Extensional Rheometer for Polymer Melt Elongation

What is the Extensional Rheometer for Polymer Melt Elongation?

An extensional rheometer stretches a polymer melt sample between rotating or moving fixtures to measure elongational viscosity and strain hardening. It is used where fibre spinning, blow moulding or film blowing performance must be predicted from melt behaviour.

Overview

Extensional flow dominates in fibre spinning, film blowing, blow moulding and foaming, and it cannot be derived from shear data alone. Instruments generally fall into two families: fixtures mounted on a rotational rheometer that wind a rectangular strip between counter-rotating drums, and dedicated devices with opposed pistons or belts that stretch the sample at a controlled strain rate. Both measure the tensile force and the sample dimensions to calculate transient elongational viscosity against time, revealing strain hardening, a property strongly influenced by long-chain branching. Strain rates span roughly 0.001 to 20 s⁻¹ with Hencky strain up to about 4, and sample thickness must be uniform because the calculation assumes a rectangular strip of constant initial dimensions and a constant volume during stretching.

How does it work?

A rheometer applies a controlled deformation to a polymer sample and measures the resulting force or torque. Rotational instruments use parallel plates, cones or concentric cylinders and can work in steady shear, giving shear viscosity against shear rate, or in oscillation, giving storage and loss moduli against frequency. Capillary instruments force melt through a die of known diameter and length at a controlled piston speed, measuring pressure to calculate apparent shear viscosity at shear rates that reach those of real extrusion. Torque rheometers mix material in a heated chamber and record the torque needed to turn the rotors, which tracks gelation, fusion and degradation in a compounding context. The choice depends on the shear rate range, the sample form and whether elasticity or only viscosity is required.

What are the key features?

• Counter-rotating drum fixtures or opposed piston design
• Transient elongational viscosity measurement
• Strain hardening and Hencky strain evaluation
• Strain rates from 0.001 to 20 s⁻¹
• Sample strip moulding accessories
• Temperature control up to 300 °C

What are the technical specifications?

A typical extensional rheometer for polymer melt elongation configuration offers strain rate range of 0.001–20 s⁻¹, maximum Hencky strain of About 4, force range of 0.001–20 N, force resolution of 0.1 mN, temperature range of Ambient to 300 °C and sample strip dimensions of Approx. 20 × 10 × 0.7 mm. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Extensional Rheometer for Polymer Melt Elongation is used in fibre spinning and film blowing formulation work, strain hardening evaluation of branched polyolefins, blow moulding grade comparison and foaming processability assessment. Across the wider Rheometers class, laboratories also deploy it for molecular weight and molecular weight distribution comparison, processing behaviour prediction for extrusion and moulding and filler and additive effect on melt viscosity.

Which standards and certifications apply?

Relevant standards and methods include ISO 3219, ISO 11443, ISO 6721-10, ASTM D3835, ASTM D4440, ASTM D2538 and IEC 61010-1. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

Sample preparation dominates repeatability: dry hygroscopic polymers, compression-mould discs of defined thickness for rotational testing, and ensure filled samples are homogeneous. Set the gap and check it with a zero-gap routine before each series, because a small gap error changes the calculated viscosity. In capillary work, verify piston speed and pressure transducer calibration, and apply the Bagley and Rabinowitsch corrections when true viscosity is required. Purge the oven with nitrogen for high-temperature or oxidation-sensitive polymers. Clean dies and plates thoroughly, since degraded residue changes the surface and the measured torque, and record temperature, geometry and gap with every result.

How do I select the right Extensional Rheometer for Polymer Melt Elongation?

Choose extensional testing when shear data cannot explain differences in spinning, blowing or foaming performance, which is typical for branched polyolefins and for recycled streams of mixed origin. Confirm whether the fixtures mount on an existing rheometer or require a stand-alone unit, and check the strain rate window against your process, since film blowing operates at high rates while foaming is often studied at low rates.

How does it compare with other Rheometers variants?

Match the instrument to the shear rate range that governs your process. Rotational rheometers cover low shear rates and are the only option when viscoelastic data, yield stress or normal force are required, making them suitable for molecular weight comparisons and additive studies. Capillary rheometers reach the high shear rates of extrusion and injection moulding and are more tolerant of filled and highly viscous materials. Torque rheometers are used when the question concerns compounding behaviour, gelation or thermal stability under shear rather than a fundamental viscosity value. Inline instruments are selected for continuous process monitoring and never replace laboratory characterisation, because they see only one fixed measurement point.