Self-extinguishing epoxy nanocomposites containing industrial biowastes as flame retardant additives

Polymeric wastes can be found everywhere around the world. The improper treatment and recycling of such materials is causing irreversible damage to both human health and the environment. Therefore, it is getting urgent the development of more waste-to-wealth routes, enabling the preparation of polymer-based products containing small amounts of synthetic and toxic components (e.g., flame retardants (FRs)). Owing to their excellent physico-chemical properties, flame retarded aliphatic epoxy resins are widely employed in the aerospace industry. However, these resins usually contain halogen-based compounds or high concentrations of phosphorus (P) in the epoxy matrix, which makes the recycling of final products even more complicated and fosters the depletion of natural resources. To overcome these drawbacks, the scientific community is proposing the exploitation of industrial biowastes as sustainable FRs in epoxy-based composites, allowing for the use of very low loadings of P and other functional additives [1]. Herein, we discuss two feasible applications (Figure 1), where biochars, one derived from the pyrolysis of spent coffee grounds and another obtained from the hydrothermal liquefaction (HTL) of civil sludges, are employed as green FRs in aliphatic epoxy-based composites. These biochars can be crucial for the obtainment of self-extinguishing (i.e., no dripping V-0 rating in UL 94 tests) hybrid epoxy materials showing a significant decrease (up to ~65%) in the peak heat release rate, especially in combination with other synergists and sol-gel-derived nanostructures. Part of the research activities concerning the use of biochar from HTL were carried out in the framework of two ongoing projects