Sự oxy hóa là nguyên nhân phổ biến gây hư hỏng cho vật liệu đàn hồi. Các yếu tố như nhiệt độ, ánh sáng và hàm lượng kim loại trong cao su tăng nhanh quá trình oxy hóa này. Nội dung này được trình bày cụ thể bên dưới.

EFFECTS OF HEAT

As noted, a small percent of oxygen, combined with an elastomer, can seriously degrade the physical properties of that elastomer. Heat can greatly increase the rate at which oxygen reacts with the polymer. The rate of reaction approximately doubles for each 10oC increase in temperature. About a 50-fold increase in reaction rate occurs between room temperature and 70oC. Another perspective would be that a change in physical properties, observed at a service temperature of 150oC, would require about 8000 times longer for the same change to occur at ambient temperature [9].

EFFECTS OF LIGHT AND WEATHERING

UV light promotes free radical oxidation at the rubber surface which produces a film of oxidized rubber. Heat and humidity then accelerate the formation of a crazing or alligatoring effect, and this oxidized layer can be rubbed off—giving a chalking appearance.

EFFECTS OF METALS

Heavy metal (principally cobalt, copper, manganese, iron) ions are believed to catalyze oxidative reactions in elastomers by influencing the breakdown of peroxides in such a way as to accelerate further attack by oxygen. The first corrective approach is to eliminate all sources of harmful metals. Compounds of copper and manganese, such as stearates and oleates, which are directly soluble in rubber, are particularly active, since they provide a direct source of heavy metal ions. Even the less soluble forms such as the oxides can cause problems by reacting with fatty acids used in compounding to produce more soluble forms.

Although some antioxidants are active against catalyzed oxidation of rubber, in general, the standard antioxidants do not give protection against the heavy metal ions. Since the activity of the metal depends on its being in an ionic form, it is possible to protect compounds by incorporating substances which react with ionic metals to give stable coordination complexes.

Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 435 – 436

(vtp-vlab-caosuviet)




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