Abstract
During the last ten years, photocatalytic nanocomposites combining titania nanoparticles with silicon-based matrices have received increasing attention in the stone conservation research field, because they offer an effective multifunctional approach to the issue of stone protection. However, much work still has to be done in studying the behaviour of these nanocomposites in real environmental conditions and understanding to what extent they are able to retain their effectiveness and compatibility once applied on outdoor surfaces. The latter is a key information that should lie at the basis of any successful conservation and maintenance campaign. The present study provides insight into this relevant topic trough laboratory testing by assessing the artificial ageing of two silane-based photocatalytic nanocomposites, previously selected through an accurate testing on different natural stones. Three accelerated ageing procedures, based on artificial solar irradiation, heating and rain wash-out, allowed simulating about two years of outdoor exposure to some of the weathering factors to which stones are normally subjected. The results provided quite accurate information about the long-term behaviour of the products and on the role that the stone properties play therein. It was shown that, when the products are able to penetrate deeply enough inside the stone pores, they retain much of their hydrophobising and photocatalytic properties and maintain a good compatibility with the stone substrates, even after partial chemical degradation of the alkyl-silica matrices has occurred on the very stone surface.
Highlights
The protection of natural stones used in historical buildings has been traditionally fulfilled through the application of suitable water-repellent polymers, which are known to modify the interface between the stone and the outer environment and, limit the capillary absorption of condensed water into the porous stone network [1,2].During the last ten years, a significant advance in the field of stone protection has been made through the development of nanocomposite treatments, which rely on the combination of traditional polymer matrices with inorganic nanoparticles
In a previous article [37], which discussed some aspects of the chemical degradation of WNC and ANC when applied on glass slides and subjected to solar irradiation for 600 h, it was shown that, while silane matrices alone do not undergo a degradation of the organic functional groups, the addition of
Once the two products were applied on stones and subjected to the same solar irradiation, the result was that their effectiveness in preventing the absorption of water into stones was retained to a very good extent
Summary
The protection of natural stones used in historical buildings has been traditionally fulfilled through the application of suitable water-repellent polymers, which are known to modify the interface between the stone and the outer environment and, limit the capillary absorption of condensed water (with atmospheric pollutants and salts dissolved therein) into the porous stone network [1,2].During the last ten years, a significant advance in the field of stone protection has been made through the development of nanocomposite treatments, which rely on the combination of traditional polymer matrices (mostly polyacrylates or siloxanes) with inorganic nanoparticles. Nanomaterials such as ZnO and TiO2 , which proved to be effective in the photocatalytic oxidation and removal of particulate deposit or in the prevention of biofilm growth on stone artefacts [6,7,8,9], were combined with polymer matrices to obtain multifunctional treatments with hydrophobising and photocatalytic/antifouling properties [10,11,12,13,14] This kind of treatments have received increasing. One issue of great relevance to the evaluation of these multifunctional treatments is their durability [16] This is true, in a general sense, for any kind of protective treatments because the planning of sustainable conservation campaigns on historical buildings and monuments requires that a reasonable prediction should be made as to how long the protective effect of the treatments will endure after their application and exposure in outdoor conditions. These studies addressed e.g., the resistance towards photo-oxidative degradation [17,18,19,20,21] and salt-induced damage [21,22]
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