Subcritical Water: A Method for Green Production of Cellulose Nanocrystals

Abstract

In the present study, an innovative method to produce cellulose nanocrystals is proposed. The conventional production of nanocrystals uses concentrated solutions of strong acids to promote the hydrolysis of cellulose amorphous regions and hemicelluloses. However, in the conventional method, long duration washing steps and the nanocrystals low temperature resistance still limit their larger industrialization and some applications in processes or end-uses that require heat resistance, like extrusion. In this context, the use of subcritical water (120 °C and 20.3 MPa for 60 min) allows higher diffusion, activity, and ionization of water. With that, partial hydrolysis of cellulose can be attended (with 21.9 wt % NCC yield). The cellulose source, the hydrolyzed cellulose, and a commercial nanocellulose were submitted to different analytical techniques to evaluate their morphology and physicochemical characteristics. The obtained cellulose nanocrystals presented a high crystallinity index (79.0% by XRD), rod-like shape with a similar aspect ratio as those known for classic cellulose nanocrystals but also a higher thermal stability even when compared with the original cellulosic source (onset around 300 °C). The exclusive use of water as a reagent is a promising process not only for its green characteristics but also for its low corrosion, low and cleaner effluent, and low cost of reagents.

Cellulosic-crystals as a fumed-silica substitute in vacuum insulated panel technology used in building construction and retrofit applications

“…This outcome may arise through manufacturing innovations reviewed [67,68]. However, it is recognised this would be a difficult considering the physical production time challenges this paper has shown.…”

“…Investment in the cellulosic-particle manufacture innovations is highly advised, as the potential benefits are clear for the application investigated and the wider global insulation industry for buildings. Going forward, cost reduction opportunities are likely via implementation of manufacture innovations reviewed; these including: the high energy ball milling method [67] and the subcritical water hydrothermal method [68].…”

“…Together, these phases accounted for the bulk of production time. Manufacture innovations were reviewed that remove the requirement of these processes including the high energy ball milling method [67] and the subcritical water hydrothermal method [68]. However it was not in the scope of this study to assess these, albeit clear opportunities exist to exploit them for the application analysed, and it is advised research be pursued.…”

“…Going forward, innovation in manufacture techniques to facilitate production with smaller cross-section and length ranges is required. Methods exist for use as a starting point [67,68]. …”

“…However, recent articles on high-efficiency separation techniques indicate these may soon be overcome [66], with innovations including the high energy ball milling [67], and the subcritical water hydrothermal method [68]. These processes provide potential to increase future market penetration on the basis of lower economic and environmental costs.…”