The critical role of Ni(III) species in MOSw@PDA@NiOOH for effective smoke suppression and flame retardancy in flexible polyvinyl chloride composites

To overcome the critical challenges of high smoke toxicity and flammability in flexible polyvinyl chloride (fPVC), this study developed a core-shell flame retardant (MOSw@PDA@NiOOH) via polydopamine (PDA) interfacial engineering and in-situ NiOOH deposition on magnesium oxysulfate whiskers (MOSw). SEM, TEM, and EDS confirm the hierarchical interface structure with Ni(III)-based NiOOH nanoparticles uniformly anchored on MOSw surfaces. Cone calorimetry tests show a 35.38 % reduction in peak smoke production rate and a 75.88 % decrease in total smoke production, respectively, for fPVC/MOSw@PDA@NiOOH compared to fPVC/MOSw composite, along with a UL-94 V-0 rating and 28.8 % limiting oxygen index. Raman, TGA, and XPS analyses reveal that: i) the strong Lewis acidity of Ni(III) facilitates chloride elimination from PVC, leading to the formation of conjugated polyene structures; ii) the powerful oxidizing capability of Ni(III) directly oxidizes unsaturated carbons such as CC, generating carbocations that subsequently promote aromatization and cross-linking reactions; iii) Ni(III) induces complete decomposition of MOSw into MgO, ultimately forming a dense carbonaceous residue layer composed of graphitic carbon, MgO, NiO, and MgCl₂. Py-GC/MS and TGA-FTIR analyses further demonstrate that the strong oxidative capacity of Ni(III) also promotes deep cleavage and oxidation of aromatic/aliphatic intermediates, which significantly reduces the accumulation of smoke precursors. Moreover, both the Ni(III) and its reduced Ni(II) possess partially filled 3d orbitals capable of scavenging gas-phase radicals (e.g., H· and HO·), thereby interrupting combustion chain reactions and imparting flame-retardant efficacy. Additionally, PDA’s amino/hydroxyl groups strengthen the MOSw-PVC interface via hydrogen bonding, increasing tensile modulus (20.9 MPa) and strength (17.8 MPa) while maintaining impact strength (83.9 kJ/m²). This work establishes a multifunctional design strategy that simultaneously achieves flame retardancy, smoke suppression, and mechanical reinforcement in fPVC composites through synergistic interfacial and catalytic effects.