Sugar‐based biorefineries have faced significant economic challenges. Biorefinery lignins are often classified as low‐value products (fuel or low‐cost chemical feedstock) mainly due to low lignin purities in the crude material. However, recent research has shown that biorefinery lignins have a great chance of being successfully used as high‐value products, which in turn should result in an economy renaissance of the whole biorefinery idea. This critical review summarizes recent developments from our groups, along with the state‐of‐the‐art in the valorization of technical lignins, with the focus on biorefinery lignins. A beneficial synergistic effect of lignin and cellulose mixtures used in different applications (wood adhesives, carbon fiber and nanofibers, thermoplastics) has been demonstrated. This phenomenon causes crude biorefinery lignins, which contain a significant amount of residual crystalline cellulose, to perform superior to high‐purity lignins in certain applications. Where previously specific applications required high‐purity and/or functionalized lignins with narrow molecular weight distributions, simple green processes for upgrading crude biorefinery lignin are suggested here as an alternative. These approaches can be easily combined with lignin micro‐/nanoparticles (LMNP) production. The processes should also be cost‐efficient compared to traditional lignin modifications. Biorefinery processes allow much greater flexibility in optimizing the lignin characteristics desirable for specific applications than traditional pulping processes. Such lignin engineering, at the same time, requires an efficient strategy capable of handling large datasets to find correlations between process variables, lignin structures and properties and finally their performance in different applications.

In this study, lodgepole pine (Pinus contorta Dougl.) bark infested by the mountain pine beetles (Dendroctonus ponderosae hopkins) was liquefied using either polyethylene glycol (PEG) or polyethylene glycol/glycerol (PEG/G) as the solvent. It was found that the addition of glycerol to PEG reduced the residue ratio during bark liquefaction. The liquefied bark fraction obtained by using PEG/G had a slightly higher hydroxyl number than that obtained by using PEG. The residue from PEG/G liquefaction contained less lignin and more cellulose than the residue from PEG liquefaction. Various polyurethane foams containing liquefied bark fractions were made, and it was found that the weight ratios of liquefied bark to pMDI used in foam formulation and bark liquefaction solvents affected the density, gel content, thermal stability, mechanical properties, and the cell structure of the resulting foams.

In this study, two types of biobased bark-derived phenol formaldehyde (PF) resins, namely, liquefied bark-PF and bark extractive-PF, were synthesized from acid-catalyzed phenol-liquefied bark and bark alkaline extractives, respectively. The biobased resins were characterized for their chemical compositions and molecular structures using the liquid-state 13C nuclear magnetic resonance (NMR) technique. The results indicated that the introduction of bark components (either as liquefied bark or as bark extractives) to the phenolic resin synthesis affected resin structures and curing performance. Methylene ether bridges were found in the bark-derived PF resins. Bark components made the formation of para–ortho-methylene linkage more favorable in bark-derived PF resins than in lab PF resins. Molecular structures of the liquefied bark-PF resin differed significantly from those of the bark extractive-PF resins. The liquefied bark-PF resin showed a higher ratio of para–para/ortho–para-methylene link (−CH2−), a higher unsubstituted/substituted hydrogen (−H/–CH2OH) ratio and a higher methylol/methylene (−CH2OH/–CH2−) ratio than the bark extractive-PF resin. The tannin components of the bark extractives accelerated the curing rate of the resulting bark extractive-PF resin. The bark extractives made the ortho position of phenol react more favorably with formaldehyde than the para position. The liquefied bark with phenolated structures had more reactive sites toward formaldehyde than the bark extractives and accelerated the curing rate of the resulting liquefied bark-PF resin.

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