**Thermoplastic Starch**

Robert Shanks and Ing Kong

*Applied Sciences, RMIT University, Melbourne Australia* 

#### **1. Introduction**

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> Thermoplastics are polymers that can flow when heated above a melting or vitrification temperature. They undergo plastic deformation, meaning viscous flow with often-complex rheology due to their large molar mass, entanglements, interactions and chain branches. Starch is a natural polymer with complex levels of structure that impinge upon thermoplastic deformation. Natural polymers are no different to synthetic polymers once some added levels of structural complexity are understood (Wunderlich, 2011). Starch is a semi-crystalline polymer that does not melt in the traditional sense to form a liquid. Starch melting does mean loss of crystallinity due to disruption of hydrogen bonds, however melting occurs in the presence of a moderate (10-30 %·w/w) water content. Starch crystals contain about 9-10 %·w/w of bound water, where bound water means water that does not freeze when cooled below 0 °C. Additional water is required for melting of starch at convenient temperatures below the boiling temperature of water and the degradation temperature of the starch.

> Starches are applicable to thermoplastic processing, unlike other polysaccharides such as cellulose and various gums. Cellulose is a natural structural polymer designed for regularity of packing, chain stiffening and strong cohesion via hydrogen bonding. Cellulose is a structural polymer forming cell walls in plants and it cannot be melted at moderate temperatures when only water is present. Cellulose has interesting solution properties of practical importance since solution processing is the method for transforming cellulose into fibres and films. Starch is an energy storage polymer designed for reversible chain propagation and reversible structure formation to allow access to enzymes for its energy forming degradation. Starch is a biopolymer that is biodegradable, suitable for green packaging materials providing that it can be formulated and readily processed into usable forms. Traditional processing equipment such as extrusion and thermoforming can be used with adaption for the specific characteristics of starch.

> Thermal processing of starch has been reviewed as to changes in microstructure, phase transitions and rheology as a consequence of processing technique, conditions and formulations (Liu, Xie, Yu, Chen, Li, 2009). Starch has a unique microstructure that contributes multiphase transitions during thermal processing that provides an illustrative model to demonstrate conceptual approaches to understanding structure-processingproperty relationships in all polymers. A unique characteristic of starches is their thermal processing properties that are considerably more complex than those of conventional

Thermoplastic Starch 97

Adjacent amylopectin branches form a double helical secondary structure that is the basis of crystallinity in starch granules. The double helical structures then associate into a tertiary structure of a superhelix of the secondary double helical configuration. Further superstructures form combined with inter-crystalline amylose and associated lipids. Figure 2 shows a starch tetramer to illustrate a stereochemical view of the primary starch structure,

representative of an amylose starch chain without branching.

Fig. 1. Diagram of starch amylopectin superstructures

Fig. 2. Chemical structure of a starch tetramer showing conformation of glucose and

stereochemistry of hydroxyl substituents and the α-1,4-glycosidic linkages

polymers, because multiple chemical and physical reactions take place during processing. Examples of phenomena taking place during processing are: water diffusion, granular expansion, gelatinization, decomposition, melting and crystallization. Of the phase transitions, gelatinization is of most importance because it is the means of conversion of starch to a thermoplastic. The starch decomposition temperature is higher than its pregelatinization melting temperature. Conventional processing techniques such as extrusion, injection molding, compression molding, thermoforming and reactive extrusion, have been adapted for processing thermoplastic starch.

Starch is a versatile biopolymer obtained from renewable plant resources such as maize, wheat and potato harvests. Starch consists of two component polymers, amylose (AM) and amylopectin (AP). Amylose is the linear polysaccharide, poly(α-1,4-glucopyronosyl). Amylopectin is poly(α-1,4-glucopyronosyl)) with many a -1,6-glucopyronosyl branches. The molar mass of AM is large, >106 g/mol, while AP is >107 g/mol. There are various crystal forms of starch, due to double helix formation of linear regions of AP. The crystalline and amorphous regions assemble in layered formations to ultimately constitute the starch granules. Starch is economically competitive with polymers derived from petroleum for manufacture of packaging materials. Starch based materials are biodegradable.

Thermoplastic starch (TPS) has attracted much attention due to its thermoplastic-like processability with temperature and shear, though the structures being disrupted are more complex than those of synthetic thermoplastics. However, TPS is no different to any other polymer with respect to linear and branched structures, molar mass, glass transition temperature, plasticiser modification, crystallinity and melting temperature. Starch is a stereo-regular polymer with chirality, directionality of the chains, branching and a high density of hydrogen bonding. Polymerisation of glucose into starch not only builds the primary chains, but forms the secondary regular packing of chains into crystals and the clustering of crystals into tertiary cell structures.

The aim of this review is to provide a framework for the transition from native starch, from its primary molecular structure, secondary structure and tertiary granules to thermoplastic starch with its properties that parallel and contrast with synthesis thermoplastics. Objectives will be to interpret chemical structure changes in forming thermoplastic starch and additives that will assist with native structure dissociation, processing and the properties of materials formed from starch compositions.
