WO2023117971A1 - Method for the purification of vinylidene fluoride polymers - Google Patents

Abstract

Process for the purification of vinylidene fluoride based polymers, said process comprising the steps of: i) providing a solid material M comprising one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, ii) contacting said solid material M with a solvent S, said solvent S comprising one or more esters, thereby dissolving said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, and forming a solution SP, iii) optionally filtering said solution SP to remove any undissolved solid material, iv) contacting said optionally filtered solution SP with a non solvent bath NS thereby causing the precipitation of said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, wherein said non solvent bath NS has a temperature at least 20°C lower than the temperature of said solution SP.

Description

Method for the purification of Vinylidene fluoride polymers

Technical Field

[0001] This application claims priority from the patent application filed on 23 December 2021 in EUROPE with Nr 21217432.0, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The present invention pertains to a method for the purification of vinylidene fluoride based polymers and copolymers (from now on referred to as “VDF polymers”). The method allows to remove most contaminants from VDF polymers, and is more environmentally friendly than currently available methods and can be used in particular to recover vinylidene fluoride based polymers and copolymers after they have reached the end of their lifecycle, providing purified polymers which have properties which are comparable with those of virgin polymers of the same class and can therefore be re-used in the same or different applications.

Background Art

[0003] VDF polymers are advantageously used in several different applications.

Some non limiting examples of industrial articles which include VDF polymers are water and dialysis filtration membranes, electrodes in secondary batteries wherein VDF polymers are employed as binders, pipes for transportation of chemical products such as petroleum based products wherein VDF polymers based pipes or VDF polymers based layers in multilayer pipes are often utilized as known in the art due to the good barrier properties of VDF polymers.

[0004] As common in the industry, all these articles comprising VDF polymers have a life cycle at the end of which such articles are discarded and/or replaced. Attempts have been made to recycle and reuse the materials making up such articles. For example several documents describe methods for recycling polyvinylidene fluoride polymers from flat and hollow fiber membranes (CN104961909, CN106589447, CN108690415). The methods described in these documents however utilize solvents which are classified as dangerous and/or harmful for the environment such as NMP or THF. Also concerning the recovery of VDF polymers used as electrode binders in the lithium battery industry, most research has been focused in using traditional non-green solvents (doi.org/10.1002/open.202100060). There is therefore a need for an improved method for purifying and recovering VDF polymers, in particular from articles which have reached the end of their lifecycle, which employs green solvents, is in general friendly to the environment and provides for purified VDF polymers which have comparable properties as those of a virgin material so that such polymers can be reused even (but not necessarily) in the same applications for which they were originally intended.

Summary of invention

[0005] The present invention relates to a process for the purification of vinylidene fluoride based polymers, said process comprising the steps of: i) providing a solid material M comprising one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, ii) contacting said solid material M with a solvent S, said solvent S comprising one or more esters, thereby dissolving said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, and forming a solution SP, iii) optionally filtering said solution SP to remove any undissolved solid material, iv) contacting said optionally filtered solution SP with a non solvent bath NS thereby causing the precipitation of said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, wherein said non solvent bath NS has a temperature at least 20°C lower than the temperature of said solution SP.

Description of embodiments [0006] As used herein, the term “vinylidene fluoride based polymers” abbreviated as “VDF polymers” indicates polymers (including copolymers) comprising at least 50 % by moles of recurring units derived from the polymerization of vinylidene fluoride (difluoro 1 ,1 -ethylene, VF2 or VDF).

[0007] As mentioned above one essential step of the method of the present invention is the provision of a solid material M comprising one or more VDF polymers. Preferably the VDF polymers which are present in the solid material M are thermoplastic.

[0008] A solid material M for use in the present invention can be any material comprising one or more VDF polymers. Such solid material M can be used in the method of the invention in any form which is suitable to achieve its dissolution, e.g. it can be provided in finely divided form such as in powders, pellets or small pieces, but also larger pieces, films, layers and complete articles can be used although, as it will be readily understood by a skilled person, as for any dissolution process, solid materials M having a lower ratio of surface to mass may dissolve more slowly in a given set of conditions.

[0009] Such solid material M is preferably derived from articles comprising VDF polymers which have reached the end of their useful life. Preferably, before being contacted with the solvent S the article comprising VDF polymers may be washed with water and optionally with an acid solution and/or with an alkaline solution, said washing solutions optionally containing surfactants and/or oxidants such as sodium hypochlorite or H2O2 to remove possible traces of greasy impurities as well as impurities mechanically trapped on the surface of the article. Examples of articles comprising VDF polymers which can be used as the source of the solid material M for use in the invention are filtration membranes, both in the form of flat membranes and/or of hollow fibers membranes. Membranes are often made of a polymeric composition which main constituents (typically more than 50% by weight of the membrane, preferably more than 90% by weight), are VDF polymers. Preferred VDF polymers for use in membranes are known in the art e.g. from EP2655452, EP2935369, EP3055048 to Solvay Specialty Polymers. As known to the skilled person and from the documents cited above, membranes containing VDF polymers are typically produced via NIPS or TIPS process and are typically used to filter fluids, such as water, chemicals, blood (in dialysis systems) and more. Also VDF polymers based membranes are used in scientific analysis for amino acid analysis and protein sequencing and many other applications as known in the art.

[0010] Membranes, during their lifecycle, mechanically filter out unwanted materials from fluids and after some time they reach a status in which they cannot be used anymore and need to be replaced. Used membranes can be washed to remove some of the impurities which are mechanically entrapped in their pores but at some point, when such membranes reach a predetermined end of life status, they are discarded. Such discarded membranes, can be used as the starting solid material M for the present invention. Membranes being very porous tend to absorb quickly the solvent, so depending on their size and shape membranes can be used in the present invention as is or cut in smaller pieces. For example hollow fiber membranes can be cut in strands of 5-40 cm, but other forms can be used and the size of membrane or portion of membrane is in general non critical. Preferably end-of-life membranes are washed with a slightly acidic water solution (e.g. 5-10% HCI or citric acid in water) before being used as solid material M in the present invention.

[0011] Other articles comprising VDF polymers which can be used as a source of said solid material M for use in the present invention are lithium batteries. Lithium batteries are an essential part of today’s industry and there is a great interest in technologies which allow separating their components and making it possible to recycle as much as possible of them. Lithium batteries often comprise VDF polymers in several parts of the battery. In particular VDF polymers are often used as electrode binders, more commonly in the cathode. Electrodes are typically an agglomerate of metallic oxides, fillers, graphite and other materials which are distributed onto a metallic substrate and kept together and adhered to said metallic substrate by a binder based on VDF polymers. The amount of binder is typically about only 0.5 to 8% of the entire electrode materials, but ,for recycling all the electrode materials, it is essential that the binder is removed because the binder entraps the powdery electrode materials in its polymeric matrix. When electrodes from a Li-ion battery are used as solid material M in the present invention, the VDF polymer based binder is dissolved by the solvent S and can be recovered with the method of the present invention, while the remaining electrode materials can be filtered out from the solution and separated and recycled independently. Once the binder is removed by the solvent S, the remaining electrode materials are in the form of free powders so that their separation and recycling is made much easier.

[0012] The process of the present invention can therefore be a part of a larger operation which allows recycling of all components of the Li battery electrodes.

[0013] VDF polymers may be present as well in other parts of a lithium battery assembly such as the separator. Any battery part which comprises VDF polymers can be used in the present invention as solid material M.

[0014] Other materials which contain VDF polymers and can be used as a source for solid material M for use in the present invention are pipes, especially those used in oil and gas operations. VDF polymers are often used in pipes because such polymers are typically easy to process in melt form and at the same time they have high resistance to chemicals (both to permeation and to aggression/corrosion). A typical application of VDF polymers is in fuel pipes and pipes for oil extraction including submarine pipes for offshore oil extraction, for example the so called flexible risers typically include a VDF polymer pipe within a metallic shell. In other cases multilayer pipes are used wherein at least the layer directly in contact with the oil is a VDF polymers layer.

[0015] Despite the low permeability, due to the extreme working conditions, these VDF polymer based pipes at some point reach their end of life and need to be replaced. At their end of life VDF polymers pipes contain significant amounts of absorbed chemicals and, in order to recover the VDF polymers contained therein these absorbed chemicals need to be removed and this can be done with the method of the present invention.

[0016] VDF polymers from end-of-life pipes can be used as a source for solid material M for use in the present invention. Before using them as solid material M for the present invention VDF polymers pipes are preferably cut in small pieces (a few mm to a few cm) and washed with a surfactant solution e.g. using a dishwashing detergent or similar agent to remove the superficial traces of oil. When pipes are multilayer pipes the VDF polymers based layer(s) should be preferably separated from the others using known techniques such as e.g. mechanical techniques. In any case the presence of small amounts of non VDF polymers (such as the typical constituents of the additional layers in oil extraction pipes such as polyolefin layers) are not problematic for the method of the invention because typical non fluorinated polyolefins are not dissolved by the selected solvents so that unwanted polymers can be simply filtered out from the solution SP.

[0017] The examples provided above are simply to be considered as possible end of life articles which can be used as the source of solid material M for use in the present invention. In practice any article comprising VDF polymers can be used as a source for solid material M for use in the present invention. Preferably such article is an end-of-life article i.e. an article which has been used in a process and which has now reached the end of its useful life and needs to be replaced. More preferably such article which is used as source for the solid material M for the present invention is selected from porous filtration membranes, pipes and electrode materials from Li-ion secondary batteries, this particularly in view of the great percentage in terms of volumes of VDF polymers that such articles represent in the current VDF polymer industry, and particularly with regard to the currently relevant concerns concerning the possibility to effectively reuse the VDF polymers which are comprised therein, when such articles reach the end of their useful life. [0018] Preferably the solid material M for use in the present invention is selected from materials wherein at least 80%, preferably at least 90%, more preferably at least 95%, even more preferably at least 99% by weight of their polymeric components are VDF polymers. When other polymers are present such as e.g. in a multilayer material, these are preferably mechanically separated from the VDF polymers before performing the process of the invention.

[0019] Preferred VDF polymers for use in the method of the present invention are those comprising 60% or more, preferably 70% or more, more preferably 80% or more moles, even more preferably 90% or more, most preferably 95% or more by moles of recurring units derived from vinylidene fluoride. For the purpose of the present invention, the vinylidene fluoride polymer may optionally comprise, in addition to the VDF monomer, recurring units different from VDF recurring units, and which are derived from the polymerization of ethylenically unsaturated monomers different from VDF (for example 0.1-20 mol%, preferably from 0.5 to 10 mol%, more preferably 0.1-5 mol% with respect to the total number of moles of the polymer). Said ethylenically unsaturated monomers may comprise at least one fluorine atom and can be hence designated as fluorinated comonomers. Still, the ethylenically unsaturated monomers can be free from fluorine atoms; examples of these non-fluorinated comonomers are notably hydrophilic (meth)acrylic monomers.

[0020] The hydrophilic (meth)acrylic monomer (MA) preferably complies to formula:

wherein each of R1 , R2, R3, equal or different from each other, is independently a hydrogen atom or a C1-C3 hydrocarbon group, and ROH is a hydrogen or a C1-C5 hydrocarbon moiety comprising at least one hydroxyl group. Non limitative examples of hydrophilic (meth)acrylic monomers (MA) are notably acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate; hydroxyethylhexyl (meth)acrylates.

[0021] The monomer (MA) is more preferably selected from:

- hydroxyethylacrylate (HEA) of formula:

- 2-hydroxypropyl acrylate (HPA) of either of formulae:

- acrylic acid (AA) of formula:

- and mixtures thereof.

[0022] Most preferably, the monomer (MA) is AA and/or HEA.

[0023] Non-limiting examples of fluorinated comonomers different from VDF, as above detailed, comprise, notably, the following:

(i) C2-C8 fluoroolefins such as trifluoroethylene (TrFE), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP);

(ii) perfluoroalkylethylenes of formula CH2=CH-Rfo, wherein Rro is a C2-C6 perfluoroalkyl group;

(ill) chloro- and/or bromo- and/or iodo-C2-Ce fluoroolefins such as chlorotrifluoroethylene (CTFE);

(iv) perfluoroalkylvinylethers of formula CF2=CFORH, wherein Rn is a Ci- Ce perfluoroalkyl group, such as perfluoromethylvinylether (PMVE) and perfluoropropylvinylether (PPVE);

(v) (per)fluorooxyalkylvinylethers of formula CF2=CFOXo, wherein Xo is a C1-C12 oxyalkyl group or a C1-C12 (per)fluorooxyalkyl group having one or more ether groups, e.g. perfluoro-2-propoxy-propyl group;

(vi) (perfluoroalkylvinylethers of formula CF2=CFOCF2ORf2, wherein Rf2 is a Ci-Ce (per)fluoroalkyl group, e.g. -CF3, -C2F5, -C3F7, or a Ci-Ce (per)fluorooxyalkyl group having one or more ether groups, e.g. -C2F5-O- CF₃;

(vii) functional (per)fluorooxyalkylvinylethers of formula CF2=CFOYo, wherein Yo is selected from a C1-C12 alkyl group or (per)fluoroalkyl group, a C1-C12 oxyalkyl group and a C1-C12 (per)fluorooxyalkyl group having one or more ether groups, Yo comprising a carboxylic or sulfonic acid group, in its acid, acid halide or salt form;

(viii) fluorodioxoles, especially perfluorodioxoles;

(ix) vinyl fluoride, and their mixtures.

[0024] Most preferred fluorinated comonomers are chlorotrifluoroethylene (CTFE), trifluoroethylene (TrFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoromethylvinylether (PMVE).

[0025] In one embodiment of the present invention, the polymer (VDF) preferably comprises, more preferably consists of:

- recurring units derived from vinylidene fluoride,

- from 0.1 % to 3% by moles, with respect to the total amount of moles of recurring units in said polymer (VDF), of recurring units derived from at least one (meth)acrylic monomer [monomer (MA)], and

- from 0.1 % to 10% by moles, preferably from 0.2% to 5% by moles, with respect to the total amount of moles of recurring units in said polymer (VDF), of recurring units derived from hexafluoropropylene (HFP). [0026] Most preferred VDF polymers for used in this process are characterized by a melt flow rate (MFR measured in accordance to ASTM D1238) from 2 g/10min to 5 g/10min (230°C, 21 ,6Kg). Also preferred VDF polymers for use in the present invention are those being partially crystalline, having therefore an enthalpy of fusion (AHf measured in accordance with ASTM D3418) between 40 and 95 J/g, preferably from 45 to 90 J/g, more preferably from 45 to 85 J/g.

[0027] In a second essential step of the present invention, said solid material M is contacted with a solvent S. The solvent S is characterized by comprising one or more esters. Esters are in general renewable and have a good environmental and safety profile. Preferably the solvent S contain at least 50%, more preferably at least 70%, even more preferably at least 80%, even more preferably at least 90% of esters, most preferably at least 95% by weight of esters. Preferred esters for use in the present invention are selected from diesters such as the methyl esters of bicarboxilic acids (e.g. those sold under the brand name of Rhodiasolv®RPDE) and cyclic esters (lactones). Most preferred ester solvents for use herein are y-valero lactone and y-butyrro lactone and mixtures thereof. One particularly preferred solvent S is y-valero lactone, another particularly preferred solvent S is y -butyrro lactone.

[0028] Preferably the solvent S is free from NMP (N-Methyl-2-pyrrolidone), DMSO (Dimethyl sulfoxide), DMF (Dimethylformamide), DMAc (N, N Dimethylacetamide), THF (Tetrahydrofuran) and alkyl phthalates. These are all commonly used solvents for VDF polymers but their presence is undesirable for environmental safety, human safety, and for the odor they impart to the polymer due to their residues. In the context of the present invention “free from X” means that X if present is present in a negligible amount, preferably less than 1 %wt, more preferably less than 0.1 % by weight, even more preferably component X is not present in the solvent S.

[0029] Preferably the second essential dissolution step is carried out using a heated solvent because it has been found that a heated ester based solvent is more effective in dissolving the VDF polymers. Moreover in some cases it has been observed that the solutions SP, if cooled below 30-40°C, depending on the nature and molecular weight of the VDF polymers, may form a gel. Even if in most cases the effect is reversible (i.e. the solution SP reverts to a fluid when heated again) a solution SP in gel form is not preferred for the method of the invention. A preferred temperature range for the solvent is 40-130°C, more preferably 45-100°C, even more preferably 45-80°C. The solution SP after bring formed is formation is preferably maintained at the same temperature to avoid the formation of gels until the moment it is put in contact with the non solvent bath.

[0030] The dissolution step can be performed in any way available to a skilled person for example by soaking and/or stirring the solid material M, thereby obtaining a solution SP. Depending of the starting solid material M, also a solid phase can be included in the solution SP as a dispersed or precipitated solid. The solvent S may dissolve VDF polymers until its saturation, however it is in general preferred that the solution SP contains from 1% to 30%, preferably from 2% to 20%, even more preferably from 5% to 15% by weight of VDF polymers based on the total weight of the solution SP.

[0031] In a subsequent optional step the solution SP may be filtered to remove an excess of undissolved solid in case they are present. If filtering is performed, the filtering step is preferably performed in mild conditions e.g. using a sieve or a filter with large enough pores to separate only relatively coarse solids and avoid removing dissolved VDF polymers. A filtering step may be only necessary when the solid material M contains a large amount of trapped solids as it can be the case when it is an electrode material, or in case other non soluble polymers are present in the solid material M. It should be noted however a relatively large amount of dispersed solids are in general not detrimental to the method of the invention as the polymer, when precipitating in the solvent bath, it does so anyway in purified form.

[0032] In a last essential step of the method of the invention the solution SP (optionally filtered) is put in contact with a non solvent bath NS thereby causing the precipitation of the VDF polymer(s). Separating VDF polymers from their solutions through the contact with a liquid bath containing a liquid which is not able to dissolve the polymer (e.g. water) is a common technique which is used e.g. in the NIPS (non solvent induced phase separation) method for the manufacturing of membranes. The amount of solution non solvent bath which comes into contact with the solution SP is not critical provided it is in an amount of at least 100%, preferably at least 150%, more preferably at least 200% of the weight of the solution SP. After contacting the solution SP and the non solvent bath NS the dissolved polymer immediately precipitates and can be filtered out from the mixture as a solid, rinsed typically with water, until pH neutrality and then dried. The solvent can be then recovered from the mixture with conventional solvent recovery techniques. Any non solvent bath used typically in the preparation of VDF polymer membranes with the DIPS method can be used herein. For example a non solvent bath NS which can be used in the present invention is water or a mixture water with a water soluble alcohol (e.g. ethanol or isopropanol) and may be acidified e.g. with HCI or citric acid. In general it has been observed that a water based non sovent bath NS optionally acidified e.g. with 1-10%wt HCI or citric acid is particularly suitable for the recovery of VDF polymers where the contaminants are predominantly inorganic, while a non solvent bath NS based on a water/alcohol mixture (typically in a weight ratio from 1 :4 to 4:1) is more suitable when also organic contaminants are present.

[0033] In one aspect of the present invention this step is characterized by the fact that the solution SP and the solvent bath NS have a temperature difference of at least 20°C, preferably 30°C, more preferably 40°C, even more preferably 50°C, wherein the non solvent bath is cooler than the solution SP when they come in contact. We have surprisingly found that this “thermal shock” has a positive effect on the quality of the recovered VDF polymers which are more pure (as seen from their whiteness index) than polymers recovered with other methods, and contain negligible traces of residual solvents. In general it is preferred that the solution SP is contacted with the non solvent bath NS at the same temperature of dissolution mentioned above i.e. 40°C and 130°C, preferably between 45°C and 100°C, more preferably 45°C to 80°C.

[0034] It should be noted that, while the method of the invention as described is suitable for purifying and recovering VDF polymers from a wide range of end of life articles, it is possible to obtain an even better purification by repeating the method twice. In other words, the recovered polymer can be used as solid material M and dissolved again in the solvent S, and recovered a second time in a non solvent bath. This is often not necessary but, as it will be apparent to the skilled person, will always lead to even purer recovered VDF polymers.

[0035] The two step process described in the former paragraph can be particularly suitable for highly contaminated solid materials M. In particular, in case a solid material M contains both organic and inorganic contaminants, it may be beneficial to use an acidified water non solvent bath NS in one step and a water/alcohol non solvent bath NS in the other.

[0036] Finally it should be note that, if a two dissolution steps process is used, even if it is preferred that in both steps the non solvent bath have a temperature which is at least 20°C, preferably 30°C, more preferably 40°C, even more preferably 50°C, lower than the solution NS, the present invention also encompasses the case wherein in one step this temperature difference is present and in the other one this temperature difference is not present (or a lower temperature difference is present).

[0037] VDF polymers recovered using this method can re-enter the life cycle and be used for the same applications as the original virgin materials so that for example they can be used to prepare VDF polymer membranes or be used as a binder for electrodes.

[0038] In another aspect, the present invention relates to a method for the purification of VDF polymers wherein the VDF polymers comprised in the solid material M are selected from VDF polymers having a relatively low crystallinity expressed and a AH of fusion below 95, preferably below 90, more preferably below 85 J/g, most preferably below 80 J/g, and the solvent S is selected from solvents consisting of diesters, cyclic esters or mixtures thereof, preferably consisting of cyclic esters, even more preferably consisting of gamma-valero lactone, gamma butyro lactone or mixtures thereof. According to this aspect of the invention the resulting solution SP is then contacted with the non solvent bath thereby causing the precipitation of the recovered polymer. While it is still preferred that the non solvent bath has a temperature which is at least 20°C, preferably 30°C, more preferably 40°C, even more preferably 50°C cooler than the solution SP, when operating in this particular embodiment wherein VDF polymers and solvent as selected as described above, this technical feature is not essential for obtaining purified VDF polymers of good quality, so that the difference in temperature between the non solvent bath and solution SP is not critical. For example according to this aspect the solution SP and the non solvent bath may have or essentially have the same temperature, or their temperature difference may be plus or minus 5, 10, or 15°C.

[0039] In another aspect the present invention also relates to VDF polymers recovered from end of life articles using the method of the invention and to articles such as for example membranes or electrodes, comprising such VDF polymers recovered from end of life articles using the method of the invention.

[0040] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0041] The invention will be now described in more detail with reference to the following examples whose purpose is merely illustrative and not limitative of the scope of the invention.

[0042] Raw materials

Solid Material M

Solid Material M1 End of life PVDF (>95%wt PVDF) hollow fibers membranes for industrial water filtration. Before using in the process of the invention the hollow fibers were dried from conservation liquid and immersed in HCI 5%vol for 20 hours. After that, they were rinsed with water until pH neutrality and dried at 100°C in a vented oven. As a final step, before being subject to the method of the invention the hollow fiber membranes were cut in segments of about 20 cm.

Solid Material M2

End of life >95%wt PVDF pipe used for crude oil from a flexible riser. Before using in the process of the invention, the pipe was cut in sections and both its outside and inside surfaces were scrubbed with a dishwashing liquid solution to remove the macroscopic oil residues. The sections were then chipped in small pieces having their larger dimension of about 1 cm.

Solvent S.

Solvent S1 - Gamma-valero lactone 100% (from Sigma Aldrich) Solvent S2 - N-Methyl Pyrrolidone 100% (from Sigma Aldrich) Solvent S3 - Rhodiasolv® RPDE Aliphatic Diesters blend (from Solvay) Solvent S4 - Cyclopentanone 100% (from Sigma Aldrich) Solvent S5 - Cyclohexanone 100% (from Sigma Aldrich)

Non solvent bath NS NS1.

Water solution of HCI at 5%wt.

NS2

Water/isopropanol mixture 1 :1 by weight.

[0043] Example 1

10 grams of solid material M1 were dissolved into 90g of Solvent S1 (Gamma-Valero-Lactone 100%) under stirring at 60°C for 90 minutes until a dark brown dispersion was obtained. This dispersion was then poured into 400 ml of non solvent bath NS1 maintained at 20°C under stirring. A solid polymer was immediately formed. The polymer was filtered out from the non solvent bath, rinsed until pH neutrality and dried at 100°C in a vented oven.

[0044] Example 2

10 grams of solid material M2 were dissolved into 90g of Solvent S1 (Gamma-Valero-Lactone 100%) under stirring at 60°C for 90 minutes until a dark brown dispersion was obtained. This dispersion was then poured into 400 ml of non solvent bath NS2 maintained at 20°C under stirring. A solid polymer was immediately formed. The polymer was filtered out from the non solvent bath, rinsed until pH neutrality and dried at 100°C in a vented oven.

[0045] Example 3

The same process of Example 2 was followed wherein solvent S3 (RPDE) was used.

[0046] Example 4

10g of polymer recovered according to Ex. 1 were further dissolved into 90g of Solvent S1 (Gamma-Valero-Lactone 100%) under stirring at 60°C for 60 minutes until a dispersion was obtained. This dispersion was then poured into 400 ml of non solvent bath NS2 maintained at 20°C under stirring. A solid polymer was immediately formed. The polymer was filtered out from the non solvent bath, rinsed and dried at 100°C in a vented oven.

[0047] Comp. Example 1

The same process of Example 1 was followed wherein solvent S2 (NMP) was used.

[0048] Comp. Example 2

The same process of Example 1 was followed wherein the non solvent bath NS1 was maintained at 60°C when the solution S was poured into it. [0049] Comp. Example 3

The same process of Example 1 was followed wherein solvent S4 (Cyclopentanone) was used.

[0050] Comp. Example 4

The same process of Example 1 was followed wherein solvent S5 (Cyclohexanone) was used.

[0051] The recovered polymer (filtered and dried) was then tested. The samples were tested to evaluate their whiteness (expressed as Yellowing Index, the lower the better), the amount of residual solvent and the amount of impurities present in the recovered VDF polymer.

[0052] The methods used are described herein:

[0053] Yellowing index Method (Yl)

5 grams of polymer powder obtained from the Examples were compression flash molded at 270°C at a pressure of 0.2 bar.

Yellow index of the films obtained was measured directly using a Gardner Colorimeter, according to ASTM E313-05, "Standard practice for calculating Yellowness and Whiteness indices from Instrumentally Measured Color Coordinates".

Rating was in the range from 0 to 100, wherein 0 a smaller number indicate a lower yellowness index.

[0054] Residual solvent Method

The residual solvent was measured by thermogravimetric analysis: in a TGA equipment under N2 flux, temperature was ramped up from 50°C to 360°C at the rate 10°C/min, then run an isothermal test at 360°C/15min. The emissions were collected to FTIR equipment to identify functional groups/fingerprint detection and then from FTIR to GC to chromatography separation followed by MS identification. The measurements were carried out on reference sample and compared to testing sample. Detection limit was determined by measuring each solvent alone in same experimental conditions.

[0055] Impurities in recovered VDF polymer

The overall purity of the VDF polymer sample, in recovered VDF polymer, were measured by thermogravimetric analysis measuring the weight loss in the temperature range 200°C to 360°C and adding the residual weight measured at 700°C. As a reference a pure, virgin VDF polymer was used which has negligible weight loss between 200°C and 360°C and negligible residues at 700°C. The TGA was performed in air, with an heating ramp of 10°C/min.

Table 1

[0056] As it can be seen, VDF polymers recovered with the method of the invention have better color (lower yellowing), less residual solvent, and less impurities than VDF polymers received with the prior art methods.

Claims

Claims

Claim 1

A process for the purification of vinylidene fluoride based polymers, said process comprising the steps of: i) providing a solid material M comprising one or more polymers, said polymers comprising at least 50% of recurring units derived from vinylidene fluoride, ii) contacting said solid material M with a solvent S, said solvent S comprising one or more esters, thereby dissolving said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, and forming a solution SP, iii) optionally filtering said solution SP to remove any undissolved solid material, iv) contacting said optionally filtered solution SP with a non solvent bath NS thereby causing the precipitation of said one or more polymers comprising at least 50% of recurring units derived from vinylidene fluoride, wherein said non solvent bath NS has a temperature at least 20°C lower than the temperature of said solution SP.

Claim 2

A process according to claim 1 wherein said solvent S comprises at least 50%, preferably at least 70%, more preferably at least 80%, even more preferably at least 90%, most preferably at least 95% by weight of esters.

Claim 3

A process according to any preceding claim wherein said esters comprised in said solvent S are selected from diesters and cyclic esters and are preferably cyclic esters. Claim 4

A process according to any preceding claim wherein said esters comprised in said solvent S are selected from y-valero lactone, y-butyrro lactone and mixtures thereof.

Claim 5

A process according to any preceding claim wherein said solvent S is free from NMP (N-Methyl-2-pyrrolidone), DMSO (Dimethyl sulfoxide), DMF (Dimethylformamide), DMAc (N, N Dimethylacetamide), THF (Tetrahydrofuran) and alkyl phthalates.

Claim 6

A process according to any preceding claim wherein said solvent S in step ii) is maintained at a T between 40°C and 130°C, preferably between 45°C and 100°C, more preferably 45°C to 80°C, while forming the solution SP.

Claim 7

A process according to any preceding claim wherein, when containg the solution SP and the non solvent bath NS, the amount of non solvent bath NS is at least 100%, preferably at least 150%, more preferably at least 200% of the weight of the solution SP.

Claim 8

A process according to any preceding claim wherein said non solvent bath NS comprises water and optionally an acidic material and/or a water soluble alcohol. Claim 9

A process according to any preceding claim wherein after step iv) a further step step v) is performed wherein said precipitated polymers are washed, rinsed and dried.

Claim 10

A process according to any preceding claim wherein said solid material M, before being contacted with the solvent S, is washed with water and optionally with an acid solution and/or with an alkaline solution said washing solutions optionally containing surfactants and/or oxydants.

Claim 11

A process according to any preceding claim wherein said solid material M is selected from end-of-life water filtration membranes, pipes used in oil extraction and/or elctrode materials from lithium batteries.

Claim 12

A process according to any preceding claim wherein said polymers comprising at least 50% of recurring units derived from vinylidene fluoride comprised in said solid material M are thermoplastic.

Claim 13

A process according to any preceding claim wherein said polymers comprising at least 50% of recurring units derived from vinylidene fluoride comprised in said solid material M have an enthalpy of fusion AHf, measured in accordance with ASTM D3418, between 40 and 95 J/g, preferably from 45 to 90 J/g, more preferably from 45 to 85 J/g. 22

Claim 14

A polymer comprising at least 50% of recurring units derived from vinylidene fluoride purified by the method of claims 1 to 13.

Claim 15

An article comprising a polymer according to claim 14.