Bonding wood with uncondensed lignins as adhesives

Abstract

Plywood is widely used in construction, such as for flooring and interior walls, as well as in the manufacture of household items such as furniture and cabinets. Such items are made of wood veneers that are bonded together with adhesives such as urea-formaldehyde and phenol-formaldehyde resins^1,2. Researchers in academia and industry have long aimed to synthesize lignin-phenol-formaldehyde resin adhesives using biomass-derived lignin, a phenolic polymer that can be used to substitute the petroleum-derived phenol^3-6. However, lignin-phenol-formaldehyde resin adhesives are less attractive to plywood manufacturers than urea-formaldehyde and phenol-formaldehyde resins owing to their appearance and cost. Here we report a simple and practical strategy for preparing lignin-based wood adhesives from lignocellulosic biomass. Our strategy involves separation of uncondensed or slightly condensed lignins from biomass followed by direct application of a suspension of the lignin and water as an adhesive on wood veneers. Plywood products with superior performances could be prepared with such lignin adhesives at a wide range of hot-pressing temperatures, enabling the use of these adhesives as promising alternatives to traditional wood adhesives in different market segments. Mechanistic studies indicate that the adhesion mechanism of such lignin adhesives may involve softening of lignin by water, filling of vessels with softened lignin and crosslinking of lignins in adhesives with those in the cell wall.

Fig. 1. Adhesion performance of adhesives prepared from different lignins.

a, Schematic illustration of preparation of plywood from wood veneers with lignins as adhesives. b, Schematic illustration of three-layer FPL-bonded plywood specimens used for adhesion performance tests (top) and wood failure of the specimen after a wet strength test (bottom). c, Adhesion performance of adhesives prepared from lignins isolated with different methods (KPL, acetone-protected lignin; DESL, deep eutectic solvent-extracted lignin; KL, kraft lignin; DL, dioxane–HCl lignin). Lignin adhesive preparation: 1:2 (w/w) lignin/water, pH 7; hot-pressing conditions: 190 °C, 8 min, 1.5 MPa and a glue application level of 100 g m⁻². d, Effects of condensation degrees of FPLs on their adhesion performances. Lignin adhesive preparation: 1:2 (w/w) lignin/water, pH 7; hot-pressing conditions: 170 °C, 8 min, 1.5 MPa and a glue application level of 100 g m⁻². The dot-dashed line at 0.7 MPa in c,d marks the minimum industrial requirement for the adhesion strength. All sample preparation and testing procedures as well as data calculation are described in the Methods. All examined bonding strengths were found to be significantly (analysis of variance (ANOVA), P < 0.01) affected by the condensation degree (or hydrogenolysis monomer yields) of lignins (Extended Data Table 1). Error bars show the s.d. of measured bonding strengths with four repeats.

Fig. 2. Effects of hot-pressing conditions on adhesion performances of lignin adhesives.

a, Response surface for the wet strengths of three-layer plywoods as a function of the hot-pressing temperature and the glue application level. b, Effects of hot-pressing temperatures and times on the adhesion performance of FPL adhesives, without acid addition. Lignin adhesive preparation: 1:2 (w/w) lignin/water; glue application level: 100 g m⁻². c, The promoting effect of acid addition on the adhesion performances of FPL adhesives. Lignin adhesive preparation: 1:2:0.1 (w/w/w) lignin/water/H₂SO₄; glue application level: 100 g m⁻². The dot-dashed line at 0.7 MPa in b,c marks the minimum industrial requirement for the adhesion strength. All examined bonding strengths were found to be significantly (ANOVA, P < 0.01) affected by hot-pressing temperatures and time as well as glue application levels (Extended Data Table 1). Error bars show the s.d. of measured bonding strengths with four repeats.

Fig. 3. Mechanical properties of seven-layer plywoods prepared from lignin adhesives.

a, Schematic illustration of preparation (top) of seven-layer plywood specimens for mechanical performance tests (bottom). b, The MOEs and MORs of seven-layer plywoods bonded with FPL, urea–formaldehyde and phenol–formaldehyde resin adhesives. The dot-dashed lines at 5,500 MPa and 32 MPa mark the minimum industrial requirements for the MOE and MOR, respectively. Lignin adhesive preparation: 1:4 (w/w) lignin (or urea–formaldehyde or phenol–formaldehyde)/water 1:4, pH 2.1 (except as indicated); hot-pressing pressure: 2.0 MPa. All examined moduli were found to be significantly (ANOVA, P < 0.01) affected by hot-pressing temperature, time and pH but not the glue application level (Extended Data Table 1). Error bars show the s.d. of measured bonding strengths with four repeats.

Fig. 4. Adhesion mechanism of lignin adhesives for bonding wood.

a, Yields of resultant aromatic monomers from hydrogenolysis of FPLs before (25 °C) and after hot pressing. The H⁺ shown in a,d indicates the addition of sulfuric acid as a crosslinking catalyst to lignin adhesives. b,c, Side-chain (b) and aromatic (c) regions in heteronuclear single quantum coherence spectra of FPL before and after hot pressing at 190 °C (δ, chemical shift; the chemical structure in the left panel of c represents an acetal-protected lignin monomeric unit). d, Optical microscopy images of the glue lines in the plywood products prepared at different hot-pressing temperatures. e, Scanning electron microscopy (left) and Fourier transform infrared (FTIR) microscopy (right) images of the glue-line region in the plywood prepared at 190 °C (the FTIR image was recorded at 1,597 cm⁻¹, a characteristic absorption peak of lignin; Extended Data Fig. 8f). See Methods for details of sample preparation and characterization.

Extended Data Fig. 1. Hydrogenolysis of lignins to monomers.

a, GC spectra of lignin monomers from hydrogenolysis of lignins. b, Yields of monomers resultant from hydrogenolysis of lignin samples that were extracted with different methods. MWL: Milled wood lignin; FPL: formaldehyde-protected lignin; KPL: acetone-protected lignin; DESL: deep eutectic solvent-extracted lignin; KL: kraft lignin; DL: dioxane-HCl lignin. M1: 2-methoxy-4-propylphenol; M2: 2,6-dimethoxy-4-propylphenol; M3: 4-(3-hydroxypropyl)-2-methoxyphenol; M4: 4-(3-hydroxypropyl)-2,6-dimethoxyphenol; M5: methylated propylguaiacol (2-methoxy-5-methyl-4-propylphenol); M6: methylated propylsyringol (2,6-dimethoxy-3-methyl-4-propylphenol); M7: methylated propanolguaiacol [4-(3-hydroxypropyl)-2-methoxy-5-methylphenol]; M8: methylated propanolsyringol [4-(3-hydroxypropyl)-2,6-dimethoxy-3-methylphenol]; M9: 2,6-dimethoxy-4-(3-methoxypropyl)phenol; M10: 4-ethyl-2,6-dimethoxyphenol; M11: 2,6-dimethoxyphenol.

Extended Data Fig. 2. Mass spectra of lignin monomers from hydrogenolysis of lignins.

All monomer samples were silylated by N, O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) prior to gas chromatography-mass spectrometry (GC-MS) analysis. See ‘Lignin characterizations’ in the Method section for details.

Extended Data Fig. 3. Comparison of adhesion performance of FPLs with different condensation degrees.

a, Schematic illustration of the preparation process of three-layer plywood. Each layer was oriented with its grain perpendicular to that of the adjoining layer. b, The photograph of a bonding strength test. c, d, Effects of formaldehyde loadings (c) and extraction temperatures and times (d) on the condensation degree and adhesion performance of FPLs. Extraction temperature and time are the reaction temperature and time required for cooking biomass in a reactor to isolate lignin from biomass. At a specific formaldehyde loading, low extraction temperature or short extraction time facilitate to reduce the condensation of isolated FPLs and improve their adhesion performance. A higher temperature (140 vs. 80 °C) facilitated FPL extraction in shorter time (20 vs. 300 min), meanwhile maintaining the relatively low condensation degree. Although the wet strength of FPL-bonded plywood went down from 1.17 ± 0.05 MPa (mean ± standard deviation; 80 °C, 300 min) to 1.00 ± 0.06 MPa (mean ± standard deviation; 140 °C, 20 min), which was still higher than 0.7 MPa. In contrast, FPL that was extracted at 160 °C for 10 min had a monomer yield of only 1.9%, showed no adhesion. e, Yields of monomers resultant from hydrogenolysis of FPLs isolated under different conditions. The structures of silylated lignin monomers (M1–M9) were given in Extended Data Fig. 2. The hot-pressing conditions of lignin adhesives used to prepare plywoods in c and d were 190 °C (hot-pressing temperature), 8 min (hot-pressing time), 1.5 MPa (hot-pressing pressure), and 100 g glue·m⁻² (glue application level).

Extended Data Fig. 4. Comparisons of MWL and FPL isolation processes.

a, The procedures of isolating MWL and FPL. b, Comparisons of isolation conditions. The MWL isolation method required time-consuming milling of wood into fine power and repeated solvent extractions, making it is an extremely energy-intensive and high-cost process. The FPL isolation method used larger particles, much less solvents, and shorter separation time, and yielded more lignin.

Extended Data Fig. 5. Mechanical performance tests of seven-layer plywood.

a, The photograph of a whole piece of prepared seven-layer plywood. b, Photograph of elasticity and rupture modulus tests. c, Photographs of piled veneers or plywoods before (top row) and after (bottom row) 7.5-Kg or 80-Kg loading. The order of the deformation degree after the 80-Kg loading was UF resin > FPL ≈ PF resin, showing the excellent mechanical properties of FPL adhesive. d, The dimensions of seven-layer plywoods prepared at different hot-pressing conditions. The FPL used in this study was isolated from eucalyptus at 80 °C (extraction temperature), 5 h (extraction time), and 400 mg formaldehyde·g⁻¹ biomass. The mass ratio of lignin, UF or PF resin to deionized water in prepared adhesive suspensions was 1: 4 (w: w). All plywoods in d were prepared under the hot-pressing conditions of 110 °C (hot-pressing temperature), 20 min (hot-pressing time), 2.0 MPa (hot-pressing pressure), and 100 g·m⁻² (glue application level).

Extended Data Fig. 6. Characterization of FPLs before and after hot-pressing.

a, GPC spectra and molecular weights of hydrogenolysis products from FPLs before and after hot-pressing. b, c, C1s in XPS spectra (b) and the contents of carbon-based linkages (c). d, Hydorgenolysis monomer yields of FPL and hot-pressed FPLs. The hot-pressed lignins were prepared by hot-pressing FPLs wrapped by aluminum foil. Hot-pressing conditions: lignin/water = 1: 2 (w: w), 190 °C (hot-pressing temperature), 8 min (hot-pressing time), pH = 7, and 1.5 MPa (hot-pressing pressure). Increased C–C and decreased C–O contents were observed after hot-pressing FPLs at 190 °C, indicating that a high temperature facilitated the occurrence of condensation or the formation of additional C–C linkages in FPLs during hot-pressing. The structures of silylated lignin monomers (M1–M9) were given in Extended Data Fig. 2.

Extended Data Fig. 7. HSQC spectra of FPL before and after hot-pressing.

a, b, Side-chain (a) and aromatic (b) regions in HSQC spectra of FPL before and after hot-pressing. c, Contents of the vacant sites on the aromatic nuclei of FPL before and after hot-pressing determined by HSQC. The hot-pressed lignins were prepared by hot-pressing FPL that were wrapped by aluminum foil under different temperatures; other hot-pressing conditions: lignin/water = 1: 2 (w: w), 8 min, pH = 7, and 1.5 MPa.

Extended Data Fig. 8. Interfacial analysis of the glueline region and thermal analysis of FPL.

a, FTIR microscopy image of the glueline region recorded at a wavenumber of 1597 cm⁻¹. b, c, SEM image (b) and schematic illustration (c) of the glue line region. These images demonstrated that the vessels at the glueline region were filled with lignin adhesives after hot pressing. d, e, DSC (d) and TG results (e) of FPL. Due to the relatively low glass transition temperature of FPL (T_g = 100–144 °C), FPL adhesives could be softened at a low hot-pressing temperature as low as 100 °C. f, The characteristic linkages in biomass and their corresponding FTIR absorption wavenumbers.

Extended Data Fig. 9. Comparison of UF and PF resins as well as a variety of lignin adhesives in adhesion performance.

a, Comparison of adhesion performances of FPL adhesives and conventional PF and UF resin adhesives. Lignin adhesive preparation in a lignin (or UF or PF)/water = 1: 2 (w: w); hot-pressing conditions: 150 °C, 8 min, 1.5 MPa, and a glue application level of 100 g·m⁻². Compared to the three-layer plywoods bonded with PF and UF resin adhesives, FPL adhesive showed comparable adhesion strength. b, Lignin adhesives prepared from different feedstocks protected with formaldehyde as well as eucalyptus lignin protected with other aldehydes. Eu: eucalyptus; Ma: Masson pine; Cs: corn stover; FPL: formaldehyde-protected lignin; EPL: ethyl aldehyde-protected lignin; PPL: propionaldehyde-protected lignin; FUPL: furfural-protected lignin; MWL, milled wood lignin. The H₃PO₄ loading was on the basis of lignin. c, Lignin adhesives prepared via mixing FPL with different solvents. Lignin adhesive preparation in b and c: lignin/water (or solvent) = 1: 2 (w: w), pH = 7; hot-pressing conditions in b and c: 190 °C, 8 min, 1.5 MPa, and a glue application level of 100 g·m⁻².

Extended Data Fig. 10. Comparison of UF and PF resins as well as a variety of lignin adhesives in weather resistance and formaldehyde emission.

a, Weather resistance. The UF-based three-layer plywoods failed the boiling water test at the first cycle as expected, suggesting the poor weather-resistance of UF resin adhesive. As the test cycle increased, the bonding strength of PF-based three-layer plywoods decreased gradually from 1.33 ± 0.05 (mean ± standard deviation) to 1.10 ± 0.05 (mean ± standard deviation) MPa while that of FPL-based plywoods slightly increased from 1.53 ± 0.09 (mean ± standard deviation) to 1.63 ± 0.11 (mean ± standard deviation) MPa. Since the synthesis and curing of PF resins are generally performed under an aqueous alkaline condition, part of the PF resin may be dissolved in water during the water boiling test, thereby leading to the gradual decrease of the strength. Different from PF resins, FPL was water-insoluble and the curing of FPL could continuously proceed in a neutral or acidic environment (e.g., the boiling water), thereby leading to the gradually increased strengths. b, Formaldehyde emission amount of three-layer plywoods prepared with FPL adhesives and UF and PF adhesives. c, Comparison of different national standards for classifying the formaldehyde emission amounts from plywoods. The test report from a specialty agent showed that the formaldehyde emission amount from the plywood bonded with UF resin was 9.98 mg∙L⁻¹, which did even not meet the E2 grade of Chinese National Standard GB/T 18580-2017. The formaldehyde emission amounts of the plywoods bonded with PF resin and FPL adhesive were 0.64 and 0.2 mg∙L⁻¹, respectively, meeting the E1 grade (≤ 1.5 mg∙L⁻¹) and the E0 grade (≤ 0.5 mg∙L⁻¹) of European standard (EN 717-1). The comparison indicates that FPL adhesive is a highly environment-friendly bio-based adhesive and safer than the traditional formaldehyde-containing wood adhesives.