The tensile strength of lifting belts exposed to long-term UV radiation deteriorates due to a combination of material aging and photochemical damage. This process involves multiple mechanisms, including fiber molecular chain breakage, changes in crystallinity, and additive degradation. Ultraviolet radiation, as high-energy electromagnetic waves, can penetrate the surface of the belt and directly affect the molecular structure of synthetic fibers, triggering photooxidation reactions and causing irreversible damage within the fibers.
Synthetic fiber belts are primarily made of polyester, nylon, and polypropylene, all of which have varying sensitivities to UV radiation. For example, the ester groups in polyester's molecular chains are susceptible to UV radiation, forming free radicals that trigger a chain reaction, shortening the molecular chains and reducing crosslinking. These microstructural changes directly weaken the fiber's load-bearing capacity, manifesting as a decrease in tensile strength. Nylon, due to its amide bonds, is more susceptible to UV radiation and generally degrades faster than polyester. Polypropylene, due to its simpler molecular structure and lacking effective UV-absorbing groups, exhibits relatively poor aging resistance.
UV radiation also causes changes in fiber crystallinity. Initially, UV rays may cause some molecules in the amorphous region to rearrange, forming a microcrystalline structure, temporarily improving the fiber's stiffness. However, as exposure time increases, the crystalline region is also damaged, resulting in a smaller and more unevenly distributed crystallites, leading to a deterioration in the fiber's overall mechanical properties. This phenomenon of initial increases in crystallinity followed by decreases explains why lifting belts in the lifting industry may not show obvious damage during initial UV exposure, but their strength rapidly decreases after prolonged use.
UV-resistant additives, such as UV absorbers and light stabilizers, are added to lifting belts during production to absorb or scatter UV rays, protecting the fiber backbone. However, these additives gradually degrade over time, especially in high temperature, high humidity, or strong UV environments, where degradation is accelerated. Once these additives have lost their effectiveness, the fiber is directly exposed to UV rays, accelerating the aging process. Furthermore, some additives may have compatibility issues with the fiber matrix and, after long-term use, may migrate to the surface, causing localized protective failure and further exacerbating strength degradation.
The synergistic effect of environmental factors and UV rays can amplify the strength loss of lifting belts. For example, humid environments promote photooxidation reactions, with water molecules acting as a reaction medium, accelerating the generation and diffusion of free radicals. High temperatures increase the thermal motion energy of molecules, making UV-induced fracture more likely. For slings used outdoors for extended periods, if simultaneously exposed to UV, rain, and high and low-temperature cycles, their tensile strength may degrade several times faster than if exposed to UV alone.
From a macroscopic perspective, UV damage is often accompanied by discoloration, surface yellowing, and fiber stiffening. The color change results from the degradation of dye molecules, while fiber stiffening is the result of the rearrangement of low-molecular-weight fragments after molecular chain breakage. While these changes do not directly equate to strength loss, they can serve as intuitive indicators of the degree of aging. Severe discoloration or localized stiffening of slings typically indicates irreversible damage to the internal structure and requires immediate removal from service.
To mitigate UV-induced strength degradation, the industry typically employs a dual strategy of material modification and process optimization. On the one hand, UV-resistant groups can be introduced through copolymerization or blending, such as by adding benzotriazole compounds, to enhance the fiber's intrinsic light resistance. On the other hand, post-finishing techniques can be used to coat the fiber surface with inorganic UV-shielding agents such as nano-titanium dioxide to form a physical protective layer. Furthermore, optimizing the lifting belt's structural design, such as increasing the weave density or adopting a double-layer structure, can also extend its service life by distributing the load.
Long-term UV exposure affects the tensile strength of lifting belts in the lifting industry gradually and irreversibly. The degradation mechanisms involve multiple factors, including molecular chain breakage, changes in crystallinity, additive failure, and environmental synergy. Understanding these mechanisms can help optimize material selection, improve production processes, and develop more scientific maintenance strategies, thereby ensuring the long-term reliability of lifting belts in complex outdoor environments.
