The Fluorescent Whitening of Textiles

Fluorescent whitening agents (FWAs) act as fluorescent dyes, though they are colorless. They absorb light in the near ultraviolet region of the spectrum, below about 400 nm, and re-emit the light, as fluorescence, in the violet-blue visible region. In addition, yellow discoloration on the fabric has an absorption tail extending into the near ultraviolet region of the spectrum, and this absorption will compete with the FWA for the irradiating energy. More important, however, unless the FWA is substantive to the fiber, little or no enhancement of the whiteness is achieved. This explains the multiplicity of FWAs for textile use. In the whitening of polyester, penetration of the FWA into the fiber is also important for complete development of whiteness. Residual yellowness causes quenching by providing molecules which absorb energy at the wavelength of the fluorescent emission. FWAs improve the brightness of pastel shades.