Fluorescent Whitening Agent KSN (also known as optical brightener KSN) is a non-ionic benzoxazole derivative used to enhance the whiteness and brightness of plastics and polyester fibers. It works by absorbing UV light and re-emitting it as visible blue-violet light, making the treated materials appear whiter and brighter. It is stable at high processing temperatures and is suitable for applications like film, injection molding, and chemical fiber dyeing.