E. Pasquier and J. Ruwoldt
Journal of Bioresources and Bioproducts 10 (2025) 325–335
Kraft lignin is not soluble in water at neutral pH, therefore, it presents as particle in suspension. Lignin powder, when dispersed in water, usually has sizes in the micron range depending on the dispersion method used ( Schneider et al., 2021 ). By reducing the size of the lignin particles, more homogeneous dispersions can be achieved ( Farooq et al., 2019 ), which leads to decreased disruption of the fiber network. The impact of lignin particle size on the properties of cellulose substrates has been overlooked so far. In this study, the goal was to further expand our work on added-lignin thermoformed pulps, elucidating various parameters such as the retention aid (flocculant type), lignin particle size, and moisture content during pressing. The research hypothesis was that optimizing these parameters could produce molded pulp materials with superior resistance to wetting. This work was hence intended to provide the fundament for further developing plastics replacements from thermoformed cellulose fibers with added lignin. Sheets were made with lignin as an internal sizing agent to strengthen the thermoformed product, especially in the wet state. Lignin was added in two different forms (powder and nanoparticles), and two different flocculants were used to improve lignin retention. A commercial flocculant was used as well as cationic starch to study their synergies with lignin. The mechanical properties of the sheets were measured in the dry and wet state to study the effect of lignin as a wet-strength agent. The moisture content during pressing was also used to tune the mechanical strength and plasticize the lignin during pressing.
2. Materials and methods
2.1. Materials
Northern bleached softwood Kraft pulp was provided by MoRe Research AB (Sweden). Pulp characterization was done in one of our previous papers ( Pasquier et al., 2023 ). Softwood kraft lignin (BioPiva 395) was purchased from UPM Biochemicals, Finland. The thermo-gravimetric analysis-differential scanning calorimetry (TGA-DSC) profile of this lignin has been published in previous works ( Heen Blindheim et al., 2024 ). Cationic starch (Perlbond 930) was acquired by Lyckeby Stärkelsen AB (Sweden). The cationic flocculant (PCB 20) was provided by Solenis, Norway. Acetone ( > 99.5 %) was purchased from Sigma-Aldrich/Merck. Deionized water was used throughout this study, if not stated otherwise.
2.2. Sample preparation
2.2.1. Lignin particle suspensions The solvent shifting method was used to produce lignin nanoparticles (LNP). For that purpose, 1.2 g of kraft lignin was dissolved in a mixture of acetone and water (9 ꞉1, V / V ). After 2 h of mixing, 180 mL of water was added to the solution. The suspension was mixed and heated to evaporate the acetone. The concentration of lignin particles in water before addition to the pulp was ∼ 10 g /L. Alternatively, lignin powder as received was dispersed in water at the concentration of 10 g /L and homogenized with a high shear disperser (Ultra-Turrax from IKA-Werke GmbH & CO. KG, Germany) with 19 mm head at 15 000 r/min for 1 min. Lignin particles and nanoparticles were characterized by dynamic light scattering (DLS) with a Zetasizer Ultra (Malvern, UK). The lignin nanoparticles had a size of 102 nm with a polydispersity index of 0.14 and a zeta-potential of (–26 ± 1) mV. The measurement of the lignin particle suspension was not valid, as the particles were too large and simply sedimented during the analysis. 2.2.2. Wet forming of the substrates The pulp was dispersed with a Lorentzen & Wettre pulp disintegrator from ABB (Sweden) at revolution of 30 000 and consistency of 15 g /L. Solutions of cationic flocculants were also prepared to improve the retention of lignin during wet forming. Cationic starch was dissolved at 1 g /L in water under heating to help dissolution. The cationic flocculant (PCB 20) was dissolved in water at 1 g /L without heating. Due to the lower charge density of the starch, a larger amount was necessary compared to PCB 20 ( Table 1 ). Four different compositions were prepared, the cellulose substrate without additives (as reference), samples containing lignin powder and PCB 20 (PL) or starch (SL), and a sample with lignin nanoparticles and starch (SLNP). Substrates were prepared following Table 1 , where the pulp concentration before filtering was always 5 g /L. Substrates with a grammage of 500 g /m 2 were prepared following our previous paper ( Pasquier et al., 2023 ). In summary, the pulp was diluted and then mixed with the flocculant, followed by adding the lignin. The suspension was filtered onto a metal mesh, and a vacuum was used to further dewater the substrate. The substrate was removed from the mesh and pressed between blotting papers at 2 MPa for 5 min. The substrates pressed at low moisture content were then air dried at 23 °C and relative humidity (RH) of 50 % until constant weight. The substrates pressed at elevated moisture content were weighted periodically until ∼ 50 % water content was reached and then stored in a plastic bag until pressing.
Table 1 Composition of substrates.
Sample
Pulp (g)
Flocculant
Lignin
Reference
6 6 6 6
–
–
PL SL
1.4 mg PCB 20 (1.4 mL) 60 mg starch (60 mL) 120 mg starch (120 mL)
LNP 1.2 g LNP 1.2 g LNP 1.2 g
SLNP
Notes: PL, cationic flocculant (PCB 20) + lignin powder; SL, cationic starch + lignin powder; SLNP, cationic starch + lignin nanoparticles; LNP, lignin nanoparticles.
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