Reactive Extrusion of Sol-Gel Silica as Fire Retardant Additive In Ethylene-Vinyl Acetate Copolymer (EVA) Composites

Ethylene-vinyl acetate copolymer (EVA) is a widely used material, particularly in the cable industry. It is frequently formulated with large quantities of inorganic filler material, such as aluminium trihydroxide (ATH). EVA is known to form a protective layer which can inhibit combustion, especially at low heating rates but, unfortunately, this effect is not observed when used in formulations with ATH. Moreover, to be efficient, a high amount of such inorganic filler is necessary (60 wt.%). This fact is an important drawback that lead to a reduction of the mechanical properties and of the processability. Aiming to improve these aspects, this research is focused on the use of sol–gel reactions to synthesize inorganic fillers in a polymeric matrix. The sol–gel method is based on a hydrolysis-condensation reaction of a metal alkoxide, which allows the synthesis of particles well dispersed in the polymer matrix. These reactions were done inside the extruder during the processing of the polymer matrix. The presence of sol-gel silica increases the Time To Ignition and also improves self-extinguishing ability, thanks to a better thermal stability at higher temperatures, and reduces the maximum Heat Release Rate in cone calorimeter tests. The flame retardant mechanism of silica in the EVA is mainly due to the physical process of silica acting as enhanced char/silica layers in the condensed phase which prevents the heat and mass transfer in the fire. A synergistic flame retardant mechanism have been evidenced by adding sol-gel silica inside EVA/ATH composites.