Effects of P–N flame retardants based on cytosine on flame retardancy and mechanical properties of polyamide 6

Two phosphorus-nitrogen flame retardants were synthesized by using chlodiphenylphosphine (CDP) or diphenylphosphinyl chloride (DPP) and cytosine (Cy), respectively. The products are marked as CCDP and CDPP, and characterized by using Fourier transform infrared spectra (FT-IR), 31P and 1H nuclear magnetic resonance and thermogravimetric analysis (TGA). The CCDP and CDPP were used to modify the flame retardancy of polyamide 6 (PA6). The results indicated that 6 wt% CCDP and CDPP both make PA6 achieve the UL-94 V-0 rating and improve its LOI value. The degradation behavior was studied by TG-IR, melt volume-flow rate, etc. The results revealed that both CCDP and CDPP promote the degradation of PA6 and improve its flame retardancy by melting away. The CCDP tends to work in the gas phase and the CDPP in the condensed phase. In addition, in order to reduce the effect of flame retardant on the mechanical properties of PA6, CDPP was coated with epoxy resin. The results showed that the flame retardancy decrease after coating. Finally, melamine cyanurate was blended with the CDPP coated by epoxy to adjust the comprehensive properties of PA6 composite, and excellent flame retardancy and mechanical properties are obtained.