Hybridization of rare earth metal hydroxide with phosphomolybdic acid hydrate as an effective substitute for ammonium octamolybdate in flexible polyvinyl chloride composite applications

It is challenging to improve the flame retardancy, smoke suppression performance, cost-effectiveness, and balance of flexible polyvinyl chloride (FPVC) materials. In this regard, a hybrid rare earth metal compound (CePM) was synthesized via the reaction between cerium hydroxide and phosphomolybdic acid. The experimental results demonstrated that the incorporation of CePM significantly enhanced the smoke suppression properties of the FPVC composites. Both flaming and non-flaming tests revealed that CePM exhibited approximate maximum specific optical density (Dsmax) but achieved a substantially prolonged Dsmax time compared to commercial ammonium octamolybdate (AOM). Furthermore, CePM displayed a 2.9 vol. % increase compared to AOM in the limiting oxygen index (LOI) of the FPVC composites, indicating superior flame-retardant performance. Cone calorimeter analysis revealed significant reductions of 70.1 %, 76.8 %, and 64.6 % in the peak heat release rate (PHRR), peak smoke production rate (PSPR), and total smoke production (TSP), respectively, following the incorporation of CePM, outperforming AOM in smoke suppression and flame retardancy. The elevated smoke suppression and flame retardancy of CePM was mainly attributed to the scavenging of hydrogen and hydroxyl radicals by the Ce⁴⁺/Ce³⁺ redox, the elimination of HCl, and stable and dense cerium-molybdenum-containing char. This work offers a promising alternative to AOM for flame retardant applications in PVC and other polymeric materials.