Preparation and Characterization of Water-Soluble Xylan Ethers
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polymers Article Preparation and Characterization of Water-Soluble Xylan Ethers Kay Hettrich 1,*, Ulrich Drechsler 2, Fritz Loth 1 and Bert Volkert 1 1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany; [email protected] (F.L.); [email protected] (B.V.) 2 Salzenbrodt GmbH & Co KG, Hermsdorfer Str. 70, D-13437 Berlin, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-331-568-1514 Academic Editor: André Laschewsky Received: 30 September 2016; Accepted: 27 March 2017; Published: 31 March 2017 Abstract: Xylan is a predominant hemicellulose component that is found in plants and in some algae. This polysaccharide is made from units of xylose (a pentose sugar). One promising source of xylan is oat spelt. This feedstock was used for the synthesis of two xylan ethers. To achieve water soluble products, we prepared dihydroxypropyl xylan as a non-ionic ether on the one hand, and carboxymethyl xylan as an ionic derivative on the other hand. Different preparation methods like heterogeneous, pseudo-homogeneous, and homogeneous syntheses were compared. In the case of dihydroxypropyl xylan, the synthesis method did not significantly affect the degree of substitution (DS). In contrast, in the case of carboxymethyl xylan, clear differences of the DS values were found in dependence on the synthesis method. Xylan ethers with DS values of >1 could be obtained, which mostly show good water solubility. The synthesized ionic, as well as non-ionic, xylan ethers were soluble in water, even though the aqueous solutions showed slight turbidity. Nevertheless, stable, transparent, and stainable films could be prepared from aqueous solutions from carboxymethyl xylans. Keywords: hemicellulose; xylan ether; solubility; etherification 1. Introduction Native polysaccharides such as cellulose, starch, and most of hemicelluloses are not soluble in cold water without preceding chemical modification. To make them soluble in water, polysaccharide ethers and esters are produced. Such polysaccharide ethers are widely used in everyday water-based applications. Typical examples are carboxymethyl cellulose (CMC), which is the most-used cellulose ether; methyl cellulose (MC), hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), as well as cationic starches, or hydroxyethyl starch (HES) and hydroxypropyl starch (HPS). Cellulose and starch ethers are used as film formers, protective colloids, stabilizers, thickening agents, adhesives or detergent additives in the food, pharmacy, paper and textile industries, as well as in the building and mining industry. Next to cellulose, hemicellulose is the second most abundant class of polysaccharides found in nature. It is the third key structural ingredient of the wooden cell-wall, besides cellulose and lignin. Furthermore, hemicellulose is found in vegetable fibers and in cell-walls of grass and corn. Thus, hemicellulose is an attractive raw renewable resource for new biobased materials and functional polymers that can be adapted by chemical reactions. In contrast to cellulose, hemicellulose is a non-crystalline heteropolysaccharide, which can be extracted from plant or wood by water and/or aqueous alkali [1–5]. Depending on the phylogeny, hemicellulose can be comprised of glucans, xyloglucans, xylans, arabinoxylans, mannans, gluco- or galactomannan with different substitution patterns [6]. Accordingly, hemicellulose may contain C5 or C6 sugar units. Polymers 2017, 9, 129; doi:10.3390/polym9040129 www.mdpi.com/journal/polymers Polymers 2017, 9, 129 2 of 13 Xylans, for example, are a prevalent hemicellulose component found in plants and in some algae, whichPolymers are made 2017, 9,from 129 units of xylose (C5 sugar units) (Figure1). Xylans compose about 10–15%2 of 13 of softwoods, about 10–35% of hardwoods and 35–40% of the total mass in the residues of annual plants, Xylans, for example, are a prevalent hemicellulose component found in plants and in some e.g., in husks or oat spelt, which are the residues of oat flake production. Xylan shows characteristic algae, which are made from units of xylose (C5 sugar units) (Figure 1). Xylans compose about 10– differences in comparison with cellulose or starch, such as a lower degree of polymerization (DP); 15% of softwoods, about 10–35% of hardwoods and 35–40% of the total mass in the residues of this meansannual lowerplants, chaine.g., in length husks or and oat lowerspelt, which molar ar masses,e the residues and theof oat absence flake production. of the primary Xylan OH-groupshows in positioncharacteristic C6. Note differences that, in thein comparison case of C6 polysaccharides,with cellulose or thestarch, primary such OH-groupas a lower atdegree position of C-6 in thepolymerization anhydroglucose (DP); unit this (AGU) means lower is the chain most length reactive and group lower inmolar comparison masses, and with the the absence two secondaryof the OH-groupsprimary at OH-group position C-2in position and C-3. C6. Therefore, Note that, the in aim the of case this of study C6 polysaccharides, was to explore ifthe it isprimary possible to modifyOH-group xylan chemically at position by C-6 methods in the comparableanhydroglucose to those unit that(AGU) are establishedis the most forreactive the modification group in of polysaccharidescomparison with at an the industrial two secondary scale, onOH-groups the one hand;at position and C-2 to achieve and C-3. water Therefore, soluble the xylanaim ofproducts this similarstudy to cellulose was to explore or starch if it derivatives,is possible to on modify the other xylan hand. chemically by methods comparable to those that are established for the modification of polysaccharides at an industrial scale, on the one hand; The structure of xylan varies depending on its herbal origin. The different structures have and to achieve water soluble xylan products similar to cellulose or starch derivatives, on the other been summarized in several reviews on hemicelluloses in wood [7,8], grass [9], and cereals [10,11]. hand. Some aspectsThe aboutstructure the of structure xylan varies of xylan depending from oaton its spelts herbal were origin. discussed The different by Saake structures et al. [12have]. Oatbeen spelts containsummarized high amounts in several of xylanreviews and on havehemicelluloses relatively in low wood lignin [7,8], content.grass [9], and Hence, cereals this [10,11]. raw material Some is an interestingaspects about source the forstructure the isolation of xylan of from xylan. oat Aspelts proposal were discussed for the structure by Saake of et xylan al. [12]. in Oat annual spelts plants is showncontain in high Figure amounts1[ 9]. of The xylan monomer and have relatively units are low linked lignin together content. byHence,β-(1 this! raw4) glycosidicmaterial is an bonds. Typicalinteresting for xylan source is the for presence the isolation of sideof xylan. chains A proposal made of for arabinofuranose, the structure of xylan xylopyranose, in annual plants rhamnose, is glucuronicshown acid,in Figure and 1 [9]. acetyl The groups.monomer Mostunits are of theselinked sugartogether residues by β-(1 → are 4) connectedglycosidic bonds. by acetal Typical bonds, whilefor the xylan acetyl isgroups the presence are connected of side bychains ester made bonds. of Typicalarabinofuranose, degreeof xylopyranose, substitution (DS)rhamnose, values of glucuronic acid, and acetyl groups. Most of these sugar residues are connected by acetal bonds, these side groups are between 0.02 and 0.2 in the case of arabinose, between 0.02 and 0.08 of glucuronic while the acetyl groups are connected by ester bonds. Typical degree of substitution (DS) values of acid, <0.02these side of all groups other sugarare between units, and 0.02 between and 0.2 0.02in the and case 0.15 of of arabinose, acetyl groups between [13]. 0.02 Usually, and 0.08 the contentof of acetylglucuronic groups acid, in extracted<0.02 of all xylan other is sugar much units, lower and because between the 0.02 ester and bonds0.15 of are acetyl cleaved groups during [13]. the alkalineUsually, extraction. the content of acetyl groups in extracted xylan is much lower because the ester bonds are Despitecleaved during promising the alkaline application extraction. possibilities, xylan derivatives have not been used in quantities large enoughDespite for promising industrial application processing possibilities, up until now.xylan derivatives Because of have the not possibility been used of in isolating quantities xylan fromlarge residues enough of annual for industrial plants, proces it is asing particularly up until cheapnow. Because biobased of rawthe possibility material whichof isolating can be xylan used for differentfrom applications. residues of annual A typical plants, example it is a particularly is oat spelt, cheap which biobased up until raw nowmaterial has which been mostlycan be used used for generationfor different of energy. applications. While A they typical have example been also is oat used spelt, as which additives up until for now animal has feedbeen similarmostly used to cereal for generation of energy. While they have been also used as additives for animal feed similar to straw, the EU Food Safety Regulation 178/2002 makes it increasingly difficult for the food industry cereal straw, the EU Food Safety Regulation 178/2002 makes it increasingly difficult for the food to deliverindustry mill to products,deliver mill such products, as oat such spelt, as oat to thespelt,