FR3013714A1 - TALC CHARGED POROUS FLUORINATED MEMBRANE IMPLEMENTED BY A STRETCHING METHOD - Google Patents

Abstract

The present invention relates to a polymer composition based on talc doped polyvinylidene fluoride (PVDF) and to porous membranes made therefrom. The invention also relates to the process for preparing porous membranes from said formulation, comprising several stretching steps. The membranes thus obtained have various applications, in particular in the filtration of water, in the field of energy storage (Li-ion batteries for example) as electrode separators, etc.

Description

FIELD OF THE INVENTION The present invention relates generally to charged porous fluorinated membranes. More specifically, the invention relates to a polymer composition based on talc doped polyvinylidene fluoride (PVDF) and to porous membranes made therefrom. The invention also relates to the process for preparing porous membranes from said composition, comprising several drawing steps. The membranes thus obtained have various applications, in particular in the filtration of water, in the field of energy storage (Li-ion batteries for example) as electrode separators. BACKGROUND OF THE INVENTION Hydrophobic microporous membranes have many advantages: excellent chemical resistance, biocompatibility, mechanical strength and good separation capabilities. They have multiple applications, especially in water filtration, dialysis, desalination or gas separation. The porous membranes used in the field of water filtration (micro, ultra and nanofiltration) are mainly polysulfone (PSU), polyethersulfone (PES), polypropylene (PP) or PVDF. In the case of battery separators, the most frequently claimed polymers are polyolefins (PP, low density polyethylene (PE), high density (HD), very low density (VLD) or very high molecular weight (UHMW). )). Among these, PVDF membranes have outstanding properties of chemical resistance, thermal resistance and radiation resistance. When used as separators in Li-ion batteries or in supercapacitors, PVDF-based membranes have the following advantages: good chemical stability to organic acids, bases, strong oxidizers, halogenated solvents; thermal stability; good mechanical strength and good affinity for liquid electrolytes.

The known PVDF microporous membranes are mainly used by the solvent route (NIPS process (Nonsolvent Induced Phase Separation) and TIPS (Thermally Induced Phase Separation) method, as described, for example, in documents EP 1 678 245 and US 5013339. This method of preparing membranes has several drawbacks: it is expensive and the large amount of solvents used (such as NMP or DMF) is problematic because it presents a high toxicological risk (for example, NMP is classified as reprotoxic whereas DMF can cause cancer in humans) and needs to be recycled. It would be desirable to have PVDF membranes made by a different process from the solvent route because of the aforementioned drawbacks. Moreover, it is known to disperse a filler, mineral or organic, in a polymer matrix and to use the composition thus obtained for the production of porous membranes. US 2011/0266707 discloses multilayer porous membranes obtained from a polyolefin solution in an organic solvent in which a heat-resistant filler has been dispersed. As stated in paragraph 53 of this document, such a charge makes it possible, in addition to improving the thermal stability of the membrane, to increase the electrolytic absorption and to initiate the formation of pores in the membrane during the treatment. drawing. The present invention proposes to provide novel compositions based on fluoropolymers, loaded with talc, capable of enabling the production of porous membranes by a drawing process. BRIEF DESCRIPTION OF THE FIGURES FIG. 1 represents the image of the membrane of Example 1, acquired by means of a transmission electron microscope (TEM), with a magnification of 10,000. SUMMARY OF THE INVENTION According to a first aspect, the invention relates to a charged polymer composition consisting of a PVDF matrix in which talc particles are dispersed. Said PVDF matrix consists of at least one PVDF homopolymer or a mixture of at least one PVDF homopolymer and at least one random copolymer of vinylidene fluoride (VDF) and hexafluoropropene (HFP). Advantageously, the VDF-HFP copolymer has an elastomeric character, namely the level of HFP being between 15% and less than 50% by weight, preferably between 20 and 45% by weight, relative to the weight of the copolymer. In the polymer matrix, PVDF homopolymer is predominant with respect to the copolymer: from 55 to 95% by weight, preferably from 60 to 90% by weight relative to the total weight of the matrix. In the composition according to the invention, the size of the talc particles dispersed in the matrix is between 50 nm and 5 μm (including limits). According to a second aspect, the invention relates to porous polymeric membranes obtained from said formulation. These membranes are characterized by their thickness, their pore size, their level of porosity and / or gas permeability, and their melting temperature. There is defined a permeability threshold at 100 above which the film is considered porous. For this type of polymer matrix, the film has a porosity if its permeability to O 2 is greater than 1500 cc.25. Tm / m 2 .24 h .atm. According to another aspect, the invention relates to a method for producing a porous polymeric membrane from the formulation according to the invention, said process comprising the following steps: the homogeneous mixture of the various constituents of the composition according to the invention; the invention. Among the mixing methods, mention may in particular be made of the melt mixture on a compounding tool such as a twin-screw extruder, a co-kneader or an internal or roll mixer. Transforming the mixture of said composition by extrusion blow molding of the sheath or "cast extrusion", resulting in a precursor film having a thickness of between 70 and 200 μm. stretching said precursor film in one or more times to obtain a microporous PVDF membrane.

According to yet another aspect, the invention relates to the various applications of the porous polymeric membranes described above, in particular for the filtration of water and as separators for Li-ion batteries or supercapacitors. For use as a separator, and in comparison with separators based on polyolefins, the membranes according to the invention have the following advantages: - good chemical stability (vis-à-vis organic acids, bases, strong oxidizers, halogenated solvents ) and thermal; - good mechanical strength; a good affinity for liquid electrolytes.

DETAILED DESCRIPTION OF EMBODIMENTS The subject of the present invention is a membrane based on PVDF doped with talc which, after stretching according to the conditions described below, has an emerging porosity.

It is considered that a polymer membrane has an "emergent" porosity if there exists within this membrane an interconnected network of pore (cavities) (or percolating) allowing a permeant (gas or liquid) ) to freely diffuse from one side to the other of the material. By "free diffusion" it is meant to diffuse out of the amorphous phase of the polymer (the crystalline phase of the polymer being considered permeable (gas or liquid)). The porous polymeric membranes formed from the formulation have the following characteristics: a thickness of 25 to 50 μm, a final porosity (after pre-stretching) of 20-50%, preferably greater than 25% and up to 50%, a pore size ranging from 50 nm to 2 μm and a melting point of 168 ° C. The talc mass content is 2 to 50%, preferably 5 to 40%, preferably 10 to 30% based on the weight of the composition. The invention will be better understood in the light of the following non-limiting examples. Products used: a homopolymer of PVDF (such as Kynar® 740 marketed by Arkema) - talc Luzenac type 20MOOS.

Compounding and Transformation: The compounding of a thermoplastic polymer composition consisting of 80% by weight of Kynar 740 and 20% by weight of talc was carried out on a twin-screw extruder of Haake type: - Machine temperature: 250 ° C. - Speed of the screw: 300 rpm - Extruder output flow: 1.6 kg / h. These compounds were then processed by a flat extrusion process. The line used is OCS type. The ratio length to diameter of screw (L / D) is 25 (the diameter of the screw is 25 mm). The material temperature was set at 230 ° C, the screw speed at 65 rpm and the line speed at 4.1 m / min. The width and opening of the die are 200 mm and 500 μm respectively. The films obtained are 100 μm thick and are drawn with a cooling roll set at a temperature of 110 ° C.

Stretching of the films The precursor film was stretched using a Zwick 4 dynamometer. The pre-stretching stage (according to MD ("Machine Direction" or direction of extrusion) or TD ("Transverse Direction Or transverse direction), carried out preferably according to MD, was carried out between -10 and 60 ° C, preferably between 10 and 30 ° C, at a biasing speed of between 10 and 2000 mm / min, preferably between 20 and 1500 mm / min and at a deformation rate of between 50 and 600%, preferably between 100 and 500%, and the degree of deformation (expressed as a%) is given by the following formula: (1-10) / lo x 100 ( 10: initial film length, 1: length of the film after deformation.) The stretching step (according to MD or TD) at higher temperature was conducted between 60 and 130 ° C, at a biasing speed between 20 and 2000 mm / min, and at a deformation rate of between 100 and 600%. the talc-loaded K740 film was stretched 100 and 120% at 40 ° C and 50 ° C respectively in the extrusion (MD) direction. The pulling speed was set at 200 mm / min.

At the end of this first bias, the pre-deformed film was biased perpendicular to the extrusion (TD) at 80 and 110 ° C (Tables 1 and 2 respectively). The results obtained are presented in Tables 1 and 2 below: Characterization: The porosity of the membranes was evaluated by density measurement. The porosity is calculated as follows: porosity (%) = (1-density porous membrane / dense membrane density) × 100 (according to ISO 1183-1). It has also been observed by transmission electron microscopy on microtomic sections of about 40 nm at -100 ° C (Fig. 1). Direction Temp. Speed Lengthening Density Stress Porosity ° C mm / min% 00 Reference - - - 1.89 - Pre-stretch MD 40 200 100 1.63 14 MD 50 200 120 1.63 14 (40 ° C * 1) TD 80 20 400 1.42 (40 ° C * 1) TD 80 100 1.40 (40 ° C * 1) TD 80 500 1.39 26 (40 ° C * 1) TD 80 1000 1.41 25 (50 ° C * 2) TD 80 1.46 23 (50 ° C * 2) TD 80 100 1.43 24 (50 ° C * 2) TD 80 500 1.44 24 (50 ° C * 2) TD 80 1000 1 , 38 27 Table 1 Direction of loading T Speed Lengthening Density Porosity t ° C mm / min% 0/0 Reference - - - - 1.89 - Pre-MD 40 200 100 1.63 14 drawing MD 50 200 120 1.63 14 (40 ° C * 1) TD 110 20 400 1.41 (40 ° C * 1) TD 110 100 1.42 (40 ° C * 1) TD 110 500 1.43 (400C * 1) TD 110 1000 1.38 27 (50 ° C * 2) TD 110 20 1.42 25 (500C * 2) TD 110 100 1.42 Table 2 * 1 Pre-stretching according to MD at 40 ° C, 5 * 2 Pre These results show, on the one hand, that the pre-stretched film according to MD at 40 ° C. or 50 ° C., at a biasing speed of 200 mm / min, exhibits a porosity of 50.degree. in the order of 14%, and secondly, that this film first pre-stretched, then stretched according to TD at different speeds (20, 100, 500 or 1000 mm / min), at 80 or 110 ° C, has a porosity of the order of 24-27%. The presence of talc thus induces an increase in the porosity of the membrane, as well as the pore size. Example 2 The talc-laden K740 film was stretched according to the following conditions: Conditions (a): Pre-stretching at 50 ° C, 200 mm / min at a strain rate of 120%, and then drawing at 110 ° C at 500 mm / min at a deformation rate of 300%. Conditions (b): - Pre-stretching at 50 ° C, 200 mm / min at 120% deformation rate, then - drawing at 110 ° C, at 20 mm / min at a deformation rate of 250% .

The porosity of the membranes was evaluated using measurements of oxygen permeability and carbon dioxide, which are given in Table 3. P02 P CO2 (cc.25um /m2.24h.atm) (cc. 25um /m2.24h.atm) Undrawn film 392 527 Stretched film according to conditions (a) 4308 6927 Stretched film according to conditions (b) 14667 23417 Table 3 These results show that films stretched under the conditions described above are more permeable with oxygen and carbon dioxide as the unstretched precursor film (by a factor greater than 10). These results coupled with density measurements account for the creation of vacuum in stretched films.

Claims (15)

REVENDICATIONS1. A charged polymer composition consisting of a polyvinylidene fluoride (PVDF) matrix in which talc particles are dispersed. The composition of claim 1 wherein said PVDF matrix is at least one PVDF homopolymer. The composition of claim 1, wherein said PVDF matrix is a blend of at least one PVDF homopolymer and at least one copolymer of vinylidene fluoride (VDF) and hexafluoropropene (HFP). 4. Composition according to claim 3 wherein the level of HFP in said copolymer is between 15 and less than 50% by weight, preferably between 20 and 45% by weight, relative to the weight of the copolymer. 5. Composition according to one of claims 3 and 4 wherein said VDF-HFP copolymer is a random copolymer. 6. Composition according to one of claims 3 to 5 wherein the mass content of PVDF homopolymer is 55 to 95% based on the weight of said matrix. 7. Composition according to one of the preceding claims wherein the mass content of talc is 2 to 50%, preferably 5 to 40%, preferably 10 to 30% relative to the weight of the composition. 8. Composition according to one of the preceding claims wherein the size of talc particles dispersed in the matrix is between 50 nm and 5 pm. 9. A porous polymeric membrane formed from the formulation according to one of claims 1 to 8. The membrane of claim 9 having a thickness of 25-50 μm and a melting temperature of 168 ° C. 11. Membrane according to one of claims 9 or 10 having a porosity ranging from 20 to 50%, preferably greater than 25% and up to 50%. 12. Membrane according to one of claims 9 to 11 having a pore size ranging from 50 nm to 21..tm. 13. Porous polymeric membrane according to one of claims 9 to 12 obtainable from the composition according to one of claims 1 to 8 by a method which comprises the following steps: a. mixing the constituents of said composition, b. transforming the resulting mixture into a precursor film; stretching said precursor film to obtain a porous membrane. 14. Use of the membrane according to one of claims 9 to 12 or the membrane obtained by the method of claim 13 for the filtration of water. 15. Use of the membrane according to one of claims 9 to 12 or the membrane obtained by the method of claim 13 as an electrode separator for Li-ion batteries or supercapacitors.