Synthesis of a novel DOPO-substituted charring agent containing triazine for reducing the fire hazard of polypropylene

A novel DOPO-substituted and triazine-based charring agent (ST-DOPO) was synthesized from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), 4,4’-diphenylmethane diisocyanate (MDI), and melamine (MEL), which is used as a synergistic flame retardant of ammonium polyphosphate (APP) for polypropylene (PP). ST-DOPO is a good char forming agent, with a residual weight up to 36.59 wt.% at 800 °C. With 30 wt.% of ST-DOPO and APP (2∶1), the PP blend achieved a V-0 rating at a vertical UL-94 test, with a limiting oxygen index (LOI) value of 35.0%. Compared to pure PP, its peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSR) were reduced by 81.2%, 34.1%, and 70.4%, respectively. Interestingly, the blend had only a 6.2% decrease in the tensile strength but a 15.1% increase in the impact strength, suggesting good compatibility of additives with the PP matrix. Moreover, the well-expanded continuous and compact char layer contributed to the improvement of the flame retardant property.