What are fluorescent whitening agents?

A fluorescent whitening agent is a nearly colourless fluorescent substance. It absorbs ultraviolet light at approximately 300–400 nm and emits violet or blue light at approximately 400–500 nm. This can compensate for the blue light absorbed by white or coloured plastic, making the material appear whiter and glossier.

An effective whitening agent should be easy to use, stable during processing and service, moderate in strength and hue, and resistant to bleeding and extraction. Only a small fraction of fluorescent organic compounds are suitable for colouring plastics.

The old-site article groups these agents into four families: thiophene bis-benzoxazole, 3-phenyl-7-triazinylamino coumarin, distyrylnaphthalene triazole, and benzotriazole phenyl coumarin types.

For white PVC, the cited dosage of the benzotriazole phenyl coumarin type is 0.01–0.05%. The article states that the other families may require roughly 1.5–3 times as much to achieve similar whiteness. Very small amounts, around 0.0025–0.005%, can mask a yellow tint in transparent PVC. The thiophene bis-benzoxazole and 3-phenyl-7-triazinylamino coumarin types have relatively good light resistance and can be used indoors for long periods, including in PVC flooring surfaces. Thiophene bis-benzoxazole types can also be used with polyolefins, polyethylene films and polypropylene.

The first three families listed above can be used in polystyrene and show good indoor stability. The article identifies the 3-phenyl-7-triazinylamino coumarin type as the most effective whitening agent for polystyrene among them.

UV absorbers and fluorescent whitening agents can absorb light in overlapping wavelength ranges. Combining them may weaken or eliminate the whitening effect. The old-site recommendation is therefore to use whitening agents only in formulations without UV absorbers.