In this study, acetone fractionation was used to increase the homogeneity of softwood kraft lignin for product development. The acetone soluble and insoluble fractions were analyzed in terms of ...elemental composition, chemical structure, and thermal properties. The phenolic hydroxyl content of the acetone soluble fraction increased by 24%. The acetone soluble fraction of kraft lignin was used to fully substitute phenol in phenolic resin formulation. The resulting formulated lignin-formaldehyde adhesive showed higher adhesion strength (4.3 MPa) compared to a lab formulated phenol-formaldehyde resin (3.4 MPa) with 100% wood failure after lap shear test. On the other hand, the acetone insoluble lignin fraction, which had 23% higher aliphatic hydroxyl content compared to the starting lignin, was used to entirely replace petroleum-based polyol in polyurethane resin formulation. Then a bio-derived solvent (dihydrolevoglucosenone, Cyrene) was used to dissolve the acetone insoluble lignin fraction to synthesize lignin-based polyurethane resin. Tailoring the structure of kraft lignin through solvent fractionation resulted in two fractions, which were used to replace 100% of two major petrochemicals (phenol and polyol) in the formulations of phenolic and PU resins, that would otherwise be challenging due to the heterogeneity of softwood kraft lignin.
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•Acetone fractionation of softwood kraft lignin resulted in two homogenous fractions.•Acetone soluble fraction used to replace 100% of phenol in phenolic adhesive.•Acetone insoluble fractions used to replace 100% of polyol in PU resin.•A bio-derived solvent (Cyrene) was used in the synthesis of lignin-based PU resin.
AbstractSoftwood kraft lignin is a major bioresource relevant to the production of sustainable bio-based products. Continued challenges to lignin valorization, however, include poor solubility in ...organic solvents and in aqueous solutions at neutral pH. Herein, an alkaline tolerant laccase was used to graft acrylate functionalities onto softwood kraft lignin, which is expected to enhance the reactivity of lignin with isocyanate when producing bio-based polyurethanes. Proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy, and high-performance liquid chromatography were used to confirm successful grafting of the acrylate monomer onto lignin and verify the importance of including tert-butyl hydroperoxide as an initiator in the grafting reaction. Laccase-mediated grafting of softwood kraft lignin under alkaline conditions produced lignin products with approximately 30% higher hydroxyl value and higher reactivity toward isocyanate. The reported enzymatic and aqueous process presents an opportunity for the sustainable valorization of softwood kraft lignin. Key points•Softwood kraft lignin displayed high phenolic hydroxyl content, polydispersity index and average molecular weight•Grafting hydroxyethyl acrylate (HEA) monomer onto kraft lignin by laccase was successful at 60 °C and alkaline conditions•Lignin-HEA grafted copolymer showed an increase in total OH value and an increase in average molecular weight
The utilization of all biorefinery products and by-products, including lignin, will be a game-changer to optimize the biorefinery concept and make corresponding processes economically viable. ...Softwood kraft lignin (SKL) is a significant bioresource that could play an essential role in achieving a sustainable bio-based economy. Due to existing aliphatic and phenolic hydroxyl groups in lignin, it could be used as biobased polyol for polyurethane (PU) application. PUs represent a large class of polymers with urethane repeating units produced by reacting polyols and diisocyanates. Major hurdles to the valorization of kraft lignins include high heterogeneity, inaccessibility of hydroxyl groups for reaction with isocyanates, and low solubility in most organic solvents. As such, technologies to enhance kraft lignin homogeneity are particularly crucial to unlocking its enormous potential and entirely replace petroleum-based polyol in PUs. To overcome the bottlenecks in valorization of SKL for applications beyond energy production, chemoenzymatic methods were established and compared to propel our knowledge of effective ways to tailor lignin structure for PU resin applications. The first method focused on fragmentation of lignin structures using ionic liquid 1-butyl-3-methylimidazolium BMIMBr treatment to gain access to the existing hydroxyl groups. However, the expected compositional changes were not detected on SKL (Indulin-AT) after IL treatment, as confirmed by phosphorus nuclear magnetic resonance spectroscopy (31P NMR) and gel permeation chromatography (GPC) analysis.The second method was a one-step acetone fractionation of LignoBoost SKL that resulted in acetone soluble and insoluble fractions, offering components suitable to entirely replace phenol and polyol in phenolic and PU resins, respectively, without the need for further chemical modification of the lignin structure. Additionally, the possibility of using a cellulose-derived solvent in PU formulation was successfully tested.Finally, to eliminate organic solvents and mimic the oxidation of lignin phenolic compounds in nature, an alkaline tolerant laccase was used to graft acrylate monomer onto LignoBoost SKL. Enzymatic grafting was confirmed by Fourier-transform infrared spectroscopy and 1H NMR techniques, and was shown to enhance the reactivity of lignin as a polyol substitute in PU resin applications. The reported enzymatic and aqueous process presents an opportunity for the sustainable valorization of SKL.
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