Challenges in Rheological Characterization of Highly Concentrated Suspensions — A Case Study for Screen-printing Silver Pastes

The overall goal of this procedure is the reliable rheological characterization of highly concentrated suspensions, which exhibit complex deformation and flow behavior in classical plate to plate rotational rheometry. This method can help answer key questions for stream printing pastes. A comprehensive characterization of different process relevant rheological parameters is essential for a tailored engineering of such pastes with improved processing and application properties.

The main advantage of this approach is the simultaneous observation of the sample deformation and recording of rheological data. Demonstrating the procedure will be Ceren Yuce, a PhD student from KIT, Institute of Mechanical Process Engineering and Mechanics. To begin the yield stress measurements, set up a parallel plate rheometer with plates twenty-five millimeters in diameter and a plate roughness of two to four micrometers.

Prepare the measurements to be taken in stepwise controlled sheer stress mode, with the sheer stress to be varied between one and three thousand pascals in thirty-five steps, with a total measuring time of one thousand fifty seconds. Mount an endoscopic camera on a tripod and equip the camera with an LED spotlight. Connect the camera to a computer and adjust the contrast and brightness as needed.

Mix a sample silver paste with a speed mixer at one thousand RPM for thirty seconds to ensure that the sample is homogeneously blended. Apply a small amount of the blended paste to the bottom plate. Bring the upper plate to just above the measurement position.

Remove excess silver paste from the plate edges with a spatula. Then, move the upper plate to the measurement position. Mark the paste with a vertical line of sit particles.

Allow the sample to sit in the instrument for five minutes to equilibrate. Once the normal forces have decayed, simultaneously start the measurement and the video recording. When the paste spills from the gap, stop the measurement and the recording.

Clean the plates with ethynol and allow the plates to air dry. Perform this measurement three times in total, blending the paste before each application, and cleaning the plates after each measurement. Next, acquire a set of measurements and recordings on a separate parallel plate rheometer using twenty millimeter plates with a roughness of one point one five micrometers.

Then, use double sided tape to attach pieces of sand paper to the twenty millimeter plates to achieve a roughness of nine micrometers. Obtain another set of measurements and recordings, replacing the sand paper after each measurement. For each measurement series, plot the deformation logarithmically against sheer stress.

Determine the yield stress of the medium by the tangent intersection point method. Prepare a vane in cup rheometer to obtain measurements in stepwise controlled sheer stress mode, with the same parameters as the parallel plate measurements. Thoroughly blend the silver paste sample, and load the paste into the rheometer cup.

Move the vane to the measuring position, and allow the instrument to equilibrate for five minutes. Then, obtain at least three measurements. Clean the vane geometry with ethynol and allow the instrument to re-equilibrate between each repetition.

Plot deformation versus sheer stress and determine the yield stress of the medium by the tangent intersection point method. Our research demonstrate that yield stress can be obtained either using a vane or plate plate geometry. In a leather case, plate roughness must be selected appropriately.

For this purpose, base behavior and the recordings must be checked carefully. To begin the stretching behavior measurements, set up a capillary breakup elongational rheometer with two cylindrical pistons six millimeters in diameter. Set the stretching velocity to seven point five millimeters per second.

Set the frame rate of a high speed camera to two hundred and fifty frames per second. Turn on the backlight and position the camera to capture the stretching measurement. Adjust the image sharpness, contrast, and brightness as needed.

Thoroughly blend the silver paste and then apply the paste to the lower piston. Bring the upper piston down to the measurement position and remove excess silver paste. Simultaneously start the camera and the measurement.

Once the filament breaks, stop the recording and measurement and clean the pistons with ethynol. Perform the measurement at this velocity three times in total, blending the paste and cleaning the pistons each time. Obtain two more sets of measurements at stretching velocities of eleven and one hundred and ten millimeters per second in this way.

Review the recordings and identify the first frame showing filament breakage for each repetition. Determine the piston position and calculate the critical stretch ratio for each velocity tested. The high printing speed, and narrow measure printings used in screen printing limit the ability to assimilate snap off point to snap lab.

Nevertheless the elongational break in these filament stretching experiments may be related to this phenomenon. The effect of plate roughness on yield stress measurements was evaluated with two silver pastes. When plates with a roughness of one point one five micrometers are used, the paste marked with sit particles sticks to the upper plate and slides on the lower plate, indicating that a plug flow has formed.

Thus, the yield stress of these silver pastes cannot be accurately measured with this system. When plates with roughness values ranging from two to four micrometers are used, a sheer deformation profile is observed from the movement of the sit marking, which is necessary for a reliable, well-defined rheological measurement. These results were validated by comparison with vane geometry measurements.

The silver pastes exhibit similar behavior for plate roughness values of one point one five in two to four micrometers. When the roughness value was increased to nine micrometers, paste B showed a sheer deformation profile at low stresses, which transitioned to sheer banding at higher stresses. However, paste A did not show the needed profile and the stress to create the observed plug flow is significantly higher than the yield stress.

The stretching behavior of both pastes was compared at various stretching velocities using a high speed camera. The textile stretch ratio at which filament breakage was observed was longer for paste A at all velocities. Once mastered, this approach can be used to characterize not only silver pastes, but also other highly filled suspensions.

However, the exponential protocol has to be adjust according to the demands of the respective material. Our experimental approach paves the way for researchers in the field of printed electronics to charge the printing properties of their pastes based on lab scanned experiments. This will help to develop pastes with superior printing properties.

After watching this video, you should have a good understanding of how to do a reliable rheological characterization of highly filled pastes, and what you have to consider, if a phenomena like wall-slip, plug flow, or sample spillage occur. You will also get an idea about the different rheological test procedures relevant to the printing process.