Ambient-pressure drying synthesis of large resorcinol–formaldehyde-reinforced silica aerogels with enhanced mechanical strength and superhydrophobicity

Abstract

Silica aerogels generally exhibit poor mechanical properties, which seriously restrict their commercial use and makes it necessary to reinforce their structures to obtain materials strong enough to withstand mechanical stresses. The incorporation of polymers into silica aerogels is a promising approach due to the enhancement in mechanical strength afforded and the possibility of imparting the hydrophobicity of the polymers to the resulting aerogels. However, the processes typically employed to prepare polymer-reinforced silica aerogels are complex, arduous and costly, requiring multiple washing and soaking steps for the infiltration of the polymer precursors after gelation and supercritical drying. Additionally, diffusion problems can result in heterogeneity in aerogel monoliths due to the use of various cross-linked polymers and the autoclave-limited dimensions of aerogels. In this study, a facile sol–gel route was developed to fabricate a large (500 cm3) polymer–silica aerogel monolith that was stable under atmospheric conditions and suitable for machining. The synthesis of the polymer–silica aerogels began with a facile one-pot reaction involving resorcinol (R), formaldehyde (F), methyltrimethoxysilane (MTMS), and N-(β-aminoethyl)-γ-aminopropyl-trimethoxysilane (AEAPTES), followed by an ambient-pressure drying process. The resulting aerogels were characterised by nitrogen adsorption–desorption, scanning electron microscopy (SEM), contact angle, and thermal conductivity measurements, and the aerogel mechanical properties were evaluated by a unidirectional compression test. The Young's modulus of compression of the aerogels was observed to increase from 3.95 to 34.88 MPa with an increase in the density of the aerogels from 0.14 to 0.26 g cm−3. Simultaneously, the aerogels were observed to be superhydrophobic, with a contact angle as high as 168°, and exhibited low thermal conductivity (0.038 W m−1 K−1) and good absorption for organic liquids, which are critical characteristics for the practical application of aerogels, especially in energy-saving and oil-/chemical-cleanup practices.