PVA-based peelable films loaded with tetraethylenepentamine for the removal of corrosion products from bronze

Highlights

• Removal of bronze corrosion patinas with traditional methods is risky and time-consuming.

• Confinement of cleaning systems allows effective and non-invasive removal.

• TEPA reduces the time needed for the formation of peelable PVA films.

• TEPA-loaded PVA films allowed the cleaning of a 16^th century bronze masterpiece.

Abstract

The removal of corrosion products from bronze artifacts is still an open challenge, in particular when stubborn corrosion patinas are found on surfaces with pronounced cavities and reliefs. Highly viscous polymeric dispersions (HVPDs) of polyvinyl alcohol (PVA) are able to adhere to highly textured 3D surfaces, forming films that can be easily peeled off. Here, PVA-based HVPDs were loaded with tetraethylenepentamine (TEPA), whose copper(II) complex has a stability constant four orders of magnitude higher than that of EDTA tetrasodium salt, traditionally used by conservators for cleaning bronze. TEPA promotes alkaline hydrolysis of acetyl groups in PVA, leading to the association of the polymer chains into more ordered structures, reducing significantly the time needed for the formation of films as compared to HVPDs loaded with water. Besides, the solubility of TEPA in most polar solvents allows to upload higher quantities of chelating agent in the HVPD, as opposed to EDTA. The confinement of TEPA inside the PVA matrix allowed the effective and progressive removal of copper corrosion products from a 16th century Italian bronze masterpiece, preserving the natural cuprite patina of the historical bronze, in times drastically shorter than traditional cleaning methods.

Introduction

The preservation of metal artifacts is an open challenge in conservation science, owing to the severe degradation processes (such as the so-called “bronze disease” [1]) that affect metallic sculptures and objects belonging to different artistic productions and ages [2]. In particular, the removal of corrosion patinas from bronze is risky and time-consuming when carried out with traditional methods. Both dry mechanical (brushes, scalpels, chisels) and wet cleaning with non-confined solvents and solutions, are invasive and scarcely controllable, potentially causing damage to the artifacts [3]. Laser ablation can grant fast cleaning action, but can lead to heating processes on the surface [4]. Alternatively, wet cleaning shows enhanced efficacy and non-invasiveness when solvents or solutions are confined in retentive matrices able to gradually release the cleaning fluids on sensitive surfaces. Besides, confined fluids have reduced volatility, which strongly decreases the health issues related to the use of solvents in art conservation [3].

Chemical and physical gels have proved to be optimal confining matrices for cleaning fluids, and in the last decades polymers such as poly(2-hydroxyethyl methacrylate) (pHEMA), polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA) have been employed to formulate systems with ideal mechanical properties and retentiveness [[5], [6], [7], [8], [9]]. In particular, both gels and highly viscous polymeric dispersions (HVPDs) have been widely employed; the first are characterized by the presence of polymer networks physically or chemically cross-linked, with rheological properties that resemble those of solids (e.g. highly viscoelastic, with storage modulus higher than loss modulus over the whole frequency span as measured in oscillatory frequency sweeps [10]). Hydrogels are able to upload aqueous solutions and, to some extent, polar solvents, as opposed to organogels that are used to confine average- or low-polarity solvents [11]. HVPDs comprise polymer dispersions or solutions that do not exhibit the aforementioned rheological behavior and are mainly used as thickeners to limit the diffusion of cleaning fluids. Typically, hydrogels are prepared as sheets with a thickness of few millimeters, whose elasticity can be tuned changing the type of polymer and the synthetic process. These systems allowed the treatment of flat surfaces and contemporary painted layers with 3D texture [12]. However bronze sculptures usually exhibit highly textured surfaces with pronounced cavities and reliefs that are hardly accessible with the aforementioned formulations. PVA-based highly viscous polymeric dispersions (HVPDs) are able to homogeneously cover highly rough surfaces, and after application they can be easily removed in one piece thanks to their viscoelasticity [13]. PVA has excellent chemical stability, biocompatibility, low toxicity and cost, and good film forming properties [[14], [15], [16]]. The characteristics of the films can be tuned by adding solvents and plasticizers to the formulation, for instance ethanol has a structuring effect on water, and thus increases the order of the structure of the polymeric network [9,13]. The adhesion of these films onto the substrate can be such that decohered and detached layers are peeled along with the film. This feature is particularly appealing when treating metal substrates, whose cohesive forces are typically much stronger than those of superficial corrosion layers [17].

Initially, HVPDs formulated for the cleaning of bronze were applied as loaded with an aqueous solution of ethylenediaminetetraacetic acid disodium salt dihydrate (Na₂EDTA) [17], a traditional chelating agent widely used for the removal of copper corrosion products [18]. Despite the good stability of EDTA-copper(II) complexes, it is not uncommon to find stubborn corrosion layers that are resistant to treatments with EDTA. Indeed, obtaining a safe, fast and effective removal of degradation products from bronze artifacts is still an open issue in art conservation. A valid alternative to EDTA is represented by tetraethylenepentamine (TEPA), whose copper(II) complex has a stability constant (logK_f = 22.8 at 25 °C) four orders of magnitude higher than that of EDTA tetrasodium salt (Y⁴⁻, logK_f = 18.8 at 25 °C) [19]. The use of TEPA, properly confined in retentive matrices, is thus expected to enhance cleaning efficacy, with unprecedented results.

Moreover, EDTA is poorly soluble in ethanol, and it was shown that only limited amounts of Na₂EDTA can be uploaded in the water-ethanol blend contained in the PVA-based HVPDs before phase separation occurs [17]. Instead, TEPA has high solubility in most polar solvents [20], which allows the upload of larger quantities of chelating agent in the dispersion, likely boosting the removal of corrosion products.

In this contribution, we formulated PVA-based HVPDs loaded with TEPA, and studied how the presence of the polyamine affects the film forming process and the physico-chemical properties of the films. In fact, diethyleneamines are usually employed as crosslinking agents of PVA, and are expected to modify the interactions among the polymer chains [21]. The HVPDs and the filmed dispersions were characterized through differential scanning calorimetry (DSC), rheological measurements, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and field emission gun scanning electron microscopy (FEG-SEM) coupled with energy-dispersive X-ray analysis (EDX). The HVPDs were then applied onto corroded bronze mock-ups, and the removal of corrosion patinas was checked at the micron scale with 2D Imaging FTIR, using a Focal Plane Array (FPA) detector. Finally, a selected HVPD formulation was used to remove stubborn corrosion layers from a 16^th century bronze masterpiece by Benvenuto Cellini.