Formation Mechanism and Process Control of Carbon Black from Tire Pyrolysis

Carbon black derived from tire pyrolysis does not originate from a single source but consists of two components. The first component is the industrial carbon black originally added as a reinforcing filler during tire manufacturing, accounting for approximately 30% of the tire's total weight. Since the pyrolysis process decomposes only the organic constituents of the rubber into volatile products, the microstructure of these original carbon black particles remains largely intact, ultimately concentrating in the solid residue. The second component consists of newly formed carbon black generated through secondary reactions of volatile matter during pyrolysis; this component is the critical variable determining both the yield and quality of the resulting carbon black.

Chemical Pathways for the Conversion of Volatiles into Carbon Black

When subjected to heat, rubber undergoes molecular chain scission, releasing large quantities of small-molecule gases and oil vapors. These volatiles comprise aromatic hydrocarbons (such as benzene, toluene, and styrene), small-molecule hydrocarbons (such as ethylene and acetylene), and various free radicals. During their residence in the high-temperature zone, these fragments are unstable and undergo a series of progressive chemical reactions.

The process begins with the evolution of small-molecule fragments into polycyclic aromatic hydrocarbons (PAHs). This transformation primarily follows the hydrogen-abstraction-acetylene-addition mechanism: free radicals abstract hydrogen atoms from benzene rings to create active sites, onto which acetylene molecules subsequently add in succession, causing the cyclic structures to expand. Concurrently, chain-like combinations and rearrangements among small-molecule radicals drive the increase in the molecular weight of the PAHs.

Once the PAHs reach a critical size, they transition from a gaseous to a condensed state. These large molecules link via carbon-carbon bonds, coalescing into fragments of two-dimensional graphene-like layers. To minimize surface energy, these fragments curve and curl to form fullerene-like core structures, eventually stacking to create nascent carbon black particles. This stage encompasses the complete physicochemical transformation from molecular nucleation to particle growth.

Key Process Parameters Influencing Carbon Black Yield

Reaction temperature is the most critical factor controlling carbon black formation. Significant carbon black generation typically commences only when temperatures exceed 1,000°C, with a marked increase in yield observed as the temperature rises to 1,300°C. Temperature not only determines whether the reaction proceeds but also influences the growth rate of polycyclic aromatic hydrocarbons (PAHs) and the final morphology of the particles.

The residence time of volatiles in the high-temperature zone is equally critical. Typically, a residence time of two to four seconds is required for volatiles to undergo the complete transformation process—from initial fragments to PAHs and subsequently to carbon black particles. Insufficient residence time results in incomplete reactions of intermediate products, thereby reducing carbon black yield; conversely, excessive residence time can lead to the over-aggregation of carbon black particles, compromising product quality.

The type of feedstock rubber also significantly affects carbon black yield. Synthetic rubbers, such as styrene-butadiene rubber (SBR) and polybutadiene rubber (BR), possess higher aromatic content in their molecular structures and exhibit higher carbon black yields during pyrolysis compared to natural rubber. This disparity stems from differences in the quantity of aromatic hydrocarbon precursors released during the thermal scission of the respective rubber molecular chains.

Process Control Strategies for Pyrolytic Carbon Black

Pyrolytic carbon black is essentially a mixture of primary industrial carbon black and carbon black generated through high-temperature secondary reactions. The latter represents the core value of pyrolysis technology in achieving resource recovery. By precisely controlling pyrolysis temperature and volatile residence time, the yield and structural characteristics of the newly formed carbon black can be tailored to meet performance requirements for various applications. Current research focuses on establishing quantitative relationships between process parameters and properties such as carbon black yield, particle size distribution, and surface activity, thereby providing a theoretical foundation for the industrial application of pyrolytic carbon black.