Waterborne Silane/Polysiloxane Hydrophobic Coating for Stone-Built Cultural Heritage Conservation

Abstract

Stone-built cultural heritage is historical evidence that represents past human events and activities. Therefore, it must be preserved for later generations since its deterioration would hardly be recovered to be the original. One of the effective ways to protect the stone-built cultural heritage from damaging factors is by applying a protective coating on the stone surface. This work aimed to prepare a waterborne protective coating with hydrophobic and self-cleaning properties. Tetraethoxysilane (TEOS) and hydroxy-terminated polydimethyl siloxane (PDMS-OH) were used to produce a protective coating due to their ability to form siloxane (Si-O-Si) bonds with inorganic substrates and to enhance a water-repellent property. The optimum weight ratio of TEOS:PDMS-OH at 80:20 was used to formulate this coating. The effect of TEOS/PDMS-OH mixture concentrations in a water medium on the properties of coated limestone substrates was studied by using coatings with different concentrations at 5, 10, 15, and 20 wt%. The TEOS/PDMS-OH coating with 5% and 10% concentrations showed good stability with no viscosity change for at least 2 months. The performance of the coated limestone was evaluated by water contact angle measurement, tape peel test, capillary water absorption, and dirt pick-up resistance test. The optimum TEOS/PDMS-OH concentration found was 10 wt% which showed significant improvement in hydrophobicity as evidenced by an increment of average water contact angle from 0° in the uncoated surface to 96° in the coated surface. The color difference (∆E*) was 1.21 and the gloss values slightly increased from 2.23 to 2.63 GU. Furthermore, the area after the peel test still maintained good hydrophobicity. The dirt pick-up resistance test revealed that the coating could protect the surface from dirt, while the uncoated surface was still covered with dirt. The results confirmed that the waterborne coating with 10 wt% TEOS/PDMS-OH is appropriate for limestone substrate protection.