Flame retardant potential of Tetra Pak®-derived biochar for ethylene-vinyl-acetate copolymers

Food packaging waste-stream is a serious threat for current lifestyle sustainability. Among all the materials, Tetra Pak® is one of the most employed to produce food and beverage containers. Its composition represents a challenge for waste management due the simultaneous presence of poly(ethylene), paper and aluminum, wrapped together into a multilayered packaging. In this work, we report on the pyrolytic conversion of Tetra Pak® to produce alumina-rich biochar (BC), to be used as a flame retardant for an ethylene-vinyl-acetate (EVA) copolymer. In particular, the obtained biochar was incorporated either into bulk EVA through compounding or just as a surface coating. For the surface approach, a masterbatch of biochar and EVA was prepared and then applied to the surface of unfilled EVA specimens. Both the strategies turned out to significantly improve the overall flame retardant features of the copolymer: as compared to unfilled EVA, the bulk approach promoted a remarkable decrease of peak of heat release rate (-45 and -65%, for the compounds containing 20 and 40 wt.% of BC, respectively) and of total heat release (-16.9% for the compounds filled with 40 wt.% of BC), combined with a significant increase of the residues at the end of forced combustion tests; conversely, the surface approach was capable of delaying the time to ignition and the time to peak of heat release rate, depending of the BC amount: more specifically, for the surface-coated EVA with the lowest BC loading (i.e. 3 wt.%), the two parameters increased by about 34 and 21%, respectively. The thermal, rheological and mechanical properties were also investigated, as well as the morphology of the BC particles and their dispersion in the copolymer matrix. In particular, increasing the biochar loading promoted an increase of the stiffness of the resulting compounds, as well as a decrease of their ductility with respect to unfilled EVA.