DOPO‐Intercalated Layered Double Hydroxide: A Synergistic Flame‐Retardant Nanohybrid for High Performance Epoxy Composite

ABSTRACT Nanohybrids between Mg/Al‐layered double hydroxides and 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO‐LDHs) were prepared at various mass ratios. A synergistic effect between the nanohybrids and polyethyleneimine‐modified ammonium polyphosphate (APP@PEI) on the flame retardancy and thermal stability of epoxy resin was observed. Among formulations, the nanocomposite loading 3 wt% DOPO‐LDH3 and 7 wt% APP@PEI (EP5) demonstrated superior flame retardancy and high thermal stability, achieving an LOI value of 32%, a UL‐94 V‐0 rating, and a high residual char content of 18.9% at 900°C. The morphology and chemical compositions of char residues of the epoxy composite were investigated. The results revealed that DOPO‐LDH nanohybrids and APP@PEI exhibited an effectively synergistic flame‐retardant effect in both condensed and gaseous phases. Notably, the formation of thermally stable spherical particles—comprising metal phosphates and polyphosphates—played a critical role in the improvement of the condensed‐phase flame‐retardant efficiency. Additionally, the EP5 nanocomposite exhibited a significant enhancement in the mechanical performance compared with the composite containing only 10 wt% APP@PEI, which is attributed to the good exfoliation and intercalation of the nanohybrid within the polymer matrix. This work highlights the potential of DOPO‐LDH nanohybrids as an effective synergistic flame‐retardant for high performance epoxy composite.