Cellulose nanocrystals (CNC)

CNC is one of the more widely researched of the three types of nanocellulose. The growth in the number of publications based on CNC is represented in Figure 3.3. This great growth in the num-ber of published papers is an indication of the great potential that CNC possesses because of the

CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES

desirable properties it can provide (high strength, optical properties, self-assembly properties etc.), (Habibi et al., 2010). Moreover, CNCs can easily be surface modified, as with other types of nanocellulose, to impart desired properties such as hydrophobicity/and electrical conductivity (Eyley and Thielemans, 2014; Trache et al., 2017). This makes them attractive as additives in electronic devices, fillers in composites and binders in inks and in paint formulations (Rusli et al., 2011; Hoeng et al., 2016; Du et al., 2017).

The general methods of CNC production involve the selective removal of the less ordered regions of the cellulose repeat unit by a hydrolysing agent, leaving behind short needle-like or rod-like crystals of the ordered regions (Azizi Samir et al., 2005; Habibi et al., 2010; Klemm et al., 2011; Trache et al., 2017). Cellulose nanocrystals are mainly prepared by the use of controlled, strong acid hydrolysis using one of sulphuric acid (Bondeson et al., 2006; Dong et al., 2016), hydrochloric acid and phosphoric acid (Habibi et al., 2010; Dufresne, 2012). Enzymes have also been used for cellulose hydrolysis (Filson et al., 2009; Yarbrough et al., 2017) and used as a mediating step to acid hydrolysis (Beyene et al., 2017). Other methods of CNC production make use of ionic liquids, of supercritical water as well as oxidants (Habibi et al., 2006; Sun et al., 2015; Zhou et al., 2018). CNC produced via the sulphuric acid hydrolysis route are surface modified with anionic sulphate groups and are able to form stable dispersions in water. However, hydrochloric acid hydrolysed CNC are not surface modified with any ionic groups and are unable to maintain a stable dispersion in water (Azizi Samir et al., 2005; Klemm et al., 2011; Dufresne, 2012).

There are various factors that can affect the dimensions of CNC. One of them is the source of cellulose used in preparation. A review on the dimensions of CNC from different sources shows that CNC from Valonia and tunicin (cellulose extracted from tunicate) could have lengths over 1000 nm and widths within 10 – 30 nm. Those from wood pulp have lengths of about 100 – 200 nm and widths of 3 – 4 nm (Dufresne, 2012; Habibi et al., 2010). Other factors that affect the morphology of CNCs include the experimental conditions such as the time, the temperature, the acid concentrations and the acid-to-pulp ratio (Habibi et al., 2010; Klemm et al., 2011; Trache et al., 2017). Studies carried out by Dong, (1998) reveal that, at a constant temperature of 45^◦C the time of hydrolysis with sulphuric acid increased from 10 min to 240 min, causing the length of the CNC to decrease from 390 nm to 177 nm, while the surface charge increased. Increasing the reaction temperature (45^◦C to 72^◦C) also results in a decrease in CNC lengths (Habibi et al.,

CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES

2010).

Using sulphuric acid (45% to 55%) at constant temperature of 45^◦C, and hydrolysis times between 10 and 30 min, followed by 5 passes through a high shear processor, Tian et al., 2016 produced what was regarded as charged CNF having interconnected nanofibrils. In another work, Sun et al., 2015 prepared what they referred to as CNC by subjecting a TEMPO-mediated oxidised cellulose to high pressure homogenisation (202 MPa) for 5 passes. The authors reported a length of 200 nm for the TEMPO-CNC. However, from the transmission emission microscopic (TEM) image provided, it was difficult to ascertain the discrete length of the fibrils as they are seen connected to each other. These studies nevertheless show how variation in the experimental conditions and methods could cause significant changes to the properties of the resulting nanocellulose, switching from CNC to CNF or vice versa.

The optimised experimental conditions to yield CNC with widths less than 10 nm and lengths between 200 nm and 400 nm from cellulose microcrystals involved a 63% - 65% sulphuric acid concentration and a 2h reaction time, which resulted to an average yield of 30% relative to the starting cellulose (Bondeson et al., 2006; Habibi et al., 2010). Above this concentration, complete hydrolysis to the glucose monomer ensues. A typical micrograph of CNC is shown in Figure 3.5. CNC is now produced on the large scale of up to 1000 kg per day, mainly by the acid hydrolysis route (Chauve and Bras, 2013; Trache et al., 2017). The associated challenges of the acid hydrolysis route include the amount of concentrated acid used during production and the fairly low yield. To reduce the amount of acid used during hydrolysis and possibly to increase the yield, the use of enzymes in a pre-treatment prior to the acid hydrolysis has been suggested (Beyene et al., 2017; Beltramino et al., 2018). However, the overall cost of the entire process would determine whether or not an additional pre-treatment process would be beneficial.

CHAPTER3. BACKGROUND ON CELLULOSE NANOMATERIALS AND COMPOSITES

Figure 3.5: Typical transmission electron micrograph of CNC derived from Sisal fibres (Gar-cia de Rodriguez et al., 2006)

CNCs having adequate surface charges and between a critical concentration (1% -10%) show optical birefringence and have the ability to self-assemble, thereby taking up a liquid crystalline arrangement (Habibi et al., 2010). This property can be useful in optical displays and security papers (Khalil et al., 2014). The mechanical properties of CNC and its reinforcing ability depends mainly on the aspect ratio. Typical aspect ratios of CNC as determined from measurements of lengths and widths from microscopic images, are between 25 to 40 (Habibi et al., 2010). While CNC from cotton linters have the lower aspect ratio (4-5) (Elazzouzi-Hafraoui et al., 2008), those from tunicin have the higher aspect ratio, up to 100 (Kimura et al., 2005). Higher aspect ratio nanomaterials are desired in composite formulation because of their efficient stress transfer from the matrix to the nanomaterial (Rusli et al., 2011). This is why higher aspect ratio CNCs, such as those from tunicin, have greater reinforcing capability than those from cotton and wood with lower aspect ratios (Klemm et al., 2011; Rusli et al., 2011; Mariano et al., 2014).