Investigating the influence of the size of cellulose aerogel beads on the physical properties: microstructure, bulk density, mechanical stability and thermal conductivity

Abstract

Cellulose aerogel beads are one of the most explored biopolymer aerogels due to the abundancy, convenient for syntheses, biocompatible and biodegradable. The physical properties of cellulose aerogel beads can be influenced by various parameters which are involved in the syntheses. We got interest in understanding how the size will be influenced if the beads are employed for thermal insulation purpose. Thus, the dependency of physical properties on the size of cellulose aerogel beads was investigated in the present study. For this purpose, cellulose was dissolved in an aqueous sodium hydroxide-urea solution. The conventional dropping techniques (using a single nozzle and multi-nozzle) were employed to prepare the cellulose beads. The gelation of the cellulose solution was induced by a neutralization reaction with acetic acid. After supercritical drying, cellulose beads having four different sizes were prepared: 1.8, 2.2, 2.7 and 3.5 mm. The physical properties such as densities, BET specific surface area, pore volume, porosity and microstructure were characterized. In general, cellulose beads did not show any differences in the microstructure. The mechanical stability of the aerogels was examined using compression tests. It was recognized that even compressions of 0.5% lead to permanent pore deformations. A compression of 40% results in only moderate pore volume loss of 19% and it was achieved with a mechanical stress of about 3 MPa. A direct dependence of the size of the beads on their thermal conductivity values was not determined. Rather, there is a dependence on the bulk porosity. By increasing the porosity from 89.4 to 92.6%, it was possible to reduce the thermal conductivity from 50.5 to 39.4 mW/m∙K. A direct dependence on particle size could not be proven for the properties investigated. However, the positive influence of mesopore size on the mechanical property was recognized. Further change in process parameters have been planned for future studies.