<p>Char layer is an important factor directly affecting the combustion behavior of polymer materials. Furthermore, it plays a key role in the fire resistance mechanism of polymer composites by preventing heat and flammable gas transfer. Therefore, in this study, the char layers, combustion behavior, and thermal properties of boric acid (H₃BO₃) and alumina (Al₂O₃)-modified epoxy composites were investigated. The thermal decomposition behavior of the powders and composites was investigated using TGA/DTG analysis; the bonding and crystal structures pre- and post-combustion were investigated using FT-IR spectroscopy and X-ray diffraction. Al₂O₃ added compositions were the most prominent in terms of residual mass after thermal analysis. The combustion behavior was investigated using the UL-94 vertical combustion test. Post-combustion SEM images of the polymer matrix composites clearly showed that H₃BO₃ and Al₂O₃ formed distinct char layers. H₃BO₃, with a lower thermal decomposition temperature, formed a glassy and continuous char layer with the polymer matrix, and no dripping was observed during combustion. Al₂O₃, with a higher thermal decomposition temperature, did not melt during combustion, maintaining its integral structure and forming a discontinuous char layer. Dripping was observed during the combustion of Al₂O₃ added polymer composites. Physical combustion test results indicate that H₃BO₃ added compositions exhibit minimal mass loss.</p> Graphical abstract <p></p>