**3.4 Stabilisers**

Starch is readily biodegradable and a natural energy storage substance in plants. It is a nutrient source for microorganisms and this makes starch materials a target for various fungal and bacterial growths. Biodegradability may begin before the required end-of-life. Additives may be included to prevent premature biodegradation though the additive level must not prevent ultimate biodegradation, nor be toxic to packaging contents and hence users.

An important area of investigation is to develop additives that decrease the susceptibility of starch materials to moisture and cyclic humidity conditions. Stabilisation against retrogradation is important, though this is usually a function of the plasticisers and filler. Thermal degradation of starch occurs by loss of water. First free water is lost, followed by weakly bound water, then bound water and the TPS no longer has the plasticizing effect of the water. During degradation water formed from dehydration reactions of the starch is eliminated and the starch degrades forming a carbonaceous residue. Figure 5 shows degradation of starch studied using thermogravimetry with water evolved from the start of the scan until about 120 °C. A sharp degradation mass loss begins at about 300 °C under the scanning conditions of 20 °C·min-1 under nitrogen with switch to air at 810 °C. After the rapid degradation, a broad temperature range degradation occurred from 350 °C as the residual organic species degraded to form a carbonaceous residue that was oxidized in air after 810 °C. Comparison is made with a starch–calcium carbonate composite that contained less water and degraded at higher temperature. The calcium carbonate decomposed to calcium oxide with elimination of carbon dioxide above 700 °C.

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

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

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

Starch is readily biodegradable and a natural energy storage substance in plants. It is a nutrient source for microorganisms and this makes starch materials a target for various fungal and bacterial growths. Biodegradability may begin before the required end-of-life. Additives may be included to prevent premature biodegradation though the additive level must not prevent

An important area of investigation is to develop additives that decrease the susceptibility of starch materials to moisture and cyclic humidity conditions. Stabilisation against retrogradation is important, though this is usually a function of the plasticisers and filler. Thermal degradation of starch occurs by loss of water. First free water is lost, followed by weakly bound water, then bound water and the TPS no longer has the plasticizing effect of the water. During degradation water formed from dehydration reactions of the starch is eliminated and the starch degrades forming a carbonaceous residue. Figure 5 shows degradation of starch studied using thermogravimetry with water evolved from the start of the scan until about 120 °C. A sharp degradation mass loss begins at about 300 °C under the scanning conditions of 20 °C·min-1 under nitrogen with switch to air at 810 °C. After the rapid degradation, a broad temperature range degradation occurred from 350 °C as the residual organic species degraded to form a carbonaceous residue that was oxidized in air after 810 °C. Comparison is made with a starch–calcium carbonate composite that contained less water and degraded at higher temperature. The calcium carbonate decomposed to

in moist atmospheres.

**3.4 Stabilisers** 

occurred at a MWCNT content of 3.8 %·w/w.

jute. The clay filled matrix did not display plastic deformation.

calcium oxide with elimination of carbon dioxide above 700 °C.

ultimate biodegradation, nor be toxic to packaging contents and hence users.

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

#### **3.5 Starch derivatives**

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

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

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

Thermoplastic Starch 109

Shear for disruption of super-structures can take place in a batch internal mixer for smallscale preparations. Extrusion is more practical for pilot scale or commercial production. After formation of TPS sheets or films final shaping may be by thermoforming. Extrusion of starch is best performed in a twin-screw extruder where custom combinations of rheological elements can be assembled along the screw. Zone of high shear will assist with disruption of granules while uncoiling of molecules can take place in less shear intensive zones. Formulations often require inlets for plasticiser, filler or other additives along the extruder barrel. Escape of volatiles such as steam will be required, without loss of other materials. At the extruder die the TPS will emerge with a higher moisture content than the equilibrium moisture content of TPS sheet or film. A drying zone will be needed before the product TPS

Extrusion or high shear is required to disrupt the native starch structure and produce a uniform composition with other components. The extrudate must be a uniform continuous stream with rheology suitable for shaping. The process is more complex than extrusion of

Starch foams can be produced using partial vaporisation of entrained water to form a cellular structure. The foams can be in the form of continuous extrudates or popcorn type

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

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

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

is wound into a coil for storage, transport or prior to further shaping.

typical thermoplastics but the outcome is similar.

crystallisation are beginning to emerge.

granules that are used as protective inserts in packaging.

**4. Processing** 

**5. Properties** 

recrystallization.

transesterification via the maleic anhydride functionality of the TPS. Confirmation of successful grafting was obtained from soxhlet extraction and infrared spectroscopy. Some blends contained 70 %·w/w PBAT therefore it is likely that PBAT was the continuous phase with dispersed TPS, though the interfacial area was high due to a fine phase morphology.

If a functional group reaction is to be used for reactive processing it must be capable of complete reaction within the residence time in the extruder and the by-products must be suited to remaining in the final product. Few organic functional groups can react fast without forming an equilibrium, which is why step growth polymerization is limited to few reactions. The more reactive substances tend to be toxic and not stable in water. The reactions summarized are suitable for batch reaction in solution.

Radical initiated derivatization of starch is another alternative. Ceric ion, hydrogen peroxide to form an alkoxy radical on starch can be used for reaction with an additive monomer. A thermal peroxide reaction such as with t-butyl peroxide is less selective for grafting than a redox system. The reactant needs not be a polymer, grafting can occur between starch and another polymer; if in an emulsion, the other polymer could be hydrophobic. Radical reactions are generally faster than functional group reactions, but residual monomer will remain unless oligomer or polymers are used as the graft. Crosslinking (gel formation) will be a problem and require a low radical concentration.

Sol-gel reactions using tetraethoxysilane (TEOS), or tetrabutyltitanate (TBuTi), provide insitu formation of the corresponding oxides, silica and titanium dioxide, gives high dispersion and bonding with starch. Borate or boric acid precursors such as borate ester form complexes that can crosslink starch. Any residual sol compound will continue to react with water in the starch until complete conversion. These composites are likely to have interlinked chains making them difficult to process, though some may be thixotropic enabling both processing and subsequent strength development.

Complexation or adsorption of starch onto surfaces can be used to modify starch. For example, alumina with a positive zeta potential, or silica such as precipitated or fumed silica with a negative zeta potential, and surface-active clays or minerals that have hydrophilic edges and hydrophobic faces such as montmorillonite, kaolin and talc can strongly adsorb starch. No reaction is required; dispersion is required and while this may result in high viscosity or gelation; the fluid dispersion is likely to be shear thinning or thixotropic.

#### **3.6 Oxidised starch**

Starch is chemically modified in various ways with oxidation being a common process. Starch has been oxidised using hypochlorite resulting in an increase in carboxylic acid and carbonyl groups (Sangseethong, Lertphanich, Sriroth, 2009). Oxidation rate depended on the alkalinity of the reaction medium and this influenced the viscosity of the oxidised starch solution, decreased the gelatinisation temperature though retrogradation was slightly increased. The light transmission was less changed with oxidised starch. Banana starch was oxidised and acetylated, then the product was used to form TPS films (Baruk, Zamudio-Flores, Bautista-Baoos, Salgado-Delgado, Arturo, Bello-Perez, 2009). Oxidation increased solubility while acetylation decreased solubility. The oxidised starch showed a high modulus and lower elongation at break that was not significantly changed by acetylation, though acetylation reduced the barrier properties.
