RU2718928C1 - Modified nanoporous polymer membrane with improved water-repellent properties for membrane contactors and method production thereof - Google Patents

Abstract

FIELD: membrane technologies.

SUBSTANCE: invention relates to membrane technologies and can be used for modification of nanoporous membranes in order to improve their hydrophobic properties to increase efficiency of membrane contactors, and can be used in membrane gas/liquid contactors to increase gas mix components extraction efficiency. Method of modifying the surface of the nanoporous membrane involves hydrophobisation of the surface of the membrane by forming a coating from compounds which form fluorine-containing functional groups.

EFFECT: technical result is creation of modified nanoporous polymer membrane with improved hydrophobic properties.

3 cl, 8 dwg, 1 tbl, 8 ex

Description

Technical field

The invention relates to the field of membrane technologies, in particular surface modification of nanoporous polymer membranes by non-destructive membrane structure, fluorination and perfluoroalkylation, in order to improve their water-repellent properties to increase the performance of membrane contactors.

State of the art

The use of nanoporous polymer membranes in membrane gas / liquid contactors makes it possible to increase the selectivity and productivity of the extraction of components of gas mixtures due to a substantial increase in the contact area of the gas phase with the liquid phase of the absorbent. In this case, the membrane performance is determined by the rate of transfer of the extracted components through the nanopores of the membrane, and the diffusion rate of the extracted components in the nanopores in the gas phase is significantly higher compared to the diffusion of components in a liquid aqueous absorbent solution. However, during the operation of the contactor, it is possible to wet the membrane and gradually fill the pores with the liquid phase of the absorbent, which negatively affects the performance of the membrane contactors. The solution to this problem is associated with preventing the wetting of the nanoporous membrane and improving its water-repellent properties. In this regard, the development of methods for producing porous polymer membranes with a modified surface without changing the porous structure during the modification in order to increase their hydrophobicity is an urgent task.

It is known to use nanoporous polymer membranes for extracting components of gas mixtures by a pertraction method using a gas / liquid membrane contactor based on hollow fibers made of polypropylene. The use of nanoporous hollow fiber polypropylene membranes, characterized by a pore size of 100 × 500 nm, allowed to increase the performance of gas / liquid membrane contactors by more than 10 times compared with traditional contactors [RU 2626645, this solution was chosen as a prototype for a nanoporous polymer membrane]. However, the gradual wetting of polypropylene membranes with an absorbent solution at excess pressure from the liquid phase, which leads to a decrease in the diffusion and transfer rates of the components when the pores are filled with the liquid phase and, during prolonged use, the liquid absorbent enters the gas phase, as well as the degradation of the membrane material, is their main disadvantage.

In this regard, non-destructive modification of the surface of polypropylene membranes in order to improve their water-repellent properties by creating on the surface of the membrane, including the inner walls of the pores, coatings of hydrophobic functional groups is an urgent task. One of the possible ways to create hydrophobic coatings on the surface of a polymer membrane is direct fluorination using atomic and / or molecular fluorine. A known method of modifying polymers by direct fluorination of the polymer surface with gaseous molecular fluorine, which improves their hydrophobic properties [Direct fluorination of polymer products - from basic research to practical use / A.P. Kharitonov, B.A. Loginov. // Russian Chemical Journal (ZhRLO named after D.I. Mendeleev). - 2008. - T. 52, No. 3. - S. 106-111]. The disadvantages of this approach are the complexity and energy consumption of the process. In addition, the use of this method of hydrophobization leads to the destruction of the structure of the polymer membrane and the contamination and / or filling of pores with polymer decomposition products.

A known method of modifying polymers using plasma polymerization of fluorine-containing monomers on their surface, which allows to improve their hydrophobic properties [US 4404256]. However, this approach requires complex hardware design and does not allow to modify the inner surface of the pore walls of the membrane.

A known method of modifying the surface of polymers based on its processing by pulsed plasma sputtering of a graphite target [RU 2325192, RU 2153887]. The disadvantages of this method are the high cost, the possible destruction of the microstructure of the membrane material, as well as a significant reduction in contact angles during deposition of the carbon film in the pores and on the surface of the polymer membrane.

A known method for the modification of polymeric materials, which consists in processing the material with a solution of perfluoro-2,4-dimethyl-3-ethyl-3-pentyl radical in perfluorinated solvents at temperatures> 80 ° C [RU 2303609]. The disadvantages of this method are the need to use inert perfluorinated solvents and purification from the original liquid reagents.

The closest in technical essence and the achieved result to the claimed method for producing a nanoporous polymer membrane is a method of modifying the surface of polymeric materials based on attaching fluorine-containing fragments to the structural elements of the polymer chain [US 4489201]. However, its disadvantage is the need for the polymer material to have hydroxyl groups and / or amino groups, which greatly narrows the range of nanoporous polymeric materials used. In addition, the materials obtained by this method have not been tested to modify nanoporous polymer membranes used in a gas / liquid contactor.

Disclosure of Invention

The technical problem is the creation of a nanoporous polymer membrane with improved water-repellent properties to increase the performance of membrane contactors and the development of a technological method for its production, which does not require complex hardware design, long and energy-intensive technological operations.

The technical problem was solved by creating on the surface of the nanoporous membrane, including the inner walls of the pores, coatings of fluorine-containing functional groups, contributing to the improvement of the water-repellent properties of nanoporous polymer membranes.

The technical result of the invention is to improve the water-repellent properties of a nanoporous polymer membrane, characterized by an increased contact angle of water wetting by at least 10 ° and an increase in their selectivity in membrane contactors for components of gas mixtures extracted into the absorbent by not less than 1.5 times.

The specified technical result is achieved by hydrophobization of the surface of the nanoporous membrane by creating on its surface, including the inner walls of the pores, coatings of fluorine-containing functional groups —CF ₃ , —CHF ₂ , —C ₂ F ₅ , —C ₃ F ₇ , —C ₄ F ₉ , by fluorination with XeF ₂ at 60-160 ° C and / or perfluoroalkylation, initiated thermally at 100-160 ° C and / or under ultraviolet radiation, using perfluoroalkyl iodides, including CF ₃ I, C ₂ F ₅ I, n-C ₃ F ₇ I, n-C ₄ F ₉ I.

The claimed method of surface modification, including the inner walls of pores, nanoporous polymer membranes is applicable to various objects and materials (bulk products, thin films, hollow fibers, etc., made of polytetrafluoroethylene, polypropylene, polysulfone, polyethersulfone, polyvinylidene fluoride, etc. ) Of particular interest is the use of this method for hydrophobization of the surface of hollow fiber polypropylene membranes, which are most often used in gas / liquid membrane contactors.

The modification of nanoporous polypropylene membranes in the proposed method is carried out by fluorination using XeF ₂ or perfluoroalkylation using perfluoroalkyl iodides, including CF ₃ I, C ₂ F ₅ I, n-C ₃ F ₇ I, n-C ₄ F ₉ I.

During fluorination, nanoporous polypropylene membranes are placed in a Teflon autoclave, the required amount of XeF _{2 is} added (based on ~ 20 mg of XeF ₂ per 1 m ^{2 of the} surface of the polypropylene membrane) and kept at a temperature of 60-140 ° C for 4-5 hours.

In the case of perfluoroalkylation, nanoporous polypropylene membranes are placed in a single or two-section ampoule, the required amount of perfluoroalkyl iodide is added (at the rate of ~ 4 mmol of perfluoroalkyl iodide per 1 m ^{2 of the} surface of the polypropylene membrane), the ampoule is soldered and kept in a gradient furnace at 100-160 ° С and / or under UV irradiation (incident radiation power of 100 mW / cm ² ) for 4-5 hours.

, C₂F₅

, C₃F₇

, C₄F₉

при нагревании и/или УФ-облучении соответствующих перфторалкилиодидов, способных реагировать с поверхностью полимерных мембран и расщеплять имеющиеся в структуре связи С-Н с образованием радикальных центров, расположенных в полимерной цепи, последующая рекомбинация которых с соответствующими радикальными частицами приводит к образованию модифицированной поверхности.The formation of a coating of fluorine-containing functional groups on the surface of the polymer membrane, including the inner walls of the pores, is due to the formation of molecular fluorine upon heating of XeF ₂ and radical particles CF ₃

, C ₂ F ₅

, C ₃ F ₇

, C ₄ F ₉

upon heating and / or UV irradiation of the corresponding perfluoroalkyl iodides capable of reacting with the surface of polymer membranes and cleaving the CH bonds in the structure with the formation of radical centers located in the polymer chain, the subsequent recombination of which with the corresponding radical particles leads to the formation of a modified surface.

At the end of fluorination / perfluoroalkylation, an autoclave / ampoule is opened and polypropylene membranes with a surface modified with fluorine-containing functional groups are removed.

Studies of the water-repellent properties and structure of modified polypropylene membranes showed an increase in contact angles of contact with water by at least 10 ° while maintaining a membrane structure with a characteristic pore size of 20 to 500 nm. Thus, according to the data obtained, the claimed modification method allows efficiently and without destroying the microstructure of nanoporous polymer membranes to modify their surface, including the inner walls of the pores, creating membranes with improved water-repellent properties.

The use of a modified nanoporous hollow fiber polypropylene membrane with improved water-repellent properties in gas / liquid membrane contactors leads to an increase in the productivity and selectivity of the components of gas mixtures extracted into the absorbent with respect to non-recoverable components of gas mixtures by at least 1.5 times in comparison with the membrane before it modifications due to the increased diffusion rate of components in the gas phase compared with their diffusion in a liquid aqueous absorbent solution.

Brief Description of the Drawings

The invention is illustrated by drawings, where in FIG. 1. presents a two-section ampoule for perfluoroalkylation when heated in a furnace (a), a single-section ampoule for perfluoroalkylation under UV irradiation (b); in FIG. 2. - a system for preparing the ampoule (a), a perfluoroalkylation unit when heated in a gradient furnace (b), a perfluoroalkylation unit under UV irradiation (c) and an autoclave for fluorination (g); in FIG. 3. - contact angles of the outflow and outflow of the original (a, b) perfluoroalkylated with CF ₃ I (c, d) and n-C ₄ F ₉ I (e, f) fluorinated with XeF ₂ (g, h) polypropylene membrane; in FIG. 4. - micrographs of the initial (a) perfluoroalkylated with CF ₃ I (b), n-C ₄ F ₉ I (c) and fluorinated with XeF ₂ (g) polypropylene membrane; in FIG. 5. - contact wetting angles of the original and modified nanoporous polypropylene membranes; in FIG. 6. - X-ray photoelectron spectroscopy results for a nanoporous hollow fiber polypropylene membrane modified by fluorination using XeF ₂ ; in FIG. 7. - the results of IR spectroscopy; in FIG. 8. - The results of using a modified nanoporous hollow fiber polypropylene membrane in a membrane contactor.

Table 1 presents the surface modification conditions for nanoporous hollow fiber polypropylene membranes, contact angles of wetting with water, and the efficiency of water vapor removal on modified nanoporous hollow fiber membranes in a gas / liquid membrane contactor.

The implementation of the invention

The present invention is illustrated by specific examples of execution, which, however, are not the only ones possible.

Examples 1-8. Obtaining modified nanoporous hollow fiber polypropylene membranes by fluorination and / or perfluoroalkylation initiated thermally or by UV irradiation.

The perfluoroalkylation of the surface of nanoporous hollow fiber polypropylene membranes using perfluoroalkylating agents — CF ₃ I or n-C ₄ F ₉ I — is carried out in a two-section ampoule (Fig. 1, a) when heated in the range from 100 to 160 ° C (examples 1- 3) or in a single-section quartz ampoule (Fig. 1, b) under UV irradiation (examples 4-5) for 4-5 hours. The perfluoroalkylation ampoules are made of thick-walled Schott Duran tubes (molybdenum glass) or quartz with an inner diameter of 5 mm and wall thickness 1.5 mm. Ampoules made of such glass withstand internal pressure up to 10-15 atm. The geometric parameters of the ampoules are shown in FIG. 1. In the manufacture of ampoules, 3-5 cm are additionally added to their total length for the manufacture of technical constrictions for connecting gaseous reagents to the evacuation / inlet system and subsequent desoldering. When perfluoroalkylation of nanoporous hollow fiber polypropylene membranes initiated by heating in a gradient furnace, the part of the ampoule containing perfluoroalkyl iodide is bent at an angle of 30-60 °, so that the liquid reagent at room temperature does not enter the heated section of the ampoule. The ampoule length is selected so that the part with polypropylene membranes when placed in a gradient furnace is at maximum temperature, and the bend area is at the outlet of the furnace. Nanoporous hollow fiber polypropylene membranes are placed in an ampoule and connected to a system for evacuation / inlet of gaseous reagents (Fig. 2, a), vacuum until a pressure of ~ 1 mm Hg is reached. and heated at 100-120 ° C to remove oxygen and water for 10-20 minutes adsorbed on the surface of the fibers. Then, gaseous CF ₃ I is charged or ~ 0.3-0.4 ml of liquid n-C ₄ F ₉ I is added to the system. The amount of emitted CF ₃ I is monitored by a manometer, stopping the gas flow upon reaching a pressure in the system of ~ 0.8 atm. Since the geometric parameters of the installation are constant, the predetermined pressure CF ₃ I allows you to control the amount of reagent introduced into the ampoule in the range from 3 to 5 mmol (0.3-0.4 ml per 1 m2 ^of nanoporous polymer material). After filling / adding CF ₃ I / n-C ₄ F ₉ I into the system, it is condensed into an ampoule, placing part of it with polypropylene membranes in liquid nitrogen. After condensation of CF ₃ I / n-C ₄ F ₉ I, the system is again evacuated (CF ₃ I / n-C ₄ F ₉ I in the ampoule was frozen) to remove non-condensed residues CF ₃ I / n-C ₄ F ₉ I, then produce a tap. After cooling the ampoule, CF ₃ I / n-C ₄ F ₉ I is condensed from the part where the polypropylene membranes are located to the bent part, which is cooled over liquid nitrogen. Excess liquid CF ₃ I / n-C ₄ F ₉ I in the bent part of the ampoule does not affect the reproducibility of perfluoroalkylation, because Saturated CF ₃ I / n-C ₄ F ₉ I pairs interact with the surface of polypropylene membranes at a pressure of ~ 5 atm. An ampoule with polypropylene membranes prepared in this way is placed in a gradient furnace (FIG. 2, b) at 100-160 ° C for 4-5 hours or irradiated with a 150 W UV lamp (incident radiation power of 100 mW / cm ² ) (FIG. 2, c). At the end of perfluoroalkylation, the ampoules are opened, having previously not frozen in liquid nitrogen, which has not reacted CF ₃ I / n-C ₄ F ₉ I, and polypropylene membranes with a perfluoroalkyl-modified surface and improved water-repellent properties are removed.

Nondestructive fluorination of the surface of polypropylene membranes is carried out using XeF ₂ as a fluorinating agent (~ 40 mg sample, based on ~ 20 mg of XeF ₂ per 1 m ^{2 of} nanoporous polymeric material) in a 100 ml Teflon autoclave in the temperature range from 20 to 140 ° С (Fig. 2, g).

The effectiveness of perfluoroalkylation and fluorination of the surface of nanoporous hollow fiber polymer membranes was assessed based on contact wetting angles, as well as studies of nanoporous hollow fiber polypropylene membranes by IR spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and also based on the results of testing the modified membrane in a membrane contact.

According to the contact wetting angle and scanning electron microscopy (Fig. 3-5), the modification of nanoporous polymer membranes by forming on the surface of the membrane, including the inner walls of the pores, coatings of fluorine-containing functional groups as a result of perfluoroalkylation using perfluoroalkyl iodides and fluorination using XeF ₂ allows increase the contact angle of wetting by at least 10 ° without destroying the microstructure of the membrane.

The analysis of modified nanoporous hollow fiber polypropylene membranes by X-ray photoelectron spectroscopy confirmed the presence of a coating of fluorine-containing functional groups on the membrane surface. According to x-ray photoelectron spectroscopy, the content of carbon atoms associated with fluorine is not less than 6% of the total number of carbon atoms (Fig. 6).

According to the results of the analysis of modified nanoporous hollow fiber polypropylene membranes by IR spectroscopy (Fig. 7), we can conclude that the integral intensity of the vibration maximum corresponding to OH groups decreases during membrane modification by perfluoroalkylation using perfluoroalkyl iodides and fluorination using XeF ₂ , which is consistent with the results of measuring the contact angle of contact and indicates a decrease in the amount of water adsorbed on the membrane surface (more e than 10 times) and improving the hydrophobic properties in comparison with the original polypropylene membrane.

Modified nanoporous polypropylene membranes were tested during the drying of moisture-saturated air at 25 ° C at a working pressure of 6 atm and an air flow of 150 l / min in a gas / liquid membrane contactor (the design of the membrane contactor is described in patent RU 2626645) using absorbent material triethylene glycol with a water content of less than 0.5%. The total membrane area in the contactor was 0.7 m ² . According to the data obtained, using modified nanoporous polypropylene membranes with improved water repellent properties allows the quantitative removal of water vapor from the gas mixture, while the use of an unmodified membrane under completely similar conditions makes it possible to recover no more than 50% (example 7, Fig. 8). An increase in membrane productivity is achieved due to the absence of wetting of the materials of the modified membrane with the absorbent solution and the absence of filling the porous structure with a liquid phase. Moreover, the high diffusion rate of the components in the gas phase compared with their diffusion in the absorbent liquid solution determines an increase in the membrane productivity.

Discrepancies in the contact wetting angles of water of modified nanoporous polypropylene membranes made similarly using CF ₃ I, C ₂ F ₅ I, n-C ₃ F ₇ I, n-C ₄ F ₉ I, and their selectivity in membrane contactors for extractable absorbent components of gas mixtures does not exceed 10%.

Claims (3)

1. A modified nanoporous polymer membrane for membrane contactors, characterized by a pore size of from 20 to 500 nm, characterized in that the membrane has a coating comprising fluorine-containing functional groups —CF ₃ , —CHF ₂ , —C ₂ F ₅ , —C ₃ F ₇ , -C ₄ F ₉ , while polypropylene-based hollow fiber membranes are used as nanoporous polymer membranes. 2. A method of producing a modified nanoporous polymer membrane for membrane contactors according to claim 1, comprising hydrophobizing the surface of the membrane by creating a coating of fluorine-containing functional groups —CF ₃ , —CHF ₂ , —C ₂ F ₅ , —C ₃ F ₇ , —C ₄ F ₉ , by fluorination with XeF ₂ and / or perfluoroalkylation using perfluoroalkyl iodides, including CF ₃ I, C ₂ F ₅ I, n-C ₃ F ₇ I, n-C ₄ F ₉ I, by UV irradiation of the membrane surface a radiation power of 100 mW / cm ² and / or heating at a temperature of 60-160 ° C in a gradient furnace for 4-5 hours. 3. The method according to p. 2, characterized in that as nanoporous polymer membranes using membranes with pore diameters from 20 to 500 nm.

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