Hur fiberväggens struktur på nanometernivå påverkas av massa-processer och vilken inverkan det har på mekaniska egenskaper (1039/21)

The understanding of the fiber cell wall on a nanometer scale is fundamental for the development of new tailor-made materials. The nanostructure is important because it affects intrinsic properties such as fiber strength and accessibility of chemicals in the fiber wall. The type of and the conditions in the delignification processes will influence the chemical composition of the pulp fibers and likely also the nanostructure of the fibers.

Kraft cooking, oxygen delignification and bleaching are some of the processes that have delignification as the main goal. Even though kraft cooking has been available on an industrial scale since the 19 th century, a deep understanding of the delignification effects on fiber cell wall nanostructure is still missing. Mechanical treatment of fibers by refining modifies the interconnections between fibrils in the fiber wall (internal and external fibrillation), however the modifications are only crudely characterized by measuring the effect on water retention ability. The lack of reliable methods for fiber cell wall characterization at the nanometer level made this project relevant. SAXS (Small Angle X-ray Scattering) is a method that can be used to measure the nanostructure of a material which is an important puzzle piece to understand the mechanical and chemical properties of the final products. A SAXS measurement results in data that is averaged over several size scales, typically from 1 nanometer to about 100 nanometers. An interpretation model was previously developed (Larsson et al., Cellulose 29, 117–131 (2022), which can extract quantitative structural information from a SAXS measurement performed on cellulose-rich pulps. In the present work, the intention was to use that model to evaluate pulps from different delignification processes, kraft cooking and oxygen delignification, as well as the effect of refining. The procedure and results are described in detail in the above-mentioned manuscript.

The results from this study showed that, compared with kraft cooked pulps, subsequent oxygen delignification did not induce any significant change in the fiber wall nanostructure, the measured apparent average particle size (AAPS) remained quite similar when the kraft cooked pulp was further
delignified by oxygen delignification. This suggests that fibril aggregation is less likely to occur during oxygen delignification, even with higher degrees of delignification during the oxygen delignification step. The pulp samples were also evaluated with and without PFI-refining, and the results showed
significant increase in the abundance of nanometer sized cavities as the result of PFI-refining. Results from modelling suggests that PFI-refining can break apart aggregated particles also on the fiber wall nanoscale levels. We are now a step closer to understand how the process operations affects fiber
wall nanostructure and how it can be modified to obtain, for example, a fiber wall that is more accessible to chemicals, easier to delignified, etc.

A manuscript from this work is to be submitted in February to Cellulose journal, entitled as: “Pulp delignification and refining – impact on the supramolecular structure of softwood fibers”, Cláudia
Esteves, Elisabet Brännvall, Jasna Stevanic-Srndovic and Per Tomas Larsson. A draft version of the manuscript is included in the PhD Thesis “Pulp strength enhancement by oxygen delignification” of Cláudia Esteves that was defended and approved at KTH on 16 Dec 2022.

Tomas Larsson (
Elisabet Brännvall (
Jasna Stevanic-Srndovic (
Cláudia Vicente Esteves (

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