Effect of Fatty Acid Esterification on the Thermal Properties of Softwood Kraft Lignin

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 (C₂), octanoyl (C₈), lauroyl (C₁₂), and palmitoyl (C₁₆) chlorides at various molar ratios with respect to the total hydroxyls present. Quantitative ³¹P 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 C₈–C₁₆ acyl chlorides showed distinct enhanced reactivity toward the aliphatic hydroxyl groups, whereas C₂ 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.