Phosphorus-doped carbon dots as an effective flame retardant for transparent PVA composite films with enhanced UV shielding property

In order to endow polyvinyl alcohol (PVA) film with the functionalities of highly-effective flame retardancy and optical properties, the strategy of doping phosphorus into carbon quantum dots (CQDs) and then using the P-containing CQDs as flame retardant is worthy investigating. In this work, m-phenylenediamine (MPD), 2-carboxyl ethyl(phenyl)phosphinic acid (HPP) and phytanic acid (PA) were used as carbon source and phosphorus sources to synthesize three kinds of CQDs, which were named as MPD-CQDs, MPD-HPP-CQDs and MPD-PA-CQDs respectively. Subsequently, the CQDs-PVA composite films were prepared by blending CQDs with PVA solution through casting process, the flame retardancy and UV shielding properties of the PVA-CQDs composite films were tested by kinds of techniques such as vertical burning tests (UL-94), limited oxygen index (LOI), UV–visible spectroscopy and so on. In addition, the residual char structure of the composite films was investigated by SEM and EDS. The results showed that compared with phosphorus-free MPD-CQDs, the two P-containing CQDs, both MPD-HPP-CQDs and MPD-PA CQDs, produced better flame-retardant effect on the PVA film. As the percentage of phosphorus increased, the flame retardancy of CQDs-PVA composite film greatly enhanced. When 15% MPD-HPP-CQDs and 15% MPD-PA-CQDs were added to PVA, the LOI value of MPD-HPP-CQDs-PVA and MPD-PA-CQDs-PVA was 27.8% and 31.5% respectively, meanwhile, the UL-94 rating reached VTM-1 and VTM-0, respectively. Due to the intact layer of P-containing condensed phase, the gas diffusion and oxygen transportation were hindered, which slowed down the combustion and promoted the further carbonation. In addition, both MPD-HPP-CQDs-PVA and MPD-PA-CQDs-PVA composite films exhibited good UV shielding properties without affecting the transparency of PVA itself.