Design-for-recycling of laminated films: Effects of polyol type used in PUR adhesives on the recyclability of multilayer polyethylene films

Multilayer films, in which a single type of material is used across all layers, are gaining attention due to their prospected adequacy to closed-loop recycling, especially multilayer monomaterial polyethylene-based (MM-PE) laminates. However, these films often use a polyurethane (PUR) adhesive as an interlayer during the lamination process of the individual layers. However PUR is thermodynamically immiscible with PE, which can compromise the film’s recyclability. This study systematically examines the role of the polyol used for the synthesis of PUR adhesives in the closed-loop mechanical recycling of MM-PE films. Five PUR adhesives were synthesized, which presented either (i) diverse polyol chemical structure (polyether, aliphatic polyester, aromatic polyester) at identical molecular weight ( M w = 1000 g/mol) or (ii) aromatic polyester-based polyols with different molecular weights ( M w = 1000, 2500, and 10,000 g/mol). All PURs were synthesised from a TDI-TMP prepolymer, maintaining the isocyanate index equal to 1.63. The resultant adhesives were characterized by FTIR, DSC, TGA, DMA, and tensile testing, and the effects of the characteristics of the polyol used on recyclability and re-processability mechanically recycled materials were evaluated. The results show that the film’s recyclability is primarily determined by the molecular weight of the polyol rather than its chemical structure. PURs containing low- M w polyols (1000 g/mol) presented reduced melt strength, leading to process instabilities and low tensile properties, showing PUR particles (gel-like, thermoset) poorly dispersed into the PE matrix. In contrast, higher- M w polyols improved dispersion of the crosslinked PUR particles, maintained melt strength, and produced recycled laminates with improved mechanical performance. Furthermore, the thermal stability of the PE-phase was not affected by the PUR type. These findings indicate that the recyclability of MM-PE films can be improved through careful design-for-recycling (D4R) of the PUR adhesive.