**3.1 Plasticisers**

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

Thermoplastic Starch 103

Gelatinization temperature range depended on glycerol content and lipid–amylose complexes were formed depending on stearic acid and moisture content. Extrusion caused fragmentation of the starch that was detected by size exclusion chromatography and cross-

Substituted starch can be extrusion gelatinised in the presence of added hydrophilic polymer with water as the only plasticizer. Without addition of glycerol, or other alcohol, the gelation and plasticity of the starch is only dependent on water content. The subsequent equilibrium water content controls the flexibility, tensile strength and elongation. The added hydrophilic polymer contributes to the strength, and together with the substituted groups, restricts retrogradation. In addition to the chemical processes contributing to gelation, the extrusion conditions including shear, residence time and temperature physically provide gelation and homogenisation. Polymers that have been blended with TPU are poly(vinyl alcohol) pure or with some residual acetate, biopolyesters such as poly(lactic acid), poly(hydroxyl alkanoate)s; poly(butylene succinate) and poly(butylene adipate). Other polar thermoplastics, such as poly(N-vinylpyrrolidone) and poly(caprolactam), are suitable

Starch–poly(caprolactone) (PCL) blends have been produced by extrusion with an in-line rheometer slit die (Belard, Dole, Averous, 2009). A multilayer structure could be generated with a PCL outer skin that was formed depending on the molar mass of the PCL. The phase separation between starch and PCL showed that the polymers were not miscible in the blend, yet they were expected to be compatible. Starch–poly(lactic acid) (PLA) blends have been filled with nano-clay (Cloisite 30B) to form nano-composite blends (Lee, Hanna, 2009). The clay was found to be present as tactoids that influenced melting temperature, onset degradation temperature, radial expansion ratio, density, bulk compressibility and bulk

TPS requires that starch be a continuous phase and exhibit flow and processability. In some starch blends the starch is dispersed like filler in a continuous phase of a second polymer. This is the case with starch–polyethylene blends where starch is dispersed in the polyethylene, usually with compatabilisation via a maleic anhydride grafted polyethylene or use of a substituted starch. Such starch blends enhance biodegradablilty of the matrix polymer through degradation of the starch leaving a porous framework of synthetic polymer that then slowly degrades. Degradation is usually too slow to be classified as

Composites of TPS are prepared for the same advantages as for synthetic thermoplastics. Typical mineral fillers include talc, clays, silica and cellulose fibres particularly microcrystalline cellulose and cellulose nano-fibres. The filler can be dispersed in starch during the gelatinisation extrusion process. It is advantageous if a twin-screw extruder with multiple inlet ports is used so that gelatinisation, plasticiser and filler addition can be

polarisation magic angle spinning nuclear magnetic resonance spectroscopy.

though biopolymers are preferred to maintain overall biodegradability.

biodegradable according to international standard definition.

separated processing steps within the one extrusion cycle.

**3.2 Starch–Polymer blends** 

spring index.

**3.3 Composites** 

occur when the plasticiser has a higher affinity for water than does starch, thus the plasticiser– water coupling lowers the total plasticiser available to hydrogen bond with starch. This causes an increase in the *Tg*, gelatinisation temperature and increased brittleness.

Plasticiser activity has been related to water sorption isotherms that give molar sorption enthalpies of starch–plasticiser systems. Competitive plasticisation has been assessed using glass transition temperature models. The study validated the anti-plasticization limit for glycerol to be ∼10–15 %·w/w, however for xylitol, its anti-plasticization limit did not manifest until 20 %·w/w (Liu, Chaudhary, Ingram, John, 2011). A new hydrogen bonding highly polar plasticizer, N,N-bis(2-hydroxyethyl)formamide, has been synthesized and used to prepare TPS from corn starch (Dai, Chang, Yu, Ma, 2008). Hydrogen bonding between the plasticizer and starch was confirmed using Fourier transform infrared (FTIR) spectroscopy and SEM showed granules to be completely disrupted after gelatinization with the new plasticizer. Comparison of the plasticizer with glycerol was made and at low relative humidity, modulus was higher while at high relative humidity extension at break was higher when the new plasticizer was included compared with glycerol.

Wheat starch plasticization in the water–glycerol combination was used to study the time– temperature regime for melt processing of TPS using excess plasticizers (Li, Sarazin, Favis, 2008). Gelatinization occurred between 1 min and 3 min with an excess of plasticizers with a rapid reduction of granule phase size.

Plasticisers for starch function the same as plasticiser in synthetic polymers. An additional function of starch plasticisers is that they form a complex via hydrogen bonding with the starch and prevent retrogradation, which will cause embrittlement due to gradual recystallization. Where the plasticiser is a polymer the correct term is an allomer, which then is a polymer blend as treated in the next section. Retrogradation is more severe in high amylose starch since it is the amylose that forms the V-type crystals. However high amylose starch is the most suitable for processing to form TPS. Additives are required to stabilise TPS against retrogradation to ensure reliable properties over time.

Fatty acids such as stearic acid are mostly hydrophobic, however the hydrophobic chain preferentially resides inside of starch helical coils forming a complex. A similar complex is formed with iodine giving a red to blue colour depending on the length of the coil and the number of iodine molecules aligned inside the coil. The interior of starch coils is more hydrophobic that the exterior due to the stereo-aligned pendant hydroxyl groups. The iodine complexes with glycans have been used to elucidate the structure of dispersed starch molecules, where iodine can bind with corn starch with only 8 %·w/w water content. Complexes with iodine and different starches (corn and potato) of varying water content were compared and potato starch where amylose was more closely involved with crystal structures, was found to form more effective complexes based upon formation of a more intense colour (Saibene D, Zobel H F, Thompson D B, Seetharaman K, 2008). Layered films based on lipophilic starch and gelatin were formed with varying amounts of fatty acids (palmitic, lauric, myristic, capric, caproic and caprylic) and sorbitol plasticizer (Fakhouri, Fontes, Innocentini-Mei, Collares-Queiroz,, 2009). The addition of fatty acids decreased opacity and elongation while decreasing tensile strength and water vapor permeability.

Native corn starch was plasticized with water, glycerol and stearic acid and gelatinized in a twin-screw extruder (Pushpadass, Kumar, Jackson, Wehling, Dumais, Hanna, 2009).

occur when the plasticiser has a higher affinity for water than does starch, thus the plasticiser– water coupling lowers the total plasticiser available to hydrogen bond with starch. This causes

Plasticiser activity has been related to water sorption isotherms that give molar sorption enthalpies of starch–plasticiser systems. Competitive plasticisation has been assessed using glass transition temperature models. The study validated the anti-plasticization limit for glycerol to be ∼10–15 %·w/w, however for xylitol, its anti-plasticization limit did not manifest until 20 %·w/w (Liu, Chaudhary, Ingram, John, 2011). A new hydrogen bonding highly polar plasticizer, N,N-bis(2-hydroxyethyl)formamide, has been synthesized and used to prepare TPS from corn starch (Dai, Chang, Yu, Ma, 2008). Hydrogen bonding between the plasticizer and starch was confirmed using Fourier transform infrared (FTIR) spectroscopy and SEM showed granules to be completely disrupted after gelatinization with the new plasticizer. Comparison of the plasticizer with glycerol was made and at low relative humidity, modulus was higher while at high relative humidity extension at break was

Wheat starch plasticization in the water–glycerol combination was used to study the time– temperature regime for melt processing of TPS using excess plasticizers (Li, Sarazin, Favis, 2008). Gelatinization occurred between 1 min and 3 min with an excess of plasticizers with a

Plasticisers for starch function the same as plasticiser in synthetic polymers. An additional function of starch plasticisers is that they form a complex via hydrogen bonding with the starch and prevent retrogradation, which will cause embrittlement due to gradual recystallization. Where the plasticiser is a polymer the correct term is an allomer, which then is a polymer blend as treated in the next section. Retrogradation is more severe in high amylose starch since it is the amylose that forms the V-type crystals. However high amylose starch is the most suitable for processing to form TPS. Additives are required to stabilise

Fatty acids such as stearic acid are mostly hydrophobic, however the hydrophobic chain preferentially resides inside of starch helical coils forming a complex. A similar complex is formed with iodine giving a red to blue colour depending on the length of the coil and the number of iodine molecules aligned inside the coil. The interior of starch coils is more hydrophobic that the exterior due to the stereo-aligned pendant hydroxyl groups. The iodine complexes with glycans have been used to elucidate the structure of dispersed starch molecules, where iodine can bind with corn starch with only 8 %·w/w water content. Complexes with iodine and different starches (corn and potato) of varying water content were compared and potato starch where amylose was more closely involved with crystal structures, was found to form more effective complexes based upon formation of a more intense colour (Saibene D, Zobel H F, Thompson D B, Seetharaman K, 2008). Layered films based on lipophilic starch and gelatin were formed with varying amounts of fatty acids (palmitic, lauric, myristic, capric, caproic and caprylic) and sorbitol plasticizer (Fakhouri, Fontes, Innocentini-Mei, Collares-Queiroz,, 2009). The addition of fatty acids decreased opacity and elongation while decreasing tensile strength and water vapor permeability.

Native corn starch was plasticized with water, glycerol and stearic acid and gelatinized in a twin-screw extruder (Pushpadass, Kumar, Jackson, Wehling, Dumais, Hanna, 2009).

an increase in the *Tg*, gelatinisation temperature and increased brittleness.

higher when the new plasticizer was included compared with glycerol.

TPS against retrogradation to ensure reliable properties over time.

rapid reduction of granule phase size.

Gelatinization temperature range depended on glycerol content and lipid–amylose complexes were formed depending on stearic acid and moisture content. Extrusion caused fragmentation of the starch that was detected by size exclusion chromatography and crosspolarisation magic angle spinning nuclear magnetic resonance spectroscopy.

#### **3.2 Starch–Polymer blends**

Substituted starch can be extrusion gelatinised in the presence of added hydrophilic polymer with water as the only plasticizer. Without addition of glycerol, or other alcohol, the gelation and plasticity of the starch is only dependent on water content. The subsequent equilibrium water content controls the flexibility, tensile strength and elongation. The added hydrophilic polymer contributes to the strength, and together with the substituted groups, restricts retrogradation. In addition to the chemical processes contributing to gelation, the extrusion conditions including shear, residence time and temperature physically provide gelation and homogenisation. Polymers that have been blended with TPU are poly(vinyl alcohol) pure or with some residual acetate, biopolyesters such as poly(lactic acid), poly(hydroxyl alkanoate)s; poly(butylene succinate) and poly(butylene adipate). Other polar thermoplastics, such as poly(N-vinylpyrrolidone) and poly(caprolactam), are suitable though biopolymers are preferred to maintain overall biodegradability.

Starch–poly(caprolactone) (PCL) blends have been produced by extrusion with an in-line rheometer slit die (Belard, Dole, Averous, 2009). A multilayer structure could be generated with a PCL outer skin that was formed depending on the molar mass of the PCL. The phase separation between starch and PCL showed that the polymers were not miscible in the blend, yet they were expected to be compatible. Starch–poly(lactic acid) (PLA) blends have been filled with nano-clay (Cloisite 30B) to form nano-composite blends (Lee, Hanna, 2009). The clay was found to be present as tactoids that influenced melting temperature, onset degradation temperature, radial expansion ratio, density, bulk compressibility and bulk spring index.

TPS requires that starch be a continuous phase and exhibit flow and processability. In some starch blends the starch is dispersed like filler in a continuous phase of a second polymer. This is the case with starch–polyethylene blends where starch is dispersed in the polyethylene, usually with compatabilisation via a maleic anhydride grafted polyethylene or use of a substituted starch. Such starch blends enhance biodegradablilty of the matrix polymer through degradation of the starch leaving a porous framework of synthetic polymer that then slowly degrades. Degradation is usually too slow to be classified as biodegradable according to international standard definition.

#### **3.3 Composites**

Composites of TPS are prepared for the same advantages as for synthetic thermoplastics. Typical mineral fillers include talc, clays, silica and cellulose fibres particularly microcrystalline cellulose and cellulose nano-fibres. The filler can be dispersed in starch during the gelatinisation extrusion process. It is advantageous if a twin-screw extruder with multiple inlet ports is used so that gelatinisation, plasticiser and filler addition can be separated processing steps within the one extrusion cycle.

Thermoplastic Starch 105

thus shear thinning and due to the time required for flocculation, they are thixotropic. A second mechanism is where the solvated clay layers exfoliate providing greatly increased surfaces for solvation and associated rheological behavior. Clay with inter-gallery sodium ions, or other alkali metals ions, can exfoliate under high shear force, though the resulting surface energy increase is unfavorable. Ion exchange with alkylammonium ions creates hydrophobic clay gallery surfaces that are exfoliated with lower shear and the lower surface energy of the layers provides a more stable system. After shearing has stopped, the layers can associate through ionic end groups or through hydrophobic interactions between surfaces. The alkylammonium treated clays are shear thinning and thixotropic agents, and

Clay is swelled in water and other polar media by intercalcation due to solvation of the gallery sodium ions and the anionic silicate surfaces. The binding of the clay layers by the gallery sodium ions is overcome by the solvation energy gained minus the entropy lost by the solvating molecules. Exfoliation of the clay increases surface energy extensively due to the huge surface area of the many exfoliated clay layers. Thin layers are an unstable state because their mass is nearly all surface. If the layer were fluid, they would contract into spherical beads. The clay layers are of sub-nanometre thickness making the surface to volume extremely high. This huge surface area must be reduced by solvation if the exfoliated state is to be thermodynamically supported. Shear can provide energy for exfoliation, but after the shearing to process the layers must be stabilized by solvation or recombination. Starch is hydrophilic and able to adsorb to the layers and assist with stabilization with less loss of entropy than required by many water molecules. Thus, exfoliated sodium montmorillonite is unstable even with polar molecules or

An alternative is where the gallery sodium ions have been exchanged by alkylammonium ions. Ionic neutrality is maintained but the clay layers are separated by alkyl chains and bound by dispersive forces from the alkyl chains. The shear force required to separate the layers is low and the exfoliated sheets have relatively low hydrocarbon surface energy. The surface energy required to be supported for stabilization of the clay layers is low and their tendency to recombine is low. Solvation by water or polar molecules can occur along the layer edges where polar silicates are exposed. The clay layers are analogous to layered surfactants with both polar and non-polar regions. Starch has hydrophobic as well as hydrophilic functionality. The starch hydrophobic groups can stabilize the low surface generated by exfoliation of treated clay with minimal loss of entropy compared with solvation by many small molecules. Creation of thin layers of any material requires an accompanying mechanism for reduction of surface energy with minimal entropy loss to the system. Reinforcement, diffusion reduction and rheological action of clays will be enhanced

All-starch nano-composites have been formed from cassava starch reinforced with waxy starch nano-crystals with glycerol plasticizer (Garcia, Ribba, Dufresne, Aranguren, Goyanes, 2009). A reinforcing effect was found in the composites, along with increased equilibrium water content. The reinforcement was interpreted as due to strong hydrogen bonding interactions between filler and matrix, and the nano-crystals were well dispersed, which provided decreased permeability. Similarly, nano-composites have been formed by

by hydrophobic modification, even in aqueous or polar environments.

can further be describes as hydrophobic thickening agents.

macromolecules adsorbed onto the surfaces.

Composites are used to reinforce, limit properties dependence on moisture or humidity, as well as restrain retrogradation. Composites tend to be more brittle, which is a problem for TPS sheets. Combinations of composites with plasticiser are used to arrive at optimum properties where inert fillers assist with reduction of problems associated with the starch phase.

A starch film with poly(vinyl alcohol) and nano-silica was prepared (Huali-Tang, Shangwen-Tang, Peng-Zou, 2009). The nano-silica was found to be evenly distributed and the films exhibited improved mechanical properties, water resistance and optical clarity when nano-silica was present. A nano-composite was prepared from starch and unmodified montmorillonite and it was found that water resistance and water vapour permeability decreased when only small amounts of the clay were added. (Maksimov, Lagzdins, Lilichenko, Plume, 2009). The permeability of water determined as a function of orientation of the platelet clay layers calculated by a method of orientation averaging of permeability characteristics of separated structural elements with planar orientation of lamellar filler particles. A comparison of nano-composites prepared from three modified montmorillonite clays (Cloisite Na+, Cloisite 30B and Cloisite 10A) was made under 60 % and 90 % relative humidity (Perez, Alvarez, Mondragun, Vazquez, 2008). The nano-composite matrix consisted of a blend of starch and PCL. Each of the nanocomposites showed decreased water diffusion coefficient considered to be due to a tortuous path for diffusion. Elongation at break increased though in general mechanical properties decreased upon exposure to water.

TPS materials blended with poly(vinyl alcohol) and reinforced with sodium montmorillonite were prepared and found to have highly exfoliated clay structures (Dean K M, Do M D, Petinakis E,Yu, 2008). PVAlc was found to be important for expanding intergallery spaces in the clay. Composites containing both PVAlc and clay were found to have superior increases in tensile modulus and strength, due to both enhanced interfacial interactions and retardation of retrogradation.

Layered clays like monmorillonite have been much studied in starch composites. As described above Cloisite clays are featured in the literature. Swelling of clays with water involves migration of the small water molecules into clay galleries where the water solvate intergallery sodium ions and the silicate anions on surfaces. Solvation causes the layer spacing to increase against the ionic attractions, with stabilisation due to the solvation, though with loss of entropy by the water molecules. Swelling occurs similarly with many polar organic molecules such as alcohols, amines, carbonyls and ethers. Swelling occurs with polar polymers such as poly(oxyethylene) (POE). POE ether groups solvate sodium ions and through hydroxy end-groups the surface silicate anions are solvated. Migration of the larger POE into galleries will be slower and solvation will be less strong than with water, but the loss of entropy is less for polymer adsorption onto a surface than it is with water. A special case of POE is crown ethers where the cyclic structure has a cavity surrounded by ether groups that is the correct size to accommodate a sodium ion. Ion-exchange of intergallery sodium ions with alkylammonium ions is an efficient way to retain the ionic balance while introducing hydrophobic groups into the galleries. The hydrophobic groups reduce the interlayer adhesion of the clay and when clay layers separate the surface energy of the layers is reduced to that of hydrocarbons. Rheological properties of clays are twofold. The first is due to absorption of water, or other polar molecules, from the liquid phase and binding them by solvation. Solvated clay is suspended as colloidal particles that flocculate to form high viscosity and under shear separate to form reduced viscosity. Clay particles are

Composites are used to reinforce, limit properties dependence on moisture or humidity, as well as restrain retrogradation. Composites tend to be more brittle, which is a problem for TPS sheets. Combinations of composites with plasticiser are used to arrive at optimum properties

A starch film with poly(vinyl alcohol) and nano-silica was prepared (Huali-Tang, Shangwen-Tang, Peng-Zou, 2009). The nano-silica was found to be evenly distributed and the films exhibited improved mechanical properties, water resistance and optical clarity when nano-silica was present. A nano-composite was prepared from starch and unmodified montmorillonite and it was found that water resistance and water vapour permeability decreased when only small amounts of the clay were added. (Maksimov, Lagzdins, Lilichenko, Plume, 2009). The permeability of water determined as a function of orientation of the platelet clay layers calculated by a method of orientation averaging of permeability characteristics of separated structural elements with planar orientation of lamellar filler particles. A comparison of nano-composites prepared from three modified montmorillonite clays (Cloisite Na+, Cloisite 30B and Cloisite 10A) was made under 60 % and 90 % relative humidity (Perez, Alvarez, Mondragun, Vazquez, 2008). The nano-composite matrix consisted of a blend of starch and PCL. Each of the nanocomposites showed decreased water diffusion coefficient considered to be due to a tortuous path for diffusion. Elongation at break increased though in general mechanical properties decreased upon exposure to water. TPS materials blended with poly(vinyl alcohol) and reinforced with sodium montmorillonite were prepared and found to have highly exfoliated clay structures (Dean K M, Do M D, Petinakis E,Yu, 2008). PVAlc was found to be important for expanding intergallery spaces in the clay. Composites containing both PVAlc and clay were found to have superior increases in tensile modulus and strength, due to both enhanced interfacial

Layered clays like monmorillonite have been much studied in starch composites. As described above Cloisite clays are featured in the literature. Swelling of clays with water involves migration of the small water molecules into clay galleries where the water solvate intergallery sodium ions and the silicate anions on surfaces. Solvation causes the layer spacing to increase against the ionic attractions, with stabilisation due to the solvation, though with loss of entropy by the water molecules. Swelling occurs similarly with many polar organic molecules such as alcohols, amines, carbonyls and ethers. Swelling occurs with polar polymers such as poly(oxyethylene) (POE). POE ether groups solvate sodium ions and through hydroxy end-groups the surface silicate anions are solvated. Migration of the larger POE into galleries will be slower and solvation will be less strong than with water, but the loss of entropy is less for polymer adsorption onto a surface than it is with water. A special case of POE is crown ethers where the cyclic structure has a cavity surrounded by ether groups that is the correct size to accommodate a sodium ion. Ion-exchange of intergallery sodium ions with alkylammonium ions is an efficient way to retain the ionic balance while introducing hydrophobic groups into the galleries. The hydrophobic groups reduce the interlayer adhesion of the clay and when clay layers separate the surface energy of the layers is reduced to that of hydrocarbons. Rheological properties of clays are twofold. The first is due to absorption of water, or other polar molecules, from the liquid phase and binding them by solvation. Solvated clay is suspended as colloidal particles that flocculate to form high viscosity and under shear separate to form reduced viscosity. Clay particles are

where inert fillers assist with reduction of problems associated with the starch phase.

interactions and retardation of retrogradation.

thus shear thinning and due to the time required for flocculation, they are thixotropic. A second mechanism is where the solvated clay layers exfoliate providing greatly increased surfaces for solvation and associated rheological behavior. Clay with inter-gallery sodium ions, or other alkali metals ions, can exfoliate under high shear force, though the resulting surface energy increase is unfavorable. Ion exchange with alkylammonium ions creates hydrophobic clay gallery surfaces that are exfoliated with lower shear and the lower surface energy of the layers provides a more stable system. After shearing has stopped, the layers can associate through ionic end groups or through hydrophobic interactions between surfaces. The alkylammonium treated clays are shear thinning and thixotropic agents, and can further be describes as hydrophobic thickening agents.

Clay is swelled in water and other polar media by intercalcation due to solvation of the gallery sodium ions and the anionic silicate surfaces. The binding of the clay layers by the gallery sodium ions is overcome by the solvation energy gained minus the entropy lost by the solvating molecules. Exfoliation of the clay increases surface energy extensively due to the huge surface area of the many exfoliated clay layers. Thin layers are an unstable state because their mass is nearly all surface. If the layer were fluid, they would contract into spherical beads. The clay layers are of sub-nanometre thickness making the surface to volume extremely high. This huge surface area must be reduced by solvation if the exfoliated state is to be thermodynamically supported. Shear can provide energy for exfoliation, but after the shearing to process the layers must be stabilized by solvation or recombination. Starch is hydrophilic and able to adsorb to the layers and assist with stabilization with less loss of entropy than required by many water molecules. Thus, exfoliated sodium montmorillonite is unstable even with polar molecules or macromolecules adsorbed onto the surfaces.

An alternative is where the gallery sodium ions have been exchanged by alkylammonium ions. Ionic neutrality is maintained but the clay layers are separated by alkyl chains and bound by dispersive forces from the alkyl chains. The shear force required to separate the layers is low and the exfoliated sheets have relatively low hydrocarbon surface energy. The surface energy required to be supported for stabilization of the clay layers is low and their tendency to recombine is low. Solvation by water or polar molecules can occur along the layer edges where polar silicates are exposed. The clay layers are analogous to layered surfactants with both polar and non-polar regions. Starch has hydrophobic as well as hydrophilic functionality. The starch hydrophobic groups can stabilize the low surface generated by exfoliation of treated clay with minimal loss of entropy compared with solvation by many small molecules. Creation of thin layers of any material requires an accompanying mechanism for reduction of surface energy with minimal entropy loss to the system. Reinforcement, diffusion reduction and rheological action of clays will be enhanced by hydrophobic modification, even in aqueous or polar environments.

All-starch nano-composites have been formed from cassava starch reinforced with waxy starch nano-crystals with glycerol plasticizer (Garcia, Ribba, Dufresne, Aranguren, Goyanes, 2009). A reinforcing effect was found in the composites, along with increased equilibrium water content. The reinforcement was interpreted as due to strong hydrogen bonding interactions between filler and matrix, and the nano-crystals were well dispersed, which provided decreased permeability. Similarly, nano-composites have been formed by

Thermoplastic Starch 107

Fig. 5. Thermogravimetry of TPS and a TPS–calcium carbonate composite

Starch is a potentially great biomaterial resource due to its natural abundance and biodegradability. However, some serious problems also exist in starch-based materials, such as poor long-term stability caused by water absorption, high hydrophilicity, poor mechanical properties and poor processability. To improve the properties of starch-based

Many reactions have been used to derivatize starch due to available hydroxyl groups. The hydroxyl groups on starch are slightly more acidic than typical alcohol hydroxyl groups. This is why base catalysis is effective for most of the reactions summarized in this section. Carboxylic anhydrides, such as maleic anhydride are used for vinyl functionalization. Acyl halide via a Schotten-Bowmann reaction using base catalysis, or sulfonyl chloride. Reaction with aldehyde for acetal formation, such as crosslinking with glutaraldehyde. Vinylsulfone reaction via a Michael reaction is used for adding an ethyl sulfone derivative. Azinyl chloride (cyanuryl trichloride) can be used to link reactive dyes or with an alkyl group. Epoxy ring opening is used to form 2-hydroxypropyl starch or in general 2-hydroxylalkyl starch. Lactone ester ring opening such as caprolactone has been used to form polycaprolactone grafts. Oxazoline (ring opening) has been used to form a convenient linking group. Etherification is used to form methylated starch, such as with iodomethane or dimethyl sulfate. Carboxylation using chloroacetic acid has been used to form carboxymethyl starch. Isocyanate reaction has been to form a urethane link to starch,

Maleated TPS has been used in reactive extrusion melt blending with poly(butylene adipate*co*-terephthalate) (PBAT) for blown-film application (Raquez, Nabar, Narayan, Dubois, 2008). Maleated TPS was formed by reaction of maleic anhydride and corn starch with glycerol plasticizer in an extruder, followed by addition of PBAT further along the extruder screw to form the complete reaction compatabilitzed blend in a single step. Grafting was via

materials, extensive studies have been focused on chemical modification of starch.

however isocyanate is toxic though it will be likely react to completion.

**3.5 Starch derivatives** 

reinforcing waxy starch films with starch nanocrystals, using sorbitol as plasticizer by formation from solution by casting and evaporation to form films (Viguie, Molina-Boisseau, Dufresne, 2007). Thermal and mechanical characterization was performed after conditioning in moist atmospheres.

Advanced nano-composites of TPS have been formed with multi-walled carbon nano-tubes (MWCNT) with glycerol plasticizer for potential application as electroactive polymers (Ma, Yu, Wang, 2008). The composites displayed restrained retrogradation, increased modulus and tensile strength, with decreased toughness. The composites were conductive though conductivity was sensitive to water content. The percolation threshold for conductivity occurred at a MWCNT content of 3.8 %·w/w.

TPS biocomposites have been formed by blending with PLA and reinforcement with coir cellulose fibres with maleic anhydride as compatabiliser or coupling agent (Lovino, Zullo, Rao, Cassar, Gianfreda, 2008). The composites showed a high degree of biodegradability as determined by carbon dioxide production upon composting. SEM showed crazing patterns on the surface and the growth of bacteria on the surfaces was observed using optical microscopy. Hybrid composites of TPS blends with PVAlc plasticized with glycerol and reinforced with layered clay and jute fabric (Ray, Sengupta, Sengupta, Mohanty, Misra, 2007). The hybrid composites were prepared by a solution casting method. The combined filler of clay and jute are of vastly differing dimension scales. The mechanical properties were superior and fracture surfaces demonstrated strong bonding between the matrix and jute. The clay filled matrix did not display plastic deformation.
