Thermoplastic polyurethane (TPU) is any of a class of polyurethane

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

s with many properties, including

elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the cl ...

, 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 (1) diisocyanates with short-chain

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

s (so-called chain extenders) and (2) diisocyanates with long-chain

s. 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.

Morphology

A TPU resin consists of linear polymeric chains in block-structures. Such chains contain low

polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of ord ...

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 miscibility of the hard and soft segments in TPU depends on the differences in their glass transition temperature (Tg) which occurs at the onset of micro-Brownian segmental motion, identifiable by dynamic mechanical spectra. For an immiscible TPU, the loss modulus spectrum typically shows double peaks, each of which is assigned to the Tg of one component. If the two components are miscible, the TPU will be characterized by a single broad peak whose position lie between that of the two original Tg peaks of the pure components. 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 Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

injection molding Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...

, and

calender A calender is a series of hard pressure rollers used to finish or smooth a sheet of material such as paper, textiles, rubber, or plastics. Calender rolls are also used to form some types of plastic films and to apply coatings. Some calende ...

ing 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 uses. TPU is also a popular material found in flexible outer cases of devices like mobile phones and keyboard protectors. TPU is well known for its applications in wire and cable jacketing, hose and tube, in adhesive and textile coating applications, and as an impact modifier of other polymers. It is also used in high-performance films, such as high impact resistant glass structures. TPU is the thermoplastic elastomer used in fused filament deposition (FFD) 3D printing. The absence of warping and lack of need for primer makes it an ideal filament for 3D printers when objects need to be flexible and elastic. Since TPU is a thermoplastic, it can be melted by the 3D printer's extruder, printed, then cooled back into an elastic solid. TPU powders are also used for other 3D printing processes, like

selective laser sintering Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power and heat source to sinter powdered material (typically nylon or polyamide), aiming the laser automatically at points in space defined b ...

(SLS) and 3D inkjet printing. It's also used in large vertical injection or extrusion moulding machines to print directly without the intermediate step of filament extrusion or powder preparation.

Overview of TPU on the market

Properties of commercially available TPU include: * high abrasion resistance *low-temperature performance *high shear strength *high elasticity *transparency *oil and grease resistance The currently available TPUs can be divided mainly in two groups, based on soft segment chemistry: # polyester-based TPUs (mainly derived from

adipic acid Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: about 2.5 billion kilograms of this white crystalline powder are produced annually, ...

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

s) # polyether-based TPUs (mainly based on

tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...

(THF)

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be c ...

s). 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) TPU is the right choice when a flexible at low temperatures and/or abrasion resistance

TPE TPE may refer to: Places * Taipei (ISO 3166 code: TW-TPE), the capital of Taiwan * " Chinese Taipei" (IOC and FIFA code: TPE), the name used in some international organizations and competitions for Taiwan Science and Technology * TPE (cable system ...

is requested. Polyether-based TPU in cases where additional excellent

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

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 In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...

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.

Safety

TPU may contain siloxanes, some of which are considered

substances of very high concern A substance of very high concern (SVHC) is a chemical substance (or part of a group of chemical substances) concerning which it has been proposed that use within the European Union be subject to authorisation under the Registration, Evaluation, Au ...

by the European Union.

References

{{reflist Thermoplastic elastomers