Surface characterization and effectiveness evaluation of anti-graffiti coatings on highly porous stone materials

Surface characterization and effectiveness evaluation of anti-graffiti coatings on highly porous stone materials

Applied Surface Science 288 (2014) 466–477 Contents lists available at ScienceDirect Applied Surface Science journal homepage:

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Applied Surface Science 288 (2014) 466–477

Contents lists available at ScienceDirect

Applied Surface Science journal homepage:

Surface characterization and effectiveness evaluation of anti-graffiti coatings on highly porous stone materials Mariateresa Lettieri ∗ , Maurizio Masieri Institute of Archaeological Heritage – Monuments and Sites, CNR–IBAM, Prov.le Lecce-Monteroni, 73100 Lecce, Italy

a r t i c l e

i n f o

Article history: Received 20 February 2013 Received in revised form 8 October 2013 Accepted 10 October 2013 Available online 18 October 2013 Keywords: Anti-graffiti Stone protection ATR-FTIR Color Wettability

a b s t r a c t In this study, two commercial sacrificial anti-graffiti systems, provided as water emulsion, were applied on a highly porous stone. The behavior of the anti-graffiti treatments was investigated by means of differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR), colorimetric tests, and water static contact angle measurements. The presence of a protective coating enhanced the removal of paint sprayed on the stone. However, penetration of the staining agent below the surface, due to the high porosity of the substrate, caused difficulties in eliminating the paint. In fact, repeated cleaning procedures, involving hot water, mechanical action, and chemical removers, did not allow a complete removal of the paint. The examined systems behaved against graffiti in different ways. No affinity between the wax-based product and the paint was observed; nevertheless, this behavior did not result in good anti-graffiti performances. On the contrary, the penetration of the paint into the fluorine-based coating yielded a good anti-graffiti effectiveness, since the stain was easily eliminated from the surfaces. The anti-graffiti coatings survived in limited areas after the cleaning processes, although the studied compounds are suggested as sacrificial products. Such behavior may affect the maintenance activities, when the surface is no longer protected and the coating need to be renewed, since compatibility problems, as well as harmful accumulation, could occur because of further treatments on these surfaces. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The term “graffiti” means any unwanted painting, drawing, lettering or scribble painted, written or carved on a surface. In recent years, graffiti, especially spray paintings, have been regarded as a form of art and they are becoming increasingly frequent. However, when graffiti are performed on cultural heritage buildings, they still remain a vandalism. The impact of graffiti on artifacts of historical and artistic relevance is heavy in social and economic terms, because of degradation and devaluation of the affected areas, as well as of high cleaning costs. Graffiti removal is usually carried out by cleaning methods based on abrasive mechanical action or chemical substances [1–3]. These procedures may cause severe damage, therefore they should be avoided on historic surfaces. In such cases, preventive actions are to be preferred. Anti-graffiti measures consist in applying protective coatings on the surfaces to preserve. The final topcoat should prevent the contact between the staining agents and the substrate and reduce

∗ Corresponding author. Tel.: +39 0832 422 219; fax: +39 0832 422 225. E-mail addresses: [email protected] (M. Lettieri), [email protected] (M. Masieri). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.

the adhesion of dirt. To this aim, products with a low surface free energy, thus a reduced surface wettability, are generally chosen [4–8]. The production of such products is fast developing and sacrificial or permanent anti-graffiti systems are tailored. The sacrificial products are removed during the cleaning process and they need to be renewed; the permanent systems, on the other hand, can withstand repeated cleaning cycles. Formulations for application to stone materials have been based on waxes [9], polysaccharides [9], polyurethanes [10,11], silicon resins [9,12,13], and fluorinated polymers [5,9,12,14–16]; recently, also organic–inorganic hybrid materials have been proposed as anti-graffiti systems [17–19]. As requested to any other protective treatment for stone materials, the anti-graffiti products should not alter the appearance of the treated surfaces or hinder water vapor exchanges between the substrate and the environment, at least. They should impart water-repellency and oleophobicity to the treated surfaces, to avoid or limit penetration of the stain, and, in addition, they should not deteriorate under environmental conditions [20]. Finally, the employment of environment-friendly products, safe for both users and the environment, is increasingly encouraged. The effects of anti-graffiti systems have been sufficiently investigated on compact stone substrates [3,21–28], whereas only few studies in literature are devoted to stone materials with a high

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porosity [24,29–31]. In addition, most of the published papers deal with products’ properties and procedures for stain removal [4,22,32–37], while the anti-graffiti efficacy is not discussed in depth. Porous stone surfaces affected by graffiti may be very difficult to clean, because these substrates can absorb the staining agents, sometimes even in large amounts [31]. Moreover, an incomplete elimination of the protective coating – especially in the case of sacrificial products – may affect the surface’s properties and the maintenance arrangements. Although such aspects can be particularly significant for porous substrates, once again, little attention has been paid to these materials. Starting from these issues, an experimental work aimed at improving knowledge about performances and behavior of antigraffiti systems on highly porous stone materials, was undertaken. Two commercial sacrificial anti-graffiti systems were selected. The first is based on polymer waxes to use directly on stone surfaces; the second is composed of waxes and acrylic-fluorinated resins to apply on surfaces previously treated with a proper primer (based on acrylic-fluorinated copolymers). These anti-graffiti systems were chosen on account of the limited impact on both the environment and the stone materials expected from their use. All the products analyzed in this work are provided, in fact, as water emulsion. Furthermore, graffiti removal by means of hot water is suggested in the technical sheets, while chemical removers are recommended only to enhance unsatisfactory results of the cleaning process. In an early study [38] these anti-graffiti products were investigated to evaluate harmlessness on “Lecce stone”, a calcarenite characterized by a very high porosity (above 30%). Negligible variations in esthetic properties, tolerable reductions in water vapor permeability and improved superficial hydrophobicity of the treated samples, were found. The studied products were, consequently, assumed as good candidates for anti-graffiti treatments of this highly porous stone. The present paper focuses on the effects of a staining action, performed with a dye spray paint, and the outcome of the subsequent cleaning process. Firstly, both the anti-graffiti products and the paint were subjected to a thermal and spectroscopic characterization by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR), respectively. In a second stage of the study, the stone surfaces were investigated before and after the anti-graffiti treatments; to this aim, ATR-FTIR analyses, colorimetric tests, and water static contact angle measurements, were performed. The same investigations were repeated after both the staining and the cleaning processes. To simulate graffiti, an aerosol spray paint was applied, because this is the most used staining agent in real practice. The appearance of the surfaces was also inspected with a stereomicroscope.


sheet, the removal of graffiti drawings from surfaces treated with AG1 can be performed using hot water (at about 80 ◦ C); a commercial remover, hereinafter called R1, may help to eliminate residual stains. R1 product (Mapei S.p.A., Italy) is a glycol ether-based solution for the cleaning of graffiti-damaged surfaces. It is a ready-to-use gelatinous product that can be applied by brush. It acts within 10 min, then, it can be easily removed with water. AG2a product (GEAL – Bel Chimica Srl, Italy) is based on acrylicfluorinated copolymers in water emulsion. It can be used, as supplied, on natural and artificial stone materials, employing from 125 to 170 ml/m2 . Its application as a primer is strongly recommended in order to facilitate the adhesion of the anti-graffiti product (AG2b, described below) to the surface. AG2b product (GEAL – Bel Chimica Srl, Italy) is based on polymer waxes added to acrylic-fluorinated resins in water emulsion. Application of 65–125 ml/m2 , without any dilution, on stone surfaces previously treated with the AG2a product, is suggested. The suppliers advise to clean the surfaces protected with the AG2 system (that is, the AG2a primer and the AG2b protective) using hot water at about 80 ◦ C and a stiff-bristled brush first. In case of inadequate cleaning results, they recommend a special commercial remover (R2), illustrated in the next point. R2 product (GEAL – Bel Chimica Srl, Italy) is a mixture of surfactants and solvents specifically tailored for the cleaning of stained surfaces protected with the AG2 system. This remover is ready-touse and it can be applied by either brush or spraying. A few minutes after the application, the remover has to be emulsified with water; then, it can be removed with a stiff-bristled brush, rinsing with water. The staining agent used in this study is a commercial aerosol spray paint (Briolux Spray by CP Italia), orange-colored, provided in a pressurized can. A preliminary selection was performed and the chosen paint was preferred because it was easily detected by infrared spectroscopy; additionally, it was clearly distinguishable from both the stone support and the anti-graffiti products. For commercial reasons, additional details on the chemical composition and the structure of both the anti-graffiti systems and the spray paint are not available. The anti-graffiti systems were tested on a highly porous calcarenite, named “Lecce stone”. This stone is widely employed since long time as construction material in the southeastern Italy and it is typical of the baroque monuments and buildings of the region. In addition, it can be considered as representative of the porous materials used for historic and civil buildings in many countries of the Mediterranean basin. Calcite is its principal constituent, along with very small quantities of clay and other non-carbonate minerals [39–41]. “Lecce stone” exhibits a very high porosity, usually ranging from 30 to 45% [29,42–45] and a unimodal porosity distribution with a radius mainly between 0.5 and 4 ␮m [39,46]. 2.2. Preparation of samples

2. Experimental 2.1. Materials Two commercial anti-graffiti systems were studied. Both are suggested for reversible protection of porous and non-porous stone materials against spray-painted graffiti. However, the manufacturers strongly recommend preliminary examinations to test the products for each specific application. The following are the details of composition and information about all the used products. AG1 product (Mapei S.p.A., Italy) is a ready-to-use water emulsion of polymer waxes. It can be directly applied on clean and dry stone surfaces in quantities ranging from 30 to 150 g/m2 , depending on the porosity of the substrate. As reported in the technical

The anti-graffiti products were separately applied by casting on glass slides, acting as an inert substrate, in order to characterize the materials. Similarly, two coats of paint were sprayed on the glass support. stone specimens with dimensions of Prismatic 5 cm × 5 cm × 1 cm were cut from a quarry block. Prior to use, the samples were smoothed with abrasive paper (180-grit silicon carbide), cleaned with a soft brush and washed with deionized water in order to remove dust deposits, according to UNI10921 standard protocol [47]. The stone specimens were completely dried by a cyclical procedure: 22 h in oven at 60 ◦ C, followed by 2 h in a desiccator with silica gel (relative humidity R.H. = 10 ± 5%) at room temperature. The samples were weighted at the end of


M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Table 1 Procedures performed on the stone samples. Samples

Treatment Protection



Control 1

Water at 60 ◦ C + R1 cleaner

Control 2


AG1 anti-graffiti (100 g/m2 by brush) AG2a primer (140 g/m2 by brush)

Acrylic paint (2 coats by spraying) Acrylic paint (2 coats by spraying) Acrylic paint (2 coats by spraying) Acrylic paint (2 coats by spraying)

Water at 60 ◦ C + R2 cleaner


AG2b anti-graffiti (80 g/m2 by brush)

each cycle. It was assumed that the dry weight was achieved when two consecutive weight measures differed no more than 0.1%. The samples were stored in a desiccator with silica gel (R.H. = 15%) at 23 ± 2 ◦ C. The procedures carried out on the stone samples are summarized in Table 1. 2.2.1. Protective treatments Before the application of the anti-graffiti products, the stone specimens were kept for 24 h in laboratory conditions (at 23 ± 2 ◦ C and 50 ± 5% R.H.). The treatments were carried out following the recommendations reported in the technical sheets. The appropriate quantity of anti-graffiti product to apply was controlled by weight measurements. Each product was applied by brush to sets of five stone specimens; only one 5 cm × 5 cm side was treated. The brushing procedure was preferred because this is the most common method used in real working conditions. The treatment with AG2b was performed 24 h after the application of AG2a. At the end of the treatments, all the samples were kept in the laboratory for 8 months. During this period, the average values of temperature and humidity were 25 ± 2 ◦ C and 45 ± 5% R.H., respectively. 2.2.2. Staining method Staining with the spray paint was carried out on protected stone samples kept for 24 h in the laboratory conditions (23 ± 2 ◦ C and 50 ± 5% R.H.). Two coats of paint were sprayed on specimens placed on a 45◦ tilted surface. The distance between the sample surface and the nebulizer was about 15 cm. In order to avoid stain deposition on the lateral sides of the specimens, these areas were protected with a PET film. The same staining procedure was performed on 10 untreated stone specimens to use as control samples for comparison purposes. After the application of the paint, the samples were stored for 3 days in the laboratory at 23 ± 2 ◦ C and 50 ± 5% R.H. In order to determine the amount of paint applied on the stone samples, the specimens were dried in oven at 40 ◦ C, using the cyclical procedure previously described, until dry weight was achieved. A temperature lower than that used for drying the untreated samples, was set, because at higher temperatures the anti-graffiti products could begin to melt, as resulted from the DSC analyses described in the discussion of results (see Section 3.1). The amounts of paint sprayed on the stained stone samples were evaluated by weight measurements. 2.2.3. Cleaning procedures The cleaning of the stained samples was performed 15 days after the application of the paint. Following the instructions given by the suppliers, all the stone surfaces were cleaned under tap water at 60 ◦ C using a soft brush

Water at 60 ◦ C + R2 cleaner Water at 60 ◦ C + R1 cleaner

first. Actually, this procedure was totally ineffective. Consequently, the recommended chemical removers were applied. R1 was employed to clean the AG1 samples and five control samples (i.e., the “control 1” samples); the AG2 samples and the remaining five control samples (i.e., the “control 2” samples) were cleaned with R2. The removers were applied by brush and removed after 15 min using running tap water at 60 ◦ C and the brush. The cleaning procedures were repeated twice; the samples were, then, dried and stored for 10 days in the laboratory environment (at 23 ± 2 ◦ C and 50 ± 5% R.H.). 2.3. Analytical techniques and measurements The characterization of both the anti-graffiti products and the paint applied on glass was performed by means of Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). A Thermo-Nicolet Continuum IR microscope was used to collect the FTIR spectra in attenuated total reflectance (ATR) mode with a Si crystal (refractive index = 3.4; incident angle = 45◦ ). Each spectrum was acquired in the range of 4000–650 cm−1 , with a resolution of 4 cm−1 and 200 scans for each measurement. In order to minimize the signals due to atmospheric carbon dioxide and water vapor, a background spectrum was acquired in air before each analysis and it was automatically subtracted (by software) from the sample spectrum. However, since the employed instrument was not purged with dry and CO2 -free air, the absorption bands of both CO2 (appearing as a doublet around 2340 cm−1 ) and water vapor (resulting in sharp and very close peaks over 3700 cm−1 ) are still present in the reported spectra. Five spectra were acquired on the treated glass slides and on the untreated stone specimens. Nine points of acquisition were investigated on each treated stone sample and the results are reported as the average of 45 spectra, unless otherwise specified. To ensure reproducibility and uniformity, the contact between the ATR crystal and the sample surface was automated and computer controlled. The ATR-FTIR data were processed with the OMNIC 8.1 software (Thermo Fisher Scientific Inc). The DSC analyses were performed carrying out thermal dynamic scans between 10 ◦ C and 100 ◦ C, with a heating rate of 10 ◦ C/min, under nitrogen atmosphere. A DSC 822 Mettler calorimeter was used and the thermal properties (glass transition temperature and peak temperature) were calculated as the average of three measurements. Since the thermal characterization of the products was aimed at selecting the appropriate water temperature to use when cleaning the surfaces, a limited range of temperature was examined. During both the preparation of the specimens and the treatments, the environmental conditions were monitored by means of a humidity–temperature logger. A Digitron MonoLog (Mod. MLH) was used to this aim. It can collect temperature data in a range from −20 ◦ C to 50 ◦ C (with an accuracy of ±2 ◦ C) and relative humidity data in a range from 5% to 100% (with a precision of ± 5%).

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Weight measurements were registered using an analytical balance (Sartorius, Model BP 2215) with an accuracy of ±0.1 mg. Before and after each protective treatment, FTIR analyses, as well as color and water-stone static contact angle measurements, were performed on the stone surfaces. The same investigations were repeated after both the staining and the cleaning processes. In addition, the stained and the cleaned surfaces were examined under a stereomicroscope. Color measurements [48] were performed with a Minolta Chroma Meter CR300 tristimulus colorimeter, using CIE Standard illuminant C. Ten measurements were performed on each specimen and the colorimeter was recalibrated to a calibration plate at ∗ ) the start of each measurement session. The color changes (Eab were calculated through the L*a*b* (CIE 1976) [49] system, using the following formula: 2 1/2

∗ Eab = [(L∗ ) + (a∗ ) + (b∗ ) ] 2


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). Water-stone static contact angle measurements [50] were carried out using a Costech apparatus. For each specimen, 30 microdrops of deionized water were deposited with a syringe on different positions of the surface. The shape of the drop was recorded with a camera and its contact angle was calculated by means of the “anglometer 2.0” software (Costech). The reported data are the averaged values measured on five specimens. Our attention was mainly devoted to changes occurred after the cleaning procedure rather than to the hydrophobic features of the surfaces. This evaluation, in fact, is expected to give information about the permanence of the applied products (both anti-graffiti and paint) on the stone surface, because the contact angle cannot be measured on the untreated Lecce stone [44]. The visual inspection of the surfaces was performed through a binocular stereomicroscope (Zeiss, mod. Stemi SV11) at magnifications of up to 100×. 3. Results and discussion 3.1. Characterization of the materials As previously stated, the stone material used in this study has a high content in calcium carbonate (above 90% [42]). In the ATRFTIR spectrum (Fig. 1) acquired on the untreated stone specimens,

Fig. 1. ATR-FTIR spectrum of the untreated stone surfaces.

Fig. 2. ATR-FTIR spectrum of the AG1 coating obtained by casting on a glass slide.

in fact, the bands at 1398, 872, and 713 cm−1 , ascribable to calcite [51], were the main peaks; the weaker signal centered around 1042 cm−1 was due to a certain amount of silicate minerals. The ATR-FTIR spectrum of the AG1 product is depicted in Fig. 2. Very strong peaks appeared in the range of 2840–2920 cm−1 , due to the stretching vibrations of aliphatic chains, typical of synthetic polymer waxes [51,52]. An additional strong and sharp peak, located at 1109 cm−1 , could be related to the presence of C–O–C groups in the polymer structure [52]. These signals are not coincident with those of the stone; the AG1 product can be, therefore, easily detected when applied on the stone surface. Regarding to the AG2 treatment, either AG2a or AG2b, separately spread on the glass slides, were analyzed by ATR-FTIR. The obtained spectra are reported in Fig. 3. In both cases, dominant IR bands in the range of 1000–1300 cm−1 arose from the different deformation modes of C F bonds [52–54], while the peak at 1735 cm−1 was due to the acrylic component [52]; however, both compounds exhibited well different profiles in the fingerprint region (between 650 and 1800 cm−1 ). Moreover, additional strong absorptions in the range of 2840–2920 cm−1 , due to the vibrations of aliphatic chains of the wax component, were recorded in the AG2b spectrum. Finally,

Fig. 3. ATR-FTIR spectra of the AG2 coatings obtained by casting on a glass slide.


M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Fig. 4. ATR-FTIR spectrum of the paint sprayed on a glass slide.

neither AG2a nor AG2b showed FTIR bands overlapping the signals of the stone. Therefore, by using ATR-FTIR analysis, these products were not difficult to distinguish from each other, as well as from the stone support. The FTIR spectrum acquired on the paint sprayed onto the glass support is reported in Fig. 4. This FTIR profile matched the standard reference spectrum (HR Nicolet Sampler Library, Thermo Fisher Scientific Inc.) of a methyl-methacrylate resin. The peaks at 1729, 1452, and 1147 cm−1 were chosen to detect the paint on the stained stone samples. A thermal characterization of both the anti-graffiti products and the spray paint was performed in order to select the appropriate water temperature to use when cleaning the stained surfaces. The DSC traces of the anti-graffiti products are drawn in Fig. 5. The thermal analysis of AG1 revealed a glass transition at 41 ± 2 ◦ C and a melting process ending around 83 ± 2 ◦ C (peak centered at 65 ± 4 ◦ C). The calorimetric investigation on AG2a evidenced a glass transition at 41 ± 2 ◦ C; a melting peak, centered at 65 ± 4 ◦ C and closed at 83 ± 2 ◦ C, was also observed. From the DSC experiments performed on the AG2b product, a glass transition temperature (Tg ) of 35 ± 1 ◦ C was measured; a melting process was experienced in the analyzed range of temperature, as proved by the presence of a peak with the maximum at 58 ± 1 ◦ C and closing at 72 ± 2 ◦ C.

Fig. 5. DSC traces of AG1, AG2a, and AG2b coatings obtained by casting on a glass slide.

Fig. 6. ATR-FTIR spectrum of the stone surfaces protected with AG1 anti-graffiti system.

The suggested cleaning at 80 ◦ C would be able to completely melt the coatings, thus facilitating stain removal by washing. However, products in the liquid form have an increased mobility that could promote their penetration into the stone, especially in the case of highly porous materials. This effect can be very harmful in the cleaning of stained surfaces, where the dirt as well can move inside the stone, making more difficult the elimination. Taking into account these data and the above mentioned considerations, water at 60 ◦ C was used during the cleaning procedure. In this condition, only a limited amount of the applied product can melt. In addition, the melting process likely affects only the most external layer of the protective coating, which can be subsequently washed away along with the staining materials. In samples treated with the AG2 system, the primer coating (i.e., the AG2a product), applied directly on the surface, can hinder penetration within the stone of both the protective product and the dirt. Finally, throughout the DSC scan from 10 ◦ C to 100 ◦ C performed on the spray paint, no thermal transition was observed; on the other hand, methyl-methacrylate polymers usually exhibit Tg values above 100 ◦ C [55], that is, over the investigated range of temperatures. 3.2. Protected surfaces The ATR-FTIR spectra of AG1 on stone (Fig. 6) almost exactly matched those obtained for the AG1 product alone (Fig. 2). All the spectra acquired on the AG1 samples exhibited a similar profile. The very weak signals at 1412 and 873 cm−1 were ascribed to calcite of the stone. The presence of a polymer coating on the stone surface, able to almost completely hide the substrate to the spectroscopic detection, can explain this result. In a previous study [38], observations of treated stone surfaces through ESEM (Environmental Scanning Electron Microscope) evidenced a superficial coating having an average thickness of 5 ␮m; on the same samples, calcium at the surface, measured by EDX (Energy Dispersive X-Ray) microanalyses and identified as arising only from the stone support, drastically decreased in content as a consequence of the polymer coating on the stone. These findings suggest that the applied product mainly remained onto the treated surface and a superficial sacrificial layer was realized. This latter, working as a barrier, was able to isolate the stone material from usual staining products. The ATR-FTIR analyses performed on the AG2 samples after the protective treatment are illustrated in Fig. 7.

M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477


Table 3 Amounts (g/m2 ) of paint sprayed on the stone surfaces in two consecutive coats. Samples

Deposited paint (g/m2 )

Control 1 Control 2 AG1 AG2

51 50 51 52

± ± ± ±

7 9 6 5

3.3. Stained surfaces

Fig. 7. ATR-FTIR spectrum of the stone surfaces protected with AG2 anti-graffiti system.

The AG2a primer was easily identified, because peaks at 1735, 1204, and 1148 cm−1 were detected, even if the bands due to the stone (1398, 872, and 712 cm−1 ) were the strongest signals. All the collected spectra were comparable in shape and intensity; therefore, it seems reasonable to assume that a uniform and continuous coating, without the formation of a thick layer, was realized on the surface. When the anti-graffiti AG2b was applied on these samples, the signals related to this product were recognized. Bands at 1085, 1039, 2919, and 2848 cm−1 appeared and the peak at 1735 cm−1 increased in intensity. At the same time, because of the additional material on the surface (i.e., the AG2b product), the bands due to both AG2a and stone lessened in height. Nevertheless, differently from the results obtained on AG1, the bands due to the stone were still the main peaks. Hence, it can be stated that the AG2 treatment just created a thin coating on the stone surface. After the AG2 treatment, in fact, the EDS analyses, carried out in our previous study [38], revealed only a slight reduction in calcium content at the surface, as a consequence of an anti-graffiti coating of low thickness. In addition, since all the spectra collected on the AG2 samples were very similar, the formation of a homogeneous polymer coating, without areas exposed to the dirty agents, can be assumed. In Table 2, the water-stone contact angles measured on the treated surfaces are listed. Both AG1 and AG2 were able to impart hydrophobicity to the stone, as already reported in the previous paper [38]. The AG1 surfaces were less hydrophobic, even if this result was unexpected, since the presence of fluorine in the polymer composition should give rise to a lower wettability of the treated surfaces, that is, a greater water-repellency [53,56].

Table 2 Water contact angle (expressed in degrees) measured on stone surfaces before and after the treatments. For each data set the standard deviation is reported. Samples

Control 1 Control 2 AG1 AG2

Treatments Without treatment



Not determinable Not determinable Not determinable Not determinable

– – 120 ± 6 99 ± 7

123 128 103 101

± ± ± ±

4 12 3 4

Cleaning 84 97 67a 86

± ± ± ±

21 21 30 18

a Value calculated on 116 measurements; the remaining 34 measures were not determinable.

Comparable quantities of stained paint were sprayed on different sets of specimens, as reported in Table 3. Interesting findings were gained by the examination of the stained samples through the stereomicroscope, since the color of the paint was easily detectable and distinguishable from the substrate. The observation of the surfaces revealed dissimilar conditions for each treatment (Fig. 8). On the un-protected control surfaces the paint appeared into the hollows of the stone, while many superficial micro-areas were free from color. On the AG1 stained samples, the paint coated the most part of the surface, but some paler spots were still evident. This effect could indicate a restricted affinity between the paint matrix and the antigraffiti polymer. The protective coating seemed to repel the paint, which was cracked, mainly gathered within the irregularity of the surface and limited to definite points. Although a certain repellency was noticed, the presence of paint inside the pores undoubtedly may render the stain removal very difficult. For the AG2 samples a different behavior was experienced. The stained specimens appeared flat and uniformly covered by the paint. In this case, the effects of repellency were not evident. On the contrary, on the AG2b coating, the absence of any visible uncoated point can be the consequence of a marked wettability by the paint’s components. This feature may have promoted the spreading of the paint on the stone surface. Actually, the presence of fluorine in the composition of the protective (as in AG2 products) was expected to reduce the surface free energy and the wettability of the coating thereof. However, other researchers [5,7,10,24,57,58] found an increased penetration of the graffiti into protective films with high chain flexibility or less molecularly dense, rather than in coatings with increased fluorine content. In the case of AG2b, the Tg (35 ◦ C) close to the ambient temperature accounts for a polymer containing a large free volume: therefore, a certain ability of graffiti to enter the protective coating can be expected. The observed results were supported by the colorimetric data, summarized in Table 4. As discussed in the previous study [38], the investigated antigraffiti treatments caused just negligible color variations on the stone. After the paint application, the color parameters dramat∗ values were calculated. In ically changed and very high Eab examining the effect of the staining procedure on AG1, AG2, and unprotected surfaces, comparable L* values were measured on all the samples, while dissimilar results were found for a* and b*. The AG2 stained samples showed the highest a* and b* values; the limited deviations of these data indicated a homogeneous surface covering. The colorimetric parameters measured on the control and the AG1 stained samples were lower and more scattered, likely as a consequence of the light areas observed to the visual inspection. In Fig. 9, the ATR-FTIR spectra acquired on the stained stone samples are reported. The spectroscopic analyses revealed that the paint masked the underlying surface. In all the cases, the peaks at 1728, 1452, and 1146 cm−1 , due to the paint coating, were very strong and


M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Fig. 8. Images of the stained surfaces taken by means of the stereomicroscope: comparison between the unprotected surfaces (control samples) and the specimens previously treated with the anti-graffiti systems (AG1 and AG2 samples).

dominant. In particular, only these bands were detected on the AG1 stained surfaces. The signals of the paint were strong on the unprotected (control) stained surfaces as well, even if, in this latter case, also the signals due to the calcite of the stone exhibited high intensity. Intermediate behavior was noticed on the AG2 stained samples: in the spectra acquired on these surfaces the signals of the paint were the strongest peaks; also the bands due to the stone were detected, but in a very weak intensity. As can be seen from the inset box in Fig. 9, where the range between 850 and 900 cm−1 is expanded and compared, the peak at 872 cm−1 (just due to calcite of the stone) was strong in the spectrum of the stained control specimens, it exhibited reduced height and area on the AG2 samples, while it was absent in the case of the AG1 surfaces. These different results can be ascribed to a deeper penetration of the paint into the porous structure of the unprotected stone. On the contrary, where the anti-graffiti products were applied, the staining agent was confined to the surface and just the limited thickness of the AG2 protective coating made it possible to detect the underlying stone substrate.

The results of the contact angle measurements after the staining are reported in Table 2. The stained surfaces of all the control samples exhibited an elevated hydrophobicity, because the waterstone contact angles were higher than 120◦ . On the other hand, a strong reduction of the contact angle was measured on the AG1 stained specimens. Finally, the values obtained on the AG2 stained surfaces remained unchanged if compared to those measured before the paint application. In the light of these findings, dissimilar interactions between the paint and the differently treated surfaces can be inferred. 3.4. Cleaned surfaces The visual inspection, to the naked eye, of the surfaces subjected to the cleaning procedure already revealed an incomplete removal of the paint. A comparison of the cleaned surfaces observed under the stereomicroscope is shown in Fig. 10. In Fig. 11(a)–(c), the evolution of the color parameters as a consequence of the treatments is reported.

Table 4 ∗ ) determined before and after the treatments. For each data set the standard deviation is reported. Colorimetric coordinates (L*, a* and b*) and global color difference (Eab Color data


Treatment Without treatment





Control 1 Control 2 AG1 AG2

81.92 ± 0.34 81.71 ± 0.41 82.00 ± 0.96 81.82 ± 0.45

– – 81.54 ± 0.95 81.90 ± 0.20

54.64 ± 1.18 54.71 ± 1.21 55.66 ± 1.13 55.41 ± 0.58

57.60 ± 1.66 63.83 ± 1.50 69.79 ± 2.85 79.13 ± 1.56


Control 1 Control 2 AG1 AG2

0.80 ± 0.13 0.71 ± 0.20 0.96 ± 0.19 0.78 ± 0.19

– – 0.95 ± 0.14 0.87 ± 0.19

41.28 ± 1.22 40.17 ± 1.21 38.29 ± 2.47 46.52 ± 0.60

21.51 ± 1.57 29.82 ± 2.73 16.09 ± 3.85 3.87 ± 1.88


Control 1 Control 2 AG1 AG2

15.87 ± 0.53 15.96 ± 0.63 15.50 ± 0.93 15.77 ± 0.68

– – 16.41 ± 1.27 15.52 ± 0.26

42.87 ± 0.41 42.88 ± 0.52 42.47 ± 0.68 45.45 ± 0.28

36.80 ± 1.74 37.63 ± 2.63 29.54 ± 3.44 20.04 ± 2.20

∗ Eab

Control 1 Control 2 AG1 AG2

55.78 54.87 53.05 60.58

40.53 38.06 23.98 5.92

– – – –

– – 1.02 0.28

M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Fig. 9. ATR-FTIR spectra of the stone surfaces sprayed with the paint. In the inset box, the range between 850 and 900 cm−1 is expanded and compared.

The elimination of the staining agent was wholly ineffective on the unprotected stone samples. The color parameters of the con∗ trol surfaces, in fact, changed slightly after the cleaning and Eab remained high (Fig. 11(a) and Table 4). Small variations can be ascribed to an abrasion of the paint coating, mainly due to the mechanical action performed during the cleaning procedure. As already stated, the paint easily penetrated inside the unprotected stone, thus its removal was strongly restricted. A more marked, although still partial, elimination of the paint was achieved on the AG1 samples. In this case, the color parameters turned toward the values of the untreated stone, but never reaching them (Fig. 11(b) and Table 4). The cleaning was more successful on the samples protected with the AG2 system, where only sporadic points with color were observed (Fig. 10). On the other hand, chromatic values very close to those measured on the samples before the treatments, were measured (Fig. 11(c) and Table 4).


Following a classification reported by other researchers [20], ∗ values are lower than 5, the color changes cannot when the Eab ∗ values increase up to be perceived by a human eye; as the Eab 10, the color variations can be seen but they are still tolerable; on ∗ values higher than 10 produce clearly visible the other hand, Eab color changes that cannot be accepted. According to these considerations, the colorimetric investigations confirmed a satisfactory cleaning only on the AG2 samples. Fig. 12 illustrates the results of the spectroscopic analyses carried out on the cleaned control samples. No significant variation in surface composition was observed in comparing the specimens before and after the cleaning procedure: the signals of the stone were the main peaks but the bands due to the paint were still very strong, confirming that the paint was not removed. The ATR-FTIR spectra collected on the cleaned AG1 surfaces are reported in Fig. 13. The results highlighted an inhomogeneous composition of the investigated surfaces. The acquired spectra can be, in fact, classified into three different groups: (1) spectra where only the calcite signals, due to the stone support, were identified; (2) spectra where weak bands (at 2921 and 2848 cm−1 ) related to the AG1 product were detected together with very strong peaks of the calcite; and (3) spectra coincident with those of the paint alone, as found on the un-cleaned surfaces. It is to point out that more than half of the acquired spectra (i.e., 24 of 45) fell into the third group, proving an incomplete removal of the paint in large areas. The detection in some points of signals related to the anti-graffiti should be focus of attention. This finding showed that the protective coating may be not completely removed from the surfaces, although the anti-graffiti product is suggested as a sacrificial layer: thus, problems may arise if maintenance activities are required. Referring to the AG2 cleaned samples, all the ATR-FTIR spectra obtained on these surfaces were identical. In these spectra, averaged in Fig. 14, the absorption bands of calcite were strong in intensity, while no signal due to the paint was found. Isolated spots of color were, in fact, observed only into the hollows of the surface (see Fig. 10), where the contact between the surface and the ATR crystal was precluded. As a consequence, the very limited amount of paint still remaining on the samples

Fig. 10. Images of the cleaned surfaces taken by means of the stereomicroscope: comparison between the unprotected surfaces (control 1 and control 2 samples) and the specimens previously treated with the anti-graffiti systems (AG1 and AG2 samples).


M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Fig. 11. L*–a*–b* graphs representing original color data and color data after the staining and after the cleaning procedures for (a) control samples, (b) AG1 samples, and (c) AG2 samples.

Fig. 12. ATR-FTIR spectra acquired on the control samples after the cleaning procedure.

cannot be revealed. In the spectra collected on the AG2 specimens, very weak peaks at 2919, 2850, 1735, 1150, and 1040 cm−1 , due to the anti-graffiti coating, were detected. Therefore, once again the possibility of an incomplete removal of the anti-graffiti products should be taken into account in further treatments of the surfaces. These findings were in agreement with the results of the contact angle measurements. A reduction of the water-stone contact angle values was found on all the cleaned specimens (Table 2), supporting the perception that the paint was removed only to a limited degree. In particular, the lowest values were found on the AG1 samples. Here, a significant number of measurements (around 23%) cannot be acquired, because the water drop was suddenly absorbed into the stone. This behavior was the consequence of areas where the stone was totally uncovered, as already detected by the FTIR analyses. Values similar to those measured before the staining procedure were found on the AG2 cleaned samples, confirming the presence of the protective layer on the surface. Finally, high standard deviations account for a non-homogeneous composition of the cleaned surface.

M. Lettieri, M. Masieri / Applied Surface Science 288 (2014) 466–477

Fig. 13. ATR-FTIR spectra acquired on the AG1 samples after the cleaning procedure. Group 1 is the average of 10 spectra; group 2 is the average of 11 spectra; group 3 is the average of 24 spectra.

Fig. 14. ATR-FTIR spectrum acquired on the AG2 samples after the cleaning procedure.

4. Conclusions In the present study, the behavior of two commercial sacrificial anti-graffiti systems on a highly porous stone was analyzed and discussed. The penetration of the staining agent into the pores of the substrate, as a consequence of the high porosity of the stone, complicated the graffiti removal. The application of the protective coatings allowed to improve the removal of an acrylic paint sprayed on the surfaces. In fact, on control samples, where no treatment was performed, the cleaning was unsuccessful; in such cases, neither mechanical action nor chemical removers were effective in eliminating the paint from the stone. The experimental results suggested that the investigated systems behaved against graffiti differently. The wax-based product (i.e., the AG1 product) repelled the sprayed paint, but the limited wettability did not result in good anti-graffiti properties, since the stain mainly remained within the irregularity of the substrate and color was still clearly visible after repeated cleaning cycles. On the other hand, the paint sprayed on the fluorine-based coating (i.e., the coating realized using the AG2 system) was almost completely


removed after the cleaning. The penetration of the graffiti into the protective coating can occur. This effect can be due to the high free volume fraction of the polymer film, consequence of Tg values close to the ambient temperature. Although the stain penetration is usually undesirable, in the studied case a very positive effect on the anti-graffiti effectiveness was obtained because the stain incorporated into the coating was easily eliminated from the surfaces. It should be taken into account, however, that the application of a primer coating (i.e., the AG2a coating) avoided the penetration of the stain within the porous structure of the stone, making possible a more effective cleaning. The feasibility of cleaning with hot water, as suggested by the manufacturers, was strongly precluded. The thermal analysis of the anti-graffiti revealed that the melting process of the used products completes around 80 ◦ C. As a consequence, an increase of temperature of the coating (e.g. using hot water) could be able to melt the applied products which can be potentially removed along with the stain. However, two fundamental aspects, that should be taken into consideration, emerged from the experimental results. First of all, only the sprayed paint was found as a coating on all the stained surfaces: the melting process, at initial stages merely involves the superficial layer, therefore, it can barely affect the underlying protective film. On the other hand, the acrylic paint selected for this study does not melt or soften at temperatures below 100 ◦ C. Secondly, especially in the case of highly porous materials, the coating in a liquid form penetrates more easily within the stone, together with dirt and/or stain, where they exist. This effect is detrimental in cleaning the stained surfaces, because the dirty agents become extremely difficult to eliminate. The application of a primer coating (as in the case of the AG2 system) directly on the stone surface can help to hinder penetration of both the protective product and the dirt within the stone. Finally, it is worth remarking that the spectroscopic investigations performed on the cleaned samples detected traces of the protective products. In addition, in most of the cases, the values of contact angle decreased but remained measurable, where they cannot be acquired before the treatments. Therefore, the anti-graffiti products still existed in limited areas of the surfaces after the cleaning procedure, although the studied compounds are suggested as sacrificial coatings. These results were likely influenced by penetration of the applied products into the porous substrate. In fact, also repeated cleaning procedures, involving hot water, mechanical action and chemical removers, did not allow the complete removal of both the stain and the protective coatings. The presence of residual anti-graffiti should be taken into account in planning maintenance activities: the stone is no longer protected and the coating need to be renewed but compatibility problems, as well as harmful accumulation, could occur because of further treatments on these surfaces. Acknowledgement The authors thank Prof. Mariaenrica Frigione (Department of Engineering for Innovation, University of Salento, Lecce) for granting access to the Differential Scanning Calorimeter used for thermal analysis. References [1] M.J. Whitford, Getting Rid of Graffiti. A Practical Guide to Graffiti Removal and Anti-graffiti Protection, E & FN Spon, London, 1992. [2] S. Goidanich, L.a. Toniolo, S. Jafarzadeh, I. Odnevall Wallinder, Effects of waxbased anti-graffiti on copper patina composition and dissolution during four years of outdoor urban exposure, J. Cult. Herit. 11 (2010) 288–296. [3] A. Gardei, O. Garcia, M. Riedl, I. Vanhellemond, J. Strupi Suput, M.L. Santarelli, Performance and durability of a new anti-graffiti system for cultural heritage – The EC Project GRAFFITAGE, in: J.W. Łukaszewicz, P. Niemcewicz (Eds.), 11th














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