Birch xylan modification by lactide grafting

Abstract: 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.