Itaconic-Based Copolyesters as Potential Replacement of Epoxy-Based Coatings in Metallic Food Contact Materials

Replacement of Bisphenol A (BPA) based coatings in food contact materials has been of industrial interest for decades due to health concerns and consumer perception. Although polyester- and acrylic-based resins have been introduced into the market as potential replacements, matching the chemical, mechanical, and thermal properties of BPA-based resins remain a challenge. Specifically, their remarkable performance into delaying the onset of corrosion in metal packaging, which protects the safety and quality of foods and beverages during their shelf-life.<br/>Unsaturated polyesters (UP) are versatile, low molecular-weight thermoplastics that can undergo cross-linking reactions to yield thermosets or be functionalized to tailor their properties for potential use as additives in coating formulations. In an effort to improve the sustainability of petroleum-based UP thermosets, the incorporation of biomass-derived molecules in UPs, such as itaconic acid derivatives or short-length chain diols, has been explored. However, the thermal and mechanical properties of the resulting UPs is often negatively affected, limiting their potential use as coatings.<br/>In this research, we present the synthesis of novel itaconic-based unsaturated copolyesters with improved mechanical and thermal properties by copolymerization with monomers of high structural rigidity (i.e. cyclic, substituted, or aromatic). We found that the nature of the catalyst played a role in favoring side reactions during the polycondensation affecting the structure and size of synthesized UPs. Zinc acetate as the melt-polymerization catalyst, yielded UPs with molar mass in the range of 3,000 to 4,000 g/mol and controlled and narrower molar-mass distributions (PDI &lt; 2) in comparison to dibutyltin oxide (PDI &gt; 10). Additionally, the backbone-integrated unsaturated moieties and the hydroxy chain-end groups were better preserved for post-functionalization.<br/>Cross-linking of UPs was performed with branched thiol-functionalized or biomass-derived cyclic vinyl monomers by UV-initiated radical polymerization. Thiol-functionalized monomers yielded thermosets with a glass transition temperature of up to 60 °C, due to the high degree of cross-linking promoted by the introduction of branching to the linear UPs and high efficiency of thiol-ene reactions. Further, only UPs containing branched or cyclic diols were able to form flexible, thin, free-standing films (15 – 20 μm) while formulations containing aliphatic diols yielded brittle films.<br/>Ongoing work involves the characterization of tensile properties and protective performance against corrosion over time in saline and alcoholic standard solutions by electrochemical characterization to assess their potential use as coatings.<br/>Upon completion of this work, an industrially attractive and scalable UP-based thermoset capable of enhanced corrosion protection is developed as alternative to epoxy based resins. This research addresses the need for high-performance and environmentally sustainable materials for the food packaging industry in their quest to maintain the quality and safety of foods and beverages.