Fabrication of a cinnamaldehyde-based bi-DOPO flame retardant with excellent glass transition temperature, fire safety and mechanical properties for epoxy resins

Manufacturing fire-resistant, tough, and durable epoxy resin (EP) is a challenging task. Traditional flame retardants derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) tend to aggregate and plasticize, causing a decline in glass transition temperature (T_g) and mechanical properties of EP. To overcome this issue, a bulky aromatic DOPO-based derivative called PPCANT was synthesized successfully using biomass-derived cinnamaldehyde (CA). The steric hindrance and intermolecular interactions (π-π stacking) of PPCANT's mitigated T_g deterioration and significantly improved mechanical properties. Incorporating 5 wt% PPCANT into the EP (EP/PPCANT-5) achieved a limiting oxygen index (LOI) of 32.6% and UL-94 V-0 rating, reducing both total heat release and smoke production. Moreover, PPCANT endowed the EP with exceptional char-forming ability. Analysis of the flame retardant mechanism indicated that the PPCANT structure imparted an ideal flame quenching mechanism, characterized by the instantaneous extinction of flames caused by the blowout of PO·/PO₂ radicals, and a dilution effect of free radicals from non-flammable gases. In addition, the incorporation of nitro and amino groups in PPCANT facilitates the formation of hydrogen bonds with the hydroxyl groups generated in the ring-opened EP, leading to a significant decrease in the hydrophilicity of EP. These findings highlight PPCANT as a promising flame retardant that simultaneously enhances flame retardancy, mechanical strength, and hydrophobicity in EP materials.