Thiourea Dioxide

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Formamidine Sulphinic Acid


Formamidine Sulphinic Acid (FAS), also called Thiourea Dioxide, is a chlorine-free reducing bleach used in papermaking to enhance brightness, especially for mechanical (TMP, CTMP) and recycled pulp. Unlike oxidative agents (e.g., hydrogen peroxide), FAS works by reducing chromophores, gently removing color without damaging fibers. It operates effectively in neutral to slightly alkaline conditions (pH 6–9) and is often used in the final bleaching stage for deinked recycled pulp or high-yield mechanical pulps.