Thermoplastic polyurethane (TPU)
2-15 usd/kgThermoplastic polyurethane (TPU) has the characteristics of both plastic and rubber. Unlike polyurethane resin (PUR), it is a melt-processable thermoplastic elastomer (TPE), which means it can be injection moulded, extruded and recycled. This makes it easier to process than thermosetting material. It is durable, flexible, strong and suitable for demanding applications in automotive, sports and textiles. Examples range from colourful moulded shoe soles to industrial non-marking caster wheels.
As with PUR, it is built by mixing a polyol with diisocyanate. There are two main types of TPU: polyester- and polyether-polyol based. This defines the flexible part of the polymer, and the diisocyanate the rigid part. The most commonly used diisocyanate for TPU is methylene diisocyanate (MDI). While polyester types offer superior abrasion resistance, heat resistance and gas permeability (film applications), polyether TPU gives better low temperature flexibility, water resistance and microbial resistance. A third group is based on polycaprolactone (PCL), which combines the toughness and resistance of polyester types with improved low temperature performance and water resistance.
By varying the type and ratio of these blocks, the physical properties of the TPU can be tailored to a range of applications. An advantage of this polymer structure, compared to polyvinyl chloride (PVC) for example, is that it does not require a plasticiser to be flexible. Plasticiser can migrate to the surface over time, reducing the elastomeric effect and potentially releasing hazardous substances.
TPU can be further divided into aromatic (closed rings of atoms) and aliphatic (straight or branched chains of atoms) types. Aromatic TPU are based on MDI and the most common. They are used in applications that require flexibility, strength and toughness. Aliphatic TPU offer superior resistance to UV, colour fading and offer exceptional optical clarity.
It is possible to produce partially bio-based TPU, by replacing one of the elements with prepolymer derived from biomaterial. An example is Apilon by Trinseo.
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Thermoplastic polyurethane (TPU) has a desirable mix of properties which make it particularly well suited to low temperature applications where flexibility is critical. Compared to other elastomers – such as polyvinyl chloride (PVC), neoprene and polyamide (PA), nylon – TPU has superior resistance to abrasion. It has excellent resistance weathering (particularly aliphatic types) and UV, oil and grease, performs well across a wide temperature range, with impressive elasticity and toughness.
It is primarily used in parts that require flexibility and resistance, such as footwear and shoe soles; hose, pipe and tubing; mobile phone covers; interior and exterior automotive parts; to name a few. It is injection moulded and over-moulded as soft touch skin. In automotive interior applications, it is also applied as a slush moulded skin or foil, which is insert moulded to make trim and instrument panels (dashboards), for example. Polyether-based TPU maintain excellent flexibility at a wide range of temperatures and have high resistance to water and microbes, making them very useful in medical applications.
TPU is suitable for several 3D printing techniques, including fused deposition modelling (FDM), selective laser sintering (SLS) and 3D inkjet printing.
Aliphatic TPU is available in water-clear grades, which are suitable for injection moulding and extrusion. It is used in applications ranging from moulded footwear to extruded pipes and hoses for use in automotive, aircraft and medical applications.