*2.5.1 Aging mechanism of natural rubber*

The aging process is: under the action of ultraviolet rays, the oxidation reaction of natural rubber starts from α-H, first is attacked by oxygen to generate hydroperoxide, and then further reacts to generate aldehydes and ketones, and at the same time the main chain is broken [50, 51]. The free radicals generated by chain scission may combine with double-free radicals, or they may be further attacked by oxygen to continue oxidation; in this process, double bonds addition reaction occurs, and the attack of free radicals on double bonds reduces the content of olefinic hydrogen. The early stage of the reaction is dominated by oxidative degradation, which is manifested as the surface of the aged sample is sticky, and cross-linking occurs in the later stage, the surface of the sample is dried and hardened, and cracks appear.

Ling Ding et al. [52]'s 1H-NMR and FT-IR absorption spectroscopy experimental results proved that the products obtained in the aging process of natural rubber in artificial ultraviolet light aging and natural aging are roughly the same. They found that natural rubber has poor resistance to ultraviolet light aging, and its molecular structure changes significantly when exposed to ultraviolet light for 2 h. Under

natural aging conditions, the structure of natural rubber changes significantly after 30 days. Artificial UV aging for 7 days is equivalent to the effect of natural aging for 3 months.
