Effects of bridged DOPO derivatives on the thermal stability and flame retardant properties of poly(lactic acid)

Three bis P-C bridged 9,10-dihydro-9-oxa-10-phosphaphe nanthrene-10-oxide (DOPO) derivatives including ethyl-(DiDOPO1), phenethyl-(DiDOPO2) and naphthalene-(DiDOPO3), were synthesized and incorporated in poly(lactic acid) (PLA). Then, the flame retardant and thermal properties of the obtained PLA composites were investigated in detail. At 10 wt% addition of the derivatives, all PLA composites achieved a V-0 rating (UL-94, 3.2 mm), and the LOI values of PLA/DiDOPO1, PLA/DiDOPO2 and PLA/DiDOPO3 increased from 20.0% (pure PLA) to 36.4%, 27.1% and 24.3%, respectively. The introduction of DiDOPO1, DiDOPO2, and DiDOPO3 decreased the peak heat release rate value of PLA composites from 561 kW/m2 (pure PLA) to 490, 488 and 434 kW/m2, respectively. In addition, the derivatives significantly enhanced the thermal stability of PLA especially under air condition. It was found that the bridged DOPO derivatives shows a dominant flame inhibition role in PLA. Furthermore, the investigation of the residue micromorphology of PLA composites revealed that a cross-linked structure likely formed with the increased of aromatic groups in the molecule.