Freeze-Thaw and UV Resistance in Building Stone Coated with Two Permanent Anti-graffiti Treatments

Abstract

Cultural heritage weathering by traditional agents (water, air pollutants, biological agents and so on) is aggravated by direct human action in the form of graffiti. New protective treatments have been developed to counter the visual impact and deterioration induced by such vandalism. While these coatings prevent paint from penetrating in the pore systems of construction materials or from adhering to their surface by lowering surface energy, their application to historic buildings and monuments is restricted. This study aims to contribute to the acquis on the performance and compatibility of these protective treatments in the context of the conservation of historic building materials. More specifically, it explores the durability of two stones (granite and limestone) protected with two permanent anti-graffiti coatings (a fluorosiloxane and a Zr-ormosil) when exposed to a total of 30 freeze-thaw cycles and 2,000 h of UVA radiation with water spray in a QUV weathering tester. The results of the two durability tests conducted revealed that both coatings accelerated limestone deterioration by obstructing the egress of water trapped in the pore systems during successive thawing episodes. By contrast, granite showed no external signs of damage. Colour analysis detected no visible decay in either of the coated limestone surfaces after UV radiation (no yellowing or darkening). The chromatic heterogeneity of granite rendered similar analysis impractical.