Meaning
The measure of internal friction within a fluid undergoing rapid deformation identifies how a liquid behaves under the intense pressure of a spray or slot die. High-shear viscosity is the critical parameter for applications where fluids are forced through small orifices at high speeds. It differs from the viscosity measured at rest because molecular structures often align or break down under stress.
This measurement is typically taken at shear rates above ten thousand reciprocal seconds.
Application Rate
Determining the correct pump pressure for a coating line requires an accurate measurement of the fluid resistance at the actual speed of the die. If the high-shear viscosity is too high, the coating will not spread evenly and may cause the substrate to tear. Pilot tests must replicate these shear conditions to predict production performance.
Pressure Drop
Fluids that exhibit shear thinning will flow more easily as the speed increases, reducing the load on the delivery system. Conversely, shear thickening materials can cause a sudden pressure spike that damages seals and gaskets. Understanding these transitions prevents unexpected downtime during the transition from batch mixing to continuous application.
Molecular Stress
Subjecting a polymer or a slurry to extreme shear can permanently change its properties by breaking long chains. This degradation reduces the quality of the final product even if the initial application looks successful. Monitoring the stability of the fluid under stress is a requirement for high volume manufacturing.
The high-shear viscosity value determines the maximum throughput of the coating head.
