Solvent-Free vs Solvent-Based Intrinsic Viscosity Testing: Choosing the Right Tool for PET

• July, 16 2025
• Category: Intrinsic Viscosity Analysis, Dilute Solution Viscosity, Automation

As sustainability goals push the plastics industry toward greener processes, solvent-free Intrinsic Viscosity (IV) testing – based on melt-flow analysis – has gained attention. Such methods promise fast, eco-friendly testing with results equivalent to solvent-based IV methods.

But are the results truly equivalent in all scenarios? And is the process genuinely faster when everything is considered?

Melt-flow based IV testing certainly has its place. In this article, we break down the capabilities and limitations of these systems to explain when it’s safe to rely on them, and when only an Intrinsic Viscosity Analyzer – based on solution viscosity – can deliver the accuracy, consistency, and confidence your operation demands.

MFI-Based IV Testing: What’s Really Happening?

Melt Flow Index (MFI) or Melt Flow Rate (MFR) test is a rheological method that pushes molten polymer through a die under set conditions. The flow rate of the polymer can be correlated to an estimated intrinsic viscosity by applying an empirically derived equation.

But there’s a catch, this method only works reliably under very specific and narrow conditions, being:

1. Moisture control

The MFI of PET changes drastically with drying quality, affecting IV estimates. PET absorbs moisture from the atmosphere, and even trace levels degrade the polymer chains during melt testing.

The MFI-based test will only give the same IV as the solvent-based method if all moisture is removed from the polymer before testing. Once completely dry, the polymer needs to be transferred to the MFI tester in a way that ensures that the polymer does not come into contact with any external air.

❗ Insufficient drying is the most common problem, which leads to low reproducibility of results. But drying the sample for too long will result in the sample starting to polymerize.

✅ Solvent-based methods are not influenced by moisture.

2. Thermal Sensitivity

PET is sensitive to thermal degradation, so the melt temperature and residence time must be accurately controlled to minimize sample degradation.

❗ Degradation during the melt process will lead to artificially low IV values.

✅ Solvent-based IV avoids thermal stress by operating at ambient or moderate temperatures.

3. Grade-Specific Calibration

Solvent-free testers do not measure IV directly but infer it through a correlation with MFI. This MFI-IV correlation must be reestablished for every new PET grade, blend, or additive. Each formulation requires a new calibration curve, which in turn requires obtaining the Intrinsic Viscosity of the reference samples by solvent-based methods.

❗ This limits flexibility and introduces uncertainty for R&D and diverse feedstock environments (recycled PET).

✅ Solvent-based methods measure the actual intrinsic viscosity – no calibration needed.

“Plug and Play?”: The Hidden Costs of MFI-Based IV Testers

Despite claiming “fast” test cycles, MFI-based IV testers require additional sample conditioning steps. To make an educated decision, the following hidden costs should be taken into consideration:

1. Sample Conditioning Requirements

Moisture significantly affects melt-flow rates in PET. To achieve consistent results, PET pellets must be thoroughly dried for several hours depending on pellet size and moisture content. This requires additional equipment like vacuum dryers and cryogenic grinders not typically included in the tester’s base package.

❗ Solvent-free systems are slower than automated solvent-based IV analyzers once the sample preparation time is factored in.

✅ Automated solvent-based IV systems don’t require grinding or drying equipment, and can complete the full test cycle – including dissolution, analysis, and cleaning – in around 45 minutes for 1 sample.

2. Equipment Cleaning

The barrel, die, and other components of the MFI tester that are in contact with the molten polymer require manual cleaning of polymer residue at high temperatures after each analysis.

❗ Health and safety hazards and added operator time.

✅ Automated solvent-based systems minimize operator input. The entire process – from solvent dispensing to instrument cleaning – is done by the instrument.

Solvent-free vs solvent-based IV: it all depends on the application.

When solvent-free MFI testers work well:

• For Quality Control in stable production of virgin PET bottle and preform grades, where moisture is well-controlled and grade consistency is high.
• When precision isn’t critical, and solvent-free operation is prioritized.

When a solvent-based IV instrument is essential:

• Recycled, blended, or filler-containing PET, where additives and higher variability undermine MFI accuracy, and MFI-to-IV correlation breaks down.
• R&D, new grades, or copolymers, where MFI-to-IV correlation curves don’t apply “off the shelf” and detailed viscosity behavior is needed.
• QC labs with critical and fine-tolerance specs.
• Cross-lab reproducibility, IV values from solvent-based methods are standardized and internationally recognized (i.e. ISO 1628-5).

Conclusion: The Right Tool for the Job

While solvent-free IV testing offers value in tightly controlled environments, it’s not universally applicable. For R&D, recycled feedstock, or any application where precision, reliability, and compliance matter, solvent-based Intrinsic Viscosity remains the gold standard.