Sản phẩm cao su lưu hóa sẽ phân hủy nhanh khi tiếp xúc với các yếu tố như oxygen, ozone, ánh sáng mặt trời, nhiệt độ cao. Sự phân hủy cao su xảy ra khi tiếp xúc với ozone được cho là do ozone phản ứng với các liên kết đôi của polymer chưa bão hòa. Phần tài liệu bên dưới sẽ trình bày cụ thể quá trình ozone hóa cao su và cơ chế bảo vệ của các chất chống ozone.

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Ozone, a degradant of vulcanized rubber, occurs naturally and is formed in the earth’s atmosphere by the action of the sun’s ultraviolet light on atmospheric oxygen. The ozone is carried into the atmosphere by winds and, depending on the seasons and geographic locations, can be found on the earth in normal concentrations of 6 parts per hundred million to concentrations of as high as 25 parts per hundred million.

The effects of ozone on vulcanized diene rubber are best noted when the rubber is stressed or stretched in use. A series of cracks develop, in time, which are perpendicular to the applied stress. Further exposure to ozone of these cracked surfaces causes the cracks to become wider and deeper until the rubber fails.

The detrimental effects of ozone are noted also on nonstressed rubber surfaces and manifest themselves as frosting, which is the exposure of the non-black fillers resulting from the formation of minute cracks on the rubber surface formed by the severing of the molecular chains. This phenomenon is quite common in footwear compounds [1].

The mechanism proposed by Criegee best describes the degradation initiated by ozone called ozonolysis. Ozone, a very reactive material, reacts at the surface, across the double bond, in an unsaturated polymer to form a trioxolane structure. This structure undergoes decomposition to give a carbonyl compound and a zwitterion, resulting in a severed molecular chain. The zwitterion can recombine to form either an ozonide, diperoxide, or higher peroxide.

Many theories have been proposed to explain the mechanism of how antiozonants protect elastomeric articles from the effects of ozone. Most support the scavenger model, and indicate that an ideal antiozonant should be capable of migrating in a rubber matrix to the surface whenever the equilibrium of the antiozonant concentration in the compound is upset by the formation of ozonized antiozonant at the surface, but yet not freely migrate to the surface and volatilize out of the elastomer without first reacting with ozone at the surface. On the other hand, the antiozonant should not be so slow in migrating through the elastomer matrix that it arrives at the surface after the ozone has already reacted with the elastomeric polymer. It has been stated that ozone is 200 times more reactive with an antiozonant than with the double bonds in an elastomer.

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Trích đăng từ sách Handbook of Specialty Elastomers, Robert C. Klingender, CRC Press, 2008, trang 430 – 431

(vtp-vlab-caosuviet)