Reactive extrusion of sol-gel silica as fire retardant synergistic additive in ethylene-vinyl acetate copolymer (EVA) composites

Silica derived from sol-gel approach was used as co-filler in aluminium hydroxide (ATH) or hydromagnesite (HM) ethylene-vinyl acetate (EVA) formulations. In this bottom-up approach, the silica network is formed inside the extruder in a first process step. The 3D silica structure is investigated by solid-state nuclear magnetic resonance (ssNMR) spectroscopy and a combined TGA-FTIR analysis revealing a partially incomplete reaction to form silica particles. In addition, 29Si CPMAS NMR showed the formation of chemical Si–C bonds between EVA and silica networks. The performance enhancement of the fire retardant properties was established by comparing with composites prepared by a top-down approach, i.e by adding preformed silica. It is confirmed that the preformed silica acts as a synergistic agent with both ATH or HM, increasing the flame retardant properties (limiting oxygen index, UL-94 and cone calorimeter tests) of EVA composites, however, the sol-gel silica shows the best performances in reducing both heat release and smoke production. The role of the sol-gel structure was deeply investigated in correlation with combustion test.