Surface Treatment. The challenge of protecting outdoor exposed monuments from atmospheric attack: experience and strategy

Abstract

Natural and synthetic waxes, acrylic and siloxane resins, perfluoropolyethers, fluorinated polyolefin and fluoroelastomers are some of the most used commercial products employed as protective coatings for stone materials. The application of these products is aimed to prevent the attack of the environmental pollutants and the penetration of water, with the connected risks of freezing and thawing cycles and the crystallization of salt solutions, in order to slow the stone degradation processes. The common and easiest way to pursue this goal is through the deposition, on the stone surface, and possibly deeper beyond the surface, of a coating layer able to modify the interaction of the stone with water, turning the partial hydrophilic character of the stone surface into a high hydrophobic interface. This modification should be achieved by affecting only the thermodynamic superficial potentials, but leaving unaltered, as long as possible, the superficial aspect, morphology, open porosity and roughness. Hence the challenge is avoiding the change of the substrate colour, the natural water vapour permeability and do not react with the stone material in order to respect its natural chemistry. The use of polymers, as protective coatings for stone materials, started in the sixties as a consequence of the expansion of macromolecular chemistry and the successful diffusion of synthetic resins in many different market fields but in most cases of conservation they have been applied without adequate knowledge of the properties of the polymer/stone system and, moreover, without a satisfactory optimization of the molecular structures for protection purposes. It is necessary to point out that the various commercial products rarely have been studied and developed expressly for applications in this field, but they have been merely transferred from different technological applications with higher economic advantages. The way these products are working, the reasons of some failures and the most common problems that can occur, have been reported here. For instance, the influence of the substrates’ nature (open porosity, superficial roughness) on the protective performances and the behaviour of water dispersed systems for the stone conservation have been investigated and reported. Stability and resistance to the photoxidative and thermal ageing of the different polymeric classes are other crucial problems: often a long term chemical integrity does not correspond to a similar protection efficacy. When a polymer is applied on a stone surface, physical rearrangements could occur to the protective layer, reducing the shielding efficacy but leaving the chemical structure unaltered. In a similar way the reversibility, even when assessed for a polymer in laboratory conditions, is practically lost when the protective is applied on a stone material with high open porosity and the product can penetrate deeply in the bulk of the system. This paper aims at being a short but complete overview of the issues that have to be considered and the problems that have to be faced when the protection of an outdoor exposed stone material is concerned.