Thermoplastic polyurethane (TPU) is any of the

polyurethane Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

s that are

thermoplastic A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains as ...

; that is, they become pliable when heated and harden when cooled. This is in contrast to most polyurethanes, which are

thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...

s, hardening irreversibly. Thermoplastic polyurethanes (TPUs) reveal vast combinations of both physical properties and processing applications. Usually, they are flexible and elastic with good resistance to impact, abrasion and weather. With TPUs, there is the possibility for colouring as well as fabrication using a wide range of techniques. The incorporation of TPUs could therefore improve the overall durability of many products. Thermoplastic polyurethanes have many desirable properties, including elasticity, transparency, and resistance to oil, grease, and abrasion. Technically, they are

thermoplastic elastomer Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers (TPR), are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) that consist of materials with both thermoplastic and elastomeric prop ...

s, consisting of linear segmented

block copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...

s 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 may also be called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting gro ...

s (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 segments which are rather long (called soft segments), alternating with shorter, high polarity segments (called hard segments). Both types of segments are linked 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 The glass–liquid transition, or glass transition, is the gradual and Reversible reaction, reversible transition in amorphous solid, amorphous materials (or in amorphous regions within Crystallinity, semicrystalline materials) from a hard and rel ...

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 lies 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 A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

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-link In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

s, 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 section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...

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 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 calender rolls are ...

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, fire hoses, buffer weight tips 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 used 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 one 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 hotend, printed, then cooled into an elastic solid. TPU powders are also used for other

3D printing processes A variety of processes, equipment, and materials are used in the production of a three-dimensional object via additive manufacturing. 3D printing is also known as additive manufacturing, because the numerous available 3D printing process tend to ...

, such as

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 ...

(SLS) and 3D inkjet printing. It is 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 into 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 C6H10O4. It a white crystalline powder at standard temperature and pressure. From an industrial perspective, it is the most important dicarboxylic acid at about 2.5 billion ...

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

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 ...

(THF)

ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

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 flexibility at low temperatures and/or an abrasion resistant

(TPE) 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 where oil and grease 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 (in which all ...

TPU based on aliphatic

isocyanate In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

s is used.

BASF BASF SE (), an initialism of its original name , is a European Multinational corporation, multinational company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters are located in Ludwigshafen, Ge ...

has pioneered crosslinking during TPU transformation, made possible by adding liquid crosslinkers or using a solid granulated additive

masterbatch A masterbatch is a concentrated mixture of pigments and / or additives blended and extruded together in a carrier matrix, such as resin or wax, that is used to add these mixed additives to a final plastic product. The additives may be used for col ...

. Plant-based bio TPU has been developed for green

applications by BASF, Merquinsa-Lubrizol and GRECO, marketed as Elastollan N, Pearlthane ECO and Isothane respectively.

Safety

TPU may contain

siloxane In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae and . Siloxanes ...

s, some of which are considered substances of very high concern by the European Union.

References

{{reflist Thermoplastic elastomers