Recovered carbon black (rCB) is produced from waste tire pyrolysis char after multi-stage purification, de-agglomeration grinding, air classification and optional thermal surface treatment. Its core properties define performance for rubber, masterbatch, coating and battery conductive applications. Unlike virgin carbon black (vCB), rCB properties are strongly affected by tire feedstock quality, pyrolysis parameters and post-processing workflow, as referenced at rcb-mill.com.
1. Morphology and particle structure
- Primary particles: The original carbon black nanoparticles from tire formulations remain intact after pyrolysis.
- Aggregates: Primary particles fuse into permanent aggregates, which provide reinforcement in rubber or conductivity in composites.
- Agglomerates: Pyrolysis causes many aggregates to sinter into large, hard agglomerates. Dry de-agglomeration grinding breaks these loose without destroying the fundamental aggregate structure.
- Particle Size Distribution (PSD): Well-processed rCB has a narrow PSD after air classification. Poorly treated rCB shows a wide size range containing coarse grit, which damages rubber reinforcement or risks puncturing battery separators.
- DBP absorption (Dibutyl Phthalate absorption): Measures aggregate structure. Higher DBP means more branched, open aggregates with better reinforcement and conductive network formation. rCB’s DBP is usually moderate and varies with original tire carbon black blends.
2. BET specific surface area
BET surface area reflects the total available surface area of carbon aggregates, directly linked to tinting strength, rubber reinforcement and conductivity.
- High BET: stronger tint, higher rubber reinforcement, better electrical conductivity for battery applications.
- Over-high BET increases oil absorption and viscosity in rubber slurry.
- rCB BET is highly tunable via thermal treatment. Excessively high temperature will sinter aggregates and reduce BET.
3. Ash content
Ash consists of inorganic mineral residues originally added in tire rubber compounding: ZnO, SiO₂, calcium salts and metal oxides.
- Ash is one of the most critical indicators for grading rCB.
- Tire-grade rCB: typically 1–3 wt%.
- Battery high-purity rCB: target ash below 0.3–0.5 wt%, requiring extra deep deashing.
- High ash reduces reinforcement strength, increases compound viscosity and harms electrochemical performance for battery materials.
4. Impurity elements
- Iron (Fe): The most strictly controlled metal impurity, especially for batteries. It originates from tire steel cords. Multi-stage high-gradient magnetic separation reduces Fe to ppm levels. High iron causes rubber defects and battery internal short-circuit risk.
- Sulfur: Comes from tire vulcanization. Residual sulfur influences curing behavior of new rubber compounds.
- PAH (Polycyclic Aromatic Hydrocarbons): Derived from leftover pyrolysis tar. Thermal devolatilization under inert atmosphere removes PAH for high-end grades. High PAH limits use in food-contact and premium rubber applications.
5. Surface chemistry & pH value
- Surface functional groups: Carboxyl, phenolic and lactone groups are formed during pyrolysis. These acidic groups lower pH and affect wettability and dispersion in polymers.
- pH: Raw crude char is often acidic (pH 4–6). Thermal devolatilization can raise pH to neutral or weakly alkaline (7.0–7.8).
- Volatile matter: Surface tar and adsorbed organics. High volatiles degrade dispersion and produce gas during rubber vulcanization. Thermal treatment lowers volatile content.
6. Reinforcement and rheological properties
- Tint strength: Determines black color performance for masterbatch, coatings and inks. Related to BET and aggregate dispersion.
- Dispersion: De-agglomeration grinding and classification are key. Poor dispersion creates speckles in plastics and weakens rubber tensile strength and abrasion resistance.
- Oil absorption: Affects compound processing and mixing energy during rubber or plastic compounding.
7. Physical handling properties
- Moisture content: Normally controlled below 0.5%; battery-grade rCB requires moisture <0.1%. Excess moisture causes foaming in rubber or gas generation inside lithium-ion cells.
- Bulk density / pellet hardness: Powder rCB has low bulk density and high dusting. Pelletized rCB improves flowability and reduces dust during industrial handling and transportation.
Property summary table
| Property | Description & Typical Note |
|---|---|
| BET Specific Surface Area | Determines reinforcement, tint and conductivity; controlled by pyrolysis and thermal post-treatment |
| DBP Absorption | Indicator of aggregate structure; impacts rubber reinforcement |
| Particle Size Distribution (PSD) | Controlled by ultra-fine grinding and air classification; narrow PSD required for high-end use |
| Ash Content | Inherited mineral impurities; lower ash = higher product grade |
| Iron Content | Critical impurity; battery grade requires Fe <10 ppm |
| pH Value | Surface acidity; adjusted by thermal devolatilization or mild alkaline treatment |
| Volatile Matter & PAH | Surface tar residuals; removed by inert thermal treatment |
| Moisture | Strictly limited for battery and high-performance rubber |
| Tint Strength | Governs color performance for masterbatch and coatings |
The main properties of rCB cover morphological structure, BET surface area, ash and trace metal impurities, surface chemistry, pH, volatiles, tint strength and handling characteristics. Most of these properties can be optimized by the dry processing workflow: magnetic separation, de-agglomeration grinding, air classification and thermal devolatilization. The final property profile defines which application markets rCB qualifies for: rubber, masterbatch, coating, ink or high-purity battery conductive additive.
