Extrusion of Thermoplastic Starch

Besides plasticizer features, properties of the starch itself such as the amylose/ amylopectin ratio, also have an influence on the final material. As mentioned above, amylose and amylopectin recrystallize in different ways. Amylose will form the more crystaUine part of a thermoplastic and amylopectin the amorphous component the opposite behavior is found in the native starch granule, in which amylopectin is predominantly the crystalHne part. The source of the starch determines the ratio and the molar mass distribution of amylose and amylopectin, and consequently the properties of the thermoplastics made out of it [7]. In this chapter the effects of and relations between the plasticizer, the amylose/amylopectin ratio, and the mechanical properties of four starches from different sources-potato, pea, wheat, and waxy maize-are described and discussed. Besides an overview of the properties, this should finally lead to better insight into the behavior of starch materials during processing. 

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Starch is the major carbohydrate reserve in higher plants and has been a material of choice since the early days of human technology. Recently starch gained new importance as a raw material in the production of plastics, in particular, for the synthesis of monomers to produce polymers such as polydactic acid) and, after chemical modification (e.g. esterification) and thermomechanical processing, to produce thermoplastic starch. This chapter gives a general overview of the most recent research on the development of materials from starch, focusing on thermoplastic starch and the perspectives for future development in this field. A brief review on reactive extrusion of thermoplastic starch is also provided. 

In order to adjust the properties of these starch-based materials to the desired application, it is necessary to combine starch with other polymers, as frequently done in the plastic industry. The need for tuneable properties may also require starch modifications, such as esterification or etherification, grafting and reactive or melting extrusion of thermoplastic starch (TPS). The main forms of starch utilization as a polymer are (i) starch grafted with vinyl monomers, (ii) starch as a filler of other polymers and (iii) plasticized starch (PLS), commonly known as TPS. 

Chaudhary, A., Miler, M., Torley, R, Sopade, R, and Halley, P. J. (2008). Amylose content and chemical modification efiects on the extrusion of thermoplastic starch from maize. Carbohydr. Polym., 74,907-913. 

In extrusion of thermoplastic starches, both heat of conduction and heat of dissipation are generally important in the process. In small machines the Brinkmann number is relatively small, but in larger machines the dissipation becomes more dominant and the process becomes more adiabatic. Because the thermal problems... 

Addition of starch has a nucleating effect, which increases the rate of crystallisation. The rheology of starch/PCL blends depends on the extent of starch granule destruction and the formation of thermoplastic starch during extrusion. Increasing the heat and shear intensities can reduce the melt viscosity, but enhance the extrudate-swell properties of the polymer. 

Starch is one of these renewable resources. Starch is widely available and is especially suited for the production of foamed thermoplastic materials due to the intrinsic presence of a blowing agent. Destructurization of starch by means of extrusion compounding will facilitate the formation of expandable beads or foams of thermoplastic starch. 

Hietala, M., Mathew, A. P, Oksman, K. (2013). Bionanocomposites of thermoplastic starch and cellulose nanofibers manufactured using twin-screw extrusion., (4), 950-956. 

Moscicki L, Mitrus M, Wojtowicz A, Oniszczuk T, Rejak A, Janssen L. Appbcation of extrusion-cooking for processing of thermoplastic starch (TPS). Food Res Int 2012 47 291-299. 

Hietala M, Mathew AP, Oksman K (2013) Bionanocomposites of thermoplastic starch and cellulose nanofibers manufactured using twin-screw extrusion. Eur Polym J 49 950-956 Hoover R, Hughes T, Chung HJ, Liu Q (2010) Composition, molecular structure, properties and modification of pulse starches a review. Food Res Int 43 399-413 Hotza D (1997) Colagem de Folhas Ceramicas. Tape Casting Ceramica 159-166 Huneault MA, Li H (2012) Preparation and properties of extruded thermoplastic starch/polymer blends. J Appl Polym Sci 126 96-108... 

Nanocomposites based on starch/PVOH/silicate clay are not widely studied (Dean and others 2008 Vasile and others 2008 Majdzadeh-Ardakani and Nazari 2010). Dean and others (2008) produced a series of thermoplastic starch/PVOH/montmoril-lonite (MMT) micro and nanocomposites which exhibit intercalated and exfoliated structures through extrusion processing. A small amormt of PVOH (up to 7 wt%) and MMT (up to 5 wt%) were used and the improvement of tensile strength (up to 67% increase) and tensile modulus (up to 85% increase) were reported. Vasile and others (2008) prepared starch/PVOH/MMT composites by the melt mixing method. They revealed the morphology and thermal behavior changes for the nanocomposites and reported a slight increase in thermal stability for starch. 

In the patent literature the term destructurized starch has been used [71-85] to refer to a form of thermoplastic starch described as molecularly dispersed in water [86]. Other patents [17, 87-91] use the term destructured starch to describe a thermoplastic starch which is treated by extrusion cooking to distinguish it from gelatinized starch, processed in presence of high amounts of water. Furthermore a recent USDA s paper [92] on this subject concludes that destructurized starch is not a new or novel entity but is merely another term used to describe the well-known disordering of starch chains and the melting of crystallites that take place when starch is heated in the presence of limited amounts of water . Henceforth extrusion-cooked starch will be reported as thermoplastic starch. 

The main use of thermoplastic starch alone is in foam applications. Starch-based foams have been found to be an effective alternative to PS foam in loose-fill protective packaging. Starch-based foams offer the advantage that they are readily biodegradable if they escape into the environment. They also offer superior antistatic properties. Starch-based foam, however, has some disadvantages it is brittle and the density is higher than PS. Foamed, starch-based articles are prepared by heating starch in an extruder in the presence of water with subsequent extrusion. 

Figure 4.8 illustrates the influence of repeated extrusion on the Tg of thermoplastic starch for blends with 25% glycerol. A slight drop in the Tg can be noted on reprocessing thermoplastic starch. The maximum decrease in the glass transition temperature did not surpass 0.15 °C. However, it should be mentioned that... 

Figure 4.8 Influence of multiple extrusions on the Tg of thermoplastic starch.

Research has shown that repetition of extrusion influences the value of the Young modulus (Figure 4.21). Regardless of the glycerol content in a mixture, the Young modulus value rises - that is, the TPS elasticity increases - with repetition of extrusion. These results are in accordance with the findings made in investigations of the viscoelastic properties of thermoplastic starches. 

Mitrus, M. (2005) Changes of specific mechanical energy during extrusion cooking of thermoplastic starch. TEKA Komisji Motoryzacji i Energetyki Rolnictwa,... 

Scaling-up of thermoplastic starch extrusion suffers from the same general problems that are encountered in many other processes in the process industry ... 

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