EN ISO 2431 is the international standard for measuring the viscosity of liquid coatings, including those used in paper and paperboard surface treatments. It specifically defines a flow time-based method using standardized flow cups (also known as Ford cups or DIN cups) to evaluate the rheological behavior of low- to medium-viscosity fluids like:
Paper coating formulations (e.g., kaolin/calcium carbonate slurries)
Barrier coatings (e.g., acrylics, starches, bio-polymers)
Primers, varnishes, and printing inks
Adhesive formulations for extrusion coating or laminating
For paper mills, converters, and R&D labs, EN ISO 2431 provides a simple, fast, and reproducible way to control coating performance, especially for application consistency, film formation, and drying behavior.
📜 Full Title
EN ISO 2431 – Paints and varnishes — Determination of flow time by use of flow cups
This standard is harmonized with DIN 53211 and ASTM D1200 but uses ISO-calibrated flow cups and precise temperature control.
🎯 Purpose of EN ISO 2431 in Paper Coating
The standard measures how long it takes for a specific volume of coating fluid to flow through an orifice under gravity. This time correlates directly with the dynamic viscosity, which affects:
Coating layer thickness
Surface uniformity
Ink adhesion
Coating holdout
Runability on blade, rod, or curtain coaters
🧪 How the Test Works
Equipment:
ISO flow cup (ISO 3, 4, 5, 6, or 7) with standardized orifice sizes
Stopwatch or automated timing device
Thermometer (measurement at 23 ± 0.5°C is critical)
Stand and leveling surface
Procedure:
Fill the cup with the liquid paper coating (stirred and bubble-free).
Remove the orifice stopper to allow flow to begin.
Measure the time (in seconds) it takes for the continuous stream to break.
Report the flow time and cup number (e.g., “60 s in ISO Cup 4 @ 23°C”).
📏 ISO Flow Cup Sizes and Ranges
Cup No.Orifice Diameter (mm)Recommended Viscosity Range (mPa·s)Flow Time Range (s)
ISO 33.0 mm3–7020–105
ISO 44.0 mm10–30030–150
ISO 55.0 mm50–70030–100
ISO 66.0 mm100–100020–80
ISO 77.0 mm200–120020–60
Use higher-numbered cups for thicker coatings like barrier dispersions or starch slurries.
📦 Applications in Paper and Board Coating
Use CaseWhy Viscosity Control Matters
Blade/rod coatingAffects coat weight and surface smoothness
Barrier coating (e.g., PVOH, PLA)Viscosity affects pinhole resistance and layer integrity
Printable coatingsViscosity impacts gloss, ink holdout, and print clarity
Sustainable coatings (e.g., bio-starch, protein)Flow time ensures consistent application and drying
Corrugated pre-coatingEnsures proper film formation without board warping
🧰 Best Practices for EN ISO 2431
Calibrate flow cups annually or after heavy use
Pre-condition coating samples to 23°C ± 0.5°C before testing
Stir coating gently to avoid air bubbles
Run test within minutes of sampling to avoid viscosity drift
Use in conjunction with Brookfield rotational viscosity when broader shear range data is needed
🌍 Regulatory and Industry Relevance
Supports ISO 28719 (coated paper performance)
Used in EN 13432 testing when biodegradable coatings are applied
Common in FMCG and food packaging QA protocols to control coat weight uniformity
Final Takeaway
EN ISO 2431 is a fast, effective method for controlling the viscosity of paper coating formulations, ensuring the material performs consistently on press and meets end-use demands. Whether you’re running aqueous barrier coatings or printable topcoats, this standard helps you manage flow, film quality, and production efficiency—right from the lab bench to the coater roll.
