Public opinion, government policy and legislation steer science to find renewable
and sustainable chemicals to reduce the burden on the finite resources of the
petrochemical industry. There is growing interest in using natural polysaccharides that are widely distributed in nature to replace man-made polymers. Two typical classes of polysaccharides are starch and hemicelluloses, each containing free hydroxyl groups that are responsible for many of the physical and chemical properties of the substrate, that we have sought to alter by modification. In the field of polysaccharides, the most commonly studied reactions of the hydroxyl group can be broadly classified as esterification, etherification and graft copolymerisation. The common feature in all these reactions is the substitution of a hydrogen atom from the hydroxyl group (OH) of the polysaccharides, and the retention of the carbon-oxygen (C- 0) bond. In view of this, it is interesting from three standpoints to study reactions involving the hydroxyl group.
l. To seek a suitable reaction medium for modification of polysaccharides;
2. To study to what extent complete reaction is possible;
3. To investigate what changes in physical and chemical properties arise from such a reaction.
This project was concerned with the above objectives.
Chemical modification of starches and hemicelluloses focused initially on the
development of a solubilisation technique, since it is known that they hardly dissolve using conventional techniques in organic solvents. A water/solvent exchange method, developed for starches and hemicelluloses has been successfully implemented resulting in the formation of homogeneous reaction solutions.
Subsequent modifications revealed that the degree of substitution of derivatised
materials could be controlled easily by adjusting the amounts of reagent and catalyst.
A series of modified polysaccharides was produced, these derivatives showing the
following advantages: increased hydrophobic character, improved thermal stability and enhanced solubility in organic solvents. A high DS-value lauroylated high amylase starch formed pliable, transparent, lustrous, colourless films, which have IV shown potential for use in plastics, especially biodegradable plastics, films and coatings.
An investigation of graft polymerisation to determine the potential of a starch-bonded acryloyl group as a suitable coupling agent with vinyl monomers was carried out. Potassium persulfate was used as a free-radical initiator using water as the reaction solvent. This work revealed the ability of water as a reaction medium. In this case, perhydroxyl and hydroxyl radicals were easily formed from persulfate to produced free radicals to activate the styrene monomer towards reaction with acryloylated potato starch.
Etherification of polysaccharides has been carried out m anhydrous dimethyl
sulfoxide as a reaction medium, using methyl iodide as a reagent, with sodium
hydroxide as a catalyst.
In addition, a novel method for the extraction and isolation of straw emicelluloses
has been investigated. Employment of an environmentally acceptable hydrogen
peroxide delignification step has replaced the use of chlorinated chemicals. The
preferred method results in isolation of straw hemicelluloses with a low lignin
content, which are bright and white in appearance and which may serve as polyvinyl alcohol substitutes in, for example, the decorative coatings industry.