ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

51KGK · 06 2020www.kgk-rubberpoint.de

Rubber · Chip & Cut wear · laboratory testing · tire testing

Tire treads are exposed to rolling and sliding impacts leading to damages, which are known as Chip and Cut (CC) effects. A reliable prediction of the CC behaviour of new designed tire tread compounds is very difficult without field tests. In this paper a first compari-son between the real tire tread wear, appearing under high severity condi-tions, with a fast laboratory testing method is presented. We predict the CC behaviour of a series of truck tire tread compounds when using a laboratory In-strumented Chip & Cut Analyser (ICCA) which operates under realistic practice-like conditions. As a result, the carbon black filled NR/BR/SBR-blends used in this study show an identical trend of CC behaviour as observed during the tire field tests.

Chip & Cut-Verschleiß von LKW-Reifenlaufflächen: Vergleich zwischen Labor- und Reifenprüfung Gummi · Chip & Cut-Verschleiß · Labor-prüfung · Reifentest

Reifenlauflächen sind unter rollenden und gleitenden Beanspruchungen Stö-ßen ausgesetzt, die zu Schädigungen führen, die als Chip- und Cut (CC)-Ver-schleiß bekannt sind. Eine zuverlässige Vorhersage des CC-Verhaltens neuer Rei-fenlauflächenmischungen ist ohne Feld-test sehr schwierig. In dieser Arbeit wird ein erster Vergleich zwischen dem tat-sächlichen Reifenlauflächenverschleiß, der bei harschen Einsatzbedingungen auftritt, und einer schnellen Labortest-methode durchgeführt. Wir verwenden einen neuen Instrumentierten Chip & Cut Analyzer (ICCA), der das CC-Verhal-ten von LKW-Reifenlauflächenmischun-gen unter realistischen Praxisbedingun-gen vorhersagt. Wir konnten zeigen, dass die in dieser Studie verwendeten mit Ruß gefüllten NR/BR/SBR-Kaut-schukmischungen den gleichen Trend beim CC-Verhaltens im Labor aufweisen, wie er beim Feldtest an den realen Rei-fenlaufflächen beobachtet wurde.

Figures and Tables: By a kind approval of the authors.

IntroductionVery recently, we reported about results from the German DFG transfer projects HE4466/26-1 (G. Heinrich) and KL1409/9-1 (M. Klüppel) regarding im-provements of dynamic laboratory pre-dictions of crack propagation behaviour and establishing correlations to wear phenomena of truck tire treads [1]. Pri-mary aim of this transfer project was transferring of test methods, developed and optimized in a former DFG research network group (DFG-FOR 597), together with transfer of scientific findings on crack growth mechanisms, into industri-al applications. Based on the comparison of laboratory tear fatigue data - using an instrumented Tear and Fatigue Analyzer (TFA) - with corresponding tire test re-sults, we identified a rather good correla-tion between the ranking list of crack growth speeds at high values of tearing energies (T > 1 N/mm) with the corre-sponding ranking list of the Chip & Cut (CC) protocols after tire testing.

Usually, Chip & Cut (CC) wear refers to the detachment or breakage of rubber material from tire treads when riding on a rough road surface (e.g., gravel roads, roots, stalks). An example for Chip & Cut wear is shown in Fig. 1. Although cutting and chipping is most commonly associ-ated with off-the-road tires (OTR), light truck (LTT) and SUV tires, this phenome-non is also observed with highway truck as well as all-season passenger tires. This leads to damage symptoms which are CC effects as well, but in a smaller scale. Reduced tire life is a consequence of tear-ing and CC behavior of tread rubbers.

Although the above mentioned tear fatigue analysis (TFA) was executed as a laboratory test (usually by estimating the complete or partial Paris-Erdogan plot within the regime of stable crack propagation (see, e.g. [1]), it is still a rela-tively time-consuming and laborious procedure. A much faster wear evalua-tion approach with data from a new special designed CC testing device, la-belled Instrumented Chip & Cut Analyzer (ICCA) and manufactured by Coesfeld GmbH & Co. KG, Germany, has been de-

veloped with a special focus on quantify-ing CC resistance. Additionally, an ICCA test generates wear pattern on small test wheel specimen similar like surfaces as tire treads show after having been oper-ated in the field for a while. This addi-tional wear pattern information cannot be supplied from the TFA test. Further on, all relevant measurements and the data recording are continuously conducted while the test runs. By using a mathe-matical algorithm, fed with the measure-ments, realistic and reliable answers on CC behaviour for the rubber are achieved in a short time with very low efforts. De-tails about the ICCA have been reported in the previous papers, e.g. [2-6].

In this work we report about a com-parison between the CC laboratory tests of four rubber compounds with the (ex-pensive) CC evaluation of the corre-sponding four truck tires operating un-der off-the-road conditions. We will show that rather good correlation is realized between the compound ranking list of CC laboratory tests with the correspond-ing ranking list of the tire’s CC protocol. This is the first time were a correlation between CC tire evaluation and direct fast CC laboratory testing with the new ICCA equipment is reported in literature, additionally to the comparison between tire tests and TFA evaluation in [1].

Chip & Cut Wear of Truck Tire Treads: Comparison between laboratory and real Tire Testing

AuthorsR. Stoček, Zlin, Czech Republic, G. Heinrich, A. Schulze, Dresden, M. Wunde, M. Klüppel, C. Vatterott, J. Tschimmel, J. Lacayo-Pineda, Hannover, R. Kipscholl, Dortmund, Germany

Corresponding Author: Prof. Dr. Gert HeinrichLeibniz-Institut für Polymer- forschung Dresden e.V.Hohe Straße 601069 Dresden, GermanyE-Mail: gheinrich@ipfdd.de

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

52 KGK · 06 2020 www.kgk-rubberpoint.de

We note here that in the past some of the authors already tried to identify oth-er simple and fast laboratory rubber tests that might correlate with the CC tire performance. There has been per-formed few previous studies, e.g. [7, 8]. Other scientists tried an identical effort, reported e.g. in [9-11], using traditional lab CC testing methods. However, from the mentioned publications it is visible, that the traditional CC test methods used in laboratories employ simple de-vices, where realistic loading conditions cannot be sufficiently and reproducible applied. Only the sample weight loss and the damaging total energy are the avail-able results after these tests. This is not sufficient to describe the complexity of CC phenomenon in practice. Also stand-ard compound physicals and dynamic-mechanical data did not previously yield any satisfying correlation to the CC tire performance (CCTP). One of the authors (G. H.) followed reference [12] where some correlations of CCTP to dynamic-mechanical rubber properties were pro-posed. One could confirm – within a set of laboratory tests with six varying com-pounds and with the available corre-sponding off-the-road tire evaluation data - that CCTP correlates tentatively with the loss factor tanδ (60°C, 20Hz, 10% dynamic amplitude) and with indi-cators derived from strain sweep (Payne effect) rubber tests, e.g. Ψ{60°C} or Ψ{90°C}. Here, Ψ{T[°C]} ≡ 100·(G’(10%, T)/G’(1%, T)) where G’(x%, T) is the low

Fig. 1: Chip & Cut wear of truck tire tread [1].

1

frequency storage shear modulus at x% strain amplitude at temperature T. Ap-parently, a good CCTP can be realized with tread rubbers of high values of Ψ [13]. Furthermore, we tried to analyze the correlation between CCTP and fast notched instrumented tensile-impact tests. The crack toughness behavior of the compounds under impact-like load-ing conditions was estimated by using instrumented tensile-impact test equip-ment RESIL IMPACTOR Junior (CEAST, Ita-ly). From load-extension diagrams one could estimate the quantities Fmax, lmax, Jd and Kd which describe the maximum value of load (Fmax) resp. extension (lmax), critical J-integral value (Jd) and the value of the critical stress-intensity factor (Kd - also called fracture toughness). Appar-ently, according these investigations a good CCTP correlates with low values of the ratio f = Fmax/lmax and with high values of the ratio k = Jd/Kd [13]. However, the disadvantage of all these described ap-proaches is the rather large distance of the applied laboratory test situations from the real wear generating situations appearing under real off-the-road and CC provoking operating conditions of the tire. Furthermore, the non-destructive dynamic-mechanical laboratory tests and the impact tests do not yield any re-alistic wear pattern of the laboratory samples. The found comparable tenden-cies between the discussed lab test pre-dictors and tire test results do not under-lie any rational reason.

In the following, we will see that even the eye-inspected wear pattern and, moreover, physical CC damage impact parameter P values, ascribed to the small laboratory test wheel specimen of the ICCA, generate first approximate impres-sions about the final CCTP.

Experimental and Materials:For the scientific experimental investiga-tions respective laboratory tests, near-practice laboratory truck tire tread com-pounds were mixed in an industrial in-ternal mixer (Werner & Pfleiderer GK 5 E) and crosslinked in a hot press at 150 °C until the t90 time (90% of the maximum torque from vulcanization curves). The blends consist of natural rubber NR (SVR CV 60) veneered with different propor-tions BR (Buna CB 24, Lanxess) and / or SBR (Buna VSL 4526, Lanxess).

Crosslinking was semi-efficient (SEV) with CBS and sulfur. The reinforcing filler used was 50 phr of carbon black (N339). The samples were mixed with the vul-canization aids stearic acid and zinc ox-ide (ZnO) and protected against aging by IPPD. Table 1 shows the complete com-pound formulations. The samples were prepared by conventional mixing tech-nique, in which first the rubbers were blended and then the carbon black add-ed. The crosslinking system is then mixed in on the two-roll mill. In addition, the tire compounds with equal formulation for the tire tests were produced by Conti-nental Reifen Deutschland GmbH, using only a slightly modified vulcanization system, which is used for the tire tread production. The tires were finally tested under field conditions. More details are described in section Tire Tests below.

The compounds in Table 1 were cho-sen based on already existent practical experiences and in accordance with the TFA tests published in [1]. The 100% NR-based compound exhibited better failure properties and fatigue crack resistance than that of the NR/BR or NR/SBR blend systems. This means that a NR/BR-based compound can more easily show CC wear phenomena because of the lower fatigue resistance compared to that of the NR based compound. From the previ-ous work [3] it is evident that the relative ranking for CC resistance based on using of larger amount of butadiene rubber followed the fatigue crack growth resist-ances of the materials but was opposite to the ranking of DIN abrasion resist-ance. This provides evidence that CC damage from impact by mm-scale as-

1 Used rubber formulasComposition in phr

Samples NR BR SBR CB CBS Sulphur IPPD ZnO Stearic acida - NR 100 - - 50 2,5 1,7 1,5 3 1b - N55B45 55 45 - 50 2,5 1,7 1,5 3 1

c - N55S45 55 - 45 50 2,5 1,7 1,5 3 1d - N55B23S23 55 22,5 22,5 50 2,5 1,7 1,5 3 1

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

53KGK · 06 2020www.kgk-rubberpoint.de

perities and abrasion of rubber against ym-scale asperities exhibit distinct char-acteristics in rubber.

Laboratory chip and cut testingLaboratory chip and cut testing was per-formed on the rubber specimens using the new Instrumented Chip & Cut Ana-lyzer (ICCA) manufactured by Coesfeld GmbH & Co. KG, Germany [2-6]. Testing with the ICCA involves rotating the rub-ber sample at a selected rotation speed and impacting the sample with a stain-less-steel tool with specified frequency. The rotation speed, impact normal force, frequency, and contact (sliding) time with the rubber surface can all be inde-pendently controlled. Figure 2 shows the ICCA and the sample geometry.

We refer to recent references [2-6] re-garding the impacting device, impactor geometry and description of the inde-pendent control of impact frequency and sliding time.

As the tangential force is the resulting answer on impact force, which is as-sumed to grow proportional with in-creasing roughness of the penetrated surface, it is taken to calculate the degree of damage every load cycle. Thus, the key characterization factor evaluated from a multi-channel data acquisition is the CC damage, which is calculated using a nu-merical algorithm. The tangential force shows an increasing scattering with in-creasing time according to increasing roughness of the sample surface. For this reason an enwrapping curve is calculated which is numerically integrated from a certain starting point C(0) to a cycle count of interest, C(n). Each sum will be divided by the corresponding cycle num-ber c(k) (0 < k< n). This (scalar) value quantifies the CC damage at cycle count k and is called “CC damage, P”. At least the curve P vs. cycle count (k; k = 0 to n) is a description of the CC behaviour under the given load conditions.

More details on the numerical calcula-tion of the CC damage parameter, P, can be found in previous papers [3, 4].

The CC behavior of the investigated four rubbers were tested at room tem-perature with the ICCA under the test conditions listed in the Table 2, whereas three replicates of each compound have been analyzed. The tests run until 5,000 impact cycles and as impactor the tool of an impacting radius 2.5 mm has been used.

The CC damage parameter, P, versus impacting cycles, n, is shown in Figure 3

and demonstrates the CC resistance trend of a > c > d > b, which is consistent with the well-established ranking for these polymers in tire applications in terms of CC tendency.

Surfaces of the rubber specimens af-ter ICCA testing after 5000 cycles are shown in Figure 4. The photographs demonstrate that the damage character-istics of the rubbers resemble qualita-tively the CC phenomenon in tire treads (Figure 5). One single sample, represent-ing each compound, is exemplary visual-ised from the three replicates analysed for each rubber. There are clear differ-ences between the extent and features

of the damage of the four compounds. Furthermore, the damage appearance respective topology of abraded surfaces correspond to the above mentioned trend from lower up to higher abrasion in the order a > c > d > b.

Tire testsThe tire tests were carried out by Conti-nental Deutschland GmbH. Tires in the dimension 205/75 R17.5 CHS3 124 L were built and heated at 145 °C for about 35 minutes. For each CC test, four tires were mounted on the rear axle of a vehi-cle. The tires were driven at a filling pres-sure of 7.5 bar and a load of 1300 kg at

Fig. 2: Photograph of Instrumented Chip & Cut Analyser (left) and diagram of rubber sample geometry with the thickness 13 mm (right).

2

2 ICCA loading conditionsRotation Speed

[rpm]Impact Normal

Forces [N]Impacting frequency

[Hz]Impact time

[ms]150 90 5 50

Fig. 3: CC damage P versus impact cycles n for all four com-pounds. The results are average values for 3 test specimens of each compound.

3

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

54 KGK · 06 2020 www.kgk-rubberpoint.de

Fig. 5: Photographs of tire treads of all analyzed compounds after CC real field testing.

5

a

c

b

d

Fig. 4: Photographs of ICCA test specimens after impacting for the indicated number of cycles.

4

a

c

b

d

about 40 km/h. The test track consisted of stones between 10 and 30 mm in size - mainly from sharp-edged and hard rock.

The evaluation of the tread surfaces was based on the wear after the test. Photographs of the tread surfaces after the tests are shown in Figure 5. Accord-ing internal evaluation criteria of the tire company, the ranking of the CC resist-ance - from the highest up to lowest de-gree – follows the order a > c > d > b.

ConclusionIn this work we reported a good correla-tion between the CC laboratory tests of four truck tire rubber compounds with the corresponding CC evaluation of the four truck tires operating under off-the-road conditions. The results clearly showed the identical ranking between the visible damage on the abraded sam-ple surfaces after the ICCA test, the nu-merically evaluated CC damage P param-

eters and the final evaluation criteria from real tire tests. In our case the rank-ing of the CC resistance is from the high-est up to lowest in order a > c > d > b. This rating also correlates well with the crack growth rates from TFA at high tearing energies [1].

In the present work the capabilities of a new laboratory test procedure, respec-tively testing instrument (ICCA), for measuring cut and chip resistance were clearly approved. This dynamic rubber impact testing approach allows the cut and chip behaviour to be quantified us-ing a new physical parameter which is the CC damage P. Results for P yield gen-erally the ranking NR > SBR > BR for the CC resistance.

AcknowledgementThe authors thank to Deutsche Forschun-gsgemeinschaft (DFG) for funding the projects HE4466/26-1 (G. Heinrich) and KL1409/9-1 (M. Klüppel). R. Stocek ac-knowledges for support from the (Czech Republic) project CPS - Strengthening Research Capacity (reg. number: CZ.1.05/2.1.00/19.0409) - as well as from Program NPU I (LO1504). We thank Continental Reifen Deutschland GmbH (Hannover, Germany) for supporting the projects.

References[1] M. Wunde, M. Klüppel, C.Vatterott, J. Tschim-

mel, J. Lacayo-Pineda, A. Schulze, G. Heinrich: Verbesserung der Laborvorhersagen zum Ris-swachstum und Verschleiß von LKW-Reifen-laufflächen, Kautschuk Gummi Kunststoffe 72, (2019) 72.

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

55KGK · 06 2020www.kgk-rubberpoint.de

[2] R. Stoček, W. V. Mars, C. G. Robertson, R. Kip-scholl: Characterizing rubber‘s resistance against chip and cut behaviour, Rubber World 257, (2018) 38.

[3] R. Stoček, W. V. Mars, R. Kipscholl, C. G. Rob-ertson: Characterisation of cut and chip be-haviour for NR, SBR and BR compounds with an instrumented laboratory device, Plastics, Rubber and Composites 48, (2019) 14.

[4] R. Kipscholl, R. Stoček: Quantification of chip and cut behaviour of basic rubber (NR, SBR), RFP Rubber Fibres Plastics, (02/2019) 88.

[5] C. G. Robertson, J. D. Suter, M. A. Bauman, R. Stoček, W. V. Mars: Finite Element Modeling and Critical Plane Analysis of a Cut-and-Chip Experiment for Rubber, Tire Science and Tech-nology, In-Press 2020.

[6] E. Euchler, H. Michael, M. Gehde, O. Kratina, R. Stocek: Wear of technical rubber materials under cyclic impact loading conditions, Kautschuk Gummi Kunststoffe 69, (2016) 22.

[7] R. Stoček, P. Ghosh, R. Mukhopadhyay, R. Kip-scholl, G. Heinrich: Fracture behavior of rub-ber-like materials under classical fatigue crack growth vs. Chip & cut analysis, Consti-tutive Models for Rubber VIII - Proceedings of the 8th European Conference on Constitutive Models for Rubbers, ECCMR 2013, (2013) 323-328, ISBN: 978-1-138-00072-8.

[8] O. Kratina, R. Stocek, E. Euchler: The Influence of Thermal Ageing of Natural Rubber/Styrene Butadiene Rubber Vulcanizates on Steady state and Dynamic Wear Behaviour, Kautschuk Gummi Kunststoffe 69, (2016) 43.

[9] J. Beatty, B. Miksch: A Laboratory Cutting and Chipping Tester for Evaluation Off-the-road and Heavy-duty Tire Treads, Rubber Chemis-try and Technology 55, (1982) 1531.

[10] H. Liang, Y. Fukahori, A. G. Thomas, J. J. C. Bus-field: The steady state abrasion of rubber: Why are the weakest rubber compounds so good in abrasion?, Wear 268, (2010) 756.

[11] D. Manas, M. Manas, M. Staněk, V. Pata: Wear of tyre treads, JAMME (Journal of Achievements in Materials and Manufac-turing Engineering) 37, (2009) 538.

[12] M. L. Engelhardt, Ki Do Kim, Jung Ho Sun: Improving cutting, chipping resistance of tire treads, in ITEC ’96 Select – Rubber & Plastics News, Sept. 1997, 12.

[13] G. Heinrich, G., unpublished results.

Forschungsprojekt: Stärker elastische TPS für höhere Temperaturen

Die Projektpartner bündeln er-neut ihre Expertisen in den Be-reichen Fasern und technische Kunststoffe.

Bild

quel

le: S

KZ

KURZFASERVERSTÄRKTE TPE Das Kunststoff-Zentrum, Würz-burg, und das Thüringische Ins-titut für Textil- und Kunststoff-Forschung, Rudolstadt, for-schen gemeinsam im IGF-For-schungsprojekt ´Kurzfaser- verstärkte TPE´. Ziel ist, die An-wendung styrolbasierter TPE in der Dichtungstechnik durch ei-nen gezielten Einsatz von Ver-stärkungsfasern für Anwen-dungen zu erweitern. TPE ver-binden viele Eigenschaften der Gummiwerkstoffe mit denen der Thermoplaste. Sie ermögli-chen das Mehrkomponenten-spritzgießen von Elektronik-

oder Pumpengehäusen mit Dichtungen oder werden als optisch und haptisch wirksa-mer Werkstoff großflächig über Strukturbauteile gespritzt. Der Gruppe der TPS – ein Blend aus einem besonders elastischen Polymer in einer thermoplasti-schen Matrix – wird innerhalb der unterschiedlichen TPE das stärkste Wachstum prognosti-ziert. Die Ziele der neuen TPS-Rezepturen: Sie sollen die Rück-stell-Performance bei höheren Temperaturen verbessern, aber gleichzeitig möglichst viele der heute für die Anwendung ver-einbarten Anforderungen an

Härte, Dichte, Weiterreißfestig-keit oder Dehnungsverhalten erfüllen. Sie sollen ihre guten Verarbeitungseigenschaften behalten und keine aufwändi-

gen Änderungen in der Werk-zeugtechnik bedingen. Bei der Charakterisierung der neuen Werkstoffe wollen die Forscher noch deutlich weiter gehen: Die bisher hauptsächlich im Be-reich der Elastomere eingesetz-te Methode ´Temperature Scanning Stress Relaxation´ soll das Gesamtbild des Werkstoff-verhaltens ergänzen und das Verständnis für ihre Leistungs-fähigkeit beim Konstrukteur erweitern. n

KONTAKTKunststoff-Zentrum, Würzburg www.skz.de

Umweltfreundliche Schmierstoffadditive aus nachwachsenden Rohstoffen

PHAT Im Projekt Phat fördert das Bundesministerium für Bil-dung und Forschung seit Okto-ber 2017 ein Konsortium aus vier Partnern mit rund 1,25 Mio. EUR. Die Experten aus Industrie und Wissenschaft – das Fo-schungsinstitut Fraunhofer Umsicht, Fritzmeier Umwelt-technik, Unavera Chem Lab, Fuchs Schmierstoffe und Fuchs Lubritech – entwickeln nachhal-tige Verdicker und Bindemittel für die Schmierstoffindustrie und setzen dafür biobasierte Rohstoffe aus Mikroorganismen ein. Im letzten Projektjahr ange-

kommen, offenbaren mehrere Anwendungstests bereits be-achtliche Ergebnisse. Schmier-stoffe werden eingesetzt, um bei Maschinen Reibung und Verschleiß zu verringern. Außer-dem können sie zur Kühlung beitragen, finden als Dichtungs-mittel Anwendung oder schüt-zen vor Korrosion. Ebenso kom-men Gleitlacke vielfältig zum Einsatz. Die besondere Eigen-schaft von Gleitlacken besteht darin, dass sie eine dünne tro-ckene Gleitschicht bilden, die die Reibung und den Verschleiß aneinanderstoßender Oberflä-

chen verringert. Damit Schmier-mittel im Allgemeinen auch in unterschiedlichsten Einsatzfel-dern eine optimale Leistung er-bringen, werden ihnen Additive wie Verdickungs- oder Binde-mittel zugesetzt. Diese sind je-doch in der Regel aktuell noch Erdöl-basiert. Die Projektpart-ner entwickeln umweltverträg-liche Verdicker und Bindemittel auf Basis einer Stoffklasse von natürlich vorkommenden Bio-polymeren, sogenannten Poly-hydroxyalkanoaten. Die PHA stammen dabei aus speziellen Mikroorganismen, die diese

Moleküle als Speicherstoff her-stellen. Nach vielen Optimie-rungsschritten der PHA-Produk-tion haben die Projektpartner inzwischen die Schwelle zum halbtechnischen Maßstab er-reicht, der in absehbarer Zeit wirtschaftlich durchgeführt werden kann. n

KONTAKTIndustrielle Biotechnologie Bayern Netzwerk, München www.ibbnetzwerk-gmbh.com