Antiozonants, rubber under dynamic conditions

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. 

It should be noted that under dynamic conditions, the protective wax film breaks down, after which the antiozonant system in the rubber formulation will serve as the primary stabilizer or protection mechanism. Waxes are used to ensure protection against ozone for products in storage, such as tires in a warehouse. 

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). 

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. 

Chemical antiozonants have been developed to protect rubber against ozone under such dynamic conditions. Several mechanisms have been proposed to explain how chemical antiozonants protect rubber. The scavenging mechanism, the protective film mechanism, or a combination of both are nowadays the most accepted mechanisms. 

Rubbers having main chain unsaturation can undergo ozone cracking when test pieces are exposed to atmospheric traces of ozone at an elongation above a characteristic threshold strain [63]. For most diene rubbers this strain is typically around 5% in the absence of any protective agent, although under dynamic strain conditions it can be reduced to less than 1%. Antiozonant materials are added to rise the threshold strain above the maximum strain encountered in service life or alternatively, as is usually the case in dynamically strained applications, to reduce the rate of crack growth. 

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. 

Under service conditions of dynamic flexing, wax alone will be inadequate to protect the rubber against ozone attack. The flexing action will rupture the surface film of the wax and thus provide an avenue of entry for the attacking ozone. Thus dynamic flexing requires the joint use of wax plus chemical antiozonant to attain proper protection against ozone attack. 

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. 

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