Novel bis-catechol-tri-DOPO flame retardant for enhancing flame retardancy and mechanical properties of epoxy resins

This study reports the synthesis of a novel flame retardant (TDOPO) via a one-pot nucleophilic addition reaction, in which 3,4-Dihydroxybenzaldehyde (PH) was grafted onto 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) at different oxidation states. The chemical structure of TDOPO was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR).The effects of TDOPO dosage on the flame retardancy, thermal properties, and mechanical properties of epoxy resin (EP) composites were systematically investigated and compared with those of pure epoxy resin. At a TDOPO content of 10 wt% (EP/TDOPO10), the epoxy resin composite achieved a Limiting Oxygen Index (LOI) of 33.6% and a UL-94V-0 rating, demonstrating outstanding flame retardancy. Cone calorimeter tests revealed that, compared to pure epoxy resin, the EP/TDOPO10 composite exhibited reductions of 41.0%, 18.6%, and 20.4% in peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP), respectively. These improvements are attributed to the abundant hydroxyl groups in the resorcinol structure, which promote the formation of a denser and more continuous char layer. Additionally, the phosphorus element in TDOPO catalyzes char formation, leading to a phosphorus-rich three-dimensional network char layer in the condensed phase during combustion. The substantial char residue effectively impedes heat and oxygen transfer to the underlying polymer matrix. Thus, this flame retardant imparts superior flame retardancy and mechanical properties to epoxy resins, positioning it as a promising candidate for developing functional thermoset flame retardants.