CZ201316A3 - Process for preparing heterogenous ion-exchange membranes - Google Patents

Abstract

The subject of the invention is a method of preparing heterogeneous ion exchange membranes. The preparation of membranes according to the present invention is characterized in that the polymeric inert binder based on polypropylene or propylene copolymer is homogenized with small particles of ion exchange resin in a device suitable for homogenizing polymer blends and the resulting mixture is then subsequently used to prepare the membrane by compression, extrusion or rolling. The homogenization of the mixture and its subsequent formation into the resulting membrane are carried out at a temperature higher than the melting point of the polymeric inert binder but lower than the degradation temperature of the ion exchange particles. The resulting membrane is then immersed in an aqueous solution of hydrogen peroxide, whereby the binder based on polypropylene or propylene copolymer partially degrades and pores are formed in the surface layer of the membrane to allow increased ion transport through the membrane and thus higher ionic conductivity of the membrane.

Description

The invention relates to a process for the preparation of heterogeneous ion-exchange membranes suitable as a semipermeable barrier in the electrolysis of water, as a solid electrolyte in fuel cells and other devices using ion-exchange membranes.

BACKGROUND OF THE INVENTION

Ion exchange membranes are used in laboratory and industrial scale. The most important applications are mainly electrochemical desalination of sea and brackish waters, separation of electrolytes from nonelectrolytes, membrane fuel cells and electrolysis of aqueous solutions. Ion-exchange membranes are divided into homogeneous membranes (the membrane consists of only one polymer carrying ion-exchange groups) and heterogeneous membranes (consisting of a matrix of polymer without ion-exchange groups in which small particles of ion-exchange ion exchange material are dispersed). The advantage of heterogeneous membranes is their production from commercially available and cheap starting materials and their very good mechanical properties.

One method of preparing heterogeneous membranes consists in dispersing small particles of ion exchange material in a polymer solution, casting the dispersion in the form of a thin film and:. 7 by solvent removal (J. Membr. Sci. 240, 211-219, 2004; J. Membi. Sci. 243, 327-333, 2004; J. Appl. Polym. Sci. 98, 494-499, 2005; Desalination 172 , 257-265, 2005; J. Membr. Sci. 269, 177-186, 2006). The second method for the preparation of heterogeneous membranes is by heat treatment without solvents. The preparation of heterogeneous ion-exchange membranes of this type consists in homogenizing the polymeric inert binder with small particles of ion-exchange material in a device suitable for homogenizing polymer mixtures such as calenders, kneaders or extruders at a temperature higher than the melting point of the polyolefinic inert binder. than the degradation temperature of the ion exchange particles. This mixture is then used to produce a membrane, generally 100 to 600 µm or more in thickness, by pressing, extruding or rolling again at a temperature above the melting point of the polymeric inert binder but below the degradation temperature of the ion exchange particles.

The polymeric inert binder may be any polymer having thermoplastic and film-forming properties or a mixture of such polymers. The polymer must have a melting point below the degradation temperature of the ion exchange material, i. it must have a melting point of less than 200 ° C for the treatment of material with cationic groups and 150 ° C for the treatment of material which contains partially or totally anionic groups. These polymers are preferably linear polyethylene, low density polyethylene and copolymers of ethylene with propylene. These polyolefins, depending on their type, melt at temperatures of 90 to 120 ° C.

The ion exchange material (ion exchange particles) may be any acidic cationic resin or a basic anionic ion exchange resin. Preferably, cationic resins with sulfo groups on a cross-linked copolymer of styrene with divinylbenzene or an anionic resins with quaternary ammonium groups on the same copolymer are used. The particles must be smaller than the resulting membrane thickness, but preferably 10 to 80 µm particles are used. Heterogeneous membranes of this type typically contain 45 to 75 wt. ion exchange particles, the lower limit being given by the interconnection of the particles (percolation threshold) and the upper limit by the mechanical strength of the membrane. The higher the particle concentration, the higher the ion conductivity of the resulting membrane.

Thus, mixtures with particles with cationic groups are generally treated at temperatures of 90 to 200 ° C, mixtures with particles with anionic groups are generally treated at temperatures of 90 to 150 ° C, but depending on the melting and degradation temperatures of the components used.

The heterogeneous ion-exchange membranes are conditioned by alternate immersion in aqueous alkaline and acidic solutions prior to use. Most often, the anionic membranes are conditioned by alternating immersion in excess of 1M aqueous NaOH, 1M aqueous HCl and 1M aqueous NaOH prior to use, and the cationic membranes are conditioned alternately by immersion in excess 1M aqueous HCl, 1M aqueous NaOH and 1M aqueous HCl prior to use. . In each solution, the membrane is optimally 24 hours followed by washing with demineralized water.

The preparation of heterogeneous ion-exchange membranes by heat treatment is described in numerous publications and patents: Encyclopedia of Polymer Science and Technology, Vol. 8, JohnWiley, New York, 1968, pp. 63; J. Membr. Sci. 250, 151-157 (2005); J. Membr. Sci.

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401-402, 83-88 (2012); čs. author's certificates Nos 235 ^ 184 and 247 ^ 833; ' _(US Patent 5,346,924; US Patent 5 948 _r? 826; US Patent 6,632,848 B2; US Patent 6,830,671 B2, WO 00/78 849-Al.

A typical property of heterogeneous membranes prepared by heat treatment is the presence of a thin coating of less than 1 µm thickness on both sides of the membrane, where the concentration of ion exchange particles is much lower than inside the membrane (sometimes immeasurably small) (J. Colloid Interface Sci. 285, 247-258, 2005 and J. Appl. Electrochem. 2009, published on Sep. 4, 2009). The existence of a surface layer and the low concentration of particles in this layer are a general property of all polymeric films filled with small particles of another material (Encyclopedia of Polymer Science and Technology).

: Kb Polymer Blends, Wiley-Interscience, New York, 2005; Macromol. Symp. 214, 115-134

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2004; Macromol. Mater. Eng. 286, 719-730 (2001); Polym. Eng. Sci. 44, ^from 528- 540, 2004), the morphology of the layer depends to some extent on the rheological properties of the polymer matrix and the film preparation conditions. The surface layer of heterogeneous ion-exchange membranes improves the mechanical properties of the membrane, but at the same time substantially deteriorates (due to the barrier properties of the predominant inert matrix polymer therein) the ionic conductivity of the membrane. It is therefore desirable to increase the permeability of this layer for ions without substantially reducing the mechanical strength of the membrane. The present process for preparing heterogeneous membranes utilizes, for breaking the surface layer of the inert binder, a volume change of ion exchange particles as the membrane swells in water, resulting in cracking of the compact surface layer and formation of pores in the layer. However, the compact surface layer can only be destroyed by ion exchange particles having a diameter greater than half the thickness of the membrane. Due to the wide particle size distribution, the surface layer is not destroyed over the entire surface and some ion exchange particles remain inaccessible. The membranes thus prepared are of low ionic conductivity (J. Colloid Interface Sci. 285, 247-258, 2005).

SUMMARY OF THE INVENTION

The present invention provides a process for preparing heterogeneous ion-exchange membranes. The preparation of the membranes of the present invention consists of the following steps: (1) homogenizing a polymeric inert filler comprising propylene polymer units with small particles of ion exchange material at a temperature higher than the melting point of the polymeric inert filler but below the degradation temperature of small particles of ion exchange material; ) forming the prepared homogenised mixture into a membrane shape again at a temperature above the melting point of the polymeric inert binder, but below the degradation temperature of the small particles. ί β · I Li '* _t <t <* · · · · · · ^of ion exchange material (3), - introducing step 3 distinguishes the process of this invention from previously described methods of preparing heterogeneous membranes - immersion in % aqueous hydrogen peroxide solution under conditions such that a portion of the polymeric inert binder containing the propylene polymer units particularly in the membrane coating degrades, the degradation products are washed out into the aqueous peroxide solution, and pores are formed in the membrane membrane throughout the membrane area and (4) alternating washing membranes in aqueous alkaline and acidic solutions.

The pores in the surface layer are shown in the accompanying figures 1b and 1c.

The polymer inert binder used in the preparation of the membranes of the present invention must be degradable in an aqueous hydrogen peroxide solution. This condition is met by polypropylene and any copolymer of propylene with other olefins or olefins such as ethylene, 1-butene, 1-hexene, 1-octene or 5-methylene-2-norbornene, as the propylene polymer units are known to be oxidatively unstable ( J. Appl. Polym. Sci., 122, 2071-2080, 2011). A limiting condition is that the melting point of the polypropylene or propylene copolymer used must be less than 200 ° C when processing mixtures with cationic groups and less than 150 ° C when processing mixtures with anionic groups.

The homogenization of the polymeric inert binder and small ion exchanger particles according to the invention is carried out in a device suitable for homogenizing polymer mixtures, such as calenders, kneaders or extruders. The homogenization is carried out at a temperature above the melting point of the polymeric inert binder, but below the degradation temperature of the ion exchange particles. The concentration of the ion exchange particles in the starting mixture is 45 to 75% by weight, preferably 60 to 70% by weight, based on the weight of the whole mixture. The concentration of polymeric inert binder in the starting mixture is 25 to 55% by weight, based on the weight of the total mixture. The homogenized mixture is then used to produce a membrane of 100 to 800 µm by compression, extrusion or rolling, again at a temperature above the melting point of the polymeric inert binder, but below the degradation temperature of the ion exchange particles. Compositions with particles with cationic groups are treated at temperatures below 200 ° C and mixtures with particles with anionic groups are treated at temperatures below 150 ° C, which are the degradation temperatures of the ion exchangers used. In ion exchange particles with cationic groups at temperatures above 200 ° C, the aromatic nuclei to which functional groups are bonded are degraded. In ion exchange particles with anionic groups, the functional group consisting of a quaternary ammonium salt also degrades at temperatures above 150 ° C.

For the preparation of heterogeneous ion exchange membranes with cationic groups, strongly acidic ion exchangers with a sulfo-functional group attached to a styrendivinylbenzene carrier are most commonly used. The use of these ion exchangers in the preparation of heterogeneous membranes is described in a number of articles and patents (F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962; J. Membr. Sci. 49, 121, 1990; JP1181119 (A) (1999) )).

For the preparation of heterogeneous ion exchange membranes with anionic groups, the most commonly used are strongly basic type I ion exchangers with a quaternary ammonium functional group attached to a styrene-divinylbenzene carrier. The use of these ion exchangers in the preparation of heterogeneous membranes is described in a number of articles and patents (J. Membr. Sci 167, 1, 2000; PV 2007-723; US5,446-424 (B1) (1994)).

The prepared membranes are immersed in a 2 to 30% aqueous hydrogen peroxide solution at 20 to 45 ° C for 20 minutes to 48 hours prior to conditioning. Preferably, the membranes are immersed in a 5 to 30% solution for 24 to 48 hours at room temperature. h. When operating at an elevated temperature of 35 to 45 ° C, the membrane is preferably immersed in a 5 to 10% solution for 0.5 to 1 hour. The membranes are conditioned before use.

The membranes of the present invention are useful as a semipermeable barrier in the electrolysis of water, as an ion-conducting membrane in fuel cell membranes and in other devices utilizing ion exchange membranes.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an electron scanning microscope image of a membrane surface: (a) a heterogeneous membrane in which the peroxide bath prepared according to Example A of Example 1 of the present invention was not used; b) a heterogeneous membrane washed for 24 h in a 5% aqueous hydrogen peroxide solution prepared according to Example 2 of the present invention; c) a heterogeneous membrane washed A h in a 30% aqueous hydrogen peroxide solution prepared according to Example 3 of the present invention.

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DETAILED DESCRIPTION OF THE INVENTION

Example 1 (comparative)

A mixture of 18.7 g of poly (ethylene-co-propylene) copolymer (ExxonMobil, Vistalon 722) and 36.3 g of anionic resin particles based on a poly (styrene-co-divinylbenzene) copolymer carrying sulfo groups in sodium form (average particle size 40 µm) , the ion exchange capacity of 8.2 meq / g dry polymer) was mixed on a kneader at 150 ° C until the mixture was optically homogeneous and the torque reached a constant value (12 minutes). A 250 µm membrane was pressed from the prepared homogeneous mixture at 150 ° C and 10 MPa. The membrane was conditioned by alternate immersion in an excess of 1M aqueous HCl, 1M aqueous NaOH, and 1M aqueous HCl. (Each solution was 24 h and each solution was followed by a water wash (1 hour)). The ion conductivity of the membrane at 35 ° C was 3.8 S / m. The laboratory oxygen hydrogen fuel cell with this membrane at 35 ° C achieved a maximum power of 0.06 W / cm ² .

Example 2

The membrane was prepared according to the procedure and materials of Example 1,

However, before conditioning, it was immersed in a 5% aqueous hydrogen peroxide solution for 24 h at 25 ° C. The ion conductivity of the membrane at 35 ° C was 4.9 S / m. The laboratory oxygen hydrogen fuel cell with this membrane at a temperature of 35 ° C achieved a maximum power of 0.10 W / cm ² .

Example 3

The membrane was prepared according to the procedure and materials of Example 1, but before immersion it was immersed in a 30% aqueous hydrogen peroxide solution for 24 h at 25 ° C. The ion conductivity of the membrane at 35 ° C was 5.4 S / m. The laboratory oxygen hydrogen fuel cell with this membrane at a temperature of 35 ° C reached a maximum output of 0.14 W / cm ² .

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Example 4

The membrane was prepared according to the procedure and with the materials of Example 1, o

but prior to conditioning it was immersed in a 5% aqueous hydrogen peroxide solution for 1 hour at 40 ° C. The ion conductivity of the membrane at 35 ° C was 5.2 S / m. The laboratory oxygen-fuel cell with this membrane at a temperature of 35 ° C reached a maximum power of 0.12 W / cm ² .

Example 5

The membrane was prepared according to the procedure and with the materials of Example 1, but was immersed in a 30% aqueous hydrogen peroxide solution for 0.5 h prior to conditioning.

-j at 40 ° C. The ion conductivity of the membrane at 35 ° C was 5.1 S / m. The laboratory oxygen hydrogen fuel cell with this membrane at a temperature of 35 ° C reached a maximum power of 0.11 W / cm ² .

Example 6 (comparative)

The membrane was prepared according to the procedure and materials of Example 1, but instead of poly (ethylene-co-propylene) was used L-used-poly (ethylene $ -co-propylene <-co-5-methylene <-2-norbomenone) ( Aldrich, 25% w / w propylene units, 70% w / w ethylene units and 5% norbornene units). The ion conductivity of the membrane at 35 ° C was 3.9 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane at 35 ° C achieved a maximum power of 0.07 W / cm ² .

Example 7

The membrane was prepared according to Example 6, but was immersed in 30% aqueous hydrogen peroxide solution for 24 hours at 25 ° C prior to conditioning. The ion conductivity of the membrane at 35 ° C was 5.4 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane reached a maximum power of 0.14 W / cm at 35 ° C.

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Example 8 (comparative)

The membrane was prepared according to the procedure and materials of Example 1, but instead of poly (ethylene-co-propylene), poly (propylene-co-1-butene) (Aldrich, 12wt%, 1-butene units) could be used. . The ion conductivity of the membrane at 35 ° C was 4.1 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane reached a maximum power of 0.08 W / cm at 35 ° C.

Example 9

The membrane was prepared according to Example 8, but before conditioning it was immersed in 9% to 30% aqueous hydrogen peroxide solution for 24 hours at 25 ° C. The ion conductivity of the membrane at 35 ° C was 5.5 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane at a temperature of 35 ° C reached a maximum power of 0.15 W / cm ² .

Example 10 (comparative)

The membrane was prepared according to the procedure and materials of Example 1, but poly (propylene) was used instead of poly (ethylene-co-propylene) (ExxonMobil ACHIEVE 1605, mp 149 ° C). The ion conductivity of the membrane at 35 ° C was 3.7 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane at 35 ° C achieved a maximum power of 0.06 W / cm ² .

Example 11

The membrane was prepared according to Example 10 but was immersed in a 30% aqueous hydrogen peroxide solution for 24 h at 25 ° C prior to conditioning. The ion conductivity of the membrane at 35 ° C was 5.2 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane at 35 ° C achieved a maximum power of 0.12 W / cm.

Example 12 (comparative)

A mixture of 18.7 g of poly (ethylene-co-propylene) with a propylene content of 27.5% (melt index 6 g / 10 min at 230 ° C / 2.16 kg) and 36.3 g of anionic resin particles based on

^The% f «· poly (styrene-co-divinylbenzene) carrying trimethylbenzylammonium groups in the chloride form (average particle size 40 microns, ion exchange capacity 3.5 meq / g dry polymer) was mixed in a kneader at 140 ° C until the mixture was not optically homogeneous and the torque did not reach a constant value (12 minutes). A 350 µm membrane was pressed from the prepared homogeneous mixture at 140 ° C and 10 MPa. The membrane was conditioned by alternate immersion in an excess of 1M aqueous NaOH, 1M aqueous HCl, and 1M aqueous NaOH. (Each solution was 24 h and each solution was followed by a water wash (1 hour)). The ion conductivity of the membrane at 30 ° C was 1.38 S / m. The membrane was incorporated into a laboratory electrolyzer with nickel foam electrodes and an electrolyte consisting of a 10% aqueous potassium hydroxide solution. At a temperature of 30 ° C and an applied voltage of 2.5 V, a current density of 0.28 A / cm ² was achieved.

Example 13

The membrane was prepared according to Example 12 but was immersed in a 10% aqueous hydrogen peroxide solution for 24 h at 25 ° C prior to conditioning. The ion conductivity of the membrane at a temperature of 30 ° C was 3.60 S / m. The membrane was incorporated into a laboratory electrolyzer with nickel foam electrodes and an electrolyte consisting of a 10% aqueous potassium hydroxide solution. At a temperature of 30 ° C and an applied voltage of 2.5 V, a current density of 0.40 A / cm ² was achieved.

The ion conductivity of the membrane at 35 ° C was 2.4 S / m. The laboratory oxygen-hydrogen fuel cell with this membrane reached a maximum power of 0.32 W / cm at 35 ° C.

Industrial applicability

The heterogeneous ion-exchange membranes of the present invention are useful as a semipermeable barrier in water electrolysis, as an ion-conducting membrane in fuel cell membranes and other ion exchange membrane devices.

Claims (6)

PATENT CLAIMS A process for the preparation of heterogeneous ion-exchange membranes containing 25 to 55 wt. a polymeric inert binder consisting of a polypropylene or polypropylene copolymer which is degradable by an aqueous peroxide solution in a concentration range of 2 to 30%, and 45 to 75 (w / w) ion exchange particles, the membranes being prepared by heat treatment and before they are conditioned by immersion in aqueous acidic and alkaline solutions, characterized in that the membranes are immersed in an aqueous solution of hydrogen peroxide at a concentration of 2 to 30% at a temperature of 20 to 45 [deg.] C. for 20 minutes prior to their conditioning. up to 48 hr. A process for the preparation of heterogeneous ion-exchange membranes according to claim 1, characterized in that the membrane is immersed in a solution of hydrogen peroxide at a concentration of 5 to 30% for 24 to 48 ^hours at 25 ° C. A process for the preparation of heterogeneous ion-exchange membranes according to claim 1, characterized in that the membrane is immersed in a solution of 5 to 10% hydrogen peroxide for 0.5 to 1 hour at 45 ° C. Method for preparing heterogeneous ion-exchange membranes according to claims 1 and 3, characterized in that the temperature during the heat treatment of the mixture of inert binder and ion exchanger, during homogenization and subsequent forming, is higher than the melting point of the polymeric binder but lower than degradation of own ion exchange particles. A process for the preparation of heterogeneous ion-exchange membranes according to claim 4, characterized in that the melting point of the polypropylene or propylene copolymer used is less than 200 ° C when processing mixtures containing ion exchange particles with cationic groups. 6. A process for the preparation of heterogeneous ion exchange membranes according to claim 4, characterized in that the melting point of the polypropylene or propylene copolymer used is less than 150 [deg.] C. when the mixtures containing ion exchange particles with anionic groups are processed.