Nanostructured silica used in super-insulation materials (SIM), hygrothermal ageing followed by sorption characterizations

Abstract

Several commercial silica powders (precipitated, fumed, hydrophobized) and aerogels were aged in climatic chambers. Four different conditions of temperature [50–70 °C] and relative humidity [70–90%RH] were used to identify the influence of both parameters and the underlying mechanisms. Samples were characterized by sorption measurements (nitrogen and water) giving access to the evolution of specific surface area (BET), pore size distribution (BJH) and hydrophilicity. Structurally, a common trend along with silica ageing is the reduction of the specific surface area and a shift in the pore size distribution (loss of the smallest mesopores). However, the ageing scenario and intensities observed differ according to the type of silica (precipitated, fumed, hydrophobic, aerogels). On the one hand, the hydrophilicity of a precipitated silica decreases with ageing due to the reduction of the specific surface area. On the other hand, the hydrophilicity of a fumed silica first increases then decreases with ageing, as a result of two competing effects: the increase of local hydrophilicity (siloxane hydrolysis) and the decrease of the specific surface area. Surface chemistry appears to be a key parameter governing the extent of these evolutions: a precipitated silica is more sensitive than a fumed silica whereas the hydrophobic silica studied exhibited the strongest resistance. Therefore, the main mechanism of structural evolution, probably implying mass transfer in a dissolution/precipitation process, requires water on the surface of silica and thus hydrophilic groups to adsorb it. Short term ageing (24 days) on precipitated and fumed silicas showed that temperature is the most important parameter in activating a mechanism, the intensity of which is thereafter (96 days) governed by relative humidity. The significant influence of humidity was also observed for aerogels, for which the ageing scenario and intensity appears to be product specific.