Homogeneous methylation of wood pulp cellulose dissolved in LiOH/urea/H2O

doi:10.1016/j.eurpolymj.2010.05.009

European Polymer Journal

Volume 46, Issue 8, August 2010, Pages 1726-1735

https://doi.org/10.1016/j.eurpolymj.2010.05.009 Get rights and content

Abstract

Spruce sulphite cellulose (number average degree of polymerization 620) dissolved in an aqueous solution of 8% (w/w) LiOH*H₂O and 12% (w/w) urea was methylated with dimethyl sulphate (DMS). By varying the reaction temperature between 22 and 50 °C, the molar ratio between 9 and 15 mol DMS per mol anhydroglucose unit, and the reaction time from 4 to 24 h, methyl cellulose (MC) with degree of substitution (DS) values in the range of 1.07 and 1.59 was prepared. The chemical structure of MC was analysed by FTIR and ¹H NMR spectroscopy. The turbidity (given in nephelometric turbidity units, NTU) of the aqueous solution of MC reached an optimum of 10 NTU for a product obtained with 12 mol DMS/mol AGU at 50 °C. GPC measurements revealed polymer degradation to a certain extent. The intrinsic viscosity and the Huggins constant k of the MC samples increased with increasing DS value. The MC samples possess k values higher than 0.8, indicating association of the polymer chain. The zero-shear viscosity decreased with increase of both temperature and the amount of methylation agent due to the depolymerization. During the heating/cooling cycle (20–90 °C) of the aqueous solutions of MC, it was observed that samples synthesized at 22 °C with DS values lower than 1.3 did not undergo phase separation in aqueous solution. Phase separation hysteresis with a precipitation temperature up to 80 °C was obtained for aqueous solutions of MC with DS values between 1.07 and 1.66 synthesized at higher temperatures. The functionalization pattern determined by GLC of the corresponding partially methylated glucitol acetates is close to randomness and comparable with those of commercial MC samples.

Graphical abstract

Methyl cellulose (MC) of different degree of substitution was prepared by conversion of cellulose dissolved in aqueous solution containing urea and lithium hydroxide. GLC analyses as well as enzymatic degradation experiments revealed a nearly random functionalization pattern that is comparable with those of commercially available MC. The samples are water soluble and possess a characteristic thermal flocculation behaviour.

Introduction

Cellulose is the most abundant natural polymer. Due to its origin from renewable resources and its non-toxicity, derivatives of the polymer have found applications in many fields. Methyl cellulose (MC) is widely used as a binder, thickener, as emulsifier, in glues, paints, and construction materials [1]. Recently, MC has become even more interesting as “intelligent” materials due its ability to exhibit thermo-reversible phase separation like gelation or flocculation. Commercial MC is synthesized under heterogeneous conditions applying methyl chloride in the presence of sodium hydroxide, which yields a statistically functionalized polymer. However, homogeneous derivatization possesses the enormous advantage of full availability of the hydroxyl groups with respect to their inherent reactivity. Thus, a better control of the degree of substitution and a uniform distribution of the functional groups along the polymer chain can be achieved. Typical solvents used in the cellulose chemistry are N,N-dimethyl acetamide combined with LiCl [2], dimethyl sulphoxide/tetrabutylammonium fluoride trihydrate [3] and very recently ionic liquids, in particular 1-N-butyl-3-methylimidazolium chloride [4]. Moreover, the dissolution of cotton cellulose in aqueous LiOH or NaOH in combination with urea is found to be a good solvent for this biopolymer [5], [6], [7]. Cotton is very pure cellulose without hemicellulose and lignin. These characteristics contribute to the relatively easy dissolution in aqueous LiOH or NaOH in combination with urea. Cellulose ethers like methyl- and hydroxyethyl cellulose are synthesized in this new solvent [8], [9]. However, commercially available cellulose derivatives are synthesized from wood cellulose that may contain other components beside cellulose, namely hemicelluloses and lignin. Therefore, the dissolution of cellulose from spruce in the solvent alkali hydroxide/urea/water seems to be more difficult. Thus, it is a challenge to study the dissolution and the subsequent derivatization of wood cellulose using the LiOH/urea/H₂O solvent.

This work reports about dissolution and methylation of spruce sulphite pulp cellulose in LiOH/urea/H₂O using dimethyl sulphate. The methyl celluloses obtained are characterized by means of FTIR- and NMR spectroscopy, GPC, turbidity measurements, and determination of zero-shear viscosity. Also temperature dependent phase separation measurements and enzymatic hydrolysis of MC to determine the substitution pattern along the polymer backbone are carried out.

Section snippets

Materials

The spruce sulphite pulp used was kindly provided by Dow Wolff Cellulosics and possesses a number average degree of polymerization (DP_n) of 620 measured by GPC in tetrahydrofuran (THF) after percarbanilation. The hemicellulose content is about 5% obtained after acidic depolymerization followed by HPLC measurement [10]. Cellulase from Trichoderma reesei and cellobiase from Aspergillus niger were purchased from Sigma–Aldrich. All other reagents were supplied by Fluka and used as received.

Measurements

Dissolution of spruce sulphite pulp cellulose

The alkali hydroxide/urea/H₂O mixture possesses a higher dissolution power in case of LiOH compared to NaOH [5]. Preliminary studies revealed that the mixture containing NaOH does not dissolve the spruce sulphite cellulose studied. Therefore, LiOH was chosen as base in order to ensure the complete dissolution of the cellulose. The optimum concentration of LiOH was found to be 8% (w/w) combined with 12% urea (w/w). Cellulose was dissolved by adding the biopolymer to the frozen LiOH/urea/H₂O

Conclusion

Spruce sulphite pulp cellulose with a DP_n of 620 could be dissolved in water containing 8% (w/w) LiOH and 12% (w/w) urea via the ballooning phase. Methylation was carried out with dimethyl sulphate (DMS) at different temperature (22–50 °C) and reaction time (4–24 h). The methyl cellulose (MC) samples obtained possess DS_NMR values from 1.07 to 1.59 (DS_GLC 1.19–1.66) and are water soluble with a low content of insolubles. The turbidity of aqueous solution of MC samples reached an optimum of 10

Acknowledgements

The authors thank Dow Wolff Cellulosics and the European Polysaccharide Network of Excellence (EPNOE) for financial support. K.V. and P.M. thank the Federal Ministry for Education and Research, Germany (BMBF), FKZ 0330837A, for financial support.

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¹: Member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu.

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Homogeneous methylation of wood pulp cellulose dissolved in LiOH/urea/H2O

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Measurements

Dissolution of spruce sulphite pulp cellulose

Conclusion

Acknowledgements

Carbohydr Polym

Carbohydr Res

Carbohydr Res

J Petroleum Sci Eng

Carbohydr Polym

Carbohydr Res

Entwicklungen in der Herstellung und Anwendung von celluloseethern

Papier

Solution studies of cellulose in lithium chloride and N,N-dimethylacetamide

Macromolecules

Effective preparation of cellulose derivatives in a new simple cellulose solvent

Macromol Chem Phys

Acylation and carbanilation of cellulose in ionic liquids

Green Chem

Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions

Macromol Biosci

Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Cellulose

Multifilament fibers based on dissolution of cellulose in NaOH/urea aqueous solution: structure and properties

Adv Mater

Homogeneous methylation of wood pulp cellulose dissolved in LiOH/urea/H₂O