Recyclability of Multilayer Monomaterial Laminated Polyethylene Films: Effects of Polyurethane Isocyanate Type and Index

Multilayer monomaterial polyethylene (MM-PE) laminated films are increasingly adopted in flexible packaging due to their potential for closed-loop mechanical recycling. However, the recyclability of these films is often limited by the presence of polyurethanes (PURs) used as lamination adhesives, which are thermoset materials thermodynamically immiscible with polyethylene and can hinder processability and compromise the physical properties of recycled films. In this study, we examine how the type of isocyanate and isocyanate index used to synthesize PUR adhesives influence the mechanical recyclability of MM-PE laminates. PURs formulated from aromatic (3TDI, MDI) or aliphatic (HDI, IPDI) isocyanates and presenting different isocyanate indexes (1.36, 1.63, and 2.1 for the HDI-based PURs) were characterized by FTIR, TGA, and DSC. The effects of PUR composition on the processability of recycled MM-PE films were evaluated through film-blowing extrusion, melt strength measurements, and morphological analysis, while the performance of the recycled films was assessed by their mechanical and optical properties. Overall, the results indicate that increasing the fraction of hydrogen-bonded urethane groups (Xb) present in the PUR adhesive substantially affects the recyclability MM-PE films, inducing to the formation of larger PUR gel particles and reducing the film-blowing process stability. Furthermore, an increase in the hard phase weight fraction (Hp) of PUR led to lower impact resistance and higher haziness. The increase in the isocyanate index resulted in an increase in Hp and Xb, being detrimental to the recyclability of the laminated films. The sample based on 3TDI showed intermediate values of Hp and Xb, presenting somewhat limited recyclability. In contrast, MDI- and IPDI-based adhesives exhibited lower values of Xb and Hp with a morphology characterized by smaller PUR gels finely dispersed within the PE matrix, resulting in better processability, higher melt strengths, and an impact resistance reaching up to 80% of that of the control film. These findings demonstrate that the type of isocyanate and its index used in PUR lamination adhesives affect the recyclability and circularity of MM-PE films, providing guidelines for designing lamination adhesives that enhance film circularity, reduce reliance on virgin plastics, and advance sustainable practices in the flexible packaging industry.