Evaluating alkali pre-impregnation for enhancing pulp brightness and selectively depolymerizing lignin in chemi-thermomechanical pulping of poplar woodchips

The structure of wood poses significant challenges for Chemi-thermomechanical pulp (CTMP) pulping process, particularly in achieving efficient chemical penetration and uniform distribution within woodchips. While alkali pre-impregnation is known to enhance pulp bleachability, this work advances prior work by systematically correlating lignin’s structural evolution under optimized pretreatment conditions with the bleaching performance of poplar woodchips (PWCs)-derived CTMP. The results show that the content of the main linkages, e.g. , β - O -4 and β - β , in lignin were significantly reduced 29.58 % and 21.00 %, respectively, after alkali pre-impregnation, and the number of aromatic regions including guaiacyl (G), syringyl (S) and para-coumaric acid ester (PCE) units reduced, confirming the partial depolymerization of lignin. The optimum pre-impregnation conditions, including 6 % alkali charge at 70 °C for 30 min, were beneficial to achieving a satisfied pulp brightness (74.73 % ISO). This work would highlight the critical role of alkali pre-impregnation techniques in CTMP pulping and bleaching processes. • A screw extrusion technique enhanced alkali solution penetration in pre-impregnation. • Optimization of alkali pre-impregnation conditions significantly improved pulp brightness. • Selective lignin depolymerization facilitated pulp bleachability and reactivity.