E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
2.3. Thermopressing
Pressing of the substrates was done with a laboratory press (Fontijne, Netherlands) at 175 °C. The substrates with low moisture content (Reference 10, PL 10, SL 10, and SLNP 10) were pressed at 50 MPa for 10 min. The substrates that were pressed at the elevated moisture content (Reference 50, PL 50, SL 50, and SLNP 50) were pressed in two steps, i.e., first at 5 MPa for 5 min sandwiched between blotting papers and then for 5 min at 50 MPa sandwiched between steel plates. The pressing parameters were detailed in Table 2 . Two substrate specimens were made and analyzed for each pressing condition.
2.4. Characterizations
All substrates were stored in a room at 23 °C and relative humidity of 50 % for at least 24 h before characterization. The only exception to this was estimating the lignin content.
2.4.1. Lignin content Mass balancing was done to estimate the lignin content of the substrates following Eq. (1) :
− 𝑚
− 𝑚
𝑚
dry
r efer ence
f locculant
Lignin percentage =
× 100%
(1)
𝑚
dry
where m dry is the weight of the substrates after being pressed at 175 °C, m reference is the dry mass of the substrates without lignin, and m flocculant is the weight of flocculant. We assumed that the flocculants were fully retained on the fibers, so it was calculated from the theoretical mass of the added flocculant. It should be noted that the addition of flocculant might increase the retention of fines, which would give an overestimation of the lignin percentage.
2.4.2. Apparent density The apparent density ( d ) of the substrates was measured following Eq. (2) :
𝑚 Thickness × Area
𝑑 =
(2)
where m stands for the mass of the sample measured at 23 °C and relative humidity of 50 % (g), and the area and thickness were measured in cm 2 and μm, respectively.
2.4.3. Tensile testing The dry and wet mechanical properties of the substrates were measured with a Zwick material tester in tensile mode. The distance between the clamps was set to 40 mm, and wide strips of 15 mm were cut from the substrates. The elongation speed was set at 100 mm/min, and the load cell could measure up to a maximum of 2.5 kN. Three strips from each substrate were measured in the dry and wet state, respectively, and the results present the average of the two duplicate substrates, i.e., the average of six measurements. The dry measurement corresponded to the samples conditioned at 23 °C and RH of 50 %. For the wet measurements, the strips were
Table 2 Pressing conditions of substrates.
Sample alias
Pressure (MPa)
Approximate moisture content before pressing (%)
Reference 10 Reference 50
50
10 50 10 50 10 50 10 50
5 + 50
PL 10 PL 50 SL 10 SL 50
50
5 + 50
50
5 + 50
SLNP 10 SLNP 50
50
5 + 50
Notes: Reference 10, cellulose substrate without additives with a moisture content of 10 % before pressing; Ref- erence 50, cellulose substrate without additives with a moisture content of 50 % before pressing; PL 10, cellulose substrate with added lignin particles and cationic polymer retention aid (PCB 20) with a moisture content of 10 % before pressing; PL 50, cellulose substrate with added lignin particles and cationic polymer retention aid (PCB 20) with a moisture content of 50 % before pressing; SL 10, cellulose substrate with added lignin particles and cationic starch retention aid with a moisture content of 10 % before pressing; SL 50, cellulose substrate with added lignin particles and cationic starch retention aid with a moisture content of 50 % before pressing; SLNP 10, cellulose substrate with added lignin nanoparticles and cationic starch retention aid with a moisture content of 10 % before pressing; SLNP 50, cellulose substrate with added lignin nanoparticles and cationic starch retention aid with a moisture content of 50 % before pressing.
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