Starch polymer
As starch polymers are all generally polymers referred to the naturally occurring polysaccharide starch build. While the polymer of starch itself is also referred to as starch polymer in the field of biochemical research, in technical parlance this term primarily refers to plastics and other materials that are manufactured on the basis of starch and are used as biomaterials .
Structure and properties of strength
→ Main article: Starch , starch as a renewable raw material
As a polysaccharide, starch is a natural biopolymer that is stored in the cells of plants in the form of starch grains as an energy storage substance. It consists of α-D- glucose units ( monomers ) that are linked to one another via glycosidic bonds. This results in the chemical formula (C 6 H 10 O 5 ) n , where C stands for the carbon contained , H for hydrogen and O for oxygen . A starch molecule normally consists of 10 4 to 10 6 glucose units, of which there are two different types:
- 20–30% of the starch consists of amylose , linear chains with a helical (screw) structure that are only linked α-1,4-glycosidically
- 70–80% consists of amylopectin , highly branched structures, with α-1,6-glycosidic and α-1,4-glycosidic linkages.
Due to the OH groups and the glycosidic bond of the individual monomers, starch can be chemically modified in different ways and thus made usable for different purposes. Thus, through various substitutions, starch ethers (starch-OR) or starch esters (starch-O-CO-R) can be obtained, and different molecules can be linked to the starch polymer (RO-starch-OR and / or R-OC-O-starch- O-CO-R). Oxidized starches and starch molecules with increased COOH or CHO groups can be obtained by oxidation of the primary OH groups ; oxidation of the secondary OH groups results in increased CHO and CO groups and ring formation of the molecules C 2 and C 3 . During thermolysis and pyrolysis , laevoglucosan can be obtained with the separation of water , which can be used as a raw material for a number of products.
The glycosidic bonds can be broken by hydrolysis with the help of enzymes or acids , whereby various modified starches , dextrins and starch-based sugars are formed.
Starch-based materials
→ Main article: bio-based plastic
Starch plays a role in the manufacture of bio-based plastics . The most important starch-based bioplastics are extruded thermoplastic starches (TPS) and starch blends as well as polylactic acid (PLA) (in order of their market share). All other bioplastics such as polyhydroxyalkanoates (PHA) together make up less than 5%.
While thermoplastic starch, starch blends and starch-filled polyolefins represent a direct use of starch or modified starches, PHAs and PLA are produced through fermentation . The fermentation substrate can - as shown above - build on different raw materials. At the moment, maize starch is mainly used for the production of PLA ( NatureWorks in the USA).
Thermoplastic strength
→ Main article: Thermoplastic starch
Thermoplastic starch (TPS) is usually only one of the components from which modern bioplastics are made on the basis of starch due to their negative property of absorbing water . 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.
With a market share of around 80 percent, thermoplastic starch is currently the most important and most widely used representative of bioplastics.
Starch blends
→ Main article: Starch blend
Starch blends are mixtures of thermoplastic starch (TPS) and plastics , which can be based either on petroleum or on renewable raw materials. Internationally, starch blends represent one of the largest proportions of bio-based plastics and their properties vary depending on their composition.
Starch-filled materials
In the case of the starch-filled composite materials, the starch in particulate or granular form is used as a filler . The filling can improve the technical material properties and the degradation properties of the materials.
The latter is due, on the one hand, to the direct biological degradation of the starch, but also to the increased surface area of the matrix plastic from the starch filling. However, matrix polymers such as polyethylene are not biodegradable, but macroscopic material decomposition occurs due to the breakdown of the starch granules.
Materials produced by fermentation
The biopolymers produced by fermentation, i.e. by fermentation of starch, sugar and other raw materials by various microorganisms , include above all the polyhydroxyalkanoates (especially polyhydroxybutyrate (PHB)) and the polylactides based on lactic acid produced by fermentation . Since the structure of the starch is completely rebuilt by the metabolism of the organisms, these materials are usually not referred to as starch polymers, but instead only as starch-based polymers.
supporting documents
- ↑ Article Starch, Chemical Properties. In: Hans Zoebelein (Ed.): Dictionary of Renewable Resources. 2nd edition, Wiley-VCH, Weinheim and New York 1996; Pages 266-267. ISBN 3-527-30114-3 .
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