Thermoplastic polyurethane


Thermoplastic polyurethane is any of a class of polyurethane plastics with many 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.

Chemistry

TPU is a block copolymer consisting of alternating sequences of hard and soft segments or domains formed by the reaction of diisocyanates with short-chain diols and 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, alternating with shorter, high polarity 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 molding 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 wire and cable jacketing, hose and tube, in adhesive and textile coating applications, as an impact modifier of other polymers.. Also used in performance films, for example transparent TPU is used in high demanding transparent film applications like high impact resistant glass structures
TPU is the thermoplastic elastomer used in FFD fused filament deposition 3D printing. The absence of warping and the fact no primer is needed, make it ideal for filament 3D printers when objects need to be flexible and elastic. The fact that TPU is a thermoplastic allows those filaments to be melted again by the 3D printer "extrusion" head, and then cooled back into the solid-elastic piece.
TPU powders are also used for other 3D printing processes, like LASER Sintering and 3D inkjet printing. It's also possible to use in big equipments vertical or to directly print, without the intermediate step of filament extrusion or powder preparation, just choosing the adequate TPU granulates.

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
  2. polyether-based TPUs.
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.
PropertyPolyester-based TPUPolyether-based TPU
Abrasion resistanceAA
Mechanical propertiesAB
Low temperature flexibilityBA
Heat agingBD
Hydrolysis resistanceDA
Chemical resistanceAC
Microbial resistanceDA
Adhesion strengthBD
InjectabilityBB

TPU is the right choice when a flexible at low temperatures and/or abrasion resistance TPE is requested. Polyether-based TPU in cases where additional excellent hydrolysis and microbial resistance is required, as well as in cases where extreme low-temperature flexibility is important. Ester-based TPU in cases the oil and greases resistance is more relevant.
When stable light colour and non-yellowing performance are required, aliphatic TPU based on aliphatic isocyanates is used.
BASF has pioneered crosslinking during TPU transformation, made possible by adding liquid crosslinkers or using a solid granulated additive masterbatch. Plant-based bio TPU has been developed for green thermoplastic elastomer applications by BASF, Merquinsa-Lubrizol and GRECO, marketed as Elastollan N, Pearlthane ECO and Isothane respectively...

Trade names

Key commercial brands available are: