Unraveling the Yellowing Mechanism in Poly(ethylene furanoate) Induced by Trace Impurities of 2,5-Furandicarboxylic Acid

The development of bio-based polymers as alternatives to traditional petroleum-based polyesters is essential for sustainable recycling in the global economy. Nevertheless, a crucial but formidable challenge is the yellowing of polyesters caused by the insufficient purity of bio-based monomers. In this study, poly(ethylene furanoate) (PEF) chips with varying color phases were synthesized via direct esterification using 2,5-furandicarboxylic acid (FDCA) of different qualities and ethylene glycol (EG). An in-depth analysis was conducted on both the polymer chips and their corresponding monomers. Gas chromatography–mass spectrometry (GC-MS) and desorption electrospray ionization mass spectrometry (DESI-MS) were employed to identify organic impurities in the FDCA monomer. Five impurities were detected, including 2-furoic acid, pantolactone, 5-chloro-2-furoic acid, 5-formyl-2-furancarboxylic acid, and 5-hydroxymethyl-2-furancarboxylic acid. Metal ion residues in the FDCA monomers were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES), demonstrating the presence of Al, Ca, Fe, K, and Na. The content of decarboxylated compounds in the FDCA/EG esterification product and the chip alcoholysis product was analyzed by ultrahigh-performance liquid chromatography/high-definition mass spectrometry (UPLC-HDMS). Density functional theory (DFT) calculations and thermogravimetric analysis (TGA) indicated that residual Na+ in the monomer facilitated decarboxylation reactions, resulting in the yellowing of PEF. This study analyzed and monitored trace components throughout the process from FDCA monomer to PEF, providing a scientific and theoretical foundation for producing high-quality bio-based PEF.