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Final Report Summary - CLEANING HERITAGE (REAL LONG-TERM WORKING CONDITIONS OF ANTI-GRAFFITI AND SELF-CLEANING COATINGS FOR THEIR IMPLEMENTATION IN THE PROTECTION OF THE EUROPEAN CULTURAL HERITAGE)

EXECUTIVE SUMMARY

CLEANING HERITAGE (short name for “Real long-term working conditions of anti-graffiti and self-cleaning coatings for their implementation in the protection of the European cultural heritage”) is a two-year project (2014-2016) funded by the European Union's Seventh Framework Programme (FP7-PEOPLE-2013-IEF) and carried out at the School of Geography and the Environment of the University of Oxford (UK).
Since graffiti and dirt removal are often expensive procedures that are not always effective and in many cases can alter already weathered and porous building materials, commercially available anti-graffiti and self-cleaning and coatings could be an effective solution on built heritage where skills and financial resources are limited. However, up to date knowledge about their performance and durability is still limited for their safe application on conservation studies.
The aim of this project is to implement the use of these protective/preventive treatments on European built heritage taking into account the special requirements of porous construction materials through a detailed study of the interactions of these products with heritage building material substrates and their performance under real long-term working conditions. To produce and improve more precise knowledge, a selection of these products (3 anti-graffiti and 1 self-cleaning coatings) was applied on different building materials (concrete, limestone, sandstone, granite).
During the two year project we have studied whether or not these coatings fulfil the demanding requirements for conservation of historic materials, i.e. that they should provide efficient protection with minimal modification of the historic substrate and without encouraging future damage. Results on the compatibility of these materials have proved to be clearly dependant on the type of substrate; e.g. the sacrificial coating was fully compatible with the stones but not with concrete as it caused significant gloss changes. Efficiency and durability of these protective treatments was assessed over the long-term by periodical cleaning or self-cleaning episodes which took place during 1 year of outdoor exposure in the south of England and during 2000 hours of accelerated weathering in a UVB chamber. We found that the anti-graffiti coatings were originally effective in facilitating graffiti removal on porous building materials, but that after both ageing trials efficiency was reduced because of the deterioration of the coatings (mainly loss of adhesion of the permanent anti-graffiti coating and loss of water-repellency of the sacrificial one). Furthermore, cleaning procedures with a pressurized water spray on concrete and sandstone coated with the permanent coating partially or totally removed the anti-graffiti protection and increased the roughness of the surfaces which favoured graffiti adhesion in subsequent painting-cleaning episodes. Results revealed that the photocatalytic activity of the self-cleaning coating is lost regardless of the amount of product applied on stone materials after both ageing trials due to the loss of adhesion of TiO2 nanoparticles. Conversely, satisfactory results were obtained on concrete surfaces.

SUMMARY DESCRIPTION OF PROJECT CONTEXT AND OBJECTIVES

CLEANING HERITAGE was conceived with the main aim of extending the present limited knowledge of the performance of anti-graffiti and self-cleaning coatings on European built heritage that would help improve their implementation.
Graffiti and soiling can be very severe on historic materials, not only because of their aesthetic impact but because cleaning methods presently in place can be also very aggressive on them, as well as being expensive and sometimes ineffective. Preventive methods such as these treatments could be an option to tackle this problem and provide efficient and long lasting protection without altering historic substrates.
Anti-graffiti coatings act as protective low energy barriers that hinder the adhesion of spray paints and pens whereas self-cleaning coatings are able to decompose dirt (air pollutants) because of the photocatalytic effect of TiO2 crystalline nanoparticles
Until now there have been few previous studies on the potential use of anti-graffiti coatings and self-cleaning coatings on heritage building materials (García and Málaga 2012; Carmona-Quiroga et al. 2010; Lettieri and Masieri 2014; Moura et al. 2016, Gherardi et al. 2016; Munafò et al. 2014). Their weathering resistance has been even less explored (Munafò et al. 2014; Carmona-Quiroga et al. 2015; Graziani et al. 2014), mostly under laboratory conditions which cannot be as precise as outdoor exposure. Because of the great variety of these products on the market, it is not clear which should be selected for conservation of historic surfaces where these treatments have to fulfil more demanding requirements by not altering the aesthetic and hydro-properties (water-repellence, water-vapour permeability, etc) of the substrates.
For these reasons, commercial and popular permanent and sacrificial anti-graffiti products (fluoroalkylsiloxane, fluorinated polyurethane and micro wax) and an aqueous TiO2 dispersion as self-cleaning product were selected to test their suitability on a number of heritage building materials: granites, limestone, sandstones and concrete. Coated substrates were exposed to one year of outdoor exposure in the South of England or submitted to 2000 hours of UVB/condensation cycles in a chamber to assess their weathering resistance and efficiency with periodical episodes of painting and graffiti removal or photodegradation of dirt (stains).
Within this context, the project focussed on the following objectives for the implementation of anti-graffiti and self-cleaning coatings in conservation works:

- Evaluation of the compatibility of treatments and substrates following guidelines of built heritage protection
- Assessment of durability performance of the applied protective treatments and correlation of indoor and outdoor test results
- Assessment of the cleaning efficiency of anti-graffiti and self-cleaning coatings before, during and after the ageing trials with special attention to different graffiti removal procedures

References:

- Carmona-Quiroga, P.M., Martínez-Ramírez, S.,Sánchez-Cortés, S., Oujja, M., Castillejo, M., Blanco-Varela, M.T. J. (2010) J. Cult. Heritage 11, 297–303
- Carmona-Quiroga, P.M., Blanco-Varela, M.T., Martínez-Ramírez, S. (2015). Ed. Lollino, G., Giordan, D., Marunteanu, C., Christaras, B., Yoshinori, I., Margottini, C. Springer International Publishing Switzerland
- Carvalhão, M., Dionísio, M. (2015) J. Cult. Heritage. 16, 579–590
- García, O., Malaga, K. (2012) J. Cult. Heritage. 13, 77-82
- Gherardi, F., Colombo, A., D'Arienzo, M., Di Credico, B., Goidanich, S., Morazzoni, F., Simonutti, R., Toniolo, L. (2016) Microchem. J. 126, 54-62
- Graziani, L, Quagliarini, E., Bondioli, F, D’Orazio. (2014) Build. Environ. 71, 193-203
- Lettieri, M., Masieri, M. (2014). Appl. Surf. Sci. 288, 466-477
- Licchelli, M., Malagodi, M., Weththimuni, M., Zanchi, C. (2014). Appl. Phys. A-Mater. 116, 1525-1539
- Moura, A., Flores-Colen, I., De Brito, J. (2016). Constr. Build. Mater. 107, 157-164
- Munafò, P., Goffredo, G.B. and Quagliarini, E. (2015). Constr. Build. Mater. 84, 201-218.
- Munafò, P., Quagliarini, E., Goffredo, G.B., Bondioli, F. and Licciulli, A. (2014) Constr. Build. Mater. 65, 218-231

DESCRIPTION OF THE MAIN S&T RESULTS/FOREGROUNDS

The research carried out in the CLEANING HERITAGE project has found that prior to weathering most of the anti-graffiti and self-cleaning coatings selected fulfil the compatibility principle in conservation of built heritage by not altering the aesthetic and the physical properties of the substrates. However, these results are clearly dependant on the type of substrate.
The self-cleaning coating has been found to be fully compatible with concrete, sandstone and limestone, but can slightly whiten the surfaces of the coloured surfaces. The sacrificial coating (micro wax) is also fully compatible with the stones, but not with concrete by increasing its superficial gloss like the fluororalkysiloxane on a polished granite. The fluorinated polyurethane (permanent anti-graffiti coating), in spite of not changing colour and gloss of the building materials and generating the lowest energy surfaces to facilitate graffiti (highest contact angle), negatively decreases the water vapour permeability of the stones, but not of concrete.
Originally, all of the coatings are efficient in aiding either graffiti removal or photodegradation of dirt (stains). On samples coated with the fluorinated polyurethane, of the two cleaning procedures carried out, detergent with soft brush and detergent with pressurized water spray (80 bars), the former is more effective in removing spray paints and pen and does not alter the superficial roughness of the substrates unlike the latter. In fact, pressurized water spray also removes the anti-graffiti protection (partially on concrete and totally on sandstone). The sacrificial coating following the manufacturer´s guidelines can only be removed alongside the graffiti with hot pressurized water spray (80 bars). In spite of being a reversible treatment, preferred in conservation studies, the cleaning procedure has negative effects by raising the roughness of the materials to a significant extent that adversely favour graffiti adhesion.
Following visual inspection of the samples supported by colorimetric measurement, the best cleaning results were obtained on the least rough material (concrete), followed by sandstone and limestone where paint retained in the pores is hard to remove. Furthermore, on two types of granite varieties there were no differences between cleaning fresh (3 days) or old graffiti (30 days).
By studying photodegradation of artificial dirt (rhodamine B or methylene blue stains) with UVA light, the self-cleaning ability of TiO2 on the substrates was confirmed, with concrete being more photocatalytic than the stones.
Investigations on the durability of coatings have revealed that both type of treatments aged under accelerated weathering conditions with UVB and condensation cycles and after one year of outdoor exposure in the south of England (Wytham, Oxford). As a result, the sacrificial coating lost water-repellency, the TiO2 nanoparticles gradually lost adhesion to the substrates, and the permanent increasingly deteriorated by getting yellow and dark and ultimately by losing adhesion with coating suffered accelerated ageing. Not only for the permanent but also for the self-cleaning coating, accelerated ageing is more aggressive than 1 year of outdoor exposure.
Consequently, graffiti removal was less effective and colour changes (ΔE*>8) were deemed unacceptable for the fluorinated polyurethane and the micro-wax, for the former changes were gradually reported after four painting and cleaning episodes were performed. Self-cleaning coating on concrete is the exception since TiO2 nanoparticles are well adhered to its surface as demonstrated by scanning electron microscopy.
Regarding the durability of Portland limestone, only the most water repellent treatment (fluoroalkylsiloxane permanent anti-graffiti coating; contact angle of around 140º) has been found to accelerate deterioration during freeze-thaw and salt crystallization cycles by hindering the egress of water. Biocolonization studies on this building material have shown that none of the treatments (fluoroalkylsiloxane, micro wax or TiO2 nanoparticles) favours the growing of fungi and cyano-bacteria biofilms two months after their inoculation. However, on the surfaces coated with the two anti-graffiti coatings the removal of the biofilm with H2O2 was more effective because the treatments hindered its penetration.
The durability issues found in this study on selected protective treatments under both artificial and natural weathering trials hinder their safe application on historic surfaces. However, individualized research studies on the compatibility, effectiveness and durability of the great variety of products available on the market on different substrates are highly recommended followed the methods used here, since this project has reported differences in their performance from one building material to another.

POTENTIAL IMPACT (INCLUDING THE SOCIO-ECONOMIC IMPACT AND THE WIDER SOCIETAL IMPLICATIONS OF THE PROJECT SO FAR) AND THE MAIN DISSEMINATION ACTIVITIES AND EXPLOITATION OF RESULTS

This research addresses the problem of graffiti and soiling in European cities particularly on built heritage which carries social, economic and cultural impacts because of the degradation and devaluation of the building materials and the expensive and sometimes ineffective cleaning procedures. This project strongly contributes to increasing the scientific knowledge of preventive ways to tackle these problems with anti-graffiti and self-cleaning coatings. Their compatibility with various substrates, efficiency and long-term performance were investigated and the results have highlighted durability issues on all the selected treatments that currently could hinder their application on some built heritage materials, but represent a step forward for future possible implementation of similar products with eventual benefits for building materials conservation.
These findings will have a positive impact on researchers and conservators that can contribute to the successful application of these coatings, and on manufacturers, by enhancing product formulations. The results of this project have been disseminated in five international conferences devoted to conservation of Cultural heritage (TECHNART 2015 (Catania, Italy), SEAHA 2015 (London, UK), SEAHA 2016 (Oxford, UK), ChemCH 2016 (Brussels, Belgium), 13th International Congress on the Deterioration and Conservation of Stone (Glasgow, UK)) and in three submitted papers to peer reviewed international SCI journals (Construction and Building Materials, PLOS ONE and Coatings).

Address of the project public website, if applicable as well as relevant contact details;
Web-page: http://www.oxrbl.com/current-projects/
Main researcher: Dr. Paula Maria Carmona-Quiroga; e-mail: paula.carmona-quiroga@ouce.ox.ac.uk; paulacq@hotmail.es

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Gill Wells, (Head of European Team)
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Fax: +44 1865 289801
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Record Number: 194277 / Last updated on: 2017-02-07
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