Biodegradable Materials Made from Thermoplastic Starch and Polylactic Acid

2 Biodegradable Materials Made from Thermoplastic Starch and Polylactic Acid 

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. 

Cooked but unexpanded thermoplastic starches (TPS) have wider applications, because of the fact that approx. 8% of water can be removed after the cooking process by devolatilization at 300 mbar, corresponding to a cooling by approx. 70 °C down to 100°C. That means that the exiting strand does not expand but yields a glassy, plastic-like pellet that can be dried in a fluidized-bed dryer. 

The most common process is a combination of these two technologies from the food and plastics sectors (Fig. 14.24). Here, the starch is first cooked, then a biodegradable 

The ZSK process is about 40% less costly to run than single-screw extrusion with pre-drying times measured in hours. In this process, the residua] moisture is removed during melting 

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