The impact of polyurethane adhesives on the recyclability of polyethylene-based laminated films

Laminated plastic films, especially mono-material multilayer polyethylene-based (MM-PE) films, have attracted strong interest within the packaging industry over the past decade. Towards closing the material loop, MM-PE films should be recycled in a film-to-film mechanical recycling process, while preserving their desired performance. However, MM-PE laminates typically contain polyurethane (PUR) adhesives between PE film layers, which can affect their recyclability. Herein, three MM-PE films containing different PUR adhesives were studied to assess the effect of the PUR characteristics on the films' recyclability. The processability of MM-PE laminated films produced via film-blowing extrusion was evaluated, followed by structural, rheological, mechanical, and optical characterization of the recycled films. The composition of the PUR adhesive substantially affects the closed-loop recyclability of the films, and even a small PUR content (2 wt%) can hinder the recycling process. Overall, PUR adhesives reduce the zero-shear viscosity and increase the heterogeneity of the recycled laminates, reducing toughness and increasing haziness. By assessing the morphology and the physicochemical characteristics of the PUR adhesives, it is possible to estimate the impact of these materials on the recyclability of MMPE laminates. PURs presenting a higher degree of mixing between hard and soft phases, as indicated by a lower content of hydrogen-bonded groups among hard segments, are preferred for improving the recyclability of MMPE laminates.