Polymers 2021 , 13 , 2485
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treated 40 ms in the nip and 23 sec in after hold. This difference appears as a more closed surface for the 190 ◦ C sample, despite its slightly higher overall porosity (see Section 3.1).
Figure5. SEM images of the structures for three different pressing temperatures for the material, ( a ) unpressed reference, ( b ) pressed at 190 ◦ C, and ( c ) pressed at 270 ◦ C. SEM images of the cross-sections polished with an argon ion miller for three different pressing temperatures for the material, ( d ) unpressed reference, ( e ) pressed at 190 ◦ C, and ( f ) pressed at 270 ◦ C. SEM images of the freeze-dried cross-sections of the fibre wall for three different pressing temperatures for the material, ( g ) unpressed reference, ( h ) pressed at 190 ◦ C, and ( i ) pressed at 270 ◦ C. The working distance for the samples was in the range from 5 to 7 mm.
The porosity differences in different samples are best visible in SEM cross-sections of these structures, obtained after polishing the samples with an argon ion milling machine. In addition to inter-fibre pores, also fibre lumens stay partly open for the unpressed sheets (Figure 5d). On the other hand, the highest 270 ◦ C temperature causes an almost complete disappearance of lumen space due to thermal softening (Figure 5f), whereas most of the collapsed lumens are still visible at the lower 190 ◦ C temperature (Figure 5e). In order to look closer at the nano-/microstructure inside the fibre wall, cross-sections were prepared also by freeze-drying the material prior to breaking the sheets. However, in these cross-sections (Figure 5g–i), it is not possible to observe any substantial differences in the porous structure when comparing the unpressed sample and the ones pressed at high temperatures. This suggests that lignin and other matrix polymers are not extracted from the fibre wall to the same extent as for some chemical treatments of wood fibres [24],
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