Polymers 2021 , 13 , 2485
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Figure 2. Visualisation of the X-ray microtomograph (CT) image of a sample pressed at 190 ◦ C temperature (grayscale), edges of segmented regions (red) and surfaces of the sheet (blue). The surfaces of the paper sheets were defined using the Carpet method [20] which works by lowering a surface following quenched noise Edwards-Wilkinson dynamics to- wards the segmented paper sheet. The bright pixels corresponding to the paper eventually slow down and stop the evolution of the surface. The paper surface is then defined by the position where the motion of the dynamic surface stops. An example of the surfaces is shown in Figure 2. The total volumes of the sheet, pores, and fibres were determined by counting the number of pixels classified to each material phase. The pore size distribution was deter- mined using the local thickness algorithm [21]. Image analysis was performed using the freely available software pi2 (https://www.github.com/arttumiettinen/pi2, accessed on 28 July 2021), and 3D visualisations were created using MeVisLab (MeVis Medical Solutions AG, Bremen, Germany).
3. Results 3.1. Porosity of the Fibre Networks from X-ray Microtomography
Hot-pressing narrows the pore-size distribution of a sheet significantly as can be seen in Figure 3. This effect is strongest at very high temperatures. Still, the mean pore size in all cases is several micrometres and thus clearly higher than the resolution of X-ray imaging. Therefore, it is reasonable to assume that the measurement of total pore volume gives reliable results.
Figure 3. Pore-size distributions for the unpressed reference sheet and hot-pressed sheets with temperature of 190 ◦ C (cylinder press) and 270 ◦ C (steel belt press). The average porosity, 0.74, is quite high for the unpressed reference sheet. In this case, lumens are still partly open, and the above value for porosity is similar as in earlier
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