Kraft Lignin with Improved Homogeneity Recovered Directly fromBlack Liquor and Its Application in Flexible Polyurethane Foams

An effective method that can produce a large amount of Kraft ligninwith improved homogeneity is strongly desired for Kraft lignin's high-value applicationsand scientific advancements. Herein, a one-pot acid-catalyzed liquefaction method wasdeveloped to recover Kraft lignin directly from black liquor. The recovery rate andproperties of the recovered lignin were affected by the reaction time, reactiontemperature, moisture content (MC), pH, and acid categories. The highest ligninrecovery rate of 75% was achieved when the concentrated black liquor (MC = 25%)reacted with methanol at pH = 7 and 160 degrees C for 10 min using acetic acid as thecatalyst. Most of the recovered lignin from this method showed an average molecularweight (Mw) value less than 2000 Da and a polydispersity (PDI) value less than 2.0.Such a PDI value was lower than that of current acid precipitated lignin (around 2.2-5.4). The recovered lignin was directly used to replace 20% of the petroleum-basedpolyol in the formula of aflexible polyurethane (PU) foam, and it was found that themolecular weight characteristics of the lignin affected the physical and mechanical properties of theflexible PU foams. The recoveredlignin with the Mw value of 1600 Da and the PDI value of 1.8 was able to maintain the major physical and mechanical properties oftheflexible PU foams. This study provided a promising way to recover lignin with improved homogeneity from black liquor with thepotential to customize lignin properties to meet the requirements of downstream processes.