**4. Processing**

108 Thermoplastic Elastomers

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

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

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

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

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,

viscosity or gelation; the fluid dispersion is likely to be shear thinning or thixotropic.

reactions summarized are suitable for batch reaction in solution.

enabling both processing and subsequent strength development.

be a problem and require a low radical concentration.

though acetylation reduced the barrier properties.

**3.6 Oxidised starch** 

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

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 typical thermoplastics but the outcome is similar.

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 granules that are used as protective inserts in packaging.
