Assembly characteristics of xylan rich extracts from birch
Licentiate thesis, 2005
The greenhouse effect is one of the most important environmental problems that mankind has to solve. One solution could be the utilization of biomass to create new materials and fuels. Traditionally wood has been used as timber and as a raw material for pulp and paper production. In traditional pulping, lignin and non cellulosic polysaccharides, so called hemicelluloses, are degraded, which is unfortunate since it is possible to use these polymers to create new materials, fuels and special chemicals. In order to increase biomass utilisation, new innovative methods for the isolation and fractionation of wood components must be developed as alternative processes to traditional pulping.
The aim of this thesis has been to isolate hemicellulose fractions from birch and study their assembly characteristics. Xylan, the main hemicellulose in hardwood, was isolated using a two step extraction method. The first step consisted of a prehydrolysis where temperature (100°C-140°C) and time (30-60 min) were varied. The prehydrolysis mediums used were water, acetic acid (0.01-0.1M) or sulphur dioxide (2-6w/w%). The second step was an extraction with sodium hydroxide performed at room temperature. It has been shown that the sample prehydrolysed at 140°C for 1h with 0.01M AcOH gave the best results regarding high yield and a relatively high molecular weight. In addition, this extraction method did not contribute to a great weight loss of the remaining wood. This fraction consisting mainly of Glucuronoxylan and lignin, which contributes to aggregate formation, was used to study the influence of the composition on the behaviour in solution. In order to investigate the influence of lignin, a lignin extraction was performed. Based on results from UV-spectroscopy and NMR, it was concluded that the lignin extraction was successful since the lignin extracted fraction had the same xylose/glucuronic acid ratio as the original fraction but it contained almost no lignin. Photon correlation spectroscopy and cryo transmission electron microscopy showed that, when lignin is removed, the aggregates disappear. This work also includes an adsorption study performed using three different xylan fractions prehydrolysed with different medium (water, acetic acid and sulphur dioxide). It turned out that all fractions adsorbed onto kraft pulp fibres in different amounts. The results also indicated that xylans with less glucuronic acid and more lignin adsorbed onto the kraft pulp fibres to a higher extent. Different xylans also gave rise to different surface morphology on the cellulose surfaces. Generally, fractions containing more lignin gave rise to a rougher surface morphology.
This work contributes to the knowledge of hemicellulose extraction prior wood pulping, the solution behaviour of such fractions and finally the possibility to readsorb such fractions onto pulp. Hopefully this work could lead to a novel pulping process that better utilizes the wood. Future work will include a more thorough investigation of the solution behaviour of xylan rich fractions, since this is of great importance in order to fully understand the adsorption mechanism. When the adsorption mechanism is known it will be possible to create tailor made fibres with different fibre properties.