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AEM with improved injection molding processability and performance attributes.

Ethylene acrylic elastomers (AEM), introduced in 1974 under the brand name Vamac, are used in many automotive applications where a combination of oil and high temperature resistance with excellent damping properties is required. The trends of increasing temperatures under the hood, more aggressive automotive fluids, reduction in hydrocarbon emissions and longer part life has led to a significant increase of AEM in vehicles over the past decade. AEM has replaced many lower performance elastomers to meet these increasingly demanding requirements.

Products introduced over the years (Vamac DP, HVG, GXF, DHC, etc.) have mainly provided incremental extensions of the existing product range, with only Vamac GXF providing a significant level of differentiation or value over the standard Vamac G grade. Development efforts were initiated to look at optimization of AEM elastomer structure and chemical composition, pushing the manufacturing technology to develop step-change improved AEM elastomers. These efforts have led to several innovative AEM elastomers, providing true step-change improvement in processability, performance and customer value for targeted applications.

A majority of AEM elastomers is injection molded. Historical 1M process improvement targets for AEM elastomers have been improved compound dispersion with one-pass mixing, reduced mold fouling and improved hot tear resistance, while maintaining the good scorch resistance and longer flow paths achieved with AEM elastomers. These improvement targets were addressed, along with improvement in high temperature tensile properties and heat/oil aging.

This article introduces Vamac VMX-3040, an AEM elastomer optimized for injection molding applications with improved performance in processability and compound/part performance. Additional benefits can be found for extrusion applications as a result of the high compound green strength. Data are presented demonstrating reduced mold fouling, improved hot tear resistance, improved physical properties at elevated temperatures, heat and oil aging resistance, and abrasion resistance. Performance will be compared to current AEM and ACM products.

Experimental

The compounds used in the study are presented in table 1. Compound #1 is a standard compound formulation for seal applications using AEM G polymer and DOTG curing coagent. Compound #2 is the same compound using ACT55 instead of DOTG as the co-agent. ACT55 is an o-toluidine-free alternative to DOTG. Compound #3 is a direct replacement of AEM G with AEM VMX3040 with ACT55 co-agent to provide a direct comparison of the polymer impact. Compounds #4 and #5 have slightly lower levels of curing agent and represent low and high hardness, respectively. Compounds #6 and #7 are ACM compounds also representing low and high hardness levels, respectively. Finally, compounds #8 and #9 are from an earlier study to provide results related to the improved mold fouling and hot tear resistance of AEM VMX3040 vs. AEM G.

The compounds were mixed in a 9.2 liter internal mixer. They were extruded into approximately 3.2 mm diameter rope for the o-ring sticking test. All other testing was performed after five minutes at 180[degrees]C press cure and four hours at 175[degrees]C post cure.

The o-ring sticking test was developed by DuPont Performance Elastomers, and details are contained in the referenced paper (ref. 1). All tests were performed per ASTM procedures as applicable.

Results and discussion

Unvulcanized properties

AEM VMX3040 has a higher polymer viscosity than AEM G. This provides higher compound viscosity demonstrated by the higher ML(1+4) at 100[degrees]C, as given in table 2. Although the compound viscosity is significantly increased at low shear rates in the Mooney viscosity test, higher shear rate (approx. 1,000 [sec..sup.-1]) viscosities as typically found in injection molding processes are still low, as shown in figure 1. This maintains the benefit of longer flow path capability, while also improving the compound mixing performance. Improved compound mixing was determined by the carbon black incorporation time (BIT) listed in table 2. Table 2 also shows the small impact on compound scorch resistance, even with the higher viscosity.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

By lowering the curative level slightly, an improvement in cure time was obtained, while maintaining good cure level.

Improved processability

A compression mold was made consisting of side-by-side identical arrays of 60 o-ring cavities. The mold is shown in figure 2. The test consisted of starting with a clean tool and repeatedly molding o-rings with the compound. No mold cleaning or use of external release sprays occurred during the test. A controlled air blast was used after each cycle to remove the o-rings. The number of o-rings remaining after the air blast was counted and used as an indication of the level of mold fouling. The results were plotted as number of stuck orings vs. number of cycles.

The results for compounds #8 and #9 are given in figure 3. Low durometer compounds were selected, as mold fouling and hot tear issues tend to increase with decreasing hardness. The lower level of o-rings sticking to the mold with each repeated molding cycle shows the reduced mold fouling capabilities of AEM VMX3040.

To measure hot tear resistance, an RPA 2000 cure plus strain sweep test was used. In the test, the compound was first pre-cured in the RPA for two minutes at 190[degrees]C. After the cure step, the material was tested in a strain sweep mode to 250% strain, and the loss shear modulus (G") was measured as a function of strain. The lower the build-up of G", the better the compound will resist hot tear during demolding from the press. Figure 4 shows the lower G" values for AEM VMX3040 vs. the current AEM G.

The improved processability of AEM VMX3040 has been confirmed in customer trials, where improvements in scrap rates and intervals between mold cleanings were achieved.

Improved compound performance

Table 3 provides the cured compound properties. The use of ACT55 instead of DOTG (Compound #2 vs. #1) shows a negative impact on properties such as ultimate elongation, tear strength and compression set. When using ACT55 and AEM VMX3040, the compound properties are regained, along with an improvement in abrasion resistance. The remaining compounds show the expected performance level of both low and high hardness compounds.

[FIGURE 4 OMITTED]

The improvement in properties with AEM VMX3040 can be seen in elevated temperature properties (150[degrees]C), as well in the final optimized compounds (#4 and #5) where elongations well above 100% were obtained and tear strengths were above ACM. This is important for low hardness compounds for thermoplastic cam cover seals, where maintaining ultimate elongation values at 150[degrees]C above 100% prevents premature splitting.

Additional improvements were obtained in heat aging with AEM VMX3040. The change in ultimate elongation after heat aging (500 hours at 175[degrees]C) improved from 51.8% with AEM G (Compound #2) to 37.2% with AEM VMX3040 (Compound #5). CSR testing (figure 5) shows the improvement with AEM VMX3040 (Compound #3) vs. AEM G (Compound #2) and ACM (Compound #7).

Conclusion

An AEM elastomer has been developed that provides significant injection molding processing improvements and final compound performance attributes. These improvements have been confirmed in the marketplace and provide customers with not only significant processing cost improvements, but offer ability to perform in more demanding applications due to improvements in abrasion resistance, elevated temperature physicals and heat aging resistance.

[FIGURE 5 OMITTED]

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

Reference

(1.) S. Bowers, "Advanced polymer architecture peroxide curable fluoroelastomers," Kautschuck Gummi Kunststoffe 55, Jahrgang, Nr 6/2002.

by Douglas King, Klaus Kammerer and Laurent Lefebvre, DuPont Performance Elastomers (www.dupontelastomers.com/Products/Vamac/)
Table 1--compound formulations

Compound: #1 #2 #3 #4 #5 #6 #7 #8 #9

AEM G 100 100 -- -- -- -- -- 100 --
AEM VMX3040 -- -- 100 100 100 -- -- -- 100
ACM AR12 -- -- -- -- -- 100 100 -- --
Naugard 445 2 2 2 2 2 2 2 2 2
Armeen 18D 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
Vanfre VAM 1 1 1 1 1 1 1 1 1
Spheron SO N550 65 65 65 40 65 62 85 30 30
Rhenosin W 759 10 10 10 10 10 5 10 0 0
DIAK No. 1 1.5 1.5 1.5 1.2 1.2 0.6 0.6 1.5 1.5
Ekaland DOTG/C 4 -- -- -- -- 2 2 4 4
Vulcofac ACT 55 -- 2 2 2 2 -- -- -- --

Table 2--compound mixing and uncured
properties

Compound: #1 #2 #3 #4

Internal mixer (40 rpm)
B.I.T. (seconds) 48 48 40 30
Temperature at B.I.T. ([degrees]C) 75 77 75 65
ML (1+4) 100[degrees]C 39.3 41.8 67.4 39.1
M. scorch 121[degrees]C, T5 (min.) 8.54 7.52 9.15 11.37
MDR 180[degrees]C/0.5 deg./12 min.
ML (dNm) 0.44 0.51 0.82 0.37
MH (dNm) 13.6 13.2 16.9 9.9
Ts2 (min.) 0.88 0.88 0.81 1.08
tc10 (min.) 0.72 0.70 0.73 0.74
tc50 (min.) 2.01 2.15 2.38 2.14
tc90 (min.) 5.91 6.82 7.07 6.18
Peak rate 4.5 4.1 4.7 3.1
Time to reach 5 dNm (sec.) 90 92 77 126

Compound: #5 #6 #7

Internal mixer (40 rpm)
B.I.T. (seconds) 39 105 100
Temperature at B.I.T. ([degrees]C) 75 62 74
ML (1+4) 100[degrees]C 66.4 43.4 61.2
M. scorch 121[degrees]C, T5 (min.) 8.58 8.04 7.10
MDR 180[degrees]C/0.5 deg./12 min.
ML (dNm) 0.84 1.56 2.41
MH (dNm) 14.6 9.9 13.9
Ts2 (min.) 0.81 0.68 0.55
tc10 (min.) 0.68 0.44 0.43
tc50 (min.) 2.07 1.37 1.32
tc90 (min.) 6.38 5.56 5.19
Peak rate 4.7 5.0 6.9
Time to reach 5 dNm (sec.) 77 64 30

Table 3--cured compound initial and aged properties

Compound: #1 #2 #3 #4

Hardness (duro. A) 75 78 79 61
Modulus 50% (MPa) 2.62 3.72 3.07 1.33
Modulus 100% (Mpa) 5.98 8.52 7.05 2.84
Tensile strength (Mpa) 14.7 16.3 18.1 17.7
elongation at break 286 195 261 409
Delft tear Fmax (N/mm) 24.1 21.8 23.8 18.8
Compression set (%)
 70 hr./150[degrees]C 13 21 15 12
 168 hr./150[degrees]C 16 23 17 24
VW comp. set PV3307 (%)
 94 hr./150[degrees]C, after 5 sec. 42 46 49 38
Sandpaper abrasion (mm3) 167 164 132 147
Properties at 150[degrees]C
Modulus 50% (Mpa) 2.05 2.85 2.59 1.17
Change in M50 (5%) -21.8 -23.4 -15.6 -12
Modulus 100% (Mpa) 4.94 - 6.19 2.59
Change in M100 (5%) -17.4 - -12.2 -8.8
Tensile strength (MPa) 5.78 5.97 6.28 4.13
Change in tensile (%) -60.8 -63.4 -65.2 -76.6
elongation at break 121 93 103 144
Change in elongation (%) -57.7 -52.3 -60.5 -64.8
Delft tear Fmax (N/mm) 4.28 4.00 4.98 3.30
Change in tear Fmax (%) -82.2 -81.7 -79.0 -82.5
Heat aging 504 hours at
 175[degrees]C (new air oven)
Hardness (duro. A) 90 92 89 67
Hardness change (pts.) 15.1 13.7 10.2 6.0
Modulus 50% (Mpa) 6.02 7.14 5.27 1.57
Change in M50% (%) 129.8 91.9 71.7 18.0
M 100% (Mpa) 10.37 -- 9.32 3.24
Change in M100% (%) 73.4 -- 32.2 14.1
Tensile strength (Mpa) 10.77 11.11 12.26 9.69
Change in tensile (%) -26.9 -32.0 -32.1 -45.1
% elongation at break 109 94 150 264
Change in elongation (%) -61.9 -51.8 -42.5 -35.5

Compound: #5 #6 #7

Hardness (duro. A) 78 65 79
Modulus 50% (MPa) 2.97 1.57 2.49
Modulus 100% (Mpa) 6.31 3.89 5.31
Tensile strength (Mpa) 17.6 8.7 8.1
elongation at break 290 232 191
Delft tear Fmax (N/mm) 26.7 14.4 13.9
Compression set (%)
 70 hr./150[degrees]C 12 8 10
 168 hr./150[degrees]C 19 13 16
VW comp. set PV3307 (%)
 94 hr./150[degrees]C, after 5 sec. 65 47 72
Sandpaper abrasion (mm3) 133 284 286
Properties at 150[degrees]C
Modulus 50% (Mpa) 2.23 1.34 1.79
Change in M50 (5%) -24.9 -14.6 -28.1
Modulus 100% (Mpa) 5.30 3.34 4.10
Change in M100 (5%) -16.0 -14.1 -22.8
Tensile strength (MPa) 6.79 4.63 5.01
Change in tensile (%) -61.5 -47.0 -38.0
elongation at break 127 130 127
Change in elongation (%) -56.2 -44 -33.5
Delft tear Fmax (N/mm) 5.22 3.48 3.94
Change in tear Fmax (%) -80.5 -75.9 -71.6
Heat aging 504 hours at
 175[degrees]C (new air oven)
Hardness (duro. A) 88 72 91
Hardness change (pts.) 10.6 7.4 12.8
Modulus 50% (Mpa) 4.56 1.86 4.10
Change in M50% (%) 53.5 18.5 64.7
M 100% (Mpa) 7.73 3.43 5.30
Change in M100% (%) 22.5 -11.8 -0.2
Tensile strength (Mpa) 11.17 6.39 5.64
Change in tensile (%) -36.6 -26.9 -30.2
% elongation at break 182 230 147
Change in elongation (%) -37.2 -0.9 -23.0
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Author:King, Douglas; Kammerer, Klaus; Lefebvre, Laurent
Publication:Rubber World
Date:Dec 1, 2008
Words:2303
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