Zn/Al-NO3 layered double hydroxide as a substitute activator for nitrile rubber vulcanization: effect of accelerator sulfur ratio on vulcanization kinetics and network properties

Research Background and Objective: Reducing zinc content in rubber formulations has always been an environmental issue of concern in the industry. Zinc oxide (ZnO), as an important component of vulcanization systems, has been explored as a low zinc activator due to its environmental concerns. Zinc containing layered double hydroxides (Zn/Al LDHs) provide an alternative approach by introducing zinc into the layered structure, which has previously been studied as a sulfurization activator and filler. However, the vulcanization behavior of unmodified Zn/Al-NO3 LDH in traditional nitrile rubber (NBR) is still not fully understood. To this end, researchers evaluated the feasibility of Zn/Al-NO3 LDH as a low zinc sulfide activator for NBR and as a substitute for ZnO/stearic acid system, and compared its performance under traditional sulfide system (accelerator/sulfur ratio A/S=0.6) and high-efficiency sulfide system (A/S=3.0).

Key experimental data

In traditional sulfurization systems, the apparent activation energy (Ea) of LDH containing formulations decreased from 90.81 kJ · mol ⁻¹ to 70.95 kJ · mol ⁻¹, with a decrease of about two orders of magnitude in the pre exponential factor.

. The tensile strength remains equivalent, but the tear strength decreases by about 14%. Thermogravimetric analysis showed that the initial degradation temperature of the LDH containing formula slightly increased. According to the nominal composition calculation, replacing the ZnO/stearic acid activation system with Zn/Al-NO3 LDH resulted in a reduction of approximately 71% in zinc loading. In an efficient vulcanization system, the LDH containing formula maintains a comparable apparent vulcanization rate constant, but its mechanical properties have decreased.

Significance for the polymer additive industry

This study provides new experimental evidence for exploring low zinc vulcanization systems in the tire and rubber industries.

. Under the traditional sulfurization system, Zn/Al-NO3 LDH maintains its tensile strength while reducing its apparent activation energy, indicating its potential as an activator for sulfurization systems to replace ZnO/stearic acid systems. However, the decrease in tear strength suggests the need to balance performance in practical formulations. For other rubber additives such as anti-aging agents and antioxidants, changes in the vulcanized network structure may affect aging behavior, but this study did not involve relevant evaluations. In terms of reinforcing fillers, LDH itself has filler properties, and its interaction with reinforcing fillers such as white carbon black deserves further attention. It should be emphasized that the study clearly did not evaluate zinc release, bioavailability, and ecotoxicological effects, therefore a 71% reduction in zinc loading cannot be directly equated with environmental benefits. When promoting low zinc vulcanization systems in the industry, it is still necessary to combine a complete environmental risk assessment and tire lifecycle performance for judgment.

Information source:Zn/Al‐NO3 Layered Double Hydroxide as Alternative Activator for Nitrile Rubber Vulcanization: Effect of the Accelerator‐to‐Sulfur Ratio on Curing Kinetics and Network Properties， Organized by Herun Chemical.