The invention of sticky tape has obviously simplified our everyday life. A damaged document can be easily fixed within a matter of seconds using a strong and transparent strip of tape. But how many times have you managed to succeed in removing tape from paper without damaging its surface? This process turns into Russian roulette: either the tape comes off without any sign of damage, or it takes away a whole layer of cellulose with it. Now think about removing tape from the front side of a paper artwork, possibly attributed to Michelangelo Buonarotti. No one would gamble with a responsibility of that scale. That was exactly the reason why N. Bonelli et al. applied their research to this issue.
Pressure-sensitive tape mainly consists of a backing and a pressure-sensitive adhesive. The most common materials that are used for backing are paper, cellophane, cellulose acetate, and oriented polypropylene. The most widely used materials for pressure-sensitive adhesives are natural and synthetic rubbers, acrylic copolymers, and silicones. Adhesives are susceptible to degradation and tend to turn into dark and oily substances that may penetrate and affect the whole paper layer. For efficient removal of tape from paper, conservators would ideally find a solvent that is able to penetrate the backing of the tape and soften the adhesive without going into the pores of the underlying paper. Additionally, the solvent has to be applied in such a manner that it does not affect the paper around the tape; this means it should be possible to apply it strictly on the surface of the tape.
With the consideration of all aforementioned requirements, Bonelli et al. devised a method to apply a nanostructured fluid as a solvent by loading it into a polymer network to form a hydrogel. The nanostructured fluid consists of nano-sized droplets of organic solvent dispersed into water. Although the researchers predicted that the nanostructured fluid would soften the adhesive, they didn’t want the fluid to seep into the paper; therefore, they used a hydrogel to immobilize and trap the fluid (like a nano-scale sponge!). A hydrogel is a polymeric material with a hydrophilic three-dimensional structure that can absorb a large amount of water. A schematic representation of a hydrogel loaded with solvent is illustrated in Figure 1. Based on their experience of developing new types of hydrogels, Bonelli et al. chose a semi-interpenetrating polymer network composed of poly(hydroxyethyl methacrylate) and polyvinylpyrrolidone, which makes a resulting hydrogel highly hydrophilic, transparent, and mechanically stable. Then, the polymer network was loaded with a nanostructured fluid termed EAPC (consisting of water, sodium dodecyl sulfate, 1-pentanol, ethyl acetate, and polypropylene carbonate).
The researchers first tested the nanostructured fluid (both alone and loaded into a hydrogel) on three pressure-sensitive tape model systems: Magic Tape; Filmoplast P (which is a tape with a backing based on cellulose and is designed specifically for the paper restoration); and ‘ordinary tape’ (commonly used for domestic purposes). Both applications (nanostructured independent fluid and fluid-loaded a hydrogel loaded with nanostructured fluid) showed consistent results: the nanostructured fluid was able to penetrate the backing of Magic Tape and Filmoplast P and soften the adhesive, while no penetration was observed for the polypropylene backing of ‘ordinary tape’.
Based on these results, the researchers applied the hydrogel loaded with nanostructured fluid to three different works of art:
1) a 16th-century drawing of a figure from Ascesa dei Beati of Michelangelo Buonarroti’s Sistine Chapel;
2) a 20th-century drawing with the recto by Maria Helena Vieira da Silva and the verso by Helen Philips Hayter;
3) a 20th-century ballpoint pen and tempera drawing by Lucio Fontana, Untitled, 1956.
All of these artworks contained tape with cellulose-based backing, so the penetration and softening was somewhat predictable based on experiments with Magic Tape and Filmoplast. Removing the tape from the drawing of Michelangelo’s figure revealed the inscription “di mano di Michelangelo”, which means “from the hand of Michelangelo”. Although, this inscription is thought to have been covered by a sneaky art collector, who wanted to cover the dubious “proof” of authenticity. The attribution of the artwork is still under investigation. Figure 2 shows some of the results of the described tape removal process.
In conclusion, this work is an example of the development of an innovative strategy to tackle difficult problems. It is rather a generic solution that can be applied beyond these few case studies. The above-described material was applied to clean three artworks and appears to be a promising candidate for preserving many more tape affected paper drawings or documents. The researchers designed the material on the molecular level that would have desired large-scale properties necessary for a challenging application.
All figures reproduced/adapted with permission from: Bonelli, N., Montis, C., Mirabile, A., Berti, D., Baglioni, P., Restoration of paper artworks with microemulsions confined in hydrogels for safe and efficient removal of adhesive tapes. PNAS 115(23), 2018.