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 In this study, the phenol portion of phenol formaldehyde (PF) resin has been replaced by an agricultural‐based lignin, which was produced as a byproduct of a cellulosic bioethanol process through dilute‐acid pretreatment and enzymatic hydrolysis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 (³¹P 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. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45124.

In this study, a biobased phenolic adhesive was successfully developed by entirely substituting both petroleum-based phenol and formaldehyde with an unmodified corn stover biorefinery lignin and glyoxal (a biobased dialdehyde), respectively. Lignin-glyoxal resins were synthesized using an alkaline catalyst with a molar ratio of lignin to glyoxal of 1:2. Chemical, thermal, and mechanical properties of the lignin, lignin-based resins, and final adhesives were assessed following appropriate standard test methods. The analysis of lignins and lignin-based resin molar mass was performed using gel permeation chromatography. The lignin-glyoxal resin was found to have a 3-fold higher average molecular weight than the starting lignin, demonstrating the successful integration of lignin into the polymeric resin network. The curing of the formulated adhesives was studied using differential scanning calorimetry and dynamic mechanical analysis. Although the lignin-glyoxal resin had a higher curing temperature (167 °C) than a conventional phenol-formaldehyde resin (142 °C) and the formulated lignin-formaldehyde resin (146 °C), the rate and degree of cure were similar or better than the other two resins. The adhesion strengths of the formulated adhesives were determined using single-lap-joint veneer samples cured according to recommended press parameters for commercial adhesives. The lignin-glyoxal adhesive had a relatively high dry adhesion strength (3.9 MPa), with over 90% wood failure, but failed the wet adhesion test (boiling water test). Although the formulated lignin-glyoxal adhesive failed the boiling water test, it had excellent stability at room temperature water, remaining intact after 1 week during the water immersion test. The high dry adhesion strength makes this class of lignin-based formaldehyde-free adhesives a unique biobased glue for the production of interior grade plywood and oriented strand boards.

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.