Quantitative-MRI tests on fluorinated compounds to preserve Cultural Heritage porous media and safe for the environment

The internal distribution of water-repellent products applied to surfaces to make a porous material repellent is of great interest in the field of the safeguard of cultural heritage items such as stone buildings, statues and frescos. Water is the main cause of degradation of porous materials exposed to rain, moisture and air pollutants. In previous papers[1], it has been shown how MRI allows one to evaluate and compare the performance of products and treatments used. Those products were commercial ones, made for different uses than cultural heritage items protection. In the middle of the ‘80s, the group of CNR in Florence started to synthesize mono-biand tetra-functionalized perfluoropolyetheric compounds for the specific use in the protection of stone, with the following properties: (i) stable to chemical agents, heating and UV irradiation; (ii) high water-repellency; (iii) ability to perform treatments in such a way that the natural permeability of stone to air and water vapor is not significantly reduced; (iiii) chemically inert with the stone substrate, and therefore completely reversible. Unfortunately, the best products synthesized were soluble in Chlorofluorocarbons. Now, new products containing a perfluoropolyetheric and hydro-alcoholic solvents, but not soluble in water or Chlorofluorocarbons. Their hydrophobic and penetration properties have been investigated by MRI images, taken on biocalcarenite samples treated with commercial and new fluorinated compounds, at increasing times during capillary water absorption. The presence of water inside each sample could be visualized and the height reached by the wetting front quantified as a function of time. The images were analyzed by an in-house software able to identify the front by an objective criterion. The images show the different protection efficiency of products and treatments. Good fits to the data were obtained with theoretical and empirical models of absorption kinetics, starting from the Washburn model. The data analysis gives us parameters useful to quantify the effects of the treatments on the capillary properties. The results demonstrate the better performance of the new fluorinated compounds able to preserve stones and environment. [1] M. Gombia et al., J Appl Phys, 103, 094913 (2008).