Polyurethane chemistry can yield diverse sets of polymeric materials exhibiting a wide range of properties for various applications and market segments. Utilizing lignin as a polyol presents an opportunity to incorporate a currently underutilized renewable aromatic polymer into these products. In this work, we will review the current state of technology for utilizing lignin as a polyol replacement in different polyurethane products. This will include a discussion of lignin structure, diversity, and modification during chemical pulping and cellulosic biofuels processes, approaches for lignin extraction, recovery, fractionation, and modification/functionalization. We will discuss the potential of incorporation of lignins into polyurethane products that include rigid and flexible foams, adhesives, coatings, and elastomers. Finally, we will discuss challenges in incorporating lignin in polyurethane formulations, potential solutions and approaches that have been taken to resolve those issues.

Lignin, produced as a byproduct of pulp and paper and bioethanol industries, is a polyphenolic compound that has excellent potential to be used as phenol replacement in phenolic adhesive formulation. In this study, the phenol portion of phenol formaldehyde (PF) resin has been replaced by an agricultural-based lignin, which was produced as a byproduct of a cellulosic bioethanol process through dilute-acid pretreatment and enzymatic hydrolysis from corn stover. The PF resol resin was formulated using isolated lignin under alkaline condition. Chemical, physical, and thermal properties of the isolated lignin, PF resin and adhesive were measured using advanced analytical techniques such as Fourier transformed infrared spectroscopy (FTIR), size exclusion chromatography (SEC), phosphorous nuclear magnetic resonance spectroscopy ( 31 P NMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The developed 100% lignin-based adhesive and a commercially formulated phenol resorcinol formaldehyde (PRF, as reference) were used to prepare single-lap-joint samples for mechanical testing. The plywood samples were pressed under exactly the same conditions (time, temperature, and pressure) as what recommended for the commercial PRF formulation. According to two-way ANOVA results, statistically there was no significant difference between the shear strengths of plywood samples made with 100% lignin-based adhesive and those made with the commercial PRF resin.

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Divanillin (DV), which can be facilely synthesized via vanillin dimerization, was employed as a building block to formulate epoxy resin. DV was synthesized through a novel approach in hot water in only 30 min with a yield of 87.5%. The process involved FeSO4-catalyzed Na2S2O8-based oxidative coupling of vanillin without any purification, followed by treatment with biobased epichlorohydrin. Epoxidized-divanillin (EDV) was cured with the petroleum-based, commercially available hardener isophorone diamine (IPDA) and a biobased-diamine (GX-3090). Complete curing of the mixture was confirmed by Fourier transform infrared (FTIR) spectroscopy and statistical heat resistant-indices (Ts), which indicated the formation of cross-linked networks with a thermostability similar to materials prepared with diglycidyl ether bisphenol A (DGEBA, the commercial BPA-based resin). The epoxy resin developed with this new formulation had comparable storage moduli (1.7–2.3 GPa) and similar glass transition temperatures as commercial resins. The epoxy networks exhibited good solvent resistance, while the presence of aldehyde groups in EDV yielded in more readily cleavable ester and amide bonds during the cross-linking process, yielding a resin with improved degradation under acidic conditions. Almost 40% of the segments in networks cured with EDV/IPDA were solubilized in acetone after treatment with 1 M HCl at room temperature in 24 h.

Thirteen unmodified lignin samples from different biomass sources and isolation processes were characterized and used to entirely replace bisphenol A (BPA) in the formulation of solubilized epoxy resins using a developed novel method. The objective was to measure the reactivity of different lignins toward bio‐based epichlorohydrin (ECH). The epoxy contents of various bio‐based epoxidized lignins were measured by titration and 1H NMR spectroscopy methods. A partial least square regression (PLS‐R) model with 92 % fitting accuracy and 90 % prediction ability was developed to find correlations between lignin properties and their epoxy contents. The results showed that lignins with higher phenolic hydroxy content and lower molecular weights were more suitable for replacing 100 % of toxic BPA in the formulation of epoxy resins. Additionally, two epoxidized lignin samples (highest epoxy contents) cured by using a bio‐based hardener (Cardolite GX‐3090) were found to show comparable thermomechanical performances and thermal stabilities to a petroleum‐based (DGEBA) epoxy system.