Dynamics of diffusion, evaporation, and retention of organic solvents in paints by unilateral NMR and HR-MAS NMR spectroscopy

Transport of organic solvents in polymeric matrices plays a central role in many processes of interest to the coatings industry. Previous studies have focused on individual processes taking place in the solvent-paint system. In the present study, we report results on the molecular dynamics in a solvent/paint system during the entire sequence of absorption, diffusion, swelling, and evaporation processes, obtained non-invasively, in situ, and in real time using unilateral NMR relaxometry. The associated chemical structures were investigated using HR-MAS NMR spectroscopy. Four distinct ranges of molecular motion in the solvent/paint film were identified based on proton NMR transverse relaxation time (T₂) data. The fast motion of the freely moving solvent molecules was discriminated from the slower motion of the so-called bound phase of the solvent molecules physically and chemically interacting with the cross-linked polymer network. In addition, two distinct domains of the cross-linked and mobile phases of the polymerized oil were observed, allowing the study of both the dynamics of the swelling-contraction process of the polymer and of the solubilization process. Such non-invasive analyses provide significant information on transport phenomena of the solvent in the paint film, as well as on the effects of various solvents applied to oil paintings.