A novel charring agent containing caged bicyclic phosphate and its application in intumescent flame retardant polypropylene systems

A novel charring agent, bis(2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-1-oxo-4-hydroxymethyl) phenylphosphonate (BCPPO), was synthesized and characterized by FT-IR, 1H NMR and 31P NMR spectroscopies. Moreover, the novel charring agent combining with ammonium polyphosphate (APP) as well as melamine (MA) was adopted as intumescent flame retardants (IFR) to impart flame retardance and dripping resistance to polypropylene (PP). Flammability and thermal behaviors of the treated PP were investigated by Limiting Oxygen Index (LOI), Vertical Burning Test (UL-94), Thermogravimetric Analysis (TGA) and Cone Calorimeter test. Results indicate that the intumescent flame retardant with the novel charring agent shows both excellent flame retardance and anti-dripping abilities for PP when the three main components of IFR coexist at appropriate proportions, and the optimum flame retardant formulation is APP:MA:BCPPO = 3:1:1 that gives an LOI of 30.3 and UL-94 V-0 rating, moreover, both the heat release rate (HRR) and the total heat release (THR) of IFR-PP with the optimum formulation decrease significantly relative to PP from cone calorimeter analysis. The residues from decomposition of the IFR-treated PP was characterized by FT-IR spectroscopy and SEM micrograph, and it was found that the char yield as well as char properties have direct effects on the flame retardance and anti-dripping behaviors of the treated PP.