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.
…
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.
…
Trích
đăng từ sách Handbook of Specialty
Elastomers, Robert C. Klingender,
CRC Press, 2008, trang 434 – 435
(vtp-vlab-caosuviet)
