Carbon Black Iodine Number, OAN, NSA and STSA Explained
What does each value on a carbon black certificate of analysis mean, from the carbon black iodine number to OAN, NSA and STSA? We explain the ASTM test methods, their practical meaning and their effect on purchasing decisions.
2026-08-25 · 9 min read
At first glance, a carbon black certificate of analysis (CoA) is just a table of numbers. Yet every row says something concrete about how the product behaves. This guide teaches you to read those rows.
Particle and surface: measuring reinforcement
Iodine adsorption number, ASTM D1510
Shows the amount of iodine (g/kg) adsorbed on the surface per unit weight. It is an indirect measure of particle size and, in practice, the fastest comparison tool. High value = small particle = strong reinforcement.
Example: N220 121 g/kg, N330 82 g/kg, N772 30 g/kg.
NSA, nitrogen surface area, ASTM D6556
The total surface area (m²/g) measured by nitrogen adsorption, including the pores of the particle.
STSA, external surface area, ASTM D6556
Measures the external surface only. The difference between NSA and STSA shows porosity. In rubber applications, the polymer chain cannot enter the pores, so STSA is what really matters.
Structure: measuring compound behavior
OAN, oil absorption number, ASTM D2414
The volume of oil absorbed by 100 grams of carbon black (ml/100 g). It shows how branched the aggregates are, in other words the "structure".
High structure brings higher modulus, better dispersion, smoother extrusion, less swell and an easier conductive network.
COAN, compressed oil absorption number, ASTM D3493
OAN measured after the sample has been compressed under pressure. Aggregates break during mixing; COAN shows the structure that survives the breakage and is closer to real application behavior than OAN.
Color and consistency
Tint strength, ASTM D3256
A measure of the grayness produced when mixed with a white pigment. It shows how narrow the particle size distribution is and is one of the best indicators of batch consistency.
Toluene discoloration, ASTM D1618
The light transmittance of the impurities extracted with toluene. A high value means a clean product. It is critical in food packaging and sensitive applications.
Impurities and physical form
- Ash content (D1506): The inorganic residue left after combustion. Low is better; especially important in conductive and food-contact applications.
- Sulfur content (D1619): Must be included in the cure balance; cannot be ignored in low-sulfur systems.
- Heating loss (D1509): Moisture content. High moisture causes bubbles and surface defects in extrusion.
- Sieve residue (D1514): Coarse particles and foreign matter. Vital in inkjet inks and thin-film applications.
- Pellet hardness (D5230): Pellets must stay intact during transport and feeding, yet break easily in the compound. That is why both the average and the maximum value are given.
- Fines content (D1508): The proportion of dust. High dust means both a workplace safety problem and a feeding problem.
- Pour density (D1513): Determines storage and transport volume.
- pH (D1512): Indicates surface chemistry; affects dispersant choice in water-based systems.
How to use the certificate
When evaluating a new supplier, look not at individual values but at how the values relate to each other. For example, of two products with the same iodine value, the one with the higher OAN will give a stiffer compound. Also compare consecutive batches from the same supplier: consistency matters more than the absolute value.
