2004
Cellulose (2019) 26:1995–2012
Fig. 6 Viscose fiber: a smooth and straight structure, b cross-section
Fig. 7 Tensile test of untreated BSKP fiber: a the fiber mounted in the tester before the test, b the tested fiber after the test
than 190 l m 2 were classified as thick-walled fibers. The average dimensions of the investigated individual fibers are presented in Table 3. The number of repeats was low for some classes, but the validity of the classification can be verified from the images supplied in the electronic supplementary material S1-S3.
Average tensile properties of the tested individual BSKP fibers are presented in Table 2 and Fig. 8a, b. The untreated, HC and HC ? LC treated BSKP fibers were classified as thin-walled, thick-walled and aver- age (containing the two previous) groups using the cross-section images and areas. The number of repeats was low for some classes (e.g., HC treated thick- walled BSKP fiber). However, the relevance of the classification can be verified from the electronic supplementary materials S1-S3.
Illustration of single fiber tensile test
The tensile test of individual untreated BSKP fiber is described in Fig. 7. Figure 7a, b present the fiber mounted in the tester before and after the test. The force-strain curve of the individual untreated BSKP fiber was concave upwards, i.e., the slope increased as a function of strain. The breaking force of the individual fiber was 160 mN, which was close to the reported value for latewood fiber of softwood kraft pulp (192 mN and 206 mN for Scots pine and Norway spruce respectively, see Table 1 in the Supplementary material S1). The strain at break of the fiber was * 20%, which was high compared to the reported values for similar softwood pulp fiber (see Table 1). Side and cross-section microscope images and the force strain curves of all measured softwood fibers are presented in the electronic supplementary materials S1-S3.
Thick-walled fibers versus thin-walled fibers
Strain at break, breaking force, slope of force, and work to break of thick-walled fibers were significantly higher compared to the respective values of thin- walled fibers, which is according to expectations. HC andHC ? LC treatment decreased the breaking force of thick-walled fibers and did not affect the average breaking force of thin-walled BSKP fibers. Also, the observed reduction in breaking force due to the mechanical treatment is an expected result. There are clear differences between the tensile properties of thick-walled and thin-walled fibers.
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