A resin is an insoluble matrix normally in the form of small (0.5-1 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous, providing a high surface area. The trapping of ions occurs with the accompanying releasing of other ions; thus the process is called ion-exchange. There are multiple types of ion-exchange resin. Most commercial resins are made of polystyrene sulfonate.
Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification. In many cases ion-exchange resins were introduced in such processes as a more flexible alternative to the use of natural or artificial zeolites. Also, ion exchange resins are highly effective in the biodiesel filtration process.