Oligoamide grafted with perfluoropolyether blocks: a potential protective coating for stone materials

In order to mitigate the decay phenomena induced by liquid water adsorption and penetration, new protective coating agents with high hydrophobicity, photo-oxidative stability and good solubility in non-toxic solvents are needed. In the present study, a partially fluorinated oligoadipamide bearing pendant PFPE segments (FAD) together with two diamides i.e., ethylenediamide (DC2) and hexamethylenediamide (DC6) incorporating perfluoropolyether (PFPE) segments, were synthesized via condensation reactions. The diamides are only soluble in chlorofluorocarbons, while the oligoadipamide is soluble in alcohols and hydro-alcoholic solvents e.g., ethanol, 2-propanol. The polymerization degree, together with the molecular weight and molecular weight distribution of oligoethylene-adipamide evaluated by 1H-NMR spectroscopy, was found very low which allowed to obtain a partially fluorinated amide with a high fluorine content (57%). To determine the protective performance of the new FAD, a series of standard tests were conducted on Lecce stone and Carrara marble samples. Regarding water repellence, compared with a commercial fluoroelastomer tested as a reference material, FAD demonstrated much higher water repellence when applied on Carrara marble samples, and comparable hydrophobic effect when applied on Lecce stone samples. In comparison with the two diamides, tested on Lecce stone, the oligoamide manifested similar protective efficacy with a much less amount applied (15 g/m2 versus 154/150 g/m2). In addition to its high hydrophobicity, FAD also preserved the original vapor permeability and chromatic features of coated surfaces. With these favorable properties, this new oligomeric product is ought to be proposed as protective coating material for stone heritage.