Thermoplastic polyurethane

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.

==Chemistryshort-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.

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 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.

Overview of TPU on the market

Among the features of commercially available TPU are:

• excellent abrasion resistance
• outstanding low-temperature performance
• excellent mechanical properties, combined with a rubber-like elasticity
• very good tear strength
• high elasticity
• high transparency
• good oil and grease resistance

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. (+ + 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.

Trade names

Key commercial brands available are:

• Elastollan (BASF & Elastogran)
• Pearlthane (Merquinsa)
• Desmopan (Bayer)
• Estane (Lubrizol)
• Pellethane (Lubrizol)
• Irogran (Huntsman)
• Exelast EC (Shin-Etsu Polymer Europe B.V.)
• Laripur (COIM SpA)