Thermoplastic polyurethanes (abbreviated TPU) are a class of polyurethane plastics with many useful properties, including elasticity, transparency, and resistance to oil, grease and abrasion. Technically, they are thermoplastic elastomers consisting of linear segmented block copolymers composed of hard and soft segments.
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TPU are formed by the reaction of: (1) diisocyanates with short-chain diols (so-called chain extenders) and (2) diisocyanates with long-chain bifunctional diols (known as polyols). The practically unlimited amount of possible combinations producible by varying the structure and/or molecular weight of the three reaction compounds allows for an enormous variety of different TPU. This allows urethane chemists to fine-tune the polymer’s structure to the desired final properties of the material.
The final resin consists of linear polymeric chains in block-structures. Such chains contain low polarity segments which are rather long (called soft segments), alternating with shorter, high polarity segments (called hard segments). Both types of segments are linked together by covalent links, so that they actually form block-copolymers.
The polarity of the hard segments creates a strong attraction between them, which causes a high degree of aggregation and order in this phase, forming crystalline or pseudo crystalline areas located in a soft and flexible matrix. This so-called phase separation between both blocks can be more or less important, depending on the polarity and the molecular weight of the flexible chain, the production conditions, etc. The crystalline or pseudo crystalline areas act as physical cross-links, which account for the high elasticity level of TPU, whereas the flexible chains will impart the elongation characteristics to the polymer.
These "pseudo crosslinks", however, disappear under the effect of heat, and thus the classical extrusion, injection moulding and calendering processing methods are applicable to these materials. Consequently, TPU scrap can be reprocessed.
TPU is very commonly used for electronic device cases, such as the iPhone. It provides protection for the devices, yet keeps a soft texture, which many consumers enjoy.
Among the features of commercially available TPU are:
The currently available TPUs can be divided mainly in two groups, based on soft segment chemistry:
Table 1: Main differences between polyester- and polyether-based TPU. (+ + excellent; + good; o acceptable; - poor; - - very poor)
| Property | Polyester-based TPU | Polyether-based TPU |
|---|---|---|
| Abrasion resistance | ++ | 0 |
| Mechanical properties | ++ | + |
| Low temperature flexibility | 0 | ++ |
| Heat ageing | + | - |
| Hydrolysis resistance | -- | ++ |
| Chemical resistance | ++ | - |
| Microbial resistance | -- | + |
| Adhesion strength | + | - |
| Injectability | ++ | 0 |
In other words, polyether-based TPU is used only in cases where excellent hydrolysis and microbial resistance is required, as well as in cases where extreme low temperature flexibility is important.
When stable light colour and non-yellowing performance are required, aliphatic TPU based on aliphatic isocyanates is used.
Recently, BASF has pioneered crosslinking during TPU production, made possible by adding liquid crosslinkers and using a masterbatch. Plant-based bio TPU has been developed for green thermoplastic elastomer applications by Merquinsa, marketed as Pearlthane ECO.
Key commercial brands available are:
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