Can a Melt Flow Indexer Be Used to Measure Intrinsic Viscosity?

Q: Can I use a melt flow indexer to directly measure the intrinsic viscosity (IV) of a polymer?

No — a melt flow indexer (MFI) does not directly measure intrinsic viscosity. Instead, it measures melt flow rate (MFR) or melt volume rate (MVR), which describe how easily a polymer melts and flows under heat and pressure. Intrinsic viscosity, by contrast, is measured in solution and reflects the molecular weight of the polymer.

Q: How are MFR and intrinsic viscosity related?

While they are different tests, there can be a correlation between MFR and intrinsic viscosity, especially for homogeneous polymer types like PET, PLA, or nylon. Typically:

• High MFR (faster flow) → Lower IV → Lower molecular weight
• Low MFR (slower flow) → Higher IV → Higher molecular weight

This relationship can be empirically calibrated. By testing a series of polymer samples with known IVs and measuring their MFRs, you can create a calibration curve.

Q: What does a calibration curve look like?

Here is a simplified example of a typical inverse relationship between MFR and IV (note the log scale on the x-axis):

This allows MFR to be used as a quick proxy for IV — but only within a specific polymer type and grade family. The curve will differ significantly between materials and must be verified with regular solution-based IV measurements.

Q: Is this method suitable for quality control?

Yes – many manufacturers use melt flow indexers to monitor relative molecular weight during processing. However, for precise molecular characterization or regulatory reporting, intrinsic viscosity must still be measured using solution viscometry.

Q: Why is IV an important parameter for PET manufacturers?

Intrinsic viscosity (IV) is a critical parameter for PET (polyethylene terephthalate) manufacturers because it directly relates to the molecular weight of the polymer — which influences how the material performs during processing and in final applications.

Q: How do I use the MFR300 to correlate my MFR results to Intrinsic Viscosity?

The Method B test on the MFR300 includes algorithmic calculation of Apparent Intrinsic Viscosity (using a formula), as well as algorithmic calculation of Shear Stress (Pa), Shear Rate (l/sec) and Viscosity (Pa/sec). In the test setup, there is an “I.V. Corre. Factor” option:

Once a user has identified a reliable IV value for their material, they can run a series of Melt Flow tests on the MFR300 to see how much the Apparent IV value given in the results differs by. The % difference is the figure that needs to be entered as the I.V. Corre. Factor.

This will then allow the user to track the change in IV that occurs as Melt Flow Rate changes for the specific material in question during subsequent testing.

Q: Why is pre-drying PET essential before determining Intrinsic Viscosity on a Melt Flow Indexer, and does the testing method vary between instrument brands?

PET is highly hygroscopic, meaning it readily absorbs moisture from the air. Even small amounts of retained moisture will cause hydrolytic degradation during melt processing, lowering molecular weight and artificially reducing the measured Intrinsic Viscosity (IV). To obtain accurate, repeatable IV results, PET must be pre-dried to the manufacturer’s specified moisture content before testing. Regardless of brand, all Melt Flow Indexers use the same underlying algorithmic approach to calculate IV from melt flow data, so the preparation steps and testing methodology remain consistent across instruments.

Q: How should PET be dried before Intrinsic Viscosity testing on a Melt Flow Indexer?

PET must be thoroughly dried before testing to prevent moisture-related degradation, which can artificially lower the measured Intrinsic Viscosity. Drying should be carried out in accordance with the resin supplier’s recommendations, using suitable equipment to reach the specified moisture level. It’s important to handle dried PET in a way that avoids re-absorption of moisture — typically by transferring it directly to the instrument or storing it in sealed, desiccated conditions until use. Consistent and correct drying practices are essential for obtaining reliable and repeatable results.

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