Impact of nanoparticles on the environmental sustainability of polymer nanocomposites based on bioplastics or recycled plastics - A review of life-cycle assessment studies

Adding nanoparticles to a host polymer can lead to performance improvements that can be twice as beneficial to the environment: first, sustainable nanocomposites based on bioplastics or recycled plastics could replace ubiquitous petroleum-based polymers; second, substantial plastic saving could be achieved by profiting from the superior specific properties of the nanocomposites. Nevertheless, the inherent environmental burden of nanoparticles can compromise the expected benefits. Here we address the controversial issue of the environmental sustainability of "green" polymer nanocomposites based on bioplastics and recycled plastics. A critical review of life-cycle assessment studies regarding nanocomposites and their individual constituents is presented. Nanoparticles have a remarkable environmental impact despite their typically low content. Except for organo-clays and graphene, the production of common nanofillers (nanocellulose, titanium dioxide, silver and, above all, carbon nanotubes) emits relevant amounts of greenhouse gases and requires high energy, nullifying the advantages of using green polymer matrices. Reaching high performance becomes hence crucial to make polymer nanocomposites truly sustainable through material saving. For this purpose, increasing the content of nanoparticles or functionalizing them to enhance their dispersion in the host polymer can unexpectedly entail environmental benefits.