1. Home
2. All Testing
3. Rheology
4. Capillary Rheology / Capillary Rheometry

Capillary Rheology / Capillary Rheometry Testing of Materials

Significance and Purpose

Capillary Rheology / Capillary Rheometry is a key technique for characterizing the flow behavior of polymer melts and other high-viscosity materials under shear and extensional conditions. This technique simulates real-world polymer processing conditions, such as extrusion and injection molding, by forcing a material through a capillary die and measuring the pressure, flow rate, and resulting rheological properties.

Capillary rheology / capillary rheometry is essential for:

• Material Characterization: Understanding flow behavior under high shear and extensional stresses.
• Processing Optimization: Identifying material behavior relevant to processes such as extrusion, injection molding, and film blowing.
• Material Development: Designing polymers, composites, and other materials with tailored flow and elasticity properties.
• Quality Control: Ensuring batch-to-batch consistency in melt viscosity and elasticity.
• Failure Analysis: Detecting instabilities such as melt fracture, die drool, or flow-induced defects in polymers during processing.

Relevant ASTM and ISO Standards

The technique is standardized under the following:

• ASTM D3835: Standard Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer.
  • Focuses on measuring the flow properties of polymer melts, including viscosity, flow stability, and melt elasticity.
• ISO 11443: Plastics — Determination of Fluidity of Plastics Using Capillary and Slit-Dies.
  • Addresses broader applications, including extensional flow and high-shear behavior, in both round and slit-die configurations.

These standards provide detailed guidelines for instrument setup, testing procedures, and data corrections to ensure consistent and reliable results.

DatapointLabs Tests for Capillary Rheology / Capillary Rheometry

Tests in the DatapointLabs test catalog that reference capillary rheology testing / capillary rheometry testing are as follows:

Capillary Rheology / Capillary Rheometry Testing Specific to Plastics, Filled Plastics, Adhesives, Coatings

Test	Test Description	Standards
D-015	Melt Density	ASTM D3835 / ISO 11443
R-011	Capillary Rheology (1 Temperature)	ASTM D3835 / ISO 11443
R-012	Capillary Rheology with Bagley Correction (1 Temperature)	ASTM D3835 / ISO 11443
R-013	Very High Shear Capillary Rheology (1 Temperature)	ASTM D3835
R-014	Cogswell Extensional Viscosity	ASTM D3835 / ISO 11443
R-015	Thermal Flow Stability	ASTM D3835
R-016	Mooney Slip	ASTM D3835
R-018	No-Flow Temperature	DPL R-018N*
R-019	Rheotens Melt Elasticity	DPL R-019N*
R-030	Die Swell Measurements by Laser Micrometry	ASTM D3835
R-032	Static Die Swell/Sag by Laser Micrometry	ASTM D3835
R-035	Slit-Die Rheology (1 Temperature, 1 Die)	ASTM D3835
R-042	Peldom Elongational Viscosity	ASTM D3835

* Internal DatapointLabs Standard

Principle of Operation

The capillary rheology process works as follows:

1. Melting the Sample: A small amount of polymer or viscous material is melted in the barrel of the capillary rheometer.
2. Extrusion through a Capillary or Slit Die: The material is forced through a narrow die (round or slit geometry) at controlled piston speeds to generate a specific flow rate or shear rate.
3. Measurement of Pressure and Flow Rate: A pressure transducer measures the force required to extrude the material, and the flow rate is calculated based on piston movement.
4. Viscosity Calculation: Using the measured pressure drop and flow rate, the apparent shear viscosity is calculated. Corrections (e.g. Bagley, Weissenberg-Rabinowitsch, Mooney slip) are applied to account for non-idealities.
5. Extensional Flow: Special dies or methods (e.g. Cogswell, PELDOM) are used to evaluate extensional viscosity and melt elasticity.
6. Die Swell Measurement: The diameter or width of the extrudate is measured using laser micrometry to assess elastic recovery.

Typical Procedure

1. Sample Preparation:
   • Condition the sample (e.g. drying for hygroscopic materials).
   • Load a representative sample (typically pellets or powders) into the rheometer barrel.
2. Instrument Setup:
   • Select the appropriate capillary or slit-die geometry.
   • Calibrate the instrument using standard reference materials, if necessary.
3. Test Conditions:
   • Heat the sample to the target temperature (typically 150–400 °C for polymers).
   • Define a range of shear rates (e.g. 10–10,000 s⁻¹ by adjusting the piston speed.
4. Data Collection:
   • Measure pressure drop across the die and the volumetric flow rate.
   • Collect data at different shear rates and temperatures.
5. Data Analysis and Corrections:
   • Apply corrections for entrance/exit effects (Bagley correction), shear-thinning behavior (Weissenberg-Rabinowitsch correction), and wall slip (Mooney slip).

Specimen Types

Specimens used by DatapointLabs in capillary rheology testing / capillary rheometry testing are as follows:

Specimen Type	DatapointLabs Test IDs
Pellets [Details]	R-020, R-021, R-022, R-023, R-024, R-025, R-026

Characterization Measurements

Melt Density (ρ_melt)

• Melt density (ρ_melt) is calculated from the flow rate and volumetric displacement of the piston at the test temperature.
• Important for converting volumetric flow rates into shear rates and for calculating extensional viscosity.

Melt Flow Stability

• Monitors pressure and flow rate fluctuations during extrusion to detect melt instabilities (e.g. melt fracture, die drool).
• A stable melt flow is critical for high-quality polymer processing.

Shear Viscosity (η) vs. Shear Rate (γ̇)

• Shear Viscosity (η):
  $\mathrm{\eta }=\frac{\mathrm{\tau }}{\dot{\mathrm{\gamma }}}=\frac{(\mathrm{P}\bullet \mathrm{R})}{2\mathrm{L}\bullet \dot{\mathrm{\gamma }}}$

  where τ is shear stress, γ̇ is shear rate, P is pressure, R is die radius, and L is die length. The following corrections are typically applied to derive the true shear rate from the apparent shear rate:

  • Weissenberg-Rabinowitsch Correction: Adjusts the apparent shear rate for shear-thinning (pseudoplastic) materials for which the shear rate at the wall is higher than would be given by a parabolic (Newtonian) velocity profile.
  • Bagley Correction: Adjusts the apparent wall shear stress to account for entrance and exit pressure losses in the die.
  • Mooney Slip: Adjusts the apparent shear rate to correct for slip at the die wall when polymer chains do not fully adhere to the surface, particularly at high shear rates.

The corrected data enables accurate modeling of shear-dependent viscosity behavior.

True Viscosity  (η) vs. Shear Rate (γ̇) Under Slit-Die Configuration

• Slit dies allow the calculation of true viscosity using a rectangular flow geometry, enabling measurements for processes like film extrusion.
• Ideal for studying planar deformation and high-shear processing conditions.

Shear Viscosity (η) at Very High Shear Rates

• Capillary rheology excels at measuring viscosities at extremely high shear rates (e.g. >10⁴ s⁻¹), relevant to injection molding and extrusion.

Extensional Viscosity (η_E)

• Extensional viscosity (η_E) quantifies resistance to stretching flows, which are critical for processes such as blow molding and film stretching.
  • Cogswell Method: Calculates extensional viscosity from pressure drop in a converging die.
  • PELDOM Method: Uses a hyperbolic die to directly measure elongational flows.

Rheotens Melt Elasticity

• A Rheotens device is used to measure the tensile force required to stretch a polymer melt extrudate.
• Provides insights into melt strength and elasticity, crucial for film blowing and fiber spinning.

Die Swell via Laser Micrometry

• Die Swell Ratio (DSR):
  $\mathrm{DSR}=\frac{\mathrm{Extrudate\; Diameter}}{\mathrm{Die\; Diameter}}$
  Measured using a laser micrometer, die swell quantifies the elastic recovery of the melt.
• Die Swell vs. Shear Rate: Indicates elastic recovery after extrusion.

Typical Data Reported (see test descriptions for exact details)

• Shear Viscosity (η): As a function of shear rate (γ̇), with corrections.
• Extensional Viscosity (η_E): As a function of extensional strain rate.
• Die Swell Ratio (DSR): Elastic recovery as a function of shear rate.
• Melt Density: At processing temperatures.
• Flow Stability: Pressure and flow rate consistency.
• Corrected Data: Incorporating Weissenberg-Rabinowitsch, Bagley, and Mooney slip corrections.

Suitable Material Types

• Polymers: Thermoplastics (e.g. PE, PP, PET, PVC), thermosets, and elastomers.
• Filled Polymers: Glass-filled or fiber-reinforced materials.
• Adhesives and Coatings: For flow behavior under shear.
• Biomaterials: Such as gels, pastes, and molten materials.

Suitable Applications

• Extrusion and Injection Molding: Optimizing shear flow and extensional flow.
• Film Blowing and Stretching: Studying extensional behavior and die swell.
• Material Development: Customizing flow properties.
• Quality Control: Ensuring consistency in melt behavior.
• Additive Manufacturing: Evaluating flow properties of 3D printing materials.

Conclusion

Capillary rheology / capillary rheometry is an essential technique for characterizing polymer melts and other viscous materials under high shear and extensional stresses. By providing insights into viscosity, elasticity, flow stability, and die swell, it is indispensable for material development, process optimization, and quality control in industries such as plastics, coatings, and advanced manufacturing. Adhering to ASTM D3835 and ISO 11443 standards ensures reproducible and accurate results for a broad range of applications across various industries.