Anlauf- und Relaxationsverhalten von dispersen Systemen, die Thixotropiemittel enthalten

2009

Polym Eng Sci

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.

Section: Introductionmentioning

confidence: 99%

“…The shear stress that does not change with the increasing shear rate is practically accepted to be the yield stress [14,[27][28][29][30][31][32][33][34].…”

Section: Introductionmentioning

confidence: 99%

Model suspensions with rheological additives: Systems with plastic flow behavior

2009

Polym Eng Sci

“…PDMS are utilized as defoaming agent also in the food industry as E900. PDMS are used for breast implants, as a calibration fluid,1 and as matrix fluid for model suspensions in the rheology 2–11. Several authors studied the rheological behavior of polydimethylsiloxane 12–18.…”

Section: Introductionmentioning

confidence: 99%

“…The time‐related flow behavior is widely used for the characterization of the rheological behavior, for example, of polyethylene melts;25 suspensions;26–30 lyotropic hydroxypropylcellulose solutions;31 thixotropy;32, 33 rheopexy;34 and polydimethylsiloxane 15, 35…”

Section: Introductionmentioning

confidence: 99%

Determination of the onset of shear thinning of polydimethylsiloxane

J of Applied Polymer Sci

2008

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.

“…Consequently, we can extend the definition of the yield stress to include all rheological systems-the rheological systems must have a three-dimensional network at low shear rates, but it is not necessary to be an elastic solid. The yield stress can be determined from the region of the flow curve, where the shear stress does not change significantly with the increase of the shear rate [19,[21][22][23][24][25][26][27] by extrapolation of the shear stress to the y-axis.…”

Section: Determination Of the Yield Stress Of Disperse Systems 150mentioning

confidence: 99%

Determination of the Yield Stress of Disperse Systems

2018

JMSE-A

The yield stress is defined as the stress below which the substance is an elastic solid, while above it is a liquid with a plastic viscosity. This yield stress is the stress at the yield point. We will report of probably the only system with yield stress according to this definition. The yield stress will be determined from the first visible difference between shear stress and residual shear stress. Shear flow start-up experiments and following stress relaxation will be performed. The relative residual shear stress indicates the strengthening of the structure. It will be reported about systems with pronounced one and two yield stress regions, where the shear stress changes insignificantly with the shear rate. The yield stress regions of these systems can be determined from the flow or viscosity curves and from the residual shear stress curve. System with flow curves without a pronounced yield stress region will be investigated and furthermore the conditions to have a yield stress will be determined. The definition for the yield stress will be modified/extended.