Published December 1979
This report is concerned with the manufacture of synthetic ion exchange resins, particularly those based on styrene-divinylbenzene (DVB) copolymers. Most modern ion exchange resins, or ion exchangers, consist of a synthetic polymer backbone or matrix to which is attached a functional group that gives each ion exchanger its specific proper- ties. Ion exchange resins are produced in various physical forms depending on the end use for the resin. Most commonly, they are used as small spherical beads or granules, but they can be made into membranes, fibers, tubes, cloth, or foams. By special manufacturing techniques, the polymers, especially in the bead form, may be made with porous structures instead of the conventional solid gel resin structure. Such resins are called macroporous or macroreticular resins.
The functional groups that are distributed throughout the resin structure contain fixed electric charges or ion-active groups, each of which is associated with a mobile counter ion of opposite charge. These mobile ions are capable of reacting with or exchanging with other ions of like sign when they are in contact with a solution containing such ions. It is important that ion exchange resins swell to a certain extent in aqueous solution so that the solution can diffuse into the resin and come into contact with the active sites.
When the fixed electrical charges within the resin matrix are negative (when the fixed functional group is a sulfonic group, for example), the mobile ions are cations and the resin is said to be a cation exchange resin. Conversely when the fixed groups are positively charged, the mobile ions are anions and the resin is an anion exchanger.
The polymer matrices are usually cross-linked to make them insoluble and to give them mechanical strength and stability. The extent of cross-linking must be controlled so as to give good mechanical properties to the resin while permitting enough water absorption and swelling to ensure good ion exchange activity. The most common resins are styrene-divinylbenzene (DVB) copolymers in which DVB is the cross-linking agent, typically present in 4-12X concentration. Other polymers include acrylates, methacrylates, phenol formaldehyde condensates, and epichlorohydrin amine condensates.
Ion exchange has been defined as the reversible interchange of ions between a solid and a liquid phase in which there is no permanent change in the structure of the solid. This means that ion exchangers are not consumed by ordinary usage, but when they are exhausted, they can be regenerated or reconverted to their original state and reused. Ion exchange is regarded as a unit process in chemical engineering and it has many applications. One of the best known and largest applications is water softening, in which calcium and magnesium ions, which cause water hardness, are removed from the water and exchanged for sodium ions from the resin. When the resin is exhausted, it is brought back to its original state by treatment with a sodium chloride solution. By a more complex process, water may be not only softened, but completely deionized. Ion exchange resins are widely used to treat boiler feed water, process water, and to perform a large number of separations and reactions in the manufacture of chemicals, foods, pharmaceuticals, electronic devices, and many other products.
Ion exchange is a widespread phenomenon in nature, occurring in living cells and in soils, for example. Ion exchange materials include silicates, phosphates, flourides, humus, wool, proteins, cellulose, alumina, glass, and many others. The first industrial ion exchangers were probably inorganic aluminum silicates, used for softening water and treating sugar solutions. Later on, it was discovered that sulfonated coal was a relatively effective ion exchange material, but such materials were fragile and were useful only under restricted operating conditions. In the United States nearly all ion exchange applications use synthetic polymer resins.
