Regulating Effects of Nitrogenous Bases on the Char Structure and Flame Retardancy of Polypropylene/Intumescent Flame Retardant Composites

In this paper, four kinds of nitrogenous bases, adenine (A), guanine (G), cytosine (C), and uracil (U), were used as biobased gas sources to regulate the efficiency of an intumescent flame retardant (IFR) in polypropylene (PP). The flame retardant properties of PP composites were evaluated by using the limiting oxygen index (LOI), the vertical burning (UL-94) test, an infrared thermal imager, etc. The thermal degradation behaviors and char morphology were studied by using thermogravimetric analysis (TGA) and scanning electron microscopy. It is found that U and C play great roles in improving the flame retardancy of PP/IFR composites. The PP sample containing 17 wt % IFR and 1 wt % U (or C) achieves the UL-94 VO rating without melt-dripping and has a LOI value of > 27.9%, while the samples with equal amounts of A or G are not classified in the UL-94 test. TGA results showed that U (or C) can react with an IFR, but the interaction between A (or G) and an IFR is weak. U (or C) accelerates the formation of char and regulates its space structure at the right content. They induce the formation of a cellular and intumescent char layer that decreases the surface temperature quickly after ignition and protects the underlying resin from flame, thus improving the efficiency of PP/IFR composites.