the layer of cured polymer in step c) by a discriminating layer application station, and the resultant composite membrane is collected at a collecting station, wherein the manufacturing unit comprises a means for moving the porous support from the radiation-curable composition application station to the irradiation source and to the discriminating layer application station and to the composite membrane collecting station.

11. The process according to claim 10 wherein step a) and/or step b) is or are performed by curtain coating, meniscus type dip coating, kiss coating, pre-metered slot die coating, reverse or forward kiss gravure coating, multi roll gravure coating, spin coating and/or slide bead coating.

12. The process according to claim 1, wherein step a) and/or step b) is or are performed by curtain coating, meniscus type dip coating, kiss coating, pre-metered slot die coating, reverse or forward kiss gravure coating, multi roll gravure coating, spin coating and/or slide bead coating.

13. The process according to claim 1 wherein: (i) the partially crosslinked, radiation-curable polymer is free from phenyl siloxane groups; and (ii) the discriminating layer comprises a polyimide comprising trifluoromethyl groups; which process further comprises the step of preparing the partially crosslinked, radiation-curable polymer by thermally curing a composition comprising one or more curable components, at least one of which comprises a dialkylsiloxane group.

14. The process according to claim 13 wherein: the radiation-curable composition is applied continuously to the porous support in step a) by means of a manufacturing unit comprising a radiation-curable composition application station, step b) is performed using an irradiation source located downstream from the radiation-curable composition application station, the discriminating layer is formed on the layer of cured polymer in step c) by a discriminating layer application station, and the resultant composite membrane is collected at a collecting station, wherein the manufacturing unit comprises a means for moving the porous support from the radiation-curable composition application station to the irradiation source and to the discriminating layer application station and to the composite membrane collecting station.

15. The process according to claim 14 wherein step a) and/or step b) is or are performed by curtain coating, meniscus type dip coating, kiss coating, pre-metered slot die coating, reverse or forward kiss gravure coating, multi roll gravure coating, spin coating and/or slide bead coating.

16. A composite membrane comprising: a. a porous support; b. a layer of radiation-cured polymer of thickness 20 to 400 nm present on the porous support; c. a discriminating layer present on the layer of radiation-cured polymer; and d. optionally a protective layer present on the discriminating layer; wherein the layer of radiation-cured polymer comprises dialkylsiloxane groups.

17. The composite membrane according to claim 16 wherein the discriminating layer comprises a polyimide, cellulose acetate, polyethyleneoxide or polyetherimide.

18. The composite membrane according to claim 16 wherein the discriminating layer comprises a polyimide comprising trifluoromethyl groups.

19. The composite membrane according to claim 16 wherein the layer of radiation-cured polymer is free from phenyl siloxane groups.

20. A cartridge comprising the composite membrane according to claim 16 wherein the cartridge is of plate-and-frame, spiral-wound, hollow-fibre, tubular or envelope type.

Description

EXAMPLES

(1) Preparation of the PCP Polymer

(2) Radiation-curable polymers were prepared by reacting the components shown in Table 1 under the conditions stated in Table 1 (except for UV-9390c which is added later). The viscosity of RCC1 (before dilution) and RCC2 are shown in Table 1.

(3) The resultant mixture was then cooled to 20 C. and diluted with n-Heptane to a polymer concentration as mentioned in Table 1. The solution was then filtered over a 2.7 m filter paper. The photo-initiator (UV-9390c) was then added in a concentration as mentioned in Table 1. The resultant radiation-curable compositions are referred to as RCC1 and RCC2.

(4) TABLE-US-00001 TABLE 1 RCC1 RCC2 UV9300 (w/w %) 75.00 100.00 TiiPr (w/w %) 2.80 0 n-Heptane (w/w %) 22.20 0 Reaction temperature ( C.) 95.0 0 Reaction time (h) 105 0 Viscosity (mPas at 25 C.) 64300 at 0.0396 s.sup.1 310 at 40 s.sup.1 Polymer concentration (w/w %) 5.00 100.00 UV-9390c (w/w %) 0.50 2.0
Preparation of Composition Used to Form a Discriminating Layer

(5) Compositions DLS1 to DLS4 were prepared by mixing the components shown in Table 2. The solution was then filtered over a 2.7 m filter paper.

(6) TABLE-US-00002 TABLE 2 DLS1 DLS2 DLS3 DLS4 PI1 (w/w %) 2.00 1.50 0 0 PI2 (w/w %) 0 0 2.00 0 CA (w/w %) 0 0 0 1.00 CH (w/w %) 6.00 0 6.00 3.00 MIBK (w/w %) 0 4.50 0 0 THF (w/w %) 0 7.50 0 0 MEK (w/w %) 92.00 86.50 92.00 96.00

(7) In the following examples Ex1 to Ex8 and comparative example CEx1, the radiation-curable compositions were applied to PAN by the methods indicated in Table 3. Irradiation (step b)) was performed using a Light Hammer LH10 from Fusion UV Systems fitted with a D-bulb and irradiating with an intensity of 16.8 kW/m (70%). The discriminating layers were formed (step c)) using the compositions DSL1 to DSL4 indicated in Table 3 using the method indicated in Table 3. After steps a) to c) had been completed, the resultant composite membranes were dried. The thickness of the dry layers of cured polymer and discriminating layer were measured by Scanning Electron Microscope (determined from the surface of the porous support or the surface of the cured polymer layer outwards) and the results are shown in Table 3.

(8) TABLE-US-00003 TABLE 3 Example Ex1 Ex2 Ex3 Ex4 Ex5 Ex6 Ex7 Ex8 CEx1 Radiation-curable RCC1 RCC1 RCC1 RCC1 RCC1 RCC1 RCC1 RCC2 RCC2 Composition Coating method 4 2 4 4 4 3 4 5 2 Coating speed (m/min) 10 10 10 10 10 5000 rpm 10 22.9 20 Coating amount (ml/m.sup.2) N.A. 15.1 N.A. N.A. N.A. N.A. N.A. 0.9 g/m.sup.2 4.8 Dry layer thickness of 195 180 97 195 249 120 195 167 4000 gutter layer (nm) Q.sub.O2 of porous 2.2 10.sup.6 3.3 10.sup.6 4.0 10.sup.6 2.2 10.sup.6 2.9 10.sup.6 6.5 10.sup.6 2.2 10.sup.6 4.5 10.sup.6 9.4 10.sup.7 support + gutter layer (m.sup.3(STP)/m.sup.2 .Math. kPa .Math. s) .sub.O2/N2 of porous 2.23 2.14 2.10 2.23 2.25 2.13 2.23 2.08 2.11 support + gutter layer Discriminating layer DLS1 DLS2 DLS2 DLS2 DLS2 DLS2 DLS3 DLS4 DLS4 composition Coating method 4 2 1 1 1 3 4 1 1 Coating speed (m/min) 10 10 10 10 10 10 10 10 10 Coating amount (ml/m.sup.2) 10.0 10.0 12.6 12.6 12.6 12.6 10.0 30.0 30.0 Dry layer thickness of 112 73 126 126 118 112 115 175 172 discriminating layer (nm) Q.sub.O2 of composite 8.1 10.sup.8 1.4 10.sup.7 8.3 10.sup.8 7.2 10.sup.8 6.1 10.sup.8 2.2 10.sup.7 7.0 10.sup.8 1.7 10.sup.8 membrane (m.sup.3(STP)/m.sup.2 .Math. kPa .Math. s) .sub.O2/N2 of composite 5.56 5.03 5.17 5.06 5.01 4.89 4.50 4.64 membrane Molar ratio Sb:Ti in 0.41 0.41 0.41 0.41 0.41 0.41 0.41 0 0 the gutter layer

(9) An .sub.O2/N2 of about 2 indicates a good quality gutter layer. The O.sub.2 flux is preferably higher than 2.0 10.sup.6.

(10) After coating the discriminating layer the .sub.O2/N2 is preferably higher than 4, more preferably higher than 4.8, especially higher than 5. The O.sub.2 flux is preferably higher than 5 10.sup.8.

(11) The flux and selectivity of example CEx1 could not be determined due to a high amount of defects in the discriminating layer formed after coating. The thickness of CEx1 was determined by SEM.

Comparative Examples CEx2 and CEx3

(12) DLS1 and DLS4 were coated directly on the PAN porous support using coating method 4, giving a dry coating thickness of 420 and 442 nm respectively. However no flux and selectivity data could be determined due to the high amount of defects.

Example 9 and Comparative Example CEx4

(13) Example 9 and Comparative Example CEx4 were prepared as described in Table 4. The data show that a better selectivity and a higher flux are achieved with a thinner gutter layer.

(14) TABLE-US-00004 TABLE 4 Example Ex9 CEx4 Radiation-curable Composition RCC1 RCC1 Coating method 4 4 Coating speed (m/min) 10 10 Gutter layer thickness (nm) 246 473 Discriminating layer composition DLS1 DLS1 Coating method 1 1 Coating speed (m/min) 10 10 Discriminating layer thickness (nm) 75 72 Q.sub.CO2 of composite membrane 5.7 10.sup.7 5.2 10.sup.7 (m.sup.3(STP)/m.sup.2 .Math. kPa .Math. s) .sub.CO2/CH4 (ratio 13/87) of composite membrane 24.9 21.1