Assessment of plasma torches as innovative tool for cleaning of historical stone materials

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Original article

Assessment of plasma torches as innovative tool for cleaning of historical stone materials Stefano Voltolina a,∗∗ , Luca Nodari b,c,∗ , Cristina Aibéo d , Ellen Egel d , Marisa Pamplona e , Stefan Simon f , Emanuele Verga Falzacappa g , Paolo Scopece g , Arianna Gambirasi b , Monica Favaro b , Alessandro Patelli h a

Veneto Nanotech scpa, Via delle Industrie 5, 30175 Venezia, Italy Institute for Energetics and interphases, CNR, Corso Stati Uniti 4, 35127 Padova, Italy c INSTM Padova Research Unit, Italy d Rathgen-Forschungslabor–Staatliche Museen zu Berlin, Schloßstrasse 1A, 14059 Berlin, Germany e Deutsches Museum, Museumsinsel 1, 80538 München, Germany f Institute for the Preservation of Cultural Heritage Center for Conservation & Preservation, Yale University, USA g Nadir srl, via F. Zugno 9, 35134 Padova, Italy h Dipartimento di Fisica, Universita’ degli Studi di Padova, via Marzolo 8, 35121 Padova, Italy b

a r t i c l e

i n f o

Article history: Received 7 September 2015 Accepted 2 May 2016 Available online xxx Keywords: Atmospheric plasma Historical stone cleaning Epoxy Acryl and siloxane coatings Infrared spectroscopy

a b s t r a c t Cleaning of historical stone surfaces has always been a challenging task, moreover in the last decades arose new restorations issues such as the need to remove aged conservation polymeric materials to avoid further damage. Different cleaning methodologies flourished in the past, mostly based on chemical, mechanical methods and on laser technology too. Nevertheless, these methodologies could not be so efficient in the removal of epoxy resins, acrylic polymers and hydrophobic siloxanes, because of their low solubility in solvents when aged or their high adhesion with the substrate. More recently, atmospheric plasma has been tested for such application even if it is not yet widely applied due to the lack of knowledge about possible side-effects on the artefacts. In the present work, assessment of three commercial atmospheric plasma devices (plasma torches) illustrated the potentialities and drawbacks of polymers’ removal from stone surface. Commercial epoxy resins, acrylic polymers and hydrophobic siloxanes were chosen for the removal test by plasma devices. Physical and chemical effects on the stone surface and the process efficiency were investigated by means of macro- and microscopic observations, preferring, when possible, non-invasive techniques and consolidated methodologies in the field of Stone Conservation Science. An introductory experimentation on coated Si specimen has allowed to find the proper working parameters, i.e. working distance, exposure time, to have an effective removal. The experimentation conducted on different lithic substrate, coated with the commercial protective, has showed that commercial devices are effective in the removal of epoxy and acrylic coatings via chemical and physical interactions. On the contrary, the removal of siloxane products is incomplete, because of the high stability of the bond Si–O in the back bone, which is not affected by the plasma. In general, the present trials highlighted that DBD apparatus used does not promote any macroscopic effects on the polymeric coating, while arc discharge ones guarantee satisfactory results. According to these preliminary trials, it was clearly evidenced that plasma is a potential cleaning tool, despite DBD systems need higher power or arc discharge needs treatment temperature mitigation and to avoid the deposition of metallic drops on the surface of the object due to electrode deterioration. © 2016 Elsevier Masson SAS. All rights reserved.

1. Research aims

∗ Corresponding author. Institute for Energetics and interphases, CNR, Corso Stati Uniti 4, 35127 Padova, Italy. Tel.: +390498295919. ∗∗ Co-corresponding author. E-mail address: [email protected] (L. Nodari).

The atmospheric pressure plasma instruments are already available on the market for several purposes in manufacturing processes, as packaging or automotive industry, for surface cleaning and activation or even in medical applications such as the wounds disinfection. In the field of cultural heritage, successful results were

http://dx.doi.org/10.1016/j.culher.2016.05.001 1296-2074/© 2016 Elsevier Masson SAS. All rights reserved.

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already obtained for organic layer removal with plasma in vacuum chambers, for example for soot removal or disinfection of paper. Few literature sources focus on the use of atmospheric low temperature plasma torches for the removal of organic coatings. This study was carried out with the aim of assessing the use of such type of plasma for the removal of polymers from stone surfaces demonstrating the potentials and the drawbacks of the new method. 2. Introduction Outdoor stone monuments and buildings often undergo deterioration both due to natural and anthropogenic effects. Among all, water is responsible for most of the decay processes as, besides causing erosion by abrasion, it is the carrier of soluble salts (marine or pollutant-derivatives); it enables bio-colonisation, freeze-thaw phenomena or wet-dry expansion [1]. These physical-chemical effects induce erosion, fissuring and cracking, which are the major patterns of a deteriorated stone surface. To prevent the penetration of water and its effects or to reduce the fissures of deteriorated stones, several organic materials have been used. Natural products as linseed oil, bee wax and natural resins among others were mainly applied until the middle of the 20th century. Afterwards, a wide range of commercially available polymers both for consolidation and water proofing purposes were massively used for stone conservation purposes [2–4]. With time, ageing of polymers by exposure to oxygen, pollutants, light and heat causes the loss of their fundamental properties such as transparency, hydrophobicity and adhesion [5,6], requiring in many cases, their removal to ensure a better conservation [7]. Physical and/or chemical methods are usually employed to remove aged coatings. Chemical methods usually involve the use of solvents applied by compresses or poultices that cause swelling or dissolution of the polymers with their consequent dispersion on the surface or in depth migration. Moreover, the solubility of aged polymers could be lower than that of freshly applied ones [8,9]. If chemical cleaning is not effective, physical methods might be also considered. While the use of rotating abrasive discs or brushes is not advisable, because they might damage the original surface, microblasting or cryoblasting are frequently applied in the cleaning of historic architecture [10]. The latter methodologies guarantee quick results, good removal and the possible damage induced on the surface is considerable smaller than with the former ones. Among the physical methods, the laser technology has also been widely applied. The main advantages are the limited invasiveness, high precision and good selectivity on black or dark layers, while a careful selection of experimental conditions has to be made for heat sensitive materials [11–14]. A recent innovation in the field of conservation is the use of plasma, which enables the removal of otherwise intractable materials. Plasma is an ionised gas containing highly reactive species according to the gas used (air, oxygen, hydrogen, etc.) and its current industrial applications rely on chemical and physical etching, coatings deposition or ion implantation, etc. often at room temperature [15–17]. Plasma technique in the field of cultural heritage has been firstly applied to the conservation of metals, particularly archaeological iron artefacts and silver objects in vacuum conditions [18–21], allowing the removal of chlorides and the reduction of silver sulphide corrosion products. Disinfection and consolidation of bio-deteriorated paper [22], soot removal on paintings damaged by smoke [23] and the removal of superficial organic coatings from paintings [24] by means of plasma demonstrated the challenging potentials of this technique. Nevertheless, all these applications were carried out under vacuum conditions, suitable for cleaning small and firm objects, which fit into the vacuum chamber and withstand low pressure. Plasma can also be obtained

at atmospheric pressure, but in this case a high density and low temperatures are difficult to combine. To obtain the best compromise between both features, different ignition mechanisms are used [25]. Few examples of atmospheric plasma as cleaning tool can be found in literature, i.e. the removal of soot from canvas and marble was tested with one of the first patented atmospheric plasma devices (working at atmospheric pressure instead of vacuum conditions) [26]. Later, atmospheric plasma was tested in the activation of polymeric surfaces in modern art to enhance the adhesion between a non-polar polymer substrate and a polar paint layer [27], and finally, the potentials of a corona discharge and a DBD jet plasma device for the removal of polymers and natural varnishes were explored [28] as well as for the treatment of oxidised metal surfaces or altered by the presence of sulphide [29]. In order to assess the applicability of atmospheric plasma as cleaning tool for historical stone surfaces, especially as an alternative or complementary technique to more invasive or low-effective traditional methodologies, different commercial plasma devices were tested for the removal of polymers used as protective coatings for stone. The selection of polymers to be removed relied on their widespread use in the field and their difficult removability with conventional techniques. Taking also into account the conservation restraints, the potentials and limitations of these plasma devices in such an application are reported in details. 3. Materials and methods Different stone substrates and polymeric coatings were used to assess the cleaning performance of commercial plasma torches. The substrates chosen for the present study were Istria limestone, Serena sandstone and Carrara marble. The selection of the stone lithotypes relied on those which were widely used for monuments and sculptures [30–34]. The main petrological and physical features of the stone are reported in Table 1. Carrara marble used for the present tests was thermally aged according to the marmo cotto procedure [35], in order to reproduce as accurately as possible the natural weathering of outdoor marble surfaces. Artificial ageing was not used for Istria and Serena specimens because, as reported in the literature [36], it does not give any satisfactory results. The polymers used for the trials were an epoxy resin, a silicon-base polymer and an acrylic emulsion. These chemicals are used for stone protection (siloxane, acrylics), filling missing parts and adhesive purposes of detached pieces (acrylics and epoxy resins). The tested materials were reported in Table 1. Firstly, trials were performed to test the effect of plasma on sound stone. Subsequently, the products were all applied by brush on one flat side of the stone specimens, sized 5 × 5 × 1 cm and/or round slices 5 × 1 cm, and removal trials were carried out after complete drying of the polymer (constant mass of the samples). Three different commercial atmospheric plasma torches, as listed in Table 2, were tested. When possible, all the apparatuses were tested in similar conditions, i.e. working distance and exposure time. Moreover, in the removal of epoxy and acryl coating, PVATePla was tested by using two tips with different diameter, hereafter defined as standard (hereafter Plasmapen ST) and narrow (hereafter Plasmapen NT) tips. Plasma effects on stone surface and polymer removal efficacy were assessed by complementary techniques: • Optical Microscopy (VHX-500FD from Keyence, Axio Imager A2 m (Zeiss) equipped with a digital camera (ProgRes and Olympus BX51); • FT-IR spectroscopy (Spectrum One from Perkin-Elmer); • External Reflection IR spectroscopy (ER-FTIR, Alpha-R/BC Spectrophotometer from Bruker); • Scanning Electron Microscopy (FEG-ESEM-EDS, FEI Quanta 200F).

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Table 1 Materials used in the experimentation. b*

␪ (◦ )

w (s)

0.7

11.1

63

2241

63.6 90.2

−0.5 −0.5

4.9 3.9

10 23

100 500

Coating and application features

L*

a*

Araldite® AY 103-1/HY 991 (Huntsman International LLC) commercial two components (resin and hardener) epoxy adhesive. The two components were mixed in the ratio 10/5 by volume before application a water emulsion of an ethylacrylate-co-methylmethacrylate polymer supplied by CTS srl. The product was applied without any dilution (Wacker Chemie AG), a solvent-free silicone concentrate based on “alkoxysilane” (Si(OR)4 )/siloxane (R2 SiO-)n mixture. SILRES® BS 280 was diluted in Petroleum Ether 1/11 (w/w) as indicated by the producer

79.1

Substrate

Petrological features

L*

Istria limestone

74.8

Serena sandstone Carrara marble (marmo cotto)

Microcrystalline limestone with microfossils and veins with spathic calcite, quartz, and limonite Hard sandstones with quartz, feldspar, calcite and phylloretin Metamorphosed liimestone with high calcite content and dolomite

Coating Araldite® on Istria

Acril 33 on Serena ®

Silres BS 280 on carrara

a*

b*

␪ (◦ )

w (s)

0.3

12.9

97

2316

58.2

0.4

5.5

71

3900

91.9

−0.3

1.9

135

770

Petrological features, typology of coating and application modality, colorimetric coordinates (CieL*a*b*), contact angle (◦ ) and water micro-drop absorption (w) of the studied lithic substrate. Errors on CieL*a*b*, ␪ and w are on the last digit. Table 2 Description of the commercial plasma torches used and related working conditions. Plasma Torch

KinPen from Neoplas Plasmapen from PVATePla Blaster from Tigres Dr. Gerstenberg GmbH

Characteristics

Gases

Ignition type

Power (W)

Gas flow (L/h)

DBD Arc discharge Arc discharge

8 100 250

180–480 1275 2400

Moreover, physical modifications induced by polymers application and their removability by plasma were assessed by ␮-water drop absorption, w, contact angle, ␪, and colorimetric measurements (CieL*a*b* coordinates), following the standard methodologies for cultural heritage assets [37,38]. Working parameters of the commercial torches, such as the power, the composition and flux of the gas, the distance between the substrate and the plasma nozzle and the exposure time were optimised during trials of polymer removal from a silicon (100) wafer. The choice of using Si (100) substrate makes easier the analysis of the plasma on the coating since it is flat, homogenous and IR transparent. In such way, a removal rate can be obtained, monitoring the modification of the polymer exposed to the plasma by means of FTIR. However, Si is thermally conductive and allow a faster dissipation of the heat load in particular of the Blaster apparatus, in fact arc discharges are thermal plasma and gas temperature can reach thousands of degrees inside the torch, therefore warm up to hundreds degrees the treated surfaces as a function of exposure time and distance [39]. The evaluated polymer removal on Si does not take into account melting and spreading of the coating on the side or inside substrates pores or polymer degradation due to heat, but based on these constrains allows a better comparison between torches’ cold effects that are mainly chemically induced by reactive species produced by the plasma.

4. Results and discussion 4.1. Removal rate assessment To highlight the chemical effect of the plasma on the coatings, the removability tests were firstly performed on Si substrates beforehand brush-coated (1–5 ␮m in thickness) with the commercial polymeric formulation. Once defined, the removal rate as the relative amount of polymer removed per time unit, the chemical evolution of the polymers were monitored by FT-IR. Selecting the proper functional group, marker, FT-IR allows a qualitative, ready

Ar/O2 99/1 Compressed air Compressed air

accessible visualization of the plasma effect on the organic coating. The selected markers are: • the bending out of plane of the C–H in the aromatic ring for the epoxy resin; • the C=O stretching band for the acrylic polymer; • (concerning siloxane) the aliphatic CH3 (side chain methyl bonded to the Si in the main siloxane chain); • the Si–O stretching bands. All the experiments were performed by using Blaster plasma device, by fixing the working distance at 10 mm. 4.1.1. Removal of the epoxy Concerning the epoxy coating (Fig. 1), the intensity of marker, centred at 841 cm−1 [40], decreases gradually increasing the exposure time and it disappears after 300 s (Fig. 1 a–b, VII). In correspondence with that decrease (Fig. 1b), a consistent variation in the C–OH, C–N, and C=C absorption bands is given in Fig. 1c–d. The presence of a C–N stretching bands, at 1296 and 1237 cm−1 , is attributable to the hardener, while the C–OH and the C=C stretching bands, centred respectively at 1241 cm−1 and at 1504 and 1608 cm−1 , are due to the polymer itself [41]. The modification of the crosslinked structure of the polymer becomes evident after 30 s of application. For this exposure time, the C–N band decreases in intensity and it disappears when the exposure exceed 60 s (spectrum II and III in Fig. 1c). These spectral modifications can be representative of the breaking of the crosslinks among the epoxy. It is worthy to note that, starting after 60 s of exposure time (IV to VII) a decrease in the C–OH stretching band intensity is observed. That evidence can be related to an oxidation promoted by the plasma with the consequent removal of the OH group from the polymeric network after 300 s of exposure. As showed in Fig. 1d, the C=C band seems to be unaffected by the plasma until exposure time exceed 105 s; increasing further the exposure time, the C=C band progressively decreases and then transforms in a shoulder after 150 s of exposure (II to V). According with these evidences, the modifications induced by plasma seem to involve the breaking

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Fig. 1. FTIR spectra of the epoxy Araldite® AY 103-1/HY 991 (Huntsman International LLC) with increasing plasma exposure: (a) whole spectra; (b) aromatic C–H stretching; (c) C–N stretching; (d) aromatic C–H bending. I: before plasma treatment, II: 30 s, III: 60 s, IV: 90 s, V: 105 s, VI: 150 s, VII: 300 s plasma treatment.

of the bridge among the epoxy chains with a subsequent oxidation of the whole polymer. 4.1.2. Removal of the acrylics In comparison to the epoxy coating, the removal of acryl needs longer exposure times. As highlighted by the FTIR spectrum (Fig. 2 a–b, IV), the acryl marker, centred at 1740 cm−1 , disappears almost completely after 1500 s. Increasing further the exposure, a decrease in intensity of the overall spectrum (Fig. 2 b–c) is observed, as consequence of the oxidation of organic groups to the volatile compound. 4.1.3. Removal of siloxane The FTIR characterization (Fig. 3 I and II) of Silres BS280 has confirmed that this commercial protective is a hydrophobic methyl-siloxane. Both fresh and thermally aged (80 ◦ C for 72 hours) coatings show the typical signals due to symmetric and asymmetric C–H stretching of methyl groups, in 3050–2800 cm−1 region, to CH3 deformation, at 1271 cm−1 , to the Si–O–Si and Si–O–C absorptions in the 1200–970 cm−1 and to Si–C stretching

absorptions in 890–740 cm−1 region together with the sharp band at 960 cm−1 , attributable to the for Si–O–C stretching of ethoxy groups. Regarding the removal of Silres BS280, the experimentation has showed that plasma acts differently with the organic and inorganic components of the polymer. The exposure to the plasma plume in fact has promoted the disappearance of the CH3 signals and the formation of new bands, in the 1200–1000 cm−1 region, attributable to the Si–O stretching mode in silica [42], as reported in Fig. 3 III and IV. The effect of oxidative plasma on siloxanes is a well-known issue in literature in particular when used as precursor for coatings deposition. In vacuum the reaction processes can be clearly explained as a function of plasma density, pressure and precursors and oxygen fluxes [43]. In fact, reactive oxygen species are able to oxidize the methyl groups in siloxanes allowing the deposition of ceramic coatings. Also, at atmospheric pressure, it is possible to replicate the same ceramic deposition process by siloxane precursors oxidation [44], also if the precursor is inserted in liquid phase as aerosol in the process and even if not directly in the plasma area [45]. Therefore, also in our case, the oxidative plasma is able to remove the methyl groups by

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Fig. 2. FTIR spectra of Acryl 33 (CTS srl) with increasing plasma exposure: (a) whole spectra; (b) C=O stretching; c) low wavenumber region. I: before treatment, II: 240 s, III: 480 s, IV: 1500 s plasma treatment.

the deposited coating and leaves the Si–O–Si backbone on the surface. 4.1.4. Removal rates The efficiency in coating removal could be qualitatively estimated by plotting the decreasing of the marker signal versus the exposure time. Concerning epoxy coating, the removal efficiency curve (Fig. 4a) shows an evident change in slope between 60 and 90 s. Considering the decreasing in the marker intensity, i.e. the decreasing of the C–H bending out-of-the-plane signal, and the modification of the whole spectrum as function of exposure time, the variation in slope can be associated to two different degradation processes. The former, that takes place when the exposure time is lower than 90 s, could be related to the breaking of the crosslinked structure, while the latter could be related to the oxidation of the polymeric network. A drastic variation in the removal rate is also present in the removal of acrylic coating, as reported in Fig. 4b. As for epoxy, the removal rate increase significantly when the exposure time exceeds 100 s but, in this case, the variation in slope cannot be directly connected with two different degradations of the polymeric matrix. On the basis of the FTIR spectra, the only modification of the coating is connected with the decreasing in the marker intensity, i.e. the intensity of the C=O absorption band. This behaviour could be related to the coexistence of a physical process and a chemical one. The former is predominant at low exposure time and attributable to an increasing of the temperature at the surface, it

is concerned with a softening of the polymer and/or to a surface modification induced by the plasma plume. The latter is recognizable by the increase in removal rate and it is due to the oxidation of the polymeric matrix. Concerning the siloxane coating, the plasma, as mentioned above, promotes only the decomposition of the alkyl groups directly bonded to the siloxane backbone and in a few seconds, only silica is present on the surface, as illustrated in Fig. 4c. 4.2. Assessment of the effects of plasma on stone surfaces An effective removal of the undesired materials and the safeguarding of the original historical substrates are mandatory requirements while cleaning works of art. Therefore, it was firstly assessed whether the plasma might have any detrimental effect on the stone surface itself: the colour and the hydrophobicity (w and ␪) were measured before and after the plasma treatment. The exposure time was fixed at 300 s and, with the exception of KinPen, a working distance of 1 cm was used. KinPen, less powerful, was tested with a working distance of ca. 3 mm. Concerning the colour variation, it has been observed that the DBD torch (KinPen) does not promote any surface changes, while arc discharge torches promote small variations in the stone hues. Optical and electron microscopy investigation have showed the presence of metallic micro-aggregates on the specimens treated with such torches, as depicted in Fig. 5. The examinations have highlighted that the amount of micro-aggregates increase increasing the exposure time; EDS analyses have showed Cu, W, Bi, Ni and

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Fig. 3. FTIR spectra of siloxane (Silres BS 280, Wacker) FTIR spectra of acryl with increasing plasma exposure: (a) whole spectra, (b) C–H stretching region, (c) Si–O stretching region; I: fresh Silres, II: after thermal ageing, III: 5 s and IV: 10 s plasma.

Fe on the specimens treated with Plasmapen, while Cr, Mn, Cu, Zn were identified on samples treated Blaster device. In both cases, the compositions of the metal droplets correspond to the ones of the torch electrodes. Therefore, it can be assumed that micro arcs melt the electrode sputtering metallic micro-drops over the stone surfaces, slightly affecting their colour. Considering the hydrophobic properties of the stones, DBD apparatus does not promote any significant variation in ␪ and w while the arc discharge ones seems to increase their hydrophobicity. These variations could be related to the metal sputtering, that modify the original roughness of the substrate, increasing the surface area and the contact angle. 4.3. Removal of epoxy resin from Istria limestone The removal of epoxy resin from Istria limestone depends strongly on the plasma device used, as illustrated in Fig. 6. In fact, Plasmapen ST and Kinpen do not affect the epoxy significantly, even for prolonged exposure time. On the other hand, Blaster and Plasmapen NT remove the coating within 660 and 300 s respectively. Nevertheless, the removal is combined with a darkening of the stone surface, probably due to the presence of sputtered metals and to the presence of highly oxidised organic particles, as will be discussed below. According to colour, ␪ and w measurements, the most effective cleaning was obtained by Blaster. The properties of the stone surface, after the treatment, show little variations, as highlighted by  values, reported in Table 3.  quantifies the difference between a physical quantity after the application of the plasma and the same quantity in the reference stone. If the removal is

successful, the  will be close to 0, that is close to the ones referred to the reference stone. The effect of the plasma on the polymer can be rationalized by *. In fact, * expresses the difference between the properties of the coated surface treated with plasma and the properties of the coated surface before the treatment. As reported in Table 3, all the treatments promote a ␪*< 0 and a w* > 0. The former can be associated with a loss in hydrophobicity of the surface, that could be due to the removal of the polymer. The latter could indicate a decrease in water penetration velocity, probably induced by the presence of residual material on the surface. Concerning the chemical transformations, ER-FTIR allows to investigate directly on the spot treated by plasma, highlighting the chemical transformation on the specimens’ surface. The untreated sample shows a ERFTIR spectrum (Fig. 7a, spectrum I), characterized by the typical derivative-like distortions [46], especially in the aliphatic C–H and the C–C aromatic stretching bands (centred respectively at 2910, 2841 cm−1 and 1609, 1514 cm−1 in the transmission spectrum) in the amide I band and the C–H bending out of plane of the aromatic ring (respectively at 1645 and 841 cm−1 in the transmission spectrum). Increasing the exposure time, a decrease in the C–H stretching absorption intensity together with a transformation in the 1800–1000 cm−1 region are evident. When the plasma exposure exceeds 30 s (Fig. 7a II), the amide derivativelike band, which inflection is initially centred at 1662 cm−1 , decreases drastically in intensity. Short exposure times promote also a formation of a new band, centred at 1700 cm−1 , ascribable to oxidized species, such as –C=O groups, as a consequence of the polymer degradation [47] and the increasing of the C–C aromatic stretching at 1543 cm−1 (Fig. 6b). The increasing in the intensity of

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Fig. 4. Removal rate curves for (a) epoxy coating; (b) acrylic coating; (c) siloxane coating. At t equal to 0, the coating is unaffected by the plasma action, increasing the exposure time the intensity of the characteristic signal (i.e. C–H bending in epoxy coating, C=O stretching in acryl coating and aliphatic C–H stretching in siloxane) decreases and the removal rate increases. In Fig. 3c, the diamond full dots represent the Si–O stretching signal, unaffected by plasma action, whereas the crossed dots represent the breaking of the Alkyl–Si bonds.

the aromatic C–C stretching band can be attributed to the overlap between the pristine signal and the formation of new C=N species as suggested by literature [47,48]. The formation of new C=O and C=N species together with an evident “burnt-effect” suggest that the plasma effect induced both chemical and thermal modifications leading to the thinning of the coating until its whole disappearance. The weak softening of the coating are qualitative clues for a surface temperature close to 50 ◦ C, which is the glass transition of the resin [49]. Increasing further the exposure time, the coating start to darkening, the marker bands decrease and, when exposure time exceeds

60 s, the polymeric film become thinner. The thinning of the film is highlighted by the presence of the CO3 = overtones and stretching of the substrate (respectively 2500, 1797 and 1400 cm−1 ) in the ERFTIR spectrum. Reaching 660 s, no more absorption bands due to the polymer are detected and the surface appears clean (Fig. 6e). In conclusion, although no traces of polymer are detected by EF-FTIR, ␪ and w remain higher than in the reference stone. These evidences could be ascribed to the metal sputtering from the electrode, or to the presence of degradation products occluding the surface porosity.

Fig. 5. Stone treated with plasma: SEM BSE image (5000 ×) of metal drops on stone surface treated with Blaster Tigres (a); SEM SE image (1000 ×) of a metal drop on stone surface treated with Plasmapen PVATePla (b).

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Fig. 6. Images of the epoxy coating after plasma treatment at the most effective treatment time: (a) after 300 s of Plasmapen narrow tip; (b) after 600 s of Plasmapen standard tip and (c) after 660 s of Blaster. Table 3 Modifications induced by plasma on the different lythic substrates coated with different polymers when the exposure time is maximum. The modifications of the plasma treated surface are quantified by the evaluation of the difference in CieL*a*b* coordinates (E), contact angle (␪) and water micro-drop absorption (w) in relation to the lythic substrate and in relation to the lythic substrate coated with the polymer (E*, ␪*, w*). Apparatus

Substrate

Variation induced by the Epoxy coating Istria + Epoxy Blaster Istria + Epoxy Plasmapen ST Plasmapen NT Istria + Epoxy Istria + Epoxy KinPen Variation induced by the Acryl33 coating Blaster Serena + Acryl33 Plasmapen ST Serena + Acryl33 Serena + Acryl33 Plasmapen NT Serena + Acryl33 KinPen Variation induced by the Silres coating Blaster Carrara + Silres Carrara + Silres Plasmapen ST Carrara + Silres KinPen

E

Exposure time (s) 660 660 300 1380 720 660 660 660 600 600 600

5 1.8 33 8.2 5.9 4.9 8.9 11.3 9.7 6.0 2.6 2,8 3,5 3,9

4.4. Removal of acrylic polymer from Serena sandstone As seen for epoxy coating removal, Kinpen turned out to be not effective in the removal of commercial acryl resin, even after prolonged exposure times. Plasmapen apparatus, equipped with ST or NT, removes partially the coating in 60 s to 660 s, as highlighted by  and * values (Table 3). Similarly, also Blaster seems to be unable to remove entirely the coating, even for prolonged exposure

E* 6.2 35.4 12 6.9 5.3 6.8 5.51 1.7 0,42 0,88 1,28

␪ (◦ ) 34 18 19 20 22 61 46 36 41 69 136 9 15 8

␪* (◦ ) −16 −15 −27 −12 −15 −25 −20 −2 −52 −47 −52

w (s) 75 476 1296 999 1784 3800 1150 697 112 2900 492 312 648 906

w* (s) 401 1221 924 1709 −2650 −3103 −3688 −900 570 906 1164

time. Macroscopic images of the surfaces after treatment with the different torches show a diffuse bleaching of the coated surface and a ruffling of the coating. In general, the surface seems to be modified by the plasma treatment. In fact, there is a light increase in E, while ␪ and w decrease in comparison with those of the untreated sample (Table 3). The properties of the cleaned stone are anyway far from those of the reference one, as indicated by  > 0. This evidence seems to be in contrast with the ␪* and

Fig. 7. Removal of epoxy coating from Istria limestone using Blaster apparatus. (a) Evolution of the ER-FTIR spectrum increasing plasma exposure time: I coated surface before treatment, II surface after 30 s, III after 300 s and IV after 660 s. (b) Magnification of the 1900–1400 cm−1 region, straight line refers to spectrum I, dashed line to spectrum II and dotted line to spectrum III. Macro-images of the surface, (c) before plasma treatment (20 ×), (d) and (e) after the 300 and 660 sec of treatment (20 × and 10 × respectively).

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Fig. 8. ER FTIR spectra of Serena sandstone with acryl removed with the Blaster apparatus. Left: (a) whole spectra; I: reference stone (uncoated); II: coated one; III: 30 s and IV: 720 s, (b) magnification of the C=O band during the plasma exposure, straight line before the plasma application, dotted line after 30 s and dash line after 720 s. Right: macro images (16 ×) of the surface: (c) before the application of plasma; (d) 60 s and (e) 720 s.

w* values. In fact, if a negative ␪* could be representative of a decreasing in hydrophobicity, while w* < 0, could be explained in terms of an increase in the speed of penetration of the water micro-drop. This apparent contrast can be explained by monitoring the removal process by ER-FTIR. As evidenced by Fig. 8a I, the C=O stretching band, a derivative-like band with the inflection centred at 1736 cm−1 , decreases gradually with increasing exposure time. Although after 60 s, the signal decreases drastically, losing the second derivative shape thanks to the thinning of the acrylic layer, it does not disappear even after long exposure time (Fig. 8 III, IV). This phenomenon can be due to the cooperative chemical and thermal action of the plasma stream on polymeric film together with the porous nature of the lythic substrate. Under the action of the plasma, the acryl chains are chemically modified, leading also to chain oxidation and scission (as evidenced by the intensity reduction and broadening of the carbonyl peaks in the ER-FTIR spectra). On the other hand, plasma exposure induced also an increase of the surface temperature over the glass transition temperature of the polymer (40 ◦ C) promoting a softening of the film. Those synergic effects cause the diffusion of melted polymer inside the porous network of the stone. These competitive processes turned out in a partial removal of the coating from the stone surface and a contemporary penetration of short polymer chains inside the stone due to the thermal effect of the plasma. This hypothesis is also confirmed by literature data on plasma removal of Paraloid B72 [50]. Paraloid B72 was easily removed from a compact stone surfaces using the Blaster/Tigres, because the low porosity did not allow the penetration of soften polymer favouring its chemical and thermal deterioration induced by the plasma. There is an apparent discrepancy between the data obtained on Si substrate and those obtained on the Serena sandstone. That difference can be explained in terms of heat transfer and diffusion. Si substrate, as mentioned above, has a higher thermal conductivity in comparison with Serena sandstone (sandstone: 1,30–1,75 W/mK; silicon 148 W/mK), therefore the heat transferred by the plasma plume is rapidly dissipated, enhancing the chemical modifications in place of the thermal ones.

4.5. Removal of hydrophobic siloxane In order to confirm the hypothesis formulated in the removal of siloxane from Si substrate, several trials in removing siloxane coating from marmo cotto were made. As the siloxane coating

is completely invisible, transparent and not glossy, the effect of plasma could not be evaluated by naked eye observation as easily as on the samples treated with epoxy and acrylic products. Plasma treatment, independently of the equipment used, leads small modification concerning the aesthetic features of the stone, but promote an evident decreasing of the properties connected with the superhydrophobicity of the coating itself, as reported in Table 3. The application of SILRES BS 280 induces an imperceptible colour change that is then reduced due to plasma treatment on the coating, as reported in Table 3. Concerning the hydrophobicity of the coating, even the less powerful instrument (Kinpen/Neoplas) affects significantly the ␪ values. This could be ascribed to the cleavage of Si–C bonds and removal of methyl side groups responsible for the high hydrophobicity of the siloxane coating, as already evidenced in tests performed on silicon slides. Micro-drop absorption measurements agree with evidences of the formation of the silica-like layer. Plasma treated samples showed water absorption times higher than those of uncoated and coated marble as consequence of the formation of the silica-like layer, which hindered the absorption of the water micro-drop. Only the samples treated with Blaster have a faster micro-drop absorption with respect to those treated with PVATePla and Kinpen. This could be due to the high power of the torch, which can cause the formation of cracks on the silica-like layer, as reported by Hillborg et al. [51], so that water can be easily be absorbed.

5. Conclusions Mandatory requirements for any practice related to conservation of Cultural Heritage are that they must be effective without any direct or future detrimental effects on the original historical surfaces. This paper reports the potentials and drawbacks of a novel cleaning technique: the atmospheric plasma for the removal of polymers (epoxy, acrylic and siloxane) from stone works of art. Different commercial plasma devices have been tested, demonstrating that coating removal depends both on the set-up of the plasma device and on the polymeric feature of the coatings. Concerning the tested apparatus, we observed that DBD-based torches are not efficient as cleaning devices for polymers probably due to its low power design. On the contrary, the arc discharge devices may remove epoxy and acrylic superficial coating by means of a progressive oxidation of the polymeric network. Siloxane-based coating

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cannot be fully removed by atmospheric plasma, as it can oxidize the alchyl group leaving the Si–O backbone. This phenomena clearly indicates that ions and radical created in the plasma induce cleavage in alchyl, esters and epoxy polymers allowing their oxidation to volatile compounds, while being completely ineffective towards siloxanes that cannot be further oxidised and whose oxides are not volatile. The experiments performed on coated stone mockup samples demonstrate also that the thermal effect induced by plasma exposure with arc discharges torches plays a significant role in the removal efficacy. Although the removal of epoxy and Acryl 33 on Istria and Serena stone is faster in comparison to the one measured on silicon slide, the stone surface temperature may increase promoting their swallowing inside the stone porosity. Furthermore, an uncontrolled increase of the temperature on stone surface may threaten sensitive materials such as deteriorated historical artefact. Finally, it has been demonstrated that arc discharge apparatuses suffer from electrode consumption followed by a metal sputtering on the stone surface, after plasma ignition. Obtained results clearly demonstrated the potentials of the atmospheric plasma as a cleaning device in alternative to less eco-friendly chemicals, which are barely effective towards aged polymers. In particular for epoxy resins, it has been observed how plasma breaks firstly the polymer crosslinking, allowing therefore an easier removal with solvents. Notwithstanding the promising results, to overcome their drawbacks, new devices need to be developed addressing the needs of CH conservation matching efficiency, low temperature and damaging. DBD design, MHz frequency or nanopulse regimes can be probably considered to avoid any particle deposition on the other side plasma pulsing and a suitable power range can help in controlling surface temperature allowing an efficient removal of the polymer from the surfaces. A new atmospheric plasma prototype is under development in the frame of the UE project PANNA and its application as cleaning tool is currently under investigation. Acknowledgements The work reported in this paper was carried out in the frame of EU-PANNA Project (FP7/2007-2013 grant agreement no. 282998), which aims at the development of a conservation protocol using Atmospheric Pressure Plasma Jet (APPJ) technology and at the construction of an APPJ device dedicated to CH applications. References [1] M. Steiger, A.E. Charola, Weathering and Deterioration, in: S. Siegesmund, R. Snethlage (Eds.), Stone in Architecture, Properties, Durability, 4th Edn, Springer, 2011. [2] C.V. Horie, Materials for conservation, Butterworth-Heinemann Ltd, Oxford, 2010. [3] L. Borgioli, Polimeri di sintesi per la conservazione della pietra, Il Prato, Padova, 2002. [4] G.G. Amoroso, Trattato di Scienza della Conservazione dei Monumenti, ALINEA, Firenze, 2002. [5] M.J. Melo, S. Bracci, M. Camaiti, O. Chiantore, F. Piacenti, Photodegradation of acrylic resins used in the conservation of stone. Polym. Degrad. Stabil. 66 (1999) 23–30. [6] C. Selwitz, Epoxy Resins in Stone Conservation, The Getty Conservation Institute, 1992. [7] H.R. Sasse, R. Snethlage, Methods for the evaluation of stone conservation treatments, in: N.S. Baer, R. Snethlage (Eds.), Report of the Dahlem Workshop, Saving our Architectural Heritage–The Conservation of Historic Stone Structures, Berlin, 3-8 March 1996, John Wiley and Sons, Chichester, 1997, pp. 223–243. [8] M. Favaro, R. Mendichi, F. Ossola, U. Russo, S. Simon, P. Tomasin, P.A. Vigato, Evaluation of polymers for conservation treatments of outdoor exposed stone monuments. Part I: photo-oxidative weathering, Polym. Degrad. Stabil. 91 (2006) 3083–3096. [9] M. Favaro, R. Mendichi, F. Ossola, S. Simon, P. Tomasin, P.A. Vigato, Evaluation of polymers for conservation treatments of outdoor exposed stone monuments. Part II: Photo-oxidative and salt-induced weathering of acrylic–silicone mixtures, Polym. Degrad. Stabil. 92 (2007) 335–351.

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