ISO 2431:2019

Overview

ISO 2431:2019 - Paints and varnishes: Determination of flow time by use of flow cups - specifies a standardized method to measure the flow time of paints, varnishes and related products for consistency and viscosity control. The standard defines four fixed-dimension flow cups with orifice diameters of 3 mm, 4 mm, 5 mm and 6 mm, gives temperature and apparatus requirements, and provides procedures and checks to ensure reliable, comparable flow-time measurements.

Key topics and requirements

• Flow cups specified: Four ISO flow cups (3, 4, 5, 6 mm orifices). The internal jet diameter is critical (flow time varies with the fourth power of jet diameter).
• Materials & finish: Jet material shall be stainless steel or sintered carbide; cup body must be corrosion-resistant. Interior finish equivalent to Ra ≤ 0.5 µm to avoid flow disturbances.
• Construction & protection: Protective sleeve recommended to avoid accidental jet damage; cups with temperature-control jackets are preferred.
• Measurement conditions: Reference test temperature (23.0 ± 0.5) °C (alternative agreed temperatures allowed); samples and cups must be conditioned and tested free from drafts. Testing in a fume cupboard must be reported.
• Scope limitations: Method applies only where the breakpoint of the flow stream can be determined with certainty (works best for Newtonian or near-Newtonian materials). Materials with flow times near the upper measurement limit (~100 s), thixotropic or strongly non‑Newtonian materials can give uncertain results.
• Verification & precision: Two methods for checking flow cups for wear and tear are given (Annex A). Precision data including repeatability and reproducibility limits are provided in Clause 9. Annex B addresses conversion of flow times between temperatures.

Practical applications and users

• Quality control and consistency checks in paint, varnish, resin and related product production lines.
• Formulation development to monitor application consistency during R&D.
• Laboratories and QC departments in coatings manufacturers, contract labs, and suppliers of raw materials.
• Note: For non-Newtonian or thixotropic products, ISO 2431 is not a substitute for rheometers or viscometers (see ISO 3219 for viscometric methods).

Related standards

• ISO 1513 - Examination and preparation of test samples
• ISO 4618 - Terms and definitions for paints and varnishes
• ISO 15528 - Sampling of paints, varnishes and raw materials
• ISO 3219 - (alternative) Determination of viscosity by viscometers

Keywords: ISO 2431:2019, flow cups, flow time, paints and varnishes, viscosity control, consistency testing, 3 mm 4 mm 5 mm 6 mm flow cup, flow cup wear, repeatability, reproducibility.