Synergistic Flame Retardancy and Smoke Suppression in Flexible PVC Enabled by Magnesium Hydroxide With a Dual‐Layer Coating and Fe( OH ) 3 Decoration

Abstract

To improve the flame retardancy and smoke suppression of magnesium hydroxide (MH) in flexible PVC, a hierarchical flame retardant (MH‐P2@Fe) was designed via dual‐layer coating of polydopamine (PDA) and polydichlorophosphazene (PDCP) followed by Fe(OH) 3 loading. The introduction of Fe(OH) 3 enhanced the thermal stability of the PDCP coating, raising its onset decomposition temperature ( T 05 ) to 343.6°C. Adding 20 phr of MH‐P2@Fe to PVC gave a limiting oxygen index of 25.6%, a UL‐94 V‐0 rating, and reductions in peak heat release rate (20.7%), total heat release (8.89%), peak smoke production rate (37.5%) and total smoke production (28.6%) compared to PVC/MH. Thermogravimetric analysis (TGA)‐based quantitative analysis showed that iron oxides (Fe 2 O 3 /Fe 3 O 4 ) derived from the decomposition of Fe(OH) 3 possess a strong catalytic char‐forming capacity of 8.44 per unit mass, promoting early dehydrochlorination and deep aromatization to form a continuous, highly graphitized inorganic–organic char layer (MgO and Fe 2 O 3 /Fe 3 O 4 ). In the gas phase, phosphorus radicals and water vapor quenched active radicals and diluted combustible gases. Moreover, the dual‐layer coating improved interfacial adhesion, resulting in higher impact strength and tensile modulus than neat PVC, although tensile strength decreased. This work provides an effective halogen‐free flame retardant for PVC with balanced performance.