Mechanistic Insight into Thermo-Oxidative De-Cross-linking of Waste Tire Rubber: A Comparative Study of Biobased and Petroleum-Based Swelling Agents

The recycling of waste tire rubber is severely limited by its cross-linked thermoset structure, creating a critical need for efficient de-cross-linking strategies to enable high-value upcycling. This study elucidates the chemical mechanism of swelling-agent-assisted thermo-oxidative de-cross-linking, comparing a sustainable biobased solvent (soybean oil, SO) against a traditional petroleum-based solvent (naphthenic oil, NO). Microstructural evolution and reaction kinetics were probed using Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and Horikx analysis. Results reveal that the de-cross-linking process is governed predominantly by main-chain scission rather than selective cross-link cleavage, driven by radical-induced oxidative aging. While NO exhibited higher de-cross-linking efficiency due to the “like-dissolves-like” principle─facilitating deeper oxygen diffusion into the nonpolar rubber matrix─the biobased SO pathway demonstrated sufficient network disruption to overcome phase separation challenges. Specifically, the SO-treated rubber achieved a critical storage-stability threshold (softening-point difference <2.0 °C) when upcycled into asphalt binders, comparable to the NO-treated counterparts. These findings provide molecular-level insights into the trade-off between solvent polarity and degradation efficiency, validating bio-oil-assisted de-cross-linking as a viable, sustainable route for converting recalcitrant rubber waste into storage-stable engineered materials.