Oligomeric Curing Activators Enable Conventional Sulfur-Vulcanized Rubbers to Self-Heal

Abstract

When introducing self-healing properties to elastomers, it is often difficult to balance their ability to recover properties after damage with a good mechanical strength prior to damage. We demonstrate that by replacing the activator system used in conventional accelerated vulcanization (CV) chemistry, from the traditional zinc oxide (ZnO) and stearic acid to a complex formed between ω-propenyl functional oligomers of poly(zinc methacrylate) (pZnMA/ZnO), the self-healing properties of vulcanized natural rubbers are enhanced while maintaining good tensile strengths. The pZnMA oligomers, as synthesized by catalytic chain transfer polymerization (CCTP), act as an activator for the sulfur curing system, while also forming an ionic network in the rubber. The addition of 20 phr of pZnMA/ZnO to a CV system resulted in a cured natural rubber with a tensile strength of 7.47 ± 0.64 MPa, which recovered 86.7% after self-healing at 80 °C for 2 h. Further addition of 40 phr of carbon black N234 unexpectedly enhanced the self-healing efficiency of these vulcanized rubbers to 92.2% under the same conditions and also improved the self-healing at room temperature. Finally, dynamic mechanical thermal analysis indicated that the natural rubber formulations containing pZnMA/ZnO showed improved wet traction but with higher rolling resistance to a standard formulation. These results point to an interesting direction for further research into the performance of self-healing composites in vehicle tire applications.