Textile Knowledge: Use of Whitening agents in spinning of Man-made Fibre

Fluorescent Whitening Agents are also called Optical brightener.
These additives are used 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

This additive center is designed to help you learn more about Fluorescent whitening agents, the mode of action and example of applications.

Application:

FWAs are effective in a variety of polymer substrates such as engineering plastics (e.g. polyesters, polycarbonate, polyamides and acrylics) thermoplastic polyurethane, polyvinylchloride, styrene homo- and copolymers, polyolefins, adhesives, and other organic substrates. Main applications include fibers, molded articles, films and sheets.

The effectiveness of a fluorescent whitener is dependent upon the type of substrate, processing conditions and possible interactions with other components in the formulation such as white pigments or UV absorbers. In general, fluorescent whiteners are effective at very low concentrations.

Titanium dioxide pigments (TiO₂) absorb light in the same UV wavelength range as fluorescent whiteners and thus generate lower whiteness degrees.

Mode of Action:

Optical brighteners or fluorescent whitening agents (FWA) are colorless to weakly colored organic compounds that in solution or applied to a substrate absorb ultraviolet light and re-emit most of the absorbed energy as blue fluorescent light between 400-500 nm.

Material that evenly reflect most of the light at all wavelengths strinking their surface appear white to the human eye. Natural fibers, for example, generally absorb more light in the blue region of the visible spectrum ('blue defect') than in others because od impurities (natural pigments) they contain. As a result, natural fibers also have this yellowish cast. Synthetic fibers also have this yellowish cast although not as pronounced. Whiteness in substrate can be improved by :

1) Increasing reflection
2) Compensating the blue defect.

Before the advent of FWA comon practice was to apply small amounts of blue or violet dyes to boost the visual inspression of whiteness. These dyes absorb light in the green-yellow region of the spectrum, thereby reducing lightness.

But, since at the same time they shift the shade of the yellowish material towards blue, the human eye perceives an increase of whiteness.Unlike dyes, FWAs offset the yellowish cast and at the same time improve lightness because their bluing effect is not based on subtracting yellow-green light, but rather on adding blue light.

FWAs are virtually colorless compounds which, when present on a substrate, absorb primarily invisible ultraviolet light in the 300-400 nanometer (nm) range and re-emit in the visible violet-to-blue fluorescent light.
This ability of FWAs to absorb invisible short wavelength radiation and re-emit in the visible blue light which imparts a brilliant whiteness to the light reflected by a substrate, is the key to FWAs effectiveness.