Characterization and degradation of poly(vinyl acetate)-based adhesives for canvas paintings

The modifications occurring to two well-established synthetic formulations (Mowilith DMC2®, Mowilith DM5®), upon accelerated degradation, were investigated. Degraded films of copolymers were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Differential Thermogravimetry (DTG), Atomic Force Microscopy (AFM) and colorimetry. The study highlighted the varied degradation of the two adhesives, which exhibit yellowing, alteration of their thermal and spectroscopic features and changes in their surface morphology and viscoelasticity. The main degradation route is characterized by the loss of acetyl groups, resulting in conjugated double bonds, as suggested by colorimetry and confirmed by FTIR. The loss of volatile organic acids represents a conservation issue when these adhesives are used for the lining of canvas paintings, since acidity promotes the hydrolysis of canvas cellulose. Rearrangement of the adhesives' molecular structure, and a competition between depolymerization and cross-linking were suggested by AFM and thermal analyses (changes in molecular weight are known to affect the solubility of copolymers and the possibility of their removal from works of art). The representativeness of the accelerated degradation protocol was positively assessed through comparison with naturally degraded adhesives.