Super-Hydrophobic Polymer/Nanoparticle Composites For The Protection Of Marble Monuments

Abstract

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 162o and 6o respectively, indicating that nanoparticles enhance the hydrophobic nature of the film. We note that the corresponding angles were 108o and 20o 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 micro-and nano-structure of the composite film as a function of the particle concentration. Enhanced hydrophobicity was explained by the Cassie-Baxter model.