**2. Thermoplastic starch**

Starch biodegrades to carbon dioxide and water in a relatively short time compared with most synthetic polymers. Considering some drawbacks of the existing technologies of biodegradable materials manufacture, in the recent years there have been started large-scale researches to increase amount of starch in starch-plastic composites to the highest possible level. The final objective of these investigations is to obtain commercial items for one-time use, produced from pure starch and to exclude synthetic polymers from the formulation. Thermoplastic starch (TPS) seems to be a perfect solution because it can be processed with conventional technologies used in synthetic plastic manufacture (extrusion, injection moulding) (Shogren et al., 1993; Wiedmann & Strobel, 1991).

To obtain thermoplastic starch, thermal and mechanical processing should disrupt semi crystalline starch granules. As the melting temperature of pure starch is substantially higher than its decomposition temperature there is a necessity to use plasticizers, for example water. Under the influence of temperature and shear forces, disruption of the natural crystalline structure of starch granules and polysaccharides form a continuous polymer phase is reported (Avérous et al., 2001; Nashed et al., 2003; Shogren et al., 1993; Van Soest et al., 1996).

TPS produced from starch plastified only with water becomes very brittle at room temperature. To increase material flexibility and improve processing other plasticizers are also used, e.g. glycerol, propylene glycol, glucose, sorbitol and others (Nashed et al., 2003; Van Soest et al., 1996).

To improve the mechanical properties of TPS based materials also other additives can be applied, like emulsifiers, cellulose, plant fibres, bark, kaolin, pectin and others (Avérous et al., 2001; Ge et al., 2000).

Thermoplastic starch can be obtained by the proper treatment with temperature and pressure in the presence of a plasticizer, like water, glycerol or sorbitol. The plasticizer penetrates into the starch granules and disrupts the initial crystallographic structure. Due to temperature and shear forces the material undergoes a melting process and forms a continuous amorphous mass that does not exhibit diffraction anymore. If the total thermal and mechanical energy provided to the starch is insufficient the product will show

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

Starch biodegrades to carbon dioxide and water in a relatively short time compared with most synthetic polymers. Considering some drawbacks of the existing technologies of biodegradable materials manufacture, in the recent years there have been started large-scale researches to increase amount of starch in starch-plastic composites to the highest possible level. The final objective of these investigations is to obtain commercial items for one-time use, produced from pure starch and to exclude synthetic polymers from the formulation. Thermoplastic starch (TPS) seems to be a perfect solution because it can be processed with conventional technologies used in synthetic plastic manufacture (extrusion, injection

To obtain thermoplastic starch, thermal and mechanical processing should disrupt semi crystalline starch granules. As the melting temperature of pure starch is substantially higher than its decomposition temperature there is a necessity to use plasticizers, for example water. Under the influence of temperature and shear forces, disruption of the natural crystalline structure of starch granules and polysaccharides form a continuous polymer phase is reported

TPS produced from starch plastified only with water becomes very brittle at room temperature. To increase material flexibility and improve processing other plasticizers are also used, e.g. glycerol, propylene glycol, glucose, sorbitol and others (Nashed et al., 2003;

To improve the mechanical properties of TPS based materials also other additives can be applied, like emulsifiers, cellulose, plant fibres, bark, kaolin, pectin and others (Avérous et

Thermoplastic starch can be obtained by the proper treatment with temperature and pressure in the presence of a plasticizer, like water, glycerol or sorbitol. The plasticizer penetrates into the starch granules and disrupts the initial crystallographic structure. Due to temperature and shear forces the material undergoes a melting process and forms a continuous amorphous mass that does not exhibit diffraction anymore. If the total thermal and mechanical energy provided to the starch is insufficient the product will show

(Avérous et al., 2001; Nashed et al., 2003; Shogren et al., 1993; Van Soest et al., 1996).

foams with lower densities and increased water resistance.

moulding) (Shogren et al., 1993; Wiedmann & Strobel, 1991).

**2. Thermoplastic starch** 

Van Soest et al., 1996).

al., 2001; Ge et al., 2000).

unmolten starch granules of clear crystallographic structure. Similarly, an insufficient amount of plasticizer may result in incomplete destruction of the crystallographic structure of starch (Souza & Andrade, 2002; Van Soest & Knooren, 1997).

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

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

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 different measurement conditions.

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

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

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 Tg dropped to around 0°C.

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 plasticizer and aid materials.

Starch Protective Loose-Fill Foams 83

The investigations on water absorption by starch without a plasticizer revealed that amylopectin absorbs less water than amylose. The influence of a glycerol addition on water

At low relative ambient moisture, i.e. below 50%, water content for both, amylopectin and amylose turned out to be lower in the blends with/without glycerol addition. This effect is probably connected with the replacement of strongly structurally immobilised water by

At the relative ambient moisture higher than 50% the mixtures with highest glycerol concentration showed the highest water content. When ambient moisture exceeded 70%, water content in the starch plastifyied by glycerol proved higher than in starch without glycerol.

Although both starch components behave alike at water absorption, in case of high ambient moisture amylose absorbs greater amount of water compared to amylopectin because the crystallisation reduces water absorption of hydrophilic polymers (Nashed et al., 2003,

Starch offers a structural platform to manufacture sustainable, biodegradable foam packaging. As a basic material can be mixed with other polymers or with plasticizers and other additives. This blends can be produced with conventional technologies used in

Starch-based loose-fill can be manufactured, in a one-step process, via an extrusion cooking process. Granular starch and water are fed into an extruder, usually a twin screw, where heat and shear causes the starch to gelatinize. Water, released as steam at the die of the extruder, is the primary blowing agent. Complete expansion or density reduction takes place immediately after the product exits the extruder (Tatarka & Cunningham, 1998).

Application of extrusion-cooking technique to process starch-plasticizer mixtures is one of the most economical and efficient way to produce TPS loose-fill foams. The process conditions are to be very stable and strictly determined according to the expected quality of the extrudates. Also raw materials used (starchy components mixed with the plasticizers) have to be fitted properly. All together need many trials and detailed measurements to find optimal process conditions which can guarantee the best quality of the product. One of the most interesting question connected with extrusion-cooking is energy consumption. Specific Mechanical Energy (SME) consumption is defined as the amount of energy that is consumed per kilogram product. Knowledge of SME is not only important for design purposes, but it is also an indication of the mechanical forces on the material and consequently of degradation an viscous heating during the process. The specific mechanical energy for extrusion cooking of thermoplastic starch are in the order of 2,52·105 J/kg, which is equivalent to 0,07 kWh/kg. This rather low SME value depends on material composition of

Harper (1981) gave a detailed description on the mechanics of starch extrusion. The phenomenon of starch foaming involves the physicochemical properties of starch, which are

the mixtures, temperature and rotation speed (Mitrus & Moscicki, 2009).

absorption is similar for both starch components (Myllarinen et al., 2002).

glycerol. Similar behaviour was detected for starch plastifyied with sorbitol.

Myllarinen et al., 2002).

**3. Methods of production** 

synthetic plastic manufacture.

The most common plasticizers like, glycerol, glycol or sorbitol possess the same hydroxyl groups as those appearing in starch, thus being compatible with starch macrogranules.

Increase of the plasticizer content brings about a decrease in tensile strength of thermoplastic starch, whereas the elongation at break increases. Starch is a natural polymer containing numerous hydrogen bonds between the hydroxyl radicals in its molecules, therefore it manifests substantial tensile strength values. Glycerol, sorbitol or glycol behave like diluents and decrease the interaction between molecules and consequently, they diminish tensile strength. At the same time they act as placticizers that improves macromolecular mobility and leads to a rise in elongation at break (Yu et al., 1998).

The increase of elongation at break at increasing plasticizer content occurs at some ranges of glycerol only. If this content surpasses 35%, a decrease of the elongation at break is noted. This effect is caused by too high percentage of glycerol, and therefore the molecular interactions are so weak that some interactions between starch molecules are replaced by the interactions between glycerol and starch molecules (Liu et al., 2001; Shogren, 1993; Yu et al., 1998).

An increase of water content in the blend induces an decrease of the tensile strength of the TPS and increase of the elongation at break. Yet, if water content exceeds 35% there is detected drop in the elongation at break of the TPS (Shogren, 1993).

Additon of filler materials like cellulose fibres, flax, kaolin or pectin increase the tensile strength but decrease the elongation at break. In turn, urea or boric acid addition improve the elongation at break but decreases the tensile strength (Fishman et al., 2000; You et al., 2003; Yu et al., 1998).

During storage of TPS some recrystallisation of amylose and amylopectin occurs. Together with a longer storage period, and consequently TPS crystallinity, tensile strength increases and elongation at break decreases. The increase of moisture content of the starchy materials storage conditions brings about intensification of their mechanical properties changes (Van Soest & Knooren, 1997).

On the basis of investigations made by Mitrus (2009) it was found out that with glycerol content increase in material mixture, a decrease of maximal stress formed at granulate compression was detected. Besides, blend moisture was also noted to affect on maximum stress generated in a compressed granulate. Along with mixture moisture growth the formed stresses are greater, however a faster drop of stress values was recorded together with glycerol increase. In the case of granulate produced from mixtures with glycerol content over 27% the stresses generated in granulate from more humid blends are lower than those are for more dry ones. The results emphasise that glycerol percentage growth is accompanied by lower tensile strength of thermoplastic starch. Although, water content increase may improve the strength of the obtained material, still it is most probable that some boundary value of total plasticizer content exists and when it has been surpassed the material becomes soften.

The excessive expansion and pore presence abates TPS tensile strength. Investigating of granulate from corn and wheat starch with 20% of glycerol content there were detected very low values of stress. The presence of pores had a significant impact on tensile strength weakening the obtained extrudate. In case of trials with 25% of glycerol the highest tensile strength values were recorded for the materials from corn starch, while the lowest for those containing wheat starch.

The most common plasticizers like, glycerol, glycol or sorbitol possess the same hydroxyl groups as those appearing in starch, thus being compatible with starch macrogranules.

Increase of the plasticizer content brings about a decrease in tensile strength of thermoplastic starch, whereas the elongation at break increases. Starch is a natural polymer containing numerous hydrogen bonds between the hydroxyl radicals in its molecules, therefore it manifests substantial tensile strength values. Glycerol, sorbitol or glycol behave like diluents and decrease the interaction between molecules and consequently, they diminish tensile strength. At the same time they act as placticizers that improves

The increase of elongation at break at increasing plasticizer content occurs at some ranges of glycerol only. If this content surpasses 35%, a decrease of the elongation at break is noted. This effect is caused by too high percentage of glycerol, and therefore the molecular interactions are so weak that some interactions between starch molecules are replaced by the interactions

An increase of water content in the blend induces an decrease of the tensile strength of the TPS and increase of the elongation at break. Yet, if water content exceeds 35% there is

Additon of filler materials like cellulose fibres, flax, kaolin or pectin increase the tensile strength but decrease the elongation at break. In turn, urea or boric acid addition improve the elongation at break but decreases the tensile strength (Fishman et al., 2000; You et al.,

During storage of TPS some recrystallisation of amylose and amylopectin occurs. Together with a longer storage period, and consequently TPS crystallinity, tensile strength increases and elongation at break decreases. The increase of moisture content of the starchy materials storage conditions brings about intensification of their mechanical properties changes (Van

On the basis of investigations made by Mitrus (2009) it was found out that with glycerol content increase in material mixture, a decrease of maximal stress formed at granulate compression was detected. Besides, blend moisture was also noted to affect on maximum stress generated in a compressed granulate. Along with mixture moisture growth the formed stresses are greater, however a faster drop of stress values was recorded together with glycerol increase. In the case of granulate produced from mixtures with glycerol content over 27% the stresses generated in granulate from more humid blends are lower than those are for more dry ones. The results emphasise that glycerol percentage growth is accompanied by lower tensile strength of thermoplastic starch. Although, water content increase may improve the strength of the obtained material, still it is most probable that some boundary value of total plasticizer

The excessive expansion and pore presence abates TPS tensile strength. Investigating of granulate from corn and wheat starch with 20% of glycerol content there were detected very low values of stress. The presence of pores had a significant impact on tensile strength weakening the obtained extrudate. In case of trials with 25% of glycerol the highest tensile strength values were recorded for the materials from corn starch, while the lowest for those

macromolecular mobility and leads to a rise in elongation at break (Yu et al., 1998).

between glycerol and starch molecules (Liu et al., 2001; Shogren, 1993; Yu et al., 1998).

detected drop in the elongation at break of the TPS (Shogren, 1993).

content exists and when it has been surpassed the material becomes soften.

2003; Yu et al., 1998).

Soest & Knooren, 1997).

containing wheat starch.

The investigations on water absorption by starch without a plasticizer revealed that amylopectin absorbs less water than amylose. The influence of a glycerol addition on water absorption is similar for both starch components (Myllarinen et al., 2002).

At low relative ambient moisture, i.e. below 50%, water content for both, amylopectin and amylose turned out to be lower in the blends with/without glycerol addition. This effect is probably connected with the replacement of strongly structurally immobilised water by glycerol. Similar behaviour was detected for starch plastifyied with sorbitol.

At the relative ambient moisture higher than 50% the mixtures with highest glycerol concentration showed the highest water content. When ambient moisture exceeded 70%, water content in the starch plastifyied by glycerol proved higher than in starch without glycerol.

Although both starch components behave alike at water absorption, in case of high ambient moisture amylose absorbs greater amount of water compared to amylopectin because the crystallisation reduces water absorption of hydrophilic polymers (Nashed et al., 2003, Myllarinen et al., 2002).
