Comparative study of reactive flame retardants based on 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide

Two new 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-containing compounds, (6,6′-(1-methylethylidene)-bis(9,10-dihydro-9-oxa-10-phosphaphenantrene-10-oxide)) (bis-DOPO) and 2,4,6-tris-(9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide)-1,3-diacetoxybenzene (DOPO-RA), were synthesized and characterized by nuclear magnetic resonance and Fourier transform infrared spectroscopy. These new flame retardants were incorporated into epoxy formulations to produce a copper-clad laminate. When fully cured, the DOPO-groups of bis-DOPO become a part of the epoxy chains, whereas the DOPO-groups of DOPO-RA remain pendant to the epoxy polymer network. Despite their higher phosphorus content, bis-DOPO-based laminates showed a poorer flame-retardant performance compared to the DOPO-RA laminates. It is believed that the pendant position of the DOPO group with respect to the polymer chains is essential for a gas-phase flame-retardant action.