*2.1.2 Cellulose nanocrystals from cellulose*

Hierarchical structure of cellulose fibers is at the origin of cellulosic nanomaterials, the nanocelluloses, having at least one dimension of nanometric scale as their name suggests. Indeed, from elementary fibrils, two types of nanocellulose can be obtained differing by their isolation procedure, as well by their properties and applications. **Figure 4** shows their different morphologies. Briefly, cellulose nanofibrils are obtained by applying high shear mechanical treatment to a cellulose suspension, and recovered nanofibrils have a length between 500 nm and 10 μm and a width between 5 and 50 nm, according to their preparation method, their source, and potential chemical treatment.

First investigation on cellulose nanocrystal (CNC) was reported by Ranby et al. in 1950 [9, 10]: after carrying out a sulfuric acid hydrolysis to wood cellulose fibers, he observed rodlike particles with two nanoscale dimensions. Indeed, by hydrolyzing cellulose fibers, most of the amorphous parts of the cellulose are disintegrated, and final nanomaterials are highly crystalline. Cellulose nanocrystals have a length between 100 and 500 nm and a width between 2 and 15 nm depending on the cellulose source and the chemical treatment applied. Indeed, even if the use of sulfuric acid for hydrolysis is the most common process, other research groups have investigated the use of other acids, leading to CNC with different properties. In any case, washing steps are essential to remove any chemicals and to well-disperse the

*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents DOI: http://dx.doi.org/10.5772/intechopen.89878*

#### **Figure 4.**

visualization of the different scales inside the cellulose fiber. In addition to being environmentally relevant, cellulose fibers present interesting mechanical properties, ability for further surface modification, low toxicity, low cost, and other properties making them outstanding materials for a lot of traditional as well as

*Schematization of a simplified (a) composition of cellulose fiber (extracted and adapted from [7]) and (b)*

*arrangements of crystalline and amorphous domains in cellulose chains (extracted from [8]).*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

Hierarchical structure of cellulose fibers is at the origin of cellulosic nanomaterials, the nanocelluloses, having at least one dimension of nanometric scale as their name suggests. Indeed, from elementary fibrils, two types of

nanocellulose can be obtained differing by their isolation procedure, as well by their properties and applications. **Figure 4** shows their different morphologies. Briefly, cellulose nanofibrils are obtained by applying high shear mechanical treatment to a cellulose suspension, and recovered nanofibrils have a length between 500 nm and 10 μm and a width between 5 and 50 nm, according to their preparation method,

First investigation on cellulose nanocrystal (CNC) was reported by Ranby et al. in 1950 [9, 10]: after carrying out a sulfuric acid hydrolysis to wood cellulose fibers, he observed rodlike particles with two nanoscale dimensions. Indeed, by hydrolyzing cellulose fibers, most of the amorphous parts of the cellulose are disintegrated, and final nanomaterials are highly crystalline. Cellulose nanocrystals have a length between 100 and 500 nm and a width between 2 and 15 nm depending on the cellulose source and the chemical treatment applied. Indeed, even if the use of sulfuric acid for hydrolysis is the most common process, other research groups have investigated the use of other acids, leading to CNC with different properties. In any case, washing steps are essential to remove any chemicals and to well-disperse the

innovative applications.

**Figure 3.**

**80**

*2.1.2 Cellulose nanocrystals from cellulose*

their source, and potential chemical treatment.

*TEM images of (a) microfibrillated cellulose (MFC), (b) TEMPO-oxidized nanofibrillated cellulose (NFC), and (c) wood cellulose nanocrystals (CNC) (extracted from [8]).*

CNC. Concerning their industrialization, around 10 CNC producers can be recorded, with annual production up to 400 tons/year. These productions are significantly lower than those of CNF, but requirement of more chemicals and difficult industrial production steps (washing, dialysis, and sonication) can easily explain this difference. **Table 1** shows the non-exhaustive list of CNC producers


#### **Table 1.**

*Main CNC producers and their location and annual production capacity (data extracted from Refs. [12, 13]).*

and their annual production capacity. Note that the leader and pioneer CelluForce© has recently announced new strategy of efficient industrialization [11].

cellulosic source is not totally pure, previous steps are required. Indeed, alkali treatment (generally NaOH) and bleaching steps (generally acetic acid, aqueous chlorite) are essential to remove impurities, especially lignin and hemicelluloses when starting directly from biomass or even biowaste. Cellulose content of raw material is thus drastically increased. Note that a lot of studies have investigated the production of CNC from less conventional sources like rice, soy, and others in order to valorize food and organic waste [20, 21]. Final yield and morphology of CNC are really dependent on the cellulosic source and on the hydrolysis conditions. Indeed, optimization and control of the acid hydrolysis have been the subject of several publications. If common parameters are the hydrolysis with 64 wt% sulfuric acid at 40–45°C during about 30 min, it has been proved that variation of one of the parameters can largely influence the reaction yield as well as CNC properties. For example, by increasing of 10 min the time of hydrolysis, it has been shown by Flauzino Neto et al. that crystalline parts are destructed inducing a significant decrease in length [20]. Beck et al. [22] have confirmed this point, admitting that too long times of reaction induce degradation of cellulose but that too short times induce only large and non-dispersible fibers and large aggregates. Only specific reaction times yield to a well-dispersed colloidal suspension of CNC. Chen et al. [17] have confirmed that best yield and CNC properties are obtained with previously mentioned standard conditions. Moreover, the importance of acid concentration relative to cellulose fibers is highlighted too, since a too high concentration could be too drastic and a too low concentration insufficient for the hydrolysis efficiency. At the end of the reaction, mixture media are first diluted with distilled water to quench the reaction, then submitting to several separation steps with centrifugation cycles and filtrations and washed by dialysis against distilled water for several days, in order to remove unreacted compounds and chemicals. In some cases, they use also NaOH to neutralize pH which can modify the crystallinity and the surface ions. After dialysis, a final centrifugation cycle or another filtration process aims to remove aggregates. CNC suspension is finally sonicated in order to well disperse the nanocrystal and obtain colloidal suspension thanks to dimensions and sulfate half

*Cellulose Nanocrystals: From Classical Hydrolysis to the Use of Deep Eutectic Solvents*

*DOI: http://dx.doi.org/10.5772/intechopen.89878*

In addition to their nanometric size, CNC are unique biodegradable and renewable nanomaterials. Moreover, they result from a previously described optimized acid hydrolysis applied on abundant sources of cellulose and exhibit many other interesting properties. **Figure 6** summarizes the main CNC properties as well as their applications. Regarding the surface properties of CNC, they generally exhibit

negative charges are sufficient to induce repulsive forces between nanomaterials and thus colloidal stability in aqueous media. Moreover, as presented in **Figure 6**, due to isolation process, other charged groups can be present on CNC surface, like carboxyl groups (▬COO), aldehyde groups (▬CHO), and others [23], leading to different charge properties inducing different CNC properties. Moreover, numerous hydroxyl groups (three groups in each AGU units) are at reactive surface sites for the introduction of new functional groups via hydroxyl groups' functionalization. Regarding the physical properties of CNC, they have a low density

for CNC extracted from cotton and around 70 for those extracted from tunicate

morphological and surface properties are highly dependent on their source as well

[15]), and a high surface area (between 150 and 800 m<sup>2</sup> g<sup>1</sup>

) on their surface after being treated with sulfuric

), these

). Note that all their

is pretty low (about 50–200 μmol g<sup>1</sup>

), a high aspect ratio length/width (e.g., varying between 10 and 30

ester groups bearing by CNC.

half-sulfate ester groups (▬SO3

(1.606 g cm<sup>3</sup>

**83**

acid. Even if the amount of ▬SO3

*2.2.2 Properties of cellulose nanocrystals*
