Cũng giống như hầu hết vật liệu khác, vật liệu đàn hồi cũng bị oxy hóa bởi oxy trong không khí, gây ra hư hỏng. Mức độ oxy hóa phụ thuộc vào loại polymer và môi trường tiếp xúc. Quá trình oxy hóa cao su xảy ra theo hai cơ chế khác nhau, quyết định tính chất của sản phẩm cao su bị oxy hóa khác nhau. Nội dung này sẽ được trình bày cụ thể bên dưới.
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Elastomers, like most organic materials, are subject to atmospheric oxidation, even at moderate temperatures. The ease of susceptibility to degradation depends, to a large degree, upon structure and environmental exposure. For example, saturated polymers are inherently more stable than unsaturated polymers because of their stronger bonds, or lack of double bonds in their backbone. Therefore, it would hold that EPDM and butyl rubber would be more stable than SBR or NR against oxidative degradation.
Oxidation is a complex process involving many reactions, each influenced by prevailing conditions such as:
1. Singlet oxygen
2. Ozone
3. Mechanical shear
4. Heat
5. Light
6. Metals
7. Fatigue
Most elastomers are subject to oxidation and it is known that the addition of only 1%–2% combined oxygen will render a rubber article useless.
Oxidation proceeds by two mechanisms:
1. Chain scission: Results from the attack of the polymer backbone which causes softening and weakness. It is the primary mechanism observed for natural rubber and butyl oxidation.
2. Cross-linking: Brittle compounds result because of radical cross-linking reactions, resulting in the formation of new cross-links and a stiffer material. This reaction occurs predominantly with SBR, polychloroprene, NBR, and EPDM.
In most cases, both types of attack occur and the one which prevails determines the final compound properties. It has been found that loss of elongation is the most sensitive criterion for aging measurements regardless of the mechanism, and it is favored over the measurement of tensile loss.
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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 434 – 435
(vtp-vlab-caosuviet)