Scattering as part of the protection afforded by nanoparticulate filters - An example with tris-biphenyl triazine.

Objective: Tris-biphenyl triazine (TBPT) is an organic nanoparticulate UV filter for use in sunscreens. Thanks to its chemical structure and particulate properties, the effectiveness of TBPT is associated with both absorption and scattering processes. In our study, we evaluated how much of the UV protection of TBPT is linked to scattering, including forward and backward scattering.

Methods: We employed a two steps approach. At first, we reproduced an earlier experimental design based on transmittance measurements of an aqueous dispersion of TBPT in cuvettes using a UV/vis-spectrometer with and without integrating sphere. Since the dispersion of TBPT is intended to be used in a finished sunscreen product, in a second phase, the supplied TBPT dispersion was added to a formulation, which was applied on a substrate plate to mimic the real use scenario. In this experimental setup, the scattering contribution was evaluated using transmittance measurements with an integrating sphere, both with closed and with open backward window.

Results: The two approaches produced the same results for the experiments conducted in cuvettes and on plates. The percentage of scattering contribution is dependent on the wavelength, which is connected to the absorbance profile of TBPT. In the UVB range the efficacy of TBPT is primarily provided by absorption with little scattering. In the UVAI range (340-400 nm), where TBPT is not absorbing the protection is completely attributed to scattering. Also, the use concentration of TBPT was shown to impact the percentage of scattering. A decrease of the scattering effect is observed when the concentration of TBPT exceeds 3% (w). This can be attributed to the fact that in concentrated systems absorption becomes the main effect as the likelihood of scattered light hitting and being absorbed by another TBPT molecule increases due to the closer proximity of the molecules.

Conclusion: This method can be used in the future to assess the scattering contribution of other particulate organic or inorganic UV filters or cosmetics particulate ingredients added in cosmetic formulations.