Carboxylated Lignin-Containing Cellulose Microfibrils Produced from Wood at High Solids through Maleic Acid Hydrotropic Fractionation

Producing lignin-containing cellulose microfibrils (L-CMF) at high solids remains challenging due to strong fibril–fibril interactions and the lignin “glue” effect in plant cell walls. This study demonstrated the high-solid production of lignin-containing cellulose microfibrils (L-CMF) from thermomechanical pulp (TMP) of birchwood through maleic acid (MA) hydrotropic fractionation (MAHF). The fractionated water-insoluble solids (WIS) were ball-milled at solid contents from 2% to 28% to produce L-CMFs. The extents of fibrillation of the resultant L-CMFs were evaluated by microscopy, water retention value (WRV), and suspension sedimentation tests. The results indicate that increasing MAHF severity improved fibrillation due to improved delignification and hemicellulose dissolution. MA also esterified cellulose and lignin in WIS, introducing surface carboxylation and an electrostatic repulsion force among fibrils to enhance the lignin lubrication effects and thereby improve fibrillation. The esterification also facilitated the breaking-up of the residual lignin (as glue) in the WIS to form individually separated lignin particles. Lignin in the WIS/L-CMF lowers the solids-loading threshold to 5% for the interfiber/fibril frictional and tensional interactions─the secondary fibrillation mechanism through external peeling off fibril separation─to effectively promote fibrillation, which is significantly lower than the 17–28% observed in ball-milling bleached wood fibers.