Kraft Cooking Kinetics of Poplar Wood Chips Pretreated via a Mechano-Enzymatic Method for Further Nanocellulose Production

Kraft cooking kinetics of poplar wood (Populus deltoides and Populus nigra) were investigated using chips of controlled size, and various enzymatic/mechanical pretreatments of the chips were attempted to observe the effects on pulp quality. Batch kraft cooks of small amounts of untreated wood chips were carried out in mini-autoclaves at a range of 140–165 °C. Delignification rate constants and apparent activation energy during the bulk phase were determined. Various mechanical pretreatments, using a modular screw device (MSD) with or without xylanase impregnation prior to the cooks, were also investigated. Cooks were monitored through pulp yield, kappa number, fiber morphology, viscosity-average degree of polymerization (DPv), and residual effective alkali (REA). The results showed a variable, temperature-dependent activation energy of about 100 kJ/mol for the bulk delignification with a non-linear Arrhenius behavior suggesting a shift from reaction-controlled to diffusion-controlled kinetics as the temperature increased. The effectiveness of xylanase impregnation was limited due to chip thickness that limited enzyme penetration. MSD and MSD-Xylanase pretreatments significantly reduced fiber length (from 871 µm to 845 µm and 784 µm, respectively) and increased fines content (from 20.3% to 26.3% and 29.1%, respectively), indicating degraded fiber quality after the cooks of pretreated chips. Variability in pulp yield and REA was too high to reach statistical significance. Finally, it was shown that neither of these pretreatments could improve pulp quality in terms of fiber morphology and pulp strength properties, but this study opens the way toward the utilization of pretreatments to improve pulp fibrillation for nanocellulose production.