Ozone mechanism

FIGURE 15.7 Ozonation mechanism for aryl alkyl-PPDs. (From Hong, S.W. and Lin, C.-Y., Rubber World, 36, August 2000.)... 

It is not difficult to see that ozone initially forms from the oxygen present in the air. Chapman [115] introduced the photochemical model of stratospheric ozone and suggested that the ozone mechanism depended on two photochemical and two chemical reactions ... 

Degradation of organics by indirect ozone mechanism occiu s at basic conditions and involves generation of OH radicals and their further reaction with present organic compormds in water. Hydroxyl radicals are generated by the reaction of ozone with hydroxyl ions present in water [109] ... 

Compared to ozonation of alkenes, much less is known about the ozonation of alkynes,710 which yields 1,2-dicarbonyl compounds, carboxylic acids, and anhydrides. 1,2,3-Trioxolene (91), analogous to 74 in alkene ozonation mechanism (Scheme 9.14), and zwitterionic intermediates (92) were formulated on the basis of IR studies and trapping experiments ... 

The ozonization of olefins yields not only the ozonides (which were first isolated by Harries,82 and whose structures were subsequently elucidated by Staudinger83 and Rieche),2,23 but also other cyclic peroxides. The formation of these products is explained by the ozonization mechanism proposed by R. Criegee64 In 1958, Bailey8 published a review of ozonization reactions in general. 

Products Studies in Antiozonant Mechanisms. Experimental proof of the individual antiozonant theories based on product studies are scarce. Using a very sophisticated instrumental analytical approach, the composition of a very complicated mixture of ozonation products of two technically important antiozonants, i.e., DOPPD and HPPD has been revealed and the reactivity pathways with ozone have been established (24, 40). Influence of the character of N-substituents on the ozonation mechanism has been evidenced. Some important... 

Earlier studies revealed a better antiozonant efficiency of PD substituted with. sec-alkyls than with n-alkyls or tert-alkyls [3,18,247]. A recent elucidation shows that the behaviour of A, 7V -di( -octyl)-l,4-PD closely parallels ozonation mechanism of 11a (Scheme 28), except that the formation of the base 155 was reduced in importance [18,247], N, A"-bis( 1,1 -Dimethylethyl)-1,4-PD bearing /cr/-alkyls was rather unstable and was oxidized with oxygen even in the absence of 03. 

Comprehending corrosion as failure of materials under the action of the environment, it would be justified to relate this term to fracture of polymer materials under the action of ambient factors, including elevated temperatures, ionizing radiation, UV radiation, ozone, mechanical loads, chemical and biological matter. 

Reverse 2-stage 1-stage Ozonation Mechanical osmosis AC filter AC filter filter... 

The most common antiozonant from the class, A -alkyl-A -aryl-p-phenylenedia-mine, is A -(l,3-dimethylbutyl)-A -phenyl-p-phenylenediamine (6PPD), and the ozonation mechanism reported in the literature for this class of antiozonant is shown on the following page in a simplified form. The antiozonant reacts with ozone. Step (1), and proceeds through the formation of an unstable intermediate which rearranges to... 

The ozonation mechanism for the iV-iV -diaryl-p-phenylenediamine antiozonants is identical to the above described mechanism except that the reaction terminates with the formation of the nitrone and the release of oxygen after reacting with the ozone, Step (3) [4]. 

The Ozonation Mechanism for an iV-AIkyl-iV -Aryl-/ -Phenylenediamine [(iV-(l,3-Dimethylbutyl)-A -Phenyl-/>-Phenylenedianiine] (6PPD)... 

The Ozonation Mechanism for an -iV -Dialkyl-p-Phenylenediamine, [Af-Af -Bis (l,4-Dimethylpentyl)-p-Phenylenedianiine] (77PD)... 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Fig. 2. Examples of ozone isopleths generated using four different chemical mechanisms (a) EPA (b) ESM (c) CBII (d) ELSTAR NMOC = nonmethane...

Aqueous Phase. In pure water, the decomposition of ozone at 20°C iavolves a complex radical chain mechanism, initiated by OH and propagated by O2 radical ions and HO radicals (25). O3 is a radical ion. 

The two main termination steps for neutral solutions are HO + HO — H2O2 + 2 O3 and HO + HO3 — H2O2 + O3 + O2. An alternative mechanism has been proposed that does not involve HO and HO but has a different initiation step (26). Three ozone molecules are destroyed for each primary event. In the presence of excess HO radical scavengers, ie, bicarbonate, the pseudo-first-order rate constant at 20°C for the initiation step is 175 X. This yields an ozone half-hfe of 66 min at pH 8. In distilled water = 50 mmol/L), the half-hfe is significantly lower, ie, 7 min. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The following mechanisms are more important for explaining ozone destmction at lower altitudes, where the avadabiLity of oxygen atoms is limited ... 

The prime installation method is mechanically fastened but fully adhered and ballasted appHcations can also be used. CSPE exhibits strong resistance not only to weathering but also to a broad range of chemicals and pollutants it is also inherently ozone-resistant. It can be produced in many colors and the sheet widths are typically 5—6.5 ft (1.5—1.65 m). The physical characteristics of a CSPE sheet have been described (17) (see Elastomers, SYNTHETIC-Cm OROSULFONATED POLYETHYLENE). 

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

The hterature suggests that more than one mechanism may be operative for a given antiozonant, and that different mechanisms may be appHcable to different types of antiozonants. All of the evidence, however, indicates that the scavenger mechanism is the most important. All antiozonants react with ozone at a much higher rate than does the mbber which they protect. The extremely high reactivity with ozone of/)-phenylenediamines, compared to other amines, is best explained by their unique abiUty to react ftee-tadicaHy. The chemistry of ozone—/)-PDA reactions is known in some detail (30,31). The first step is beheved to be the formation of an ozone—/)-PDA adduct (32), or in some cases a radical ion. Pour competing fates for dissociation of the initial adduct have been described amine oxide formation, side-chain oxidation, nitroxide radical formation, and amino radical formation. 

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). 

The first use for butyl mbber was ia inner tubes, the air-retention characteristics of which contributed significantly to the safety and convenience of tires. Good weathefing, ozone resistance, and oxidative stabiUty have led to appHcations ia mechanical goods and elastomeric sheeting. Automobile tires were manufactured for a brief period from butyl mbber, but poor abrasion resistance restricted this development at the time. 

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