Thermoplastic polyurethane

Chemistry

TPU is a block copolymer consisting of alternating sequences of hard and soft segments or domains formed by the reaction of: (1) diisocyanates with short-chain diols (so-called chain extenders) and (2) diisocyanates with long-chain diols. By varying the ratio, structure and/or molecular weight of the reaction compounds, an enormous variety of different TPU can be produced. This allows urethane chemists to fine-tune the polymer’s structure to the desired final properties of the material. For example, a greater ratio of hard to soft segments will result in a more rigid TPU, while the reverse is also true.

Morphology

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 pieces 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.

Uses

TPU has many applications including automotive instrument panels, caster wheels, power tools, sporting goods, medical devices, drive belts, footwear, inflatable rafts, and a variety of extruded film, sheet and profile applications. TPU is also a popular material found in outer cases of mobile electronic devices, such as mobile phones. It is also used to make keyboard protectors for laptops. TPU is well known for its applications in performance films, wire and cable jacketing, hose and tube, in adhesive and textile coating applications and as an impact modifier of other polymers. . TPU pellets are used as Adidas' most recent cushioning technology, known as Boost. Several thousands of TPU pellets are bound together to create a comfortable sole for the shoe.

Overview of TPU on the market

Properties of commercially available TPU include:

The currently available TPUs can be divided mainly in two groups, based on soft segment chemistry:

1. polyester-based TPUs (mainly derived from adipic acid esters)
2. polyether-based TPUs (mainly based on tetrahydrofuran (THF) ethers). The differences between these two groups are outlined in the table below:

Table of properties

Table 1: Main differences between polyester- and polyether-based TPU.

(A = excellent; B = good; C = acceptable; D = poor; F = very poor)

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 and GRECO, marketed as Pearlthane ECO and Isothane respectively.

Trade names

Key commercial brands available are:

Zythane TPU Data Sheet

Boost (adidas)