Cellulose (2021) 28:5807–5826
5821
CTMP
PCC-CTMP
CTMP
PCC-CTMP
100
40
90
30
y = 0.13x - 24.9 R² = 0.981
80
20
70
10
y = 0.062x - 8.8 R² = 0.989
60
0
0
200
400
600
0 min
30 min
60 min
Sheet density [kg/m³]
the interaction with Ca 2 ? and the partial negative charge of the hydroxyl group (due to polarization) (Dalas et al. 2000), but the hydroxyl groups on the fibers require alkaline conditions to ionize (Sjostrom 1989) and contribute to the fiber surface charge. The fiber charge at neutral pH range is commonly attributed to carboxyl groups found e.g. in hemicel- luloses instead of cellulose (Laine 2007; Sjostrom 1989), and CaCO 3 has been reported to precipitate onto surfaces via an ACC precursor that attaches onto the surface containing carboxyl groups (Stephens et al. 2011). Lignin is known to form complexes with metals (Schmidt 2010) and has been reported to have a high affinity towards Ca 2 ? (Torre et al. 1992). The mechanism of metal uptake of lignin is however not well understood (Guo et al. 2008; Suhas et al. 2007). It has been linked with proton displacement (Christ et al. 2002; Guo et al. 2008) and carboxylic and phenolic sites, of which phenolic groups are thought to be dominant (Guo et al. 2008). The hydroxyl groups of cellulose may undergo metal-ion facilitated deproto- nation especially at high pH (Al-Sogair et al. 2011), whereas it has been reported that lignin exhibits the opposite behavior, i.e. a decreasing deprotonation with increasing pH (Christ et al. 2002). This could explain why cellulose was ‘‘more effective than its charge’’ in initiating PCC precipitation under the conditions used in our work, especially if the charges are compared at neutral pH. Another possible reason for the increase in ash content with increasing beating and increasing surface Fig. 16 Opacity of the handsheets prepared using fractions of CTMP and CTMP-PCC. n/f indicates sheets made using unfractionated pulp
Fig. 14 Tensile index versus sheet density of fractionated sheets. The lines show the linear regression
CTMP
PCC-CTMP
80
70
60
50
40
0 min
30 min
60 min
Fig. 15 Brightness (R457C) of the handsheets prepared using fractions of CTMP and CTMP-PCC. n/f indicates sheets made using unfractionated pulp
secondary wall S 1) are known to be richer in charged species (such as lignin) than the inner layers of the fibers, which in turn are richer in less charged cellulose (Hubbe 2006). The study showed an approximately linear correlation between the ash content of the PCC- CTMP fractions and the anionic surface charge density of the corresponding CTMP fraction ( R 2 of liner fit was 0.849, see Fig. 17). Assuming that the metal-binding ability of the functional groups correlates with their charge, this is partly in conflict with the hypothesis that the func- tional metal-binding groups steer the precipitation onto the fibers. The situation is however more complex. The dominating interaction is stated to be
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