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

## TL;DR

This paper compares methods to create hydrophobic phenolic/silica aerogels for thermal insulation, finding one method produces stronger, more weather-resistant materials.

## Contribution

A novel methyl modification method (RA-IS) is shown to produce aerogels with superior hydrophobicity, strength, and weather resistance.

## Key findings

- RA-IS aerogels have a thermal conductivity of 32.2 mW·m−1·K−1, much lower than other methods.
- RA-IS shows higher compression strength (3.3 MPa) and Young’s modulus (19.2 MPa) than existing aerogels.
- RA-IS is incombustible and maintains microstructure stability under flame exposure.

## Abstract

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.