Superhydrophobic Coatings Based on Siloxane Resin and Calcium Hydroxide Nanoparticles for Marble Protection

Abstract

Calcium hydroxide (Ca(OH2)) nanoparticles are produced following an easy, ion exchange process. The produced nanoparticles are characterized using transmission electron microscopy (TEM) and Fourier- transform infrared spectroscopy (FTIR) and are then dispersed in an aqueous emulsion of silanes/siloxanes. The dispersions are sprayed on marble and the surface structures of the deposited coatings are revealed using scanning electron microscopy (SEM). By adjusting the nanoparticle concentration, the coated marble obtains superhydrophobic and water repellent properties, as evidenced by the high static contact angles of water drops (> 150°) and the low sliding angles (< 10°). Because Ca(OH)2 is chemically compatible with limestone-like rocks, which are the most common stones found in buildings and objects of the cultural heritage, the produced composite coatings have the potential to be used for conservation purposes. For comparison, the wetting properties of another superhydrophobic and water repellent coating composed of the same siloxane material and silica (SiO2) nanoparticles, which were commonly used in several previously published reports, were investigated. The suggested siloxane+Ca(OH)2 composite coating offers good protection against water penetration by capillarity and has a small effect on the aesthetic appearance of marble, according to colorimetric measurements.

TEOS-Based Superhydrophobic Coating for the Protection of Stone-Built Cultural Heritage

“…As described in Section 2.3 m uw and m cw were measured after putting the specimens in contact with liquid water for various periods t i ranging from 60 to 120 min. The RC% results are provided in Figure 4 and correspond to a mean value of 74.3% ± 3.4%, which is remarkably close to the corresponding data reported for other superhydrophobic and water-repellent coatings [40,44]. In particular, a composite coating consisted of polysiloxane, an organic fluoropolymer, and silica nanoparticles was deposited on marble and reduced the amount of absorbed water by 75.1% [40].…”

“…Six brush stokes were applied, with each covering being applied to the opposite direction of the previous one after the previous one had dried. The airbrush system (Paasche Airbrush, Chicago, IL, USA)) was operated using a nozzle that was 660 µm in diameter, as described in detail elsewhere [35,43,44]. For the dip coating process, marble samples were partially immersed by 1 cm in the sol-solution for 50 h. After coating deposition, the marble samples were left to dry for 24 h at room temperature, then further cured at 110 • C for 24 h and finally left to cool.…”

“…These extreme, non-wetting properties are highly desirable for their ability to reduce the rate of water-induced degradation in natural stone. For this reason, several investigations have been carried out in the last decade to impart enhanced hydrophobicity (120 • < CA < 150 • ) [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] or, even better, superhydrophobicity (CA > 150 • ) [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] onto stone surfaces. However, in most of these previously published reports, nanoparticles have been used that were made of silicon [8,11,17,23,24,27,28,30,31,…”

Fluorosilane Water-Repellent Coating for the Protection of Marble, Wood and Other Materials

“…Superhydrophobicity is defined by the very large static water contact angle (typically > 150 • ), whereas a water-repellent surface is characterized by the low sliding angle of water drop (typically < 10 • ). Nanocomposites, which consist of nanoparticles embedded in polysiloxane networks, are a new class of advanced materials, designed for the protection of stone [6][7][8][9][10][11][12][13][14][15][16]. Extreme hydrophobicity or even superhydrophobicity is achieved on the surfaces of these nanocomposites, as the additives-nanoparticles promote surface roughness [6][7][8][9][10][11][12][13][14][15][16].…”

“…Nanocomposites, which consist of nanoparticles embedded in polysiloxane networks, are a new class of advanced materials, designed for the protection of stone [6][7][8][9][10][11][12][13][14][15][16]. Extreme hydrophobicity or even superhydrophobicity is achieved on the surfaces of these nanocomposites, as the additives-nanoparticles promote surface roughness [6][7][8][9][10][11][12][13][14][15][16]. According to the Cassie-Baxter model, which is commonly used to describe non-wetting phenomena, the hydrophobic character of any material is enhanced with surface roughness [17].…”