Nitrogen and phosphorus co-doped carbon dots for developing highly flame retardant poly (vinyl alcohol) composite films

In this work, nitrogen and phosphorus co-doped fluorescent carbon dots (N,P-CDs) were synthesized from m-phenylenediamine and phosphoric acid in a one-step hydrothermal method. Then, novel flame retardant PVA composite films (PVA/N,P-CDs) were prepared by introducing N,P-CDs into the PVA matrix via solvent mixing. Subsequently, the flame retardancy, thermal stability, mechanical properties and transparency of the PVA/N,P-CDs films were investigated through vertical burning tests (UL-94), thermogravimetric analysis (TGA), mechanical property tests and UV–visible spectroscopy, respectively. In addition, the structure of the residual char of the composite films was studied by SEM, EDS, in situ FTIR and Raman spectroscopy, and the corresponding flame retardant mechanisms were analysed. It was demonstrated that the addition of a small amount of N,P-CDs endows films with remarkable anti-dripping performance. The limiting oxygen index (LOI) of the PVA/N,P-CDs composite films increased to more than 30% and achieved a UL94 VTM-0 rating when the N,P-CDs solution (the N,P-CDs are diluted 30 times) was higher than 20%. Moreover, the introduction of N,P-CDs solution increased the residual char from 12.4% for neat PVA to 36.4% for PVA/N,P-CDs. These improvements are attributed to the N,P-CDs in the N,P-CDs solution, which are contribute to the formation of a larger char barrier layers, and a less phosphoric acid in the N,P-CDs solution can effectively promote the dehydration, crosslinking and cyclization of PVA matrix to form a close and continuous char barrier layers. Importantly, N,P-CDs considerably improve the high temperature thermostability and mechanical properties of PVA films without affecting transparency and endow these films with UV absorption ability.