Cellulose (2019) 26:1995–2012
2005
Fig. 8 a Breaking force over breaking strain and b slope of force over work to break with 95% confidence intervals plotted from the data in Table 2. The arrows show the influence of increasing mechanical treatment
Fiber elongation
tangential modulus of thin- and thick-walled fibers (Table 2). However, the subsequent mechanical LC treatment reduced the tangential moduli of the thick- walled BSKP fibers, whereas the tangential moduli of the thin-walled BSKP fibers were slightly improved. Microfibril angle (MFA) has been reported to dictate the elastic moduli of thin-walled (presumably early- wood) and thick-walled (presumably latewood) fibers (Schniewind 1966; Borodulina et al. 2015). Therefore, the investigated fibers may also have contained varied microfibril angles. Tangential modulus of the viscose fibers was 2.6 GPa, which was less than half that of the BSKP fibers. Unexpectedly, the influence of mechan- ical treatment on BSKP fibers’ tangential stiffness was small.
The high elongation values obtained for the BSKP fibers are higher that the most results presented in the literature. However, these fibers were from industrial pulp and gone through harsher treatments than labo- ratory processed pulps. Additionally the BSKP was made of chips from younger trees originating from thinnings that typically have higher fibril angle fibers (Groom et al. 2002). Elongation at break of thick- walled fibers was not significantly changed by the mechanical treatments, only the variance grew. On the other hand, strain at break of thin-walled fibers decreased from the original 17.7% value to 14.5% and 12.1% due to the HC and HC ? LC treatments, respectively. The microscope photographs (see electronic supplementary material S1-S3) show that the investigated untreated BSKP fibers had a significant amount of small-scale deformations, which was slightly unexpected. Elongation of the viscose fiber, at 34.3%, was significantly higher compared to the BSKP fibers. Unexpectedly, the individual BSKP fiber elongation was not improved after the mechan- ical treatments.
Individual fiber strength
The HC and HC ? LC treatments decreased the tensile strength of BSKP pulp fibers on average, but the decrease concentrated on the thick-walled fibers, whereas strength changes of thin-walled fibers were within the 95% confidence intervals. Tensile strength in MPa units (force scaled with the cross-section area) of the BSKP fibers varied from 600 to 840 MPa, which is comparable to the results obtained in other studies (Hardacker and Brezinski 1973; Duncker and Nord- man 1965; Page et al. 1972; Groom et al. 2002). The origin of the fiber strength is the strength of natural cellulose I polymer in the cell wall. A few estimates, varying from 297 to 800 MPa, have been given for the strength of crystalline and amorphous cellulose I polymer (Zeng et al. 2013; Lundahl et al. 2016), but on
Fiber tangential stiffness
The tangential moduli of average BSKP fibers (con- taining thin- and thick-walled fibers) were in quite a narrow range from 5.5 to 6.3 GPa, but the values were also only 30–50% of the values presented from the literature in Table 1 in the supplementary material S1. The mechanical HC treatment seemed to improve the
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