**3. Thermoplastic starch**

Formation of soluble starch or thermoplastic starch requires disruption of starch granules and their supramolecular structures, dissociation of complexes with lipids and melting of crystals with the assistance of added water. Figure 4 shows an environmental scanning electron microscope (SEM) image of corn starch granules. Though there is bound water within starch that varies with ambient humidity, water is typically added. A water concentration of 25 %·w/w will give a gelatinisation temperature with a range of 60-70 °C. Gelatinisation is assisted by shear. An extruder, either a twin-screw or single-screw, is suitable for continuous shear processing. Alternatively batch mixers with a wiping action can be used.

Fig. 4. Environmental SEM of corn starch granules

A starch solution can be formed by thoroughly mixing starch with cold water to form a uniform suspension. The suspension will be a milky colour with an relatively high viscosity. The suspension can then be heated without coagulation. Gradual heating produces a clear solution of increased viscosity. If the solution is stored for several days it will gradually become opalescent through to milky. Starch solutions are not stable because hydrogen bonds within and between starch molecules are more stable than the hydrogen bonds with water that keep the starch in solution. Formation of starch–starch hydrogen bonds accompany ordering of starch molecules into crystalline structures different from the original starch granules. The ordering can be considered as a lyophilic liquid crystalline

Thermoplastic Starch 101

Gelatinisation of cassava and potato starches at low water levels (>20 %·w/w) in the presence of sodium chloride caused a decrease in gelatinisation temperature (Farahnaky, Farhat, Mitchell, Hill, 2009). A range of moisture contents was studied and the decrease in

Retrogradation of waxy corn starch has been studied using differential scanning calorimetry (DSC) and it was found that an endotherm developed after 2-5 h and increased with time (Liu, Yu, Tong, Chen, 2010). Two-phase transitions were identified; the Gr transition was due to helix-helix dissociation while the M1r transition was due to a helix-coil transition. This indicated reformation of mainly the amylopectin molecules due to an original structural memory. The retrogradation in this high amylopectin starch was due to reforming of the amylopectin double helix. The process was inhibited by increased amylose content. Cassava starch retrogradation has been studied using X-ray diffraction and DSC and it was found that with this starch the gelatinisation procedure has no significant effect on the melting enthalpy upon retrogradation, while the storage temperature was a factor

Starch with about 15-25 %·w/w water shows a gelatinisation peak in differential scanning calorimetry (DSC) at 60-70 °C. This endotherm is a double peak with this lower temperature peak being due to disruption of starch–lipid complexes and the high temperature peak being due to melting of starch crystals. The endotherm has a relatively small area, heat of fusion. The term melting is used to describe disruption of crystals in starch, though it does not mean melting in the sense of crystals becoming a flowable melted liquid. The gelatinised starch is a fluid that can flow under shear because of the presence of water, so it could be

There is uncertainty in the position of the glass transition temperature. When *Tg* is plotted against Tm for many synthetic polymers a trend is observed that *Tg* ~ 2/3·*Tm* (temperatures in Kelvin). Therefore *Tg* of starch should be below 0 °C. Some careful DSC and dynamic mechanical analysis (DMA) measurements have shown *Tg* to be in a range -10 to 0°C. In this range *Tg* can be obscured or uncertain due to the large melting endotherm of free water in the processing of starch. The glass transition of waxy maize starch and water (1:2 w/w) has been studied using DSC after different degrees of gelatinisation (Sang, Alavi, Shi, 2009). A three-phase model with an amorphous phase, a rigid amorphous phase and a crystalline phase was proposed to interpret the data. Gelatinisation had an onset of 61.5 °C with a maximum of 70.3 °C and an end temperature of 81.7 °C. The *Tg* temperature and detection depended upon prior thermal history and gelatinisation, however a distinct subzero *Tg* of

Water is the main plasticiser, however it is too transient to be used as the sole plasticiser for TPS. Other less volatile plasticisers are glycerol, pentaerythritol, polyols, sugar alcohols, poly(oxyethylene)s, poly(oxypropylene)s, non-ionic and anionic surfactants. Hydrogen bonding polar liquids are most suitable. Plasticisers with a high affinity for water, such as glycerol, can exhibit anti-plasticisation at particular concentrations, typically low concentrations depending on water content. Anti-plasticiser activity has been found, for different reasons in dioctyl phthalate plasticised poly(vinyl chloride). Anti-plasticisation may

gelatinisation temperatures was 30 °C at a water content of 11 %·w/w for example.

(Rodriguez-Sandoval, Fernandez-Quintero, Cuvelier, Relkin, Bello-Perez, 2008).

annealed native starch granules was observed after annealing at -7 °C.

described as a concentrated starch–water solution.

**3.1 Plasticisers** 

behaviour. Excessive heating of a starch in water solution will cause the starch to flocculate. This will be experienced as a continuous increase in viscosity until gelation and then formation of a strengthening gel structure. This gelation is consistent with lower critical solution temperature (LCST) behaviour. LCST behaviour is observed with solutions/solubility of other highly hydrogen bonding polymers in water, such as poly(methacrylic acid).

Conversion of starch into a packaging material requires disruption of the starch granules and their constitutive crystals into a flowable thermoplastic. High AM content is preferred for preparation of thermoformable starch due to the better flow properties of the linear polymer and its decreased tendency to crystallise. High amylose corn starch is the major source of amylose-rich starch and it can have amylose contents of up to 80 %. Typically high AM starch can be processed in an extruder if it is first equilibrated with water and any other plasticisers, normally alcohols such as glycerol. Water must diffuse into the granules and hydrogen bond with the starch. Upon heating the granules undergo destructurisation and the crystals melt to form a uniform viscous starch fluid. Processing, strength and stability of the starch is assisted by addition of a hydrophilic polymer such as poly(vinyl alcohol), poly(oxyethylene), poly(caprolactam) or poly(vinylpyrrolidone). After formation of a starch film the added polymer stabilises the amorphous structure by hydrogen bonding with the starch. This prevents recrystallization of the starch to form V-type crystals in a process called retrogradation. Another way to limit retrogradation is to use substituted starch such as hydroxyethyl, hydroxypropyl or acetoxy starches. The substituents interrupt the structural regularity of the starch and thereby restrict crystallization.

The viscosity increases considerably as the molecules are freed from their ordered superstructures and the gelatinous mass becomes transparent. The gelatinised starch is amorphous and referred to as thermoplastic starch (TPS) since it can be reprocessed so long as water content is retained or replenished. The glass transition temperature of starch is uncertain, but likely to be about -50 to -10 °C with the water content suitable for gelatinisation and subsequent processing. TPS has properties expected of any thermoplastic though water content is required for them to be revealed. TPS has a glass transition temperature, it can flow under shear when heated and it can slowly crystallise. TPS sheets are transparent, though depending on the extent of shear mixing there can be traces of starch granules recognisable as faint outlines. These are called ghost granules and they disturb the clarity of sheets and films of TPS, which is a problem for some packaging applications.

Disruption of the ordered structures within starch is necessary to prepare thermoplastic starch. Heat is required along with shear to disrupt native starch structures to form a homogeneous thermoplastic starch that can flow or form into shapes. Water content, shear degradation by chain cleavage and thermal degradation by dehydration or bond rupture are limitations in the processing of TPS. Residual crystallinity and re-crystallisation formed by the retrogradation process causes loss of film clarity and embrittlement. After processing amylopectin enhances the properties of TPS and retards retrogradation in TPS. Retrogradation involves the more thermodynamically and kinetically favourable crystallisation of the linear amylose to form V-type crystals that are distinct from native starch crystals.

behaviour. Excessive heating of a starch in water solution will cause the starch to flocculate. This will be experienced as a continuous increase in viscosity until gelation and then formation of a strengthening gel structure. This gelation is consistent with lower critical solution temperature (LCST) behaviour. LCST behaviour is observed with solutions/solubility of other highly hydrogen bonding polymers in water, such as

Conversion of starch into a packaging material requires disruption of the starch granules and their constitutive crystals into a flowable thermoplastic. High AM content is preferred for preparation of thermoformable starch due to the better flow properties of the linear polymer and its decreased tendency to crystallise. High amylose corn starch is the major source of amylose-rich starch and it can have amylose contents of up to 80 %. Typically high AM starch can be processed in an extruder if it is first equilibrated with water and any other plasticisers, normally alcohols such as glycerol. Water must diffuse into the granules and hydrogen bond with the starch. Upon heating the granules undergo destructurisation and the crystals melt to form a uniform viscous starch fluid. Processing, strength and stability of the starch is assisted by addition of a hydrophilic polymer such as poly(vinyl alcohol), poly(oxyethylene), poly(caprolactam) or poly(vinylpyrrolidone). After formation of a starch film the added polymer stabilises the amorphous structure by hydrogen bonding with the starch. This prevents recrystallization of the starch to form V-type crystals in a process called retrogradation. Another way to limit retrogradation is to use substituted starch such as hydroxyethyl, hydroxypropyl or acetoxy starches. The substituents interrupt the

The viscosity increases considerably as the molecules are freed from their ordered superstructures and the gelatinous mass becomes transparent. The gelatinised starch is amorphous and referred to as thermoplastic starch (TPS) since it can be reprocessed so long as water content is retained or replenished. The glass transition temperature of starch is uncertain, but likely to be about -50 to -10 °C with the water content suitable for gelatinisation and subsequent processing. TPS has properties expected of any thermoplastic though water content is required for them to be revealed. TPS has a glass transition temperature, it can flow under shear when heated and it can slowly crystallise. TPS sheets are transparent, though depending on the extent of shear mixing there can be traces of starch granules recognisable as faint outlines. These are called ghost granules and they disturb the clarity of sheets and films of TPS, which is a problem for some packaging

Disruption of the ordered structures within starch is necessary to prepare thermoplastic starch. Heat is required along with shear to disrupt native starch structures to form a homogeneous thermoplastic starch that can flow or form into shapes. Water content, shear degradation by chain cleavage and thermal degradation by dehydration or bond rupture are limitations in the processing of TPS. Residual crystallinity and re-crystallisation formed by the retrogradation process causes loss of film clarity and embrittlement. After processing amylopectin enhances the properties of TPS and retards retrogradation in TPS. Retrogradation involves the more thermodynamically and kinetically favourable crystallisation of the linear amylose to form V-type crystals that are distinct from native

structural regularity of the starch and thereby restrict crystallization.

poly(methacrylic acid).

applications.

starch crystals.

Gelatinisation of cassava and potato starches at low water levels (>20 %·w/w) in the presence of sodium chloride caused a decrease in gelatinisation temperature (Farahnaky, Farhat, Mitchell, Hill, 2009). A range of moisture contents was studied and the decrease in gelatinisation temperatures was 30 °C at a water content of 11 %·w/w for example.

Retrogradation of waxy corn starch has been studied using differential scanning calorimetry (DSC) and it was found that an endotherm developed after 2-5 h and increased with time (Liu, Yu, Tong, Chen, 2010). Two-phase transitions were identified; the Gr transition was due to helix-helix dissociation while the M1r transition was due to a helix-coil transition. This indicated reformation of mainly the amylopectin molecules due to an original structural memory. The retrogradation in this high amylopectin starch was due to reforming of the amylopectin double helix. The process was inhibited by increased amylose content. Cassava starch retrogradation has been studied using X-ray diffraction and DSC and it was found that with this starch the gelatinisation procedure has no significant effect on the melting enthalpy upon retrogradation, while the storage temperature was a factor (Rodriguez-Sandoval, Fernandez-Quintero, Cuvelier, Relkin, Bello-Perez, 2008).

Starch with about 15-25 %·w/w water shows a gelatinisation peak in differential scanning calorimetry (DSC) at 60-70 °C. This endotherm is a double peak with this lower temperature peak being due to disruption of starch–lipid complexes and the high temperature peak being due to melting of starch crystals. The endotherm has a relatively small area, heat of fusion. The term melting is used to describe disruption of crystals in starch, though it does not mean melting in the sense of crystals becoming a flowable melted liquid. The gelatinised starch is a fluid that can flow under shear because of the presence of water, so it could be described as a concentrated starch–water solution.

There is uncertainty in the position of the glass transition temperature. When *Tg* is plotted against Tm for many synthetic polymers a trend is observed that *Tg* ~ 2/3·*Tm* (temperatures in Kelvin). Therefore *Tg* of starch should be below 0 °C. Some careful DSC and dynamic mechanical analysis (DMA) measurements have shown *Tg* to be in a range -10 to 0°C. In this range *Tg* can be obscured or uncertain due to the large melting endotherm of free water in the processing of starch. The glass transition of waxy maize starch and water (1:2 w/w) has been studied using DSC after different degrees of gelatinisation (Sang, Alavi, Shi, 2009). A three-phase model with an amorphous phase, a rigid amorphous phase and a crystalline phase was proposed to interpret the data. Gelatinisation had an onset of 61.5 °C with a maximum of 70.3 °C and an end temperature of 81.7 °C. The *Tg* temperature and detection depended upon prior thermal history and gelatinisation, however a distinct subzero *Tg* of annealed native starch granules was observed after annealing at -7 °C.
