Rheological properties are an important consideration in formulating paints and coatings. For many applications, it is necessary to use rheological modifiers to create a shear-thinning or pseudo-plastic viscosity profile. This property is widely understood in coatings formulation and is straightforward to measure. In addition, many applications will require a coating with a yield stress, a property equally as important as shear thinning but more challenging to characterize. A material with a yield stress will behave as a solid at rest and will not flow until a sufficient amount of stress has been applied. This characteristic is highly relevant to a coating's end-use properties. Suspensions may require a sufficient yield stress to prevent settling over time. Application behavior is also affected. Yield stress may be beneficial in preventing drips or bleed-over in an ink. High yield stress may also cause problems. Dispensing a material, whether by pouring, pumping or spraying, requires a critical shear stress to achieve flow. High yield stress may slow or prevent these processes. Due to the complex demands on the rheological performance of the coatings, characterization and understanding of yield stress is necessary in product development and in quality control. While yield stress is a crucial rheological property it is difficult to quantify. There are multiple rheological testing methods for this measurement, and the choice of tests and specific conditions can produce significantly different results. The work presented here will compare different rheological testing methods for measuring yield stress, including steady shear in both stress-controlled and rate-controlled modes, creep-recovery, oscillation, and newer techniques such as orthogonal superposition.
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