Thermal superinsulating and monolithic cellulose fiber – silica aerogel composites via ambient drying

Abstract : Ambient pressure drying of the silica-based alcogels is used to produce lower cost thermal superinsulating materials and to have more flexibility in the manufacturing process regarding to sample shape and size. In this work we demonstrate that short cellulose fibers incorporated into silica aerogels allow making monolithic, not brittle and thermal superinsulating ambient dried aerogels. We have compared the effects of the different cellulosic fibers on the properties of silica-based composite aerogels produced both via ambient drying and the supercritical (sc) CO2 drying methods. The aerogels with 0.7to 3 final vol% of fibers were synthesized and after hydrophobization of silica phase, were dried via either ambient pressure or sc drying methods. The subsequent materials showed thermal conductivities as low as 0.016 W/m.K in room conditions. The presence of the fibers in the silica matrix restrained drastically the material shrinkage during the evaporative drying, prevented silica breakage into particles and allowed maintaining initial monolithicity and volume of the composite gels due to “spring-back” effect. The bulk densities of ambient and sc CO2 dried samples were similar. The addition of cellulose fibers significantly increased the Young’s modulus of the aerogels. While pure silica aerogels are very brittle (strain at break of few %), aerogel composites, both sc and ambient dried, are not brittle. This original aerogel preparation method was also successfully demonstrated with man-made cellulose fibers as well as natural fibers from wood and flax. This is a significant breakthrough in the field.