AEM - extended agings in selective automotive fluids.

Changes in the automotive industry are increasing the demands for higher performance seal and gasket materials in both the engine and transmission. Engineers are requiring higher quality products with both improved performance and greater life expectancy. Many of the materials used historically, such as cork and rubber gaskets and formed-in-place gaskets, are unable to meet the requirements of increasing underhood temperatures, more aggressive engine oils, and longer service life and warranty periods. Molded rubber gaskets, carrier gaskets and molded-in-place technologies have become the preferred sealing systems.

The vast majority of these newer gasket designs, especially for the engine, are made from silicone (VMQ) rubber. Although silicone has shown adequate performance in these applications, it has several inherent deficiencies. Properties such as initial strength, long term durability in actual service fluids, hot oil migration and noise transmission are all areas of concern. Therefore the prolonged applicability and performance of silicone gaskets can be questioned. Long term durability in actual service fluid will be discussed in detail in this article.

In R.E. Vaiden's 1992 SAE paper, he outlines four critical criteria for an effective engine or transmission gasket material (ref. 1):

* The material should have a compression set no greater than 30% after 70 hours at 150[degrees]C.

* The tear strength of the material should be at least 25 dn/m (using ASTM D-624, Tear Die B), to minimize damage during normal factory handling.

* The material should show a positive volume swell of 0-25% with a hardness drop no greater than 25 points. The positive volume swell compensates for the compression set that is experienced immediately after installation.

* While initial tensile strength, modulus and elongation are of secondary importance, the percent retention of these properties after extended exposure to actual service fluids is critical to gasket performance, especially in high mileage vehicles.

The use of ethylene/acrylic (AEM) elastomers in powertrain applications has steadily increased in recent years, mainly because of their good balance of high and low temperature properties and oil resistance. This family of elastomers was expanded in 1992 with the commercial introduction of three new types of polymer. Before that time, all existing grades of AEM elastomers were based on a single gum polymer which was defined as a terpolymer of 55% methyl acrylate, ethylene and a cure site monomer. This product line expansion included a higher methyl acrylate offering, AEM-LS, which has reduced volume swell in motor oils and ATF fluid, while sacrificing only 5-6[degrees]C in low temperature properties. A new family of peroxide cured dipolymers was also introduced which exhibited good compression set properties without a postcure. The dipolymer family includes AEM-D and AEM-DLS. AEM-D has similar swell and low temperature properties to AEM-G1, the standard ethylene/acrylic elastomer, and AEM-DLS has similar swell and low temperature characteristics to AEM-LS[2].

In this article, these three new AEM polymers, along with AEM-G1, are evaluated against this criteria for an effective gasket material. They will be compared to silicone (VMQ) and polyacrylate (ACM), two other materials widely used in powertrain gasket and sealing applications. Extended aging studies in both engine oil and automatic transmission fluid will be used to demonstrate the merits of AEM polymers over the long term for powertrain gaskets.

Experimental

Compounding ingredients and definitions

The AEM vulcanizates tested were prepared using commercial grades of Vamac ethylene/acrylic elastomers from Du Pont. The four grades used were AEM-G1, AEM-LS, AEM-D and AEM-DLS. G1 and LS have ethylene/methyl acrylate/cure site monomer with an amine cure system. The D and DLS monomers are ethylene/methyl acrylate using a peroxide cure system. The two VMQ compounds are commercial gasket compounds available from Dow Coming STI. The 40 durometer VMQ compound, 24008-V, is used in engine gaskets at Ford and the 60 durometer VMQ compound, Q4-2918, is used in engine gaskets at General Motors (GM). The ACM grade is a general all purpose polymer, Hycar 4052EP, which uses a no postcure cure system. A complete listing of the compounding ingredients used in these vulcanizates is available.

Mixing, molding and testing

The AEM and ACM compounds were mixed in a lab internal mixer. A standard upside-down mix procedure with a drop temperature of 100[degrees]C was used. Typical gasket formulations were used. The VMQ compounds were obtained as mixed compounds.

All test procedures conformed to the appropriate ASTM test methods. Compression set tests were run on plied discs. Test specimens were die cut from slabs prepared by compression molding. All compounds were molded 10 minutes at 177[degrees]C. The AEM-G1 and AEM-LS compounds were posteured four hours at 175[degrees]C in an air-circulating oven. No postcure was necessary for the other compounds.

The test fluids used for the extended testing were `factory fill' motor oils and automatic transmission fluids (ATF) specified by Ford and GM. The GM motor oil was changed weekly in an attempt to simulate how frequently oil is changed in the average automobile. The Ford motor oil and the ATF fluids, on the other hand, remained the same throughout the 3,000 hour test. Samples were tested at 168, 504, 1,008, 1,512. 2,016 and 3,024 hour intervals. The Ford ATF fluid was also tested at 3,528 and 4,032 hours. Since the useful life of an elastomer is generally defined as above a 50% absolute elongation, testing was stopped when elongation at break went below the 50% level.

Discussion

Physical properties and compression set properties

In table 1, the four ethylene/acrylic (AEM) based compounds show good physical properties at room temperature. They all easily meet the 25 dN/m minimum tear requirement. The 70 hour at 150[degrees]C compression set is excellent for all four compounds, well below 20%. The ACM and VMQ compounds also show good physical properties but slightly lower tensile strength and the ACM compound is borderline on tear. In figure 1, long term compression sets were measured out to 2.016 hours. All the compounds performed quite well under this extremely rigorous test. Note that the four AEM polymers have the lowest values for compression set at 2,016 hours. Also, the postcured AEM-GI and AEM-LS compounds consistently gave the lowest compression set values over the test period.

Fluid resistance

Short term agings

Short term fluid agings in ASTM #3 oil, Ford and GM engine oil, and Ford and GM automatic transmission fluid for the four AEM types are shown in figure 2. In ranking the ethylene/acrylic elastomers from the most resistant to the least resistant polymer in these hydrocarbon based fluids: AEM-DLS is slightly lower in swell than AEM-LS, which is 1/3 to 1/2 lower in swell than AEM-G1 which is just slightly lower than AEM-D. Figure 3 shows short term fluid agings in the same fluids for the five compounds which were not postcured. The ACM polymer consistently shows the lowest volume swell in all fluids tested and the AEM-DLS is the next lowest. Silicone rubber shows a lower swell than the AEM-D in the popular test fluid, ASTM #3 oil. However, in actual service fluids, both motor oil and ATF fluid, both silicone compounds swell significantly more than any of the other compounds tested. Silicone volume swells after 168 hours in ATF fluid ranged from 41-61% and swells in motor oil ranged from 25-38%.

Long term agings in automatic transmission fluid

Long term aging data in both GM and Ford `factory fill' ATF fluids was studied for the four AEM elastomers and the five compounds which were not postcured. These five compounds include the two AEM dipolymers, the ACM and the two VMQ compounds. For many of the new gasket applications, which include carrier gaskets and molded-in-place gaskets, it is impractical to postcure so it is important to compare the `no-postcure' candidates to one another. For the lowest compression set, a postcure is still recommended for most of these compounds.

Test data show that low swell ethylene/acrylic compounds, based on AEM-DLS and AEM-LS, maintain 85-120% of their original tensile strength after aging 3,000 hours at 150[degrees]C in the GM ATF fluid and 4,000 hours in the Ford ATF fluid. The standard AEM polymers, AEM-D and AEM-G1, also show excellent retention of tensile strength, varying between 60-100% during the same test conditions. An even more important measurement for an elastomer is the retention of elongation. All four AEM compounds maintained good elongation throughout the test period and all were well above the 50% absolute elongation which is used to gauge service life of an elastomeric compound. In general, the two dipolymer compounds, based on AEM-D and AEM-DLS, tended to retain a slightly higher percentage of their original elongation than the terpolymer compounds during this testing. This is not unexpected since the cure site has been removed from the dipolymer composition making it less susceptible to the amine-based additive packages found in many ATF fluids. The Ford fluid seems to be slightly more aggressive towards the polymers than the GM fluid. All four AEM polymers maintain a constant volume swell throughout the test. Therefore, the rating from lowest to highest volume swell does not change from that which was reviewed in the short term aging section. AEM-DLS has the lowest swell, varying between 8-12%. and AEM-D the highest swell, varying between 20-25%.

In comparing the five `no-postcure' compounds, the ACM compound was shown to be the most fluid resistant of the group with the AEM-DLS compound being nearly equivalent. Volume swell of the ACM compound ranged from -3 to 4% during testing. In fact, because the volume swell of an ACM compound is so low, Vaiden reported that ACM compounds typically do not satisfy current gasket design parameters (ref. 1). In contrast to ACM, the two silicone compounds exhibited extremely high volume swells in ATF fluid. The 40 durometer compound swelled 61% after only one week in the Ford ATF. The 60 durometer compound was better, but still swelled 48%. Somewhere between 1,500 and 2,000 hours, the silicone compounds began to lose volume. The ACM compound has similar retention of tensile strength to AEM-DLS, which is excellent. Both silicone compounds, in contrast, show a dramatic decrease in tensile strength in just the first week of testing and this trend continued through 1,000 hours. Both silicone compounds showed dramatic decreases in elongation through the first 1,000 hours of testing. Testing was stopped at 2,000 hours when the absolute elongation dropped below 50%. The ACM compound retained good elongation throughout the test, however, it did lose elongation at a faster rate than either of the AEM compounds.

Long term agings in motor oil

Long term aging data was obtained for both GM and Ford `factory fill' motor oils. A different test procedure was used for testing the two motor oils. The GM motor oil was changed weekly to more closely simulate how frequently the oil is changed in the average automobile. The Ford motor oil was kept the same throughout the test.

The changes in properties of the compounds using AEM polymers was plotted through 3,000 hours. All four AEM polymers showed a consistent tensile strength throughout testing in the Ford motor oil. The low swell polymers, AEM-LS and AEM-DLS, retained high percentages of their original tensile strength throughout the test period, while the AEM-GI and AEM-D compounds maintained approximately 75% of their original tensile strength. Changing the motor oil weekly, as was done with the GM motor oil, proves to be a much more severe test. The low swell polymers, i.e. AEM-DLS and AEM-LS, dropped in tensile strength in the first 1,000 hours of testing before stabilizing between 60-70% of their original strength. The AEM-GI and AEM-D continued to drop in tensile strength out to 2,000 hours before leveling off at around 40% of their original strength. The percent retained elongation data almost directly mirrors the trends seen in the tensile data. In reviewing the volume swell data, the AEM-DLS compound had the lowest swell measuring consistently between 7-8%. The AEM-D compound showed the greatest volume increase at a consistent value of 22% in the Ford motor oil and gradually increasing from 17-25% in the GM motor oil test.

When comparing the ACM and silicone compounds to the two AEM dipolymer compounds, similar trends are seen to the results seen with the ATF testing. The retained tensile strength of the polyacrylate compound is shown to be similar to the AEM-DLS compound. In contrast, the silicone compound deteriorates very quickly in the GM motor oil test. Dramatic drops are seen in both tensile and elongation in the first 500 hours of testing and the material became too brittle to test in less than 1,000 hours. The tensile strength and elongation also significantly dropped in the Ford motor oil, but the material failed later, between 1,000 and 1,500 hours. When looking at the percent retained elongation, the ACM and both AEM compounds had similar retention of elongation. The Ford motor oil test was less aggressive, allowing all three compounds to retain between 80-100% of their original elongation. This was reduced to 40-60% in the GM motor oil test. The ACM compound is the most resistant to motor oil, exhibiting volume swells of -1 to 1% in both motor oils. Once again, this swell is so low it does not meet the minimum swell criteria for gasket designs. The AEM-DLS swells around 7-8% in this test. In contrast, both silicone compounds swell much greater than any of the other compounds tested. In the GM motor oil test, silicone starts to significantly decrease in volume between 500 and 1,000 hours.

Conclusion

The extended agings carried out for this article have shown some very interesting trends. By conducting the testing for periods as long as 3,000 and 4,000 hours, it is possible to see where a material begins to deteriorate. In the case of the AEM and ACM polymers, their properties have been shown to level off after a certain amount of property loss. However, the silicone polymers lost all properties by 2,000 hours in ATF fluid and 1,500 hours in motor oil.

When looking at the volume swell data, the silicone shows very high swell, which may be considered too high, especially in ATF fluid, for gasket designs. In contrast, the ACM has very low swells, which could be considered too low for gasket designs. The volume swells of the four AEM polymers remained intermediate and consistent throughout the extended aging period. The volume swells of the four types of AEM polymers can be ranked as follows: DLS < LS G1 < D.

Overall, the best combination of properties to meet gasket design criteria was exhibited by the AEM compounds. The compounds based on ethylene/acrylic elastomers were the most consistent in their percent retention of tensile and elongation, and had low volume swell, at the optimal level to seal properly.

References

[1.] R.E. Vaiden, "Elastomeric materials for engine and transmission gaskets, " SAE paper no. 920132, presented at the SAE International Congress & Exposition, Detroit, MI, February, 24-28, 1992. [2.] TM. Dobel and J.R. Harrell, "New developments in ethylene/acrylic elastomers," paper no. 28, presented at a meeting of the Rubber Division, American Chemical Society, Detroit, MI, October 8-11, 1991.