Effect of preparation conditions on the physical and hydrophobic properties of two step processed ambient pressure dried silica aerogels

Abstract

The experimental results on the physical and hydrophobic properties of the ambient pressure dried silica aerogels as a function of sol-gel and drying conditions, are reported.The aerogels have been produced by a two stage (acidic and basic) catalytic sol-gel process using tetraethylorthosilicate (TEOS) precursor, oxalic acid (OXA) and ammonium hydroxide (NH4OH) catalysts, ethanol (EtOH) solvent and hexamethyldisilazane (HMDZ) silylating agent at 200∘C.The molar ratios of HMDZ/TEOS (M), OXA/TEOS (A) NH4OH/TEOS (B), acidic H2O/TEOS (Wa) basic H2O/TEOS (Wb), EtOH/TEOS (S) were varied from 0.09 to 0.9, 3.115 × 10− 5 to 3.115 × 10−3, 4 × 10− 3 to 8 × 10− 2, 2 to 9, 1.25 to 5 and 1 to 16 respectively. The physical properties such as the percentage (%) of volume shrinkage, density, thermal conductivity, percentage of porosity, the percentage of optical transmission and contact angle have been found to be strongly dependent on the sol-gel parameters. It was found from the FTIR spectra of the aerogels that with the increase of M, the bands at 3500 and 1600 cm− 1 corresponding to H-OH and Si-OH respectively decreased and the bands at 840 and 1250 cm− 1 due to Si-C and 2900 and 1450 cm−1 due to C-H increased. The best quality silica aerogels in terms of low density, low volume shrinkage, low thermal conductivity, high hydrophobicity and high optical transmission have been obtained with the molar ratio of TEOS:EtOH:acidicH2O:basicH2O:OXA:NH4OH:HMDZ at 1:8:3.75:2.25:6.23 × 10− 5: 4 × 10− 2:0.36 respectively, by ambient pressure dried method.