Encapsulated inorganic pigments in epoxy composite microspheres using emulsion synthesis

Inorganic pigments are a commonly used colorant with various applications and promising prospects. However, the implement of organic pigments is hindered by poor dispersion stability in coatings due to non-uniform morphology and size. Herein, encapsulated pigments of Cobalt Chrome Green CoCr₂O₄ (PG 26), Cobalt Blue CoAl₂O₄ (PB 28), Titanium Chrome Yellow TiCrSbO₆ (PB 24) in epoxy microspheres are obtained via an oil-in-water emulsion polymerization. The morphology, particle size, thermal properties, glass transition temperature (T_g) and color performance of the composite microspheres were characterized by different analytical techniques. The average particle size varied with pigments types and loadings. When the content of CoCr₂O₄ and CoAl₂O₄ is in the range of 15 wt%-30 wt%, and TiCrSbO₆ content is between 15 wt%-25 wt%, the composite microspheres exhibit a narrow particle size distribution, regular morphology, and smooth surface. The size followed the rule of CoAl₂O₄/epoxy (20–26 μm) > CoCr₂O₄/epoxy (1₄-16μm) > TiCrSbO₆/epoxy (6–9 μm). Besides, according to the aggregation-induced emission (AIE) principle, the relationship between fluorescence intensity and reaction time was investigated, thus deducing the curing process of the epoxy groups with m-xylenol dimethylamine (MXDA). The kinds of three inorganic fillers exhibit varying effects on the diffusion process of MXDA. TiCrSbO₆ promotes the progress of the epoxy-amine curing reaction. Notably, CoCr₂O₄ microspheres exhibited stable green value (a*) and color saturation (C*), TiCrSbO₆ microspheres showed increased yellow value (b*) and saturation (C*), and CoAl₂O₄ microspheres enhanced blue value (b*) with minimal change in lightness (L*). Overall, this study provides a comprehensive understanding of the properties of inorganic pigment/epoxy composite microspheres and highlights their potential for improved dispersion stability in coating applications.