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Cellulose (2021) 28:5807–5826
the 400 mesh wire when the CTMP was fractionated. Reduced interparticle interactions can explain the increase in fines content during precipitation of PCC and the loss of fines during fractionation. This is in good agreement with the report that water removal from a MNFC-containing furnish is increased when PCC-fiber composite is created on the MNFC (Ran- tanen et al. 2015), although the increase in the amount of mobile fines is expected to decrease the water removal from the sheet (Hubbe and Heitmann 2007). Since the PCC was found to be attached mainly onto the fines, which increases the light scattering of mechanical pulp (Leskela¨ 1998), the increase in s r was probably also low for the fines fraction. Therefore, assuming that PCC is enriched on the fibrils of externally fibrillated fibers similarly than it attaches onto fines and damaged CTMP fibers, a carefully executed refining of the pulp prior to precipitation phase could be beneficial for pulp intended for PCC-fiber composite preparation. By controlling external fibrillation it could be possible to tailor PCC-fiber composites, for example by steering the precipitation onto larger fiber fractions by the use of external fibrillation. The use of selected fractions in pulp blends could also be of interest, although fractionation as such may not be the most alluring industrial approach.
affecting the bonding was that the PCC was precip- itated in (split fiber) lumens and perhaps inside the fibrillated structures of the more well-fibrillated CTMP, which is expected to stiffen the fibers, to make them less conformable and less collapsible (Kumar et al. 2011). Such factors are desirable when high bulk is required, but they tend to have a negative effect on bonding. There was significantly less debonding with the separate fractions than with the unfractionated PCC- CTMP. The large-fiber fractions retained their strength properties better than the small-fiber frac- tions. A comparison of the fiber-weight-based indices for PCC-CTMP and CTMP sheets showed only a slight decrease on strength for most fractions, and the tensile stiffness index of R400 even increased although filler is known to disturb the formation of a fiber network (Hubbe and Gill 2016). The conserved strength could depend on the distribution of PCC in the sheets (different perhaps to that of traditionally filled sheets, as reported by Silenius (2002)), or due to differences in segment activation compared with a typical sheet. The results suggest, however, that the fines fractions may be capable of containing a considerable amount of PCC without losing their bonding potential to a proportional degree. Based on the SEM micrographs, most the PCC present was expected to form a PCC-CTMP compos- ite, i.e. the yield of CTMP and Ca(OH) 2 to PCC- CTMP was considered to be high. This suggests excellent retention during sheet-making, but this may not be so since the fines easily passed wires during fractionation. Fines were expected to grow in size due to attachment of PCC. This may partially explain why the fines content increased during precipitation of PCC onto CTMP (i.e. they were better detected by the fiber analyzer), but the increase in size does not explain the increased tendency for the fines to pass the wires. We suggest that the loss of fines during fractionation was due to the precipitated PCC that reduced the tendency of the fines to attach onto other particles. Such result has been reported by Klungness et al. (1996): In their pilot trial, when using a PCC-fiber composite the total first pass retention was negatively affected, though first-pass retention of filler increased. The PCC attached onto the fines appeared to prevent the fines from forming a gel-like, water- removal-resisting structure, which was observed on
Conclusions
Fractionation of the PCC-CTMP composite showed that the ash content increased with decreasing fraction size. The PCC was attached well onto the fiber, as shown by its tendency to remain in the samples and by SEM micrographs in case of the ‘pass’ fraction. The reason for the preferred precipitation onto fines is unclear, but a correlation was found between the anionic surface change density of the CTMP fraction and the ash content of the PCC-CTMP fraction. SEM micrograph data suggests that the PCC prefers to precipitate to the areas typically associated a lower charge than the primary wall. This may, however, be explained by chemical changes such as metal-ion facilitated deprotonation in cellulose due the higher pH. The correlation is therefore thought to support the suggestion that the interparticle (fiber-ACC) interac- tions, perhaps partially originating from cationic-
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