Ozone, surface cracking

This transitory behavior was observed to arise from all the weathering agents considered in this study except ozone. Instead, test coupons exposed to ozone exhibited an initial decline in the crosslink density of the silicone with the formation of surface cracks, which were difficult to distinguish with the naked eye. With continued exposure to ozone, however, the material would begin to crosslink. We proposed that ozone s greatest affinity... 

Ozone attack occurs mainly at the olefinic double bond of a diene rubber and, if not protected against, will result in loss of physical integrity for thin sectioned articles and surface cracking on larger mass products. 

The ozone concentration in the atmosphere is only a few pphm. In certain chemical plants as in electrolytic mercury cell houses in the chloralkali industry, the ozone concentration is higher. Although the atmospheric ozone level is low, it reacts with rubber double bonds rapidly and causes cracking of rubber products. Especially when rubber is under stress (stretching and bending as in the case of flexible cell covers), the crack development is faster. Neoprene products resist thousands of parts per hundred million of ozone for hours without surface cracking. This nature of neoprene is quite suitable for cell house application in chlor-alkali industries. Natural rubber will crack within minutes when subjected to ozone concentration of only 50 pphm. 

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

Typical examples of stress corrosion are crack formation in strained rubber vulcanizates under the influence of ozone, hair cracks in PE under stress in the presence of a surface active agent (see also 7.4.2), crack formation in PC, when exposed to e.g. CCI4, within a few seconds after the application of a small stress. 

Experimental 1x6 inch test specimens of the unprotected and protected vulcanizates were elongated 20% and ozonized for 15 minutes at room temperature, using a stream of ozonized oxygen flowing at 0.10 cubic meter per hour and furnishing approximately 1 X 10 mole of ozone per hour. The test strips were examined visually during ozonization to determine the time required to obtain initial surface cracking. 

Due to the high reactivity of ozone with unsaturated hydrocarbons moieties, surface cracking of stressed or flexed NR, BR, NBR, and SBR vulcanizates arises. Rubber goods designed for outdoor applications must therefore be stabilized against both C>2 and 0 attacks. Antioxidant protection mechanisms have been discussed in detail ( 1). Discussions dealing with antiozonant mechanism involve some contradictory experimental observations. 

The role of ozone is also very important in the degradation of rubber. It was found that on exposing a stressed rubber sample to ozone, small cracks formed on the surface. These cracks are perpendicular to the direction of applied stress [450]. The velocity of cracking depends on the chemical structure of the polymer, time of exposure, magnitude of applied stress, content of plasticizers, and so on [13, 14, 111, 186, 239, 360, 361, 553, 554]. Several workers [13, 184] have indicated that the ozonization of diene elastomers is accompanied by an autocatalytic... 

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). 

In general, polymeric materials which have double bonds in their polymer chain are susceptible to ozone attack. The results of optical microscope studies with the ozone treated sample specimens did not reveal any surface crack(Table 3). 

Neoprene displays excellent resistance to sun, weather, and ozone. Because of its low rate of oxidation, products made of neoprene have high resistance to both outdoor and indoor aging. Over prolonged periods of time in an outdoor environment, the physical properties of neoprene display insignificant change. If neoprene is properly compounded, ozone in atmospheric concentrations has little effect on the product. When severe ozone exposure is expected, as for example around electrical equipment, compositions of neoprene can be provided to resist thousands of parts per million of ozone for hours without surface cracking. Natural rubber will crack within minutes when subjected to ozone concentrations of only 50 ppm. 

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... 

Initially transparent polybutadiene develops micron-sized surface cracks when stretched and exposed to ozone. The... 

While exponential crack growth occurs at increased eccentricity, a linear relationship exists between static deformation and the characteristics of crack condition, showing that, after three months of weathering, ozone crack depth increases with an increase in static strain from 10 to 20%. Approx. 1 mm deep surface cracks replace the hairline cracks. Figure 5.107 [698]. 

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. 

Typically, ozone cracking initiates at sites of high stress (flaws) on the mbber surface. Thus, in general, mbber articles should be designed to rninirnize potential sites of high elongation such as raised lettering. Similarly, the use of clean molds helps to reduce the incidence of surface flaws. 

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. 

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). 

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