E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
Fig. 6. (a) Ultimate tensile strength; (b) Elastic modulus of the samples measured after 24 h in water.
Fig. 7. (a) Swelling in thickness of substrate after 24 h in water; (b) Wet strength retention calculated by ratio of wet and dry ultimate strength.
addition of lignin or with the high moisture pressing, while it was multiplied by > 10 when both were applied. As for the wet strength, the wet-strength retention was significantly improved in the presence of lignin and with the pressing with high moisture ( Fig. 7 b). Apart from the elevated tensile strength and modulus of SLNP 10, no significant effect was observed from varying the lignin particle size or the type of flocculant added. This would also entail that the proposed synergies between lignin and starch did not materialize, the latter one acting as a plasticizer for lignin. This could be due to either the plasticization effect of the starch not improving the mechanical strength, or the concentration of starch being too low to induce an effect. Fig. 7 a shows the swelling of the substrates after 24 h in water. Both lignin and elevated moisture during pressing limited the swelling. At high temperatures, lignin was softened, which allowed the deformation of the network that could stiffen again when cold. Moreover, self-condensation of lignin could potentially occur during hot pressing, which might improve the stability of the network. This effect was enhanced by the presence of moisture during pressing ( Zhao et al., 2020 ). Swelling of the substrate without lignin was also reduced when pressed at high humidity. This could be due to the lower stiffness and higher flexibility of the fibers at high moisture, which potentially led to better packing of the fibers during pressing. Higher wet strength was correlated with lower substrate swelling partially due to the reduction in thickness, but also be- cause the mechanisms responsible for the decreased swelling potentially induced a strengthening of the substrates. Indeed, the hardening of lignin can create a network around the fibers ( Fig. 5 ) which can prevent the substrate from swelling. In addition, Zhao et al. (2020) showed that while pressing at high moisture content and high temperature, degradation of lignin and condensa-
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