| Abstract: | SUMMARIES. The performance of a water-repellent treatment applied to a stone surface is generally evaluated without considering the depth of penetration and uniformity of distribution of the active product in the treated porous material. Magnetic resonance imaging (MRI), a non-destructive methodology, has recently made it possible to visualize the presence and distribution of different hydwphobic polymers in rocks, thanks to their water-repellent properties. The authors show, through the use of this technique, how the penetration depth and the distribution of a water-repellent material in a stone arc affected, and therefore may to some extent be directed, by the application method. Two commercial Hydrophobic polymers (Paraloid B-12 and Silirain 50) frequently used in the restoration/conservation of stone artifacts have been tested on a biocalcarenite (pietra di Lecce).
CONCLUSION. 1H-NMR imaging proved to be a valid technique to investigate the penetration depth and the distribution of a hydrophobic product in a stone material. Such parameters, and also the performance of the hydrophobic treatments, were proved to depend on the application method of the active product. For polymers characterized by high molecular weights, the influence of the method of application of the treatment is particularly evident, especially if the solvent has a low boiling point. In the case of the acrylic resin Paraloid B-72 in chloroform solution, the performance of the treatment is affected both by the technique of application of the product (by brush or by capillary absorption) and by the concentration of the solution. A treatment applied by brush led to a superficial distribution of the polymer, while the treatments by capillary absorption gave penetration up to about one centimetre. Deeper penetration was achieved with more dilute solutions. When pure solvent (chloroform) was applied to the treated surface, the resin penetrated further into the inner layers of the stone, but the superficial protective efficacy became poorer. In the present work it was confirmed that the performance of treatments with the acrylic resin Paraloid B-72 diminishes due to the action of wetting-drying cycles, in particular when a thin superficial layer of polymer is applied (Figure 6). In the case of Silirain 50, a low molecular weight alkyl-alkoxy silane, the performance of the treatment proved to be less affected by the application method, thanks to the low molecular weight of the active product. Applications both by brush and by capillary absorption let the product penetrate in depth into the stone and showed good performance when a sufficient amount of product was applied. |
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