The research on hardwood lignin is quite limited for lignin-based adhesives due to its heterogeneity and chemical complexity compared with those of softwood and non-wood lignin. Herein, a novel ...approach of producing entirely bio-based and nontoxic wood adhesives by self-crosslinking of industrial hardwood kraft lignin (HKL) is presented to replace formaldehyde-based resin adhesives. The key issues of converting HKL to bio-wood adhesive are their low reactivity for crosslinking capacity and high heterogeneity. For this, HKL extracted from industrial black liquor was acetone fractionated to reduce its heterogeneity, resulting in acetone-soluble HKL (AS-HKL) and acetone-insoluble HKL (AI-HKL), which were then glyoxylated to facilitate crosslinking and form a three-dimensional network structure. Lignin samples were analyzed by gel permeation chromatography (GPC), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), 31P nuclear magnetic resonance (31P NMR), solid state 13C cross-polarization/magic angle spinning (13C CP/MAS) NMR, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The adhesive strength of lignin-based adhesives was evaluated by determining the tensile shear strength of plywood. In this current research the chemical structure changes especially the crosslinked structure and adhesion performance were investigated. Results showed that the self-crosslinking was successfully formed by ether bonds formation between glyoxalated lignin. Also, the self-crosslinked acetone insoluble hardwood kraft lignin fractionation (AI-HKL) showed the best adhesion performance. FTIR, XPS and 13C CP/MAS NMR spectra revealed new peaks and ether bonds in the glyoxylated lignin. Therefore, these results indicate a successful self-crosslinking of the hardwood kraft lignin after acetone fractionation and glyoxalation for adhesives via network formation. The glyoxalated AI-HKL (AI-HKL–GLY) exhibited the highest viscosity and provided the best plywood adhesion strength (0.8MPa) among the adhesives, and showed a good potential as wood adhesives. Thus, the results suggest that the self-crosslinking of hardwood kraft lignin after a proper fractionation and glyoxalation is a promising approach of developing lignin-based bio-adhesives for wood bonding.
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•Totally green adhesives were developed by self-crosslinking of fractionated and glyoxalated hardwood kraft lignin.•Self-crosslinking was successfully formed by ether bonds between the glyoxalated lignin.•The self-crosslinked lignin showed better thermal stability than the control.•The self-crosslinked lignin from acetone insoluble fraction showed the best adhesion in plywood.
Refining of industrial lignin to produce homogeneous fractions is essential for high-value applications. However, the understanding of key interactions between a variety of solvents with lignin ...polymer is still uncertain. In this work, single-step fractionation of industrial hardwood kraft lignin (HKL) using organic solvents of different polarities – ethanol, acetone, diethyl ether and hexane – was investigated by combining an experimental and theoretical approach. Experimental results revealed that higher polarity solvents (ethanol and acetone) exhibited higher solubility yield compared to moderate and low polarity solvents. The chemical differences between lignin fractions were proven by pyrolysis gas chromatography mass spectrometry and near infrared spectroscopy. Density functional theory (DFT) results indicated that ethanol presented higher interaction energy followed by acetone, diethyl ether and hexane, which was consistent with experimental findings. Hydrogen bond and non-covalent interaction results from DFT demonstrated that the predominant interaction was found for high polarity of ethanol over other solvents and γ-OH in the lignin model is the key site.
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•Single-step fractionation was carried out for hardwood kraft lignin using 4 solvents.•Higher polarity solvents exhibited higher solubility yield.•Fractionation with diethyl ether leaded to homogeneous low molecular weight fraction.•Quantum chemical calculations showed that ethanol exhibited higher interaction energy.•γ-OH in the lignin model is the key site for strong hydrogen bonding.
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•Crosslinking with glyoxal led to networks formed by amide, imine, and ether bonds in the lignin structure.•DETA- and EDA-aminated lignin comprised higher nitrogen content.•Green ...adhesives were developed by the crosslinking of HKL fractions.•DETA-aminated acetone-insoluble fraction exhibited higher shear strength.•Aminated acetone-soluble fraction satisfied the standard for interior application.
Hardwood kraft lignin from the pulping industry is burned or discarded. Its valorization was conducted by subjecting fractionation, amination with ethylenediamine, diethylenetriamine, and monoethanolamine, and crosslinking with formaldehyde or glyoxal to obtain bio-based wood adhesives. Acetone-soluble and insoluble hardwood kraft lignin were prepared and subjected to amination and then crosslinking. Fourier transform infrared, 13C NMR, 15N NMR, and X-ray photoelectron spectroscopy results revealed successful amination with amide, imine, and ether bonds and crosslinking of all samples. Hardwood kraft lignin aminated with diethylenetriamine/ethylenediamine and crosslinked using glyoxal exhibited excellent results in comparison with samples crosslinked using formaldehyde. Acetone-insoluble hardwood kraft lignin aminated and crosslinked using diethylenetriamine and formaldehyde, respectively, exhibited excellent adhesion strength with plywood, satisfying the requirements of the Korean standards. The amination and crosslinking of industrial waste hardwood kraft lignin constitute a beneficial valorization method.
Kraft lignin, an industrially available by-product from the pulp and paper industry, has revealed enormous potential to be valorised into a wide range of chemicals and biomaterials in the last two ...decades. However, the understanding of lignin chemistry remains challenging due to its chemical complexity. The goal of this work was to investigate the effect of drying temperature on the chemical, physical, and hygroscopic properties of hardwood kraft lignin isolated from industrial black liquor and elucidate the molecular interactions occurring between water and kraft lignin. Sorption-desorption isotherms determined by dynamic vapour sorption (DVS) technique revealed that the drying process considerably affected the hygroscopicity of the lignin polymer. Moreover, analytical pyrolysis (Py–GC–MS), dynamic NIR spectra collected as a function of relative humidity (0–95%) during sorption-desorption cycles and principal component analysis (PCA), evidenced chemical differences between lignin dried at room (25 °C) temperature and mild oven (55 °C) conditions. The main spectral changes associated with the water sorption in kraft lignin samples were analyzed using difference spectrum technique. 2D NIR spectral correlation analysis provided water sorption mechanism of lignin polymer, disclosing for the first time the sequential order in which water vapour molecules interact with active sorption sites in kraft lignin.
Lignins, naturally occurring aromatic polymers with phenylpropane units, are promising bio-based alternatives for petroleum-based products. Resole-type phenol formaldehyde (PF) adhesive is commonly ...used in wood composites requiring durability and weather-proofness. However, PF adhesive is a petroleum-based product. The objective of this study is to transform the low-reactivity hardwood kraft lignin (KL) as the phenol substitute in the PF adhesive formulation by acidic phenolation. The variations in the molecular weights, chemical structures, and functional groups in lignins were investigated before and after the phenolation. The results indicate that the KL can be cleaved, and phenols are crosslinked onto KL to produce phenolated kraft lignin (PKL) under the suitable phenolation condition, heating 3/5 (w/w) of KL/phenol at 90 °C for 2 h with 5% H2SO4 as the catalyst. Resole-type PKL-PF adhesives can be directly synthesized after the phenolation in the same reactor. Plywood laminated with this adhesive obtains satisfactory strength and low formaldehyde emission. This not only reduces the usage of petroleum-based phenol but also increases the reactivity and applications for hardwood KL.
Studies on lignin have garnered great interest for increasing the value of lignin-added products, especially lignin-based adhesives. However, even with the advantage of abundant aromatic materials in ...lignin, its aromaticity and heterogeneity require its fractionation with various solvents. This study used hardwood kraft lignin (HKL) extracted from black liquor and was acetone-fractionated to produce acetone-soluble HKL (AS-HKL) and acetone-insoluble HKL (AI-HKL). The lignins were crosslinked with different amounts of epichlorohydrin (ECH; 5%, 10%, and 15%) for the synthesis of lignin-based adhesives with a three-dimensional network. The crosslinked HKLs were characterized with gel permeation chromatography (GPC), Fourier transform infrared (FT-IR), nuclear magnetic resonance (
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C-NMR), and differential scanning calorimetry (DSC) to investigate their molecular weight, chemical structure, and thermal curing behavior. AI-HKL exhibited higher molecular weight than that of AS-HKL. As expected, the FT-IR and
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C-NMR spectra showed successful crosslinking of HKL by ECH with the intensity reduction of − OH, increased C-O stretching, and new peak appearance. From the DSC results, the exothermic peak temperature for crosslinking was 100 °C-130 °C. HKL and AI-HKL plywoods showed good tensile shear strength with minimal variations when they were crosslinked with 10 and 15% ECH. These results suggest the potential of high molecular weight HKL as lignin-based adhesives for wood bonding.
Heterogeneity of kraft lignin is one of the main limitations for the development of high-performance applications. Therefore, refining lignin using organic solvents is a promising strategy to obtain ...homogenous fractions with controlled quality in terms of structure and properties. In this work, one-step refining processes for hardwood kraft lignin using nine organic solvents of different chemical nature and polarity were carried out with the aim of investigating and understanding the effect of the type of organic solvent on the quality of resulting fractions. Structural features of both soluble and insoluble lignin fractions were assessed by GPC, Py-GC-MS, and FTIR linked to PCA analysis. Moreover, functional properties such as physical appearance, hygroscopicity, antioxidant capacity, and thermal properties were evaluated. The results evidenced the relationship between the nature and polarity of the solvents and the properties of the obtained soluble and insoluble fractions.
The dyeing characteristics of hardwood kraft lignin (KL) were investigated on cotton fabrics, assessing its potential as a sustainable and environmentally friendly dye. Dyeability was evaluated by ...varying the KL concentrations, temperature, and time. An aqueous solution of Na2CO3 (1.0%) as the dye bath and a fabric-to-liquor ratio of 1:50 were used. Cationizing effects were studied using (3-chloro-2-hydroxypropyl) trimethylammonium chloride (CHPTAC), and the mordanting effects of various mordants were also evaluated. Post-mordanting was performed with FeSO4, FeCl2, Fe-lactate (Fe-lac), MgSO4, CuCl2, CuSO4, and Al2(SO4)3. A mordant concentration of 1 mM o.w.f. (0.5 mM for Al2(SO4)3), with a liquor ratio of 1:30, at 60 °C for 30 min were employed. Suitable dyeing conditions were 2.0% o.w.b., 90 °C, and 90 min, resulting in a brownish color of the cotton fabric. The fabrics exhibited a range of light brown to light grayish brown colors and showed lighter colors than the untreated fabrics when mordanted with MgSO4 and Al2(SO4)3. The color difference (ΔE) between cationized and uncationized cotton fabrics was 3.48. From the colorfastness assessment, KL-dyed cotton fabric showed good rubbing and washing fastness for staining, but poor light and washing fastness for fading.
Lignin has shown a great potential to produce fuels, value-added chemicals, and functional materials due to its high-energy density and intrinsic aromatic-based structure. Yet, the lignin ...precipitation of different biomasses needs investigation because most of the work has been performed on softwood and much less is known about hardwoods. In fact, the lignin from these two wooden biomasses vary in composition and pulping performance, which can reflect on lignin precipitation. Therefore, the present study investigated the precipitation and composition of 40 distinct kraft lignins obtained from pine, acacia, sweetgum, and eucalyptus black liquors. Two lignin fractions were precipitated at different pHs, according to known industrial lignin separation practices (pH = 9.5 and 2.5) from black liquors taken at different levels of pulping. Overall, lignin recovery increased with increasing lignin concentration in the black liquor, i.e., higher amounts of lignin were obtained at higher levels of delignification. In addition, pine lignins showed superior yields than the hardwoods and were around five times purer. Among the hardwoods, lignin recovery increased with the S–G ratio of the native lignin, and eucalyptus showed the best performance by achieving the highest yields and purities. Finally, the present work compared the lignin recovery yield and the purity of softwood and different hardwood lignins in a systematic way, which will increase awareness of this underutilized green material and could potentially increase the interest in establishing new lignin plants across the globe.
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