**2.5 Rubber**

The rubber material is a polymer material with high elasticity and viscoelasticity. Among them, natural rubber (NR) is widely used in tires, conveyor belts, hoses, adhesive tapes, adhesives, sponges, and rubber products because of its excellent comprehensive properties. Production of various rubber products such as shoes, sports equipment, wires and cables, medical equipment, aircraft, and auto parts.

In the past 30 years, the average annual growth rate of global natural rubber consumption has exceeded 5%. The consumption has increased from 4.35 million tons in 1985 to 11.98 million tons in 2015 [42, 43].

In the actual storage and application process, rubber products are often in various environments and will be affected by oxygen, ozone, ultraviolet rays, and heat [44–47].

Weng et al. [47] have investigated Vulcanized natural rubber (NR) under quiescent thermal oxidation aging and high temperature fatigue loading with small strain amplitude. The vulcanized NR was simultaneously subject to high temperature (85°C) and cyclic loading. Under these conditions, their scanning electron microscope (SEM) images confirmed the appearance of nanoscale cracks and voids which are initiated by the combined impact of high temperature and cyclic loading. As the cracks evolve, nano-scale voids appear. They concluded that further evolution of the smaller voids leads to the larger spherical voids with their diameters varying between a few hundred nanometers and several microns.

In addition, seawater [48] will also affect rubber products. The above-mentioned various factors lead to the destruction of rubber composition and structure, gradually losing its original excellent performance, and even loss of use-value, the phenomenon of rubber aging [49]. "Aging" is a common problem of all polymer materials, especially natural rubber with unsaturated bonds on the major chain, which is more susceptible to aging by oxygen and free radicals. Once aging occurs, it will seriously affect its use value and application range, and even a major accident with dire consequences.
