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Cellulose (2020) 27:6961–6976
Fig. 3 SEM images of a V-fines, b CMF, and c TCNF using equal magnification to illustrate the size difference between the components. The measure bar shows the length of 1 l m
values of the materials. Photographs of foam-formed fibre materials are shown in Fig. 5. In thin foam-formed sheets obtained by removing the foam by vacuum, fibres usually orient themselves mainly to the planar direction, in parallel to the foam flow (Ja¨rvinen et al. 2018). It is not obvious that similar preferred orientation could be found inside thick fibre networks drained by gravity. In addition to foam flow and drainage, the orientation may also depend on the air content and bubble size and employed raw materials together with boundary effects caused by the mould walls. Tomography images were produced to capture possible preferred fibre orientations in the formed materials.
Fig. 4 Estimated size scale of the fibre and fine components employed based on microscopy
reached the target of 50–60%. The fibre foam was poured along a plate into a deep mould with a wire bottom, drained by gravity for 15 min, and removed from the mould on the wire. The wet material (dry matter content around 12%) was inserted into a laboratory oven and dried at 70 C. Drying to a remaining moisture content below 10% was complete within 24 h. In a special trial point (TP11) made with LBG, the drainage was extremely slow. Therefore, the samples were dried in the oven at a higher temperature (100 C). Table 1 shows the most essential foaming parameters for the trial points. The final material density level was adjusted by pressing the wet foam after the drainage of the free water and/or after the drying of the material. The once- dried materials were rewetted with water to the dry matter content of 50% and pressed between two plates with spacers to the desired final thickness and dried again in an oven (70 C). Both panel surfaces were densified due to the dewatering in one direction and the pressing procedures. The bulk density values presented in the Tables 2 and 3 are the average density
Material testing methods
Basis weight was measured by weighing the material at 50% RH and 23 C and by measuring the material dimensions. Thickness was measured with a laser beam. Compression stress was measured with univer- sal material tester (Lloyd) at 10% deformation, according to EN 826 (2013). The compression test was continued to 50% deformation level after 1 min of relaxation. Compression modulus was defined from the slope of the stress–strain curve in the elastic (linear) deformation region. Specific compression stress was defined by dividing the compression stress with density. Materials were imaged with a tomo- graphic scanner (RX Solutions, France). Tube voltage 40 kV and current 300 l A were used. The voxel size was 25 l m.
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