Abstract
In order to protect a material belonging to Cultural Heritage (i.e., stone, wood) from weathering, and in turn to preserve its beauty and historical value for the future generations, the contact with external harmful agents, particularly water, must be avoided, or at least limited. This task can be successfully obtained with the use of a protective organic coating. The use of nano-metric reinforcing agents in conventional polymeric coatings demonstrated to be a successful route in achieving better protective performance of the films and improved physical properties, even in extreme environments. The present paper would, therefore, review the more recent findings in this field. Generally speaking, when a hydrophobic product is applied on its surface, the stone material will absorb less water and consequently, less substances which may be harmful to it. An efficient organic coating should also supply wear and abrasion resistance, resistance to aggressive chemicals, excellent bond to the substrate; finally, it should be also able to guarantee vapor exchange between the environment and the material interior, i.e., the material should keep the same water vapor permeability as if it was un-protected. To regard to the conservation of wood artifacts, protective treatments for wood will preserve the material from environmental agents and biological attack. Hence, potential advantages of hybrid (organic–inorganic) nano-composite coatings for stone/wood have been found to be: Enhanced mechanical properties in comparison to the pure polymeric matrix, due to the reinforcing effect of the nano-filler; superior barrier properties (the presence of the nano-filler hinders the ingress of water and/or potentially harmful chemicals); optical clarity and transparency. It has been found that the efficacy of a nano-filled coating strongly depends on the effectiveness of the method used to uniformly disperse the nano-filler in the polymeric matrix. Furthermore, the presence of nano-particles should not impair the viscosity of the organic matrix, in order to employ the conventional techniques of application for coatings.
Highlights
As recommended by conservation professionals of historic monuments and building, a protective product for stone surfaces must be able to: protect the stone from external agents and guarantee a high level of hydrophobicity, avoiding in particular the ingress of water, considering that water and water-soluble salts represent the main causes of degradation mechanisms; allow the transpiration of the stone, in order to avoid that the water already present in the substrate can cause further degradation; not alter the color and other optical characteristics—like gloss—of the substrate; be reversible, to permit an easy removing of the coating when it will result no more effective with no damage of the substrate
Addition of silica nano-particles at low concentrations (1–2% w/w) results in the formation of film on stone with a continuous structure having nano/micro-scale roughness (Figure 7), sometimes exhibiting a two-length-scale hierarchical structure on the surface which is the reason for the superhydrophobic nature of these coatings [39]
Contact angle values obtained on surface treated with nanosilica hybrids obtained via sol-gel processes showed enhancement in hydrophobicity, up to super-hydrophobicity
Summary
As recommended by conservation professionals of historic monuments and building, a protective product for stone surfaces must be able to: protect the stone from external agents and guarantee a high level of hydrophobicity, avoiding in particular the ingress of water, considering that water and water-soluble salts represent the main causes of degradation mechanisms; allow the transpiration of the stone, in order to avoid that the water already present in the substrate can cause further degradation; not alter the color and other optical characteristics—like gloss—of the substrate; be reversible, to permit an easy removing of the coating when it will result no more effective with no damage of the substrate. Polymeric coatings, produced starting mainly from acrylic/methacrylic monomers, unsaturated polyesters, fluorinated polymers, epoxy resins, silanes and siloxanes, are able to form hydrophobic and completely transparent films, capable to limit the ingress of fluids into the stone. Among other advantages, their possibility to develop high mechanical resistance upon curing and hardening under a wide range of environmental conditions, low weight, due to the small thicknesses required for an efficient protection, wear and abrasion resistance, good bond exerted to stone [1,2]. Enhanced mechanical superficial characteristics (resistance against abrasion and wear), impact strength and fracture toughness, improved resistance to flame, fire and moisture can be achieved with the introduction of an inorganic nano-filler in a protective organic coating [6]
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