Thermoplastic elastomers

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Thermoplastic elastomers (abbreviation  TPE or TPR from Thermoplastic Rubber, English for '' thermoplastic rubber ' , sometimes also called elastoplasts ) are plastics that behave similarly to classic elastomers at room temperature , but can be plastically deformed when heat is applied and thus have a thermoplastic behavior demonstrate.

properties

“Normal” elastomers are chemically wide-meshed network molecules . The cross-links cannot be dissolved without decomposition of the material.

Thermoplastic elastomers are materials in which elastic polymer chains are embedded in thermoplastic material. They can be processed in a purely physical process in a combination of high shear forces, the action of heat and subsequent cooling. Although there is no need for chemical crosslinking through time-consuming and temperature-consuming vulcanization as is the case with elastomers, the parts produced have rubber-elastic properties due to their special molecular structure. Renewed exposure to heat and shear forces leads to melting and deformation of the material. At the same time, this means that the TPE are far less thermally and dynamically resilient than standard elastomers. The TPE are therefore not a "successor product" of conventional elastomers, but a supplement that combines the processing advantages of thermoplastics with the material properties of elastomers.

In some areas, thermoplastic elastomers have physical cross-linking points ( secondary valence forces or crystallites) that dissolve when heated without the macromolecules decomposing. Therefore they can be processed much better than normal elastomers. Plastic waste can also be melted down again and processed further.

However, this is also the reason why the material properties of thermoplastic elastomers change non-linearly over time and temperature. The two main measurable physical material properties are compression set and stress relaxation . Compared to ethylene propylene diene rubber (EPDM), they have poorer material properties in terms of their short-term behavior, and the raw material is more expensive. In the long-term behavior, however, the picture is reversed compared to EPDM.

Since the manufacturing process is basically the same as that of thermoplastics, similarly short cycle times are possible. In manufacturing, thermoplastic elastomers are increasingly being used in body seals for automobiles and in components. They can be extruded, injection molded or blow molded and are usually purchased ready-to-use.

Classification

A distinction is made between copolymers and elastomer alloys based on their internal structure.

Copolymers are used either as random or as block copolymers. The former consist of a crystallizing (and thus physically crosslinking) main polymer such as B. polyethylene , the degree of crystallization by a randomly installed along the chain comonomer such. B. vinyl acetate is reduced to such an extent that the crystallites (= the hard phase) in the finished material (in the example EVA ) no longer have direct contact. As in conventional elastomers, they then act as isolated cross-linking points.

In block copolymers the hard and soft segments are sharply separated in one molecule (e.g. SBS, SIS). With TPEs, below a certain temperature, the material separates into a continuous and a discontinuous phase. As soon as the latter falls below its glass transition temperature Tg (the Tg of the continuous phase is well below the later application temperature), it again acts as a crosslinking point.

Elastomer alloys are polyblends , i.e. mixtures (mixtures) of finished polymers , i.e. the plastic consists of several types of molecules. Different mixing ratios and additives result in tailor-made materials (for example polyolefin elastomer made of polypropylene (PP) and natural rubber (NR) - depending on the proportion, they cover a wide range of hardnesses).

A distinction is made between the following categories:

Categories with

Short name

outdated

Short name

Explanation Trade names
TPA TPE-A Thermoplastic polyamide elastomers PEBAX ( Arkema )
TPC TPE-E Thermoplastic copolyester elastomers Hytrel ( Du Pont ), Keyflex ( LG Chem )
TPO TPE-O Thermoplastic elastomers based on olefin , predominantly PP / EPDM Elastron TPO, Saxomer TPE-O ( PCW )
TPS TPE-S Thermoplastic styrene block copolymers (SBS, SEBS, SEPS, SEEPS and MBS) Elastron G and Elastron D, Kraton ( Kraton Polymers ), Septon ( Kuraray ), Styroflex ( BASF ), Thermolast ( Kraiburg TPE ) ALLRUNA (ALLOD Material GmbH & Co.KG) or Saxomer TPE-S ( PCW )
TPU TPE-U Urethane-based thermoplastic elastomers Elastollan ( BASF ) or Desmopan, Texin, Utechllan ( Covestro )
TPV TPE-V Thermoplastic vulcanizates or cross-linked thermoplastic elastomers based on olefins , mainly PP / EPDM Elastron V, Sarlink ( DSM ), Santoprene ( Exxon )
TPZ - Unclassified thermoplastic elastomers of any composition or structure other than the categories already mentioned. ---

advantages

Thermoplastic elastomers are elastomers that behave like classic elastomers at room temperature, but become deformable when heated. Usually these are copolymers that consist of a “soft” elastomer and a “hard” thermoplastic component. The properties of elastoplasts lie between those of elastomers and thermoplastics. Examples are block copolymers made from styrene and polyolefins developed by Shell from 1965 onwards .

A great advantage of these elastic plastics is the ability to weld them in order to create watertight connections.

Web links

Individual evidence

  1. ↑ a b c What are thermoplastic elastomers? .
  2. ↑ Thomas Hirth; Polymer Engineering: Technologies and Practice, pp. 167-176, ISBN 978-3540724025 .
  3. ↑ DIN EN ISO 18064: 2015-03: Thermoplastic elastomers - nomenclature and abbreviations (ISO 18064: 2014); German version EN ISO 18064: 2014 . Ed .: DIN German Institute for Standardization e. V. Beuth Verlag GmbH, Berlin March 2015.
  4. ↑ Martin Bonnet: Plastics in engineering applications: properties, processing and practical use of polymer materials. Vieweg + Teubner Verlag, 2008, ISBN 978-3-8348-0349-8 , p. 55.
  5. ↑ W. Woebcken, K. Stoeckhert, HBP Gupta: plastic lexicon. 9th edition, Hanser Verlag, 1998, ISBN 978-3-446-17969-1 , p. 256.