Filling Effects of BaSO4 and CaF2 on Non-linear Rheological Behaviours and Mechanical Performances in Fluoroelastomer

Correlations between filling amount and effects in FKM of two fillers, BaSO4 and CaF2, were investigated to clarify differences in dispersion, binding, and reinforcement. Rheological behaviors of FKM mixtures were first explored via LAOS tests, indicating that the increase in filling content of BaSO4 had relatively monotonous impacts on the non-linear viscoelasticity responses, while those of CaF2 presented a complex three-stage alteration pattern. It was hypothesized that an approximate CaF2-FKM “mesophase” was formed with filler content up to the 25–30 phr range due to the specific binding properties of CaF2 to the FKM matrix. Next, investigations on FKM vulcanizate corroborated the conjecture provided by the LAOS test, where differences in filler aggregation morphology under SEM images and G ' from DMA measurements were observed. Engineering tests were then conducted for FKM vulcanizates, with which the discrepancies of BaSO4 and CaF2 in the filling effects on FKM were further discussed. Results illustrated that BaSO4 had poor dispersion and binding in FKM, while CaF2 possessed more significant reinforcing effects due to its good wetting and dispersion, and the difference in performances of vulcanizate with two types of filler was most prominent in the filling range of 20–30 phr.