Method for the preparation of a porous thermoplastic polyolefin membrane

Abstract

The invention relates to a method for preparing a porous thermoplastic polyolefin membrane by bulk blending a thermoplastic polyolefin, a water-soluble inorganic filler and a water-soluble polymer, wherein 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 shaped into a film, which is washed with water to remove all of the water-soluble inorganic filler and the water-soluble polymer from said polymer film in order to obtain the porous thermoplastic polyolefin membrane.

Description

[0001] Process for the preparation of a porous thermoplastic polyolefin membrane

[0002] 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.

[0003] 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 extruding 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. Also, US 20060003214 may be mentioned, 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 weight of less than 10,000, followed by extraction, using another solvent, of the organic or inorganic compound and the low-volatility solvent to produce a porous membrane.

[0004] 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 modifying the volume fraction of the porosity of the membrane.

[0005] Thus, a first subject 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 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 0.3, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing from the polymer film all of the water-soluble inorganic filler and the water-soluble polymer by bringing the polymer film into contact with water to obtain the porous thermoplastic polyolefin membrane.,

[0006] Another subject of the invention is a porous thermoplastic polyolefin membrane capable of being obtained by the process according to the invention.

[0007] Detailed description of the invention

[0008] 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. Similarly, they may also come from the recycling of materials already in use, 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.

[0009] 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.

[0010] In the present invention, as is well known, filler is understood to mean a material which is generally in the form of particles, whether aggregated or not.

[0011] In the present invention, the term inorganic to describe a chemical substance, in particular a filler, refers in a well-known manner to a substance which does not contain a carbon-hydrogen bond.

[0012] In the present invention, water-soluble polymers are 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.

[0013] The terms "membranes" and "films" are well known to those skilled in the technical field. It is recalled that, as is well known, a membrane is a structure as defined by IUPAC, in "IUPAC recommendations 1996". Also as is well known and in accordance with the definition given in IUPAC, the term "film" is understood according to the definition given by IUPAC in "IUPAC recommendations 1996".

[0014] In the present application, the term thermoplastic polyolefin membrane means a membrane which consists entirely of a thermoplastic polyolefin.

[0015] As is well known, polymers in a blend are considered incompatible when the blend 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 blend of two incompatible polymers typically exhibits two glass transition temperatures.

[0016] 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.

[0017] In the mixing step of step a), i.e. for preparing the mixture of step a), the volume of thermoplastic polyolefin used is greater than the volume of 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 0.3, preferably less than 0.3.

[0018] 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.

[0019] 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. The ratio between the volume of the water-soluble polymer and the volume of the water-soluble inorganic filler is advantageously less than 0.25, more advantageously less than 0.2.

[0020] The respective application of the preferred ranges which concern 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.

[0021] 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 press-pressing, 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.

[0022] 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.

[0023] 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).

[0024] 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).

[0025] Step c) typically involves a selective extraction process using water to selectively dissolve the water-soluble portions of the film. The 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. The removal of both the water-soluble inorganic filler and the water-soluble polymer creates a network of pores in the polymer film in which 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 all 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.

[0026] 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 ambient atmosphere (air). Drying may be accelerated if 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 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.

[0027] Thermoplastic polyolefins are well known to those skilled in the art who may 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 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)). The most suitable polyolefins are polyethylenes, even more preferably high-density polyethylenes, known by the abbreviation HDPE or PE-HD.

[0028] 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. Examples of water-soluble polymers that may be mentioned include those from the family of polyethylene oxides (PEO), polyvinyl alcohols (PVOH), polyvinylpyrrolidones (PVP), and polysaccharides. Based on 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.

[0029] 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 the polymer, additives traditionally used such as antioxidants, flame retardants, antistatic agents. 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.

[0030] Water-soluble inorganic filler typically comes 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.

[0031] 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.

[0032] The quantities of D50, D10 and D90 are quantities commonly used by those skilled in the art to characterize particle sizes and their volume distribution. Dx is the diameter for which particles with a diameter less than Dx constitute x% of the total volume of the particles. In other words, the volume of particles with a diameter less than D50 is equal to 50% of the total volume of the particles. The D10 is such that the volume of particles with a diameter less than D10 is equal to 10% of the total volume of the particles; the D90 is such that the volume of particles with a diameter less than D90 is equal to 90% of the total volume of the particles. 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.

[0033] 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. The membrane according to the invention is typically characterized by a porosity network in which all of the pores are connected to each other.

[0034] 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).

[0035] In summary, the invention is advantageously implemented according to any one of the following embodiments 1 to 22:

[0036] Method 1: 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 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 0.3, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing from the polymer film all of the water-soluble inorganic filler and the water-soluble polymer by bringing the polymer film into contact with water to obtain the porous thermoplastic polyolefin membrane.,

[0037] 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.

[0038] 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 0.3.

[0039] 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.

[0040] Mode 5: Process according to any one of modes 1 to 4 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 0.25. Mode 6: Process according to any one of modes 1 to 5 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 0.2.

[0041] Mode 7: Process according to any one of modes 1 to 6 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.

[0042] Mode 8: Process according to any one of modes 1 to 7 in which the mixing of step a) is thermomechanical mixing.

[0043] 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 200 nm to 100,000 nm.

[0044] 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 500 nm to 100,000 nm.

[0045] Mode 11: Process according to any one of modes 1 to 10 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 1000 nm to 100000 nm.

[0046] Mode 12: Process according to any one of modes 1 to 11 in which the water-soluble inorganic filler consists of particles having a median diameter D50 ranging from 1000 nm to 50000 nm.

[0047] Mode 13: A method according to any one of modes 7 to 12 in which 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.

[0048] Mode 14: A process according to any one of modes 1 to 13 wherein the thermoplastic polyolefin has a glass transition temperature or a melting temperature greater than 80°C.

[0049] Method 15: Process according to any one of methods 1 to 14 in which the thermoplastic polyolefin is chosen from the group consisting of polyethylenes, polypropylenes, polybutenes, polymethylpentenes.

[0050] Mode 16: Process according to any one of modes 1 to 15 in which the thermoplastic polyolefin is a polyethylene.

[0051] Mode 17: Process according to any one of modes 1 to 16 in which the thermoplastic polyolefin is a high density polyethylene. Mode 18: Process according to any one of modes 1 to 17 in which the water-soluble polymer is a polyethylene oxide.

[0052] Mode 19: Method according to any one of modes 1 to 18 in which the polymer film has a thickness greater than or equal to 1 pm and less than or equal to 200 pm.

[0053] Mode 20: A method according to any one of modes 1 to 19 wherein the porous thermoplastic polyolefin membrane has a total volume porosity ranging from 50% to 80% of the membrane volume.

[0054] Mode 21: Method according to any one of modes 1 to 20 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.

[0055] Mode 22: Porous thermoplastic polyolefin membrane capable of being obtained by the process defined in any one of modes 1 to 21.

[0056] 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.

[0057] Examples

[0058] 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.

[0059] Table 1:

[0060] Thermoplastic polyolefin: high density polyethylene ERACLENE MP90U from ENI VERSALIS, Tg -110°C and Tf 137°C;

[0061] Water-soluble inorganic filler: Sodium chloride SIGMA-ALDRICH (cas: 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 particle size analyzer;

[0062] 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.

[0063] Membranes M1, M2 and M3 are prepared according to the following procedure.

[0064] 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 cm 3 , which is 75% full and is equipped with Roller rotors.

[0065] Table 2

[0066] The components are introduced into the tank, which has been preheated to 150°C. The procedure is as follows:

[0067] 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.

[0068] The mixture is then pressed at 160°C and 100 kN to obtain a 1 mm thick layer. This layer is then cut into 5 identical surfaces, which are then stacked on top of each other. The whole 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.

[0069] A 3 cm diameter pellet is taken from the 1 mm thick mixing plate. This pellet is placed between two aluminum sheets, framed by 0.1 mm thick steel shims, then the assembly is pressed at 160°C and 200 kN for 3 minutes. This produces a 0.1 mm thick mixing film.

[0070] The whole after pressing is cooled on a cooling plate at 23°C for 3 minutes. Once the whole has cooled, the upper aluminum foil is removed, then the film, still on the lower aluminum foil, is immersed for one hour in a distilled water bath heated to 50°C in order to carry out the extraction step of the water-soluble phase, in the absence of agitation of the water bath.

[0071] 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.

[0072] 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.

[0073] Membranes M2 and M3 are prepared according to a process according to the invention and are therefore in accordance with the invention. Since the volume fraction of the water-soluble part in compositions C2 and C3 is 70% and the water-soluble part is extracted in step c), the pores created by the extraction of the water-soluble part represent 70% by volume of the membrane: membranes M2 and M3 have a porosity of 70%. Since porosity is defined in a known manner as the ratio of the volume of the voids of a material to the total volume, it corresponds to the 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.In other words, in the examples the volume fraction of the water-soluble part being 70% in the films before extraction of the water-soluble part and the water-soluble part being extracted in step c), the porosity of the membrane is equal to the volume fraction of void in the membrane which corresponds to the volume fraction of the water-soluble part extracted, i.e. 70%.

[0074] For each of the prepared membranes, its elongation at break is measured according to the method described below.

[0075] Method for determining elongation at break:

[0076] 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.

[0077] The results are shown in Table 3.

[0078] Table 3

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 0.3, b) Forming the mixture into a film in the absence of solvent to form a polymer film, c) Removing from the polymer film all of the water-soluble inorganic filler and the water-soluble polymer 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. Method according to claim 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 0.

3.

4. Method according to any one of claims 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.

5. Method according to any one of claims 1 to 4 in which the mixing of step a) is thermomechanical mixing.

6. Method according to any one of claims 1 to 5 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 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 nm, even more preferably ranging from 1000 nm to 50,000 nm.

8. A method according to claim 7 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.

9. Method 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 polyethylene.

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

12. Method according to any one of claims 1 to 11 in which the water-soluble polymer is a polyethylene oxide.

13. Method according to any one of claims 1 to 12 in which the polymer film has a thickness greater than or equal to 1 μm and less than or equal to 200 μm.

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

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

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