UV Fluorescent Epoxy Adhesives from Noncovalent and Covalent Incorporation of Coumarin Dyes

Epoxies are commonly used in art conservation as adhesives for artifact reconstruction and repair. However, with the development of colorless epoxies, it has become more difficult to detect repair work. Fluorescent epoxies would allow for easy detection of the epoxy joints by simple visual inspection under UV light while remaining unnoticeable under normal display lighting. Coumarins are natural dyes that can be added in very small amounts to make thermosets fluoresce. Depending on the functionality of the coumarin used, the dye may be physically encapsulated in the cross-linked polymer or it may be bound to the polymer through covalent bonds. In this paper, we examine the efficacy of coumarin (1) and coumarin 480 (2) as physically encapsulated dyes and 7-hydroxycoumarin (3) and 7-glycidyloxycoumarin (4) as covalently bound dyes in a commercial epoxy thermoset, Epo-Tek 301. All four dyes could be used to make the epoxy fluorescent, but coumarins 1 and 2 slightly reduced the lap shear strength of the thermoset and could be extracted with solvent. In contrast, coumarins 3 and 4 had little effect on the mechanical properties of the epoxy and only minute amounts could be extracted.