UpcyclingRecycled PET into High-Performance PolyurethaneCoating and Engineering Materials: Effect of NCO Index on Mechanical,Stability, and Surface Properties

Upcycling of polyethylene terephthalate (PET) waste into high-value polymeric materials offers a promising pathway toward circular and sustainable polymer engineering. In this work, polyurethane (PU) films were synthesized from recycled PET (rPET)-based polyols, and the influence of the isocyanate index (NCO index: 100–200) on the network structure and functional properties of the resulting materials was investigated. FT-IR results supported the formation of urethane linkages between the rPET-based polyol and polymeric methylene diphenyl diisocyanate (PMDI), indicating successful network formation. Increasing the NCO index significantly enhanced cross-link density, reflected by an increase in gel content from 54.97 to 93.46% and a decrease in swelling ratio from 254.58% to 133.30%. This structural evolution led to improved mechanical performance, with tensile strength and Young’s modulus reaching 7.26 and 5.29 N/mm2, respectively. Thermal analysis further revealed an increase in glass transition temperature (−32.05–0.93 °C) and enhanced thermal stability with higher char yields (17.57–24.71%). Surface analyses demonstrated smoother, denser, and more hydrophobic morphologies at elevated NCO indices. An optimal balance of mechanical, thermal, and surface properties was achieved at NCO indices of 150–175, highlighting the potential of rPET-based PU films as sustainable coating and engineering materials for advanced applications.