Design of Experiments‐Based Optimization of Recovered Carbon Black–Reinforced Isobutylene–Isoprene Rubber in Comparison With Conventional Carbon Black Systems

ABSTRACT Recovered carbon black (rCB) obtained from end‐of‐life tire pyrolysis is a promising sustainable alternative to conventional carbon black (CB) in elastomer formulations; however, its lower reinforcing efficiency often limits direct substitution in industrial applications. Previous studies have shown that iso‐mass and iso‐volume replacement of CB by rCB in isobutylene–isoprene rubber (IIR) weakens filler networking due to high inorganic content and altered surface chemistry, reducing tensile performance. In this work, rCB‐filled IIR composites were optimized through formulation design, without physicochemical modification of the rCB. A central composite design was employed to evaluate the effects of rCB loading, plasticizer content, and sulfur level on Shore A hardness and index rupture energy (IRE). Shore A hardness exhibited a predominantly linear response with excellent model fit ( R 2 = 98.13%), whereas IRE showed significant curvature and interaction effects, requiring a quadratic response surface model ( R 2 _adj = 90.44%). Multi‐response optimization identified rCB–IIR formulations achieving Shore A hardness comparable to the CB‐filled reference and substantially improved IRE relative to non‐optimized rCB systems. Under the investigated conditions, full CB replacement by rCB in IIR composites can be achieved without compromising static mechanical properties; however, validation through dynamic and aging performance remains necessary for industrial implementation.