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Is there a diffusion of alkali in the activation of dissolving cellulose pulp at low NAOH stoichiometric excess?
Umeå University, Faculty of Science and Technology, Department of Chemistry.
AkzoNobel, Stenungsund, Sverige.
AkzoNobel, Arnhem, The Netherlands.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2019 (English)In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 26, no 2, p. 1297-1308Article in journal (Refereed) Published
Abstract [en]

We conducted a quantitative study, following the degree of activation (i.e. the transformation to alkali cellulose, denoted as DoA) over time for dissolving cellulose pulp treated with different [NaOH] at low NaOH/anhydroglucose unit stoichiometric ratio (denoted as (r) ≤ 2.6). Our quantitative approach was based on Raman spectroscopy data, evaluated by partial least squares regression modelling. The results show strong influence of the (r) on DoA (increasing from DoA= 45% at (r) = 0.8, to DoA = 85% at (r) = 2.6), and its complex dependence on [NaOH]. At (r) = 0.8 the highest DoA (DoA ≳ 60%) was found at 30% [NaOH], while at (r) =1.3 it was found at 20% [NaOH] (DoA ≳ 80%). Although activation of cellulose happens in minutes at the studied temperature (30 °C), it was found that the reaction may be slow when a low (r) is used. A gradual increase of the DoA from ≈ 30% to ≈ 70% in time was seen when samples were activated with 30% [NaOH] at (r) = 0.8. At the same (r), a similar increase of DoA from ≈ 30 % to ≈ 60 % was also observed when 40% [NaOH] was used. Slow diffusion of NaOH through poorly swollen cellulose fibres is proposed as an explanation for this phenomenon. Lastly, solid-state CP/MAS NMR measurements suggest that at a fixed temperature, the Na-Cell allomorph mostly depends on [NaOH]. However, in the transition area between Na-Cell I and Na-Cell II, its influence might be affected by (r). 

Place, publisher, year, edition, pages
Springer, 2019. Vol. 26, no 2, p. 1297-1308
Keywords [en]
Alkali cellulose, Activation, Polymorphs, Cellulose ether, Diffusion
National Category
Other Chemistry Topics Materials Engineering
Identifiers
URN: urn:nbn:se:umu:diva-153345DOI: 10.1007/s10570-018-2104-9ISI: 000459455200045OAI: oai:DiVA.org:umu-153345DiVA, id: diva2:1263776
Projects
Bio4EnergyAvailable from: 2018-11-16 Created: 2018-11-16 Last updated: 2019-08-30Bibliographically approved
In thesis
1. Experimental study of alkalinisation of cellulose in industrial relevant conditions
Open this publication in new window or tab >>Experimental study of alkalinisation of cellulose in industrial relevant conditions
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mercerisation of cellulose pulp with a solution of NaOH is the first step of manufacturing cellulose-based value-added products, e.g. viscose fibres and cellulose ethers. During this process, cellulose transforms into a swollen crystalline structure, alkali cellulose (Na-Cell). This increases the reactivity of the cellulose and makes it more accessible for reagents to penetrate and react with the hydroxyl groups. The mercerisation conditions are known to affect the degree of alkalinisation of cellulose as well as the final products. The main objective of this thesis is to investigate how the alkalinisation of softwood sulphite dissolving cellulose pulp is influenced by the co-variation of process variables in the mercerisation in industrial relevant conditions, for both viscose and cellulose ether production. This objective was achieved by quantitative analysis of the effect of simultaneous variation of a set of key parameters on the degree of alkalinisation (i.e. degree of activation, DoA) of the chosen dissolving pulp. Quantitative measurements were performed using Raman spectroscopy data, evaluated by partial least squares (PLS) regression. For mercerisation at viscose production conditions, the effect of studied variables on mass yield was also considered. In the case of mercerisation at ether processing conditions, formation of alkali cellulose at a fixed temperature was included. The knowledge obtained on mercerisation under ether processing conditions was then applied for preparation of the ionic cellulose ether carboxymethylcellulose (CMC).

The overall results show that temperature has a strong effect on DoA and mass yield for mercerised samples under steeping conditions. Measured DoA decreases as the temperature increases from 20 to 70 °C. Mass yield correlates positively with the temperature up to 45-50 °C in the PLS model, after which the relation is negative. The [NaOH] and reaction time show a complex dependence of other variables. At mercerisation conditions for cellulose ether production, the NaOH/AGU stoichiometric ratio, denoted as (r), shows to be very important for DoA, with a positive correlation. At these mercerisation conditions, temperature shows no effect on DoA. The influence of the [NaOH] (which also refers to the concentration of water) shows a complex dependence on (r). As (r) increases and [NaOH] decreases, the measured DoA increases. Prolonged mercerisation time shows no significance in the modelled DoA. However, a gradual increase of the DoA over time was seen when mercerisation was performed with 30% and 40% [NaOH] at (r) = 0.8, suggesting a slow diffusion of NaOH and Na-Cell formation. 13C CP-MAS NMR measurements of samples produced at room temperature show that formation of the Na-Cell allomorph is mainly determined by the [NaOH]. However, in the transition area between Na-Cell I and Na-Cell II, (r) also seems to be of importance.  An increase of DS in the produced CMC samples also shows to be consistent at such conditions with the increase in the measured DoA and with increased (r) and decreased [NaOH]. However, these conditions also favour the formation of by-products. In the synthesised CMC samples, a DS of up to 0.7 was achieved. Measured solubility was lower than expected for any given DS. This, along with the non-statistical distribution of monomer units in the polymer chains, indicates high heterogeneity in the synthesised samples. The distribution of substituents within the AGU shows attachment to hydroxyl oxygens in the order O3 < O2 ≈ O6. The relative importance of the substitution at O3 indicates an increase at this position when [NaOH] increases.

The models presented in this thesis will hopefully serve as a basis for predicting the effects of the studied variables on the DoA, as well as on the mass yield of cellulose pulp when mercerisation conditions are adjusted. Moreover, it is believed that the presented studies can give a better understanding of mercerisation at cellulose ether conditions, hence enabling further development of this process step.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2018. p. 81
Keywords
Mercerisation, Alkalinisation, Cellulose I, Cellulose II, Alkali cellulose, Viscose, Cellulose ether, Carboxymethylcellulose, Sodium glycolate, Sodium diglycolate, Raman spectroscopy
National Category
Chemical Engineering Polymer Chemistry
Identifiers
urn:nbn:se:umu:diva-153412 (URN)978-91-7601-987-0 (ISBN)
Public defence
2018-12-14, KB.E3.01 (Lilla hörsalen), KBC-huset, Umeå, 10:00 (English)
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Supervisors
Available from: 2018-11-23 Created: 2018-11-19 Last updated: 2019-01-10Bibliographically approved

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Albán Reyes, Diana CarolinaSundman, Ola

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