Antiozonant or antioxidant?

Three kinds of ageing

Oxygen ageing slowly affects all of the rubber; heat accelerates this process and the rubber hardens or softens. Ozone attacks only surfaces under strain and opens cracks perpendicular to the strain. Dynamic fatigue is crack growth through repeated deformation.

Each needs different protection. For this reason, tire and technical part compound formulations usually contain two or three antidegradants together.

Antioxidants

TMQ, benzimidazoles (MB, MBZ) and diphenylamine derivatives (KY-405) slow oxygen and heat ageing. They work inside the rubber.

TMQ is the most common and economical. MB is weak on its own but works synergistically with amine antioxidants and does not stain. Low-volatility types such as KY-405 are not lost in parts working at high temperature.

Antiozonants

p-Phenylenediamines (6PPD, IPPD, 77PD) migrate to the surface and react with ozone before the rubber does. They also protect against fatigue and heat.

In static parts, microcrystalline wax is added for ozone protection: the wax forms a physical layer on the surface. In dynamic parts the wax cracks; there a chemical antiozonant is essential.

Color

p-Phenylenediamines change color with light and stain the surfaces they touch. This is not a problem for black compounds. In light-colored products, phenolic antioxidants (SP), benzimidazoles and waxes are used; ozone protection remains more limited.

6PPD-quinone

When 6PPD reacts with ozone, 6PPD-quinone forms. This substance has been shown to be toxic to some fish species; an environmental debate continues over tire wear residues.

At present there is no widely available substitute that fully matches the performance of 6PPD. The industry and regulators are researching alternatives; developments need to be followed.

Summary