Cellulose (2021) 28:5807–5826
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Fig. 8 SEM micrographs of fractionated PCC-CTMP samples. a (30 min R200): a band of outer fiber layers essentially free from PCC (indicated with the arrow), surrounded by more PCC rich areas. b (0 min R200): heavily fibrillated fiber with high
content of PCC and a more intact fiber with substantially no PCC. c (0 min ‘pass’ fraction): fibrils and fines extremely rich in PCC. Image d (60 min R200): split or cleaved fiber with PCC deposited in lumen and outer layer essentially free from PCC
part of them), and these objects containing more PCC were removed during fractioning. The surfaces of the larger CTMP fibers were however also affected by fractionation: some of the PCC residing on the fiber surface was removed during fractionation.
The PCC was located mainly on fiber regions where the outer layer of the fibers had been removed (Fig. 8a) or where the fiber surface was damaged or subjected to fibrillation (Fig. 8b). The tendency of the PCC to attach to fibrils or to damaged regions of the fiber was clearly evident in the fractionated samples with the highest amount of fines, i.e. the R400 and in ‘pass’ fractions (Fig. 8c). Split fibers had enriched PCC deposits in the lumen, while the outer surface appeared to be essentially free from PCC (Fig. 8d). This type of finding was usually seen in R100 and R200, but occasionally also in R30. When the fractions of PCC-CTMP were compared with the unfractionated PCC-CTMP (Fig. 9), it was evident that a larger portion of the fibers were substantially free from PCC in the fractionated pulps than in the unfractionated sample. Some of this difference could be explained by the hypothesis that the fines in the unfractionated pulp were attached or otherwise located on larger fibers (thus appearing a
Handsheet properties
Physical properties
The physical properties of the handhseets are shown in Table 4 and in Figs. 10, 11, 12 and 13. The sheets had an average grammage of 68.6 g/m 2 (standard devia- tion 5.8 g/m 2 ). The ash content of the sheets was assumed to be close to that of the pulp or fraction used in sheet preparation (Table 1), since the auxiliary wire improved mechanical retention. The density of PCC-CTMP handsheets from unfractionated pulps was higher than that of CTMP handsheets free from PCC. The difference in density
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