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. 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.