Antiozonants

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. 

Antiozonants are a very large part of the rubber antidegradant market. Many times antiozonants are used together with antioxidants in rubber compounds to achieve the best synergistic protection from oxidative degradation while the rubber product is in use. Virtually all chemical antiozonants can act, at least to some extent, as antioxidants. 

Antiozonants are important for rubber products, especially when they are subjected to periodic deformations. Ozone directly destroys the surface of rubber. The attack of ozone on polydienes seems to involve reactions such as below. 

The most important antiozonants are aromatic amines with structures such as 

The chemical mechanism of the antiozonants would seem like antioxidants to be free radical in nature. 

Diene-type rubbers are particularly sensitive to ozone attack and should be protected by antiozonants. Common antiozonants are aromatic diamines capable of direct ozone scavenging by adduct formation. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

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. 

A physical antiozonant must provide an effective barrier against the penetration of ozone at the mbber surface. A chemical antiozonant, on the other hand, must first of all be extremely reactive with ozone. 

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 antiozonant should be effective under both static and dynamic conditions. 

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. 

Since the mid-1950s several materials have been found effective in combating ozone-initiated degradation, in particular certain p-phenylenediamine derivatives. The actual choice of such antiozonants depends on the type of polymer and on whether or not the polymer is to be subject to dynamic stressing in service. Since antiozonants are not known to have any use in plastics materials, even those which may have certain rubber particles for toughening, they will not be dealt with further here. Anyone interested further should consult references 3-5. 

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