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

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.