Encapsulation of an ultraviolet filter into cationic polystyrene particles via emulsion polymerization with poly(vinyl alcohol)

Highly effective organic ultraviolet (UV) filters, such as diethylamino hydroxybenzoyl hexyl benzoate (DHHB), are widely used in sunscreens; however, they have remarkably low solubility in water, which limits their use in water-based cosmetics. In this study, DHHB-encapsulated cationic polystyrene particles (PS-DHHB) dispersed in water were prepared via emulsion polymerization. Before polymerization, an emulsifying styrene monomer phase containing dissolved DHHB and a water phase containing low-molecular-weight poly(vinyl alcohol) were prepared, and subsequent polymerization yielded PS-DHHB particles. Crucially, the addition of polystyrene (PS) to the monomer phase contributed to a high solid content, and PS served as a stabilizer, enhancing the encapsulation efficiency of DHHB. Notably, the synthesized PS-DHHB exhibited UV absorption in a range similar to that of DHHB. Furthermore, even when aqueous alcohol (used as an antibacterial agent) was added, no DHHB was leached from the PS-DHHB particles. Finally, we found that the cationic PS-DHHB particles suppressed hair protein loss induced by UV light (320–400 nm) more effectively than cationic PS particles without DHHB. In summary, water-dispersed PS-DHHB particles exhibiting effective UV absorption and adsorption onto hair were developed, and these particles have applications in polymer science and cosmetics.