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ANJA OLTMANNS

NORBERT VENNEMANN

JOCHEN MITZLER

Due to their unique properties –high abrasion resistance, highmechanical strength,and very good

resistance to media – thermoplasticpolyurethane elastomers (also called ther-moplastic polyurethanes,TPU) have beenused in special applications for more than40 years. Previously, however, in caseswhere higher demands were placed ontemperature resistance, the applicationpossibilities of standard TPU were limit-ed due to the relatively low softening point(140°C to 180°C). Moreover, it had notbeen possible to combine the elastic prop-erties and the high recoverability with si-multaneous low Shore hardness, e.g. as ischaracteristic for natural rubber.

Now, the new cross-linked TPU (TPU-X) Elastollan X-Flex (manufacturer: Elas-togran GmbH) adds pronounced rubber-elastic capabilities and good performanceat higher temperatures to the good pro-cessing and outstanding mechanicalproperties of thermoplastic polyur-ethanes. The combination of these fea-tures, coupled with excellent bonding totechnical plastics, opens up an enormousapplication potential for this material inthe automotive and mechanical engi-neering industries.

Clear Processing AdvantagesCompared to ConventionalMethods

At the K 2007, the joint development proj-ect of the partners Elastogran GmbH,KraussMaffei Technologies GmbH, and

Mues Products & Moulds GmbH (see boxon page 21) was presented on the exhibi-tion stand of the Munich-based machinebuilder KraussMaffei (Fig. 1). Thanks tothe integration of extrusion, injectionmolding, and reactive processing in an in-jection molding compounder (IMC)(Fig. 2, manufacturer: KraussMaffei), andby using an innovative cross-linking stage– the so-called X-Form process – it is pos-sible to produce the TPU-X in a singlemanufacturing cell. Cross-linking occursat the mold temperatures and within thecycle times encountered in normal injec-tion molding techniques [1].

Compared to the manufacture of con-ventional rubber-metal composites, thenew process presents the user with con-siderable advantages, because apart fromprimers, the vulcanization and other sub-sequential stages are saved (e.g. finishingand calibrating), plus offering a greaterfreedom of design. Moreover, continuousdirect melting and the uniform process-ing conditions during injection moldingensure highly consistent quality.

What’s New about TPU-X?

TPU involves copolymers that are syn-thesized by means of polyaddition. Here-by, the strength properties are determinedby the crystallizing hard phase that acts asa physical network, whilst the amorphoussoft phase is responsible for flexibility andthe elastomeric properties (Fig. 3).

Reaction into polyurethane is an equi-librium reaction and is therefore re-versible – which must be taken into ac-count during thermoplastic processing. Ifa certain temperature limit is exceeded,isocyanate groups will be regenerated,which can recombine with OH groups af-ter processing, resulting in an increase ofmolar mass (Fig. 4).

This equilibrium reaction can be uti-lized for cross-linking into TPU-X. Un-der the usual processing conditions dur-ing injection molding, the molecularchains of the TPU are split up. The cross-linking agent added to the melt reactswith the free ends of the molecular chains,whereby skilful process control permits

© Carl Hanser Verlag, Munich Kunststoffe international 3/2008

Online Cross-linking. First introduced at

the K, a new processing concept integrates

extrusion, injection molding, and reactive processing in a single manufacturing

cell. The application is centered around a thermoplastic polyurethane with rubber-

elastic properties that is produced in an injection molding compounder by means

of online cross-linking, and is also suitable for applications at higher tempera-

tures.

� PE104206

Keeping Track ofElastomer

Fig. 1. The part “Torque converter bearing“ consists of a hard (PA66 + 35% GF) and a soft componentTPU-X (pictures except (2): Elastogran)Translated from Kunststoffe 3/2008, pp. 38–42

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this process to be managed up to finalcooling of the melt.

TPU with Pronounced Rubber-elastic Properties

When substituting conventional elastomermaterials (rubber) with thermoplasticelastomers (TPE), the elastomer-specificproperties have a special relevance. Apartfrom the standard testing methods such ashardness, tensile strength, and reboundelasticity during material selection, it istherefore a frequent practice to apply thecompression set test (DIN 53517 or DINISO 815) to assess the recovery behaviorafter constant compression.

In order to examine the rubber-elasticproperties of the new TPU-X, the mate-rial was subjected to a series of tests in thePlastics Laboratory at the OsnabrückUniversity of Applied Science. Whencomparing the stress/elongation curves

(Fig. 5) of various elastomers, the stan-dard, non-linked TPU (Elastollan) is dis-tinguished by a very high elongation atbreak (>1,000 %) and a relatively hightensile strength (~30 MPa). Cross-linkedTPU-X (Elastollan X-Flex) has a some-what lower value of 600 to 650 % for elon-gation at break, but this is still higher thanthe typical values of carbon black-filledelastomer materials such as NR, SBR, andNBR. The additional cross-linking of thenew TPU-X leads to even higher strength

values (above 40 MPa) than standardTPU. Consequently, the strength of Elas-tollan X-Flex is superior to that of con-ventional elastomers and other TPE ma-terials of comparable hardness.

A disadvantage of commercial TPEwhen compared with rubber is reflectedin the starting region of the stress/elon-gation curve. Whilst the curves of typicalrubber materials are relatively flat in thestarting region, and only become steeperat higher elongations, most TPEs usuallyexhibit a steeper strain increase in thestarting region due to the thermoplasticphase. Amongst others, this is one reasonwhy TPE often have a stiffer “feel” thanrubber materials, even though the meas-ured hardness is the same. The enlargeddetail in Figure 5 shows clearly that in thestarting region of the stress/elongationcurve, the behavior of Elastollan X-Flexhardly differs from that of conventionalelastomers.

Compression Set after ConstantDeformation

Determination of the compression set isintended to provide information abouthow much of the elastic properties ofelastomers are maintained after a longperiod of constant deformation. For thispurpose, tests were conducted in accor-dance with DIN ISO 815 at differenttemperatures, as well as in accordancewith the VDA Guideline 675 216(Method B at 100°C). Hereby, and con-trary to the more frequently usedMethod A, the sample is initially cooledto room temperature in the deformedstate, and then the stress is relieved. Thisprocedure involves tougher testing con-ditions, and usually leads to poorer val-ues than the more usual strain relief inthe warm state.

An examination of the compression setvalues (Fig. 6) determined in this mannerproves that the additional cross-linkingin TPU-X clearly improves the elasticproperties when compared with standardTPU. In spite of tougher testing condi-tions, the compression set values obtained

Fig. 2. The X-Form process demonstrated on an injection molding compounder is a combination ofreactive compounding and multi-component injection molding (photo: KraussMaffei)

Fig. 3. The crystallizing hard phase determines the strength, the amorphous soft phase, flexibility,and the elastomeric properties

T10 [°C] T50 [°C] T90 [°C] TSSR-index

TPU (Elastollan) 57.4 94.3 150.1 0.60

TPU-X (Elastollan X-Flex) 108.3 145.2 167.0 0.83

TPO (EPDM+PP) 39.1 54.4 102.0 0.51

TPV (EPDM-X+PP) 49.0 100.7 79.2 0.61

EPDM 118.7 164.9 184.3 0.84

NR (natural rubber) 115.5 161.5 217.1 0.73

Table 1. The resultsof the TSSR measure-ment of non-linkedand cross-linked TPUare compared withselected TPEs andelastomer materials

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are partially lower than for other elas-tomer materials or TPE.

However, it is known that the results ofcompression set tests are prone to con-siderable uncertainties, and do not alwayspermit sufficient differentiation betweensimilar materials.For this reason, two newtesting methods were recently developed,which permit a more extensive assess-ment of the elastomer-specific properties[2, 3]. Within the scope of this project,both methods were also applied to eval-uate the TPU-X samples, and will there-fore be described briefly in the following.

Intermittent Strain/ElongationMeasurements

Earlier work has shown that the differ-ences in reversibility of the deformationbehavior between TPE and rubber be-come particularly clear if these are exam-ined with intermittent elongation strainin the stepped hysteresis test using in-creasing deformation amplitudes [4, 5].For these measurements, a test specimenis deformed by tensile strain at a constant

elongation speed (here: 50 mm/min) upto a defined elongation limit (here: ε1 =20 %), and is relieved completely imme-diately after at the same speed.After strainrelief, the residual elongation is deter-mined,and the procedure is repeated with

a simultaneous increase of the elongationlimit by the amount Δε (here: Δε = 20 %).This sequence is repeated until the sam-ple tears or the elongation limit reaches aspecified maximum value.

The deformation behavior’s reversibil-ity – one of the most important elas-tomer-specific properties – can be as-sessed particularly well if the residualelongation values determined after the in-dividual deformation cycles are repre-sented as a function of the elongation lim-it. The measurement curves (Fig. 7) showthat the behavior of SBR-based elastomeris almost ideal, i.e. residual elongation islow – also at high elongation limits. Onthe other hand, commercial thermoplas-tic elastomers such as TPV (EPDM-X +PP) or standard TPU behave differently.Here, and with low elongation limits,residual elongation is greater than withrubber, and even increases dispropor-tionately above a critical elongation.When compared with standard gradeTPV or TPU materials, Elastollan X-Flexexhibits a significantly better recovery be-havior, which is comparable with an elas-tomer based on EPDM.

Anisothermal Test of StrainRelaxation

As the physical cross-linking of TPE isthermally reversible, the mechanical be-havior under thermal stress is of majorsignificance. By means of the newly de-veloped TSSR method (temperaturescanning stress relaxation), this behaviorcan be analyzed in a simple manner andwith high reproducibility. Hereby, theelastomer-specific properties in particu-lar are reflected in the test results.

Fig. 4. The reaction to polyurethane is an equilibrium reaction and therefore reversible – an impor-tant aspect for thermoplastic processing

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Fig. 5. Cross-linkedTPU-X is strongerthan conventionalelastomers and otherTPE materials withcomparable hardness

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Fig. 6. Compared tostandard TPU, the ad-ditional cross-linkingin TPU-X improves itselastic properties

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The TSSR method is based on a strainrelaxation test, which is conducted underanisothermal conditions. A detailed de-scription of the testing method is foundin earlier papers [3, 6, 7].Within the scopeof this project, the tests were conductedwith a TSSR meter supplied by Braben-der Messtechnik GmbH & Co. KG, Duis-burg, Germany. Details on the equipmentare given in [8]. The following parame-ters can be determined from the meas-ured force/temperature curve.

The temperature limit Tx indicates atwhich temperature force F has been re-duced by x %, referred to the initial forceF0. Normally, the temperature limits T10,T50 and T90 are defined. Hereby, the val-ue T90 must be seen as the theoretical

maximum value for the operating tem-perature range, whilst T10 describes thetemperature at which strain relaxation

processes overcompensate the entropyelastic behavior. Value T50 can serve as acomparative value for the mechanical-thermal behavior of TPE and elastomers.Extensive investigations with thermo-plastic vulcanisates (TPV) have shownthat this value correlates with the valuefor compression set [6], and thus repre-sents an alternative to the time-consum-ing and often poorly reproducible testmethod.

The TSSR index is a relative measurefor the elastomer-like temperature be-havior of a TPE or elastomer. Hereby, thetheoretical behavior of an idealized elas-tomer material serves as reference, whichexhibits no strain reduction (strain re-laxation) even at increasing temperature.For this, the surface area below the stan-dardized (F/F0) force/temperature curveis determined, and put into relationship V

Kunststoffe international 3/2008

Materials technology:Elastogran GmbHElastogranstr. 60D-49448 LemfördeGermanyE-Mail: elastomere@elastogran.dewww.elastogran.de

Mold technology:Mues Products & Moulds GmbHGewerbepark Conradty 1D-83059 KolbermoorGermanywww.mues-pm.de

Plant technology:KraussMaffei Technologies GmbHKrauss-Maffei-Str. 2D-80997 MünchenGermanyE-Mail: info@kraussmaffei.comwww.kraussmaffei.com

Project Partnersi

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Fig. 7. Compared tomaterials such asstandard TPU or TPV,TPU-X exhibits sig-nificantly better re-covery behavior,which is comparablewith an EPDM-basedelastomer

© Kunststoffe

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Standardized Force/Temperature Curves

Fig. 8. The TSSRforce/temperaturecurves show the me-chanical behaviorunder simultaneousthermal stressing ofstandard TPU andTPU-X when com-pared with other TPEand elastomer mate-rials

© Kunststoffe

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Fig. 9. The TSSR index, represented here as a function of temperature limit T50, shows that the prop-erties of TPU-X are similar to those of conventional elastomers of EPDM or natural rubber

© Kunststoffe

TSSR Index

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with the surface area of the idealized elas-tomer material. The greater the TSSR in-dex value is, the more elastomer-like is thetemperature behavior of the examinedmaterial. Extensive investigations on con-ventional elastomers have shown thatthese come very close to the ideal behav-ior, and have TSSR indices of >0.8. Sig-nificantly lower values in the range ofabout 0.5 to 0.7 result for TPE.

The standardized (F/F0) force/temper-ature curves (Fig. 8) show that the chem-ically cross-linked elastomer materialsbased on natural rubber (NR) and EPDMexhibit the highest temperature limits (e.g.T50 >160°C). Contrary to this, simpleblends – like the TPO based on EPDM+PPdescribed here – already exhibit a pro-nounced strain reduction at relatively lowtemperatures, so that only low T50 valuesresult in the range of 50 to 60°C.

In comparison, and due to dynamiccross-linking of the elastomer phase, farbetter behavior is shown by TPV, for ex-ample the EPDM-X + PP described here,for which a significant increase of thetemperature limit T50 to values above100°C can be observed. Similarly goodbehavior is found with TPU, includingElastollan. Nonetheless, a relatively largedifference to conventional elastomers canstill be ascertained by all commercialTPEs, which is also reflected in the corre-sponding performance characteristics.

However, comparing the curve ofTPU-X (Fig. 8) with the results deter-mined for typical rubber samples (NRand EPDM) reveals that the difference isconsiderably less than for the other TPEs.Similarly, a comparison of the parame-ters derived from the TSSR measurements(Table 1) shows that cross-linked Elastol-lan X-Flex exhibits better values than thestandard TPU (Elastollan) and other TPEmaterials (TPV and TPO). In terms ofthese properties, it practically corre-sponds to conventional elastomers basedon EPDM and NR. Also the TSSR index,a measure for a material’s compliancewith the behavior of ideal elastomers, andshown in Figure 9 as a function of tem-perature limit T50, makes clear that theproperties of TPU-X are similar to thoseof conventional elastomers based onEPDM or natural rubber (NR).

Summary

The material Elastollan X-Flex representsa novel integration of thermoplasticpolyurethane (TPU) and cross-linkingagent. It combines the good processingproperties of thermoplastic materials

with the rubber-elastic properties of anelastomer. Simultaneously, the outstand-ing mechanical properties of TPU, and itsresistance to media and ozone are main-tained.

The clearly improved behavior ofcross-linked TPU at higher temperaturesopens up a range of new applications thatwere previously impossible with standardTPU. Due to the combination of theseproperties, TPU-X is a highly interestingalternative to the rubber compounds usedso far – not only in the automotive in-dustry. ■

ACKNOWLEDGEMENT

Special thanks are due to Markus Seidl, Mues Prod-ucts & Moulds GmbH, for the realization of this proj-ect.

REFERENCES

1 Mitzler, J.; Hilmer, K.; Seidl, M.: A Simple Alterna-tive to Rubber-Metal Composites. KunststoffeInternational (10/2007) 10, pp. 126–130

2 Vennemann, N.; Hündorf, J.; Kummerlöwe, C.;Schulz, P.: Phasenmorphologie und Relaxationsver-halten von SEBS/PP-Blends. Kautsch. GummiKunstst. 54 (2001), pp. 362–367

3 Vennemann, N.: Praxisgerechte Prüfung von TPE.Kautsch. Gummi Kunstst. 55 (2003), pp. 242–249

4 Eisele, U.: Spezifische Merkmale von Gummi imVergleich zu anderen Werkstoffen. Kautsch. Gum-mi Kunstst., Sonderdruck Celle (1987), pp. 17–22

5 Vennemann, N.; Leifheit, S.; Schulz, P.: TPE Testfor Automotive Engineering. Kunststoffe plasteurope 90 (2000) 8, pp. 46–48

6 Reid, C.G.; Cai, K.G.; Tran, H.; Vennemann, N.: Poly-olefin TPV for Automotive Interior Applications.Kautsch. Gummi Kunstst. 57 (2004), pp. 227–234

7 Barbe, A.; Bökamp, K.; Kummerlöwe, C.; Soll-mann, H.; Vennemann, N.; Vinzelberg, S.: Investi-gation of Modified SEBS-Based ThermoplasticElastomers by Temperature Scanning StressRelaxation Measurements. Polymer Engineering &Science 45 (2005), pp. 1498–1507

8 Fuchs, F.: Grenzen aufzeigen – anisotherme Span-nungsrelaxionsmessung. Kautsch. Gummi Kunstst.59 (2006), pp. 302–303

THE AUTHORS

DIPL.-ING. ANJA OLTMANNS, born in 1966, worksfor Elastogran GmbH, Lemförde, Germany, in Salesand Technical Service of the Automotive EuropeanBusiness Unit Injection Molding ThermoplasticPolyurethanes.

PROF. DR. NORBERT VENNEMANN, born in 1953,is head of the Plastics Laboratory in the Faculty ofEngineering and Computer Science at the OsnabrückUniversity of Applied Science; contact: www.ecs.fh-osnabrück.de

DIPL.-ING. (FH) JOCHEN MITZLER, born in 1973, ishead of Product and Technology Management atKraussMaffei Technologies GmbH, Munich, Germany.

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