Plastic waste accumulation poses significant environmental challenges, underscoring the importance of closing the plastic life cycle by converting waste into valuable products. Pyrolytic char, a promising byproduct, offers diverse industrial applications. However, little attention has been given to converting hydrocarbon-based plastics into solid carbon due to their low fixed carbon content. This study compares sulfonation and thermal oxidation stabilization techniques for carbonizing waste polyethylene and polyvinyl chloride. Results indicate sulfonation yields higher pyrolytic char formation and improves surface properties, surpassing Al2O3 in catalyst support characteristics. Thermal oxidation increases fixed carbon content but adversely affects char yield and surface properties. Both methods effectively enhance fixed carbon content, but sulfonation emerges as more advantageous, suggesting potential use of plastic-derived char as a catalyst support material, contributing to circular economy development.