Oil swell and low temperature properties of AEM and impact on seal performance.

Vamac ethylene acrylic elastomers (AEM) have been commercially available for over 30 years and, since their introduction, have been adopted in a wide variety of automotive seal, hose, duct and dynamic applications. Cured AEM compounds provide an excellent balance of good oil resistance, heat resistance, low temperature flexibility and high damping properties.

The current principal polymer grades are based on either dipolymers of ethylene and methyl acrylate or terpolymers of ethylene, methyl acrylate and an acidic cure site monomer. Compounds with AEM terpolymers use a diamine curing agent and require a post cure to develop full performance properties. AEM dipolymer compounds are peroxide cured and usually are not post-cured. The post-cured AEM terpolymers generally have better properties than the non-post-cured AEM dipolymers.

There are two AEM terpolymer grades commercially available, with different levels of methyl acrylate providing different levels of oil swell performance. The higher MA content AEM terpolymer (GLS) has about half the swell level as the medium MA content grade (G) for a given fluid. These two elastomers can be blended to provide any intermediate level of swell resistance desired. The low temperature performance is impacted as MA content is increased, with the higher MA content grade having a 5-7[degrees]C disadvantage compared to the medium MA grade (G).

Plasticizers can be used to improve the low temperature performance of AEM elastomers, with just 10 phr of an etherester plasticizer regaining the 5-7[degrees]C temperature loss of the high MA content AEM elastomer (GLS). Further improvements in low temperature performance can be obtained with larger additions of plasticizers, but the loss of other physical properties, as well as the thermal stability of the plasticizer itself, begin to have negative impacts (ref. 1) (figure 2).

[FIGURES 1-2 OMITTED]

A new AEM polymer was developed to provide improved low temperature performance without the need for heavy use of plasticizers. This new AEM polymer (LTX) has a lower glass transition temperature (Tg), but also results in higher fluid swell, than the current AEM grades. The main targeted application (automotive boots) uses high viscosity greases, and the higher fluid swell is acceptable. This new AEM polymer provides the same heat resistance as current grades and is blendable with AEM terpolymers. It provides a means to improve the low temperature performance of AEM elastomer seal and gasket compounds without using plasticizers.

It is a misconception that lower fluid swell is always better for a seal and gasket application, when in fact some swell is beneficial for a static seal; as oil swell provides an increase in sealing pressure and an improvement in lower temperature properties as the fluid plasticizes the compound. Problems can occur when excessive swell levels create strains in the seal design that exceed the maximum material capabilities and the seal splits, or changes in other properties are unacceptable. In general, the swell levels of AEM compounds in commercial powertrain fluids are well within design limits, especially as these fluids become more synthetic, which reduces the swell in AEM compounds.

Results are presented on the performance of AEM terpolymers, with and without plasticizers, and in blends with the new low temperature AEM polymer. Data are presented on oil swell, low temperature properties, oil and air aging, compression set and compressive stress relaxation (CSR).

Experimental

Compounds were mixed in either a 1.5 liter, 1.75 liter or 3.7 liter internal mixer and the mixed compound sheeted on a two-roll mill. Test specimens were compression molded for five minutes at 175[degrees]C and then post-cured four hours at 175[degrees]C.

All tests were performed per ASTM procedures for elastomers. CSR testing was performed using modified Wykeham-Farrance fixtures and molded buttons.

Results and discussion

Compounds evaluated

Compounds were standard formulations for seals and gaskets ranging in hardness from 63 to 74 durometer A. Table 1 lists the compound formulations.

Effect of MA content and plasticizer stability

Figure 3 shows the impact of MA content on Tg and volume increase (VI) in IRM903 oil after 70 hours at 150[degrees]C aging. As the MA content (polarity) increases, the Tg increases and volume change decreases. The addition of 10 phr of an ether-ester plasticizer (TP759) decreases the Tg and lowers the volume increase. The high MA content polymer (GLS) has roughly half the volume increase of the medium MA content (G) polymer, but a 7[degrees]C disadvantage in Tg. With 10 phr of plasticizer added, the higher MA content AEM GLS now has a low volume increase and a Tg 1[degrees]C lower than the unplasticized, medium MA content, AEM G. Tg and volume increase can be varied by adjusting MA content (by blending of the AEM polymers) and addition of plasticizer.

[FIGURE 3 OMITTED]

The level of fluid swell is determined by the polarity of the AEM polymer, as well as the polarity of the fluid. IRM903 is an aromatic fluid that is highly polar. While useful as a reference fluid, it is not indicative of current, commercial power-train fluids. Figure 4 compares the AEM volume increase in IRM903 to SF105 (engine oil reference fluid), Dexron VI (commercial ATF) and Mobil Supersyn 0W-40 (a fully synthetic engine oil). Given the lower polarity of these fluids compared to IRM903, the volume increases are significantly lower; with single digit volume increases achievable with the high MA content (GLS) AEM polymer.

[FIGURE 4 OMITTED]

The benefit of fluid swell level on seal performance can be shown in figure 5. While compression set in air is relatively similar for the AEM compounds of varying MA levels, when tested in ASTM SF105 oil, all the compounds show improvement in compression set. The greatest improvement occurred with the AEM G compound that had the highest level of swell. CSR testing in oils also showed improved results with higher volume increases. Table 2 lists volume increase and CSR results after six weeks of aging in the various fluids at 150[degrees]C. The impact of VI on CSR performance is dependent on the VI difference, with greater impact seen with SF105 oil (where the VI difference is 14.2%) compared to Dexron VI (where the VI difference is 9.8%). The results in Mobil Supersyn did not show a correlation of VI to CSR results, even though the VI difference was the same as in Dexron VI. The Mobil Supersyn data point for the G/GLS blend compound seems high compared to the other two compounds, and may be erroneous.

[FIGURE 5 OMITTED]

While plasticizers provide a means to improve the low temperature performance of a compound initially, they do lose their effectiveness during aging. Figure 6 shows the Tg of the plasticized and unplasticized AEM G compounds during heat aging at 150[degrees]C. While the unplasticized compound had a relatively stable Tg over the six weeks aging period, the plasticized compound lost its original 6.8[degrees]C advantage and it was reduced to only 2[degrees]C after six weeks aging. This loss of original Tg advantage occurs during fluid aging, but with a different mechanism. In figure 7, we see that the Tg of the plasticized compound stayed relatively stable. It is not believed that this is due to the plasticizer having better stability in the oil, but that the plasticizer is simply replaced by the oil during aging. The unplasticized compound shows an improvement in Tg as the higher fluid swell plasticizes the compound. These two effects combine to eliminate the original advantage of the plasticized compound in a relatively short period.

[FIGURES 6-7 OMITTED]

The same effects can be shown in compression set and CSR testing. Figures 8 and 9 show the compression set results in air and in ASTM SF105 oil at 150[degrees]C, respectively. In air, the unplasticized compound shows better performance due to heat stability impact of the plasticizer; and in oil, the unplasticized compound shows improved performance due to its higher fluid swell. Figure 10 shows the CSR results, again showing the improved results with an unplasticized AEM compound.

[FIGURES 8-10 OMITTED]

Blends of AEM G and LTX

It has been shown that using plasticizers to improve the low temperature flexibility of AEM compounds introduces aging stability issues that can impact the performance. It was also shown that higher levels of fluid swell actually provide beneficial results for oil seal applications. Results are presented next on blending a new AEM polymer with AEM G to improve the low temperature performance while maintaining, and even improving, other performance aspects.

Figure 11 shows the volume increase in various fluids and the improvement in Tg with blend ratios of AEM LTX and AEM G varying from 0 to 50%. A Tg improvement of approximately 10[degrees]C can be obtained over this range, with modest VI increases of 10% in SF105 Oil and 7% in Dexron VI or Mobil Supersyn.

[FIGURE 11 OMITTED]

Figure 12 shows the Tg stability of AEM LTX blends during heat aging and the improvement of the Tg during fluid aging. There is essentially no change in Tg after six weeks of air aging at 150[degrees]C, regardless of the level of AEM LTX, while an improvement in Tg is shown for all the compounds as a result of the fluid swell. The improvement in Tg upon fluid aging is similar for these blend levels as a result of the small difference in volume increase.

[FIGURE 12 OMITTED]

Another test for low temperature performance for seal applications is low temperature compression set. The sample is compressed 25% at room temperature, aged for 22 hours at low temperatures, released and maintained at the low temperature to see the amount of recovered height after 30 minutes. Figure 13 shows the cold temperature compression set for the various blend levels of AEM LTX. Improvement in cold compression set can be seen with increasing blend levels of AEM LTX.

[FIGURE 13 OMITTED]

Performance of the blends in standard compression set for six weeks at 150[degrees]C in air and the various fluids is provided in figure 14. Here, all the compounds have very good performance, with generally a slight improvement in compression set in oil vs. air aging.

[FIGURE 14 OMITTED]

CSR performance was similar to compression set in that all the compounds provided similar good results, with no correlation to blend level. Figures 15, 16 and 17 show the CSR results in SF105, Dexron VI and Mobil 1, respectively. Since the CSR testing was performed at elevated temperature (150[degrees]C), the low temperature performance advantage of the AEM LTX blends was not measured.

[FIGURES 15-17 OMITTED]

Conclusions

While plasticizers can improve the low temperature performance of AEM compounds, they were shown to have a negative impact on aging performance. Fluid swell was shown to have a beneficial impact on low temperature and sealing tests performance, such as compression set and CSR.

A new AEM polymer was introduced, providing an ability to improve low temperature performance of AEM compounds without the use of plasticizers. In blends with another AEM polymer, this new AEM polymer was shown to lower the Tg and improve cold compression set, while maintaining the excellent oil aging compression set and CSR performance of AEM compounds.

This article is based on a paper presented at a meeting of the Rubber Division, ACS, (www.rubber.org).

Reference

(1.) "From the voice of the customer to new products: Development of a new AEM polymer with improved low temperature properties," K. Kammerer, et al, DKT Poster Session, 2006.

Table 1--AEM compound formulations

Designation           G     G+TP    G/GLS    G/GLS+    GLS    GLS+
                                               TP              TP

Vamac G              100    100      50        50
Vamac GLS                            50        50      100    100
LTX polymer
TP759 plasticizer            10      10        10              10

Designation          15%    30%    50%
                     LTX    LTX    LTX

Vamac G               85     70     50
Vamac GLS
LTX polymer           15     30     50
TP759 plasticizer

All compounds contain 2 phr Naugard 445 (a), 0.5 phr Armeen 18D (b), 1
phr stearic acid, 1 phr Vanfre VAM (c), 50 phr N5500 carbon black, 1.5
phr HMDC (d) and 4 phr DOTG (e).

((a)--Naugard 445--Chemtura; (b)--Armeen 18D--Lion Akzo; (c)--Vanfre
VAM--R.T. Vanderbilt; (d)--Hexamethylenediamine carbamate;
(e)--di-o-tolyiguanidine)

Table 2--AEM compound VI vs. CSR
performance at six weeks at 150[degrees]C

               SF105          Dexron VI         Mobil

AEM            VI    CSR       VI    CSR       VI    CSR

compound
G           28.4%    82%    18.7%    45%    19.6%    59%
G/GLS        22.0     76     14.4     42     15.1     68
GLS          14.2     68      8.9     40     10.3     50