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

ABSTRACT 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.