Characterization of molecular environments and chemical exchange in acrylic paints via single-sided NMR

Abstract

Both oil and acrylic paints consist of pigments bound within a polymeric network; however, acrylic paints dry by evaporation of aqueous solvent, rather than curing chemically like oil paints. In this study, we used single-sided 1H NMR relaxometry to characterize the molecular environments in both dry and wet acrylic paints by their molecular mobilities. We measured the transverse relaxation behaviors of paints with various pigment concentrations and conducted T2–T2 correlation measurements on wet paint to monitor exchange. In dry paint, we identified two transverse relaxation behaviors, indicating regions of greater and lesser mobility, which we attributed to amorphous and crystalline domains of the semi-crystalline acrylic polymer in the film. These polymer domains were not affected by pigment concentration. In wet paint, we characterized two different relaxation behaviors, which we hypothesize to originate from unique molecular environments in the acrylic binding medium. Additionally, we characterized two simultaneous exchange processes involving aqueous solvent in wet paint and showed the effects of pigment concentration on the exchange kinetics. Through non-invasive, non-destructive relaxometry measurements, we characterized the unique molecular environments in both dry and wet acrylic paints, which can be compared to molecular environments characterized by others in oil paints and can provide new insights into the physical properties of acrylics relevant to art conservation.