JAIC 2004, Volume 43, Number 1, Article 2 (pp. 03 to 21)
JAIC online
Journal of the American Institute for Conservation
JAIC 2004, Volume 43, Number 1, Article 2 (pp. 03 to 21)

POLYMER TREATMENTS FOR STONE CONSERVATION: METHODS FOR EVALUATING PENETRATION DEPTH

FRANCESCA CASADIO, & LUCIA TONIOLO

ABSTRACT—This study examines the measurement of penetration depths for three polymer treatments for stone conservation—acrylic, partially fluorinated acrylic, and siloxane polymers. The treatments were tested on three types of calcareous stones of increasing porosity, widely present in historic architecture and monuments in Italy. Various approaches were explored, including direct determination with various Fourier transform infrared spectroscopic techniques (micro-attenuated total reflection, diamond cell transmission, and hyphenated with thermogravimetry) as well as indirect methods such as static angle measurements. Micro-attenuated total reflection on cross sections of treated specimens proved to be a particularly powerful technique for detection of polymers, with fairly good spatial resolution and sensitivity. The degree of penetration of polymers inside the porous matrix of stone is an important parameter when evaluating the efficacy and durability of treatments. Therefore, developing reliable testing methods for determining penetration depth is crucial.

TITRE—Le traitement de la pierre à l'aide de polymères: méthodes pour évaluer le degré de pénétration. RÉSUMÉ—Cette étude examine le degré de pénétration de trois différents polymères utilisés pour traiter la pierre: l'acrylique, l'acrylique partiellement fluoré et le siloxane. Les traitements ont été effectués sur trois types de pierres calcaires de porosité variée, qui sont fréquemment utilisées dans les édifices et monuments historiques en Italie. Diverses approches ont été testées, dont la détermination directe avec différentes techniques spectroscopiques infrarouges par transformée de Fourier (réflexion totale micro-atténuée, transmission en utilisant une cellule à enclumes de diamant et détermination progressive par thermogravimétrie), ainsi que des méthodes indirectes telles que des mesures statiques des angles. L'étude par réflexion totale micro-atténuée sur des coupes stratigraphiques de spécimens traités s'est avérée être une technique particulièrement efficace pour la détection des polymères, avec une résolution spatiale et une sensibilité adéquate. Le degré de pénétration des polymères à l'intérieur de la matrice poreuse de la pierre est un paramètre important pour évaluer l'efficacité et la longévité des traitements. Par conséquent, il est crucial de développer des méthodes fiables pour déterminer le degré de pénétration.

TITULO—Tratamientos con polímeros para la conservación de la piedra: métodos para la evaluación de la profundidad de penetración. RESUMEN— Este estudio examina las mediciones de profundidad de la penetración de tres tratamientos con polímeros utilizados en la conservación de piedra—acrílico, acrílico parcialmente fluorinado y polímeros de siloxano. Estos tratamientos fueron realizados sobre tres tipos de piedras calcáreas cada una de mayor porosidad que la anterior, las cuales son encontradas muy frecuentemente en monumentos y arquitectura histórica en Italia. Se exploraron varias formas de enfocar esta investigación, incluyendo una determinación directa con varias técnicas de espectroscopía infrarroja con transformadores de Fourier—FTIR (reflexión total micro atenuada, transmisión de célula de diamante, y termogravimetría en tándem con FTIR), tanto como métodos indirectos tales como las medidas de ángulo estático. La reflexión total micro atenuada sobre cortes estratigráficos de las muestras tratadas resultó ser una técnica particularmente potente para la detección de polímeros, con una resolución espacial y sensibilidad bastante buena. El grado de penetración de los polímeros dentro de la matriz porosa de la piedra es un parámetro importante cuando se evalúan la eficacia y durabilidad de los tratamientos. Por lo tanto, el desarrollo de métodos de examen que sean confiables es crucial para determinar la profundidad de penetración.

TÍTULO—Tratamentos com polímero para conservação de pedra: métodos para avaliar a profundidade de penetração. RESUMO—Este estudo trata da medição da profundidade de penetração em três tratamentos com polímeros para conservação de pedras ‐ acrílico, acrílico parcialmente fluoretado e polímeros siloxanos. Os tratamentos foram testados em três tipos de pedras calcáreas de porosidade crescente, largamente usadas na arquitetura histórica e monumentos na Itália. Foram analisados vários procedimentos, inclusive a determinação direta através de várias técnicas espectroscópicas de infravermelho com transformada de Fourier (reflexão total micro-atenuada, transmissão em célula de diamante e hifenado com termogravimetria), assim como a utilização de métodos indiretos como medições de ângulo estático. A reflexão total micro-atenuada em secções transversais de espécimens tratados provou ser uma técnica particularmente poderosa para detecção de polímeros, com resolução espacial e sensibilidade razoavelmente boas. O grau de penetração dos polímeros no interior de matriz porosa de pedra é um parâmetro importante quando se avalia a eficácia e a durabilidade dos tratamentos. Por esta razão, é crucial desenvolver métodos confiáveis de teste para determinação da profundidade de penetração.


1 INTRODUCTION

Procedures for testing stone materials treated with polymer products are available in the literature, formulated by both national (in Italy, the NorMaL Commission) and international committees (RILEM; working groups 25 PEM and 59 TPM). Recommended tests primarily involve assessing the performance of polymers and measuring detectable variations in the macroscopic properties of the treated substrates, such as, for example, superficial colorimetric characteristics, porosity, capillary water absorption, water vapor absorption, superficial water repellency, and strength. Very little attention has been given, however, to penetration depths achieved by various polymer products or blends of products available and widely used for conservation purposes. D. Honeyborne reported “as long ago as 1932” that “a common cause of failure of stone preservatives is that, even in porous materials, and under the most favorable conditions, the preservative penetrates only to a relatively small depth, and a surface skin is formed which differs in physical properties from the underlying material” (quoted in Ashurst and Dimes 1990, 158). Similarly, in a detailed review of current research on stone conservation in 1996, Price (1996, 19) stated that “little attention has been given to the distribution of products within stone at the microscopic level. Little is known about the bonding, if any, that takes place between treatment and the substrate, and much is left to chemical intuition.” Still another discussion of the issue of polymer penetration is presented by Charola (1995, 13), who argues that

with regards to the depth of impregnation, there is no consensus. Some consider that durability is improved by a deeper impregnation (Nägele 1985), while others recommend only a surface spraying (Sramek 1993). Furthermore, there is no clear delimitation between the treated and untreated area (Roth 1988), and it has been established that the effectiveness of the treatment is not equal in depth (Wendler et al. 1992).

The depth of penetration of solutions of polymers within the stone matrix is strictly correlated to:

  • porosimetric features of the stone material (i.e., total open porosity and pore size distribution);
  • specific surface, wettability, and superficial polarity of the stone substrate;
  • properties of the polymer solution;
  • mode of application of the protective treatment (by poultice, brush, spray, or impregnation by total immersion at atmospheric pressure or under vacuum);
  • microclimatic conditions of curing of the treated samples (temperature, relative humidity, whether the atmosphere is saturated with vapors of the solvent, etc.).

This article addresses the issue of measuring the actual penetration depth achieved by polymer treatments, reporting the development and evaluation of different testing methods. To explore the capabilities of the various methods proposed, polymers belonging to two very widely employed classes—acrylics and siloxanes—were studied. In particular, an experimental partially fluorinated acrylic polymer (TFEMA/MA; 2,2,2 trifluoroethylmethacrylate/methylacrylate copolymer), whose synthesis, characteristics, and applications have been described in detail in previous studies (Alessandrini et al. 2000a; Ciardelli et al. 2000), was tested with respect to its penetration depth and compared to its nonfluorinated homologue Paraloid B-72 (EMA/MA; ethylmethacrylate/methylacrylate copolymer).

Paraloid B-72 has been extensively used in Italy since the early 1970s both as a consolidant and as protective treatment (Roby 1996), although its water repellency cannot be considered fully satisfactory. Acrylics are very interesting materials for applications in conservation, however, because they remain almost soluble, and therefore removable, after curing and over time. Indeed, TFEMA/MA has been developed with the aim of enhancing the water repellency of an easily fine-tunable acrylic structure so as to obtain a fairly good protective material. The introduction of fluorine in a short side chain of the macromolecule, in fact, guarantees very good performance and satisfactory durability (Alessandrini et al. 2000b; Toniolo et al. 2002). Among the siloxanes, poly-dimethylsiloxane, Wacker 290, was selected. This product achieves good protective performance thanks to the adhesion properties and stability of its cross-linked structure (van Hees et al. 1997).

All the polymers were applied on stone materials by capillary absorption, which offers controlled and reproducible conditions of treatment. The concentration and solvents in the polymer solutions were the same as those currently used in conservation practice.


1.1 OVERVIEW OF CURRENT RESEARCH ON POLYMER PENETRATION

Two approaches are viable for determining the depth of penetration of polymers within a porous stone substrate:

1. direct methods, involving either staining tests (with 1, 5-diphenylthiocarbazone, iodine vapor, Rhodamine B, and other fluorescent dyes) or direct detection of the applied product by instrumental chemical analysis, primarily with analytical techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy‐energydispersive x-ray spectrometry (SEM-EDX), and x-ray photoelectron spectroscopy (XPS)

2. indirect methods, involving assessment of the polymer treatment's modification of stone materials.

For protective products, such modifications are connected to variation in water repellency and physical properties related to interaction with water. The methods include measurement of contact angle, water drop absorption time, acid etching, capillary water absorption, and water absorption through a pipe. If the product shows consolidating properties, indirect methods of analysis can also include measuring physical‐mechanical parameters such as bending strength, splitting tensile strength, modulus of elasticity, drilling resistance, and ultrasonic velocity. The penetration depth values thus obtained must be considered specific to each method, and data are complementary. For example, the water repellency and consolidating effect might be detected at different depths within the stone material, and, in addition, the polymer might be chemically identifiable even in areas where such effects are not evident.

In general, published methods for determining penetration of polymers in stone matrix are indirect methods and are characterized by varying degrees of operational complexity. Pioneering work by Domaslowski (Domaslowski and Lehmann 1972; Domaslowski and Kesy-Lewandowska 1985; Domaslowski 1988) and Lewin (Lewin and Papadimitriou 1981) investigated the impregnation behavior of solutions of polymers in different solvents by immersing slices of the treated stone in hydrochloric acid (HCl) and assuming that portions insoluble in the acid were those reached by the polymer. The method is, of course, very approximate; the spatial resolution achievable is very low, and there is no indication as to what concentration of polymer within the stone matrix is sufficient for effectively shielding the inorganic substrate from acid attack. Kumar and Ginell (1997) published a simple procedure involving exposing cut sections of treated limestone to iodine vapor and visually assessing the penetration depths achieved by various polymers. This method is fast and requires only an easily accessible, inexpensive apparatus, but errors can result if it is not skillfully performed. Exposure time, for example, needs to be adjusted depending on product concentration, and if the test is executed prior to complete evaporation of the solvent, there is a risk that the solvent's penetration depth may be observed and recorded, rather than the polymer's. Staining methods using various dyes have been reported by many authors: Golikov and Zharikova (1990) described the use of fluorescent dyes for locating treatments within the stone, but the nature of the reagents is patented and so not explained clearly, and the method seems applicable to very few materials. Kumar and Ginell (1997) performed some tests with aqueous solutions of Rhodamine B, but this dye worked only with epoxy-resin treated materials.

More recently a European interlaboratory research group published a comparative study of different methods for determining penetration depths of polymers in porous stone. This study was part of the European Community Hardrock project that developed and validated an innovative methodology for evaluating the mechanical characteristics of monumental stones and related materials (Leroux et al. 2000). Among the methods evaluated were spot tests with 1, 5-diphenylthiocarbazone (reacting with tin contained in catalysts for many silicone resins, but interference from other metals produced frequent false positives); wetting of treated surfaces and visual evaluation of hydrophobicity; microdrop absorption (a very simple and easy method, but not a very accurate one); acid etching of stone slices (applicable only to carbonate stone); microdrilling resistance (ineffective with marble); ultrasonic velocity tracing (more effective with sandstone). The results obtained by these authors indicate how indirect methods of analysis can establish penetration depth values. The study also revealed that the effectiveness of various tests depends on the nature of the stone substrate. That different laboratories obtained different values highlights the problem of reproducibility.

More sophisticated indirect techniques are described by Giorgi et al. (2000), in which nuclear magnetic resonance (NMR) imaging is used to visually determine distribution of water inside treated porous stone, thus implying the distribution profile of protective polymers. In the past few years, numerous authors have evaluated the potential of this technique for mapping the distribution of polymers inside stone, but the results are still somewhat ambiguous (Piacenti et al. 1999; Alesiani et al. 1999, 2001; Borgia et al. 2000, 2001; Appolonia et al. 2001).

As for direct methods for determining the depth of penetration of polymers within a porous stone substrate, a few publications describe applications of FTIR spectroscopy (Lemaire et al. 1996, 1997), either in transmission mode on samples only a few µm thick or in photoacoustic mode on unaltered samples taken from historic buildings subjected to hydrophobic treatments. FTIR spectroscopy in reflection mode was also used on sections of various stones treated with silicone resins and coordinated with measurements of water uptake of the same sections in order to link the hydrophobic properties of the treated stone to the presence of the polymer (Franke et al. 1997). It is worth noting, however, that the FTIR techniques employed by these researchers can be used to analyze only certain kinds of substrates. Photoacoustic spectroscopy, for example, can be used to investigate only the sample's uppermost layers (approximately the first 20 µm): it is not suitable for depth-profiling. Reflection FTIR analysis is successful only with certain types of stone and is generally ineffective for porous substrates, as their rough surfaces give rise to poor specular reflectance. Finally, transmission FTIR spectroscopy requires samples to be so thin that they are transparent, only a few microns thick, supported on infrared absorption-free substrates. Thus sample preparation is a delicate and time-consuming operation, and some stone samples are likely to crumble to pieces in the process.

Data on the chemical or physical-chemical interaction between porous stone and polymer product remain very scarce (Danehey et al. 1992; Elfving and Jäglid 1992; Kumar 1995; Spoto et al. 2000; Rizzarelli et al. 2001).


1.2 RESEARCH AIMS AND EXPERIMENTAL APPROACH

The work described in this article explores various approaches using FTIR accessories and equipment to understand whether and how polymers penetrate inside the porous matrix of stone. Micro-attenuated total reflection (ATR), diamond cell transmission, and thermogravimetry-FTIR were evaluated. Depth-profiling of polymers inside stone substrates by direct determination with vibrational spectroscopic techniques has the advantage of providing information on the actual molecular structure of the macromolecule deposited inside the porosity of the stone. This information can help to verify possible modifications induced by humidity, solvent residues, salts, or particular chemical compounds and to evaluate the presence of chemical or electrostatic interactions between polymer and stone or to assess the occurrence of chromatographic separation effects during treatment. A systematic study of the potential and limitations of different infrared techniques for the investigation of stone-polymer systems has been carried out with the aim of setting up a reproducible procedure that can be used either to control conservation treatments or during maintenance operations on suitable samples of stone collected from actual historic surfaces.

It must be borne in mind that concentration of polymers inside stone materials is highly dependent on stone type but is always very low, ranging from a few grams to a maximum of approximately 500 g per square meter. These quantities, especially with lowporosity stone (such as marble), are often very close to the minimum detection limits of analytical instrumentation. Nevertheless, it is likely that the polymer can display its water-repellent effect even well below the instrumental detection limits. Therefore the analytical problems to be solved were particularly challenging. Indeed, the various instrumental setups and sample manipulations described here were developed in response to the need for detecting the presence and distribution of very small amounts of different polymers within the porous matrix of stone, with a widely used and well-established technique such as FTIR spectroscopy.


2 MATERIALS


2.1 STONE SPECIMENS

The project focused on lithotypes of varying porosity, widely occurring in historic architecture and monuments in Italy. In particular, results are reported regarding substrates listed below:

  • Noto stone, a highly porous calcareous stone with total open porosity ranging from 25% to 35%, widely present in the historic architecture of the famous baroque town of Noto, near Syracuse, Sicily.
  • Yellow Angera stone, a Triassic dolomitic sedimentary rock containing trace amounts of clay and iron hydroxide (responsible for the yellow color), characterized by 11.5% total open porosity, widely present in structures in Lombardy.
  • Candoglia marble, a metamorphic, compact calcareous stone characterized on average by 1% total open porosity, largely employed in building and sculpture: it is the construction material of the Cathedral of Milan (14th‐19th century).


2.2 POLYMER TREATMENTS

The resins used for treatment of the stone specimens were:

  • Paraloid B-72 (Rohm & Haas), ethylmethacrylate/methylacrylate copolymer
  • TFEMA/MA, a partially fluorinated, experimental, 2,2,2 trifluoroethylmethacrylate/methylacry-late copolymer (Ciardelli et al. 2000), basically a fluorinated homologue of Paraloid B-72;
  • Wacker 290 (Wacker Chemie), a water repellent based on oligomeric alkylalkoxysiloxanes.

Details concerning these polymers' characteristics (including molecular weights and glass transition temperatures [Tg]), efficacy, and UV resistance are reported elsewhere (De Witte et al. 1993; Melo et al. 1999; Chiantore and Lazzari 2000; Alessandrini et al. 2000a, 2000b).


3 METHODS


3.1 STONE TREATMENT

Suitable stone samples, measuring 50 x 50 x 20 mm, were smoothed with abrasive carborundum paper (180 grit), washed with deionized water, and dried until constant weight was reached. Then the samples were treated by capillary rise absorption with solutions of the three different products: a 5% w/w solution of Paraloid B-72, a 5% w/w solution of TFEMA/MA, both in ethyl acetate, and an 8% w/w solution of Wacker 290 in white spirit. Treatment time varied from six seven hours, depending on the porosity of substrate. The quantity of polymeric material impregnating the stone specimens as a result of the treatment is reported in table 1.


3.2 TECHNIQUES OF ANALYSIS

Depending on the type of investigation performed, samples were prepared in two different ways for depth-profiling: treated samples were cut with a diamond precision saw (Struers Minitom) either as sections measuring 10 x 15 x 2 mm with plane surfaces parallel to the impregnated outer surface, or as cross sections, sawing along the direction of capillary rise of the solution, i.e., perpendicular to the treated uppermost surface (fig. 1).

Cross sections were investigated with the following techniques:

  • micro-ATR spectroscopy: performed with a FTIR Spectrophotometer Perkin Elmer Spectrum 2000 equipped with nitrogen-cooled MCT detector and microscope with video camera, motorized stage, and germanium (Ge) crystal micro-ATR accessory
  • static contact angle measurement: executed with a Lorentzen and Wettre instrument, modified with tilting sample holder. Sections were investigated with the following techniques:
  • diamond cell FTIR spectroscopy: performed with a FTIR Spectrophotometer Perkin Elmer 1725x equipped with DTGS detector and Graseby-Specac diamond cell accessory
  • TG-FTIR: carried out with DuPont Instruments thermal balance 951 coupled to a FTIR Spectrophotometer JASCO FT/IR 420, equipped with interchangeable DTGS and MCT detectors and White's gas-cell with an optical path length of 120 cm.


4 RESULTS AND DISCUSSION


4.1 MICRO-ATR SPECTROSCOPY

Examination was performed on cross sections placed on the motorized microscope stage and subjected to repeated spectral recordings at increasing depths. The technique is very promising because it allows direct, nondestructive probing of the examined surface, coupling microscopic visible-light observation with infrared spectra as a function of Cartesian coordinates with a spatial resolution of 50‐100 μm, without further manipulation or preparation of the sample. The major drawback of the method is the need for perfect contact between the ATR probe crystal and the investigated micro-area (100 μm in diameter) to obtain a reasonable signal and an acceptable spectrum, a condition not easily achieved when investigating rough stone surfaces. In addition, the technique, like other infrared applications, is generally qualitative, allowing at best semi-quantitative assessment of the polymer's presence at different depths.

Results obtained on highly porous calcareous

Table . Quantity of Polymeric Meterial Impregnating the stone Sample as result of Treatment Application
Fig. 1. Schematic representation of the preparation of treated stone samples into sections prior to analysis
stone (Noto stone) were particularly promising, since different impregnation behaviors toward the same stone matrix were noted for the polymers investigated. In particular, not only could the presence or absence of treatment be ascertained by following a characteristic absorption band chosen as polymer tracer, but an evaluation of the areas of marker peaks at different depths allowed determination of the relative dose of polymeric treatment. After normalization of all the spectra with respect to the band centered at 873 cm-1 (due to out-of-plane bending vibration of the carbonate ion [#2CO3] in calcium carbonate [CaCO3] [Gadsden Aric 1975]), a sharp and scarcely interfered peak, generally constant in absorbance throughout all measurements), areas of the selected marker band at different depths were calculated. Assuming that the polymer dose at the top surface of the specimen was 100%, the ratio between peak areas was calculated, and polymer doses at different depths were assessed. The results obtained (summarized in table 2. and showed in fig. 2) evidenced that:
  • The presence of Paraloid B-72 within the porous matrix was detectable down to 2.5 mm below the top surface. However, the amount of product reaching inner regions of the treated stone was fairly low. In fact, a significant decrease in the concentration of polymer beneath the top surface was evident, with levels dropping to 14% of the initial value just 1.5 mm underneath the upper surface.
  • TFEMA/MA was detected down to 4.5 mm inside the treated stone. In addition, doses of the fluorinated acrylic polymer showed a less abrupt drop, going down to 66% of the value of the uppermost surface at 1.5 mm of depth, 18% at 2.5 mm, and 14% at 4.5 mm from the top surface.
  • Wacker 290 was shown to impregnate the entire thickness (20 mm) of the tested samples with a homogeneous distribution. A gradual decrease in polymer dose was observed, with values equal to 50% of the surface value still present down to 4.5 mm inside the stone; and from 8.5 mm to the very bottom of the samples, fairly constant doses were observed, approximately equal to 23% of the surface value.

In the case of marble, a lithotype characterized by low total open porosity (< 1%), positive identification of the different polymers by the micro-ATR mapping technique was confined to the first few microns (around 100 µm) of the top surface.


4.2 DIAMOND CELL FTIR

Since the main difficulty in direct determination of polymers in treated stones is often the dominating signal of the inorganic substrate, which masks the weaker signals of organic compounds, one analytical strategy involves eliminating the calcareous stone matrix by acid attack with 0.2 normal (N) HCl solution followed by analysis of the residues with diamond cell FTIR spectroscopy. By removing all interfering CaCO3, whose intense absorption bands often mask peaks belonging to the polymeric material, the presence of the different treatments can be unambiguously detected. Instead of probing a single cross section surface of a sample, this methodology

Table . Depth Profiling of Polymer Doses Resulting from Semiquantitative Treatment of Micro-ATR Spectrophotometric Data (calculated as percentages of polymers with respect to an uppermost value of 100%)
has the advantage of allowing detection of the polymer in a greater volume of the treated stone; thus it is more reliable about the actual presence of the polymeric material. Although the method is applicable only to calcareous stone substrates, it is very effective both for silicone and acrylic resins because treatment with diluted acid does not affect the polymers, as no variations were observed in the spectra of polymers subjected to acid attack under the same experimental conditions. The method is very sensitive and requires careful manipulation of the sample. The spatial resolution achievable is limited by the need to cut the samples and by the loss of stone material involved in the cutting operation, corresponding to the blade thickness (see fig. 1).

Acid attack was performed on powders ground from different sections of the samples to obtain information about the depth to which the polymer had penetrated. The technique proved to be more effective with stone substrates characterized by medium-high porosity values than with low-porous stones such as marble, since high-porous stones contained significantly higher quantities of polymeric materials. Samples of yellow Angera stone and Noto stone treated with Paraloid B-72 confirmed the tendency for the acrylic resin to form a film, as it was detected only in the uppermost section examined for both (i.e., within the first 2 mm). Figure 3 demonstrates how closely the spectrum obtained on the residue, after acid treatment, of the powder obtained from the uppermost section of a treated Noto stone sample matches the reference standard spectrum for Paraloid B-72. The only extraneous bands in the spectrum are those characteristic of silicates and iron oxides and hydroxides present in Noto stone and of calcium chloride (CaCl2) (as the bi-or hexahydrate salt, formed during treatment of CaCO3 with HCl), the latter centered at 1640 cm-1 in association with the broad band due to stretching modes of hydroxyl (OH) groups, centered at 3427 cm-1 (Nyquist and Kagel 1997).

On the other hand, investigation of samples treated with Wacker 290 confirmed in-depth impregnation of both substrates by the siloxane oligomers. In fact, traces of the silicone resin were still detectable in sections corresponding to all levels inside the stone samples (fig. 1: positive detection down to section Z = 18‐20 mm). Figure 4 shows the spectrum obtained after acid dissolution of a section corresponding to 12‐14 mm inside Noto stone treated with Wacker 290. It is interesting to note that

Fig. 2. Decrease of polymer doses within the treated stone as a function of distance from the uppermost surface of the sample
the diamond cell FTIR technique allows complete fingerprinting of the organic compound, instead of detecting primarily a characteristic marker band, as happens with the micro-ATR technique, whose spectra invariably show prominent features of the inorganic matrix.

In the case of marble, positive identification of polymers was possible only in the powders obtained by scraping the top surface of the stone samples with a scalpel. The treatment was invariably located in the uppermost region of the first cut section and, when sections cut from deeper regions were attacked with acid, no residue of polymer was recovered.


4.3 TG-FTIR

The combination of thermogravimetric and infrared analyses provides precise quantitative data on the various components of the investigated systems along with step-by-step infrared fingerprinting of volatile compounds emitted during thermal degradation. It is thus possible to ascribe each individual weight loss to the emission of certain gases, therefore identifying without any ambiguity the compound responsible for such weight loss.

For the analysis, sections of the treated stone samples were ground and 30 mg of the resulting powder was placed in the platinum crucible of the thermobalance and submitted to heating from 40°C to 900°C at a speed of 10°C/min in helium flux.

In the examined samples, the technique was capable of discriminating between organic polymer and inorganic matrix. Moreover, the method has the advantage of allowing precise quantitative determination of the doses of polymers, can highlight eventual interaction between the organic and inorganic phases, and has the benefit that, following cutting of the samples into sections, it requires very simple preparation. The main disadvantages of the technique are the limited spatial resolution due to the sample cutting procedure (see fig. 1) and its inappropriateness for resins with a complex degradation pattern (such as highly cross-linked resins) that leave a significant carbonized residue after degradation. As an example, no significant results were obtained from specimens treated with the siloxane Wacker 290 because of its

Fig. 3. Diamond cell FTIR spectra of: (a) reference sample of Paraloid B-72; (b) residue recovered after acid attack of powder obtained from the uppermost section (0‐2 mm) of a specimen of Noto stone treated with Paraloid B-72 (asterisks mark bands characteristic of calcium chloride hexahydrate [CaCl26H2O]).
Fig. 4. Diamond cell FTIR spectra of: (a) reference sample of Wacker 290; (b) residue recovered after acid attack of powder obtained from an inner section (Z = 12‐14 mm) of a sample of Noto stone treated with Wacker 290 (asterisks mark bands characteristic of CaCl26H2O).
highly cross-linked structure and its degradation pattern that does not produce a sharp, definite weight loss. On the other hand, the method was successfully applied to the investigation of treatments with acrylic resins. In fact, both the acrylic and the fluorinated acrylic resins exhibit a weight loss due to the polymer's chain scission, which is caused by thermally induced “unzipping” depolymerization that produces a complete volatilization of the degradation products, with no charred residues (Kashiwagi et al. 1985, 1986; Manring 1988, 1989, 1991; Manring et al. 1989; McNeill 1992). In the case of Paraloid B-72, such weight loss has a maximum in its differential thermogravimetric (DTG) curve at 385°C, corresponding to the evolution of ethylmethacrylate monomer and CO2 (fig. 5), clearly distinguishable from the weight loss due to decomposition of the carbonate matrix of the stone, which has a maximum in the DTG at 777°C. In the case of TFEMA/MA, the weight loss due to the degradation of the polymer takes place at 392°C, with the evolution of trifluoroethylmethacry-late monomer, small quantities of methylacrylate, CO2, aldehyde compounds, and methane.

Differences in penetration depths reached by Paraloid B-72 and its fluorinated homologue in Candoglia marble and Noto stone were determined. On one hand, both resins were observed only in the first section of treated marble samples, corresponding to the first 2 mm of the uppermost surface. On the other hand, when applied on highly porous substrate such as Noto stone, TFEMA/MA was found to be present in two sections, corresponding to 5 mm from the top surface, while Paraloid B-72 was confined only to the first section. Moreover, the quantity of Paraloid B-72 impregnating the first section of the Noto stone sample was found to be equal to 5.7% of the total weight of sample analyzed, while it was equal to 0.43% in Candoglia marble. In the case of TFEMA/MA, 8.01% of the polymer was contained in the uppermost section of Noto stone, and 1.15% in the uppermost marble section table 3.

It is interesting to note that the thermal degradation of the polymeric chains, corresponding to a maximum in the DTG curves, takes place at lower temperatures for the polymers applied on stone, with respect to temperatures observed for the pure polymers used as standard reference. This observation led to the hypothesis that polymers establish some electrostatic interaction with the stone substrate, resulting in slightly lower electronic density in proximity to the backbone of the polymer, which is, therefore, more susceptible to thermal degradation (i.e., slightly less energy is required to break the polymer chain).

Fig. 5. (a) TG and DTG curves relative to the analysis of an uppermost section of Noto stone treated with Paraloid B-72; (b) FTIR spectrum of gases evolved in correspondence of the 385°C weight loss (showing characteristic absorptions of ethylmethacrylate [EMA] and CO2).

Table . Percentages of Acrylic Polymers (calculated with respect to total weight of sample analyzed) Impregnating the First Sections (0‐2 mm from top surface) of Treated Noto Stone and Candoglia Marble, as Determined with Thermogravimetric Analysis


4.4 STATIC CONTACT ANGLE

The determination of superficial water repellency of stone-polymer systems by measuring static contact angles can indicate the presence of polymers on porous substrates. When water repellency is measured on the surface of cross sections of treated samples, data obtained can be related to penetration depths of polymers inside the stones (Biscontin et al. 1993; Littmann et al. 1993; Delgado Rodrigues et al. 1996; Leroux et al. 2000). This very simple and lowtech method was used to check the general reliability of results obtained by various infrared spectroscopic approaches. Major advantages are the ease of measurement and the effective visualization of the hydrophobization of the substrate, which, in turn, indicates the actual polymer's presence within the stone material. Major limitations are low spatial resolution (due to the relatively large dimensions of the microdrops) and the nature of indirect measurements, which are sometimes prone to more ambiguity than direct determinations.

The impregnating behavior of the three tested resins applied on medium (Angera stone) and high (Noto stone) porous stone was compared. Figure 6 reports values of contact angle measured on cross sections of treated Angera stone samples as a function of depth profile. It is evident that, while Wacker 290 evenly impregnated the whole sample, providing widespread water repellency, Paraloid B-72 was confined in the upper sections, since the stone immediately absorbed drops deposited 3 mm below the uppermost surface.

Measurements of static contact angles at different depths were also associated with photographs, taken through a stereomicroscope, of the water-repellency properties of the tested surfaces. Water microdrops were deposited on the whole length of cross sections, and a photograph was taken after two minutes in order to record which, among the drops, retained their spherical shape because of the hydrophobicity of the surface and which, instead, had been absorbed. Results are shown in figure 7‐11. They clearly indicate

Fig. 6. Comparison of measured static contact angle values for cross sections of Angera stone treated with Paraloid B-72 and Wacker 290 versus depth profile
that, both in Angera and in Noto stone, Paraloid B-72 is less penetrating than Wacker 290. In addition, a comparison of the behaviors of the nonfluorinated and fluorinated acrylics when applied to the same stone substrate shows that the fluorinated compound penetrates deeper. As an example, figure 9 shows how application of TFEMA/MA on Noto stone gives hydrophobicity to the first 2 mm of the cross section, as opposed to Paraloid B-72, whose treatment confers water repellency only to the first millimeter of the treated Noto stone cross section (fig. 8).

In the case of marble, static contact angles measured on the treated surface generally showed lower values than those measured on samples of more porous substrates treated with the same polymer material. Surface roughness is, in such measurements, the critical parameter directly correlated with the static angle values (Della Volpe et al. 2000) and, in the case of marble surfaces, the angles' values generally match those observed for the polymer itself when applied as a film on an inert and smooth substrate (glass or metal slides). Moreover, the testing of the marble cross sections did not allow visualization of water drops beneath the uppermost surface of the specimens, thus confirming the limited penetration of the various polymers on this substrate.


5 CONCLUSIONS

The results presented help to evaluate the potential of a wide range of infrared spectroscopic techniques, coupled to appropriate sampling methods, in identifying and characterizing, selectively and with good sensitivity, the depth of penetration of polymeric materials applied as protective treatments on stone substrates with different porosities. The experiments confirm that texture and porosity of the stone together with chemical nature and quantity of applied polymers can affect the pattern of resin distribution

Fig. 7. Photomicrograph of the hydrophobization properties of the surface of a cross section of yellow Angera stone treated with Paraloid B-72 (photograph taken two minutes after deposition of microdrops; each bar indicates 1 mm)
Fig. 8. Photomicrograph of the hydrophobization properties of the surface of a cross section of Noto stone treated with Paraloid B-72 (photograph taken two minutes after deposition of microdrops; each bar indicates 1 mm)
Fig. 9. Photomicrograph of the hydrophobization properties of the surface of a cross section of Noto stone treated with TFEMA/MA (photograph taken two minutes after deposition of microdrops; each bar indicates 1 mm)
and provide useful guidelines for selection of the most appropriate methodology to evaluate a treatment's penetration.

In general, micro-ATR is considered the most powerful and widely applicable technique for characterizing types of polymers and substrates. Micro-ATR has the highest spatial resolution and precision among the evaluated methods when good contact between the ATR crystal and the examined surface is obtained.

The other testing methods used in this study are also valuable, but they are affected by poor spatial resolution (due to the necessary sample preparation and cutting), and/or they are suitable for only a limited number of specific classes of polymers and stones. For example, TG-FTIR is effective for substrates treated with acrylics but not silicone resins, while diamond cell FTIR analysis of the residues of acid attack is useful only for calcareous stone, not for sandstones.

Application of these methods to the depth-profiling and mapping of polymer distribution within treated stones allows differentiation of various polymer behaviors. For medium-to-high porous stones (Angera limestone and Noto calcarenite), the following conclusions can be drawn:

  • The partially fluorinated acrylic copolymer generally achieves greater penetration depths with respect to the nonfluorinated homologue, which, in turn, shows the greatest tendency to form a superficial coating on treated stones.
  • The siloxane polymer has a great ability to impregnate the porous structure of treated stones.

With regard to stone substrates with very low porosity (such as marbles, where penetration depths are limited, i.e., <1 mm), the precise determination of penetration depths requires application of analytical techniques displaying higher spatial resolution. These will allow discrimination among polymers penetrating a range of a few hundreds microns of depth.

Fig. 10. Photomicrograph mosaic showing the hydrophobization properties of the surface of a cross section of yellow Angera stone treated with Wacker 290 (photograph taken two minutes after deposition of microdrops; each bar indicates 1 mm)

Fig. 11. Photomicrograph mosaic showing the hydrophobization properties of the surface of a cross section of Noto stone treated with Wacker 290 (photograph taken two minutes after deposition of microdrops; each bar indicates 1 mm)


ACKNOWLEDGEMENTS

The authors wish to thank Professor Franco Cariati for setting up the TG-FTIR instrument and for scientific advice given throughout the research work. The Fondazione Laboratorio Materie Plastiche is thanked for kindly making available the micro-ATR equipment. The Progetto Finalizzato Beni Culturali is also gratefully acknowledged.



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AUTHOR INFORMATION

FRANCESCA CASADIO received her Ph.D. in chemistry from the Università degli Studi di Milano, Italy. She was graduate intern in the Science Department at the Getty Conservation Institute, Los Angeles, then research fellow at the ICVBC-CNR “Gino Bozza” in Milan. Since July 2003 she has been an A.W. Mellon conservation scientist at the Art Institute of Chicago. Address: Art Institute of Chicago, 111 South Michigan Ave., Chicago, Ill. 60603-6110; e-mail:fcasadio@artic.edu

LUCIA TONIOLO graduated with a degree in chemistry from Università degli Studi di Pisa. Since 1988 she has been a scientific researcher for the Italian National Research Council. Since 1998 she has been senior researcher of the Diagnostic Laboratory of the Centre “Gino Bozza” per lo studio delle cause di deperimento e dei metodi di conservazione delle opere d'arte, Milan. She has authored more than 100 publications in the field of stone materials conservation and pigment analysis. Address: ICVBCCNR Sezione di Milano “G. Bozza,” P. za L. Da Vinci 32, 20133 Milan, Italy; e-mail:lucia.toniolo@polimi.it

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