Overview
ISO 20965:2021 - Plastics - Determination of the transient extensional viscosity of polymer melts - specifies a standardized method to measure the transient extensional viscosity (also called the tensile stress growth coefficient) of polymer melts. The method stretches melt specimens uniaxially at a constant Hencky strain rate and constant temperature to determine time- and strain-dependent extensional behaviour. Typical test conditions covered include Hencky strain rates from about 0.01 s–1 to 1 s–1, Hencky strains up to approximately 4, temperatures up to about 250 °C, and transient extensional viscosity ranges around 10^4 Pa·s to 10^7 Pa·s.
Key Topics and Requirements
- Measurement principle: Constant strain-rate uniaxial extension with simultaneous force and deformation measurement to derive net tensile stress and transient extensional viscosity.
- Strain representation: Uses Hencky (true) strain and Hencky strain rate as primary descriptors.
- Apparatus types: Defines four instrument configurations (rotating clamps - Types A/B, translating clamps - Types C/D) and specifies requirements for force, strain and strain-rate determination.
- Temperature control: Allows silicone oil bath or temperature-controlled chamber (forced gas) for isothermal testing; notes support/ buoyancy issues and possible oil absorption.
- Specimen handling: Sampling, specimen preparation, mounting, and pre-conditioning procedures are described to ensure reproducibility.
- Calibration & accuracy: Instrument calibration, force sensor resolution and specimen shape retention are highlighted as limits on the lowest measurable viscosities; annexes discuss uncertainty and checking for specimen swelling.
- Analysis & reporting: Procedures for calculating tensile stress growth coefficient, data analysis for different instrument types, precision and required test report contents.
Practical Applications
- Characterizing extensional rheology for polymer formulation and compounding decisions (e.g., melt strength, strain-hardening behaviour).
- Supporting process design and optimization for plastic processing operations that involve stretching flows (film blowing, fiber spinning, thermoforming, blow molding).
- Quality control and verification of material consistency during production and R&D comparisons between grades or additives.
- Instrument manufacturers and calibration laboratories use the standard to validate extensional rheometers and test methods.
Who Should Use ISO 20965:2021
- Polymer rheologists and materials scientists measuring extensional viscosity and viscoelastic response.
- R&D teams developing polymers and additives where melt strength and extensional behaviour matter.
- Quality/control laboratories in plastics processing and compound manufacturing.
- Test laboratories and instrument vendors seeking compliance with an international extensional rheology method.
Related Standards
- ISO 472, Plastics - Vocabulary (normative reference used in ISO 20965:2021)
- Standards and guidance from ISO/TC 61 (Plastics) are relevant for broader plastics property testing and terminology.
Keywords: ISO 20965:2021, transient extensional viscosity, polymer melts, Hencky strain, extensional rheology, melt strength, extensional viscosity testing, silicone oil bath.