The Melting Process in Thermoplastic Starches

Thermoplastic Starch. Edited by Leon P.B.M. Janssen and Leszek Moscicki 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32528-3 

Wojtowicz (in print) reports on mixing tests performed in a two-bladed counterrotating batch mixer (Do-Coder E330, Brabender OHG, Germany) interfaced with 

---

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

Starch is made thermoplastic at devated temperatures in the presence of water as a plasticizer, allowing melt processing alone or in blends with other thermoplastics (192—194). Good solvents such as water lower the mdt-transition temperature of amylose, the crystalline component of starch, so that processing can be done well bdow the decomposition—degradation temperature. 

Another proposed process employed injection molding in which starch and limited amounts of plasticizing water are heated under pressure to temperatures above the Tg and Tm to transform the native starch into a homogenous, destructured, thermoplastic melt. The process melt is then cooled to below the Tg of the system before pressure release to maintain the moisture content. Additives include natural and synthetic polymers, plasticizers and lubricants.136-139 159 160 The technology has been used to prepare pharmaceutical capsules and shaped objects, such as disposable cutlery, straws and pens. 

The thermoplastic starch beads are foamed on a Demag D60 injection molding machine equipped with a standard PE screw (compression ratio 1 2). Beads with the desired moisture content (10-15% b.w.) are fed into the injection imit, melted and injected in the same fashion as in a regular injection molding process. The exception, however, is that the melt is injected into free air instead of a mould. The temperature used for foaming ranges from 150 to 200°C. 

Particularly suitable polyesters considered in the past have been poly-e-caprolactone (PCL) and its copolymers. Nevertheless, films made of thermoplastic starch and PCL are tacky as extruded, rigid, and have low melt strength at temperatures over 130 °C. Moreover, due to the slow crystallization rate of such polymers, the complete cooling process needs a long time after production of the finished articles, giving an undesirable change of properties with time. 

Attempts have been made to process natural starch on standard equipment and existing technology known in the plastic industry. Since natural starch generally has a granular structure, it needs to be destructurized and modified before it can be melt processed fike a thermoplastic material (1). 

---