• protective coating
• PLA nanocomposite
• PLA functionalization
• PLA
• Cultural Heritage
• stone conservation

Stone materials outdoor exposed slowly but inevitably undergo a degradation process, either by natural or artificial causes. The rate of degradation has recently experienced an important acceleration due to natural and anthropic changes in the climatic conditions. It is well known that water plays a central role in the mechanisms of decay (through chemical and physical action), because it is involved in the transport of air pollutants, biodeteriogen growth, transport and crystallization of soluble salts inside the stone and freezing/thaw cycles. In the last decades synthetic polymers have been widely employed for treatment of stone materials as protective coatings, examples being polyacrylates, polysiloxanes and synthetic waxes. However these polymers were chosen among commercial products already available, that often did not meet the requirements for application on artifacts of historical and artistic interest (e.g. minimum alteration, durability and reversibility).
Following the principles of green chemistry, in this work a new class of synthetic polymers derived from polylalctic acid has been developed and tested as protective materials for stone surfaces. By copolymerization of substituted glycolides and lactones, such as lactide, mandelide and salicylide, copolymers with tailor made properties were prepared. Most notably Tg, hydrophobicity and UV barrier properties were improved over conventional PLA. A high and controlled Tg, far from ambient temperature, is particularly important for a polymer coating, so it never stays in the sticky-rubbery state and is less affected by temperature variations during outdoor exposition. The introduction of perfluorinated chain segments, either by functionalization with perfluoro-alcohols or diols, allowed to improve the hydrorepellency and durability of the coatings. Fluorine containing polymers showed almost no signs of degradation in the accelerated ageing tests, showing excellent stability to photo-oxidative deterioration and negligible color changes.
Another approach to these goals was the synthesis of polylactic acid/silica nanocomposite coatings for the protection of building stones. These organic-inorganic hybrid materials showed improved hydrophobic properties, with water contact angles as high as 137° on stone and glass surfaces, keeping the water barrier properties and durability of fluorinated polylactic acid. A very simple procedure was developed for the synthesis of these materials, by mixing the organic and inorganic components in an organic solution, stirring and directly applying the solution on the surface to be protected. Microscopic analysis (SEM, HAADF-STEM) showed an uniform coating constituted by single particles and larger aggregates covered by the polymer matrix. AFM showed a marked increase of surface roughness, increasing the three-phases contact area between water, air and solid substrate, effectively preventing the penetration of water in the pores of the matrix. Due to the high hysteresis (about 22°) water droplets remained stuck on the surface instead of rolling off, an effect very similar to the natural one occurring on some plants (e.g. rose petals). A reversibility test was carried out by treatment of coated surfaces with organic solvent. The results (AFM, SEM, EDX) indicated that the coating was completely removed, even with the highest silica content, confirming the reversibility of the treatments.