Rheological characterization of a polyethylene melt, filled with different concentrations of fumed silica at 453 K

Abstract

Fumed silica is widely used as filler / thickening agent to increase the viscosity or to change the flow behavior type of a suspension. Strong hydrogen bonds between silica-silica and silica-matrix can lead to a suspension with a yield stress. We wanted to know if fumed silica can also form a three-dimensional network in a polyethylene melt at 453 K. The systems with the polyethylene LDPE 1810H (LDPE) filled with different concentration of the fumed silica Cab-o-sil TS720 (silica) were investigated with the MCR300 rheometer from Anton Paar at 453 K. The shear stress τ and the 1st normal stress difference were measured simultaneously in stress growth experiments. The points of the flow curves represent the steady state values from the start-up experiments with a constant shear rate. Just after the steady state values are reached, the stress deformation is stopped, and the stress relaxation begins. The relative residual shear stress is an important value. The absence of residual shear stress means that the structure of the system is destroyed in the stress growth experiment and the system has shear-thinning flow behavior. The systems with high residual shear stress form a strong network that leads to a system with plastic flow behaviour with a yield stress. The shear stress changes insignificantly in the yield stress region with the shear rate. The yield stress can be determined also from the relative residual shear stress curve.