Preparation and characterization of a novel oligomeric charring agent and its application in halogen-free flame retardant polypropylene

A novel charring agent (CNCO-HA) containing triazine rings was synthesized by a series of polycondensation reactions, and characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance spectroscopy (13C solid-state NMR), and thermal gravimetric analysis (TGA). The synergistic effects between APP and CNCO-HA on flame retardancy, thermal degradation, flammability properties and mechanism of polypropylene (PP) were investigated by the limited oxygen index (LOI), vertical burning test (UL-94), thermogravimetric analysis (TGA), cone calorimeter teat test (CCT), Laser Raman spectroscopy analysis (LRS) and Scanning electron microscopy (SEM). The TGA results showed that CNCO-HA had a good char forming ability, and a high initial temperature of thermal degradation; the char residue of CNCO-HA reached 36.2% and 6.2% at 600 °C and 700 °C under air; ammonium polyphosphate (APP) could improve the char residue of the APP/CNCO-HA system, the char residue reached 39.8% and 25.5% at 600 °C and 700 °C under air. The results from LOI and UL-94 showed that the intumescent flame retardant (IFR) containing CNCO-HA and APP was very effective in flame retardancy of PP. When the mass ratio of APP and CNCO-HA was 4:1, and the IFR loading was 30%, the IFR showed the best effect, and the LOI value reached 40.2%. It was also found that when the IFR loading was only 25%, the flame retardancy of PP/IFR could still pass V-0 rating in UL-94 tests, and its LOI value reached 34.3%. The CCT results demonstrated that IFR could clearly change the decomposition behavior of PP and form a char layer on the surface of the composites, consequently resulting in the efficient reduction of the flammability parameters, such as heat release rate (HRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP) and mass loss (ML). The morphological structures observed by digital photos and SEM demonstrated that IFR could be promoted to form the continuous and compact intumescent char layer. The LRS analysis results indicated that an appropriate graphitization degree of the residue char was formed to improve strength and shield properties of the char layer to obtain better flame retardant properties of the composite.