Development of an innovative and sustainable coating for the protection of outdoor bronze artworks

Every outdoor bronze artwork is a one-of-a-kind creation with a distinctive history in terms of composition, production process and external exposure to the environment. However, they face a common problem: corrosion phenomena occurring over the surface promoted by degradation agents in the atmosphere. So, there is the need to choose the proper conservation method not to alter the aesthetic and the stability of these precious objects. The most widely used practice for protecting metals is applying a coating on the surface. In general, coatings for conservation are applied to avoid the contact of the metal-patina layer with the actively corroding agents present in the environment. Moreover, an ideal protective coating should prevent the object from degradation without modifying its appearance. Eventually, the properties of these treatments should assure total compatibility with the surfaces involved, good protective performance, reversibility, long-term durability and low-cost maintenance. The most frequently applied coating for bronze protection is Incralac, based on an acrylic resin, namely Paraloid ® B44, containing the inhibitor benzotriazole (BTA) as a corrosion inhibitor that has the role of slowing down the corrosion processes but also exhibits some drawbacks related to lack of stability, poor permanence on the surface and toxicity. Due to these limitations, the search for alternative products to Incralac is of great interest. This PhD thesis presents a novel sustainable protective coating and a new system for encapsulating corrosion inhibitors that ensure long-lasting permanence over time. Additionally, a novel and easy-to-interpret chemometric approach has been developed to monitor and study the evolution of coatings. A multi-analytical study on Paraloid® B44 formulations with 5-mercapto-1-pheniltetrazole (MPT) or 2-amino-5-ethyl-1,3,4-thiadiazol- (AEDTA) corrosion inhibitors and UV stabilizers—namely Tinuvin® 312 or Tinuvin® 5050—identified the best combination, with better performance than Incralac. Particular attention was given to the stability of corrosion inhibitors, addressed by encapsulating AEDTA in a complex with methyl-β-cyclodextrin (Me-β-CD), which improved its retention and stability. Simultaneously, an innovative chemometric approach using Principal Component Analysis (PCA) was developed, demonstrating its effectiveness as a tool for monitoring and interpreting the degradation processes of coatings. Finally, the project explored the optimal application methods and protective properties of the newly formulated acrylic coatings on bronze patinated mock-ups. It was found out that applying two layers of Paraloid ® B44 coatings at different concentration by brush resulted in enhanced protection compared to the bare patina. Furthermore, the most effective formulation was determined to be one that included AEDTA and Tinuvin® 312.