Isothermal Titration Calorimetry Reveals Entropy-Driven Bisphenol A Epoxy Resin Adhesion to Metal Oxide Surfaces

Polymer-coated metals are ubiquitous in multiple industries as a corrosion protection strategy. Particularly in food and beverage packaging, bisphenol A (BPA)-based epoxy coatings provide an excellent barrier and strong adhesion to metals. There is, however, a need to design safer, alternative coatings with similar adhesion as BPA-epoxies due to environmental and health concerns associated with BPA. Limited critical information exists on epoxy-metal interactions and the effect of interfacial functional group concentration on overall adhesion due to the constraints of most experimental methods, which typically probe the interface only within a few nanometers in situ. Herein, we use isothermal titration calorimetry (ITC) and molecular dynamics simulations to characterize the thermodynamics of epoxy-metal oxide binding in the liquid phase and identify the influence of epoxy resin structure and metal oxide surface chemistry in dictating the binding process. Across a series of epoxy resins and three metal oxides, we reveal a previously unreported dominant role of entropy in the binding process, primarily facilitated by the release of bound solvent molecules from the epoxy/metal interface with possible contributions from dispersive OH-pi interactions between the benzene rings of the resin and the -OH groups on the metal oxide surface. Enthalpy-favored hydrogen bonding between the -OH groups of the resin and the metal oxide plays a supporting role in the binding, with its participation dependent on the interfacial -OH group concentration. ITC therefore offers key molecular insights into the relative functional group contributions to the adhesion mechanism and informs the rational design of next-generation polymer coatings.