ABSTRACT:The shear stress and the first normal stress difference were simultaneously measured in shear flow start-up experiments and a subsequent stress relaxation. The measured polydimethylsiloxane show shear thinning flow behavior with a first Newtonian region up to the shear rate at the onset of shear thinning. We found that the shear stress and the first normal stress difference have an equal shear rate at the onset of shear thinning. The shear stress at the onset of shear thinning does not depend on the molecular weight of the polymer and on temperature. Similar to the viscosity master curve, it is possible to create a normal stress master curve, outgoing from the first normal stress coefficient. The shear rate at the onset of shear thinning is estimated from the viscosity master curve and the shear rate at the onset of normal stress thinning from the normal stress master curve. The shear stress and the first normal stress difference have similar transient behavior with startup stress curves without maximum in stress growth experiments with shear rates from the first Newtonian region. The shear flow start-up experiment with the shear rate at the onset of shear thinning leads to the first indication of an overshoot of the start-up curve. The shear flow start-up experiments with shear rates from the shear thinning region show start-up curves with a maximum for the shear stress and for the first normal stress difference. It was found that the shear rate at the onset of shear thinning and the shear rate at the onset of normal stress thinning, determined by the shear flow start-up measurements, are equal to the values obtained from the master curves.

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Model suspensions with different concentrations of the rheological additives Aerosil 380 in silicone oil M20 000 and Bentone 27 in epoxy resin Araldite GY260 were researched. The shear stress and the first normal stress difference were measured simultaneously with shear flow start-up experiments followed by stress relaxation. At higher concentrations, the rheological additives build a strong three-dimensional (3D) structure that leads to systems with plastic flow behavior. It was established that the structure of 7.5% Aerosil 380 in silicone oil M20 000 is strong and stiff due to the big difference between shear stress and normal stress at small shear rates. This solid-like system exhibits only one yield stress region. It was found that the suspensions with a strong 3D structure and comparable values of shear stress and first normal stress difference at small shear rates have a first and a so-called second yield stress regions. In the transition section, between the two yield stress regions, there occurs a break of the distortion and a rearrangement of the structure. The decrease and increase of the first normal stress difference also belongs to the rearrangement of the structure. POLYM. ENG. SCI., 50:811-818, 2010.

The rheological behavior of model suspensions with the silicone oil M20000 and different concentrations of Cab-o-sil TS 720 resp. Durcal 5 are compared. The increase of the Cab-o-sil concentration changes the flow behavior of the suspension from shear-thinning, to pseudoplastic, and to plastic flow behavior. The first normal stress difference rises at the same time at certain shear rate. The disperse systems with Durcal 5 keep the structural viscous behavior of the silicone oil even with a filler concentration of 40.5 wt%. The dependence of the first normal stress difference on shear rate represents for suspensions with Durcal 5 only one straight line with a slope of n = 2. The normal stress has double the amount of the silicone oil M20000 at given shear rate and is independent of the used Durcal 5 concentration. It was established that suspensions with the silicone oil M20000 have a first normal stress difference that can, depending on the filler type, either increase (with Cab-o-sil TS 720) or decrease (with Durcal 5) at certain shear stress with increasing filler concentration. It is to be supposed that the decrease of the normal stress at a given shear stress, with increasing Durcal concentration, is a softening effect, caused by the filler.