WO2020008152A1

WO2020008152A1 - Hollow polymer fibers as filtration membranes

Info

Publication number: WO2020008152A1
Application number: PCT/FR2019/051665
Authority: WO
Inventors: François Beaume; Thomas VROMAN; Jean-Christophe Remigy; Jean-Christophe Rouch; Pierre Aimar
Original assignee: Arkema France; Universite Toulouse Iii - Paul Sabatier; Centre National De La Recherche Scientifique; Institut National Polytechnique De Toulouse
Priority date: 2018-07-06
Filing date: 2019-07-04
Publication date: 2020-01-09
Also published as: TW202005709A; FR3083458B1; FR3083458A1

Abstract

The invention relates to a porous hollow fiber comprising a copolymer comprising units derived from vinylidene fluoride and units derived from at least a second comonomer, having a Young's modulus ranging from 2 to 30 MPa and in which the ratio of the mean external diameter thereof to the mean internal diameter thereof ranges from 1.2 to 2. The invention also relates to a filtration module comprising such a hollow fiber, to a process for manufacturing such a hollow fiber, to the use of such a hollow fiber for treating effluent, and also to a process for declogging such a hollow fiber.

Description

HOLLOW POLYMERIC FIBERS AS MEMBRANES OF

FILTRATION

FIELD OF THE INVENTION

The present invention relates to porous hollow polymer fibers useful as a filtration membrane, in particular for microfiltration or ultrafiltration, and methods of manufacturing such hollow fibers. TECHNICAL BACKGROUND

Membrane filtration, in particular ultrafiltration, is an important part of natural water or aqueous effluent treatment operations. It allows, for example, to eliminate clay particles and microorganisms present in river water and thus produce clear and disinfected water. In the field of water treatment, filtration is carried out in particular using polymer membranes of the hollow fiber type.

However, the accumulation of objects retained by the membrane gradually induces a clogging of the membrane, resulting in a decrease in its production capacities. The membranes must therefore be regularly unclogged and cleaned. The deposits which clog the membrane are typically of two kinds: the deposits weakly fixed on the membrane, also called reversible clogging, which are generally eliminated by a physical cleaning like backwashing, and the deposits strongly fixed or adsorbed on the membrane, also called irreversible clogging, which must be treated by chemical cleaning.

In order to improve physical cleaning by backwashing, various solutions have been considered.

For example, the article by Y. Ye et al., Evolution of fouling deposition and removal on hollow fiber membrane during filtration with periodical backwash, Desalination, vol. 283, pp. 198-205, 2011, teaches that the use of an “air scouring” process (or air washing) in combination with backwashing increases the effectiveness of the latter, in particular because the clogging detached by backwashing is driven away from the surface of the membrane, which limits its immediate re-deposition on the membrane.

Document WO 2016/01 1,128 describes membrane modules comprising a nonwoven substrate as well as an elastic microfiltration membrane, for example made of polyether-based polyurethane, framed by two perforated supports.

The document US Pat. No. 4,816,160 describes a filtration process using an elastic hollow fiber polypropylene membrane in which the hollow fibers of the membrane are cooled before being subjected to a reverse flow of clarified liquid and then of gas.

Document US Pat. No. 5,643,455 describes a filtration process on a membrane of polypropylene elastic hollow fibers comprising a step of removing the solids retained in the fibers by the passage of a pressurized liquid and then of a pressurized gas through the pores of the fibers .

Document US Pat. No. 6,159,373 discloses a gas backwashing process in which an explosive decompression is created in order to dislodge the clogging agents in and on the elastic hollow fibers of the membrane.

Other strategies include improving resistance to chemical cleaning of membranes.

For example, the document US 2009/0101600 describes membranes having a high resistance to the chemicals used in chemical cleaning. These membranes comprise, as majority component, a resin of PVDF (polyvinylidene fluoride) having a degree of crystallization and a specific surface area.

There is a real need to provide filtration membranes to improve the elimination of reversible clogging and in particular to increase the efficiency of backwashing.

SUMMARY OF THE INVENTION

The invention relates firstly to a porous hollow fiber comprising a copolymer comprising units originating from vinylidene fluoride and units originating from at least one second comonomer, and having a Young's modulus of less than 40 MPa

According to embodiments, the invention relates to a porous hollow fiber comprising a copolymer comprising units from vinylidene fluoride and units from at least one second comonomer, having a Young's modulus of 2 to 30 MPa and in which the report of his average outside diameter relative to its average inside diameter is 1.2 to 2.

According to embodiments, the hollow fiber consists essentially, or consists, of a copolymer comprising units derived from vinylidene fluoride and units derived from at least one second comonomer.

According to embodiments, the second comonomer is chosen from the group consisting of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, 1-chloro-1-fluoroethylene, tetrafluoroethylene and combinations thereof, the second comonomer being preferably hexafluoropropylene.

According to embodiments, the units derived from vinylidene fluoride are present in the copolymer in a mass amount of at least 50% by weight, preferably at least 75% by weight, more preferably at least 85% by weight, and up to 99% by weight.

According to embodiments, the hollow fiber has a Young's modulus from 1 to 30 MPa, preferably from 2 to 20 MPa, even more preferably from 3 to 10 MPa.

According to embodiments, the hollow fiber has a Young's modulus of 3 to 13 MPa and the ratio of its mean outside diameter to its mean inside diameter is from 1.4 to 1.8.

According to embodiments, the hollow fiber has pores having an average diameter of 1 nm to 2 µm, preferably from 1 nm to 100 nm.

According to some embodiments, the hollow fiber has an average outside diameter of 500 to 2000 microns, preferably from 800 μm to 1400 microns.

The invention also relates to a filtration module comprising at least one hollow fiber as described above.

The invention also relates to a method for manufacturing a hollow fiber as described above comprising:

- the dissolution of a copolymer comprising units from vinylidene fluoride and units from at least one second comonomer in a solvent to form a collodion;

- the shaping of hollow collodion fiber;

- the coagulation of the shaped collodion.

According to embodiments, the coagulation of the shaped collodion is carried out by bringing the shaped collodion into contact with a coagulation solution containing a non-solvent for the copolymer. According to embodiments, the coagulation of the shaped collodion is carried out by a change in temperature of the shaped collodion, preferably a decrease in temperature.

The invention also relates to the use of a hollow fiber as described above or of a filtration module as described above, for the treatment of effluents.

The invention also relates to a method for unclogging a hollow fiber as described above, comprising:

- subjecting the hollow fiber to mechanical stress so as to cause deformation of said hollow fiber;

- optionally backwashing the hollow fiber.

According to embodiments, the mechanical stress is the pressure exerted by the backwashing liquid and / or a stretching of the hollow fiber.

The invention also relates to a process for filtering an effluent, comprising:

- bringing the effluent into contact with a hollow fiber as described above;

- the unclogging of the hollow fiber according to the unclogging process as described above.

The present invention makes it possible to meet the need expressed above. It more particularly provides a reversible deformable hollow fiber making it possible to make the physical cleaning of the membranes more effective, therefore increasing the productivity of the filtration process and reducing operating costs, in particular by reducing the water consumption beforehand filtered, and consumption of cleaning chemicals.

This is accomplished thanks to the presence, in the hollow fiber, of a copolymer comprising units originating from vinylidene fluoride and units originating from at least one second comonomer, and to the Young's specific modulus of the fiber. Such a fiber, reversibly deformable, allows the induction of macroscopic deformation of the surface of the fiber under the effect of mechanical stress. The deformation of the surface of the fiber causes shearing at the interface between the fiber and the deposit of sealant which covers it, thus facilitating the detachment of the sealant adhering to the surface of the fiber (and not only of the sealant present in the pores). In addition, the elastic nature of such fibers can allow a reversible enlargement of the diameter of the pores of the fiber under the effect of stress. mechanical, thus improving the dislodgement of clogging present in the pores. Reversible enlargement of the pores can also allow the use of a larger flow during backwashing.

In other embodiments, when the hollow fiber according to the invention deforms under the effect of mechanical stress, the diameter of its pores remains constant or almost constant. This makes it possible to maintain good selectivity of the membrane over time by preventing a potential irreversible enlargement of the pores from occurring over time. This can in particular be accomplished by the association, in the fiber, of a specific Young's modulus (from 2 to 30 MPa) with a specific ratio of mean outside diameter / mean inside diameter (from 1.2 to 2).

The invention also has one or preferably several of the following advantageous characteristics:

- the deformation of the fiber is reversible, so that, in certain embodiments, the pores regain their initial size when the mechanical stress ceases, which makes it possible to maintain good performance of the fiber in terms of separation power;

- the fiber returns to its initial size when the mechanical stress ceases;

- the pore size is well controlled;

- the fiber has a high permeability to water;

- the fiber has long-term resistance to chemicals used during high chemical cleaning.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 A to 1 F are snapshots extracted from a video recorded by a video camera during the backwashing described in the example below. These pictures correspond to different times from the start of the backwash:

- Figure 1A at 0 s;

- Figure 1 B at 3 s;

- Figure 1 C at 6 s;

- Figure 1 D at 9 s;

- Figure 1 E at 12 s;

- Figure 1 F at 15 s.

On all these pictures, the bundle of hollow fibers of polyvinylidene fluoride homopolymer (reference membrane) is located on the right of the photograph and the bundle of hollow fibers of a fluoride copolymer vinylidene and hexafluoropropylene (elastic membrane according to the invention) is located on the left of the photograph.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is now described in more detail and without limitation in the description which follows.

Unless otherwise indicated, all percentages for quantities are mass percentages.

In the present application, the term “a copolymer comprising units derived from vinylidene fluoride and units derived from at least one second comonomer” should be understood to mean “one or more copolymers comprising units derived from vinylidene fluoride and motifs from at least one second comonomer ”.

In the present application, the copolymer comprising units derived from vinylidene fluoride and units derived from at least one second comonomer is also called "vinylidene fluoride copolymer".

Hollow fibers and filtration modules

A first object of the invention consists of a porous hollow fiber useful for filtration. The hollow fiber comprises a copolymer comprising units originating from vinylidene fluoride and units originating from at least one second comonomer. It has a Young's modulus of less than 40 MPa. Such a Young's module gives the fiber an ability to deform under relatively low stress.

More particularly, the fiber preferably has a Young's modulus equal to from 2 to 30 MPa and the ratio of its average external diameter relative to its average internal diameter is 1.2 to 2. Such a Young's modulus associated with this ratio outer diameter / inner diameter gives the fiber an ability to deform under a relatively low stress while maintaining an almost constant pore diameter.

By "mean outside diameter" and "mean inside diameter" is meant the mean outside and inside diameter of the hollow fiber when the fiber is not subjected to any mechanical stress.

Young's modulus can be measured by a tensile test on 50 mm hollow fiber test pieces. The stress-strain curve of the hollow fiber is then determined. The stress (expressed in units of pressure) is the ratio between the tensile force and the surface of the section of the specimen, and the deformation (dimensionless) is the relative elongation of the specimen. The Young's modulus is given by the initial slope (i.e. the slope of the elastic domain) of the stress-strain curve.

Preferably, the second comonomer is chosen from the group consisting of hexafluoropropylene, fluoroethylene or vinyl fluoride, chlorofluoroethylenes (1-chloro-1-fluoroethylene and 1 -chloro-2-fluoroethylene), trifluoroethylene, chlorodifluoroethylenes ( in particular 1-chloro-2,2-difluoroethylene), 1 -bromo-2,2-difluoroethylene, bromotrifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, trifluoropropenes (in particular 3,3,3-trifluoropropene), tetrafluoropropenes (in particular the

2.3.3.3-tetrafluoropropene or 1, 3,3,3-tetrafluoropropene), chlorotrifluoropropenes (in particular 2-chloro-3,3,3-trifluoropropene), pentafluoropropenes (in particular 1, 1, 3,3,3 -pentafluoropropene or the

1 .2.3.3.3-pentafluoropropene), perfluoroalkylvinylethers of general formula Rf-0-CF-CF ₂ , Rf being an alkyl group, preferably C1 to C4, and in particular PPVE (perfluoropropylvinylether) and PMVE (perfluoromethylvinyl ether) as well than combinations of these. In a particularly preferred manner, the copolymer is a copolymer of vinylidene fluoride and hexafluoropropylene (that is to say a copolymer consisting of units derived from vinylidene fluoride and units derived from hexafluoropropylene).

Advantageously, said copolymer contains at least 50% by weight, preferably at least 75% by weight, preferably at least 85% by weight of units derived from vinylidene fluoride and up to 99% by weight of units derived from vinyl fluoride polyvinylidene. These copolymers can be obtained by known polymerization methods such as solution, emulsion or suspension polymerization. According to embodiments, the copolymer is prepared by an emulsion process in the absence of fluorinated surfactant.

The hollow fiber can have a Young's modulus of less than 1 MPa, or ranging from 1 to 5 MPa; or from 5 to 10 MPa; or from 10 to 15 MPa; or from 15 to 20 MPa; or from 20 to 25 MPa; or from 25 to 30 MPa; or from 30 to 35 MPa; or from 35 to 40 MPa. Ranges from 1 to 30 MPa, or from 2 to 20 MPa, or from 3 to 10 MPa.

More advantageously, the hollow fiber has a modulus of Youg ranging from 2 to 5 MPa; or from 5 to 10 MPa; or from 10 to 15 MPa; or from 15 to 20 MPa; or from 20 to 25 MPa; or from 25 to 30 MPa. Ranges from 3 to 13 MPa, or from 4 to 8 MPa are particularly preferred. The hollow fiber according to the invention preferably has an average outside diameter of 500 μm to 600 μm, or from 600 μm to 700 μm, or from 700 μm to 800 μm, or from 800 μm to 900 μm, or from 900 μm to 1000 pm, or from 1000 pm to 1100 pm or from 1100 pm to 1200 pm, or from 1200 pm to 1300 pm, or from 1300 pm to 1400 pm, or from 1400 pm to 1500 pm, or from 1500 pm to 1600 pm, or from 1600 pm to 1700 pm, or from 1700 pm to 1800 pm, or from 1800 pm to 1900 pm, or from 1900 pm to 2000 pm. Preferably, the fiber has an average outside diameter of 500 to 2000 μm, more preferably from 800 to 1400 μm, or from 1000 to 1400 μm. The average outside diameter can be measured on photographs obtained by scanning electron microscopy or by optical microscopy of sections of hollow fibers previously cryofractured.

The hollow fiber according to the invention preferably has an average internal diameter of 20 μm to 100 μm, or from 100 μm to 200 μm, or from 200 μm to 300 μm, or from 300 μm to 400 μm, or from 400 μm to 500 pm, or from 500 pm to 600 pm, or from 600 pm to 700 pm, or from 700 pm to 800 pm, or from 800 pm to 900 pm, or from 900 pm to 1000 pm, or from 1000 pm to 1100 pm or 1,100 pm to 1,200 pm, or 1,200 pm to 1,300 pm, or 1,300 pm to 1,400 pm, or 1,400 pm to 1,500 pm, or 1,500 pm to 1,600 pm. The average internal diameter can be measured on photographs obtained by scanning electron microscopy or by optical microscopy of sections of hollow fibers previously cryofractured.

The average thickness of the fiber wall is advantageously from 10 pm to 50 pm, or from 50 pm to 100 pm, or from 100 pm to 150 pm, or from 150 pm to 200 pm, or from 200 pm to 250 pm , or from 250 pm to 300 pm, or from 300 pm to 350 pm, or from 350 pm to 400 pm, or from 400 pm to 450 pm, or from 450 pm to 500 pm. Preferably, the average thickness of the wall of the fiber is from 100 to 400 μm, more preferably from 150 to 250 μm. The average thickness of the fiber wall can be measured on photographs obtained by scanning electron microscopy or by optical microscopy of sections of previously cryofractured hollow fibers.

The ratio of the mean outside diameter of the hollow fiber to the mean inside diameter of the hollow fiber can be from 1.08 to 2.5, more preferably from 1.6 to 2. In embodiments, the ratio of the outside diameter mean of the hollow fiber over the mean inside diameter of the hollow fiber is from 1.08 to 1.1, or from 1.1 to 1.2, or from 1.2 to 1.3, or from 1.3 to 1, 4, or 1, 4 to 1, 5, or 1, 5 to 1, 6, or 1, 6 to 1, 7 or 1, 7 to 1, 8, or 1.8 to 1, 9, or 1, 9 to 2, or 2 to 2.1, or 2.2 to 2.3, or 2.3 to 2.4, or 2.4 to 2.5.

More preferably, the ratio of the mean outside diameter of the hollow fiber to the mean inside diameter of the hollow fiber is from 1.2 to 2, even more preferably from 1.4 to 1.8. In embodiments, the ratio of the average outside diameter of the hollow fiber to the average inside diameter of the hollow fiber is 1, 2 to 1, 3, or 1, 3 to 1, 4, or 1, 4 to 1, 5, or from 1, 5 to 1, 6, or from 1, 6 to 1, 7, or from 1, 7 to 1, 8, or from 1, 8 to 1, 9, or from

1 .9 to 2.

Preferably, the hollow fiber has a Young's modulus of 3 to 13 MPa and a ratio of its mean external diameter to its mean internal diameter being 1.4 to 1.8, and more preferably a Young's modulus being

4 to 8 MPa and a ratio of its average outside diameter to its average inside diameter of about 1.6. In embodiments, the hollow fiber has a Young's modulus of 6 to 30 MPa and a ratio of its mean outside diameter to its mean inside diameter of 1.2 to 1.4; or a Young's modulus of 4 to 13 MPa and a ratio of its mean outside diameter to its mean inside diameter of 1.4 to 1.6; or a Young's modulus of 3 to 8 MPa and a ratio of its mean outside diameter to its mean inside diameter of 1.6 to 1.8; or a Young's modulus of 2 to 6 MPa and a ratio of its mean outside diameter to its mean inside diameter of 1.8 to 2.

The hollow fiber according to the invention may comprise pores having an average diameter ranging from 1 nm to 2 μm, and in particular from 1 nm to 5 nm, or from

5nm to 10nm, or from 10nm to 20nm, or from 20nm to 50nm, or from 50nm to 80nm, or from 80nm to 100nm, or from 100nm to 200nm, or from 200nm at 300nm, or from 300nm to 400nm, or from 400nm to 500nm, or from 500nm to 600nm, or from 600nm to 700nm, or from 700nm to 800nm, or from 800nm to 900 nm, or from 900 nm to 1 miti, or from 1 miti to 1,5 miti, or from 1,5 miti to 2 miti. In a particularly preferred manner, the hollow fiber may comprise pores having an average diameter ranging from 1 to 100 nm, for example when the hollow fiber is intended to be used as an ultrafiltration membrane. The hollow fiber may also advantageously include pores having an average diameter of 100 nm to 1 µm, for example when the hollow fiber is intended to be used as a microfiltration membrane. The average pore diameter can be measured on photographs of the external surface of the fibers obtained by scanning electron microscopy. The copolymer comprising units originating from vinylidene fluoride and units originating from at least one second comonomer may be the only polymer component of the hollow fiber.

Alternatively, the hollow fiber may include another polymer. In this case, the vinylidene fluoride copolymer is preferably present, relative to the total weight of polymers, in an amount of at least 50% by weight, preferably at least 60% by weight, more preferably of at least 70% by weight. Examples of other polymers which can be used, alone or in combination, are polyvinylpyrolidone (PVP) and / or polyethylene glycol (PEG), in particular as pore-forming and / or hydrophilizing agent, acrylic polymers homopolymers or copolymers, and fluoropolymers, for example a homopolymer of polyvinylidene fluoride.

In a particularly preferred manner, the hollow fiber consists essentially, or consists, of a copolymer comprising units derived from vinylidene fluoride and units derived from at least one second comonomer.

By “the hollow fiber essentially consists of a copolymer comprising units derived from vinylidene fluoride and units derived from at least one second comonomer”, it is meant that the hollow fiber comprises at least 80% by weight of the vinylidene fluoride copolymer , preferably at least 90% by weight, or at least 95% by weight, or at least 99% by weight.

In other embodiments, the hollow fiber comprises at least 50% by weight, preferably at least 60% by weight, even more preferably at least 70% by weight, of vinylidene fluoride copolymer, relative to the total weight hollow fiber.

The hollow fiber may comprise one or more additives, in particular chosen from the group consisting of poly (methyl methacrylate) (PMMA), poly (ethylene glycol) (PEG), polyvinylpyrrolidone (PVP), partially saponified polyvinyl alcohols , inorganic salts such as lithium chloride (LiCI), magnesium chloride (MgCh), zinc chloride (ZnCh), oligomers, polymers (such as those mentioned above), surfactants, copolymers with amphiphilic blocks, (nano) metallic and / or ceramic particles, and carbon nanotubes, or a combination thereof. These additives are preferably present in the fiber in an amount of 0 to 50% by weight, even more preferably from 0 to 40% by weight, even more preferably from 0 to 30% by weight, and even more preferably from 0 to 20% by weight or from 0 to 10% by weight or from 0 to 5% by weight.

According to another aspect, the invention relates to a filtration module, in particular microfiltration or ultrafiltration, comprising at least one hollow fiber as described above. The filtration module can include from 1 to 1,000,000 hollow fibers. The filtration module can be a bundle module, a flat module or a plate module, or can have any other shape.

Another object of the invention consists in the use of a hollow fiber or a filtration module as described above for the treatment of effluents, in particular water. The filtration is preferably carried out from the outside of the fiber to the inside of the fiber.

Another object of the invention is a method of unclogging a hollow fiber as described above, comprising subjecting the hollow fiber to mechanical stress so as to cause deformation of said hollow fiber. The unclogging process can also include backwashing the hollow fiber. This backwashing is preferably carried out simultaneously with the step of submitting the hollow fiber to mechanical stress. Backwashing involves the passage of a fluid, preferably a liquid, from the inside of the fiber to the outside of the fiber.

The mechanical stress can be of all types. It may for example consist of a pressure exerted by a fluid, preferably a liquid, more preferably the backwashing liquid, and / or a stretching of the fiber.

The pressure exerted by the liquid can be from 0.01 to 0.5 MPa, preferably from 0.03 to 0.2 MPa. In embodiments, the pressure exerted by the liquid is from 0.01 MPa to 0.05 MPa, or from 0.05 MPa to 0.1 MPa, or from 0.1 MPa to 0.15 MPa, or 0.15 MPa to 0.2 MPa, or from 0.2 MPa to 0.25 MPa, or from 0.25 MPa to 0.3 MPa, or from 0.3 MPa to 0.35 MPa, or from 0, 35 MPa to 0.4 MPa, or from 0.4 MPa to 0.45 MPa, or from 0.45 MPa to 0.5 MPa.

When stretching the fiber, the fiber can stretch from 1% to 20%, preferably from 3 to 7%.

Preferably, under the effect of mechanical stress, the mean outside diameter of the hollow fiber increases from 2 to 17%, preferably from 4 to 8%, for example from 2 to 4%, or from 4 to 5%, or 5 to 6%, or 6 to 7%, or 7 to 8%, or 8 to 10%, or 10 to 12%, or 12 to 14%, or 14 to 17%. Another object of the invention is a method of filtering an effluent, comprising:

- bringing the effluent into contact with a hollow fiber as described above;

- the unclogging of the hollow fiber according to a unclogging process as described above.

Manufacturing process

The hollow fibers according to the invention can be prepared by a phase inversion process.

Another object of the invention is therefore a process for preparing the fibers as described above, comprising:

- the dissolution of a copolymer comprising vinylidene fluoride units and units derived from at least one second comonomer in a solvent to form a collodion;

- the shaping of hollow collodion fiber;

- the coagulation of the shaped collodion.

Suitable solvents are for example dimethylacetamide (DMAc), N-methyl-pyrrolidone (NMP), dimethyl formamide (DMF), dimethylsulfone (DMSO2), dimethylsulfoxide (DMSO), trialkylphosphate, tetrahydrofuran (THF), acetone, benzaldehyde, acetophenone, benzophenone, hexamethylphosphoramide (HMPA), tetramethylurea (TMU), triethylphosphate (TEP), trimethylphosphate (TMP) or a combination thereof. However, the solvent can be any other solvent, or combination of solvents, capable of dissolving the vinylidene fluoride copolymer.

The collodion after dissolution of the vinylidene fluoride copolymer preferably comprises from 10 to 40% by weight of vinylidene fluoride copolymer.

The collodion can include additives, preferably in a mass amount of 0% to 50%. These additives can be, for example, poly (methyl methacrylate) (PMMA), poly (ethylene glycol) (PEG), polyvinylpyrrolidone (PVP), a partially saponified polyvinyl alcohol, inorganic salts such as lithium chloride ( LiCI), magnesium chloride (MgCh), zinc chloride (ZnCh). They can also be non-solvents of the vinylidene fluoride copolymer, for example water, ethanol, methanol, glycerol or propylene glycol, oligomers, polymers, surfactants, copolymers with amphiphilic blocks, additives type (nano) metallic and / or ceramic particles, or carbon nanotubes. It can also be a combination of two or more of the additives mentioned above.

The hollow fiber shaping of the collodion is preferably carried out by extruding the collodion through an annular die. For example, the annular die can have an outside diameter between 0.25 and 5 mm, preferably between 0.5 and 2.5 mm, and an inside diameter between 0.1 and 2 mm, preferably between 0, 2 and 1 mm. The flow rate can be between 0.1 and 72 ml / min, more particularly between 6 and 11 ml / min. Advantageously, a so-called internal liquid solution, making it possible to form the internal volume of the hollow fibers, is coextruded with the collodion. This internal liquid preferably contains a non-solvent for the vinylidene fluoride copolymer, such as water, methanol, ethanol, glycerol, propylene glycol, and, if appropriate, one or more solvents for the copolymer of vinylidene fluoride, preferably identical to those contained in the collodion, for example NMP in an amount by weight of between 0 and 100%, in particular between 15 and 70%, and one or more additives, such as LiCl, or of the PEG. Its temperature can be between 10 and 100 ° C, in particular between 30 and 80 ° C. Its extrusion rate can be between 0.1 and 18 mL / min, more particularly between 1 and 7 mL / min.

Advantageously, the shaping of hollow fiber of the collodion is assisted by drawing of the fiber. If no modification of the drawing by stretching is carried out, the drawing speed is equal to the extrusion speed of the collodion. If a change in the formatting of the collodion by stretching is carried out, the stretching speed is different from the extrusion speed, in particular greater than the extrusion speed. The drawing speed can be between 1 and 40 m / min, more particularly between 7 and 23 m / min

The coagulation of the shaped collodion is preferably carried out by bringing the shaped collodion into contact with a coagulation solution containing a non-solvent for the copolymer. This non-solvent can be in liquid form, in vapor form or in successively vapor and then liquid form. It can in particular be water, methanol, ethanol, glycerol and / or propylene glycol. The coagulation solution may also contain one or more solvents of the vinylidene fluoride copolymer, preferably identical to those contained in the collodion, for example NMP at a concentration by weight of between 0 and 50% by weight, in particular between 0 and 10% by weight, and one or more additives, such as LiCI or PEG. Alternatively, or in addition to the treatment with a non-solvent, the coagulation of the shaped collodion can be carried out by a change in temperature of the shaped collodion, preferably by a decrease in its temperature. By way of example, the temperature of the collodion can in particular be between 10 and 130 ° C, more particularly between 50 and 80 ° C; the temperature of the coagulation bath can in particular be between 0 and 100 ° C. more particularly between 40 and 70 ° C.

Generally, during the manufacture of the fibers, the ratio of the mean outside diameter of the hollow fiber to the mean inside diameter of the hollow fiber may be reduced:

- by reducing the collodion flow;

- by increasing the flow of internal liquid;

- by combining a decrease in the flow of the collodion with an increase in the flow of the internal liquid;

- by combining an increase in the internal liquid flow with an increase in the drawing speed;

- by combining a decrease in the collodion flow with an increase in the drawing speed.

Generally, during the manufacture of the fibers, the Young's modulus of the hollow fiber can be reduced:

- by reducing the drawing speed; by increasing the porosity of the membrane, for example by reducing the amount of vinylidene fluoride copolymer or by increasing the amount of additives in the collodion.

EXAMPLES

The following examples illustrate the invention without limiting it.

Preparation of hollow fibers

Hollow fibers are prepared from a copolymer of vinylidene fluoride and hexafluoropropylene comprising 11% by weight of hexafluoropropylene, by phase inversion.

The operating conditions used for the manufacture of these fibers are as follows:

Composition of the collodion:

^■ 20% copolymer of vinylidene fluoride and hexafluoropropylene comprising 11% by weight of hexafluoropropylene ^■ 12% Poly (ethylene) glycol of average molar mass by weight Mw = 10,000 g. mol ¹

^■ 68% Dimethylsulfoxide (DMSO)

Composition of the internal liquid: water

Composition of the coagulation bath: water

Temperature :

^■ Collodion: T = 50 ° C

^■ Internal liquid: T = 49 ° C

^■ Coagulation bath: T = 49 ° C

Debit :

^■ Collodion: Q = 6.24 mL.min ¹

^■ Internal liquid: Q = 6.0 mL.min ¹

Drawing speed: 22 m.min ¹

Flow-to-stretch ratio: 1.39

Spinning height: 20 cm.

A membrane module consists of the grouping of 7 hollow fibers in a bundle (hereinafter called “elastic membrane”)

The Young's modulus of this membrane is 7 MPa. The membrane has a breaking load of 0.56 N and a breaking stress of 1.46 MPa.

A reference membrane is also produced (hereinafter referred to as a "reference membrane"). Hollow fibers are prepared from a polyvinylidene fluoride homopolymer, by the formation of a collodion comprising 15% by weight of polyvinylidene fluoride homopolymer, 3% by weight of lithium chloride and 82% of NMP.

The operating conditions used for the manufacture of the reference fibers are as follows:

Composition of the collodion:

^■ 15% polyvinylidene fluoride homopolymer (Kynar® PVDF HSV900)

^■ 3% Lithium chloride (LiCI)

^■ 82% N-Methyl-2-pyrrolidone (NMP)

Composition of the internal liquid:

^■ 85% water

^■ 15% N-Methyl-2-pyrrolidone

Composition of the coagulation bath: water

Temperature :

^■ Collodion: T = 50 ° C ^■ Internal liquid: T = 49 ° C

^■ Coagulation bath: T = 49 ° C

Debit :

^■ Collodion: Q = 8.5 mL.min ^{· 1}

^■ Internal liquid: Q = 2.1 mL.min ^{· 1}

Drawing speed: 8 m.min ^{· 1}

Flow-to-stretch ratio: 1.14

Spinning height: 18 cm.

The Young's modulus of this membrane is 55 MPa. It has a breaking load of 2 N and a breaking stress of 8 MPa.

The properties of the elastic membrane and the reference membrane are summarized in the table below.

Backlash study

The elastic membrane and the reference membrane are used to filter a suspension of bentonite in water so that an identical deposit is formed on the different fibers of the two membranes. Filtration is carried out from the outside of the fiber towards the inside, the clogging settling on the outside surface of the fibers. The two membranes are then simultaneously subjected to backwashing: water is passed from the inside of the fibers to the outside, at a pressure of 0.9 bar, and the elimination of the clogging of the fibers over time is observed by video camera.

Snapshots from the video taken by the camera, at different backwashing times are shown in Figures 1A to 1F. The elastic membrane is on the left on the pictures, the reference membrane is on the right. The clogging separates from the hollow fiber in the form of filaments.

It is observed that at t = 0 s (FIG. 1 A), only the two fiber bundles are visible, no unclogging is observed.

At t = 3 s (FIG. 1 B), some filaments of clogging begin to peel away from the fibers of the elastic membrane. No filament of clogging detaches from the reference membrane.

Between t = 6 s and t = 12 s (Figures 1C, 1D and 1 E), clogging filaments continue to peel away from the fibers of the elastic membrane, while no unclogging is observed on the reference membrane.

At t = 15 s (FIG. 1F), some rare filaments still detach from the elastic membrane while on the reference membrane the deposit remains adherent.

It is therefore found that the hollow fibers according to the invention unclog faster, and more efficiently, than the rigid fibers of polyvinylidene fluoride homopolymer.

In addition, the water permeability of the elastic backwashing membrane, that is to say from the inside of the fiber to the outside, as well as the outside diameter of the fiber, are measured as a function of the pressures. backwashing applied.

The results are presented in the tables below.

The relative pore radius is given with respect to the pore radius at the backwashing pressure of 0.3 bar and is estimated by the permeability measurement, considering that this is proportional to the radius at the power 4.

It can be seen that, overall, the outside diameter of the fibers of the elastic membrane increases with the backwashing pressure, but that the diameter of the pores remains almost constant.

Claims

1. Porous hollow fiber comprising a copolymer comprising units from vinylidene fluoride and units from at least one second comonomer, having a Young's modulus of 2 to 30 MPa and in which the ratio of its mean outside diameter by compared to its average internal diameter is worth 1, 2 to 2.

2. Hollow fiber according to claim 1, consisting essentially or consisting of a copolymer comprising units from vinylidene fluoride and units from at least one second comonomer.

3. Hollow fiber according to claim 1 or 2, in which the second comonomer is chosen from the group consisting of hexafluoropropylene, trifluoroethylene, chlorotrifluoroethylene, 1-chloro-1-fluoroethylene, tetrafluoroethylene and combinations thereof. ci, the second comonomer preferably being hexafluoropropylene.

4. Hollow fiber according to one of claims 1 to 3, wherein the units derived from vinylidene fluoride are present in the copolymer in a mass amount of at least 50% by weight, preferably at least 75% by weight. weight, more preferably at least 85% by weight, and up to 99% by weight.

5. Hollow fiber according to one of claims 1 to 4, having a Young's modulus of 3 to 13 MPa and in which the ratio of its mean outside diameter to its mean inside diameter is from 1.4 to 1.8 .

6. Hollow fiber according to one of claims 1 to 5, comprising pores having an average diameter of 1 nm to 2 µm, preferably from 1 nm to 100 nm.

7. Hollow fiber according to one of claims 1 to 6, having an average outside diameter of 500 to 2000 microns, preferably from 800 pm to 1400 pm.

8. Filtration module comprising at least one hollow fiber according to one of claims 1 to 7.

9. Method for manufacturing a hollow fiber according to one of claims 1 to 7 comprising:

- the shaping of hollow collodion fiber;

- the coagulation of the shaped collodion.

10. The method of claim 9, wherein the coagulation of the shaped collodion is carried out by bringing the shaped collodion into contact with a coagulation solution containing a non-solvent for the copolymer.

11. The method of claim 9 or 10, wherein the coagulation of the shaped collodion is effected by a change in temperature of the shaped collodion, preferably a decrease in temperature.

12. Use of a hollow fiber according to one of claims 1 to 7 or a filtration module according to claim 8, for the treatment of effluents.

13. Method for unclogging a hollow fiber according to one of claims 1 to 7 comprising:

- optionally backwashing the hollow fiber.

14. The unclogging method according to claim 13, wherein the mechanical stress is the pressure exerted by the backwashing liquid and / or a stretching of the hollow fiber.

15. Process for the filtration of an effluent, comprising:

- bringing the effluent into contact with a hollow fiber according to one of claims 1 to 7;

- the unclogging of the hollow fiber according to the unclogging method of claim 13 or 14.