**1. Introduction**

78 Thermoplastic Elastomers

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polyethylene-starch blends. In Degradable materials :Perspectives, Issues and Opportunities, Barenberg S. A., Brash J. L., Narayan R. and Redpath A. E. (eds), Shredded newsprint and cardboard, popcorn, flour, starch and expanded polystyrene (EPS) are the most common materials used to protective packing products. Their function is to provide cushioning, protection, and stabilization of articles packaged for transport. EPS-based loose-fill foam products have enjoyed a steady growth in this application over the last decades, but became targeted recently in the solid waste disposal debate. Huge quantities of plastic is used for refuse and retail bags, egg cartons, packaging film, and loose fillers.

Fig. 1. Extruded starch foams.

Starch Protective Loose-Fill Foams 81

unmolten starch granules of clear crystallographic structure. Similarly, an insufficient amount of plasticizer may result in incomplete destruction of the crystallographic structure

One of the major drawbacks connected with starchy materials is their brittleness. This is related to a relatively high glass transition temperature Tg. This temperature marks the transition from a highly flexible state to a glassy one. Tg is considered the most important parameter for determining the mechanical properties of amorphous polymers and for the

The effect of starch plastification with water was repeatedly studied and various techniques for glass transition temperature were compared. The method of differential scanning calorimetry DSC is used most commonly, but the glass transition temperature found by DSC can be 10 – 30°C higher than the Tg value obtained by NMR (nuclear magnetic resonance) or DMTA (dynamic mechanical thermal analysis). The analysis of the influence of water on the Tg of amylose and amylopectin showed that the very branched amylopectin had a slightly lower glass transition temperature than the amylose. On the grounds of published researches and practical observation it can be stated that starchy material containing water is generally in the glassy state and therefore brittle under natural conditions (De Graaf et al.,

However, the results of measurements published by various authors are inconsistent to a high degree due to complex changes that occur in starch due to high temperatures and

Zeleznak & Hoseney (1997) found that the glass transition temperature of wheat starch with 13 - 18,7% moisture varies between 30 and 90°C and that Tg is likely to be lower than room temperature if the starch humidity increases above 20%. Van Soest et al. (1996) detected a Tg of 5°C for extruded potato starch with 14% moisture content, while at higher moisture Tg could not be determined. Shogren showed that the glass transition temperature for starch

Myllarinen et al. (2002) indicated that Tg of amylose and amylopectin may equal the room temperature when the water content in a blend is 21%, however, at the same glycerol content it goes up to 93°C. This leads to the conclusion that glycerol is a less effective plasticizer than water. Moreover, on the basis of calculations they found out that a glycerol

Also for other plasticizers, like sorbitol the Tg of TPS decreases at increasing concentrations. Yu et al. (1998) hold that TP maize starch with 10% moisture and 25-35% glycerol shows a Tg running from 83 – 71°C. Van Soest & Knooren (1997) proved that potato TPS with 11% moisture and 26% glycerol had a Tg = 40 °C, whereas for the materials of higher moisture and glycerol content it fell below 20°C. Lourdin et al. (1997) reported that potato starch of 13% moisture with 15% glycerol content had the Tg around 25°C, while at 25% glycerol the

The TPS mechanical properties depend on the temperature of starch production, water content as well as quantity and type of added plasticizers and aid materials. The most considerable influence on the changes in mechanical properties proved to be the amount of

of starch (Souza & Andrade, 2002; Van Soest & Knooren, 1997).

control of their crystallisation process (De Graaf et al., 2003).

2003; Moates et al., 2001; Myllarinen et al., 2002).

with 7 - 18% moisture ranged from 140 – 150°C (Shogren, 1993).

content as high as 35% is required to let Tg drop to the room temperature.

different measurement conditions.

Tg dropped to around 0°C.

plasticizer and aid materials.

As an alternative to very popular polymer foams such as expanded polystyrene (EPS), loosefill (foamed chips for filling space around goods within a packing box) extruded from starch (fig. 1) is probably the most successful application of starch-based material in cushion packaging. Several patents on the extruded foams based on starch and blends of starch with various additives have been filed (Bastioli et al., 1998a, 1998b; Bellotti et al., 2000; Xu & Doane, 1998) and the material is commercially available. Considerable effort has been made to study the influence of extrusion conditions, moisture content and composition on the physical properties of starch-based foams (Bhatnagar & Hanna, 1995; Tatarka & Cunningham, 1998).

Extruded starch foams are generally water soluble, and their properties are sensitive to moisture content. Greatest expansion and lowest densities are generally achieved through the use of modified high amylase starches. Various synthetic polymers, like poly(vinyl alcohol) or polycaprolactone, have been blended with unmodified starches to produce foams with lower densities and increased water resistance.
