Devulcanization is a core technology for end-of-life rubber recycling, designed to selectively break sulfur crosslinks in vulcanized rubber networks while preserving rubber polymer chains to produce reusable reclaimed rubber. As the primary reinforcing filler in tire and industrial rubber compounds, carbon black typically accounts for 20–30% of tire formulations, and its structural evolution during devulcanization directly determines the performance of devulcanized rubber compounds, as well as the quality of carbon black recovered from devulcanized feedstock.
Unlike pyrolysis, which thermally decomposes rubber polymers completely to produce raw pyrolysis char, devulcanization operates at milder temperatures (typically 120–250 °C) and targets selective scission of S–S and C–S crosslinks. Its impact on carbon black structure is therefore distinct from thermal pyrolysis, with effects concentrated on aggregate networks, surface interfacial layers, surface chemistry and adsorption properties.
1. Aggregate Structure and Filler Network Evolution
Carbon black exists in vulcanized rubber on two structural levels: primary aggregates covalently fused during production, and secondary filler networks formed by physical van der Waals attraction between aggregates. Devulcanization affects these two structures to different degrees.
Breakdown of the secondary filler network
In cured rubber compounds, carbon black aggregates form a continuous three-dimensional filler network, which is the structural origin of the Payne effect and contributes significantly to material modulus. During devulcanization, combined thermal energy and mechanical shear disrupt the weak physical interactions between aggregates, causing the secondary filler network to disassemble.
This breakdown is reflected in a measurable reduction in the Payne effect of devulcanized compounds. As inter-aggregate attractions weaken, filler-filler interactions diminish, which improves the processability of reclaimed rubber but also reduces the elastic modulus contribution from the filler network.
Partial primary aggregate breakage under high shear
Under intense mechanical shear — as occurs in mechanochemical devulcanization via two-roll mills or reactive extrusion — even the covalently bonded primary aggregates can undergo partial fracture. Studies have confirmed that carbon black aggregates break up during high-shear devulcanization, generating fresh fracture surfaces with high surface energy and reactive active sites.
These newly exposed active sites readily react with devulcanization aids (such as diphenyl disulfide) and rubber radicals generated during crosslink scission. For mild thermo-chemical devulcanization with low shear input, however, the primary aggregate structure remains largely intact, preserving the native reinforcing morphology of the carbon black.
2. Bound Rubber Layer and Interfacial Structure Changes
In vulcanized rubber, a layer of tightly bound rubber forms on the carbon black surface through chemical adsorption and molecular chain entanglement. This bound rubber layer is the critical interfacial structure responsible for carbon black’s reinforcing effect, and it undergoes characteristic changes during devulcanization.
Selective decoupling of crosslinked interface chains
Devulcanization breaks the sulfur crosslinks within and around the bound rubber layer, loosening the entangled rubber molecular chains and allowing partial desorption from the carbon black surface. However, the innermost rubber fraction, which is chemically bonded to the carbon black surface, cannot be removed by mild devulcanization.
As a result, devulcanized carbon black retains a stable core-shell structure: a carbon black core wrapped in a thin residual organic rubber shell. This residual organic layer is the defining structural difference between devulcanization-recovered carbon black, virgin carbon black and pyrolysis-derived recovered carbon black (rCB).
Impact on interfacial compatibility
The residual rubber shell alters the surface energy of carbon black particles. On one hand, it improves compatibility with fresh rubber matrices during revulcanization; on the other hand, it prevents direct contact between the carbon black surface and the new polymer, leading to slightly lower reinforcement efficiency compared with virgin carbon black of the same grade. Research on carbon black separated from oil-desulfurized crumb rubber confirms that the residual surface organic layer is a major reason for its lower reinforcing performance relative to virgin N330 carbon black.
3. Surface Chemistry and Active Site Modification
Devulcanization is not only a physical process but also a chemical one, and it induces measurable changes in the surface chemistry of carbon black.
Generation and reaction of surface active sites
Primary aggregate fracture creates fresh carbon surfaces with abundant unsaturated bonds and free radicals. These highly reactive sites readily couple with rubber macroradicals and devulcanization reagent radicals formed during crosslink scission, forming new covalent bonds on the carbon black surface.
Studies on thermo-chemical devulcanization have found that reactive radicals generated during the process react with active sites on the carbon black surface to form complex gel structures. This side reaction consumes free radicals and reduces devulcanization efficiency, and it also permanently modifies the surface chemical composition of the carbon black.
Process-dependent surface functional group evolution
Different devulcanization technologies introduce different surface modifications:
- Sulfur-containing devulcanization aids such as diphenyl disulfide introduce aromatic and sulfide groups onto the carbon black surface, which can improve interfacial bonding with hydrocarbon rubber matrices.
- In microwave devulcanization, carbon black acts as a strong microwave absorber, generating rapid localized heating. This selective thermal effect can cause mild surface oxidation, increasing the content of oxygen-containing functional groups and altering surface wettability.
4. Pore Structure and Adsorption Performance
The effect of devulcanization on the internal pore structure of carbon black is significantly different from that of high-temperature pyrolysis.
Preserved intrinsic microporosity
Since devulcanization temperatures are far below the graphitization and thermal decomposition temperature of carbon black, the native microporous structure inside carbon black particles is almost completely preserved. This is a potential structural advantage of devulcanization-derived carbon black over pyrolysis rCB, as high-temperature pyrolysis often causes micropore collapse and structural shrinkage via carbon condensation.
Reduced accessible surface area
Despite preserved internal pores, the residual bound rubber layer and organic deposits on the particle surface block micropore entrances and cover external surface area. As a result, the measured iodine number and BET specific surface area of as-separated devulcanized carbon black are consistently lower than those of the corresponding virgin grade. The full adsorption potential of the material is only revealed after deep purification to remove surface organic residues.
5. Variations Across Devulcanization Technologies
The extent of structural change depends strongly on the devulcanization method employed:
- Mechanochemical devulcanization applies the highest shear force, resulting in the most thorough breakdown of secondary filler networks and the highest proportion of primary aggregate breakage. It causes the greatest structural damage but also produces the most reactive carbon black surfaces.
- Thermo-chemical devulcanization operates under mild conditions with minimal shear. It preserves primary aggregate structure almost completely and produces a uniform residual rubber layer, making it the least structurally damaging route for carbon black.
- Microwave/ultrasonic devulcanization achieves selective crosslink scission through volumetric heating or cavitation. It causes minimal damage to bulk carbon black structure, but localized hot spots may induce slight surface oxidation.
6. Implications for Recycled Carbon Black Production
Devulcanization represents an alternative pathway for recovering functional carbon black from waste rubber, producing a material with different structural characteristics than pyrolysis-derived rCB. While its intrinsic structure is well preserved, residual organic coatings and re-agglomeration require targeted post-processing to unlock its full application value.
Ultra-fine grinding and high-precision air classification technologies can effectively break residual agglomerates in devulcanized carbon black, achieve tight particle size control at D90 < 10μm, and improve dispersion performance. When combined with purification steps to remove excess surface organics, the finished product can deliver stable iodine values and purity suitable for medium-grade rubber and polymer filler applications. As a leading provider of rCB grinding and classification systems, JACAN Powder Equipment’s multi-parameter intelligent optimization platforms can be adapted to devulcanization-derived feedstocks, delivering consistent, application-grade recycled carbon black with preserved structural integrity. Devulcanization does not fundamentally destroy the primary structural framework of carbon black, but it modifies its secondary aggregation state, surface interfacial layer and chemical properties through mechanical shear, thermal action and chemical reaction. Well-controlled mild devulcanization preserves most of carbon black’s intrinsic reinforcing structure, making it a valuable recyclable functional material. With appropriate post-grinding, classification and purification processing, devulcanization-derived carbon black can serve as a sustainable, structure-preserving alternative to medium-grade virgin carbon black across a wide range of industrial applications.
