A study on the laser-induced phenomena of commercial dammar, ketone-based and poly(isobutyl methacrylate) resin artists’ varnishes upon Er:YAG laser irradiation

This research aims to comprehensively analyse the interaction between a 2940 nm wavelength Er:YAG laser and several commercial varnishes commonly used in the mid-twentieth century. A selection of triterpenoid, poly(cyclohexanone), poly(cyclohexanol) and poly(isobutyl methacrylate) resins was chosen to make dammar, Ketone Resin N, MS2A and Paraloid B67 varnishes. Before creating varnish films, thermogravimetric analysis (TGA) was performed to determine the thermal stability of the resins as received from the manufacturer. The resins were measured at six different heating rates, and the kinetic activation energies (E_a) were calculated using the Flynn-Wall-Ozawa method. Also, differential scanning calorimetry (DSC) was performed to determine the resins' glass transition temperatures (Tg). After a 170.6 klux·h dose of radiation (λ>310 nm) simulating daylight through window glass of approximately 53 museum years, the varnish films were irradiated with an Er:YAG laser. The laser was used in the Very Short Pulse (VSP ≃ 100 µs) and Short Pulse (SP ≃ 300 µs) modes. The 2940 nm laser beam with a 4 mm diameter was fired onto dry and prewetted varnishes using a working distance of 20 cm. Single laser pulses with fluences ranging between 0.5 and 2.7 J/cm² were used. A prewetting solution of 1 % v/v Tween®20 surfactant in deionised water was applied to the varnish films. Er:YAG transmission through the aged varnishes was monitored in real time and showed that the transmitted energy increased almost linearly with fluence, except for the Paraloid B67 films which did not interact with the laser. Chemical changes were monitored using Attenuated Total Reflectance/Fourier Transform Infrared (ATR/FT-IR) spectroscopy. A reduction in hydroxy groups and carbon-hydrogen bonding was observed as a function of fluence for dammar and Ketone N films, while no change was observed for the dry and prewetted MS2A films. FTIR was also employed to experimentally determine the absorption coefficient of the varnish films. Optical Microscopy and low-vacuum Scanning Electron Microscopy (SEM) in the Backscattered Electron (BSE) mode were used to compare the dry and prewet irradiated spot areas. These analyses revealed that laser spots were less noticeable on prewetted varnishes than on dry-irradiated films.