D90 is a common particle‑size distribution (PSD) index widely used in rCB quality control and milling process tuning. In recovered carbon black processing, D90 means 90% of the total particle volume is smaller than the stated value; only 10% of the volume consists of larger particles.
Simple Definition
For example, if rCB is labeled D90 = 10 μm:
- 90% of the powder volume is made up of particles smaller than 10 μm;
- 10% of the powder volume is particles larger than 10 μm.
Important: D90 describes agglomerate size, not the primary carbon black sphere size. The primary carbon nanoparticles in rCB are nanoscale, tightly bonded into porous agglomerates. Grinding and classification break these agglomerates apart; D90 measures the size of these de‑agglomerated clusters.
Why D90 Is Critical for rCB
D90 is one of the key specifications for buyers and mill operators.
- Rubber / tire compound performance: Large agglomerates act as hard defects in rubber. A high D90 (too many big clusters) causes poor dispersion, weak tensile strength and surface defects in finished rubber parts.
- Milling process control: Operators adjust classifier speed, gas pressure and feed rate to control D90. Raising classifier wheel speed in an ACM or fluidized bed jet mill reduces D90 and produces finer powder.
- Equipment protection: D90 monitoring can detect abnormal milling conditions. A sudden rise in D90 indicates incomplete de‑agglomeration, often caused by overfeeding, insufficient grinding energy or worn mill components.
Typical D90 Ranges for Commercial rCB
- Standard tire‑grade rCB: D90 around 8–15 μm
- Fine rubber / plastic masterbatch grade: D90 4–8 μm
- Ultra‑fine premium rCB (jet mill processed): D90 <4 μm
How D90 Is Measured
Laser particle size analyzers are the standard instrument for rCB PSD testing. A small powder sample is dispersed, then laser light scattering calculates the volume‑based particle size distribution and outputs D10, D50, D90 together.
D50 is the median size (50% volume below this value), while D90 is the “coarse tail” indicator. For rCB, D90 is often more important than D50 because even a small volume fraction of large agglomerates can ruin reinforcement performance.
Relationship to Milling Operation
For both ACM classifier mills and fluidized bed jet mills, the integrated dynamic classifier wheel directly controls the D90 cut point.
- Higher classifier speed → lower D90 (finer powder, less coarse tail)
- Lower classifier speed → higher D90 (coarser powder, higher throughput)
D90 is a volume‑based particle size metric: 90% of rCB powder by volume is smaller than the D90 value. It quantifies the coarse tail of agglomerates, which is the critical parameter for rCB end‑use performance in rubber and plastics. During grinding, operators tune mill parameters to hit the target D90 specification and limit oversized agglomerates.
