Cellulose aerogels

Hoepfner, S., Ratke, L., and Milow, B. (2008). Synthesis and characterization of nanoflbrillar cellulose aerogels, Celh jQ 15,121-129. 

Both native and regenerated cellulose can be used for the preparation of cellulose aerogels. Cellulose in native form, generally nanofibrillar such as bacterial cellulose and microfibrillated cellulose, has been proposed for the preparation of aerocellulose [77, 266]. Aerogels based on NFC can offer advantages from an environmental point of view, because NFC is obtained from renewable resources and no harmful solvents are required during the processing. 

Cai J, Kimura S, Wada M, Kuga S, Zhang LN (2008) Cellulose aerogels from aqueous alkali hydroxide-urea solution. ChemSusChem 1 149-154... 

Hoepfher S, Ratke L, Milow B (2008) Synthesis and characterisation of nanofibrillar cellulose aerogels. Cellulose 15 121-129... 

Liebner F, Potthast A, Rosenau T, Haimer E, Wendland M (2008) Cellulose aerogels highly porous, ultra-lightweight materials. Holzforschung 62 129-135... 

Jin H, Nishiyama Y, Wada M, Kuga S (2004) Nanofibrillar cellulose aerogels. Colloid Surf A Physicochem Eng Asp 240 63-67... 

Surapolchai W, Schiraldi DA (2010) The effects of physical and chemical interactions in the formation of cellulose aerogels. Polym Bull 65 951-960... 

Litschauer M, Neouze MA, Haimer E, Henniges U, Potthast A, Rosenau T, Liebner F (2010) Silica modified cellulosic aerogels. Cellulose 18 143-149... 

Several methods are described in the hterature to prepare cellulose aerogels and still many new methods are developed, partly depending on the raw material used, since hemicellulose needs a different process than raw cellulose or lignocellulosic polymer mixtures. 

Jin and co-workers [10] developed another technique to produce high-quality cellulose aerogels. Their technique avoids the utilization of toxic isocyanates and allows in contrast to the method of Tan [8] to use lower amounts of cellulose. Their technique is based on semicrystalline raw cellulose whose morphology can be well described by a mixture of regions with highly crystalline order and unordered intermediate areas connecting the ordered ones as described above. 

Figure 9.2. Schematic of the process developed by Jin and co-workers [10] to produce cellulose aerogels.

Figure 9.3. Strength of cellulose aerogels as prepared by Jin and co-workers [10] using a salt-hydrate melt to dissolve cellulose (calcium thiocyanate/water) and regeneration of the cellulose in alcohols (redrawn from [10] different units used here).

Figure 9.4. Microstructure of a cellulose aerogel as prepared by Jin and co-workers [10] (with permission of Elsevier). The samples shown have a cellulose concentration of 2 wt%. The figure in the left panel shows a regularly freeze dried sample, in the right a sample rapidly freeze dried as described above with gel casting onto a copper plate kept at liquid nitrogen temperature.

Figure 9.5. Density of cellulose aerogels prepared by Innerlohinger et al. [19] using NMMO as a solvent and supercritical drying with carbon dioxide (redrawn on the basis of their data using different units).

Figure 9.6. Cellulose aerogels with different concentrations of cellulose as prepared by Hoepfner and co-workers [24] using the procedure of Jin [10]. All cellulose aerogels exhibit a white appearance. They are soft and easily compressible by gentle pressure.

Cai et al. [28] were able to prepare transparent cellulose aerogels using an aqueous alkali hydroxide/urea solution as the dissolving and gelling agent. Figure 9.9 shows one of their impressive results, a transparent cellulose aerogel. 

Figure 9.10. Density and specific surface area of cellulose aerogels prepared with aq. LiOH/urea and regenerated at 20°C. The cellulose concentration was 6 wt% after [30].

Aaltonen and Jauhiainen [32] prepared cellulose aerogels from microcrystalline cellulose, spruce wood, and from mixtures of cellulose, lignin, and xylan using an imiic... 

Cellulose is one of the oldest materials used to produce fibers, filaments, and yams, from which fabrics of all kinds are manufacmred. Today s filament and yam production is a field for both natural and synthetic polymers [31]. All fibers used today have a compact microstracture and a very large aspect ratio. In contrast to these, Schmenk et al. [34] and Hacker et al. [35] produced an open porous, nanostmctured filament of cellulose aerogel for the first time using a sol-gel routine as described above for mmioliths and different spinning techniques. 

Figure 9.11. Light microscopic picture of a filament of cellulose aerogel after supercritical drying having a dry density of 124 kg/m. ...

Cellulose aerogels are at the beginning of intensive research and owing to their fascinating structure and properties they promise to have many applications. Various routes have been tested to produce aerogels. There is ample space for improvement and refinement as well as fundamental research on the kinetics of gelation in a solution of the flexible... 

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