Cellulosic aerogels as ultra-lightweight materials. Part 2: Synthesis and properties

Ultra-lightweight cellulose aerogels can be obtained in three steps: (1) preparation of a cellulose solution in molten N-methylmorpholine-N-oxide monohydrate (NMMO center dot H2O) at 110-120 degrees C and casting of the viscous mass into moulds; (2) extraction of the solidified castings with ethanol to initiate cellulose aggregation and to remove NMMO center dot H2O so that the fragile, fine-porous texture of cellulose II is largely retained; and (3) drying of the lyogel using supercritical carbon dioxide (scCO(2)). According to this approach, cellulosic aerogels were prepared from eight commercial cellulosic materials and pulps and analysed for selected chemical, physicochemical and mechanical parameters. The results reveal that all aerogels obtained from 3% cellulose containing NMMO center dot H2O melts had a largely uniform mesoporous structure with an average pore size of similar to 9-12 nm, surface area of 190-310 m(2) 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 behaviour, with Young's modulus ranging from similar to 5 to 10 N mm(-2). Doubling the cellulose content in the NMMO center dot H2O melt from 3% to 6% increased Young's modulus by one order of magnitude. Shrinkage of the fragile cellulose bodies during scCO(2) drying was still considerable and is subject to further investigations. Influencing parameters such as scCO(2) pressure, cellulose content, regenerating solvent and the number of regenerating baths were optimised.