S. Ceccherini: Pulp reactivity during dissolution: from assessment to activation

Abstract:

The market of man-made cellulosic fibers (MMCF) is expanding. From 2000 to 2019, the global production of these fibers has increased by ca. 4400 TMT, and a further increase is expected in the future. However, the dissolution of cellulosic natural fibers, which is a fundamental step for the manufacturing of MMCF, is hindered by the recalcitrance of cellulose. If the dissolution is not sufficiently thorough, the undissolved matter can slow down the downstream process and affect the quality of the final fibers. "Reactivity" is a somewhat ill-defined, but important, characteristic of pulp that relates to its dissolution behavior in a compatible solvent.This thesis studies whether it is possible to assess pulp reactivity by monitoring the rheological behavior of fiber suspensions during dissolution. Secondly, the pulp features that affect reactivity are examined. Finally, methods to increase pulp reactivity are investigated.The Dissolution-Based Torque Reactivity (DTR) test was developed to measure pulp reactivity by tracking the torque of a fiber suspension while dissolving in cupriethylenediamine at a constant shear rate. The dissolution produces a rheogram with torque as a function of time which reaches a plateau when the dissolution is complete. The rheogram was analyzed in terms of dissolution time (DT) and torque plateau. Faster dissolutions corresponded to higher reactivity. The DT could rank the pulps consistently with the current industrial understanding of pulp reactivity. The average coefficient of variation of DT and plateau measured ca. 8 and 3%, respectively.Plateau and DT increased with the molar mass of pulp. The DT increased with hornification (lowered swelling). The plateau was affected by the xylan content and the amount of undissolved matter. The DTR test was compared to the Treiber test for some dissolving pulps. The DTR test was less sensitive than the Treiber test for undissolved material, and the two tests are not interchangeable. However, for pulps with similar molar mass, the plateau increased with the decrease of the Kr filterability value, indicating a certain correlation between the methods. Moreover, the DTR test is much faster and less laborious than the Treiber test.Enzymatic treatments targeting the selective depolymerization of cellulose or hemicellulose could shorten the DT of paper- and dissolving-grade pulps. Cellulose was depolymerized with endoglucanase and lytic polysaccharide monooxygenase (LPMO), while hemicelluloses with mannanase and xylanase. Activation by endoglucanase and LPMO was due to cellulose depolymerization and the increase in porosity and surface area. Activation was most effective when hydrolysis was performed at high solids content. TrCel45A endoglucanase shortened the dissolution of softwood Kraft pulp more than TrAA9A LPMO, but even shorter dissolution was achieved with TrCel45A and TrAA9A together. Xylanase and mannanase reduced the dissolution time of dissolving pulp even if the changes to the molar mass distribution were modest and the removal of part of the hemicelluloses caused hornification.

Follow the remote defence: https://aalto.zoom.us/j/66157965152

Opponents: Professor Monica Ek, KTH Royal Institute of Technology, Sweden

Supervisor: Professor Thaddeus Maloney,  Aalto University, School of Chemical Engineering, Department of Bioproducts and Biosystems

Link to electronic thesis:  Pulp reactivity during dissolution: from assessment to activation