The changes in oil resistance of oil-resistant rubber heat shrink tubing under mechanical stress are a complex interplay involving material structure, mechanical forces, and the chemical environment. Oil resistance fundamentally depends on the interaction between rubber molecular chains and oil molecules. Mechanical stress indirectly influences this process by altering the material's microstructure, ultimately manifesting as dynamic changes in oil resistance.
Under static conditions, the oil resistance of oil-resistant rubber heat shrink tubing is primarily determined by the molecular structure of the rubber substrate. For example, the fluorine atoms in fluororubber molecular chains have strong electronegativity, forming a dense molecular barrier that effectively blocks the penetration of oil molecules. The acrylonitrile groups in nitrile rubber inhibit oil swelling through polar interactions. In the unstressed state, the molecular chains of these materials are naturally coiled, forcing oil molecules to overcome a high energy barrier to penetrate the material, resulting in stable oil resistance.
When mechanical stress is applied to heat shrink tubing, two key changes occur within the material: conformational adjustments of the molecular chains and the evolution of microscopic defects. Under tensile stress, rubber molecular chains stretch along the stress direction, straightening previously curled segments and increasing the free volume between molecules. This structural change lowers the energy barrier for oil molecules to penetrate, making it easier for oil to penetrate the material. For example, under repeated bending stress, microcracks may form on the surface of heat shrink tubing. These defects serve as "pathways" for oil penetration, accelerating the swelling process.
The impact of mechanical stress on oil resistance is also closely related to the type of stress. Static tensile stress causes continuous deformation of the material, keeping the molecular chains in a high-energy state for a long time, and the oil penetration rate increases over time. Dynamic fatigue stress (such as alternating stress) can cause irreversible damage to the material's internal structure. Under dynamic loading conditions, rubber molecular chains may break or slip, disrupting the crosslinking network. This damage reduces the material's density, allowing oil molecules to penetrate deeper, leading to increased swelling and performance degradation.
The synergistic effect of temperature and mechanical stress further amplifies changes in oil resistance. At high temperatures, the thermal motion of rubber molecular chains is intensified, making mechanical stress more likely to induce molecular chain slip and rearrangement. For example, when heat shrink tubing is simultaneously subjected to high temperatures and tensile stress, the permeation rate of oil molecules can increase exponentially. This is because high temperature reduces the interactions between molecular chains, while stress provides an additional driving force. The combined effect of these two factors significantly reduces the material's oil resistance.
Material design can mitigate the negative impact of mechanical stress on oil resistance through structural optimization. For example, reinforcing a rubber matrix with nanofillers (such as carbon nanotubes and silica) creates a "filler-rubber" network structure. This structure not only improves the material's mechanical strength but also inhibits molecular chain slippage through interfacial interactions between the filler and rubber, thereby reducing stress-induced swelling. Furthermore, adjusting the crosslink density can achieve a balance between material strength and oil resistance. Moderate crosslinking can restrict molecular chain motion, but excessive crosslinking can increase material brittleness and exacerbate stress damage.
Mechanical stress indirectly affects the oil resistance of oil-resistant rubber heat shrink tubing by altering the conformation and microstructure of the rubber molecular chains. This effect is closely related to the type of stress, temperature, and material design, requiring structural optimization and standardized testing to achieve performance control.
