Super-hydrophobic Polymer/nanoparticle Composites for the Protection of Marble Monuments

Water contributes substantially to the degradation of monuments through freezing and thawing cycles and also because it is a carrier of pollutants and salt solutions. Water repellency can be promoted by the application of a thin organic, hydrophobic, film. The efficacy of simple polymer coatings for monument protection is, however, still debated. We demonstrate that a polymer/nanoparticle composite film, composed of a siloxane resin and silica nanoparticles has superior properties. The film was applied on specimens of the Greek marble “Penteliko”, which is used for the restoration of the Akropolis of Athens. The equilibrium contact angle and the contact angle hysteresis were found to be 162 and 6 respectively, indicating that nanoparticles enhance the hydrophobic nature of the film. We note that the corresponding angles were 108 and 20 when pure siloxane resin was applied (no particles). Instead of the traditional film application by brush (or capillary absorption), a simple spraying technique was employed. Water vapour permeability, water capillary absorption and colourimetry measurements of the treated marble specimens were also carried out, to reveal the effect of the nanoparticles on these important parameters. The study was then extended to two more types of Greek marbles, with similar results. SEM and AFM were employed to reveal the microand nano-structure of the composite film as a function of the particle concentration. Enhanced hydrophobicity was explained by the Cassie-Baxter model. INTRODUCTION Water contributes substantially to the degradation of monuments through freezing and thawing cycles and also because it is a carrier of pollutants and salt solutions. Water repellency can be promoted by the application of a thin organic, hydrophobic, film. The use of simple polymer coatings for monument protection is, however, still debated because of undesirable side effects developed upon ageing. Nevertheless, research on the protection efficacy of such coatings is still carried out, in the light of recent achievements which provide some very promising results. Several investigations have recently suggested strategies which can impart superhydrophobicity to thin polymer films [1-9]. Many of these strategies however, have some important disadvantages which make them inappropriate for the treatment of large surfaces such as buildings and monuments. For example the use of sophisticated instruments raises the cost of the application method, which therefore cannot be utilized for the treatment of large surfaces. In a previous work, we reported that a simple spraying technique, shown in figure 1, can be used to apply polymer-particle dispersions on large calcium carbonate surfaces [10]. In this study, silica nanoparticles were dispersed in solutions of a functionalized perfluorinated polyether and poly(methyl methacrylate). Dispersions were sprayed on calcium carbonate blocks with the aid of a specific nozzle (figure 1). The resulting films appeared to have an enhanced hydrophobic character [10]. In the present work we apply this strategy using a commercial siloxane resin and silica nanoparticles for the preparation of the dispersions, which are subsequently sprayed on three Greek marbles: Pentelic, Naxos marbles and Thassos. The wettability of these surfaces is discussed in the light of the Cassie-Baxter model. The applicability of the latter is investigated in detail using (instead of marble) glass surfaces as substrates. Water vapour permeability, water capillary absorption and colorimetry 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt.net/search/docs.php3?MainSource=65