Polymers 2021 , 13 , 2485
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similar measurements [22]. During hot-pressing, density increases and porosity decreases significantly as lumina collapse and fibres soften (Figure 4). Moreover, a slight decrease in sheet porosity is also observed when temperature and pressure are increased, from 0.34 at 190 ◦ C and 6 MPa (cylinder press) to 0.32 at 270 ◦ C and 8 MPa (steel belt press), despite the much shorter pressing time in the latter case. Thus, plastic fibre deformations take place very rapidly at high temperatures when the polymer components of fibres soften.
( a ) ( c ) Figure4. 3D visualisations of the CT images of samples. ( a ) Unpressed reference, ( b ) pressed at 190 ◦ C, and ( c ) pressed at 270 ◦ C. ( b )
Table 1 shows the resulting sheet densities. The values obtained from 3D structural images are higher than those obtained with standard sheet density measurements mainly because of the surface roughness volume excluded when calculating the effective value from the CT data. Table 1. Effective density of the TMP sheets obtained from the X-ray microtomography (CT) com- pared with the standard measurement.
Sample
Effective Sheet Density (CT)
Sheet Density (ISO 534)
367kg/m 3 955kg/m 3 1000kg/m 3
313kg/m 3 694kg/m 3 734kg/m 3
Unpressed
Pressed 190 ◦ C Pressed 270 ◦ C
The effective fibre density ρ f can be measured from sheet grammage G , area of sample A , and total volume of fibres V f determined from the 3D images,
GA V f
(1)
ρ f =
Equation (1) gives the values 1440 kg/m 3 (unpressed reference), 1450 kg/m 3 (190 ◦ C, cylinder press) and 1460 kg/m 3 (270 ◦ C, steel belt press) for the density of the fibres. The wall density without lumen is about 1500 kg/m 3 for natural wood fibres [23]. The slightly lower values can be explained by a small total volume of pores whose size is below the imaging resolution. However, the main conclusion is that the hot-pressing does not induce any noticeable density change in the fibre walls, despite a large reduction in the network porosity and mean pore size. 3.2. Visual Observations on Pressing-Induced Changes in Fibres Figure 5 shows SEM images of the TMP paper sheets pressed at different temperatures. The unpressed sample (Figure 5a) has a porous structure, with fibres having their charac- teristic oval shape. For the sheets pressed at higher temperatures, 190 ◦ C (Figure 5b), and 270 ◦ C (Figure 5c), fibres consolidate into ribbon-like structures with an almost perfectly closed lumen. The sample pressed at 190 ◦ C (cylinder press) is treated for a much longer time, 1.5 s in the nip and 70 s after hold, compared to the sample at 270 ◦ C (steel belt press),
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