ASTM D1765 Carbon Black Classification: Grade Codes Explained
How the ASTM D1765 carbon black classification works: what the letter N means, what the digits tell you, and how hard and soft grades differ. Reading the code is the first step to choosing the right product.
2026-09-01 · 6 min read
The first thing you hear when talking to a carbon black supplier is the grade code: N330, N550, N220. These codes are not random; they are part of a system defined by the ASTM D1765 standard, and once you learn to read them, they tell you a great deal about the product.
The code has three parts
The letter: cure rate
The letter at the start of the code shows the effect of the carbon black on the cure rate of the rubber. N means "normal" and S means "slow". Today, almost all commercial use is in the N class.
The first digit: particle size
This is the most informative part of the code. The first digit represents the average particle size range, and a lower digit means a smaller particle:
| Series | Particle size | Character |
|---|---|---|
| N1xx | 11–19 nm | Highest reinforcement, highest abrasion resistance |
| N2xx | 20–25 nm | High reinforcement, tire treads |
| N3xx | 26–30 nm | General-purpose hard grades |
| N5xx | 40–48 nm | Semi-reinforcing, extrusion |
| N6xx | 49–60 nm | Carcass and sidewall |
| N7xx | 61–100 nm | High loading, low reinforcement |
A smaller particle means a larger surface area, and a larger surface area means more contact with the rubber and a stronger bond. The price is harder processing and higher heat build-up.
The last two digits: structure and other properties
This serial number distinguishes grades within the same particle size class. For example, N330 and N339 are in the same particle class (26–30 nm) but have different structures: the OAN of N330 is 102 ml/100 g and that of N339 is 120 ml/100 g. This difference gives N339 a markedly higher modulus and better extrusion behavior.
Hard versus soft
In practice, the industry divides grades into two groups:
- Hard grades, N100 to N300 series: High surface area, strong reinforcement, high abrasion resistance. Tire treads, conveyor belt covers, high-performance mechanical parts.
- Soft grades, N500 to N700 series: Low surface area, easy processing, high filler capacity, low heat build-up. Carcass, hoses, profiles, seals, masterbatch.
Outside the code
There are also specialty products positioned outside the ASTM system. SHP50, for example, is a medium-structure reinforcing grade with a profile between N550 and the hard grades. For products like these, the code itself tells you nothing; you need to look at the technical data table.
The code does not tell you everything
A grade code is a classification, not a full specification. There can be small differences between manufacturers under the same code. That is why, when changing brands, the right approach is to compare certificates of analysis rather than rely on a code match.
