A Phosphoramide-Sulfonate Flame Retardant at Ultra-Low Loading for Thin-Walled Transparent Polycarbonate Composites With Enhanced Flame Retardancy

Flame-retardant polycarbonates (PCs) are in demand for a diversity of applications. However, it remains challenging to achieve both high transparency and flame retardancy in thin-walled PC without compromising its overall performance. In this study, a sulfonate- and phosphoramide-containing flame retardant (KSPH) was synthesized and applied to PC. The results demonstrate that a minimal addition of KSPH can impart excellent flame-retardant properties to PC. The limiting oxygen index (LOI) increased to 33.3%, and UL 94 V-0 rating was achieved with only 0.075 wt% KSPH in the PC matrix. The smoke production rate (SPR) was reduced by 21.7% and the CO yield (COY) decreased by 33.24% in cone calorimetry test upon addition of 0.125 wt% of KSPH. The PC composite retained high transparency owing to high compatibility and minimal loading of the additive. Analysis of the residual char indicates that the flame-retardant mechanism stems from the carbonization of sulfonate and phosphorus groups in KSPH, along with the barrier effect of a highly graphitized and intact carbon layer. Moreover, impact and tensile strength were maintained at levels comparable to pure PC. This study provides a new strategy for designing efficient sulfonate-based flame retardants and expands the potential of transparent, thin-walled PC composites.