RU2846615C1 - Method of producing a copolymer of perfluoro-4-(fluorosulphonyl)butylvinyl ether and tetrafluoroethylene - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods of producing perfluorinated copolymers with short side chain length. Disclosed is a method of producing a copolymer of perfluoro-4-(fluorosulphonyl)butylvinyl ether and tetrafluoroethylene by water-emulsion copolymerisation. Copolymerisation of tetrafluoroethylene and aqueous emulsion of perfluoro-4-(fluorosulphonyl)butylvinyl ether (hereinafter sulphomonomer), obtained by emulsification in an inert atmosphere at temperature 10-20 °C with volume ratio of sulpho-monomer : water phases equal to 1.0:(8.0-9.2) in the presence of an emulsifier – salts of perfluorinated oxoacids, in amount of 0.68-0.95 mol. % of the sulpho-monomer, a medium pH regulator – phosphate buffer solution and a special additive that limits the formation of a tetrafluoroethylene homopolymer, is carried out in a steel autoclave in presence of an ammonium persulphate initiator in amount of 1.8-3.6 mol. % of sulphonomer, at mixing rate of 450-500 rpm and operating temperature of 50-60 °C by feeding tetrafluoroethylene under layer of reaction mass at working pressure of tetrafluoroethylene 0.7-0.9 MPa for 9 hours to form copolymer latex with the required ratio of monomer links, the extraction of which is carried out by coagulation in an aqueous electrolyte solution, multiple washing with water and drying in a vacuum at temperature of 50-90 °C.

EFFECT: method for water-emulsion copolymerisation of perfluoro-4-(fluorosulphonyl)butylvinyl ether and tetrafluoroethylene, which enables to obtain a copolymer with given physical and chemical properties, while preventing formation of a tetrafluoroethylene homopolymer.

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Description

The present invention relates to the field of high-molecular chemistry, in particular to a method for producing a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether (hereinafter referred to as sulfomonomer) of the structural formula FSO ₂ -(CF ₂ ) ₄ -O-CF=CF ₂ and tetrafluoroethylene, which belongs to the group of perfluorinated copolymers with shortened side chains (SSC) containing sulfonyl fluoride groups. This copolymer is a precursor of proton-conducting membranes with improved performance characteristics, which are successfully used in the manufacture of hydrogen fuel cells or other electrolytic cells used in a wide variety of industries.

LEVEL OF TECHNOLOGY

The proton-conducting membrane obtained from a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene, known under the trade name "3M Ionomer", was first obtained by the American company "3M". The properties and characteristics of this membrane were described in the article "The Development of New Membranes for Proton Exchange Membrane Fuel Cells" published in the journal ECS Transaction, 11 (1) 3-14 (2007). The authors of this article provide a comparative characteristic of two different types of membranes "3M™ Ionomer" (SSC) and "Nafion®" (LSC), which clearly shows the advantages of the first in terms of service life, performance and the ability to function at higher operating temperatures without losing mechanical strength and ion-exchange capacity.

A number of methods are known for producing a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene using water-emulsion copolymerization.

According to a known method for copolymerization of perfluorinated sulfomonomers with tetrafluoroethylene (US 7071271, 04.07.2006), an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether prepared by emulsification at a speed of 24,000 rpm for 2 minutes with the addition of LiOH in an amount of 0.02-0.3 mol.% of the sulfomonomer is copolymerized with tetrafluoroethylene at a temperature of 60°C and an operating pressure of 6-8 bar in an inert atmosphere with initiation by an oxidation-reduction system: sodium metabisulfite - ammonium persulfate.

The resulting latex is coagulated by freezing, washed four times with deionized water and dried for 15 hours at a temperature of 130°C. The conversion of the sulfomonomer reaches 83%. The yield of the converted sulfomonomer is 94%.

A method is known for copolymerizing perfluorinated sulfomonomers with tetrafluoroethylene (US 7214740, published 08.05.2007), which consists of preliminary dispersion of an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether at a speed of 24,000 rpm for 2 minutes with the addition of LiOH and direct copolymerization of the resulting emulsion and tetrafluoroethylene, initiated by sodium metabisulfite and ammonium persulfate in the presence of potassium chloride at a temperature of 60°C, an operating pressure of 8 bar and a stirring speed of 240 rpm. The resulting latex is coagulated by freezing at a temperature of minus 18°C and dried at a temperature of 130°C. The conversion of the sulfomonomer reaches 86%. The yield of the converted sulfomonomer is 95.2%.

The disadvantages of these methods are: the use of LiOH to obtain the lithium salt of the sulfomonomer, which acts as an emulsifier in the preparation of an aqueous emulsion, which reduces the conversion of the sulfomonomer, as well as the use of highly efficient refrigeration equipment at the stage of isolating the copolymer, which increases the overall energy consumption of the process.

A method is known for copolymerizing perfluorinated sulfomonomers with tetrafluoroethylene (US 2005/0107488. published 19.05.2005), which includes preparing an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether in the presence of an emulsifier - ammonium salt of perfluorocaprylic acid by emulsification at a speed of 24,000 rpm for 2 minutes and its copolymerization with tetrafluoroethylene in the presence of initiators sodium metabisulfite and ammonium persulfate at a temperature of 50°C and an operating pressure of 6 bar. The copolymer is not isolated from the latex. The degree of conversion of the sulfomonomer and the yield of the copolymer are not indicated.

The disadvantage of this method is the use of perfluorinated aliphatic carboxylic acid salts as an emulsion stabilizer, which are prohibited for use by the Stockholm Convention on Persistent Organic Pollutants.

Thus, there is a need to develop a method for copolymerizing perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene that is free from the disadvantages inherent in known methods.

DISCLOSURE OF THE ESSENCE OF THE INVENTION

The technical task facing the authors was to develop a technologically optimal method for producing a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene, which would make it possible to obtain a copolymer with specified physicochemical properties, excluding the formation of a homopolymer of tetrafluoroethylene.

The stated technical task is achieved by the claimed invention - the proposed method for producing a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene by the method of water emulsion copolymerization.

The claimed method is characterized by a set of essential features and consists of copolymerization of tetrafluoroethylene and an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether obtained by emulsification in an inert atmosphere at a temperature of 10-20°C at a volume ratio of the sulfomonomer:water phases equal to 1.0:(8.0-9.2) in the presence of an emulsifier - salts of perfluorinated oxoacids in an amount of 0.68-0.95 mol.% of the sulfomonomer, a pH regulator - a phosphate buffer solution and a special additive that limits the formation of a homopolymer of tetrafluoroethylene (fluoroplastic), in an inert gas atmosphere, in a steel autoclave in the presence of an initiator of ammonium persulfate in an amount of 1.8-3.6 mol.% of the sulfomonomer, at a stirring speed of 450-500 rpm and an operating temperature 50-60°C by feeding tetrafluoroethylene under a layer of reaction mass at a working pressure of tetrafluoroethylene of 0.7-0.9 MPa for 9 hours with the formation of a copolymer latex with the required ratio of monomer units, the isolation of which is carried out by coagulation in an aqueous electrolyte solution, repeated washing with water and drying in a vacuum at a temperature of 50-90°C.

An aqueous solution of ammonium salts of perfluorinated oxoacids of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ (n=2, 3) was used as an emulsifier in the preparation of the aqueous emulsion of the sulfomonomer. In a preferred embodiment of the present invention, an equimolar mixture of salts of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ (n=2, 3) in the form of a 50% aqueous solution is used. The use of perfluorinated oxoacid salts in an amount of 0.68-0.95 mol.% of the sulfomonomer as an emulsifier makes it possible to obtain an emulsion whose stability is not lost throughout the entire copolymerization cycle. A phosphate buffer of monosubstituted and disubstituted sodium phosphates is used as a pH regulator of the medium.

According to the claimed method, the process of preparing an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether is carried out by emulsification at a speed of 25,000 rpm for 25 minutes in an argon atmosphere. Inorganic peroxides are used as a copolymerization initiator: ammonium or potassium persulfates, preferably ammonium persulfate. The reagents are consumed in a molar ratio of perfluoro-4-(fluorosulfonyl)butylvinyl ether: emulsifier: ammonium persulfate equal to 1:(0.0068-0.0095):(0.018-0.036).

In conditions of achieving a high degree of conversion of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene, the formation of a homopolymer of tetrafluoroethylene inevitably occurs, the presence of which in the product has a negative effect on the final properties of the ion-exchange membrane, such as loss of elasticity, film heterogeneity, reduced ion-exchange activity, etc. The process of fluoroplastic formation can be inhibited by adding water-soluble organic chain transfer agents as a special additive. According to the claimed invention, low-molecular aliphatic alcohols, in particular methanol, are used as such additives. Moreover, the amount of the additive introduced should be the minimum necessary to level the process of formation of tetrafluoroethylene homopolymer, preferably 3.1 mass. % of the sulfomonomer.

In the process of copolymerization of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene, maintaining the temperature in the range of 50-60°C, the stirring speed at 450-500 rpm and feeding tetrafluoroethylene under the layer of reaction mass ensure a high yield of the copolymer and a high degree of conversion of perfluoro-4-(fluorosulfonyl)butylvinyl ether.

The copolymer is separated from the latex by adding a coagulant, which is an aqueous solution of an electrolyte. An aqueous solution of inorganic salts is used as the electrolyte - sodium chloride, sodium sulfate, calcium chloride, etc. In a preferred embodiment of the present invention, an aqueous solution of calcium chloride is used. The coagulated copolymer is washed with water and dried in a vacuum at a temperature of 50-90 ° C to obtain a white amorphous powder.

The proposed method for obtaining a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene is carried out as follows:

1. Passivation of the reactor surface.

The internal mirror surface of the autoclave made of 12X18N10T stainless steel is treated with a 5% aqueous solution of ammonium persulfate at a temperature of 90-95°C for 2 hours.

2. Preparation of an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether.

Perfluoro-4-(fluorosulfonyl)butylvinyl ether is added to an aqueous solution of an emulsifier, which is an equimolar mixture of ammonium salts of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ (n=2, 3), a pH regulator - a phosphate buffer (a mixture of sodium dihydrogen phosphate and hydrogen phosphate) and methanol, and emulsified at a speed of 25,000 rpm in an argon atmosphere at a temperature of 10-20°C.

3. Copolymerization process.

The prepared aqueous emulsion is loaded into an autoclave in an inert atmosphere, an aqueous solution of the initiator - ammonium persulfate is added and at a temperature in the reactor of 50-60 °C, excess pressure is built up, opening the supply of tetrafluoroethylene through a siphon into the lower part of the reaction mass. The copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene is obtained using a four-blade stirrer at a speed of 450-500 rpm. Tetrafluoroethylene is fed using an electromagnetic valve in the pressure range of 0.7-0.9 MPa. The amount of absorbed tetrafluoroethylene is controlled by the decrease in the weight of the cylinder. After absorbing a certain amount of tetrafluoroethylene necessary to achieve the pre-calculated equivalent molecular weight of the copolymer, the supply of tetrafluoroethylene is stopped, the excess pressure is released, the resulting latex is purged with nitrogen and unloaded from the reactor.

4. Isolation of the copolymer.

The resulting latex is added to a 0.1% solution of calcium chloride in deionized water while stirring, followed by coagulation. The coagulated copolymer is then repeatedly washed with deionized water. The filtered product is dried in a vacuum oven at a temperature of 50-90°C to a constant weight.

5. Conversion of the copolymer into sulfonic acid form (sample preparation).

A sample of the dried copolymer is boiled in 2M NaOH. The salt form of the copolymer is isolated and treated with 1M H ₂ SO ₄ until the sulfonic acid form of the copolymer is obtained. The copolymer in the sulfonic acid form is washed with deionized water until the pH of the aqueous extract is neutral and dried in a vacuum at a temperature of 130°C.

The properties of the copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene obtained by the claimed method are determined as follows:

1. The melt flow index (MFI) is determined according to GOST 11645-73 on a plastometer of the IIRT-M type at 270±0.5°C, a load of 2.16 kg, a capillary diameter of 2.095±0.005 mm, and a sample holding time in the extrusion chamber of 5 minutes.

2. The ion exchange capacity (IEC) of the copolymer is determined by titrating with 0.1 M NaOH the sulfonic acid form of the copolymer sample (m) after treatment with 0.5 M NaCl:

IEC (meq/g)=[V NaOH ⋅ T NaOH] / m

3. The equivalent molecular weight (EW) is calculated based on the determined ion exchange capacity (IEC) of the copolymer:

EW=1000/IEC

4. The conversion of perfluoro-4-(fluorosulfonyl)butyl vinyl ether is calculated based on the values of the equivalent molecular weight (EW), the yield of the copolymer (W), the amount of perfluoro-4-(fluorosulfonyl)butyl vinyl ether loaded into the reaction mixture (W1), according to the formula:

Conversion (%) = [380⋅W⋅100] / [W1⋅EW],

where: 380 is the molecular weight of perfluoro-4-(fluorosulfonyl)butylvinyl ether.

5. The IR spectrum of the copolymer is recorded on a Bruker Vertex 80 IR Fourier spectrometer in the range of 500-4000 cm ^-1 . Characteristic bands of the IR spectrum of the copolymer (cm ^-1 ): 1466 (-SO ₂ F); 1004-1344 (C-O-C; -CF ₂ ).

6. The glass transition temperature of the copolymer (T _g ) is determined by differential scanning colorimetry (DSC) on a DSC 3+/700/2305 device (Mettler Toledo, Switzerland) in the temperature range of 25-200°C (heating rate 5°C/min, flow rate 30 ml/min).

Thus, the analysis of known methods for obtaining a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene did not allow us to find a solution that completely coincides in terms of the set of features with the claimed one, which may indicate its novelty.

Information confirming the possibility of implementing the invention, including in accordance with the requirement of “industrial applicability”, but not limiting it, is illustrated by the following examples.

EXAMPLES

Perfluoro-4-(fluorosulfonyl)butylvinyl ether is used as a sulfomonomer of the claimed copolymerization method, the method for producing which is disclosed by the authors in patent RU 2800857, published on July 31, 2023.

Example 1.

In 1000 ml of deionized water, 4.5 g (3.75 mmol) of an equimolar mixture of ammonium salts of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ (m = 2.3), 2.95 g of sodium dihydrogen phosphate, 2.6 g of sodium hydrogen phosphate and 6.5 g of methanol are dissolved. To the resulting solution in an argon atmosphere, 210 g (0.553 mol) of perfluoro-4-(fluorosulfonyl)butylvinyl ether are added and the emulsion is prepared using a Unidrive X1000D disperser for 25 minutes at a temperature of 10-20°C. The resulting emulsion is placed in a 2000 cm ³ steel autoclave, a solution of 4.6 g (0.02 mol) of ammonium persulfate in 10 ml of distilled water is added, the reactor is successively purged with nitrogen at a rate of 1.5 l/min for 10 minutes and with tetrafluoroethylene at a rate of 0.5 l/min for 15 minutes. Then the contents of the reactor are heated to a temperature of 50-60 °C and, with stirring at a rate of 450-500 rpm, tetrafluoroethylene is fed to a pressure of 0.9 MPa. After initiation begins and the tetrafluoroethylene pressure drops to 0.7 MPa, the reaction mass is fed with tetrafluoroethylene to the initial pressure of 0.9 MPa using an electromagnetic valve and the synthesis is carried out in the pressure range of 0.7-0.9 MPa. At the end of the synthesis, the tetrafluoroethylene pressure is reduced to 0.3 MPa. Over the course of 9 hours, the amount of tetrafluoroethylene reacted was 176 g. The resulting latex was unloaded from the autoclave and poured into 6 liters of 0.1% aqueous calcium chloride solution for 20 minutes, coagulated with vigorous stirring for 30 minutes, and filtered under vacuum. The coagulated crude product was washed with 5 liters of deionized water for 30 minutes at 50°C and filtered under vacuum. The washing operation was repeated 3 times. The washed copolymer was dried in vacuum at 50-90°C to constant weight.

The yield of copolymer is 373 g (96.6%).

The degree of conversion of perfluoro-4-(fluorosulfonyl)butylvinyl ether is 93.8%.

Melt flow index (MFI) (270°C, 2.16 kg, 2.095 mm) - 0.58 g/10 min. Ion exchange capacity (TEC) - 1.370 meq/g (EW 730). Glass transition temperature (T _g ): 140.7°C.

Example 2.

The process of obtaining the copolymer is carried out similarly to example 1, but when preparing the emulsion, 1150 ml of deionized water, 6.3 g (5.25 mmol) of an equimolar mixture of ammonium salts of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ (n = 2, 3), 2.95 g of sodium dihydrogen phosphate, 2.6 g of sodium hydrogen phosphate and 6.5 g of methanol were used, and during initiation, an aqueous solution of 3.2 g (0.014 mol) of ammonium persulfate was added.

Within 10 hours, the amount of tetrafluoroethylene reacted was 190 g.

Copolymer yield: 390 g (97.5%).

The conversion rate of perfluoro-4-(fluorosulfonyl)butylvinyl ether is 94.6%. The melt flow index (MFI) (270°C, 2.16 kg, 2.095 mm) is 0.42 g/10 min. The ion exchange capacity (IEC) is 1.289 meq/g (EW 776). The glass transition temperature (T _g ): 136.5°C. Example 3.

The process of obtaining the copolymer is carried out similarly to example 2.

Within 12 hours, the amount of tetrafluoroethylene reacted was 228 g.

The yield of copolymer is 426 g (97.3%).

The conversion rate of perfluoro-4-(fluorosulfonyl)butylvinyl ether is 94.3%. The melt flow index (MFI) (270°C, 2.16 kg, 2.095 mm) is 0.32 g/10 min. The ion exchange capacity (IEC) is 1.225 meq/g (EW 816). The glass transition temperature (T _g ): 134.4°C. Example 4.

The process of obtaining the copolymer is carried out similarly to example 1, but during initiation, an aqueous solution of 2.3 g (0.01 mol) of ammonium persulfate was added.

Over the course of 11.5 hours, the amount of tetrafluoroethylene that reacted was 224 g.

The yield of copolymer is 421 g (97.0%).

The conversion rate of perfluoro-4-(fluorosulfonyl)butylvinyl ether is 93.8%. The melt flow index (MFI) (270°C, 2.16 kg, 2.095 mm) is 0.25 g/10 min. The ion exchange capacity (IEC) is 1.176 meq/g (EW 850).

Glass transition temperature (Tg): 132.3°C.

References.

1. Emery M. et al. "The Development of New Membranes for Proton Exchange Membrane Fuel Cells", ECS Transaction, 11 (1) 3-14 (2007).

2. Patent US 7071271, 07/04/2006.

3. Patent US 7214740. 05/08/2007.

4. Patent US 2005/0107488, 05/19/2005.

Claims (5)

1. A method for producing a copolymer of perfluoro-4-(fluorosulfonyl)butylvinyl ether and tetrafluoroethylene, which consists in the fact that the copolymerization of tetrafluoroethylene and an aqueous emulsion of perfluoro-4-(fluorosulfonyl)butylvinyl ether (hereinafter referred to as the sulfomonomer), obtained by emulsification in an inert atmosphere at a temperature of 10-20°C at a volume ratio of the sulfomonomer:water phases equal to 1.0:(8.0-9.2) in the presence of an emulsifier - salts of perfluorinated oxoacids, in an amount of 0.68-0.95 mol.% of the sulfomonomer, a pH regulator of the medium - a phosphate buffer solution and a special additive that limits the formation of a tetrafluoroethylene homopolymer, is carried out in a steel autoclave in the presence of an initiator of ammonium persulfate in an amount of 1.8-3.6 mol.% of the sulfomonomer, at a stirring speed of 450-500 rpm and an operating temperature of 50-60°C by feeding tetrafluoroethylene under a layer of reaction mass at an operating pressure of tetrafluoroethylene of 0.7-0.9 MPa for 9 hours with the formation of a copolymer latex with the required ratio of monomer units, the isolation of which is carried out by coagulation in an aqueous electrolyte solution, repeated washing with water and drying in a vacuum at a temperature of 50-90°C. 2. The method according to item 1, characterized in that the production of an aqueous emulsion of a sulfomonomer is carried out by emulsification at a speed of 25,000 rpm at a temperature of 10-20°C. 3. The method according to item 1, characterized in that a 50% aqueous solution of an equimolar mixture of ammonium salts of perfluorinated oxoacids of the composition CF ₃ CF ₂ [CF ₂ OCF(CF ₃ )] _n COONH ₄ , where n=2, 3, is used as the emulsifier. 4. The method according to paragraph 1, characterized in that methanol is used as a special additive that limits the formation of tetrafluoroethylene homopolymer. 5. The method according to item 1, characterized in that an aqueous solution of calcium chloride is used as an electrolyte during coagulation of the copolymer.