# Fabricating Sustainable All-Cellulose Composites
## 2 Cellulose
### Nanocellulose  
The production of nanoscale cellulose has captured great attention due to its biodegradationity, renewability, high mechanical properties and low density [49]. Nanocellulose, as a general term, applies to a wide range of nano- and micro-sized fibrils and crystalline particles (sometimes also referred to as whiskers). Different terminologies have been used for the various types and forms. Because of inconsistency in terminology, the Technical Association of the Pulp and Paper Industry (TAPPI) has proposed standard terms for various types of nanocellulose based on their size and form (Table 1). Nanocellulose can be processed from plant-derived cellulose, but it is also found natively in tunicates and bacteria. The feedstock used in nanocellulose manufacture is highly important, as it affects the properties and size of the extracted cellulose [42]. The biosynthesis of cellulose forms continuous microfibrils, which self-organises fibre bundles with crystalline and amorphous domains [22]. The term cellulose microfibrils (MFC) was first introduced in 1983 by [PERSON] et al. [51] when he and his co-workers developed a new method to prepare highly fibrillated cellulose from wood fibres by high-pressure homogenization treatment in water. The extraction of cellulose microfibrils require great precision in order to minimize damage to the fibrillary structure and to keep the long axial length of the microfibrils to preserve the large aspect ratio. The other native features of the cellulose, such as crystallinity and degree of polymerization, must also be secured [52]. An ideal production method should produce large amounts of high-quality material with low energy consumption. To meet these requirements, a number of pre-treatments can be used to improve the conversion efficiency. Different chemical, mechanical and enzymatic treatments are used for removal of non-cellulosic constituents and loosen the cell wall structure of the cellulose fibres to ease further processing [53]. Cellulose nanofibrils (CNFs), or their microfibril bundles, are typically produced using a variety of mechanical processes, including grinding/refining, high-pressure homogenization, cryocrushing, high-intensity ultrasonic treatments, electrospinning, steam explosion and microfluidization [50,54]. Chemical modification methods such as TEMPO mediated oxidation [55], ammonium persulfate oxidation [56], carboxymethylation [57] and cationization [58] have also emerged as promising methods for CNF production. 'Rod-like' cellulose microcrystals (MCC) and cellulose nanocrystals (CNC) can be obtained from different cellulosic sources such as wood, cotton, bacteria or tunicates with varied dimensions [59,60]. Their dimensions depend on the origin of the cellulose fibre or fibril and the employed conditions. Isolated nanoscale fibres or crystals from natural fibres show much higher strength and stiffness in comparison to the original source [49]. The elastic moduli of solid wood, single pulp fibre, microfibrils and crystallites have been measured at 10, 40, 70 and 250 GPa, respectively [61]. Cellulose nano- and micro-crystals can be produced from nano- and microfibrils by applying chemical treatments \begin{table}
\begin{tabular}{c c c c c} \hline
**NAME** & **ABBREVIATION** & **WIDTH** & **LENGTH** & **L/D RATIO** \\ \hline Cellulose & \multirow{2}{*}{CNC (NCC)} & (I) 3–10 nm & \multirow{2}{*}{(II) 50–500 nm} & \multirow{2}{*}{(I) \textgreater{5}} \\ nanocrystal & & (II) 3–20 nm & & \\ \hline Cellulose & \multirow{2}{*}{CNF (NFC)} & (I) 5–30 nm & \multirow{2}{*}{(II) 0.5–2 μm} & \multirow{2}{*}{(I) \textgreater{5}} \\ nanofibril & & (II) 3–100 nm & & \\ \hline Cellulose & \multirow{2}{*}{CMC (MCC)} & (I) 10–15 μm & \multirow{2}{*}{(II) 10–50 μm} & \multirow{2}{*}{(I) \textless{2}} \\ microcrystal & & (II) 10–50 μm & & \\ \hline Cellulose & \multirow{2}{*}{CMF (MFC)} & (I) 10–100 nm & \multirow{2}{*}{(I) 0.5–50 μm} & \multirow{2}{*}{(I) \textless{2}} \\ microfibril & & (II) 10–100 nm & & \\ \hline \end{tabular}
\end{table}