Synthesis and characterization of cyclic bisphenol A (phenylene phosphonate) oligomer and its flame retardancy application

A novel solid flame retardant cyclic bisphenol A (phenylene phosphonate) (CPBA) oligomer was synthesized in high yields by the reaction of phenylphosphonic dichloride with bisphenol A under pseudo-high-dilution conditions via polycondensation. Detailed structural characterizations of this oligomer were conducted using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, 1H NMR, 31P NMR and FTIR. Furthermore, the flame-retardant efficiency of this oligomer in polybutylene terephthalate (PBT) was investigated using limiting oxygen index (LOI) and UL-94 tests. After the addition of 25 wt.% CPBA, the LOI increased from 20.2% to 28.6% and the UL-94 V-0 rating was achieved. Thermogravimetric analysis, FTIR, and Pyrolysis/GC/MS were employed to elucidate the pyrolysis behavior of CPPA and PBT/CPBA. Results showed that the CPBA oligomer could change the degradation path of PBT and improved the char formation of the PBT/CPBA systems. The residual morphologies of residues of the PBT/CPBA25% system were investigated by scanning electron microscopy (SEM). SEM investigations revealed that the residual chars containing polyphosphoric or phosphoric acid, which plays an important role in the process of carbonization. The influence of CPBA content on the mechanical properties of PBT/CPBA blends was also discussed.