Moderate Chemical Aggregation of Phenolated Phosphaphenanthrene Groups via Methylene Bridge Toward Simultaneously Enhanced Flame Retardancy and Toughness of Epoxy Thermosets

ABSTRACT

To validate moderate chemical aggregation of flame‐retardant groups as an effective strategy for obtaining advanced flame‐retardant molecules, moderate‐aggregated linear oligomer B‐PDHQ (trimer‐dominant) and high‐aggregated crosslinked macromolecule C‐PDHQ were synthesized via the polymerization between the 10‐(2,5‐Dihydroxyphenyl)‐10H‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO‐HQ) monomer and formaldehyde. By propagating linear polymerization aggregation of phenolated phosphaphenanthrene (PDOPO) groups, the moderate‐aggregated B‐PDHQ exhibits outstanding superiority in combustibility suppression efficiency and toughness enhancement effectiveness of epoxy thermoset (EP), compared with the highly aggregated C‐PDHQ and non‐aggregated monomer DOPO‐HQ. Especially, 3%B‐PDHQ/EP passed UL94 V‐0 rating, 3%DOPO‐HQ/EP only passed UL94 V‐2 rating, while 3%C‐PDHQ/EP failed to pass any rating of UL94 vertical burning test. Furthermore, moderate‐aggregated B‐PDHQ also showed leading efficiency in the limited oxygen index, combustion heat inhibition, smoke emission reduction, and char formation enhancement of EP. The behavior of linear oligomer B‐PDHQ that triggers the flame‐retardant groups aggregation effect was revealed from the condensed‐phased char‐forming behavior, char layer morphology investigation, and the gas‐phased thermal decomposition volatile tracing. In addition, the moderate polymerization aggregation of PDOPO groups in linear oligomer B‐PDHQ still enable EP matrix with a higher glass transition temperature and impact toughness. The superiority of moderate‐aggregated B‐PDHQ provides a practical and efficient route for designing and manufacturing high‐performance reactive flame‐retardant molecules.