Synthesis and characterization of two-component acidic ion intercalated layered double hydroxide and its use as a nanoflame-retardant in ethylene vinyl acetate copolymer (EVA)

Abstract

A series of two-component acidic ion intercalated layered double hydroxides (MgAlZn-X₁/X₂-LDHs) were synthesized by coprecipitation under microwave crystallization, and were further used as novel nanoflame-retardants for ethylene vinyl acetate (EVA). Various measurement methods have been employed to characterize the structure, morphology, thermal stability and combustible properties of the MgAlZn-X₁/X₂-LDHs and MgAlZn-X₁/X₂-LDHs/EVA nanocomposites. The results revealed that MgAlZn-LDHs were intercalated by different two-component acidic ions successfully, and nano-sized particles of MgAlZn-X₁/X₂-LDHs were achieved. The incorporation of MgAlZn-X₁/X₂-LDHs into EVA not only can significantly improve the thermal stability of EVA but also improve the thermal oxidative resistance. Compared with pure EVA (LOI = 19, no char yield), the limit oxygen index (LOI) values and char yield of MgAlZn-SnO₃²⁻/CO₃²⁻-LDHs/EVA (40/60) nanocomposite increased, reaching 29% and 28.40%, respectively. Meanwhile, the max smoke density (MSD) values and underwriters laboratories test (UL-94) of MgAlZn-SnO₃²⁻/CO₃²⁻-LDHs/EVA nanocomposite decreased by 2.98 and reached a UL-94 V-0 rating, respectively. The improvement of flame retardancy and smoke suppression of MgAlZn-SnO₃²⁻/CO₃²⁻-LDHs/EVA nanocomposite can be attributed to the intercalation of flame retardant elements into the LDH interlayer and laminate, and the synergistic flame retardant effects between two-component acidic ions, evidenced by the dramatically reduced peak heat release (68.5% reduction), total heat release values (22.2% reduction) and peak smoke production rate (71.2% reduction) obtained from cone calorimeter tests, as compared with that of pure EVA. As such, MgAlZn-SnO₃²⁻/CO₃²⁻-LDHs could be applied as a promising high-efficiency flame retardant with the function of smoke suppression.