Ultraviolet (UV) filters, as the heart of sunscreen technology, are the key ingredients in sunscreen formulations providing protection to skin. There are two types of UV filters available on the market, organic UV absorbers and particulate UV filters. Organic UV absorbers enjoy a long history of use and have higher efficiency with recent new development such as Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine (BEMT), Diethylamino Hydroxybenzoyl Hexyl Benzoate (DHHB), and Ethylhexyl Triazone (EHT) which are high molecular weight hydrophobic compounds with improved photostability and safety profiles, as well as reduced environmental impact. However, such high molecular weight organic UV absorbers, usually crystalline solids at ambient temperature, require proper solvents to facilitate the formulation process. In addition, crystalline substances tend to recrystallize if not properly solubilized and stabilized in the formulation, giving rise to a negative effect on the effectiveness of sunscreen products. Although traditional liquid state organic UV absorbers namely Ethylhexyl Methoxycinnamate (EHMC), Octocrylene (OCR), and Homosalate (HMS) can work as solvents for crystalline organic UV absorbers, studies have revealed their endocrine disrupting properties as well as potential adverse impacts on ecosystems. Therefore, it’s crucial to find emollients that exhibit excellent solubilization for crystalline organic UV absorbers while preventing them from recrystallizing in the formulation to maximize the performance of organic UV absorbers in commercial products.
On the other hand, there is a major boom in the use of inorganic particulate UV filters such as Titanium Dioxide (TiO2) and Zinc Oxide (ZnO) in sunscreen products because of their relatively low impact on environment and skin sensitivity. Nevertheless, these insoluble particulates are difficult to handle and tend to agglomerate in formulations, posing a major formulation challenge for effective sunscreen development. Finding emollients to efficiently disperse the UV filter particles and ensure the incorporation of adequate inorganic particulates into the formulation has always been a challenge.
In this study, a variety of emollient esters were selected to test the solubility for BEMT, DHHB, EHT, and Butyl Methoxydibenzoylmethane (BMDBM) respectively. Polarized microscope analysis was chosen to determine the maximum solubility and monitor the recrystallization over time. Dispersion tests of inorganic particulate UV filters (hydrophobically coated TiO2, coated and uncoated ZnO) in emollient esters were conducted where viscosity and particle size of the dispersions were measured to determine the dispersion properties of the emollients. Microstructure of the dispersions was determined by microscope analysis.
Furthermore, simple formulations were made to test the stability and performance of individual organic UV absorber by in vitro SPF/UVAPF analysis. Since UV protection of inorganic UV particulate-based sunscreen formulation normally depends on the quality of the dispersion, the spreadability and consistence of inorganic UV particulate in simple formulations was evaluated by both in vitro SPF/UVAPF analysis and rheometer. The correlations between dispersion-ability and solubilization of emollient esters versus their physicochemical properties were investigated.
Results show that in vitro SPF/UVAPF performance of UV filters were closely related to the solubilization and dispersion-ability of emollient esters. The polarity of emollients plays an important role in solubilizing organic UV absorbers. Recrystallization may happen in well-solubilized samples upon storage. Organic UV absorbers responds differently to emollients with relatively high polarity. The dispersion properties of particulate UV filters display a correlation between the coating material and emollient ester’s structure and consistency.
Effective UV filter solubilizers and dispersants are proposed respectively, and sample sunscreen formulations are given as references with in vitro SPF/UVAPF indications. Proposed tools to study solubility, recrystallization, dispersion, and in vitro SPF/UVAPF are provided to formulators who want to assess the performance of emollient esters when developing effective and stable products for sun protection.