Thermoplastic polyurethane (TPU): The wear-resistant elastomer for high mechanical stress

TPU (Thermoplastic Polyurethane) is a thermoplastic elastomer based on segmented polyurethane block copolymers and is referred to as TPE-U within the TPE-Materials classified.

TPU exhibits very high abrasion, tear and cut resistance as well as good resistance to mineral oils, fats and ozone. TPU can be processed in a hardness range of 60 Shore A to 75 Shore D, ranging from medium hardness to almost „plastic-hard“. Compared to other elastomers such as NBR or EPDM TPU is significantly more mechanically durable and is processed not by vulcanization but by melting and cooling, like thermoplastics. Due to the thermal processing, TPU has a lower temperature resistance and tends to remain deformed at higher temperatures.

Typical applications include wear-resistant wiper blades, sealing lips, roller coatings and Buffer in which there is little chemical and no special thermal stress.

1. What is TPU?

TPU belongs to the thermoplastic elastomers (TPE) and therefore behaves rubber-like at room temperature, but melts like a thermoplastic when heated and can be injection molded or extruded. Within the TPEs, TPU is considered the most mechanically efficient variant: tensile strength, tear resistance, and abrasion resistance are above those of most vulcanized rubbers. The properties can be adapted within a wide range through the choice of raw materials and the ratio of hard to soft segments – from soft and flexible to virtually rigid structural materials.

What is the difference between TPU, TPE-U, PUR and trade names such as Elastollan or Desmopan?

TPU and TPE-U denote the same material. TPU is the abbreviation valid according to ISO 18064; TPE-U is an older spelling that continues to be widely used in the German-speaking world. PUR or PU, on the other hand, is the general term for all polyurethanes – from hard foam to paints to elastomers. This also includes cast polyurethanes as well as the solid-state urethane rubbers AU (polyester-based) and EU (polyether-based). These are chemically crosslinked and cannot be melted back after molding. TPU, on the other hand, is physically crosslinked and therefore processable by injection molding. Elastollan (BASF), Desmopan (Covestro), Estane and Pellethane (Lubrizol) are trade names for TPU types from various manufacturers and are not specific material categories. Possible differences between the products result from the polyol base, the hardness, the addition of various additives, and the production of the raw material.

How is TPU chemically structured?

TPU is formed by polyaddition from three basic building blocks: a polyol, a diisocyanate, and a chain extender. Long-chain polyols (P), such as polyester, polyether, polycaprolactone, or polycarbonate diol, are linked to a diisocyanate (D), such as hexamethylene diisocyanate (HDI), or methylene diphenyl diisocyanate (MDI). 1,4-butanediol is often used as a short-chain chain extender (C). A urethane group forms at each linking point between the diisocyanate and the polyol or chain extender, giving the material its name.

Figure 1: Structure of a TPU polymer chain made of flexible segments (polyol P and diisocyanate D) and rigid segments (diisocyanate D and chain extenders C)

Figure 1 shows a section of a TPU polymer chain. In the left section, polyol and diisocyanate alternate (D–P–D–P–D). The long, mobile polyol chains form flexible segments, also called soft segments, while diisocyanate and chain extenders follow one another at short intervals (C–D–C–D–C) and form a solid segment of compact, rigid units. Along the entire chain, this sequence repeats many times, which is why TPU is considered a block copolymer.

The solid segments of adjacent chains aggregate to form crystalline hard segments, which are stabilized by hydrogen bonding between the urethane groups. These hard segment domains act as physical crosslinking points embedded within a soft, elastic matrix of the flexible segments (cf. morphology in the TPE contribution and Figure 2).

Figure 2: TPU forms crystalline hard segments, surrounded by a flexible matrix

Upon heating, the hard segment domains dissolve, the polymer chains become mobile, the TPU melts and becomes liquid. Upon cooling, the solid segments reorganize into domains. This thermally reversible process distinguishes TPU from vulcanized elastomers with irreversibly chemically bonded sites.

The ratio of rigid to flexible segments (in Figure 1, the proportion of the right-hand segments relative to the left-hand ones) determines the material behavior. The higher the proportion of rigid segments, the higher the hardness, stiffness, and heat resistance. A higher proportion of soft segments, in turn, increases elasticity and cold flexibility.

2. Properties of TPU

TPU is characterized by a combination of very high mechanical strength, good elasticity, and resistance to oils, fats, and ozone. However, at high temperatures, TPU melts and is therefore not suitable for use in hot water or polar solvents. TPU is also resistant to many chemicals:

High durability	Conditional durability
Abrasion and wear	UV radiation (aromatic types)
Mineral oils and fats	Water at room temperature (polyester-TPU)
Ozone	Diluted acids and alkalis
Aliphatic fuels	Aromatic fuels
Oxygen and aging	Alcohols

Table 1: Chemical resistance of TPU

Depending on the hardness, polymer base and additives, mechanical TPU achieves tensile strengths of approximately 30 to 60 MPa and tear elongation of 400 to 700 %. The abrasion loss according to ISO 4649 is often in the range of 20 to 40 mm³, which is significantly lower than the values of many vulcanized elastomers. In addition, TPU has high load-carrying capacity and good damping properties, which makes the material interesting for buffer and damping elements as well as, for example, for shoe soles. The density, at approximately 1.1 to 1.2 g/cm³, is slightly above that of TPE-S or EPDM. TPU exhibits good electrical insulation properties and is therefore often used for cable sheaths. The material is hygroscopic and absorbs moisture from the environment, which is why a preliminary drying is required especially during processing.

At temperatures close to the melting point and under prolonged stress, TPU tends to have a higher residual compressive deformation than chemically crosslinked elastomers. Therefore, TPU is suitable for dynamic, wear-resistant seals at moderate temperatures, but less suitable for static seals under sustained pressure at high temperatures.

Shore hardnesses for TPU

The available hardness range of TPU typically ranges from 60 Shore A to 75 Shore D and is adjusted based on the ratio of hard to soft segments. It is therefore more like FKM or HNBR in the case of harder base polymers, with the added benefit that TPU is also used in the much harder Shore-D range. TPU in the range of approximately 60 to 80 Shore A is used for applications requiring high flexibility, such as hoses, films, membranes, sealing lips and soft-touch surfaces. However, soft types are more demanding to process and tend to stick more in the mold. Hardness ranges of approximately 85 to 95 Shore A are used for technical components such as hydraulic and pneumatic seals, wiper blades, roller bearings, buffers and cable sheaths. In this range, a balanced ratio of elasticity, wear resistance and residual compressive deformation results. Hard TPU types in the range of approximately 50 to 75 Shore D are used when high stiffness is required together with high toughness, for example in gears, sliding and guiding elements, coupling elements or support rings.

Purity and cleanliness requirements for TPU

In terms of purity and cleanliness requirements, TPU offers a structural advantage over vulcanized elastomers. Since no Vulcanisation Because TPU does not contain any additional components that could cause irritation or allergies, it is generally considered safe for use in contact with food. Instead, it is made from a combination of natural and synthetic materials that are combined to form a homogeneous material. This means that the material does not contain any additional components that could cause irritation or allergies. In addition, it does not require any additional FDA approval, as it is released through the additives included in the mixture. This means that additives such as stabilizers, lubricants, and flame retardants that can migrate to the surface and then burst into flame must be tested under special conditions before being used.

3. Processing & Manufacturing of TPU

TPU is processed in thermoplastic manufacturing processes and does not require vulcanization. For molded parts, the most common use is Injection moulding Extruded tubing, profiles, films, and cable sheaths are used for applications. In addition, TPU can be blown molded, calendared, and processed with additive manufacturing, for example for prototypes using the FDM or SLS process.

Since TPU is hygroscopic, the granules must be % dried to a residual moisture of less than 0.02 before processing, typically in a dry air dryer for 2 to 3 hours, depending on the hardness. Excessive residual moisture leads to bubbles and striations in the finished component as well as a reduction in molecular weight, which permanently degrades the mechanical properties of the component.

Figure 3: TPU Granulate

In injection molding, the processing temperatures of the raw material range from approximately 180 to 230 °C depending on hardness and type, and the cooled tools usually range between 30 and 60 °C. Soft types tend to cool more quickly, while hard types are processed more quickly. Since the urethane bond is thermally degraded when overheated or for too long, temperature control and cycle time must be closely monitored.

After forming, TPU components can be tempered, typically for several hours at 80 to 100 °C. Tempering eliminates the formation of hard segment domains, reduces inherent stresses, and improves the pressure-deformation residual and the mold stability in particular. Due to its thermoplastic base, TPU is very suitable for multi-component injection molding and overmolding. In many cases, a direct bond without adhesion agents can be achieved on technical plastics such as PC, ABS, or PA, which simplifies the production of plastic composite parts. Economically, TPU processing offers short cycle times, low post-processing, and the possibility of reusing inserts to a limited extent as regenerant. This is offset by higher tool costs for injection molding tools, which is why TPU injection molding is used primarily for medium to high-volume production.

Polyurethane resins and vulcanized urethane rubber (AU/EU) are excluded from the TPU processing. Cast PU is liquidized and injected into open or closed molds, chemically crosslinked, which is particularly suitable for large-volume components. AU and EU mixtures are similar to conventional Rubbers in compression or Transfer presses They are processed and vulcanized and are therefore usually the cheaper alternative for smaller batch sizes.

4. Material comparison: TPU vs. other elastomers

For TPU, comparisons with NBR, HNBR and EPDM are particularly meaningful, as these materials are often discussed as alternatives for seals, wipers and wear-resistant molded parts.

Material	Thermo-plastic polyurethane	Nitrile rubber	Hydrated nitrile rubber	Ethylene-propylene-diene rubber
International abbreviation	TPU	NBR	HNBR	EPDM
Hardness range (in Shore)	60A – 75D	20A – 75D	50A – 95A	20A – 95A
Mechanical properties at RT
Tear resistance
Elongation at break
Back-reflection elasticity
Further tearing resistance
Abrasion resistance
Compression set
at maximum continuous operating temperature
at room temperature
Thermal behaviour
Cooling behaviour (Tg) up to °C	−40	−30	−30	−50
Max. continuous operating temperature up to °C	80	110	150	130
Resistance to
Petrol
Mineral oil (at 100 °C)
Acids (aqueous inorganic acids at RT)
Alkalis (aqueous inorganic alkalis at RT)
Water (at 100 °C, distilled)
Weather and ozone

Table 2: Material comparison of TPU (thermoplastic polyurethane) with NBR (nitrile-butadiene rubber), HNBR (hydrated Nitrile-butadiene rubber) and EPDM (ethylene-propylene-diene rubber). TPU values for polyester-TPU of medium hardness; Polyether-TPU exhibits better values in terms of water and cold resistance.

What does TPU cost?

The cost of TPU is in the middle range of technical elastomers: above that of EPDM and NBR, but significantly below that of high-performance materials such as HNBR or FKM. Within the TPEs, TPU is one of the more expensive types. The specific price depends on the polyol base, the hardness, the color requirements, the certifications, and the quantity of the order.

Material	EPDM	NBR	CR	TPE/TPU	Silicone (LSR)	Silicone (HTV)	HNBR	FKM
Cost factor	× 1,0	× 1,0	× 1,2	× 1,3	× 1,4	× 1,8	× 2,9	× 3,7

Table 4: Cost comparison of TPU with other common elastomers

The material price alone is not decisive for the component costs. The elimination of vulcanization, short cycle times, and minimal post-processing often compensate for the higher raw material price when dealing with larger quantities.

What are the differences between TPU and NBR?

TPU and NBR are both resistant to mineral oils and fats. However, TPU offers significantly higher abrasion and tear resistance and is therefore superior in dynamic and abrasive-loaded seals. NBR, on the other hand, can be used up to approximately 110 °C and has a lower residual compressive deformation. In static seals under continuous pressure, it is the more economical choice.

What are the differences between TPU and HNBR?

HNBR achieves strengths similar to those of TPU and can be used up to approximately 150 °C, while maintaining good resistance to oil and ozone. TPU remains superior in abrasion and tear strength and is significantly more cost-effective. HNBR is therefore chosen when higher temperatures occur in addition to mechanical stress.

What are the differences between TPU and EPDM?

EPDM is highly resistant to hot water, steam, weather, and ozone; however, it foams strongly in mineral oils and fuels. TPU behaves in almost the opposite way: high oil and wear resistance, but limited resistance to hot water. Accordingly, EPDM is used for water-bearing and outdoor applications, while TPU is used for mechanically stressed components with oil and grease contact.

What are the differences between TPU and other TPEs?

Compared to TPE-S and TPE-O, TPU is significantly more mechanically durable and oil-resistant, but more expensive and more complex to process because it must be dried. TPE-V has a lower residual compressive deformation compared to TPU and better resistance to hot water. Therefore, TPE-V is always chosen when wear resistance and mechanical durability are the primary considerations.