Ozone cracking reaction

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

Under some circumstances it is possible to observe very low 5 values for polymer solids, namely when the energy to fracture chemical bonds is provided by chemical reaction. An excellent example of this is the ozone cracking of unsaturated hydrocarbon elastomers where crack propagation occurs at threshold values of as low as 0.05 Jm Under such low stresses the toss function of Eq. (5) is effectively... 

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. 

Rubber that is not strained undergoes reaction with ozone to form oxidized film, but it does not show the characteristic ozone cracking. ... 

A review is presented of the literature on the protection of rubber against ozone. Particular attention is paid to the historical background, ozone formation, chemistry of the ozone-rubber reaction, physical requirements for ozone cracking, physical methods of ozone protection, chemical antiozonants, chemical antioxonants for polychloroprene, mechanism of action of chemical antiozonants, chemistry of the reaction of ozone and p-phenylenediamine, free-radical mechanism, and critical stress and antiozonants. 88 refs. USA... 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Although the cracking of mbbers is related to the reaction of ozone on the double bond, it must be mentioned that ozone reacts also with sulfur cross-links. These reactions, however, are much slower. The reaction of ozone with di- and polysulfides is at least 50 times slower than the corresponding reaction with olefins [49]. 

The length and amount of cracks is assessed according to the Bayer method [72,73]. The ISO standard ozone test conditions involve a test temperature of 40°C zE 1°C and an ozone level of 50 5 pphm, with a test duration of 72 h. Testing is done under static [72] and/or dynamic strain [73]. These are accelerated tests and should be used for the relative comparison of compounds, rather than for the prediction of long-term service life. The method is rather complicated and demands a long duration of ozone exposure. Therefore, in some cases the rate constants of the antiozonants reaction with ozone in solution are used instead to evaluate the efficiency of different antiozonants [74]. 

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. 

Recently, a number of municipalities have become so concerned with the problem that they have installed automatic ozone recorders at strategic locations. Some of these instruments are based on the chemical determination of ozone by oxidation of potassium iodide, and colorimetric or electrometric measurement of the extent of the reaction. Others are spectrophotometric instruments a few are based on rubber cracking. The value of the chemical determinations of ozone in the presence of the oxidizing or reducing substances present in polluted atmospheres is questionable. Spectrophotometric methods require a light path of a few hundred feet and cannot be moved easily from one location to another. Determination by rubber cracking is... 

---