Insights into the stabilization of photolabile UV-absorbers in sunscreens

Sunscreens are used to protect human skin against harmful UV radiation. Today there is a trend towards high sun protection factors (SPF) and good UVA protection. Methods for the assessment of SPF and UVA protection involve irradiation of the product, and the photostability properties of the sunscreen have an influence on its performance. Sunscreens often contain more than one UV filter. Some photolabile UV absorbers may be stabilized by the presence of other photostable UV-absorbers. Stabilization can be achieved just by a certain optical density due to the presence of such UV-filter substances. However, photostabilization may also be caused by quenching mechanisms, such as singlet-singlet or triplet-triplet energy transfer. Investigation of butyl methoxy dibenzoylmethane and ethylhexyl methoxycinnamate as photolabile sunscreens in the presence of either octocrylene or bis ethylhexyloxyphenol methoxyphenyl triazine showed that both mechanisms may apply. With the systems butyl methoxy dibenzoylmethane plus octocrylene and ethylhexyl methoxycinnamate plus bis ethylhexyloxyphenol methoxyphenyl triazine the quenching mechanism appears to be predominant.