Flame-retardant Properties of 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide Derivatives in Poly(lactic acid)

2-(6-Oxido-6H-dibenz < c,e >< 1,2 > oxaphosphorin-6-yl)-methanol (ODOPM) was synthesized by reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) with methanal, and its structure was characterized by FTIR, H-1-NMR, P-31-NMR and MS. The flame-retardant performance of ODOPM in poly(lactic acid) (PLA) was explored and compared with those of 2-(6-oxido-6H-dibenz < c,e >< 1,2 > oxaphosphorin-6-yl)-1,4-benzenediol (DOPO-HQ). The flame-retardant properties of PLA-based ODOPM and DOPO-HQ were further investigated by determining the limiting oxygen index (LOI) as well as conducting vertical burning (UL-94) and cone calorimeter tests; their thermal-degradation behavior was studied by thermogravimetric analysis (TG) measurements. The results indicated that both flame retardants could decrease the peak heat release rate (PHRR), improve LOI, and promote the thermal stability and char formation. At flame-retardant content of 15 wt%, PHRR was decreased from 571 kW/m(2) of the neat PLA to similar to 430 kW/m(2) for both PLA/ODOPM and PLA/DOPO-HQ composites, while the former showed a high LOI of 34.4%, 72% increase compared with neat PLA (LOI of 20%). Moreover, V-0 rating in UL-94 test was achieved by PLA/ODOPM composite with filler concentration at 5 wt%, but PLA/DOPO-HQ required 15 wt% filler to reach the same rating with only 28.8% for LOI. In addition, all the flame retardant PLA systems decomposed earlier than neat PLA under N-2 condition, but the incorporation of ODOPM could increase the maximum degradation temperaeture (T-max). Especially, T-max of PLA containing 5 wt% ODOPM was 18 degrees C higher than that of neat PLA. However, PLA/ODOPM had generally lower T-max than neat PLA did unless the fill concentration reached 15 wt%. Characterization of char morphology by Raman spectroscopy and scanning electron microscopy (SEM) showed that incorporation of DOPO-HQ was conducive to the formation of a compact cross-linked char layer. TG-FTIR results suggested that both two flame retardants exerted a gas-phase-flame inhibition effect. Energy-dispersive spectroscopy (EDS) demonstrated an increased P content in the carbon layer along with the increasing flame-retardant content, possibly attributed to polyphosphate formation. Rheological analysis indicated that PLA/ODOPM possessed weaker viscoelasticity compared with PLA/DOPO-HQ. Finally, the mechanical measurements on neat PLA and PLA composites, including tensile strength, flexural strength, and notch impact strength, showed that PLA/DOPO-HQ possessed better mechanical properties than PLA/ODOPM did.