Effects of heating and shearing on the rheological properties of poly(tetra fluoro-ethylene) suspensions in silicone oil

The effects of heating and shearing on the rheological properties of poly(tetra fluoro-ethylene) (PTFE, 0.2 μm particles) suspensions in silicone oil 1000 cSt were investigated. The suspensions were heated at 140 °C for 1 day and then sheared at a rate of 1000 s−1 for 30 min. Four suspensions were prepared: the unheated and unsheared suspension, the heated and unsheared suspension, the unheated and sheared suspension, and the heated and sheared suspension. Hysteresis loops, stress-strain (S-S) sweep curves, oscillatory dynamic moduli, and steady state shear flow curves of the suspensions were measured at 25 °C. No hysteresis was observed in the hysteresis loops for any of the suspensions. Two yield stresses were observed in the S-S sweep curves of the unsheared suspensions. Shearing caused the lower yield stress to fade but retained the higher yield stress. The S-S sweep curves of the sheared suspensions were nearly superimposable regardless of heating. The movement of the position of a target in the sheared suspensions occurred at a larger strain than that in the unsheared suspensions. Shearing caused changes in the oscillatory dynamic moduli of storage (G′) and loss (Gʺ) as a function of strain: the unsheared suspensions behaved as solid-like viscoelastic matter, whereas the sheared suspensions gave similar magnitudes of the G′ and Gʺ values in the linear regions. Beyond the linear region, the Gʺ value showed strain overshoot regardless of the treatment. The steady state shear flow curves of the suspensions were compared with the Herschel-Bulkley equation; their fits were comparable and the calculated parameters in the equation were well correlated to the historical treatments of the suspensions.