Cracking by Ozone

In an atmosphere containing ozone, stretched samples of unsaturated elastomers develop surface cracks that grow in length and depth until they eventually sever the test piece. Even when they are quite small, they can cause a serious reduction in strength and fatigue life. Quite small concentrations of ozone will induce cracking, given sufficient time. The effect is so sensitive, in fact, that it is the basis for an ozone detection system (Mott and Roland, 1999). 

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 that 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 controlling. 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. (10.32) is then dominant for values of (pr greater than about 10 s that is, at temperatures more than 25°C above Tg (Gent and McGrath, 1965). 

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Degradation of rubbers and resins can also be produced by ozone attack. Ozone directly reacts with, and cleaves, the carbon-carbon double bonds of rubbers and resins. Thus only polymers with backbone unsaturation will be cracked by ozone. Unlike oxidation, ozone attack cannot be accelerated by increasing the... 

In contrast to this, very little is known about the ozonolysis of olefins which bear halogen substituents at the double bond. This is somewhat surprising since compounds containing vinyl halide moieties are important technical products whose properties could be adversely affected by ozone degradation. A case in point is neoprene rubber, whose performance as an elastomer could suffer considerably by ozone attack and concurrent crack formation (1). 

NR/EPDM blends are also used for making rubber boots for agriculture. These boots will have more resistance against cracks caused by ozone. A ratio of 25 75 to 40 60 parts by wt of NR/EPDM is used for preparing these kinds of boots. 

Because of their chemically saturated structure, FKM exhibit excellent weathering resistance to sunlight and especially to ozone. After 13 years of exposure in Florida in direct sunlight, samples showed little or no change in properties or appearance. Similar results were experienced with samples exposed to various tropical conditions in Panama for a period of 10 years. Products made of this elastomer are unaffected by ozone concentrations as high as 100 ppm. No cracking occurred in a bent loop test after one year of exposure to 100 ppm of ozone in air at 100°F (38°C) or in a sample held at 356°F (180°C) for several hundred hours. This property is particularly important considering that standard tests, for example, in the automotive industry, require resistance only to 0.5 ppm ozone. 

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. 

B) Ozone cutting or ozone checking (that is, a series of interlacing cracks produced by ozone on rubber under mechanical stress) ... 

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