Process for preparing a porous thermoplastic polyolefin membrane.

Abstract

The invention relates to a method for preparing a porous thermoplastic polyolefin membrane by mass mixing a thermoplastic polyolefin, a water-soluble inorganic filler, and a water-soluble polymer. The thermoplastic polyolefin is insoluble in water, and the thermoplastic polyolefin and the water-soluble polymer are incompatible in the mixture. The resulting mixture is formed into a film, washed with water to remove all or part of the water-soluble inorganic filler and the water-soluble polymer from the polymer film, thus obtaining the porous thermoplastic polyolefin membrane.

Description

Title of the invention: Process for preparing a porous thermoplastic polyolefin membrane.

[0001] The field of the invention is that of processes for preparing porous membranes based on thermoplastic polyolefins as well as that of porous membranes based on thermoplastic polyolefins.

[0002] The synthesis of porous membranes based on thermoplastic polymers is described in the state of the art relating, for example, to filtration devices, electrochemical devices, textiles. For example, WO 2021176080 may be cited, which describes the preparation of a film produced by extrusion of a mixture of a thermoplastic polymer such as a polyolefin, a polyamide or a polyester and calcium carbonate particles, followed by the extraction of the calcium carbonate particles by washing with water to produce a porous membrane. Mention may also be made of document US 20060003214 which describes the preparation of a film by mixing a polymer, a low volatility solvent and a compound, either organic or inorganic, with a molecular mass of less than 10,000, followed by extraction, using another solvent, of the compound, organic or inorganic, and the low volatility solvent to produce a porous membrane.

[0003] The Applicant has developed a new process which makes it possible to improve the mechanical properties of a porous membrane made of a thermoplastic polyolefin without the volume fraction of the porosity of the membrane being modified.

[0004] Thus, a first object of the invention is a process for preparing a porous thermoplastic polyolefin membrane, which process comprises the following steps a), b) and c): (a) Preparing a mixture which contains a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer by mass blending a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer, thermoplastic polyolefin being insoluble in water, the thermoplastic polyolefin and the water-soluble polymer being incompatible in the mixture, in the mixing step the volume of the thermoplastic polyolefin being greater than the volume of the water-soluble polymer, the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer being greater than 50% and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and water-soluble polymer and the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler being less than or equal to 0.5, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing all or part of the water-soluble inorganic filler and the water-soluble polymer from the polymer film by contacting the polymer film with water to obtain the porous thermoplastic polyolefin membrane.

[0005] Another subject of the invention is a porous thermoplastic polyolefin membrane capable of being obtained by the process according to the invention. Detailed description of the invention

[0006] The polymers mentioned in the description may be of fossil or bio-sourced origin. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. In the same way, they may also come from the recycling of materials already used, that is to say they may be, partially or totally, derived from a recycling process, or obtained from raw materials themselves derived from a recycling process.

[0007] In the present invention, the term "water-soluble inorganic filler" means an inorganic filler which has a solubility of at least 10 g / L in distilled water (pH 7) at a temperature of 20°C.

[0008] In the present invention, as is well known, the term “filler” means a material which is generally in the form of particles, whether aggregated or not.

[0009] In the present invention, the term inorganic to qualify a chemical substance, in particular a filler, refers in a well-known manner to a substance which does not contain a carbon hydrogen bond.

[0010] In the present invention, by water-soluble polymers is meant polymers which, in solution in distilled water (pH 7) with a concentration of between 0.01% and 50%, at a temperature of approximately 20°C, do not give macroscopic phase separation.

[0011] The terms "membranes" and "films" are well known to those skilled in the technical field. It is recalled that, in a well-known manner, a membrane is a structure as defined by IUP AC, in "IUP AC recommendations 1996". Also in a well-known manner and in accordance with the definition given in IUP AC, the term "film" is understood according to the definition given by IUP AC in "IUP AC recommendations 1996".

[0012] In the present application, the term thermoplastic polyolefin membrane is understood to mean a membrane which is made entirely or partly of a thermoplastic polyolefin, preferably a membrane which is made entirely of a thermoplastic polyolefin.

[0013] As is well known, polymers in a mixture are considered incompatible when the mixture exhibits phase segregation. Phase segregation can be observed by various methods well known to those skilled in the art, such as microscopy analysis or differential scanning calorimetry (DSC) analysis. As is well known, a mixture of two incompatible polymers typically exhibits two glass transition temperatures.

[0014] According to the invention, in step a) a mixture is prepared consisting of a thermoplastic polyolefin, a water-soluble polymer and a water-soluble inorganic filler. The preparation of the mixture is carried out in bulk, typically by thermomechanical mixing which can be carried out in an internal mixer, in a screw machine such as an extruder. The mixing is preferably carried out at a temperature greater than or equal to the glass transition or melting temperature which is the highest among the glass transition and melting temperatures of the thermoplastic polyolefin and the water-soluble polymer. Preferably, the thermoplastic polyolefin is the polymer of the mixture which has the glass transition temperature or, when the thermoplastic polyolefin has a melting temperature, the highest melting temperature.

[0015] In the mixing step of step a), that is to say for the preparation of the mixture of step a), the volume of the thermoplastic polyolefin used is greater than the volume of the water-soluble polymer used, the sum of the volumes used of the water-soluble inorganic filler and the water-soluble polymer is greater than 50% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer and the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is less than or equal to 0.5, preferably less than or equal to 0.3. Preferably, the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer is greater than 60% and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer. Preferably, the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is greater than or equal to 0.01 and less than or equal to 0.3.

[0016] The respective application of the preferential ranges which relate to the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer and the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler leads to obtaining a membrane having the best performance compromise between the porosity rate and the elongation at break.

[0017] Once prepared, the mixture is transformed into a film in step b). Between step a) and step b), the mixture can be stored, in particular at room temperature (23°C). The film can be prepared by any means known to those skilled in the art for producing films, such as placing under a press, calendering, extrusion-blowing, etc. Those skilled in the art use one of the known means for producing a film, taking into account the target dimensions of the film, in particular its thickness, the physical properties of the film, in particular the ductility of the mixture and the glass transition temperature or, where appropriate, the melting temperature of the thermoplastic polyolefin and the water-soluble polymer. They also adapt the operating conditions for transformation, such as the temperature to which the mixture is brought during step b), depending on the mixture to be converted into a film.Preferably, like step a), step b) is carried out at a temperature greater than or equal to the glass transition or melting temperature which is the highest of the glass transition and melting temperatures of the thermoplastic polyolefin and the water-soluble polymer.

[0018] The mixture is converted into a film whose thickness is defined by the intended use of the membrane prepared according to the invention. The thickness of the film prepared in step b) also determines the thickness of the membrane, the two thicknesses being practically identical. The polymer film preferably has a thickness greater than or equal to 1 μm and less than or equal to 200 μm.

[0019] Once the film has been formed, step c) is carried out. Step c) may be preceded by a step of cooling the formed film to bring the film to a temperature lower than the temperature to which the mixture was brought in step b). For example, the film may be cooled to a temperature close to room temperature (23°C), simply by storing the film at room temperature before it is subjected to step c).

[0020] Step c) is typically a film washing step to extract the water-soluble parts of the film, namely the water-soluble inorganic filler and the water-soluble polymer. It can be carried out by immersing the film in a bath containing water, preferably brought to a temperature greater than or equal to room temperature (23°C).

[0021] Step c) typically uses a selective extraction process using water to selectively dissolve the water-soluble portions of the film. Removal of the water-soluble inorganic filler and the water-soluble polymer creates pores in the polymer film to form the porous thermoplastic polyolefin membrane. Removal of both the water-soluble inorganic filler and the water-soluble polymer creates a network of pores in the polymer film in which some or all of the pores, preferably all of the pores, are interconnected. The inventors believe that the thermoplastic polyolefin forms one continuous phase and the water-soluble portions form a second continuous phase. It is the removal of the second continuous phase that forms interconnected cavities in the polymer film.The method according to the invention results in a porous film of a thermoplastic polyolefin, referred to in the present application as a porous thermoplastic polyolefin membrane and characterized by a porosity network in which some or all of the pores, preferably all of the pores, are connected to each other. The method according to the invention thus makes it possible to create a membrane with a particular porosity which is different from that obtained by the methods known from the state of the art not implementing the sequence of steps a), b) and c). Moreover, the membranes according to the invention have improved mechanical strength compared to thermoplastic polyolefin membranes which have the same volume fraction of porosity as the membranes according to the invention, but which are not produced according to the method according to the invention.

[0022] Step c) may be followed by a drying step. Drying may be carried out under an inert atmosphere such as nitrogen or argon, or under an ambient atmosphere (air). Drying may be accelerated if it is carried out at a temperature above ambient temperature (23°C), for example at a temperature ranging from 60°C to 100°C. It may also be accelerated if it is carried out under the scavenging of a gas such as nitrogen, argon or air. Drying may be carried out until a constant weight of the membrane to be dried is obtained.

[0023] Thermoplastic polyolefins are well known to those skilled in the art who can refer to encyclopedias or works known as "Handbooks" relating to the properties of polymers. Polyolefins are generally insoluble in water. Preferably, the thermoplastic polyolefin has a glass transition temperature or a melting temperature greater than 80°C. The glass transition temperature (Tg) and the melting temperature (Tf) are measured in a known manner by DSC (Differential Scanning Calorimetry) according to standard ASTM D3418 (2015). The thermoplastic polyolefin is preferably chosen from the group consisting of polyethylenes, polypropylenes, polybutenes (or poly(but-1-ene)), polymethylpentenes (or poly(4-methylpent-1-ene)). Polyolefins which are more preferably suitable are, for example, polyethylenes, even more preferably high density polyethylenes, known by the abbreviation HDPE or PE-HD.

[0024] Water-soluble polymers are also known to those skilled in the art, who may also refer to works known as "Handbooks" relating to the properties of polymers. As water-soluble polymers, examples that may be mentioned are those of the family of polyethylene oxides (PEO), polyvinyl alcohols (PVOH), polyvinylpyrrolidones (PVP), and polysaccharides. From their general knowledge supplemented by the data in the aforementioned "Handbooks", those skilled in the art know how to choose a suitable polymer that is both water-soluble and incompatible with the thermoplastic polyolefin. The water-soluble polymer is preferably a polyethylene oxide.

[0025] The thermoplastic polyamide and the water-soluble polymer may be commercially available products or polymers synthesized to order for the purposes of the invention. The polymers useful for the purposes of the invention may contain, in minor quantities, generally less than 5% of the weight of polymer, conventionally used additives such as antioxidants, flame retardants, antistatic agents.

[0026] The water-soluble inorganic filler useful for the purposes of the invention may be any inorganic filler provided that it is water-soluble. As a water-soluble inorganic filler, mention may be made of salts from the families of alkali metals, alkaline earth metals, and halides. In choosing a water-soluble salt, a person skilled in the art may refer to works known as "Handbooks" relating to the properties of inorganic substances. The inorganic filler is preferably a halide salt, more preferably a halide of an alkali metal or an alkaline earth metal, even more preferably a chloride of an alkali metal, better still sodium chloride.

[0027] The water-soluble inorganic filler is typically in powder form. It can be a commercially available product or be custom-synthesized. It can be obtained mechanically, for example by reducing an inorganic solid substance to a more or less fine powder, or chemically, depending on the desired particle size of the inorganic filler.

[0028] The water-soluble inorganic filler preferably consists of particles having a median diameter D50 ranging from 200 nm to 100,000 nm, preferably from 500 nm to 100,000 nm, more preferably from 1000 nm to 100,000 nm, even more preferably from 1000 nm to 50,000 nm. Advantageously, for any of the D50 ranges, the particles have a D10 which is not less than 0.1 times the D50 value and a D90 which is not greater than 10 times the D50 value. The sizes D50, D10 and D90 are commonly used sizes by skilled person to characterize the particle sizes and their volume distribution. Dx is the diameter for which particles with a diameter less than Dx constitute x% of the total particle volume. In other words, the volume of particles with a diameter less than D50 is equal to 50% of the total particle volume. D10 is such that the volume of particles with a diameter less than D10 is equal to 10% of the total particle volume; D90 is such that the volume of particles with a diameter less than D90 is equal to 90% of the total particle volume. The values ​​of D10, D50 and D90 are measured in a well-known manner using a laser particle size analyzer using the ISO 13320:2009 standard.

[0029] The porous thermoplastic polyolefin membrane according to the invention which can be obtained by the process according to the invention can have a porosity corresponding to a volume fraction of up to 80%, this volume fraction resulting from the sum of the volume fractions of the water-soluble inorganic filler and the water-soluble polymer used in the preparation of the membrane and extracted in the process according to the invention. Preferably, the membrane according to the invention has a total volume porosity ranging from 50% to 80% of the volume of the membrane. Preferably, the membrane according to the invention is characterized by a porosity network in which all of the pores are connected to each other.

[0030] The membrane has a thickness which is defined by that of the film. It preferably has a thickness greater than or equal to 1 μm and less than or equal to 200 μm. The other dimensions of the membrane may be those of the film or different depending on whether or not the film has undergone a cutting operation between step b) and step c).

[0031] In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 19:

[0032] Method 1: Process for preparing a porous thermoplastic polyolefin membrane, which process comprises the following steps a), b) and c): (a) Preparing a mixture which contains a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer by mass blending a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer, thermoplastic polyolefin being insoluble in water, the thermoplastic polyolefin and the water-soluble polymer being incompatible in the mixture, in the mixing step the volume of the thermoplastic polyolefin being greater than the volume of the water-soluble polymer, the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer being greater than 50% and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer and the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler being less than or equal to 0.5, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing all or part of the water-soluble inorganic filler and the water-soluble polymer from the polymer film by contacting the polymer film with water to obtain the porous thermoplastic polyolefin membrane.

[0033] Mode 2: Process according to mode 1 in which the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer is greater than 60% and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer.

[0034] Mode 3: Process according to mode 1 or 2 in which the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is less than or equal to 0.3.

[0035] Mode 4: Process according to any one of modes 1 to 3 in which the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is greater than or equal to 0.01 and less than or equal to 0.3.

[0036] Mode 5: Process according to any one of modes 1 to 4 in which steps a) and b) are carried out at a temperature greater than or equal to the glass transition or melting temperature which is the highest among the glass transition and melting temperatures of the thermoplastic polyolefin and the water-soluble polymer.

[0037] Mode 6: Process according to any one of modes 1 to 5 in which the mixing of step a) is thermomechanical mixing.

[0038] Mode 7: Process according to any one of modes 1 to 6 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 200 nm to 100,000 nm.

[0039] Mode 8: Process according to any one of modes 1 to 7 in which the water-soluble inorganic filler consists of particles having a median diameter D50, ranging from 500 nm to 100,000 nm.

[0040] Mode 9: Process according to any one of modes 1 to 8 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 1000 nm to 100,000.

[0041] Mode 10: Process according to any one of modes 1 to 9 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 1000 nm to 50,000 nm.

[0042] Mode 11: A method according to any one of modes 7 to 10 wherein the particles have a D10 which is not less than 0.1 times the value of the D50 and a D90 which is not greater than 10 times the value of the D50.

[0043] Model 2: Process according to any one of modes 1 to 11 in which the thermoplastic polyolefin has a glass transition temperature or a melting temperature greater than 80°C.

[0044] Method 13: Process according to any one of methods 1 to 12 in which the thermoplastic polyolefin is chosen from the group consisting of polyethylenes, polypropylenes, polybutenes, polymethylpentenes.

[0045] Mode 14: Process according to any one of modes 1 to 13 in which the thermoplastic polyolefin is a high density polyethylene.

[0046] Mode 15: Process according to any one of modes 1 to 14 in which the water-soluble polymer is a polyethylene oxide.

[0047] Mode 16: Method according to any one of modes 1 to 15 in which the polymer film has a thickness greater than or equal to 1 μm and less than or equal to 200 μm.

[0048] Mode 17: Method according to any one of modes 1 to 16 in which the porous thermoplastic polyolefin membrane has a total volume porosity ranging from 50% to 80% of the volume of the membrane.

[0049] Mode 18: Method according to any one of modes 1 to 17 in which the porous thermoplastic polyolefin membrane has a thickness greater than or equal to 1 μm and less than or equal to 200 μm.

[0050] Method 19: Porous thermoplastic polyolefin membrane capable of being obtained by the process defined in any one of methods 1 to 18.

[0051] The above-mentioned characteristics of the present invention, as well as others, will be better understood upon reading the following description of several exemplary embodiments of the invention, given for illustrative and non-limiting purposes. Examples

[0052] Three porous thermoplastic polyolefin membranes, M1 to M3, are prepared. Membranes M1 to M3 are prepared from the respective mixtures, compositions C1 to C3, described in Table 1.

[0053] Table 1: Composition Cl C2 C3 Thermoplastic polyolefin (%vol) 30 30 30 Water-soluble inorganic filler (%vol) 70 65 55 Water-soluble polymer (%vol) 5 15 Water-soluble phase fraction (%vol) 70% 70% 70%

[0054] Thermoplastic polyolefin: high density polyethylene ERACLENE MP90U from ENI VERSALIS, Tg -110°C and Tf 137°C; Water-soluble inorganic filler: SIGMA-ALDRICH sodium chloride (case: 7647-14-5), 99% purity from SIGMA-ALDRICH, reference S9888, ground with an IKA-WERK mill for 1 minute and then sieved through a 20 pm sieve (mesh 625); The size population thus obtained is defined as follows: D10: 14 pm; D50: 29 pm; D90: 50 pm. The values ​​of D10, D50 and D90 are measured by laser diffraction which allows the particle size distribution to be determined in volume percentage using a dry granulometer; Water-soluble polymer: polyethylene oxide from SIGMA-ALDRICH, reference 181986 (Mv 100000; viscosity = 12 to 50 cPs at 5% in water at 25°C), Tf 63°C.

[0055] Membranes M1, M2 and M3 are prepared according to the following procedure. The constituents, weighed beforehand according to the masses indicated in Table 2, are first mixed in a HAAKE PolyLab internal mixer with an internal useful volume of 69 cm3, which is filled to 75% and which is equipped with Roller rotors.

[0056] [Tables2] Mass introduced (g) Ml M2 M3 ERACLENE MP90U 14.7 14.7 14.7 NaCl S9888 77.8 72.3 61.1 PEO 181986 0.0 3.2 9.7

[0057] The constituents are introduced into the tank previously heated to 150°C. The procedure is as follows: The thermoplastic polyolefin is introduced into the mixer, then mixed for 30 seconds at a speed of 30 revolutions per minute, then the water-soluble fraction is introduced. In the case of M1, only the water-soluble inorganic filler is introduced; in the case of M2 and M3, the water-soluble inorganic filler and the water-soluble polymer are introduced. Once all the components have been introduced, the mixing speed is increased to 50 revolutions per minute for 3 minutes. During this time, every minute the mixer chamber is opened by the upper cylinder for 5 seconds, then the chamber is closed. After these 3 minutes, the mixture is removed from the mixer and cooled to room temperature.

[0058] Then, the mixture is pressed at 160°C and 100 kN to obtain a layer 1 mm thick. This layer is then cut into 5 identical surfaces, which are then stacked on top of each other. The assembly formed by the stacking of the layers is pressed again. This step is carried out 5 times. A 1 mm thick mixing plate is obtained.

[0059] A 3 cm diameter pellet is taken from the 1 mm mixing plate thick. This pellet is placed between two aluminum sheets, framed by 0.1 mm thick steel shims, then the whole is pressed at 160°C and 200 kN for 3 minutes. This produces a 0.1 mm thick mixture film.

[0060] The assembly after pressing is cooled on a cooling plate at 23°C for 3 minutes.

[0061] Once the assembly has cooled, the upper aluminum sheet is removed, then the film, still on the lower aluminum sheet, is immersed for one hour in a bath of distilled water heated to 50°C in order to carry out the step of extracting the water-soluble phase, in the absence of agitation of the water bath.

[0062] Finally, the film from which the water-soluble phase has been extracted is peeled off the aluminum foil, then rinsed under a stream of distilled water for one minute and left to dry in an oven at 60°C for 2 hours. The 0.1 mm thick porous thermoplastic membrane is recovered.

[0063] Since the mixture (composition C1) does not contain a water-soluble polymer, the membrane M1 is prepared according to a process not in accordance with the invention and is therefore a membrane not in accordance with the invention.

[0064] Membranes M2 and M3 are prepared according to a process in accordance with the invention and are therefore in accordance with the invention.

[0065] For each of the prepared membranes, its elongation at break is measured according to the method described below.

[0066] Method for determining elongation at break: Elongation at break is determined by tensile tests. Unless otherwise stated, tensile tests are carried out in accordance with French standard ASTM D882-10, using a strip-type specimen, with a tensile speed of 50 mm / min. Elongations at break (in %) are measured at 23°C ± 2°C.

[0067] The results are shown in Table 3.

[0068] [Tables3] Membrane Ml M2 M3 Elongation at break 50% 130% 100%

Claims

Claims

1. A method for preparing a porous thermoplastic polyolefin membrane, which method comprises the following steps a), b) and c): a) Preparing a mixture which contains a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer by mass mixing a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer, the thermoplastic polyolefin being insoluble in water, the thermoplastic polyolefin and the water-soluble polymer being incompatible in the mixture, in the mixing step the volume of the thermoplastic polyolefin being greater than the volume of the water-soluble polymer, the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer being greater than 50% and less than 85% of the sum of the volumes of the thermoplastic polyolefin,of the water-soluble inorganic filler and the water-soluble polymer and the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler being less than or equal to 0.5, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing all or part of the water-soluble inorganic filler and the water-soluble polymer from the polymer film by bringing the polymer film into contact with water to obtain the porous thermoplastic polyolefin membrane.,

2. The method of claim 1 wherein the sum of the volumes of the water-soluble inorganic filler and the water-soluble polymer is greater than 60% and less than 85% of the sum of the volumes of the thermoplastic polyolefin, the water-soluble inorganic filler and the water-soluble polymer.

3. A method according to claim 1 or 2 wherein the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is less than or equal to 0.

3.

4. A method according to any one of claims 1 to 3 wherein the ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is greater than or equal to 0.01 and less than or equal to 0.

3.

5. A method according to any one of claims 1 to 4 wherein the mixing of step a) is thermomechanical mixing.

6. A method according to any one of claims 1 to 5 wherein steps a) and b) are carried out at a temperature greater than or equal to the glass transition or melting temperature which is the higher of the glass transition and melting temperatures of the thermoplastic polyolefin and the water-soluble polymer.

7. Method according to any one of claims 1 to 6 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 200 nm to 100,000 nm, preferably ranging from 500 nm to 100,000 nm, more preferably ranging from 1000 nm to 100,000, even more preferably ranging from 1000 nm to 50,000 nm.

8. The method of claim 7 wherein the particles have a D10 that is not less than 0.1 times the value of the D50 and a D90 that is not more than 10 times the value of the D50.

9. Process according to any one of claims 1 to 8 in which the thermoplastic polyolefin is chosen from the group consisting of polyethylenes, polypropylenes, polybutenes, polymethylpentenes.

10. A method according to any one of claims 1 to 9 wherein the thermoplastic polyolefin is a high density polyethylene.

11. A method according to any one of claims 1 to 10 wherein the water-soluble polymer is polyethylene oxide.

12. A method according to any one of claims 1 to 11 wherein the polymer film has a thickness greater than or equal to 1 pm and less than or equal to 200 pm.

13. A method according to any one of claims 1 to 12 wherein the porous thermoplastic polyolefin membrane has a total volume porosity ranging from 50% to 80% of the membrane volume.

14. A method according to any one of claims 1 to 13 wherein the porous thermoplastic polyolefin membrane has a thickness greater than or equal to 1 pm and less than or equal to 200 pm.

15. Porous thermoplastic polyolefin membrane obtainable by the process defined in any one of claims 1 to 14.