PLASMA REMOVABLE COATING
SUPER HYDROPHOBIC, PLASMA REMOVABLE COATING FOR THE PROTECTION OF CULTURAL HERITAGE ASSETS
After cleaning (by chemical and/or physical methods) cultural heritage assets are protected by organic or hybrid coatings. The removability of such a coating is paramount as it must always be possible to bring the artifacts back to their original state. The coatings that are currently being used are mostly solvent removable and not very hydrophobic (contact angle < 100°). This lack of water repellency causes the water to accumulate on the surface where it will collect dirt and pollutant that will inevitably attack the coating and the underlying artifact (soot deposition, corrosion, black crust…).
The new coating, developed by ChemStream, during the PANNA project, funded by the European commission, displays super-hydrophobic qualities when applied. A water contact angle larger than 155° causes the water droplets to ‘bounce’ off the surface, even at very low sliding angles. During the PANNA project, the performance of the coating was field tested by applying it on several cultural heritage assets, such as the metopes of the Doge’s Palace in Venice (Figure 1).
Figure 1: Metopes on the Doge’s Palace in Venice
Additionally, the ChemStream coating remains completely removable by using an atmospheric plasma torch. This state-of-the-art technique allows pin-point non-contact removal of the coating so that specific areas of the surface may be treated while leaving adjacent areas untouchedBy regulating the power of the plasma torch and the distance between the coating surface and the torch, the rate of removal can be controlled. These parameters influence the required treatment time for obtaining the complete removal of a coating. In this case a HTS (high throughput screening) methodology was used for performing plasma removal of the organic coating using a Tigres Plasma Blaster.
Figure 2: Plasma treatment of coated marble
As an example a 4 um Paraloid B67 based coating was removed using the plasma torch at full power (200W/nozzle). The working distance was varied from 10 to 4 mm and the plasma treatment times varied from 1 to 10 min. Each of the plasma treated samples was evaluated by FTIR-analysis that allowed us to calculate the percentage of remaining Paraloid B67.
In Figure 3 the percentage of remaining coating is plotted as a function of treatment time for the varying working distances. Logically, as the treatment time increases more of the coating is removed.
Figure 3: The % of remaining Paraloid B67 measured by FTIR analysis as a function of treatment time
The optimal combination of working distance and treatment time for this Paraloid B67 coating was found to be 4 mm of working distance and 6 min of treatment time.
In this video, the plasma removable process of the ChemStream super hydrophobic coating applied on fresco simulants, is demonstrated. This video was a promotional tool designed during the PANNA project, to promote the full life cycle process.