Antiozonants, rubber under static conditions

The effect of ozone is complicated in so far as its effect is largely at or near the surface and is of greatest consequence in lightly stressed rubbers. Cracks are formed with an axis perpendicular to the applied stress and the number of cracks increases with the extent of stress. The greatest effect occurs when there are only a few cracks which grow in size without the interference of neighbouring cracks and this may lead to catastrophic failure. Under static conditions of service the use of hydrocarbon waxes which bloom to the surface because of their crystalline nature give some protection but where dynamic conditions are encountered the saturated hydrocarbon waxes are usually used in conjunction with an antiozonant. To date the most effective of these are secondary alkyl-aryl-p-phenylenediamines such as /V-isopropyl-jV-phenyl-p-phenylenediamine (IPPD). 

Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. For serving conditions which involve continuous flexing, /j-phenylenediamines (A, A -alkyl-aryl derivatives) can be added. These chemical antiozonants scavenge the ozone before it reacts with the rubber. A barrier of ozonized products is created which protects both the rubber and antiozonant from further attack. However, p-phenylenediamines are staining compounds. Whenever colour is an important concern, blends of elastomers can be used elastomers loading should be higher than 30 phr to provide sufficient effectiveness. 

Rubbers can be protected against ozone by use of chemical antiozonants and via several physical methods. The chemical antiozonants protect rubber under both static and dynamic conditions, whereas the physical methods are more related towards protection under static conditions. 

Rubber is protected against ozone attack by addition of physical and/or chemical antio-zonants. Hydrocarbon waxes are the most conunon type of physical antiozonants, and p-phenylenediamines derivatives are the prevalent chemical antiozonants. Waxes bloom to the rubber surface and form a protective barrier. Since this bloom is brittle, it is broken by flexing. Therefore, waxes only protect under static conditions. A connnon loading for antiozonants in rubber formulations is 1.5 to 3 phr. Combinations of waxes and chemical antiozonants are generally used. 

Blends of petroleum waxes are commonly used as antiozonants in the rubber industry. Just as 6PPD gives excellent protection against ozone attack of the surface of the rubber under dynamic conditions, waxes will bloom to the surface of the rubber to impart excellent resistance to ozone attack under static conditions. The wax actually exudes (or blooms) to the surface of the rubber article to create a physical protective barrier to protect the rubber surface against ozone attack, which can create cracking of the rubber. 

The requirements for static ozone resistance vs. dynamic ozone resistance are very different, so that choiee of antiozonant depends greatly on the expected service of the rubber produet. Static protection is provided by petroleum waxes, usually paraffin and/or microcrystalline waxes. The waxes work by blooming to the rubber surface to form a physical barrier to ozone attack. The choice of wax or wax blend is based upon migration temperature where mobility and solubility of the wax in the rubber are balanced so that sufficient bloom occurs for optimum protection. Because the wax film is inextensible, it will rupture under deformation and expose the elastomer. Waxes protect only under static conditions. 

Of the two general categories of waxes— paraffin and micro-crystalline—the latter are more strongly held to the surface. However, the use of waxes alone to provide protection against ozone attack is rather well restricted to static conditions of service. Whenever constant flexing is present, even the more strongly held microcrystalline waxes flake off and protection is lost. Combinations of waxes and chemical antiozonants are therefore used to provide protection under both static and dynamic conditions of service. In fact, it is felt that waxes can aid in the diffusion of chemical antiozonant to the rubber surface. 

Photo-oxidation proceeds relatively slowly. That is why elastomers made from chlo-roprene rubber exhibit considerably better resistance to ozone attack and weather influences than those made from NR, SBR, and NBR. Moreover, chloroprene rubbers respond well to anti-aging and antiozonant agents so that they can be recommended for use under difficult climatic conditions, e.g., in the tropics. By extrapolating mechanical property changes after several years of weathering in ozone-rich atmosphere, the current technology allows the prediction that chloroprene rubber products will exhibit sufficient strain for most static applications even after 50 years in service [697]. 

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See also in sourсe #XX -- [ , ]

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