How to Read Carbon Black ASTM N-Numbers (N220, N330, N550) — and Why It Matters for Masterbatch
Open almost any carbon black datasheet and you will see a code like N220, N330, or N660. Buyers often treat these as opaque product names, or assume the number is a quality ranking — a higher number meaning better. Neither is true. The ASTM N-number is a compact classification that tells you specific things about the carbon black, and just as importantly, it leaves other things out. This article decodes each part of the number, clears up a common myth about the last two digits, and explains why the whole system comes from the rubber industry — and what that means when you are buying black masterbatch for plastics.
The Short Answer
An ASTM N-number has three parts: a letter, a first digit, and two more digits. The letter (N or S) describes how the black affects the cure rate of rubber. The first digit places the black into a surface-area group — which, because surface area and particle size move in opposite directions, also tells you roughly how fine the particles are. The last two digits are assigned arbitrarily and carry no built-in meaning. Notably, the number says nothing directly about structure (the branching and void volume of the aggregate), which you must read separately from the oil-absorption value. The classification comes from ASTM D1765, the standard for carbon blacks used in rubber.
The Letter: N or S
The first character is a letter, and in practice you will almost always see N. It refers to the black's effect on vulcanization: N denotes a normal curing rate, while S denotes a slow curing rate. Slow-curing behavior is characteristic of channel blacks and of surface-oxidized blacks, because oxygen groups on the particle surface interfere with cure chemistry. For a plastics buyer this letter carries no processing meaning on its own — but it is a useful reminder that the whole scheme was built around rubber cure behavior, not plastics coloring.
The First Digit: Surface Area (and Therefore Particle Size)
This is the digit that actually carries information. Under ASTM D1765, the nitrogen-surface-area range of carbon blacks is divided into ten groups, and the first digit names the group. Because surface area rises as particles get smaller, a low first digit means fine particles and high surface area, while a high first digit means coarse particles and low surface area. In other words, an N1xx black is among the finest, and an N9xx (thermal) black is among the coarsest.
| First digit | Nitrogen surface area group | Example grades |
|---|---|---|
| 0 | > 150 m²/g | specialty high-area blacks |
| 1 | 121–150 m²/g | N110 (SAF), N115, N134 |
| 2 | 100–120 m²/g | N220 (ISAF), N234 |
| 3 | 70–99 m²/g | N326, N330 (HAF), N339 |
| 4 | 50–69 m²/g | furnace blacks in this range |
| 5 | 40–49 m²/g | N539, N550 |
| 6 | 33–39 m²/g | N650, N660, N683 |
| 7 | 21–32 m²/g | N762, N774 |
| 8 | 11–20 m²/g | coarse furnace blacks |
| 9 | 0–10 m²/g | N990, N991 (thermal black) |
The surface-area group boundaries in this table are the exact ranges defined in ASTM D1765; grade names are shown for orientation, with typical nitrogen-surface-area values taken from carbon black manufacturer technical data.
Surface area matters because, for a given amount of carbon black, finer particles give deeper blackness (jetness), higher tint strength, and better UV protection — but they are also harder to disperse and tend to raise viscosity. That trade-off is the heart of choosing a black.
The Last Two Digits Are Assigned Arbitrarily
Here is the point that surprises many buyers. A widespread belief is that the last two digits encode structure — so that, say, N330 and N375 differ in a coded, readable way. In the standard itself, that is not how it works: after the first digit, the remaining two digits are assigned arbitrarily by the ASTM committee. They act as an identification number within a surface-area group, not as a formula you can decode. Two grades with the same first digit share a surface-area class, but you cannot rank their structure or performance just by reading the last two digits. For that, you have to look at the measured properties.
What the Number Does Not Tell You: Structure
The single most important property the N-number omits is structure — how much the primary particles are fused and branched into open, high-void aggregates. Structure is measured separately by oil absorption (the OAN or DBP number, ASTM D2414). High-structure blacks build viscosity and conductivity and can disperse more readily; low-structure blacks pack more densely and can reach higher gloss and jetness. Two blacks in the same surface-area group can have quite different structure, so a serious specification always pairs the N-number (or a trade grade) with the OAN/DBP value, the nitrogen surface area (ASTM D6556 or the iodine number, D1510), and often tint strength (D3265). The N-number is a starting label, not a full fingerprint.
This Is a Rubber System — What Plastics Buyers Should Read
ASTM D1765 is, by its own title, the classification for carbon blacks used in rubber products, and it names more than forty standardized rubber grades from the N100 through N900 series. Those grades were designed to reinforce tires and industrial rubber, where surface area and structure drive abrasion resistance and modulus. Colour and jetness were never the point.
Carbon black used to make black masterbatch for plastics is a different world. High-jetness pigment blacks are typically finer than most rubber-reinforcing grades, and they are usually identified by manufacturer trade grades rather than N-numbers. What carries over is the underlying physics: whether you read an N-number or a pigment trade grade, the properties that decide performance are the same — particle size, surface area, structure (OAN), surface chemistry, and how well the black is dispersed. So if a plastics supplier quotes you an N-number, treat it as a surface-area clue, then ask for the parameters that actually govern a black masterbatch: particle size, OAN/structure, and dispersion quality.
How This Connects to Your Masterbatch
Reading the number correctly turns into practical decisions. If you need maximum jetness and UV protection — cable jacketing, agricultural film, high-appearance parts — you are pushing toward finer, high-surface-area blacks, and dispersion becomes the deciding factor, because fine blacks are exactly the ones that resist dispersing. If you need economical opacity in thicker sections, a coarser black at a sensible loading may be the better value. A capable masterbatch maker chooses the carbon black grade to the job and then compounds it so the fine, hard-to-disperse blacks still let down cleanly on your line.
ZFH selects its carbon black by application — finer pigment grades where jetness and weathering matter, balanced against dispersibility — and treats dispersion as a controlled quality attribute rather than an afterthought.
Frequently Asked Questions
Does a higher N-number mean a better carbon black?
No. The number is a classification, not a quality score. A higher first digit simply means lower surface area and coarser particles. Better depends entirely on the application.
Can I compare two grades just by their N-numbers?
Only partly. Grades with the same first digit share a surface-area class, but the last two digits are arbitrary and structure is not encoded at all. Compare the measured OAN/DBP, surface area, and dispersion instead.
My masterbatch supplier does not quote an N-number. Is that a problem?
Not at all — it is normal for plastics. Pigment blacks for masterbatch are usually identified by trade grade. Ask instead for particle size, surface area, structure (OAN), and a dispersion result such as a filter pressure value.
Sources
The classification framework and the surface-area group boundaries in this article are defined in ASTM D1765, Standard Classification System for Carbon Blacks Used in Rubber Products, which also sets out the letter designation, the surface-area digit, and the arbitrarily assigned last two digits. The related characterization methods referenced are: structure by oil absorption number, ASTM D2414 (OAN/DBP); nitrogen surface area, ASTM D6556 (N2SA); iodine adsorption number, ASTM D1510; and tinting strength, ASTM D3265. Representative nitrogen-surface-area figures for named grades (for example N220 ≈ 114 m²/g, N326 ≈ 78 m²/g, N660 ≈ 35 m²/g, N990 ≈ 8 m²/g) are typical values published in carbon black manufacturers' technical datasheets and vary slightly between producers.
The N-number is a useful clue, but a black masterbatch is defined by how well the carbon black is dispersed and matched to your resin. To compare grades on what actually matters on your line, see ZFH standard PE/EVA black masterbatch and high color black masterbatch, or ask how we characterize and disperse our blacks on the quality and compliance page.