Physical aging of a glassy polymer in cultural heritage conservation

Abstract

Artworks, particularly easel paintings, are multi-component materials intended to last indefinitely. The protective coatings applied to these artworks often consist of amorphous glass-like polymers, which undergo slow physical aging and structural recovery below their glass transition temperature. This process alters key physical properties such as mechanical strength, optical clarity, and thermal stability over extended periods. This study investigates the time-dependent evolution of these properties in Laropal A81, a widely used synthetic resin for cultural heritage conservation, particularly as a replacement for ancient varnishes. The investigation involves characterization of enthalpy recovery via differential scanning calorimetry, refractive index evolution via refractometry, and creep compliance evolution via rheological measurements. The aging behavior of Laropal A81 is further analyzed using the Kohlrausch–Williams–Watts function and the Tool–Narayanaswamy–Moynihan model, enabling the quantification of critical dynamic parameters such as activation energy, nonlinearity, partition coefficients, and non-exponentiality. The gained insights into the long-term behavior of this coating's properties can be valuable for improving preservation and restoration strategies for cultural heritage.