1998
Cellulose (2019) 26:1995–2012
Table 1 Average length weighted fiber properties of the pulps and viscose fibers
Untreated HC HC ? LC Viscose
LW fiber length of pulp (mm) (FiberMaster)
1.83
1.87 1.80
LW fiber length of pulp (mm) (Fiber Tester Plus)
1.73
1.66 1.22
5.54
LW fiber width of pulp ( l m) (FiberMaster) LW fiber width of pulp ( l m) (Fiber Tester Plus)
26.6
27.0 27.1
27.4
25.5 24.2
17.2
Shape factor (%) (FiberMaster)
83.7
80.3 83.3
Form (shape factor) (%) (Fiber Tester Plus) Kink per mm (mm - 1 ) (Fiber Tester Plus)
84.8
80.4 83.5
88.7
0.64
1.15 0.90
0.014
clear influence of WD treatment on the average fiber properties of the pulp, e.g. shape factor and kinks, and the strength properties of hand sheets were observed. In this study, the objective was to apply mechanical treatment methods of pulp fibers and chemical strength additives in order to maximize the strength and elongation of individual fibers and sheets. HC treatment of BSKP is known to increase both small and large-scale fiber deformation, whereas LC treat- ment straightens the fibers without significantly removing small-scale deformations (Seth 2005). The fiber samples were selected from untreated, HC (high consistency) treated and HC ? LC(high ? low consistency)-treated pulps. Viscose fibers [Kelheim GmbH (Danufil , 1.7 dTex and staple length 6 mm)] were used as a reference (see Bernt 2011 for more details). Fiber properties of the pulps were measured using the STFI FiberMaster and L&W Fiber Tester Plus (the fibers were measured in swollen state). The length weighted (LW) average fiber lengths and widths, as well as the shape factors, are presented in Table 1.
of two larger research projects: ‘Tailored fibre–fibre interactions for boosted extensibility of bio-based fibre networks (ExtBioNet)’ supported by the Academy of Finland and the ‘Advanced Cellulose to Novel Prod- ucts (ACel)’ program of the Finnish Bioeconomy Cluster CLIC Innovation. The objective of both projects was to improve the extensibility of the pulp fiber network for thermoforming applications.
Experimental
Raw materials
The raw material was a dried bleached softwood kraft pulp (BSKP), a mixture of spruce and pine, from a Nordic pulp mill. The BSKP was soaked overnight in tap water and then disintegrated for 30 min using a Valley beater without load. The BSKP was refined at 40% solids content (high consistency, HC treatment) using a wing defibrator (WD) exactly according to Khakalo et al. (2017a). WD is a high-intensity single stage batch wood chip refiner equipped with four rotating blades. Additional information on the refining conditions for HC treatment can be found in Zeng et al. (2013). The HC treatment was followed by Valley beating (LC treatment) according to the standard procedure (SCAN-C 25:76). The same batch of BSKP is described in detail in Khakalo et al. (2017b) and was utilized in Khakalo et al. (2017a) and Kouko et al. (2018). The SR numbers (ISO 5267-1) of the untreated andHC ? LC treated BSKPs were 12 and 25 and the WRV (ISO 23714) 1.08 g/g and 1.47 g/g, respec- tively. The SR number of the HC treated BSKP was approximately the same as untreated BSKP, because the influence of the HC treatment on SR number was minor as can be seen from Zeng et al. (2013) and Khakalo et al. (2017a). However, in both studies a
Paper samples
The pulps were made into 60 g/m 2 hand sheets according to EN ISO 5269-1. All hand sheets were wet pressed at 350 kPa for 5 ? 2 min with one exception, which is described later. Two drying methods were used to vary the shrinkage of the sheets: restrained drying after wet pressing according to the EN ISO 5269-1 and unrestrained drying between two forming wires that were supported by a grid and separated by a 5 mm thick rod. The elastic modulus and thus the strain to break of paper is known to vary considerably depending on whether the drying shrink- age is free or restrained, see, e.g., Ma¨kela¨ (2009) and Kouko and Retulainen (2018). Untreated and HC
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