Thermoplastic Elastomers (TPEs): Properties, Production and Applications
Thermoplastic elastomers (TPEs) have unique properties that have become essential in automotive, manufacturing and industrial applications. This has led them to replace traditional elastomers in the manufacture of tubes, hoses, insulation coatings, gaskets and seals. Thermoplastic materials that exhibit elasticity and flexibility similar to that of vulcanised rubbers are generally identified as TPEs.
What is a thermoplastic elastomer?
TPEs, also referred to as thermoplastic rubbers, represent a polymer class that exhibits both thermoplastic and elastomeric properties. TPEs are flexible, rubber-like materials with a low modulus of elasticity that are capable of stretching up to twice their original length and reverting back to their near initial dimensions when stress is removed. They can be melted at elevated temperatures and are processable. TPEs are widely used in many industries because of their recyclability and low processing cost. Typical rubbers cannot be reprocessed due to their thermosetting characteristics, whereas TPEs can be moulded, extruded and reused. TPEs do not necessarily require stabilisers or reinforcing agents, making them more consistent in batch-to-batch processing [1].
Thermoplastic elastomers are classified into these polymer classes [2]:
- Styrenic block copolymers (S-TPEs)
- Olefinic blends (TPOs)
- Thermoplastic vulcanizates (TPVs)
- Polyurethanes (TPUs)
- Copolyesters (COPEs)
- Polyamides (PEBAs)
Properties of thermoplastic elastomers
Phase structure
Thermoplastic elastomers are essentially phase-separated systems. A TPE is a biphasic material at its solid-state; with a hard-crystalline domain, and a soft amorphous domain. These phases are bonded chemically by block or graft polymerisation [2].
The hard phase contributes to the strength, chemical resistance and physical cross-linking of TPEs. It gives the material plastic properties such as [1]:
- Thermoplastic processability
- High tensile strength
- High tear resistance
On the other hand, the soft phase of TPEs offers elastomeric properties such as:
- High hardness
- Slip resistance
- Flexibility
- Shock absorption
- Compression resistance
Flow behaviour
Thermoplastic elastomers are generally made by melting processes similar to plastics that mostly rely on the flow of melted material at increased temperatures. Rheology, or the study of material flow, is essential in the success of processing TPEs. The rheology of TPEs is complex due to the melt viscosity’s dependence on temperature and shear rate. Thermoplastic elastomers are non-Newtonian, with high viscosities due to their long polymer chain structures [1].
Thermoplastic elastomers are mostly processed through melting techniques similar to plastics. Some of these melting processes include, but are not limited to:
- Injection moulding
- Extrusion
- Blow moulding
- Rotational lining
Recent methods have explored the use of TPEs as flexible filament materials in additive manufacturing that allows manufacturers to produce complex and futuristic designs.
Shrinkage
TPEs are compressible; and when cooled, they typically shrink in size. Cast moulds for thermoplastic elastomers are designed to be dimensionally larger, in anticipation of the shrinking and warping. In some cases, TPEs may require fillers, stabilisers, or reinforcements in order to compensate for the shrinkage [3].
Thermoplastic elastomer material production
Each of the thermoplastic elastomer classes is either blended mechanically or through dynamic vulcanisation.
Mechanical blend
As in the case of Polyolefin TPE, the blend is prepared mechanically, mixing the hard polymer to the elastomer on high-shear compounding equipment or a continuous mixer. The viscosities of the two materials must be matched at the temperature and shear rate of mixing. The proportion and solubility parameters of the two materials must be considered in blending [1].
Dynamically vulcanised blend
In dynamically vulcanised blends, the elastomeric phase is discontinuous and cross-linked. This is done by partially vulcanising the soft elastomer phase at a high shear rate and an elevated temperature above the melting point of the thermoplastic in order to activate and complete the vulcanisation. The compatibility of the material, particle size, and degree of cure are some parameters that must be considered in order to achieve optimum properties [4].
What are thermoplastic elastomers used for?
Automotive
The automotive industry is the largest segment – at nearly 40% application share – that utilises TPE materials. Auto manufacturers need strong elastic materials that are capable of withstanding weathering, high temperatures, as well as chemical and abrasion resistance. TPEs are an excellent solution for many parts such as shock-absorbing seals, bumper stops, vibration dampeners and other weather-stripping elements [1].
Consumer goods
Following the automotive industry, in terms of volumetric consumption of TPEs, is the consumer goods sector. TPEs are found in many products such as footwear soles, appliances, sports equipment and leisure goods.
Construction
TPEs are widely used in the construction industry across many applications such as an additive material in asphalting roads, electrical wiring insulations and in adhesives, sealants and coatings [1].
Medical
The biocompatibility of TPEs mean the materials can be used in the medical industry in a range of applications. You can read more about their uses here: Thermoplastic Elastomers for Medical Applications
Sources
[1] J.G. Drobny, 2014, Handbook of Thermoplastic Elastomers, Elsevier
[2] R.J. Spontak, N.P. Patel, 2000, “Thermoplastic elastomers: fundamentals and applications.” Current Opinion in Colloid & Interface Science, 5, pp. 333–40.
[3] “Thermoplastic Elastomer (TPE) FAQS,” n.d., from: https://www.polyone.com/products/thermoplastic-elastomers/tpe-knowledge-center/tpe-faqs
[4] S. Abdou-Sabet, R. C. Puydak, C. P. Rader, 1996, “Dynamically Vulcanized Thermoplastic Elastomers.” Rubber Chemistry and Technology, 69, no. 3, pp. 476-94.