Project Abstract

<p>                <b>ABSTRACT</b></p><p> Gum Arabic exudate was collected from Acacia senegal trees around Zaria metropolis, purified in 95% ethanol and its physical and chemical modifications carried out. Physical modification of the gum involved plasticization of the gum with glycerol and ethylene glycol. The chemical method was performed by acid hydrolysis, acetolysis and acetate formation. Appearance of both modifications was observed after three days of drying. Acetic anhydride (AAN), acetolysis (ACT) and ethylene glycol (EGL) modifications became hard and solid, and were ground to powder. Glycerol (GLY) turned very sticky and acid hydrolysis (AHY) turned into a viscous liquid. From characterization of the samples, all modifications were found to be less dense than the pure gum Arabic sample (PGM). AHY sample was found to be more turbid and has the highest conductivity value followed by AAN sample. pH of all samples was found to be below 7.0, indicating acidic nature of the gums. Melting point measurements showed that all test samples have lower melting point values than the pure gum. FTIR and GC-MS spectra of the pure and chemically modified samples were studied. It was found that there were shifts and absorptions at different frequencies, indicating some degree of interaction between the gum and the modifying solvents. The gums were all mixed with PVC at different compositions and cast into films using tetrahydrofuran as solvent. The films produced were subjected to mechanical tests. <br></p>

Project Overview

<p>INTRODUCTION<br>Polysaccharides have been described as high molecular weight polymers formed by condensation of many monosaccharide units or their derivatives. They have also been defined as polymeric substances, the building blocks of which are monosaccharide. From the foregoing, polysaccharides could be said to be long chain carbohydrate molecules built from some monosaccharides such as glucose, rhamnose, galactose, etc. or their derivatives. Polysaccharides could be classified based on their chemical compositions. In this regard a polysaccharide which yields only one type of monosaccharide on hydrolysis is called homoglycan e.g starch, while those which yield two or more types of monosaccarides are called heteroglycan e.g. gum Arabic (Varki et al, 2008).<br>1.1 STRUCTURE OF POLYSACCHARIDES<br>Polysaccharides are normally isolated from their natural environments without much degredation for study. When monosaccharides come together to form polysaccharide, an anomeric hydroxyl group of one monose unit can condense with any hydroxyl group other than that of the former. The few monoses which occur as components of polysaccharides are D-glucose, D-mannose, D-fructose, D-galactose, D-xylose, L-arabinose, D-glucosamine and D-glucuronic acid.<br>1.2 STRUCTURAL MODIFICATION OF POLYSACCHARIDES<br>1.2.1 Starch Acetate<br>Starch acetate derivative has been reportedly prepared by the treatment of starch with acetic anhydride or vinyl acetate (Rutenberg, 1968). The products so formed are low<br>2<br>substituted starch esters with good hydrating capacity, flow, filming and viscosity stability. The process by which this reaction occurs is still not clear, it is however possible that the branches are being substituted by the acetate group. This would lead to smaller side chains which might enhance chain flexibility.<br>1.2.2 Hydroxylethyl ethers<br>Hydroxylethyl starch ethers are products of reaction of ethylene oxide and starch (Rutenberg, 1968). These ethers have properties similar to starch acetates including flexible film formation. The close resemblance of starch and gum Arabic makes the acetate and ether derivatives formation likely to reduce the brittleness of gum Arabic.<br>1.2.3 Acid hydrolysis<br>Acid hydrolysis of polysaccharides normally breaks polysaccharides down to their monomers, dimers and some oligosaccharides. The products of this hydrolysis are low viscous liquids. The acid hydrolysis is believed to fragment the molecules by cleaving bonds other than the (1-6) which is the most resistant to acid hydrolysis (Guthrie 1974; Bochkov, 1979). Acid catalysed hydrolysis of O-glycosides yield corresponding alcohols and some reducing sugars via some protonated intermediates (Bochkov, 1979). This hydrolysis of monosaccharides is a slow reaction with rate constant values in the range of 10-4S-1 to 10-6S-1 at acid concentrations within the range of 0.01M to 0.5M HCl or H2SO4. The rate constant was also shown to increase with increasing acid concentration. It is therefore believed that at very high concentration, the reaction will be very fast.<br>3<br>1.2.4 Acetolysis<br>Acetolysis has been described as the treatment of polysaccharides with a mixture of acetic acid and acetic anhydride in equal weights in the presence of concentrated sulphuric acid. Products of this reaction have their (1-6) link cleaved contrary to acid hydrolysis in which the (1-6) link is very resistant (Govorchenko, 1973). This link is the least resistant to acetolysis (Guthrie, 1974). It is therefore believed that for a polysaccharide which has the (1-6) link as its branching points, the product of such acetolysed polysaccharide will be an almost linear polysaccharide.<br>1.3 PLASTICIZATION<br>Plasticization involves incorporation of plasticizer molecules into a polymer. Plasticizer molecules enter between molecules and separate the polymer molecules. Polar plasticizers, compatible with polysaccharides are attracted to polysaccharide molecules by their polar groups. The bulkiness, configuration and polarity of such plasticizers are such that arrangement of polymer molecule is affected to give room for slide chains and bulky side groups, and allow easy slippage of polymer molecules past one another.<br>PVC can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, electrical cable insulation, inflatable products and many other applications in which it would originally have replaced rubber (Titow, 1984). As an amorphous polymer, PVC resin is extensively formulated to produce an extremely large variety of compounds (Blanco, 2000). Also due to its inexpensive nature and flexibility it is used in plastic pressure pipes systems for pipelines in the water industries. The capability of PVC to perform such diverse<br>4<br>functions is due to its ability to incorporate various additives to suit the numerous applications (Titow, 1984).<br>1.4 BRIEF HISTORY OF PVC<br>The first mention of poly (vinyl chloride) was in 1872 by Baumann when he described formation of a white powder by the action of sunlight on vinyl chloride contained in a sealed tube. The first commercial interest in PVC was shown by the Carbide and Carbon Chemical Corporation, Du Pont and IG Farben who independently filed patents in 1928 (Brydson, 2000). At that stage it was only possible to process homopolymer in the melt state, at temperatures, where high decomposition rates occurred, whereas copolymers could be processed at lower temperatures. Effective so called external plasticization of the PVC homopolymer by incorporating plasticizers was first discovered around 1930. When compounding with dibutyl phthalate and other esters reduction in the softening point PVC occurred, this resulted in rubber-like properties at room temperature (Brydson, 2000). The historical development of PVC is highlighted below according to Titow, (1990). Table 1.1 Historical background of PVC<br>Year<br>History<br>1835<br>Vinyl chloride monomer was first prepared by Regnault<br>1872<br>Baumann discussed the reaction of vinyl halides and acetylene in a sealed tube<br>1921 <br></p><p> 1.7 MECHANISM OF FORMATION<br>Vinyl chloride is a relatively un-reactive monomer, but it responds to free radical<br>initiators of the usual type to form a high polymer. The steps involved in the polymerization<br>are initiation, propagation, chain transfer and termination. The end result of these processes<br>is high molecular weight poly (vinyl chloride), which is quite uniform in a chemical sense<br>but contain a variety of molecular weight species.<br>1.7.1 Initiation<br>There are a series of organic compounds that are readily dissociated at reasonable<br>temperatures to produce free radicals (Nass, 1976). For the polymerization of vinyl chloride,<br>the free radical can be generated from hydroperoxides and Azo compounds. The peroxides<br>decompose by a first-order reaction, giving rise to two radicals, as indicated in the following<br>equation:<br>Examples of peroxides used are dilauryl peroxide dicaproxyl peroide, diisopropyl<br>peroxide and benzoyl peroxide.<br>O<br>O<br>O<br>O<br>O<br>O<br>+ + C<br>CO2<br>When the free radical as produced above is added to the monomer, a new free radical<br>is produced. Phenyl radical adds on the vinyl chloride monomer in the following manner,<br>11<br>C + H2C<br>Cl<br>CH<br>Cl<br>1.7.2 Propagation<br>The new free radical produced in initiation adds on to another monomer molecule to<br>form a new free radical which further adds on another fresh monomer molecule to form<br>another free radical and so on.<br>+<br>H2C<br>Cl<br>CH<br>Cl Cl<br>CH<br>Cl<br>In the course of the reaction, hundreds or even thousands of monomers are added<br>and incorporated into the polymer chain. These molecules, in principle may be added in one<br>of two ways, ―head to tail‖ or head to head‖ <br></p>