Hydrophobic Phenolic/Silica Hybrid Aerogels for Thermal Insulation: Effect of Methyl Modification Method

Hydrophobic phenolic/silica hybrid aerogels were synthesized via different methyl modification methods including in situ polymerization (RA-IS), surface grafting (RA-SG), and vapor deposition (RA-VD). All the methods achieved good hydrophobicity, with a water contact angle around 140°, and the hydrophobic mechanisms were clarified. RA-IS possesses the highest specific surface area and nanopore volume, and the lowest bulk density. Therefore, it exhibits much lower thermal conductivity (32.2 mW·m^-1·K^-1) at 25 °C than RA-SG, RA-VD and other reported phenolic/silica hybrid aerogels. The compression strength (3.3 MPa) and Young's modulus (19.2 MPa) of RA-IS are higher than those of its state-of-the-art counterparts. The methyl groups in RA-IS are linked in the matrix by a covalent bond, leading to excellent weather resistance under thermal, hygrothermal, and ultraviolet aging conditions. The methyl species in RA-SG and RA-VD are loaded on the surface via a covalent bond and physical adsorption, exhibiting poor weather resistance. RA-IS is incombustible and its microstructure is stable on an alcohol flame. This study provides new insights into the hydrophobicity of phenolic/silica hybrid aerogels, and offers significant guidance for developing aerogels with high strength, hydrophobicity, flame resistance, weather resistance, and insulation performance for building insulation.