How to read a black masterbatch CoA: certificate of analysis
The data sheet tells you what a grade was designed to be. The CoA tells you what is actually in the bag you received. Buyers who read the CoA properly catch drifting quality months before it becomes a defect claim. Here is what each line means — and what to do when a number moves.
By Zoe Lin, Export Manager, E-LUCK (Guangdong Yijuyuan Plastic Industry Technology Co., Ltd.)
1. CoA vs data sheet: which document answers which question
A data sheet is a promise: carbon content, melt flow, carrier resin, recommended let-down. It is written once for a grade and rarely changes. A certificate of analysis is a measurement: the results recorded for the specific lot in front of you, tied to a lot number and a test date.
When two shipments behave differently on the same line, the CoA is where you look first. If the supplier cannot produce a lot-specific CoA, you are buying on faith — and "typical values" are not a substitute.
2. Carbon black content: the number that moves your cost
Carbon content is usually measured by pyrolysis or ash method and reported as a percentage by weight. A "40% grade" might carry a tolerance of ±1–2 points, so a lot measuring 38.5% is within spec for many suppliers — but at a fixed let-down it delivers roughly 4% less carbon into your part than the nameplate number.
What to do with it:
- Compare the lot value against the grade's stated tolerance, not just the nameplate percentage.
- Watch the trend across lots. One lot at 38.9% is noise. Four consecutive lots drifting down is a process change you should ask about — politely, with the CoAs attached.
- Recalculate let-down on the trend if your part has a color or UV target you must hold.
3. Melt flow index (MFI): will it mix on your line
MFI describes how the carrier resin flows at a standard temperature and load. For masterbatch the practical question is whether the melt will blend evenly with your natural polymer. A masterbatch whose MFI is far below the carrier resin of your process tends to mix poorly at high let-down; one that is far above can lubricate the melt and change cycle behavior.
The usual convention is that the masterbatch carrier should be compatible with — and often somewhat higher flowing than — the resin it colors. Check the CoA value against the grade's nominal MFI and against your own process window, and treat a large lot-to-lot swing the same way you would treat swing on your own incoming resin.
4. Filter pressure value (FPV): the dispersion predictor
Filter pressure value measures how quickly a dispersion clogs a standard filter — it is the closest thing a paper number has to "will this give me gels and die lines." Lower is better. As a rule of thumb, film and fiber grades need the lowest FPV, injection grades tolerate more, and thick-wall pipe and conduit the most.
Three practical uses:
- Lot acceptance: set an internal FPV ceiling with your supplier based on your thinnest product, and reject lots above it before they reach the floor.
- Supplier comparison: FPV is one of the few numbers that lets you compare dispersion quality between two suppliers without running trials.
- Early warning: rising FPV across lots usually means carbon feedstock or compounding conditions changed upstream — ask before it becomes a screen-pack problem on your extruder.
5. Moisture and volatiles: the silent defect source
Moisture content looks like a formality until a black masterbatch that has absorbed humidity starts producing porosity, streaks or bubbles — particularly in PET, PA and other moisture-sensitive processes. Check the moisture line on the CoA, but also check the packaging date and condition when the container is opened. A CoA is a snapshot at the plant gate; sea freight in summer is not the plant gate.
6. A 30-minute incoming inspection protocol
For each new lot, before it goes to the floor:
- Match the lot number on the bag, the CoA and your purchase order. Mismatches happen more often than anyone admits.
- Check the test date is close to the production date. A CoA dated months after the lot was made is a reconstructed document.
- Compare three numbers against your spec band: carbon content, MFI and FPV (or the dispersion measure your supplier reports). Log them — a simple spreadsheet, one row per lot.
- File the CoA with the retention sample. If a defect claim ever needs evidence, this is the evidence.
Ten lots in, you will have a better picture of your supplier's real process control than any audit questionnaire.
7. Red flags on a CoA
- Identical values across every lot — real measurements vary slightly; photocopy CoAs do not.
- No lot number, no test date, no method reference.
- Tolerances quoted only verbally. The band should be on the document.
- Missing FPV or equivalent dispersion measure on a grade sold for film.
8. What we put on ours
Every E-LUCK lot ships with a CoA reporting carbon content, MFI, moisture and dispersion data against the grade's stated tolerance, tied to the lot number and production date. If you need the raw test trace or a third-party verification for a specific lot, ask — SGS test reports for standard grades are already on file and available to serious inquiries.
9. What a CoA is — and what it is not
Buyers occasionally treat a CoA as a guarantee, and suppliers occasionally allow them to. It is worth being precise about the document, because the difference shows up the one time it matters.
A certificate of analysis is a record of what was measured on a specific lot, produced by the supplier against a stated method and reported against a stated tolerance. That is all it is — and that is a great deal. What it is not:
- It is not a warranty of fitness for your application. A lot can sit perfectly inside its declared band and still be wrong for your line, because the band was written for a general case. Suitability is settled by a trial on your equipment, not by a CoA.
- It is not necessarily a third-party result. Most CoAs are the producer's own in-house testing. That is normal and useful — but where your customer or your own quality system requires independent verification, that is a separate document. Ask for it rather than assume the CoA covers it.
- It is not the specification. The TDS states what the grade is designed to be; the CoA states what a particular lot measured. Reading one as the other is the most common confusion in incoming inspection.
- It is only as good as the method behind it. A dispersion figure means nothing unless you know the method and can compare it with your own. Where methods differ between supplier and buyer, two correct numbers can look like a mismatch.
The practical rule: agree the method, agree the tolerance band in writing, then use the CoA to check the lot against that agreement — not to discover the specification for the first time.
10. Reading a CoA against your own incoming test
The CoA and your incoming inspection answer different questions, and the comparison between them is where the value sits.
| Check | What the CoA tells you | What your own test adds |
|---|---|---|
| Carbon content | What the supplier measured on the lot, against the grade band | Whether the lot you actually received matches the lot the document describes — the check that catches a mix-up in despatch |
| MFI | The flow the grade was compounded to | Whether it flows as expected on your machine, at your temperature profile — the lab figure is only a proxy for this |
| Moisture | The figure at the plant gate | What it is after weeks in a container and a humid warehouse. The number most likely to differ, and the one most likely to cause a defect |
| Dispersion (FPV) | The supplier's dispersion result, by their method | Whether that dispersion survives your let-down and your shear history — dispersion is not fixed for all time |
| Ash | Filler content, which drives both cost and surface | Whether the ash level suits the surface the part is actually judged on |
Two habits make this comparison useful rather than ceremonial. Keep the file per lot, not per purchase order — defects are usually discovered months after delivery, and by then the question is which bag the part came from. And record your own numbers, not just pass/fail — a moisture reading that is inside tolerance but drifting upward across three lots tells you about your storage long before it causes a visible defect.
Request a CoA sample with your quote
Questions about reading a lot certificate
A certificate of analysis is only useful if you know which lot it describes and which values were measured rather than promised.
What must a black masterbatch CoA contain to be useful?
The lot number, the production date, and measured values rather than specification ranges. Ranges tell you what the supplier promises; only measured values tell you what is in your silo. Useful additions are carbon black or pigment content, sieve residue, dispersion or filter pressure value with its method stated, moisture, and melt mass flow rate. A lot number is the minimum — without it, the document cannot be matched to the bags you receive.
How do I match a CoA to the shipment I received?
Compare the lot number on the certificate with the lot number printed on the bags, and check the production date against the delivery date. A certificate dated well before the shipment describes a batch that may no longer be the one you have. If the lot numbers do not match, ask for the certificate covering your lot rather than accepting the one that arrived by email.
Why do two suppliers quote very different dispersion values for the same grade?
Because filter pressure value is method-dependent. Different mesh, different filter medium, different test speed and different sample preparation produce different numbers on identical material. A value quoted without its method and unit is not comparable to anything. This is the single most common source of false disputes in black masterbatch sourcing, and it is why we publish a free white paper on how to compare dispersion numbers properly.
What is a sieve residue value and what should I look for?
Sieve residue is the fraction of the compound that does not pass a stated mesh, which is a direct proxy for foreign material and oversized agglomerates that could block a die or a screen pack. Look for the mesh stated on the certificate: a low value on a fine mesh is a stronger claim than the same number on a coarse one. If your line has a known screen pack, the certificate is more useful when the mesh matches what you actually run.
Should a first order be accepted without a CoA?
No. A supplier who cannot produce a lot-specific certificate on request is buying material rather than compounding it, and you have no visibility of the lot you are actually receiving. Ask for the CoA covering your first lot, compare the values against your specification, and keep it. Six months later, when a quality dispute starts, that document is the only factual basis either side has.
Continue reading
- White paper: measuring dispersion and filter pressure value — why two suppliers' FPV numbers are not comparable, and the eight items to fix in a spec.
- Carbon black content testing in PE — why two correct labs can report different numbers for the same lot.
- PET black masterbatch: gels and high filter pressure — what a filter pressure value does and does not predict.
- How to calculate let-down ratio for black masterbatch — the other number that decides what the bag really costs.
- Black masterbatch defects: gels, streaks and specks — what bad dispersion looks like when it reaches the line.
- Carbon black price outlook 2026 — why carbon content and price drift together.
Related pages
- Is black masterbatch a dangerous good? — the shipping classification, and the UN numbers that get confused with it
- SGS test reports — third-party verification for standard grades
- Black masterbatch pricing — what drives cost per kg
- Film grades — where FPV matters most
- All black masterbatch — full grade range and selection guide
- H6045 grade page — a full TDS laid out line by line, the way we present lot data