Antiozonant mechanisms

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. 

Antiozonant Properties. Aromatic secondary diamines are the only class of organic chemicals able to reduce efficiently the ozone crack growth of vulcanizates under dynamic conditions and be acceptable at the same time from both the technical and toxicological points of view. The presence of a secondary aromatic amine moiety itself in a molecule is not a sufficient condition to attain antiozonants efficiency. (E.g., secondary monomaines are only antioxidants and flex-crack inhibitors without appreciable antiozonant activity. On the other hand, all N,N -disubstituted PD antiozonants are also efficient antoxidants and most of them also act as flex-crack inhibitors (1). Both these stabilization activities have to be considered in the complex antiozonant mechanism, together with some metal deactivating activity. 

An extensive screening of structure-activity relations revealed ( 1 ) the outstanding properties of N,N -disubstituted PD. It is generally accepted that the presence of N-sec.alkyls accounts for better antiozonant protection than that of N-prim. and N-tert.alkyls or N-aryls (21). This may be one of the clues to decipher the chemical pathways of the antiozonant mechanism. The final effect is moreover fully dependent on the composition of the vulcanizate. The structure of commercially used antiozonants is an optimum compromise of efficiency, physical properties and toxicity. N, N -Disec.alkyl-1-4-PD are used in the U.S.A., N-sec.alkyl-N -aryl-l,4-PD are preferred in Europe. N,N -Diaryl derivaties are not applied as antiozonants in NR, BR, IR, or SBR. One of the reasons may be their low solubility in rubber vulcanizates (22). It does not allow them to reach a concentration level in the rubber bulk which is able to act as a long-term operative store of a stabilizer ready to supply the rubber surface slowly but continuously with active compounds by migration and to maintain the protective effect without inefficient quick blooming of an incompatible PD. A chemical reason accounting for the minority antiozonant role of N,N -diaryl PD is discussed later. 

Antiozonant Mechanism. No simple model approach to the explanation of the antiozonant activity of PD is applicable. Most ideas were influenced by the fact that the rubber ozonation is a surface process, not exceeding a thickness of about 40 molecular diameters (23). The reaction of ozone with an antiozonant in the vulcanizate surface layer and replenishment of the consumed stabilizer by means... 

In spite of many objections, ozone scavenger and protective film theories cannot be neglected because of serious experimental evidence. They should be considered as an important part of the overall antiozonant mechanism. Their role prevails in rubber solution or in very thin rubber films. It is very probable that a part of an antiozonant is wastefully depleted just because of direct ozonation. 

Involvement of the Ozonized Rubber Moieties in Antiozonant Mechanism. The rubber chain relinking theory (30) is consistent in part with the self-healing film formation theory (37) a reaction between an antiozonant or some of its transformation products and ozonized elastomer is considered. Either scission of ozonized rubber is prevented in this way or severed parts of the rubber chain are recombined (i.e., relinked). A "self-healing" film resistant to ozonation is formed on the rubber surface. Such a film formed by the contribution of nonvolatile and flexible fragments of the rubber matrix should be more persistent than any film suggested in the protective film theory. 

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... 

Formation of the protective film from ozonized rubber and AOZ creates a barrier against penetration of 03 into rubber [3,4]. Reactions of AOZ with ozonides 3, zwitterions 4 and aldehydic fragments 8 are envisaged. The respective reactivities are lower than the direct ozonation and the relevant contribution to the antiozonant mechanism is therefore inferior to the ozone scavenging. The reactivity of the ozonized rubber with AOZ accounts for rubber chain repairing, classified also as chain relinking or selfhealing mechanisms [4,21,247-249]. This contributes to formation of relaxed surface films. 

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). 

In the 1950s it became recognised that one type of antioxidant also often behaved as an antiozonant. These were the branched alkyl, unsubstituted aryl-/7-phenylenediamines typified by A-isopropyl-A -/ -phenylenediamine (IPPD). The mechanism of their action is still not fully understood but it is to be noted that they are often improved by being used in conjunction with small amounts of hydrocarbon waxes. 

Chemical antiozonants have been developed to protect rubber against ozone under such dynamic conditions. Several mechanisms have been proposed to explain how chemical antiozonants protect rubber. The scavenging mechanism, the protective film mechanism, or a combination of both are nowadays the most accepted mechanisms. 

The scavenging mechanism states that antiozonants function by migrating towards the surface of the rubber and, due to their exceptional reactivity towards ozone, scavenge the ozone before it can react with the rubber [60]. The scavenging mechanism is based on the fact that all antiozonants react much more rapidly with ozone than do the double bonds of the rubber molecules. This fact distinguishes antiozonants from antioxidants. 

The protective film mechanism states that the rapid reaction of ozone with the antiozonant produces a film on the surface of the rabber, which prevents attack on the mbber, like waxes do [63]. 

This mechanism is based on the fact that the ozone uptake of elongated rubber containing a substituted p-phenylene diamine type of antiozonant is very fast initially and then decreases rather rapidly with time and eventually stops almost completely. The film has been studied spectroscopically and shown to consist of unreacted antiozonant and its ozonized products, but no ozonized rubber is involved [64], Since these ozonized products are polar, they have poor solubility in the rubber and accumulate on the surface. 

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. 

Solvent wiping. Rubbers tend to swell by application of solvents and the mechanical interlocking of the adhesive is favored. Although chlorinated hydrocarbon solvents are the most effective, they are toxic and cannot be used toluene and ketones are currently the most common solvents. The treatment with solvents is effective in the removal of processing oils and plasticizers in vulcanized mbbers, but zinc stearate is not completely removed and antiozonant wax gradually migrates to the mbber/polyurethane adhesive interface. Table 27.1 shows the moderate increase in adhesion produced in SBR by MEK wiping. 

Almost all commercial polymers are susceptible to the attack of oxygen or traces of ozone in the atmosphere. Oxidation is the major cause of their discoloration, impairment of mechanical properties, and subsequent failure. It is accelerated by heat or sunlight. Antioxidants and antiozonants are added to the polymers to extend their useful temperature ranges and service lives and to allow outdoor application. Their 1967 consumption for various polymers is given in Table II. 

The relationships of oxidation potential to radical reactivity index Sr and nucleophilic reactivity index Sn illustrated in Figure 4 are very similar to those with antioxidation and antiozonization, where the maximum values were observed at 0.4 and 0.25 volt. Therefore, antioxidation seems to proceed by a radical mechanism in contrast to the nucleophilic type of antiozonization. Indeed, the antioxidation effect of amines toward NR, SBR, BR, and HR is well correlated with radical reactivity as shown in Figures 5-8. The protection of SBR solution by amines from oxidative degradation and the termination of chain reaction in the oxygen-Tetralin system are also shown as functions of Sr in Figures 9 and 10. 

Provides antiozonant and antioxidant properties with high temperature, fatigue and flex resistance to rubber compounds. Used in pneumatic tire components, solid tires, belts, hoses, cables, automotive mounts, bushings and general mechanical products that are exposed to continuous and intermittent dynamic operating conditions that require protection from ozonation. 

Nitroxides and benzoquinonediimines are formed from aromatic amines and diamines respectively as a consequence of amine involvement in antioxidant and/or antiozonant processes. Their participation in antioxidant regenerative mechanisms is suggested. Features of phenylenediamine involvement in antiozonant processes are discussed in relation to contemporary theories. 

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. 

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