Cellulose dissolution in aqueous NaOH–ZnO : effect of pulp pretreatment at macro and molecular levels

Funding Information: This work was done in collaboration between UPM-Kymmene Corporation and Aalto University. The research team is thanked for the fruitful discussions at the project meetings. In addition, we are grateful for the support from Business Finland, Walter Ahlström foundation and the FinnCERES Materials Bioeconomy Ecosystem. Finally, M.Sc. Rubina Ajdary is highly thanked for her work in optimizing the dissolution system and Mrs. Rita Hatakka for her assistance with the HPAEC measurements. Funding Information: Open access funding provided by Aalto University. This work was a part of a joint research project between UPM-Kymmene Oyj and Aalto University. In addition, the authors would like to acknowledge the financial support from Business Finland, Walter Ahlström foundation and the FinnCERES Materials Bioeconomy Ecosystem. Publisher Copyright: © 2021, The Author(s). Copyright: Copyright 2021 Elsevier B.V., All rights reserved.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.Peer reviewe

Interaction of divalent cations with carboxylate group in TEMPO-oxidized microfibrillated cellulose systems

Cellulose-based films can potentially replace non-biodegradable plastic films in various applications such as food packaging. In this work we produced and studied films made of mixtures of chemical pulps and catalytically oxidized microfibrillated cellulose. The films were prepared on a support which was then soaked in solutions of CaCl 2 and MgCl 2 to exchange the sodium ions originally present in the film to divalent metal cations. We assumed that the electrostatic interaction of the anionic pulp fibers and the fibrils with Ca 2+ and Mg 2+ would promote internal bonding of the fiber-fibril network that would then reflect positively on the film properties. The immersion of the wet film into aqueous CaCl 2 or MgCl 2 solidified the film with time. When the solidified films were dried with an excess of the salt, elastic, skin-like materials were formed. Rewetting in water and redrying the materials produced paper-like films with improved mechanical properties in comparison with films prepared without the divalent cation salts. SEM imaging of the fracture surfaces provided support for the increased internal film strength by the divalent cations. The new knowledge on their role could be utilized in tailoring cellulosic film properties for specific uses.Peer reviewe

Time-triggered calcium ion bridging in preparation of films of oxidized microfibrillated cellulose and pulp

One of the main trends in developing bio-based materials is to improve their mechanical and physical properties using MFC derived from sustainable natural sources and compatible low-cost chemicals. The strength of anionic MFC based materials can be increased with addition of multivalent cations. However, direct mixing of solutions of multivalent cations with oxidized MFC may result in immediate, uncontrollable fibril aggregation and flock formation. The aim of this study was to design a method where Ca 2+ ions liberate from solid CaCO 3 particles on bleached hardwood (birch) kraft pulp, which was mixed with oxidized MFC and crosslink it to tailor the mechanical properties of the dried structure. In few minutes after adding acetic anhydride, pH of the wet film dropped from 7.3–4.8 through liberation of acetic acid and CaCO 3 particles solubilized releasing Ca 2+ . The novel method could be applied on industrial scale for improving the performance of packaging materials.Peer reviewe

Insights into the borohydride reduction of dialdehyde cellulose: the dilemma of competing reduction and β-elimination reactions

Funding Information: Open access funding provided by University of Natural Resources and Life Sciences Vienna (BOKU). The financial support by Business Finland, UPMKymmene Oyj (Finland), the Austrian Biorefinery Center Tulln (ABCT), and the doctoral school “Advanced Biorefineries: Chemistry & Materials” (ABC&M) is gratefully acknowledged. Publisher Copyright: © 2023, The Author(s).Borohydride reduction of dialdehyde cellulose (DAC) is a promising strategy to generate dialcohol cellulose as bio-based alternative to petroleum-based materials. However, the degradation of the polymer backbone according to β-elimination mechanisms limits the practical applications of the reaction. Therefore, we aimed at optimizing the process to suppress degradation reactions by varying reaction time, pH, and reagent stoichiometry. The degree of oxidation (DO) of the DAC intermediates significantly impacts the yields and molecular weights of the isolated dialcohol celluloses, with a “leveling-off” effect at higher DO values. Increasing the amount of sodium borohydride can minimize—but not entirely prevent—chain scissions. Lowering the pH value during reduction slows down the degradation but results in incomplete conversion of the aldehyde functionalities. Our study provides valuable insights into the consequences of side reactions during borohydride reduction of DAC as well as into chemistry and analysis ofthe dialdehyde cellulose/dialcohol cellulose system. Graphical abstract: About a dilemma in cellulose chemistry: Dialcohol cellulose derived by periodate oxidation and subsequent borohydride reduction of cellulose has received increasing attention in the development of sustainable thermoplastic materials. The present study highlights the challenge of suppressing β-elimination and favoring the reduction pathway to optimize reaction conditions and minimize chain degradation.Peer reviewe

Reductive Amination of Dialdehyde Cellulose: Access to Renewable Thermoplastics

Funding Information: We kindly thank Business Finland and UPM-Kymmene Oyj (Finland) for their financial support and UPM-Kymmene Oyj for providing the softwood kraft pulp. We also thank Flavia Fröhlich (IMC University of Applied Sciences, Austria) for her help in preparing the diamine celluloses. The BOKU doctoral school ABC&M and the Austrian Biorefinery Center Tulln (ABCT) are gratefully acknowledged for their support. Publisher Copyright: © 2022 The Authors. Published by American Chemical Society.The reductive amination of dialdehyde cellulose (DAC) with 2-picoline borane was investigated for its applicability in the generation of bioderived thermoplastics. Five primary amines, both aliphatic and aromatic, were introduced to the cellulose backbone. The influences of the side chains on the course of the reaction were examined by various analytical techniques with microcrystalline cellulose as a model compound. The obtained insights were transferred to a 39%-oxidized softwood kraft pulp to study the thermal properties of thereby generated high-molecular-weight thermoplastics. The number-average molecular weights (Mn) of the diamine celluloses, ranging from 60 to 82 kD, were investigated by gel permeation chromatography. The diamine celluloses exhibited glass transition temperatures (Tg) from 71 to 112 °C and were stable at high temperatures. Diamine cellulose generated from aniline and DAC showed the highest conversion, the highest Tg (112 °C), and a narrow molecular weight distribution (Døµ of 1.30).Peer reviewe

Debugging periodate oxidation of cellulose: Why following the common protocol of quenching excess periodate with glycol is a bad idea

Funding Information: We kindly thank Business Finland and UPM-Kymmene Oyj (Finland) for their financial support. We also thank Lisa Ruetz for her help in sample preparation and data collection. The BOKU doctoral school ABC&M and the Austrian Biorefinery Center Tulln (ABCT) are gratefully acknowledged for their support. Publisher Copyright: © 2023 The AuthorsPeriodate oxidation of cellulose to produce “dialdehyde cellulose” (DAC) has lately received increasing attention in sustainable materials development. Despite the longstanding research interest and numerous reported studies, there is still an enormous variation in the proposed preparation and work-up protocols. This apparently reduces comparability and causes reproducibility problems in DAC research. Two simple but prevalent work-up protocols, namely glycol quenching and filtration/washing, were critically examined and compared, resulting in this cautionary note. Various analytical techniques were applied to quantify residual iodine species and organic contaminations from quenching side reactions. The commonly practiced glycol addition cannot remove all oxidising iodine compounds. Both glycol and the formed formaldehyde are incorporated into DAC's polymeric structure. Quenching of excess periodate with glycol can thus clearly be discouraged. Instead, simple washing protocols are recommended which do not bearthe risk of side reactions with organic contaminants. While simple washing was sufficient for mildly oxidised celluloses, higher oxidised samples were more likely to trap residual (per)iodate, as determined by thiosulfate titration. For work-up, simple washing with water is proposed while determining potential iodine contaminations after washing with a simple colorimetric test and, if needed, removal of residual periodate by washing with an aqueous sodium thiosulfate solution.Peer reviewe

Surface analysis of tissue paper using laser scanning confocal microscopy and micro-computed topography

Abstract Tissue paper softness relies on two major factors, the bulk softness, which can be indicated by the elasticity of the sheet, and surface softness. Measurement of surface softness is complicated and often requires a multi-step process. A key parameter defining surface softness is the topography of the surface, particularly the crepe structure and its periodicity. Herein, we present a novel approach to measure and quantify the tissue paper surface crepe structure and periodicity based on the detection of waviness along the sample using laser scanning confocal microscopy (LSM) and X-ray tomography (XRT). In addition, field emission scanning electron microscope (FESEM) was used to characterize the tissue paper surface. We demonstrate that surface topography is directly correlated to the erosion of the doctor blade, which is used to remove the dry tissue paper from the Yankee cylinder. Because of its accuracy and simplicity, the laser confocal microscopy method has the potential to be used directly on the production line to monitor the production process of the tissue paper. XRT revealed more structural details of the tissue paper structure in 3D, and it allowed for the reconstruction of the surface and the internal structure of the tissue paper