WP4 - Active and passive protective coatings
Metal objects are affected by atmospheric aggressive agents that induce corrosion processes (e.g. “bronze disease” of copper-based objects) and deactivation of traditional anticorrosion agents. Many plastic materials, found in art collections, show instability, e.g. yellowing and embrittlement. Thus, regardless the nature of materials, artworks require protective treatments in order to avoid rapid degradation, which is often due to autocatalytic processes whose early stages are not easily perceivable by curators.
Up to now the most widespread approach for corrosion protection of metallic artefacts is the application of protective polymeric coatings. However, extra functionalities have to be imparted to the polymeric coating in order to provide long-term protection to the substrate. The aim of WP4 “Protection of surfaces” was to develop, optimize and validate a new generation of multifunctional coatings, by exploiting the concept of nanoparticle and nanocarriers which possess also ACTIVE feedback in response to changes in the local environment, so that they are capable of hampering the degradation process in addition to the merely passive protection provided by traditional approaches.
- 8 protective coatings: multifunctional protective systems, both active (able to delay the degradation mechanisms by interfering with the chemical degradation pathways), passive (able to delay the diffusion of gases pollutants towards the substrate), and multilayer (active+passive) were developed for the preservation of metal artifacts and polymer-based rapid prototyping objects. Active/passive/multilayer systems (6 formulations) for metal surfaces overcome commercial benchmarks from the restoration practice, in terms of long-lasting durability, safety and sustainability. The active and passive coatings for plastics (2 formulations) introduced a knowledge advancement in the field, as no benchmark currently exists for the protection of polymers.
Traditional protectives (e.g. acrylics or wax films) exhibit the following critical issues: they can induce changes in the appearance of the treated surfaces, use toxic corrosion inhibitors or harmful organic solvents, or lack safe application and removal protocols. The new “active” coatings are based on environmentally friendly polymeric transparent matrices that embed nanomaterials (.ie. Layered double hydroxide and alkaline nanoparticles as calcium carbonate and calcium hydroxide), to protect the surface of metal artifacts. The active coatings developed for plastic-based artworks are based on silver clusters (i.e. aggregates of 5 silver atoms) which are safely deposited on the polymeric substrates. The “passive” protective systems have better gas barrier properties than the current best practice. The new “active” and “passive” protective layers can be combined in protective multilayer structures to provide reliable and long-lasting protection.
For plastic artifacts, no benchmark exhisted before NANORESTART, where the exploration of innovative active and passive coatings was carried out.
How do they work?
Active coatings for metal substrates are based on green-chemicals, such as chitosan and High Amouprhous PolyVinyl Alcohol (HAVOH), and contain nanoparticles consisting of Layer Double Hydroxide modified with anionic anticorrosion additive able to be released upon external corrosion-related stimuli (e.g. increase of acidity and presence of chloride ions) alongside with alkaline ananoparticles. Silylated cellulose ether-based formulations were used to grant hydrophobic properties along with active protection.
Active coatings for polymeric substrates are based on waterborne dispersion of silver clusters able to substract radicals which generate on the surface of plastic artworks when exposed in aggressive environment.
Improved passive protective coatings based on polyurethane as well as high dense inorganic matrix can work as high barrier against water, oxygen, heat, and UV light.
How are they applied?
The coatings can be applied by brushing or spraying.
Case studies and application
These systems have been used on modern and contemporary metal and plastic surfaces. The multilayer coating (consisting of an active layer covered by a passive layer) prevents interactions of Cu-based alloy artifacts with the atmospheric aggressive agents that induce corrosion processes (e.g. “bronze disease”) and protects the anticorrosion additives through a stimuli-responsive mechanism involving its realese when chloride ions migrate in the coating. The developed multilayer coatings applied on plastics are able to reduce the amount of oxidation induced by light aging.