Recovered carbon black (rCB) is mainly generated by pyrolysis of waste rubber-containing feedstocks. The feedstock composition directly affects rCB purity, ash content, metal impurities, particle structure and final reinforcing performance. Waste tires are the dominant commercial raw material, while other rubber wastes can also be processed, each with unique advantages and limitations for rCB production.
1. End-of-life tires (ELT / waste tyres)
End-of-life tires are the most widely adopted and preferred feedstock for industrial rCB manufacturing.
- Composition: Natural rubber, synthetic rubber (SBR, BR), virgin carbon black, fillers (zinc oxide, clay, calcium carbonate), steel cords, textile fibers and curing additives.
- Benefits: Stable carbon black microstructure, relatively consistent rubber polymer ratio, well-established supply chain. The rCB derived from waste tires retains primary particle structure suitable for rubber reinforcement, making it usable in tires, hoses, seals and rubber compounds.
- Challenges: Contains steel wire and fiber contaminants that require pre-treatment (shredding, magnetic separation, fiber removal). Inorganic fillers increase ash content, which must be reduced by downstream purification. Different tire types (passenger car tires, truck tires, OTR off-road tires) yield rCB with distinct DBP absorption, particle size and ash levels.
Note: Truck tires generally produce higher-quality rCB than passenger tires due to higher loading of reinforcing carbon black. OTR tires have large rubber volume but higher mineral filler content.
2. Waste rubber products (non-tire rubber waste)
This category covers post-consumer and post-industrial rubber goods made from cross-linked elastomers.
- Examples: conveyor belts, rubber hoses, rubber mats, rubber seals, gaskets, shoe soles, rubber rollers, and rubber flooring.
- Advantages: Diverse local supply sources; some industrial rubber scrap has lower metal contamination.
- Limitations: Highly variable formulation. Many products contain high levels of mineral fillers, plasticizers, flame retardants or colored pigments. These additives raise ash and heteroatom content, leading to rCB with unstable reinforcing properties. Some waste rubber contains halogenated additives, which introduce corrosion and emission risks during pyrolysis. This feedstock usually produces lower-grade rCB for non-critical applications such as plastic filling or low-performance rubber.
3. Rubber manufacturing scrap (pre-consumer rubber waste)
This is offcut, flash, rejected green rubber and cured rubber scraps directly generated from tire and rubber product factories.
- Advantages: Cleaner feedstock with less dirt, stone or external impurities; consistent formulation controlled by the rubber plant. Lower pre-processing cost and more predictable rCB quality. It is an excellent premium feedstock for high-grade rCB.
- Limitations: Limited supply volume, often reserved for in-house recycling; price is usually higher than end-of-life tires.
4. Other potential feedstocks (limited commercial use for rCB)
These materials contain carbon but are not mainstream for high-performance rCB production:
- Waste rubber powder: Pre-shredded fine rubber powder from tire recycling. It shortens pyrolysis heat transfer time but is prone to carry high fine ash.
- Mixed polymer waste: Blends of rubber and plastic. Plastics generate extra wax and tar during pyrolysis, creating heavy coke deposits on rCB char and severely degrading reinforcement quality. Mixed plastic-rubber feed is generally avoided for premium rCB grades.
- Waste latex products: Cross-linked latex waste. High moisture and high protein content; low carbon black content, not economical for rCB.
Feedstock factors that affect final rCB quality
When selecting feedstock for rCB production, producers evaluate these key characteristics:
- Carbon black loading in the original rubber
- Type and quantity of inorganic fillers (ash precursors: CaCO₃, silicates, ZnO)
- Contaminants: steel, textile, dirt, stones, paint, halogen compounds
- Rubber polymer type (NR, SBR, BR or blends)
- Moisture content
- Variability between batches
End-of-life tires, especially truck tires, remain the primary feedstock for commercial high-grade rCB. Pre-consumer rubber factory scrap is a premium alternative. Non-tire post-consumer rubber can be used for low-spec rCB, while mixed rubber-plastic waste is generally not recommended for high-performance recovered carbon black. Feedstock sorting and pre-treatment (shredding, metal and fiber removal, cleaning) are essential upstream steps to minimize ash and impurities before pyrolysis, reducing the burden on downstream grinding, classification and purification processes.
