Ion Exchange Resins

An ion exchange resin is a plastic, often a co-polymer of styrene and divinylbenzene.  In a water softener, the resin is formed into porous beads, of about 1 mm or less in diameter.

Resins can be produced to selectively exchange with either positive or negative ions by treatment of the plastic bead material.  In order to produce a resin that can remove water hardness (Mg²+ or Ca²⁺ ions), the resin must be treated to produce negative sites to which the positive ions can attach.  These sites are typically either carboxyl ( -COO^-) or sulfonate (SO₃H^-) groups which can be created by treatment of the resin with acetic or sulfuric acids.   The resins are then washed with sodium hyrdoxide, which neutralizes the acid, and produces the sodium salts of the treated resin molecules: R-COONa or R-SO₃HNa

Resin beads gradually break down.  This can be because of mechanical degradation, or because chlorine in the water will gradually oxidize the cross-linkages in the polymer.    Therefore the resin in a water softener must be periodically replaced (perhaps every 5 - 10 years in home use).