Multifunctional TiO2 coatings for Cultural Heritage

Progress in Organic Coatings 74 (2012) 186–191 Contents lists available at SciVerse ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat Multifunctional TiO2 coatings for Cultural Heritage Mauro F. La Russa a , Silvestro A. Ruffolo a,∗ , Natalia Rovella a , Cristina M. Belfiore b , Anna M. Palermo c , Maria T. Guzzi a , Gino M. Crisci a a b c Dipartimento di Scienze della Terra, Università della Calabria, Via Pietro Bucci, cubo 12B, 87036 Arcavacata di Rende (CS), Italy Dipartimento di Scienze Biologiche, Geologiche ed Ambientali – Sezione di Scienze della Terra, Università di Catania, Corso Italia 57, 95129 Catania, Italy Dipartimento di Ecologia, Università della Calabria, Via P. Bucci, cubo 6/B, 87036 Arcavacata di Rende, Cosenza, Italy a r t i c l e i n f o Article history: Received 4 June 2011 Received in revised form 4 December 2011 Accepted 13 December 2011 Available online 4 January 2012 Keywords: Titanium dioxide Restoration Cultural Heritage Biocides Photodegradation a b s t r a c t Environmental pollution arising from industrial implants and urban factors is constantly increasing, causing aesthetical and durability concerns to urban structures exposed to the atmosphere. Nanometric titanium dioxide has become a promising photocatalytic material owing to its ability to catalyze the complete degradation of many organic contaminants and environmental toxins. This work deals with the preparation system that could take advantage of functionalized building materials in order to improve the quality of urban surfaces, with particular regard to Cultural Heritage. TiO2 -containing photoactive materials represent an appealing way to create self-cleaning surfaces, thus limiting maintenance costs, and to promote the degradation of polluting agents. Titanium dioxide dispersed in polymeric matrices can represent a coating technology with hydrophobic, consolidating and biocidal properties, suitable for the restoration of building stone materials belonging to our Cultural Heritage. Mixtures were tested on marble and limestone substrates. Capillary water absorption, simulated solar aging, colorimetric and contact angle measurements have been performed to evaluate their properties. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Deterioration of stone materials used in artistic/architectural field (lime-based wall paintings, calcareous stones) is one of the most serious problems facing conservation today. Air pollution, soluble salts and biodeterioration are the main causes of decay, and the existing literature includes many papers concerning the investigation of their mechanisms of action [1–5]. In the last years, various synthetic polymers have been widely used in the treatment of construction materials of historical monuments for consolidation and conservation of such structures [6]. However, protection of monuments by using polymeric coatings has created serious challenges for the surface science and technology. Photocatalytic oxidation has a strong potentiality as being an effective process for removing and destroying low-level pollutants in the air. Most recently, the area of interest is shifting into practical and technological applications, like self-cleaning construction materials and antimicrobial photocatalytic coatings [7]. The latter area has attracted our attention for the increasing loss of efficacy of the conventional methods to achieve higher biocidal efficiency. When irradiated, photocatalytic particles are in direct contact with or close to microbes, hence the microbial surface becomes the primary target of the initial oxidative attack [8] and, in the case of microbial cells, results in a decrease of the respiratory activity and cell death [9]. Photocatalytic treatments of environmental pollutants using various semiconductors are well known [10–14]. TiO2 is one of the main photocatalysts used in paints, cements, or in other products for sterilizing, deodorizing and anti-fouling properties. Furthermore, Matsunaga and coworkers [15,16] reported that microbial cells in water could be killed by contact with a TiO2 –Pt catalyst upon illumination. This work deals with an experimental investigation of the properties of an organic coating (in which TiO2 was dispersed) applied on two carbonatic lithotypes. The aim is to verify if this coating technology has biocidal and hydrophobic features, suitable for the restoration of stone materials belonging to our Cultural Heritage. For this purpose, biological experiments, along with capillary water absorption, simulated solar radiation, contact angle and colorimetric measurements, have been performed. In addition, self-cleaning features were evaluated by methylene blue degradation test by colour variation measurements. Penetration of the oxide within the stone materials was assessed by means of SEM–EDS analyses. 2. Materials and methods ∗ Corresponding author. E-mail address: silvestro.ruffolo@unical.it (S.A. Ruffolo). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.12.008 Fosbuild FBLE 200 is a commercial product distributed by Steikos srl (Italy). It consists of a nanopowdered TiO2 (anatase M.F. La Russa et al. / Progress in Organic Coatings 74 (2012) 186–191 187 Fig. 1. Cross section of treated (a) limestone and (b) marble samples. Concentration measurements have been performed within the squares. crystalline phase with particle mean diameter of 20 nm) dispersed in an aqueous suspension of an acrylic polymer (polymer 4wt%, TiO2 0.3wt%). The product was tested on two different carbonatic lithotypes: Carrara marble (with a porosity of about 1%) and a limestone (whose porosity ranged between 20 and 30%). The application of the product on the stone samples has been made by brushing, in two different amounts for each lithotype, depending on the stone porosity: 2 and 4 g/m2 for the marble sample (ML and MH series), 20 and 40 g/m2 for the limestone (CL and CH series). All the treated samples underwent laboratory procedures with the aim of assessing some specific properties of the coating in relation to the substrate. Specifically, the tests performed include: (i) SEM observations of the treated samples through a FEI Quanta 200F Philips scanning electron microscope, coupled with EDS. EDS microanalysis was performed in order to obtain information on the penetration dept of the TiO2 within the samples. All the SEM–EDS analyses were carried out using an acceleration voltage of 20 kV and under low vacuum conditions (10−3 mbar pressure). (ii) The biocidal efficiency of treatments was assessed by a semiquantitative method observing the growth of Aspergillus niger colonies on the stone surface. The fungal species was placed in a liquid culture medium, inside a climatic chamber at 20 ◦ C for three days. Once the microorganisms had developed, 500 ␮l of suspension was put on the treated surface of samples (dimensions 2 cm × 2 cm × 1 cm) and then they were put directly in outdoor environment. For greater accuracy, three tests were performed for each treatment. The Petri dishes were left for 12 days and checked daily for the microbial growth of fungal species. For a semi-quantitative evaluation of the fungal colonisation, the biological growth on the stone samples was subjected to a specific conversion. In detail, biological colonisation was subdivided into four levels, as follows: 1 = no colonisation were observed on the sample in each Petri dish; 2 = colonisation’s start on the surface; 3 = colonisation cover less than half surface; 4 = colonisation high was observed on the sample. (iii) Measurements of water absorption were performed by capillarity test in order to evaluate the amount of water absorbed by a stone specimen per surface unit (Qi) over time, before and after a treatment [17]. Qi is defined as: Qi = (mt − m0 )/S, where S is the area of the base of sample, mt and m0 are the sample weights measured during the test, respectively, at the time t and the time 0. (iv) Contact angle measurements were carried out in order to determine the wettability. In all the experiments, the first step of the measurement consisted in the placement of a water drop of defined volume (10 ␮l) on the solid sample surface. Drop shape was recorded with a camera and automatically evaluated in terms of contact angle, which represents the angle between the substrate surface and the tangent from the edge to the contour of the drop. (v) Accelerated aging tests were performed through light emitted by a 300-W OSRAM Ultravitalux light with a UV-A component, whose UV intensity was 2 W/cm2 . Such tests were conducted to evaluate the stability of the hydrophobic property of the protective film and the variability of the contact angle. This light source was also used to evaluate the increase in photooxidation of the blue methylene (0.1wt% aqueous solution) in presence of the TiO2 -containing product. (vi) Colorimetric tests were carried out using a CM-2600d Konica Minolta spectrophotometer, to assess chromatic variations. Chromatic values are expressed in the CIE L*a*b* space, where L* is the lightness/darkness coordinate, a* the red/green coordinate (+a* indicating red and −a* green) and b* the yellow/blue coordinate (+b* indicating yellow and −b* blue). 3. Results and discussion 3.1. SEM/EDS analysis Penetration of nanoparticles within the bulk sample was investigated by SEM–EDS (Fig. 1). In particular, EDS microanalysis carried out on cross sections revealed the distribution of TiO2 in depth (Fig. 2). It is evident that the product shows decreasing values of Ti content from the surface to the bulk. For the marble sample, this value dramatically decreases after 200 ␮m of depth (Fig. 2a), due to the low porosity of the material. It could be an advantage, because nanoparticles are mostly located in the surface, the correct place to act as photocatalyst. In the case of limestone, measurable amounts are detected at 3 mm from the surface (Fig. 2b), thus signifying that most of nanoparticles will be inactive as photocatalysts. 188 M.F. La Russa et al. / Progress in Organic Coatings 74 (2012) 186–191 Fig. 2. TiO2 distribution at different depths in both marble (a) and limestone (b) samples. In the marble, the Ti content significantly decreases after a depth of 200 ␮m, whereas in the limestone, measurable amounts have been detected at a depth of 3 mm. 3.2. Evaluation tests 3.2.1. Biocidal and photodegradation efficiency In Table 1, the results obtained by A. niger growth test are reported (Fig. 3). After five days, observations revealed a diffuse growth of colonies on untreated specimens. In particular, limestone samples seem to be slightly more susceptible to biological attack, probably due to the greater roughness of surface. Several authors have demonstrated that the bioreceptivity of building materials is highly variable and is largely determined by surface roughness, initial porosity and mineralogical characteristics [18,19]. The mineralogical and petrographic differences between limestone and marble are known by literature; the first is a sedimentary rock, the second is a metamorphic rock [20,21]. Treatments on stones induce an inhibition in cell growth and this effect seems to be similar for the two lithotypes. Moreover, despite the semi-quantitative nature of the test, it seems that a greater amount of nanopowder applied on samples does not improve the efficiency of the biocidal feature. In order to evaluate the photodegradation effect of the coating, treated and untreated samples, stained with methylene blue, were posed on UV lamp and monitored through colour measurements (Fig. 4). In Fig. 4 the trends of colour variation during time are reported for marble (Fig. 5a) and limestone (Fig. 5b) samples. Fig. 3. Fungal colonisation on stone samples. (a) Treated marble sample after one day since inoculation; (b) treated; (c) untreated marble samples after 8 days; (d) microphotograph of the colonised marble surface; (e) treated limestone sample after one day since inoculation; (f) treated and (g) untreated limestone samples after 8 days; (h) microphotograph of the colonised limestone surface. Fig. 4. (a) Sample stained with methylene blue, (b) sample coated with 2 g/m2 of product and irradiated by UV lamp for 5 days, and (c) sample coated with 4 g/m2 of product and irradiated by UV lamp for 5 days. M.F. La Russa et al. / Progress in Organic Coatings 74 (2012) 186–191 189 Table 1 Temporal evolution of A. niger growth. After five days, a diffuse growth of colonies on untreated specimens of marble and limestone can be observed. Treatments on the surface of both the lithotypes cause a similar inhibition in the cell growth. Time (days) 1 2 5 8 12 Marble Limestone Untreated ML MH Untreated CL CH 1 2 3 3 3 0 1 1 1 1 0 1 1 1 1 1 2 4 4 4 1 2 2 2 2 1 2 2 2 2 Fig. 5. Colour variation values measured on stained surface during the time. The initial value represents the average difference between samples after and before the methylene blue application. The untreated marble samples (Fig. 5a) show a linear decrease of E value, while the coated specimens display a faster degradation of organic dyestuff. Coated limestone (Fig. 5b) shows a faster degradation than the untreated one, but its increasing rate seems to be smaller than the coated marble. This can be due to the penetration of nanoparticles within the bulk, or to the partial penetration of the dyestuff. It is interesting to notice that although the amount of TiO2 on limestone is one order of magnitude greater than the amount on marble, the latter seems to be more efficient in terms of photodegradation, since most of the oxide is located on the surface. 3.2.2. Colour variations and hydrophobic properties Treated marble and limestone surfaces were investigated in order to assess the colour variations with respect to untreated samples and the hydrophobic properties of the coating. The colour modification (E) was calculated using the following relation: E =  L∗2 + a∗2 + b∗2 where L*, a* and b* represent the difference between the value of each chromatic coordinate in treated samples and the value in untreated ones. This parameter is important for aesthetic reasons, since a coating should not induce E greater than 5 [22], in order to preserve the original colour of surfaces. E values recorded after coating application and aging are reported in Table 2. After treatment and aging, negligible colour variations were observed, thus confirming the suitability of the product for restoring purposes. Contact angle formed by water on the samples surface was measured in order to assess the decrease of wettability after treatment as well as its variation after UV irradiation. The state of superhydrophilicity, ascribable to the photoactivity of the surface, has been observed for photocathalytic oxides [23–25] and it could interfere with the hydrophobic properties induced by the polymer. Beside this, there is the effect of the polymer degradation that can lead to a significant alteration of the original features. Table 3 shows the contact angle measured before and after 1000 h of exposure to UV light. The percentage variations of the contact angle, referred to its initial value, are also reported. For each sample, the contact angles both before and after irradiation were calculated as an Fig. 6. Capillarity water absorption of (a) marble and (b) limestone specimens. average over at least 20 drops of water put on the surface. Contact angle for untreated samples was also calculated as reference. Although contact angle is influenced by surface irregularity [26–28], roughness measurements of surfaces were not performed since we paid attention especially on differences of contact angle before and after radiation. For this, it is reasonable to suppose that roughness does not change significantly during radiation. On marble samples, after treatment, a smaller increase in the contact angle is revealed; furthermore, after solar radiation, treated surfaces seem to behave like the untreated ones. Similar results are obtained for both amounts of applied product. For the limestone samples, results are different, since after treatment the average contact angle increases from 0 to more than 100. After aging only a slight decrease of values (about 20%) was recorded. Even in this case, there is no significant difference between the different amounts of applied product. Water is one of the most important abiotic factors of decay in porous materials [29,30]. Once it penetrates into the pores by capillary force, water carries out its deteriorating effect through the chemical dissolution of the carbonatic component of the stone, through physical phenomena such as freezing/thawing cycles, salt crystallization and deposition, and through microorganisms growth, due to deteriorative processes connected to their colonisation. For this reason, the hydrophobic property of the multipurpose products was tested by capillary absorption. Analyses were carried out on both untreated and freshly treated samples and after accelerated aging by UV radiation, to simulate the coating behaviour after a certain period of solar irradiation, which may lead to a decreased water resistance, due to alterations in the polymer film [31,32]. Absorption curves shown by the specimens are reported in Fig. 6. The marble samples after treatment show a decrease in the water absorption (Fig. 6a), especially in the time range up to 150 s1/2 ; after that, an increase of the Qi values is revealed. At the equilibrium, a slight difference between treated and untreated samples is M.F. La Russa et al. / Progress in Organic Coatings 74 (2012) 186–191 190 Table 2 Colour variations after coating application and aging. Sample Marble Limestone  (Treated–untreated) ML MH CL CH  (Aged–untreated) L* a* b* E L* a* b* E 1.0 1.2 −1.4 −1.1 −0.3 −0.3 0.5 1.3 0.1 0.3 0.5 0.7 1.1 1.2 1.5 1.8 1.1 1.2 −1.3 −1.0 −0.5 −0.4 0.0 0.2 0.2 0.4 0.6 1.8 1.3 1.3 1.4 2.1 Table 3 Values of contact angle of water on samples before and after 1000 h of exposure to UV light. The percentage variations of the contact angle, referred to its initial value, are also reported. Sample Contact angle (◦ ) after treatment Contact angle (◦ ) after solar radiation % Variation Untreated marble ML MH Untreated limestone CL CH 32 ± 84 ± 90 ± 0 106 ± 117 ± 32 ± 32 ± 36 ± 0 83 ± 85 ± 0 −62 −60 − −22 −27 7 6 5 7 8 5 8 7 8 8 observed. After aging, the treated samples behave similarly to the untreated ones, thus revealing a loss of the hydrophobic features of the organic polymer. For comparison, in Fig. 6 the water absorption curves of a sample just treated with an acrylic polymer without any additives and aged, were reported. It is worth to note that after radiation the acrylic polymer displays a good behaviour, while in the Fosbuild mixture, a decrease of the hydrophobic performance has been revealed. Although both products are similar, they behave in very different ways after aging, probably due to TiO2 nanoparticles that catalyze the degradation of the polymer [33–36]. The limestone samples behave in a very different way (Fig. 6b). After the product application, water absorption decreases by one order of magnitude and there is no relevant variation after the aging process. Results are very similar to those obtained by treating the sample only with the acrylic polymer. It can be stated that treatment in limestone shows a better behaviour. This can be partially explained with the different amounts applied on the two lithotypes, being the quantity of product on limestone one order of magnitude greater. In addition, the different porosity which characterizes the two stones plays a main role, since the porosity of limestone is ten times greater than that of marble, so that the product can penetrate more deeply in the bulk. In Fig. 2 it can be noticed that, close to the surface, the Ti concentration is comparable for the two stones, so it is reasonable to assume the same behaviour for the organic polymer. conversely, should concentrate on the surface to lead to the photocatalytic effect. Moreover, the roughness of the limestone surface may promote the cell growth. Hydrophobic measurements have been performed before and after treatment, as well as after solar radiation, in order to simulate the aging of the coating. Results have shown a good water repellence after treatments. After aging, the behaviour shown by the two lithotypes was slightly different. Limestone treated surfaces seem to be not affected by solar radiation; conversely, for marble, the coating is almost ineffective after aging. For all tests, no significant difference between the two different amounts of product applied in each stone have been detected. In terms of multifunctional features, hydrophobic and photoactive, the coating seems to give the best performance for limestone. 4. Conclusions References In this work, the biocidal and hydrophobic properties of an organic-TiO2 coating have been tested. Furthermore, the suitability of this material for restorative and conservative purposes of Cultural Heritage has been assessed. The product tested consists of an aqueous dispersion of an acrylic polymer and anatase nanoparticles which was applied on marble and limestone specimens. SEM–EDS analyses revealed the different penetration depth of the oxide within the stone materials, due to their diverse porosity. Specifically, in the limestone traces of the oxide have been found up to about 3 mm from the surface, while in the marble nanoparticles were mostly located close to the surface. The biocidal effectiveness of the titanium dioxide against the fungus A. niger was assessed. Results have shown a great growth inhibition efficiency on both lithotypes. Photodegradation tests revealed a good efficiency in increasing the rate of oxidation of methylene blue stains. In these tests (biocidal and oxidation), the treatment on limestone has shown a slightly worse behaviour, probably due to the deeper penetration of the oxide, which, [1] B.O. Fobe, G.J. Vleugels, E.J. Roekens, R.E. Van Grieken, B. Hermosin, J.J. OrtegaCalvo, A.S. Del Junco, C. Saiz-Jimenez, Environ. Sci. Technol. 29 (1995) 1691. [2] D. Camuffo, M. Del Monte, C. Sabbioni, O. Vittori, Atmos. Environ. 16 (1982) 2253. [3] M. Del Monte, in: N.S. Baer, C. Sabbioni, A.I. Sors (Eds.), Proceedings of the European Symposium Science, Technology and European Cultural Heritage, Bologna, Italy, 1991, p. 78. [4] K. Zehnder, A. Arnold, J. Cryst. Growth 97 (1989) 513. [5] P.S. Griffin, N. Indictor, R.J. Koestler, Int. Biodeterior. 28 (1991) 187. [6] M. Favaro, R. Mendichi, F. Ossola, U. Russo, S. Simon, P. Tomasin, P.A. Vigato, Polym. Degrad. Stab. 91 (2006) 3083. [7] M.R. Hoffman, S.T. Martin, W. Choiand, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [8] S.H. Lee, S. Pumprueg, B. Moudgil, W. Sigmund, Colloids Surf. B: Biointerfaces 40 (2005) 93. [9] T. Matsunaga, Y. Namba, T. Nakajima, Bioelectrochem. Bioenerg. 13 (1984) 393. [10] K. Pirkanniemi, M. Sillanpa, Chemosphere 48 (2002) 1047. [11] A. Mills, S.L. Hunte, J. Photochem. Photobiol. A. 101 (1997) 1. [12] D.S. Bhatkhande, V.G. Pangarkar, A.A. Beenacker, J. Chem. Technol. Biotechnol. 77 (2001) 102. [13] J.K. Reddy, K. Lalitha, V.D. Kumari, M. Subrahmanyam, Catal. Lett. 121 (2008) 131. [14] S.A. Ruffolo, M.F. La Russa, M. Malagodi, C. Oliviero Rossi, A.M. Palermo, G.M. Crisci, Appl. Phys. A 100 (2010) 829. Acknowledgements We would express our gratitude to the anonymous reviewers for the constructive comments and suggestions provided that certainly contributed to increase the quality of the manuscript. This research was supported by a request for a national project entitled NaTeBeCu nanotechnology applied to Cultural Heritage supported by the Italian Ministry for Economic Development. M.F. La Russa et al. / Progress in Organic Coatings 74 (2012) 186–191 [15] T. Matsunaga, R. Tomada, T. Nakajima, H. Wake, FEMS Microbiol. Lett. 29 (1985) 211. [16] T. Matsunaga, R. Tomoda, Y. Nakajima, N. Nakamura, T. Komine, Appl. Environ. Microbiol. 54 (1988) 1330. [17] UNI 10859 protocol “Cultural Heritage”. Natural and artificial stones. Water Repellent-Application on Samples and Determination of their Properties in Laboratory, 2001. [18] B. Prieto, B. Silva, Int. Biodeterior. Biodegrad. 56 (2005) 206. [19] B. Silva, B. Prieto, T. Rivas, M.J. Sanchez-Biezma, G. Paz, R. Carballal, Int. Biodeterior. Biodegrad. 40 (1997) 191. [20] M. Matteini, Scienza e Restauro, Nardini Editore, 2002. [21] L. Morbidelli, Le Rocce e i Loro Costituenti, Bardi Editore, 2003. [22] G. Vigliano, Graffiti and Antigraffiti Project, 2002, http://www.icr. beniculturali.it. [23] A. Nakajima, S. Koizumi, T. Watanabe, K. Hashimoto, J. Photochem. Photobiol. A 146 (2001) 129. [24] R. Wang, N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem. B 103 (1999) 2188. 191 [25] M.V. Diamanti, M. Ormellese, M. Pedeferri, Cem. Concr. Res. 38 (2008) 1349. [26] K.H. Guan, Surf. Coat. Technol. 191 (2005) 155. [27] C. Della Volpe, A. Penati, R. Peruzzi, S. Siboni, L. Toniolo, C. Colombo, J. Adhes. Sci. Technol. 14 (2000) 277. [28] M. Brugnara, E. Degasperi, C. Della Volpe, D. Maniglio, A. Penati, S. Siboni, L. Toniolo, T. Poli, S. Invernizzi, V. Castelvetro, Colloids Surf. A 241 (2004) 299. [29] G.G. Amoroso, V. Fassina, Stone decay and conservation, Elsevier, Amsterdam/New York, 1983, p. 453. [30] T. Warscheid, J. Brahms, Int. J. Biodeterior. Biodegrad. 46 (2000) 343. [31] M.J. Melo, S. Bracci, M. Camaiti, O. Chiantore, F. Piacenti, Polym. Degrad. Stab. 66 (1999) 23. [32] O. Chiantore, M. Lazzari, Polymer 42 (2001) 17. [33] J.K. Pandey, K.R. Reddy, A.P. Kumar, R.P. Singh, Polym. Degrad. Stab. 898 (2005) 234. [34] X.D. Chen, Z. Wang, Z.F. Liao, Y.L. Mai, M.Q. Zhang, Polym. Test. 26 (2007) 202. [35] L. Zan, L. Tian, Z. Liu, Z. Peng, Appl. Catal. A: Gen. 264 (2004) 237. [36] N. Nkayama, T. Hayashi, Polym. Degrad. Stab. 92 (2007) 1255.