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LITERATURE REVIEW 2.1 Biopolymers
Biodegradability is dependent on the chemical nature of the material and the constitution of the final product. Biodegradable plastics can be synthetic or natural polymers. Natural biodegradable polymers are based on renewable resources, whereas synthetic biodegradable polymers are petroleum-based. Biodegradation is degradation caused by biological activity, such as enzyme action, which leads to major changes in the chemical structure in a given time period into simple molecules, such as carbon dioxide and water [Billmeyer, 1966].
Starch structure and propertie
Native starch is the term used to describe starch in the form in which it occurs in plants, such as potatoes, wheat, cassava, rice and maize. In plants, starch occurs in the form of granules. The granules vary in shape, size and relative proportions of amylose and amylopectin, depending on the source of the starch. Starch is therefore described by its plant source as cornstarch, potato starch, tapioca starch, etc. [Souza & Andrade, 2001].
Starch is composed of carbon, hydrogen and oxygen in the ratio of 6:10:5 [C6H10O5], placing it in the class of carbohydrate organic compounds. Starch is considered to be a polymer of glucose, with the linkages between the glucose units being formed as if condensation has taken place. The glucose units are connected through an oxygen atom, connecting through carbon atom 1 of one glucose unit to carbon atom 4 of the next glucose unit, forming a long chain of interconnected glucose units. This linkage of one glucose unit to another one through the C-1 oxygen atom is called the glycoside bond [Souza & Andrade, 2001].
Starch in its native state consists of a mixture of two polysaccharides: amylose and amylopectin. Amylose is a linear polymer, while amylopectin is highly branched. The glucose units in amylose are connected to each other through 1-4 linkages [Hulleman et al., 1998]. The relative amounts of these two polymers in a particular type of starch determine the properties of the starch. The molecular structures of amylose and amylopectin are given in Figures 1 and 2 respectively [Bello-Perez & Paredes-Lopez, 1995]. Typical starch compositions are given in Table 1.
Starch gelatinisation
In its native from, granular starch is partially crystalline. When dry starch granules are heated, thermal degradation occurs before the granular crystalline melting point is reached. As a result, starch cannot be melt-processed in its native form. In order to melt-process native starch, the hydrogen bonds holding the starch molecules together have to be reduced. The reduction of starch hydrogen bonding can be achieved in the presence of a solvent, such as water. When starch is heated in an aqueous medium, the phase transition forms an ordered to disordered state called gelatinisation [Kim et al., 1997].
The properties of starch in water are the bases on which starch can be melt-processed. When starch is heated with the solvent at a critical temperature, the solvent interacts with the starch hydroxyl groups, thus reducing the hydrogen bonding among the starch molecules. This allows individual chains to move freely relative to each other, thus allowing starch to be melt-processed. The critical temperature at which this phenomenon occurs is called the gelatinisation temperature [Willett & Doane, 2002].
Starch modification
Starch modifications are carried out in order to provide products with the required end-use properties. These modifications are aimed at changing the gelatinisation characteristics, solids–viscosity relationship, gelling tendency of starch dispersions and hydrophilic character, and at introducing ionic character [Souza & Andrade, 2001].
Modifications aimed at changing the amylose/amylopectin content
Hybrid breeding has been the most successful way of developing starches containing mainly amylopectin. These waxy starches have been available since 1942.
Modification by controlled degradation
This process involves the scission of the starch molecules to fragments of lower molecular weight. Commercially, the conversion is carried out by the action of oxidising agents, acids and/or heat.
Oxidation
Native starch can be treated with a variety of oxidising agents. The agent most commonly used is sodium hypochlorate. In the first stage of oxidation, the starch chain is hydrolysed into shorter fragments, thus reducing the molecular mass. This results in a reduction in the viscosity of the system. The starch hydroxyl groups are oxidised to aldehyde and ketones. The introduction of the carbonyl group into the amylose molecules reduces retrogradation [African Products, s.a.].
Acid-converted starches
In this process starch is treated with a mineral acid at low temperature, below the starch gelatinisation temperature, to keep the granules intact. Under these conditions, the acid hydrolyses the starch, breaking the linkages between glucose monomers to yield shorter chains. Hydrolysis initially takes place at the branching points of amylopectin, producing a starch that has a higher proportion of linear molecules. Since the polymer chains have been shortened, the starches have a lower molecular mass than unmodified starches, as well as low viscosities [African Products, s.a.].
Pregellatinisation
Pregellatinised starches are prepared by cooking and drying starch slurries in heated drums or by means of extrusion. Since the granular structure has been disrupted, this process produces starches that swell in cold water [Souza & Andrade, 2001].
Cross-linking
The gellatinisation and swelling properties of the starch granule can be modified by the addition of a cross-linking agent. This is done by reacting a starch with chemicals containing more than one functional group, which are able to react with at least two hydroxyl groups. The most common cross-linking agents used in starch are linear dicarboxylic acid anhydrides (e.g. adipic acid) or phosphates (e.g. phosphorous oxychloride trimetaphosphates) [Souza & Andrade, 2001].
Cationisation
Starch cationisation is performed by chemical means in changing the electrical charge from slightly negative to positive. The cationisation process is done by substituting the hydrogen atoms on the starch molecules with quaternary ammonium chemical groups. The cationic activity in the starch derivative results from the positive charge on the ammonium ion. The number of cationic groups per glucose molecule determines the degree of substitution. The degree of substitution varies from 0.01 to 0.10 [Valle et al., 1991]. Cationic starches are of large-scale importance in industry due to their affinity for negatively charged substrates (cellulose and other fibres) [Souza & Andrade, 2001].
Acetylating
Starch esterification proceeds either by direct reaction with carboxylic acids or by indirect reaction with carboxylic acid derivatives. Common reagents used in the esterification of starch are acetic anhydride, acetic anhydride-pyridine, ketene, vinyl acetate, acetic acid and acetic anhydride–acid. Direct acid esterification is proton-catalysed, with the formation of a starch ester and water. Indirect esterification uses nucleophilic substitution at the unsaturated carbon atom. Acetylated starches of commercial importance are the derivatives of low substitution, since the process preserves the granular structure of starch molecules [Souza & Andrade, 2001].
ABSTRACT
ACKNOWLEDGEMENTS
LIST OF FIGURES
LIST OF TABLES
DEFINITIONS .
LIST OF ABBREVIATIONS.
1 INTRODUCTION
1.1 SCOPE OF THE WORK
2 LITERATURE REVIEW
2.1 BIOPOLYMERS
2.2 STARCH STRUCTURE AND PROPERTIES
2.3 STARCH GELATINISATION.
2.4 STARCH MODIFICATION
2.5 STARCH AS A THERMOPLASTIC MATERIAL
2.6 STARCH-FILLED PLASTICS
2.7 STARCH BLENDS
2.8 STARCH-BASED NANO-COMPOSITES
3 EXPERIMENTAL
3.1 EXPERIMENTAL DESIGN
3.2 MATERIALS
3.3 SAMPLE PREPARATION
3.4 CHARACTERISATION
4 RESULTS AND DISCUSSION
4.1 THERMOPLASTIC STARCH
4.2 TPS-PVB BLENDS
4.3 PVB-POLYAMIDE BLENDS
4.4 TPS-POLYAMIDE BLENDS..
4.5 TPS–PVB-POLYAMIDE SYSTEMS .
4.6 TPS–PVB-ANHYDRIDE SYSTEMS
5 CONCLUSIONS
REFERENCES
APPENDICES
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