Flame-retardant microcapsules were prepared using a urea–melamine–formaldehyde (UMF) shell and boric acid-crosslinked ammonium polyphosphate (APP) as the core to improve the dispersion stability and processing compatibility of phosphorus-based flame retardants. Thermal analysis showed that the microcapsules exhibited initial mass loss near 80 °C due to moisture evaporation and shell relaxation, while APP-related degradation occurred at higher temperatures, indicating delayed release of the core and enhanced thermal resistance through encapsulation. Scanning electron microscopy confirmed the formation of microcapsules, and morphological changes before and after combustion suggested the development of protective char layers. Boron-containing residues are expected to contribute to char stabilization through the formation of B–O–P structures during heating. The flame-retardant properties were evaluated using limiting oxygen index, smoke density, and vertical burning tests. Although the limiting oxygen index slightly decreased due to reduced accessible APP content, stable burning behavior was maintained, and characteristic char formation was observed after combustion. These results indicate that the UMF/APP microcapsules can improve thermal stability and handling of phosphorus-based flame retardants. The microencapsulation approach presented here may provide practical advantages for polymer processing and surface-coating applications.
Lee et al. (Fri,) studied this question.