Thermoplastic strength

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Thermoplastic starch packaging chips

Thermoplastic starch (TPS) is a thermoplastic biopolymer in which the starch granules used as raw material are destructured. With a market share of around 80 percent, thermoplastic starch is currently the most important and most common representative of bioplastics and is classified as a starch polymer due to its raw material basis .

Raw material starch

Section from an amylose polymer
Part of an amylopectin polymer

Main article: Starch , starch as a renewable raw material

Starch occurs in various tissues and varying proportions of all green plants . The tissues of the seeds , tubers , bulbs and rhizomes, as well as the wood rays and the wood parenchyma in the wood body of the trees, are very rich in starch .

Starch molecules consist of D - glucose units that are linked to one another via glycosidic bonds. Strength is too

  • 20–30% amylose , linear chains with a helical (screw) structure that are only linked α-1,4-glycosidically and
  • 70–80% amylopectin , highly branched structures, with α-1,6-glycosidic and α-1,4-glycosidic links.

The reserve starch differs in its large grain size from the fine-grained starch occurring in the assimilating tissue. It forms the basis for obtaining starch from starch-rich plants. The most important plants that are used to obtain starch as raw material are maize , wheat and potatoes in Europe, Africa and North America, and tapioca in Asia. The raw mass is cleaned of by-products such as proteins , vegetable oils and vegetable fibers and prepared accordingly for use.

production method

The destructuring of the starch granules during production in the extruder is mainly thermomechanical and depends on the amount of water added, the shear forces and the temperature. In addition, there is a thermochemical gelatinization that takes place depending on the water content due to the water absorption of the starch molecules.

The polarity of the components amylose and amylopectin leads to molecular interrelationships in the destructured starch, which lead to difficult extrudability and a brittle material. In order to counteract this, auxiliary substances such as water and plasticizers , e.g. glycerine , are added to the starch to make it plasticizable . The glass transition temperature T g , which for pure thermoplastic starch is at a temperature of about 80 ° C., can be greatly reduced in this way, the material becomes tougher and thus less brittle. Mixtures with other polymers represent further optimization options, which result in starch blends .

Starch blend

Main article: Starch blend

Thermoplastic starch, due to its negative property of absorbing water, is usually just one of the components from which modern bioplastics are made on the basis of starch . The second basic component of these plastic blends consists of water-repellent, biodegradable polymers such as polyesters , polyester amides , polyester urethanes or polyvinyl alcohol .

A plastic blend is therefore composed of two phases, the continuous and the hydrophobic polymer phase , and the disperse and hydrophilic starch phase. During the melting process in the extruder , the water-soluble, disperse starch phase and the water-insoluble, continuous plastic phase combine to form a water-resistant starch plastic.

Use of thermoplastic starch

Pure starch has the property of absorbing moisture and is therefore mainly used in the pharmaceutical sector to produce drug capsule shells, although it has largely been replaced by hard gelatine. Natural softeners and plasticizers such as sorbitol and glycerine are added to make the readily available starch thermoplastic . These additives allow a specific change in the material properties of the so-called thermoplastic starch, which is adapted to the intended use.

Starch blends and compounds are developed and produced individually for further use in the plastics processing industry, depending on the area of ​​application. As granulates , they can be processed into foils, thermoformable flat foils, injection molded articles or coatings on the existing systems. Examples of this are carrier bags, yoghurt or drinking cups, plant pots, cutlery, diaper foils, coated paper and cardboard. Starch can also be thermoplastically modified by chemical changes such as conversion to starch esters or starch ethers with a high degree of substitution. However, because of the high costs involved, these methods have not yet caught on.

literature

  • Hans-Josef Endres, Andrea Siebert-Raths: Technical biopolymers. Hanser-Verlag, Munich 2009. ISBN 978-3-446-41683-3 .
  • Jürgen Lörcks: Bioplastics. Plants - raw materials, products. Fachagentur Nachwachsende Rohstoffe eV, Gülzow 2005 ( PDF download ).
  • P. Eyerer, P. Elsner, T. Hirth (Eds.): The plastics and their properties. 6th edition, Springer Verlag, Heidelberg 2005; Pages 1444-1448. ISBN 3-540-21410-0 .
  • Jörg Müssig, Michael Carus: Bio-polymer materials as well as wood and natural fiber reinforced plastics. In: Market Analysis of Renewable Raw Materials Part II. Agency for Renewable Raw Materials eV, Gülzow 2007 ( PDF download ).