ISO 6721-2:2019

Overview

ISO 6721-2:2019 specifies the torsion-pendulum method for determining the linear dynamic mechanical properties of plastics. The standard defines two test methods (Method A and Method B) to measure the storage (G') and loss (G'') components of the torsional modulus as a function of temperature for small deformations within the frequency range 0.1 Hz to 10 Hz. Temperature-dependent measurements (for example −50 °C to +150 °C for many commercial plastics) reveal polymer transition regions such as the glass transition, melting transitions and the onset of plastic flow.

Key topics and requirements

• Test scope: Linear dynamic mechanical properties of plastics (not suitable for rubbers; see ISO 4664-2) and not for non‑symmetrical laminates (see ISO 6721-3).
• Two pendulum arrangements:
  • Method A: inertial element suspended from specimen; lower end excited.
  • Method B: inertial element suspended from a wire/counterweight; upper end excited (wire becomes part of the oscillating system).
• Frequency and temperature: Measurements over 0.1–10 Hz; temperature runs can use a single inertial element (frequency varies with temperature) or interchangeable inertial elements to keep frequency approximately constant.
• Apparatus essentials:
  • Inertial element with recommended moment of inertia (≈ 3 × 10^−5 kg·m^2 for standard specimens; higher for filled polymers).
  • Self-aligning clamps, thermally non‑conducting connecting rod, temperature‑controlled chamber, gas supply and accurate temperature measurement (per ISO 6721-1).
  • Oscillation-inducing device limited to ~1.5° amplitude (3° for low‑modulus materials).
  • Frequency and amplitude recording accurate to ±1% (±5% within transition region).
• Specimen geometry (recommended): free length 40–120 mm (preferably 50 mm); width 5–11 mm (preferably 10 mm); thickness 0.13–2 mm (preferably 1 mm).
• Results: Calculation of logarithmic decrement and conversion to torsional storage modulus G' and loss modulus G'' to characterize viscoelastic behavior.

Applications - who uses this standard

• Materials scientists and polymer engineers assessing temperature-dependent viscoelasticity of plastics.
• Quality control and R&D teams evaluating transition temperatures, damping properties and the onset of flow.
• Test laboratories performing standardized dynamic mechanical analysis (torsional mode) for material selection, product development and failure analysis.
• Manufacturers of plastics, composites and laboratory test equipment specifying apparatus and procedures to ensure reproducible torsional modulus data.

Related standards

• ISO 6721-1 - General principles for determination of dynamic mechanical properties
• ISO 6721-3 - Bending methods (non‑symmetrical laminate considerations)
• ISO 4664-2 - Methods for testing rubbers (when plastics methods are not applicable)

Keywords: ISO 6721-2, torsion-pendulum method, plastics dynamic mechanical properties, torsional modulus, storage modulus G', loss modulus G'', temperature dependence.