Transforming waste cooking oil into zinc soap: a sustainable approach to multifunctional additives for enhancing natural rubber composites

Recycled or used cooking oil (UCO) was utilized as a raw material to synthesize zinc soaps, which were subsequently employed as processing aids in silica-filled natural rubber (NR) compounds. Their performance was compared with zinc soaps derived from coconut and palm oils, as well as a commercial processing aid. The results revealed that zinc soaps significantly reduced Mooney viscosity and enhanced stress relaxation rates in NR compounds, attributed to their lubricating and plasticizing properties, which improved flowability and processability. These zinc soaps also influenced curing characteristics, resulting in higher torque differences and crosslink densities, thereby enhancing mechanical strength. Furthermore, the incorporation of in-house synthesized zinc soaps shortened cure and scorch times while accelerating the cure rate, underscoring their synergistic role in promoting crosslinking reactions. Additionally, these zinc soaps improved the dispersion of solid particles within the NR matrix, fostering greater uniformity and optimizing compound properties. The inclusion of zinc soaps also boosted rubber elasticity, as indicated by a higher rubber index (RI), and enhanced thermal stability, evidenced by extended T₉₀ values, higher peak positions, and larger peak areas in stress relaxation profiles.