Paints and coatings are usually highly structured fluids that consist of several different components. Pigments and dyes provide color and contribute to the final strength of the applied material. Binders are the film forming components that surround the pigments to prevent aggregation, and that influence properties such as gloss, durability, flexibility and toughness. Solvents can be used as transport media which do not become part of the final paint film. Different kinds of additives can be used to modify the surface tension, optimize the thixotropic behavior, or improve finish appearance of the paint film. All these components contribute to the flow behavior of the final paint product.
During processing, transport and application, paints and coatings are exposed to a wide range of shear rates. The pumpability of a material correlates strongly with viscosity at medium and higher shear rates. The lowest shear rates and the yielding behavior are of interest for maintaining shelf life. The evaluation of the rheological performance of a paint requires comprehensive test methods to simulate usage of different tools like brushes, rolls or spray guns. Measuring the change of the microstructure at high shear rates and the recovery at rest or under lower stresses and strains can be a useful tool to optimize formulations and evaluate final products.
Different kinds of commercially available paints have been tested using a rheometer in combination with a Peltier temperature control unit for concentric cylinders and a CC25 DIN measuring geometry. The tested materials include standard wall paint, primer paint and a lacquer for smooth surfaces.
A common test method for the investigation of thixotropic behavior of paints and coatings is a loop experiment including a Controlled Rate (CR) ramp from low to high shear rates, followed by a steady shear rate element at the highest shear rate and a CR ramp back to the low shear rate. One of the major disadvantages of the thixotropic loop experiment is that it doesn’t provide any information about the material’s recovery time after being exposed to a high shear rate, nor to what degree it recovers in a certain time period. This information can be obtained from the so-called shear recovery experiments, where the change in viscosity over time is monitored after the material has been exposed to a high shear rate. However, the recovery of a material can only be measured if the applied shear stress or shear rate is small. First, the viscosity of a material with an intact structure is measured, followed by a high shear rate period to break down the microstructure of the sample. Finally, the applied forces are reduced to the initial value again, to monitor the recovery after a high shear impact.
The shear recovery test can be performed in two different ways. The first and the third step can be performed at a low shear rate (CR) to get a baseline for the material and to measure the time dependent recovery. Both elements can also be performed at a low shear stress. This method simulates the effects of gravity acting on a paint after it was applied to a surface better than the shear rate experiment. However this test can only be conducted with a rheometer which is able to work in Controlled Stress (CS) mode. For both methods the middle part is performed in CR at an elevated shear rate.
Read Investigating the Thixotropic and Shear Recovery Behavior of Paints and Coatings Using the Thermo Scientific HAAKE Viscotester iQ to see the results of a thixotropic loop experiment for lacquer, wall paint and primer paint, the results of three-step shear recovery test for wall paint using different settings, and the results of a three-step shear recovery test for wall paint, lacquer and primer paint using the CS method.