Abstract
Extending the use of novel anti-graffiti coatings to built heritage could be of particular interest providing the treatments are efficient enough in facilitating graffiti removal and long-lasting to maintain their protective properties without interfering with the durability of the substrates. However, studies of the durability of these coatings are scarce and have been mainly carried out under accelerated weathering conditions, the most common practice for assessing the durability of materials but one that does not reproduce accurately natural working conditions. The present study aimed to assess the durability of the anti-graffiti protection afforded by two anti-graffiti treatments (a water dispersion of polyurethane with a perfluoropolyether backbone and a water based crystalline micro wax) on Portland limestone and Woodkirk sandstone after 1 year of outdoor exposure in the South of England with periodic painting and cleaning episodes taking place. A parallel study under artificial weathering conditions in a QUV chamber for 2000 hours was also carried out. Changes to the coatings were assessed by measuring colour, gloss, water-repellency, roughness and microstructure, the latter through micro-Raman and optical microscope observations, periodically during the experiments. The results show that both anti-graffiti treatments deteriorated under both artificial and natural weathering conditions. For the polyurethane based anti-graffiti treatment, artificial ageing produced more deterioration than 1 year of outdoor exposure in the south of England due to loss of adhesion from the stones, whereas for micro wax coating there were no substantial differences between the two types of weathering.
Highlights
Coatings and stones have been linked from ancient times; from rock coatings that have contributed to the preservation of cultural assets by protecting the underlying surface of stones from weathering or have enhanced their deterioration [1] to current coatings formulations that are used in stone conservation [2] such as consolidants, water repellents, biocides, etc
Protective barriers that facilitate graffiti removal by generating low energy surfaces that make the substrate water- and oil-repellent have been applied to modern buildings [4]
On the AG1 coated samples, the combination of detergent and brush was the preferred cleaning procedure since pressurized water spray increases the roughness of the materials which favours graffiti adhesion in subsequent painting-cleaning episodes and even on sandstone the removal of the anti-graffiti protection
Summary
Coatings and stones have been linked from ancient times; from rock coatings that have contributed to the preservation of cultural assets by protecting the underlying surface of stones from weathering (patinas, heavy metals skins) or have enhanced their deterioration (soiling, sulfate crusts) [1] to current coatings formulations that are used in stone conservation [2] such as consolidants, water repellents, biocides, etc. Anti-graffiti coatings have been used for only the last 40 years, since the phenomenon of deliberate marking of surfaces with paints and PLOS ONE | DOI:10.1371/journal.pone.0172347. Protective barriers that facilitate graffiti removal (with pressurized water spray and/or solvents) by generating low energy surfaces that make the substrate water- and oil-repellent have been applied to modern buildings [4]. Current advances of anti-graffiti coatings formulations with nanoparticles [7,8,9], powder technology, etc. for building retrofitting stand as proof of their increasing interest in the engineering field
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