Composite membrane comprising layer of perfluoropolyether on hydrophilic substrate

Abstract

The invention relates to a composite comprising a porous substrate at least partially coated with a coating layer prepared from curing a coating composition (C), the coating composition (C) comprising at least one curable perfluoropolyether (PFPE) polymer. The invention further relates to a process for manufacturing a composite as afore-described, comprising the steps of: (a) providing a coating composition (C) comprising at least one curable perfluoropolyether (PFPE) polymer; (b) depositing said coating composition (C) on a porous substrate; and (c) crosslinking said coating composition (C) to form a porous substrate at least partially coated.

Claims

1. A composite comprising a porous hydrophilic substrate which is a membrane having a nominal pore size in a range of 0.1 ?m to 0.5 ?m, at least partially coated with a hydrophobic coating layer prepared from curing a coating composition (C), the coating composition (C) comprising at least one curable functional perfluoropolyether compound (E), said compound (E) comprising a (per)fluoropolyalkylene chain (R.sub.f) and at least one unsaturated moiety, wherein said chain (R.sub.f) complies with formula:
(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.q(CFYO).sub.r(CF.sub.2CFYO).sub.s(CF.sub.2(CF.sub.2).sub.zCF.sub.2O).sub.t wherein Y is a C.sub.1-C.sub.5 perfluoro(oxy)alkyl group, z is 1 or 2; and p, q, r, s, t are integers?0, selected such that the molecular weight of said chain (R.sub.f) is more than 500 g/mol and less than 4000 g/mol, wherein the hydrophilic substrate is selected from cellulose, modified PVDF, polysulphone and polyethersulphone.

2. A composite according to claim 1, wherein the coating composition (C) further comprises at least one crosslinking initiator.

3. A composite according to claim 2, wherein the PFPE polymer is radiation curable, and wherein the crosslinking initiator is a photo initiator selected from the group consisting of following families: alpha-hydroxyketones; phenylglyoxylates; benzyldimethyl-ketals; alpha-aminoketones; and bis acyl-phosphines.

4. A composite according to claim 1, wherein compound (E) is present in an amount in the range of 5% to 100% wt with respect to coating composition (C).

5. A composite according to claim 4, wherein coating composition (C) further comprises at least one nonfluorinated compound (M) having at least one unsaturated moiety, provided that said nonfluorinated compound (M) has at least two unsaturated moieties if compound (E) has one unsaturated moiety.

6. A composite according to claim 4, wherein compound (E) is selected from the group consisting of: (1) acrylate derivatives of formula: ##STR00009## wherein A and A, equal or different from each other, are independently a bond or a divalent, trivalent or tetravalent bonding group; n is 1 when A is a bond or a divalent group, n is 2 when A is a trivalent group, and n is 3 when A is a tetravalent group; m is 1 when A is a bond or a divalent group, m is 2 when A is a trivalent group, and m is 3 when A is a tetravalent group; R.sub.f is a (per)fluoropolyalkylene chain of formula:
(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.q(CFYO).sub.r(CF.sub.2CFYO).sub.s(CF.sub.2(CF.sub.2).sub.zCF.sub.2O).sub.t wherein Y is a C1-C5 perfluoro(oxy)alkyl group, z is 1 or 2; and p, q, r, s, t are integers?0, selected such that the molecular weight of R.sub.f is more than 500 g/mol and less than 4000 g/mol; and R.sub.H, R.sub.H, equal or different from each other, are independently H or a C.sub.1-C.sub.6 alkyl group; (2) acrylamide-urea derivatives of formula: ##STR00010## wherein A and A, equal or different from each other, are independently a bond or a divalent, trivalent or tetravalent bonding group; n is 1 when A is a bond or a divalent group, n is 2 when A is a trivalent group, and n is 3 when A is a tetravalent group; m is 1 when A is a bond or a divalent group, m is 2 when A is a trivalent group, and m is 3 when A is a tetravalent group; R.sub.f is a (per)fluoropolyalkylene chain of formula:
(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.q(CFYO).sub.r(CF.sub.2CFYO).sub.s(CF.sub.2(CF.sub.2).sub.zCF.sub.2O).sub.t wherein Y is a C1-C5 perfluoro(oxy)alkyl group, z is 1 or 2; and p, q, r, s, t are integers?0, selected such that the molecular weight of R.sub.f is more than 500 g/mol and less than 4000 g/mol; and R.sub.H, R.sub.H, equal or different from each other, are independently H or a C.sub.1-C.sub.6 alkyl group; (3) acrylate-urethane derivatives of formula: ##STR00011## wherein A and A, equal or different from each other, are independently a bond or a divalent, trivalent or tetravalent bonding group; n is 1 when A is a bond or a divalent group, n is 2 when A is a trivalent group, and n is 3 when A is a tetravalent group; m is 1 when A is a bond or a divalent group, m is 2 when A is a trivalent group, and m is 3 when A is a tetravalent group; R.sub.f is a (per)fluoropolyalkylene chain of formula:
(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.q(CFYO).sub.r(CF.sub.2CFYO).sub.s(CF.sub.2(CF.sub.2).sub.zCF.sub.2O).sub.t wherein Y is a C1-C5 perfluoro(oxy)alkyl group, z is 1 or 2; and p, q, r, s, t are integers?0, selected such that the molecular weight of R.sub.f is more than 500 g/mol and less than 4000 g/mol; R.sub.H, R.sub.H, equal or different from each other, are independently H or a C.sub.1-C.sub.6 alkyl group; and each of R.sup.B, equal to or different from each other, is a divalent, trivalent or tetravalent group selected from the group consisting of C.sub.1-C.sub.20 aliphatic group, C.sub.5-C.sub.40 cycloaliphatic group, C.sub.6-C.sub.50 aromatic, alkylaromatic or heteroaromatic group; and (4) urethane-amide-acrylate derivatives of formula: ##STR00012## wherein A and A, equal or different from each other, are independently a bond or a divalent, trivalent or tetravalent bonding group; n is 1 when A is a bond or a divalent group, n is 2 when A is a trivalent group, and n is 3 when A is a tetravalent group; m is 1 when A is a bond or a divalent group, m is 2 when A is a trivalent group, and m is 3 when A is a tetravalent group; R.sub.f is a (per)fluoropolyalkylene chain of formula:
(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.q(CFYO).sub.r(CF.sub.2CFYO).sub.s(CF.sub.2(CF.sub.2).sub.zCF.sub.2O).sub.t wherein Y is a C1-C5 perfluoro(oxy)alkyl group, z is 1 or 2; and p, q, r, s, t are integers?0, selected such that the molecular weight of R.sub.f is more than 500 g/mol and less than 4000 g/mol, and R.sub.H, R.sub.H, equal or different from each other, are independently H or a C.sub.1-C.sub.6 alkyl group; and each of R.sup.B, equal to or different from each other, is a divalent, trivalent or tetravalent group selected from the group consisting of C.sub.1-C.sub.20 aliphatic group, C.sub.5-C.sub.40 cycloaliphatic group, C.sub.6-C.sub.50 aromatic, alkylaromatic or heteroaromatic group.

7. A composite according to claim 4, wherein compound (E) is a compound of formula:
A-NHCOOCH.sub.2CF.sub.2(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.qCF.sub.2CH.sub.2OCONH-A wherein p and q are selected so that the molecular weight of chain (R.sub.f) is more than 500 and less than 4000.

8. A composite according to claim 4, wherein compound (E) is a compound of formula:
A-NHCOOCH.sub.2CF.sub.2(CF.sub.2O).sub.p(CF.sub.2CF.sub.2O).sub.qCF.sub.2CH.sub.2OCONH-A wherein: ##STR00013## and p and q are selected so that the molecular weight of chain (R.sub.f) is more than 500 and less than 4000.

9. A process for providing a composite of claim 1, the process comprising: depositing a coating composition (C) comprising at least one curable compound (E) on a porous membrane; and curing said coating composition (C) to form a porous membrane at least partially coated with a hydrophobic coating layer.

10. The process according to claim 9, wherein curing said coating composition (C) comprises submitting the coating composition (C) to UV radiation or heating the coating composition (C), to crosslink said PFPE polymer.

11. The process according to claim 9, further comprising: washing the coated porous substrate with a solvent to remove un-crosslinked PFPE polymer.

12. A method for separating water from aqueous solutions of inorganic solutes, the method comprising contacting the aqueous solution with the composite according to claim 2, wherein the composite comprises a composite membrane.

13. A composite according to claim 7, wherein p and q are selected so that the molecular weight of chain (R.sub.f) is between 1200 and 3000.

14. A composite according to claim 7, wherein p and q are selected so that the molecular weight of chain (R.sub.f) is between 1500 and 2500.

15. A composite according to claim 8, wherein p and q are selected so that the molecular weight of chain (R.sub.f) is between 1200 and 3000.

16. A composite according to claim 8, wherein p and q are selected so that the molecular weight of chain (R.sub.f) is between 1500 and 2500.

Description

EXAMPLES

(1) Materials

(2) Membrane 1: Polyethersulfone (PES) membrane 100H Thin from Pall with a nominal pore size of 0.1 micron, a porosity of about 0.74, a contact angle towards water of the top side of 67 degrees and of 84 degrees of the bottom side, a liquid entry pressure (L.E.P.) of 0.3 bar and a N2 flux of 0.43 L/(min cm2).

(3) Membrane 2: Regenerated Cellulose (RC) membrane from Sartorius with a nominal pore size of 0.45 micron and a porosity of about 0.74, a drop penetration time of less than 5 sec on both sides, a L.E.P. of 0 bar and a N2 flux of 0.75 L/(min cm2).

(4) Membrane 3: hydrophilic polyvinylidene fluoride (PVDF) membrane Durapore? GVWP 0.22 from Millipore with a nominal pore size of 0.22 micron, a porosity of about 0.65, a L.E.P. of 0 bar, a nitrogen flux of 0.39 L/(min cm2) and a drop penetration time of less than 5 sec on both membrane sides.

(5) Fluorolink? MD 700 from Solvay Specialty Polymers is an oligo urethane methacrilate with a PFPE backbone whose Mw is about 1500.

(6) ##STR00007##

(7) Fluorolink? AD 1700 from Solvay Specialty Polymers with a PFPE backbone whose Mw is about 4000.

(8) ##STR00008##

(9) Darocur? 1173 from Ciba is a liquid photoinitiator which is commonly used to initiate the photopolymerisation of chemically unsaturated acrylate.

(10) Analytical Methods

(11) Measurement of Contact Angle and of the Drop Penetration Time

(12) For the purpose of the present invention, a material is considered hydrophobic, or non-wetted, when the contact angle of a water droplet on its surface is higher than 90?. The contact angle towards water was evaluated at 25? C. by a Dataphysics OCA 20, according to ASTM D 5725-99. Contact angle measurements were taken on virgin (i.e. un-treated) membranes and coated (i.e. UV cured) membranes using 2 ?L water drops. In the case of highly hydrophilic membranes, however, the water contact angle cannot be measured because the water drop quickly penetrates into the membrane; in this case, a water penetration time is reported to evaluate the membrane hydrophilicity.

(13) Measurement of Water Permeability

(14) Water permeability of different membranes is measured according to the technique known in the art. The water permeability of tested membranes was measured with a dead-end stainless steel apparatus under an atmospheric pressure of 1 bar. The surface area of the membranes was about 11 cm.sup.2.

(15) Measurement of L.E.P.

(16) Hydrophobic porous membranes do not allow water to pass until a pressure exceeding a certain minimum value is applied thereon. The liquid entry pressure (L.E.P.) used herein is defined as the minimum pressure that must be applied on the liquid before it penetrates into the pores and subsequently produce a flow. This L.E.P. value is related to the interfacial tension of the liquid, the surface energy of the material and the shape and size of the membranes pores.

(17) It is worth noting that L.E.P. takes into account only active pores (pores which connect both sides of the membrane) since a liquid cannot pass through closed pores (or inactive pores). The experimental apparatus and the procedure to measure L.E.P. were identical to what described in GARCIA-PAYO, M. C., et al. Wetting Study of Hydrophobic Membranes via Liquid Entry Pressure Measurements with Aqueous Alcohol Solutions. Journal of Colloid and Interface Science. 2000, vol. 230, p. 420-431, and the measurement was carried on flat sheet (dry) membranes. The pressures applied during the measurements ranged from 0.1 to about 5.5 bar and the liquid used was Milli-Q water. In the case of hydrophilic membranes, a L.E.P of 0 bar was reported, indicating that the water spontaneously flows through the membrane.

(18) Nitrogen Flux

(19) The permeation flux of nitrogen through dry membranes was measured at a trans-membrane pressure of 0.12 bar, following the ASTM standard F316-70. This test allows the determination of membrane permeance under examination and from that it is possible to evaluate the decrease in gas and vapor transport induced by any surface coating. The procedure is well-known in the art and is described, for example, in BROUGHTON, J., et al. Porous cellular ceramic membranes: a stochastic model to describe the structure of an anodic oxide membrane. Journal of Membrane Science. 1995, vol. 106, p. 89-101.

(20) Measurement of Porosity

(21) Porosity of the membrane is defined as the volume of the pores divided by the total volume of the membrane. The porosities were measured using IPA (isopropyl alcohol) as wetting fluid according to the procedure described for example in SMOLDERS, K., et al. Terminology for Membrane Distillation. Desalination. 1989, vol. 72, p. 249-262. Specifically, this measurement method is relied on the fact that IPA penetrates into the pores of the membrane and water does not penetrate into the pores of the membrane.

(22) Firstly, the density of the polymer material (?-.sub.pol) of the membrane was measured using the formula below:

(23) ${?}_{\mathrm{pol}}=\frac{{?}_{\mathrm{IPA}}\mathrm{wt}\mathrm{.3}}{\mathrm{wt}\mathrm{.1}+\mathrm{wt}\mathrm{.3}-\mathrm{wt}\mathrm{.2}}$
wherein wt. 1=weight of the pyknometer with IPA; wt. 2=weight of the pyknometer with IPA and membrane; wt. 3=dry weight of the membrane. Secondly, in the same way, the density of the membrane (?-.sub.m) was calculated by the following formula:

(24) ${?}_m=\frac{{?}_w\mathrm{wt}\mathrm{.3}}{\mathrm{wt}\mathrm{.1}+\mathrm{wt}\mathrm{.3}-\mathrm{wt}\mathrm{.2}}$
wherein wt. 1=weight of the pyknometer with water; wt. 2=weight of the pyknometer with water and membrane; wt. 3=dry weight of the membrane. Subsequently, the membrane porosity (?) can be calculated by the formula below:

(25) $\mathrm{.Math.}=1-\frac{{?}_m}{{?}_{\mathrm{pol}}}$

(26) UV Irradiation

(27) Membranes were cured by exposure to UV light from a Fusion system model VPS 1600 curing unit with variable power supply (240 watts/cm), H lamp 13 mm and variable focal distance. This unit is mounted on a variable speed conveyor (velocity from 10 to 60 m/min) and is able of delivering a dose of 0.28 J/cm.sup.2 to 1.68 J/cm.sup.2 in a single pass as measured with a UV process supply compact radiometer EIT PawerMap?.

(28) FT-IR Analysis

(29) The presence of the crosslinked Fluorolink? oligomers on the membrane surface was detected by the peaks of urethanes at 1694 cm.sup.?1 and 1725 cm.sup.?1 in the IR spectrum obtained in reflection.

(30) Measurement of Direct Contact Membrane Distillation (DCMD) Flux

(31) Water flux across the membrane in DCMD was measured according to a conventional technique known in the art. The apparatus used is described in A. CRISCUOLI. Evaluation of energy requirements in membrane distillation. Chemical Engineering and Processing. 2008, vol. 47, no. 7, p. 1098-1105.

Example 1

(32) A piece of Membrane 1 was irradiated twice with UV light having a radiation energy of 1.68 J/cm.sup.2. Then, the membrane was dipped in a Butyl acetate solution containing 5% w/w of Fluorolink? MD 700 and 0.1% w/w Darocur? 1173 for 2 minutes. After the impregnation, the membrane was removed from the solution and held horizontally to be dried in a fume hood for 12 hours. Then, the upper side was again UV irradiated with a total radiation energy of 1.68 J/cm.sup.2. Finally it was washed for three times, each time with 100 ml of Butyl acetate to wash out the un-reacted Fluorolink? material from the membrane, and thereby obtaining a composite product (or more specifically, a composite membrane).

(33) FT-IR spectroscopic analyses of the composite membrane revealed the presence of crosslinked Flurolink? MD 700 only on the UV-irradiated (upper) surface of the membrane, not the un-irradiated lower membrane surface. The porosity and nitrogen flux of the composite membrane were nearly unchanged compared to the un-coated Membrane 1. The upper side of the composite membrane gave a water contact angle of 126 degrees, and the water contact angle on the lower side of the membrane remained substantially unchanged. In addition, an increased L.E.P. of 2.5 bar was measured on the upper side of the composite membrane.

Example 2

(34) Another piece of Membrane 1 was first dipped in HFE 7100/Butyl acetate 30/70 v/v solution containing 5% w/w of Fluorolink? AD 1700 and 0.1% w/w Darocur? 1173 (initiator) for 2 minutes, before it was removed from the solution and held horizontally to be dried in a fume hood for 12 hours. Subsequently, the upper side of the dried membrane was irradiated for three times by UV light with a radiation energy of 1.68 J/cm.sup.2. Finally, it was washed for three times, each with 100 ml of HFE? 7100/Butyl acetate 30/70 solution, to wash out the un-reacted Fluorolink? material from the membrane, and thereby obtaining a composite product (or more specifically, a composite membrane).

(35) FT-IR spectroscopic analyses of the composite membrane revealed the presence of crosslinked Flurolink? AD 1700 only on the UV-irradiated (upper) surface of Membrane 1, not its un-irradiated lower membrane surface. The porosity and the nitrogen flux of the composite membrane were substantially unchanged compared to the un-coated Membrane 1. The upper side of the composite membrane gave a water contact angle of 125 degrees, and the water contact angle on the lower side of the membrane remained substantially unchanged. In addition, an increased L.E.P. of 2.7 bar was measured on the upper side of the composite membrane in Example 2.

Example 3

(36) A piece of Membrane 2 was dipped in HFE 7100/Butyl acetate 30/70 v/v solution containing 5% w/w of Fluorolink? AD 1700 and 0.1% w/w Darocur? 1173 (initiator) for 2 minutes, before it was removed from the solution and held horizontally to be dried in a fume hood for 12 hours. Then, the upper side of the dried membrane was irradiated for ten times by UV light with a radiation energy of 1.68 J/cm.sup.2. Finally, it was washed for three times, each time with 100 ml of HFE 7100/Butyl acetate 30/70 solution to wash out the un-reacted Fluorolink? material from the membrane, and thereby obtaining a composite product (or more specifically, a composite membrane).

(37) FT-IR spectroscopic analyses of the composite membrane revealed the presence of crosslinked Flurolink? AD 1700 only on the UV-irradiated (upper) surface of the Membrane 2: not on its un-irradiated lower membrane surface. A slightly decreased porosity of 0.70 was measured in the composite membrane, but the nitrogen flux of the composite membrane was substantially unchanged compared to the un-coated Membrane 2. The upper side of the composite membrane gave a large water contact angle of 131 degree, and an increased L.E.P. value of 2.2 bar. In comparison, water readily penetrated the composite membrane from its bottom side, with a drop penetration time of less than 5 secondssubstantially the same as that measured in the un-coated Membrane 2.

Example 4

(38) The same procedure as detailed in Example 3 was followed for a piece of Membrane 3. After the UV irradiation treatment and subsequent solution wash, FT-IR analyses of the treated Membrane 3 revealed the presence of crosslinked Flurolink? AD 1700 only on the UV-irradiated (upper) surface of the membrane: not on its un-irradiated lower surface. A slightly decreased porosity of 0.60 was observed for the composite membrane, whose nitrogen flux was substantially unchanged compared to the un-coated Membrane 3. The upper side of the composite membrane gave a water contact angle of 136 degrees, and an increased L.E.P. value of 2.2 bar. In comparison, water readily penetrated into the composite membrane from its bottom side, with a drop penetration time of less than 5 secondssubstantially the same as that measured in the un-coated Membrane 3.

(39) TABLE-US-00001 TABLE 1 Measured parameters of un-treated membranes used in Examples 1-4 Membrane 1 Membrane 2 Membrane 3 Membrane Material PES RC PVDF Porosity 0.74 0.74 0.65 Nitrogen Flux [L/(min .Math. cm.sup.2)] 0.43 0.75 0.39 L.E.P. (bar) 0.3 0 0 Water contact angle (?) ? at the upper side (?) 67 ~0 ~0 ? at the lower side (?) 84 ~0 ~0 Drop penetration time (d.p.t.) d.p.t at the upper side (s) n.m.* <5 <5 d.p.t at the lower side (s) n.m.* <5 <5 *n.m.= not measured

(40) TABLE-US-00002 TABLE 2 Measured parameters of composites obtained in Examples 1-4 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Un-coated substrate Membrane Membrane Membrane Membrane (material) 1 (PES) 1 (PES) 2 3 (PVDF) (RC) Nitrogen Flux 0.40 0.41 0.75 0.39 [L/(min .Math. cm.sup.2)] Porosity 0.74 0.74 0.70 0.60 L.E.P. (bar) 2.5 2.7 2.2 2.2 Water contact angle (?) ? at the upper side (?) 126 125 131 136 ? at the lower side (?) 85 83 ~0 ~0 Drop penetration time (d.p.t.) d.p.t at the upper side n.m.* n.m.* n.m.* n.m.* (s) d.p.t at the lower side (s) n.m.* n.m.* <5 <5 *n.m.= not measured

Example 5

(41) A piece of composite membrane as prepared in Example 1 was immersed in a saline solution (containing NaCl at 3.5% w/w) at 80? C. for 15 days. Afterwards, the composite membrane was removed from saline, repeatedly rinsed in the distilled water, and subsequently dried in a vacuum oven at 40? C. for 24 hours. The nitrogen flux and the L.E.P. value of the thus dried membrane were found to be substantially unchanged, indicating an outstanding thermal stability and physical durability of the composite made according to the present invention.

Example 6

(42) A piece of composite membrane as prepared in Example 2 was immersed in saline solution (containing NaCl at 3.5% w/w) at 80? C. for 15 days. Afterwards, the composite membrane was removed from saline, repeatedly rinsed in the distilled water, and subsequently dried in a vacuum oven for 24 hours at 40? C. The nitrogen flux and the L.E.P. value of the thus dried membrane were found to be substantially unchanged, indicating an outstanding thermal stability and physical durability of the composite made according to the present invention.

Example 7

(43) The same procedure as in Example 5 or 6 was followed in Example 7, for treating a composite membrane as prepared in Example 3. Similarly, the nitrogen flux and the L.E.P. value of the oven dried composite membrane in Example 7 were found to be substantially unchanged compared to the product of Example 3, indicating that the hydrophobic coating provided by the present invention also has an outstanding thermal stability and physical durability when applied on a hydrophilic membrane substrate.

Example 8

(44) The same procedure as in Example 5 or 6 was followed in Example 8, for treating a composite as prepared in Example 4. Similarly, the nitrogen flux and the L.E.P. value of the oven dried membrane in Example 8 were found to be substantially unchanged compared to the product of Example 4, indicating that the hydrophobic coating provided by the present invention also has an outstanding thermal stability and physical durability when applied on a hydrophilic membrane substrate.

Comparative Example 9

(45) A piece of membrane was prepared as in Example 1 except that no UV irradiation was applied to the membrane surface. The membrane porosity and nitrogen flux of the thus obtained membrane were substantially unchanged compared to the un-coated Membrane 1. The thus obtained membrane gave a water contact angle of 70 degrees on its upper side, and 90 degrees on its lower side. In addition, a substantially unchanged L.E.P. of 0.4 bar was measured on the upper side of the membrane obtained in Example 9, much lower than that measured on the upper side of the coated membrane obtained in Example 1.

(46) Without wishing to be bound by the theory, the applicant thinks that in this comparative example, the PFPE moiety could not stay on the membrane surface without the UV irradiation treatment, and was almost completely removed by the washing procedure.

Comparative Example 10

(47) The same procedure as detailed in Example 4 was followed for a piece of Membrane 3 except that, in this case, the upper side of the membrane was irradiated for only once by UV light with a radiation energy of 1.68 J/cm.sup.2. Nitrogen flux of the thus obtained membrane was substantially unchanged compared to the original Membrane 3. The thus obtained membrane gave an upper side with a relatively large water contact angle of 78 degrees, and a bottom side from where water could readily penetrate the membranewith a drop penetration time of less than 5 seconds. In addition, a L.E.P. of only 0.2 bar was measured on the upper side of the membrane obtained in Example 10. Without wishing to be bound by the theory the applicant thinks that, in this comparative example, the UV treatment was not enough to guarantee the right hydrophobicity necessary for the application to the membrane.

Example 11

(48) A piece of composite membrane as prepared in Example 2 was tested in a DCMD apparatus which used distilled water as the feed liquid. The feed and the distillate flow rate were respectively set at 230 and 200 L/h; the surface area of the membrane is 40 cm.sup.2 and its thickness is 65 ?m. Three feed temperatures were tested, and at each feed temperature, the test ran for at least 3 hours with no apparent variation in membrane flux. The measured fluxes are reported in Table 3.

Comparative Example 12

(49) A hydrophobic commercial membrane made of polypropylene having a pore size of 0.2 ?m and a thickness of 92 ?m was tested under the same DCMD conditions as in Example 11. Lower membrane fluxes have been obtained than with the membrane prepared according to Example 2, as shown in Table 3.

(50) TABLE-US-00003 TABLE 3 Transmembrane Fluxes Obtained in DCMD Tf [? C.]* Td [? C.]* Flux [Kg/m2h] Ex. 11 40.3 17.4 11.8 50.0 17.5 20.9 60.1 17.8 35.3 Ex. 12 40.0 17.3 6.0 (Comp.) 50.1 17.5 12.3 59.9 17.5 20.5 *T.sub.f and T.sub.d respectively represent the temperature of the feed and the distillate

(51) It is to be understood that variations and modifications of the present invention may be made without departing from the scope of the invention. It is also to be understood that the scope of the invention is not to be interpreted as limited to the specific embodiment disclosed herein, but only in accordance with the appended claims when read in light of the foregoing disclosure.