Biopolymers-based treatments for the protection of stone in the cultural heritage

The guiding thread of the research is testing biopolymers-based treatments for the protection of stone in cultural heritage against water and salt crystallization. The idea developed is that the reversibility requirement can be fulfilled by bio-polymeric treatments, owing to a “reversibility by biodegradation” once their efficacy has finished. The first part aimed at investigating the performance of two poly(hydroxyalkanoate)s-based (PHAs-based) formulations, namely solutions of poly(3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxyvalerate) (PHBVV), as hydrophobic coating. These formulations were applied to sandstone, limestone and marble, in order to investigate their behaviour on different substrates. Their effectiveness and compatibility were evaluated and the samples wettability after application of biopolymers and of two commercial products was compared. Promising results in terms of efficacy and compatibility were obtained, although the present PHAs-based formulations still need improvement to increase their durability and on-site applicability. Among the strategies proposed to mitigate the damage caused by salt crystallization, the modification of the surface of the pore wall, by means of a polymer coating able to minimize the repulsive disjoining force normally acting between the stone and the crystallizing salt, is particularly promising. Thus, some polyectrolyte biopolymers (alginic acid sodium salt, chitosan and tannic acid) were selected and compared with a polymer (polyacrylic acid sodium salt) for preliminary screening tests, designed and performed in order to assess whether the selected polymers fulfil some basic requirements such as good adsorption onto the stone substrate, film-forming ability, affinity and interaction with sodium sulfate. Moreover, in order to reduce calcite dissolution in water that may trigger the effect of the polymer coating, a diammonium hydrogen phosphate (DAP) water-based treatment was applied as coupling layer on carbonate substrates. Then, the effectiveness of the coatings was evaluated by two macroscale crystallization tests on limestone with promising results especially for chitosan and alginic acid coupled with DAP.