Cellulose dissolution in aqueous NaOH\u2013ZnO: effect of pulp pretreatment at macro and molecular levels

Saija Väisänen,Harri Kosonen,Tapani Vuorinen,Matti Ristolainen

doi:10.1007/s10570-021-03779-w

Saija Väisänen, Harri Kosonen + Show 2 more

Open Access

https://doi.org/10.1007/s10570-021-03779-w

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Abstract

This paper discusses the effect of hydrolytic pretreatments on pulp dissolution in the aqueous NaOH–ZnO solvent system. Eight samples were studied. They consisted of a never-dried softwood kraft pulp that was hydrolyzed under seven different conditions as well as the pulp without hydrolysis as a reference. The dissolution of the pulps was evaluated both at the macro level as well as at the molecular level based on their reactivity with 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxo-piperidium (4-AcNH-TEMPO+). The fiber properties (i.e. the extent of fibrillation, amount of fines and fiber width, coarseness, and length) as well as the chemical composition (hemicellulose and cellulose contents) and the viscosity of the pulps was investigated. The results show that hydrolysis at medium consistency (10%) was successful in increasing the solubility of cellulose. Hydrolysis at high consistency (50%), on the other hand, increased the solubility only to some extent. With extended treatment time the fibers formed aggregates and their dissolution became poor. This phenomenon could be overcome by mechanically refining the fibers after the hydrolysis. Moreover, comparison of the viscosity of the pulp over the degree of oxidation revealed that the viscosity needed to decrease below ca. 400 ml/g in order for the outer layers of the fibers to dissolve. Finally, when pulps with similar viscosities where compared against each other, the ones with the higher glucomannan contents formed gels over time. This was true also for the pulp with the lowest viscosity and the highest solubility of the studied samples.

Highlights

As renewable, recyclable, biocompatible and nontoxic substance, cellulose is a highly interesting raw material for manufacturing bio-based products
The dissolution of the pulps is evaluated both visually with an optical microscope as well as at the molecular level based on their reactivity with 4-acetamido2,2,6,6-tetramethylpiperidine-1-oxo-piperidium (4AcNH-TEMPO?)
The dissolution of the pulps in the NaOH–zinc oxide (ZnO) solvent system was evaluated both visually with optical microscopy and at the molecular level based on their reactivity with 4-AcNH-TEMPO?

Summary

Introduction

Recyclable, biocompatible and nontoxic substance, cellulose is a highly interesting raw material for manufacturing bio-based products. Cellulose dissolution in aqueous NaOH and the stability of the solution against gelation can be enhanced with certain additives, such as zinc oxide (ZnO) (Kihlman et al 2013; Liu et al 2011; Yang et al 2011). Based on more recent studies, Cuissinat and Navard (2008) have proposed the mechanism for dissolution by ballooning to be the following: when the solvent permeates the fibers, cellulose in the secondary wall dissolves causing heterogeneous swelling of the primary wall and the formation of balloons bursting them followed by the dissolution of the unswollen sections of the fibers and, in the end, the balloon membrane scraps. Pulp dissolution was evaluated in relation to the fiber properties (i.e. the extent of fibrillation, amount of fines and fiber width, coarseness, and length) as well as the hemicellulose and cellulose contents and the viscosity of the pulps

Materials and methods

Results and discussion

Conclusions