Key role of mild sulfonation of pine sawdust in the production of lignin containing microfibrillated cellulose by ultrafine wet grinding

Cheaper methods and raw materials are currently being sought for the production of microfibrillated cellulose. Presently microfibrils with high lignin content have attracted increasing interest. Wood residues, like sawdust, are good candidates due to their abundancy and low price but the recalcitrant nature of the lignocellulose complex is an obstacle for the mechanical separation of microfibrils directly from wood. The purpose of this study was to investigate whether the mild sulfonation pretreatment of pine sawdust with sodium sulfite in neutral and mild alkaline conditions could enhance mechanical fibrillation in ultrafine wet grinding. Sawdust sulfonated at pH 7 and 9 resulted in a lignin sulfonation degree of 0.5–0.6% with a yield loss of 5–6%, approximately half of which was due to the dissolution of extractives from the initial content of 4.7% to 1.3–1.5%. Sawdust (with and without sulfonation) was pre-ground first in a twin-screw extruder, followed by ultrafine grinding with a friction grinder at a temperature of 80 °C. The viscosity and size distribution of the microfibrillated sample suspensions were determined. Additionally, the samples were filtered on a membrane, dried, and the mechanical properties of the sheets made from the samples were measured. The results showed that the sulfonation of softwood sawdust is a promising economic and environment-friendly method to produce microfibrillated cellulose with high lignin content (above 28%), having excellent strength properties, low viscosity, and fast dewatering rates. The sheets made from the sulfonated samples yielded an almost twofold increase in tensile strength and Young’s modulus (100 MPa and 7–7.5 GPa, respectively) compared to the reference sample without sulfonation (52 MPa and 4.3 GPa) at an applied net grinding energy of 7 MW h per ton. It was found that the size of the fibrils was at a similar level with and without sulfonation, but the bonding ability of the sulfonated samples was cle