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Cellulose (2019) 26:1995–2012
the other hand the strength of cellulose should probably be greater than the strength of BSKP fiber. The tensile strength (310 MPa) of the viscose fibers, which are formed from cellulose II polymer chains, was less than half of that of the BSKP fibers. However, according to Bledzki and Gassan (1999), the mechan- ical properties of the man-made cellulosic fibers depend on their structure on different levels, and the tensile strength of viscose fibers has been shown to be strongly influenced by the length of molecules and straining in the manufacturing process. The lower tangential stiffness and the higher plasticity of viscose fibers, in respect to the BSKP fibers (shown in Fig. 10a–d), are some of the probable reason for the higher elongation and lower strength. The BSKP fibers contained some visible small-scale deformation and because of the removed lignin and hemicellulose (from the inner cell walls S 1 and S 2 ), their fiber-wall was possibly more porous than that of viscose fiber. The small-scale deformations in the BSKP fibers did not seem to have any role for the strength in comparison to the structurally more homogeneous viscose fibers. As expected, a clear decrease in BSKP fiber strength was observed due to the mechanical treatments. On the other hand, the observed decrease in work to break (i.e. toughness) due to the mechanical treatment on BSKP was unexpected.
Fig. 9 Breaking force and strain at break of the single BSKP fibers as a function of cleavage index of the pulps with 95% confidence intervals
the number of cleavages per length-unit in viscose fiber was lower compared to BSKP. Results indicate that viscose fibers are less homogenous than expected and may also exhibit defects. Also the mechanically untreated BSKP fibers had a significant number of cleavages per fiber.
Stress–strain curves of the fibers
Stress–strain curves of the individual fibers are presented in Fig. 10a–d. Stress–strain curves of thick-walled fibers are presented in red and thin- walled in light blue. The markers with the 95% confidence intervals present average values of thick- walled, thin-walled, and all fibers according to Table 3. All force-strain curves of the individual fibers are presented in the electronic supplementary material S1-S4. The tensile curves of the individual untreated BSKP fibers (Fig. 10a) were either smoothly and bi-linearly concave upward, apparently linear, or slightly concave downward, but none of them were highly concave downward, which is a typical shape for a tensile curve of paper (see Fig. 11a, b). The larger parts of the tensile curves were formed from two or even three phases that were mostly slightly concave upward. Increasing slopes may indicate the presence of dislo- cations or other fiber deformations, which were pulled straight during tensile testing. The large variation between the single BSKP fiber stress–strain curves in terms of shape, in addition to the variation of elongation from 10 to 32%, can also be regarded as an important finding. The single fiber tensile curves
Cleavage of fibers
A cleavage test using HCl treatment was used to estimate the number of fiber defects (Ander et al. 2005, 2008; Zeng et al. 2012). The HCl treatment mainly cuts the fibers at the dislocation areas, which can be considered as weak points in the fibers. The high consistency (HC) treatment of BSKP increased the cleavage and the LC treatment reduced the cleavage, as presented in Table 3. Both breaking force and elongation of the BSKP fibers formed a linear relation with negative slope as a function of cleavage index and with each other as shown in Fig. 9. This suggests that both, fiber strength and fiber extensibility decrease with increasing mechanical treatment due to fiber damage. Additionally, the results indicate that the cleavage test may be a useful indirect method of estimating the influence of mechanical treatments on the strength and elongation of individual pulp fibers. Surprisingly, the viscose fibers showed several cleavages per fiber. However,
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