Highly Efficient NIR-Reflective CrAl₂O₄‑Based Polymer Microcapsules for Advanced Thermal Management Coatings

Near-infrared (NIR)-reflective materials are of interest for reducing solar heat gain in coated surfaces. In this study, CrAl₂O₄ particles were investigated as a novel NIR-reflective pigment. A spinel CrAl₂O₄ structure with an average particle size of 1 μm demonstrates high NIR reflectance. The CrAl₂O₄ particles were first modified with 3-methacryloxypropyl trimethoxysilane (MPS-CrAl₂O₄) to improve their compatibility and dispersion in the polymer matrix. The MPS-CrAl₂O₄ particles were encapsulated in polymer microcapsules using UV-initiated microsuspension polymerization. Microcapsules based on methyl methacrylate (MMA), butyl acrylate (BA), and ethylene glycol dimethacrylate (EGDMA) were prepared, and the effects of monomer composition and pigment loading on morphology, thermal behavior, NIR reflectance, and coating performance were examined. A monomer ratio of MMA: BA: EGDMA = 50:40:10 yielded spherical microcapsules with good colloidal stability. The NIR reflectance increased with increasing MPS-CrAl₂O₄ loading, reaching 87% at 40 wt %, which was comparable to that of the original CrAl₂O₄ particles. Increasing the BA content lowered the glass transition temperature of the microcapsules and promoted coating formation on the glass substrate without an additional binder. Under the specific test conditions used in this work, the microcapsule-coated glass showed a lower interior temperature, with a maximum reduction of about 16 °C, whereas bare glass showed a smaller temperature difference of about 2-3 °C. The results demonstrate that the developed CrAl₂O₄ particles exhibited effective NIR reflection and can be integrated into coating systems by polymer encapsulation while retaining their NIR-reflective behavior.