Development of lychee-like core-shell chlorophyll/epoxy@SiO₂ microspheres via Pickering emulsion polymerization for enhanced photostability

Chlorophyll (Chl), a natural pigment with significant applications in functional coatings, food processing and biomedical fields, suffers from poor photostability under light and oxygen exposure, limiting its practical use. Herein, a novel chlorophyll/epoxy@SiO₂ (Chl/EP@SiO₂) microsphere with a lychee-like core-shell structure was synthesized via Pickering emulsion polymerization. The modification level and amount of SiO₂ were systematically optimized, yielding microspheres with an average particle size of ∼15 μm and a shell thickness ranging from 0.9 to 1 μm. Two distinct preparation strategies were investigated: (i) incorporating Chl during emulsification step by premixing with bisphenol A diglycidyl ether (DGEBA) to produce ChlD/EP@SiO₂ microspheres, and (ii) introducing Chl during curing phase by mixing with m-xylylenediamine (MXDA) to form ChlM/EP@SiO₂ microspheres. ChlD/EP@SiO₂ microspheres exhibited uniform particle size distribution, with Chl showing no interference with the diffusion of MXDA or the curing reaction. Conversely, the diffusion of Chl in the curing process disrupted the SiO₂ adsorption equilibrium, leading to particle rearrangement, surface roughness, and non-uniform particle size distribution in ChlM/EP@SiO₂. Reflectance spectra and colorimetric analysis during photostability tests demonstrated that ChlD/EP@SiO₂ microspheres, used as fillers in polyurethane coatings, enhanced Chl photostability by 11-fold compared to unencapsulated Chl. Furthermore, encapsulating Chl within an epoxy core and SiO₂ shell improved its dispersion in waterborne polyurethane coatings. This lychee-like core-shell composite microsphere provides a promising platform for stabilizing and dispersing organic pigments in coating applications.