Anti-Ozonant

The term “antiozonant” refers to any additive that protects rubber from ozone deterioration. The protective effect typically arises from a reaction with ozone, known as a chemical antiozonant. Ozone is naturally produced through electrical discharge and solar radiation in the stratosphere, resulting in ground-level ozone concentrations of 1-5 parts per hundred million (pphm). However, in urban environments, ozone levels can significantly increase, reaching up to 25 pphm due to the ultraviolet photolysis of pollutants. Even a small amount of ozone in the air, just a few parts per hundred million, can cause rubber cracking, rendering elastomer products ineffective. Desirable properties of antiozonant additives include:

A physical antiozonant should create an effective barrier on the rubber surface to prevent ozone penetration. This barrier should be continuous, unreactive, impermeable to ozone, and capable of self-renewal if damaged, such as through abrasion. It should also exhibit flexibility and extensibility under dynamic stress conditions. Conversely, a chemical antiozonant must exhibit high reactivity with ozone. If a compound is not reactive enough, it will not provide sufficient protection. However, an antiozonant should not be overly reactive with oxygen, as it needs to persist in the rubber to offer long-term ozone protection.
The antiozonant should possess suitable solubility and diffusivity characteristics. Since ozone attack primarily occurs at the surface, the antiozonant must migrate to the rubber surface to provide protection. Insufficient solubility in rubber can lead to problems such as excessive bloom or inadequate levels of antiozonant for long-term protection. The rate of diffusion to the exposed surface should be high enough to counteract the influx of ozone while maintaining a reservoir within the bulk of the rubber throughout the product’s useful lifespan. Antiozonants that are bound and unable to continuously diffuse and replenish themselves on the surface are not suitable.
The antiozonant should not negatively impact the rubber processing characteristics, including mixing, fabrication, vulcanization, and physical properties. This is particularly important for general-purpose rubbers, as the antiozonant must be compatible with sulfur curing systems.
The antiozonant should effectively protect the rubber under both static and dynamic conditions, spanning a wide range of extension and temperature conditions.
The antiozonant should persist in the rubber throughout its entire life cycle. It should demonstrate resistance to oxidation, vaporization, or extraction by water or other solvents.
For non-carbon black-filled rubbers, the antiozonant must not cause discoloration or staining.
The antiozonant should have low toxicity and be nonmutagenic.
The antiozonant should be economically viable, with low manufacturing costs, and usable at low bulk concentration levels.

To protect rubber against ozone attack, physical and/or chemical antiozonants are added. Hydrocarbon waxes are commonly used as physical antiozonants, while p-phenylenediamine derivatives are prevalent as chemical antiozonants.

Ozone degradation can lead to the deterioration of rubbers and resins. Ozone reacts directly with the carbon-carbon double bonds in these materials, causing chain scission and the formation of decomposition products. Ozone cracking occurs when the polymer is stretched, exposing the double bonds to ozone. However, without stretching, cracks do not develop as the underlying double bonds remain unexposed.

Different rubbers exhibit varying degrees of resistance to ozone. Highly unsaturated rubbers like natural rubber, styrene-butadiene rubber, butyl rubber, and nitrile rubber are more susceptible to cracking, while deactivated double carbon-carbon bond rubbers like polychloroprene rubber demonstrate moderate ozone resistance.

To effectively prevent ozone cracking, antiozonants should fulfill two key functions: reducing the rate of crack growth in the rubber and increasing the critical stress value at which crack growth occurs. Therefore, desirable properties of antiozonants include:

Physical antiozonants should create a continuous, unreactive, and impermeable barrier on the rubber surface, capable of self-renewal if damaged.
Chemical antiozonants should exhibit high reactivity with ozone while avoiding excessive reactivity with oxygen.
Antiozonants should have suitable solubility and diffusivity characteristics, allowing them to migrate to the rubber surface for protection. Poor solubility can lead to excessive bloom.
Antiozonants should not negatively impact rubber processing.
Antiozonants should be effective under both static and dynamic conditions.
Antiozonants should persist in the rubber throughout its entire lifespan.
Antiozonants should not cause discoloration or staining.
There are three methods to prevent ozone attack on rubber: coating the surface, adding chemical antioxidants, and relieving internal stresses by incorporating ozone-resistant polymers.

Hydrocarbon waxes are commonly used as physical antiozonants, forming a protective barrier by blooming to the rubber surface. However, their protection is limited to static conditions as the wax barrier can be broken by flexing. In situations where continuous flexing is expected, chemical antiozonants such as derivatives of p-phenylenediamine can be incorporated. These antiozonants scavenge ozone, creating a barrier of ozonized products that protect both the rubber and the antiozonant. However, p-phenylenediamines are known to cause staining. In cases where color is important, blends of elastomers with higher loadings (above 30 phr) can be used to ensure sufficient effectiveness.

Typical loading for antiozonants in rubber formulations ranges from 1.5 to 3 phr. Combinations of waxes and chemical antiozonants are employed when service conditions involve both static and dynamic stresses over extended periods.