*2.2.3 Various applications of cellulose nanocrystals and their industrialization*

As exposed in **Figure 6**, CNC found applications in various fields. Indeed, thanks to their outstanding morphological, mechanical, and rheological properties as well as their colloidal stability and high surface reactivity. All these properties added to their biodegradability and renewability make them highly interesting and innovative materials with many potential applications. **Table 2** summarizes CNC applications and corresponding exploited properties, as well as some literature references.

Nanocomposite field is an emerging research area which finds applications in several domains like food packaging, medical devices, and building. Renewable aspect of CNC is particularly interesting since it correlates with the development of bio-based and biodegradable polymers as mentioned in the first part of this chapter. Moreover, these same properties are just as interesting in other application fields, from coatings, electronics, filtration, and biomedical devices to energy, cosmetics, and security. Note that for applications that may enter in contact with food or human body and for any industrialization, toxicity of CNC is a key challenge to investigate. Indeed, even if cellulose is known to be a nontoxic polymer, CNC are nanomaterials—and the "nano" prefix can be frightened for media and population

#### **Figure 7.**

as their isolation process and conditions [8, 22]. Moreover, CNC exhibit highly interesting mechanical properties. Indeed, in addition to their high crystallinity (between 54 and 88% according to the source [24]), their high elastic modulus (≈150 50 GPa) and tensile strength (≈7.5 0.5 GPa) [25] make them interesting materials as mechanical reinforcement in polymer matrices, for example. For comparison, their mechanical properties are similar to Kevlar® fibers [26] widely used

*Main surface and physical properties of cellulose nanocrystals and inherent main applications.*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

and thus between each nanocrystals, CNC water suspension is in the form of a translucent gel. Rheological properties of CNC are outstanding and concentration dependent. Indeed, at low concentration (<3 wt%), CNC suspension presents shear thinning behavior at high shear rate, and at higher concentration (>3 wt%), the suspension presents shear thinning behavior explained by the nanocrystals alignment in the flow direction at a critical shear rate [27]. Source and isolation of CNC influence these rheological properties too. Besides all these properties, CNC selforganize in ordered structure, especially to form a nematic phase. Revol et al. [28] described in the 1990s this self-organization of CNC in water suspension into stable chiral nematic phases. These last exhibit liquid crystalline properties, which when

At low solid content (<3 wt%), due to hydrogen bonds between cellulose chains

in composite field.

**Figure 6.**

**84**

*(a) Translucent gel-like CNC suspension at 15 wt% in water (extracted from 57), (b) birefringence with shear-inducing observed for an aqueous CNC suspension at 0.6 wt% in cross-polarized light (extracted from [57]), (c) solvent-casted CNC film in diffuse light, normal to the surface (on the left part) and oblique to the surface (on the right part) (extracted from [29]), and (d) schematic representation of CNC orientation in isotropic and anisotropic phases (This scheme was extracted from an unpublished work (PhD manuscript of R. Bardet, 2014)).*


Deep eutectic solvents (DESs) are a new class of green organic solvents; they are in the continuity of molten salt and ionic liquid solvent, but they are less toxic and easier to use. A deep eutectic solvent is composed of at least two compounds, a Lewis or Brönsted acid and a base. According to its constituents, a DES can be

In the case of type III, it is accepted that the self-association occurs via hydrogen bonding interactions between the hydrogen bond donor and the hydrogen bond

> **Time Temp. (°C)**

1:2 2 h 100 88

ChCl:OAD 1:2 1 h 100 80.0 l = 152.7

ChCl:OAD 1:3 1 h 100 81.6 l = 122.4

3 min 90 62.4

**Yield (%)**

1:1 2 h 120 73 l = 353 16

3 min 80 74.2 l = 100–350

3 min 100 57.8 l ≈ 150

6 h 80 73 l = 270 92

6 h 80 71 l = 258 54

6 h 80 88 l = 5726 3856

AH:glycerol 1:2 10 min 70 d = 5.7 1.3 [54]

ChCl:citric acid 1:2 6 h 85 l = 25–37 [55]

T

*ChCl, choline chloride; OAD, oxalic acid dihydrate; t, toluenesulfonic; AH, aminoguanidine hydrochloride; GH,*

**Dimension (nm)**

d = 13.6 1.1

d = 13.8 0.7

d=3–8

d = 9.6 2.9

d = 6.1 1.2

d = 4.7 2.2

d=3–25

d < 17

80 d = 5.6 1.4 [56]

7 h 80 86 l = 5152 3328 [53]

**References**

[49]

[50]

[51]

[52]

The association of these compounds with a specific ratio forms an eutectic

classed in one of the four existing categories listed in **Table 3**.

**Cellulose source Pretreatment**

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

Cotton fiber ChCl:OAD

Bleached eucalyptus kraft

Bleached birch Kraft pulp

Empty fruit bunch

**Table 4.**

**87**

pulp

**(DES, etc.)**

ChCl:levulinic acid

> *Microwave assisted*

ChCl: OAD *+ catalysis, FeCl3*.*6H2O (mmol/gDES)*

Dissolving pulp GH:APA 1:2 24 h Room

*guanidine hydrochloride; APA, anhydrous phosphoric acid.*

mixture with a melting temperature far below than of its constituents.

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

**Molar ratio**

Dissolving pulp ChCl:OAD 1:1 2 h 100 68 l = 390 25

Dissolving pulp ChCl:OAD 1:1 30 min 100 l = 50–350

Cotton fiber ChCl:OAD 1:1 1 h 100 79.8 l = 194.1

1:1 *800 W*

1:1 *800 W*

1:1 *800 W*

> 1:4 *0*

1:4 *0.15*

> 1:4 *0.3*

1:1 *0.15*

*Overview of the different DES pretreatments tested for cellulose nanocrystals obtention.*

ChCl:p–t 1:1 2 h 60 70

#### **Table 2.**

*Main market applications of CNC and corresponding properties and literature references.*

—with specific morphology and surface properties. A recent review of Roman et al. [44] explored CNC toxicity. Results of this study correlate with previous results of Lin et al. [39] and Kovacs et al. [45], demonstrating that CNC are not toxic by ingestion or dermal contact and for aquatic organisms. However, pulmonary and cytotoxicity are less ideal since their toxicity depends on CNC properties and form (especially if they are in powder form, since they are more volatile). In any case, toxicity of CNC is low, especially when they are in wet-state or in composites, films, or coatings, for example, not constraining development of new CNC-based products.
