Evaluating Fatigue Characteristics and Heat Generation

in Silica ‘Green Tire Recipe’ and Conventional

Carbon Black Filled Rubber

Keywords: ASTM D623, Goodrich Flexometer, Heat Generation, Flexing Fatigue, Heat Build-up, HBU, Heat Blow-out EF033

INTRODUCTION Three specimen of 20 mm x 20 mm (Diameter x Height) from each

of four formulations were used for each test. The dimensions dier
In recent years, trends in tire manufacturing have shifted to high from ASTM D623 specications but were consistent throughout
performance rubber compounds that oer lower fuel consumption. the study for a relevant comparison to each other.
Dynamic mechanical performance of these rubber compounds is
primarily inuenced by the mechanical properties of the matrix The materials were selected based on similar shore hardness to
polymer and its interaction with the ller.1 The two main llers provide a relevant and fair comparison. The formulations used
added to rubber compounds are carbon black and precipitated for this study are summarized in Table 1. In addition to silica and
silica. Carbon black has long been the primary reinforcement for carbon black as varying ingredients, butyl rubber was changed
rubber compounds and is used to improve tear strength, modulus, from linear to branched molecular structure. If neodymium is used
and abrasion resistance.2 On the other hand, silica provides as a catalyst during the polymerization reaction, butyl rubber with a
unique capabilities of tear strength, abrasion, and aging resistance narrow molecular weight distribution with low degree of branching
as well as better wet traction and improved low rolling resistance is obtained whereas if cobalt is employed, butyl rubber with a
compared to carbon black.3 It has been estimated that a reduction broad molecular weight distribution and a high degree of branching
in rolling resistance by 10 % can lead to up to 2% improvement can be produced. Same silica compounds were previously also
in fuel eciency.4 Therefore, silica is sometimes known as the investigated for their processing, pre-cure properties by Heinz et
eco-friendlier, more green ller for tire tread compounds due to its al using Rubber Process Analyzer from TA Instruments.8
low environmental impact by reducing fuel consumption and CO2
emissions.5, 6 Heat Build Up tests were carried out per ASTM D623 test method
on ElectroForce 3330 instrument with oven. The recommended
This article investigates the exing properties and heat generation mean load on the specimen is 644, 990 or 1970 kPa and a dynamic
of rubber mixtures of styrene butadiene rubber and butyl rubber, peak-to-peak strain of 17.8%, 22.84% or 25.4%. Phenolic platens
with silica and carbon black as llers, using Goodrich Flexometer were used to avoid any heat loss during dynamic tests under high
test or Heat Build-up.7 loads with the lower platen tted with a thermocouple to measure
the specimen’s base temperature. A schematic of the phenolic
MATERIALS AND METHODS platen is shown in Figure 1.

Compound Designation
Lin-Si Lin-C
Components Br-Si (phr) Br-C (phr)
(phr) (phr)
Styrene
Butadiene 96.25 96.25 96.25 96.25
Rubber
30 30 30 30
Butyl Rubber
(Linear) (Linear) (Branched) (Branched)
Silica 80 - 80 -
Figure 1. Phenolic platens (Left: side view of the clamps with black
Carbon Black specimen in the center, right: top view, dotted line indicates specimen
- 73 - 73
(N-234) positioning and central dot is a thermocouple)

Silane (Si-266) 5.8 - 5.8 - WinTest® HBU Application software was used to maintain a mean
DPG-80 2.5 - 2.5 - compressive load of 990 kPa and an equivalent total dynamic stroke
of peak-to-peak 17.8% strain was implemented simultaneously at
Vulkacit CZ
1.6 1.6 1.6 1.6 a frequency of 30 Hz. Before the test starts, DMA measures the
(CBS) specimen height at a given pre-load which serves as a basis for
Perkacit 0.2 - 0.2 - subsequent strain calculations. The ambient oven temperature
was set to 50 ºC. The test ran for a total of 25 minutes with a
Sulphur 2 1.4 2 1.4
data collection rate of 10kHz. The HBU application also includes
A/S ratio 2.15 1.14 2.15 1.14 common DMA measurements including complex modulus,
storage modulus, and loss modulus as well as tangent delta, also
Table 1. Summer tire tread formulations

1 EF033
 known as loss factor. These results are obtained from the precise The eect of higher heat generation can be seen in the form of
measurement of the magnitude and phase relationships between increased specimen temperature. Maximum heat build-up was
force and displacement signals as the material deteriorates under seen in the branched compound with carbon black ller and the
high dynamic and mean forces. least amount of heat build-up was seen in the branched compound
with silica.
8

Storage modulus E’ (MPa)

6

Loss modulus E” (MPa)

M04-Lin-Si
M07-Lin-C
4 M11-Br-Si
M14-Br-C

0
0 300 600 900 1200 1500
Time t (s)

Figure 4. Progression of storage modulus and loss modulus during Heat

Build Up test
Figure 2. ElectroForce 3330 with oven and tensile fixture
The rapid change in mechanical properties for all the compounds
Before the experiment, the specimen height was measured with
was observed in the rst few minutes (Figure 4 and Figure 5).
a vernier caliper. After the experiment, the specimen was allowed
The storage moduli decreased for the rst ve minutes and then
to cool to room temperature for 1 hour and the nal height was
started to increase. This change was higher and more drastic in
measured. Compression set was calculated per following equation:
carbon lled compounds compared to silica lled compounds.
The increase in storage modulus is an indication of buckling of
(ho - hi) the specimen and a higher tangent delta (loss factor) for same
CA = x 100 compounds and reinforces the previous increased temperature
ho rise data. Silica, on the other hand, showed negligible buckling
Where: and lower loss factor.
0,30
CA = compression set as a percentage of original height
M04-Lin-Si
ho = original height M07-Lin-C
0,25
hi = nal height M11-Br-Si
M14-Br-C
Tan (delta) tan (δ)

RESULTS AND DISCUSSION 0,20

A thermocouple mounted in the insulating platens measured the

sample’s base temperature. The base temperature change is 0,15
plotted in Figure 3.
100 0,10

0,05
0 300 600 900 1200 1500
Temperature T (ºC)

80 Time t (s)

Figure 5. Tan Delta or Loss Factor during Heat Build Up test

The heat Build-Up experiment was continued only for 25 minutes

60 per ASTM D623. Longer durations should show a continuously
Lin-Si
increasing temperature for carbon lled polymer whereas steady-
Lin-C
Br-Si
state temperature for silica lled polymer was reached within
Br-C 25 minutes. Approximately 5 ºC temperature dierence was
40 observed for carbon black between the linear and branched
0 300 600 900 1200 1500
compound compared to only 2.5 ºC average between the similar
Time t (s)
silica-containing formulations. Total temperature rise of the
Figure 3. Temperature increase as measured by base thermocouple during compounds with Carbon black is ~40 ºC and almost twice as high
HBU as of Silica compounds (Figure 6). This suggests heat generation

2 EF033
 of the rubber compounds was inuenced to a greater extent by CONCLUSIONS
the type of ller and perhaps accelerator/sulfur ratio compared to
the branching of the polymer. Surface properties of the ller and Dynamic performance of greener tire recipe with silica lled
ller network interaction9 can also have an inuence in the heat rubber has been demonstrated to be better than carbon black
buildup behavior. lled compounds. Branched and linear butyl rubber in styrene
butadiene rubber matrix with llers were tested for their dynamic
50.0 mechanical response. No signicant dierence between the
46.0
41.1 polymer architecture of butyl rubber was observed. However,
reasonable dierence was illustrated between silica and carbon
Temperature Difference (ºC)

40.0
lled compounds. Carbon lled compounds which also have
lower accelerator to sulfur ratio showed twice as much increase
30.0 in damping at higher strains compared to silica lled compounds
21.1 21.0 under high strain and high frequency loading. Heat buildup and
20.0 compression set were also found to be higher for carbon lled
compounds. The data presented in this study show that silica lled
compounds oer better dynamic mechanical performance and
10.0 heat generation than carbon black lled compounds. This shows
silica’s potential for a more sustainable tire by oering improved
0.0 fuel economy and reduced environmental impact.
Lin-Si Lin-C Br-Si Br-C
Sample FUTURE OUTLOOK

Figure 6. Average temperature increase from 50 ºC with standard deviation The current study indicates the dierence between carbon lled
bars
and silica lled compound which could either stem from the llers/
matrix interactions or due to variation of accelerator to sulphur
The compression set also showed a similar trend to temperature
ratio. It will be benecial if the rubber compounds with similar
rise between silica and carbon as illustrated in Figure 7. The
A/S ratio or with dierent llers are tested to help identify which
carbon compounds tested have a higher permanent compression
variable most impact these properties.
set compared to the silica samples. Less compression set was
observed in linear polymer than branched polymer for carbon ller. REFERENCES
However, an equal amount of compression set was seen in silica
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Noordermeer, J. W. (2003). Mechanistic aspects of the role
8.0
6.4 of coupling agents in silica–rubber composites. Composites
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of crosslinks on the physical properties of natural rubber.
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4.0 2.9
2.8 3. Rattanasom, N., Saowapark, T. A., & Deeprasertkul, C. (2007).
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Reinforcement of natural rubber with silica/carbon black hybrid
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Lin-Si Lin-C Br-Si Br-C
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Figure 7. Average compression set measured after HBU test. Vertical bars
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as the accelerator to sulfur ratio increases, aging resistance is
interaction and reinforcement in tire tread compounds.
improved. This was also observed in the temperature increase
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in each specimen in Fig. 6 and the compression set in Fig. 7,
respectively. Since silica lled polymers have approximately twice 6. Sarkar, P., & Bhowmick, A. K. (2018). Sustainable rubbers and
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less likely to plastically shear against each other, hence less heat
generation.

3 EF033
7. ASTM D623-07(2019) e1, Standard Test Methods for
Rubber Property—Heat Generation and Flexing Fatigue In
Compression, ASTM International, West Conshohocken, PA,
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ACKNOWLEDGEMENT

The authors would like to express their sincerest gratitude to

Evonik Operations GmbH, Smart Materials for providing cured tire
tread formulations for completing this investigation.

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