Durability of silica aerogels dedicated to superinsulation measured under hygrothermal conditions

Energy efficiency in buildings is a crucial lever for lowering greenhouse gas emissions and a mass market for silica aerogel as retrofitting material. Their durability is therefore a major concern. When applying the precautionary principle, industrial companies have difficulties in estimating the service lifetime of innovative materials such as aerogel. Thus, their costs remain high and market penetration low until the durability of the materials they produce is confirmed. In this paper, commercial silica aerogels were exposed to accelerated ageing over long periods. Microstructural evolutions were tracked by nitrogen and water sorption while thermal conductivities were characterized by heat fluxmeter measurements. 96 and 384 days at 70 degrees C and 90%RH were sufficient to produce noticeable changes in the physico-chemical properties of certain products but almost none in others. The first change observed was an increase of hydrophilicity, followed by a loss of specific surface area, and a shifted and enlarged pore size distribution. The measurements confirmed that microstructure changes induce higher thermal conductivities. Depending on the specific type of aerogel product, the accelerated ageing tests lead to a significant, but limited increase of thermal conductivity of up to 2,5 mW/(m.K). The main hypotheses for explaining this phenomenon are the growth of neck size between two silica particles and the presence of water in the pores. In addition, it is shown for the first time for a wide range of aerogels that pore size distribution and hydrophobization are the key parameters for tuning aerogel durability in severe conditions. Consequently, the mesoporous community has a framework of tests and mechanisms to design aerogels for their specific efficiency/cost/durability requirements.