Method for making a nanofiltration membrane, and resulting membrane

The invention concerns a method for producing a nanofiltration membrane. At least one face ( 30 ) of a porous membrane ( 3 ) is put in the presence of a grafting composition comprising at least one monomer grafted by radical polymerisation and at least one crosslinking agent, that is however free from photoinitiator, and of light radiation capable of forming free radicals during a predetermined period adapted so as to obtain nanofiltration properties. The invention also extends to the nanofiltration membrane obtained in this way of which the properties may be precisely predetermined and which resist ageing.

The installation shown in FIG. 1 enables a method for producing a nanofiltration membrane according to the invention to be implemented in a discontinuous manner. This installation comprises a cylindrical vessel 1 receiving a grafting solution 2 . A microporous microfiltration or mesoporous ultrafiltration supporting membrane 3 with a rectangular shape is immersed in the solution 2 , rolled up on itself and pressed against the wall of the vessel 1 , and held upright for example with the aid of a ring 4 . An ultraviolet lamp 5 is immersed axially (along the axis of symmetry of the vessel 1 so as to be equidistant from the different zones of the inner face 30 of the membrane 3 ) within the liquid solution 2 so as to emit opposite the grafting face 30 of the supporting membrane 3 . This ultraviolet lamp 5 is preferably a quartz lamp cooled by a water circuit 6 formed of a DURAN 50® glass tube. A conduit 7 enables gaseous nitrogen to be bubbled into the bottom of the vessel 1 so as to deoxygenate the solution 2 . A stirrer 8 is advantageously provided at the bottom of the vessel 1 . In order to produce a nanofiltration membrane made of polysulfone, the starting point is a microporous or mesoporous membrane made of polysulfone and a grafting composition is used containing at least one grafting monomer such as acrylic acid and at least one crosslinking agent such as methylene bisacrylamide, in suitable concentrations. The membrane 3 is placed in the vessel 1 , the lamp 5 being left outside the bath. Nitrogen is bubbled into the solution 2 with stirring until the concentration of dissolved oxygen falls to a value of 0.25 mg/l of oxygen. This dissolved oxygen concentration may be measured with the aid of a conventional probe 15 . When this threshold concentration is reached, the lamp 5 is immersed in the bath facing the membrane. The lamp 5 is illuminated sufficiently in advance before being immersed in the bath so that it reaches its permanent operating state. The membrane is then irradiated for a predetermined period and is then withdrawn from the reactor and washed with distilled water. FIG. 2 a illustrates an installation for carrying out a similar process continuously, the supporting membrane 3 being formed of a continuous strip 9 moving forward in a bath 10 of liquid grafting solution. A source of ultraviolet radiation 11 is positioned above the bath 10 opposite the grafting face 12 of the membrane 9 to be treated. The bath 10 is formed in a vessel 13 which also receives a conduit 14 for bubbling gaseous nitrogen into the bath 10 . The variant of FIG. 2 b differs from the preceding one in that the source of light radiation 11 is applied downstream from the bath 10 after the membrane 9 has been removed from the bath 10 . This is in actual fact impregnated with the grafting liquid solution and application of the radiation is sufficient to bring about polymerisation and crosslinking of the polymer, at the same time as it is grafted on the face 12 of the membrane. In the case of hollow fibres, the light source 11 may be formed of one or more cylindrical ultraviolet ovens through which the fibre passes axially so as to ensure uniform peripheral illumination. The rate of passage of the membrane 9 is adapted according to the desired duration for the application of radiation onto the grafting face 12 , with a view to obtaining a nanofiltration membrane. In continuous installations, a water-washing unit 16 is also provided downstream from the treatment with ultraviolet radiation. Nevertheless, it should be noted that no drying step is necessary. The treatment temperature enabling nanofiltration membranes to be obtained according to the invention is ambient temperature. In practice, this temperature may vary between 10° C. and 50° C. The installation in FIG. 1 was used to prepare examples 1 and 2 described below. For these examples, a mesoporous ultrafiltration supporting membrane 3 was used made of polysulfone marketed by the POLYMEM company, with a hydraulic permeability equal to 10 −3±10 −4 l·h −1 m −2 ·Pa −1 , and initially not offering any retention of ionic inorganic species. The supporting membrane 3 was placed at a distance of the order of 16 mm from the lamp 5 , and the irradiated area was approximately 100 cm 2 . The volume of the liquid grafting solution was 650 cm 3 and the lamp 5 was a Hanau Heraeus TQ 150 ultraviolet lamp of which the cooling tube was made of DURAN 50® glass. This lamp 5 emitted at different wavelengths according to table 1 below. 1 TABLE 1 Wavelength +HL,8 (nm) 254 313 366 436 546 Light flux 0.0 2.5 5.8 3.6 4.6 with glass tube (W) The grafting solution was an aqueous solution of acrylic acid (as a monomer) and methylene bisacrylamide, as the crosslinking agent. In order to measure hydraulic permeabilities, a cylindrical cell is used (for example AMICON 8050®) with a capacity of 50 cm 3 , and with an internal diameter of 43 mm. The working area of the membrane was 13.2 cm 2 . The hydraulic permeability was measured with the aid of osmosis-purified water. FIG. 3 represents the installation used for measuring the degree of retention of ionic inorganic species (or the degree of ionic retention). This degree of ionic retention was measured with reference to the calcium ion Ca 2&plus; with the aid of a synthetic solution of calcium chloride containing 50 mg/l of calcium ions prepared with osmosis/purified water. The installation comprised a reservoir 20 pressurised by nitrogen introduced into the upper part under a pressure of 4.10 5 Pa through a conduit 21 . The solution was placed in the reservoir 20 which was closed at its lower part by the membrane 22 to be tested, placed above a mesh 23 which emerged into a collector 24 collecting the liquid passing through the membrane 22 and the mesh 23 . The assembly formed the cylindrical cell. The collector 24 poured the collected solution (permeate) into a vessel 25 placed on an electronic balance 26 for determining the mass of permeate collected as a function of time. This measurement made it possible to calculate the hydraulic permeability by dividing the mass of permeate by the filtering area, pressure and the measurement time. The concentrations of calcium C 0 of the initial solution placed in the vessel 20 , and C 1 of the permeate, were measured with the aid of a plasma torch. The concentration C 1 of the permeate was measured for a predetermined volume factor (ratio of the initial volume of the solution in the vessel over its final volume), in particular equal to 15. The degree of ionic retention Tr was calculated according to the following equation: Tr&equals; 1 −C 1 /C 0 
 EXAMPLE 1 In this example, a concentration of 2.5% (by mass) of acrylic acid was used in the grafting solution. The concentration of methylene bisacrylamide was 0.0267% (by mass) in the solution, corresponding to approximately 1.25 molar % of the quantity of acrylic acid. In this example, the ultraviolet irradiation time was held fixed at 5 min. Nevertheless, the permeability of the supporting membrane 3 was varied. The following table indicated the results obtained concerning the permeability of the modified nanofiltration membrane as well as its degree of calcium retention. 2 TABLE 2 Degree of calcium Permeability of retention Permeability of membrane modified (synthetic supporting according to the solution membrane invention containing 50 mg/l (l.h −1 .m −2 .Pa −1 ) (l.h −1 .m −2 .Pa −1 ) of Ca 2&plus; 10 −3 0.38.10 −5 57% 5.10 −3 0.8.10 −5 25% 6.10 −3 1.4.10 −5 23% 
 EXAMPLE 2 In this example, the mass concentration of acrylic acid in the solution was varied as well as the mass concentration of the crosslinking agent. A comparative example was also carried out using a solution of acrylic acid in the presence of a photoinitiator (benzoin) and without the crosslinking agent. The ultraviolet irradiation time was varied from 3.5 to 7 min. Ageing of the membrane was performed by soaking it in osmosis-purified water at 60° C. for a predetermined period. A test was also carried out on a nanofiltration membrane according to the invention using, for measuring the degree of calcium retention, not a solution made with osmosis-purified water, but one made with tap water having a concentration of 31.3 mg/l of calcium ions. The following table 3 gives the results obtained: 3 TABLE 3 Methylene bis- Acrylic acrylamide Degree of acid conc.(mass Benzoin Ca conc. % of conc. (mass retention (mass % solution) % of (Ca of Cross- solution) Ageing time, solution solution) linking Photo- Irradiation days (water Permeability with 50 Monomer agent initiator time (mm) 60° C.) (l/h/m 2 /bar mg/l Ca 2&plus; ) Support 0 0 0 0 0 100 0% membrane Conventional 2.5 0 0.1 5 0 0.94 8% membrane &plus; photoinitiator Aged convent- 2.5 0 0.1 5 7 13.46 0% ional membrane &plus; photoinitiator Present 2.5 0.027 0 3 0 0.87 18% invention Present 2.5 0.027 0 5 0 0.38 57% invention Present 2.5 0.027 0 7 0 0.34 85% invention Present 1.0 0.0021 0 5 0 2.4 4% invention Present 7.0 0.015 0 5 0 0.16 18% invention Present 2.5 0.027 0 3 0 0.85 15% invention Present 2.5 0.0535 0 3 0 0.47 14% invention Present 2.5 0.027 0 3 7 0.86 17% invention, aged Present 2.5 0.027 0 5 7 0.38 56% invention, aged Present 2.5 0.027 0 7 7 0.34 86% invention, aged Present 2.5 0.027 0 5 0 0.3 74% invention, (tap waterwith 31.3 mg/l Ca 2&plus; The first three lines of table 3 show comparative examples. As can be seen, the fact of using a photoinitiator in the absence of a crosslinking agent considerably reduces, on the one hand, the final degree of calcium retention obtained, but above all destroys any resistance-to-ageing property of the membrane. On the contrary, by means of the invention, the membrane obtained not only has a high degree of calcium retention with satisfactory permeability, but above all these properties are maintained after ageing for seven days with water at 60° C. The membranes used in this example were prepared discontinuously as mentioned above with the installation of FIG. 1 . The first test mentioned in table 3 (concentrations of acrylic acid of 2.5% by mass and methylene bisacrylamide of 0.023% by mass, 3 min irradiation without ageing) was repeated by soaking for 15 min in a deoxygenated solution in the absence of ultraviolet, and then by removing the grafting solution from the vessel 1 before inserting the UV lamp. This test was thus representative of the continuous process in which the membrane was extracted from the bath before being irradiated by ultraviolet. The same results were obtained as those mentioned in table 3. 
 EXAMPLE 3 In this example, a hollow nanofiltration fibre was produced continuously with an installation similar to that represented in FIG. 2 b. The starting point was a hollow ultrafitration fibre made of polysulfone rolled on a reel which was passed continuously through a bath of grafting composition degassed with nitrogen, identical to that of example 1, and then in two Hoenle FOZFR 250® UV ovens, 290 nm<&lgr;<600 nm, mounted in series, and then in a three-cylinder device known as a “tricylinder”, providing a constant rate of progression. The fibre was then washed with osmosis-purified water. The following table 4 gives the results obtained. 4 TABLE 4 Irradiation time (s) 0 2 3 6 Permeability 100 22 7 5 (l.h −1 .m −2 .bar −1 ) Degree of calcium 0% 2.5% 6% 16% retention, Synthetic solution of calcium chloride with 50 mg/l Ca 2&plus; It will be seen that a nanofiltration membrane was obtained in the form of a hollow fibre with a short treatment time, in a simple and economical manner, that could be carried out on the industrial scale. The invention may be the subject of many alternative methods of implementation with respect to the embodiments and examples mentioned above.