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Mechanical properties

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Desmopan® and Texin® can be described by a wide range of mechanical properties, from stiffness to tensile behavior under pressure and many more.

Desmopan® and Texin® grades range in hardness from 60–98 Shore A to 30–75 Shore D. They are characterized by high values for tensile stress and elongation at break, but these properties are only of interest when describing the materials. For practical purposes, the stress-strain values in the small deformation range are key. Desmopan® and Texin® offer outstanding resistance to wear by virtue of its high tear and tear propagation resistance combined with its elasticity. Under certain conditions, e.g. wet abrasion, Desmopan® and Texin® can even attain a longer service life than high-grade steels.

Stiffness

The high level of variation that can be achieved in polyurethane chemistry makes it possible to produce Desmopan® with a very wide range of Young’s moduli. The range up to 1000 MPa is covered by non-reinforced grades. Reinforced thermoplastic polyurethanes (RTPU) (e.g. Desmovit®) can attain considerably higher moduli.
The graphic below shows the Young’s modulus range for thermoplastic polyurethanes and RTPU compared to other engineering plastics.
Young’s moduli for different engineering plastics

Young’s modulus (modulus of elasticity) is established by conducting the tensile test in accordance with DIN 53504 on an S1 specimen defined by DIN EN ISO 3167, employing a testing rate of 2 mm/min. Young’s modulus is calculated as the ratio of stress to strain along the initial slope of the stress-strain curve.

For plastics, laboratory testing has shown that Young’s modulus depends on the following parameters:
• Orientation of the macromolecules and fillers
• Temperature
• Moisture content
• Stress rate and duration
• Specimen geometry
• Test setup

Stiffness as a function of temperature
The relationship between stiffness and temperature is shown in the following two diagrams for different raw materials and hardness values.

Moduli of Desmopan® as a function of temperature for 300 series grades
Moduli of Desmopan® as a function of temperature for 85 Shore A grades

Shore hardness A and D

Shore hardness is a characteristic value for the material properties of elastomers and plastics. It is specified by the DIN 53505 and DIN EN ISO 868 standards. The “Shore durometer” which is used to test Shore hardness consists of a spring-loaded indenter, and the resulting elastic indentation is inversely related to the Shore hardness of the material. The hardness scale ranges from 0 to 100. A high figure indicates a high hardness.
The thickness of the part to be tested should be at least 4 mm (based on experience, 6 mm is preferable). It is permissible to stack some thinner parts, but they must maintain close contact. As for the values, a hardness of Shore A applies to soft elastomers. The hardness tester for this range has a blunted tip. If values above 90 Shore A are measured, the hardness tester for Shore D should be used. A hardness of Shore D applies to harder elastomers or plastic materials. The tester for this range has a pointed tip. If values below 20 D are read, the Shore A hardness tester should be used. Plasticizer-free Desmopan® can be obtained in a hardness range of 60 Shore A to 75 Shore D. Please note that additive coatings such as wax on surfaces lead to a penetration, rather than indentation, of the surface. Such additives can therefore produce incorrect results. We advise cleaning specimens with alcohol prior to testing. After the specimen is fully dry, the test can be conducted under standard conditions (23°C/50% rel. hum.).

Glass transition temperature

The glass transition temperature Tg (also known as the glass temperature or glass transition temperature) is characterized by the conversion of an amorphous, glass-like or semi-crystalline polymer specimen, with a greater or lesser degree of hardness, into a rubber-like or viscous phase. The transition to this phase does not take place spontaneously. Instead, the polymer has a so-called principal softening range of around 50 K. This phenomenon of the glass transition temperature is brought about by the freezing or thawing of longish chain segments (20 to 50 chain atoms) in the macromolecules. The macroconformation, by contrast, does not undergo any change when the glass transition temperature is attained.

Dynamic mechanical analysis (DMA)

Dynamic Mechanical Analysis (DMA) has various applications, including the determination of the following:
• Viscoelastic material properties, such as various moduli and the loss factor tan (δ)
• Temperatures that characterize viscoelastic properties
• Damping
• Glass transition temperature, in particular (for which DMA is the most sensitive method)
• Frequency-dependent mechanical properties of materials
In DMA (analysis of the tensile modulus/complex modulus of elasticity), a very small fluctuating mechanical sinusoidal stress is applied to the test specimen (which is under a constant preload, which may be dependent on the storage modulus), with variation of temperature over time. This setup results in deformation of the specimen in phase. The parameters measured are the force amplitude, deformation amplitude and phase displacement Δ φ between the force and deformation signals. The preload is used to keep the specimen sufficiently tensioned at the time of negative dynamic deformation amplitude. The DMA yields the complex modulus of elasticity of the specimen. A prerequisite here is that the specimen is not subject to stress outside the linear elastic region (Hooke’s region); see Figure 1. The specimens may react in one of three ways:
• Perfectly elastic specimens react immediately to the applied force, the phase angle φ = 0. They oscillate without loss.
• Perfectly viscous specimens reach their deformation maximum at the crossover point of the force. Their phase angle φ = π/2 (90°). They convert the excitation energy entirely into heat.
• Viscoelastic materials, such as Desmopan®, exhibit a certain lag in the deformation of the specimen after the application of force. Therefore, for the phase angle Δ φ, 0 < φ < π/2. The greater the phase angle, the more pronounced the damping of the oscillation.
Fig. 1 Test setup for DMA
Fig. 1 Test setup used to determine:
• Softening behavior, heat resistance
• Glass transition temperatures
• Plasticizing effects
• Niveau of modules
• Phase characteristics
• Morphology, crystallization and melting
• Flow process

Test parameters:
• Heating at 0.1 K/min up to 10 K/min; adjustable
• Temperature range: -150°C to +300°C
• Frequency: 0.01 to 20 Hz in 1-2-5 sequences, adjustable
• Thickness of specimen: 0.1 to 2 mm

Fig. 2 Storage moduli curves for different polymer types
Figure 2 shows the storage moduli curves for different polymer types. If the storage modulus falls to just slightly above the glass transition temperature and remains at this level up to the decomposition temperature, i.e., the material remains comparatively rigid over the entire temperature range in which it is used above the glass transition temperature, the material can be assumed to be an extremely tough, crosslinked, short chain polymer.

If the storage modulus drops substantially into the region of the glass transition temperature and then remains at a more or less high level up to final softening, it is an elastomer, crosslinked to varying degrees, or a thermoplastic elastomer (e.g., thermoplastic polyurethane), depending on whether the final softening involves melting without decomposition or a decomposition process. If the storage modulus falls further with varying rapidity after passing through the glass transition temperature, the material is a non-crosslinked polymer with a low or high molecular weight.

This is the standard method for the (mechanical/thermal) determination of glass transition temperature, plasticizing effects, phase behavior or multiphase systems and the corresponding morphological structures, the miscibility of polymers and the melting and crystallization properties of partially crystalline phases. In particular, it permits an analysis of the different reasons for softening and an attribution to molecular processes (the glass process) of an amorphous phase or melting (crystalline phases).
 Fig. 3 E-module measurement for commercially available TPU grade
Figure 3 shows the modulus for a commercially available TPU. Measurements were carried out using forced oscillations at a constant measurement frequency of 1 Hz at different temperatures.

The glass transition at the maximum of the loss modulus E” is apparent; the glass transition temperature can be determined with a precision of 0.1°C since it is defined here by a maximum. The two-stage softening of the TPU material is attributable to the melting of a partially crystalline soft phase, which follows a broad softening of an amorphous hard phase. In molecular terms, the glass process differs substantially from the melting of a material. The glass process is a cooperative molecular relaxation process of main chain segments of the polymer, which has an activation energy based on the time-temperature-equivalence principle and therefore proceeds at different excitation frequencies at different temperatures. The temperature at which softening is observed upon melting of a material, however, is only dependent on its specific melting temperature, and not on the frequency of its dynamic excitation.

In Figure 4 below, the modulus of elasticity of a selected Desmopan® grade is compared to that of a cast PU and a hard thermoplast (ABS).
Fig. 4 Comparison of Young’s modulus as a function of temperature for different polymers
The relationship between modulus of elasticity and Shore A hardness, as well as the limits of a linear plot, are shown in Figure 5. It is apparent that the modulus of elasticity (or “stiffness”) is a more accurate means of assessing the behavior of a material (in particular using the finite element method or FEM) than determining Shore A hardness, which is in widespread use because it is easier to determine.
Fig. 5 Correlation between moduli of elasticity and Shore A hardness

Tensile strength

Tensile strength testing is employed to assess the behavior of plastics under uniaxial tensile stress. The advantage of the tensile test is that it allows even ductile materials to be tested up to the point of complete fracture. Young’s modulus is the parameter used to compare different materials and constitutes a measure of stiffness. The test is generally performed at 23°C. Desmopan® attains very high values for tensile strength. Even the softest grades almost always achieve 25 MPa. For medium to hard grades, tensile strengths of 50 to 60 MPa are possible. As a typical TPU, Desmopan® clearly outperforms all other classes of TPE.

Test results for Desmopan®:
Stress and strain data, including:
• Modulus of tension Et [MPa]
• Modulus at 100% strain (MPa)
• Modulus at 300% strain (MPa)
• Tensile stress at break σB [MPa]
• Elongation at break εB [%]
• Other characteristic values on request

Standards: DIN 53504, measured on an S1 test specimen with a displacement rate of 200 mm/min.

Tear propagation strength

The tear propagation test is used to establish structural strength.
The test is carried out according to DIN ISO 34-1Bb on angled test specimens with an incision. These are pulled, perpendicular to the incision, at a velocity of 500 mm/min until they are torn apart. The tear propagation strength [kN/m] is the quotient of the highest force attained and the specimen thickness.
Results of the test method:
• Tear propagation strength [kN/m specimen thickness]
In terms of its tear propagation strength Desmopan® is far superior to the majority of plastics. Very soft grades attain around 30 – 40 kN/m, while very hard grades can achieve values of more than 200 kN/m.
The test setup for the tear propagation strength test is shown in the diagram below.
Tear propagation strength test

Compression set

The compression set indicates the deformation component of the test material. Several test methods for elastomers, such the tensile strength test, provide information (to a greater or lesser extent) on the quality and properties of a test specimen. The compression set, by contrast, is a key factor which needs to be taken into account before the material is employed for a specific purpose. For essential parts such as seals and shims, for example, the residual deformation – or compression set – is a crucial parameter.
As laid out in DIN ISO 815, the compression set is measured with a constant deformation. To determine the compression set, a cylindrical test specimen is compressed by a percentage that is calculated on the basis of the material’s hardness. The specimen is then stored at a given temperature for a certain period of time in the compressed state. The temperature used for the compression set test depends on the material to be tested, its intended purpose and the test setup (e.g. 24 hours at 70°C for seals). 30 minutes after the pressure has been removed, the height of the specimen is measured again and, based on this value, the residual deformation established.
A compression set of 0% means that the specimen has regained its full original thickness (which is only possible in theory), while a compression set of 100% indicates that the specimen underwent a complete deformation during the test and shows no sign of recovery.
The calculation is based on the formula:

CS (%) = (L0 – L2) / (L0 – L1) x 100%, where:
• CS = compression set in %
• L0 = height of specimen prior to the test
• L1 = height of the specimen during the test (spacer)
• L2 = height of the specimen after the test

To illustrate the significance of this test, take a rubber component like a seal, for example: When the seal is fitted, it is compressed to a certain thickness and exerts compression on the sealing surfaces. The strength of compression goes down over time, since the seal undergoes plastic deformation. If this plastic variable, i.e. the compression set, is too high, the compressive force and hence the sealing effect will be reduced to such an extent that leaks can occur (either at individual points or over the full surface of the seal), particularly if the seal is poorly sized. As a rule of thumb, the compression set for flat seals should not exceed 40% at the continuous service temperature. The following diagram shows the compression set that can be attained on average when using different types of chain extenders. This is one of the means by which it is possible to influence the crystallinity of the rigid segment and the phase separation – as the most important parameters. While Desmopan® grades in the standard ester series (100–300 series) generally have a compression set ranging between 45 and 55% (24h/70°C), considerably more favorable compression sets between 30 and 35% (24h/70°C) can be achieved through the use of special raw materials in the special ester series (400 series). The first Desmopan® products with a technically serviceable compression set at 100°C or 120°C are currently under development.
Compression set of MDI-TPU (60 Shore D) based on different chain extenders

Impact strength

The Charpy impact test is a simple and rapid test that can be used to conduct a comparative assessment of different materials. It is employed, among other purposes, to study the effect of changes in formulation conditions, compounding conditions and injection molding conditions on test specimens. In the Charpy impact test, specimens are subject to a loading configuration similar to that in the three-point bending test and, if they are sufficiently brittle, they will fracture. Notched specimens must be used to achieve fracture in ductile materials. The test can be carried out within a temperature range of –40°C to +23°C. Compared with other materials, Desmopan® attains very high impact strength and notched impact strength values. On account of the elastic properties of TPU, there is no breakage above the glass transition temperature (Tg). The following table shows a number of characteristic values for reinforced Desmopan® grades:
Impact and notched strength values for various Desmopan grades
Results of the test method conducted according to DIN EN ISO 179:
• Impact strength aiU [KJ/m2]
• Notched impact strength aiN [KJ/m2]
• Plus, an assessment of specimen fracture at the corresponding temperature and, where appropriate, a tough/brittle transition.

Friction

For plastics, it is virtually impossible to obtain a meaningful assessment of friction behavior, since, in practice, friction processes are accompanied by effects that cannot be comprehensively defined. The friction behavior of Desmopan® products is determined by their hardness and surface finish. Friction increases as the hardness of the material decreases. “Stick-slip” effects thus develop with flexible products. Friction behavior is also dependent on the surface against which the friction is tested, as well as on the condition of the surface (e.g. wet, dry). The following table provides an impression of the friction behavior of a leather-grained Desmopan® specimen paired with polished steel based on the requirements in DIN-EN-ISO-8295 (further details see below):
Friction behavior of Desmopan® to TM 900040
Test details:
Specimen: leather-grained flat specimen 50 x 50
Friction partner: polished steel
Test weight: 2000 g
Mean sliding friction: µD
Friction behavior of Desmopan® under varying test conditions

Poisson’s ratio

Poisson’s ratio is the ratio of the transverse strain contraction ey,nom to the longitudinal strain ex,nom expressed by the formula:
unom = -ey,nom / ex,nom,
whereby the nominal elongations represent the relative change in length (DL) or in width (DB) with reference to the initial dimension of the measuring region according to
ex,nom = DL/L0 ey,nom = DB / B0.
Poisson’s ratio can also be used to determine whether a material maintains a constant volume when deformed or whether the volume increases under deformation, e.g. due to the formation of microcracks and voids.
Rather than using the nominal Poisson’s ratio (see above) for this purpose, it makes sense to use the so-called Hencky ratio:
uH = -ln (1 + ey,nom) / ln (1 + ex,nom),
as this remains constant in the case of volume change with constant form, while the nominal Poisson’s ratio declines logarithmically (e.g. at constant volume).
For small elongations, in particular in the linear-elastic deformation region, which is relevant for normal simulations, the Hencky elongation and the nominal elongation have the same value, so the corresponding ratios are also identical within this region.
Determination of the Poisson’s ratio is carried out on a tensile test machine using the gray value correlation method. This involves tracking changes in the local elongation using an arbitrary dot pattern. The following table shows the Hencky ratios for selected types of Desmopan® – whereby it should be noted that this ratio is identical to the nominal Poisson’s ratio in the linear-elastic region.
Compressive behavior of Desmopan® grades expressed by Poisson’s ratio
The selected grades of Desmopan® demonstrate Poisson’s ratios between 0.45 and 0.5. A Poisson’s ratio of 0.5 means that the volume remains constant under tensile or compressive stress. A Poisson’s ration slightly below 0.5 means that the material experiences a slight increase in volume when subjected to tensile stress and a slight decrease in volume when subjected to compressive stress.

Abrasion

This method, as described in ISO 4649, can be used to test the resistance of surfaces and coatings to wear or abrasion. Two rollers covered with emery paper rub against the surface of a rotating specimen. By varying the grain size and the contact pressure of the emery paper, different degrees of loading can be simulated. The abrasion is specified in mm³, with an allowance for the density of the test specimen. Generally, data obtained from this test can only be applied to parts in the real world if the test conditions tally with those encountered in operation. Otherwise, this test method is intended to provide a comparative assessment of different materials. Together with other TPUs, Desmopan® counts as one of the most abrasion-resistant elastomeric materials under actual service conditions. Its abrasion loss is frequently a fraction of other materials, particularly when compared with other TPE classes. Desmopan® polyester grades that are frequently employed in abrasive applications display an abrasion loss of no more than 20 to 30 mm³. For the sports shoe industry, additional special grades in the S043 series offer particularly low abrasion loss. If processed carefully, these grades can even cut abrasion loss down to between 15 and 20 mm³. TPU is considered to be the elastomeric material that offers the highest resistance to abrasion under the conditions encountered in practice. This requires the granules to have been dried sufficiently prior to processing.
Degradation due to moisture

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