Starch as Source of Polymeric Materials

Department of Materials Engineering Engineering School of Sao Carlos, University of Sao Paulo 

Starch is the major carbohydrate reserve in higher plants and has been a material of choice since the early days of human technology. Recently starch gained new importance as a raw material in the production of plastics, in particular, for the synthesis of monomers to produce polymers such as polydactic acid) and, after chemical modification (e.g. esterification) and thermomechanical processing, to produce thermoplastic starch. This chapter gives a general overview of the most recent research on the development of materials from starch, focusing on thermoplastic starch and the perspectives for future development in this field. A brief review on reactive extrusion of thermoplastic starch is also provided. 

Keywords Starch, thermoplastic starch, biodegradable polymers, reactive extrusion 

Susheel Kalia and Luc Avctous (eds.) Biopolymers Biomedical and Environmental Applications, (81-98) Scrivener Publishing LLC 

Starch can be obtained in a great variety of crops, the choice of the botanical resource depending mainly on geographic and climatic factors and on the desired functional properties of the extracted starch [5]. It is always possible to find a highly productive plant to produce starch whatever the climate and agricultural conditions, such as maize in temperate and subtropical zones, cassava (the same as manioc or tapioca) in tropical regions, rice in inundated areas, and wheat or potatoes in temperate and cold climates. The main plant sources are maize, rice, wheat, potatoes and cassava [6]. In the year 2005, worldwide starch production accounted for approximately 58 million tons. 

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Starch is used today in a large number of products with very diverse properties. The roles played by starch include load bearing polymeric material, a component of biodegradable blends, a filler in biodegradable materials, and even a role of plasticizer. Its popularity stems from a low price, being derived from a renewable source, being familiar component fonnd in the natural environment and as such easily convertible by natural means, chemically reactive, and compatible with many materials. 

This chapter reviews the general context of starch as a material. After a survey of the major sources of starch and their characteristic compositions in terms of amylase and amylopectin, the morphology of the granules and the techniques applied to disrupt them are critically examined. The use of starch for the production of polymeric materials covers the bulk of the chapter, including the major aspect of starch plasticization, the preparation and assessment of blends, the processing of thermoplastic starch (TPS), the problems associated with its degradation and the preparation of TPS composites and nanocomposites. The present and perspective applications of these biodegradable materials and the problems associated with their moisture sensitivity conclude this manuscript. 

Besides a source of energy, organisms require a source of materials for biosynthesis of cellular matter and products in cell operation, maintenance and reproduction. These materials must supply all the elements necessary to accomplish this. Some microorganisms utilize elements in the form of simple compounds, others require more complex compounds, usually related to the form in which they ultimately will be incorporated in the cellular material. The four predominant types of polymeric cell compounds are the lipids (fats), the polysaccharides (starch, cellulose, etc.), the information-encoded polydeoxyribonucleic acid and polyribonucleic acids (DNA and RNA), and proteins. Lipids are essentially insoluble in water and can thus be found in the nonaqueous biological phases, especially the plasma and organelle membranes. Lipids also constitute portions ofmore complex molecules, such as lipoproteins and liposaccharides. Lipids also serve as the polymeric biological fuel storage. 

The over growing environmental pressure caused by the wide spread consumption of petroleum based polymers and plastics has hastened the development of biodegradable and environmentally acceptable materials. Biopolymers derived from various natural resources such as proteins, cellulosics, starch and other polysaccharides are regarded as the alternate materials. Biodegradable polymeric materials derived from renewable sources are the most promising materials because of their easy availability and cost effectiveness. Biodegradable modified polysaccharides have been found to possess varied applications such as salt resistant absorption of water [109]. 

There are various ways to classify polymers. A simple way is to distinguish polymers with respect to their origin in synthetic and natural polymers. Natural polymeric materials such as shellac, cellulose, and natural rubber have been used for centuries. Natural polymers are a class of polymers derived from renewable biomass sources, such as plants, vegetable oil, com starch, pea starch. Generally, natural polymers (or biopolymers) are used after modification reactions. Some biopolymers are designed to biodegrade. Table 2.1.1 and Fig. 2.1.1 give examples of natural polymers and modified natural polymers. 

The naturally biodegradable polymers such as starch, chitosan and cellulose derived from natural sources have produced a number of interesting NR blends and IPNs. These blended systems have an advantage in that they create fewer waste disposal problems compared to the petroleum based polymeric materials. The use of stareh blends to enhance the biodegradability of conventional plastics has been reported by many researchers in order to reduce the environmental impaet of petroleum based plastic products and waste. The NR/maize stareh blends exhibited a decrease in their mechanical strength due to the speeifie properties of starch. However, the blended polymers showed a low interfaeial interaetion between the two phases due to the different polarity behaviour of the hydrophobic NR and the hydrophilic starch. 

There are three basic routes to produce polymers from renewable resources feedstock. Direct extraction yields polymer materials such as cellulose, starch, fibres, oils and proteins from which plastic materials can be developed. The second pathway is to convert raw materials first into biomonomers by hydrolysis, and then to polymers by chemical synthesis. A good example is PLA, the most commercialised so far. The third route is to obtain polymeric materials directly by microbial way from carbon sources through biosynthesis (fermentation). A typical example is the production of PHAs by bacteria. 

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