Esterification of kraft lignin inherently addresses its potential for thermoplastic applications either on its own or as a component of polymer blends. In this effort, we have investigated the selectivity of softwood kraft lignin toward esterification via acylation. LignoBoost kraft lignin was esterified with acetyl (C 2 ), octanoyl (C 8 ), lauroyl (C 12 ), and palmitoyl (C 16 ) chlorides at various molar ratios with respect to the total hydroxyls present. Quantitative 31 P NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC) were used to evaluate the selectivity and efficiency of these reactions on the various hydroxyl groups present. The C 8 −C 16 acyl chlorides showed distinct enhanced reactivity toward the aliphatic hydroxyl groups, whereas C 2 acyl chloride was found to react uniformly with any available OH irrespective of their chemical nature. The effects of long chain acylation on the polymer and material properties were also examined using solution viscosity, thermal, and rheological measurements. Polymer blends were also produced and studied by melt extrusion. The long aliphatic chains when installed on the lignin displayed peculiar association effects in solution and enhanced the melt flow characteristics of the lignin−polymer blends.

Lignin, esterified with palmitic and lauric acid chloride, has been studied for the application as coating on fiber-based packaging material. The aim was to improve the barrier properties against water vapor and oxygen of paperboard. The esterification was followed by Fourier transform infrared spectroscopy, 31 P nuclear magnetic resonance spectroscopy, and gel permeation chromatography measurements. The lignin esters were applied on paperboard and formed a continuous film. The moisture barrier property of the coated paperboards was characterized by the water vapor transmission rate (WVTR). A significant decrease in WVTR was observed, for example, 40 g m -2 (for 24 h) for a paperboard coated with 10.4 g m -2 hardwood kraft lignin palmitate. The contact angle of water on the lignin ester coatings was high and stable. For all paperboard samples coated with lignin esters, a significant decrease in oxygen transmission rate was observed. Accordingly, lignin palmitate and laurate have a high potential as a barrier materials in packaging applications.

The kraft lignin's low molecular weight and too high hydroxyl content hinder its application in bio-based carbon fibers. In this study, we were able to polymerize kraft lignin and reduce the amount of hydroxyl groups by incubating it with the white-rot fungus Obba rivulosa. Enzymatic radical oxidation reactions were hypothesized to induce condensation of lignin, which increased the amount of aromatic rings connected by carbon− carbon bonds. This modification is assumed to be beneficial when aiming for graphite materials such as carbon fibers. Furthermore, the ratio of remaining aliphatic hydroxyls to phenolic hydroxyls was increased, making the structure more favorable for carbon fiber production. When the modified lignin was mixed together with cellulose, the mixture could be spun into intact precursor fibers by using dry-jet wet spinning. The modified lignin leaked less to the spin bath compared with the unmodified lignin starting material, making the recycling of spin-bath solvents easier. The stronger incorporation of modified lignin in the precursor fibers was confirmed by composition analysis, thermogravimetry, and mechanical testing. This work shows how white-rot fungal treatment can be used to modify the structure of lignin to be more favorable for the production of bio-based fiber materials.

Lignocellulosic biorefineries produce lignin-rich side streams with high valorization potential concealed behind their recalcitrant structure. Valorization of these residues to chemicals, materials, and fuels increases the profitability of biorefineries. Fractionation is required to reduce the lignins' structural heterogeneity for further processing. We fractionated the technical biorefinery lignin received after steam explosion and saccharification processes. More homogeneous lignin fractions were produced with high β-O-4′ and aromatic content without residual carbohydrates. Non-toxic biodegradable organic solvents like ethanol and formic acid were used for fractionation and can be adapted to the existing biorefinery processes. Macromolecular properties of the isolated fractions were carefully characterized by structural, chemical, and thermal methods. The ethanol organosolv treatment produced highly soluble lignin with a reasonable yield, providing a uniform material for lignin applications. The organosolv fractionation with formic acid and combined ethanol-formic acid produced modified lignins that, based on thermal analysis, are promising as thermoresponsive materials.