**3. Methods of production**

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 synthetic plastic manufacture.

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

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

Starch Protective Loose-Fill Foams 85

available starch-based foams (CLEAN GREEN, ENVIROFIL, ECO-FOAM, FLO-PACK BIO 8, RENATURE and STAR-KORE), manufactured in extrusion cooking process. They reported that all starch-based foams have higher open-cell content than either EPS-based foam. Considering the manufacturing process used, it is not surprising that starch-based foams have more open cells. The expansion is attributable to the escape of water as steam during the extrusion process, resulting between 96 and 99% open cells. Steam can easily rupture the cell walls because thermoplastic starches have poor melt strength. After exposure to high humidities and temperatures, most foams exhibited a statistically significant, but trivial increase, about 1,0%, in open-cell content. Commercial starch-based foams have an open cellular structure. This differs from patents that claim hydroxypropylated high amylose foams as having a closed-cell structure, but the method

Bhatnagar and Hanna (1995) tested commercial polystyrene (PS) loose-fills, commercial starch-based loose-fills and starch-based plastic loose-fills with addition of polystyrene and poly(methyl methacrylate) (PMMA). Tests showed that the PS loose-fills and the starchbased loose-fills had uniform cell size. The commercial starch foams and starch-based PS foams had cells larger than the commercial PS foams. The commercial starch foams also had fewer large-diameter cavities. These cavities can help to reduce the density, but also affect

Willet and Shogren (2002) reported that surface of starch-based foams have many small holes, suggesting that the starch outer wall burst during extrusion foaming. This reflects the low melt strength and elasticity of starch melts and is consistent with previous studies indicating that starch foams have open cells (Tatarka & Cunningham, 1998). Surface of starch foams containing poly(lactic acid) (PLA) or poly(hydroxyester ether) (PHEE) had fewer or no holes. This suggests that these foams have greater melt strength and resistance to bubble rupture. The average cell size is much larger for foams containing addition of polymer, reflecting the

According with Zhang and Sung (2007a) foams of PLA/starch were successfully prepared by using water as a blowing agent in the presence of talc, which acted as an effective nucleation agent. Water concentration, foaming temperature, nucleation agent concentration, screw speed, and die nozzle diameter were factors influencing foam forming, cell size, and cell distribution. Water was a good blowing agent for the PLA/starch system. The foam structure was dramatically affected by solid inorganic fillers that acted as nucleation agents. With 0,5% talc the expansion ratio of the foam was dramatically reduced by almost 50%. The expansion ratio was further reduced with talc concentration. At 3% talc, the foam expansion was 11,2, <25% of the expansion ratio of the foam without talc. In contrast, the bulk density remained the same and increased slightly at a talc concentration of 3%. With a reduction in the expansion ratio by talc, the foam cellular structure was also changed dramatically. Compared with foams without talc, the texture of the foam with talc became uniform and fine. Cell size distribution became narrower as talc content increased from 0,5 to 2,0%. Majority cell size (MCS) was also reduced. For example, the MCS was between 0,6 and 1,2 mm for the foam with 0,5% talc, whereas the MCS was about 0,4 mm with 2% talc. At 3% talc, the cell size

higher volume expansion of these foams than the starch-based foams.

becomes larger and distribution becomes broader again.

used to make this assessment was not disclosed.

other functional characteristics of the foams.

modified during extrusion (Moscicki, 2011). The rheological properties of the starch plastic are in turn reliant on these physicochemical properties (Mercier et al.,1998), which affect the quality attributes of the foamed product. Extrusion process parameters, such temperature, screw speed, feed rate and moisture have direct influence on density, expansion ratio and other physical properties of extruded foams. Extrusion temperature depends on processed blend composition and ranged from 100°C to 180°C (Bhatnagar & Hanna, 1995; Cha et al., 2001; Shogren et al., 2002).

Moulded foam trays have been developed based on baking technology (Shogren et al., 2002) and are commercially available. Batter is foamed up and dried within heated moulds to foam thin-shelled containers similar to the process for making ice cream cones. The foam structure is featured by a highly porous centre sandwiched by much denser skin layers. The technology is somewhat limited by the slow processing rate necessary to dry off the moisture in the batter, which in turn restricts the maximum wall thickness of the foams.

Technologies for producing bulk starch forms have also been developed. Corrugated foam planks (Lye et al., 1998) made by extrusion foaming of modified cornstarch have been shown to have good cushion performance. The high foam density and cost of the materials, however, have somewhat restricted their widespread applications in packaging. Block foams have been made by combination of extrusion foaming and adhesion technology (Wang et al., 2001). The foams are of lower density and made from low cost wheat flours. When combined with other materials to form lightweight sandwich composites, mechanical properties and resistance to water attack can be drastically enhanced (Song, 2005).

Moulded starch block foams are highly desirable in order to provide biodegradable counterparts to moulded polymeric foams. Recently, a microwave foaming process has been described for making moulded starch foams from extruded pellets (Zhou et al., 2006). This involves converting starch-based raw materials into pellets by extrusion processing and foaming the extruded pellets by microwave heating. The microwaveable starch pellets are compact for transportation and storage and can be expanded using microwave when needed. They may be formulated to produce microwaveable snacks in food industry. In non-food applications, free-flowing foamed balls may be produced for loose-fill packaging. When the pallets are foamed in a mould, lightweight mouldings can be produced in forms of containers, end caps, edge or corner cushion pads for protective packaging, which are difficult to produce with extrusion foaming technology.
