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Thermoplastic polyurethane (TPU) is any of a class of
polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from ...
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 adapta ...
s with many 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, 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 properties. ...
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 ...
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)
diisocyanate 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 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
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. 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 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 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, macros ...
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-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 cal ...
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 (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 annuall ...
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 ...
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—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 ...
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 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, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...
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 ...
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 has pioneered crosslinking during TPU transformation, made possible by adding liquid crosslinkers or using a solid granulated additive
masterbatch Masterbatch (MB) is a solid additive used for coloring (color masterbatch) or imparting other properties (additive masterbatch) to plastics. Masterbatch is a concentrated mixture of pigments and/or additives which is manufactured by encapsulation du ...
. Plant-based bio TPU has been developed for green
thermoplastic elastomer Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, 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 properties. ...
applications by BASF, Merquinsa-Lubrizol and GRECO, marketed as Elastollan N, Pearlthane ECO and Isothane respectively.


Safety

TPU may contain
siloxane A siloxane is a functional group in organosilicon chemistry with the Si−O−Si linkage. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H(OSiH2)''n''OH and (OSiH2)n. Siloxanes also include branched compound ...
s, some of which are considered substances of very high concern by the European Union.


See also

*
Polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily work ...
*
Silicone A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cookin ...
* Mobile phone cover


References

{{reflist Thermoplastic elastomers