Optical Brightener - Properties

Properties

The most common classes of chemicals with this property are the stilbenes and older, non-commercial fluorescent chemical such as umbelliferone, which absorb energy in the UV portion of the spectrum and re-emit it in the blue portion of the visible spectrum. A white surface treated with an optical brightener can emit more visible light than that which shines on it, making it appear brighter. The blue light emitted by the brightener compensates for the diminishing blue of the treated material and changes the hue away from yellow or brown and toward white.

There are approximately 400 brightener types listed in the Colour Index, but less than 90 are actually produced commercially and only a handful are commercially important. Generically, the C.I. FBA number can be assigned to a specific molecule, however, some are duplicated, since manufacturers apply for the index number when they produce it. The global OBA production for paper, textiles and detergents is dominated by just a few di- and tetra-sulfonated triazole-stilbenes and a di-sulfonated stilbene-biphenyl. These are subject to fading when exposed long term to UV, due to the formation of optically inactive stilbene cis-isomers found at the center of the molecule. Exposed to gases and especially oxygen they will fade too, like most dye colorants. All brighteners have extended conjungation and/or aromaticity, allowing for electron movement. Some non-stilbene brighteners are used in more permanent applications such as whitening synthetic fiber.

Basic class types of brighteners include:

1. Triazine-stilbenes (di-, tetra- or hexa-sulfonated)
2. Coumarins
3. Imidazolines
4. Diazoles
5. Triazoles
6. Benzoxazolines
7. Biphenyl-stilbenes

Brighteners can be "boosted" by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.