Optical Brighteners

are also called white shaders which are used to prevent yellowing and greying of plastics. These substances absorb the invisible ultra-violet radiation of about 300 – 400 nm and irridiate it as visible long wave radiation of 400 – 500 nm. Thus unprepossessing yellow and grey shades become sparkling white. This phenomenon is caused by p-electron-systems which enable a p-p transition which radiates slightly blue fluorescence, that is the complementary colour of the yellowness. As optical brighteners are used organic fluorescent dyes such as coumarone, oxadiazole and derivates of 4,4‘-Diamino-2,2‘-stilbendisulphonic acid, 4,4‘-distyrylbiphenylene, methylumbelliferon, coumarone, dihydroquinoline, 1,3-diarylparyzoline, naphthal acid imide, by CH=CH-bonds linked benzoxazol-, benzisoxazol- and benzimidazol-systems, pyrene derivates substituted by heterocycles.