Navigating Viscosity: A Guide to Viscometry Techniques

Viscosity is a cornerstone in coatings analysis and influences formulation precision, product quality, and process efficiency. In the realm of paint and coatings formulation, the assessment and control of viscosity are paramount to achieving desired application characteristics and product performance.

As coatings play an essential role in many industries, from automotive to aerospace, understanding and controlling viscosity becomes increasingly crucial. Understanding techniques from traditional methods to innovative technologies and applications equips coatings professionals with the knowledge and insights needed to navigate the challenges of formulation optimization, quality control, process control, and troubleshooting with precision and confidence.

1. Understanding Viscosity, Impacts, and Importance in Paint and Coatings

Viscosity is a fundamental property that dictates fluid behavior, a fluid’s resistance to flow, and measures in terms of centipoise (cP) or poise (P). Higher viscosity fluids flow more slowly and resist deformation under stress, while lower viscosity fluids flow more easily. Viscosity directly impacts how smoothly the material can be applied, its ability to form a uniform film formation and its overall performance.

Factors such as temperature, shear rate, and formulation can influence viscosity and require careful monitoring and control. Precise viscosity measurement and control are essential for ensuring product quality, meeting regulatory standards, and achieving customer satisfaction. By understanding viscosity and its implications, coatings professionals can optimize formulations, ensure consistent product quality, and meet the diverse requirements of end-users.

2. Various Traditional Viscometry Techniques

Traditional viscosity testing techniques are foundational for coatings professionals, providing insights into formulations’ flow behavior and consistency. Conventional methods, such as Rotational Viscometry, Cone and Plate Viscometry, KREBS Viscometry, Capillary Viscometry, and Viscosity Cup Viscometry, offer valuable means of quantifying viscosity across a spectrum of formulations, from low-viscosity solutions to highly thixotropic coatings.

By understanding and employing these traditional techniques, coatings manufacturers can optimize formulations, ensure consistent quality, and meet the diverse needs of end-users across various applications.

A. Rotational Viscometry

Rotational viscometers, also known as Vane viscometers, are widely employed for viscosity measurement in paint and coating formulations and are used to measure absolute viscosity. This technique involves rotating a spindle or bob immersed in a cup of sample fluid and then determining the viscosity by measuring the torque on a vertical shaft that rotates the spindle, which is required to overcome the fluid’s resistance to flow.

The fluid’s viscosity is then calculated based on the rotational speed and the observed torque. Rotational viscometry provides accurate viscosity measurements over a wide range of shear rates and is suitable for Newtonian and non-Newtonian fluids. Digital viscometers that apply a precise shear rate are necessary for non-Newtonian fluids. Rotational viscometers can measure all types of liquids with viscosities from a near water-like viscosity to a very high viscosity range as high as 320 million centipoises for epoxy, sealants, and paste. From determining flow behavior to assessing coating performance, rotational viscometry offers invaluable insights for coatings professionals.

• Advantages:
o Rotating the spindle at different speeds allows for analyzing shear-dependent behavior 
o Analysis of time-dependent fluids can be measured by rotating the spindle continuously over time
o Test both Newtonian and non-Newtonian fluids

• Typical Applications:
o  Low Viscosity (L) Range: 20 to 2,000,000 centipoiseAdhesives (solvent); Chemicals; Cosmetics; Hot Waxes; lithographic Inks; Latex paints; Coating systems; Polymers; Rubber solutions; Solvents
o  Medium Viscosity (R)
Range: 100 to 13,000,000 centipoise
Paints; Inks (screen printing); Surface coatings; Varnishes; Adhesives (hot melt); Ceramic slurries; Gums; Paper coatings; and Plastisols
o  High Viscosity (H)
Range: 200 to 104,000,000 centipoise
Sealants; Sheet molding compounds; Epoxies; Gels; Inks; Caulking compounds; Pastes; Putty; Roofing compounds;
and Asphalt

Rotational viscometers have multiple speeds and interchangeable spindles that provide many viscosity ranges for flexibility in application. The correct model choice will ensure maximum accuracy and sensitivity in the measured viscosity range. Some factors to consider when selecting a viscometer are the viscosity range of the samples, sample size, and the ability to monitor temperature or record the viscosity values.

B. Cone and Plate Viscometry

The Cone and Plate viscometer is a rotational viscometer that includes shear rate-defined geometries and measures viscosity under a high shear force. The cone and plate viscometer is versatile, which makes it practical for R&D or QC labs that require quick and easy testing of materials. Cone and Plate viscometers determine viscosity by measuring the shear stress and shear rate when the cone rotates against the plate. Use the resulting data to calculate the viscosity.

• Advantages:
o  Highly accurate and versatile
o  Suitable for a wide range of liquids as well as high-viscosity materials
o  Provides for viscosity measurements at very well-defined high shear rates
o  Requires a small volume of the sample liquid
o  Reading at different timed intervals ensures accurate results with thixotropic fluids

• Typical Applications:
o  Simulating brush, roller, and spray application effects of paints in the lab
o  Automotive exterior paints
o  The study of rheological characteristics of unfilled resins and etching gels
o  Testing the viscosity of inks for inkjet printers

C. KREBS Viscometry

KREBS viscometry, also called Stormer Viscometers, are precision instruments used to measure the viscosity of paint in KU values and cP units of Newtonian and non-Newtonian liquid samples. It is easy to use and has a fixed-speed motor, a paddle-style spindle, and a single-chip microcomputer for reading the KU values directly from the instrument. The Stormer viscometer complies with the ASTM method D562.

• Advantages:
o  Displays and calculates Krebs units (KU
o  Real-time high-resolution graphing of test samples
o  Highly stable motor speed control for accuracy and repeatability of measurement

•  Typical Applications:
o  Measuring the consistency of paints, inks, varnishes, and coatings
o  Measuring the consistency of detergents and related samples
o  Tests the viscosity of foods and beverages to chemicals or adhesives

D. Capillary Viscometry

Capillary viscometers, called U-tube viscometers, come in Ostwald and Ubbelohde variants. Determining viscosity with a capillary viscometer is established by measuring the time it takes for a given volume of sample fluid to travel through the capillary tube, either by gravity or a driving force. Then, determine viscosity using the measured flow rate, applied pressure, and tube dimensions. The time it takes for a specific fluid volume to flow a particular distance through a tube is a function of the fluid’s resistance to flow. The more viscous the fluid, the longer it takes to travel the distance.

• Advantages:
o Measures precise viscosities for many diverse fluids
o Detects viscosities at different shear rates
o Requires only small volumes of sample fluid

• Typical Applications:
o Measures the viscosity of a wide range of diverse fluids, including petroleum products, lubricants, adhesives, and sealants

E. Flow Cup Viscometry

Viscosity or flow cups are considered kinematic viscometers that do not apply a shear force to measure viscosity. They are standard flow cups with a defined volume and a calibrated orifice diameter adapted to solvent paints. Flow cups are the most common kinematic viscometers, including Zahn Cups, Ford Cups, ISO Cups, and DIN Cups.

Kinematic viscosity is the measurement of a fluid’s capability to resist flow under the influence of gravity. Assess this property by timing the duration, in seconds, for a set volume of fluid to travel a known distance via gravity through a capillary within a calibrated viscometer, all while maintaining a tightly regulated temperature. Such viscometers are user-friendly, cost-effective, and well-suited for production oversight, field assessments, or laboratory analyses.
• Advantages:
o Easy to use
o Reasonably priced
o Portable

• Typical Applications:
o Production control
o Field inspection
o Laboratory use

3. Advanced Viscometry Techniques

Advanced viscometry techniques include oscillatory viscometry and rheology profiling and offer comprehensive advancements in viscosity techniques and the extensive characterization of flow properties.

A. Oscillatory Viscometry

Oscillatory viscometry represents a significant advancement in viscosity measurement, which provides insights into complex fluids’ rheological behavior. From assessing viscoelastic properties to predicting coating performance, oscillatory viscometry offers a deeper understanding of coating behavior under varying conditions. Use oscillating viscometers to study the rheological properties of paint and coatings formulations and to quantify their thixotropic properties.

• Advantages:
o Rapid, accurate measurements
o Flexibility in sample handling

• Typical Applications:
o Formulation optimization
o Quantifying thixotropic properties
o Compatibility Testing
o Cure monitoring
o Additive evaluation

B. Rheology Profiling

Rheology profiling goes beyond traditional viscosity measurements, comprehensively characterizing a fluid’s flow properties. By incorporating rheological profiling into workflows, coatings professionals can gain deeper insights into the complex rheological behavior of coating systems.

• Advantages:
o Comprehensive coating characterization
o Formulation Optimization
o Process Optimization
o Troubleshooting
o Quality Control

• Typical Applications:
o Formulation and Quality Control of paint and coatings
o Optimization of application properties such as viscosity, leveling, and sag resistance
o Designing adhesives and sealants with specific flow characteristics
o Formulation of construction materials such as concrete, mortars, and grouts

4. Importance in Formulation Optimization:

Viscosity testing is vital in the formulation optimization process for paints and coatings. By accurately measuring viscosity, formulators can adjust the composition of formulations to achieve desired flow properties, rheological behavior, and application characteristics. Optimization ensures smooth, even coating application with the appropriate thickness, enhancing performance and durability.

5. Quality Control:

Viscosity testing is critical to quality control protocols within paint and coatings manufacturing facilities. Consistent viscosity measurements throughout the production process enable manufacturers to monitor batch-to-batch consistency, identify deviations from specifications, and take corrective actions, as necessary. By maintaining tight control over viscosity parameters, manufacturers can minimize product defects, reduce waste, and uphold the brand’s reputation for delivering high-quality coatings.

6. Application Consistency:

In the paint and coatings market, achieving uniform application consistency ensures satisfactory results for end-users. Viscosity testing allows manufacturers to determine the optimal viscosity range for various application methods, such as brushing, spraying, or dipping. By calibrating viscosity to meet application requirements, coati ngs can be applied smoothly and evenly, resulting in uniform film thickness and appearance across surfaces.

7. Regulatory Compliance:

Viscosity testing is also crucial in ensuring regulatory compliance within the paint and coatings industry. Many regulatory agencies, such as the Environmental Protection Agency and the European Chemicals Agency, have established viscosity standards for coatings formulations to minimize environmental impact and ensure user safety. By conducting viscosity testing according to regulatory guidelines, manufacturers can demonstrate compliance with relevant regulations and avoid potential fines or penalties.

8. Customer Satisfaction:

Ultimately, the importance of viscosity testing in the paint and coatings market lies in its impact on customer satisfaction. Coatings products that exhibit consistent viscosity, optimal application characteristics, and reliable performance are more likely to meet or exceed customer expectations. Whether used for residential, commercial, or industrial applications, coatings that provide superior finish quality, durability, and ease of application contribute to positive user experiences and long-term
customer loyalty.

Different models of viscometers are necessary for applications with high, medium, and low viscosity. These designations are intended only as guidelines. Multiple speeds and interchangeable spindles extend the viscosity ranges of rotational-type viscometers for application flexibility.

Choosing the correct model will ensure maximum sensitivity and accuracy in the appropriate viscosity range. Some factors to consider when selecting a viscometer are the viscosity range of the samples, sample size, temperature monitoring, and the need to record the viscosity values.

Viscosity testing plays a multifaceted role in the paint and coatings market, influencing formulation optimization, quality control, application consistency, regulatory compliance, and customer satisfaction. By leveraging precise viscosity measurement techniques and incorporating viscosity testing into manufacturing processes, coatings manufacturers can ensure the production of high-quality products to meet the diverse needs of their customers while maintaining competitiveness in the global marketplace. 

Sherri Thompson, marketing manager for the Paul N. Gardner Company, has a BA in commercial art and a minor in marketing from East Carolina University. She has over 20 years of experience in marketing, management, and application of physical test instrumentation in the coatings and inspection equipment industries.