The Science Behind Fluid Quality
Same formulation. Same raw materials.
So why does product performance vary from batch to batch?
The same formulation and raw materials can produce very different results. Paint may develop orange peel or sagging during application. Sealants may string during dispensing. Creams may have inconsistent textures, while determining the endpoint of a resin polymerization reaction can be challenging.
Viscosity is a routine quality control parameter in many industries, and most manufacturers test it regularly. However, the impact of testing conditions and procedures is often overlooked. As a result, measurements may be inaccurate or difficult to compare, even when tests are performed. Repeatedly adjusting the formulation may still fail to resolve batch-to-batch variations.
In this article, we'll explore the key aspects of viscosity testing and how reliable viscosity data can help ensure consistent product quality.
What Is Viscosity, and Why Does It Matter?
Simply put, viscosity measures a fluid's resistance to flow. It is commonly expressed in cP (mPa·s). In the architectural paint industry, KU (Krebs Units) is also widely used. It is a scale specific to Stormer viscometers and is used to assess paint consistency for brushing and rolling applications.
- Incoming Quality Control: Test raw materials such as resins, emulsions, adhesive bases and food pastes to identify batch variations and ensure consistent incoming material quality.
- Process Monitoring: Track viscosity changes during polymerization, emulsification and blending to help determine reaction endpoints in both production and R&D.
- Finished Product Quality Control: Viscosity affects application and processing methods such as spraying, dispensing, coating, filling and spreading, directly influencing the end-user experience.
- Stability Assessment: Monitor viscosity changes to identify potential separation, settling, gelation or deterioration and evaluate product storage stability.
A quick reminder: Inaccurate viscosity measurements can undermine even a well-designed formulation, leading to inconsistent product quality, rework, waste and customer complaints.
Getting Inaccurate Viscosity Readings? 5 Common Testing Mistakes to Avoid
Inconsistent readings from repeated tests on the same sample don't necessarily mean the instrument is faulty.
From a measurement perspective, factors affecting viscosity results generally fall into three categories:
- Intrinsic fluid properties: These are inherent rheological characteristics of the material, not measurement errors.
- Instrument and measurement method: These can introduce systematic errors and consistently skew the results.
- Operator technique and test conditions: These can affect measurement repeatability, leading to inconsistent readings.
Here are five common testing issues to watch out for:
- Temperature
Temperature has a significant effect on viscosity. In general, viscosity decreases as temperature increases.
A temperature difference of just 0.5°C can cause viscosity readings to vary by more than 5% for some fluids. For temperature-sensitive samples, temperature control accessories are recommended.
- Spindle and Container Selection
The spindle and measuring container must be properly matched. Using an incorrect combination can lead to significant measurement errors.
- Spindle Immersion Depth
The spindle must be immersed to the correct depth for accurate measurements. Always follow the immersion depth specified in the instrument manual.
- Air Bubbles
Air bubbles in the sample can interfere with the measurement and lead to inaccurate readings.
- Instrument Maintenance
Clean the spindle and sample container promptly after each test. Dried material, scratches or wear on the spindle can introduce ongoing measurement errors.
Choosing the Right Viscosity Testing Method
Viscosity testing generally falls into two categories: quick checks on the production floor and precision measurements in the laboratory.
Each serves a different purpose, from routine quality control in manufacturing to research and development in universities and laboratories. The best choice depends on your application and testing requirements. There's no need to invest in a high-end instrument if a simpler solution meets your needs.
- Flow Cups
How they work: Viscosity is estimated by measuring the time it takes for a fixed volume of liquid to flow through a standard orifice.
Key advantages: Affordable and easy to use, making them ideal for routine quality checks on the production floor.
- Rotational Viscometers (The Mainstay of Laboratory Testing)
How they work: A spindle rotates in the sample, and the torque required to maintain rotation is used to determine viscosity. Common types include:
General-Purpose Rotational Viscometers: Cover a wide viscosity range, from low-viscosity liquids to highly viscous pastes.
KU Stormer Viscometers: Designed for architectural paints and coatings, with results reported in KU.
Cone-and-Plate Viscometers: Require only a small sample volume and offer temperature control, making them suitable for R&D and rheological studies.
In-line Viscometers: Installed directly in production lines to monitor viscosity continuously without the need for manual sampling.
BEVS Viscosity Measurement Solutions
BEVS Viscometer Series covers a wide range of applications, from quick checks on the production floor to precision laboratory testing and real-time in-line monitoring. Designed to meet major GB, ASTM and ISO standards, the series simplifies testing and makes measurement data easier to track and trace.
Applications by Industry
Testing Needs
Control viscosity during coating and dispensing, evaluate stringing and open time, and optimize sealant dilution to ensure consistent application.
Key Benefits
Customizable test programs
High-shear dispensing simulation
Wide viscosity range
Excellent repeatability
Testing Needs
Monitor reaction endpoints, measure resin viscosity, assess dilution compatibility, and ensure batch consistency.
Key Benefits
Wide viscosity measurement range
Efficient, high-frequency testing
Standardized test reports
Testing Needs
Control KU viscosity in latex paints, assess rheology, leveling, and atomization, and verify dilution for high-solids coatings. Supports both production QC and formulation development.
Key Benefits
ASTM/ISO compliant
Multiple shear rates
Easy operation
One-touch data export for QC and R&D
Testing Needs
Assess the thixotropy of lotions, creams, and gels, evaluate storage stability and skin feel, and test temperature-sensitive formulations with minimal sample volumes.
Key Benefits
Temperature-controlled micro-sample testing
Touchscreen and voice control
Clear rheology curves
Testing Needs
Control the consistency of suspensions, ointments, and sauces to ensure reliable filling and application, with traceable data for compliance.
Key Benefits
Exportable test reports
Precise temperature control
Detailed rheology curves
Traceable test data
Viscosity testing may seem simple, but consistent, reliable, and traceable data is essential for formulation development, quality control, and process optimization.
BEVS offers comprehensive viscosity and rheology solutions, from quick checks on the production floor and precision laboratory testing to in-line monitoring and digital laboratory management. With integrated support for instruments, consumables, and digital solutions, we help you ensure product quality and optimize production.
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