Cellulosic aerogels as ultra-lightweight materials. Part 2: Synthesis and properties: 2nd ICC 2007, Tokyo, Japan, October 25–29, 2007

Abstract

Ultra-lightweight cellulose aerogels can be obtained in three steps: (1) preparation of a cellulose solution in molten N-methylmorpholine-N-oxide monohydrate (NMMO.H₂O) at 110-120°C and casting of the viscous mass into molds; (2) extaction of the solidified castings with ethanol to initiate cellulose aggregation and to remove NMMO.H₂O so that the fragile, fine-porous texture of cellulose II is largely retained; and (3) drying of the lyogel using supercritical carbon dioxide (scCO₂). According to this approach, cellulosic aerogels were prepared from eight commercial cellulosic materials and pulps and analyzed for selected chemical, physicochemical and mechanical parameters. The results reveal that all aerogels obtained from 3% cellulose containing. NMMO.H₂O melts had a largely uniform mesoporous structure with an average pore size of approximatively 9-12 nm, surface area of 190-310 m² g⁻¹, and specific density of 0.046-0.069 g cm⁻³, but rather low mechanical stability expressed as compressive yield strain of 2.9-5.5%. All samples showed viscoelastic behavior, with Young's modulus ranging from approximatively 5 to 10 N mm⁻². Doubling the cellulose content in the NMMO.H₂O melt from 3% to 6% increased Young's modulus by one order of magnitude. Shrinkage of the fragile cellulose bodies during scCO₂ drying was still considerable and is subject to further investigations. Influencing parameters such as scCO₂ pressure, cellulose content, regenerating solvent and the number of regenerating baths were optimized.