Long-lasting hydrophobization of monolithic phenolic gels via silylation

Abstract

The nature of a material’s surface is of great importance for industrial applications, and various physical and chemical methods are used to adapt it to the respective requirements. Hydrophobic materials are particularly suitable for applications where the surface needs to be protected from moisture or corrosion. Due to the presence of free aromatic hydroxyl groups, phenolics are naturally hydrophilic materials. Phenolic aerogels and xerogels are particularly sensitive to the absorption of water and other polar liquids due to their high porosity. This behavior is also typical for organic gels based on resorcinol-formaldehyde (RF). Among the few literature-known methods for the chemical modification of RF is its silylation using trimethylsilyl reagents. However, the method described is restricted to powdered specimen and provides the material with a temporary hydrophobicity of a few weeks. The research objective of this work was to develop methods for the hydrophobization of RF aerogels using silylation processes that offer longer-lasting protection. For the purpose of increasing reactivity of silyl reagents, electronically activated reagents (solutions of silyl chlorides and triflates) were employed as well as suitable amine bases. In order to decrease lability of the resulting silyl ethers towards hydrolysis, reagents based on sterically demanding silyl moieties were investigated. Elemental analysis, X-ray photoelectron spectroscopy, pycnometry, absorption analysis and scanning electron microscopy using electron scattering X-ray spectroscopy were used to structurally characterize the resulting gels. Furthermore, the wetting behavior of the modified gels was investigated on the basis of the contact angle. Despite only slight incorporation of the silylation reagents, highly hydrophobic gels were formed that maintained the wetting behavior with contact angles of >130° over the course of several months. The hydrophobic behavior of the monoliths was also retained over several months in acidic or base environments. As envisioned, monoliths with sterically demanding silyl groups showed a higher stability to aqueous acid than trimethylsilylated RF gels.