Cellulose (2019) 26:959–970
967
the kraft pulps most likely contained lipophilic extractives that had survived the harsh acidic condi- tions of CNC production. Extraction of the three kraft pulps with an appropriate solvent was required in order to confirm this theory. The three kraft pulps were extracted with a mixture of acetone, water, and acetic acid. The content of lipophilic wood pitch components in extracts were analyzed according to standard laboratory procedure (O¨ rsa˚ and Holmbom 1994). It was seen that all of the kraft pulps contained lipophilic components to some extent (Table 3). The three pulps did not contain noticeable amounts of triglycerides or steryl esters, since these components had been hydrolyzed into fatty acids and sterols by the strong alkaline conditions during the kraft cook (Back 2000). The dominating components in the eucalyptus and birch kraft pulp were fatty acids and sterols, as expected (Ekman and Holmbom 2000). It is known that the water-solubil- ities of long-chained fatty acids are very limited even at alkaline pH, and can be severely decreased at high conductivity values (Sundberg et al. 2009; MacNeil et al. 2011). This fact makes it very difficult to completely remove fatty acids by alkaline washing, even after kraft pulping. Traces of resin acids were also noticed in the hardwood pulps, which may have been added to aid deresination during washing at the pulp mills. The dominating components in the soft- wood kraft pulp were fatty acids and resin acids (Table 3). The detectable content of lipophilic extrac- tives in the three kraft pulps were quite low overall, only from 48 to 188 l g/g o.d. pulp. Still, these amounts were significant enough to be detectable by FCM analyses of the final CNCs. It is known that lipophilic extractives are not degraded by acidic conditions, which has previously been seen in analyses of extractives throughout an acidic sulfite pulping line (Sithole et al. 2010a, b). This meant that the fatty acids, resin acids, and sterols observed in the kraft pulps were able to withstand the harsh acidic
hydrolysis step of CNC production, and remained with the CNCs through the rest of the production steps. When reexamining the visible hydrophobic particle populations in Fig. 2, it was clear that the highest content of hydrophobic particles was seen in the CNCs made from birch kraft pulp. There were significantly less hydrophobic particles in the eucalyptus CNC, and only a very low amount in the softwood CNC. It was expected that the content of lipophilic particles in the kraft pulps would correlate directly with the observed hydrophobicity of the CNC. However, eucalyptus kraft pulp had the highest content of lipophilic components, while birch kraft pulp contained slightly less (Table 3). In order to understand this, the hydrophobicity of the different groups of extractives need to be taken into account. The pH of the CNC suspensions after dialysis was roughly 5.5. It is known that a fraction of the fatty acids may have been deprotonated, formed soaps, and have been dissolved into the aqueous phase even at this moderate pH (Stro¨m 2000; Sundberg et al. 2009). However, the large and bulky carbon skeletons of sterols, and the lack of carboxylic acid groups, indicate that these are very hydrophobic compounds (Sjo¨stro¨m 1981; Ekman and Holmbom 2000; Nisula 2018). Sterols have also been shown to be quite troublesome in deresination of sulfite pulp, unlike resin and fatty acids (Sithole et al. 2010a). When taking the surface-active nature, and pH-dependent water-solubility of fatty acids into account, it is clear that fatty acids need to be considered significantly less hydrophobic than sterols, at least at pH values higher than 3. The sterols most likely influenced the particle hydrophobicity to a higher extent than fatty acids. A more satisfactory correlation was found between the hydrophobic pop- ulations in the CNCs and the sterol content of the three different kraft pulps. The least hydrophobic particles were produced from the raw material that contained the lowest amount of lipophilic components, i.e. the softwood kraft pulp. Some of the birch kraft pulp was
Table 3 Quantified amounts of extractives found in the aceton:water:acetic acid extracts of the different kraft pulps. The amounts are expressed as l g per o.d. g of pulp (uncertainty ± 5%) Kraft pulp Fatty acids and fatty alcohols ( l g/g) Resin acids ( l g/g) Sterols ( l g/g) Other* ( l g/g) Lipophilics ( l g/g)
Eucalyptus 134
8
46
22
188
Birch
79
1
56
50
136
Softwood
38
10
0
26
48
Other* various identified degradation products. Lipophilics the sum of fatty acids, fatty alcohols, resin acids, and sterols
123
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