Removing polymeric coatings with nanostructured fluids: influence of substrate, nature of the film, and application methodology

Cleaning is one of the most important and delicate operations in the conservation of cultural heritage, and, if not correctly performed, may irreversibly damage works of art. The removal of aged or detrimental polymeric coatings from works of art is a common operation in conservation, and nanostructured fluids (NSFs), such as aqueous swollen micelles and oil-in-water (o/w) microemulsions, are used as an alternative to non-confined organic solvents that pose a series of non-negligible drawbacks. NSFs effectiveness in removing polymeric coatings has been thoroughly demonstrated in the last decades, while their cleaning mechanism is still under investigation. The present work deepens the knowledge on the removal mechanisms of NSFs, studying the interaction of a four-component NSF with four different types of acrylic and vinyl polymer films cast from solutions or aqueous polymer latexes on three substrates (glass, marble and polystyrene) with different hydrophilicity and wettability. NSFs were applied either as non-confined or confined in cellulose poultices (traditionally employed by conservators), or in highly retentive chemical gels, observing the influence of the confining matrix on the removal process. It was found that the NSF/polymer film interaction is greatly dependent on the film structure and composition. Films formed from solvent solutions can be swollen by water/organic solvents mixtures or dewetted when a surfactant is added to the cleaning fluid; films formed from polymer latexes, on the other hand, are generally swollen even just by water alone, but poorly dewet. Abstract Cleaning is one of the most important and delicate operations in the conservation of cultural heritage, 16 and, if not correctly performed, may irreversibly damage works of art. The removal of aged or detrimental polymeric coatings from works of art is a common operation in conservation, and nanostructured fluids (NSFs), such as aqueous swollen micelles oil-in-water microemulsions, are used as an alternative to non-confined organic solvents that pose a series of non- negligible drawbacks. NSFs effectiveness in removing polymeric coatings has been thoroughly 21 demonstrated in the last decades, while their cleaning mechanism is still under investigation. The 22 present work deepens the knowledge on the removal mechanisms of NSFs, studying the interaction of a four-component NSF with four different types of acrylic and vinyl polymer films cast from 24 solutions or aqueous polymer latexes on three substrates (glass, marble and polystyrene) with different hydrophilicity and wettability. NSFs were applied either as non-confined or confined in 26 cellulose poultices (traditionally employed by conservators), or in highly retentive chemical gels, 27 observing the influence of the confining matrix on the

Cleaning is one of the most important and delicate operations in the conservation of cultural heritage, 16 and, if not correctly performed, may irreversibly damage works of art. The removal of aged or 17 detrimental polymeric coatings from works of art is a common operation in conservation, and 18 nanostructured fluids (NSFs), such as aqueous swollen micelles and oil-in-water (o/w) 19 microemulsions, are used as an alternative to non-confined organic solvents that pose a series of non-20 negligible drawbacks. NSFs effectiveness in removing polymeric coatings has been thoroughly 21 demonstrated in the last decades, while their cleaning mechanism is still under investigation. The 22 present work deepens the knowledge on the removal mechanisms of NSFs, studying the interaction 23 of a four-component NSF with four different types of acrylic and vinyl polymer films cast from 24 solutions or aqueous polymer latexes on three substrates (glass, marble and polystyrene) with 25 different hydrophilicity and wettability. NSFs were applied either as non-confined or confined in 26 cellulose poultices (traditionally employed by conservators), or in highly retentive chemical gels, 27 observing the influence of the confining matrix on the removal process. It was found that the 28 NSF/polymer film interaction is greatly dependent on the film structure and composition. Films 29 formed from solvent solutions can be swollen by water/organic solvents mixtures or dewetted when a 30 surfactant is added to the cleaning fluid; films formed from polymer latexes, on the other hand, are 31 generally swollen even just by water alone, but poorly dewet. The substrate also plays an important 32 role in the removal of polymer films formed from solutions, for instance the removal of an acrylic 33 polymer from polystyrene could be achieved only through highly selective cleaning using NSF-34

Introduction 38
Cleaning of works of art generally consists in the selective removal of materials that promote the 39 degradation of the artifacts or alter their readability and appearance. Among these materials, aged or 40 detrimental polymeric coatings are often found on works of art, and their removal is a common 41 operation in art conservation. Synthetic polymers have been largely employed in the traditional 42 restoration practice as varnishes, adhesives, protectives and consolidating agents. The presence of 43 polymeric coatings on the surface of porous inorganic substrates (wall paintings, stone, mortars) 44 drastically reduces water permeability and enhances the degradation induced by salts, up to 45 consistent loss of the artifacts' surface layers (Carretti and Burnstock and Kieslich, 1996). 51 The vast amount of different solvent-sensitive materials found in classic and contemporary art poses 52 continuous challenges to the safe removal of detrimental coatings (Kavda et al., 2017). Traditionally, 53 restorers and conservators rely on the use of organic solvents to dissolve or swell unwanted materials. 54 Solvents are typically applied either as non-confined, using cotton swabs, or thickened in viscous 55 polymeric solutions and solvent gels (Baglioni et al., 2012c;Burnstock and Kieslich, 1996; 56 Burnstock and White, 2000). However, these methods exhibit poor control and scarce selectivity, or 57 involve the presence of residues from the cleaning system. In addition, health concerns arise from the 58 toxicity of most solvents used in restoration. 59 Alternatively, nanostructured fluids (NSFs) such as aqueous swollen micelles and oil-in-water (o/w) 60 microemulsions were proposed in the late 1980s for the removal of hydrophobic matter from porous 61 inorganic substrates (Borgioli et al., 1995). Their effectiveness in removing polymeric coatings from 62 different types of surfaces has been thoroughly demonstrated in the last decades (Baglioni et al., 63 2010(Baglioni et al., 63 , 2012b(Baglioni et al., 63 , 2015a(Baglioni et al., 63 , 2015b(Baglioni et al., 63 , 2016(Baglioni et al., 63 , 2018cCarretti et al., 2003Carretti et al., , 2007 either as non-confined or confined in cellulose poultices (traditionally employed by conservators), or 100 in highly retentive chemical gels, observing the influence of the confining matrix on the removal 101 process. 102 The NSF is composed of water, an alcohol ethoxylate nonionic surfactant, 2-butanol (BuOH), and 2-103 butanone (methyl ethyl ketone, MEK), and is representative of formulations actually employed in the 104 conservation practice for the cleaning of real works of art. 105 The three selected substrates were coated with the four different polymers, obtaining a set of 12 106 samples, which were then exposed to the NSF, investigating the interaction mechanism for each 107 combination. The effects of the NSF on the films were studied by means of optical microscopy and 108 micro-reflectance infrared Fourier Transform spectroscopy 2D mapping of the areas of interest, 109 which provides spatial resolution down to the micron-scale. 110

Substrates 142
The three different substrates selected for this work, i.e., glass, marble and polystyrene, were selected 143 as they exhibit different wettability and are representative of artistic substrates frequently found in 144 classic or contemporary art production. Glass slides of 5 × 5 × 0.3 cm 3 , marble tiles of 5 × 5 × 1 cm 3 , 145 and polystyrene slides of 5 × 5 × 0.2 cm 3 were used.

NSF preparation 163
The NSF selected for this work is composed as follows (w/w): H 2 O, 65.9%; C 9-11 E 5,5 , 3.5%; BuOH, 164 9.7%; MEK, 20.9%. In order to better understand the role of each component in the interaction with 165 the polymer coatings, other liquid systems were also used, obtained removing some of the 166 components from the complete NSF formulation. Namely, a water/C 9-11 E 5,5 surfactant solution, and a 167 water/BuOH/MEK solvents mixture were used. In both systems, the ratio between each component 168 was the same as in the complete NSF formulation. It is worth noting that both BuOH and MEK are 169 partly water-miscible (12.5% and 24% at 20˚C, respectively (Verschueren, 2001)) and this made 170 possible to obtain a single-phase stable mixture of water and solvents even in the absence of 171 surfactants. 172 173

Small-Angle Neutron Scattering 174
Small-angle neutron scattering (SANS) experiments were performed on the spectrometer V4 (Bensc-175 Helmholtz Zentrum Berlin). Two different configurations were employed (i.e., sample-to-detector 176 I n r e v i e w This is a provisional file, not the final typeset article distances, SD = 2 or 8 m) to cover a range of wave vectors q (q = (4π/λ)sin(θ/2), where λ is the 177 wavelength of the incident neutron beam and θ the scattering angle) from 0.007 to 0.28 Å -1 . For each 178 configuration a 6 Å neutron wavelength was used and the wavelength resolution, Δλ/λ, was less than 179 10%. Samples were contained in 1 mm thick quartz cells and kept at 20 ± 2 °C during the 180 measurements. The scattering intensity was corrected for the empty cell contribution, transmission, 181 and detector efficiency and was normalized to the absolute scale by direct measurement of the 182 intensity of the incident neutron beam. The integration of the normalized 2D intensity distribution 183 with respect to the azimuthal angle yielded the 1D scattering intensity distribution, I(q), in cm −1 . The 184 reduction of the data was performed using standard BENSC procedures for small-angle isotropic 185 scattering. The background from the incoherent scattering coming from each sample was determined 186 from analysis of the Porod asymptotic limit and subtracted from the normalized spectra. 187 Experimental data normalized to absolute scale were fitted using Igor routines (NCNR_SANS_ 188 package_6.011) (Kline, 2006) available from NIST, National Institute for Standard and Technology, 189 Gaithersburg, MD, running on Igor Pro© (Wavemetric Inc., Lake Oswego, Oregon; Version 6.22). The Fourier transform infrared (FTIR) 2D imaging of the treated surfaces was carried out using a 244 Cary 620-670 FTIR microscope, equipped with an FPA 128 ×128 detector (Agilent Technologies). 245 The spectra were recorded directly on the surface of the samples in reflectance mode, with open 246 aperture and a spectral resolution of 8 cm -1 , acquiring 128 scans for each spectrum. Each analysis 247 produces an IR map of 700 × 700 µm 2 (128 ×128 pixels), with a spatial resolution of 5.5 µm (i.e. 248 each pixel has dimensions of 5.5 × 5.5 µm 2 and is associated to an independent spectrum). In each 249 map, the intensity of a characteristic peak of each polymer, e.g. the C=O ester stretching typical of 250 both acrylics and vinyls, at about 1730 cm -1 , was shown with a chromatic scale, following the order 251 red > yellow > green > blue. The four polymeric coatings were characterized by means of FTIR-ATR and contact angle 256 measurements. Table 2 reports the values for the contact angle of water droplets laid on the surface 257 of the polymer films, or of the selected substrates (glass, marble, polystyrene). The substrates exhibit 258 a range of contact angles passing from hydrophilic (glass) to hydrophobic surfaces (polystyrene). 259 Regarding the polymers, it can be noticed that the acrylics are more hydrophobic than the vinyls, and 260 the films deriving from polymer latexes are more hydrophilic than those coming from solutions. This 261 is likely due to the presence of surfactants, stabilizers and other polar additives in the latexes 262 emulsions, which affect the wettability of the film. 263

NSF Characterization 292
The selected NSF for this work is composed of water, a nonionic surfactant and two solvents, i.e., 293 BuOH and MEK, which are partly miscible with water. The structure of this NSF was never studied 294 before, thus acquiring information on the micelles' size and shape, and on the location of each 295 component in the NSF, was deemed a preliminary step to help understanding the interaction of the 296 fluid with the polymeric films. To this aim, SANS measurements were performed on four D 2 O based 297 samples: 1) the D 2 O/C 9-11 E 5.5 binary mixture; 2) the D 2 O/C 9-11 E 5.5 /  to two different models. The supramolecular aggregates, in the case of the binary surfactant/water 305 and the water/surfactant/BuOH systems, were modeled as non-interacting polydisperse core-shell 306 spheres, defined by two contrasts, i.e. bulk/shell and shell/core. On the other hand, the best fitting for 307 the complete NSF was obtained by modeling the micelles as non-interacting prolate core-shell 308 ellipsoidal particles, again defined by a double contrast. The scattering length density (SLD) of bulk, 309 shell and core, i.e., respectively, ρ bulk , ρ shell and ρ core , were calculated according to the SLD for 310 neutrons of each chemical included in the formulations, as reported in Table 3. For globular micelles 311 of homogeneous scattering length density, the total scattered intensity I(q) (cm -1 ) is given by (Liu et 312 al., 1995;Sheu and Chen, 1988): 313 where N p is the number density of the scattering particles (cm -3 ), V p is the volume (cm 3 ), Δρ is the 315 contrast term (cm -2 ), P(q) is the form factor and S(q) is the structure factor. In this case S(q) = 1, as 316 the particles were considered to be non-interacting. In the case of spherical core-shell aggregates, the 317 particle scattering intensity is expressed as follows (Kline, 2006): 318 where j(x) is a spherical Bessel function and is expressed as: 320 and where φ is the volume fraction of the micellar phase, V c is the core volume, r c is the core radius, 322 r s = r c + t (t is the shell thickness). Since this model takes into account a polydisperse core, which 323 follows the Schultz distribution, the form factor calculated in equation (2) is normalized by the 324 average particle volume: 325 and z is the width parameter of the Schultz distribution (Degiorgio et al., 1985): 329 being σ 2 the variance of the distribution. The polydispersity index (PDI), reported in Table 4 is  331 defined as σ/〈r c 〉 (see equation (6)) and its value is comprised between 0 and 1. 332 In the case of monodisperse non-interacting prolate ellipsoids, on the other hand, when modeling 333 asymmetric micelles with a core-shell scattering length profile, P(q) included in equation (1) is 334 usually calculated as an orientationally-averaged normalized form factor, ( ). First, the orientation-335 I n r e v i e w dependent form factor F(q,µ), is defined as follows (where µ is the cosine between the direction of 336 the symmetry axis of the ellipsoid and the Q vector): 337 where j(x) is the same spherical Bessel function defined in eq. (3) and u and ν are expressed as: 339   after the addition of BuOH (see Figure 2-bottom). In fact, BuOH was found to be partitioned 366 between the micellar and the aqueous bulk phase in a 30:70 ratio, meaning that most of the solvent is 367 mixed with water. The fraction solubilized in the micelles is preferentially located in the shell, 368 replacing D 2 O hydration molecules, and thus affects the micellar size and shape only slightly. The 369 main effect of the inclusion of BuOH is to double the polydispersity of the core radius with respect to 370 the binary water/surfactant system, even though the value is perfectly suitable for a micellar solution. 371 The results of the contrast variation experiments performed on the complete NSF formulation 372 showed that the inclusion of a significant amount of MEK in the system completely alters both the 373 size and shape of the aggregates (see Figure 2-

NSF/polymer interaction 392
The interaction between the NSF and the four polymer films was initially investigated simply by 393 immersing the coated specimens into four different liquid systems, i.e., water, water/surfactant, 394 water/solvents, and the complete NSF. The micromorphology of the film was observed by optical 395 microscopy at different times during the total 10 minutes of incubation of the specimens in the 396 liquids, while 2D micro-FTIR mapping was performed on the polymer surface before exposure to the 397 fluids and after 10 minutes of incubation. Finally, a removal test was performed on the polymers 398 using wet cotton swabs, in order to check their removability. 399 400 role in determining the behavior when the film is exposed to the same NSF. 2D microFTIR mapping 413 was crucial to confirm the location and distribution of the polymers on the micron-scale. In 414 particular, when a film is dewetted (see Figures 3 and 4) it was shown that no polymer residues are 415 present (above the instrumental detection limit) outside the droplets on the substrate surface. 416 Polymer coatings formed from aqueous emulsions are significantly more sensitive to the action of 417 water (a well known issue when emulsion-based acrylic paint layers are exposed to aqueous cleaning This is justified by the presence of a non-negligible amount of hydrophilic additives and surfactants 420 in the polymer emulsions, which remain in the film after drying and are able to interact with water 421 molecules, favoring the swelling of the film. The presence of hydrophilic additives can also explain 422 why the swollen films tend to dewet less easily. The energetic balance of dewetting can be described 423 by the spreading coefficient S, which for a polymer film on a glass surface, immersed in a liquid, is 424 defined as (Baglioni and Chelazzi, 2013): 425 where γ LG is the interfacial tension between glass and the liquid, γ PG is the interfacial tension between 427 glass and the polymer, and γ LP is the interfacial tension between the liquid and the polymer. When S 428 is negative, dewetting is energetically favored and occurs spontaneously unless an activation energy 429 barrier hinders the process kinetically. Hydrophilic additives in the polymer film might lower the 430 values of both γ PG and γ LP , making S less negative for a given liquid/polymer/glass set. On the other 431 I n r e v i e w This is a provisional file, not the final typeset article hand, the presence of surfactant additives may lower the glass transition temperature (T g ) of the 432 polymer films (as in the case of V E -see Table 1), making polymer chains more mobile, thus  433 possibly decreasing the energy costs related to the formation of new interfacial regions during the 434 detachment of the film, and lowering the activation energy necessary to initiate dewetting (Baglioni  435  et  Overall, in the case of the polymer films from emulsions investigated here, the films' thermodynamic 437 stability seems to prevail on the kinetic drive of the dewetting process. However, the swollen films 438 are softened and easily removable from glass and marble even with water and water/ C 9-11 E 5.5 , 439 meaning that the adhesion to the solid surface was sensibly reduced, while in the case of polystyrene 440 the removal is slightly more difficult. When a polymer film formed from a solvent solution is 441 exposed to a cleaning fluid, a more diversified behavior is observed. Water and water/ C 9-11 E 5.5 are 442 partly or completely ineffective in removing the films. Instead, dewetting processes are induced by 443 water/solvents mixtures or, more efficiently, by the synergistic action of solvents and surfactants 444 (Baglioni et al., , 207, 2018bGentili et al., 2012;Xu et al., 2012). This is clearly exemplified by 445 A S on marble, where water alone produces no visible effects on the polymer. The nonionic surfactant 446 micellar solution is able to induce some swelling of the film, but not sufficient to grant its easy 447 removal. A water/solvents mixture, on the other hand, produces a partial dewetting pattern, and the 448 film can be partly removed using some mechanical action. This means that solvents are able to swell 449 the polymer to such an extent that the T g is lowered below room temperature, starting the dewetting 450 process. However, it is only with the synergistic action of a surfactant, which lowers the interfacial 451 tension at the liquid/substrate and liquid/polymer interfaces, that dewetting proceeds further, leading 452 to easy and complete polymer removal. The substrate's chemical nature is a key factor. Increasing the 453 hydrophobicity of the substrate, the affinity of the polymer film with the substrate increases, leading 454 to less efficient polymer removal, culminating with the limit case of A S on polystyrene, which 455 resulted completely irremovable.    The interaction between the complete NSF and the four polymer films was also monitored through 492 CLSM imaging, as described in Section 2.7.2. Homogeneous and reproducible 2-4 µm thick films on 493 coverglasses were obtained by spin coating. Figure 7 summarizes the results of the investigation, 494 which were in perfect agreement with what was observed during the immersion tests reported above. 495 A S and V S , i.e. the polymer films cast from solutions, were completely and quickly dewetted by the 496 NSF, while A E and V E , i.e. the films cast from polymer latexes, were just swollen. Figure 7 shows 497 that some small cracks and/or holes (of few microns) are visible in the V E and A E swollen films, 498 which nonetheless maintain their overall coherence on a macro-scale. These holes possibly form in 499 correspondence of previous film defects, which can be present on such thin films. Overall, the 500 behavior of polymer films cast from aqueous latexes can be seen as the physical process stopped at 501 the very early stages of dewetting. This is in agreement with what observed during previous 502 I n r e v i e w experiments, and seems to enforce the hypothesis that the amphiphilic additives in these films play a 503 key role in inhibiting the dewetting process, which otherwise would likely occur. 504 The inhomogeneous aspect of the V E and A E films at t = 0 is due to the fact that the hydrophobic 505 fluorescent dye was not evenly distributed in the aqueous polymer latex. As the films is swollen by 506 the penetration of the organic solvents, the dye evenly spread through the film. 507 CLSM investigations, overall, allowed to confirm the results of the immersion tests on macroscopic 508 samples, where the polymer thickness could not be directly measured and not accurately controllable. 509 510 511 I n r e v i e w This is a provisional file, not the final typeset article Recently, SAXS and rheology studies showed that these gels act as "sponges", able to load different 525 NSFs without being altered or dramatically alter the properties of the fluids (Baglioni et al., 2018a). 526 The two systems were applied for 15 minutes on the surface of the films, and then micrographs of the 527 treated area were taken before checking the coating removability via gentle mechanical action with a 528 wet cotton swab. As visible in Figure 8, the areas treated with the NSF-loaded poultice are generally 529 more inhomogeneous than the ones treated with the NSF-loaded hydrogel. Moreover, several 530 cellulose fibers were spotted on the samples that were in contact with the poultice, indicating the 531 permanence of paper or cellulose pulp residues on the treated areas. Dewetting patterns could be 532 clearly highlighted in the areas of A S on glass and marble treated with the NSF-loaded hydrogel, 533 indicating a more controlled and reliable cleaning action. In any case, after the removal of either the 534 compress or the gel, complete and easy removal could be obtained via a gentle mechanical action 535 using wet cotton swabs, for all the specimens except A S on polystyrene. In this case, the acrylic 536 polymer could not be removed after 15 minutes of application of either the NSF-loaded poultice or 537 hydrogel (Figures 9A and 9B). We hypothesized that in this case the application time was too long, 538 causing the migration of solvents through the AS coating up to the polymer/substrate interface, where 539 they interacted with polystyrene creating a sort of joint or adhesion layer between the polymer and 540 the substrate. This issue is easily overcome by using multiple applications of shorter length, assuming 541 the confining matrix of the NSF is enough retentive to release gradually the uploaded fluid on short 542 time scales, which is the case of the pHEMA/PVP gels. In fact, repeated shorter applications (2 +2 +2 543 minutes) of the NSF loaded in the hydrogel allowed complete removal of the coating (as confirmed 544 by the 2D microFTIR mapping), while uneven polymer residues were left from the same application 545 using cellulose poultices (see Figures 9C-F) The cartoon in Figure 9 summarizes the removal 546 process in the two cases: using the highly retentive hydrogel it is possible to perform a gradual 547 action, which proceeds layer by layer, controlling the interaction of the NSF with the substrate. This 548 approach was recently used to perform highly selective removal of overpaintings and vandalism 549 (Giorgi et al., 2017).  This study focused on unveiling some key aspects of the NSFs/polymer coatings interaction 578 depending on several different factors: i) the chemical nature and, thus, hydrophilicity of the 579 substrate; ii) the chemical nature and physical structure of the polymeric films to be removed; iii) the 580 influence of the application methodology on the cleaning outcome. A series of systematic tests was 581 performed and a coherent and clear picture emerged. A water/C 9-11 E5.5/BuOH/MEK NSF was 582 selected for this study and was firstly characterized by means of SANS measurements, which showed 583 that rod-like nonionic micelles are dispersed in a water/BuOH/MEK mixture close to the system 584 cloud point. These features possibly make this NSF particularly effective in polymer removal. Then, 585 glass, marble, and polystyrene specimens were coated with four different polymers, including two 586 vinyls and two acrylics, each applied either as a solvent solution or as an aqueous emulsion. It was 587 found that the NSF/polymer film interaction is greatly dependent on the film structure and 588 composition. Films formed from solvent solutions can be swollen by water/organic solvents mixtures 589 or dewetted when a surfactant is added to the cleaning fluid; films formed from polymer latexes, on 590 the other hand, are generally swollen even just by water, but they tend not to dewet. This happens 591 independently from the chemical nature of the polymer, and is a direct consequence of its structure 592 and composition, which includes a significant amount of amphiphilic additives. These substances 593 alter the energetic balance of the liquid/polymer/solid system and stabilize the film, which does not 594 dewet. However, these films are easily removable from the substrates, meaning that the action of the 595 cleaning fluid induces loss of adhesion, similarly to what happens during the first stages of the 596 dewetting process that occurs for films cast from polymer solutions. The substrate also plays an 597 important role in the removal of polymer films formed from solutions. In this case, the more the film 598 is affine to the substrate, the harder its removal. In the limit case, the removal of an acrylic polymer 599 from polystyrene could be achieved only through selective cleaning action using a NSF-loaded 600 highly retentive chemical hydrogel, which grants significantly more controlled performances than 601 traditional cellulose pulp poultices. These results have twofold relevance: they deepen the knowledge 602 of the physico-chemical processes that underpin phenomena of daily conservation practice, and 603 provide conservators with innovative solutions to face new challenges in art preservation. 604 605

Conflict of Interest 606
The authors declare that the research was conducted in the absence of any commercial or financial 607 relationships that could be construed as a potential conflict of interest.