Cellulose (2021) 28:5807–5826
5815
CTMP R30 CTMP R100 CTMP R200 CTMP R400
PCC-CTMP R30 PCC-CTMP R100 PCC-CTMP R200 PCC-CTMP R400
Zeta potential and surface charge density
The zeta potential and surface charge density data are shown in Table 3; Fig. 7. Although the samples were either anionic (CTMP and most PCC-CTMP samples) or essentially non-charged (some of the PCC-CTMP samples), the unfractionated PCC-CTMPs showed positive zeta potentials. Refining of the CTMP increased the surface charge density but reduced the zeta potential, presumably due to the creation of more cellulose rich areas on the CTMP fiber surfaces (Hubbe 2006). This was observed particularly for the R30 CTMP samples. Increasing refining did not affect the surface charge of the R400 fractions in an obvious manner. The charge development detected for CTMP is in good agreement with the literature. Sundberg and Holmbom (2004) reported that flake-like CTMP fines had a higher total charge than fibril-like fines, both of which had a total charge more than twice that of the CTMP fibers. Mosbye and Laine (2002) reported that the primary fines of mechanical pulp had a higher total charge than secondary fines, although the difference was greatly diminished by alkali treatment used to imitate the effect of bleaching. In our case the pulp was bleached. Although the charge of PCC is commonly cationic due to an excess of positive calcium (Ca 2 ? ) (Laine 2007), all fractions of PCC-CTMP, including the extremely PCC rich ‘pass’ fraction, had negative or essentially neutral surface charges. This may have been due to adsorbed anionic dissolved (and colloidal) material without significant incorporation of the materials into crystal as described by Jada and Verraes (2003) for a different polymer or the ‘‘cleaning effect’’ of the in-situ precipitation of PCC, in which dissolved (and perhaps colloidal) material is absorbed onto the PCC as reported by Matula et al. (2018). The positive zeta potentials of unfractionated PCC- CTMP were probably an artifact caused by small, PCC-rich particles found on the ‘pass’ fraction of the unfractionated pulps, as they were poorly immobilized during the zeta potential measurement and were able to pass the wire and disturb the measurement. Another possible explanation was over-emphasizing of possi- bly cationic PCC residing on fiber surfaces of unfrac- tionated samples due to the flow conditions and ion
25
20
15
10
5
0
50
500
5000
0.05 0.1 0.2 0.5 1 2
5
Fiber length class [mm]
Fig. 5 Fiber length frequency distributions for CTMP and CTMP-PCC refined for 30 min. Note the logarithmic x-axis
CTMP R30 CTMP R100 CTMP R200 CTMP R400
PCC-CTMP R30 PCC-CTMP R100 PCC-CTMP R200 PCC-CTMP R400
25
20
15
10
5
0
5
10 20 30 40
50 Fiber width class [μm]
100 200
Fig. 6 Fiber width frequency distribution for CTMP and CTMP-PCC refined for 30 min. Note the logarithmic x-axis
123
Made with FlippingBook Online newsletter maker