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.