Bonding wood with uncondensed lignins as adhesives

The study addresses the challenge of developing cost-effective, high-performance, and green wood adhesives by directly using lignin from biomass. Traditional wood adhesives (urea-formaldehyde and phenol-formaldehyde resins) dominate due to cost and performance. Attempts to incorporate isolated lignins into phenol-formaldehyde resins face issues: high molecular weight and extensive condensation of commercial lignins limit reactive sites, yielding highly viscous, dark adhesives requiring severe curing, which are unattractive to industry. The authors hypothesize that lignins with no or limited condensation can be directly used as wood adhesives and undergo self-crosslinking during hot pressing, potentially offering an industrially viable alternative that promotes sustainable adhesives and profitable biorefining.

Prior work on lignin-based adhesives includes lignin-phenol-formaldehyde and lignin-glyoxal systems aiming to reduce formaldehyde and replace petroleum-derived phenol. However, commercial lignins (kraft, soda, biorefinery) often undergo condensation during extraction, reducing reactive sites and increasing molecular weight, negatively impacting adhesive performance and processing (viscosity, color, curing). Reviews highlight lignin’s role and the challenge of condensation in biomass processing. Protection strategies during extraction (e.g., aldehyde or ketone protection) and the use of deep eutectic solvents have been explored to stabilize lignin and improve monomer yields upon depolymerization. Despite advances, cost and performance disadvantages versus UF and PF resins remain barriers to adoption.

• Lignin sourcing and isolation: Evaluated lignins separated from eucalyptus wood particles using multiple methods yielding different condensation degrees: milled wood lignin (MWL), formaldehyde-protected lignin (FPL), acetone-protected lignin (KPL), deep eutectic solvent-extracted lignin (DESL), kraft lignin (KL), and dioxane-HCl lignin (DL). Condensation degree was inferred inversely from aromatic monomer yields after hydrogenolysis (higher yield = less condensed).
• Adhesive preparation: Simple suspensions by mixing isolated lignin with deionized water. Typical ratio 1:2 (w/w) lignin:water at pH 7 for three-layer plywood tests. For catalyst-enhanced curing, 1:2:0.1 (w/w/w) lignin:water:H2SO4.
• Plywood fabrication (three-layer): Veneers bonded using lignin suspensions at glue application level 100 g m−2. Hot-pressing conditions varied: temperatures 100–190 °C, pressures 1.5 MPa (three-layer) and 2.0 MPa (seven-layer), and pressing times 2–20 min. Systematic variation of temperature, time, glue application level, and acid addition to optimize performance.
• FPL condensation control: Produced FPLs at varying formaldehyde loadings (0–500 mg g−1 biomass) and extraction temperatures/times to modulate condensation; assessed impact on bonding.
• Multilayer plywood tests: Seven-layer plywood prepared to assess mechanical properties (MOE, MOR) under various curing temperatures (100–170 °C), times (10–20 min), glue application levels (100 or 200 g m−2), and pH (2.1 vs 4.3). Comparative controls with urea-formaldehyde (UF) and phenol-formaldehyde (PF) adhesives prepared at 1:4 (w/w) adhesive:water.
• Performance evaluation: Measured dry and wet adhesion strengths for three-layer plywood (GB/T 9846-2015 minimum requirement 0.7 MPa). For seven-layer, measured MOE and MOR versus national standards (MOE ≥ 5,500 MPa; MOR ≥ 32 MPa). Conducted weather-resistance tests and formaldehyde emission assessments.
• Mechanistic characterization: Hydrogenolysis to quantify monomer yields pre- and post-hot-pressing; gel permeation indicators (molecular weights); HSQC NMR to observe side-chain/aromatic signal changes; optical microscopy and FTIR microscopy to visualize glue lines and lignin distribution; SEM imaging of glue-line regions; assessment of flowability and vessel filling at different temperatures.

• Adhesion performance depends strongly and inversely on lignin condensation degree. Slightly condensed or protected lignins (MWL, FPL, acetone-protected) achieved dry and wet strengths meeting or exceeding 0.7 MPa under 190 °C, 1.5 MPa, 8 min, with wood failure observed. Severely condensed lignins (KL, DL) showed negligible adhesion.
• Hydrogenolysis monomer yields correlated with performance: MWL 50.6% and FPL 44.5% (good adhesion) versus KL 5.8% and DL 2.9% (failed adhesion).
• Tuning FPL condensation via formaldehyde loading (0–400 mg g−1 biomass) increased monomer yields (19.0% → 44.9%) and improved bonding at 170 °C: wet 0.32 ± 0.10 → 0.99 ± 0.18 MPa; dry 0.69 ± 0.10 → 1.28 ± 0.17 MPa. Increasing to 500 mg g−1 had negligible additional benefit.
• Higher hot-pressing temperatures (e.g., 190 °C) enabled more condensed FPLs to meet the 0.7 MPa requirement; longer press times can compensate for lower temperatures (e.g., at 170 °C, wet strength increased from 0.46 ± 0.05 MPa at 2 min to 1.00 ± 0.12 MPa at 15 min).
• Acid catalysis (H2SO4) substantially reduced curing severity, enabling wet strengths ≥ 0.7 MPa at 100 °C and 2 min, lowering energy and time requirements.
• Seven-layer plywoods bonded with FPL met standards: with 200 g m−2 glue, all prepared at 110–170 °C for 20 min achieved MOE ≥ 5,500 MPa and MOR ≥ 32 MPa. With 100 g m−2, MOE 6,716 ± 907 MPa and MOR 32 ± 4 MPa achieved at 110 °C for 20 min. Performance comparable to UF and PF under the same curing conditions.
• Lower pH (2.1) improved stiffness versus pH 4.3: at 130 °C, 20 min, MOE decreased from 8,344 ± 382 MPa (pH 2.1) to 5,736 ± 629 MPa (pH 4.3), indicating acid-facilitated crosslinking enhances adhesion.
• Mechanistic evidence: Post-press FPLs showed decreased hydrogenolysis monomer yields, increased molecular weights, increased C–C and decreased C–O contents, and diminished HSQC signals (G5, G6), supporting self-crosslinking during curing.
• Microscopy indicated temperature-dependent vessel filling; even limited mechanical interlocking at 100 °C still yielded good adhesion, suggesting interfacial crosslinking with cell wall lignins.
• Broader applicability: FPL adhesives prepared using different solvents (dioxane, acetone) and from different feedstocks (Masson pine, corn stover) as well as lignins protected with other aldehydes (acetaldehyde, propionaldehyde, furfural) achieved qualified adhesion (>0.7 MPa). Plywood showed weather resistance and low formaldehyde emissions suitable for interior use.

The results validate the hypothesis that uncondensed or slightly condensed lignins can function as effective wood adhesives without further chemical modification. Adhesion strength scales inversely with condensation degree, indicating that preserving native-like lignin structures (via protective extraction with aldehydes/ketones) enables self-crosslinking under hot pressing. Acid-catalyzed curing accelerates crosslinking, allowing significant reductions in temperature and time, improving energy efficiency and productivity for manufacturers. Mechanistic analyses (hydrogenolysis yields, HSQC NMR, microscopy) support a model where water softens lignin, facilitating flow and vessel filling, followed by crosslinking both within the adhesive and potentially at the interface with cell wall lignins. The approach delivers plywood mechanical properties comparable to UF and PF adhesives while offering advantages in sustainability and potentially lower formaldehyde emissions, highlighting relevance for various wood products and market segments.

This work introduces a practical, scalable strategy to use uncondensed or slightly condensed lignins—particularly formaldehyde-protected lignins—as standalone wood adhesives. By controlling lignin condensation during extraction and optimizing pressing conditions (with optional acid catalysis), the adhesives achieve industrially required wet and dry strengths and deliver seven-layer plywood MOE/MOR comparable to UF and PF adhesives. The adhesives exhibit weather resistance and low formaldehyde emissions, indicating suitability for interior and exterior applications. Future work should elucidate interfacial bonding and penetration mechanisms at the cell wall level and extend the approach to other pressed wood products (particleboards, fibreboards) and continuous manufacturing processes, while refining extraction and curing parameters for cost and performance optimization.

• MWL, though performant, is not industrially practical due to energy-intensive milling, long separation time, and low yield.
• Effective performance requires low condensation degree; maintaining this may necessitate protective extraction (e.g., with aldehydes) and process control.
• Multilayer products face heat transfer limitations, necessitating longer press times unless acid catalysis is used.
• The proposed adhesion mechanism involving interfacial crosslinking and penetration into cell walls requires further experimental validation.
• Acidic curing improves processability but introduces pH control and potential corrosion/handling considerations for industrial implementation.