Ion-exchange membranes are ion-exchange resins in sheet form. There are, however, significant differences between ion-exchange resins and membranes as far as the mechanical properties and especially the swelling behavior are concerned. Ion-exchange resins are mechanically weak or tend to swell drastically in diluate electrolyte solutions . The most common solution to this problem is the preparation of a membrane with a backing of a stable reinforcing material, which gives the necessary strength and dimensional stability. Two techniques are used today for the preparation of ion-exchange membranes. One leads to a more heterogeneous and the other to a more homogeneous structure. Both preparation procedures are described in great detail in the patent literature [15-17].
3.1.1 Preparation of heterogeneous ion-exchange membranes
Ion-exchange membranes with a heterogeneous structure consist of fine ion-exchange particles embedded in an inert binder polymer such as polyethylene, phenol resins, or polyvinylchloride. Heterogeneous ion-exchange membranes are characterized by the discontinuous phase of the ionexchange material. The efficient transport of ions through a heterogeneous membrane requires either a contact between the ion-exchange particles or an ion-conducting solution between the particles. Heterogeneous ionexchange membranes can easily be prepared by mixing an ion-exchange powder with a dry binder polymer and extrusion of sheets under the appropriate conditions of pressure and temperature or by dispersion ofion-exchange particles in a solution containing a dissolved film-forming binder polymer, casting the mixture into a film and then evaporating the solvent.
Heterogeneous ion-exchange membranes with useful low electrical resistances contain more than 65% by weight of the ion-exchange particles. Membranes that contain significantly less than 65wt% ion-exchange particles have high electric resistance and membranes with significantly more resin particles have poor mechanical strength. Furthermore, heterogeneous membranes develop water filled interstices in the polymer matrix during the swelling process, which affects both the mechanical properties as well as the permselectivity.
The ion-exchange capacity of heterogeneous membranes is in the range of 1—2eqkg_1 dry membrane and thus significantly lower than that of homogeneous membranes, which is between 2 and 3eqkg_1 dry resin. In general, heterogeneous ion-exchange membranes have higher electrical resistances and lower permselectivity than homogeneous membranes.
3.1.2 Preparation of homogeneous ion-exchange membranes
Homogeneous ion-exchange membranes can be prepared by polymerization of monomers that contain a moiety that either is or can be made anionic or cationic, or by polymerization of a monomer that contains an anionic or a cationic moiety, or by introduction of anionic or cationic moieties into a polymer dissolved in a solvent by a chemical reaction, or grafting functional groups into a preformed polymer film.
A method of preparing both cation- and anion-exchange membranes, which is used for the preparation of commercial cation-exchange membranes, is the polymerization of styrene and divinylbenzene and its sulfonation according to the following reaction scheme :
h2c=ch polymerization sulfonation
In a first step, styrene is partially polymerized and cross-linked with divinylbenzene and then in a second step sulfonated with concentrated sulfuric acid. The obtained membranes show high ion-exchange capacity and low electrical resistance. To increase the mechanical strength, the membrane is cast on a support screen.
A homogeneous anion-exchange membrane can be obtained by introducing a quaternary amine group into polystyrene by a chloromethylation procedure followed by an amination with a tertiary amine according to the following reaction scheme:
The membrane structures and their preparation described above are just two examples. There are many variations of the basic preparation procedure, resulting in slightly different products. Instead of styrene, often substituted styrene such as methylstyrene or phenylacetate is used and instead of divinylbenzene monomers such as divinylacetylene or butadiene are used .
More recently cation-exchange membranes with good mechanical and chemical stability and well controlled ion-exchange capacity are prepared by sulfonation of dissolved polysulfone . The sulfonation is carried out with chlorosulfonic acid according to the following scheme:
To obtain membranes with different ion-exchange capacity the sulfonated polysulfone can be mixed with unsulfonated polymer in a solvent such as N-methylpyrrolidone. By changing the ratio of the sulfonated to unsulfonated polymer, the fixed charge density can easily be adjusted to a desired value. The sulfonated polysulfone can be cast as a film on a screen. After the evaporation of the solvent, a reinforced membrane with excellent chemical and mechanical stabilities and good electrochemical properties is obtained.
Anion-exchange membranes based on polysulfone can be prepared by halomethylation of the backbone polymer and subsequent reaction with a tertiary amine.
For the preparation of cation-exchange membranes also, polyether-etherketone is used as the basic polymer. It can very easily be sulfonated with concentrated sulfuric acid according to the following scheme:
Sulfonation occurs on one polyetheretherketone block only and is thus very easy to control. To obtain membranes with different ion-exchange capacities, the sulfonated polyetheretherketone can be mixed with poly-ethersulfone in a solvent such as N-methylpyrrolidone. By changing the ratio of the sulfonated polyetheretherketone to polyethersulfone, the fixed charge density can easily be adjusted to a desired value.
In addition to the monopolar membrane described above, a large number of special property membranes are used in various applications such as low-fouling anion-exchange membranes used in certain wastewater treatment applications or composite membranes with a thin layer of weakly dissociated carboxylic acid groups on the surface used in the chlorine/ alkaline production, and bipolar membranes composed of laminate of an anion- and a cation-exchange layer used in the production of protons and hydroxide ions to convert a salt in the corresponding acids and bases. The preparation techniques are described in detail in the literature [2,19,20].
One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stabilities and they are the key component in the chlorine/alkaline electrolysis as well as in most of today's fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today . The various preparation techniques are described in detail in the patent literature.
Today's commercially available perfluorocarbon membranes have the following basic structure:
The synthesis of the perfluorocarbon membranes is rather complex and requires a multistep process. In addition to the various perfluorinated cation-exchange membranes also, perfluorinated anion-exchange membranes have been developed. The anion-exchange membranes have similar chemical and thermal properties as the cation-exchange membranes.
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