<p>This study presents a sustainable approach for upcycling waste polyethylene terephthalate (PET) into a functional binder for ballpoint pen ink. PET was depolymerized via glycerolysis under systematically optimized conditions: PET-to-glycerol mass ratio of 1:1.1, catalyst loading of 0.6 wt%, temperature of 230 °C, and reaction time of 4 h. Comprehensive characterization by FT-IR, HPLC, and GPC elucidated the structural features and molecular composition of the depolymerization products. FT-IR confirmed the presence of hydroxyl end groups and preservation of ester linkages, providing evidence for a transesterification-based depolymerization mechanism. GPC analysis revealed a number-average molecular weight (M_n) of approximately 1,800 g/mol, confirming successful cleavage of PET chains into oligomeric fragments suitable for binder applications. HPLC analysis of the reaction effluent further corroborated the proposed reaction pathway, offering insights into the by-products generated during depolymerization. The formulated ink exhibited excellent performance characteristics: appropriate viscosity, rapid drying (≤ 19 s), thermal stability across − 20 to 40 °C, UV resistance exceeding 72 h, and storage stability over 340 days. These properties meet critical industry standards, demonstrating the practical viability of the upcycled product. This study successfully establishes a practical route to converting PET waste into commercially valuable materials, providing a cost-effective and sustainable alternative to conventional petrochemical binders in the stationery industry while contributing to plastic pollution reduction.</p> Graphical abstract <p></p>