Thermal and optical characterization, as well as photostability assessment of sunscreens by using photoacoustic and photopyroelectric techniques

Sunscreens are fundamental public health tools for mitigating the harmful effects of ultraviolet (UV) radiation on the skin. UVA radiation reaches the Earth's surface in greater proportion, contributing to photoaging and skin carcinogenesis. In this context, the efficacy of sunscreens depends not only on their spectral absorption capacity but also on their thermal behavior and photochemical stability under irradiation. In this work, six commercial cosmetic formulations, including sunscreens and makeup products with and without sun protection factor, were characterized using photoacoustic spectroscopy and front configuration of photopyroelectric techniques. Absorption spectra were obtained to evaluate protection in the UVB, UVA, and visible regions, and thermal transport properties, such as thermal diffusivity and conductivity, were determined. The results show that the analyzed formulations exhibit significant absorption in the UVB and UVA regions. Formulations with pigments extend this absorption into the visible region, providing additional protection against high-energy visible light. Thermal analysis revealed higher diffusivity and thermal conductivity values in sunscreens compared to makeup bases, suggesting greater heat dissipation efficiency. Furthermore, photostability tests revealed significant differences between formulations, with some products showing a decrease in absorption consistent with significant photodegradation processes after irradiation. These results provide a basis for designing and evaluating photoprotective formulations with improved performance under real-world exposure conditions. These results demonstrate that combining optical and photothermal techniques allows for a comprehensive performance evaluation of photoprotective formulations, highlighting the relevance of thermal properties, photochemical stability, and spectral coverage to the actual efficacy of these products.