E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
Fig. 3. Scanning electron microscope images of the surface of the substrates after pressing (The scale bar in the top left is the same for all pictures).
Fig. 4. Ultimate dry tensile strength measured at standard conditions.
lignin that appeared to be melted and spread on the surface. Lignin softening occurs at temperatures around 160 °C and is dependent on the water content ( Back and Salmén, 1982 ; Bouajila et al., 2006 ). The pressing temperature used here was above the softening temperature of lignin, which explains the spreading of lignin on the surface. All the samples showed a partial coverage of the lignin on the surface, which was due to the distribution of the particles in the substrates. There appeared to be a slight difference, however, with the LNPs providing a more homogeneous coverage. This could be expected, considering the different sizes of the LNPs compared to the macroscopic kraft lignin particles.
3.2. Dry strength
The strength of the substrates was first measured at standard conditions (50 % RH and 23 °C). The size of the lignin particles, the flocculant, and the moisture content during pressing were varied, and their effect on the tensile strength was studied. Fig. 4 shows the ultimate strength of the samples and Fig. S3 shows the tensile stress-strain curves. It could be noted that the addition of lignin had a positive effect on all samples compared to the references. 3.2.1. Effect of flocculant To improve the retention of lignin particles, flocculants were used during the wet forming process. Cationic starch and a commercial flocculant for pulping wastewater were used. The addition of the flocculant was done prior to the addition of lignin to maximize its adsorption on the fibers ( Maximova et al., 2001 ). The PL 10 and SL 10 had similar strength of (39 ± 4) and (38 ± 5) MPa, respectively. The type of flocculant did not seem to have an impact on the mechanical strength of the samples when pressed at low moisture. The combination of starch and lignin has previously shown synergies by, for example, increasing the density of thermopressed products
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