The synergistic enhancement of flame retardancy and mechanical properties of polypropylene by phosphorus/nitrogen-modified nano-calcium carbonate

Polypropylene (PP) is a commonly used non-metallic material in automotive lightweighting. To meet the requirements of carbon neutrality and green environmental policies and to promote energy conservation and emission reduction in the automotive sector, this study focuses on balancing the flame retardancy and mechanical properties of PP composites. To this end, nano-CaCO 3 was synergistically modified with phosphorus and nitrogen using three routes: a one-step sodium hexametaphosphate (SHMP)/urea method, a two-step SHMP/Urea method, and a two-step SHMP/aminotrimethylene phosphonic acid (ATMP) method. After subsequent hydrophobic modification, the P/N-modified nano-CaCO 3 was blended with an intumescent flame retardant (IFR) and PP to prepare flame-retardant PP composites. The results indicate that the modification successfully introduced phosphorus- and nitrogen-containing groups onto the surface of nano-CaCO 3 without altering its calcite crystal structure. Among these methods, the SHMP/ATMP two-step route showed the highest modification efficiency, resulting in superior dispersibility and thermal stability. With the appropriate incorporation of modified nano-CaCO 3 into the PP/IFR system, the composites exhibited a marked synergistic flame-retardant effect. When the IFR content was 20wt%, the UL 94 rating was improved from V-1 to V-0, while the elongation at break was significantly increased. Concurrently, the modified nano-CaCO 3 promoted the formation of a dense and stable char layer, increasing the char yield at 600°C from 12.9% to 23.0%. These results indicate that modified nano-CaCO 3 serves as a highly effective synergistic additive that combines both flame-retardant and toughening properties. This study provides an effective strategy for simultaneously enhancing both the flame-retardant and mechanical properties of PP composites in low-additive flame-retardant systems.