Compatibility Study of Fluorinated Elastomers in Automatic Transmission Fluids 2008-01-1619
A compatibility study was conducted on fluorinated elastomers (FKM and FEPM) in various Automatic Transmission Fluids (ATF). Representative compounds from various FKM families were tested by three major FKM raw material producers - DuPont Performance Elastomers (DPE), Dyneon and Solvay. All involved FKM compounds were tested in a newly released fluid (ATF-A) side-by-side with conventional transmission fluids, at 150°C for various time intervals per ASTM D471. In order to evaluate the fluid compatibility limits, some FKM's were tested as long as 3024 hrs, which is beyond the normal service life of seals. Tensile strength and elongation were monitored as a function of ATF exposure time.
The traditional dipolymers and terpolymers showed poor resistance to the new fluid (ATF-A). Both types demonstrated significant decreases in strength and elongation after extended fluid exposure at 150°C. Both dipolymers (Type 1) and terpolymers (Type 2) have been successfully used for transmission sealing applications in conventional transmission fluid (ATF-B). Terpolymers (Type 2) behaved slightly better than dipolymers (Type 1). All base resistance fluoroelastomers (Type 4, 5, and FEPM) and the low temperature flexible FKM grade (Type 3) demonstrated excellent fluid resistance in ATF-A.
Another deterioration of the traditional dipolymers or terpolymers was evidenced with surface cracking. Cracking displayed a temperature and time dependence. A 180° looping test and a mandrel bend flexibility test were used to evaluate the tendency of surface cracking. The tests were performed on specimens after being exposed to ATF's at three temperatures to evaluate the impact of temperature and fluid. The bend test differentiated the aggressiveness of conventional transmission fluid (ATF-B) and new fluid (ATF-A), using the past history of successful performance (no cracking) of FKM Type 1 bisphenol-AF cured in conventional transmission fluid (ATF-B) for 1008 hrs at 150°C.
Aeration is favorable to the fluid resistance of FKM materials in ATF-A. When samples were aged side by side in: (1) Aerated ATF-A; (2) Fresh ATF-A; (3) Fresh ATF-B, Types 1 and 2 saw similar property losses in Aerated ATF-A and Fresh ATF-B. Fresh ATF-A remained aggressive. Fluid aging at 150°C per ASTM D471 is usually more severe than real transmission application environments, where temperatures are in most cases below 150°C and aeration is always expected. The results, however, do provide good material selection guideline by relative comparisons.
Publications1,2,3,4 suggested that the poor base resistance of FKM Type 1 and 2 was believed to be associated with the high acidic hydrogen on the vinylidene fluoride unit next to a hexafluoropropylene (HFP) unit on the fluorinated elastomer polymer chain. The high acidic hydrogen can be easily attacked by chemicals with electron donating groups, such as amines or amine derivatives. Replacing HFP with other monomers significantly improves FKM material's resistance to base natured substances, and the resulted products are re-categorized as FEPM, Type 4, Type 5, or low temperature flexible fluorinated elastomers (Type 3).