Natural rubber is widely used in dynamic applications because of the way several properties counterbalance each other, which can-not be said of other synthetic elastomers. These properties are:   • modulus;   • good adhesion to metal;   • low creep;   • excellent fatigue resistance under high amplitude; and   • good dynamic behavior, with good resilience. Dynamic parts include engine mounts and bushings that filter vibrations and absorb shocks. Table 1 shows the relationship between properties needed on parts and rubber compound properties. We will start with a review of required rubber compound properties, and then we will review the evolution of rubber formulations related to the increase in under-hood tempera-tures for cars.

NR compound requirements for anti-vibration applicationsAdhesion to metal Rubber to metal bonding results from the formation of strong links between the rubber compound and an adhesive during vulcanization. The adhesive film is made by dipping or spray-ing on metal parts after cleaning or degreasing and special metal treatment (grit blasting, phosphate deposit, electrolytic zinc deposit, etc.). Metal treatment ↓ Adhesive deposit ↓ Drying ↓ Molding We generally use a two-coat layer consisting of a primer which creates links with metal and an adhesive top coat which creates links with the rubber compound. Adhesive thickness is about 20µ. The adhesion test is performed with specific samples:   • peel test ASTM D 429 (B); and  • double-lap shear specimen ASTM D 945. Good adhesion allows for cohesive strength in the rubber and it is noted “R” as rubber failure. Poor adhesion results in a failure at the interface, noted “RC” as rubber to cement or “M” as failure in metal.

Dynamic properties Usually measured on a button in compression, dynamic prop-erties represent the rubber compound’s ability to damp sinu-soidal stressing. We measure rubber response, characterized by elastic modulus E’, in phase with stressing and viscous modulus E’’ which is out of phase with stressing. Damping is

characterized by the ratio E’’/E’, known as tangent delta. Dynamic properties are measured in compression on a button H 10 mm/Ø 10 mm. Static stiffness Ks is measured at a compres-sion of 10%. We generally measure two characteristic points:   • K15 stiffness at 15 Hz with an amplitude of 2%; and   • K155 stiffness at 155 Hz with an amplitude of 0.1%. Ratio t = K155/K15 is the coefficient of dynamic stiffening. A rubber compound with a low tangent delta will be resil-ient and will offer low dynamic stiffening, whereas a rubber compound with a high tangent delta will offer high damping with high dynamic stiffening at high frequency.

Creep Creep is the loss of height of a rubber sample under constant stress and temperature over a given time. Because of high di-mensional stability, low creep or low compression set is re-quired. Compression set (ASTM D 395-85) is a simple test which characterizes the material well. For natural rubber, tests are done over a period of 72 hours at temperatures ranging from 70°C to 100°C.

Hot air aging Hot air aging is characterized by the loss of mechanical prop-erties (tensile strength and elongation at break) versus time and temperature (ASTM D 573681). For natural rubber, tests are often performed at seven days

NR in automotive dynamic applications

16 RUBBER WORLD

Table 2 - MRPRA EDS 12 formulation

Natural rubberZinc oxideStearic acidCarbon blackProcessing oilAntioxidant paraphenylene diamineWax SulfurSulfenamide (CBS)

1003.5

245

522

2.50.7

Table 1 - part requirements vs. compound properties

Properties on partStiffness (deformation under stress)Damping at low frequencyLow stiffening at high frequencyFatigue life

Adhesion to metalDurabilityCreep resistance

Rubber compound propertiesModulus

High loss angleLow loss angle

Tear strengthResistance to cut increase

Adhesion to metalAging resistance (O2, O3)

Compression set

Tech Service by Benoit Le Rossignol, Yann Fromont and Frederic Gomez, Hutchinson

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17

or 14 days for temperatures from 70°C to 100°C.

Fatigue life resistance Fatigue life resistance is the time or number of cycles required to destroy the part under alternative stress. The part destruction is due to crack propagation until complete failure of the rubber occurs. There are many fatigue test methods, but the most important ones are:   • FTFT (fatigue to failure tester);  • crack initiation; and   • crack propagation speed. As a rule, in order to determine fatigue behavior of a rubber compound, we have to establish a relationship between sev-eral applied stresses and the number of cycles to failure (Wöhler curves).

Compound developments versus temperature increase Over the last 20 years, the under-hood temperature for cars has sharply increased from 70°C to 100°C. The main reasons for this temperature increase are the increase of engine power, wide use of turbo for diesel engine, downsizing and encapsula-tion of the engine for noise reduction. In order to manage this increase, the rubber industry has had to improve its natural rubber compounds. For dynamic applications with an operating temperature of 70°C, a natural rubber compound is usually based on the for-mulation shown in table 2. This is a "classic" curing system called CV for classic vul-canization with ratio of accelerator/sulfur = 0.28. This compound has the following level of properties (table 3). To improve the aging resistance at higher temperatures, we

NOVEMBER 2009

have to change the curing system from classic vulcanization system (ratio accelerator/sulfur = 0.1 to 0.6 consisting of large and flexible polysulfide links) to the semi-efficient curing system noted SEV (ratio of accelerator/sulfur = 0.7 to 2.5 ) at 85°C and then, to the efficient curing system noted EV (ratio of accelerator/sulfur > 2.5 forming short and stiff mono sulfide links) at 100°C. These are shown on an evolution basis in figure 1. These developments in the curing system have not only changed aging resistance, but also affect the balance between other properties like compression set, fatigue resistance and damping or dynamic properties. The following diagrams (figure 2) illustrate the relative po-sition of the different curing systems versus properties. We can state that, although aging resistance and compres-sion set improve when going from the classic to the efficient system, conversely, fatigue resistance is lower when going from CV to EV. This clearly shows that the balance has been modified. It is also important to notice that adhesion to metal

Figure 1 - evolution of under-hood temperature versus time

140

120

100

80

60

40

20

01930 1940 1950 1960 1970 1980 1990 2000 2010

Tem

pera

ture

CVSEV

EV?

Figure 2 - cure system position vs. properties

110

100

90

80

70

60

50Compression set

T °c

EV

SEV

CV

110

100

90

80

70

60

50Tg Delta 15 Hz

T °c

EV

SEV

CV

110

100

90

80

70

60

50Fatigue life

T °c

EV

SEV

CV

Table 3 - MRPRA EDS 12 properties

Hardness ISO 27 (IRHD)Tensile strength (ASTM D 412-87)Elongation at break (ASTM D 412-87)Compression set 22 h. at 70°C (ASTM D 395-85)Tangent delta 2% 15 hz at 23°C

6025 MPa

590%26%

0.082

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RUBBER WORLD

is more critical with the EV system with a lower level of sulfur in the compound. Until 2008, Hutchinson used these three curing systems to meet car manufacturers’ technical requirements. To bring new and better solutions, to break fresh ground and to anticipate new needs in terms of aging at higher temperatures over 100°C, Hutchinson has developed a new concept natural rub-ber formulation.

New concept SHTC (super high temperature compound) The main idea of this research is to offer a natural rubber com-pound with dynamic behavior and fatigue resistance compa-rable to the EV compound, but with improved hot air aging and compression set. The challenge here is to get the same compression set that the EV compound has at 100°C, but with this new material, at a temperature of 115°C. The target of this development is to offer a natural rubber compound which will have:   • the same compression set as the EV compound at 100°C, but measured at 115°C for the SHTC compound with;  • better aging resistance than the EV material. Aging is measured at 115°C on 2 mm flat samples. Aging time is the time required to reach a 50% loss of elongation at break; and   • the same resistance to fatigue as the EV compound, al-though it should remain stable after aging at 115°C. As a first step, work has been carried out with a 55 durom-eter compound for engine mount applications. We therefore

optimized a curing and protection package to improve aging behavior of NR.

Mechanical properties Table 4 shows a comparison of EV vs. SHTC compounds. The elongation at break evolution was performed over a period of 21 days (figure 3). We notice a significant improvement in the time to reach a 50% loss of elongation at break. Duration is about twice as much for the SHTC compound.

Dynamic properties Table 5 shows dynamic properties before and after aging. These results clearly show noticeable improvements in terms of aging resistance and good property stability for the SHTC compound.

Resistance to fatigue The resistance to fatigue test has been performed at the initial state and after aging at 115°C over a period of 28 days. The test sample is a rubber to metal bonded sample tested in com-pression. Results are shown in table 6. We can note great stability of fatigue life for the SHTC com-pound. Even if fatigue resistance is slightly lower than the EV compound, the number of cycles after aging is stable for the SHTC compound, while fatigue resistance decreases dramati-cally for the EV compound. This result confirms that the behav-ior of the SHTC compound improves after aging at 115°C.

Adhesion to metal The adhesion test has been performed on a peel test sample at the initial state and after aging in hot glycol (24 hours at 95°C). Adhesion results show a slight advantage for SHTC com-pound, especially after aging in glycol, as notedin table 7.

Industrial validation An industrial validation has been performed to test all the steps of the process, from compounding to molding of the part. The part chosen for this study is a hydraulic engine mount, cur-rently manufactured with the EV compound.

18

Table 5 - dynamic properties before and after aging

Initial stateKsK15K155T = K155/K15Tg delta 15 HzAfter hot air aging 28 days at 115°CKs changeK15 changeK155 change

N/mmN/mmN/mm

EV class 100°C3104206801.62

0.150

+110+130+145

SHTC class 115°C3104105201.27

0.106

+25+35+35

Table 4 - comparison of EV vs. SHTC compounds

HardnessTensile strength TS (ASTM D 412-87)Elongation at break EB (ASTM D 412-87)Compression set (ASTM D 395-85)Aging in hot air (ASTM D 573-81)TS changeEB change

Durometer AMPa

%

94 h. at 100°C, %94 h. at 115°C, %

7 days at 115°C

%%

EV class 100°C

5520.4

379

3842

-50%-40%

SHTC class 115°C

5520

390

3237

-40%-30% Figure 3 - aerobic aging improvement

SHTC vs. EV

600

500

400

300

200

100

00 100 200 300 400 500

Eb

(z)

Aerobic aging improvement (115°C)

Hours

EV SHTC

+9 days

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NOVEMBER 2009 19

Conclusion This new compound development constitutes a considerable step forward in terms of aging resistance of natural rubber, and places natural rubber at a level never reached to date. We can now use natural rubber at 115°C without any loss in resistance to fatigue. This new concept may be used in a large variety of hardnesses and can cover the range of industrial applications for cars, trucks, buses and aerospace applications. Industrial application of SHTC is in progress with several car manufacturers for engine mounts and also for aircraft en-gine mounts, where balance between aging, resistance to fa-tigue and compression set are required.

The customer requires a continuous aging temperature at 115°C and a peak temperature of 130°C. We tested the SHTC compound versus the current EV ma-terial under the following conditions:   • 500 hours at 110°C;   • 500 hours at 120°C; and  • 500 hours at 130°C. Then, we subjected the part to a fatigue test. Once more, results confirm that the SHTC compound be-haves better in terms of aging resistance (figure 4).

Table 6 - fatigue properties before and after aging

Fatigue at initial stateNumber of cycles to failure*Fatigue after hot air aging 28 days at 115°CNumber of cycles to failure* Base 100 for EV compound

EV class 100°C

100

10

SHTC class 115°C

85

75

Table 7 - adhesion to metal before and after aging

Initial stateAfter aging in glycol

EV class 100°C

Peeling strength

(daN)2120

EV class 100°C

Failure surface

R 100%R96%

RC 4%

SHTC class 115°C

Peeling strength

(daN)20.7

21

SHTC class 115°C

Failure surface

R 100%R98%

RC 2%

Figure 4 - part durability (kilocycles) after heat aging at 110, 120 and 130°C

After heat aging 500 h.

at 110°C

After heat aging 500 h.

at 120°C

After heat aging 500 h.

at 130°C

Durability

EV SHTC

600

500

400

300

200

100

0

Kcs

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