Surface cracking by ozone

In an atmosphere containing ozone, stretched samples of unsaturated elastomers develop surface cracks which grow in length and depth until they eventually sever the test piece. Even when they are quite small, they can cause a 

As the stress level is raised above the minimum value, numerous weaker stress raisers become effective and more cracks form. Actually, a large number of small, mutually interfering cracks are less harmful than a few widely separated cracks which develop into deep cuts, so that the most harmful condition is just above the critical stress. 

The rate at which a crack grows when the critical energy condition is satisfied depends on two factors the rate of incidence of ozone at the crack tip and the rate of segmental motion in the tip region. When either of these processes is sufficiently slow, it becomes rate controfiing.The overall rate R of crack growth is thus given approximately by 

For a typical outdoor atmosphere, C is of the order of 10 mg/L, and the second term in Eq. (30) is then dominant for values of (pj greater than about 10 sec that is, at temperatures more than 25°C above Fg [79]. 

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Corona k9- ro-n9 [L, garland, crown, cornice] (1563) n. Cracks on the surface caused by ozone, usually spearing at right angles to a stress, checking, cutting or cracking. 

Ozone cracking - The surface cracks, checks or crazing caused by exposure to an atmosphere containing ozone. 

It was found advantageous to use as ozone measure the sum of the depth of all cracks as measured imder the microscope (lOOx) using an ocular micrometer. The measurement of crack depth was made on freshly cut surfaces exposed by making longitudinal cuts through the test strip 1 mm. from the edges. 

Antiozonants are special antidegradants used by the rubber industry to protect cured rubber articles from attack by atmospheric ozone, which can be particularly potent if the rubber is unprotected. Those elastomers (NR, SBR, BR, and so on) with carbon-carbon double bonds in the polymer chain are the most susceptible to ozone attack. Ozone attack is more acute with rubber articles that in service are undergoing rapid and repeated deformation (flexing). This attack is worst when the deformation is elongation, and it manifests itself in the formation of surface cracks in the rubber. In service, these cracks will progressively grow and finally result in premature failure of the rubber article. Though observed and suffered for more than a century, this problem was only understood and remedied in the mid-twentieth century. 

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

Waxes function as antiozonants because they tend to be incompatible with the polymer or polymers with which they are combined, and as a result, migrate to the surface to form a barrier which is impermeable to ozone. Hence, the ozone is prevented from reacting with the compound polymers until the wax barrier is broken or cracked by some dynamic performance of the elastomeric compound. Therefore, waxes are generally employed in elastomeric compounds, intended for static apph-cations, but when used in elastomeric compounds, intended for dynamic applications, they are employed for short-term protection only. 

One of the major causes of failure in mbber is the development of cracks at the surface. The growth of these cracks under repeated deformation, or fatigue, leads to catastrophic failure. This fatigue failure is initiated at minute flaws where stresses are high and mechanical mpmre at such points can lead to the development of cracks. Similarly, attack by ozone can cause cracks to occur at the surface whose rate of growth is directly proportional to the ozone concentration. 

Rubber products may be protected against ozone attack by the use of a highly saturated rubber molecule, the use of a wax inhibitor which will "bloom" to the surface, and the use of paper or plastic wrappings to protect the surface. Despite these efforts, rubber products still crack more on the West Coast than on the East Coast of the United States. 

When diene rubbers are exposed to ozone under stressed conditions cracks develop which are perpendicular to the direction of stress. Whilst ozone must react with unstressed rubber no cracking occurs in such circumstances nor when such rubber is subsequently stressed after removal of the ozone environment. For many years such rubbers were protected by waxes which bloomed on to the surface of the rubber to form an impermeable film. This was satisfactory for static applications but where the rubber was operating under dynamic conditions the wax layer became broken and hence less effective. 

Table 11.14 gives critical stress, critical strain, and critical stored energy of NR-EPDM blends for initiation of ozone cracking. All the properties show an increase on addition of TOR, especially the critical stored energy. Stored energy is a strong indicator of ozone resistance and shows an increase by about 80% on addition of 20 parts of TOR to the compound. This was confirmed by SEM pictures of surface ozone cracks and the results of dynamic mechanical moduli and tan d measurement. 

A significant number of papers take advantage of the fact that oxidation of PDMS (either by exposure to UV-ozone or to oxygen plasma) can create a stiff silica-like material [17, 40, 43], During the oxidation process, up to 50% of PDMS in the topmost layer is converted into silica [40], Oxygen plasma treatment was first applied by B owden and coworkers [ 16] to create such a surface layer and use a thermal shrinkage process to create wrinkles. They found patterns with wavelengths between fractions of a micron and 10 p.m. They also reported the appearance of cracks which arise due to the brittle nature of the silica material. 

Antidegmdents. This group of chemicals is added to prevent undesirable chemical reactions with the polymer network. The most important are the antioxidants, which trap free radicals and prevent chain scission and cross-linking. Antiozonants are added to prevent ozone attack on the rubber, which can lead to the formation and growth of cracks. Antiozonants function by diffusion of the material to the surface of the rubber, thereby providing a protective film. Certain antioxidants have this characteristic, and waxes also are used for this purpose. 

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