Glycidyl polyhedral oligomeric silsesquioxane-enhanced flexible aminosiloxanes to protect sandstone monuments

To protect weathered ancient stone monuments from surface cracking and surface damage by powders, strong materials containing flexible chains are required. Herein, a facile strategy was proposed to produce hybrid octa-glycidyl polyhedral oligomeric silsesquioxane (GPOSS)-enhanced flexible siloxanes by the ring-opening reaction of GPOSS using three aminosiloxanes (bis(3-aminopropyl) terminated poly (dimethyl siloxane) (NH₂-PDMS), 3-aminopropyltriethoxysilane (APTS), and N-(6-aminohexyl) aminopropyl trimethoxysilane (AHAPTMS). This produced three different protective materials (POSS-PDMS, POSS-APTS, and POSS-AHAPTMS) that greatly improved the weather resistance of sandstone monuments, especially their resistance to water, salt, temperature, and humidity. All three protective materials presented an adhesion of up to 2.4 MPa. The resistance of freeze-thaw aging cycles was much higher for the protected sandstone (104–133 cycles) compared with the unprotected sandstone (20 cycles). The three protective materials improved the salt weather resistance of protected sandstone (9–60 cycles) compared with unprotected sandstone (3 cycles), with the long-chain aminosiloxane AHAPTMS providing the best protection. The hydrophobicity of three protective materials on the sandstone surface was improved. Furthermore, the water absorption, water vapor permeability, pore size distribution, mechanical strength, light transmittance, and glass transition temperature (T_g), were all improved. The three GPOSS-enhanced flexible siloxanes are novel and eco-friendly materials for protecting sandstone monuments.