Development of Polyphenol-containing Konjac Glucomannan Hydrogels for Cleaning Historical Wood Surfaces

This study, in collaboration with the Arvedi Laboratory of Non-Invasive Diagnostics (University of Pavia) at the Museo del Violino of Cremona, aimed at developing a new hydrogel system for the cleaning of historical wooden surfaces, with a particular focus on string musical instruments. The cleaning of historical wooden instrument is traditionally performed using free water-based solvent, which are able to remove pollutants like dirt, soil and human sweat. However, due to the hydrophilic character of wooden surfaces, the loading of water should be localized in specific areas, avoiding excessive solvent leaching and the consequent damage risks. Konjac glucomannan (KGM), a biopolymer of natural origin, was chosen for our study because of its ability to absorb and retain significant amounts of water through its high content of hydroxyl groups. Hydrogel formulation as proposed in previous studies [1], consisting of KGM crosslinked with borax (sodium tetraborate decahydrate), was optimized, and an improved crosslinking method involving boric acid and sodium hydroxide (NaOH) was introduced, resulting in uniformly crosslinked gels with enhanced moisture retention and elasticity. KGM polymer matrix was also subsequently modified with natural polyphenols (PPs), in order to provide an alternative sustainable crosslinking option alongside the introduction of antioxidant properties and protection against UV radiation [2]. Three PPs have been selected to implement the modification: softwood kraft lignin (SKL), Vitis Vinifera tannin (Vv) and tannic acid (TA). PPs have additionally been functionalized with epichlorohydrin (ECH) to allow for a covalent bonding between the PP and KGM and to induce self-polymerization of the PP for reducing leaching especially of smaller PPs from the gel to the treated surface. Various analytical techniques were employed to characterize the starting PPs and their derivatives, and to analyze the morphology and moisture properties of the hydrogels comprising the PP-modification. Based on the overall analysis results, two representative gels of the newly developed systems, i.e., the KGM gel modified with the ECH-modified Vv derivative crosslinked with borax and the KGM gel modified with Vv crosslinked with boric acid and NaOH, were chosen for cleaning trials on mock-ups replicating both Western and East Asian string instruments’ surfaces, contaminated with synthetic soil and sweat mixtures. [3] The cleaning efficacy was assessed through non-invasive analyses, i.e., stereomicroscopy, X-ray fluorescence and external reflection Fourier Transform IR spectroscopy, which were performed before and after cleaning trials. Results were compared with the non-modified KGM gel crosslinked with borax and the gold standard in conservation, i.e., agar-agar-based hydrogel. The results showed that generally, KGM-based hydrogels, which were characterized by softer texture, showed better cleaning efficiency than the rigid agar-agar gels. Lastly, a direct relation between the moisture properties of the gel and its cleaning efficacy was found, with hydrogels that maintained better retention capabilities performing more effectively, like the simple KGM gel. In conclusion, in this study it was possible to develop hydrogels with tuned moisture properties from sustainable materials, suggesting future exploration of mechanical, antioxidant, and UV-protection features.