Effect from Multiple Factors

The contrast between the reaction mechanisms of oxygen and ozone lies in their spatial characteristics. Ozone primarily undergoes a surface reaction, whereas oxygen can engage in both surface and internal reactions.

Effect of Oxidation:

Upon oxidation, polymer systems can undergo either softening and tackiness or hardening and brittleness. This is a result of either polymer chain scission (leading to softening) or cross-linking (resulting in hardening). Natural and synthetic polyisoprene, butyl, and “G” type neoprene polymers tend to experience chain scission and become soft and tacky. Nitrile, SBR, and PBR are examples of polymer systems that tend to harden or undergo cross-linking.

Non-polar polymer systems such as EPR/EPDM, silicone/fluorosilicone, and FKM have limited reactive sites available for oxidation or ozonation to occur. Generally, these saturated polymer systems do not require the addition of anti-degradants. In fact, non-polar polymers can serve as anti-degradants themselves. When a non-polar polymer is blended with a polar polymer system, the non-polar polymer offers enhanced protection to the rubber article against oxidation. For instance, blending 25 to 30 phr EPDM with natural or synthetic polyisoprene significantly delays the oxidative process.

Effect of Heat:

Elevated temperatures play a significant role in the degradation of rubber products. Heat has detrimental effects on rubber and accelerates oxidation. Generally, two conditions contribute to increased temperatures in rubber articles: curing and dynamic flex.

Curing: Certain antidegradants are specifically designed to protect uncured rubber during mixing and storage. However, these antidegradants exhibit reduced efficacy at elevated curing temperatures. For instance, phosphite antioxidants, which are highly efficient as polymer stabilizers, are consumed during higher vulcanization temperatures.
Dynamic Flex: This refers to the stress applied to a rubber product during its intended use. Many antiozonants and specific amines are formulated to provide anti-flex cracking properties to rubber products.

Elevated temperatures can limit the free radical scavenging ability of certain groups of antidegradants. For example, hindered phenols are capable of providing long-term thermal stability within a wide temperature range of 0–575ºF. On the other hand, phosphites, hydroxylamines, and thiosynergists lose their effectiveness during vulcanization. Thiosynergists demonstrate high effectiveness in scavenging free radicals or decomposing hydroperoxides during polymer processing, but they are not suitable for long-term thermal stabilization. By sacrificially reacting during the curing process, thiosynergists alleviate the burden on the more thermally stable antidegradants.

Effect of UV – Light & Weathering:

UV light induces free radical oxidation on the surface of rubber, leading to the development of an oxidized rubber film known as “Frosting.”
This process is expedited by elevated temperatures and high humidity levels.
UV light has a greater impact on light-colored rubber products than on black-colored ones, primarily because carbon black, which is present in black rubber, functions as a UV light absorber.
Rubber products with a thin cross-section experience more pronounced UV light attack.

Effect of Metallic Ions:

Metal ions have the potential to react with oxygen radicals and adversely affect rubber by acting as “poisons.” Among these metal ions, copper, manganese, cobalt, and iron are particularly known to catalyze the oxidative process in rubber. They facilitate the breakdown of peroxide radicals, thereby accelerating oxidation. Nevertheless, the detrimental catalytic effect of metal ions can be counteracted by employing metal deactivators.. Certain antidegradants possess the ability to chelate metal ions, reducing their capacity to generate radicals. These antidegradants, known as “metal deactivators,” can be either antioxidants or antiozonants.

Effect of Ozone:

Longer-wavelength UV light undergoes photolysis of Nitrogen Dioxide, yielding Oxygen atoms (O) and Nitrogen Oxide. Subsequently, the Oxygen atoms combine with atmospheric Oxygen molecules to generate Ozone. In unpolluted regions, ozone concentration typically ranges from 2 to 5 parts per hundred million (pphm), while in more polluted areas, it can reach levels of 40 to 50 pphm.

Ozone is also formed in the stratosphere through the action of short-wavelength UV light on Oxygen. Although the upper atmosphere absorbs most of the flux of short-wavelength UV light, the presence of Nitrogen Oxide ensures a substantial ozone concentration in the troposphere.

Oxygen atoms released during the photolysis of oxygen molecules also combine with additional oxygen molecules to form ozone. Ozone concentration in the atmosphere peaks around midday due to the photolytic reaction and diminishes during nighttime.

The absorption of ozone by elastomers occurs at a linear rate. The degree of ozone absorption is proportional to the ozone concentration. When the rubber surface is not stressed, it reacts with ozone to form an oxidized film but does not exhibit typical ozone cracks.

Ozone-induced crack growth occurs only when the stress exceeds a specific threshold known as the “Critical Stress Value.” At stress levels just above this threshold, the number of ozone cracks is limited but they tend to be large in length and depth. As stress increases to higher levels, the number of ozone cracks increases while their size becomes finer.

Ozone reacts not only with the numerous double bonds present in highly unsaturated rubbers but also with saturated polymers and polysulfide chains, albeit at a slower rate. Unsaturated polymers containing electron-donating groups (e.g., methyl groups in NR) are more susceptible to ozone attack. Conversely, unsaturated polymers containing electron-withdrawing groups (e.g., chlorine in CR, bromine in BIIR) are less vulnerable to ozone attack due to the deactivating effect of halogen atoms on the double bonds.

The reaction between ozone and double bonds in rubber molecules leads to chain scission, resulting in the formation of surface cracks perpendicular to the applied strain.