**5. Properties**

Starch crystal structures are characterised using wide-angle X-ray scattering (WAXS). A Kratzky powder camera is used on starch specimens that have been cryo-ground into powder. Figure 6 shows a WAXS pattern for waxy potato starch with diffraction peaks characteristic of B type crystals; camera type: powder, source 0.154 nm, power 4.0 kW, aperture 200 µm, range 5-35°, count time 10 s-1, interval 0.05 data/°. Figure 7 shows a WAXS pattern for corn starch captured under the same conditions shown above. Figure 8 shows a WAXS pattern for TPS, that is gelatinised starch where the lower curve has been aged, compared with the upper curve, and small peaks indicating onset of retrogradation crystallisation are beginning to emerge.

The modulus of TPS compositions is typically high compared with synthetic thermoplastics. Elastic properties at low strain are measurable however TPS has low, moisture dependant, elongation at break. The high modulus of TPS has dependence upon moisture, other plasticisers, fillers and recrystallization. TPS has high high strength, that is break stress for brittle materials or yield stress for ductile materials, due to inter- and intra-molecular hydrogen bonding, and strong adhesion to blended polymers and fillers. Fracture of starch materials tend to be brittle and increasingly brittle with time due to moisture and recrystallization.

Dynamic mechanical properties (modulated force thermomechanometry (mf-TM)) is used for detection of damping peaks, elastic and loss modulus changes with temperature. Mf-TM was performed using a Perkin-Elmer Diamond DMA in tensile mode. A synthetic frequency consisting of 0.5, 1, 2, 10, 20 Hz was applied with constant amplitude of 10 µm. Fourier

Thermoplastic Starch 111

Fig. 8. WAXS patterns for TPS where the lower curve has been aged compared with the

Fig. 9. Thermoplastic starch sheet, mf-TM tensile mode at 10 °C·min-1

1 Hz 2 Hz 4 Hz 10 Hz 20 Hz

TPS sheets are often brittle and mf-TM is performed better using single cantilever bend mode instead of tensile mode. Figure 10 shows such an analysis performed using a TA Instruments Q800 DMA with 0.2 % strain, 1 Hz frequency and heating at 2 °C·min-1. The specimen was coated with paraffin oil prior to clamping in the instrument to minimize moisture loss during the scan. The glass transition denoted by the peak of loss modulus or damping factor is at a higher temperature than that shown in Figure 9 since at the slower scan rate loss of moisture during the scan made the specimen more brittle with higher glass

20 30 40 50 60 70 80 90 100 110

**Temperature /°C**

upper curve

**Tan()**

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

analysis deconvoluted the data into the five constituent frequencies. A tan() curve is shown for each heating rate scan since these curves showing the best resolution. Examples of storage and loss curves are shown for heating at 10 °C·min-1. The specimen was coated with paraffin oil and a higher heating rate than normally used for mf-TM (typically 2 °C·min-1 used) to minimise moisture loss during the scan. The damping curves in Figure 6 show a phase change at about 90 °C interpreted to be due to melting of crystals formed by retrogradation.

Fig. 6. WAXS pattern for waxy potato starch (B type crystals)

Fig. 7. WAXS pattern for corn starch (A type crystals)

analysis deconvoluted the data into the five constituent frequencies. A tan() curve is shown for each heating rate scan since these curves showing the best resolution. Examples of storage and loss curves are shown for heating at 10 °C·min-1. The specimen was coated with paraffin oil and a higher heating rate than normally used for mf-TM (typically 2 °C·min-1 used) to minimise moisture loss during the scan. The damping curves in Figure 6 show a phase change at about 90 °C interpreted to be due to melting of crystals formed by

Fig. 6. WAXS pattern for waxy potato starch (B type crystals)

Fig. 7. WAXS pattern for corn starch (A type crystals)

retrogradation.

Fig. 8. WAXS patterns for TPS where the lower curve has been aged compared with the upper curve

Fig. 9. Thermoplastic starch sheet, mf-TM tensile mode at 10 °C·min-1

TPS sheets are often brittle and mf-TM is performed better using single cantilever bend mode instead of tensile mode. Figure 10 shows such an analysis performed using a TA Instruments Q800 DMA with 0.2 % strain, 1 Hz frequency and heating at 2 °C·min-1. The specimen was coated with paraffin oil prior to clamping in the instrument to minimize moisture loss during the scan. The glass transition denoted by the peak of loss modulus or damping factor is at a higher temperature than that shown in Figure 9 since at the slower scan rate loss of moisture during the scan made the specimen more brittle with higher glass

Thermoplastic Starch 113

Similar to moisture content other plasticisers in TPS can bleed or become leached or extracted. Small molecule plasticisers such as glycerol are relatively involatile though they may migrate to the surface with water transfer or become extracted by adjacent materials such as packaging contents. Other plasticisers may separate depending upon changes in water content or due to their compatibility with TPS. Combinations of plasticisers create

Humidity responsive starch-poly(methyl acrylate) films were formed as graft copolymers that were plasticized with water and urea (Willett, 2008). The films display significant shrinkage that was composition independent in relative humidities greater than 50 % and in relative humidities above 75 % shrinkage was correlated with the urea:starch weight ratio that determines the equilibrium moisture content of the films. A master curve was constructed by shifting shrinkage data with respect to a reference relative humidity, demonstrating that relaxation processes in the starch phase control film shrinkage that was accompanied by loss of orientation within the starch. While moisture sensitivity is to be avoided in typical TPS applications and it is an undesired property, it can be exploited in materials that are sensors to

A severe property change is retrogradation caused by re-crystallisation of amylopectin or crystallisation of amylose to for new V-type crystals. Retrogradation causes embrittlement of TPS and loss of optical clarity often accompanies this. Retrogradation is usually more rapid in high amylose starches, since the linear chains are most mobile to form single helix V-type crystals. Re-crystallisation of amylopectin occur by reforming double helical crystals by

Further changes in TPS with time can arise from mould growth due to presence of combined

Starch is a nutrient for many organisms and since water is present in the final structure the starch is readily biodegraded. Water can easily be absorbed by starch resulting in disintegration of the material by partial solubility. Partially solubilised starch is even more readily biodegraded by enzymes principally from microorganisms. Starch–poly(vinyl alcohol) blends were plasticised with glycerol and reinforced with unripe coconut fibres to form fully biodegradable composites (Rosa, Chiou, Medeiros, Wood, Mattoso, Syed, Inam, 2009)). Degradation rate slowed when fibre content increased, though crystallinity was not changed in the composites. The blend and composites degraded slower in composite that the pure TPS.

Commercialisation of thermoplastic starch and starch-based polymers has been progressing. Due to the inherent biodegradability of starch materials they are allied mostly to packaging as films or sheet that can be subsequentially shaped by thermoforming into custom packaging requirements. Starch are most suited for packaging of dry products., otherwise there will be a diffusion transfer and equilibrium established between the TPS package and its contents. TPS foams are suited for damping impacts to protect fragile products. TPS solve the issue of disposal of packaging materials because they are biodegradable into environmentally friendly

fragments, depending on the nature of non-starch components of TPS compositions.

moisture and nutrients, however this form of change is specific to the next section.

complex phase diagrams that may involve crystallinity and mutual attractions.

humidity when controlled shrinkages are designed into the structure.

inter-coiling of adjacent chain branches.

**6. Biodegradation** 

**7. Applications** 

transition temperature. Scans were performed at 1, 2, 5 and 10 °C·min-1 and 10 °C gave the best scan though with highest thermal lag. Thermomechanical analysis requires a compromise between moisture loss and thermal lag of the instrument.

Fig. 10. Thermoplastic starch sheet, mf-TM single cantilever bend mode at 2 °C·min-1

Dielectric properties of TPS are a sensitive to changes in structure due to the density of polar groups and water content. Thermally stimulated depolarization current (TSDC) has been used to study the dielectric relaxations in cassava starch on semi-crystalline and amorphous variations (Laredo, Newman, Bello, Muller, 2009). Three secondary relaxation modes were detected and interpreted as due to short-range orientations of polar groups or to main chain restricted motion. Moisture plasticization contributed to relaxation Vogel-Tammann-Fulcher parameters that confirmed cooperative relaxation. A heterogeneous amorphous phase resulted in a bimodal distribution of relaxation times.

Vapour transmission – oxygen, moisture, other volatiles, tends to be low compared with other polymers due to density of hydrogen bonds and polarity to restrain diffusion of small molecules. Vapour transmission is dependant upon moisture content and any crystal structures remaining or formed. Moisture sorption is a problem, however moisture does not readily pass through TPS sheets.

#### **5.1 Property changes with time**

Thermoplastic starch structure changes with time, temperature and humidity. These changes are the greatest limitation for adoption of TPS in commercial applications. TPS is hydrophilic and during ambient humidity changes the water content of the starch varies. There is a water content hysteresis depending upon whether the humidity is increasing or decreasing because desorption of hydrogen bonded water is delayed compared with adsorption of water. Moisture and other TPS components exchange with packaging contents, either water is absorbed into the TPS, softening it, while drying the package contents or water is absorbed into the package contents drying and embrittling the TPS package.

transition temperature. Scans were performed at 1, 2, 5 and 10 °C·min-1 and 10 °C gave the best scan though with highest thermal lag. Thermomechanical analysis requires a

Fig. 10. Thermoplastic starch sheet, mf-TM single cantilever bend mode at 2 °C·min-1

resulted in a bimodal distribution of relaxation times.

into the package contents drying and embrittling the TPS package.

readily pass through TPS sheets.

**5.1 Property changes with time** 

Dielectric properties of TPS are a sensitive to changes in structure due to the density of polar groups and water content. Thermally stimulated depolarization current (TSDC) has been used to study the dielectric relaxations in cassava starch on semi-crystalline and amorphous variations (Laredo, Newman, Bello, Muller, 2009). Three secondary relaxation modes were detected and interpreted as due to short-range orientations of polar groups or to main chain restricted motion. Moisture plasticization contributed to relaxation Vogel-Tammann-Fulcher parameters that confirmed cooperative relaxation. A heterogeneous amorphous phase

Vapour transmission – oxygen, moisture, other volatiles, tends to be low compared with other polymers due to density of hydrogen bonds and polarity to restrain diffusion of small molecules. Vapour transmission is dependant upon moisture content and any crystal structures remaining or formed. Moisture sorption is a problem, however moisture does not

Thermoplastic starch structure changes with time, temperature and humidity. These changes are the greatest limitation for adoption of TPS in commercial applications. TPS is hydrophilic and during ambient humidity changes the water content of the starch varies. There is a water content hysteresis depending upon whether the humidity is increasing or decreasing because desorption of hydrogen bonded water is delayed compared with adsorption of water. Moisture and other TPS components exchange with packaging contents, either water is absorbed into the TPS, softening it, while drying the package contents or water is absorbed

compromise between moisture loss and thermal lag of the instrument.

Similar to moisture content other plasticisers in TPS can bleed or become leached or extracted. Small molecule plasticisers such as glycerol are relatively involatile though they may migrate to the surface with water transfer or become extracted by adjacent materials such as packaging contents. Other plasticisers may separate depending upon changes in water content or due to their compatibility with TPS. Combinations of plasticisers create complex phase diagrams that may involve crystallinity and mutual attractions.

Humidity responsive starch-poly(methyl acrylate) films were formed as graft copolymers that were plasticized with water and urea (Willett, 2008). The films display significant shrinkage that was composition independent in relative humidities greater than 50 % and in relative humidities above 75 % shrinkage was correlated with the urea:starch weight ratio that determines the equilibrium moisture content of the films. A master curve was constructed by shifting shrinkage data with respect to a reference relative humidity, demonstrating that relaxation processes in the starch phase control film shrinkage that was accompanied by loss of orientation within the starch. While moisture sensitivity is to be avoided in typical TPS applications and it is an undesired property, it can be exploited in materials that are sensors to humidity when controlled shrinkages are designed into the structure.

A severe property change is retrogradation caused by re-crystallisation of amylopectin or crystallisation of amylose to for new V-type crystals. Retrogradation causes embrittlement of TPS and loss of optical clarity often accompanies this. Retrogradation is usually more rapid in high amylose starches, since the linear chains are most mobile to form single helix V-type crystals. Re-crystallisation of amylopectin occur by reforming double helical crystals by inter-coiling of adjacent chain branches.

Further changes in TPS with time can arise from mould growth due to presence of combined moisture and nutrients, however this form of change is specific to the next section.
