Thermoplastic polyurethanes
Encyclopedia
Thermoplastic polyurethanes (abbreviated TPU) are a class of polyurethane
plastic
s with many useful properties, including elasticity
, transparency
, and resistance to oil, grease and abrasion. Technically, they are thermoplastic elastomer
s consisting of linear segmented block copolymers composed of hard and soft segments.
s (so-called chain extenders) and (2) diisocyanates with long-chain bifunctional diols (known as polyol
s). 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.
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 crystal
line 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
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
, injection moulding
and calendering
processing methods are applicable to these materials. Consequently, TPU scrap can be reprocessed.
The currently available TPUs can be divided mainly in two groups, based on soft segment chemistry:
(+ + excellent; + good; o acceptable; - poor; - - very poor)
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 isocyanate
s 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.
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s with many useful properties, including elasticity
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
, transparency
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
, and resistance to oil, grease and abrasion. Technically, they are thermoplastic elastomer
Thermoplastic elastomer
Thermoplastic elastomers , sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers which consist of materials with both thermoplastic and elastomeric properties...
s consisting of linear segmented block copolymers composed of hard and soft segments.
Chemistry
TPU are formed by the reaction of: (1) diisocyanates with short-chain diolDiol
A diol or glycol is a chemical compound containing two hydroxyl groups A geminal diol has two hydroxyl groups bonded to the same atom...
s (so-called chain extenders) and (2) diisocyanates with long-chain bifunctional diols (known as polyol
Polyol
A polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
s). 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 crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line 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
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
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
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
, injection moulding
Injection moulding
Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity...
and calendering
Calendering
Calendering is a finishing process used on cloth where fabric is folded in half and passed under rollers at high temperatures and pressures. Calendering is used on fabrics such as moire to produce its watered effect and also on cambric and some types of sateens.In preparation for calendering, the...
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
- high shear 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:
- polyester-based TPUs (mainly derived from adipic acidAdipic acidAdipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...
esterEsterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s) - polyether-based TPUs (mainly based on tetrahydrofuranTetrahydrofuranTetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
(THF) etherEtherEthers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
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.(+ + 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
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
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
Aliphatic compound
In organic chemistry, aliphatic compounds are acyclic or cyclic, non-aromatic carbon compounds.Thus, aliphatic compounds are opposite to aromatic compounds.- Structure :...
TPU based on aliphatic isocyanate
Isocyanate
Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...
s is used.
Recently, BASF has pioneered crosslinking during TPU production, made possible by adding liquid crosslinkers and using a masterbatch
Masterbatch
Masterbatch is a solid or liquid additive for plastic used for coloring plastics or imparting other properties to plastics .-Applications of additive masterbatches:...
. Plant-based bio TPU has been developed for green thermoplastic elastomer
Thermoplastic elastomer
Thermoplastic elastomers , sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers which consist of materials with both thermoplastic and elastomeric properties...
applications by Merquinsa, marketed as Pearlthane ECO.
Trade names
Key commercial brands available are:- Elastollan (BASFBASFBASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...
& Elastogran) - Pearlthane (Merquinsa)
- Desmopan (BayerBayerBayer AG is a chemical and pharmaceutical company founded in Barmen , Germany in 1863. It is headquartered in Leverkusen, North Rhine-Westphalia, Germany and well known for its original brand of aspirin.-History:...
) - Estane (LubrizolLubrizolThe Lubrizol Corporation, a Berkshire Hathaway company, is a specialty chemical company that produces and supplies technologies, which are designed to improve the quality and performance of products in the global transportation, industrial and consumer markets...
) - Pellethane (Lubrizol)
- Irogran (Huntsman)
- Exelast EC (Shin-Etsu Polymer Europe B.V.Shin-Etsu Chemicalis the largest chemical company in Japan, ranked No. 9 in Forbes Global 2000 for chemical sector. Shin-Etsu has the largest global market share for polyvinyl chloride, semiconductor silicon and photomask substrates....
) - Laripur (COIM SpA)
- Avalon (Huntsman) -
- Isothane (GrecoGrecoGreco may refer to:* Greco , a list of people with this surname* El Greco , a Greek-Spanish painter, sculptor and architect* El Greco , a composition by Vangelis...
)