ISO 19403-3:2017

Overview

ISO 19403-3:2017 specifies an optical test method for measuring the surface tension of liquids used in paints and varnishes by the pendant drop method (drop contour analysis). The standard defines the principle (Young–Laplace equation), required apparatus, sampling, test conditions and evaluation criteria for characterizing liquid coating materials. The method is widely used in wettability and surface-energy assessments but can be restricted for non‑Newtonian liquids.

Key Topics and Requirements

• Principle: Surface tension is calculated from the shape of a pendant drop using the Young–Laplace equation and drop contour analysis.
• Apparatus: State-of-the-art drop contour analysis system with digital image capture, dosing unit and circular‑cylindrical needle. Needle outside diameter range: 0.5 mm to 2.5 mm (selected based on the quotient σ/Δρ).
• Imaging & lighting: Optimize camera zoom and brightness; recommended grey values near interface ≤ 40 (of 256) and outside drop 170–200 for best contrast.
• Calibration & inputs: Enter local acceleration of gravity into the device software; measure liquid and ambient phase densities (ISO 2811) to determine Δρ.
• Test conditions: Conduct tests at 23 ± 2 °C and 50 ± 5 % RH and ensure sample thermal equilibrium.
• Sample handling: Take representative samples per ISO 15528 and avoid contamination/bubbles during dosing.
• Evaluation criteria: Axial symmetry of the drop is mandatory. Key quality metrics:
  • Shape parameter B should be within 0.60 ± 0.06.
  • Fit error (deviation between measured and calculated contour) should be < 1 µm per measuring point.
• Precision: Repeatability r ≈ 0.7 mN/m; reproducibility R ≈ 1.6 mN/m for test liquids and 2.2 mN/m for coating materials.
• Reporting: Report must include identification, reference to ISO 19403-3, shape parameter B for each drop, fit error, number of drops, mean surface tension ± SD, densities and device/type/operator/date plus any deviations.

Applications and Who Uses It

• Users: R&D chemists, quality-control laboratories, coatings formulators, surface scientists and instrument manufacturers.
• Applications:
  • Characterizing surface tension of paints, varnishes, solvents and coating raw materials.
  • Input for adhesion and wettability studies, surface-energy calculations, and optimization of coating formulations and application processes.
  • Validation and calibration of drop shape analysis instruments.

Related Standards

• ISO 19403-1 (terminology and principles), ISO 19403-4 / ISO 19403-5 (polar/disperse fractions)
• ISO 2811 (density), ISO 4618 (terms), ISO 15528 (sampling), ISO 3270 (test conditions)
• Alternative methods: EN 14370, ISO 1409

Keywords: ISO 19403-3, pendant drop method, surface tension, drop contour analysis, paints and varnishes, wettability, drop shape analysis.