and Frederic Gomez, Hutchinson Tech Service NR in automotive dynamic applications Natural rubber is widely used in dynamic applicationsbecause of the way several properties counterbalance each other, which cannot 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 temperatures for cars. NR compound requirements for anti-vibration applications Adhesion 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 spraying 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 properties represent the rubber compound’s ability to damp sinusoidal 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 by Benoit Le Rossignol, Yann Fromont 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 compression 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 resilient 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 dimensional stability, low creep or low compression set is required. 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 properties (tensile strength and elongation at break) versus time and temperature (ASTM D 573681). For natural rubber, tests are often performed at seven days Table 1 - part requirements vs. compound properties Rubber compound properties Properties on part Modulus Stiffness (deformation under stress) High loss angle Damping at low frequency Low loss angle Low stiffening at high frequency Tear strength Fatigue life Resistance to cut increase Adhesion to metal Adhesion to metal Aging resistance (O2, O3) Durability Compression set Creep resistance Table 2 - MRPRA EDS 12 formulation Natural rubber Zinc oxide Stearic acid Carbon black Processing oil Antioxidant paraphenylene diamine Wax Sulfur Sulfenamide (CBS) 100 3.5 2 45 5 2 2 2.5 0.7 16 RUBBER WORLD 28. As a rule. the rubber industry has had to improve its natural rubber compounds. although aging resistance and compression set improve when going from the classic to the efficient system. downsizing and encapsulation of the engine for noise reduction. The main reasons for this temperature increase are the increase of engine power.Table 3 . fatigue resistance is lower when going from CV to EV. For dynamic applications with an operating temperature of 70°C. In order to manage this increase. Compound developments versus temperature increase Over the last 20 years. Fatigue life resistance Fatigue life resistance is the time or number of cycles required to destroy the part under alternative stress. to the efficient curing system noted EV (ratio of accelerator/sulfur > 2. properties 110 100 90 T °c 80 70 60 50 110 100 90 T °c 80 70 60 50 110 100 90 T °c 80 70 60 50 Fatigue life EV SEV CV Tg Delta 15 Hz SEV CV EV Compression set EV SEV CV NOVEMBER 2009 17 . at 70°C (ASTM D 395-85) 26% 0. we have to establish a relationship between several applied stresses and the number of cycles to failure (Wöhler curves). We can state that. conversely. It is also important to notice that adhesion to metal Figure 2 . but the most important ones are: • FTFT (fatigue to failure tester). wide use of turbo for diesel engine. These developments in the curing system have not only changed aging resistance. and • crack propagation speed. in order to determine fatigue behavior of a rubber compound. The part destruction is due to crack propagation until complete failure of the rubber occurs.5 ) at 85°C and then.7 to 2. There are many fatigue test methods. The following diagrams (figure 2) illustrate the relative position of the different curing systems versus properties. This clearly shows that the balance has been modified. fatigue resistance and damping or dynamic properties.MRPRA EDS 12 properties 60 Hardness ISO 27 (IRHD) 25 MPa Tensile strength (ASTM D 412-87) 590% Elongation at break (ASTM D 412-87) Compression set 22 h. To improve the aging resistance at higher temperatures. the under-hood temperature for cars has sharply increased from 70°C to 100°C. we Figure 1 . These are shown on an evolution basis in figure 1. This compound has the following level of properties (table 3).cure system position vs. • crack initiation.evolution of under-hood temperature versus time 140 120 Temperature 100 80 60 40 20 0 1930 1940 1950 1960 1970 1980 1990 2000 2010 CV SEV EV ? have to change the curing system from classic vulcanization system (ratio accelerator/sulfur = 0. a natural rubber compound is usually based on the formulation shown in table 2. This is a "classic" curing system called CV for classic vulcanization with ratio of accelerator/sulfur = 0. but also affect the balance between other properties like compression set.5 forming short and stiff mono sulfide links) at 100°C.082 Tangent delta 2% 15 hz at 23°C or 14 days for temperatures from 70°C to 100°C.1 to 0.6 consisting of large and flexible polysulfide links) to the semi-efficient curing system noted SEV (ratio of accelerator/sulfur = 0. dynamic properties before and after aging EV class 100°C SHTC class 115°C Initial state 310 310 N/mm Ks 420 410 N/mm K15 680 520 N/mm K155 1. to break fresh ground and to anticipate new needs in terms of aging at higher temperatures over 100°C. SHTC compounds EV class SHTC class 100°C 115°C Durometer A 55 55 MPa 20.62 1. at 115°C.4 20 379 38 42 -50% -40% 390 32 37 -40% -30% optimized a curing and protection package to improve aging behavior of NR. as notedin table 7. Dynamic properties Table 5 shows dynamic properties before and after aging. The elongation at break evolution was performed over a period of 21 days (figure 3). SHTC compounds. Results are shown in table 6. Adhesion results show a slight advantage for SHTC compound. New concept SHTC (super high temperature compound) The main idea of this research is to offer a natural rubber compound with dynamic behavior and fatigue resistance comparable to the EV compound. Aging time is the time required to reach a 50% loss of elongation at break. especially after aging in glycol. These results clearly show noticeable improvements in terms of aging resistance and good property stability for the SHTC compound. % Aging in hot air 7 days at 115°C (ASTM D 573-81) TS change % EB change % Table 5 . As a first step. We notice a significant improvement in the time to reach a 50% loss of elongation at break. EV Aerobic aging improvement (115°C) 600 500 Eb (z) 400 300 200 100 0 0 100 200 Hours 300 400 500 +9 days EV SHTC Hardness Tensile strength TS (ASTM D 412-87) Elongation at break EB % (ASTM D 412-87) Compression set 94 h. Until 2008. This result confirms that the behavior of the SHTC compound improves after aging at 115°C. The part chosen for this study is a hydraulic engine mount. at 100°C. the number of cycles after aging is stable for the SHTC compound. Hutchinson has developed a new concept natural rubber formulation.aerobic aging improvement SHTC vs. The test sample is a rubber to metal bonded sample tested in compression.27 T = K155/K15 0. 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. and • the same resistance to fatigue as the EV compound. Aging is measured at 115°C on 2 mm flat samples. 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. from compounding to molding of the part.150 0. at a temperature of 115°C. Even if fatigue resistance is slightly lower than the EV compound. Figure 3 . We therefore Table 4 . We can note great stability of fatigue life for the SHTC compound. although it should remain stable after aging at 115°C. while fatigue resistance decreases dramatically for the EV compound. The challenge here is to get the same compression set that the EV compound has at 100°C. but with improved hot air aging and compression set. Hutchinson used these three curing systems to meet car manufacturers’ technical requirements. work has been carried out with a 55 durometer compound for engine mount applications. currently manufactured with the EV compound. • better aging resistance than the EV material. To bring new and better solutions. but measured at 115°C for the SHTC compound with. but with this new material. Duration is about twice as much for the SHTC compound.is more critical with the EV system with a lower level of sulfur in the compound. Industrial validation An industrial validation has been performed to test all the steps of the process. % (ASTM D 395-85) 94 h. Mechanical properties Table 4 shows a comparison of EV vs.comparison of EV vs.106 Tg delta 15 Hz After hot air aging 28 days at 115°C +110 +25 Ks change +130 +35 K15 change +145 +35 K155 change 18 RUBBER WORLD . 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). We can now use natural rubber at 115°C without any loss in resistance to fatigue. Conclusion This new compound development constitutes a considerable step forward in terms of aging resistance of natural rubber. Then. and • 500 hours at 130°C. Once more.adhesion to metal before and after aging EV class EV class 100°C 100°C Failure Peeling strength surface (daN) 21 R 100% Initial state R96% 20 After aging in RC 4% glycol SHTC class SHTC class 115°C 115°C Peeling Failure strength surface (daN) 20. Industrial application of SHTC is in progress with several car manufacturers for engine mounts and also for aircraft engine mounts. buses and aerospace applications. This new concept may be used in a large variety of hardnesses and can cover the range of industrial applications for cars. at 130°C Durability Table 7 . at 110°C After heat aging 500 h. trucks.7 R 100% 21 R98% RC 2% SHTC The customer requires a continuous aging temperature at 115°C and a peak temperature of 130°C. at 120°C EV After heat aging 500 h. We tested the SHTC compound versus the current EV material under the following conditions: • 500 hours at 110°C. we subjected the part to a fatigue test. resistance to fatigue and compression set are required. 120 and 130°C 600 500 400 Kcs 300 200 100 0 After heat aging 500 h.Table 6 .part durability (kilocycles) after heat aging at 110. where balance between aging. results confirm that the SHTC compound behaves better in terms of aging resistance (figure 4). • 500 hours at 120°C. and places natural rubber at a level never reached to date.fatigue properties before and after aging EV class 100°C Fatigue at initial state 100 Number of cycles to failure* Fatigue after hot air aging 28 days at 115°C 10 Number of cycles to failure * Base 100 for EV compound SHTC class 115°C 85 75 Figure 4 . NOVEMBER 2009 19 . . users may print. download. 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