Poor dispersion of rCB in rubber or polymer compounds is one of the most common quality issues. The root cause is almost always unbroken agglomerates, but there are multiple contributing factors from milling, pre‑treatment, storage and compounding. Below are the main causes, sorted by likelihood, plus practical checks.
1. Incomplete de‑agglomeration (most frequent reason)
The pyrolysis char forms hard, porous clusters. If grinding / turbo classification does not fully break these agglomerates, residual large clusters remain in the final powder.
- Symptom: Visible specks, grit or “fish eyes” in cured rubber; higher D97 than your specification.
- Causes:
- Insufficient classifier (turbo wheel) speed; coarse agglomerates leak through the classifier.
- Overfeeding the mill; too much material reduces residence time and incomplete grinding.
- Worn mill hammers, liners or turbo blades, which weaken grinding and create coarse bypass.
- Test: Run laser PSD. If D97 is high, this confirms coarse agglomerates are present.
2. Moisture and re‑agglomeration after milling
rCB is highly hygroscopic. Even well‑milled fine rCB will re‑clump if it absorbs moisture (>1–2%).
- Moisture makes carbon clusters stick together; these secondary agglomerates are very hard to break during mixing.
- Check: Moisture content; powder caking in bags or silos.
- Fix: Keep dryer outlet moisture <1%, use sealed silo storage and avoid humid environments.
3. High ash / inorganic residue
Tire pyrolysis char contains silica, metal oxides and mineral ash. High ash can create hard mineral agglomerates that resist dispersion and act as rigid grit.
- Ash also increases oil absorption and changes surface chemistry, making wetting harder.
4. Residual steel / metal contaminants
Embedded steel fragments from tire wire are not only abrasive; fine iron oxide particles can create hard micro‑agglomerates and disrupt compound wetting.
- This is why magnetic separation is required after pre‑crushing, before fine grinding.
5. Surface chemistry of rCB (BET, DBP, volatile content)
Virgin carbon black has well‑controlled surface activity. rCB surface differs after pyrolysis:
- Low volatile content / low surface activity → poor wetting by rubber polymer.
- Irregular porous surface structure → higher oil absorption, requiring longer mixing time or extra dispersant.
- Even if PSD looks good, poor surface activity can still lead to incomplete dispersion.
6. Milling classification creating “over‑fine” fraction
Too much sub‑micron ultra‑fine dust increases inter‑particle cohesion. These ultra‑fines stick together and form soft agglomerates that are hard to separate during mixing.
- The PSD span becomes too wide; lots of ultra‑fines plus residual coarse agglomerates worsen dispersion.
7. Compounding / mixing process issues (not the rCB itself)
Sometimes the powder quality is fine, but mixing parameters are wrong:
- Insufficient mixing time, too low mixing temperature, wrong rotor speed.
- Incorrect order of ingredient addition; insufficient oil / processing aid to wet rCB.
- Too high loading of rCB in the formulation.
Quick Troubleshooting Checklist
- Test PSD (D97 first): If D97 exceeds target → grinding / turbo classifier problem.
- Check moisture: >1.5% is a red flag for re‑agglomeration.
- Check ash and residual iron content.
- Examine powder under microscope: distinguish hard primary agglomerates (from bad milling) vs soft cake agglomerates (from moisture).
- Run a simple lab dispersion test to separate powder‑related issues from mixing recipe issues.
Poor rCB dispersion is usually caused by unbroken hard agglomerates escaping the turbo classifier, followed by moisture-induced re‑agglomeration, high ash, or low surface activity. First measure D97 and moisture to quickly isolate whether the problem originates in the milling line or in powder storage / compounding.
