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Fluoroelastomers and modern engine fluids.

Fluoroelastomers with their associated high temperature resistance are increasingly utilized in both spark ignition (SI) and diesel (CI or compression ignition) engines. Routine operating temperatures of 150 [degrees] C (302 [degrees] F) and higher for both coolants and lubricants indicate more extensive use of this class of elastomer. This article reviews fluoroelastomers of two categories, FKM a dipolymer of vinylidene fluoride/ hexafluoropropylene and FEPM a dipolymer of tetrafluoroethylene/propylene, in both an engine coolant and a hydrocarbon lubricant. Initial evaluation was performed by monitoring long-term compressive stress relaxation (CSR) at 150 [degrees] C (302 [degrees] F). Data is reported for the first 1,008 hours; testing is ongoing.

Experimental

The CSR test specimen is essentially a rubber ball (sphere) of 6.35 mm (0.25 in) diameter with a molded thin-walled centering ring (Bussem, Seals Eastern, Inc. figure 1). The spherical shape offers several advantages, e.g. maximum volume with minimum surface area, contact area and contact pressure similar to an o-ring, deformable in essentially a linear section of a stress-strain plot (figure 2).

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The test jig (figure 3) can be economically produced in large quantities for use in environmental chambers, ovens and pressure vessels. The test specimen is centralized in the test jig by the locating ring; when compressed by tightening the stripper bolt screws, the deformation is 25%. The spherical seal is now sealing the 0.8 mm (0.03 in) pressure port.

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The pressure test fixture (figure 3) uses any convenient source of gas pressure (typically 3.5 MPa or 508 psi). By having a gage and/or pressure transducer between the test jig and pressure source, when the line is charged and the pressure source isolated, the test jig/CSR specimen will relieve the pressure until the rubber ball contact stress is equivalent to the system pressure. Equilibrium is normally achieved within 120 seconds. The pressure value is recorded and the fixture removed for additional aging if desired. The size is sufficiently compact as to allow a multiple number of jigs to be incorporated in an environmental test chamber so both high and low temperature sealability can be determined. The use of dissimilar metals in the fluid aging vessel will also introduce legitimate electrochemical effects.

Test fluids were selected to represent reasonable current practice, e.g. ASTM reference oil TMC 1006 represents SAE 5W-30 engine oil and is the recommended test fluid in D471-98, identified as service liquid 105 (SF-105). Engine coolants are currently undergoing a globalization of environmentally acceptable formulations. The organic acid technology as represented by Texaco's "Extended Life Coolant" (TELC) was selected to be representative of this group.

FKM was compounded in accordance with AMS-R-83248. FEPM was compounded using proprietary in-house technology.

Results

Figures 4, 5 and 6 illustrate compressive stress decay graphically. SF-105 (figure 4) is particularly aggressive on the FKM sample; this would be the result of the nucleophilic stabilizers dehydrofluorinating VF2 sites with subsequent polymer cleavage. The FKM testing was discontinued after 336 hours. FEPM reaches equilibrium after the same time period (7.5% drop in compressive stress between 500 and 3,000 hours).

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TELC (figure 5) is even more aggressive on FKM with testing discontinued after 168 hours. The rapid stress decay is probably due to hydrolysis of the crosslinks. FEPM is beginning to stabilize after 504 hours and remains at a virtual plateau through 4,000 hours of aging (2.6% drop in compressive stress between 500 and 4,000 hours).

Dry heat (figure 6) is aggressive on the FKM, although the rapid decay is not predictable from the excellent compression set resistance reported by FKM suppliers. Testing of FKM was discontinued after 168 hours while the FEPM reaches a plateau after approximately 500 hours and remains nearly flat through 4,000 hours (6.3% drop in compressive stress between 500 and 4,000 hours).

Discussion

FKM elastomers have had great success over many years in critical applications. However, this test program was done using fluids we should correctly refer to as aqueous electrolytes (TELC) and non-aqueous electrolytes (SF-105). In both of these liquids we have a strong nucleophilic (electron donor) presence. To preserve the integrity of the test program, both materials (FKM and FEPM) were tested for 70 hours in spectrophotometer grade "Nujol," a pure white paraffin oil (figure 7). Note that while FKM was essentially flat, FEPM showed the typical initial rapid decline in retained stress.

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This phenomenon is a function of our particular compounding approach and is essentially the same as the "Mullins Effect" common to engineered dynamic rubber components.

Conclusions

The proposed test geometry appears to offer a rapid screening test for better understanding of the compressive stress relaxation behavior of various materials when subjected to virtually any desired service conditions (aging time, fluid and temperature).

Summary

Compressive stress relaxation is essentially the process of "dying" a mechanically loaded elastomeric component undergoes. Having knowledge of the rate and an actual value (particularly for seals) of retained stress is very helpful. Modifications of the test apparatus should allow development of actual low-temperature sealing values.

References

(1.) "Engineering Design with Natural Rubber." NR Technical Bulletin. The Malaysian Rubber Producers' Research Association. ISSN 0956-3856.
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Copyright 2001, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Bussem, Hermann
Publication:Rubber World
Date:Feb 1, 2001
Words:867
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