AEM dipolymer with improved cure rate and compression set.

Ethylene acrylic elastomers (AEM) have been commercially available for almost 30 years and, since their introduction, have been adopted in a wide variety of automotive seal, hose 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 do not require a post-cure. Compression set properties of post-cured AEM terpolymers are typically better than non-post-cured AEM dipolymers. Injection molding cycle times are longer for the dipolymer, as terpolymers only need to be cured to a dimensionally stable state with full properties developed during the post-cure.

The first AEM dipolymer was introduced in 1991 to provide a peroxide curable AEM elastomer for injection molding large automotive elastomer gaskets where a post cure operation was undesirable. Other applications soon followed where post cure is undesirable, such as parts with plastic inserts, extruded hose and wire and cable. Elimination of post cure operations is desired not only from a cost perspective but also to reduce manufacturing steps that can introduce quality problems. Most post cure operations are batch processes and limit the ability to adjust manufacturing rate to the customer needs in a JIT or lean manufacturing process.

Development efforts with AEM dipolymers have been focused on improving the compression set and cure rate to more closely approach the properties and press cycle times obtained with post cured AEM terpolymers. These efforts have been in two areas, including new AEM dipolymer development and gaining a better understanding of the effects of peroxide/co-agent systems. A new AEM dipolymer, DHC, is being introduced with improvements in cure state and rate versus the current AEM dipolymer, DP.

Results are presented on the performance of the new DHC dipolymer versus the current DP dipolymer and a standard AEM terpolymer, Vamac G. The effects of various peroxide/co-agent systems are also presented.

Experimental

Compounds were mixed in an internal mixer (either 1.5, 1.75 or 3.7 liter) and the mixed compound sheeted on a two-roll mill.

Test specimens were compression molded. The AEM terpolymer compound was press cured for five minutes at 175[degrees]C and then post cured four hours at 175[degrees]C. All of the dipolymer compounds were press cured only l0 minutes at 180[degrees]C. All tests were performed per ASTM procedures for elastomers.

Results and discussion

Compounds evaluated

Compounds were standard formulations ranging in hardness from 65 to 73 durometer A. Table 1 lists the compound formulations used. Variations were made in the dipolymer peroxide/co-agent systems and are designated cure system 1, 2, 3 and 4, as shown in table 2. These cure systems were selected based on previously presented data indicating unique performance differences (ref. 1).

Cure system 1 with the combination of DBDB peroxide (a-a-bis(t-butyl peroxy) diisopropylbenzene) and N,N'-m-phenylene dimaleimide is the most common system used for AEM dipolymers and provides a good balance of scorch and tensile properties. Cure system 2 uses a faster peroxide (DCR dicumyl peroxide). Cure systems 3 and 4 use the faster DCP, but then incorporate other co-agents (1,3-butadiene and TMPTMA), which provide other property enhancements.

Cure and rheological properties

Table 3 gives results on Mooney viscosity, Mooney scorch and MDR testing of the various compounds. The dipolymer compounds were slightly lower in viscosity than the terpolymer compound, with only a slight difference in compound viscosity between the dipolymers DP and DHC. One of the major advantages of the peroxide cured dipolymers is the improved scorch safety vs. the diamine-cured terpolymer. The data for Mooney scorch at 121[degrees]C shows t3 and t10 values of greater than 20 minutes for most of the dipolymer compounds.

Maximum torque (MH) values show the improved cure state of the DHC compounds vs. the DP compounds. MH values were 15-20% higher with the DHC dipolymer. Improved cure rate was indicated by higher slope values of the MDR cure curve.

AEM terpolymers have faster injection molding cycle times, as they are only cured to a "dimensionally stable" state in the press, with final properties established with the post cure. Time to 50% cure, t(50), is close to the cure time needed for the dimensionally stable state. AEM dipolymers must be fully cured in the molding cycle, as their advantage is the elimination of the post cure cycle. Therefore, to meet the press cycle times of an AEM terpolymer, the t(90) time of the dipolymer compound must be close to the terpolymer t(50) time without a significant loss in scorch safety.

All of the dipolymer compounds had adequate scorch safety as indicated by the ts2 times being greater than 0.5 minutes. Comparison of the terpolymer G compound with the first dipolymer DP compound (cure system 1) showed that, although the post cure operation was eliminated, the molding time was double. Using a faster peroxide in cure systems 2 through #4 provided for a significant improvement in dipolymer cure times, yielding press cycle times similar to AEM terpolymers.

Compound properties

Table 4 lists the compound properties. The higher state of cure for DHC gave slight increases in hardness, modulus and tensile strength, with a corresponding loss in ultimate elongation. Cure system 3 provided the smallest difference in tensile properties.

Figures 2 and 3 show the compression set performance at one and six weeks at 150[degrees]C, respectively. DHC provided improvement in compression set performance with cure system 1. The use of the faster peroxide in cure systems 2-4 resulted in a significant improvement in compression set for both dipolymers, providing performance similar to post cured terpolymer G.

[FIGURES 2-3 OMITTED]

Figures 4 and 5 provide the change in hardness and ultimate elongation after aging for six weeks at 150[degrees]C. There was a loss in air aging performance with DHC vs. DR with greater increases in hardness and greater change in ultimate elongation. The obtained values are still considered good performance levels for automotive applications.

[FIGURES 4-5 OMITTED]

Performance in oil was significantly better, as indicated in figures 6-8. Samples were aged six weeks in ASTM SF105 oil at 150[degrees]C, and the figures show the change in hardness, ultimate elongation and volume swell, respectively. There was a noticeable improvement in volume swell for DHC.

[FIGURES 6-8 OMITTED]

For SF105 oil, the volume swells were 6% lower than DP and better than the G compound. Compared to the results from air aging, DHC showed an improvement in the change in hardness and less loss of elongation. Similar results were obtained in Dexron III automatic transmission fluid.

All AEM compounds exhibited excellent compression stress relaxation performance. Figure 9 shows the CSR performance in ASTM SF 105 oil at 1500C for the terpolymer G compound and the dipolymer compounds using cure system 1. All three compounds provided excellent CSR results.

[FIGURE 9 OMITTED]

Conclusions

A new AEM dipolymer, DHC, was introduced that provides performance improvements in oil swell, cure state and cure rate performance. The differences from using various peroxide/co-agent systems with the AEM dipolymers were also provided. With these two items, more options are now available to optimize molding and compound performance with AEM dipolymers without a post cure.

Table 1--AEM compound formulations

DP or DHC       100
G                       100
Naugard 445     1.0     2.0
Armeen 18D      0.5     0.5
Stearic acid    0.5     1.5
Vanfre VAM      0.5     1.0
FEF, N550        55     55
Peroxide       Varied
Co-agent       Varied
Diak No.1               1.5
DOTG                    4.0

Table 2--peroxide/co-agent systems

                1     2     3     4

Vulcup 40KE    5.0
Dicup 40C            5.0   9.0   9.0
HVA #2         2.0   2.0
Sartomer 350                     3.0
Ricon 152                  3.0

Table 3--compound rheological properties

AEM polymer:                            G              DP

Cure system                     (Diamine)      1      2      3      4

ML (1 +4) @ 100[degrees]C, mu        47.9   43.2   44.0   39.6   36.9
MS @ 121[degrees]C/20 mins.
  Min. visc.                         16.3   14.2   14.6   13.4   12.5
t3, mins.                             7.5    >20    >20    >20    9.1
t10, mins.                           11.8    >20    >20    >20    9.9

MDR @ 177[degrees]C/20 mins.
ML, in.-lb.                           0.4    0.6    0.6    0.6    0.6
MH, in.-lb.                          12.9   13.8   10.2   15.3   12.2
ts2, mins.                            1.1    0.8    0.8    0.6    1.0
t(50), mins.                          2.5    1.6    1.2    1.5    1.7
t(90), mins.                          9.5    5.2    3.1    4.0    3.7
Slope                                 1.1    2.2    2.8    3.3    3.0

Table 4--compound original properties

AEM polymer:                  G                      DP

Cure system               (Diamine)       1       2       3       4

Hardness, duro. A, pts.        72.6    69.1    66.5    69.2    65.0
Modulus @ 501%, psi             380     373     289     385     288
Modulus @ 1001%, psi            905   1,146     747     992     812
Tensile strength, psi         2,631   2,750   2,433   2,622   2,540
Ultimate elongation, %          309     206     264     213     246
Tear die C, ppi                 216     195     225     204     193

AEM polymer:                             DHC

Cure system                   1       2       3       4

Hardness, duro. A, pts.    71.1    67.0    71.0    67.3
Modulus @ 501%, psi         448     310     420     331
Modulus @ 1001%, psi      1,409     911   1,092     940
Tensile strength, psi     2,777   2,541   2,673   2,407
Ultimate elongation, %      176     231     205     218
Tear die C, ppi             178     206     195     179

Reference

(1.) D. King, et al, "No post cure developments with AEM ethylene acrylic elastomers," paper presented at a meeting of the Rubber Division, ACS, October 14-1Z 2003.