Chemical recycling of PET waste and utilization of recycled product for synthesis of hybrid unsaturated polyester–urethane coatings

Chemical recycling of polyethylene terephthalate (PET) waste has gained importance in last few decades due to its dramatic consumption through its variety of applications. This study employed conventional heating-mediated glycolysis of PET waste with excess ethylene glycol, catalyzed by zinc acetate, to achieve the monomeric product bis(2-hydroxyethyl terephthalate) (BHET). Further, three different hybrid unsaturated polyester–urethane resins were synthesized using recycled product BHET, urethane diol which was synthesized using nonisocyanate route of urethane synthesis, maleic anhydride and varying concentration of dimer fatty acid, citric acid and phthalic anhydride. These synthesized novel resins, hybrid unsaturated polyester–urethane possessed the unsaturated ester as well as urethane linkages in their polymer backbone. Comprehensive characterization of the synthesized hybrid resins and monomeric urethane diol was performed using established techniques. This included determination of hydroxyl value, iodine value and molecular weight via gel permeation chromatography (GPC). Additionally, Fourier-transform infrared spectroscopy and ¹H nuclear magnetic resonance spectroscopy were employed for structural elucidation. Subsequently, the formulated hybrid polyester–urethane resins were cured at room temperature using a styrene monomer, methyl ethyl ketone peroxide initiator and cobalt salt accelerator. A rigorous evaluation of the cured coatings was conducted to determine their optical properties, mechanical performance and chemical resistance. These properties are crucial for understanding the films' potential applications. Finally, thermal behavior was investigated using differential scanning calorimetry and thermogravimetric analysis.