Viscosity measurement of LDPE samples with the same MFR value

Significance of the topic

The melt flow rate (MFR) test is a simple, widely used quality-control metric in the plastics industry for characterizing polymer flowability. However, MFR reports a single data point at low shear and therefore can fail to predict materials’ behavior under actual processing conditions where shear rates vary widely. This application report demonstrates how capillary (extruder) rheometry reveals differences in flow behavior between two LDPEs that share the same MFR but behave differently during manufacturing, underscoring the need for shear‑rate‑resolved rheological testing for reliable processability assessment.

Objectives and overview of the study

The study aimed to investigate why two low‑density polyethylene (LDPE) grades with identical MFR values (4.0 g/10 min) showed different performance in production. Using an extruder capillary rheometer setup, the authors measured the viscosity function across a range of apparent shear rates to determine whether the materials truly had the same rheological behavior beyond the single MFR point.

Materials and test conditions

The key sample and test conditions reported:

• Polymer: LDPE; two commercial grades from different suppliers, both labelled MFR = 4.0 g/10 min.
• Extruder temperature profile: feed/center/nozzle zones 180°C / 240°C / 280°C.
• Melt pump and downstream temperatures: melt pump 280°C; die 280°C.
• Extruder screw speed: 100 rpm; melt pump speed varied from 5 to 60 rpm to set different volume flows.
• Extruder feed zone: liquid cooled.
• Capillary (slit) die dimensions: 0.8 mm by 2.0 mm.

Used instrumentation

The experimental setup and instrumentation included:

• Torque rheometer system: Thermo Scientific HAAKE PolyLab OS Torque Rheometer with HAAKE RheoDrive 7 OS drive unit.
• Capillary analysis software: Thermo Scientific HAAKE PolySoft OS Capillary Software.
• Laboratory single‑screw extruder: Rheomex 19/25 OS with screw length L = 25 × D and compression ratio 2:1.
• Melt pump with bypass valve to control flow to the capillary die.
• Melt‑pressure sensors to record pressure drop across the capillary.

Methodology

Polymer pellets were melted and homogenized in the extruder and the melt was routed through a bypass valve to a metering melt pump that produced defined volumetric flows (Q). For each pump speed the apparent shear rate was calculated from the slit capillary geometry (width W, height H) and Q. Pressure transducers measured the pressure drop (Δp) across the capillary; from geometry and pressure drop the shear stress (τ) was computed. Apparent viscosity (η) was then obtained as η = τ / (apparent shear rate). By stepping the melt pump speed from low to high, the investigators obtained a viscosity vs. shear‑rate curve (viscosity function) for each LDPE grade.

Results and discussion

Measured viscosity functions of both LDPE samples coincided at low apparent shear rates, consistent with the identical MFR values that reflect low‑shear behavior. However, as shear rate increased the viscosity curves diverged: one material exhibited significantly different shear‑thinning behavior compared with the other. These high‑shear differences are directly relevant for processing operations such as extrusion and melt pumping where shear rates are typically much higher than those sampled by the MFR test. The capillary rheometer therefore exposed rheological distinctions that the MFR single‑point measurement could not resolve, explaining the practical processing issues observed with one of the grades.

Key findings and implications

• MFR provides a single, low‑shear reference point and may not reflect material performance under real processing shear rates.
• Capillary rheometry using an extruder/melt‑pump arrangement yields a full viscosity function and can reveal differences in shear‑dependent viscosity and shear‑thinning behavior between materials with identical MFR.
• Detecting these differences allows proactive selection or modification of grades to avoid production problems related to melt viscosity at processing shear rates.

Benefits and practical applications of the method

The extruder capillary rheometer approach delivers several practical benefits:

• Better processability prediction: viscosity versus shear‑rate data help anticipate behavior in extrusion, injection molding, and blow molding.
• Improved quality control: discrimination between nominally similar grades enables more informed supplier selection and incoming inspection protocols.
• Troubleshooting: identifying rheological causes of production defects or instability supports targeted formulation or processing adjustments.

Future trends and potential applications

Opportunities to extend and modernize rheological characterization include:

• Integration with in‑line and at‑line sensors to monitor rheology in real time during manufacturing.
• Expansion to extensional rheometry or combined shear/extensional tests to emulate complex flows encountered in processing.
• Use of rheological fingerprints in digital twins and process models to predict behavior across processing windows.
• Application of machine learning to correlate full viscosity functions with processing outcomes and optimize grade selection.
• Development of standardized protocols linking capillary rheometry metrics to specific processing performance criteria.

Conclusion

This application report demonstrates that two LDPEs with identical MFR can exhibit markedly different viscosity behavior at processing‑relevant shear rates. Single‑point MFR testing is insufficient to capture such differences; capillary rheometry using an extruder and melt pump provides the shear‑rate‑resolved viscosity function necessary to predict processing performance and avoid operational issues. Implementing shear‑resolved rheological testing in QC and formulation workflows improves reliability in polymer processing.

References

• Jährling M. Application report LR33: Viscosity measurement of LDPE samples with the same MFR value. Thermo Fisher Scientific; 2024.