2008
Cellulose (2019) 26:1995–2012
the best results. For unrestrained drying the stress– strain curves indicate that the increase in strain is more due to an increase in bonding and less to dislocations and other fiber deformations. Unexpectedly, the results show that fiber bonding has an important role not only in determining the strength but also the elongation of fiber networks, which is in accordance with Seth (2005) and its references. In other words, a large increase in elongation can be reached only by strengthening fiber–fiber bonding (as demonstrated by the sheets with starch and CMF). Also, LC refining with 2000 revolutions in the PFI mill raised sheet extensibility to levels similar to HC and HC ? LC treatment in the case of both drying methods (unre- strained and restrained). This is untypical, as HC refining usually results in higher elongation at break (see e.g. West 1964). The results suggest that HC refining can have a small positive effect on the elongation of paper but not on the elongation at break of single fibers. This apparent contradiction may be due to the fact that the stress individual fibers are subjected to in paper is, on average, much lower than their fracture stress. Paper fracture is determined by factors related to the development of stress concentrations that trigger the final failure in the fiber network. Nevertheless, the results indicate that microcompressions and ultimate strain at break of individual fibers may play a smaller role in sheet extensibility than has been previously believed. The result show that high elongation of paper can be promoted by enabling shrinkage of the fiber network and securing high bonding ability of fibers. Additionally, unrestrained drying seems to be much more effective tool for increasing paper extensibility than the fiber deformations caused by mechanical treatment. As a summary, unrestrained drying of BSKP fiber sheets in combination with increased fiber bond strength may be a promising approach for increasing paper extensibility.
tensile curves were similar to the typical concave downward shape of a paper sheet. The shape of the stress–strain curve of the viscose fibers also varied considerably, but to a much smaller extent than the pulp fibers (Fig. 10d). Tensile curves of viscose fibers typically consisted of three phases, i.e., an initial phase with steep slope was followed by a concave downward or concave upward phase and the last phase was concave downward. The strain at break of the viscose fibers varied from 20 to 50%, which was larger than expected. However, the variation of breaking force of viscose fibers was clearly smaller than that of the BSKP fibers. The relatively small variation of the breaking force of viscose fibers compared to BSKP fibers may be caused by the less heterogeneous cross-section area and the smaller number of defects.
Tensile properties of fiber networks
Tensile tests were performed for both BSKP sheets dried under restraint and without restraint. The tensile test results are presented in Table 4 and the stress– strain curves in Fig. 11a, b. There was a large difference in the tensile stiffness and elongation at break values between the restrained and unrestrained dried sheets. This can be contributed to the difference in drying shrinkage. Unrefined pulps gave the lowest strength and elongation at break values. Increased inter-fiber bonding by application of starch and cellulose microfibrils (CMF) and utilization of higher wet pressing pressure of the unrefined pulp improved elongation and tensile strength significantly and close to those of the HC refined pulps, which had approx- imately only 1 percentage point higher elongation. However, increasing the number and area of fiber contacts by increasing the wet pressing pressure may be connected to water removal and is therefore limited. Applying LC refining to HC refined pulp increased both strength and elongation further. LC refining is known to make the fibers more straight and increase fiber swelling and subsequent shrinkage (Zeng et al. 2012, 2013). The HC ? LC treatment in combination with unrestrained drying yielded the best elongation of 9.3% and tensile strength of 30 MPa of the studied BSKP samples, whereas in the case of individual fibers, the untreated BSKP fibers, on average, yielded
Conclusions
The tensile tests of individual fibers and paper sheets from bleached kraft pulp containing Scots pine ( Pinus sylvestris ) and Norway spruce ( Picea abies ) were conducted in order to investigate the effect of small-
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