Upcycling Recycled PET into High-Performance Polyurethane Coating 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.