Bad Dilatancy – The Importance of Viscosity in High-Speed Coating
Abstract
In high-speed coating operations, achieving consistent film quality requires precise control over fluid viscosity—particularly at high shear rates. Dilatancy, or shear thickening behavior, can lead to sudden viscosity increases under high-speed conditions, resulting in coating defects, process instability, and equipment strain.
This article explores the mechanisms of dilatancy, the importance of measuring high shear viscosity, and tools like the ACA AX-100 capillary viscometer according to TAPPI T-582 standard that support robust formulation and process control.
Introduction: Why Viscosity Matters
In high-speed coating, where substrates can move at 1000 m/min or more, fluid behavior under shear is critical. Viscosity—resistance to flow—must be carefully controlled to ensure smooth film formation. However, many industrial fluids are non-Newtonian, meaning their viscosity changes with shear rate.
While shear thinning is often desirable, some fluids exhibit dilatancy, where viscosity increases under shear. This is especially dangerous in coating processes, where sudden thickening can lead to defects and operational problems.
For example, a blade coating system operating at 1000 m/min with a target coat weight of 10 g/m² generates shear rates far beyond the capability of conventional rheometers. It’s under these extreme conditions that dilatant behavior becomes a serious liability.
Understanding Dilatancy in Coating Fluids
Dilatancy arises in concentrated suspensions when particles interact under shear to form transient structures or friction that resist flow. This shear-induced jamming increases viscosity sharply, especially in systems with high solids content or poor dispersion stability.
In high-speed coating, this behavior causes several problems:
Film break-up and poor leveling due to increased resistance,
Misting and air entrainment from unstable flow,
Increased equipment stress from high back pressure or torque, and
A narrower process window, increasing the risk of defects.
Importantly, dilatancy may not appear in low-speed lab testing. Fluids can pass basic QC tests yet fail at full production speeds due to unrecognized high shear behavior.
Figure 1: Two coating colors measured with ACA AX-100 capillary rheometer. Dilatancy is clearly visible in yellow curve. Viscosity at 500 000 1/s four times higher (163 mPas vs. 44 mPas) than the reference (blue line)
The Need for High Shear Viscosity Measurement
To control these risks, it's essential to measure viscosity at relevant shear rates. Traditional viscometers often fall short, typically maxing out around 10,000 s⁻¹—far below industrial needs.
The ACA AX-100 high shear viscometer addresses this gap, offering reliable measurements up to 500,000 s⁻¹ and beyond. This enables direct simulation of production conditions and allows formulators to:
Detect dilatant tendencies early,
Optimize additives and particle systems,
Guide scale-up from lab to line, and
Set meaningful quality control thresholds.
Supporting this, TAPPI Standard T-582 defines a standardized method for high shear viscosity measurement. Designed for paper and coating applications, it provides a common framework for formulation development and production control.
Conclusion
Dilatancy is a hidden but critical challenge in high-speed coating. Without understanding how viscosity changes under extreme shear, even well-designed formulations can fail during application. Tools like the ACA AX-100 and TAPPI T-582 offer the industry practical ways to test, control, and design around high shear behavior.
Ultimately, managing high shear viscosity is not just good practice—it’s essential for ensuring stable processes, reducing waste, and achieving the consistent quality that modern coating operations demand.
Curious how high shear measurement could benefit your process?
Get in touch with our team to learn more about the ACA AX-100 and how it can support your formulation and quality control work.