Carbon black content testing in PE: TGA, ashing and which method to use
Carbon black content is the figure buyers use to compare grades, price a bag, and check a delivery. It is also the figure most often compared across methods that do not measure the same thing — which is how two laboratories, both correct, produce two different numbers for the same material.
This is what the common methods actually measure, where they diverge, and how to set up a comparison that means something.
What the number is supposed to tell you
Before the method, be clear about the question, because three different questions get answered with one word:
- "How much carbon black is in this masterbatch?" — a formulation question, used to price the grade and verify the label.
- "How much carbon black reached the finished part?" — a process question, which depends on let-down and on how well the masterbatch dispersed.
- "How much UV protection does this part have?" — an application question. Carbon content is an input to it; dispersion and carbon type decide whether the loading does its job.
The methods below answer the first and second well. None of them answers the third, and buying on carbon content alone is how a part that "tests at spec" still fails in the field.
Method 1: thermogravimetric analysis (TGA)
A small sample is heated under a controlled atmosphere while its mass is recorded continuously. The polymer decomposes and leaves as volatiles; what remains at the end of the programme is the inorganic residue.
What it gives you: a reproducible figure on a small sample, with the mass-loss curve as a bonus — the shape of the curve often tells you more than the final number, because different components leave at different temperatures.
Where it needs care: the residue is not necessarily all carbon black. A filled grade leaves mineral filler behind, and a grade containing other inorganic additives adds to the residue. If the method does not distinguish, a "carbon content" figure on a filled grade quietly becomes a carbon-plus-filler figure. Ask what the residue contains before comparing it with a specification written for carbon black alone.
Instrumented TGA is the version most often cited in lab work, and ASTM D1603 is the classic tube-furnace relative for polyolefins. They are close relatives of the same idea: drive off the polymer, weigh what is left.
Method 2: ashing in a muffle furnace
The older, simpler and still very common route: weigh a sample, burn it off at high temperature, weigh the residue. It costs far less than a TGA instrument and needs no specialist operator, which is why it remains standard practice in incoming inspection.
What it gives you: a fast, cheap figure adequate for routine lot checking — particularly for unfilled grades where the residue really is close to the carbon black.
Where it needs care:
- Ash is not the same as carbon black. In a filled grade the two diverge, and the divergence is the size of the filler loading. This is the single most common source of a "supplier is wrong" dispute that turns out to be a method mismatch.
- Oxidation conditions matter. Carbon black can be partly consumed under the wrong atmosphere or hold time, which biases the figure low — and does so inconsistently between labs.
- Sample size is small. A few grams taken from one bag is not the lot. Sampling discipline matters more here than in most tests.
Method 3: dissolution and filtration
The polymer is dissolved in a suitable solvent and the insoluble carbon black is filtered, dried and weighed. Less common in routine QC, but useful where thermal methods are ambiguous — for instance where you need to separate carbon black from a filler chemically rather than by temperature.
What it gives you: a cleaner separation of carbon black from other solids than a thermal route, where the solvent is chosen well.
Where it needs care: solvent choice, filtration losses and drying completeness all introduce their own error, and the method is slower and more operator-dependent than the thermal routes. It is a laboratory tool rather than a production one.
Why two correct labs report different numbers
This is the practical heart of the matter, and it has five sources:
| Source of difference | Effect | How to close it |
|---|---|---|
| Different methods | Thermal residue vs. ash vs. dissolution are not the same measurement | Name the standard in the purchase specification, not just the figure |
| Filler counted or not | A filled grade reports higher by roughly the filler loading | State whether the figure is carbon black or total inorganic residue |
| Sampling | One pellet from the surface of a bag does not represent the lot | Agree a sampling plan — number of increments, locations, sample size |
| Moisture and volatiles | Retained water leaves as mass loss and reads as lower carbon | Condition samples identically before weighing |
| Atmosphere and hold time | Carbon black partly oxidised under the wrong conditions | Follow the standard's parameters, not a house shortcut |
None of these is exotic. Every one of them shows up in practice, and all five are closed by a single habit: agree the method and the sampling plan in writing before figures are compared. A specification that names a figure but not a method is not a specification — it is a number waiting to be argued about.
Testing a finished part: the HDPE tarpaulin case
Buyers of woven HDPE tarpaulin often want to check the black content of the fabric rather than of the masterbatch, which is the right instinct — the masterbatch figure tells you about the input, not the article you paid for. Two things change when you move from pellets to fabric:
- The formulation is no longer just carbon black and polymer. A serviceable tarpaulin typically carries UV stabiliser and may carry filler. Whatever the residue contains, the figure has to be read against that formulation, not against a masterbatch label.
- Sampling is harder. Woven fabric is not uniform, and a sample from the edge or from a print area is not representative. Take a defined area from the body of the fabric, and take more than one.
The practical sequence for a tarpaulin check:
- Agree the target as a specification figure for the finished fabric, with the method named.
- Confirm what the formulation contains that will show up in the residue — filler especially.
- Sample from defined positions, in more than one place, and condition identically.
- Report the method alongside the result, so the next test is comparable.
And keep the framing straight: carbon content tells you whether you received the formulation you specified. It does not tell you the UV life of the tarpaulin, which also depends on carbon type, dispersion and the stabiliser package. The relationship between loading and UV service life is here.
What we report, and how to check it
Our lot CoAs report carbon black content with the method stated, and — where the grade is filled — distinguish carbon black from the filler so the figure is comparable with an unfilled grade. On a filled grade such as 508C, the ash figure and the carbon figure are reported separately for exactly this reason.
If you want to verify a lot on receipt, the honest way is a spot check against the method named in the specification, with a sampling plan agreed in advance. If your check and our CoA disagree, send us both sets of numbers and the methods — in our experience the gap is nearly always a method or sampling difference rather than a production one, and it is usually resolved in a single exchange. The incoming inspection protocol is here.
Questions buyers ask
What is the standard test method for carbon black content?
Two families dominate. Thermogravimetric methods heat the sample in a controlled atmosphere and measure the mass loss as the polymer decomposes, leaving the carbon black and any mineral filler behind. ASTM D1603 is the classic tube-furnace version for polyolefins, and ISO 6964 covers pyrolysis-based determination for polyethylene pipe applications. Ashing in a muffle furnace is the older manual equivalent and still common in incoming inspection. The important thing is not which standard your supplier cites, but that supplier and buyer are using the same one.
Why do my numbers differ from the supplier's CoA?
Four common reasons. Different methods, since a thermogravimetric result and a muffle-furnace ash figure are not the same measurement. Ash versus carbon black, because a filled grade leaves mineral filler behind that an ash test counts as residue. Sampling, because a single pellet taken from the surface of a bag is not representative of the lot. And moisture or volatiles retained in the sample, which leave as mass loss in any thermal method. Agree the method and the sampling plan in writing before comparing figures, otherwise the two numbers are measuring different things.
How do I test carbon black content in a finished part like HDPE tarpaulin?
The same thermal approach applies, with two additional questions. First, what else is in the formulation — a woven tarpaulin may carry UV stabiliser, filler and colour, all of which affect what the residue contains. Second, whether you are checking total residue or carbon black specifically, because those differ in a filled product. Take a representative sample from the fabric rather than from the edge, condition it, and run the method your specification names. If the target is a specification figure for the finished article, test the article; testing the masterbatch tells you about the input, not the output.
Can a supplier's carbon content figure be trusted?
Treat it the way you would treat any lot figure: as a claim to be verified against an agreed method, not as a settled fact. Ask what method was used, at what sample size, and whether the residue includes filler. A supplier reporting a method and a tolerance is more useful than one reporting a bare number. Verification on receipt, even on a spot-check basis, is what turns a CoA from a document into a control.
Related pages
- Carbon black peaked: what the reversal means — why ash is the check to run when carbon gets cheap again
- How to read a black masterbatch CoA — the full lot report, figure by figure
- A 30-minute incoming inspection protocol — where carbon content fits in the receiving routine
- Carbon black loading for UV service life — why loading alone does not predict field performance
- All 12 black masterbatch grades — carbon content and ash by grade