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.