Polyamide MFI Measures Your Dryer, Not Your Polymer
Polyamide purchasing specifications often contain the line "melt flow index (MFI) — no more than X g/10 min". Suppliers send commercial offers, two of them show a "matching" MFI — yet on the production floor the two materials behave differently: one holds the profile in extrusion, the other runs. Not because anyone lied. The reason is that for polyamide a single MFI point is a metric with a large built-in error: the test result depends on the residual moisture of the specimen, so without a fixed preparation protocol, comparing two data sheets by this number is unreliable. The industry has long measured polyamides by a different value — the viscosity number per ISO 307. This article explains what exactly breaks down in MFI on polyamide, what VN is, and how to rewrite the specification requirement so that two commercial data sheets can be compared honestly.
Every numeric value below is either referenced to a standard or a public manufacturer document, or marked "typical for the class — verify against the TDS of the specific grade". This is an article about a specification method, not about pricing.
What ISO 1133 Actually Measures
The method is ascetically simple. Pellets are poured into a heated barrel, a piston with a load is placed on top, and after a dwell time the melt is extruded through a standard capillary. You weigh how many grams came out in 10 minutes — that is MFR (mass flow rate, g/10 min); if volume is measured — MVR (cm³/10 min). Test conditions are tied to the material: for polyamides in European data sheets a typical condition is, for example, MVR at 275 °C and a 5 kg load, as in the Ultramid® product range.
Note three things built into the design of the method:
- it is a single point — one temperature and load mode, one shear rate (a low one: in the plastometer capillary — on the order of units to tens of s⁻¹, depending on material and load; in the nozzle and runners of an injection molding machine — several orders of magnitude higher);
- the specimen sits in the hot barrel for minutes — and all that time chemistry is happening in it, if the material has anything to react with;
- the result is integral: it does not distinguish whether the polymer "flows" because of shorter chains or because the formulation has changed.
For polyethylene or polypropylene these assumptions work: polyolefins do not hydrolyze, and time in the barrel changes almost nothing. MFI grew up as a working metric of exactly that chemistry. Polyamide is a different chemistry.
Why the Number Drifts on Polyamide: Hydrolysis in the Instrument Barrel
The amide group is a reversible product of a polycondensation reaction. In the melt at processing temperatures (typical for the class — around 250–300 °C, verify against the TDS of the grade) the equilibrium is alive: excess moisture shifts it toward hydrolysis — chains break, molecular mass drops, melt viscosity decreases, and the instrument honestly shows a higher MFI. The mechanism is well described in the polyamide processing literature: the more moisture in the pellet, the deeper the drop in molecular mass during the residence time in the melt. A very dry pellet under a long dwell can shift the equilibrium the opposite way — post-condensation slowly builds the chain up. The direction of causality is stable both ways; the specific numbers depend on grade, moisture, and time.
The practical consequence: polyamide MFI is largely a measurement of the specimen state, not of the polymer. Two samples of the same batch, dried differently, will give different numbers. Two data sheets with "the same MFI" may have been taken on specimens with different residual moisture — and then matching numbers do not mean matching material.
This is not our editorial thesis — it is the position of the standards system itself. For materials sensitive to moisture and thermal history there is a separate part of the standard — ISO 1133-2: it normalizes the barrel temperature and the time the material spends at that temperature more strictly, precisely to reduce the scatter of results. And the interlaboratory comparison of MFR measurements of moisture-sensitive plastics published in Polymer Testing became part of the rationale for that standard: without strict control of specimen preparation, the reproducibility of the method on such materials is unsatisfactory. If measuring "simple" flowability correctly requires a separate standard with minute-level tolerances — that is the diagnosis of the metric as applied to polyamides.
Viscosity Number per ISO 307: How the Industry Really Sorts Polyamides
The industry found its standard answer to this problem long ago: do not measure polyamide in the melt at all. ISO 307 defines the viscosity number (VN) of a dilute polyamide solution: a weighed sample is dissolved in 96% sulfuric acid (or 90% formic acid, or m-cresol) at a concentration of 0.005 g/ml, and the flow times of the solution and the pure solvent through a capillary viscometer at 25 °C are compared. No melt — no hydrolysis during the measurement. The influence of specimen moisture is minimized by the method itself: the standard normalizes the moisture of the weighed sample (over-drying the specimen or correcting the weight for measured moisture), so the result is not distorted by the state of the pellet.
VN is classical dilute-solution viscometry, i.e., a direct function of the polymer's molecular mass. That is why it works as the sorting metric of a product line: in the public Ultramid® range overview the PA6 and PA66 grades are arranged by VN in a range of roughly 130 to 325 cm³/g — from easy-flowing injection grades to high-viscosity extrusion ones. A longer chain — higher melt viscosity, higher melt strength in extrusion, usually better part impact toughness; a shorter one — easier filling of thin walls. It is the same information the buyer is trying to extract from MFI, only measured in a way that does not destroy what is being measured.
Two important notes on the method worth knowing when reading data sheets:
- the solvent must be stated. The VN of one grade in sulfuric and in formic acid are different numbers; and additives (flame retardants, modifiers) can shift the result upward in one solvent and downward in the other, depending on the specific additive and its content — this is explicitly cautioned in the text of ISO 307. Only VN values in the same solvent can be compared;
- the method is not applicable to crosslinked and anionically polymerized polyamides (cast caprolactam) — they do not dissolve completely.
Tellingly, even the Soviet standards system arrived at the same conclusion: GOST 11034 "Polyamides. Method for determination of viscosity number" is a direct relative of ISO 307, and it was the viscosity number, not "flowability", that the industry documents on polyamides normalized. We took a detailed look at that lineage in the article on GOST 17648 and the mythical "SV30 analog".
Filled Compounds: One Point Does Not Describe a Shear-Thinning Fluid
For glass-filled grades the story is even harsher. A PA GF melt is markedly shear-thinning: its viscosity falls as the shear rate grows. The ISO 1133 plastometer works in the low-shear-rate zone; the nozzle, runners, and thin walls of the mold are several orders of magnitude higher. Two grades can coincide at the MFI point and diverge exactly where mold filling is decided — or vice versa.
So when the question is the processability of a compound in a specific process, the correct instrument is capillary rheometry per ISO 11443: the "viscosity — shear rate" curve in the working range (the standard covers shear rates from 1 to 10⁶ s⁻¹ and viscosities from 10 to 10⁷ Pa·s). A curve is a function; MFI is one of its values at a point that most likely has nothing to do with your process.
How to Rewrite the Specification: a Table of Methods
| Metric | Standard | What it physically measures | Why it is critical for PA |
|---|---|---|---|
| MFR / MVR (MFI) | ISO 1133-1; for moisture-sensitive materials — ISO 1133-2 | Mass/volume of melt through a capillary in 10 min, one low-shear point | The result is sensitive to residual specimen moisture: hydrolysis in the barrel shifts the number. Comparability between labs without a strict preparation protocol is weak |
| Viscosity number VN | ISO 307 | Viscosity of a dilute solution at 25 °C → a direct function of molecular mass | Measurement without melt — pellet moisture does not distort the result. Compare only within one solvent (H₂SO₄ ≠ HCOOH) |
| Viscosity curve | ISO 11443 | Viscosity as a function of shear rate (1…10⁶ s⁻¹) at the working temperature | The most correct way to compare filled grades in the shear range of the real process rather than at a single point |
| Pellet moisture | gravimetric/coulometric method per the manufacturer's TDS | Residual moisture before processing | Without fixing moisture, any "flowability comparison" compares dryers, not polymers. A typical requirement before molding PA is below ~0.1% (typical for the class — verify against the TDS of the grade) |
The specification formula comes out as follows. Instead of "MFI no more than X g/10 min" — "viscosity number per ISO 307 in [the stated solvent] within a range from… to…"; for filled grades and sensitive processes — additionally "a viscosity curve per ISO 11443 at the working temperature" or an agreed trial molding. If MFI still remains in the contract (for example, for compatibility with legacy record cards) — fix the protocol: specimen drying, residual moisture, time in the barrel, and require measurement per ISO 1133-2, not -1.
And the mirror conclusion for reading other people's data sheets: if the main molecular-mass metric in a polyamide supplier's specification is MFI alone, with no VN and no specimen preparation protocol — that is a frequent sign the specification was carried over from a polyolefin template. On how to read the rest of the data sheet lines — a separate breakdown: how to read a polymer TDS.
Expert review: 5 questions
1. Our specification has said "MFI, 2.16 kg" for PA6 for years — and supply somehow worked. Why change anything? It worked while the material came from a single source: within one grade and one laboratory, MFI as an internal trend indicator is perfectly alive. It breaks down the moment you compare two suppliers: numbers taken on specimens with different residual moisture and different barrel dwell times are not comparable with each other. If the specification is used to qualify alternative material — that is exactly when the requirement should be moved to VN per ISO 307, otherwise the decision is made on noise.
2. Why do manufacturers print MVR in polyamide TDS at all, if the metric is so weak? Because it is a quick processing reference and a tribute to market habit: MVR at 275 °C / 5 kg gives a first impression of whether a grade is "injection or extrusion", and it takes minutes to measure, not hours. Note how the major manufacturers do it: in their data sheets MVR stands next to VN, while the sorting of the line and the grade designations are built around solution viscosity. MVR is a reference number under controlled specimen preparation in the manufacturer's lab; it is not designed as an acceptance criterion between parties.
3. One supplier gives VN in sulfuric acid, another in formic. How do we compare? Carefully: these are different scales. For neat polyamides without interfering additives, ISO 307 provides conversion relations between solvents (an annex to the standard); but for filled, stabilized, and flame-retardant grades the conversion is incorrect — additives shift the result upward in one solvent and downward in the other, depending on the additive. The reliable option: ask one of the suppliers to provide the value in the second solvent (most have both) or send both materials to one laboratory for measurement under a single protocol.
4. We extrude a PA6 profile. What VN should we ask for? We will not name a number — the task class will: extrusion needs a higher molecular mass than injection molding, because the melt must hold its shape between the die and the calibrator. The practical path: take the VN of a grade that already runs stably in your process as a reference point, and specify a range around it rather than an absolute from someone else's data sheet. If there is no reference grade — select from the upper part of the manufacturer's VN scale with a trial batch; our specialist will help match Examid® line grades to the task.
5. Can polyamide MFI be useful at all? Yes, in two roles. The first is internal stability control of your own process: with a fixed dryer, a fixed protocol, and one grade, the batch-to-batch MFI trend signals degradation or material substitution (that is what ISO 1133-2 with its strict tolerances is designed for). The second is rough sorting of recycled feedstock, where VN is often not even on the table. What MFI cannot do is serve as a bridge for comparison between two data sheets from different polyamide manufacturers.
What to Check Before a Production Batch
- VN is declared, with the solvent. The specification and the batch CoA must contain the viscosity number per ISO 307 and the stated solvent. "MFI with nothing else" is a reason to ask questions.
- Pellet moisture at intake. Measure residual moisture before processing; the drying requirement is per the grade's TDS (a typical reference for molding PA is below ~0.1%, typical for the class).
- If MFI is in the contract — the protocol is fixed. Method (ISO 1133-2), specimen preparation, temperature, load, time in the barrel — otherwise the number will be impossible to reproduce or contest.
- For filled grades — a curve or a trial molding. A single flowability point does not guarantee filling of a thin-walled part; agree on an ISO 11443 curve at the working temperature or a test batch on your mold.
- Batch traceability. A CoA for every batch with the actual VN, not a "typical" one; batch-to-batch deviation is a separate specification line.
Material Wizard Grades: Entry Points into the Group
The logic above is not about one grade — it is about how to specify any polyamide. Below are three different points of the PA6 line where the "VN instead of bare MFI" requirement works differently: a reinforced grade for series molding, an unfilled base grade, and a high-flow grade for thin walls. Full navigation of the group is on the polyamides hub.
Summary
MFI is an honest method for the chemistry it was created for. Polyamide in the melt lives a life of its own: residual specimen moisture shifts the result so much that the number often says more about the dryer than about the polymer — which is exactly why moisture-sensitive materials needed a separate ISO 1133-2. The industry benchmark of polyamide molecular mass is the viscosity number per ISO 307, measured in solution without any melt; for filled compounds in sensitive processes — the viscosity curve per ISO 11443. The "MFI ≤ X" requirement for PA is an inherited polyolefin template; the "VN within a range, solvent stated" requirement is a specification you can use to compare offers and accept batches.
See also: How to read a polymer TDS · What is polyamide 6 · How much water does PA6 really absorb · PA610-DS, not SV30: what GOST 17648-83 actually requires · hub: Examid® polyamides.
Material Wizard manufactures and supplies Examid® engineering polyamides, provides a TDS and a per-batch CoA, and helps convert legacy "MFI-based" specifications into correct VN requirements — with an incoming inspection plan for your process. The company operates from Derazhnia and Kharkiv. Examid® polyamides are available with delivery across Ukraine — please check with our specialist to verify your specification.
Standards mentioned in this article: ISO 1133-1:2022 (MFR/MVR, standard method) · ISO 1133-2:2011 (MFR/MVR for materials sensitive to time-temperature history and/or moisture) · ISO 307:2019 (viscosity number of polyamides) · ISO 11443:2021 (capillary and slit-die rheometry) · GOST 11034 (viscosity number of polyamides — the historical counterpart of ISO 307).
Sources: ISO 1133-1:2022 — determination of MFR/MVR, standard method · ISO 1133-2:2011 — method for materials sensitive to time-temperature history and/or moisture · Interlaboratory comparison of melt flow rate testing of moisture sensitive plastics — Polymer Testing, 2009 · ISO 307:2019 — viscosity number of polyamides: solvents, 0.005 g/ml concentration, 25 °C, additive effects · ISO 11443:2021 — rheometry: shear-rate range 1…10⁶ s⁻¹, viscosity range 10…10⁷ Pa·s · Ultramid® (PA) — Product Range: PA6/PA66 grades sorted by VN (ISO 307), reference MVR values
The mechanism of polyamide hydrolysis in the melt and the pre-processing drying requirement are described in publicly available technical literature on polyamide processing; specific moisture limits and drying regimes are per the TDS of the respective grade. Both diagrams in this article are mechanism illustrations without numeric values, not Material Wizard measurement results. The VN values of 130–325 cm³/g are the range of another manufacturer's public product line, given as an illustration of the grade-sorting principle; the properties of Material Wizard grades are per their TDS.