Surface ozone attack

Ozone is a major atmospheric pollutant in urban areas. In addition to its damaging effect on lung tissue and even on exposed skin surfaces, ozone attacks the rubber of tires, causing them to become brittle and crack. But in the stratosphere, where ozone absorbs much of the short-wavelength UV radiation from the sun, it provides a vital protective shield for life forms on earth. 

The antiozonant should possess adequate solubiUty and diffusivity characteristics (19). Siace ozone attack is a surface phenomenon, the antiozonant must migrate to the surface of the mbber to provide protection. The antiozonant should have no adverse effects on the mbber processiag characteristics, eg, mixing, fabrication, vulcanization, or physical properties. 

The relinking (14) and self-healing film (3) theories require chemical interaction between the antiozonant and ozonized mbber. The evidence for these interactions is meager (35,36). Overall, there seems to be no clear evidence in the Hterature for PDA derivatives becoming attached to mbber chains as a result of ozone attack. Much fundamental work in this area remains to be done, however. It seems clear at this point that any antiozonant—mbber interaction must be much less important than the scavenging effect of the antiozonant. In summary, the scavenger model is beheved to be the principal mechanism of antiozonant action. Ozone—antiozonant reaction products form a surface film that provides additional protection (37). 

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. 

This situation is identical to the previous one and occurs for example when paraffin wax is mixed into rubber above the melting point of the wax. On cooling, the wax starts to crystallise, some of it forming a bloom on the rubber surface. Such a bloom assists in protecting a diene rubber from ozone attack. 

Ozone attack of rubbers can be prevented in three ways (1) coating the surface (2) adding a chemical antioxidant (3) relieving internal stresses by adding ozone-resistant polymers. 

Rubber is protected against ozone attack by addition of physical and/or chemical antiozonants. Hydrocarbon waxes are the most common type of physical antiozonants, and p-phenylenediamine derivatives are the prevalent chemical antiozonants. Waxes bloom to the rubber surface and form a protective barrier. 

The most effective antiozonants are the substituted PPDs. Their mechanism of protection against ozone is based on the scavenger-protective film mechanism [68-70]. The reaction of ozone with the antiozonant is much faster than the reaction with the C=C bond of the rubber on the rubber surface [56]. The rubber is protected from the ozone attack tUl the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the mbber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. A thin flexible film developed from the antiozonant/ozone reaction products on the mbber surface also offers protection. 

Since ozone attack on rubber is essentially a surface phenomenon, the test methods involve exposure of the rubber samples under static and/or dynamic strain, in a closed chamber at a constant temperature, to an atmosphere containing a given concentration of ozone. Cured test pieces are examined periodically for cracking. 

In our study we have found that UV-light greatly accelerates the rate of ozone attack at the polypropylene surface. Presented results were undertaken to determine mechanism of the photo-oxidation of polypropylene surface upon UV-irradiation of polymer films in ozone. 

Retardation of oxidative degeneration and the effects of ozone attack can be mitigated but not totally overcome by the use of chemicals which, unfortunately, in the case of the most effective types, carry the penalty of causing staining of the rubber compound or surfaces with which it comes into contact. 

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. 

Ozone (qv) reacts with double bonds so rapidly that it has no chance to diffuse into the mbber and therefore all action is at the surface. Thus surface-protective agents are most useflil against ozone attack. For example, waxes that bloom to the surface of mbber to form an inert film are used effectively for static protection (34). 

The ideal measure will be that the rubber lined surface is always kept wet with water in order to arrest the likely deterioration from ultraviolet rays and ozone attack during storage. 

There is strong observational evidence that tropospheric ozone can be destroyed by reactions in addition to those discussed so far. Surface ozone observations during polar sunrise in the Arctic have frequently shown the occurrence of unmeasurably low ozone concentrations, coinciding with high filterable Br" (31). Further measurements (32) identified BrO as one of the active Br compounds, which, as is well known from stratospheric measurements, may rapidly attack ozone by series of catalytic reactions, such as... 

Ozone-nicotine surface chemistry has recently been identified by Destaillats et al. (2006a). Nicotine and other products of tobacco smoking adsorb strongly to indoor surfaces. In the presence of ozone, surface nicotine can form a variety of products (Scheme 13.7). On cotton, high humidity inhibits this reaction. Ozone attacks the pyrrolidinic N, suggesting that other indoor surface bound species with this functionality may be oxidized in this manner. Not only does this chemistry generate volatile by-products, it may explain why indoor nicotine concentrations correlate poorly with exposure to tobacco smoke. This raises the question, what other, similarly functionalized, surface amines may also react with ozone in indoor environments ... 

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. 

Ozone attack on the oxidatively violated rubber surface is thus prevented (21, 27, 29, 30). Creation of the surface layer was confirmed using microscopy. The theory is in agreement with the observation that the initial rapid ozone consumption is stabilized rubber drops and may be renewed after mechanical break of the formed film 21). There is a chemical proof of the theory. Ozonation products of N,N -bis(l-methylheptyl)-l,4-PD (DOPPD) form a surface film on ozonized and DOPPD doped vulcanized NR (31) and carbon-black loaded NR (24, 32). In addition to unreacted DOPPD, many of the low-molecular weight compounds observed in the film were found also in the ozonation of pure DOPPD (24) the only difference was a... 

The requirements for an antiozonant are believed to be more stringent. Since ozone attack is essentially a surface phenomenon, it is assumed (44-46) that an antioxidant can only be effective in the surface of the rubber. However, it has been found (47) that extraction makes very little difference to the antiozonant activity of MADA-B in rubber (see Table 16). The mechanism of antiozonant action may therefore require some revision. 

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. 

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). 

Ethylene-propylene rubber is particularly resistant to sun, weather, and ozone attack. Excellent weather resistance is obtained whether the material is formulated in color, white, or black. The elastomer remains free of surface crazing and retains a high percentage of its properties after years of exposure. Ozone resistance is inherent in the polymer, and for all practical purposes, it can be considered immune to ozone attack. It is not necessary to add any special compounding ingredients to produce this resistance. 

Ozone will attack any elastomer with backbone unsaturation. Degradation results from the reaction of ozone with rubber double bonds. Unstretched rubber reacts with ozone but is not cracked. In this case, since only the double bonds at the surface are attacked, the degradation is confined to a thin surface layer (0.5 m thick). Occasional "frosting" (a white or gray bloomlike appearance) may occur in transparent unstretched rubbers, but elongation is required to induce the characteristic ozone cracking. 

As the stressed polymer chains cleave under ozone attack, new high-stress surface is exposed. The localized continuation of this process results in visible... 

Ozone attack leads to chain scission and the formation and propagation of cracks. As crack growth proceeds, fresh surfaces are continuously exposed for further ozone attack. Degradation continues until failure relieves the inherent stress. It has been calculated that only about 1 % of all the ozone that reacts with the rubber is responsible for the formation of cracks. 

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