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Birch xylan modification by lactide grafting
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymer Technology.
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The wood based biorefinery concept has led to an increasing interest in the possibility of using hemicelluloses as a source for value added products. Hemicelluloses are dissolved in large amounts in the process liquor during kraft pulping. In particular, hardwood 4-O-methylglucuronoxylan (xylan) has been shown to be highly stable towards the harsh conditions used in kraft pulping procedures and it can therefore be isolated with intact polymeric features. Xylan is the most abundant hemicellulose found in plant cells and this makes it very interesting as a novel biorefinery product. Some of the challenges of utilizing xylan in value-added applications lie in its lack of both film-forming ability and thermal processability. Chemical modification of the hardwood hemicellulose is one way to overcome these weaknesses.

In this work, birch xylan, isolated from a kraft cooking liquor, has been modified by grafting with lactide. Only a small degree of modification was sufficient to give a film-forming ability. Thin films of the modified xylan had a tensile strength of up to 48 MPa. The lactide-grafted xylan also had properties suitable for thermal processing.

The grafting was achieved by ring-opening polymerization of lactide onto the xylan backbone. Two different catalysts were employed: stannous octoate and triazabicyclodecene. With stannous octoate, the xylan could be modified into lactidegrafted xylan with oligolactide branches. By optimizing the synthesis procedure with triazabicyclodecene as catalyst, the yield was almost quantitative. This also enabled polylactide-graft-xylan copolymers with a specific branch length to be produced.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 32 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2012:18
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-93804ISBN: 978-91-7501-310-7 (print)OAI: oai:DiVA.org:kth-93804DiVA: diva2:524101
Presentation
2012-05-21, STFI-salen, Innventia, Drottning Kristinas väg 61, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120503Available from: 2012-05-03 Created: 2012-04-27 Last updated: 2012-05-03Bibliographically approved
List of papers
1. Modification of birch xylan by lactide-grafting
Open this publication in new window or tab >>Modification of birch xylan by lactide-grafting
2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 3, 518-524 p.Article in journal (Refereed) Published
Abstract [en]

4-O-methylglucuronoxylan, isolated by ultrafiltration from a birch kraft cooking liquor, was delignified and subsequently modified by grafting lactide-chains onto the hydroxyl groups via ring opening polymerization with L-lactide. The structures and average molar masses of the lactide-grafted xylan polymers were characterised by NMR and SEC respectively. By varying the lactide-to-xylan feed ratio, graft polymers with different graft levels were synthesized. The degree of substitution of the hydroxyls in the xylan back-bone ranged from 0.7 to 1.7 with lactide side-chains having an average length of 1.2 to 2.5 lactide units. The side-chain length and degree of substitution influenced the hydrophobicity, thermal properties and tensile strength properties. A glass transition could be detected for lactide-grafted xylan polymers with lactide chains longer than 1.8 lactide units. Solution-cast thin films prepared from lactide-grafted xylans exhibited strong tensile strength and high modulus with decreasing strength and increasing elongation at break as the lactide chain length was increased.

Keyword
Hemicellulose, Xylan, Grafting, Lactide, Glass transition, Tensile strength
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-85778 (URN)000311020100001 ()2-s2.0-84865223617 (Scopus ID)
Note

QC 20121214

Available from: 2012-02-13 Created: 2012-02-13 Last updated: 2017-06-14Bibliographically approved
2. Birch xylan grafted with pla branches of predictable length
Open this publication in new window or tab >>Birch xylan grafted with pla branches of predictable length
2012 (English)In: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 7, no 3, 3640-3655 p.Article in journal (Refereed) Published
Abstract [en]

Birch xylan (4-O-methylglucuronoxylan) isolated from a kraft cooking liquor was delignified and grafted with polylactide of predictable branch length. This graft copolymerization resulted in very high total yields, greater than 90%, and with less than 10% polylactide homopolymer byproducts. Mild reaction conditions (40 degrees C, 5 to 120 minutes) were used, which was believed to limit transesterification reactions and thus make it possible to reach good predictability of the polylactide branch length. The thermal properties of the polylactide-grafted xylan depended on the branch length. Short branches resulted in fully amorphous materials with a glass transition temperature of about 48 to 55 degrees C, whereas long polylactide branches resulted in semi-crystalline materials with melting points of about 130 degrees C. Using mixtures of L-lactide and D/L-lactide in the monomer feed further altered the thermal properties. The degradation temperatures of the polylactide-grafted xylans were higher than that of the unmodified xylan, with degradation temperatures of about 300 degrees C and 250 degrees C, respectively. Tensile testing showed increased elongation at break with increasing branch length. The proposed method thus enables tailor-making of copolymers with specific thermal and mechanical properties.

Keyword
Hemicellulose, Xylan, Grafting, Lactide, ROP, Glass transition, Melt transition; Tensile strength
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-93970 (URN)000307639900073 ()2-s2.0-84865748399 (Scopus ID)
Note

QC 20120924. Updated from manuscript to article in journal.

Available from: 2012-05-03 Created: 2012-05-03 Last updated: 2017-12-07Bibliographically approved

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