Next-Generation High-Performance Bio-Based Naphthalate Polymers Derived from Malic Acid for Sustainable Food Packaging

Increasing demand for safe, convenient, and affordable packaging has prompted tremendous growth in single-use plastics, with attendant increases in carbon dioxide emissions and environmental waste. This study presents a family of engineering polyesters featuring biobased naphthalate rigid segments. The proposed polyesters can serve as an eco-friendly substitute for existing packaging materials, such as poly(ethylene terephthalate) (PET). Bio-PET analogs using 2,7-naphthalate-based rigid segments of dimethyl 1,2,3,4-tetrahydronaphthalene-2,7-dicarboxylate (THN) or dimethyl 2,7-naphthalene dicarboxylate (2,7-N) were synthesized via transesterification with ethylene glycol to the bis-hydroxy ester followed by polycondensation. The proposed bionaphthalate polyesters provide unique performance advantages. In experiments, the glass transition temperature of poly(ethylene THN) was comparable to that of PET (Tg = 67.7 °C), and the glass transition temperature of poly(ethylene 2,7-N) was far higher (Tg = 121.8 °C). The thermal stability of poly(ethylene 2,7-N) far exceeded that of PET, as evidenced by its char yield of 33.4 wt % at 1000 °C. Moreover, the poly(ethylene 2,7-N) also produced 30% less acetaldehyde under typical processing temperatures at 250–300 °C. Finally, the oxygen permeability values of these naphthalate-based polymers were less than PO2 = 0.0034 barrer, which represents a 3-fold improvement over PET (0.0108 barrer). Overall, biobased naphthalate rigid segment polyesters are promising candidates for sustainable packaging materials, particularly those requiring high gas barrier performance.