Starch foaming

There have been more efforts to use blends such as Nova Chemicals Arcel (an ethylene styrene interpolymer in bead form) to create a more durable foam than EPS. EPP can be moulded into complex shapes, with the steam bonding of the beads (291). These packaging foams compete with polyolefin film products such as Jiffy foam (bubblewrap) and macrobubbles made by heat sealing 150 mm lengths of tubular LDPE film. As it is difficult or costly to recycle such foam products, in some countries there is a move towards biodegradable starch foams and moulded paperboard. 

Altieri, P. A. Tessler, M. M. (1996). Water Humidity Resistant Starch Foams, U.S. Patent 5,554,660. 

Nabar, Y., Narayan, R., Schindler, M. (2006). Twin-screw extrusion production and characterizatin of starch foam products for use in cushioning and insulation applications. Polymer Engineering and Science, 46,438 51. 

Starch foam - [DEFOAMERS] (Vol 7) -biodegradation [POLYMERS, ENVIRONMENTALLY DEGRADABLE] (Vol 19)... 

Liu, Z., Chuah, C.S.L., and Scanlon, M.G. 2003. Compressive elastic modulus and its relationship to the structure of a hydrated starch foam. Acta Materialia 51, 365-371. 

One of the first applications of biodegradable materials is based on the cooked, extruded, and expanded starch known from the food and chemical sectors (Fig. 14.23). Starch is cooked with water in the extruder and chemically modified as necessary or mixed with plasticizers, then expanded to a starch foam and dried. The extrudate is ground so that the functional properties thus created can be used in the food/chemicals sector. The foamed, cut, and dried extrudate is the end product for loose-fill packaging applications. The degree of expansion is a measure of the foam texture. It increases strongly with product temperature at the die, helped by a higher specific mechanical energy input. However, both measures increase the water-solubility of the product. 

Starch-based foam loosefill packaging shapes are also available. While starch was not traditionally considered a plastic, the technique for producing these foams involves plasticizing the starch with water (and sometimes additional plasticizing agents) in an extruder and molding it. Starch foam shapes and cushions compete with loosefill PS and molded PS cushions. 

Carr. L., Parra, D., Ponce, P., Lugao, A., Buchler, P. Influence of fibers on the mechanical properties of cassava starch foams. J. Polym. Environ. 14, 179-183 (2006)... 

The properties of foams made from pure starch are limited however mechanical properties are poor and starch foam is very sensitive to changes in relative humidity. To improve these characteristics, starch is chemically and physically modified or blended with other additives or polymers. This approach has proven to be very effective. Various starch-based foams have been introduced in the market, mainly in loose fill applications ... 

From the experiments described in the previous sections foaming conditions have been foimd that produce a starch foam with small cells and low density. The cell structure and mechanical properties of the resulting foam have been analyzed. All foam samples are conditioned at 20°C at 60% R.H. for one week prior to measurements. 

Figure 9. SEM photo of potato starch foam (magnification 400x).

Mechanical testing on the foam is performed to measure the compressive stress and the resiliency. The results from diese tests are shown in table 2, in which the properties of extruded polystyrene foam (XPS) and commercial starch-based loose-frll foams (Eco-foam and Mater-Bi) together with EPS loose-fill foam (Pelaspan Pac) are added for conq>arison. The values of XPS are obtained from tests on typical XPS retail packaging trays. The table shows that the compressive stress reached with potato starch foam is comparable with that of XPS. Through the cell structure of the potato starch foam (high cell density, very small cells) a good resiliency can be obtained, although pure starch plastics exhibit brittle fracture behavior. This brittle fracture still is present on the microscopic scale of the individual cells but due to the cell density, the foam exhibits resiliency on macroscopic scale. 

Table 2 shows also that while Eco-Foam and Mater-Bi foams have comparable properties to PS-based loose-fill materials, the properties of potato starch foams are comparable with XPS. This indicates that potato starch foams could be suitable for XPS-like applications. 

Table 2. Comparison of properties of XPS, EPS, potato starch foam, Eco-foam and Mater-Bi. Data for XPS, potato starch foam and Mater-Bi from tests in this study. Data of Eco-foam and...

Potato starch foam samples have been conditioned at several relative humidity values. In these experiments, the commercially available Eco-Foam and Mater-Bi have been tested in the same fashion for comparison. From these experiments it was found that the potato starch foam has a good humidity stability up to values of about 75 to 80%. Only at high humidity the foam absorbs that much moisture from the environment that the Tg decreases so the foam shrinks noticeable or collapses. The results are shown in Fig 11 and demonstrate that the humidity stability of the potato starch foam competes very well with these commercial foams up to 78% R.H. 

Figure II. Foam shrinkage as function of the relative humidity for 1. Eco-foam 2. Potato starch foam and 3. Mater-Bi...

Serma, M. M., Yossef, A. M., Hossam, F. M., El-naggar, A. W. M., Biodegradation of low-density polyethylene/thermoplastic starch foams before and after electron beam irradiation. Journal of Applied Polymer Science 2007,106,3273-3281. 

Starch is a renewable resource and is biodegradable. It can be an important raw material to make environmentally friendly products. The National Starch and Chemical Co. developed an extrusion process to make dry starch foam. The starch foam behaves like the styrene form and can be used as its replacement in the packing industry. After usage, the starch foam can be dumped into a sink... 

Shogren R.L., Lawton J.W., Tiefenbacher K.F., Baked starch foams Starch modifications and additives improve process parameters, structure and properties, Ind. Crops. Prod., 16, 2002, 69-79. 

Extruded starch foam is produced by the flash generation of steam as the starch leaves the die (49,50). Foam packaging peanuts, as a replacement for polystyrene, can be made by extruding starch acetate or hydroxypropylated starch and water (Fig. 2). A commercial example is EcoFoam, a water-soluble extruded hydroxypropylated starch foam manufactured by National Starch and Chemical Co. 

Fig. 8. Starch foam cup and tray coated with water-resistant PHBV. An adhesive layer of shellac, a natural resin, prevents delamination of PHBV from the starch. Courtesy of R. Shogren, U. S. D. A.

Starch-based foams can provide compostable packaging for the fast-food industry The extruded starch foam can be coated with a outer layer of acetylated starch to provide water resistance, or a foamed starch-acetylated starch hlend can be so coated (76,77). Starch has also been coated with the microbial polyester PHBV to provide resistance to hot and cold water a natural resin, shellac, prevents delamination (78) (Fig. 8). 

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