The mixtures of salicylic acid, phenol, and formalin are refluxed for 1-4 hrs. in the presence of sulfuric acid or hydrochloric acid. The resinous products obtained are separated, washed with water, and further heated at 100℃. under atomospheric pressure (or 120- 130℃., 15-20 mm.) for 1-10 hrs. (hardening process). These resins, which are prepared on various conditions, are treated with 1N sodium hydroxide, and regenerated by 1N hydrochloric acid. These resins obtained are insoluble in mild alkali, and ion exchangeable.
Sulfuric acid as catalyser gives better results, and when the molratio of salicylic acid, phenol and formaldehyde is 1 : 1 : 3, ion exchangers of high total capacity (4.5-3.3 m. eq./g.) are obtained in good yield (48-88% ). The prolongation of duration of hardening process increases yield, and decreases exchange capacity and the degree of swelling.
The some series of cation-exchange resins are prepared by two-stage process in which sulfuric acid is used as catalyser at initial condensation and then alkali such as sodium hydroxide and ammonium hydroxide is added as catalyser for further condensation. Ion exchange resins in this method are lighter-colored and of high exchange capacity (3.-4.0 m.eq/g.).