Durability assessment of nanostructured TiO2 coatings applied on limestones to enhance building surface with self-cleaning ability

摘要

Coating with nanostructured Titanium dioxide is able to enhance natural stone surface with self-cleaning and depolluting abilities; nonetheless, the durability of the coatings is a crucial issue to be assessed in relation to their potential for use in the context of the building heritage. In this paper, the resistance of coatings applied on a compact and a soft limestone, which differ in roughness, porosity and cohesion features, has been investigated. A peeling and a water impact test were performed and their effect on the coated surface was evaluated as film loss by qualitative/quantitative ESEM-EDS analyses. Self-cleaning effectiveness was assessed by performing a photodegradation test of Rhodamine B. Crack-free and conformable films showed a good anchorage to both limestones; after the durability tests the coated surfaces exhibited almost unchanged Ti/Ca ratios and self-cleaning efficiencies. Large adhesion failures and decreased Ti amounts were recorded where coatings were fissured; they especially occurred on the surface of the compact limestone, along with a reduced self-cleaning ability. High surface roughness accounted for a better adhesion of both crack-free and fissured films to the soft limestone, but some erosion of the coated surface, which affected this friable stone under the water impact, compromised their preservation. Nonetheless, good self-cleaning performances were still provided due to the retention of nanoparticles under the surface.