Fluorescent Whitening | Mechanism of Fluorescent Whitening Agent

Fluorescent Whitening Agent:
Fluorescent Whitening agents are also called Optical brightener. The Fluorescent Brightening agents operate by the phenomena of fluorescence. In order to understand the mechanism of FBA it is necessary to understand Fluorescence.
Energy Diagram of Optical Brighteners and Transitions
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation.

These additives are used in Fluorescent Whitening agents in order to :
  • Brighten colors
  • Mask natural yellowing of plastics
  • Improve initial color
  • Get brilliancy of colored or black pigmented articles
These Fluorescent whitening agents work via a fluorescent mechanism which absorbs light in the UV spectrum and emits light in the blue region of visible spectrum to yield a brighter and fresher appearance.

Suggested applications are :

  • Molded thermoplastic
  • Film and Sheets
  • Fibers
  • Adhesives
  • Synthetic Leather
Optical brighteners,optical brightening agents (OBAs), fluorescent brightening agents (FBAs) or fluorescent whitening agents (FWAs) are dyes that absorb light in the UV and violet region (usually 340‐370nm) of the EM spectrum and re‐emit light in the blue region (typically 420‐470nm). Fluorescent activity is a short term or rapid emission response, unlike phosphorescence, which is a delayed emission. These additives are often used to enhance the appearance of color of fabric and paper causing a "whitening" effect, making materials look less yellow by increasing the overall amount of blue light reflected.

Mechanism of Fluorescent Whitening Agent:
When a specimen transforms a part of the absorbed light into light of another wavelength instead of into heat (as is the case with normal dyed specimens), it is called fluorescent specimen. FBAs absorb ultraviolet light in 300-400 nm region from day light and emit it in the visible region (400-460 nm) at the blue-violet of the spectrum. The emitted blue light compensates for yellow tints of fibres and at the same time they also increase the luminosity of the goods. The emission spectrum is characteristics of a particular agent on a given substrate. Depending on the energy distribution of the spectrum, the fluorescent light emitted is blue-violet, blue, blue-green. According to Stoke's law the shape of the fluorescene band can be predicted from the shape and the position of the absorption band, and the color of the fluorescence can thus be determined. The relative distribution of the emitted light within the emission band is also important.

The mechanism of optical brightening is also explained on the basis of quantum theory of light and electronic structure of atoms and molecules. In the fluorescent substance molecules which have absorbed radiation (light) of short wavelength can pass into an excited state of higher energy. These excited molecules then return to the ground state of lower energy with re-emission of light quanta only slightly smaller than those absorbed i.e., of lower energy and longer wavelength (visible light). The average mean life of excited molecules is 10 -s to 10 -9 sec and in general the shorter this life the less difference will be there between the wavelength of fluorescence and emitted light and greater will be the fluorescence as there will be less time for dissipation of energy as heat.


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