E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
Fig. 5. Structure of substrates containing lignin powder and lignin nanoparticles before and after hot pressing at different humidity levels (Densi- fication of the fiber network occurred with high humidity pressing. Lignin particles size affected the homogeneity of the lignin in the fiber network (created with BioRender.com).).
and decreasing the surface wettability when applied as a coating ( Tanase-Opedal and Ruwoldt, 2022 ). Here, no synergies were found between starch and lignin when pressed at low moisture content. However, when pressed at elevated moisture content, SL 50 had a higher strength compared to PL 50, suggesting that there were synergies between the starch and lignin particles. This makes sense, as starch can function as a plasticizer to lignin, hence improving its material properties. It appears that the moisture played a vital role in enabling these interactions. 3.2.2. Effect of the moisture content during pressing The moisture content during pressing had a significant effect on the tensile strength. Considering the reference, the strength increased by 154 % after pressing with high moisture. The increase in tensile strength for the samples containing lignin was less, but it was still significant at 77 %, 121 %, and 39 % for the PL, SL, and SLNP, respectively. Samples with a higher strength after pressing at low moisture content of course showed a lower increase after pressing at elevated moisture content, as the initial strength was already high. The increase of strength is hence not only due to the presence of technical lignin, as the reference kraft substrate had the highest strength increase. This is in accordance with Wang et al. (2019) , they measured a linear increase in tensile strength relative to the mat humidity during the pressing of CTMP fibers. Moreover, Sanchez-Salvador et al. (2024) studied the effect of moisture during hot-pressing and noticed an increase in ultimate strength while pressing moist CTMP handsheets in the presence and absence of additional lignin. The increase in strength could be partially due to an increase in density, but this was not the case for all the samples. The water could plasticize the fibers at high humidity, which made them more flexible and could improve their packing. In addition, the fibers were swollen in water, which provided a greater surface area for adhesion. In the presence of lignin, the moisture content also impacts the glass-transition temperature ( T g ) of lignin, as water acts as a plasticizer ( Bouajila et al., 2006 ). The energy needed to melt the lignin is hence lower, and the lignin is more easily melted to flow around the fibers ( Fig. 5 ). Tuning the moisture during pressing could be used to tune the strength while keeping the pressure and temperature constant during thermopressing. Alternatively, it could be used to decrease the temperature or pressure while keeping the same strength. 3.2.3. Effect of lignin particles size The size of the lignin particles had an impact when the substrates were pressed at low humidity. The SLNP 10 had a strength of (56 ± 5) MPa, which was 50 % higher than that of SL 10. Higher dispersion of the lignin particles in the network and better homogeneity could explain the results ( Fig. 1 ). The nano size of the particles could also allow better filling of the pores and less disruption of the fiber network ( Fig. 5 ). However, when pressed at high moisture content, the difference in strength was reduced. The complete melting of the lignin, no matter its size or dispersion, in these conditions could explain the smaller strength difference.
3.3. Wet strength
The tensile strength of the samples was measured in wet conditions after immersion in water for 24 h (Fig. S4). Fig. 6 shows the results of the ultimate wet strength and the elastic modulus for the different samples. Similar trends for the dry strength were observed, while the differences between the samples became more pronounced. It can be noted that both added lignin and moisture during pressing had a positive effect on the wet strength and there are synergies between these, as the wet strength doubled with the
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