To investigate the effect of aeration on fluid-elastomer
compatibility, 4 types of elastomers were aged in three gear lubes.
The four types of elastomers include a production fluorinated
rubber (FKM) and production hydrogenated nitrile rubber (HNBR)
mixed by the part fabricator, a standard low temperature flexible
fluorinated rubber (FKM, ES-4) and a standard ethylene-acrylic
copolymer (AEM, ES-7) mixed by SAE J2643 approved rubber mixer. The
three gear lubes are Fluid a, Fluid b and Fluid c, where Fluid b is
a modified Fluid with additional friction modifier, and Fluid c is
friction modified chemistry from a different additive supplier.
The aeration effect tests were performed at 125°C for 504 hours.
The aerated fluid aging test was performed by introducing air into
fluid aging tubes as described in General Motors Company Materials
Specification GMW16445, Appendix B, side-by-side with a standard
ASTM D471 test. Shore A hardness, tensile strength, elongation,
modulus at 50% & 100% elongations and volume swell were
measured before and after the designated fluid aging period (504
hours).
The effect of aeration on fluid-elastomer compatibility showed a
strong dependence on the types of fluids and types of elastomers.
Aeration provides positive influence to fluorinated elastomers
(FKM), but became detrimental to hydrogenated nitrile rubber (HNBR)
and ethylene-acrylic copolymer (AEM). Among the three fluids, Fluid
c appeared to be more aggressive towards HNBR and AEM regardless of
whether the fluid was aerated or not. Both FKM materials showed
good resistance in all three fluids.
Aged fluid usually has a tendency toward oxidation, which in
turn produces acidic by-products evidenced with an increase of
Total Acid Number (TAN). Aeration, a process of introducing air to
fluid during aging process, is expected to accelerate oxidation
reaction, in turn generate more acidic by-products. However,
testing results from this study actually showed a decrease of TAN
of aerated fluids, even though aeration did cause catastrophic
deterioration of the material properties for HNBR and AEM.
