Optical Brighteners

(also optical bleaches), colorless or slightly colored organic compounds that absorb ultraviolet rays of wavelengths between 300 and 400 millimicrons (mμ). They subsequently convert the ultraviolet rays into blue or violet light in the 400–500 mμ range, which compensates for an insufficiency of blue rays in the light that is reflected from the brightened material. Optical brighteners are used with many materials, including cotton, synthetic fibers, paper, wool, raw silk, leather, fur, and plastics. This process imparts a high degree of brightness to colorless materials and brightness and contrast to colored articles.

Most of the optical brighteners that are used with cellulose materials are stilbene-triazine derivatives of the general structural formula

where Rʹ and Rʺ are such groupings as NHR, NR₂, NHC₂H₄-OH, N(C₂H₄OH)₂, NHAr, OCH₃, NHC₆H₄SO₃Na, NH₂, and OH (R being any alkyl radical and Ar being any aryl radical).

Optical brighteners are used during various stages in the preparation and processing of natural and synthetic materials. Optical brightening and dyeing are similar processes, especially the brightening and direct dyeing of cellulose materials. However, considerably less optical brightener than dye is needed to produce the desired effect. Optical brighteners that are insoluble in water may be used either in highly dispersed form or in solution with organic solvents.