Effect of montmorillonite type on mechanical and flame retarded properties of Al(OH)₃/ethylene vinyl acetate copolymer composites

Montmorillonite containing Ca²⁺ (Ca-MMT) and montmorillonite modified with organic group (Na-MMT) were used in flame retardant ethylene vinyl acetate copolymer (EVA) with Al(OH)₃, respectively. The effect of Ca-MMT was investigated on mechanical, thermal and flammability properties of Al(OH)₃/EVA composites, compared with Na-MMT. MMT-Al(OH)₃/EVA composites were processed by melt blending in a twin-screw extrude. XRD and TEM show that MMT achieves exfoliation in MMT-Al(OH)₃/EVA composites and the exfoliated layer of Ca-MMT is relatively smaller than Na-MMT. Ca-MMT-Al(OH)₃/EVA shows better tensile strength and thermal stability than Na-MMT-Al(OH)₃/EVA. In Cone calorimeter test, Ca-MMT-Al(OH)₃/EVA also performs better than Na-MMT-Al(OH)₃/EVA with lower heat release. Moreover, the condensed phase of Ca-MMT-Al(OH)₃/EVA exhibits higher thermal insulation through lower and flatter back-temperature curves of CONE, corresponding to the compact and intact residue char surface after CONE. All the evidences indicate that the exfoliation of Ca-MMT in Al(OH)₃/EVA is conducive to barrier effect of flame retardancy and calcium ions of Ca-MMT can promote the charring of Al(OH)₃/EVA. Finally, fire performance indexes (FPI) of MMT-Al(OH)₃/EVA composites were calculated with the ratio of time to ignition (TTI) and heat-release peak value. The FPI data verify that the loading of Ca-MMT remarkably contributes to the increase of FPI, thus fire risk of Ca-MMT-Al(OH)₃/EVA is significantly decreased.