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Table 1 The measured fiber properties of unrefined and low-consistency refined softwood kraft pulp
Unrefined ( SR 12)
ProLab refined ( SR 25)
Fiber length (l) (mm)
1.83
1.83
Fines (l) (%)
2.45
2.90
Fiber width ( l m)
28.5
29.7
Cell wall thickness ( l m)
6.2
7.6
Curl (%)
15.3
14.5
( l ) Length-averaged values
Results and discussions
weight and thickness. The air permeability of the papers was determined with a Lorentzen & Wettre, L&W Air permeance tester according to ISO 5636-3. The mechanical properties (i.e. tensile index, stretch, work, and tensile stiffness) of the paper sheets were determined with a Lloyd tensile tester (AMETEK, USA), in accordance with ISO 5270:1998. The straining speed was 100 mm/min in the tensile mea- surements. Paper samples were conditioned and all testing of the samples took place at a temperature of 23 C and at 50% relative humidity (EN ISO 5269-3). Eight parallel measurements were performed. The paper shrinkage during restrained and unrestrained drying was measured and calculated according to the procedure described by Vishtal and Retulainen (2014a). Shrinkages of restrained and unrestrained dried sheets were measured by applying marker holes with known separations in the wet sheets as described by Khakalo et al. (2017b).
Mechanical testing of model films from the polysaccharides
Model films were prepared from the different polysac- charides with added sorbitol as softener (50%). Addition of sorbitol was required in order to success- fully prepare polysaccharide films, since these type of films otherwise become too brittle to handle, and crack during drying. It has previously been reported that addition of 50% glycerol as plasticizer was needed when model films were prepared from agar and j - carrageenan in order to avoid brittleness of the films (Rhim 2012). It has also been reported that the tensile strength and Young’s modulus of oat spelt arabinoxy- lan films was slightly higher with sorbitol than with glycerol at high dosages (Mikkonen et al. 2009). Therefore, sorbitol was the softener of choice in the current experimental series. The model films from the different polysaccharides were subjected to mechanical testing. It was immedi- ately evident that the different polysaccharides gave films with very different characteristics (Fig. 3). Alginate gave the strongest model films, with a
2D formability tests
The formability strain and force of the paper samples were measured using a 2D-formability tester at VTT. A detailed description of the measurement procedure and set-up is available in literature (Vishtal and Retulainen 2014a). In this study, the velocity of the forming press was 10 mm/s and the width of the paper sample was 20 mm. The paper samples were set in the tester so that the sprayed surface was not in contact with the heated press. The 2D formability tests were repeated seven times for each trial point.
Scanning electron microscopy
Scanning electron microscopy (SEM) was performed using a JEOL JSM-IT100 instrument. Images were produced in low-vacuum mode (40 Pa) using back- scattered electrons. The accelerating voltage was 10 kV.
Fig. 3 Examples of the stress–strain curves of polysaccharide model films. To all of the displayed examples, 50% sorbitol had been added as softener
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