Imide-Containing Benzenesulfonic Acid Dicesium Salt for Improving Flame Retardancy of Transparent Polycarbonate Cast Film

To address the limitations of conventional sulfonate flame retardants in balancing fire safety and optical properties of thin-wall polycarbonate (PC), an imide-containing benzenesulfonic acid dicesium salt (FAS-Cs) was synthesized through molecular structure engineering, and a PC/FAS-Cs cast film (0.25 mm thickness) was prepared via melt-extrusion. With the incorporation of 1 wt % FAS-Cs, a limiting oxygen index of 29.8% and UL-94 VTM-0 were achieved for the PC/1%FAS-Cs cast film. Concurrently, the peak heat release rate (336.3 kW/m2) and the fire growth index (3.17 kW/m2·s) of the film was reduced by 19.9% and 24.7% as compared to those of pure PC (419.9 kW/m2 and 4.21 kW/m2·s, respectively). The presence of FAS-Cs suppressed the release of flammable combustibles and promoted the formation of continuous and dense carbon layers during combustion, thus improving the fire safety of the PC/FAS-Cs cast film. Furthermore, the film presented 84.3% retention of PC’s transmittance and achieved anisotropic mechanical enhancement including 75.2% and 153.8% improvements in transverse and longitudinal elongation at break, respectively. The dynamic rheological results disclosed that FAS-Cs is compatible with the PC matrix and the melt strength of the PC/1%FAS-Cs composite was higher than that of neat PC. The present work provided a strategy to prepare PC cast films with better fire safety, comparable transparency, and improved ductility.