Multielement Flame Retardant System Enhances the Flame Retardancy, Mechanical Properties, and Thermal Stability of Unsaturated Polyester Resin

To address the research gap in the application of organic–inorganic flame retardant systems in unsaturated polyester (UPR) and to develop highly efficient flame-retardant UPR composites, this study proposes a novel organic–inorganic hybrid flame retardant system, PNNH@KL. The effects of PNNH@KL on the flame retardancy, mechanical properties, and flame-retardant mechanisms of UPR were systematically investigated. Cone calorimetry tests revealed that the incorporation of 15 wt% PNNH and 15 wt% KL into UPR (UPR/15PNNH@15KL) resulted in a significant reduction of 39.2% and 33.0% in the peak heat release rate (pHRR) and total heat release (THR), respectively, compared to pure UPR. Moreover, the mechanical properties of the composites were enhanced, flexural strengths reaching 53.5 MPa, representing increases of 23.5% over those of pure UPR. The analysis results of the gas phase and condensed phase indicate that the catalytic carbonization effect of PNNH synergizes with KL during combustion, forming a stable thermal barrier that effectively suppresses the transfer of heat and combustible gases. The decomposition of PNNH releases phosphorus-containing free radicals (PO·/PO₂·), which quench the combustion chain reaction through a gas-phase flame inhibition mechanism. This study provides valuable theoretical insights and technical references for the design and application of organic–inorganic hybrid flame retardant systems.