Synthesis of novel phosphorus-based flame retardants containing 9,9-bis (4-hydroxyphenyl) fluorine and their reinforcements on the fire safety of polypropylene

Fabricating an effective synergism to improve the flame-retardant (FR) efficiency is recently considered to be a very promising way to prepare high-efficient FRs system. A novel FRs containing phosphorus and (4-hydroxyphenyl) fluorene have been synthesized and characterized by FTIR, ¹H NMR and MS. The FRs were then incorporated into polypropylene (PP) in different ratios, and then, ammonium polyphosphate (APP) was added to the FRs/PP system, with subsequent investigation into the synergistic effects between FRs and APP. Limited oxygen index (LOI), UL-94 test and cone calorimeter test were adopted to investigate the FR properties of the flame-retardant PP composites. The LOI value is as high as 31.0%, when FRs/APP is 2/3. Moreover, due to the synergistic FR effects between FRs and APP, the pHRR of PP7 decreases from 783 kW m⁻² (PP0) to 110 kW m⁻² (PP7), which is 85% lower than those of PP. Meanwhile, thermogravimetric analysis (TG) was used to study the thermal degradation characteristics of the PP composites. The char residues increased from 0.02 mass% to 15.85 mass% at 650 °C when FRs/APP is 3:2, indicating that there are synergistic flame-retarding effects between FRs and APP. Generally, the stable char layers of FR/APP/PP composites are responsible for the improved FRs properties, and the char layers could not only effectively prevent the release of combustion gases from releasing but also hinder the propagation of oxygen and heat into the interior substrate.