Examining the Mechanics of Polyester Coatings to Assess the Opportunity to Replace BPA in Metal Food Packaging

There is an ongoing effort to replace bisphenol A/F (BPA/BPF) based coatings in food contact metal packaging applications due to potential health risks associated with these materials. Polyester based coatings are increasingly being used as replacement options for their readily tunable properties. Mechanical properties are particularly important in high deformation applications where flat precoated metal (PCM) sheets are deformed into can bodies and lids. Here, we consider the mechanical performance of BPA-free polyester-based coatings with varied molecular weight, functionality, and cross-link density. We first compared the thermomechanical behavior of the polyesters to a popular BPF-based epoxy using dynamic mechanical analysis. The thermal properties and stress–strain character of free-standing films were then examined. The failure of coatings on aluminum (Al) sheets was also analyzed, and fracture characteristics were probed by scanning electron microscopy and microcomputed tomography (micro-CT). In the formulations studied, the polyester coatings exhibited higher glass transition temperature (Tg), stiffness, and ductility, compared to the BPF-based epoxy. Increasing the cross-link density in the polyester coatings correlated with both higher Tg and increased the crack onset strain for films coated on aluminum sheets. Furthermore, the polyester formulations tested showed distinct mechanical behavior compared to the BPF-based epoxy in two key aspects. First, the ductility of the films increases to a greater extent at elevated temperatures. The greater ductility at elevated temperatures may lead to processing advantages. Second, crazing was observed as a failure mode in the clear polyester coatings, whereas it was not observed in the epoxy coatings. Understanding the crazing failure criteria in greater detail may assist in further optimization of the mechanical performance of polyester-based coatings.