D97 is a volume‑based particle size distribution (PSD) specification for recovered carbon black. D97 means 97% of the powder by volume has agglomerate size smaller than the quoted value; only 3% of the volume consists of particles larger than this size. It is the strictest coarse‑tail control index used in commercial rCB contracts and milling QC.
Important note: D97 measures the size of rCB agglomerates, not the nanoscale primary carbon black spheres. The grinding process de‑agglomerates porous pyrolysis char clusters rather than breaking the tiny primary carbon particles.
Why D97 Is More Stringent Than D90
D90 allows 10% of volume to be oversized particles, while D97 only permits 3% oversized volume. For rubber and tire applications, even a small volume of large agglomerates creates defects in rubber compounds, poor dispersion, reduced tensile strength and surface flaws. This is why tire‑grade and high‑performance rCB usually specify D97 instead of D90.
For example: If rCB is specified D97 ≤10 μm, only 3% of the powder volume can be larger than 10 μm. This tight limit eliminates coarse agglomerates that would ruin reinforcement performance.
Typical Commercial D97 Ranges for rCB
- General‑purpose rubber / non‑critical filler grade: D97 15–25 μm
- Standard tire‑grade rCB: D97 8–15 μm
- Fine rubber & masterbatch grade: D97 4–8 μm
- Ultra‑fine premium rCB (fluidized bed jet mill): D97 ≤4 μm
How D97 Is Controlled During Milling
The dynamic classifier wheel inside ACM or fluidized bed jet mill is the primary tool to control D97:
- Increase classifier wheel speed → stronger centrifugal force, rejects more coarse agglomerates, reduces D97.
- Optimize air volume and reduce feed rate to extend particle residence time for full de‑agglomeration.
- Coarse agglomerates rejected by the classifier fall back to the grinding zone for repeated de‑agglomeration in closed‑circuit loop.
Worn mill components (hammers, liners, nozzles) or poor pre‑treatment (oversize lumps, residual steel, high moisture) will cause D97 to drift upward, failing product specifications.
Measurement Method
Laser diffraction particle size analyzers are the standard test instrument for rCB D97 measurement. The test outputs D10, D50, D90 and D97 together. Dry dispersion is preferred for rCB to represent the actual powder state in the milling system.
D97 is the key volume‑based specification limiting the coarse tail of rCB agglomerates. It is stricter than D90 and widely adopted for tire‑grade and high‑spec rCB. By tuning the dynamic classifier speed and closed‑circuit recirculation, operators can hold D97 within the target range, ensuring no excessive large agglomerates enter the final product and damage rubber compound performance.
