**2. Basics of starch plasticization**

Native starch does not have any thermoplastic properties without addition of plasticizer(s) (e.g., water, glycerol, sorbitol, etc.). The products made from native starch are readily broken into fragments when they are dried in ambient conditions due to strong intermolecular hydrogen bonding in the amylose and amylopectin macromolecular chains (Ma et al., 2007). But in the presence of plasticizers and elevated temperatures and shear, native starch readily melts and flows, allowing for its use as an extruding, injection moulding, or blowing material, similar to most conventional synthetic thermoplastic polymers (Ma et al., 2007). The role of

Retrogradation and Antiplasticization of Thermoplastic Starch 119

Zhang & Han (2010) showed that gelatinized and plasticized TPS contains about 10% crystallinity, meaning 90% of TPS is amorphous region. Starch polymers in the amorphous region are metastable and consequently they automatically retrogradate over time during storage. Mechanism of starch retrogradation has been intensively investigated. Liu & Han (2005) observed starch crystal formation of amylose and amylopectin film respectively under an inverted phase-contrast microscope. They found amylose film featured a layer of dendrites with 90-degree branching (Fig. 2a), while amylopectin film showed a network of interlinked clusters with an amorphous background (Fig. 2b). Lian et al. (2011) observed the starch retrogradation under a microscope. They found starch nuclei in 2 h and crystals in 48 h (Fig. 3a & b). Russell (1987) postulated amylose formed nuclei by congregating, and amylopectin formed crystalline lamellae by prolongating the rod-like growth of crystals. Delville et al. (2003) proposed a mechanism for formation of starch crystalline lamellae shown in Fig.4, presenting the crystalline cluster formation of amylopectin. The cluster formation begins with the formation of crystalline lamellae composed of double helices of amylopectin short chains (symbolized with rectangular boxes). Then, the packing of double helices

Tako & Hizukuri (2002) proposed starch retrogradation mechanisms at a molecular level, in which starch retrogradation occurs as a result of intermolecular hydrogen bonding between O-6 of D-glucosyl residues of amylose molecules and OH-2 of D-glucosyl residues of short side-chains of amylopectin molecules (Fig. 5). The starch retrogradation was also attributed to intermolecular hydrogen bonding between OH-2 of D-glucosyl residues of amylose molecules and O-6 of D-glucosyl residues of short side-chains of amylopectin molecules (Fig. 6). In addition to intermolecular hydrogen bonding between amylose and amylopectin, hydrogen bonding between O-3 and OH-3 of D-glucosyl residues on different amylopectin molecules also occurs (Fig. 7). The intramolecular hydrogen bonding might occur between OH-6 and adjacent hemiacetal oxygen atom of the D-glucosyl residues within an amylose molecule (Fig. 5 and 6), while the intramolecular association within an amylopectin molecule was not suggested to exist

Fig. 1. A schedule of a typical extruder (Adapted from Li et al., 2011)

**3. Mechanism of TPS retrogradation** 

forms crystalline clusters (Delville et al., 2003).

(Tako & Hizukuri, 2002).

plasticizers is to attract the water molecules around them, reduce the intermolecular interactions between the starch molecules, and then increase the flexibility of native starch (Ke & Sun 2001). This process of overcoming the brittleness in starch by softening the structure and by increasing the mobility of the macromolecular chains, resulting in a lowering of processing temperature, is termed as plasticization of starch. During the plasticization process, starch is transformed from a semicrystalline granular material into a system containing granular remnants, or to an amorphous paste with no structure at all (Smits et al., 2003).

Three theories have been proposed to account for the mechanisms of plasticization. These are lubricity theory, gel theory, and free volume theory. Lubricity theory proposes that the plasticizer acts as a lubricant and lubricates movements of the macromolecules over each other. Gel theory proposes that the plasticizer disrupts the interaction of starch chain bonds. Free volume theory proposes that plasticizer increases free volume between the starch chains and lowers its glass transition temperature (*Tg*). The commonality of these is that plasticizer is considered to interpose itself between the starch chains and reduce the forces holding the chains together (Gioia & Guilbert, 1999).

Commonly used plasticizers for TPS include water, glycerol, sucrose, fructose, glucose, glycols, urea, formamide, ethanolamine, and ethylene bisformamide, and amino acids (Huang et al., 2006; Ma & Yu, 2004; Pushpadass et al., 2008; Wang et al., 2008; Yang et al., 2006a, 2006 b; Zhang & Han, 2006a, 2006b). These chemicals are small in molecular size and are hydrophilic. Water and glycerol have traditionally been considered as the most effective plasticizers. Urea, formamide, ethanolamine, and ethylene bisformamide, which contain - CO-NH- functional groups, have recently been proven to be good plasticizers, for they are believed to suppress retrogradation and improve mechanical properties of TPS (Ma et al., 2005; Ma et al., 2006). Zullo & Iannace (2009) found that a urea/formamide mixture worked more effectively than glycerol in making homogenous and robust TPS films. Ma & Yu (2004) compared the hydrogen bond energy of the urea-starch, formamide-starch, acetamidestarch, and glycerol-starch, and concluded that urea, formamide and acetamide formed stronger hydrogen bonds with starch. Consequently urea, formamide and acetamide are more effective in plasticizing starch than polyols.

A minimum of 20% glycerol or any other suitable plasticizer is required to plasticize starch successfully (Pushpadass et al., 2008). With increasing plasticizer amount, properties of TPS, like tensile strength, Young's modulus, and glass transition temperature (*Tg*), decrease, while elongation and gas permeability increase. A TPS film which contains 25% glycerol is reported to exhibit maximum tensile strength and optimum modulus of elasticity (Pushpadass et al., 2008).

For industrial manufacturing of TPS, extrusion processing is a realistic approach. A typical extruder consists of a hopper, barrel, feed screw, thermocouples, and die (Fig. 1). Starch pellets or beads are fed from the hopper along the feed screw through the barrel chamber. As starch pellets or beads travels along the barrel, it is subject to friction, compression, and heated zones. The result is that the starch pellets homogeneously melt and mix as they traveled through the feed screw to the die. The die is precisely machined with a pattern opening such that the extruded starch mix takes the die pattern for its cross sectional area. TPS extrudates from the die solidify quickly. Before solidifying, TPS extrudates can be blown into films, sheets, or be moulded into desired shapes (Thunwall et al., 2006).

plasticizers is to attract the water molecules around them, reduce the intermolecular interactions between the starch molecules, and then increase the flexibility of native starch (Ke & Sun 2001). This process of overcoming the brittleness in starch by softening the structure and by increasing the mobility of the macromolecular chains, resulting in a lowering of processing temperature, is termed as plasticization of starch. During the plasticization process, starch is transformed from a semicrystalline granular material into a system containing granular

Three theories have been proposed to account for the mechanisms of plasticization. These are lubricity theory, gel theory, and free volume theory. Lubricity theory proposes that the plasticizer acts as a lubricant and lubricates movements of the macromolecules over each other. Gel theory proposes that the plasticizer disrupts the interaction of starch chain bonds. Free volume theory proposes that plasticizer increases free volume between the starch chains and lowers its glass transition temperature (*Tg*). The commonality of these is that plasticizer is considered to interpose itself between the starch chains and reduce the forces

Commonly used plasticizers for TPS include water, glycerol, sucrose, fructose, glucose, glycols, urea, formamide, ethanolamine, and ethylene bisformamide, and amino acids (Huang et al., 2006; Ma & Yu, 2004; Pushpadass et al., 2008; Wang et al., 2008; Yang et al., 2006a, 2006 b; Zhang & Han, 2006a, 2006b). These chemicals are small in molecular size and are hydrophilic. Water and glycerol have traditionally been considered as the most effective plasticizers. Urea, formamide, ethanolamine, and ethylene bisformamide, which contain - CO-NH- functional groups, have recently been proven to be good plasticizers, for they are believed to suppress retrogradation and improve mechanical properties of TPS (Ma et al., 2005; Ma et al., 2006). Zullo & Iannace (2009) found that a urea/formamide mixture worked more effectively than glycerol in making homogenous and robust TPS films. Ma & Yu (2004) compared the hydrogen bond energy of the urea-starch, formamide-starch, acetamidestarch, and glycerol-starch, and concluded that urea, formamide and acetamide formed stronger hydrogen bonds with starch. Consequently urea, formamide and acetamide are

A minimum of 20% glycerol or any other suitable plasticizer is required to plasticize starch successfully (Pushpadass et al., 2008). With increasing plasticizer amount, properties of TPS, like tensile strength, Young's modulus, and glass transition temperature (*Tg*), decrease, while elongation and gas permeability increase. A TPS film which contains 25% glycerol is reported to exhibit maximum tensile strength and optimum modulus of elasticity

For industrial manufacturing of TPS, extrusion processing is a realistic approach. A typical extruder consists of a hopper, barrel, feed screw, thermocouples, and die (Fig. 1). Starch pellets or beads are fed from the hopper along the feed screw through the barrel chamber. As starch pellets or beads travels along the barrel, it is subject to friction, compression, and heated zones. The result is that the starch pellets homogeneously melt and mix as they traveled through the feed screw to the die. The die is precisely machined with a pattern opening such that the extruded starch mix takes the die pattern for its cross sectional area. TPS extrudates from the die solidify quickly. Before solidifying, TPS extrudates can be

blown into films, sheets, or be moulded into desired shapes (Thunwall et al., 2006).

remnants, or to an amorphous paste with no structure at all (Smits et al., 2003).

holding the chains together (Gioia & Guilbert, 1999).

more effective in plasticizing starch than polyols.

(Pushpadass et al., 2008).

Fig. 1. A schedule of a typical extruder (Adapted from Li et al., 2011)
