E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
placed in water at 23 °C for 24 h before the measurement. The wet thickness was measured right before the tensile test and taken into account in the calculation of the ultimate wet strength. The wet strength retention ratio was calculated following Eq. (3) :
Wet t ensile st rength
Wet str ength r atio =
× 100%
(3)
Dry t ensile st rength
The swelling of the substrates was calculated following Eq. (4) :
( 𝑡
− 𝑡
)
wet
dry
Swelling =
× 100%
(4)
𝑡
dry
where 𝑡
wet represents the thickness after 24 h in water and 𝑡
dry is the thickness of the samples conditioned in air at 23 °C and RH of
50 %.
2.4.4. Surface imaging The surface of the substrates was imaged with a SU3500 (Hitachi, Japan) scanning electron microscope (SEM). The samples were cut and stuck to a sample holder, and then coated with gold by sputtering before imaging.
3. Results and discussion
3.1. Morphology
Sheets were made from bleached kraft pulp with lignin as the internal sizing agent. A reference sample was made without lignin as well as three compositions with lignin: cationic starch and lignin powder (SL), PCB 20 flocculant and lignin powder (PL), and cationic starch and lignin nanoparticles (SLNP). All mentioned compositions were pressed at low moisture content (around 10 % ( w / w ) water) and at elevated moisture content, which had around 50 % ( w / w ) water content before pressing. In the case of lignin nanoparticles, the filtrate during sheet formation was colored, which indicated that part of the lignin nanopar- ticles was not retained by the filter due to their small sizes. This is also shown during the mass balancing to determine the added lignin content ( Table 3 ). A pressure of 50 MPa and a temperature of 175 °C were used for all the samples during pressing. However, preliminary testing showed that pressing at such high pressure and temperature with elevated moisture content led to delamination of the substrates, as water vapor was trapped inside the substrate during pressing, which expanded upon pressure release (Fig. S1). Melting of the lignin and water vapor pressure release were also visible on the SEM images of the surface of the substrates (Fig. S2). Hence, the pressing conditions were adapted to the elevated moisture level and a second pressing step was introduced. The first step consisted of a low pressure (5 MPa) pressing with the presence of blotting paper to absorb the residual water, while the second step was similar to the dry substrates at high pressure (50 MPa) and without blotting paper. Images of the substrates after pressing were presented in Fig. 1 . Samples containing lignin had a brown color and there was a noticeable difference between the homogeneity of the samples containing lignin powder and the LNP. The nanoparticles had an average dynamic diameter of 102 nm and a zeta potential of (–26 ± 1) mV, which means they form stable suspensions without sedimentation. On the contrary, the lignin powder was not stable in water and sedimented. The powder consisted of micrometer- sized particles and was hence not measurable with the Zetasizer. The size difference resulted in better dispersion of the LNP in the pulp. However, even with the use of flocculant, it also led to the loss of LNP during the sheet formation, as shown by the lower lignin content in Table 3 . The visual appearance of the filtrate was independent of the type of flocculant used. Fig. 2 shows the density of the substrates after pressing. The substrates containing lignin had higher densities compared to the references. Thermoformed substrates were still porous even after high pressure had been applied. Here, the lignin could act as a filler and decrease the porosity of the substrates. The sample SLNP 50 (cellulose fibers with added lignin nanoparticles and cationic starch retention aid that were pressed at 50 % initial moisture) had a lower density, which was in agreement with the lower lignin content. Similar trends were shown with the addition of organosolv lignin to CTMP thermopressed substrates ( Tanase-Opedal and Ruwoldt, 2022 ). Other than that, there appears to be little effect of the lignin particle size or the type of flocculant used on density. Fig. 3 shows images of the microstructure of the surface. The fibers were well visible on the surface and there was also a clear difference between the samples with and without lignin. For the samples containing lignin, the surface was partially covered with
Table 3 Lignin content of the substrates with standard deviations.
Sample
Added lignin content (%, w / w )
Reference 10
–
PL 10 SL 10
17 ± 1
20.1 ± 0.2
SLNP 10
12 ± 3
Reference 50
–
PL 50 SL 50
15 ± 3 17 ± 3
SLNP 50
7 ± 1
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