Functionalized recycled carbon black from waste tires reinforced bio-high density polyethylene from sugarcane

• Ethylene functionalization of vacuum pyrolyzed recycled carbon black from waste tires. • Functionalized rCB disperses better, reducing agglomeration in composites. • Functionalized rCB reinforced composites show up to 45% higher tensile modulus. • Functionalized rCB acts as nucleating agent, enhancing polymer crystallinity and stiffness. • Provides sustainable alternative to petroleum-derived CB, supporting circular economy by enabling carbon resource valorization. This study investigates the valorization of recycled carbon black (rCB) recovered from end-of-life tires by vacuum pyrolysis through in situ surface functionalization via a new catalytic ethylene oligomerization. Short polyethylene-like chains are grafted directly onto the rCB surface, transforming an inherently incompatible, low-value recycled filler into a functionalized reinforcing agent modified for integration into a bio-based polymer matrix. The functionalized rCB (rCB-F) is incorporated into an eco-friendly thermoplastic composite using bio-based high-density polyethylene (Bio-HDPE) derived from sugarcane and characterized by a low carbon footprint. The particle size distribution and morphology of both commercial carbon black (cCB), recycled carbon black (rCB), and functionalized recycled carbon black (rCB-F), were characterized using laser diffraction, scanning electron microscopy, and transmission electron microscopy. Composites containing 3 wt% and 15 wt% CB were manufactured and analyzed using X-ray diffraction, differential scanning calorimetry, mechanical testing, and microscopy techniques. The results indicate that both filler type and concentration affect crystallinity, lamellar structure, and mechanical properties. Notably, the bio-composite containing 15 wt% rCB-F exhibited a 45% increase in tensile modulus and a 14% increase in hardness relative to neat Bio-HDPE. Crystallinity measurements obtained from both XRD and DSC were correlated with tensile performance, highlighting the influence of differences between the two analytical techniques. These findings suggest that rCB-F provides a sustainable reinforcement alternative to fossil fuel-derived cCB, offering competitive performance in environmentally responsible composite material applications.