Investigation of Protective Coatings for Bronze Using Combined Techniques

Abstract

Bronze is a metal alloy that is commonly used for statuary, but also as decorative element in architecture. Naturally, the surface of bronze monuments tends to passivation and formation of patina. This acts as protection, but can also have a decorative value. Anyhow, in certain conditions an artificial protection of bronze sculptures is also desired. Waxes and polymeric protection (Incralac, Sotter, …), also in combinations of layers, are mostly used nowadays. However, in order to prepare more effective solutions for protection, there have been performed some studies on possibilities for application of organic coatings (benzotriazole [1,2]) and various polymeric protective coatings like acrylate/methacrylate [3], fluoropolymers [1], polyurethanes [4] and sol-gel [5,6]. Since conservators demand that the coatings have to be removable, sol-gel coatings are often overlooked. Various strippers, for example on the basis of benzyl alcohol, can be applied to remove polymeric coatings from bronze surfaces. From environmental point of view, the studies of polymeric coatings are not prepared only from solvent-born formulations, but are shifted also to water-born formulations and comparison of both types of coatings [3]. Herein is reported the investigation of fluoropolymer and polyurethane protective coatings for bronze. The characterization of these coatings was made by using different analytical techniques. For structural determination, infrared (IR) and Raman vibrational spectroscopies were applied. Surface characteristics were determined by scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle measurements. Protective efficiency was analyzed using electrochemical techniques like potentiodynamic polarization and electrochemical impedance spectroscopy. As more advanced analytical techniques, there were applied ex situ IR reflection-absorption (IR RA) and in situ Raman spectroelectrochemistry. The analyzed coating is in both spectroelectrochemical techniques gradually chronocoulometrically charged at more and more positive anodic potentials to follow the eventual degradation processes that are analyzed spectroscopically after each charging. IR RA spectra are measured in near grazing incidence angle assembly using an angle of 80° and P polarized IR radiation. In such spectra longithudinal optical (LO) modes appear that are commonly shifted towards higher wavenumbers compared to usual transversal optical (TO) modes from absorbance IR spectra. The disadvantage is the contact of electrochemically treated protective coating with atmosphere. This circumstance is overcome in in situ Raman spectroelectrochemical technique. The coating on bronze is mounted in a custom-made Tephlon in situ three-electrode cell. The counter electrode is platinum grid and the reference electrode Ag/AgCl. All electrochemical experiments are performed in 0.5 M sodium chloride. Fluoropolymer coatings are prepared from resins on the basis of fluoroethylene/vinyl ether (FEVE) alternating copolymers (Asahi Glass Company, Japan). The building unit of FEVE compounds is -CF2-CFX-. Solvent- and water- born formulations are made from appropriate resins, by applying corresponding poly(isocyanate) hardener, defoamer, light stabilizers, surfactants and co-solvents. Results confirm that the higher contact angles for water are achieved for solvent-born coatings (> 100°) than for water-born coatings (< 80°). Potentiodynamic polarization measurements are in accordance with these values, suggesting higher protective effectivity of solvent-born coatings. The use of water-born coatings for outside long-term protection of bronze is questionable, since the ex situ IR RA measurements showed that after treatment at high anodic potential the coating becomes much thinner. Consequently, the band at 565 cm-1 appeared in the IR RA spectrum and can be assigned to Cu2O of the bronze substrate. Similarly, also the in situ Raman spectra showed that after application of long anodic chronocoulometric pulse the coating burned and detached from the bronze substrate. Solvent-born coatings reveal more stable behavior through in situ Raman spectroelectrochemical measurements. Solvent-born polyurethane protective coatings show hydrophobic properties and the contact angle of 110°. Relatively good protective properties are confirmed through potentiodynamic polarization curves, impedance spectroscopy and ex situ IR RA measurements. The latter technique shows quite stable response during forced degradation at high anodic potentials. The activity is carried out within the framework of the NANORESTART project which has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 646063.