POLYMER TREATMENTS FOR STONE CONSERVATION: METHODS FOR EVALUATING PENETRATION DEPTH
FRANCESCA CASADIO, & LUCIA TONIOLO
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.