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Cellulose (2019) 26:959–970
Graphical abstract
Keywords Cellulose CNC Flow cytometry Light scattering Colloids Hydrophobicity Lipophilic extractives
prolonged (Battista 1950; Ha˚kansson and Ahlgren 2005). The LODP for bleached wood pulp has been reported as 140–200 units (Dong et al. 1998; Habibi et al. 2010). The surface charge of the CNCs will be very different depending on the choice of acid; sulfuric acid introduces sulfate half ester groups to the CNC, while e.g. hydrochloric acid does not (Araki et al. 1999; Beck-Candanedo et al. 2005). The introduction of the anionically charged groups on the surface of the CNCs will greatly increase its colloidal stability and even prevent sedimentation. It was reported that initial hydrolysis of the cellulose raw material by hydrochlo- ric acid can be followed up by a separate sulfuric acid treatment to introduce sulfate half ester groups on the cellulose microcrystals (CMCs), and fine-tune the viscosity properties of the suspension (Araki et al. 1999). CNC particles and suspensions have previously been characterized with a wide array of analytical methods, such as transmission electron microscopy (TEM), atomic force microscopy (AFM), photon correlation spectroscopy (PCS), x-ray diffraction (XRD), conductometric titration, elemental analysis, viscosity measurements, sedimentation measurements (Marchessault et al. 1961; Dong et al. 1998; Beck- Candanedo et al. 2005; Ha˚kansson and Ahlgren 2005; Hirai et al. 2009; Chen et al. 2015). However, additional analysis techniques may still provide useful information, relevant to the ever-growing CNC community. Flow cytometry (FCM) is a relatively new tech- nique in the field of pulping and papermaking. The technique was adapted from medical science, where it is used mainly for the counting of cells (Shapiro 2003). FCM measures the light scattering intensity of parti- cles in suspension in forward and side direction.
Introduction
Cellulose nanocrystals (CNCs) have received a lot of attention in research due to their physical and chem- ical properties, renewability, sustainability, and use- fulness in composite materials (Favier et al. 1995a, b; Ruiz et al. 2000; Habibi et al. 2010; Wang et al. 2012). CNCs are a strong contender for various future applications in composite materials, due to their low cost and abundancy. CNCs are produced by acid hydrolysis of the amorphous and para crystalline regions of cellulose fibers, while the crystalline regions are left intact as rod-like particles (Marches- sault et al. 1961; Habibi et al. 2010). The crystallites of CNCs are, therefore, similar to the crystallites of the cellulose fiber raw materials. A wide array of different raw materials have been tested for CNC production and specific hydrolysis and separation protocols have been established for each of these (Beck-Candanedo et al. 2005; Habibi et al. 2010). The cellulose raw material is normally hydrolyzed by addition of strong acid, in combination with controlled reaction condi- tions, i.e. temperature, agitation, and time (Revol et al. 1994; Dong et al. 1998; Wang et al. 2014). The length of the cellulose crystals after hydrolysis depends on the leveling-off degree of polymerization (LODP) of the starting material. It has been reported that milder hydrolysis conditions will eventually reach the LODP of the starting material with lower losses of material, but that the reaction times need to be severely
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