A new intermediate performance specialty.

A new class of specialty elastomers is uniquely positioned for today's more demanding service conditions. As can be seen from a comparison of heat and oil resistance (figure 1), the Advanta series has been designed to provide heat resistance that exceeds that of currently available oil resistant elastomers. We believe that these specialty elastomers will provide opportunities for cost-effective upgrades from current oil resistant elastomers. Potential end-uses may include o-rings, oil seals and gaskets for the automotive powertrain and industrial applications where relatively high temperatures and low swells are important.

The first product introductions in this new class of specialty elastomers, Advanta 3320 and 3650, encompass the heat and oil resistance illustrated in figure 1 from the lower to upper regions, respectively. They consist of compatible alloys of proprietary polar ethylene copolymers and peroxide-curable fluoroelastomers. These alloys are peroxide-curable whereby the same cure system co-crosslinks both major components.

Experimental

Compounding ingredients

The vulcanizates for which test results are given in this article were prepared using commercial and developmental grades of specialty elastomers from Du Pont and other suppliers. Table 1 provides a general description of the different elastomers evaluated.

  Table 1 - elastomers evaluated
Product        General description
Advanta 3320   A new proprietary allow of a polar ethylene
               copolymer and fluoroeslastomer with
               a 1.3 specific gravity
Advanta 3650   A new proprietary alloy of a polar ethylene
               copolymer and fluoroelastomer with
               a 1.6 specific gravity
AEM-G1         Vamac G1 is a terpolymer of methyl acrylate,
               ethylene and a cure-site monomer
AEM-LS         Vamac LS is a terpolymer of methyl acrylate,
               ethylene and a cure-site monomer
               providing lower oil swell compared to
               Vamac G1
HNBR-2010      Zetpol 2010 is a fully saturated HNBR
               containing 36% bound ACN providing
               good oil, heat and ozone resistance
ACM-4052       Hytemp 4052EP is a general purpose
               polyacrylate with good low temperature
               properties
VMQ-7170       A 50/50 blend of SE-7160/SE-7180, a
               general-purpose dimethyl reinforced gum
               used in sealing applications
FKM-A401C      Viton A-401C is a bisphenol cured fluoroelastomer
               containing 66% fluorine
               designed for high performance o-ring
               sealing

Mixing and molding

Compounding was done using standard rubber lab equipment (mixer and two-roll mill). Compound formulations were typical of those recommended for sealing devices. Several were chosen from previous work (refs. 1 and 2). All materials were cured and postcured per manufacturers recommendations. Although our goal was to obtain compounded hardness of 75 +-5, a few of the compounds fell below this range.

Testing

The elastomers were characterized using the test methods shown below:

* Unaged estress-strain properties at 23 [degrees] C (ASTM D412, Method A, Die C);

* Unaged tear strength at 23 [degrees] C (ASTM D624, Die B and C);

* Compression set after 70 and 168 hours at 150 [degrees] C, 165 [degrees] C and 175 [degrees] C (ASTM D395, Method B);

* Low temperature properties as measured by temperature retraction (ASTM D1329), brittle point (ASTM D2137) and glass transition (Tg) via DSC;

* Hot air aging resistance after 1,000 hours at 150 [degrees] C, 165 [degrees] C and 175 [degrees] C (ASTM D573);

* Volume and weight change (ASTM D471).

Results and discussion

Product description

These elastomers are alloys of proprietary polar ethylene copolymers and peroxide curable fluoroelastomers, whereby the alloying technology provides a very finely dispersed microstructure. Typical properties of each are shown in table 2.

  Table 2 - typical polymer properties
Advanta type                  3320      3650
Specific gravity               1.3       1.6
Mooney viscosity,               25        50
(ML 1 + 10 @ 100 [degrees] C)
        Typical vulcanizate properties
Hardness, shore A, pts.                60-90
Tensile strength, MPa                  12-15
Elongation @ break, %                 200-400
100% modulus, MPa                       2-12
Compression set, %                     25-30
70 hr/165 [degrees] C
Low temperature properties
Tg, infection, [degrees] C            -16    -15
TR-10, [degrees] C                    -12    -11
Brittle point, [degrees] C            -20    -45
ASTM #3 oil resistance
70 hr/150 [degrees]C, vol. swell, %    20     10

Properties

The main features are heat and oil resistance. Within the ASTM D2000/SAE J200 classification system for elastomers, these materials would command an EJ to FK rating. Positioning versus other specialty elastomers is also shown in figure 1.

Heat resistance

The ASTM D20OO/SAE J200 service limits represent a short-term guide to elastomer performance in automotive applications. As for all materials, continuous exposure at high temperatures shows that there is an inevitable relationship between time and temperature. These ethylene/FKM alloys age by continued crosslinking and subsequent embrittlement rather than any process of reversion. Using the new SAE standard, J2236, for determining continuous upper temperature resistance, the 3320 would qualify as a 165 [degrees] C material, and the 3650 would qualify as a 175 [degrees] C material. This standard defines continuous upper temperature limit as the temperature at which the material retains a 50% minimum of both the original elongation and tensile at break after 1,008 hours. In addition, 3650 will function with intermittent temperature excursions to 200 [degrees] C. Current laboratory work has shown promising leads for further improvements in longterm heat resistance. Typical heat aging properties are shown in figure 2.

Fluid resistance

Compounds of these alloys provide low swell in a variety of lubricants, hydraulic fluids and greases, and emerging engine oils, transmission fluids and power steering fluids. Swells in water and ethylene glycol are also low. They are not suitable for continuous exposure with standard non-mineral oil brake fluid, esters or ketones. Swell in aromatic fuels such as gasoline is fair. Volume swell values are shown in figure 3.

Compression set resistance

The resistance to compression set of these elastomers is good, typically providing values of 25% after 70 hours at 150 [degrees] C and 30% at 165 [degrees] C. Typical set properties are shown in figure 4.

Low temperature properties

The low temperature flexibility, as determined by glass transition (Tg) and temperature retraction (TR-10) are similar to fluoroelastomers (FKM.) Brittle point values for the new elastomer span the range of -20 to -45 [degrees] C. Low temperature sealing using a static o-ring test apparatus described in earlier work (ref. 3) has confirmed sealing values of 3,320 down to minus 30 [degrees] C, essentially equivalent to FKM-A401C. Current laboratory work has shown the low temperature test values can be lowered by another 5-10 [degrees] C.

Benchmarking study

Various specialty elastomers were selected for the benchmarking study. In addition to 3320 and 3650, Vamac G1 and LS were selected since we have an established data base for these polymers. A medium-high acrylonitrile HNBR material, Zetpol 2010 was selected, since it is representative of leading HNBR polymers sold into the sealing market. Viton A-401C was selected since it is representative of state-of-the-art for FKM o-rings. A general-purpose polyacrylate, Hytemp 4052, rounds out the study.

Table 3 summarizes compound formulations, stock characteristics and vulcanizate properties for the selected elastomers. Short-term (168 hr.) and long-term (1,000 hr) agings were done in order to establish heat resistance capability. They are also reported in table 3. A broad fluid resistance comparison is shown in table 4. The fluids include ASTM reference oils and fuels, commercial automotive motor oils, transmission fluids, power steering oil, brake fluid and axle lube, as well as engine coolant and a biodegradable oil and fuel. A general purpose silicone, VMQ-7170 was included for this comparison.

[TABULAR DATA 3 & 4 OMITTED]

From the standpoint of stress strain property changes, previous work has shown that immersion testing in engine fluids requires aeration to correlate with real world engine operating conditions (refs. 4 and 5). Such immersion testing is planned for the new elastomers.

Overall fluid resistance for Advanta, as measured by volume change, looks very encouraging. The retention of stres-strain properties in motor oils is very good. Figures 5 and 6 compare elongation changes in engine oil with heat agings of 1,000 hrs. at 165 [degrees] C and 175 [degrees] C, respectively. These two figures demonstrate the outstanding balance of heat and fluid resistance that can be obtained through the use of these elastomers. Figure 6 shows that 3650 is a viable material option for 175 [degrees] C service, other than fluoroelastomer.

General compounding and processing

Compounding

The principles of compounding these elastomers are very similar to those practiced for other heat-resistant specialty elastomers. The choice of potential additives tends to be more limited, and their selection more specific. Equal attention must be given to compounding ingredients that should be avoided, due to detrimental effects on heat aging properties, primarily. A good guideline is to utilize those compounding practices that have proven successful with fluoroelastomers. To aid in compounding simplification, Advanta is offered in precompounded form.

Cure system

These elastomers must be crosslinked using a peroxide-cure system. A peroxide such as Luperco 101-XL or Varox is recommended. A coagent, such as Diak #7, is also recommended. Other peroxides and coagents have been evaluated, but these provided the best balance of properties., especially heat resistance.

Metal oxides

The addition of metal oxide is not required. We have found that zinc oxide is extremely detrimental to heat resistance.

Fillers

It has been found that SRF carbon black provides the best combination of physical properties and processing. Table 5 provides data using N762 carbon black at various levels ranging from 20 to 95 in the 3320 rubber. These levels produce durometer ranges from 60 to 90.

It is anticipated that mineral fillers could also be used successfully. Some general purpose ones that are recommended to start with are: precipitated barium sulfate (Blanc Fixe), calcium silicate (Nyad 400), precipitated calcium carbonate F3(Albagloss), and silicon dioxide (Minusil 5u). Color compounds can be obtained with organic and inorganic pigments typically used in the industry.

Plasticizers

Plasticizers should generally be avoided in Advanta formulations as they interfere with the peroxide-cure system.

Process aids

Process aids may be used at low levels (1-2 phr). The process aids that have been found to work well are Carnauba wax, Vanfre VAM and VPA #3.

Processing

Mixing

These elastomers are relatively easy materials to process. They mix very readily using eithier an internal mixer or two-roll mill. They release quite nicely from mixer and roll surfaces. The lower viscosity of 3320 (25 Mooney) suggests the material is more easily processed at lower temperatures (100 [degrees] C or below). 3320 shows a tendency to stick to itself when warm, therefore polyethylene sheet separation is recommended during sheet-off.

Extrusion and calendering

The 3320 compounds have relatively low viscosity and nerve and display a tendency towards sticking to itself. Cool barrel and die temperatures have proven successful, without showing appreciable die swell or roughness. Laboratory calendering evaluations have been very limited to date. Vanfre VAM has shown efficacy in enhancing roll release with 3320. Cool roll temperatures are recommended (i.e. 50-60 [degrees] C). 3650 has a higher viscosity and will process better at higher temperatures with less tendency towards surface tack.

Molding

These alloys may be processed using standard techniques of compression, transfer and injection molding. Process aids like Vanfre VAM, VPA #3 and Carnauba wax are effective in promoting part release. Vanfre VAM has been found to be the most effective for 3320.

Bonded parts

The production of high quality bonded parts (metal and plastic substrates) may be readily accomplished using commercial primers and bonding agents routinely recommended for fluoroelastomers.

Post-curing

In order to develop full properties and optimize compression set properties, a postcure is highly recommended -8 hrs. at 175 [degrees] C is routinely used.

Conclusions

These specialty elastomers provide a unique balance of exceptional heat resistance and low swell in a broad range of fluids. Heat aging results support positioning them as a 175 [degrees] C to 200 [degrees] C material within the ASTM D20OO/SAE J200 framework. It has superior heat aging properties compared to HNBR, ACM and AEM elastomers; only silicones, fluorosilicones and fluoroelastomers have higher temperature capabilities. The 3650 elastomer will function with intermittent temperature excursions to 200 [degrees] C.

We believe that Advanta specialty elastomers willl provide opportunities for cost-effective upgrades from currently available oil resistant elastomers. Potential end-uses may include o-rings, oil seals and gaskets for the automotive powertrain and industrial applications where relatively high temperature and low swells are important.

[Figure 1-6 ILLUSTRATION OMITTED]

References

1. T.M. Dobel and J.R. Harrell, "New developments in ethylene/acrylic elastomers," paper no. 28, 140th meeting of the Rubber Division, American Chemical Sociely, Detroit, MI, October 8-11, 1991. 2. T.M. Dobel and D.A. Kotz, "Ethylene/acrylic high performance elastomers -- extended agings in selected automotive fluids," paper no. 97, 144th meeting of the Rubber Division, American Chemical Society, Orlando, FL, October 26-29, 1993. 3. R.D. Stevens, E.W. Thomas, J.H. Brown and W.N.K. Revolta, "Low temperature sealing capabilities of fluoroelastomers," SAE paper #900194, February 26-March 2, 1990. 4. B.N. Dinzburg, "Investigation of the effect of aeration on automotive oils and rubber components," 142nd meeting of the Rubber Division, American Chemical Society, Nashville, 6 TN, November 3-6, 1992. 5. J.G. Bauerle and D.W. Bruhnke, "The effects of aeration of testfluids on the retention of physical properties of fluoroelastomer vulcanizates," SAE paper no. 890362, February 27-March 3, 1989.